NAME
gcc - GNU project C and C++ compiler
SYNOPSIS
gcc [
-c|
-S|
-E] [
-std=standard]
[
-g] [
-pg] [
-Olevel]
[
-Wwarn...] [
-Wpedantic]
[
-Idir...] [
-Ldir...]
[
-Dmacro[=
defn]...] [
-Umacro]
[
-foption...] [
-mmachine-option...]
[
-o outfile] [@
file]
infile...
Only the most useful options are listed here; see below for the remainder.
g++ accepts mostly the same options as
gcc.
DESCRIPTION
When you invoke GCC, it normally does preprocessing, compilation, assembly and
linking. The "overall options" allow you to stop this process at an
intermediate stage. For example, the
-c option says not to run the
linker. Then the output consists of object files output by the assembler.
Other options are passed on to one stage of processing. Some options control the
preprocessor and others the compiler itself. Yet other options control the
assembler and linker; most of these are not documented here, since you rarely
need to use any of them.
Most of the command-line options that you can use with GCC are useful for C
programs; when an option is only useful with another language (usually C++),
the explanation says so explicitly. If the description for a particular option
does not mention a source language, you can use that option with all supported
languages.
The
gcc program accepts options and file names as operands. Many options
have multi-letter names; therefore multiple single-letter options may
not be grouped:
-dv is very different from
-d -v.
You can mix options and other arguments. For the most part, the order you use
doesn't matter. Order does matter when you use several options of the same
kind; for example, if you specify
-L more than once, the directories
are searched in the order specified. Also, the placement of the
-l
option is significant.
Many options have long names starting with
-f or with
-W---for
example,
-fmove-loop-invariants,
-Wformat and so on. Most of
these have both positive and negative forms; the negative form of
-ffoo
is
-fno-foo. This manual documents only one of these two forms,
whichever one is not the default.
OPTIONS
Option Summary
Here is a summary of all the options, grouped by type. Explanations are in the
following sections.
- Overall Options
- -c -S -E -o file
-no-canonical-prefixes -pipe -pass-exit-codes -x
language -v -### --help[=class[,...]]
--target-help --version -wrapper @file
-fplugin= file
-fplugin-arg-name=arg
-fdump-ada-spec[-slim] -fada-spec-parent=unit
-fdump-go-spec=file
- C Language Options
- -ansi -std=standard -fgnu89-inline
-aux-info filename
-fallow-parameterless-variadic-functions -fno-asm -fno-builtin
-fno-builtin- function -fhosted -ffreestanding -fopenacc
-fopenmp -fopenmp-simd -fms-extensions -fplan9-extensions
-trigraphs -traditional -traditional-cpp -fallow-single-precision
-fcond-mismatch -flax-vector-conversions -fsigned-bitfields
-fsigned-char -funsigned-bitfields -funsigned-char
- C++ Language Options
- -fabi-version=n -fno-access-control
-fcheck-new -fconstexpr-depth=n
-ffriend-injection -fno-elide-constructors
-fno-enforce-eh-specs -ffor-scope -fno-for-scope
-fno-gnu-keywords -fno-implicit-templates
-fno-implicit-inline-templates -fno-implement-inlines
-fms-extensions -fno-nonansi-builtins -fnothrow-opt
-fno-operator-names -fno-optional-diags -fpermissive
-fno-pretty-templates -frepo -fno-rtti -fsized-deallocation
-fstats -ftemplate-backtrace-limit=n
-ftemplate-depth= n -fno-threadsafe-statics
-fuse-cxa-atexit -fno-weak -nostdinc++
-fvisibility-inlines-hidden
-fvtable-verify=[std|preinit| none]
-fvtv-counts -fvtv-debug -fvisibility-ms-compat
-fext-numeric-literals -Wabi=n -Wabi-tag
-Wconversion-null -Wctor-dtor-privacy -Wdelete-non-virtual-dtor
-Wliteral-suffix -Wnarrowing -Wnoexcept -Wnon-virtual-dtor
-Wreorder -Weffc++ -Wstrict-null-sentinel
-Wno-non-template-friend -Wold-style-cast -Woverloaded-virtual
-Wno-pmf-conversions -Wsign-promo
- Objective-C and Objective-C++ Language Options
- -fconstant-string-class=class-name
-fgnu-runtime -fnext-runtime -fno-nil-receivers
-fobjc-abi-version= n -fobjc-call-cxx-cdtors
-fobjc-direct-dispatch -fobjc-exceptions -fobjc-gc
-fobjc-nilcheck -fobjc-std=objc1 -fno-local-ivars
-fivar-visibility=[
public|protected|private|package]
-freplace-objc-classes -fzero-link -gen-decls
-Wassign-intercept -Wno-protocol -Wselector
-Wstrict-selector-match -Wundeclared-selector
- Language Independent Options
- -fmessage-length=n
-fdiagnostics-show-location=[ once|every-line]
-fdiagnostics-color=[ auto|never|always]
-fno-diagnostics-show-option -fno-diagnostics-show-caret
- Warning Options
- -fsyntax-only -fmax-errors=n
-Wpedantic -pedantic-errors -w -Wextra -Wall -Waddress
-Waggregate-return -Waggressive-loop-optimizations -Warray-bounds
-Warray-bounds= n -Wbool-compare -Wno-attributes
-Wno-builtin-macro-redefined -Wc90-c99-compat -Wc99-c11-compat
-Wc++-compat -Wc++11-compat -Wc++14-compat -Wcast-align -Wcast-qual
-Wchar-subscripts -Wclobbered -Wcomment -Wconditionally-supported
-Wconversion -Wcoverage-mismatch -Wdate-time -Wdelete-incomplete
-Wno-cpp -Wno-deprecated -Wno-deprecated-declarations
-Wno-designated-init -Wdisabled-optimization
-Wno-discarded-qualifiers -Wno-discarded-array-qualifiers
-Wno-div-by-zero -Wdouble-promotion -Wempty-body -Wenum-compare
-Wno-endif-labels -Werror -Werror=* -Wfatal-errors -Wfloat-equal
-Wformat -Wformat=2 -Wno-format-contains-nul -Wno-format-extra-args
-Wformat-nonliteral -Wformat-security -Wformat-signedness
-Wformat-y2k -Wframe-larger-than=len
-Wno-free-nonheap-object -Wjump-misses-init -Wignored-qualifiers
-Wincompatible-pointer-types -Wimplicit
-Wimplicit-function-declaration -Wimplicit-int -Winit-self -Winline
-Wno-int-conversion -Wno-int-to-pointer-cast
-Wno-invalid-offsetof -Winvalid-pch -Wlarger-than=len
-Wunsafe-loop-optimizations -Wlogical-op
-Wlogical-not-parentheses -Wlong-long -Wmain -Wmaybe-uninitialized
-Wmemset-transposed-args -Wmissing-braces
-Wmissing-field-initializers -Wmissing-include-dirs
-Wno-multichar -Wnonnull -Wnormalized=[
none|id|nfc| nfkc]
-Wodr -Wno-overflow -Wopenmp-simd -Woverlength-strings -Wpacked
-Wpacked-bitfield-compat -Wpadded -Wparentheses
-Wpedantic-ms-format -Wno-pedantic-ms-format -Wpointer-arith
-Wno-pointer-to-int-cast -Wredundant-decls
-Wno-return-local-addr -Wreturn-type -Wsequence-point -Wshadow
-Wno-shadow-ivar -Wshift-count-negative -Wshift-count-overflow
-Wsign-compare -Wsign-conversion -Wfloat-conversion
-Wsizeof-pointer-memaccess -Wsizeof-array-argument
-Wstack-protector -Wstack-usage= len
-Wstrict-aliasing -Wstrict-aliasing=n -Wstrict-overflow
-Wstrict-overflow= n
-Wsuggest-attribute=[pure|const|noreturn|format]
-Wsuggest-final-types -Wsuggest-final-methods -Wsuggest-override
-Wmissing-format-attribute -Wswitch -Wswitch-default
-Wswitch-enum -Wswitch-bool -Wsync-nand -Wsystem-headers
-Wtrampolines -Wtrigraphs -Wtype-limits -Wundef -Wuninitialized
-Wunknown-pragmas -Wno-pragmas -Wunsuffixed-float-constants
-Wunused -Wunused-function -Wunused-label -Wunused-local-typedefs
-Wunused-parameter -Wno-unused-result -Wunused-value
-Wunused-variable -Wunused-but-set-parameter
-Wunused-but-set-variable -Wuseless-cast -Wvariadic-macros
-Wvector-operation-performance -Wvla -Wvolatile-register-var
-Wwrite-strings -Wzero-as-null-pointer-constant
- C and Objective-C-only Warning Options
- -Wbad-function-cast -Wmissing-declarations
-Wmissing-parameter-type -Wmissing-prototypes -Wnested-externs
-Wold-style-declaration -Wold-style-definition
-Wstrict-prototypes -Wtraditional -Wtraditional-conversion
-Wdeclaration-after-statement -Wpointer-sign
- Debugging Options
- -dletters -dumpspecs -dumpmachine
-dumpversion -fsanitize=style -fsanitize-recover
-fsanitize-recover= style
-fasan-shadow-offset=number
-fsanitize-undefined-trap-on-error -fcheck-pointer-bounds
-fchkp-check-incomplete-type -fchkp-first-field-has-own-bounds
-fchkp-narrow-bounds -fchkp-narrow-to-innermost-array
-fchkp-optimize -fchkp-use-fast-string-functions
-fchkp-use-nochk-string-functions -fchkp-use-static-bounds
-fchkp-use-static-const-bounds
-fchkp-treat-zero-dynamic-size-as-infinite -fchkp-check-read
-fchkp-check-read -fchkp-check-write -fchkp-store-bounds
-fchkp-instrument-calls -fchkp-instrument-marked-only
-fchkp-use-wrappers -fdbg-cnt-list
-fdbg-cnt=counter-value-list
-fdisable-ipa-pass_name
-fdisable-rtl-pass_name
-fdisable-rtl-pass-name =range-list
-fdisable-tree- pass_name
-fdisable-tree-pass-name =range-list
-fdump-noaddr -fdump-unnumbered -fdump-unnumbered-links
-fdump-translation-unit[ -n]
-fdump-class-hierarchy[ -n] -fdump-ipa-all
-fdump-ipa-cgraph -fdump-ipa-inline -fdump-passes
-fdump-statistics -fdump-tree-all
-fdump-tree-original[ -n]
-fdump-tree-optimized[ -n] -fdump-tree-cfg
-fdump-tree-alias -fdump-tree-ch
-fdump-tree-ssa[- n]
-fdump-tree-pre[-n]
-fdump-tree-ccp[-n]
-fdump-tree-dce[-n]
-fdump-tree-gimple[-raw]
-fdump-tree-dom[-n]
-fdump-tree-dse[-n]
-fdump-tree-phiprop[- n]
-fdump-tree-phiopt[-n]
-fdump-tree-forwprop[-n]
-fdump-tree-copyrename[ -n] -fdump-tree-nrv
-fdump-tree-vect -fdump-tree-sink
-fdump-tree-sra[- n]
-fdump-tree-forwprop[- n]
-fdump-tree-fre[- n] -fdump-tree-vtable-verify
-fdump-tree-vrp[-n]
-fdump-tree-storeccp[- n]
-fdump-final-insns=file
-fcompare-debug[=opts] -fcompare-debug-second
-feliminate-dwarf2-dups -fno-eliminate-unused-debug-types
-feliminate-unused-debug-symbols -femit-class-debug-always
-fenable- kind-pass
-fenable-kind -pass=range-list
-fdebug-types-section -fmem-report-wpa -fmem-report
-fpre-ipa-mem-report -fpost-ipa-mem-report -fprofile-arcs
-fopt-info -fopt-info-options[=file]
-frandom-seed=string -fsched-verbose=n
-fsel-sched-verbose -fsel-sched-dump-cfg
-fsel-sched-pipelining-verbose -fstack-usage -ftest-coverage
-ftime-report -fvar-tracking -fvar-tracking-assignments
-fvar-tracking-assignments-toggle -g -glevel -gtoggle
-gcoff -gdwarf- version -ggdb -grecord-gcc-switches
-gno-record-gcc-switches -gstabs -gstabs+ -gstrict-dwarf
-gno-strict-dwarf -gvms -gxcoff -gxcoff+
-gz[=type] -fno-merge-debug-strings
-fno-dwarf2-cfi-asm
-fdebug-prefix-map=old=new
-femit-struct-debug-baseonly -femit-struct-debug-reduced
-femit-struct-debug-detailed[ =spec-list] -p -pg
-print-file-name= library -print-libgcc-file-name
-print-multi-directory -print-multi-lib -print-multi-os-directory
-print-prog-name=program -print-search-dirs -Q
-print-sysroot -print-sysroot-headers-suffix -save-temps
-save-temps=cwd -save-temps=obj -time[ =file]
- Optimization Options
- -faggressive-loop-optimizations
-falign-functions[=n]
-falign-jumps[=n] -falign-labels[=n]
-falign-loops[= n] -fassociative-math -fauto-profile
-fauto-profile[= path] -fauto-inc-dec
-fbranch-probabilities -fbranch-target-load-optimize
-fbranch-target-load-optimize2 -fbtr-bb-exclusive
-fcaller-saves -fcheck-data-deps -fcombine-stack-adjustments
-fconserve-stack -fcompare-elim -fcprop-registers
-fcrossjumping -fcse-follow-jumps -fcse-skip-blocks
-fcx-fortran-rules -fcx-limited-range -fdata-sections -fdce
-fdelayed-branch -fdelete-null-pointer-checks -fdevirtualize
-fdevirtualize-speculatively -fdevirtualize-at-ltrans -fdse
-fearly-inlining -fipa-sra -fexpensive-optimizations
-ffat-lto-objects -ffast-math -ffinite-math-only -ffloat-store
-fexcess-precision= style -fforward-propagate
-ffp-contract= style -ffunction-sections -fgcse
-fgcse-after-reload -fgcse-las -fgcse-lm -fgraphite-identity
-fgcse-sm -fhoist-adjacent-loads -fif-conversion
-fif-conversion2 -findirect-inlining -finline-functions
-finline-functions-called-once -finline-limit= n
-finline-small-functions -fipa-cp -fipa-cp-clone -fipa-cp-alignment
-fipa-pta -fipa-profile -fipa-pure-const -fipa-reference -fipa-icf
-fira-algorithm=algorithm -fira-region=region
-fira-hoist-pressure -fira-loop-pressure
-fno-ira-share-save-slots -fno-ira-share-spill-slots
-fira-verbose= n -fisolate-erroneous-paths-dereference
-fisolate-erroneous-paths-attribute -fivopts
-fkeep-inline-functions -fkeep-static-consts
-flive-range-shrinkage -floop-block -floop-interchange
-floop-strip-mine -floop-unroll-and-jam -floop-nest-optimize
-floop-parallelize-all -flra-remat -flto -flto-compression-level
-flto-partition=alg -flto-report -flto-report-wpa
-fmerge-all-constants -fmerge-constants -fmodulo-sched
-fmodulo-sched-allow-regmoves -fmove-loop-invariants
-fno-branch-count-reg -fno-defer-pop -fno-function-cse
-fno-guess-branch-probability -fno-inline -fno-math-errno
-fno-peephole -fno-peephole2 -fno-sched-interblock -fno-sched-spec
-fno-signed-zeros -fno-toplevel-reorder -fno-trapping-math
-fno-zero-initialized-in-bss -fomit-frame-pointer
-foptimize-sibling-calls -fpartial-inlining -fpeel-loops
-fpredictive-commoning -fprefetch-loop-arrays -fprofile-report
-fprofile-correction -fprofile-dir=path
-fprofile-generate -fprofile-generate=path
-fprofile-use -fprofile-use= path -fprofile-values
-fprofile-reorder-functions -freciprocal-math -free
-frename-registers -freorder-blocks -freorder-blocks-and-partition
-freorder-functions -frerun-cse-after-loop
-freschedule-modulo-scheduled-loops -frounding-math
-fsched2-use-superblocks -fsched-pressure -fsched-spec-load
-fsched-spec-load-dangerous
-fsched-stalled-insns-dep[=n]
-fsched-stalled-insns[=n] -fsched-group-heuristic
-fsched-critical-path-heuristic -fsched-spec-insn-heuristic
-fsched-rank-heuristic -fsched-last-insn-heuristic
-fsched-dep-count-heuristic -fschedule-fusion
-fschedule-insns -fschedule-insns2 -fsection-anchors
-fselective-scheduling -fselective-scheduling2
-fsel-sched-pipelining -fsel-sched-pipelining-outer-loops
-fsemantic-interposition -fshrink-wrap -fsignaling-nans
-fsingle-precision-constant -fsplit-ivs-in-unroller
-fsplit-wide-types -fssa-phiopt -fstack-protector
-fstack-protector-all -fstack-protector-strong
-fstack-protector-explicit -fstdarg-opt -fstrict-aliasing
-fstrict-overflow -fthread-jumps -ftracer -ftree-bit-ccp
-ftree-builtin-call-dce -ftree-ccp -ftree-ch
-ftree-coalesce-inline-vars -ftree-coalesce-vars -ftree-copy-prop
-ftree-copyrename -ftree-dce -ftree-dominator-opts -ftree-dse
-ftree-forwprop -ftree-fre -ftree-loop-if-convert
-ftree-loop-if-convert-stores -ftree-loop-im -ftree-phiprop
-ftree-loop-distribution -ftree-loop-distribute-patterns
-ftree-loop-ivcanon -ftree-loop-linear -ftree-loop-optimize
-ftree-loop-vectorize -ftree-parallelize-loops=n
-ftree-pre -ftree-partial-pre -ftree-pta -ftree-reassoc
-ftree-sink -ftree-slsr -ftree-sra -ftree-switch-conversion
-ftree-tail-merge -ftree-ter -ftree-vectorize -ftree-vrp
-funit-at-a-time -funroll-all-loops -funroll-loops
-funsafe-loop-optimizations -funsafe-math-optimizations
-funswitch-loops -fipa-ra -fvariable-expansion-in-unroller
-fvect-cost-model -fvpt -fweb -fwhole-program -fwpa
-fuse-linker-plugin --param name=value
-O -O0 -O1 -O2 -O3 -Os -Ofast -Og
- Preprocessor Options
- -Aquestion=answer
-A-question[ =answer] -C -dD -dI -dM
-dN -Dmacro[=defn] -E -H
-idirafter dir -include file -imacros
file -iprefix file -iwithprefix dir
-iwithprefixbefore dir -isystem dir
-cxx-isystem dir -imultilib dir
-isysroot dir -M -MM -MF -MG -MP -MQ -MT -nostdinc
-P -fdebug-cpp -ftrack-macro-expansion -fworking-directory
-remap -trigraphs -undef -U macro -Wp,option
-Xpreprocessor option -no-integrated-cpp
- Assembler Option
- -Wa,option -Xassembler
option
- Linker Options
- object-file-name -fuse-ld=linker
-l library -nostartfiles -nodefaultlibs -nostdlib -pie
-rdynamic -s -static -static-libgcc -static-libstdc++
-static-libasan -static-libtsan -static-liblsan -static-libubsan
-static-libmpx -static-libmpxwrappers -shared -shared-libgcc
-symbolic -T script -Wl,option
-Xlinker option -u symbol -z
keyword
- Directory Options
- -Bprefix -Idir
-iquotedir -iremapsrc:dst
-L dir -specs=file -I-
--sysroot=dir
- Target Options
- -V version -b machine
-Bprefix -Idir -iplugindir=dir
-iquotedir -Ldir -specs=file
-I- --sysroot=dir --no-sysroot-suffix
- Machine Dependent Options
- AArch64 Options -mabi=name
-mbig-endian -mlittle-endian -mgeneral-regs-only
-mcmodel=tiny -mcmodel=small -mcmodel=large -mstrict-align
-momit-leaf-frame-pointer -mno-omit-leaf-frame-pointer
-mtls-dialect=desc -mtls-dialect=traditional
-mfix-cortex-a53-835769 -mno-fix-cortex-a53-835769
-mfix-cortex-a53-843419 -mno-fix-cortex-a53-843419
-march=name -mcpu=name
-mtune=name
Adapteva Epiphany Options -mhalf-reg-file
-mprefer-short-insn-regs -mbranch-cost=num -mcmove
-mnops= num -msoft-cmpsf -msplit-lohi -mpost-inc
-mpost-modify -mstack-offset= num -mround-nearest
-mlong-calls -mshort-calls -msmall16 -mfp-mode=mode
-mvect-double -max-vect-align= num -msplit-vecmove-early
-m1reg- reg
ARC Options -mbarrel-shifter -mcpu=cpu -mA6
-mARC600 -mA7 -mARC700 -mdpfp -mdpfp-compact -mdpfp-fast
-mno-dpfp-lrsr -mea -mno-mpy -mmul32x16 -mmul64 -mnorm
-mspfp -mspfp-compact -mspfp-fast -msimd -msoft-float -mswap -mcrc
-mdsp-packa -mdvbf -mlock -mmac-d16 -mmac-24 -mrtsc -mswape
-mtelephony -mxy -misize -mannotate-align -marclinux
-marclinux_prof -mepilogue-cfi -mlong-calls -mmedium-calls
-msdata -mucb-mcount -mvolatile-cache -malign-call
-mauto-modify-reg -mbbit-peephole -mno-brcc -mcase-vector-pcrel
-mcompact-casesi -mno-cond-exec -mearly-cbranchsi -mexpand-adddi
-mindexed-loads -mlra -mlra-priority-none -mlra-priority-compact
mlra-priority-noncompact -mno-millicode -mmixed-code -mq-class
-mRcq -mRcw -msize-level= level -mtune=cpu
-mmultcost= num
-munalign-prob-threshold=probability
ARM Options -mapcs-frame -mno-apcs-frame
-mabi=name -mapcs-stack-check -mno-apcs-stack-check
-mapcs-float -mno-apcs-float -mapcs-reentrant
-mno-apcs-reentrant -msched-prolog -mno-sched-prolog
-mlittle-endian -mbig-endian -mfloat-abi=name
-mfp16-format= name -mthumb-interwork
-mno-thumb-interwork -mcpu=name
-march=name -mfpu=name
-mtune=name -mprint-tune-info
-mstructure-size-boundary= n -mabort-on-noreturn
-mlong-calls -mno-long-calls -msingle-pic-base
-mno-single-pic-base -mpic-register=reg
-mnop-fun-dllimport -mpoke-function-name -mthumb
-marm -mtpcs-frame -mtpcs-leaf-frame
-mcaller-super-interworking -mcallee-super-interworking
-mtp= name -mtls-dialect=dialect
-mword-relocations -mfix-cortex-m3-ldrd
-munaligned-access -mneon-for-64bits
-mslow-flash-data -masm-syntax-unified -mrestrict-it
AVR Options -mmcu=mcu -maccumulate-args
-mbranch-cost= cost -mcall-prologues -mint8
-mn_flash=size -mno-interrupts -mrelax -mrmw
-mstrict-X -mtiny-stack -nodevicelib -Waddr-space-convert
Blackfin Options -mcpu=cpu[-sirevision]
-msim -momit-leaf-frame-pointer -mno-omit-leaf-frame-pointer
-mspecld-anomaly -mno-specld-anomaly -mcsync-anomaly
-mno-csync-anomaly -mlow-64k -mno-low64k -mstack-check-l1
-mid-shared-library -mno-id-shared-library
-mshared-library-id=n -mleaf-id-shared-library
-mno-leaf-id-shared-library -msep-data -mno-sep-data -mlong-calls
-mno-long-calls -mfast-fp -minline-plt -mmulticore -mcorea -mcoreb
-msdram -micplb
C6X Options -mbig-endian -mlittle-endian -march=cpu
-msim -msdata=sdata-type
CRIS Options -mcpu=cpu -march=cpu
-mtune= cpu -mmax-stack-frame=n
-melinux-stacksize= n -metrax4 -metrax100 -mpdebug
-mcc-init -mno-side-effects -mstack-align -mdata-align
-mconst-align -m32-bit -m16-bit -m8-bit -mno-prologue-epilogue
-mno-gotplt -melf -maout -melinux -mlinux -sim -sim2
-mmul-bug-workaround -mno-mul-bug-workaround
CR16 Options -mmac -mcr16cplus -mcr16c -msim
-mint32 -mbit-ops -mdata-model=model
Darwin Options -all_load -allowable_client -arch
-arch_errors_fatal -arch_only -bind_at_load -bundle
-bundle_loader -client_name -compatibility_version
-current_version -dead_strip -dependency-file -dylib_file
-dylinker_install_name -dynamic -dynamiclib
-exported_symbols_list -filelist -flat_namespace
-force_cpusubtype_ALL -force_flat_namespace
-headerpad_max_install_names -iframework -image_base -init
-install_name -keep_private_externs -multi_module -multiply_defined
-multiply_defined_unused -noall_load
-no_dead_strip_inits_and_terms -nofixprebinding -nomultidefs
-noprebind -noseglinkedit -pagezero_size -prebind
-prebind_all_twolevel_modules -private_bundle -read_only_relocs
-sectalign -sectobjectsymbols -whyload -seg1addr -sectcreate
-sectobjectsymbols -sectorder -segaddr -segs_read_only_addr
-segs_read_write_addr -seg_addr_table -seg_addr_table_filename
-seglinkedit -segprot -segs_read_only_addr
-segs_read_write_addr -single_module -static -sub_library
-sub_umbrella -twolevel_namespace -umbrella -undefined
-unexported_symbols_list -weak_reference_mismatches -whatsloaded
-F -gused -gfull -mmacosx-version-min= version -mkernel
-mone-byte-bool
DEC Alpha Options -mno-fp-regs -msoft-float -mieee
-mieee-with-inexact -mieee-conformant
-mfp-trap-mode=mode -mfp-rounding-mode=mode
-mtrap-precision=mode -mbuild-constants
-mcpu=cpu-type -mtune=cpu-type -mbwx -mmax
-mfix -mcix -mfloat-vax -mfloat-ieee -mexplicit-relocs
-msmall-data -mlarge-data -msmall-text -mlarge-text
-mmemory-latency= time
FR30 Options -msmall-model -mno-lsim
FRV Options -mgpr-32 -mgpr-64 -mfpr-32 -mfpr-64
-mhard-float -msoft-float -malloc-cc -mfixed-cc -mdword
-mno-dword -mdouble -mno-double -mmedia -mno-media -mmuladd
-mno-muladd -mfdpic -minline-plt -mgprel-ro
-multilib-library-pic -mlinked-fp -mlong-calls -malign-labels
-mlibrary-pic -macc-4 -macc-8 -mpack -mno-pack -mno-eflags
-mcond-move -mno-cond-move -moptimize-membar
-mno-optimize-membar -mscc -mno-scc -mcond-exec -mno-cond-exec
-mvliw-branch -mno-vliw-branch -mmulti-cond-exec
-mno-multi-cond-exec -mnested-cond-exec -mno-nested-cond-exec
-mtomcat-stats -mTLS -mtls -mcpu=cpu
GNU/Linux Options -mglibc -muclibc -mbionic -mandroid
-tno-android-cc -tno-android-ld
H8/300 Options -mrelax -mh -ms -mn -mexr -mno-exr -mint32
-malign-300
HPPA Options -march=architecture-type
-mdisable-fpregs -mdisable-indexing -mfast-indirect-calls -mgas
-mgnu-ld -mhp-ld -mfixed-range=register-range
-mjump-in-delay -mlinker-opt -mlong-calls -mlong-load-store
-mno-disable-fpregs -mno-disable-indexing -mno-fast-indirect-calls
-mno-gas -mno-jump-in-delay -mno-long-load-store
-mno-portable-runtime -mno-soft-float -mno-space-regs
-msoft-float -mpa-risc-1-0 -mpa-risc-1-1 -mpa-risc-2-0
-mportable-runtime -mschedule=cpu-type -mspace-regs
-msio -mwsio -munix=unix-std -nolibdld -static
-threads
IA-64 Options -mbig-endian -mlittle-endian -mgnu-as -mgnu-ld
-mno-pic -mvolatile-asm-stop -mregister-names -msdata
-mno-sdata -mconstant-gp -mauto-pic -mfused-madd
-minline-float-divide-min-latency
-minline-float-divide-max-throughput
-mno-inline-float-divide -minline-int-divide-min-latency
-minline-int-divide-max-throughput -mno-inline-int-divide
-minline-sqrt-min-latency -minline-sqrt-max-throughput
-mno-inline-sqrt -mdwarf2-asm -mearly-stop-bits
-mfixed-range= register-range
-mtls-size=tls-size -mtune=cpu-type -milp32
-mlp64 -msched-br-data-spec -msched-ar-data-spec
-msched-control-spec -msched-br-in-data-spec
-msched-ar-in-data-spec -msched-in-control-spec -msched-spec-ldc
-msched-spec-control-ldc -msched-prefer-non-data-spec-insns
-msched-prefer-non-control-spec-insns
-msched-stop-bits-after-every-cycle
-msched-count-spec-in-critical-path
-msel-sched-dont-check-control-spec -msched-fp-mem-deps-zero-cost
-msched-max-memory-insns-hard-limit
-msched-max-memory-insns=max-insns
LM32 Options -mbarrel-shift-enabled -mdivide-enabled
-mmultiply-enabled -msign-extend-enabled -muser-enabled
M32R/D Options -m32r2 -m32rx -m32r -mdebug
-malign-loops -mno-align-loops -missue-rate=number
-mbranch-cost=number
-mmodel=code-size-model-type
-msdata=sdata-type -mno-flush-func
-mflush-func=name -mno-flush-trap
-mflush-trap=number -G num
M32C Options -mcpu=cpu -msim
-memregs=number
M680x0 Options -march=arch -mcpu=cpu
-mtune= tune -m68000 -m68020 -m68020-40 -m68020-60
-m68030 -m68040 -m68060 -mcpu32 -m5200 -m5206e -m528x -m5307
-m5407 -mcfv4e -mbitfield -mno-bitfield -mc68000 -mc68020
-mnobitfield -mrtd -mno-rtd -mdiv -mno-div -mshort -mno-short
-mhard-float -m68881 -msoft-float -mpcrel -malign-int
-mstrict-align -msep-data -mno-sep-data -mshared-library-id=n
-mid-shared-library -mno-id-shared-library -mxgot -mno-xgot
MCore Options -mhardlit -mno-hardlit -mdiv -mno-div
-mrelax-immediates -mno-relax-immediates -mwide-bitfields
-mno-wide-bitfields -m4byte-functions -mno-4byte-functions
-mcallgraph-data -mno-callgraph-data -mslow-bytes -mno-slow-bytes
-mno-lsim -mlittle-endian -mbig-endian -m210 -m340
-mstack-increment
MeP Options -mabsdiff -mall-opts -maverage -mbased=n
-mbitops -mc=n -mclip -mconfig=name
-mcop -mcop32 -mcop64 -mivc2 -mdc -mdiv -meb -mel -mio-volatile
-ml -mleadz -mm -mminmax -mmult -mno-opts -mrepeat -ms -msatur
-msdram -msim -msimnovec -mtf -mtiny=n
MicroBlaze Options -msoft-float -mhard-float -msmall-divides
-mcpu= cpu -mmemcpy -mxl-soft-mul -mxl-soft-div
-mxl-barrel-shift -mxl-pattern-compare -mxl-stack-check -mxl-gp-opt
-mno-clearbss -mxl-multiply-high -mxl-float-convert
-mxl-float-sqrt -mbig-endian -mlittle-endian -mxl-reorder
-mxl-mode- app-model
MIPS Options -EL -EB -march=arch
-mtune=arch -mips1 -mips2 -mips3 -mips4 -mips32 -mips32r2
-mips32r3 -mips32r5 -mips32r6 -mips64 -mips64r2 -mips64r3 -mips64r5
-mips64r6 -mips16 -mno-mips16 -mflip-mips16
-minterlink-compressed -mno-interlink-compressed
-minterlink-mips16 -mno-interlink-mips16 -mabi=abi
-mabicalls -mno-abicalls -mshared -mno-shared -mplt -mno-plt
-mxgot -mno-xgot -mgp32 -mgp64 -mfp32 -mfpxx -mfp64 -mhard-float
-msoft-float -mno-float -msingle-float -mdouble-float
-modd-spreg -mno-odd-spreg -mabs=mode
-mnan=encoding -mdsp -mno-dsp -mdspr2 -mno-dspr2
-mmcu -mmno-mcu -meva -mno-eva -mvirt -mno-virt
-mxpa -mno-xpa -mmicromips -mno-micromips
-mfpu=fpu-type -msmartmips -mno-smartmips
-mpaired-single -mno-paired-single -mdmx -mno-mdmx -mips3d
-mno-mips3d -mmt -mno-mt -mllsc -mno-llsc -mlong64 -mlong32 -msym32
-mno-sym32 -Gnum -mlocal-sdata -mno-local-sdata
-mextern-sdata -mno-extern-sdata -mgpopt -mno-gopt
-membedded-data -mno-embedded-data -muninit-const-in-rodata
-mno-uninit-const-in-rodata -mcode-readable=setting
-msplit-addresses -mno-split-addresses -mexplicit-relocs
-mno-explicit-relocs -mcheck-zero-division
-mno-check-zero-division -mdivide-traps -mdivide-breaks
-mmemcpy -mno-memcpy -mlong-calls -mno-long-calls -mmad -mno-mad
-mimadd -mno-imadd -mfused-madd -mno-fused-madd -nocpp -mfix-24k
-mno-fix-24k -mfix-r4000 -mno-fix-r4000 -mfix-r4400
-mno-fix-r4400 -mfix-r10000 -mno-fix-r10000 -mfix-rm7000
-mno-fix-rm7000 -mfix-vr4120 -mno-fix-vr4120 -mfix-vr4130
-mno-fix-vr4130 -mfix-sb1 -mno-fix-sb1 -mflush-func=func
-mno-flush-func -mbranch-cost=num -mbranch-likely
-mno-branch-likely -mfp-exceptions -mno-fp-exceptions
-mvr4130-align -mno-vr4130-align -msynci -mno-synci
-mrelax-pic-calls -mno-relax-pic-calls -mmcount-ra-address
MMIX Options -mlibfuncs -mno-libfuncs -mepsilon -mno-epsilon
-mabi=gnu -mabi=mmixware -mzero-extend -mknuthdiv
-mtoplevel-symbols -melf -mbranch-predict -mno-branch-predict
-mbase-addresses -mno-base-addresses -msingle-exit
-mno-single-exit
MN10300 Options -mmult-bug -mno-mult-bug -mno-am33 -mam33
-mam33-2 -mam34 -mtune=cpu-type
-mreturn-pointer-on-d0 -mno-crt0 -mrelax -mliw -msetlb
Moxie Options -meb -mel -mmul.x -mno-crt0
MSP430 Options -msim -masm-hex -mmcu= -mcpu= -mlarge -msmall
-mrelax -mhwmult= -minrt
NDS32 Options -mbig-endian -mlittle-endian -mreduced-regs
-mfull-regs -mcmov -mno-cmov -mperf-ext -mno-perf-ext
-mv3push -mno-v3push -m16bit -mno-16bit
-misr-vector-size= num -mcache-block-size=num
-march=arch -mcmodel=code-model -mctor-dtor
-mrelax
Nios II Options -G num -mgpopt=option
-mgpopt -mno-gpopt -mel -meb -mno-bypass-cache
-mbypass-cache -mno-cache-volatile -mcache-volatile
-mno-fast-sw-div -mfast-sw-div -mhw-mul -mno-hw-mul -mhw-mulx
-mno-hw-mulx -mno-hw-div -mhw-div
-mcustom-insn=N -mno-custom-insn
-mcustom-fpu-cfg=name -mhal -msmallc
-msys-crt0=name -msys-lib=name
Nvidia PTX Options -m32 -m64 -mmainkernel
PDP-11 Options -mfpu -msoft-float -mac0 -mno-ac0 -m40 -m45
-m10 -mbcopy -mbcopy-builtin -mint32 -mno-int16 -mint16
-mno-int32 -mfloat32 -mno-float64 -mfloat64 -mno-float32 -mabshi
-mno-abshi -mbranch-expensive -mbranch-cheap -munix-asm
-mdec-asm
picoChip Options -mae=ae_type
-mvliw-lookahead=N -msymbol-as-address
-mno-inefficient-warnings
PowerPC Options See RS/6000 and PowerPC Options.
RL78 Options -msim -mmul=none -mmul=g13 -mmul=rl78
-m64bit-doubles -m32bit-doubles
RS/6000 and PowerPC Options -mcpu=cpu-type
-mtune= cpu-type -mcmodel=code-model
-mpowerpc64 -maltivec -mno-altivec -mpowerpc-gpopt
-mno-powerpc-gpopt -mpowerpc-gfxopt -mno-powerpc-gfxopt
-mmfcrf -mno-mfcrf -mpopcntb -mno-popcntb -mpopcntd -mno-popcntd
-mfprnd -mno-fprnd -mcmpb -mno-cmpb -mmfpgpr -mno-mfpgpr
-mhard-dfp -mno-hard-dfp -mfull-toc -mminimal-toc -mno-fp-in-toc
-mno-sum-in-toc -m64 -m32 -mxl-compat -mno-xl-compat -mpe
-malign-power -malign-natural -msoft-float -mhard-float
-mmultiple -mno-multiple -msingle-float -mdouble-float
-msimple-fpu -mstring -mno-string -mupdate -mno-update
-mavoid-indexed-addresses -mno-avoid-indexed-addresses
-mfused-madd -mno-fused-madd -mbit-align -mno-bit-align
-mstrict-align -mno-strict-align -mrelocatable -mno-relocatable
-mrelocatable-lib -mno-relocatable-lib -mtoc -mno-toc -mlittle
-mlittle-endian -mbig -mbig-endian -mdynamic-no-pic -maltivec
-mswdiv -msingle-pic-base
-mprioritize-restricted-insns=priority
-msched-costly-dep= dependence_type
-minsert-sched-nops= scheme -mcall-sysv -mcall-netbsd
-maix-struct-return -msvr4-struct-return
-mabi=abi-type -msecure-plt -mbss-plt
-mblock-move-inline-limit= num -misel -mno-isel
-misel=yes -misel=no -mspe -mno-spe -mspe=yes
-mspe=no -mpaired -mgen-cell-microcode
-mwarn-cell-microcode -mvrsave -mno-vrsave -mmulhw
-mno-mulhw -mdlmzb -mno-dlmzb -mfloat-gprs=yes
-mfloat-gprs=no -mfloat-gprs=single -mfloat-gprs=double -mprototype
-mno-prototype -msim -mmvme -mads -myellowknife -memb -msdata
-msdata=opt -mvxworks -G num -pthread
-mrecip -mrecip=opt -mno-recip -mrecip-precision
-mno-recip-precision -mveclibabi=type -mfriz
-mno-friz -mpointers-to-nested-functions
-mno-pointers-to-nested-functions -msave-toc-indirect
-mno-save-toc-indirect -mpower8-fusion -mno-mpower8-fusion
-mpower8-vector -mno-power8-vector -mcrypto -mno-crypto
-mdirect-move -mno-direct-move -mquad-memory -mno-quad-memory
-mquad-memory-atomic -mno-quad-memory-atomic -mcompat-align-parm
-mno-compat-align-parm -mupper-regs-df -mno-upper-regs-df
-mupper-regs-sf -mno-upper-regs-sf -mupper-regs -mno-upper-regs
RX Options -m64bit-doubles -m32bit-doubles -fpu -nofpu
-mcpu= -mbig-endian-data -mlittle-endian-data
-msmall-data -msim -mno-sim -mas100-syntax
-mno-as100-syntax -mrelax -mmax-constant-size=
-mint-register= -mpid
-mno-warn-multiple-fast-interrupts -msave-acc-in-interrupts
S/390 and zSeries Options -mtune=cpu-type
-march= cpu-type -mhard-float -msoft-float -mhard-dfp
-mno-hard-dfp -mlong-double-64 -mlong-double-128 -mbackchain
-mno-backchain -mpacked-stack -mno-packed-stack -msmall-exec
-mno-small-exec -mmvcle -mno-mvcle -m64 -m31 -mdebug -mno-debug
-mesa -mzarch -mtpf-trace -mno-tpf-trace -mfused-madd
-mno-fused-madd -mwarn-framesize -mwarn-dynamicstack -mstack-size
-mstack-guard
-mhotpatch=halfwords,halfwords
Score Options -meb -mel -mnhwloop -muls
-mmac -mscore5 -mscore5u -mscore7 -mscore7d
SH Options -m1 -m2 -m2e -m2a-nofpu -m2a-single-only
-m2a-single -m2a -m3 -m3e -m4-nofpu -m4-single-only
-m4-single -m4 -m4a-nofpu -m4a-single-only -m4a-single -m4a
-m4al -m5-64media -m5-64media-nofpu -m5-32media
-m5-32media-nofpu -m5-compact -m5-compact-nofpu -mb -ml
-mdalign -mrelax -mbigtable -mfmovd -mhitachi -mrenesas
-mno-renesas -mnomacsave -mieee -mno-ieee -mbitops -misize
-minline-ic_invalidate -mpadstruct -mspace -mprefergot -musermode
-multcost= number -mdiv=strategy
-mdivsi3_libfunc= name
-mfixed-range=register-range -mindexed-addressing
-mgettrcost= number -mpt-fixed
-maccumulate-outgoing-args -minvalid-symbols
-matomic-model=atomic-model -mbranch-cost=num
-mzdcbranch -mno-zdcbranch -mcbranch-force-delay-slot
-mfused-madd -mno-fused-madd -mfsca -mno-fsca -mfsrra -mno-fsrra
-mpretend-cmove -mtas
Solaris 2 Options -mclear-hwcap -mno-clear-hwcap -mimpure-text
-mno-impure-text -pthreads -pthread
SPARC Options -mcpu=cpu-type
-mtune=cpu-type -mcmodel=code-model
-mmemory-model= mem-model -m32 -m64 -mapp-regs
-mno-app-regs -mfaster-structs -mno-faster-structs -mflat
-mno-flat -mfpu -mno-fpu -mhard-float -msoft-float
-mhard-quad-float -msoft-quad-float -mstack-bias
-mno-stack-bias -munaligned-doubles -mno-unaligned-doubles
-muser-mode -mno-user-mode -mv8plus -mno-v8plus -mvis
-mno-vis -mvis2 -mno-vis2 -mvis3 -mno-vis3 -mcbcond
-mno-cbcond -mfmaf -mno-fmaf -mpopc -mno-popc -mfix-at697f
-mfix-ut699
SPU Options -mwarn-reloc -merror-reloc -msafe-dma
-munsafe-dma -mbranch-hints -msmall-mem -mlarge-mem
-mstdmain -mfixed-range=register-range -mea32
-mea64 -maddress-space-conversion -mno-address-space-conversion
-mcache-size=cache-size -matomic-updates
-mno-atomic-updates
System V Options -Qy -Qn -YP,paths
-Ym,dir
TILE-Gx Options -mcpu=CPU -m32 -m64 -mbig-endian
-mlittle-endian -mcmodel=code-model
TILEPro Options -mcpu=cpu -m32
V850 Options -mlong-calls -mno-long-calls -mep -mno-ep
-mprolog-function -mno-prolog-function -mspace
-mtda=n -msda=n -mzda=n
-mapp-regs -mno-app-regs -mdisable-callt -mno-disable-callt
-mv850e2v3 -mv850e2 -mv850e1 -mv850es -mv850e -mv850
-mv850e3v5 -mloop -mrelax -mlong-jumps
-msoft-float -mhard-float -mgcc-abi
-mrh850-abi -mbig-switch
VAX Options -mg -mgnu -munix
Visium Options -mdebug -msim -mfpu -mno-fpu -mhard-float
-msoft-float -mcpu=cpu-type
-mtune=cpu-type -msv-mode -muser-mode
VMS Options -mvms-return-codes -mdebug-main=prefix
-mmalloc64 -mpointer-size=size
VxWorks Options -mrtp -non-static -Bstatic -Bdynamic
-Xbind-lazy -Xbind-now
x86 Options -mtune=cpu-type
-march=cpu-type -mtune-ctrl=feature-list
-mdump-tune-features -mno-default -mfpmath=unit
-masm= dialect -mno-fancy-math-387
-mno-fp-ret-in-387 -msoft-float -mno-wide-multiply -mrtd
-malign-double -mpreferred-stack-boundary=num
-mincoming-stack-boundary= num -mcld -mcx16 -msahf
-mmovbe -mcrc32 -mrecip -mrecip=opt -mvzeroupper
-mprefer-avx128 -mmmx -msse -msse2 -msse3 -mssse3 -msse4.1 -msse4.2
-msse4 -mavx -mavx2 -mavx512f -mavx512pf -mavx512er -mavx512cd
-msha -maes -mpclmul -mfsgsbase -mrdrnd -mf16c -mfma
-mprefetchwt1 -mclflushopt -mxsavec -mxsaves -msse4a -m3dnow
-mpopcnt -mabm -mbmi -mtbm -mfma4 -mxop -mlzcnt -mbmi2 -mfxsr
-mxsave -mxsaveopt -mrtm -mlwp -mmpx -mmwaitx -mthreads
-mno-align-stringops -minline-all-stringops
-minline-stringops-dynamically -mstringop-strategy= alg
-mmemcpy-strategy= strategy
-mmemset-strategy=strategy -mpush-args
-maccumulate-outgoing-args -m128bit-long-double -m96bit-long-double
-mlong-double-64 -mlong-double-80 -mlong-double-128
-mregparm=num -msseregparm
-mveclibabi=type -mvect8-ret-in-mem -mpc32 -mpc64
-mpc80 -mstackrealign -momit-leaf-frame-pointer -mno-red-zone
-mno-tls-direct-seg-refs -mcmodel=code-model
-mabi= name -maddress-mode=mode -m32 -m64
-mx32 -m16 -mlarge-data-threshold= num -msse2avx -mfentry
-mrecord-mcount -mnop-mcount -m8bit-idiv
-mavx256-split-unaligned-load -mavx256-split-unaligned-store
-malign-data= type
-mstack-protector-guard=guard
x86 Windows Options -mconsole -mcygwin -mno-cygwin -mdll
-mnop-fun-dllimport -mthread -municode -mwin32 -mwindows
-fno-set-stack-executable
Xstormy16 Options -msim
Xtensa Options -mconst16 -mno-const16 -mfused-madd
-mno-fused-madd -mforce-no-pic -mserialize-volatile
-mno-serialize-volatile -mtext-section-literals
-mno-text-section-literals -mtarget-align -mno-target-align
-mlongcalls -mno-longcalls
zSeries Options See S/390 and zSeries Options.
- Code Generation Options
- -fcall-saved-reg
-fcall-used-reg -ffixed-reg
-fexceptions -fnon-call-exceptions -fdelete-dead-exceptions
-funwind-tables -fasynchronous-unwind-tables
-fno-gnu-unique -finhibit-size-directive
-finstrument-functions
-finstrument-functions-exclude-function-list=
sym,sym ,...
-finstrument-functions-exclude-file-list=
file,file ,... -fno-common -fno-ident
-fpcc-struct-return -fpic -fPIC -fpie -fPIE -fno-jump-tables
-frecord-gcc-switches -freg-struct-return -fshort-enums
-fshort-double -fshort-wchar -fverbose-asm
-fpack-struct[=n] -fstack-check
-fstack-limit-register= reg
-fstack-limit-symbol=sym -fno-stack-limit
-fsplit-stack -fleading-underscore -ftls-model=model
-fstack-reuse= reuse_level -ftrapv -fwrapv
-fbounds-check
-fvisibility=[default|internal|hidden|
protected] -fstrict-volatile-bitfields -fsync-libcalls
Options Controlling the Kind of Output
Compilation can involve up to four stages: preprocessing, compilation proper,
assembly and linking, always in that order. GCC is capable of preprocessing
and compiling several files either into several assembler input files, or into
one assembler input file; then each assembler input file produces an object
file, and linking combines all the object files (those newly compiled, and
those specified as input) into an executable file.
For any given input file, the file name suffix determines what kind of
compilation is done:
- file.c
- C source code that must be preprocessed.
- file.i
- C source code that should not be preprocessed.
- file.ii
- C++ source code that should not be preprocessed.
- file.m
- Objective-C source code. Note that you must link with the
libobjc library to make an Objective-C program work.
- file.mi
- Objective-C source code that should not be
preprocessed.
- file.mm
- file.M
- Objective-C++ source code. Note that you must link with the
libobjc library to make an Objective-C++ program work. Note that
.M refers to a literal capital M.
- file.mii
- Objective-C++ source code that should not be
preprocessed.
- file.h
- C, C++, Objective-C or Objective-C++ header file to be
turned into a precompiled header (default), or C, C++ header file to be
turned into an Ada spec (via the -fdump-ada-spec switch).
- file.cc
- file.cp
- file.cxx
- file.cpp
- file.CPP
- file.c++
- file.C
- C++ source code that must be preprocessed. Note that in
.cxx, the last two letters must both be literally x.
Likewise, .C refers to a literal capital C.
- file.mm
- file.M
- Objective-C++ source code that must be preprocessed.
- file.mii
- Objective-C++ source code that should not be
preprocessed.
- file.hh
- file.H
- file.hp
- file.hxx
- file.hpp
- file.HPP
- file.h++
- file.tcc
- C++ header file to be turned into a precompiled header or
Ada spec.
- file.f
- file.for
- file.ftn
- Fixed form Fortran source code that should not be
preprocessed.
- file.F
- file.FOR
- file.fpp
- file.FPP
- file.FTN
- Fixed form Fortran source code that must be preprocessed
(with the traditional preprocessor).
- file.f90
- file.f95
- file.f03
- file.f08
- Free form Fortran source code that should not be
preprocessed.
- file.F90
- file.F95
- file.F03
- file.F08
- Free form Fortran source code that must be preprocessed
(with the traditional preprocessor).
- file.go
- Go source code.
- file.ads
- Ada source code file that contains a library unit
declaration (a declaration of a package, subprogram, or generic, or a
generic instantiation), or a library unit renaming declaration (a package,
generic, or subprogram renaming declaration). Such files are also called
specs.
- file.adb
- Ada source code file containing a library unit body (a
subprogram or package body). Such files are also called
bodies.
- file.s
- Assembler code.
- file.S
- file.sx
- Assembler code that must be preprocessed.
- other
- An object file to be fed straight into linking. Any file
name with no recognized suffix is treated this way.
You can specify the input language explicitly with the
-x option:
- -x language
- Specify explicitly the language for the following
input files (rather than letting the compiler choose a default based on
the file name suffix). This option applies to all following input files
until the next -x option. Possible values for language are:
c c-header cpp-output
c++ c++-header c++-cpp-output
objective-c objective-c-header objective-c-cpp-output
objective-c++ objective-c++-header objective-c++-cpp-output
assembler assembler-with-cpp
ada
f77 f77-cpp-input f95 f95-cpp-input
go
java
- -x none
- Turn off any specification of a language, so that
subsequent files are handled according to their file name suffixes (as
they are if -x has not been used at all).
- -pass-exit-codes
- Normally the gcc program exits with the code of 1 if
any phase of the compiler returns a non-success return code. If you
specify -pass-exit-codes, the gcc program instead returns
with the numerically highest error produced by any phase returning an
error indication. The C, C++, and Fortran front ends return 4 if an
internal compiler error is encountered.
If you only want some of the stages of compilation, you can use
-x (or
filename suffixes) to tell
gcc where to start, and one of the options
-c,
-S, or
-E to say where
gcc is to stop. Note
that some combinations (for example,
-x cpp-output -E) instruct
gcc to do nothing at all.
- -c
- Compile or assemble the source files, but do not link. The
linking stage simply is not done. The ultimate output is in the form of an
object file for each source file.
By default, the object file name for a source file is made by replacing the
suffix .c, .i, .s, etc., with .o.
Unrecognized input files, not requiring compilation or assembly, are
ignored.
- -S
- Stop after the stage of compilation proper; do not
assemble. The output is in the form of an assembler code file for each
non-assembler input file specified.
By default, the assembler file name for a source file is made by replacing
the suffix .c, .i, etc., with .s.
Input files that don't require compilation are ignored.
- -E
- Stop after the preprocessing stage; do not run the compiler
proper. The output is in the form of preprocessed source code, which is
sent to the standard output.
Input files that don't require preprocessing are ignored.
- -o file
- Place output in file file. This applies to whatever
sort of output is being produced, whether it be an executable file, an
object file, an assembler file or preprocessed C code.
If -o is not specified, the default is to put an executable file in
a.out, the object file for
source.suffix in
source.o, its assembler file in
source.s, a precompiled header file in
source .suffix.gch, and all
preprocessed C source on standard output.
- -v
- Print (on standard error output) the commands executed to
run the stages of compilation. Also print the version number of the
compiler driver program and of the preprocessor and the compiler
proper.
- -###
- Like -v except the commands are not executed and
arguments are quoted unless they contain only alphanumeric characters or
"./-_". This is useful for shell scripts to capture the
driver-generated command lines.
- -pipe
- Use pipes rather than temporary files for communication
between the various stages of compilation. This fails to work on some
systems where the assembler is unable to read from a pipe; but the GNU
assembler has no trouble.
- --help
- Print (on the standard output) a description of the
command-line options understood by gcc. If the -v option is
also specified then --help is also passed on to the various
processes invoked by gcc, so that they can display the command-line
options they accept. If the -Wextra option has also been specified
(prior to the --help option), then command-line options that have
no documentation associated with them are also displayed.
- --target-help
- Print (on the standard output) a description of
target-specific command-line options for each tool. For some targets extra
target-specific information may also be printed.
- --help={class|[^]qualifier}[,...]
- Print (on the standard output) a description of the
command-line options understood by the compiler that fit into all
specified classes and qualifiers. These are the supported classes:
- optimizers
- Display all of the optimization options supported by the
compiler.
- warnings
- Display all of the options controlling warning messages
produced by the compiler.
- target
- Display target-specific options. Unlike the
--target-help option however, target-specific options of the linker
and assembler are not displayed. This is because those tools do not
currently support the extended --help= syntax.
- params
- Display the values recognized by the --param
option.
- language
- Display the options supported for language, where
language is the name of one of the languages supported in this
version of GCC.
- common
- Display the options that are common to all languages.
These are the supported qualifiers:
- undocumented
- Display only those options that are undocumented.
- joined
- Display options taking an argument that appears after an
equal sign in the same continuous piece of text, such as:
--help=target.
- separate
- Display options taking an argument that appears as a
separate word following the original option, such as: -o
output-file.
Thus for example to display all the undocumented target-specific switches
supported by the compiler, use:
--help=target,undocumented
The sense of a qualifier can be inverted by prefixing it with the
^
character, so for example to display all binary warning options (i.e., ones
that are either on or off and that do not take an argument) that have a
description, use:
--help=warnings,^joined,^undocumented
The argument to
--help= should not consist solely of inverted qualifiers.
Combining several classes is possible, although this usually restricts the
output so much that there is nothing to display. One case where it does work,
however, is when one of the classes is
target. For example, to display
all the target-specific optimization options, use:
--help=target,optimizers
The
--help= option can be repeated on the command line. Each successive
use displays its requested class of options, skipping those that have already
been displayed.
If the
-Q option appears on the command line before the
--help=
option, then the descriptive text displayed by
--help= is changed.
Instead of describing the displayed options, an indication is given as to
whether the option is enabled, disabled or set to a specific value (assuming
that the compiler knows this at the point where the
--help= option is
used).
Here is a truncated example from the ARM port of
gcc:
% gcc -Q -mabi=2 --help=target -c
The following options are target specific:
-mabi= 2
-mabort-on-noreturn [disabled]
-mapcs [disabled]
The output is sensitive to the effects of previous command-line options, so for
example it is possible to find out which optimizations are enabled at
-O2 by using:
-Q -O2 --help=optimizers
Alternatively you can discover which binary optimizations are enabled by
-O3 by using:
gcc -c -Q -O3 --help=optimizers > /tmp/O3-opts
gcc -c -Q -O2 --help=optimizers > /tmp/O2-opts
diff /tmp/O2-opts /tmp/O3-opts | grep enabled
- -no-canonical-prefixes
- Do not expand any symbolic links, resolve references to
/../ or /./, or make the path absolute when generating a
relative prefix.
- --version
- Display the version number and copyrights of the invoked
GCC.
- -wrapper
- Invoke all subcommands under a wrapper program. The name of
the wrapper program and its parameters are passed as a comma separated
list.
gcc -c t.c -wrapper gdb,--args
This invokes all subprograms of gcc under gdb --args, thus the
invocation of cc1 is gdb --args cc1 ....
- -fplugin=name.so
- Load the plugin code in file name.so, assumed to be
a shared object to be dlopen'd by the compiler. The base name of the
shared object file is used to identify the plugin for the purposes of
argument parsing (See
-fplugin-arg-name-key= value
below). Each plugin should define the callback functions specified in the
Plugins API.
- -fplugin-arg-name-key=value
- Define an argument called key with a value of
value for the plugin called name.
- -fdump-ada-spec[-slim]
- For C and C++ source and include files, generate
corresponding Ada specs.
- -fada-spec-parent=unit
- In conjunction with -fdump-ada-spec[-slim]
above, generate Ada specs as child units of parent unit.
- -fdump-go-spec=file
- For input files in any language, generate corresponding Go
declarations in file. This generates Go "const",
"type", "var", and "func" declarations which
may be a useful way to start writing a Go interface to code written in
some other language.
- @file
- Read command-line options from file. The options
read are inserted in place of the original @ file option. If
file does not exist, or cannot be read, then the option will be
treated literally, and not removed.
Options in file are separated by whitespace. A whitespace character
may be included in an option by surrounding the entire option in either
single or double quotes. Any character (including a backslash) may be
included by prefixing the character to be included with a backslash. The
file may itself contain additional @ file options; any such
options will be processed recursively.
Compiling C++ Programs
C++ source files conventionally use one of the suffixes
.C,
.cc,
.cpp,
.CPP,
.c++,
.cp, or
.cxx; C++ header
files often use
.hh,
.hpp,
.H, or (for shared template
code)
.tcc; and preprocessed C++ files use the suffix
.ii. GCC
recognizes files with these names and compiles them as C++ programs even if
you call the compiler the same way as for compiling C programs (usually with
the name
gcc).
However, the use of
gcc does not add the C++ library.
g++ is a
program that calls GCC and automatically specifies linking against the C++
library. It treats
.c,
.h and
.i files as C++ source
files instead of C source files unless
-x is used. This program is also
useful when precompiling a C header file with a
.h extension for use in
C++ compilations. On many systems,
g++ is also installed with the name
c++.
When you compile C++ programs, you may specify many of the same command-line
options that you use for compiling programs in any language; or command-line
options meaningful for C and related languages; or options that are meaningful
only for C++ programs.
Options Controlling C Dialect
The following options control the dialect of C (or languages derived from C,
such as C++, Objective-C and Objective-C++) that the compiler accepts:
- -ansi
- In C mode, this is equivalent to -std=c90. In C++
mode, it is equivalent to -std=c++98.
This turns off certain features of GCC that are incompatible with ISO C90
(when compiling C code), or of standard C++ (when compiling C++ code),
such as the "asm" and "typeof" keywords, and
predefined macros such as "unix" and "vax" that
identify the type of system you are using. It also enables the undesirable
and rarely used ISO trigraph feature. For the C compiler, it disables
recognition of C++ style // comments as well as the
"inline" keyword.
The alternate keywords "__asm__", "__extension__",
"__inline__" and "__typeof__" continue to work despite
-ansi. You would not want to use them in an ISO C program, of
course, but it is useful to put them in header files that might be
included in compilations done with -ansi. Alternate predefined
macros such as "__unix__" and "__vax__" are also
available, with or without -ansi.
The -ansi option does not cause non-ISO programs to be rejected
gratuitously. For that, -Wpedantic is required in addition to
-ansi.
The macro "__STRICT_ANSI__" is predefined when the -ansi
option is used. Some header files may notice this macro and refrain from
declaring certain functions or defining certain macros that the ISO
standard doesn't call for; this is to avoid interfering with any programs
that might use these names for other things.
Functions that are normally built in but do not have semantics defined by
ISO C (such as "alloca" and "ffs") are not built-in
functions when -ansi is used.
- -std=
- Determine the language standard. This option is currently
only supported when compiling C or C++.
The compiler can accept several base standards, such as c90 or
c++98, and GNU dialects of those standards, such as gnu90 or
gnu++98. When a base standard is specified, the compiler accepts
all programs following that standard plus those using GNU extensions that
do not contradict it. For example, -std=c90 turns off certain
features of GCC that are incompatible with ISO C90, such as the
"asm" and "typeof" keywords, but not other GNU
extensions that do not have a meaning in ISO C90, such as omitting the
middle term of a "?:" expression. On the other hand, when a GNU
dialect of a standard is specified, all features supported by the compiler
are enabled, even when those features change the meaning of the base
standard. As a result, some strict-conforming programs may be rejected.
The particular standard is used by -Wpedantic to identify which
features are GNU extensions given that version of the standard. For
example -std=gnu90 -Wpedantic warns about C++ style //
comments, while -std=gnu99 -Wpedantic does not.
A value for this option must be provided; possible values are
- c90
- c89
- iso9899:1990
- Support all ISO C90 programs (certain GNU extensions that
conflict with ISO C90 are disabled). Same as -ansi for C code.
- iso9899:199409
- ISO C90 as modified in amendment 1.
- c99
- c9x
- iso9899:1999
- iso9899:199x
- ISO C99. This standard is substantially completely
supported, modulo bugs and floating-point issues (mainly but not entirely
relating to optional C99 features from Annexes F and G). See <
http://gcc.gnu.org/c99status.html> for more information. The
names c9x and iso9899:199x are deprecated.
- c11
- c1x
- iso9899:2011
- ISO C11, the 2011 revision of the ISO C standard. This
standard is substantially completely supported, modulo bugs,
floating-point issues (mainly but not entirely relating to optional C11
features from Annexes F and G) and the optional Annexes K (Bounds-checking
interfaces) and L (Analyzability). The name c1x is deprecated.
- gnu90
- gnu89
- GNU dialect of ISO C90 (including some C99 features).
- gnu99
- gnu9x
- GNU dialect of ISO C99. The name gnu9x is
deprecated.
- gnu11
- gnu1x
- GNU dialect of ISO C11. This is the default for C code. The
name gnu1x is deprecated.
- c++98
- c++03
- The 1998 ISO C++ standard plus the 2003 technical
corrigendum and some additional defect reports. Same as -ansi for
C++ code.
- gnu++98
- gnu++03
- GNU dialect of -std=c++98. This is the default for
C++ code.
- c++11
- c++0x
- The 2011 ISO C++ standard plus amendments. The name
c++0x is deprecated.
- gnu++11
- gnu++0x
- GNU dialect of -std=c++11. The name gnu++0x
is deprecated.
- c++14
- c++1y
- The 2014 ISO C++ standard plus amendments. The name
c++1y is deprecated.
- gnu++14
- gnu++1y
- GNU dialect of -std=c++14. The name gnu++1y
is deprecated.
- c++1z
- The next revision of the ISO C++ standard, tentatively
planned for 2017. Support is highly experimental, and will almost
certainly change in incompatible ways in future releases.
- gnu++1z
- GNU dialect of -std=c++1z. Support is highly
experimental, and will almost certainly change in incompatible ways in
future releases.
- -fgnu89-inline
- The option -fgnu89-inline tells GCC to use the
traditional GNU semantics for "inline" functions when in C99
mode.
Using this option is roughly equivalent to adding the "gnu_inline"
function attribute to all inline functions.
The option -fno-gnu89-inline explicitly tells GCC to use the C99
semantics for "inline" when in C99 or gnu99 mode (i.e., it
specifies the default behavior). This option is not supported in
-std=c90 or -std=gnu90 mode.
The preprocessor macros "__GNUC_GNU_INLINE__" and
"__GNUC_STDC_INLINE__" may be used to check which semantics are
in effect for "inline" functions.
- -aux-info filename
- Output to the given filename prototyped declarations for
all functions declared and/or defined in a translation unit, including
those in header files. This option is silently ignored in any language
other than C.
Besides declarations, the file indicates, in comments, the origin of each
declaration (source file and line), whether the declaration was implicit,
prototyped or unprototyped ( I, N for new or O for
old, respectively, in the first character after the line number and the
colon), and whether it came from a declaration or a definition ( C
or F, respectively, in the following character). In the case of
function definitions, a K&R-style list of arguments followed by their
declarations is also provided, inside comments, after the
declaration.
- -fallow-parameterless-variadic-functions
- Accept variadic functions without named parameters.
Although it is possible to define such a function, this is not very useful
as it is not possible to read the arguments. This is only supported for C
as this construct is allowed by C++.
- -fno-asm
- Do not recognize "asm", "inline" or
"typeof" as a keyword, so that code can use these words as
identifiers. You can use the keywords "__asm__",
"__inline__" and "__typeof__" instead. -ansi
implies -fno-asm.
In C++, this switch only affects the "typeof" keyword, since
"asm" and "inline" are standard keywords. You may want
to use the -fno-gnu-keywords flag instead, which has the same
effect. In C99 mode ( -std=c99 or -std=gnu99), this switch
only affects the "asm" and "typeof" keywords, since
"inline" is a standard keyword in ISO C99.
- -fno-builtin
- -fno-builtin-function
- Don't recognize built-in functions that do not begin with
__builtin_ as prefix.
GCC normally generates special code to handle certain built-in functions
more efficiently; for instance, calls to "alloca" may become
single instructions which adjust the stack directly, and calls to
"memcpy" may become inline copy loops. The resulting code is
often both smaller and faster, but since the function calls no longer
appear as such, you cannot set a breakpoint on those calls, nor can you
change the behavior of the functions by linking with a different library.
In addition, when a function is recognized as a built-in function, GCC may
use information about that function to warn about problems with calls to
that function, or to generate more efficient code, even if the resulting
code still contains calls to that function. For example, warnings are
given with -Wformat for bad calls to "printf" when
"printf" is built in and "strlen" is known not to
modify global memory.
With the -fno-builtin-function option only the built-in
function function is disabled. function must not begin with
__builtin_. If a function is named that is not built-in in this
version of GCC, this option is ignored. There is no corresponding
-fbuiltin- function option; if you wish to enable built-in
functions selectively when using -fno-builtin or
-ffreestanding, you may define macros such as:
#define abs(n) __builtin_abs ((n))
#define strcpy(d, s) __builtin_strcpy ((d), (s))
- -fhosted
- Assert that compilation targets a hosted environment. This
implies -fbuiltin. A hosted environment is one in which the entire
standard library is available, and in which "main" has a return
type of "int". Examples are nearly everything except a kernel.
This is equivalent to -fno-freestanding.
- -ffreestanding
- Assert that compilation targets a freestanding environment.
This implies -fno-builtin. A freestanding environment is one in
which the standard library may not exist, and program startup may not
necessarily be at "main". The most obvious example is an OS
kernel. This is equivalent to -fno-hosted.
- -fopenacc
- Enable handling of OpenACC directives "#pragma
acc" in C/C++ and "!$acc" in Fortran. When -fopenacc
is specified, the compiler generates accelerated code according to the
OpenACC Application Programming Interface v2.0 <
http://www.openacc.org/>. This option implies -pthread,
and thus is only supported on targets that have support for
-pthread.
Note that this is an experimental feature, incomplete, and subject to change
in future versions of GCC. See <
https://gcc.gnu.org/wiki/OpenACC> for more information.
- -fopenmp
- Enable handling of OpenMP directives "#pragma
omp" in C/C++ and "!$omp" in Fortran. When -fopenmp
is specified, the compiler generates parallel code according to the OpenMP
Application Program Interface v4.0 < http://www.openmp.org/>.
This option implies -pthread, and thus is only supported on targets
that have support for -pthread. -fopenmp implies
-fopenmp-simd.
- -fopenmp-simd
- Enable handling of OpenMP's SIMD directives with
"#pragma omp" in C/C++ and "!$omp" in Fortran. Other
OpenMP directives are ignored.
- -fcilkplus
- Enable the usage of Cilk Plus language extension features
for C/C++. When the option -fcilkplus is specified, enable the
usage of the Cilk Plus Language extension features for C/C++. The present
implementation follows ABI version 1.2. This is an experimental feature
that is only partially complete, and whose interface may change in future
versions of GCC as the official specification changes. Currently, all
features but "_Cilk_for" have been implemented.
- -fgnu-tm
- When the option -fgnu-tm is specified, the compiler
generates code for the Linux variant of Intel's current Transactional
Memory ABI specification document (Revision 1.1, May 6 2009). This is an
experimental feature whose interface may change in future versions of GCC,
as the official specification changes. Please note that not all
architectures are supported for this feature.
For more information on GCC's support for transactional memory,
Note that the transactional memory feature is not supported with non-call
exceptions ( -fnon-call-exceptions).
- -fms-extensions
- Accept some non-standard constructs used in Microsoft
header files.
In C++ code, this allows member names in structures to be similar to
previous types declarations.
typedef int UOW;
struct ABC {
UOW UOW;
};
Some cases of unnamed fields in structures and unions are only accepted with
this option.
Note that this option is off for all targets but x86 targets using
ms-abi.
- -fplan9-extensions
- Accept some non-standard constructs used in Plan 9 code.
This enables -fms-extensions, permits passing pointers to structures
with anonymous fields to functions that expect pointers to elements of the
type of the field, and permits referring to anonymous fields declared
using a typedef. This is only supported for C, not C++.
- -trigraphs
- Support ISO C trigraphs. The -ansi option (and
-std options for strict ISO C conformance) implies
-trigraphs.
- -traditional
- -traditional-cpp
- Formerly, these options caused GCC to attempt to emulate a
pre-standard C compiler. They are now only supported with the -E
switch. The preprocessor continues to support a pre-standard mode. See the
GNU CPP manual for details.
- -fcond-mismatch
- Allow conditional expressions with mismatched types in the
second and third arguments. The value of such an expression is void. This
option is not supported for C++.
- -flax-vector-conversions
- Allow implicit conversions between vectors with differing
numbers of elements and/or incompatible element types. This option should
not be used for new code.
- -funsigned-char
- Let the type "char" be unsigned, like
"unsigned char".
Each kind of machine has a default for what "char" should be. It
is either like "unsigned char" by default or like "signed
char" by default.
Ideally, a portable program should always use "signed char" or
"unsigned char" when it depends on the signedness of an object.
But many programs have been written to use plain "char" and
expect it to be signed, or expect it to be unsigned, depending on the
machines they were written for. This option, and its inverse, let you make
such a program work with the opposite default.
The type "char" is always a distinct type from each of
"signed char" or "unsigned char", even though its
behavior is always just like one of those two.
- -fsigned-char
- Let the type "char" be signed, like "signed
char".
Note that this is equivalent to -fno-unsigned-char, which is the
negative form of -funsigned-char. Likewise, the option
-fno-signed-char is equivalent to -funsigned-char.
- -fsigned-bitfields
- -funsigned-bitfields
- -fno-signed-bitfields
- -fno-unsigned-bitfields
- These options control whether a bit-field is signed or
unsigned, when the declaration does not use either "signed" or
"unsigned". By default, such a bit-field is signed, because this
is consistent: the basic integer types such as "int" are signed
types.
Options Controlling C++ Dialect
This section describes the command-line options that are only meaningful for C++
programs. You can also use most of the GNU compiler options regardless of what
language your program is in. For example, you might compile a file
firstClass.C like this:
g++ -g -frepo -O -c firstClass.C
In this example, only
-frepo is an option meant only for C++ programs;
you can use the other options with any language supported by GCC.
Here is a list of options that are
only for compiling C++ programs:
- -fabi-version=n
- Use version n of the C++ ABI. The default is version
0.
Version 0 refers to the version conforming most closely to the C++ ABI
specification. Therefore, the ABI obtained using version 0 will change in
different versions of G++ as ABI bugs are fixed.
Version 1 is the version of the C++ ABI that first appeared in G++ 3.2.
Version 2 is the version of the C++ ABI that first appeared in G++ 3.4, and
was the default through G++ 4.9.
Version 3 corrects an error in mangling a constant address as a template
argument.
Version 4, which first appeared in G++ 4.5, implements a standard mangling
for vector types.
Version 5, which first appeared in G++ 4.6, corrects the mangling of
attribute const/volatile on function pointer types, decltype of a plain
decl, and use of a function parameter in the declaration of another
parameter.
Version 6, which first appeared in G++ 4.7, corrects the promotion behavior
of C++11 scoped enums and the mangling of template argument packs,
const/static_cast, prefix ++ and --, and a class scope function used as a
template argument.
Version 7, which first appeared in G++ 4.8, that treats nullptr_t as a
builtin type and corrects the mangling of lambdas in default argument
scope.
Version 8, which first appeared in G++ 4.9, corrects the substitution
behavior of function types with function-cv-qualifiers.
Version 9, which first appeared in G++ 5.2, corrects the alignment of
"nullptr_t".
See also -Wabi.
- -fabi-compat-version=n
- On targets that support strong aliases, G++ works around
mangling changes by creating an alias with the correct mangled name when
defining a symbol with an incorrect mangled name. This switch specifies
which ABI version to use for the alias.
With -fabi-version=0 (the default), this defaults to 2. If another
ABI version is explicitly selected, this defaults to 0.
The compatibility version is also set by -Wabi=n.
- -fno-access-control
- Turn off all access checking. This switch is mainly useful
for working around bugs in the access control code.
- -fcheck-new
- Check that the pointer returned by "operator new"
is non-null before attempting to modify the storage allocated. This check
is normally unnecessary because the C++ standard specifies that
"operator new" only returns 0 if it is declared
"throw()", in which case the compiler always checks the return
value even without this option. In all other cases, when "operator
new" has a non-empty exception specification, memory exhaustion is
signalled by throwing "std::bad_alloc". See also new
(nothrow).
- -fconstexpr-depth=n
- Set the maximum nested evaluation depth for C++11 constexpr
functions to n. A limit is needed to detect endless recursion
during constant expression evaluation. The minimum specified by the
standard is 512.
- -fdeduce-init-list
- Enable deduction of a template type parameter as
"std::initializer_list" from a brace-enclosed initializer list,
i.e.
template <class T> auto forward(T t) -> decltype (realfn (t))
{
return realfn (t);
}
void f()
{
forward({1,2}); // call forward<std::initializer_list<int>>
}
This deduction was implemented as a possible extension to the originally
proposed semantics for the C++11 standard, but was not part of the final
standard, so it is disabled by default. This option is deprecated, and may
be removed in a future version of G++.
- -ffriend-injection
- Inject friend functions into the enclosing namespace, so
that they are visible outside the scope of the class in which they are
declared. Friend functions were documented to work this way in the old
Annotated C++ Reference Manual. However, in ISO C++ a friend function that
is not declared in an enclosing scope can only be found using argument
dependent lookup. GCC defaults to the standard behavior.
This option is for compatibility, and may be removed in a future release of
G++.
- -fno-elide-constructors
- The C++ standard allows an implementation to omit creating
a temporary that is only used to initialize another object of the same
type. Specifying this option disables that optimization, and forces G++ to
call the copy constructor in all cases.
- -fno-enforce-eh-specs
- Don't generate code to check for violation of exception
specifications at run time. This option violates the C++ standard, but may
be useful for reducing code size in production builds, much like defining
"NDEBUG". This does not give user code permission to throw
exceptions in violation of the exception specifications; the compiler
still optimizes based on the specifications, so throwing an unexpected
exception results in undefined behavior at run time.
- -fextern-tls-init
- -fno-extern-tls-init
- The C++11 and OpenMP standards allow
"thread_local" and "threadprivate" variables to have
dynamic (runtime) initialization. To support this, any use of such a
variable goes through a wrapper function that performs any necessary
initialization. When the use and definition of the variable are in the
same translation unit, this overhead can be optimized away, but when the
use is in a different translation unit there is significant overhead even
if the variable doesn't actually need dynamic initialization. If the
programmer can be sure that no use of the variable in a non-defining TU
needs to trigger dynamic initialization (either because the variable is
statically initialized, or a use of the variable in the defining TU will
be executed before any uses in another TU), they can avoid this overhead
with the -fno-extern-tls-init option.
On targets that support symbol aliases, the default is
-fextern-tls-init. On targets that do not support symbol aliases,
the default is -fno-extern-tls-init.
- -ffor-scope
- -fno-for-scope
- If -ffor-scope is specified, the scope of variables
declared in a for-init-statement is limited to the "for"
loop itself, as specified by the C++ standard. If -fno-for-scope is
specified, the scope of variables declared in a for-init-statement
extends to the end of the enclosing scope, as was the case in old versions
of G++, and other (traditional) implementations of C++.
If neither flag is given, the default is to follow the standard, but to
allow and give a warning for old-style code that would otherwise be
invalid, or have different behavior.
- -fno-gnu-keywords
- Do not recognize "typeof" as a keyword, so that
code can use this word as an identifier. You can use the keyword
"__typeof__" instead. -ansi implies
-fno-gnu-keywords.
- -fno-implicit-templates
- Never emit code for non-inline templates that are
instantiated implicitly (i.e. by use); only emit code for explicit
instantiations.
- -fno-implicit-inline-templates
- Don't emit code for implicit instantiations of inline
templates, either. The default is to handle inlines differently so that
compiles with and without optimization need the same set of explicit
instantiations.
- -fno-implement-inlines
- To save space, do not emit out-of-line copies of inline
functions controlled by "#pragma implementation". This causes
linker errors if these functions are not inlined everywhere they are
called.
- -fms-extensions
- Disable Wpedantic warnings about constructs used in MFC,
such as implicit int and getting a pointer to member function via
non-standard syntax.
- -fno-nonansi-builtins
- Disable built-in declarations of functions that are not
mandated by ANSI/ISO C. These include "ffs", "alloca",
"_exit", "index", "bzero",
"conjf", and other related functions.
- -fnothrow-opt
- Treat a "throw()" exception specification as if
it were a "noexcept" specification to reduce or eliminate the
text size overhead relative to a function with no exception specification.
If the function has local variables of types with non-trivial destructors,
the exception specification actually makes the function smaller because
the EH cleanups for those variables can be optimized away. The semantic
effect is that an exception thrown out of a function with such an
exception specification results in a call to "terminate" rather
than "unexpected".
- -fno-operator-names
- Do not treat the operator name keywords "and",
"bitand", "bitor", "compl", "not",
"or" and "xor" as synonyms as keywords.
- -fno-optional-diags
- Disable diagnostics that the standard says a compiler does
not need to issue. Currently, the only such diagnostic issued by G++ is
the one for a name having multiple meanings within a class.
- -fpermissive
- Downgrade some diagnostics about nonconformant code from
errors to warnings. Thus, using -fpermissive allows some
nonconforming code to compile.
- -fno-pretty-templates
- When an error message refers to a specialization of a
function template, the compiler normally prints the signature of the
template followed by the template arguments and any typedefs or typenames
in the signature (e.g. "void f(T) [with T = int]" rather than
"void f(int)") so that it's clear which template is involved.
When an error message refers to a specialization of a class template, the
compiler omits any template arguments that match the default template
arguments for that template. If either of these behaviors make it harder
to understand the error message rather than easier, you can use
-fno-pretty-templates to disable them.
- -frepo
- Enable automatic template instantiation at link time. This
option also implies -fno-implicit-templates.
- -fno-rtti
- Disable generation of information about every class with
virtual functions for use by the C++ run-time type identification features
("dynamic_cast" and "typeid"). If you don't use those
parts of the language, you can save some space by using this flag. Note
that exception handling uses the same information, but G++ generates it as
needed. The "dynamic_cast" operator can still be used for casts
that do not require run-time type information, i.e. casts to "void
*" or to unambiguous base classes.
- -fsized-deallocation
- Enable the built-in global declarations
void operator delete (void *, std::size_t) noexcept;
void operator delete[] (void *, std::size_t) noexcept;
as introduced in C++14. This is useful for user-defined replacement
deallocation functions that, for example, use the size of the object to
make deallocation faster. Enabled by default under -std=c++14 and
above. The flag -Wsized-deallocation warns about places that might
want to add a definition.
- -fstats
- Emit statistics about front-end processing at the end of
the compilation. This information is generally only useful to the G++
development team.
- -fstrict-enums
- Allow the compiler to optimize using the assumption that a
value of enumerated type can only be one of the values of the enumeration
(as defined in the C++ standard; basically, a value that can be
represented in the minimum number of bits needed to represent all the
enumerators). This assumption may not be valid if the program uses a cast
to convert an arbitrary integer value to the enumerated type.
- -ftemplate-backtrace-limit=n
- Set the maximum number of template instantiation notes for
a single warning or error to n. The default value is 10.
- -ftemplate-depth=n
- Set the maximum instantiation depth for template classes to
n. A limit on the template instantiation depth is needed to detect
endless recursions during template class instantiation. ANSI/ISO C++
conforming programs must not rely on a maximum depth greater than 17
(changed to 1024 in C++11). The default value is 900, as the compiler can
run out of stack space before hitting 1024 in some situations.
- -fno-threadsafe-statics
- Do not emit the extra code to use the routines specified in
the C++ ABI for thread-safe initialization of local statics. You can use
this option to reduce code size slightly in code that doesn't need to be
thread-safe.
- -fuse-cxa-atexit
- Register destructors for objects with static storage
duration with the "__cxa_atexit" function rather than the
"atexit" function. This option is required for fully
standards-compliant handling of static destructors, but only works if your
C library supports "__cxa_atexit".
- -fno-use-cxa-get-exception-ptr
- Don't use the "__cxa_get_exception_ptr" runtime
routine. This causes "std::uncaught_exception" to be incorrect,
but is necessary if the runtime routine is not available.
- -fvisibility-inlines-hidden
- This switch declares that the user does not attempt to
compare pointers to inline functions or methods where the addresses of the
two functions are taken in different shared objects.
The effect of this is that GCC may, effectively, mark inline methods with
"__attribute__ ((visibility ("hidden")))" so that they
do not appear in the export table of a DSO and do not require a PLT
indirection when used within the DSO. Enabling this option can have a
dramatic effect on load and link times of a DSO as it massively reduces
the size of the dynamic export table when the library makes heavy use of
templates.
The behavior of this switch is not quite the same as marking the methods as
hidden directly, because it does not affect static variables local to the
function or cause the compiler to deduce that the function is defined in
only one shared object.
You may mark a method as having a visibility explicitly to negate the effect
of the switch for that method. For example, if you do want to compare
pointers to a particular inline method, you might mark it as having
default visibility. Marking the enclosing class with explicit visibility
has no effect.
Explicitly instantiated inline methods are unaffected by this option as
their linkage might otherwise cross a shared library boundary.
- -fvisibility-ms-compat
- This flag attempts to use visibility settings to make GCC's
C++ linkage model compatible with that of Microsoft Visual Studio.
The flag makes these changes to GCC's linkage model:
- 1.
- It sets the default visibility to "hidden", like
-fvisibility=hidden.
- 2.
- Types, but not their members, are not hidden by
default.
- 3.
- The One Definition Rule is relaxed for types without
explicit visibility specifications that are defined in more than one
shared object: those declarations are permitted if they are permitted when
this option is not used.
In new code it is better to use
-fvisibility=hidden and export those
classes that are intended to be externally visible. Unfortunately it is
possible for code to rely, perhaps accidentally, on the Visual Studio
behavior.
Among the consequences of these changes are that static data members of the same
type with the same name but defined in different shared objects are different,
so changing one does not change the other; and that pointers to function
members defined in different shared objects may not compare equal. When this
flag is given, it is a violation of the ODR to define types with the same name
differently.
- -fvtable-verify=[std|preinit|none]
- Turn on (or off, if using -fvtable-verify=none) the
security feature that verifies at run time, for every virtual call, that
the vtable pointer through which the call is made is valid for the type of
the object, and has not been corrupted or overwritten. If an invalid
vtable pointer is detected at run time, an error is reported and execution
of the program is immediately halted.
This option causes run-time data structures to be built at program startup,
which are used for verifying the vtable pointers. The options std
and preinit control the timing of when these data structures are
built. In both cases the data structures are built before execution
reaches "main". Using -fvtable-verify=std causes the data
structures to be built after shared libraries have been loaded and
initialized. -fvtable-verify=preinit causes them to be built before
shared libraries have been loaded and initialized.
If this option appears multiple times in the command line with different
values specified, none takes highest priority over both std
and preinit; preinit takes priority over std.
- -fvtv-debug
- When used in conjunction with -fvtable-verify=std or
-fvtable-verify=preinit, causes debug versions of the runtime
functions for the vtable verification feature to be called. This flag also
causes the compiler to log information about which vtable pointers it
finds for each class. This information is written to a file named
vtv_set_ptr_data.log in the directory named by the environment
variable VTV_LOGS_DIR if that is defined or the current working
directory otherwise.
Note: This feature appends data to the log file. If you want a fresh
log file, be sure to delete any existing one.
- -fvtv-counts
- This is a debugging flag. When used in conjunction with
-fvtable-verify=std or -fvtable-verify=preinit, this causes
the compiler to keep track of the total number of virtual calls it
encounters and the number of verifications it inserts. It also counts the
number of calls to certain run-time library functions that it inserts and
logs this information for each compilation unit. The compiler writes this
information to a file named vtv_count_data.log in the directory
named by the environment variable VTV_LOGS_DIR if that is defined
or the current working directory otherwise. It also counts the size of the
vtable pointer sets for each class, and writes this information to
vtv_class_set_sizes.log in the same directory.
Note: This feature appends data to the log files. To get fresh log
files, be sure to delete any existing ones.
- -fno-weak
- Do not use weak symbol support, even if it is provided by
the linker. By default, G++ uses weak symbols if they are available. This
option exists only for testing, and should not be used by end-users; it
results in inferior code and has no benefits. This option may be removed
in a future release of G++.
- -nostdinc++
- Do not search for header files in the standard directories
specific to C++, but do still search the other standard directories. (This
option is used when building the C++ library.)
In addition, these optimization, warning, and code generation options have
meanings only for C++ programs:
- -Wabi (C, Objective-C, C++ and Objective-C++
only)
- When an explicit -fabi-version=n option is
used, causes G++ to warn when it generates code that is probably not
compatible with the vendor-neutral C++ ABI. Since G++ now defaults to
-fabi-version=0, -Wabi has no effect unless either an older
ABI version is selected (with -fabi-version=n) or an older
compatibility version is selected (with -Wabi=n or
-fabi-compat-version= n).
Although an effort has been made to warn about all such cases, there are
probably some cases that are not warned about, even though G++ is
generating incompatible code. There may also be cases where warnings are
emitted even though the code that is generated is compatible.
You should rewrite your code to avoid these warnings if you are concerned
about the fact that code generated by G++ may not be binary compatible
with code generated by other compilers.
-Wabi can also be used with an explicit version number to warn about
compatibility with a particular -fabi-version level, e.g.
-Wabi=2 to warn about changes relative to -fabi-version=2.
Specifying a version number also sets
-fabi-compat-version=n.
The known incompatibilities in -fabi-version=2 (which was the default
from GCC 3.4 to 4.9) include:
- *
- A template with a non-type template parameter of reference
type was mangled incorrectly:
extern int N;
template <int &> struct S {};
void n (S<N>) {2}
This was fixed in -fabi-version=3.
- *
- SIMD vector types declared using "__attribute
((vector_size))" were mangled in a non-standard way that does not
allow for overloading of functions taking vectors of different sizes.
The mangling was changed in -fabi-version=4.
- *
- "__attribute ((const))" and "noreturn"
were mangled as type qualifiers, and "decltype" of a plain
declaration was folded away.
These mangling issues were fixed in -fabi-version=5.
- *
- Scoped enumerators passed as arguments to a variadic
function are promoted like unscoped enumerators, causing
"va_arg" to complain. On most targets this does not actually
affect the parameter passing ABI, as there is no way to pass an argument
smaller than "int".
Also, the ABI changed the mangling of template argument packs,
"const_cast", "static_cast", prefix
increment/decrement, and a class scope function used as a template
argument.
These issues were corrected in -fabi-version=6.
- *
- Lambdas in default argument scope were mangled incorrectly,
and the ABI changed the mangling of "nullptr_t".
These issues were corrected in -fabi-version=7.
- *
- When mangling a function type with function-cv-qualifiers,
the un-qualified function type was incorrectly treated as a substitution
candidate.
This was fixed in -fabi-version=8, the default for GCC 5.1.
- *
- "decltype(nullptr)" incorrectly had an alignment
of 1, leading to unaligned accesses. Note that this did not affect the ABI
of a function with a "nullptr_t" parameter, as parameters have a
minimum alignment.
This was fixed in -fabi-version=9, the default for GCC 5.2.
It also warns about psABI-related changes. The known psABI changes at this point
include:
- *
- For SysV/x86-64, unions with "long double"
members are passed in memory as specified in psABI. For example:
union U {
long double ld;
int i;
};
"union U" is always passed in memory.
- -Wabi-tag (C++ and Objective-C++ only)
- Warn when a type with an ABI tag is used in a context that
does not have that ABI tag. See C++ Attributes for more information
about ABI tags.
- -Wctor-dtor-privacy (C++ and Objective-C++
only)
- Warn when a class seems unusable because all the
constructors or destructors in that class are private, and it has neither
friends nor public static member functions. Also warn if there are no
non-private methods, and there's at least one private member function that
isn't a constructor or destructor.
- -Wdelete-non-virtual-dtor (C++ and Objective-C++
only)
- Warn when "delete" is used to destroy an instance
of a class that has virtual functions and non-virtual destructor. It is
unsafe to delete an instance of a derived class through a pointer to a
base class if the base class does not have a virtual destructor. This
warning is enabled by -Wall.
- -Wliteral-suffix (C++ and Objective-C++ only)
- Warn when a string or character literal is followed by a
ud-suffix which does not begin with an underscore. As a conforming
extension, GCC treats such suffixes as separate preprocessing tokens in
order to maintain backwards compatibility with code that uses formatting
macros from "<inttypes.h>". For example:
#define __STDC_FORMAT_MACROS
#include <inttypes.h>
#include <stdio.h>
int main() {
int64_t i64 = 123;
printf("My int64: %"PRId64"\n", i64);
}
In this case, "PRId64" is treated as a separate preprocessing
token.
This warning is enabled by default.
- -Wnarrowing (C++ and Objective-C++ only)
- Warn when a narrowing conversion prohibited by C++11 occurs
within { }, e.g.
int i = { 2.2 }; // error: narrowing from double to int
This flag is included in -Wall and -Wc++11-compat.
With -std=c++11, -Wno-narrowing suppresses the diagnostic
required by the standard. Note that this does not affect the meaning of
well-formed code; narrowing conversions are still considered ill-formed in
SFINAE context.
- -Wnoexcept (C++ and Objective-C++ only)
- Warn when a noexcept-expression evaluates to false because
of a call to a function that does not have a non-throwing exception
specification (i.e. "throw()" or "noexcept") but is
known by the compiler to never throw an exception.
- -Wnon-virtual-dtor (C++ and Objective-C++ only)
- Warn when a class has virtual functions and an accessible
non-virtual destructor itself or in an accessible polymorphic base class,
in which case it is possible but unsafe to delete an instance of a derived
class through a pointer to the class itself or base class. This warning is
automatically enabled if -Weffc++ is specified.
- -Wreorder (C++ and Objective-C++ only)
- Warn when the order of member initializers given in the
code does not match the order in which they must be executed. For
instance:
struct A {
int i;
int j;
A(): j (0), i (1) { }
};
The compiler rearranges the member initializers for "i" and
"j" to match the declaration order of the members, emitting a
warning to that effect. This warning is enabled by -Wall.
- -fext-numeric-literals (C++ and Objective-C++
only)
- Accept imaginary, fixed-point, or machine-defined literal
number suffixes as GNU extensions. When this option is turned off these
suffixes are treated as C++11 user-defined literal numeric suffixes. This
is on by default for all pre-C++11 dialects and all GNU dialects:
-std=c++98, -std=gnu++98, -std=gnu++11,
-std=gnu++14. This option is off by default for ISO C++11 onwards (
-std=c++11, ...).
The following
-W... options are not affected by
-Wall.
- -Weffc++ (C++ and Objective-C++ only)
- Warn about violations of the following style guidelines
from Scott Meyers' Effective C++ series of books:
- *
- Define a copy constructor and an assignment operator for
classes with dynamically-allocated memory.
- *
- Prefer initialization to assignment in constructors.
- *
- Have "operator=" return a reference to
*this.
- *
- Don't try to return a reference when you must return an
object.
- *
- Distinguish between prefix and postfix forms of increment
and decrement operators.
- *
- Never overload "&&", "||", or
",".
This option also enables
-Wnon-virtual-dtor, which is also one of the
effective C++ recommendations. However, the check is extended to warn about
the lack of virtual destructor in accessible non-polymorphic bases classes
too.
When selecting this option, be aware that the standard library headers do not
obey all of these guidelines; use
grep -v to filter out those
warnings.
- -Wstrict-null-sentinel (C++ and Objective-C++
only)
- Warn about the use of an uncasted "NULL" as
sentinel. When compiling only with GCC this is a valid sentinel, as
"NULL" is defined to "__null". Although it is a null
pointer constant rather than a null pointer, it is guaranteed to be of the
same size as a pointer. But this use is not portable across different
compilers.
- -Wno-non-template-friend (C++ and Objective-C++
only)
- Disable warnings when non-templatized friend functions are
declared within a template. Since the advent of explicit template
specification support in G++, if the name of the friend is an
unqualified-id (i.e., friend foo(int)), the C++ language
specification demands that the friend declare or define an ordinary,
nontemplate function. (Section 14.5.3). Before G++ implemented explicit
specification, unqualified-ids could be interpreted as a particular
specialization of a templatized function. Because this non-conforming
behavior is no longer the default behavior for G++,
-Wnon-template-friend allows the compiler to check existing code
for potential trouble spots and is on by default. This new compiler
behavior can be turned off with -Wno-non-template-friend, which
keeps the conformant compiler code but disables the helpful warning.
- -Wold-style-cast (C++ and Objective-C++ only)
- Warn if an old-style (C-style) cast to a non-void type is
used within a C++ program. The new-style casts ("dynamic_cast",
"static_cast", "reinterpret_cast", and
"const_cast") are less vulnerable to unintended effects and much
easier to search for.
- -Woverloaded-virtual (C++ and Objective-C++
only)
- Warn when a function declaration hides virtual functions
from a base class. For example, in:
struct A {
virtual void f();
};
struct B: public A {
void f(int);
};
the "A" class version of "f" is hidden in "B",
and code like:
B* b;
b->f();
fails to compile.
- -Wno-pmf-conversions (C++ and Objective-C++
only)
- Disable the diagnostic for converting a bound pointer to
member function to a plain pointer.
- -Wsign-promo (C++ and Objective-C++ only)
- Warn when overload resolution chooses a promotion from
unsigned or enumerated type to a signed type, over a conversion to an
unsigned type of the same size. Previous versions of G++ tried to preserve
unsignedness, but the standard mandates the current behavior.
Options Controlling Objective-C and Objective-C++ Dialects
(NOTE: This manual does not describe the Objective-C and Objective-C++ languages
themselves.
This section describes the command-line options that are only meaningful for
Objective-C and Objective-C++ programs. You can also use most of the
language-independent GNU compiler options. For example, you might compile a
file
some_class.m like this:
gcc -g -fgnu-runtime -O -c some_class.m
In this example,
-fgnu-runtime is an option meant only for Objective-C
and Objective-C++ programs; you can use the other options with any language
supported by GCC.
Note that since Objective-C is an extension of the C language, Objective-C
compilations may also use options specific to the C front-end (e.g.,
-Wtraditional). Similarly, Objective-C++ compilations may use
C++-specific options (e.g.,
-Wabi).
Here is a list of options that are
only for compiling Objective-C and
Objective-C++ programs:
- -fconstant-string-class=class-name
- Use class-name as the name of the class to
instantiate for each literal string specified with the syntax
"@"..."". The default class name is
"NXConstantString" if the GNU runtime is being used, and
"NSConstantString" if the NeXT runtime is being used (see
below). The -fconstant-cfstrings option, if also present, overrides
the -fconstant-string-class setting and cause
"@"..."" literals to be laid out as constant
CoreFoundation strings.
- -fgnu-runtime
- Generate object code compatible with the standard GNU
Objective-C runtime. This is the default for most types of systems.
- -fnext-runtime
- Generate output compatible with the NeXT runtime. This is
the default for NeXT-based systems, including Darwin and Mac OS X. The
macro "__NEXT_RUNTIME__" is predefined if (and only if) this
option is used.
- -fno-nil-receivers
- Assume that all Objective-C message dispatches
("[receiver message:arg]") in this translation unit ensure that
the receiver is not "nil". This allows for more efficient entry
points in the runtime to be used. This option is only available in
conjunction with the NeXT runtime and ABI version 0 or 1.
- -fobjc-abi-version=n
- Use version n of the Objective-C ABI for the
selected runtime. This option is currently supported only for the NeXT
runtime. In that case, Version 0 is the traditional (32-bit) ABI without
support for properties and other Objective-C 2.0 additions. Version 1 is
the traditional (32-bit) ABI with support for properties and other
Objective-C 2.0 additions. Version 2 is the modern (64-bit) ABI. If
nothing is specified, the default is Version 0 on 32-bit target machines,
and Version 2 on 64-bit target machines.
- -fobjc-call-cxx-cdtors
- For each Objective-C class, check if any of its instance
variables is a C++ object with a non-trivial default constructor. If so,
synthesize a special "- (id) .cxx_construct" instance method
which runs non-trivial default constructors on any such instance
variables, in order, and then return "self". Similarly, check if
any instance variable is a C++ object with a non-trivial destructor, and
if so, synthesize a special "- (void) .cxx_destruct" method
which runs all such default destructors, in reverse order.
The "- (id) .cxx_construct" and "- (void) .cxx_destruct"
methods thusly generated only operate on instance variables declared in
the current Objective-C class, and not those inherited from superclasses.
It is the responsibility of the Objective-C runtime to invoke all such
methods in an object's inheritance hierarchy. The "- (id)
.cxx_construct" methods are invoked by the runtime immediately after
a new object instance is allocated; the "- (void) .cxx_destruct"
methods are invoked immediately before the runtime deallocates an object
instance.
As of this writing, only the NeXT runtime on Mac OS X 10.4 and later has
support for invoking the "- (id) .cxx_construct" and "-
(void) .cxx_destruct" methods.
- -fobjc-direct-dispatch
- Allow fast jumps to the message dispatcher. On Darwin this
is accomplished via the comm page.
- -fobjc-exceptions
- Enable syntactic support for structured exception handling
in Objective-C, similar to what is offered by C++ and Java. This option is
required to use the Objective-C keywords @try, @throw, @catch, @finally
and @synchronized. This option is available with both the GNU runtime and
the NeXT runtime (but not available in conjunction with the NeXT runtime
on Mac OS X 10.2 and earlier).
- -fobjc-gc
- Enable garbage collection (GC) in Objective-C and
Objective-C++ programs. This option is only available with the NeXT
runtime; the GNU runtime has a different garbage collection implementation
that does not require special compiler flags.
- -fobjc-nilcheck
- For the NeXT runtime with version 2 of the ABI, check for a
nil receiver in method invocations before doing the actual method call.
This is the default and can be disabled using -fno-objc-nilcheck.
Class methods and super calls are never checked for nil in this way no
matter what this flag is set to. Currently this flag does nothing when the
GNU runtime, or an older version of the NeXT runtime ABI, is used.
- -fobjc-std=objc1
- Conform to the language syntax of Objective-C 1.0, the
language recognized by GCC 4.0. This only affects the Objective-C
additions to the C/C++ language; it does not affect conformance to C/C++
standards, which is controlled by the separate C/C++ dialect option flags.
When this option is used with the Objective-C or Objective-C++ compiler,
any Objective-C syntax that is not recognized by GCC 4.0 is rejected. This
is useful if you need to make sure that your Objective-C code can be
compiled with older versions of GCC.
- -freplace-objc-classes
- Emit a special marker instructing
ld(1) not to statically link in the resulting
object file, and allow dyld(1) to load it in
at run time instead. This is used in conjunction with the Fix-and-Continue
debugging mode, where the object file in question may be recompiled and
dynamically reloaded in the course of program execution, without the need
to restart the program itself. Currently, Fix-and-Continue functionality
is only available in conjunction with the NeXT runtime on Mac OS X 10.3
and later.
- -fzero-link
- When compiling for the NeXT runtime, the compiler
ordinarily replaces calls to "objc_getClass("...")"
(when the name of the class is known at compile time) with static class
references that get initialized at load time, which improves run-time
performance. Specifying the -fzero-link flag suppresses this
behavior and causes calls to "objc_getClass("...")" to
be retained. This is useful in Zero-Link debugging mode, since it allows
for individual class implementations to be modified during program
execution. The GNU runtime currently always retains calls to
"objc_get_class("...")" regardless of command-line
options.
- -fno-local-ivars
- By default instance variables in Objective-C can be
accessed as if they were local variables from within the methods of the
class they're declared in. This can lead to shadowing between instance
variables and other variables declared either locally inside a class
method or globally with the same name. Specifying the
-fno-local-ivars flag disables this behavior thus avoiding variable
shadowing issues.
- -fivar-visibility=[public|protected|private|package]
- Set the default instance variable visibility to the
specified option so that instance variables declared outside the scope of
any access modifier directives default to the specified visibility.
- -gen-decls
- Dump interface declarations for all classes seen in the
source file to a file named sourcename.decl.
- -Wassign-intercept (Objective-C and Objective-C++
only)
- Warn whenever an Objective-C assignment is being
intercepted by the garbage collector.
- -Wno-protocol (Objective-C and Objective-C++
only)
- If a class is declared to implement a protocol, a warning
is issued for every method in the protocol that is not implemented by the
class. The default behavior is to issue a warning for every method not
explicitly implemented in the class, even if a method implementation is
inherited from the superclass. If you use the -Wno-protocol option,
then methods inherited from the superclass are considered to be
implemented, and no warning is issued for them.
- -Wselector (Objective-C and Objective-C++ only)
- Warn if multiple methods of different types for the same
selector are found during compilation. The check is performed on the list
of methods in the final stage of compilation. Additionally, a check is
performed for each selector appearing in a "@selector(...)"
expression, and a corresponding method for that selector has been found
during compilation. Because these checks scan the method table only at the
end of compilation, these warnings are not produced if the final stage of
compilation is not reached, for example because an error is found during
compilation, or because the -fsyntax-only option is being
used.
- -Wstrict-selector-match (Objective-C and
Objective-C++ only)
- Warn if multiple methods with differing argument and/or
return types are found for a given selector when attempting to send a
message using this selector to a receiver of type "id" or
"Class". When this flag is off (which is the default behavior),
the compiler omits such warnings if any differences found are confined to
types that share the same size and alignment.
- -Wundeclared-selector (Objective-C and Objective-C++
only)
- Warn if a "@selector(...)" expression referring
to an undeclared selector is found. A selector is considered undeclared if
no method with that name has been declared before the
"@selector(...)" expression, either explicitly in an @interface
or @protocol declaration, or implicitly in an @implementation section.
This option always performs its checks as soon as a
"@selector(...)" expression is found, while -Wselector
only performs its checks in the final stage of compilation. This also
enforces the coding style convention that methods and selectors must be
declared before being used.
- -print-objc-runtime-info
- Generate C header describing the largest structure that is
passed by value, if any.
Options to Control Diagnostic Messages Formatting
Traditionally, diagnostic messages have been formatted irrespective of the
output device's aspect (e.g. its width, ...). You can use the options
described below to control the formatting algorithm for diagnostic messages,
e.g. how many characters per line, how often source location information
should be reported. Note that some language front ends may not honor these
options.
- -fmessage-length=n
- Try to format error messages so that they fit on lines of
about n characters. If n is zero, then no line-wrapping is
done; each error message appears on a single line. This is the default for
all front ends.
- -fdiagnostics-show-location=once
- Only meaningful in line-wrapping mode. Instructs the
diagnostic messages reporter to emit source location information
once; that is, in case the message is too long to fit on a single
physical line and has to be wrapped, the source location won't be emitted
(as prefix) again, over and over, in subsequent continuation lines. This
is the default behavior.
- -fdiagnostics-show-location=every-line
- Only meaningful in line-wrapping mode. Instructs the
diagnostic messages reporter to emit the same source location information
(as prefix) for physical lines that result from the process of breaking a
message which is too long to fit on a single line.
- -fdiagnostics-color[=WHEN]
- -fno-diagnostics-color
- Use color in diagnostics. WHEN is never,
always, or auto. The default depends on how the compiler has
been configured, it can be any of the above WHEN options or also
never if GCC_COLORS environment variable isn't present in
the environment, and auto otherwise. auto means to use color
only when the standard error is a terminal. The forms
-fdiagnostics-color and -fno-diagnostics-color are aliases
for -fdiagnostics-color=always and
-fdiagnostics-color=never, respectively.
The colors are defined by the environment variable GCC_COLORS. Its
value is a colon-separated list of capabilities and Select Graphic
Rendition (SGR) substrings. SGR commands are interpreted by the terminal
or terminal emulator. (See the section in the documentation of your text
terminal for permitted values and their meanings as character attributes.)
These substring values are integers in decimal representation and can be
concatenated with semicolons. Common values to concatenate include
1 for bold, 4 for underline, 5 for blink, 7
for inverse, 39 for default foreground color, 30 to
37 for foreground colors, 90 to 97 for 16-color mode
foreground colors, 38;5;0 to 38;5;255 for 88-color and
256-color modes foreground colors, 49 for default background color,
40 to 47 for background colors, 100 to 107 for
16-color mode background colors, and 48;5;0 to 48;5;255 for
88-color and 256-color modes background colors.
The default GCC_COLORS is
error=01;31:warning=01;35:note=01;36:caret=01;32:locus=01:quote=01
where 01;31 is bold red, 01;35 is bold magenta, 01;36
is bold cyan, 01;32 is bold green and 01 is bold. Setting
GCC_COLORS to the empty string disables colors. Supported
capabilities are as follows.
- "error="
- SGR substring for error: markers.
- "warning="
- SGR substring for warning: markers.
- "note="
- SGR substring for note: markers.
- "caret="
- SGR substring for caret line.
- "locus="
- SGR substring for location information, file:line or
file:line:column etc.
- "quote="
- SGR substring for information printed within quotes.
- -fno-diagnostics-show-option
- By default, each diagnostic emitted includes text
indicating the command-line option that directly controls the diagnostic
(if such an option is known to the diagnostic machinery). Specifying the
-fno-diagnostics-show-option flag suppresses that behavior.
- -fno-diagnostics-show-caret
- By default, each diagnostic emitted includes the original
source line and a caret '^' indicating the column. This option suppresses
this information. The source line is truncated to n characters, if
the -fmessage-length=n option is given. When the output is done to
the terminal, the width is limited to the width given by the
COLUMNS environment variable or, if not set, to the terminal
width.
Options to Request or Suppress Warnings
Warnings are diagnostic messages that report constructions that are not
inherently erroneous but that are risky or suggest there may have been an
error.
The following language-independent options do not enable specific warnings but
control the kinds of diagnostics produced by GCC.
- -fsyntax-only
- Check the code for syntax errors, but don't do anything
beyond that.
- -fmax-errors=n
- Limits the maximum number of error messages to n, at
which point GCC bails out rather than attempting to continue processing
the source code. If n is 0 (the default), there is no limit on the
number of error messages produced. If -Wfatal-errors is also
specified, then -Wfatal-errors takes precedence over this
option.
- -w
- Inhibit all warning messages.
- -Werror
- Make all warnings into errors.
- -Werror=
- Make the specified warning into an error. The specifier for
a warning is appended; for example -Werror=switch turns the
warnings controlled by -Wswitch into errors. This switch takes a
negative form, to be used to negate -Werror for specific warnings;
for example -Wno-error=switch makes -Wswitch warnings not be
errors, even when -Werror is in effect.
The warning message for each controllable warning includes the option that
controls the warning. That option can then be used with -Werror=
and -Wno-error= as described above. (Printing of the option in the
warning message can be disabled using the
-fno-diagnostics-show-option flag.)
Note that specifying -Werror=foo automatically implies
-W foo. However, -Wno-error=foo does not imply
anything.
- -Wfatal-errors
- This option causes the compiler to abort compilation on the
first error occurred rather than trying to keep going and printing further
error messages.
You can request many specific warnings with options beginning with
-W,
for example
-Wimplicit to request warnings on implicit declarations.
Each of these specific warning options also has a negative form beginning
-Wno- to turn off warnings; for example,
-Wno-implicit. This
manual lists only one of the two forms, whichever is not the default. For
further language-specific options also refer to
C++ Dialect Options and
Objective-C and Objective-C++ Dialect Options.
Some options, such as
-Wall and
-Wextra, turn on other options,
such as
-Wunused, which may turn on further options, such as
-Wunused-value. The combined effect of positive and negative forms is
that more specific options have priority over less specific ones,
independently of their position in the command-line. For options of the same
specificity, the last one takes effect. Options enabled or disabled via
pragmas take effect as if they appeared at the end of the command-line.
When an unrecognized warning option is requested (e.g.,
-Wunknown-warning), GCC emits a diagnostic stating that the option is
not recognized. However, if the
-Wno- form is used, the behavior is
slightly different: no diagnostic is produced for
-Wno-unknown-warning
unless other diagnostics are being produced. This allows the use of new
-Wno- options with old compilers, but if something goes wrong, the
compiler warns that an unrecognized option is present.
- -Wpedantic
- -pedantic
- Issue all the warnings demanded by strict ISO C and ISO
C++; reject all programs that use forbidden extensions, and some other
programs that do not follow ISO C and ISO C++. For ISO C, follows the
version of the ISO C standard specified by any -std option used.
Valid ISO C and ISO C++ programs should compile properly with or without
this option (though a rare few require -ansi or a -std
option specifying the required version of ISO C). However, without this
option, certain GNU extensions and traditional C and C++ features are
supported as well. With this option, they are rejected.
-Wpedantic does not cause warning messages for use of the alternate
keywords whose names begin and end with __. Pedantic warnings are
also disabled in the expression that follows "__extension__".
However, only system header files should use these escape routes;
application programs should avoid them.
Some users try to use -Wpedantic to check programs for strict ISO C
conformance. They soon find that it does not do quite what they want: it
finds some non-ISO practices, but not all---only those for which ISO C
requires a diagnostic, and some others for which diagnostics have
been added.
A feature to report any failure to conform to ISO C might be useful in some
instances, but would require considerable additional work and would be
quite different from -Wpedantic. We don't have plans to support
such a feature in the near future.
Where the standard specified with -std represents a GNU extended
dialect of C, such as gnu90 or gnu99, there is a
corresponding base standard, the version of ISO C on which the GNU
extended dialect is based. Warnings from -Wpedantic are given where
they are required by the base standard. (It does not make sense for such
warnings to be given only for features not in the specified GNU C dialect,
since by definition the GNU dialects of C include all features the
compiler supports with the given option, and there would be nothing to
warn about.)
- -pedantic-errors
- Give an error whenever the base standard (see
-Wpedantic) requires a diagnostic, in some cases where there is
undefined behavior at compile-time and in some other cases that do not
prevent compilation of programs that are valid according to the standard.
This is not equivalent to -Werror=pedantic, since there are errors
enabled by this option and not enabled by the latter and vice versa.
- -Wall
- This enables all the warnings about constructions that some
users consider questionable, and that are easy to avoid (or modify to
prevent the warning), even in conjunction with macros. This also enables
some language-specific warnings described in C++ Dialect
Options and Objective-C and Objective-C++ Dialect Options.
-Wall turns on the following warning flags:
-Waddress -Warray-bounds=1 (only with -O2)
-Wc++11-compat -Wc++14-compat -Wchar-subscripts
-Wenum-compare (in C/ObjC; this is on by default in C++)
-Wimplicit-int (C and Objective-C only)
-Wimplicit-function-declaration (C and Objective-C only)
-Wcomment -Wformat -Wmain (only for C/ObjC and
unless -ffreestanding) -Wmaybe-uninitialized
-Wmissing-braces (only for C/ObjC) -Wnonnull
-Wopenmp-simd -Wparentheses -Wpointer-sign
-Wreorder -Wreturn-type -Wsequence-point
-Wsign-compare (only in C++) -Wstrict-aliasing
-Wstrict-overflow=1 -Wswitch -Wtrigraphs
-Wuninitialized -Wunknown-pragmas -Wunused-function
-Wunused-label -Wunused-value -Wunused-variable
-Wvolatile-register-var
Note that some warning flags are not implied by -Wall. Some of them
warn about constructions that users generally do not consider
questionable, but which occasionally you might wish to check for; others
warn about constructions that are necessary or hard to avoid in some
cases, and there is no simple way to modify the code to suppress the
warning. Some of them are enabled by -Wextra but many of them must
be enabled individually.
- -Wextra
- This enables some extra warning flags that are not enabled
by -Wall. (This option used to be called -W. The older name
is still supported, but the newer name is more descriptive.)
-Wclobbered -Wempty-body -Wignored-qualifiers
-Wmissing-field-initializers -Wmissing-parameter-type (C
only) -Wold-style-declaration (C only) -Woverride-init
-Wsign-compare -Wtype-limits -Wuninitialized
-Wunused-parameter (only with -Wunused or
-Wall) -Wunused-but-set-parameter (only with
-Wunused or -Wall)
The option -Wextra also prints warning messages for the following
cases:
- *
- A pointer is compared against integer zero with
"<", "<=", ">", or
">=".
- *
- (C++ only) An enumerator and a non-enumerator both appear
in a conditional expression.
- *
- (C++ only) Ambiguous virtual bases.
- *
- (C++ only) Subscripting an array that has been declared
"register".
- *
- (C++ only) Taking the address of a variable that has been
declared "register".
- *
- (C++ only) A base class is not initialized in a derived
class's copy constructor.
- -Wchar-subscripts
- Warn if an array subscript has type "char". This
is a common cause of error, as programmers often forget that this type is
signed on some machines. This warning is enabled by -Wall.
- -Wcomment
- Warn whenever a comment-start sequence /* appears in
a /* comment, or whenever a Backslash-Newline appears in a
// comment. This warning is enabled by -Wall.
- -Wno-coverage-mismatch
- Warn if feedback profiles do not match when using the
-fprofile-use option. If a source file is changed between compiling
with -fprofile-gen and with -fprofile-use, the files with
the profile feedback can fail to match the source file and GCC cannot use
the profile feedback information. By default, this warning is enabled and
is treated as an error. -Wno-coverage-mismatch can be used to
disable the warning or -Wno-error=coverage-mismatch can be used to
disable the error. Disabling the error for this warning can result in
poorly optimized code and is useful only in the case of very minor changes
such as bug fixes to an existing code-base. Completely disabling the
warning is not recommended.
- -Wno-cpp
- (C, Objective-C, C++, Objective-C++ and Fortran only)
Suppress warning messages emitted by "#warning" directives.
- -Wdouble-promotion (C, C++, Objective-C and
Objective-C++ only)
- Give a warning when a value of type "float" is
implicitly promoted to "double". CPUs with a 32-bit
"single-precision" floating-point unit implement
"float" in hardware, but emulate "double" in software.
On such a machine, doing computations using "double" values is
much more expensive because of the overhead required for software
emulation.
It is easy to accidentally do computations with "double" because
floating-point literals are implicitly of type "double". For
example, in:
float area(float radius)
{
return 3.14159 * radius * radius;
}
the compiler performs the entire computation with "double" because
the floating-point literal is a "double".
- -Wformat
- -Wformat=n
- Check calls to "printf" and "scanf",
etc., to make sure that the arguments supplied have types appropriate to
the format string specified, and that the conversions specified in the
format string make sense. This includes standard functions, and others
specified by format attributes, in the "printf",
"scanf", "strftime" and "strfmon" (an X/Open
extension, not in the C standard) families (or other target-specific
families). Which functions are checked without format attributes having
been specified depends on the standard version selected, and such checks
of functions without the attribute specified are disabled by
-ffreestanding or -fno-builtin.
The formats are checked against the format features supported by GNU libc
version 2.2. These include all ISO C90 and C99 features, as well as
features from the Single Unix Specification and some BSD and GNU
extensions. Other library implementations may not support all these
features; GCC does not support warning about features that go beyond a
particular library's limitations. However, if -Wpedantic is used
with -Wformat, warnings are given about format features not in the
selected standard version (but not for "strfmon" formats, since
those are not in any version of the C standard).
- -Wformat=1
- -Wformat
- Option -Wformat is equivalent to -Wformat=1,
and -Wno-format is equivalent to -Wformat=0. Since
-Wformat also checks for null format arguments for several
functions, -Wformat also implies -Wnonnull. Some aspects of
this level of format checking can be disabled by the options:
-Wno-format-contains-nul, -Wno-format-extra-args, and
-Wno-format-zero-length. -Wformat is enabled by
-Wall.
- -Wno-format-contains-nul
- If -Wformat is specified, do not warn about format
strings that contain NUL bytes.
- -Wno-format-extra-args
- If -Wformat is specified, do not warn about excess
arguments to a "printf" or "scanf" format function.
The C standard specifies that such arguments are ignored.
Where the unused arguments lie between used arguments that are specified
with $ operand number specifications, normally warnings are still
given, since the implementation could not know what type to pass to
"va_arg" to skip the unused arguments. However, in the case of
"scanf" formats, this option suppresses the warning if the
unused arguments are all pointers, since the Single Unix Specification
says that such unused arguments are allowed.
- -Wno-format-zero-length
- If -Wformat is specified, do not warn about
zero-length formats. The C standard specifies that zero-length formats are
allowed.
- -Wformat=2
- Enable -Wformat plus additional format checks.
Currently equivalent to -Wformat -Wformat-nonliteral
-Wformat-security -Wformat-y2k.
- -Wformat-nonliteral
- If -Wformat is specified, also warn if the format
string is not a string literal and so cannot be checked, unless the format
function takes its format arguments as a "va_list".
- -Wformat-security
- If -Wformat is specified, also warn about uses of
format functions that represent possible security problems. At present,
this warns about calls to "printf" and "scanf"
functions where the format string is not a string literal and there are no
format arguments, as in "printf (foo);". This may be a security
hole if the format string came from untrusted input and contains
%n. (This is currently a subset of what
-Wformat-nonliteral warns about, but in future warnings may be
added to -Wformat-security that are not included in
-Wformat-nonliteral.)
- -Wformat-signedness
- If -Wformat is specified, also warn if the format
string requires an unsigned argument and the argument is signed and vice
versa.
- -Wformat-y2k
- If -Wformat is specified, also warn about
"strftime" formats that may yield only a two-digit year.
- -Wnonnull
- Warn about passing a null pointer for arguments marked as
requiring a non-null value by the "nonnull" function attribute.
-Wnonnull is included in -Wall and -Wformat. It can be
disabled with the -Wno-nonnull option.
- -Winit-self (C, C++, Objective-C and Objective-C++
only)
- Warn about uninitialized variables that are initialized
with themselves. Note this option can only be used with the
-Wuninitialized option.
For example, GCC warns about "i" being uninitialized in the
following snippet only when -Winit-self has been specified:
int f()
{
int i = i;
return i;
}
This warning is enabled by -Wall in C++.
- -Wimplicit-int (C and Objective-C only)
- Warn when a declaration does not specify a type. This
warning is enabled by -Wall.
- -Wimplicit-function-declaration (C and Objective-C
only)
- Give a warning whenever a function is used before being
declared. In C99 mode ( -std=c99 or -std=gnu99), this
warning is enabled by default and it is made into an error by
-pedantic-errors. This warning is also enabled by
-Wall.
- -Wimplicit (C and Objective-C only)
- Same as -Wimplicit-int and
-Wimplicit-function-declaration. This warning is enabled by
-Wall.
- -Wignored-qualifiers (C and C++ only)
- Warn if the return type of a function has a type qualifier
such as "const". For ISO C such a type qualifier has no effect,
since the value returned by a function is not an lvalue. For C++, the
warning is only emitted for scalar types or "void". ISO C
prohibits qualified "void" return types on function definitions,
so such return types always receive a warning even without this option.
This warning is also enabled by -Wextra.
- -Wmain
- Warn if the type of "main" is suspicious.
"main" should be a function with external linkage, returning
int, taking either zero arguments, two, or three arguments of appropriate
types. This warning is enabled by default in C++ and is enabled by either
-Wall or -Wpedantic.
- -Wmissing-braces
- Warn if an aggregate or union initializer is not fully
bracketed. In the following example, the initializer for "a" is
not fully bracketed, but that for "b" is fully bracketed. This
warning is enabled by -Wall in C.
int a[2][2] = { 0, 1, 2, 3 };
int b[2][2] = { { 0, 1 }, { 2, 3 } };
This warning is enabled by -Wall.
- -Wmissing-include-dirs (C, C++, Objective-C and
Objective-C++ only)
- Warn if a user-supplied include directory does not
exist.
- -Wparentheses
- Warn if parentheses are omitted in certain contexts, such
as when there is an assignment in a context where a truth value is
expected, or when operators are nested whose precedence people often get
confused about.
Also warn if a comparison like "x<=y<=z" appears; this is
equivalent to "(x<=y ? 1 : 0) <= z", which is a different
interpretation from that of ordinary mathematical notation.
Also warn about constructions where there may be confusion to which
"if" statement an "else" branch belongs. Here is an
example of such a case:
{
if (a)
if (b)
foo ();
else
bar ();
}
In C/C++, every "else" branch belongs to the innermost possible
"if" statement, which in this example is "if (b)".
This is often not what the programmer expected, as illustrated in the
above example by indentation the programmer chose. When there is the
potential for this confusion, GCC issues a warning when this flag is
specified. To eliminate the warning, add explicit braces around the
innermost "if" statement so there is no way the "else"
can belong to the enclosing "if". The resulting code looks like
this:
{
if (a)
{
if (b)
foo ();
else
bar ();
}
}
Also warn for dangerous uses of the GNU extension to "?:" with
omitted middle operand. When the condition in the "?": operator
is a boolean expression, the omitted value is always 1. Often programmers
expect it to be a value computed inside the conditional expression
instead.
This warning is enabled by -Wall.
- -Wsequence-point
- Warn about code that may have undefined semantics because
of violations of sequence point rules in the C and C++ standards.
The C and C++ standards define the order in which expressions in a C/C++
program are evaluated in terms of sequence points, which represent
a partial ordering between the execution of parts of the program: those
executed before the sequence point, and those executed after it. These
occur after the evaluation of a full expression (one which is not part of
a larger expression), after the evaluation of the first operand of a
"&&", "||", "? :" or ","
(comma) operator, before a function is called (but after the evaluation of
its arguments and the expression denoting the called function), and in
certain other places. Other than as expressed by the sequence point rules,
the order of evaluation of subexpressions of an expression is not
specified. All these rules describe only a partial order rather than a
total order, since, for example, if two functions are called within one
expression with no sequence point between them, the order in which the
functions are called is not specified. However, the standards committee
have ruled that function calls do not overlap.
It is not specified when between sequence points modifications to the values
of objects take effect. Programs whose behavior depends on this have
undefined behavior; the C and C++ standards specify that "Between the
previous and next sequence point an object shall have its stored value
modified at most once by the evaluation of an expression. Furthermore, the
prior value shall be read only to determine the value to be stored.".
If a program breaks these rules, the results on any particular
implementation are entirely unpredictable.
Examples of code with undefined behavior are "a = a++;",
"a[n] = b[n++]" and "a[i++] = i;". Some more
complicated cases are not diagnosed by this option, and it may give an
occasional false positive result, but in general it has been found fairly
effective at detecting this sort of problem in programs.
The standard is worded confusingly, therefore there is some debate over the
precise meaning of the sequence point rules in subtle cases. Links to
discussions of the problem, including proposed formal definitions, may be
found on the GCC readings page, at <
http://gcc.gnu.org/readings.html>.
This warning is enabled by -Wall for C and C++.
- -Wno-return-local-addr
- Do not warn about returning a pointer (or in C++, a
reference) to a variable that goes out of scope after the function
returns.
- -Wreturn-type
- Warn whenever a function is defined with a return type that
defaults to "int". Also warn about any "return"
statement with no return value in a function whose return type is not
"void" (falling off the end of the function body is considered
returning without a value), and about a "return" statement with
an expression in a function whose return type is "void".
For C++, a function without return type always produces a diagnostic
message, even when -Wno-return-type is specified. The only
exceptions are "main" and functions defined in system headers.
This warning is enabled by -Wall.
- -Wshift-count-negative
- Warn if shift count is negative. This warning is enabled by
default.
- -Wshift-count-overflow
- Warn if shift count >= width of type. This warning is
enabled by default.
- -Wswitch
- Warn whenever a "switch" statement has an index
of enumerated type and lacks a "case" for one or more of the
named codes of that enumeration. (The presence of a "default"
label prevents this warning.) "case" labels outside the
enumeration range also provoke warnings when this option is used (even if
there is a "default" label). This warning is enabled by
-Wall.
- -Wswitch-default
- Warn whenever a "switch" statement does not have
a "default" case.
- -Wswitch-enum
- Warn whenever a "switch" statement has an index
of enumerated type and lacks a "case" for one or more of the
named codes of that enumeration. "case" labels outside the
enumeration range also provoke warnings when this option is used. The only
difference between -Wswitch and this option is that this option
gives a warning about an omitted enumeration code even if there is a
"default" label.
- -Wswitch-bool
- Warn whenever a "switch" statement has an index
of boolean type. It is possible to suppress this warning by casting the
controlling expression to a type other than "bool". For example:
switch ((int) (a == 4))
{
...
}
This warning is enabled by default for C and C++ programs.
- -Wsync-nand (C and C++ only)
- Warn when "__sync_fetch_and_nand" and
"__sync_nand_and_fetch" built-in functions are used. These
functions changed semantics in GCC 4.4.
- -Wtrigraphs
- Warn if any trigraphs are encountered that might change the
meaning of the program (trigraphs within comments are not warned about).
This warning is enabled by -Wall.
- -Wunused-but-set-parameter
- Warn whenever a function parameter is assigned to, but
otherwise unused (aside from its declaration).
To suppress this warning use the "unused" attribute.
This warning is also enabled by -Wunused together with
-Wextra.
- -Wunused-but-set-variable
- Warn whenever a local variable is assigned to, but
otherwise unused (aside from its declaration). This warning is enabled by
-Wall.
To suppress this warning use the "unused" attribute.
This warning is also enabled by -Wunused, which is enabled by
-Wall.
- -Wunused-function
- Warn whenever a static function is declared but not defined
or a non-inline static function is unused. This warning is enabled by
-Wall.
- -Wunused-label
- Warn whenever a label is declared but not used. This
warning is enabled by -Wall.
To suppress this warning use the "unused" attribute.
- -Wunused-local-typedefs (C, Objective-C, C++ and
Objective-C++ only)
- Warn when a typedef locally defined in a function is not
used. This warning is enabled by -Wall.
- -Wunused-parameter
- Warn whenever a function parameter is unused aside from its
declaration.
To suppress this warning use the "unused" attribute.
- -Wno-unused-result
- Do not warn if a caller of a function marked with attribute
"warn_unused_result" does not use its return value. The default
is -Wunused-result.
- -Wunused-variable
- Warn whenever a local variable or non-constant static
variable is unused aside from its declaration. This warning is enabled by
-Wall.
To suppress this warning use the "unused" attribute.
- -Wunused-value
- Warn whenever a statement computes a result that is
explicitly not used. To suppress this warning cast the unused expression
to "void". This includes an expression-statement or the
left-hand side of a comma expression that contains no side effects. For
example, an expression such as "x[i,j]" causes a warning, while
"x[(void)i,j]" does not.
This warning is enabled by -Wall.
- -Wunused
- All the above -Wunused options combined.
In order to get a warning about an unused function parameter, you must
either specify -Wextra -Wunused (note that -Wall implies
-Wunused), or separately specify -Wunused-parameter.
- -Wuninitialized
- Warn if an automatic variable is used without first being
initialized or if a variable may be clobbered by a "setjmp"
call. In C++, warn if a non-static reference or non-static
"const" member appears in a class without constructors.
If you want to warn about code that uses the uninitialized value of the
variable in its own initializer, use the -Winit-self option.
These warnings occur for individual uninitialized or clobbered elements of
structure, union or array variables as well as for variables that are
uninitialized or clobbered as a whole. They do not occur for variables or
elements declared "volatile". Because these warnings depend on
optimization, the exact variables or elements for which there are warnings
depends on the precise optimization options and version of GCC used.
Note that there may be no warning about a variable that is used only to
compute a value that itself is never used, because such computations may
be deleted by data flow analysis before the warnings are printed.
- -Wmaybe-uninitialized
- For an automatic variable, if there exists a path from the
function entry to a use of the variable that is initialized, but there
exist some other paths for which the variable is not initialized, the
compiler emits a warning if it cannot prove the uninitialized paths are
not executed at run time. These warnings are made optional because GCC is
not smart enough to see all the reasons why the code might be correct in
spite of appearing to have an error. Here is one example of how this can
happen:
{
int x;
switch (y)
{
case 1: x = 1;
break;
case 2: x = 4;
break;
case 3: x = 5;
}
foo (x);
}
If the value of "y" is always 1, 2 or 3, then "x" is
always initialized, but GCC doesn't know this. To suppress the warning,
you need to provide a default case with assert(0) or similar code.
This option also warns when a non-volatile automatic variable might be
changed by a call to "longjmp". These warnings as well are
possible only in optimizing compilation.
The compiler sees only the calls to "setjmp". It cannot know where
"longjmp" will be called; in fact, a signal handler could call
it at any point in the code. As a result, you may get a warning even when
there is in fact no problem because "longjmp" cannot in fact be
called at the place that would cause a problem.
Some spurious warnings can be avoided if you declare all the functions you
use that never return as "noreturn".
This warning is enabled by -Wall or -Wextra.
- -Wunknown-pragmas
- Warn when a "#pragma" directive is encountered
that is not understood by GCC. If this command-line option is used,
warnings are even issued for unknown pragmas in system header files. This
is not the case if the warnings are only enabled by the -Wall
command-line option.
- -Wno-pragmas
- Do not warn about misuses of pragmas, such as incorrect
parameters, invalid syntax, or conflicts between pragmas. See also
-Wunknown-pragmas.
- -Wstrict-aliasing
- This option is only active when -fstrict-aliasing is
active. It warns about code that might break the strict aliasing rules
that the compiler is using for optimization. The warning does not catch
all cases, but does attempt to catch the more common pitfalls. It is
included in -Wall. It is equivalent to
-Wstrict-aliasing=3
- -Wstrict-aliasing=n
- This option is only active when -fstrict-aliasing is
active. It warns about code that might break the strict aliasing rules
that the compiler is using for optimization. Higher levels correspond to
higher accuracy (fewer false positives). Higher levels also correspond to
more effort, similar to the way -O works. -Wstrict-aliasing
is equivalent to -Wstrict-aliasing=3.
Level 1: Most aggressive, quick, least accurate. Possibly useful when higher
levels do not warn but -fstrict-aliasing still breaks the code, as
it has very few false negatives. However, it has many false positives.
Warns for all pointer conversions between possibly incompatible types,
even if never dereferenced. Runs in the front end only.
Level 2: Aggressive, quick, not too precise. May still have many false
positives (not as many as level 1 though), and few false negatives (but
possibly more than level 1). Unlike level 1, it only warns when an address
is taken. Warns about incomplete types. Runs in the front end only.
Level 3 (default for -Wstrict-aliasing): Should have very few false
positives and few false negatives. Slightly slower than levels 1 or 2 when
optimization is enabled. Takes care of the common pun+dereference pattern
in the front end: "*(int*)&some_float". If optimization is
enabled, it also runs in the back end, where it deals with multiple
statement cases using flow-sensitive points-to information. Only warns
when the converted pointer is dereferenced. Does not warn about incomplete
types.
- -Wstrict-overflow
- -Wstrict-overflow=n
- This option is only active when -fstrict-overflow is
active. It warns about cases where the compiler optimizes based on the
assumption that signed overflow does not occur. Note that it does not warn
about all cases where the code might overflow: it only warns about cases
where the compiler implements some optimization. Thus this warning depends
on the optimization level.
An optimization that assumes that signed overflow does not occur is
perfectly safe if the values of the variables involved are such that
overflow never does, in fact, occur. Therefore this warning can easily
give a false positive: a warning about code that is not actually a
problem. To help focus on important issues, several warning levels are
defined. No warnings are issued for the use of undefined signed overflow
when estimating how many iterations a loop requires, in particular when
determining whether a loop will be executed at all.
- -Wstrict-overflow=1
- Warn about cases that are both questionable and easy to
avoid. For example, with -fstrict-overflow, the compiler simplifies
"x + 1 > x" to 1. This level of -Wstrict-overflow is
enabled by -Wall; higher levels are not, and must be explicitly
requested.
- -Wstrict-overflow=2
- Also warn about other cases where a comparison is
simplified to a constant. For example: "abs (x) >= 0". This
can only be simplified when -fstrict-overflow is in effect, because
"abs (INT_MIN)" overflows to "INT_MIN", which is less
than zero. -Wstrict-overflow (with no level) is the same as
-Wstrict-overflow=2.
- -Wstrict-overflow=3
- Also warn about other cases where a comparison is
simplified. For example: "x + 1 > 1" is simplified to "x
> 0".
- -Wstrict-overflow=4
- Also warn about other simplifications not covered by the
above cases. For example: "(x * 10) / 5" is simplified to
"x * 2".
- -Wstrict-overflow=5
- Also warn about cases where the compiler reduces the
magnitude of a constant involved in a comparison. For example: "x + 2
> y" is simplified to "x + 1 >= y". This is reported
only at the highest warning level because this simplification applies to
many comparisons, so this warning level gives a very large number of false
positives.
- -Wsuggest-attribute=[pure|const|noreturn|format]
- Warn for cases where adding an attribute may be beneficial.
The attributes currently supported are listed below.
- -Wsuggest-attribute=pure
- -Wsuggest-attribute=const
- -Wsuggest-attribute=noreturn
- Warn about functions that might be candidates for
attributes "pure", "const" or "noreturn".
The compiler only warns for functions visible in other compilation units
or (in the case of "pure" and "const") if it cannot
prove that the function returns normally. A function returns normally if
it doesn't contain an infinite loop or return abnormally by throwing,
calling "abort" or trapping. This analysis requires option
-fipa-pure-const, which is enabled by default at -O and
higher. Higher optimization levels improve the accuracy of the
analysis.
- -Wsuggest-attribute=format
- -Wmissing-format-attribute
- Warn about function pointers that might be candidates for
"format" attributes. Note these are only possible candidates,
not absolute ones. GCC guesses that function pointers with
"format" attributes that are used in assignment, initialization,
parameter passing or return statements should have a corresponding
"format" attribute in the resulting type. I.e. the left-hand
side of the assignment or initialization, the type of the parameter
variable, or the return type of the containing function respectively
should also have a "format" attribute to avoid the warning.
GCC also warns about function definitions that might be candidates for
"format" attributes. Again, these are only possible candidates.
GCC guesses that "format" attributes might be appropriate for
any function that calls a function like "vprintf" or
"vscanf", but this might not always be the case, and some
functions for which "format" attributes are appropriate may not
be detected.
- -Wsuggest-final-types
- Warn about types with virtual methods where code quality
would be improved if the type were declared with the C++11
"final" specifier, or, if possible, declared in an anonymous
namespace. This allows GCC to more aggressively devirtualize the
polymorphic calls. This warning is more effective with link time
optimization, where the information about the class hierarchy graph is
more complete.
- -Wsuggest-final-methods
- Warn about virtual methods where code quality would be
improved if the method were declared with the C++11 "final"
specifier, or, if possible, its type were declared in an anonymous
namespace or with the "final" specifier. This warning is more
effective with link time optimization, where the information about the
class hierarchy graph is more complete. It is recommended to first
consider suggestions of -Wsuggest-final-types and then rebuild with
new annotations.
- -Wsuggest-override
- Warn about overriding virtual functions that are not marked
with the override keyword.
- -Warray-bounds
- -Warray-bounds=n
- This option is only active when -ftree-vrp is active
(default for -O2 and above). It warns about subscripts to arrays
that are always out of bounds. This warning is enabled by
-Wall.
- -Warray-bounds=1
- This is the warning level of -Warray-bounds and is
enabled by -Wall; higher levels are not, and must be explicitly
requested.
- -Warray-bounds=2
- This warning level also warns about out of bounds access
for arrays at the end of a struct and for arrays accessed through
pointers. This warning level may give a larger number of false positives
and is deactivated by default.
- -Wbool-compare
- Warn about boolean expression compared with an integer
value different from "true"/"false". For instance, the
following comparison is always false:
int n = 5;
...
if ((n > 1) == 2) { ... }
This warning is enabled by -Wall.
- -Wno-discarded-qualifiers (C and Objective-C
only)
- Do not warn if type qualifiers on pointers are being
discarded. Typically, the compiler warns if a "const char *"
variable is passed to a function that takes a "char *"
parameter. This option can be used to suppress such a warning.
- -Wno-discarded-array-qualifiers (C and Objective-C
only)
- Do not warn if type qualifiers on arrays which are pointer
targets are being discarded. Typically, the compiler warns if a
"const int (*)[]" variable is passed to a function that takes a
"int (*)[]" parameter. This option can be used to suppress such
a warning.
- -Wno-incompatible-pointer-types (C and Objective-C
only)
- Do not warn when there is a conversion between pointers
that have incompatible types. This warning is for cases not covered by
-Wno-pointer-sign, which warns for pointer argument passing or
assignment with different signedness.
- -Wno-int-conversion (C and Objective-C only)
- Do not warn about incompatible integer to pointer and
pointer to integer conversions. This warning is about implicit
conversions; for explicit conversions the warnings
-Wno-int-to-pointer-cast and -Wno-pointer-to-int-cast may be
used.
- -Wno-div-by-zero
- Do not warn about compile-time integer division by zero.
Floating-point division by zero is not warned about, as it can be a
legitimate way of obtaining infinities and NaNs.
- -Wsystem-headers
- Print warning messages for constructs found in system
header files. Warnings from system headers are normally suppressed, on the
assumption that they usually do not indicate real problems and would only
make the compiler output harder to read. Using this command-line option
tells GCC to emit warnings from system headers as if they occurred in user
code. However, note that using -Wall in conjunction with this
option does not warn about unknown pragmas in system headers---for
that, -Wunknown-pragmas must also be used.
- -Wtrampolines
- Warn about trampolines generated for pointers to nested
functions. A trampoline is a small piece of data or code that is created
at run time on the stack when the address of a nested function is taken,
and is used to call the nested function indirectly. For some targets, it
is made up of data only and thus requires no special treatment. But, for
most targets, it is made up of code and thus requires the stack to be made
executable in order for the program to work properly.
- -Wfloat-equal
- Warn if floating-point values are used in equality
comparisons.
The idea behind this is that sometimes it is convenient (for the programmer)
to consider floating-point values as approximations to infinitely precise
real numbers. If you are doing this, then you need to compute (by
analyzing the code, or in some other way) the maximum or likely maximum
error that the computation introduces, and allow for it when performing
comparisons (and when producing output, but that's a different problem).
In particular, instead of testing for equality, you should check to see
whether the two values have ranges that overlap; and this is done with the
relational operators, so equality comparisons are probably mistaken.
- -Wtraditional (C and Objective-C only)
- Warn about certain constructs that behave differently in
traditional and ISO C. Also warn about ISO C constructs that have no
traditional C equivalent, and/or problematic constructs that should be
avoided.
- *
- Macro parameters that appear within string literals in the
macro body. In traditional C macro replacement takes place within string
literals, but in ISO C it does not.
- *
- In traditional C, some preprocessor directives did not
exist. Traditional preprocessors only considered a line to be a directive
if the # appeared in column 1 on the line. Therefore
-Wtraditional warns about directives that traditional C understands
but ignores because the # does not appear as the first character on
the line. It also suggests you hide directives like "#pragma"
not understood by traditional C by indenting them. Some traditional
implementations do not recognize "#elif", so this option
suggests avoiding it altogether.
- *
- A function-like macro that appears without arguments.
- *
- The unary plus operator.
- *
- The U integer constant suffix, or the F or
L floating-point constant suffixes. (Traditional C does support the
L suffix on integer constants.) Note, these suffixes appear in
macros defined in the system headers of most modern systems, e.g. the
_MIN/ _MAX macros in "<limits.h>". Use of
these macros in user code might normally lead to spurious warnings,
however GCC's integrated preprocessor has enough context to avoid warning
in these cases.
- *
- A function declared external in one block and then used
after the end of the block.
- *
- A "switch" statement has an operand of type
"long".
- *
- A non-"static" function declaration follows a
"static" one. This construct is not accepted by some traditional
C compilers.
- *
- The ISO type of an integer constant has a different width
or signedness from its traditional type. This warning is only issued if
the base of the constant is ten. I.e. hexadecimal or octal values, which
typically represent bit patterns, are not warned about.
- *
- Usage of ISO string concatenation is detected.
- *
- Initialization of automatic aggregates.
- *
- Identifier conflicts with labels. Traditional C lacks a
separate namespace for labels.
- *
- Initialization of unions. If the initializer is zero, the
warning is omitted. This is done under the assumption that the zero
initializer in user code appears conditioned on e.g. "__STDC__"
to avoid missing initializer warnings and relies on default initialization
to zero in the traditional C case.
- *
- Conversions by prototypes between fixed/floating-point
values and vice versa. The absence of these prototypes when compiling with
traditional C causes serious problems. This is a subset of the possible
conversion warnings; for the full set use
-Wtraditional-conversion.
- *
- Use of ISO C style function definitions. This warning
intentionally is not issued for prototype declarations or variadic
functions because these ISO C features appear in your code when using
libiberty's traditional C compatibility macros, "PARAMS" and
"VPARAMS". This warning is also bypassed for nested functions
because that feature is already a GCC extension and thus not relevant to
traditional C compatibility.
- -Wtraditional-conversion (C and Objective-C
only)
- Warn if a prototype causes a type conversion that is
different from what would happen to the same argument in the absence of a
prototype. This includes conversions of fixed point to floating and vice
versa, and conversions changing the width or signedness of a fixed-point
argument except when the same as the default promotion.
- -Wdeclaration-after-statement (C and Objective-C
only)
- Warn when a declaration is found after a statement in a
block. This construct, known from C++, was introduced with ISO C99 and is
by default allowed in GCC. It is not supported by ISO C90.
- -Wundef
- Warn if an undefined identifier is evaluated in an
"#if" directive.
- -Wno-endif-labels
- Do not warn whenever an "#else" or an
"#endif" are followed by text.
- -Wshadow
- Warn whenever a local variable or type declaration shadows
another variable, parameter, type, class member (in C++), or instance
variable (in Objective-C) or whenever a built-in function is shadowed.
Note that in C++, the compiler warns if a local variable shadows an
explicit typedef, but not if it shadows a struct/class/enum.
- -Wno-shadow-ivar (Objective-C only)
- Do not warn whenever a local variable shadows an instance
variable in an Objective-C method.
- -Wlarger-than=len
- Warn whenever an object of larger than len bytes is
defined.
- -Wframe-larger-than=len
- Warn if the size of a function frame is larger than
len bytes. The computation done to determine the stack frame size
is approximate and not conservative. The actual requirements may be
somewhat greater than len even if you do not get a warning. In
addition, any space allocated via "alloca", variable-length
arrays, or related constructs is not included by the compiler when
determining whether or not to issue a warning.
- -Wno-free-nonheap-object
- Do not warn when attempting to free an object that was not
allocated on the heap.
- -Wstack-usage=len
- Warn if the stack usage of a function might be larger than
len bytes. The computation done to determine the stack usage is
conservative. Any space allocated via "alloca", variable-length
arrays, or related constructs is included by the compiler when determining
whether or not to issue a warning.
The message is in keeping with the output of -fstack-usage.
- *
- If the stack usage is fully static but exceeds the
specified amount, it's:
warning: stack usage is 1120 bytes
- *
- If the stack usage is (partly) dynamic but bounded, it's:
warning: stack usage might be 1648 bytes
- *
- If the stack usage is (partly) dynamic and not bounded,
it's:
warning: stack usage might be unbounded
- -Wunsafe-loop-optimizations
- Warn if the loop cannot be optimized because the compiler
cannot assume anything on the bounds of the loop indices. With
-funsafe-loop-optimizations warn if the compiler makes such
assumptions.
- -Wno-pedantic-ms-format (MinGW targets only)
- When used in combination with -Wformat and
-pedantic without GNU extensions, this option disables the warnings
about non-ISO "printf" / "scanf" format width
specifiers "I32", "I64", and "I" used on
Windows targets, which depend on the MS runtime.
- -Wpointer-arith
- Warn about anything that depends on the "size of"
a function type or of "void". GNU C assigns these types a size
of 1, for convenience in calculations with "void *" pointers and
pointers to functions. In C++, warn also when an arithmetic operation
involves "NULL". This warning is also enabled by
-Wpedantic.
- -Wtype-limits
- Warn if a comparison is always true or always false due to
the limited range of the data type, but do not warn for constant
expressions. For example, warn if an unsigned variable is compared against
zero with "<" or ">=". This warning is also
enabled by -Wextra.
- -Wbad-function-cast (C and Objective-C only)
- Warn when a function call is cast to a non-matching type.
For example, warn if a call to a function returning an integer type is
cast to a pointer type.
- -Wc90-c99-compat (C and Objective-C only)
- Warn about features not present in ISO C90, but present in
ISO C99. For instance, warn about use of variable length arrays,
"long long" type, "bool" type, compound literals,
designated initializers, and so on. This option is independent of the
standards mode. Warnings are disabled in the expression that follows
"__extension__".
- -Wc99-c11-compat (C and Objective-C only)
- Warn about features not present in ISO C99, but present in
ISO C11. For instance, warn about use of anonymous structures and unions,
"_Atomic" type qualifier, "_Thread_local"
storage-class specifier, "_Alignas" specifier,
"Alignof" operator, "_Generic" keyword, and so on.
This option is independent of the standards mode. Warnings are disabled in
the expression that follows "__extension__".
- -Wc++-compat (C and Objective-C only)
- Warn about ISO C constructs that are outside of the common
subset of ISO C and ISO C++, e.g. request for implicit conversion from
"void *" to a pointer to non-"void" type.
- -Wc++11-compat (C++ and Objective-C++ only)
- Warn about C++ constructs whose meaning differs between ISO
C++ 1998 and ISO C++ 2011, e.g., identifiers in ISO C++ 1998 that are
keywords in ISO C++ 2011. This warning turns on -Wnarrowing and is
enabled by -Wall.
- -Wc++14-compat (C++ and Objective-C++ only)
- Warn about C++ constructs whose meaning differs between ISO
C++ 2011 and ISO C++ 2014. This warning is enabled by -Wall.
- -Wcast-qual
- Warn whenever a pointer is cast so as to remove a type
qualifier from the target type. For example, warn if a "const char
*" is cast to an ordinary "char *".
Also warn when making a cast that introduces a type qualifier in an unsafe
way. For example, casting "char **" to "const char **"
is unsafe, as in this example:
/* p is char ** value. */
const char **q = (const char **) p;
/* Assignment of readonly string to const char * is OK. */
*q = "string";
/* Now char** pointer points to read-only memory. */
**p = 'b';
- -Wcast-align
- Warn whenever a pointer is cast such that the required
alignment of the target is increased. For example, warn if a "char
*" is cast to an "int *" on machines where integers can
only be accessed at two- or four-byte boundaries.
- -Wwrite-strings
- When compiling C, give string constants the type
"const char[ length]" so that copying the address of one
into a non-"const" "char *" pointer produces a
warning. These warnings help you find at compile time code that can try to
write into a string constant, but only if you have been very careful about
using "const" in declarations and prototypes. Otherwise, it is
just a nuisance. This is why we did not make -Wall request these
warnings.
When compiling C++, warn about the deprecated conversion from string
literals to "char *". This warning is enabled by default for C++
programs.
- -Wclobbered
- Warn for variables that might be changed by
"longjmp" or "vfork". This warning is also enabled by
-Wextra.
- -Wconditionally-supported (C++ and Objective-C++
only)
- Warn for conditionally-supported (C++11 [intro.defs])
constructs.
- -Wconversion
- Warn for implicit conversions that may alter a value. This
includes conversions between real and integer, like "abs (x)"
when "x" is "double"; conversions between signed and
unsigned, like "unsigned ui = -1"; and conversions to smaller
types, like "sqrtf (M_PI)". Do not warn for explicit casts like
"abs ((int) x)" and "ui = (unsigned) -1", or if the
value is not changed by the conversion like in "abs (2.0)".
Warnings about conversions between signed and unsigned integers can be
disabled by using -Wno-sign-conversion.
For C++, also warn for confusing overload resolution for user-defined
conversions; and conversions that never use a type conversion operator:
conversions to "void", the same type, a base class or a
reference to them. Warnings about conversions between signed and unsigned
integers are disabled by default in C++ unless -Wsign-conversion is
explicitly enabled.
- -Wno-conversion-null (C++ and Objective-C++
only)
- Do not warn for conversions between "NULL" and
non-pointer types. -Wconversion-null is enabled by default.
- -Wzero-as-null-pointer-constant (C++ and
Objective-C++ only)
- Warn when a literal '0' is used as null pointer constant.
This can be useful to facilitate the conversion to "nullptr" in
C++11.
- -Wdate-time
- Warn when macros "__TIME__", "__DATE__"
or "__TIMESTAMP__" are encountered as they might prevent
bit-wise-identical reproducible compilations.
- -Wdelete-incomplete (C++ and Objective-C++
only)
- Warn when deleting a pointer to incomplete type, which may
cause undefined behavior at runtime. This warning is enabled by
default.
- -Wuseless-cast (C++ and Objective-C++ only)
- Warn when an expression is casted to its own type.
- -Wempty-body
- Warn if an empty body occurs in an "if",
"else" or "do while" statement. This warning is also
enabled by -Wextra.
- -Wenum-compare
- Warn about a comparison between values of different
enumerated types. In C++ enumeral mismatches in conditional expressions
are also diagnosed and the warning is enabled by default. In C this
warning is enabled by -Wall.
- -Wjump-misses-init (C, Objective-C only)
- Warn if a "goto" statement or a
"switch" statement jumps forward across the initialization of a
variable, or jumps backward to a label after the variable has been
initialized. This only warns about variables that are initialized when
they are declared. This warning is only supported for C and Objective-C;
in C++ this sort of branch is an error in any case.
-Wjump-misses-init is included in -Wc++-compat. It can be
disabled with the -Wno-jump-misses-init option.
- -Wsign-compare
- Warn when a comparison between signed and unsigned values
could produce an incorrect result when the signed value is converted to
unsigned. This warning is also enabled by -Wextra; to get the other
warnings of -Wextra without this warning, use -Wextra
-Wno-sign-compare.
- -Wsign-conversion
- Warn for implicit conversions that may change the sign of
an integer value, like assigning a signed integer expression to an
unsigned integer variable. An explicit cast silences the warning. In C,
this option is enabled also by -Wconversion.
- -Wfloat-conversion
- Warn for implicit conversions that reduce the precision of
a real value. This includes conversions from real to integer, and from
higher precision real to lower precision real values. This option is also
enabled by -Wconversion.
- -Wsized-deallocation (C++ and Objective-C++
only)
- Warn about a definition of an unsized deallocation function
void operator delete (void *) noexcept;
void operator delete[] (void *) noexcept;
without a definition of the corresponding sized deallocation function
void operator delete (void *, std::size_t) noexcept;
void operator delete[] (void *, std::size_t) noexcept;
or vice versa. Enabled by -Wextra along with
-fsized-deallocation.
- -Wsizeof-pointer-memaccess
- Warn for suspicious length parameters to certain string and
memory built-in functions if the argument uses "sizeof". This
warning warns e.g. about "memset (ptr, 0, sizeof (ptr));" if
"ptr" is not an array, but a pointer, and suggests a possible
fix, or about "memcpy (&foo, ptr, sizeof (&foo));". This
warning is enabled by -Wall.
- -Wsizeof-array-argument
- Warn when the "sizeof" operator is applied to a
parameter that is declared as an array in a function definition. This
warning is enabled by default for C and C++ programs.
- -Wmemset-transposed-args
- Warn for suspicious calls to the "memset"
built-in function, if the second argument is not zero and the third
argument is zero. This warns e.g.@ about "memset (buf, sizeof buf,
0)" where most probably "memset (buf, 0, sizeof buf)" was
meant instead. The diagnostics is only emitted if the third argument is
literal zero. If it is some expression that is folded to zero, a cast of
zero to some type, etc., it is far less likely that the user has
mistakenly exchanged the arguments and no warning is emitted. This warning
is enabled by -Wall.
- -Waddress
- Warn about suspicious uses of memory addresses. These
include using the address of a function in a conditional expression, such
as "void func(void); if (func)", and comparisons against the
memory address of a string literal, such as "if (x ==
"abc")". Such uses typically indicate a programmer error:
the address of a function always evaluates to true, so their use in a
conditional usually indicate that the programmer forgot the parentheses in
a function call; and comparisons against string literals result in
unspecified behavior and are not portable in C, so they usually indicate
that the programmer intended to use "strcmp". This warning is
enabled by -Wall.
- -Wlogical-op
- Warn about suspicious uses of logical operators in
expressions. This includes using logical operators in contexts where a
bit-wise operator is likely to be expected.
- -Wlogical-not-parentheses
- Warn about logical not used on the left hand side operand
of a comparison. This option does not warn if the RHS operand is of a
boolean type. Its purpose is to detect suspicious code like the following:
int a;
...
if (!a > 1) { ... }
It is possible to suppress the warning by wrapping the LHS into parentheses:
if ((!a) > 1) { ... }
This warning is enabled by -Wall.
- -Waggregate-return
- Warn if any functions that return structures or unions are
defined or called. (In languages where you can return an array, this also
elicits a warning.)
- -Wno-aggressive-loop-optimizations
- Warn if in a loop with constant number of iterations the
compiler detects undefined behavior in some statement during one or more
of the iterations.
- -Wno-attributes
- Do not warn if an unexpected "__attribute__" is
used, such as unrecognized attributes, function attributes applied to
variables, etc. This does not stop errors for incorrect use of supported
attributes.
- -Wno-builtin-macro-redefined
- Do not warn if certain built-in macros are redefined. This
suppresses warnings for redefinition of "__TIMESTAMP__",
"__TIME__", "__DATE__", "__FILE__", and
"__BASE_FILE__".
- -Wstrict-prototypes (C and Objective-C only)
- Warn if a function is declared or defined without
specifying the argument types. (An old-style function definition is
permitted without a warning if preceded by a declaration that specifies
the argument types.)
- -Wold-style-declaration (C and Objective-C
only)
- Warn for obsolescent usages, according to the C Standard,
in a declaration. For example, warn if storage-class specifiers like
"static" are not the first things in a declaration. This warning
is also enabled by -Wextra.
- -Wold-style-definition (C and Objective-C only)
- Warn if an old-style function definition is used. A warning
is given even if there is a previous prototype.
- -Wmissing-parameter-type (C and Objective-C
only)
- A function parameter is declared without a type specifier
in K&R-style functions:
void foo(bar) { }
This warning is also enabled by -Wextra.
- -Wmissing-prototypes (C and Objective-C only)
- Warn if a global function is defined without a previous
prototype declaration. This warning is issued even if the definition
itself provides a prototype. Use this option to detect global functions
that do not have a matching prototype declaration in a header file. This
option is not valid for C++ because all function declarations provide
prototypes and a non-matching declaration declares an overload rather than
conflict with an earlier declaration. Use -Wmissing-declarations to
detect missing declarations in C++.
- -Wmissing-declarations
- Warn if a global function is defined without a previous
declaration. Do so even if the definition itself provides a prototype. Use
this option to detect global functions that are not declared in header
files. In C, no warnings are issued for functions with previous
non-prototype declarations; use -Wmissing-prototypes to detect
missing prototypes. In C++, no warnings are issued for function templates,
or for inline functions, or for functions in anonymous namespaces.
- -Wmissing-field-initializers
- Warn if a structure's initializer has some fields missing.
For example, the following code causes such a warning, because
"x.h" is implicitly zero:
struct s { int f, g, h; };
struct s x = { 3, 4 };
This option does not warn about designated initializers, so the following
modification does not trigger a warning:
struct s { int f, g, h; };
struct s x = { .f = 3, .g = 4 };
In C++ this option does not warn either about the empty { } initializer, for
example:
struct s { int f, g, h; };
s x = { };
This warning is included in -Wextra. To get other -Wextra
warnings without this one, use -Wextra
-Wno-missing-field-initializers.
- -Wno-multichar
- Do not warn if a multicharacter constant ('FOOF') is
used. Usually they indicate a typo in the user's code, as they have
implementation-defined values, and should not be used in portable
code.
- -Wnormalized[=<none|id|nfc|nfkc>]
- In ISO C and ISO C++, two identifiers are different if they
are different sequences of characters. However, sometimes when characters
outside the basic ASCII character set are used, you can have two different
character sequences that look the same. To avoid confusion, the ISO 10646
standard sets out some normalization rules which when applied
ensure that two sequences that look the same are turned into the same
sequence. GCC can warn you if you are using identifiers that have not been
normalized; this option controls that warning.
There are four levels of warning supported by GCC. The default is
-Wnormalized=nfc, which warns about any identifier that is not in
the ISO 10646 "C" normalized form, NFC. NFC is the
recommended form for most uses. It is equivalent to -Wnormalized.
Unfortunately, there are some characters allowed in identifiers by ISO C and
ISO C++ that, when turned into NFC, are not allowed in identifiers. That
is, there's no way to use these symbols in portable ISO C or C++ and have
all your identifiers in NFC. -Wnormalized=id suppresses the warning
for these characters. It is hoped that future versions of the standards
involved will correct this, which is why this option is not the default.
You can switch the warning off for all characters by writing
-Wnormalized=none or -Wno-normalized. You should only do
this if you are using some other normalization scheme (like
"D"), because otherwise you can easily create bugs that are
literally impossible to see.
Some characters in ISO 10646 have distinct meanings but look identical in
some fonts or display methodologies, especially once formatting has been
applied. For instance "\u207F", "SUPERSCRIPT LATIN SMALL
LETTER N", displays just like a regular "n" that has been
placed in a superscript. ISO 10646 defines the NFKC normalization
scheme to convert all these into a standard form as well, and GCC warns if
your code is not in NFKC if you use -Wnormalized=nfkc. This warning
is comparable to warning about every identifier that contains the letter O
because it might be confused with the digit 0, and so is not the default,
but may be useful as a local coding convention if the programming
environment cannot be fixed to display these characters distinctly.
- -Wno-deprecated
- Do not warn about usage of deprecated features.
- -Wno-deprecated-declarations
- Do not warn about uses of functions, variables, and types
marked as deprecated by using the "deprecated" attribute.
- -Wno-overflow
- Do not warn about compile-time overflow in constant
expressions.
- -Wno-odr
- Warn about One Definition Rule violations during link-time
optimization. Requires -flto-odr-type-merging to be enabled.
Enabled by default.
- -Wopenmp-simd
- Warn if the vectorizer cost model overrides the OpenMP or
the Cilk Plus simd directive set by user. The
-fsimd-cost-model=unlimited option can be used to relax the cost
model.
- -Woverride-init (C and Objective-C only)
- Warn if an initialized field without side effects is
overridden when using designated initializers.
This warning is included in -Wextra. To get other -Wextra
warnings without this one, use -Wextra
-Wno-override-init.
- -Wpacked
- Warn if a structure is given the packed attribute, but the
packed attribute has no effect on the layout or size of the structure.
Such structures may be mis-aligned for little benefit. For instance, in
this code, the variable "f.x" in "struct bar" is
misaligned even though "struct bar" does not itself have the
packed attribute:
struct foo {
int x;
char a, b, c, d;
} __attribute__((packed));
struct bar {
char z;
struct foo f;
};
- -Wpacked-bitfield-compat
- The 4.1, 4.2 and 4.3 series of GCC ignore the
"packed" attribute on bit-fields of type "char". This
has been fixed in GCC 4.4 but the change can lead to differences in the
structure layout. GCC informs you when the offset of such a field has
changed in GCC 4.4. For example there is no longer a 4-bit padding between
field "a" and "b" in this structure:
struct foo
{
char a:4;
char b:8;
} __attribute__ ((packed));
This warning is enabled by default. Use -Wno-packed-bitfield-compat
to disable this warning.
- -Wpadded
- Warn if padding is included in a structure, either to align
an element of the structure or to align the whole structure. Sometimes
when this happens it is possible to rearrange the fields of the structure
to reduce the padding and so make the structure smaller.
- -Wredundant-decls
- Warn if anything is declared more than once in the same
scope, even in cases where multiple declaration is valid and changes
nothing.
- -Wnested-externs (C and Objective-C only)
- Warn if an "extern" declaration is encountered
within a function.
- -Wno-inherited-variadic-ctor
- Suppress warnings about use of C++11 inheriting
constructors when the base class inherited from has a C variadic
constructor; the warning is on by default because the ellipsis is not
inherited.
- -Winline
- Warn if a function that is declared as inline cannot be
inlined. Even with this option, the compiler does not warn about failures
to inline functions declared in system headers.
The compiler uses a variety of heuristics to determine whether or not to
inline a function. For example, the compiler takes into account the size
of the function being inlined and the amount of inlining that has already
been done in the current function. Therefore, seemingly insignificant
changes in the source program can cause the warnings produced by
-Winline to appear or disappear.
- -Wno-invalid-offsetof (C++ and Objective-C++
only)
- Suppress warnings from applying the "offsetof"
macro to a non-POD type. According to the 2014 ISO C++ standard, applying
"offsetof" to a non-standard-layout type is undefined. In
existing C++ implementations, however, "offsetof" typically
gives meaningful results. This flag is for users who are aware that they
are writing nonportable code and who have deliberately chosen to ignore
the warning about it.
The restrictions on "offsetof" may be relaxed in a future version
of the C++ standard.
- -Wno-int-to-pointer-cast
- Suppress warnings from casts to pointer type of an integer
of a different size. In C++, casting to a pointer type of smaller size is
an error. Wint-to-pointer-cast is enabled by default.
- -Wno-pointer-to-int-cast (C and Objective-C
only)
- Suppress warnings from casts from a pointer to an integer
type of a different size.
- -Winvalid-pch
- Warn if a precompiled header is found in the search path
but can't be used.
- -Wlong-long
- Warn if "long long" type is used. This is enabled
by either -Wpedantic or -Wtraditional in ISO C90 and C++98
modes. To inhibit the warning messages, use -Wno-long-long.
- -Wvariadic-macros
- Warn if variadic macros are used in ISO C90 mode, or if the
GNU alternate syntax is used in ISO C99 mode. This is enabled by either
-Wpedantic or -Wtraditional. To inhibit the warning
messages, use -Wno-variadic-macros.
- -Wvarargs
- Warn upon questionable usage of the macros used to handle
variable arguments like "va_start". This is default. To inhibit
the warning messages, use -Wno-varargs.
- -Wvector-operation-performance
- Warn if vector operation is not implemented via SIMD
capabilities of the architecture. Mainly useful for the performance
tuning. Vector operation can be implemented "piecewise", which
means that the scalar operation is performed on every vector element;
"in parallel", which means that the vector operation is
implemented using scalars of wider type, which normally is more
performance efficient; and "as a single scalar", which means
that vector fits into a scalar type.
- -Wno-virtual-move-assign
- Suppress warnings about inheriting from a virtual base with
a non-trivial C++11 move assignment operator. This is dangerous because if
the virtual base is reachable along more than one path, it is moved
multiple times, which can mean both objects end up in the moved-from
state. If the move assignment operator is written to avoid moving from a
moved-from object, this warning can be disabled.
- -Wvla
- Warn if variable length array is used in the code.
-Wno-vla prevents the -Wpedantic warning of the variable
length array.
- -Wvolatile-register-var
- Warn if a register variable is declared volatile. The
volatile modifier does not inhibit all optimizations that may eliminate
reads and/or writes to register variables. This warning is enabled by
-Wall.
- -Wdisabled-optimization
- Warn if a requested optimization pass is disabled. This
warning does not generally indicate that there is anything wrong with your
code; it merely indicates that GCC's optimizers are unable to handle the
code effectively. Often, the problem is that your code is too big or too
complex; GCC refuses to optimize programs when the optimization itself is
likely to take inordinate amounts of time.
- -Wpointer-sign (C and Objective-C only)
- Warn for pointer argument passing or assignment with
different signedness. This option is only supported for C and Objective-C.
It is implied by -Wall and by -Wpedantic, which can be
disabled with -Wno-pointer-sign.
- -Wstack-protector
- This option is only active when -fstack-protector is
active. It warns about functions that are not protected against stack
smashing.
- -Woverlength-strings
- Warn about string constants that are longer than the
"minimum maximum" length specified in the C standard. Modern
compilers generally allow string constants that are much longer than the
standard's minimum limit, but very portable programs should avoid using
longer strings.
The limit applies after string constant concatenation, and does not
count the trailing NUL. In C90, the limit was 509 characters; in C99, it
was raised to 4095. C++98 does not specify a normative minimum maximum, so
we do not diagnose overlength strings in C++.
This option is implied by -Wpedantic, and can be disabled with
-Wno-overlength-strings.
- -Wunsuffixed-float-constants (C and Objective-C
only)
- Issue a warning for any floating constant that does not
have a suffix. When used together with -Wsystem-headers it warns
about such constants in system header files. This can be useful when
preparing code to use with the "FLOAT_CONST_DECIMAL64" pragma
from the decimal floating-point extension to C99.
- -Wno-designated-init (C and Objective-C only)
- Suppress warnings when a positional initializer is used to
initialize a structure that has been marked with the
"designated_init" attribute.
Options for Debugging Your Program or GCC
GCC has various special options that are used for debugging either your program
or GCC:
- -g
- Produce debugging information in the operating system's
native format (stabs, COFF, XCOFF, or DWARF 2). GDB can work with this
debugging information.
On most systems that use stabs format, -g enables use of extra
debugging information that only GDB can use; this extra information makes
debugging work better in GDB but probably makes other debuggers crash or
refuse to read the program. If you want to control for certain whether to
generate the extra information, use -gstabs+, -gstabs,
-gxcoff+, -gxcoff, or -gvms (see below).
GCC allows you to use -g with -O. The shortcuts taken by
optimized code may occasionally produce surprising results: some variables
you declared may not exist at all; flow of control may briefly move where
you did not expect it; some statements may not be executed because they
compute constant results or their values are already at hand; some
statements may execute in different places because they have been moved
out of loops.
Nevertheless it proves possible to debug optimized output. This makes it
reasonable to use the optimizer for programs that might have bugs.
The following options are useful when GCC is generated with the capability
for more than one debugging format.
- -gsplit-dwarf
- Separate as much dwarf debugging information as possible
into a separate output file with the extension .dwo. This option allows
the build system to avoid linking files with debug information. To be
useful, this option requires a debugger capable of reading .dwo
files.
- -ggdb
- Produce debugging information for use by GDB. This means to
use the most expressive format available (DWARF 2, stabs, or the native
format if neither of those are supported), including GDB extensions if at
all possible.
- -gpubnames
- Generate dwarf .debug_pubnames and .debug_pubtypes
sections.
- -ggnu-pubnames
- Generate .debug_pubnames and .debug_pubtypes sections in a
format suitable for conversion into a GDB index. This option is only
useful with a linker that can produce GDB index version 7.
- -gstabs
- Produce debugging information in stabs format (if that is
supported), without GDB extensions. This is the format used by DBX on most
BSD systems. On MIPS, Alpha and System V Release 4 systems this option
produces stabs debugging output that is not understood by DBX or SDB. On
System V Release 4 systems this option requires the GNU assembler.
- -feliminate-unused-debug-symbols
- Produce debugging information in stabs format (if that is
supported), for only symbols that are actually used.
- -femit-class-debug-always
- Instead of emitting debugging information for a C++ class
in only one object file, emit it in all object files using the class. This
option should be used only with debuggers that are unable to handle the
way GCC normally emits debugging information for classes because using
this option increases the size of debugging information by as much as a
factor of two.
- -fdebug-types-section
- When using DWARF Version 4 or higher, type DIEs can be put
into their own ".debug_types" section instead of making them
part of the ".debug_info" section. It is more efficient to put
them in a separate comdat sections since the linker can then remove
duplicates. But not all DWARF consumers support ".debug_types"
sections yet and on some objects ".debug_types" produces larger
instead of smaller debugging information.
- -gstabs+
- Produce debugging information in stabs format (if that is
supported), using GNU extensions understood only by the GNU debugger
(GDB). The use of these extensions is likely to make other debuggers crash
or refuse to read the program.
- -gcoff
- Produce debugging information in COFF format (if that is
supported). This is the format used by SDB on most System V systems prior
to System V Release 4.
- -gxcoff
- Produce debugging information in XCOFF format (if that is
supported). This is the format used by the DBX debugger on IBM RS/6000
systems.
- -gxcoff+
- Produce debugging information in XCOFF format (if that is
supported), using GNU extensions understood only by the GNU debugger
(GDB). The use of these extensions is likely to make other debuggers crash
or refuse to read the program, and may cause assemblers other than the GNU
assembler (GAS) to fail with an error.
- -gdwarf-version
- Produce debugging information in DWARF format (if that is
supported). The value of version may be either 2, 3, 4 or 5; the
default version for most targets is 4. DWARF Version 5 is only
experimental.
Note that with DWARF Version 2, some ports require and always use some
non-conflicting DWARF 3 extensions in the unwind tables.
Version 4 may require GDB 7.0 and -fvar-tracking-assignments for
maximum benefit.
- -grecord-gcc-switches
- This switch causes the command-line options used to invoke
the compiler that may affect code generation to be appended to the
DW_AT_producer attribute in DWARF debugging information. The options are
concatenated with spaces separating them from each other and from the
compiler version. See also -frecord-gcc-switches for another way of
storing compiler options into the object file. This is the default.
- -gno-record-gcc-switches
- Disallow appending command-line options to the
DW_AT_producer attribute in DWARF debugging information.
- -gstrict-dwarf
- Disallow using extensions of later DWARF standard version
than selected with -gdwarf-version. On most targets using
non-conflicting DWARF extensions from later standard versions is
allowed.
- -gno-strict-dwarf
- Allow using extensions of later DWARF standard version than
selected with -gdwarf-version.
- -gz[=type]
- Produce compressed debug sections in DWARF format, if that
is supported. If type is not given, the default type depends on the
capabilities of the assembler and linker used. type may be one of
none (don't compress debug sections), zlib (use zlib
compression in ELF gABI format), or zlib-gnu (use zlib compression
in traditional GNU format). If the linker doesn't support writing
compressed debug sections, the option is rejected. Otherwise, if the
assembler does not support them, -gz is silently ignored when
producing object files.
- -gvms
- Produce debugging information in Alpha/VMS debug format (if
that is supported). This is the format used by DEBUG on Alpha/VMS
systems.
- -glevel
- -ggdblevel
- -gstabslevel
- -gcofflevel
- -gxcofflevel
- -gvmslevel
- Request debugging information and also use level to
specify how much information. The default level is 2.
Level 0 produces no debug information at all. Thus, -g0 negates
-g.
Level 1 produces minimal information, enough for making backtraces in parts
of the program that you don't plan to debug. This includes descriptions of
functions and external variables, and line number tables, but no
information about local variables.
Level 3 includes extra information, such as all the macro definitions
present in the program. Some debuggers support macro expansion when you
use -g3.
-gdwarf-2 does not accept a concatenated debug level, because GCC
used to support an option -gdwarf that meant to generate debug
information in version 1 of the DWARF format (which is very different from
version 2), and it would have been too confusing. That debug format is
long obsolete, but the option cannot be changed now. Instead use an
additional -glevel option to change the debug level for
DWARF.
- -gtoggle
- Turn off generation of debug info, if leaving out this
option generates it, or turn it on at level 2 otherwise. The position of
this argument in the command line does not matter; it takes effect after
all other options are processed, and it does so only once, no matter how
many times it is given. This is mainly intended to be used with
-fcompare-debug.
- -fsanitize=address
- Enable AddressSanitizer, a fast memory error detector.
Memory access instructions are instrumented to detect out-of-bounds and
use-after-free bugs. See <
https://github.com/google/sanitizers/wiki/AddressSanitizer> for
more details. The run-time behavior can be influenced using the
ASAN_OPTIONS environment variable. When set to "help=1",
the available options are shown at startup of the instrumended program.
See <
https://github.com/google/sanitizers/wiki/AddressSanitizerFlags#run-time-flags>
for a list of supported options.
- -fsanitize=kernel-address
- Enable AddressSanitizer for Linux kernel. See <
https://github.com/google/kasan/wiki> for more details.
- -fsanitize=thread
- Enable ThreadSanitizer, a fast data race detector. Memory
access instructions are instrumented to detect data race bugs. See <
https://github.com/google/sanitizers/wiki#threadsanitizer> for
more details. The run-time behavior can be influenced using the
TSAN_OPTIONS environment variable; see <
https://github.com/google/sanitizers/wiki/ThreadSanitizerFlags>
for a list of supported options.
- -fsanitize=leak
- Enable LeakSanitizer, a memory leak detector. This option
only matters for linking of executables and if neither
-fsanitize=address nor -fsanitize=thread is used. In that
case the executable is linked against a library that overrides
"malloc" and other allocator functions. See <
https://github.com/google/sanitizers/wiki/AddressSanitizerLeakSanitizer>
for more details. The run-time behavior can be influenced using the
LSAN_OPTIONS environment variable.
- -fsanitize=undefined
- Enable UndefinedBehaviorSanitizer, a fast undefined
behavior detector. Various computations are instrumented to detect
undefined behavior at runtime. Current suboptions are:
- -fsanitize=shift
- This option enables checking that the result of a shift
operation is not undefined. Note that what exactly is considered undefined
differs slightly between C and C++, as well as between ISO C90 and C99,
etc.
- -fsanitize=integer-divide-by-zero
- Detect integer division by zero as well as "INT_MIN /
-1" division.
- -fsanitize=unreachable
- With this option, the compiler turns the
"__builtin_unreachable" call into a diagnostics message call
instead. When reaching the "__builtin_unreachable" call, the
behavior is undefined.
- -fsanitize=vla-bound
- This option instructs the compiler to check that the size
of a variable length array is positive.
- -fsanitize=null
- This option enables pointer checking. Particularly, the
application built with this option turned on will issue an error message
when it tries to dereference a NULL pointer, or if a reference (possibly
an rvalue reference) is bound to a NULL pointer, or if a method is invoked
on an object pointed by a NULL pointer.
- -fsanitize=return
- This option enables return statement checking. Programs
built with this option turned on will issue an error message when the end
of a non-void function is reached without actually returning a value. This
option works in C++ only.
- -fsanitize=signed-integer-overflow
- This option enables signed integer overflow checking. We
check that the result of "+", "*", and both unary and
binary "-" does not overflow in the signed arithmetics. Note,
integer promotion rules must be taken into account. That is, the following
is not an overflow:
signed char a = SCHAR_MAX;
a++;
- -fsanitize=bounds
- This option enables instrumentation of array bounds.
Various out of bounds accesses are detected. Flexible array members,
flexible array member-like arrays, and initializers of variables with
static storage are not instrumented.
- -fsanitize=alignment
- This option enables checking of alignment of pointers when
they are dereferenced, or when a reference is bound to insufficiently
aligned target, or when a method or constructor is invoked on
insufficiently aligned object.
- -fsanitize=object-size
- This option enables instrumentation of memory references
using the "__builtin_object_size" function. Various out of
bounds pointer accesses are detected.
- -fsanitize=float-divide-by-zero
- Detect floating-point division by zero. Unlike other
similar options, -fsanitize=float-divide-by-zero is not enabled by
-fsanitize=undefined, since floating-point division by zero can be
a legitimate way of obtaining infinities and NaNs.
- -fsanitize=float-cast-overflow
- This option enables floating-point type to integer
conversion checking. We check that the result of the conversion does not
overflow. Unlike other similar options,
-fsanitize=float-cast-overflow is not enabled by
-fsanitize=undefined. This option does not work well with
"FE_INVALID" exceptions enabled.
- -fsanitize=nonnull-attribute
- This option enables instrumentation of calls, checking
whether null values are not passed to arguments marked as requiring a
non-null value by the "nonnull" function attribute.
- -fsanitize=returns-nonnull-attribute
- This option enables instrumentation of return statements in
functions marked with "returns_nonnull" function attribute, to
detect returning of null values from such functions.
- -fsanitize=bool
- This option enables instrumentation of loads from bool. If
a value other than 0/1 is loaded, a run-time error is issued.
- -fsanitize=enum
- This option enables instrumentation of loads from an enum
type. If a value outside the range of values for the enum type is loaded,
a run-time error is issued.
- -fsanitize=vptr
- This option enables instrumentation of C++ member function
calls, member accesses and some conversions between pointers to base and
derived classes, to verify the referenced object has the correct dynamic
type.
While
-ftrapv causes traps for signed overflows to be emitted,
-fsanitize=undefined gives a diagnostic message. This currently works
only for the C family of languages.
- -fno-sanitize=all
- This option disables all previously enabled sanitizers.
-fsanitize=all is not allowed, as some sanitizers cannot be used
together.
- -fasan-shadow-offset=number
- This option forces GCC to use custom shadow offset in
AddressSanitizer checks. It is useful for experimenting with different
shadow memory layouts in Kernel AddressSanitizer.
- -fsanitize-recover[=opts]
- -fsanitize-recover= controls error recovery mode for
sanitizers mentioned in comma-separated list of opts. Enabling this
option for a sanitizer component causes it to attempt to continue running
the program as if no error happened. This means multiple runtime errors
can be reported in a single program run, and the exit code of the program
may indicate success even when errors have been reported. The
-fno-sanitize-recover= option can be used to alter this behavior:
only the first detected error is reported and program then exits with a
non-zero exit code.
Currently this feature only works for -fsanitize=undefined (and its
suboptions except for -fsanitize=unreachable and
-fsanitize=return), -fsanitize=float-cast-overflow,
-fsanitize=float-divide-by-zero and
-fsanitize=kernel-address. For these sanitizers error recovery is
turned on by default. -fsanitize-recover=all and
-fno-sanitize-recover=all is also accepted, the former enables
recovery for all sanitizers that support it, the latter disables recovery
for all sanitizers that support it.
Syntax without explicit opts parameter is deprecated. It is
equivalent to
-fsanitize-recover=undefined,float-cast-overflow,float-divide-by-zero
Similarly -fno-sanitize-recover is equivalent to
-fno-sanitize-recover=undefined,float-cast-overflow,float-divide-by-zero
- -fsanitize-undefined-trap-on-error
- The -fsanitize-undefined-trap-on-error option
instructs the compiler to report undefined behavior using
"__builtin_trap" rather than a "libubsan" library
routine. The advantage of this is that the "libubsan" library is
not needed and is not linked in, so this is usable even in freestanding
environments.
- -fcheck-pointer-bounds
- Enable Pointer Bounds Checker instrumentation. Each memory
reference is instrumented with checks of the pointer used for memory
access against bounds associated with that pointer.
Currently there is only an implementation for Intel MPX available, thus x86
target and -mmpx are required to enable this feature. MPX-based
instrumentation requires a runtime library to enable MPX in hardware and
handle bounds violation signals. By default when
-fcheck-pointer-bounds and -mmpx options are used to link a
program, the GCC driver links against the libmpx runtime library
and libmpxwrappers library. It also passes '-z bndplt' to a linker
in case it supports this option (which is checked on libmpx
configuration). Note that old versions of linker may ignore option. Gold
linker doesn't support '-z bndplt' option. With no '-z bndplt' support in
linker all calls to dynamic libraries lose passed bounds reducing overall
protection level. It's highly recommended to use linker with '-z bndplt'
support. In case such linker is not available it is adviced to always use
-static-libmpxwrappers for better protection level or use
-static to completely avoid external calls to dynamic libraries.
MPX-based instrumentation may be used for debugging and also may be
included in production code to increase program security. Depending on
usage, you may have different requirements for the runtime library. The
current version of the MPX runtime library is more oriented for use as a
debugging tool. MPX runtime library usage implies -lpthread. See
also -static-libmpx. The runtime library behavior can be influenced
using various CHKP_RT_* environment variables. See <
https://gcc.gnu.org/wiki/Intel%20MPX%20support%20in%20the%20GCC%20compiler>
for more details.
Generated instrumentation may be controlled by various -fchkp-*
options and by the "bnd_variable_size" structure field attribute
and "bnd_legacy", and "bnd_instrument" function
attributes. GCC also provides a number of built-in functions for
controlling the Pointer Bounds Checker.
- -fchkp-check-incomplete-type
- Generate pointer bounds checks for variables with
incomplete type. Enabled by default.
- -fchkp-narrow-bounds
- Controls bounds used by Pointer Bounds Checker for pointers
to object fields. If narrowing is enabled then field bounds are used.
Otherwise object bounds are used. See also
-fchkp-narrow-to-innermost-array and
-fchkp-first-field-has-own-bounds. Enabled by default.
- -fchkp-first-field-has-own-bounds
- Forces Pointer Bounds Checker to use narrowed bounds for
the address of the first field in the structure. By default a pointer to
the first field has the same bounds as a pointer to the whole
structure.
- -fchkp-narrow-to-innermost-array
- Forces Pointer Bounds Checker to use bounds of the
innermost arrays in case of nested static array access. By default this
option is disabled and bounds of the outermost array are used.
- -fchkp-optimize
- Enables Pointer Bounds Checker optimizations. Enabled by
default at optimization levels -O, -O2, -O3.
- -fchkp-use-fast-string-functions
- Enables use of *_nobnd versions of string functions (not
copying bounds) by Pointer Bounds Checker. Disabled by default.
- -fchkp-use-nochk-string-functions
- Enables use of *_nochk versions of string functions (not
checking bounds) by Pointer Bounds Checker. Disabled by default.
- -fchkp-use-static-bounds
- Allow Pointer Bounds Checker to generate static bounds
holding bounds of static variables. Enabled by default.
- -fchkp-use-static-const-bounds
- Use statically-initialized bounds for constant bounds
instead of generating them each time they are required. By default enabled
when -fchkp-use-static-bounds is enabled.
- -fchkp-treat-zero-dynamic-size-as-infinite
- With this option, objects with incomplete type whose
dynamically-obtained size is zero are treated as having infinite size
instead by Pointer Bounds Checker. This option may be helpful if a program
is linked with a library missing size information for some symbols.
Disabled by default.
- -fchkp-check-read
- Instructs Pointer Bounds Checker to generate checks for all
read accesses to memory. Enabled by default.
- -fchkp-check-write
- Instructs Pointer Bounds Checker to generate checks for all
write accesses to memory. Enabled by default.
- -fchkp-store-bounds
- Instructs Pointer Bounds Checker to generate bounds stores
for pointer writes. Enabled by default.
- -fchkp-instrument-calls
- Instructs Pointer Bounds Checker to pass pointer bounds to
calls. Enabled by default.
- -fchkp-instrument-marked-only
- Instructs Pointer Bounds Checker to instrument only
functions marked with the "bnd_instrument" attribute. Disabled
by default.
- -fchkp-use-wrappers
- Allows Pointer Bounds Checker to replace calls to built-in
functions with calls to wrapper functions. When -fchkp-use-wrappers
is used to link a program, the GCC driver automatically links against
libmpxwrappers. See also -static-libmpxwrappers. Enabled by
default.
- -fdump-final-insns[=file]
- Dump the final internal representation (RTL) to
file. If the optional argument is omitted (or if file is
"."), the name of the dump file is determined by appending
".gkd" to the compilation output file name.
- -fcompare-debug[=opts]
- If no error occurs during compilation, run the compiler a
second time, adding opts and -fcompare-debug-second to the
arguments passed to the second compilation. Dump the final internal
representation in both compilations, and print an error if they differ.
If the equal sign is omitted, the default -gtoggle is used.
The environment variable GCC_COMPARE_DEBUG, if defined, non-empty and
nonzero, implicitly enables -fcompare-debug. If
GCC_COMPARE_DEBUG is defined to a string starting with a dash, then
it is used for opts, otherwise the default -gtoggle is used.
-fcompare-debug=, with the equal sign but without opts, is
equivalent to -fno-compare-debug, which disables the dumping of the
final representation and the second compilation, preventing even
GCC_COMPARE_DEBUG from taking effect.
To verify full coverage during -fcompare-debug testing, set
GCC_COMPARE_DEBUG to say -fcompare-debug-not-overridden,
which GCC rejects as an invalid option in any actual compilation (rather
than preprocessing, assembly or linking). To get just a warning, setting
GCC_COMPARE_DEBUG to -w%n-fcompare-debug not
overridden will do.
- -fcompare-debug-second
- This option is implicitly passed to the compiler for the
second compilation requested by -fcompare-debug, along with options
to silence warnings, and omitting other options that would cause
side-effect compiler outputs to files or to the standard output. Dump
files and preserved temporary files are renamed so as to contain the
".gk" additional extension during the second compilation, to
avoid overwriting those generated by the first.
When this option is passed to the compiler driver, it causes the
first compilation to be skipped, which makes it useful for little
other than debugging the compiler proper.
- -feliminate-dwarf2-dups
- Compress DWARF 2 debugging information by eliminating
duplicated information about each symbol. This option only makes sense
when generating DWARF 2 debugging information with -gdwarf-2.
- -femit-struct-debug-baseonly
- Emit debug information for struct-like types only when the
base name of the compilation source file matches the base name of file in
which the struct is defined.
This option substantially reduces the size of debugging information, but at
significant potential loss in type information to the debugger. See
-femit-struct-debug-reduced for a less aggressive option. See
-femit-struct-debug-detailed for more detailed control.
This option works only with DWARF 2.
- -femit-struct-debug-reduced
- Emit debug information for struct-like types only when the
base name of the compilation source file matches the base name of file in
which the type is defined, unless the struct is a template or defined in a
system header.
This option significantly reduces the size of debugging information, with
some potential loss in type information to the debugger. See
-femit-struct-debug-baseonly for a more aggressive option. See
-femit-struct-debug-detailed for more detailed control.
This option works only with DWARF 2.
- -femit-struct-debug-detailed[=spec-list]
- Specify the struct-like types for which the compiler
generates debug information. The intent is to reduce duplicate struct
debug information between different object files within the same program.
This option is a detailed version of -femit-struct-debug-reduced and
-femit-struct-debug-baseonly, which serves for most needs.
A specification has the syntax[
dir:|ind:][ord:|gen:](
any|sys|base|none)
The optional first word limits the specification to structs that are used
directly ( dir:) or used indirectly (ind:). A struct type is
used directly when it is the type of a variable, member. Indirect uses
arise through pointers to structs. That is, when use of an incomplete
struct is valid, the use is indirect. An example is struct one direct;
struct two * indirect;.
The optional second word limits the specification to ordinary structs (
ord:) or generic structs ( gen:). Generic structs are a bit
complicated to explain. For C++, these are non-explicit specializations of
template classes, or non-template classes within the above. Other
programming languages have generics, but
-femit-struct-debug-detailed does not yet implement them.
The third word specifies the source files for those structs for which the
compiler should emit debug information. The values none and
any have the normal meaning. The value base means that the
base of name of the file in which the type declaration appears must match
the base of the name of the main compilation file. In practice, this means
that when compiling foo.c, debug information is generated for types
declared in that file and foo.h, but not other header files. The
value sys means those types satisfying base or declared in
system or compiler headers.
You may need to experiment to determine the best settings for your
application.
The default is -femit-struct-debug-detailed=all.
This option works only with DWARF 2.
- -fno-merge-debug-strings
- Direct the linker to not merge together strings in the
debugging information that are identical in different object files.
Merging is not supported by all assemblers or linkers. Merging decreases
the size of the debug information in the output file at the cost of
increasing link processing time. Merging is enabled by default.
- -fdebug-prefix-map=old=new
- When compiling files in directory old,
record debugging information describing them as in
new instead.
- -fno-dwarf2-cfi-asm
- Emit DWARF 2 unwind info as compiler generated
".eh_frame" section instead of using GAS ".cfi_*"
directives.
- -p
- Generate extra code to write profile information suitable
for the analysis program prof. You must use this option when
compiling the source files you want data about, and you must also use it
when linking.
- -pg
- Generate extra code to write profile information suitable
for the analysis program gprof. You must use this option when
compiling the source files you want data about, and you must also use it
when linking.
- -Q
- Makes the compiler print out each function name as it is
compiled, and print some statistics about each pass when it finishes.
- -ftime-report
- Makes the compiler print some statistics about the time
consumed by each pass when it finishes.
- -fmem-report
- Makes the compiler print some statistics about permanent
memory allocation when it finishes.
- -fmem-report-wpa
- Makes the compiler print some statistics about permanent
memory allocation for the WPA phase only.
- -fpre-ipa-mem-report
- -fpost-ipa-mem-report
- Makes the compiler print some statistics about permanent
memory allocation before or after interprocedural optimization.
- -fprofile-report
- Makes the compiler print some statistics about consistency
of the (estimated) profile and effect of individual passes.
- -fstack-usage
- Makes the compiler output stack usage information for the
program, on a per-function basis. The filename for the dump is made by
appending .su to the auxname. auxname is generated
from the name of the output file, if explicitly specified and it is not an
executable, otherwise it is the basename of the source file. An entry is
made up of three fields:
- *
- The name of the function.
- *
- A number of bytes.
- *
- One or more qualifiers: "static",
"dynamic", "bounded".
The qualifier "static" means that the function manipulates the stack
statically: a fixed number of bytes are allocated for the frame on function
entry and released on function exit; no stack adjustments are otherwise made
in the function. The second field is this fixed number of bytes.
The qualifier "dynamic" means that the function manipulates the stack
dynamically: in addition to the static allocation described above, stack
adjustments are made in the body of the function, for example to push/pop
arguments around function calls. If the qualifier "bounded" is also
present, the amount of these adjustments is bounded at compile time and the
second field is an upper bound of the total amount of stack used by the
function. If it is not present, the amount of these adjustments is not bounded
at compile time and the second field only represents the bounded part.
- -fprofile-arcs
- Add code so that program flow arcs are instrumented.
During execution the program records how many times each branch and call
is executed and how many times it is taken or returns. When the compiled
program exits it saves this data to a file called
auxname.gcda for each source file. The data may be
used for profile-directed optimizations ( -fbranch-probabilities),
or for test coverage analysis ( -ftest-coverage). Each object
file's auxname is generated from the name of the output file, if
explicitly specified and it is not the final executable, otherwise it is
the basename of the source file. In both cases any suffix is removed (e.g.
foo.gcda for input file dir/foo.c, or dir/foo.gcda
for output file specified as -o dir/foo.o).
- --coverage
- This option is used to compile and link code instrumented
for coverage analysis. The option is a synonym for -fprofile-arcs
-ftest-coverage (when compiling) and -lgcov (when linking).
See the documentation for those options for more details.
- *
- Compile the source files with -fprofile-arcs plus
optimization and code generation options. For test coverage analysis, use
the additional -ftest-coverage option. You do not need to profile
every source file in a program.
- *
- Link your object files with -lgcov or
-fprofile-arcs (the latter implies the former).
- *
- Run the program on a representative workload to generate
the arc profile information. This may be repeated any number of times. You
can run concurrent instances of your program, and provided that the file
system supports locking, the data files will be correctly updated. Also
"fork" calls are detected and correctly handled (double counting
will not happen).
- *
- For profile-directed optimizations, compile the source
files again with the same optimization and code generation options plus
-fbranch-probabilities.
- *
- For test coverage analysis, use gcov to produce
human readable information from the .gcno and .gcda files.
Refer to the gcov documentation for further information.
With
-fprofile-arcs, for each function of your program GCC creates a
program flow graph, then finds a spanning tree for the graph. Only arcs that
are not on the spanning tree have to be instrumented: the compiler adds code
to count the number of times that these arcs are executed. When an arc is the
only exit or only entrance to a block, the instrumentation code can be added
to the block; otherwise, a new basic block must be created to hold the
instrumentation code.
- -ftest-coverage
- Produce a notes file that the gcov code-coverage
utility can use to show program coverage. Each source file's note file is
called auxname.gcno. Refer to the
-fprofile-arcs option above for a description of auxname and
instructions on how to generate test coverage data. Coverage data matches
the source files more closely if you do not optimize.
- -fdbg-cnt-list
- Print the name and the counter upper bound for all debug
counters.
- -fdbg-cnt=counter-value-list
- Set the internal debug counter upper bound.
counter-value-list is a comma-separated list of
name:value pairs which sets the upper bound of each debug
counter name to value. All debug counters have the initial
upper bound of "UINT_MAX"; thus "dbg_cnt" returns true
always unless the upper bound is set by this option. For example, with
-fdbg-cnt=dce:10,tail_call:0, "dbg_cnt(dce)" returns true
only for first 10 invocations.
- -fenable-kind-pass
- -fdisable-kind-pass=range-list
- This is a set of options that are used to explicitly
disable/enable optimization passes. These options are intended for use for
debugging GCC. Compiler users should use regular options for
enabling/disabling passes instead.
- -fdisable-ipa-pass
- Disable IPA pass pass. pass is the pass name.
If the same pass is statically invoked in the compiler multiple times, the
pass name should be appended with a sequential number starting from
1.
- -fdisable-rtl-pass
- -fdisable-rtl-pass=range-list
- Disable RTL pass pass. pass is the pass name.
If the same pass is statically invoked in the compiler multiple times, the
pass name should be appended with a sequential number starting from 1.
range-list is a comma-separated list of function ranges or
assembler names. Each range is a number pair separated by a colon. The
range is inclusive in both ends. If the range is trivial, the number pair
can be simplified as a single number. If the function's call graph node's
uid falls within one of the specified ranges, the pass is
disabled for that function. The uid is shown in the function header
of a dump file, and the pass names can be dumped by using option
-fdump-passes.
- -fdisable-tree-pass
- -fdisable-tree-pass=range-list
- Disable tree pass pass. See -fdisable-rtl for
the description of option arguments.
- -fenable-ipa-pass
- Enable IPA pass pass. pass is the pass name.
If the same pass is statically invoked in the compiler multiple times, the
pass name should be appended with a sequential number starting from
1.
- -fenable-rtl-pass
- -fenable-rtl-pass=range-list
- Enable RTL pass pass. See -fdisable-rtl for
option argument description and examples.
- -fenable-tree-pass
- -fenable-tree-pass=range-list
- Enable tree pass pass. See -fdisable-rtl for
the description of option arguments.
Here are some examples showing uses of these options.
# disable ccp1 for all functions
-fdisable-tree-ccp1
# disable complete unroll for function whose cgraph node uid is 1
-fenable-tree-cunroll=1
# disable gcse2 for functions at the following ranges [1,1],
# [300,400], and [400,1000]
# disable gcse2 for functions foo and foo2
-fdisable-rtl-gcse2=foo,foo2
# disable early inlining
-fdisable-tree-einline
# disable ipa inlining
-fdisable-ipa-inline
# enable tree full unroll
-fenable-tree-unroll
- -dletters
- -fdump-rtl-pass
- -fdump-rtl-pass=filename
- Says to make debugging dumps during compilation at times
specified by letters. This is used for debugging the RTL-based
passes of the compiler. The file names for most of the dumps are made by
appending a pass number and a word to the dumpname, and the files
are created in the directory of the output file. In case of
=filename option, the dump is output on the given file
instead of the pass numbered dump files. Note that the pass number is
computed statically as passes get registered into the pass manager. Thus
the numbering is not related to the dynamic order of execution of passes.
In particular, a pass installed by a plugin could have a number over 200
even if it executed quite early. dumpname is generated from the
name of the output file, if explicitly specified and it is not an
executable, otherwise it is the basename of the source file. These
switches may have different effects when -E is used for
preprocessing.
Debug dumps can be enabled with a -fdump-rtl switch or some -d
option letters. Here are the possible letters for use in
pass and letters, and their meanings:
- -fdump-rtl-alignments
- Dump after branch alignments have been computed.
- -fdump-rtl-asmcons
- Dump after fixing rtl statements that have unsatisfied
in/out constraints.
- -fdump-rtl-auto_inc_dec
- Dump after auto-inc-dec discovery. This pass is only run on
architectures that have auto inc or auto dec instructions.
- -fdump-rtl-barriers
- Dump after cleaning up the barrier instructions.
- -fdump-rtl-bbpart
- Dump after partitioning hot and cold basic blocks.
- -fdump-rtl-bbro
- Dump after block reordering.
- -fdump-rtl-btl1
- -fdump-rtl-btl2
- -fdump-rtl-btl1 and -fdump-rtl-btl2 enable
dumping after the two branch target load optimization passes.
- -fdump-rtl-bypass
- Dump after jump bypassing and control flow
optimizations.
- -fdump-rtl-combine
- Dump after the RTL instruction combination pass.
- -fdump-rtl-compgotos
- Dump after duplicating the computed gotos.
- -fdump-rtl-ce1
- -fdump-rtl-ce2
- -fdump-rtl-ce3
- -fdump-rtl-ce1, -fdump-rtl-ce2, and
-fdump-rtl-ce3 enable dumping after the three if conversion
passes.
- -fdump-rtl-cprop_hardreg
- Dump after hard register copy propagation.
- -fdump-rtl-csa
- Dump after combining stack adjustments.
- -fdump-rtl-cse1
- -fdump-rtl-cse2
- -fdump-rtl-cse1 and -fdump-rtl-cse2 enable
dumping after the two common subexpression elimination passes.
- -fdump-rtl-dce
- Dump after the standalone dead code elimination
passes.
- -fdump-rtl-dbr
- Dump after delayed branch scheduling.
- -fdump-rtl-dce1
- -fdump-rtl-dce2
- -fdump-rtl-dce1 and -fdump-rtl-dce2 enable
dumping after the two dead store elimination passes.
- -fdump-rtl-eh
- Dump after finalization of EH handling code.
- -fdump-rtl-eh_ranges
- Dump after conversion of EH handling range regions.
- -fdump-rtl-expand
- Dump after RTL generation.
- -fdump-rtl-fwprop1
- -fdump-rtl-fwprop2
- -fdump-rtl-fwprop1 and -fdump-rtl-fwprop2
enable dumping after the two forward propagation passes.
- -fdump-rtl-gcse1
- -fdump-rtl-gcse2
- -fdump-rtl-gcse1 and -fdump-rtl-gcse2 enable
dumping after global common subexpression elimination.
- -fdump-rtl-init-regs
- Dump after the initialization of the registers.
- -fdump-rtl-initvals
- Dump after the computation of the initial value sets.
- -fdump-rtl-into_cfglayout
- Dump after converting to cfglayout mode.
- -fdump-rtl-ira
- Dump after iterated register allocation.
- -fdump-rtl-jump
- Dump after the second jump optimization.
- -fdump-rtl-loop2
- -fdump-rtl-loop2 enables dumping after the rtl loop
optimization passes.
- -fdump-rtl-mach
- Dump after performing the machine dependent reorganization
pass, if that pass exists.
- -fdump-rtl-mode_sw
- Dump after removing redundant mode switches.
- -fdump-rtl-rnreg
- Dump after register renumbering.
- -fdump-rtl-outof_cfglayout
- Dump after converting from cfglayout mode.
- -fdump-rtl-peephole2
- Dump after the peephole pass.
- -fdump-rtl-postreload
- Dump after post-reload optimizations.
- -fdump-rtl-pro_and_epilogue
- Dump after generating the function prologues and
epilogues.
- -fdump-rtl-sched1
- -fdump-rtl-sched2
- -fdump-rtl-sched1 and -fdump-rtl-sched2
enable dumping after the basic block scheduling passes.
- -fdump-rtl-ree
- Dump after sign/zero extension elimination.
- -fdump-rtl-seqabstr
- Dump after common sequence discovery.
- -fdump-rtl-shorten
- Dump after shortening branches.
- -fdump-rtl-sibling
- Dump after sibling call optimizations.
- -fdump-rtl-split1
- -fdump-rtl-split2
- -fdump-rtl-split3
- -fdump-rtl-split4
- -fdump-rtl-split5
- These options enable dumping after five rounds of
instruction splitting.
- -fdump-rtl-sms
- Dump after modulo scheduling. This pass is only run on some
architectures.
- -fdump-rtl-stack
- Dump after conversion from GCC's "flat register
file" registers to the x87's stack-like registers. This pass is only
run on x86 variants.
- -fdump-rtl-subreg1
- -fdump-rtl-subreg2
- -fdump-rtl-subreg1 and -fdump-rtl-subreg2
enable dumping after the two subreg expansion passes.
- -fdump-rtl-unshare
- Dump after all rtl has been unshared.
- -fdump-rtl-vartrack
- Dump after variable tracking.
- -fdump-rtl-vregs
- Dump after converting virtual registers to hard
registers.
- -fdump-rtl-web
- Dump after live range splitting.
- -fdump-rtl-regclass
- -fdump-rtl-subregs_of_mode_init
- -fdump-rtl-subregs_of_mode_finish
- -fdump-rtl-dfinit
- -fdump-rtl-dfinish
- These dumps are defined but always produce empty
files.
- -da
- -fdump-rtl-all
- Produce all the dumps listed above.
- -dA
- Annotate the assembler output with miscellaneous debugging
information.
- -dD
- Dump all macro definitions, at the end of preprocessing, in
addition to normal output.
- -dH
- Produce a core dump whenever an error occurs.
- -dp
- Annotate the assembler output with a comment indicating
which pattern and alternative is used. The length of each instruction is
also printed.
- -dP
- Dump the RTL in the assembler output as a comment before
each instruction. Also turns on -dp annotation.
- -dx
- Just generate RTL for a function instead of compiling it.
Usually used with -fdump-rtl-expand.
- -fdump-noaddr
- When doing debugging dumps, suppress address output. This
makes it more feasible to use diff on debugging dumps for compiler
invocations with different compiler binaries and/or different text / bss /
data / heap / stack / dso start locations.
- -freport-bug
- Collect and dump debug information into temporary file if
ICE in C/C++ compiler occured.
- -fdump-unnumbered
- When doing debugging dumps, suppress instruction numbers
and address output. This makes it more feasible to use diff on debugging
dumps for compiler invocations with different options, in particular with
and without -g.
- -fdump-unnumbered-links
- When doing debugging dumps (see -d option above),
suppress instruction numbers for the links to the previous and next
instructions in a sequence.
- -fdump-translation-unit (C++ only)
- -fdump-translation-unit-options (C++
only)
- Dump a representation of the tree structure for the entire
translation unit to a file. The file name is made by appending .tu
to the source file name, and the file is created in the same directory as
the output file. If the -options form is used,
options controls the details of the dump as described for the
-fdump-tree options.
- -fdump-class-hierarchy (C++ only)
- -fdump-class-hierarchy-options (C++
only)
- Dump a representation of each class's hierarchy and virtual
function table layout to a file. The file name is made by appending
.class to the source file name, and the file is created in the same
directory as the output file. If the -options form is used,
options controls the details of the dump as described for the
-fdump-tree options.
- -fdump-ipa-switch
- Control the dumping at various stages of inter-procedural
analysis language tree to a file. The file name is generated by appending
a switch specific suffix to the source file name, and the file is created
in the same directory as the output file. The following dumps are
possible:
- all
- Enables all inter-procedural analysis dumps.
- cgraph
- Dumps information about call-graph optimization, unused
function removal, and inlining decisions.
- inline
- Dump after function inlining.
- -fdump-passes
- Dump the list of optimization passes that are turned on and
off by the current command-line options.
- -fdump-statistics-option
- Enable and control dumping of pass statistics in a separate
file. The file name is generated by appending a suffix ending in
.statistics to the source file name, and the file is created in the
same directory as the output file. If the -option form is
used, -stats causes counters to be summed over the whole
compilation unit while -details dumps every event as the passes
generate them. The default with no option is to sum counters for each
function compiled.
- -fdump-tree-switch
- -fdump-tree-switch-options
- -fdump-tree-switch-options=filename
- Control the dumping at various stages of processing the
intermediate language tree to a file. The file name is generated by
appending a switch-specific suffix to the source file name, and the file
is created in the same directory as the output file. In case of
=filename option, the dump is output on the given file
instead of the auto named dump files. If the -options form
is used, options is a list of - separated options which
control the details of the dump. Not all options are applicable to all
dumps; those that are not meaningful are ignored. The following options
are available
- address
- Print the address of each node. Usually this is not
meaningful as it changes according to the environment and source file. Its
primary use is for tying up a dump file with a debug environment.
- asmname
- If "DECL_ASSEMBLER_NAME" has been set for a given
decl, use that in the dump instead of "DECL_NAME". Its primary
use is ease of use working backward from mangled names in the assembly
file.
- slim
- When dumping front-end intermediate representations,
inhibit dumping of members of a scope or body of a function merely because
that scope has been reached. Only dump such items when they are directly
reachable by some other path.
When dumping pretty-printed trees, this option inhibits dumping the bodies
of control structures.
When dumping RTL, print the RTL in slim (condensed) form instead of the
default LISP-like representation.
- raw
- Print a raw representation of the tree. By default, trees
are pretty-printed into a C-like representation.
- details
- Enable more detailed dumps (not honored by every dump
option). Also include information from the optimization passes.
- stats
- Enable dumping various statistics about the pass (not
honored by every dump option).
- blocks
- Enable showing basic block boundaries (disabled in raw
dumps).
- graph
- For each of the other indicated dump files
(-fdump-rtl- pass), dump a representation of the control
flow graph suitable for viewing with GraphViz to
file. passid.pass.dot.
Each function in the file is pretty-printed as a subgraph, so that
GraphViz can render them all in a single plot.
This option currently only works for RTL dumps, and the RTL is always dumped
in slim form.
- vops
- Enable showing virtual operands for every statement.
- lineno
- Enable showing line numbers for statements.
- uid
- Enable showing the unique ID ("DECL_UID") for
each variable.
- verbose
- Enable showing the tree dump for each statement.
- eh
- Enable showing the EH region number holding each
statement.
- scev
- Enable showing scalar evolution analysis details.
- optimized
- Enable showing optimization information (only available in
certain passes).
- missed
- Enable showing missed optimization information (only
available in certain passes).
- note
- Enable other detailed optimization information (only
available in certain passes).
- =filename
- Instead of an auto named dump file, output into the given
file name. The file names stdout and stderr are treated
specially and are considered already open standard streams. For example,
gcc -O2 -ftree-vectorize -fdump-tree-vect-blocks=foo.dump
-fdump-tree-pre=stderr file.c
outputs vectorizer dump into foo.dump, while the PRE dump is output
on to stderr. If two conflicting dump filenames are given for the
same pass, then the latter option overrides the earlier one.
- all
- Turn on all options, except raw, slim,
verbose and lineno.
- optall
- Turn on all optimization options, i.e., optimized,
missed, and note.
The following tree dumps are possible:
- original
- Dump before any tree based optimization, to
file .original.
- optimized
- Dump after all tree based optimization, to
file .optimized.
- gimple
- Dump each function before and after the gimplification pass
to a file. The file name is made by appending .gimple to the source
file name.
- cfg
- Dump the control flow graph of each function to a file. The
file name is made by appending .cfg to the source file name.
- ch
- Dump each function after copying loop headers. The file
name is made by appending .ch to the source file name.
- ssa
- Dump SSA related information to a file. The file name is
made by appending .ssa to the source file name.
- alias
- Dump aliasing information for each function. The file name
is made by appending .alias to the source file name.
- ccp
- Dump each function after CCP. The file name is made by
appending .ccp to the source file name.
- storeccp
- Dump each function after STORE-CCP. The file name is made
by appending .storeccp to the source file name.
- pre
- Dump trees after partial redundancy elimination. The file
name is made by appending .pre to the source file name.
- fre
- Dump trees after full redundancy elimination. The file name
is made by appending .fre to the source file name.
- copyprop
- Dump trees after copy propagation. The file name is made by
appending .copyprop to the source file name.
- store_copyprop
- Dump trees after store copy-propagation. The file name is
made by appending .store_copyprop to the source file name.
- dce
- Dump each function after dead code elimination. The file
name is made by appending .dce to the source file name.
- sra
- Dump each function after performing scalar replacement of
aggregates. The file name is made by appending .sra to the source
file name.
- sink
- Dump each function after performing code sinking. The file
name is made by appending .sink to the source file name.
- dom
- Dump each function after applying dominator tree
optimizations. The file name is made by appending .dom to the
source file name.
- dse
- Dump each function after applying dead store elimination.
The file name is made by appending .dse to the source file
name.
- phiopt
- Dump each function after optimizing PHI nodes into
straightline code. The file name is made by appending .phiopt to
the source file name.
- forwprop
- Dump each function after forward propagating single use
variables. The file name is made by appending .forwprop to the
source file name.
- copyrename
- Dump each function after applying the copy rename
optimization. The file name is made by appending .copyrename to the
source file name.
- nrv
- Dump each function after applying the named return value
optimization on generic trees. The file name is made by appending
.nrv to the source file name.
- vect
- Dump each function after applying vectorization of loops.
The file name is made by appending .vect to the source file
name.
- slp
- Dump each function after applying vectorization of basic
blocks. The file name is made by appending .slp to the source file
name.
- vrp
- Dump each function after Value Range Propagation (VRP). The
file name is made by appending .vrp to the source file name.
- all
- Enable all the available tree dumps with the flags provided
in this option.
- -fopt-info
- -fopt-info-options
- -fopt-info-options=filename
- Controls optimization dumps from various optimization
passes. If the -options form is used, options is a
list of - separated option keywords to select the dump details and
optimizations.
The options can be divided into two groups: options describing the
verbosity of the dump, and options describing which optimizations should
be included. The options from both the groups can be freely mixed as they
are non-overlapping. However, in case of any conflicts, the later options
override the earlier options on the command line.
The following options control the dump verbosity:
- optimized
- Print information when an optimization is successfully
applied. It is up to a pass to decide which information is relevant. For
example, the vectorizer passes print the source location of loops which
are successfully vectorized.
- missed
- Print information about missed optimizations. Individual
passes control which information to include in the output.
- note
- Print verbose information about optimizations, such as
certain transformations, more detailed messages about decisions etc.
- all
- Print detailed optimization information. This includes
optimized, missed, and note.
One or more of the following option keywords can be used to describe a group of
optimizations:
- ipa
- Enable dumps from all interprocedural optimizations.
- loop
- Enable dumps from all loop optimizations.
- inline
- Enable dumps from all inlining optimizations.
- vec
- Enable dumps from all vectorization optimizations.
- optall
- Enable dumps from all optimizations. This is a superset of
the optimization groups listed above.
If
options is omitted, it defaults to
optimized-optall, which
means to dump all info about successful optimizations from all the passes.
If the
filename is provided, then the dumps from all the applicable
optimizations are concatenated into the
filename. Otherwise the dump is
output onto
stderr. Though multiple
-fopt-info options are
accepted, only one of them can include a
filename. If other filenames
are provided then all but the first such option are ignored.
Note that the output
filename is overwritten in case of multiple
translation units. If a combined output from multiple translation units is
desired,
stderr should be used instead.
In the following example, the optimization info is output to
stderr:
gcc -O3 -fopt-info
This example:
gcc -O3 -fopt-info-missed=missed.all
outputs missed optimization report from all the passes into
missed.all,
and this one:
gcc -O2 -ftree-vectorize -fopt-info-vec-missed
prints information about missed optimization opportunities from vectorization
passes on
stderr. Note that
-fopt-info-vec-missed is equivalent
to
-fopt-info-missed-vec.
As another example,
gcc -O3 -fopt-info-inline-optimized-missed=inline.txt
outputs information about missed optimizations as well as optimized locations
from all the inlining passes into
inline.txt.
Finally, consider:
gcc -fopt-info-vec-missed=vec.miss -fopt-info-loop-optimized=loop.opt
Here the two output filenames
vec.miss and
loop.opt are in
conflict since only one output file is allowed. In this case, only the first
option takes effect and the subsequent options are ignored. Thus only
vec.miss is produced which contains dumps from the vectorizer about
missed opportunities.
- -frandom-seed=string
- This option provides a seed that GCC uses in place of
random numbers in generating certain symbol names that have to be
different in every compiled file. It is also used to place unique stamps
in coverage data files and the object files that produce them. You can use
the -frandom-seed option to produce reproducibly identical object
files.
The string can either be a number (decimal, octal or hex) or an
arbitrary string (in which case it's converted to a number by computing
CRC32).
The string should be different for every file you compile.
- -fsched-verbose=n
- On targets that use instruction scheduling, this option
controls the amount of debugging output the scheduler prints. This
information is written to standard error, unless -fdump-rtl-sched1
or -fdump-rtl-sched2 is specified, in which case it is output to
the usual dump listing file, .sched1 or .sched2
respectively. However for n greater than nine, the output is always
printed to standard error.
For n greater than zero, -fsched-verbose outputs the same
information as -fdump-rtl-sched1 and -fdump-rtl-sched2. For
n greater than one, it also output basic block probabilities,
detailed ready list information and unit/insn info. For n greater
than two, it includes RTL at abort point, control-flow and regions info.
And for n over four, -fsched-verbose also includes
dependence info.
- -save-temps
- -save-temps=cwd
- Store the usual "temporary" intermediate files
permanently; place them in the current directory and name them based on
the source file. Thus, compiling foo.c with -c -save-temps
produces files foo.i and foo.s, as well as foo.o.
This creates a preprocessed foo.i output file even though the
compiler now normally uses an integrated preprocessor.
When used in combination with the -x command-line option,
-save-temps is sensible enough to avoid over writing an input
source file with the same extension as an intermediate file. The
corresponding intermediate file may be obtained by renaming the source
file before using -save-temps.
If you invoke GCC in parallel, compiling several different source files that
share a common base name in different subdirectories or the same source
file compiled for multiple output destinations, it is likely that the
different parallel compilers will interfere with each other, and overwrite
the temporary files. For instance:
gcc -save-temps -o outdir1/foo.o indir1/foo.c&
gcc -save-temps -o outdir2/foo.o indir2/foo.c&
may result in foo.i and foo.o being written to simultaneously
by both compilers.
- -save-temps=obj
- Store the usual "temporary" intermediate files
permanently. If the -o option is used, the temporary files are
based on the object file. If the -o option is not used, the
-save-temps=obj switch behaves like -save-temps.
For example:
gcc -save-temps=obj -c foo.c
gcc -save-temps=obj -c bar.c -o dir/xbar.o
gcc -save-temps=obj foobar.c -o dir2/yfoobar
creates foo.i, foo.s, dir/xbar.i, dir/xbar.s,
dir2/yfoobar.i, dir2/yfoobar.s, and
dir2/yfoobar.o.
- -time[=file]
- Report the CPU time taken by each subprocess in the
compilation sequence. For C source files, this is the compiler proper and
assembler (plus the linker if linking is done).
Without the specification of an output file, the output looks like this:
# cc1 0.12 0.01
# as 0.00 0.01
The first number on each line is the "user time", that is time
spent executing the program itself. The second number is "system
time", time spent executing operating system routines on behalf of
the program. Both numbers are in seconds.
With the specification of an output file, the output is appended to the
named file, and it looks like this:
0.12 0.01 cc1 <options>
0.00 0.01 as <options>
The "user time" and the "system time" are moved before
the program name, and the options passed to the program are displayed, so
that one can later tell what file was being compiled, and with which
options.
- -fvar-tracking
- Run variable tracking pass. It computes where variables are
stored at each position in code. Better debugging information is then
generated (if the debugging information format supports this information).
It is enabled by default when compiling with optimization ( -Os,
-O, -O2, ...), debugging information (-g) and the
debug info format supports it.
- -fvar-tracking-assignments
- Annotate assignments to user variables early in the
compilation and attempt to carry the annotations over throughout the
compilation all the way to the end, in an attempt to improve debug
information while optimizing. Use of -gdwarf-4 is recommended along
with it.
It can be enabled even if var-tracking is disabled, in which case
annotations are created and maintained, but discarded at the end. By
default, this flag is enabled together with -fvar-tracking, except
when selective scheduling is enabled.
- -fvar-tracking-assignments-toggle
- Toggle -fvar-tracking-assignments, in the same way
that -gtoggle toggles -g.
- -print-file-name=library
- Print the full absolute name of the library file
library that would be used when linking---and don't do anything
else. With this option, GCC does not compile or link anything; it just
prints the file name.
- -print-multi-directory
- Print the directory name corresponding to the multilib
selected by any other switches present in the command line. This directory
is supposed to exist in GCC_EXEC_PREFIX.
- -print-multi-lib
- Print the mapping from multilib directory names to compiler
switches that enable them. The directory name is separated from the
switches by ;, and each switch starts with an @ instead of
the -, without spaces between multiple switches. This is supposed
to ease shell processing.
- -print-multi-os-directory
- Print the path to OS libraries for the selected multilib,
relative to some lib subdirectory. If OS libraries are present in
the lib subdirectory and no multilibs are used, this is usually
just ., if OS libraries are present in
libsuffix sibling directories this prints e.g.
../lib64, ../lib or ../lib32, or if OS libraries are
present in lib/subdir subdirectories it prints e.g.
amd64, sparcv9 or ev6.
- -print-multiarch
- Print the path to OS libraries for the selected multiarch,
relative to some lib subdirectory.
- -print-prog-name=program
- Like -print-file-name, but searches for a program
such as cpp.
- -print-libgcc-file-name
- Same as -print-file-name=libgcc.a.
This is useful when you use -nostdlib or -nodefaultlibs but
you do want to link with libgcc.a. You can do:
gcc -nostdlib <files>... `gcc -print-libgcc-file-name`
- -print-search-dirs
- Print the name of the configured installation directory and
a list of program and library directories gcc searches---and don't
do anything else.
This is useful when gcc prints the error message installation
problem, cannot exec cpp0: No such file or directory. To resolve this
you either need to put cpp0 and the other compiler components where
gcc expects to find them, or you can set the environment variable
GCC_EXEC_PREFIX to the directory where you installed them. Don't
forget the trailing /.
- -print-sysroot
- Print the target sysroot directory that is used during
compilation. This is the target sysroot specified either at configure time
or using the --sysroot option, possibly with an extra suffix that
depends on compilation options. If no target sysroot is specified, the
option prints nothing.
- -print-sysroot-headers-suffix
- Print the suffix added to the target sysroot when searching
for headers, or give an error if the compiler is not configured with such
a suffix---and don't do anything else.
- -dumpmachine
- Print the compiler's target machine (for example,
i686-pc-linux-gnu)---and don't do anything else.
- -dumpversion
- Print the compiler version (for example, 3.0)---and don't
do anything else.
- -dumpspecs
- Print the compiler's built-in specs---and don't do anything
else. (This is used when GCC itself is being built.)
- -fno-eliminate-unused-debug-types
- Normally, when producing DWARF 2 output, GCC avoids
producing debug symbol output for types that are nowhere used in the
source file being compiled. Sometimes it is useful to have GCC emit
debugging information for all types declared in a compilation unit,
regardless of whether or not they are actually used in that compilation
unit, for example if, in the debugger, you want to cast a value to a type
that is not actually used in your program (but is declared). More often,
however, this results in a significant amount of wasted space.
Options That Control Optimization
These options control various sorts of optimizations.
Without any optimization option, the compiler's goal is to reduce the cost of
compilation and to make debugging produce the expected results. Statements are
independent: if you stop the program with a breakpoint between statements, you
can then assign a new value to any variable or change the program counter to
any other statement in the function and get exactly the results you expect
from the source code.
Turning on optimization flags makes the compiler attempt to improve the
performance and/or code size at the expense of compilation time and possibly
the ability to debug the program.
The compiler performs optimization based on the knowledge it has of the program.
Compiling multiple files at once to a single output file mode allows the
compiler to use information gained from all of the files when compiling each
of them.
Not all optimizations are controlled directly by a flag. Only optimizations that
have a flag are listed in this section.
Most optimizations are only enabled if an
-O level is set on the command
line. Otherwise they are disabled, even if individual optimization flags are
specified.
Depending on the target and how GCC was configured, a slightly different set of
optimizations may be enabled at each
-O level than those listed here.
You can invoke GCC with
-Q --help=optimizers to find out the exact set
of optimizations that are enabled at each level.
- -O
- -O1
- Optimize. Optimizing compilation takes somewhat more time,
and a lot more memory for a large function.
With -O, the compiler tries to reduce code size and execution time,
without performing any optimizations that take a great deal of compilation
time.
-O turns on the following optimization flags:
-fauto-inc-dec -fbranch-count-reg
-fcombine-stack-adjustments -fcompare-elim
-fcprop-registers -fdce -fdefer-pop
-fdelayed-branch -fdse -fforward-propagate
-fguess-branch-probability -fif-conversion2
-fif-conversion -finline-functions-called-once
-fipa-pure-const -fipa-profile -fipa-reference
-fmerge-constants -fmove-loop-invariants
-fshrink-wrap -fsplit-wide-types -ftree-bit-ccp
-ftree-ccp -fssa-phiopt -ftree-ch
-ftree-copy-prop -ftree-copyrename -ftree-dce
-ftree-dominator-opts -ftree-dse -ftree-forwprop
-ftree-fre -ftree-phiprop -ftree-sink
-ftree-slsr -ftree-sra -ftree-pta -ftree-ter
-funit-at-a-time
-O also turns on -fomit-frame-pointer on machines where doing
so does not interfere with debugging.
- -O2
- Optimize even more. GCC performs nearly all supported
optimizations that do not involve a space-speed tradeoff. As compared to
-O, this option increases both compilation time and the performance
of the generated code.
-O2 turns on all optimization flags specified by -O. It also
turns on the following optimization flags: -fthread-jumps
-falign-functions -falign-jumps -falign-loops -falign-labels
-fcaller-saves -fcrossjumping -fcse-follow-jumps
-fcse-skip-blocks -fdelete-null-pointer-checks
-fdevirtualize -fdevirtualize-speculatively
-fexpensive-optimizations -fgcse -fgcse-lm
-fhoist-adjacent-loads -finline-small-functions
-findirect-inlining -fipa-cp -fipa-cp-alignment
-fipa-sra -fipa-icf
-fisolate-erroneous-paths-dereference -flra-remat
-foptimize-sibling-calls -foptimize-strlen
-fpartial-inlining -fpeephole2 -freorder-blocks
-freorder-blocks-and-partition -freorder-functions
-frerun-cse-after-loop -fsched-interblock -fsched-spec
-fschedule-insns -fschedule-insns2 -fstrict-aliasing
-fstrict-overflow -ftree-builtin-call-dce
-ftree-switch-conversion -ftree-tail-merge -ftree-pre
-ftree-vrp -fipa-ra
Please note the warning under -fgcse about invoking -O2 on
programs that use computed gotos.
- -O3
- Optimize yet more. -O3 turns on all optimizations
specified by -O2 and also turns on the -finline-functions,
-funswitch-loops, -fpredictive-commoning,
-fgcse-after-reload, -ftree-loop-vectorize,
-ftree-loop-distribute-patterns, -ftree-slp-vectorize,
-fvect-cost-model, -ftree-partial-pre and
-fipa-cp-clone options.
- -O0
- Reduce compilation time and make debugging produce the
expected results. This is the default.
- -Os
- Optimize for size. -Os enables all -O2
optimizations that do not typically increase code size. It also performs
further optimizations designed to reduce code size.
-Os disables the following optimization flags: -falign-functions
-falign-jumps -falign-loops -falign-labels -freorder-blocks
-freorder-blocks-and-partition -fprefetch-loop-arrays
- -Ofast
- Disregard strict standards compliance. -Ofast
enables all -O3 optimizations. It also enables optimizations that
are not valid for all standard-compliant programs. It turns on
-ffast-math and the Fortran-specific -fno-protect-parens and
-fstack-arrays.
- -Og
- Optimize debugging experience. -Og enables
optimizations that do not interfere with debugging. It should be the
optimization level of choice for the standard edit-compile-debug cycle,
offering a reasonable level of optimization while maintaining fast
compilation and a good debugging experience.
If you use multiple -O options, with or without level numbers, the
last such option is the one that is effective.
Options of the form
-fflag specify machine-independent flags. Most
flags have both positive and negative forms; the negative form of
-ffoo
is
-fno-foo. In the table below, only one of the forms is listed---the
one you typically use. You can figure out the other form by either removing
no- or adding it.
The following options control specific optimizations. They are either activated
by
-O options or are related to ones that are. You can use the
following flags in the rare cases when "fine-tuning" of
optimizations to be performed is desired.
- -fno-defer-pop
- Always pop the arguments to each function call as soon as
that function returns. For machines that must pop arguments after a
function call, the compiler normally lets arguments accumulate on the
stack for several function calls and pops them all at once.
Disabled at levels -O, -O2, -O3, -Os.
- -fforward-propagate
- Perform a forward propagation pass on RTL. The pass tries
to combine two instructions and checks if the result can be simplified. If
loop unrolling is active, two passes are performed and the second is
scheduled after loop unrolling.
This option is enabled by default at optimization levels -O,
-O2, -O3, -Os.
- -ffp-contract=style
- -ffp-contract=off disables floating-point expression
contraction. -ffp-contract=fast enables floating-point expression
contraction such as forming of fused multiply-add operations if the target
has native support for them. -ffp-contract=on enables
floating-point expression contraction if allowed by the language standard.
This is currently not implemented and treated equal to
-ffp-contract=off.
The default is -ffp-contract=fast.
- -fomit-frame-pointer
- Don't keep the frame pointer in a register for functions
that don't need one. This avoids the instructions to save, set up and
restore frame pointers; it also makes an extra register available in many
functions. It also makes debugging impossible on some
machines.
On some machines, such as the VAX, this flag has no effect, because the
standard calling sequence automatically handles the frame pointer and
nothing is saved by pretending it doesn't exist. The machine-description
macro "FRAME_POINTER_REQUIRED" controls whether a target machine
supports this flag.
The default setting (when not optimizing for size) for 32-bit GNU/Linux x86
and 32-bit Darwin x86 targets is -fomit-frame-pointer. You can
configure GCC with the --enable-frame-pointer configure option to
change the default.
Enabled at levels -O, -O2, -O3, -Os.
- -foptimize-sibling-calls
- Optimize sibling and tail recursive calls.
Enabled at levels -O2, -O3, -Os.
- -foptimize-strlen
- Optimize various standard C string functions (e.g.
"strlen", "strchr" or "strcpy") and their
"_FORTIFY_SOURCE" counterparts into faster alternatives.
Enabled at levels -O2, -O3.
- -fno-inline
- Do not expand any functions inline apart from those marked
with the "always_inline" attribute. This is the default when not
optimizing.
Single functions can be exempted from inlining by marking them with the
"noinline" attribute.
- -finline-small-functions
- Integrate functions into their callers when their body is
smaller than expected function call code (so overall size of program gets
smaller). The compiler heuristically decides which functions are simple
enough to be worth integrating in this way. This inlining applies to all
functions, even those not declared inline.
Enabled at level -O2.
- -findirect-inlining
- Inline also indirect calls that are discovered to be known
at compile time thanks to previous inlining. This option has any effect
only when inlining itself is turned on by the -finline-functions or
-finline-small-functions options.
Enabled at level -O2.
- -finline-functions
- Consider all functions for inlining, even if they are not
declared inline. The compiler heuristically decides which functions are
worth integrating in this way.
If all calls to a given function are integrated, and the function is
declared "static", then the function is normally not output as
assembler code in its own right.
Enabled at level -O3.
- -finline-functions-called-once
- Consider all "static" functions called once for
inlining into their caller even if they are not marked "inline".
If a call to a given function is integrated, then the function is not
output as assembler code in its own right.
Enabled at levels -O1, -O2, -O3 and -Os.
- -fearly-inlining
- Inline functions marked by "always_inline" and
functions whose body seems smaller than the function call overhead early
before doing -fprofile-generate instrumentation and real inlining
pass. Doing so makes profiling significantly cheaper and usually inlining
faster on programs having large chains of nested wrapper functions.
Enabled by default.
- -fipa-sra
- Perform interprocedural scalar replacement of aggregates,
removal of unused parameters and replacement of parameters passed by
reference by parameters passed by value.
Enabled at levels -O2, -O3 and -Os.
- -finline-limit=n
- By default, GCC limits the size of functions that can be
inlined. This flag allows coarse control of this limit. n is the
size of functions that can be inlined in number of pseudo instructions.
Inlining is actually controlled by a number of parameters, which may be
specified individually by using --param
name=value. The -finline-limit=n option
sets some of these parameters as follows:
- max-inline-insns-single
- is set to n/2.
- max-inline-insns-auto
- is set to n/2.
See below for a documentation of the individual parameters controlling inlining
and for the defaults of these parameters.
Note: there may be no value to
-finline-limit that results in
default behavior.
Note: pseudo instruction represents, in this particular context, an
abstract measurement of function's size. In no way does it represent a count
of assembly instructions and as such its exact meaning might change from one
release to an another.
- -fno-keep-inline-dllexport
- This is a more fine-grained version of
-fkeep-inline-functions, which applies only to functions that are
declared using the "dllexport" attribute or declspec
- -fkeep-inline-functions
- In C, emit "static" functions that are declared
"inline" into the object file, even if the function has been
inlined into all of its callers. This switch does not affect functions
using the "extern inline" extension in GNU C90. In C++, emit any
and all inline functions into the object file.
- -fkeep-static-consts
- Emit variables declared "static const" when
optimization isn't turned on, even if the variables aren't referenced.
GCC enables this option by default. If you want to force the compiler to
check if a variable is referenced, regardless of whether or not
optimization is turned on, use the -fno-keep-static-consts
option.
- -fmerge-constants
- Attempt to merge identical constants (string constants and
floating-point constants) across compilation units.
This option is the default for optimized compilation if the assembler and
linker support it. Use -fno-merge-constants to inhibit this
behavior.
Enabled at levels -O, -O2, -O3, -Os.
- -fmerge-all-constants
- Attempt to merge identical constants and identical
variables.
This option implies -fmerge-constants. In addition to
-fmerge-constants this considers e.g. even constant initialized
arrays or initialized constant variables with integral or floating-point
types. Languages like C or C++ require each variable, including multiple
instances of the same variable in recursive calls, to have distinct
locations, so using this option results in non-conforming behavior.
- -fmodulo-sched
- Perform swing modulo scheduling immediately before the
first scheduling pass. This pass looks at innermost loops and reorders
their instructions by overlapping different iterations.
- -fmodulo-sched-allow-regmoves
- Perform more aggressive SMS-based modulo scheduling with
register moves allowed. By setting this flag certain anti-dependences
edges are deleted, which triggers the generation of reg-moves based on the
life-range analysis. This option is effective only with
-fmodulo-sched enabled.
- -fno-branch-count-reg
- Do not use "decrement and branch" instructions on
a count register, but instead generate a sequence of instructions that
decrement a register, compare it against zero, then branch based upon the
result. This option is only meaningful on architectures that support such
instructions, which include x86, PowerPC, IA-64 and S/390.
Enabled by default at -O1 and higher.
The default is -fbranch-count-reg.
- -fno-function-cse
- Do not put function addresses in registers; make each
instruction that calls a constant function contain the function's address
explicitly.
This option results in less efficient code, but some strange hacks that
alter the assembler output may be confused by the optimizations performed
when this option is not used.
The default is -ffunction-cse
- -fno-zero-initialized-in-bss
- If the target supports a BSS section, GCC by default puts
variables that are initialized to zero into BSS. This can save space in
the resulting code.
This option turns off this behavior because some programs explicitly rely on
variables going to the data section---e.g., so that the resulting
executable can find the beginning of that section and/or make assumptions
based on that.
The default is -fzero-initialized-in-bss.
- -fthread-jumps
- Perform optimizations that check to see if a jump branches
to a location where another comparison subsumed by the first is found. If
so, the first branch is redirected to either the destination of the second
branch or a point immediately following it, depending on whether the
condition is known to be true or false.
Enabled at levels -O2, -O3, -Os.
- -fsplit-wide-types
- When using a type that occupies multiple registers, such as
"long long" on a 32-bit system, split the registers apart and
allocate them independently. This normally generates better code for those
types, but may make debugging more difficult.
Enabled at levels -O, -O2, -O3, -Os.
- -fcse-follow-jumps
- In common subexpression elimination (CSE), scan through
jump instructions when the target of the jump is not reached by any other
path. For example, when CSE encounters an "if" statement with an
"else" clause, CSE follows the jump when the condition tested is
false.
Enabled at levels -O2, -O3, -Os.
- -fcse-skip-blocks
- This is similar to -fcse-follow-jumps, but causes
CSE to follow jumps that conditionally skip over blocks. When CSE
encounters a simple "if" statement with no else clause,
-fcse-skip-blocks causes CSE to follow the jump around the body of
the "if".
Enabled at levels -O2, -O3, -Os.
- -frerun-cse-after-loop
- Re-run common subexpression elimination after loop
optimizations are performed.
Enabled at levels -O2, -O3, -Os.
- -fgcse
- Perform a global common subexpression elimination pass.
This pass also performs global constant and copy propagation.
Note: When compiling a program using computed gotos, a GCC
extension, you may get better run-time performance if you disable the
global common subexpression elimination pass by adding -fno-gcse to
the command line.
Enabled at levels -O2, -O3, -Os.
- -fgcse-lm
- When -fgcse-lm is enabled, global common
subexpression elimination attempts to move loads that are only killed by
stores into themselves. This allows a loop containing a load/store
sequence to be changed to a load outside the loop, and a copy/store within
the loop.
Enabled by default when -fgcse is enabled.
- -fgcse-sm
- When -fgcse-sm is enabled, a store motion pass is
run after global common subexpression elimination. This pass attempts to
move stores out of loops. When used in conjunction with -fgcse-lm,
loops containing a load/store sequence can be changed to a load before the
loop and a store after the loop.
Not enabled at any optimization level.
- -fgcse-las
- When -fgcse-las is enabled, the global common
subexpression elimination pass eliminates redundant loads that come after
stores to the same memory location (both partial and full redundancies).
Not enabled at any optimization level.
- -fgcse-after-reload
- When -fgcse-after-reload is enabled, a redundant
load elimination pass is performed after reload. The purpose of this pass
is to clean up redundant spilling.
- -faggressive-loop-optimizations
- This option tells the loop optimizer to use language
constraints to derive bounds for the number of iterations of a loop. This
assumes that loop code does not invoke undefined behavior by for example
causing signed integer overflows or out-of-bound array accesses. The
bounds for the number of iterations of a loop are used to guide loop
unrolling and peeling and loop exit test optimizations. This option is
enabled by default.
- -funsafe-loop-optimizations
- This option tells the loop optimizer to assume that loop
indices do not overflow, and that loops with nontrivial exit condition are
not infinite. This enables a wider range of loop optimizations even if the
loop optimizer itself cannot prove that these assumptions are valid. If
you use -Wunsafe-loop-optimizations, the compiler warns you if it
finds this kind of loop.
- -fcrossjumping
- Perform cross-jumping transformation. This transformation
unifies equivalent code and saves code size. The resulting code may or may
not perform better than without cross-jumping.
Enabled at levels -O2, -O3, -Os.
- -fauto-inc-dec
- Combine increments or decrements of addresses with memory
accesses. This pass is always skipped on architectures that do not have
instructions to support this. Enabled by default at -O and higher
on architectures that support this.
- -fdce
- Perform dead code elimination (DCE) on RTL. Enabled by
default at -O and higher.
- -fdse
- Perform dead store elimination (DSE) on RTL. Enabled by
default at -O and higher.
- -fif-conversion
- Attempt to transform conditional jumps into branch-less
equivalents. This includes use of conditional moves, min, max, set flags
and abs instructions, and some tricks doable by standard arithmetics. The
use of conditional execution on chips where it is available is controlled
by -fif-conversion2.
Enabled at levels -O, -O2, -O3, -Os.
- -fif-conversion2
- Use conditional execution (where available) to transform
conditional jumps into branch-less equivalents.
Enabled at levels -O, -O2, -O3, -Os.
- -fdeclone-ctor-dtor
- The C++ ABI requires multiple entry points for constructors
and destructors: one for a base subobject, one for a complete object, and
one for a virtual destructor that calls operator delete afterwards. For a
hierarchy with virtual bases, the base and complete variants are clones,
which means two copies of the function. With this option, the base and
complete variants are changed to be thunks that call a common
implementation.
Enabled by -Os.
- -fdelete-null-pointer-checks
- Assume that programs cannot safely dereference null
pointers, and that no code or data element resides there. This enables
simple constant folding optimizations at all optimization levels. In
addition, other optimization passes in GCC use this flag to control global
dataflow analyses that eliminate useless checks for null pointers; these
assume that if a pointer is checked after it has already been
dereferenced, it cannot be null.
Note however that in some environments this assumption is not true. Use
-fno-delete-null-pointer-checks to disable this optimization for
programs that depend on that behavior.
Some targets, especially embedded ones, disable this option at all levels.
Otherwise it is enabled at all levels: -O0, -O1, -O2,
-O3, -Os. Passes that use the information are enabled
independently at different optimization levels.
- -fdevirtualize
- Attempt to convert calls to virtual functions to direct
calls. This is done both within a procedure and interprocedurally as part
of indirect inlining ( -findirect-inlining) and interprocedural
constant propagation ( -fipa-cp). Enabled at levels -O2,
-O3, -Os.
- -fdevirtualize-speculatively
- Attempt to convert calls to virtual functions to
speculative direct calls. Based on the analysis of the type inheritance
graph, determine for a given call the set of likely targets. If the set is
small, preferably of size 1, change the call into a conditional deciding
between direct and indirect calls. The speculative calls enable more
optimizations, such as inlining. When they seem useless after further
optimization, they are converted back into original form.
- -fdevirtualize-at-ltrans
- Stream extra information needed for aggressive
devirtualization when running the link-time optimizer in local
transformation mode. This option enables more devirtualization but
significantly increases the size of streamed data. For this reason it is
disabled by default.
- -fexpensive-optimizations
- Perform a number of minor optimizations that are relatively
expensive.
Enabled at levels -O2, -O3, -Os.
- -free
- Attempt to remove redundant extension instructions. This is
especially helpful for the x86-64 architecture, which implicitly
zero-extends in 64-bit registers after writing to their lower 32-bit half.
Enabled for Alpha, AArch64 and x86 at levels -O2, -O3,
-Os.
- -fno-lifetime-dse
- In C++ the value of an object is only affected by changes
within its lifetime: when the constructor begins, the object has an
indeterminate value, and any changes during the lifetime of the object are
dead when the object is destroyed. Normally dead store elimination will
take advantage of this; if your code relies on the value of the object
storage persisting beyond the lifetime of the object, you can use this
flag to disable this optimization.
- -flive-range-shrinkage
- Attempt to decrease register pressure through register live
range shrinkage. This is helpful for fast processors with small or
moderate size register sets.
- -fira-algorithm=algorithm
- Use the specified coloring algorithm for the integrated
register allocator. The algorithm argument can be priority,
which specifies Chow's priority coloring, or CB, which specifies
Chaitin-Briggs coloring. Chaitin-Briggs coloring is not implemented for
all architectures, but for those targets that do support it, it is the
default because it generates better code.
- -fira-region=region
- Use specified regions for the integrated register
allocator. The region argument should be one of the following:
- all
- Use all loops as register allocation regions. This can give
the best results for machines with a small and/or irregular register
set.
- mixed
- Use all loops except for loops with small register pressure
as the regions. This value usually gives the best results in most cases
and for most architectures, and is enabled by default when compiling with
optimization for speed ( -O, -O2, ...).
- one
- Use all functions as a single region. This typically
results in the smallest code size, and is enabled by default for
-Os or -O0.
- -fira-hoist-pressure
- Use IRA to evaluate register pressure in the code hoisting
pass for decisions to hoist expressions. This option usually results in
smaller code, but it can slow the compiler down.
This option is enabled at level -Os for all targets.
- -fira-loop-pressure
- Use IRA to evaluate register pressure in loops for
decisions to move loop invariants. This option usually results in
generation of faster and smaller code on machines with large register
files (>= 32 registers), but it can slow the compiler down.
This option is enabled at level -O3 for some targets.
- -fno-ira-share-save-slots
- Disable sharing of stack slots used for saving call-used
hard registers living through a call. Each hard register gets a separate
stack slot, and as a result function stack frames are larger.
- -fno-ira-share-spill-slots
- Disable sharing of stack slots allocated for
pseudo-registers. Each pseudo-register that does not get a hard register
gets a separate stack slot, and as a result function stack frames are
larger.
- -fira-verbose=n
- Control the verbosity of the dump file for the integrated
register allocator. The default value is 5. If the value n is
greater or equal to 10, the dump output is sent to stderr using the same
format as n minus 10.
- -flra-remat
- Enable CFG-sensitive rematerialization in LRA. Instead of
loading values of spilled pseudos, LRA tries to rematerialize
(recalculate) values if it is profitable.
Enabled at levels -O2, -O3, -Os.
- -fdelayed-branch
- If supported for the target machine, attempt to reorder
instructions to exploit instruction slots available after delayed branch
instructions.
Enabled at levels -O, -O2, -O3, -Os.
- -fschedule-insns
- If supported for the target machine, attempt to reorder
instructions to eliminate execution stalls due to required data being
unavailable. This helps machines that have slow floating point or memory
load instructions by allowing other instructions to be issued until the
result of the load or floating-point instruction is required.
Enabled at levels -O2, -O3.
- -fschedule-insns2
- Similar to -fschedule-insns, but requests an
additional pass of instruction scheduling after register allocation has
been done. This is especially useful on machines with a relatively small
number of registers and where memory load instructions take more than one
cycle.
Enabled at levels -O2, -O3, -Os.
- -fno-sched-interblock
- Don't schedule instructions across basic blocks. This is
normally enabled by default when scheduling before register allocation,
i.e. with -fschedule-insns or at -O2 or higher.
- -fno-sched-spec
- Don't allow speculative motion of non-load instructions.
This is normally enabled by default when scheduling before register
allocation, i.e. with -fschedule-insns or at -O2 or
higher.
- -fsched-pressure
- Enable register pressure sensitive insn scheduling before
register allocation. This only makes sense when scheduling before register
allocation is enabled, i.e. with -fschedule-insns or at -O2
or higher. Usage of this option can improve the generated code and
decrease its size by preventing register pressure increase above the
number of available hard registers and subsequent spills in register
allocation.
- -fsched-spec-load
- Allow speculative motion of some load instructions. This
only makes sense when scheduling before register allocation, i.e. with
-fschedule-insns or at -O2 or higher.
- -fsched-spec-load-dangerous
- Allow speculative motion of more load instructions. This
only makes sense when scheduling before register allocation, i.e. with
-fschedule-insns or at -O2 or higher.
- -fsched-stalled-insns
- -fsched-stalled-insns=n
- Define how many insns (if any) can be moved prematurely
from the queue of stalled insns into the ready list during the second
scheduling pass. -fno-sched-stalled-insns means that no insns are
moved prematurely, -fsched-stalled-insns=0 means there is no limit
on how many queued insns can be moved prematurely.
-fsched-stalled-insns without a value is equivalent to
-fsched-stalled-insns=1.
- -fsched-stalled-insns-dep
- -fsched-stalled-insns-dep=n
- Define how many insn groups (cycles) are examined for a
dependency on a stalled insn that is a candidate for premature removal
from the queue of stalled insns. This has an effect only during the second
scheduling pass, and only if -fsched-stalled-insns is used.
-fno-sched-stalled-insns-dep is equivalent to
-fsched-stalled-insns-dep=0. -fsched-stalled-insns-dep
without a value is equivalent to -fsched-stalled-insns-dep=1.
- -fsched2-use-superblocks
- When scheduling after register allocation, use superblock
scheduling. This allows motion across basic block boundaries, resulting in
faster schedules. This option is experimental, as not all machine
descriptions used by GCC model the CPU closely enough to avoid unreliable
results from the algorithm.
This only makes sense when scheduling after register allocation, i.e. with
-fschedule-insns2 or at -O2 or higher.
- -fsched-group-heuristic
- Enable the group heuristic in the scheduler. This heuristic
favors the instruction that belongs to a schedule group. This is enabled
by default when scheduling is enabled, i.e. with -fschedule-insns
or -fschedule-insns2 or at -O2 or higher.
- -fsched-critical-path-heuristic
- Enable the critical-path heuristic in the scheduler. This
heuristic favors instructions on the critical path. This is enabled by
default when scheduling is enabled, i.e. with -fschedule-insns or
-fschedule-insns2 or at -O2 or higher.
- -fsched-spec-insn-heuristic
- Enable the speculative instruction heuristic in the
scheduler. This heuristic favors speculative instructions with greater
dependency weakness. This is enabled by default when scheduling is
enabled, i.e. with -fschedule-insns or -fschedule-insns2 or
at -O2 or higher.
- -fsched-rank-heuristic
- Enable the rank heuristic in the scheduler. This heuristic
favors the instruction belonging to a basic block with greater size or
frequency. This is enabled by default when scheduling is enabled, i.e.
with -fschedule-insns or -fschedule-insns2 or at -O2
or higher.
- -fsched-last-insn-heuristic
- Enable the last-instruction heuristic in the scheduler.
This heuristic favors the instruction that is less dependent on the last
instruction scheduled. This is enabled by default when scheduling is
enabled, i.e. with -fschedule-insns or -fschedule-insns2 or
at -O2 or higher.
- -fsched-dep-count-heuristic
- Enable the dependent-count heuristic in the scheduler. This
heuristic favors the instruction that has more instructions depending on
it. This is enabled by default when scheduling is enabled, i.e. with
-fschedule-insns or -fschedule-insns2 or at -O2 or
higher.
- -freschedule-modulo-scheduled-loops
- Modulo scheduling is performed before traditional
scheduling. If a loop is modulo scheduled, later scheduling passes may
change its schedule. Use this option to control that behavior.
- -fselective-scheduling
- Schedule instructions using selective scheduling algorithm.
Selective scheduling runs instead of the first scheduler pass.
- -fselective-scheduling2
- Schedule instructions using selective scheduling algorithm.
Selective scheduling runs instead of the second scheduler pass.
- -fsel-sched-pipelining
- Enable software pipelining of innermost loops during
selective scheduling. This option has no effect unless one of
-fselective-scheduling or -fselective-scheduling2 is turned
on.
- -fsel-sched-pipelining-outer-loops
- When pipelining loops during selective scheduling, also
pipeline outer loops. This option has no effect unless
-fsel-sched-pipelining is turned on.
- -fsemantic-interposition
- Some object formats, like ELF, allow interposing of symbols
by the dynamic linker. This means that for symbols exported from the DSO,
the compiler cannot perform interprocedural propagation, inlining and
other optimizations in anticipation that the function or variable in
question may change. While this feature is useful, for example, to rewrite
memory allocation functions by a debugging implementation, it is expensive
in the terms of code quality. With -fno-semantic-interposition the
compiler assumes that if interposition happens for functions the
overwriting function will have precisely the same semantics (and side
effects). Similarly if interposition happens for variables, the
constructor of the variable will be the same. The flag has no effect for
functions explicitly declared inline (where it is never allowed for
interposition to change semantics) and for symbols explicitly declared
weak.
- -fshrink-wrap
- Emit function prologues only before parts of the function
that need it, rather than at the top of the function. This flag is enabled
by default at -O and higher.
- -fcaller-saves
- Enable allocation of values to registers that are clobbered
by function calls, by emitting extra instructions to save and restore the
registers around such calls. Such allocation is done only when it seems to
result in better code.
This option is always enabled by default on certain machines, usually those
which have no call-preserved registers to use instead.
Enabled at levels -O2, -O3, -Os.
- -fcombine-stack-adjustments
- Tracks stack adjustments (pushes and pops) and stack memory
references and then tries to find ways to combine them.
Enabled by default at -O1 and higher.
- -fipa-ra
- Use caller save registers for allocation if those registers
are not used by any called function. In that case it is not necessary to
save and restore them around calls. This is only possible if called
functions are part of same compilation unit as current function and they
are compiled before it.
Enabled at levels -O2, -O3, -Os.
- -fconserve-stack
- Attempt to minimize stack usage. The compiler attempts to
use less stack space, even if that makes the program slower. This option
implies setting the large-stack-frame parameter to 100 and the
large-stack-frame-growth parameter to 400.
- -ftree-reassoc
- Perform reassociation on trees. This flag is enabled by
default at -O and higher.
- -ftree-pre
- Perform partial redundancy elimination (PRE) on trees. This
flag is enabled by default at -O2 and -O3.
- -ftree-partial-pre
- Make partial redundancy elimination (PRE) more aggressive.
This flag is enabled by default at -O3.
- -ftree-forwprop
- Perform forward propagation on trees. This flag is enabled
by default at -O and higher.
- -ftree-fre
- Perform full redundancy elimination (FRE) on trees. The
difference between FRE and PRE is that FRE only considers expressions that
are computed on all paths leading to the redundant computation. This
analysis is faster than PRE, though it exposes fewer redundancies. This
flag is enabled by default at -O and higher.
- -ftree-phiprop
- Perform hoisting of loads from conditional pointers on
trees. This pass is enabled by default at -O and higher.
- -fhoist-adjacent-loads
- Speculatively hoist loads from both branches of an
if-then-else if the loads are from adjacent locations in the same
structure and the target architecture has a conditional move instruction.
This flag is enabled by default at -O2 and higher.
- -ftree-copy-prop
- Perform copy propagation on trees. This pass eliminates
unnecessary copy operations. This flag is enabled by default at -O
and higher.
- -fipa-pure-const
- Discover which functions are pure or constant. Enabled by
default at -O and higher.
- -fipa-reference
- Discover which static variables do not escape the
compilation unit. Enabled by default at -O and higher.
- -fipa-pta
- Perform interprocedural pointer analysis and
interprocedural modification and reference analysis. This option can cause
excessive memory and compile-time usage on large compilation units. It is
not enabled by default at any optimization level.
- -fipa-profile
- Perform interprocedural profile propagation. The functions
called only from cold functions are marked as cold. Also functions
executed once (such as "cold", "noreturn", static
constructors or destructors) are identified. Cold functions and loop less
parts of functions executed once are then optimized for size. Enabled by
default at -O and higher.
- -fipa-cp
- Perform interprocedural constant propagation. This
optimization analyzes the program to determine when values passed to
functions are constants and then optimizes accordingly. This optimization
can substantially increase performance if the application has constants
passed to functions. This flag is enabled by default at -O2,
-Os and -O3.
- -fipa-cp-clone
- Perform function cloning to make interprocedural constant
propagation stronger. When enabled, interprocedural constant propagation
performs function cloning when externally visible function can be called
with constant arguments. Because this optimization can create multiple
copies of functions, it may significantly increase code size (see
--param ipcp-unit-growth= value). This flag is enabled by
default at -O3.
- -fipa-cp-alignment
- When enabled, this optimization propagates alignment of
function parameters to support better vectorization and string operations.
This flag is enabled by default at -O2 and -Os. It requires
that -fipa-cp is enabled.
- -fipa-icf
- Perform Identical Code Folding for functions and read-only
variables. The optimization reduces code size and may disturb unwind
stacks by replacing a function by equivalent one with a different name.
The optimization works more effectively with link time optimization
enabled.
Nevertheless the behavior is similar to Gold Linker ICF optimization, GCC
ICF works on different levels and thus the optimizations are not same -
there are equivalences that are found only by GCC and equivalences found
only by Gold.
This flag is enabled by default at -O2 and -Os.
- -fisolate-erroneous-paths-dereference
- Detect paths that trigger erroneous or undefined behavior
due to dereferencing a null pointer. Isolate those paths from the main
control flow and turn the statement with erroneous or undefined behavior
into a trap. This flag is enabled by default at -O2 and
higher.
- -fisolate-erroneous-paths-attribute
- Detect paths that trigger erroneous or undefined behavior
due a null value being used in a way forbidden by a
"returns_nonnull" or "nonnull" attribute. Isolate
those paths from the main control flow and turn the statement with
erroneous or undefined behavior into a trap. This is not currently
enabled, but may be enabled by -O2 in the future.
- -ftree-sink
- Perform forward store motion on trees. This flag is enabled
by default at -O and higher.
- -ftree-bit-ccp
- Perform sparse conditional bit constant propagation on
trees and propagate pointer alignment information. This pass only operates
on local scalar variables and is enabled by default at -O and
higher. It requires that -ftree-ccp is enabled.
- -ftree-ccp
- Perform sparse conditional constant propagation (CCP) on
trees. This pass only operates on local scalar variables and is enabled by
default at -O and higher.
- -fssa-phiopt
- Perform pattern matching on SSA PHI nodes to optimize
conditional code. This pass is enabled by default at -O and
higher.
- -ftree-switch-conversion
- Perform conversion of simple initializations in a switch to
initializations from a scalar array. This flag is enabled by default at
-O2 and higher.
- -ftree-tail-merge
- Look for identical code sequences. When found, replace one
with a jump to the other. This optimization is known as tail merging or
cross jumping. This flag is enabled by default at -O2 and higher.
The compilation time in this pass can be limited using
max-tail-merge-comparisons parameter and
max-tail-merge-iterations parameter.
- -ftree-dce
- Perform dead code elimination (DCE) on trees. This flag is
enabled by default at -O and higher.
- -ftree-builtin-call-dce
- Perform conditional dead code elimination (DCE) for calls
to built-in functions that may set "errno" but are otherwise
side-effect free. This flag is enabled by default at -O2 and higher
if -Os is not also specified.
- -ftree-dominator-opts
- Perform a variety of simple scalar cleanups (constant/copy
propagation, redundancy elimination, range propagation and expression
simplification) based on a dominator tree traversal. This also performs
jump threading (to reduce jumps to jumps). This flag is enabled by default
at -O and higher.
- -ftree-dse
- Perform dead store elimination (DSE) on trees. A dead store
is a store into a memory location that is later overwritten by another
store without any intervening loads. In this case the earlier store can be
deleted. This flag is enabled by default at -O and higher.
- -ftree-ch
- Perform loop header copying on trees. This is beneficial
since it increases effectiveness of code motion optimizations. It also
saves one jump. This flag is enabled by default at -O and higher.
It is not enabled for -Os, since it usually increases code
size.
- -ftree-loop-optimize
- Perform loop optimizations on trees. This flag is enabled
by default at -O and higher.
- -ftree-loop-linear
- Perform loop interchange transformations on tree. Same as
-floop-interchange. To use this code transformation, GCC has to be
configured with --with-isl to enable the Graphite loop
transformation infrastructure.
- -floop-interchange
- Perform loop interchange transformations on loops.
Interchanging two nested loops switches the inner and outer loops. For
example, given a loop like:
DO J = 1, M
DO I = 1, N
A(J, I) = A(J, I) * C
ENDDO
ENDDO
loop interchange transforms the loop as if it were written:
DO I = 1, N
DO J = 1, M
A(J, I) = A(J, I) * C
ENDDO
ENDDO
which can be beneficial when "N" is larger than the caches,
because in Fortran, the elements of an array are stored in memory
contiguously by column, and the original loop iterates over rows,
potentially creating at each access a cache miss. This optimization
applies to all the languages supported by GCC and is not limited to
Fortran. To use this code transformation, GCC has to be configured with
--with-isl to enable the Graphite loop transformation
infrastructure.
- -floop-strip-mine
- Perform loop strip mining transformations on loops. Strip
mining splits a loop into two nested loops. The outer loop has strides
equal to the strip size and the inner loop has strides of the original
loop within a strip. The strip length can be changed using the
loop-block-tile-size parameter. For example, given a loop like:
DO I = 1, N
A(I) = A(I) + C
ENDDO
loop strip mining transforms the loop as if it were written:
DO II = 1, N, 51
DO I = II, min (II + 50, N)
A(I) = A(I) + C
ENDDO
ENDDO
This optimization applies to all the languages supported by GCC and is not
limited to Fortran. To use this code transformation, GCC has to be
configured with --with-isl to enable the Graphite loop
transformation infrastructure.
- -floop-block
- Perform loop blocking transformations on loops. Blocking
strip mines each loop in the loop nest such that the memory accesses of
the element loops fit inside caches. The strip length can be changed using
the loop-block-tile-size parameter. For example, given a loop like:
DO I = 1, N
DO J = 1, M
A(J, I) = B(I) + C(J)
ENDDO
ENDDO
loop blocking transforms the loop as if it were written:
DO II = 1, N, 51
DO JJ = 1, M, 51
DO I = II, min (II + 50, N)
DO J = JJ, min (JJ + 50, M)
A(J, I) = B(I) + C(J)
ENDDO
ENDDO
ENDDO
ENDDO
which can be beneficial when "M" is larger than the caches,
because the innermost loop iterates over a smaller amount of data which
can be kept in the caches. This optimization applies to all the languages
supported by GCC and is not limited to Fortran. To use this code
transformation, GCC has to be configured with --with-isl to enable
the Graphite loop transformation infrastructure.
- -fgraphite-identity
- Enable the identity transformation for graphite. For every
SCoP we generate the polyhedral representation and transform it back to
gimple. Using -fgraphite-identity we can check the costs or
benefits of the GIMPLE -> GRAPHITE -> GIMPLE transformation. Some
minimal optimizations are also performed by the code generator ISL, like
index splitting and dead code elimination in loops.
- -floop-nest-optimize
- Enable the ISL based loop nest optimizer. This is a generic
loop nest optimizer based on the Pluto optimization algorithms. It
calculates a loop structure optimized for data-locality and parallelism.
This option is experimental.
- -floop-unroll-and-jam
- Enable unroll and jam for the ISL based loop nest
optimizer. The unroll factor can be changed using the
loop-unroll-jam-size parameter. The unrolled dimension (counting
from the most inner one) can be changed using the
loop-unroll-jam-depth parameter. .
- -floop-parallelize-all
- Use the Graphite data dependence analysis to identify loops
that can be parallelized. Parallelize all the loops that can be analyzed
to not contain loop carried dependences without checking that it is
profitable to parallelize the loops.
- -fcheck-data-deps
- Compare the results of several data dependence analyzers.
This option is used for debugging the data dependence analyzers.
- -ftree-loop-if-convert
- Attempt to transform conditional jumps in the innermost
loops to branch-less equivalents. The intent is to remove control-flow
from the innermost loops in order to improve the ability of the
vectorization pass to handle these loops. This is enabled by default if
vectorization is enabled.
- -ftree-loop-if-convert-stores
- Attempt to also if-convert conditional jumps containing
memory writes. This transformation can be unsafe for multi-threaded
programs as it transforms conditional memory writes into unconditional
memory writes. For example,
for (i = 0; i < N; i++)
if (cond)
A[i] = expr;
is transformed to
for (i = 0; i < N; i++)
A[i] = cond ? expr : A[i];
potentially producing data races.
- -ftree-loop-distribution
- Perform loop distribution. This flag can improve cache
performance on big loop bodies and allow further loop optimizations, like
parallelization or vectorization, to take place. For example, the loop
DO I = 1, N
A(I) = B(I) + C
D(I) = E(I) * F
ENDDO
is transformed to
DO I = 1, N
A(I) = B(I) + C
ENDDO
DO I = 1, N
D(I) = E(I) * F
ENDDO
- -ftree-loop-distribute-patterns
- Perform loop distribution of patterns that can be code
generated with calls to a library. This flag is enabled by default at
-O3.
This pass distributes the initialization loops and generates a call to
memset zero. For example, the loop
DO I = 1, N
A(I) = 0
B(I) = A(I) + I
ENDDO
is transformed to
DO I = 1, N
A(I) = 0
ENDDO
DO I = 1, N
B(I) = A(I) + I
ENDDO
and the initialization loop is transformed into a call to memset zero.
- -ftree-loop-im
- Perform loop invariant motion on trees. This pass moves
only invariants that are hard to handle at RTL level (function calls,
operations that expand to nontrivial sequences of insns). With
-funswitch-loops it also moves operands of conditions that are
invariant out of the loop, so that we can use just trivial invariantness
analysis in loop unswitching. The pass also includes store motion.
- -ftree-loop-ivcanon
- Create a canonical counter for number of iterations in
loops for which determining number of iterations requires complicated
analysis. Later optimizations then may determine the number easily. Useful
especially in connection with unrolling.
- -fivopts
- Perform induction variable optimizations (strength
reduction, induction variable merging and induction variable elimination)
on trees.
- -ftree-parallelize-loops=n
- Parallelize loops, i.e., split their iteration space to run
in n threads. This is only possible for loops whose iterations are
independent and can be arbitrarily reordered. The optimization is only
profitable on multiprocessor machines, for loops that are CPU-intensive,
rather than constrained e.g. by memory bandwidth. This option implies
-pthread, and thus is only supported on targets that have support
for -pthread.
- -ftree-pta
- Perform function-local points-to analysis on trees. This
flag is enabled by default at -O and higher.
- -ftree-sra
- Perform scalar replacement of aggregates. This pass
replaces structure references with scalars to prevent committing
structures to memory too early. This flag is enabled by default at
-O and higher.
- -ftree-copyrename
- Perform copy renaming on trees. This pass attempts to
rename compiler temporaries to other variables at copy locations, usually
resulting in variable names which more closely resemble the original
variables. This flag is enabled by default at -O and higher.
- -ftree-coalesce-inlined-vars
- Tell the copyrename pass (see -ftree-copyrename) to
attempt to combine small user-defined variables too, but only if they are
inlined from other functions. It is a more limited form of
-ftree-coalesce-vars. This may harm debug information of such
inlined variables, but it keeps variables of the inlined-into function
apart from each other, such that they are more likely to contain the
expected values in a debugging session.
- -ftree-coalesce-vars
- Tell the copyrename pass (see -ftree-copyrename) to
attempt to combine small user-defined variables too, instead of just
compiler temporaries. This may severely limit the ability to debug an
optimized program compiled with -fno-var-tracking-assignments. In
the negated form, this flag prevents SSA coalescing of user variables,
including inlined ones. This option is enabled by default.
- -ftree-ter
- Perform temporary expression replacement during the
SSA->normal phase. Single use/single def temporaries are replaced at
their use location with their defining expression. This results in
non-GIMPLE code, but gives the expanders much more complex trees to work
on resulting in better RTL generation. This is enabled by default at
-O and higher.
- -ftree-slsr
- Perform straight-line strength reduction on trees. This
recognizes related expressions involving multiplications and replaces them
by less expensive calculations when possible. This is enabled by default
at -O and higher.
- -ftree-vectorize
- Perform vectorization on trees. This flag enables
-ftree-loop-vectorize and -ftree-slp-vectorize if not
explicitly specified.
- -ftree-loop-vectorize
- Perform loop vectorization on trees. This flag is enabled
by default at -O3 and when -ftree-vectorize is enabled.
- -ftree-slp-vectorize
- Perform basic block vectorization on trees. This flag is
enabled by default at -O3 and when -ftree-vectorize is
enabled.
- -fvect-cost-model=model
- Alter the cost model used for vectorization. The
model argument should be one of unlimited, dynamic or
cheap. With the unlimited model the vectorized code-path is
assumed to be profitable while with the dynamic model a runtime
check guards the vectorized code-path to enable it only for iteration
counts that will likely execute faster than when executing the original
scalar loop. The cheap model disables vectorization of loops where
doing so would be cost prohibitive for example due to required runtime
checks for data dependence or alignment but otherwise is equal to the
dynamic model. The default cost model depends on other optimization
flags and is either dynamic or cheap.
- -fsimd-cost-model=model
- Alter the cost model used for vectorization of loops marked
with the OpenMP or Cilk Plus simd directive. The model argument
should be one of unlimited, dynamic, cheap. All
values of model have the same meaning as described in
-fvect-cost-model and by default a cost model defined with
-fvect-cost-model is used.
- -ftree-vrp
- Perform Value Range Propagation on trees. This is similar
to the constant propagation pass, but instead of values, ranges of values
are propagated. This allows the optimizers to remove unnecessary range
checks like array bound checks and null pointer checks. This is enabled by
default at -O2 and higher. Null pointer check elimination is only
done if -fdelete-null-pointer-checks is enabled.
- -fsplit-ivs-in-unroller
- Enables expression of values of induction variables in
later iterations of the unrolled loop using the value in the first
iteration. This breaks long dependency chains, thus improving efficiency
of the scheduling passes.
A combination of -fweb and CSE is often sufficient to obtain the same
effect. However, that is not reliable in cases where the loop body is more
complicated than a single basic block. It also does not work at all on
some architectures due to restrictions in the CSE pass.
This optimization is enabled by default.
- -fvariable-expansion-in-unroller
- With this option, the compiler creates multiple copies of
some local variables when unrolling a loop, which can result in superior
code.
- -fpartial-inlining
- Inline parts of functions. This option has any effect only
when inlining itself is turned on by the -finline-functions or
-finline-small-functions options.
Enabled at level -O2.
- -fpredictive-commoning
- Perform predictive commoning optimization, i.e., reusing
computations (especially memory loads and stores) performed in previous
iterations of loops.
This option is enabled at level -O3.
- -fprefetch-loop-arrays
- If supported by the target machine, generate instructions
to prefetch memory to improve the performance of loops that access large
arrays.
This option may generate better or worse code; results are highly dependent
on the structure of loops within the source code.
Disabled at level -Os.
- -fno-peephole
- -fno-peephole2
- Disable any machine-specific peephole optimizations. The
difference between -fno-peephole and -fno-peephole2 is in
how they are implemented in the compiler; some targets use one, some use
the other, a few use both.
-fpeephole is enabled by default. -fpeephole2 enabled at
levels -O2, -O3, -Os.
- -fno-guess-branch-probability
- Do not guess branch probabilities using heuristics.
GCC uses heuristics to guess branch probabilities if they are not provided
by profiling feedback ( -fprofile-arcs). These heuristics are based
on the control flow graph. If some branch probabilities are specified by
"__builtin_expect", then the heuristics are used to guess branch
probabilities for the rest of the control flow graph, taking the
"__builtin_expect" info into account. The interactions between
the heuristics and "__builtin_expect" can be complex, and in
some cases, it may be useful to disable the heuristics so that the effects
of "__builtin_expect" are easier to understand.
The default is -fguess-branch-probability at levels -O,
-O2, -O3, -Os.
- -freorder-blocks
- Reorder basic blocks in the compiled function in order to
reduce number of taken branches and improve code locality.
Enabled at levels -O2, -O3.
- -freorder-blocks-and-partition
- In addition to reordering basic blocks in the compiled
function, in order to reduce number of taken branches, partitions hot and
cold basic blocks into separate sections of the assembly and .o files, to
improve paging and cache locality performance.
This optimization is automatically turned off in the presence of exception
handling, for linkonce sections, for functions with a user-defined section
attribute and on any architecture that does not support named sections.
Enabled for x86 at levels -O2, -O3.
- -freorder-functions
- Reorder functions in the object file in order to improve
code locality. This is implemented by using special subsections
".text.hot" for most frequently executed functions and
".text.unlikely" for unlikely executed functions. Reordering is
done by the linker so object file format must support named sections and
linker must place them in a reasonable way.
Also profile feedback must be available to make this option effective. See
-fprofile-arcs for details.
Enabled at levels -O2, -O3, -Os.
- -fstrict-aliasing
- Allow the compiler to assume the strictest aliasing rules
applicable to the language being compiled. For C (and C++), this activates
optimizations based on the type of expressions. In particular, an object
of one type is assumed never to reside at the same address as an object of
a different type, unless the types are almost the same. For example, an
"unsigned int" can alias an "int", but not a
"void*" or a "double". A character type may alias any
other type.
Pay special attention to code like this:
union a_union {
int i;
double d;
};
int f() {
union a_union t;
t.d = 3.0;
return t.i;
}
The practice of reading from a different union member than the one most
recently written to (called "type-punning") is common. Even with
-fstrict-aliasing, type-punning is allowed, provided the memory is
accessed through the union type. So, the code above works as expected.
However, this code might not:
int f() {
union a_union t;
int* ip;
t.d = 3.0;
ip = &t.i;
return *ip;
}
Similarly, access by taking the address, casting the resulting pointer and
dereferencing the result has undefined behavior, even if the cast uses a
union type, e.g.:
int f() {
double d = 3.0;
return ((union a_union *) &d)->i;
}
The -fstrict-aliasing option is enabled at levels -O2,
-O3, -Os.
- -fstrict-overflow
- Allow the compiler to assume strict signed overflow rules,
depending on the language being compiled. For C (and C++) this means that
overflow when doing arithmetic with signed numbers is undefined, which
means that the compiler may assume that it does not happen. This permits
various optimizations. For example, the compiler assumes that an
expression like "i + 10 > i" is always true for signed
"i". This assumption is only valid if signed overflow is
undefined, as the expression is false if "i + 10" overflows when
using twos complement arithmetic. When this option is in effect any
attempt to determine whether an operation on signed numbers overflows must
be written carefully to not actually involve overflow.
This option also allows the compiler to assume strict pointer semantics:
given a pointer to an object, if adding an offset to that pointer does not
produce a pointer to the same object, the addition is undefined. This
permits the compiler to conclude that "p + u > p" is always
true for a pointer "p" and unsigned integer "u". This
assumption is only valid because pointer wraparound is undefined, as the
expression is false if "p + u" overflows using twos complement
arithmetic.
See also the -fwrapv option. Using -fwrapv means that integer
signed overflow is fully defined: it wraps. When -fwrapv is used,
there is no difference between -fstrict-overflow and
-fno-strict-overflow for integers. With -fwrapv certain
types of overflow are permitted. For example, if the compiler gets an
overflow when doing arithmetic on constants, the overflowed value can
still be used with -fwrapv, but not otherwise.
The -fstrict-overflow option is enabled at levels -O2,
-O3, -Os.
- -falign-functions
- -falign-functions=n
- Align the start of functions to the next power-of-two
greater than n, skipping up to n bytes. For instance,
-falign-functions=32 aligns functions to the next 32-byte boundary,
but -falign-functions=24 aligns to the next 32-byte boundary only
if this can be done by skipping 23 bytes or less.
-fno-align-functions and -falign-functions=1 are equivalent
and mean that functions are not aligned.
Some assemblers only support this flag when n is a power of two; in
that case, it is rounded up.
If n is not specified or is zero, use a machine-dependent default.
Enabled at levels -O2, -O3.
- -falign-labels
- -falign-labels=n
- Align all branch targets to a power-of-two boundary,
skipping up to n bytes like -falign-functions. This option
can easily make code slower, because it must insert dummy operations for
when the branch target is reached in the usual flow of the code.
-fno-align-labels and -falign-labels=1 are equivalent and
mean that labels are not aligned.
If -falign-loops or -falign-jumps are applicable and are
greater than this value, then their values are used instead.
If n is not specified or is zero, use a machine-dependent default
which is very likely to be 1, meaning no alignment.
Enabled at levels -O2, -O3.
- -falign-loops
- -falign-loops=n
- Align loops to a power-of-two boundary, skipping up to
n bytes like -falign-functions. If the loops are executed
many times, this makes up for any execution of the dummy operations.
-fno-align-loops and -falign-loops=1 are equivalent and mean
that loops are not aligned.
If n is not specified or is zero, use a machine-dependent default.
Enabled at levels -O2, -O3.
- -falign-jumps
- -falign-jumps=n
- Align branch targets to a power-of-two boundary, for branch
targets where the targets can only be reached by jumping, skipping up to
n bytes like -falign-functions. In this case, no dummy
operations need be executed.
-fno-align-jumps and -falign-jumps=1 are equivalent and mean
that loops are not aligned.
If n is not specified or is zero, use a machine-dependent default.
Enabled at levels -O2, -O3.
- -funit-at-a-time
- This option is left for compatibility reasons.
-funit-at-a-time has no effect, while -fno-unit-at-a-time
implies -fno-toplevel-reorder and -fno-section-anchors.
Enabled by default.
- -fno-toplevel-reorder
- Do not reorder top-level functions, variables, and
"asm" statements. Output them in the same order that they appear
in the input file. When this option is used, unreferenced static variables
are not removed. This option is intended to support existing code that
relies on a particular ordering. For new code, it is better to use
attributes when possible.
Enabled at level -O0. When disabled explicitly, it also implies
-fno-section-anchors, which is otherwise enabled at -O0 on
some targets.
- -fweb
- Constructs webs as commonly used for register allocation
purposes and assign each web individual pseudo register. This allows the
register allocation pass to operate on pseudos directly, but also
strengthens several other optimization passes, such as CSE, loop optimizer
and trivial dead code remover. It can, however, make debugging impossible,
since variables no longer stay in a "home register".
Enabled by default with -funroll-loops.
- -fwhole-program
- Assume that the current compilation unit represents the
whole program being compiled. All public functions and variables with the
exception of "main" and those merged by attribute
"externally_visible" become static functions and in effect are
optimized more aggressively by interprocedural optimizers.
This option should not be used in combination with -flto. Instead
relying on a linker plugin should provide safer and more precise
information.
- -flto[=n]
- This option runs the standard link-time optimizer. When
invoked with source code, it generates GIMPLE (one of GCC's internal
representations) and writes it to special ELF sections in the object file.
When the object files are linked together, all the function bodies are
read from these ELF sections and instantiated as if they had been part of
the same translation unit.
To use the link-time optimizer, -flto and optimization options should
be specified at compile time and during the final link. For example:
gcc -c -O2 -flto foo.c
gcc -c -O2 -flto bar.c
gcc -o myprog -flto -O2 foo.o bar.o
The first two invocations to GCC save a bytecode representation of GIMPLE
into special ELF sections inside foo.o and bar.o. The final
invocation reads the GIMPLE bytecode from foo.o and bar.o,
merges the two files into a single internal image, and compiles the result
as usual. Since both foo.o and bar.o are merged into a
single image, this causes all the interprocedural analyses and
optimizations in GCC to work across the two files as if they were a single
one. This means, for example, that the inliner is able to inline functions
in bar.o into functions in foo.o and vice-versa.
Another (simpler) way to enable link-time optimization is:
gcc -o myprog -flto -O2 foo.c bar.c
The above generates bytecode for foo.c and bar.c, merges them
together into a single GIMPLE representation and optimizes them as usual
to produce myprog.
The only important thing to keep in mind is that to enable link-time
optimizations you need to use the GCC driver to perform the link-step. GCC
then automatically performs link-time optimization if any of the objects
involved were compiled with the -flto command-line option. You
generally should specify the optimization options to be used for link-time
optimization though GCC tries to be clever at guessing an optimization
level to use from the options used at compile-time if you fail to specify
one at link-time. You can always override the automatic decision to do
link-time optimization at link-time by passing -fno-lto to the link
command.
To make whole program optimization effective, it is necessary to make
certain whole program assumptions. The compiler needs to know what
functions and variables can be accessed by libraries and runtime outside
of the link-time optimized unit. When supported by the linker, the linker
plugin (see -fuse-linker-plugin) passes information to the compiler
about used and externally visible symbols. When the linker plugin is not
available, -fwhole-program should be used to allow the compiler to
make these assumptions, which leads to more aggressive optimization
decisions.
When -fuse-linker-plugin is not enabled then, when a file is compiled
with -flto, the generated object file is larger than a regular
object file because it contains GIMPLE bytecodes and the usual final code
(see -ffat-lto-objects. This means that object files with LTO
information can be linked as normal object files; if -fno-lto is
passed to the linker, no interprocedural optimizations are applied. Note
that when -fno-fat-lto-objects is enabled the compile-stage is
faster but you cannot perform a regular, non-LTO link on them.
Additionally, the optimization flags used to compile individual files are
not necessarily related to those used at link time. For instance,
gcc -c -O0 -ffat-lto-objects -flto foo.c
gcc -c -O0 -ffat-lto-objects -flto bar.c
gcc -o myprog -O3 foo.o bar.o
This produces individual object files with unoptimized assembler code, but
the resulting binary myprog is optimized at -O3. If,
instead, the final binary is generated with -fno-lto, then
myprog is not optimized.
When producing the final binary, GCC only applies link-time optimizations to
those files that contain bytecode. Therefore, you can mix and match object
files and libraries with GIMPLE bytecodes and final object code. GCC
automatically selects which files to optimize in LTO mode and which files
to link without further processing.
There are some code generation flags preserved by GCC when generating
bytecodes, as they need to be used during the final link stage. Generally
options specified at link-time override those specified at compile-time.
If you do not specify an optimization level option -O at link-time
then GCC computes one based on the optimization levels used when compiling
the object files. The highest optimization level wins here.
Currently, the following options and their setting are take from the first
object file that explicitely specified it: -fPIC, -fpic,
-fpie, -fcommon, -fexceptions,
-fnon-call-exceptions, -fgnu-tm and all the -m target
flags.
Certain ABI changing flags are required to match in all compilation-units
and trying to override this at link-time with a conflicting value is
ignored. This includes options such as -freg-struct-return and
-fpcc-struct-return.
Other options such as -ffp-contract, -fno-strict-overflow,
-fwrapv, -fno-trapv or -fno-strict-aliasing are
passed through to the link stage and merged conservatively for conflicting
translation units. Specifically -fno-strict-overflow,
-fwrapv and -fno-trapv take precedence and for example
-ffp-contract=off takes precedence over -ffp-contract=fast.
You can override them at linke-time.
It is recommended that you compile all the files participating in the same
link with the same options and also specify those options at link time.
If LTO encounters objects with C linkage declared with incompatible types in
separate translation units to be linked together (undefined behavior
according to ISO C99 6.2.7), a non-fatal diagnostic may be issued. The
behavior is still undefined at run time. Similar diagnostics may be raised
for other languages.
Another feature of LTO is that it is possible to apply interprocedural
optimizations on files written in different languages:
gcc -c -flto foo.c
g++ -c -flto bar.cc
gfortran -c -flto baz.f90
g++ -o myprog -flto -O3 foo.o bar.o baz.o -lgfortran
Notice that the final link is done with g++ to get the C++ runtime
libraries and -lgfortran is added to get the Fortran runtime
libraries. In general, when mixing languages in LTO mode, you should use
the same link command options as when mixing languages in a regular
(non-LTO) compilation.
If object files containing GIMPLE bytecode are stored in a library archive,
say libfoo.a, it is possible to extract and use them in an LTO link
if you are using a linker with plugin support. To create static libraries
suitable for LTO, use gcc-ar and gcc-ranlib instead of
ar and ranlib; to show the symbols of object files with
GIMPLE bytecode, use gcc-nm. Those commands require that ar,
ranlib and nm have been compiled with plugin support. At
link time, use the the flag -fuse-linker-plugin to ensure that the
library participates in the LTO optimization process:
gcc -o myprog -O2 -flto -fuse-linker-plugin a.o b.o -lfoo
With the linker plugin enabled, the linker extracts the needed GIMPLE files
from libfoo.a and passes them on to the running GCC to make them
part of the aggregated GIMPLE image to be optimized.
If you are not using a linker with plugin support and/or do not enable the
linker plugin, then the objects inside libfoo.a are extracted and
linked as usual, but they do not participate in the LTO optimization
process. In order to make a static library suitable for both LTO
optimization and usual linkage, compile its object files with -flto
-ffat-lto-objects.
Link-time optimizations do not require the presence of the whole program to
operate. If the program does not require any symbols to be exported, it is
possible to combine -flto and -fwhole-program to allow the
interprocedural optimizers to use more aggressive assumptions which may
lead to improved optimization opportunities. Use of -fwhole-program
is not needed when linker plugin is active (see
-fuse-linker-plugin).
The current implementation of LTO makes no attempt to generate bytecode that
is portable between different types of hosts. The bytecode files are
versioned and there is a strict version check, so bytecode files generated
in one version of GCC do not work with an older or newer version of GCC.
Link-time optimization does not work well with generation of debugging
information. Combining -flto with -g is currently
experimental and expected to produce unexpected results.
If you specify the optional n, the optimization and code generation
done at link time is executed in parallel using n parallel jobs by
utilizing an installed make program. The environment variable
MAKE may be used to override the program used. The default value
for n is 1.
You can also specify -flto=jobserver to use GNU make's job server
mode to determine the number of parallel jobs. This is useful when the
Makefile calling GCC is already executing in parallel. You must prepend a
+ to the command recipe in the parent Makefile for this to work.
This option likely only works if MAKE is GNU make.
- -flto-partition=alg
- Specify the partitioning algorithm used by the link-time
optimizer. The value is either 1to1 to specify a partitioning
mirroring the original source files or balanced to specify
partitioning into equally sized chunks (whenever possible) or max
to create new partition for every symbol where possible. Specifying
none as an algorithm disables partitioning and streaming
completely. The default value is balanced. While 1to1 can be
used as an workaround for various code ordering issues, the max
partitioning is intended for internal testing only. The value one
specifies that exactly one partition should be used while the value
none bypasses partitioning and executes the link-time optimization
step directly from the WPA phase.
- -flto-odr-type-merging
- Enable streaming of mangled types names of C++ types and
their unification at linktime. This increases size of LTO object files,
but enable diagnostics about One Definition Rule violations.
- -flto-compression-level=n
- This option specifies the level of compression used for
intermediate language written to LTO object files, and is only meaningful
in conjunction with LTO mode ( -flto). Valid values are 0 (no
compression) to 9 (maximum compression). Values outside this range are
clamped to either 0 or 9. If the option is not given, a default balanced
compression setting is used.
- -flto-report
- Prints a report with internal details on the workings of
the link-time optimizer. The contents of this report vary from version to
version. It is meant to be useful to GCC developers when processing object
files in LTO mode (via -flto).
Disabled by default.
- -flto-report-wpa
- Like -flto-report, but only print for the WPA phase
of Link Time Optimization.
- -fuse-linker-plugin
- Enables the use of a linker plugin during link-time
optimization. This option relies on plugin support in the linker, which is
available in gold or in GNU ld 2.21 or newer.
This option enables the extraction of object files with GIMPLE bytecode out
of library archives. This improves the quality of optimization by exposing
more code to the link-time optimizer. This information specifies what
symbols can be accessed externally (by non-LTO object or during dynamic
linking). Resulting code quality improvements on binaries (and shared
libraries that use hidden visibility) are similar to
-fwhole-program. See -flto for a description of the effect
of this flag and how to use it.
This option is enabled by default when LTO support in GCC is enabled and GCC
was configured for use with a linker supporting plugins (GNU ld 2.21 or
newer or gold).
- -ffat-lto-objects
- Fat LTO objects are object files that contain both the
intermediate language and the object code. This makes them usable for both
LTO linking and normal linking. This option is effective only when
compiling with -flto and is ignored at link time.
-fno-fat-lto-objects improves compilation time over plain LTO, but
requires the complete toolchain to be aware of LTO. It requires a linker
with linker plugin support for basic functionality. Additionally,
nm, ar and ranlib need to support linker plugins to
allow a full-featured build environment (capable of building static
libraries etc). GCC provides the gcc-ar, gcc-nm,
gcc-ranlib wrappers to pass the right options to these tools. With
non fat LTO makefiles need to be modified to use them.
The default is -fno-fat-lto-objects on targets with linker plugin
support.
- -fcompare-elim
- After register allocation and post-register allocation
instruction splitting, identify arithmetic instructions that compute
processor flags similar to a comparison operation based on that
arithmetic. If possible, eliminate the explicit comparison operation.
This pass only applies to certain targets that cannot explicitly represent
the comparison operation before register allocation is complete.
Enabled at levels -O, -O2, -O3, -Os.
- -fcprop-registers
- After register allocation and post-register allocation
instruction splitting, perform a copy-propagation pass to try to reduce
scheduling dependencies and occasionally eliminate the copy.
Enabled at levels -O, -O2, -O3, -Os.
- -fprofile-correction
- Profiles collected using an instrumented binary for
multi-threaded programs may be inconsistent due to missed counter updates.
When this option is specified, GCC uses heuristics to correct or smooth
out such inconsistencies. By default, GCC emits an error message when an
inconsistent profile is detected.
- -fprofile-dir=path
- Set the directory to search for the profile data files in
to path. This option affects only the profile data generated by
-fprofile-generate, -ftest-coverage, -fprofile-arcs
and used by -fprofile-use and -fbranch-probabilities and its
related options. Both absolute and relative paths can be used. By default,
GCC uses the current directory as path, thus the profile data file
appears in the same directory as the object file.
- -fprofile-generate
- -fprofile-generate=path
- Enable options usually used for instrumenting application
to produce profile useful for later recompilation with profile feedback
based optimization. You must use -fprofile-generate both when
compiling and when linking your program.
The following options are enabled: -fprofile-arcs,
-fprofile-values, -fvpt.
If path is specified, GCC looks at the path to find the
profile feedback data files. See -fprofile-dir.
- -fprofile-use
- -fprofile-use=path
- Enable profile feedback-directed optimizations, and the
following optimizations which are generally profitable only with profile
feedback available: -fbranch-probabilities, -fvpt,
-funroll-loops, -fpeel-loops, -ftracer,
-ftree-vectorize, and ftree-loop-distribute-patterns.
By default, GCC emits an error message if the feedback profiles do not match
the source code. This error can be turned into a warning by using
-Wcoverage-mismatch. Note this may result in poorly optimized code.
If path is specified, GCC looks at the path to find the
profile feedback data files. See -fprofile-dir.
- -fauto-profile
- -fauto-profile=path
- Enable sampling-based feedback-directed optimizations, and
the following optimizations which are generally profitable only with
profile feedback available: -fbranch-probabilities, -fvpt,
-funroll-loops, -fpeel-loops, -ftracer,
-ftree-vectorize, -finline-functions, -fipa-cp,
-fipa-cp-clone, -fpredictive-commoning,
-funswitch-loops, -fgcse-after-reload, and
-ftree-loop-distribute-patterns.
path is the name of a file containing AutoFDO profile information.
If omitted, it defaults to fbdata.afdo in the current directory.
Producing an AutoFDO profile data file requires running your program with
the perf utility on a supported GNU/Linux target system. For more
information, see < https://perf.wiki.kernel.org/>.
E.g.
perf record -e br_inst_retired:near_taken -b -o perf.data \
-- your_program
Then use the create_gcov tool to convert the raw profile data to a
format that can be used by GCC. You must also supply the unstripped binary
for your program to this tool. See <
https://github.com/google/autofdo>.
E.g.
create_gcov --binary=your_program.unstripped --profile=perf.data \
--gcov=profile.afdo
The following options control compiler behavior regarding floating-point
arithmetic. These options trade off between speed and correctness. All must be
specifically enabled.
- -ffloat-store
- Do not store floating-point variables in registers, and
inhibit other options that might change whether a floating-point value is
taken from a register or memory.
This option prevents undesirable excess precision on machines such as the
68000 where the floating registers (of the 68881) keep more precision than
a "double" is supposed to have. Similarly for the x86
architecture. For most programs, the excess precision does only good, but
a few programs rely on the precise definition of IEEE floating point. Use
-ffloat-store for such programs, after modifying them to store all
pertinent intermediate computations into variables.
- -fexcess-precision=style
- This option allows further control over excess precision on
machines where floating-point registers have more precision than the IEEE
"float" and "double" types and the processor does not
support operations rounding to those types. By default,
-fexcess-precision=fast is in effect; this means that operations
are carried out in the precision of the registers and that it is
unpredictable when rounding to the types specified in the source code
takes place. When compiling C, if -fexcess-precision=standard is
specified then excess precision follows the rules specified in ISO C99; in
particular, both casts and assignments cause values to be rounded to their
semantic types (whereas -ffloat-store only affects assignments).
This option is enabled by default for C if a strict conformance option
such as -std=c99 is used.
-fexcess-precision=standard is not implemented for languages other
than C, and has no effect if -funsafe-math-optimizations or
-ffast-math is specified. On the x86, it also has no effect if
-mfpmath=sse or -mfpmath=sse+387 is specified; in the former
case, IEEE semantics apply without excess precision, and in the latter,
rounding is unpredictable.
- -ffast-math
- Sets the options -fno-math-errno,
-funsafe-math-optimizations, -ffinite-math-only,
-fno-rounding-math, -fno-signaling-nans and
-fcx-limited-range.
This option causes the preprocessor macro "__FAST_MATH__" to be
defined.
This option is not turned on by any -O option besides -Ofast
since it can result in incorrect output for programs that depend on an
exact implementation of IEEE or ISO rules/specifications for math
functions. It may, however, yield faster code for programs that do not
require the guarantees of these specifications.
- -fno-math-errno
- Do not set "errno" after calling math functions
that are executed with a single instruction, e.g., "sqrt". A
program that relies on IEEE exceptions for math error handling may want to
use this flag for speed while maintaining IEEE arithmetic compatibility.
This option is not turned on by any -O option since it can result in
incorrect output for programs that depend on an exact implementation of
IEEE or ISO rules/specifications for math functions. It may, however,
yield faster code for programs that do not require the guarantees of these
specifications.
The default is -fmath-errno.
On Darwin systems, the math library never sets "errno". There is
therefore no reason for the compiler to consider the possibility that it
might, and -fno-math-errno is the default.
- -funsafe-math-optimizations
- Allow optimizations for floating-point arithmetic that (a)
assume that arguments and results are valid and (b) may violate IEEE or
ANSI standards. When used at link-time, it may include libraries or
startup files that change the default FPU control word or other similar
optimizations.
This option is not turned on by any -O option since it can result in
incorrect output for programs that depend on an exact implementation of
IEEE or ISO rules/specifications for math functions. It may, however,
yield faster code for programs that do not require the guarantees of these
specifications. Enables -fno-signed-zeros,
-fno-trapping-math, -fassociative-math and
-freciprocal-math.
The default is -fno-unsafe-math-optimizations.
- -fassociative-math
- Allow re-association of operands in series of
floating-point operations. This violates the ISO C and C++ language
standard by possibly changing computation result. NOTE: re-ordering may
change the sign of zero as well as ignore NaNs and inhibit or create
underflow or overflow (and thus cannot be used on code that relies on
rounding behavior like "(x + 2**52) - 2**52". May also reorder
floating-point comparisons and thus may not be used when ordered
comparisons are required. This option requires that both
-fno-signed-zeros and -fno-trapping-math be in effect.
Moreover, it doesn't make much sense with -frounding-math. For
Fortran the option is automatically enabled when both
-fno-signed-zeros and -fno-trapping-math are in effect.
The default is -fno-associative-math.
- -freciprocal-math
- Allow the reciprocal of a value to be used instead of
dividing by the value if this enables optimizations. For example "x /
y" can be replaced with "x * (1/y)", which is useful if
"(1/y)" is subject to common subexpression elimination. Note
that this loses precision and increases the number of flops operating on
the value.
The default is -fno-reciprocal-math.
- -ffinite-math-only
- Allow optimizations for floating-point arithmetic that
assume that arguments and results are not NaNs or +-Infs.
This option is not turned on by any -O option since it can result in
incorrect output for programs that depend on an exact implementation of
IEEE or ISO rules/specifications for math functions. It may, however,
yield faster code for programs that do not require the guarantees of these
specifications.
The default is -fno-finite-math-only.
- -fno-signed-zeros
- Allow optimizations for floating-point arithmetic that
ignore the signedness of zero. IEEE arithmetic specifies the behavior of
distinct +0.0 and -0.0 values, which then prohibits simplification of
expressions such as x+0.0 or 0.0*x (even with -ffinite-math-only).
This option implies that the sign of a zero result isn't significant.
The default is -fsigned-zeros.
- -fno-trapping-math
- Compile code assuming that floating-point operations cannot
generate user-visible traps. These traps include division by zero,
overflow, underflow, inexact result and invalid operation. This option
requires that -fno-signaling-nans be in effect. Setting this option
may allow faster code if one relies on "non-stop" IEEE
arithmetic, for example.
This option should never be turned on by any -O option since it can
result in incorrect output for programs that depend on an exact
implementation of IEEE or ISO rules/specifications for math functions.
The default is -ftrapping-math.
- -frounding-math
- Disable transformations and optimizations that assume
default floating-point rounding behavior. This is round-to-zero for all
floating point to integer conversions, and round-to-nearest for all other
arithmetic truncations. This option should be specified for programs that
change the FP rounding mode dynamically, or that may be executed with a
non-default rounding mode. This option disables constant folding of
floating-point expressions at compile time (which may be affected by
rounding mode) and arithmetic transformations that are unsafe in the
presence of sign-dependent rounding modes.
The default is -fno-rounding-math.
This option is experimental and does not currently guarantee to disable all
GCC optimizations that are affected by rounding mode. Future versions of
GCC may provide finer control of this setting using C99's
"FENV_ACCESS" pragma. This command-line option will be used to
specify the default state for "FENV_ACCESS".
- -fsignaling-nans
- Compile code assuming that IEEE signaling NaNs may generate
user-visible traps during floating-point operations. Setting this option
disables optimizations that may change the number of exceptions visible
with signaling NaNs. This option implies -ftrapping-math.
This option causes the preprocessor macro "__SUPPORT_SNAN__" to be
defined.
The default is -fno-signaling-nans.
This option is experimental and does not currently guarantee to disable all
GCC optimizations that affect signaling NaN behavior.
- -fsingle-precision-constant
- Treat floating-point constants as single precision instead
of implicitly converting them to double-precision constants.
- -fcx-limited-range
- When enabled, this option states that a range reduction
step is not needed when performing complex division. Also, there is no
checking whether the result of a complex multiplication or division is
"NaN + I*NaN", with an attempt to rescue the situation in that
case. The default is -fno-cx-limited-range, but is enabled by
-ffast-math.
This option controls the default setting of the ISO C99
"CX_LIMITED_RANGE" pragma. Nevertheless, the option applies to
all languages.
- -fcx-fortran-rules
- Complex multiplication and division follow Fortran rules.
Range reduction is done as part of complex division, but there is no
checking whether the result of a complex multiplication or division is
"NaN + I*NaN", with an attempt to rescue the situation in that
case.
The default is -fno-cx-fortran-rules.
The following options control optimizations that may improve performance, but
are not enabled by any
-O options. This section includes experimental
options that may produce broken code.
- -fbranch-probabilities
- After running a program compiled with
-fprofile-arcs, you can compile it a second time using
-fbranch-probabilities, to improve optimizations based on the
number of times each branch was taken. When a program compiled with
-fprofile-arcs exits, it saves arc execution counts to a file
called sourcename.gcda for each source file. The
information in this data file is very dependent on the structure of the
generated code, so you must use the same source code and the same
optimization options for both compilations.
With -fbranch-probabilities, GCC puts a REG_BR_PROB note on
each JUMP_INSN and CALL_INSN. These can be used to improve
optimization. Currently, they are only used in one place: in
reorg.c, instead of guessing which path a branch is most likely to
take, the REG_BR_PROB values are used to exactly determine which
path is taken more often.
- -fprofile-values
- If combined with -fprofile-arcs, it adds code so
that some data about values of expressions in the program is gathered.
With -fbranch-probabilities, it reads back the data gathered from
profiling values of expressions for usage in optimizations.
Enabled with -fprofile-generate and -fprofile-use.
- -fprofile-reorder-functions
- Function reordering based on profile instrumentation
collects first time of execution of a function and orders these functions
in ascending order.
Enabled with -fprofile-use.
- -fvpt
- If combined with -fprofile-arcs, this option
instructs the compiler to add code to gather information about values of
expressions.
With -fbranch-probabilities, it reads back the data gathered and
actually performs the optimizations based on them. Currently the
optimizations include specialization of division operations using the
knowledge about the value of the denominator.
- -frename-registers
- Attempt to avoid false dependencies in scheduled code by
making use of registers left over after register allocation. This
optimization most benefits processors with lots of registers. Depending on
the debug information format adopted by the target, however, it can make
debugging impossible, since variables no longer stay in a "home
register".
Enabled by default with -funroll-loops and -fpeel-loops.
- -fschedule-fusion
- Performs a target dependent pass over the instruction
stream to schedule instructions of same type together because target
machine can execute them more efficiently if they are adjacent to each
other in the instruction flow.
Enabled at levels -O2, -O3, -Os.
- -ftracer
- Perform tail duplication to enlarge superblock size. This
transformation simplifies the control flow of the function allowing other
optimizations to do a better job.
Enabled with -fprofile-use.
- -funroll-loops
- Unroll loops whose number of iterations can be determined
at compile time or upon entry to the loop. -funroll-loops implies
-frerun-cse-after-loop, -fweb and -frename-registers.
It also turns on complete loop peeling (i.e. complete removal of loops
with a small constant number of iterations). This option makes code
larger, and may or may not make it run faster.
Enabled with -fprofile-use.
- -funroll-all-loops
- Unroll all loops, even if their number of iterations is
uncertain when the loop is entered. This usually makes programs run more
slowly. -funroll-all-loops implies the same options as
-funroll-loops.
- -fpeel-loops
- Peels loops for which there is enough information that they
do not roll much (from profile feedback). It also turns on complete loop
peeling (i.e. complete removal of loops with small constant number of
iterations).
Enabled with -fprofile-use.
- -fmove-loop-invariants
- Enables the loop invariant motion pass in the RTL loop
optimizer. Enabled at level -O1
- -funswitch-loops
- Move branches with loop invariant conditions out of the
loop, with duplicates of the loop on both branches (modified according to
result of the condition).
- -ffunction-sections
- -fdata-sections
- Place each function or data item into its own section in
the output file if the target supports arbitrary sections. The name of the
function or the name of the data item determines the section's name in the
output file.
Use these options on systems where the linker can perform optimizations to
improve locality of reference in the instruction space. Most systems using
the ELF object format and SPARC processors running Solaris 2 have linkers
with such optimizations. AIX may have these optimizations in the future.
Only use these options when there are significant benefits from doing so.
When you specify these options, the assembler and linker create larger
object and executable files and are also slower. You cannot use
gprof on all systems if you specify this option, and you may have
problems with debugging if you specify both this option and
-g.
- -fbranch-target-load-optimize
- Perform branch target register load optimization before
prologue / epilogue threading. The use of target registers can typically
be exposed only during reload, thus hoisting loads out of loops and doing
inter-block scheduling needs a separate optimization pass.
- -fbranch-target-load-optimize2
- Perform branch target register load optimization after
prologue / epilogue threading.
- -fbtr-bb-exclusive
- When performing branch target register load optimization,
don't reuse branch target registers within any basic block.
- -fstack-protector
- Emit extra code to check for buffer overflows, such as
stack smashing attacks. This is done by adding a guard variable to
functions with vulnerable objects. This includes functions that call
"alloca", and functions with buffers larger than 8 bytes. The
guards are initialized when a function is entered and then checked when
the function exits. If a guard check fails, an error message is printed
and the program exits.
- -fstack-protector-all
- Like -fstack-protector except that all functions are
protected.
- -fstack-protector-strong
- Like -fstack-protector but includes additional
functions to be protected --- those that have local array definitions, or
have references to local frame addresses.
- -fstack-protector-explicit
- Like -fstack-protector but only protects those
functions which have the "stack_protect" attribute
- -fstdarg-opt
- Optimize the prologue of variadic argument functions with
respect to usage of those arguments.
- -fsection-anchors
- Try to reduce the number of symbolic address calculations
by using shared "anchor" symbols to address nearby objects. This
transformation can help to reduce the number of GOT entries and GOT
accesses on some targets.
For example, the implementation of the following function "foo":
static int a, b, c;
int foo (void) { return a + b + c; }
usually calculates the addresses of all three variables, but if you compile
it with -fsection-anchors, it accesses the variables from a common
anchor point instead. The effect is similar to the following pseudocode
(which isn't valid C):
int foo (void)
{
register int *xr = &x;
return xr[&a - &x] + xr[&b - &x] + xr[&c - &x];
}
Not all targets support this option.
- --param name=value
- In some places, GCC uses various constants to control the
amount of optimization that is done. For example, GCC does not inline
functions that contain more than a certain number of instructions. You can
control some of these constants on the command line using the
--param option.
The names of specific parameters, and the meaning of the values, are tied to
the internals of the compiler, and are subject to change without notice in
future releases.
In each case, the value is an integer. The allowable choices for
name are:
- predictable-branch-outcome
- When branch is predicted to be taken with probability lower
than this threshold (in percent), then it is considered well predictable.
The default is 10.
- max-crossjump-edges
- The maximum number of incoming edges to consider for
cross-jumping. The algorithm used by -fcrossjumping is O(N^2) in
the number of edges incoming to each block. Increasing values mean more
aggressive optimization, making the compilation time increase with
probably small improvement in executable size.
- min-crossjump-insns
- The minimum number of instructions that must be matched at
the end of two blocks before cross-jumping is performed on them. This
value is ignored in the case where all instructions in the block being
cross-jumped from are matched. The default value is 5.
- max-grow-copy-bb-insns
- The maximum code size expansion factor when copying basic
blocks instead of jumping. The expansion is relative to a jump
instruction. The default value is 8.
- max-goto-duplication-insns
- The maximum number of instructions to duplicate to a block
that jumps to a computed goto. To avoid O(N^2) behavior in a number of
passes, GCC factors computed gotos early in the compilation process, and
unfactors them as late as possible. Only computed jumps at the end of a
basic blocks with no more than max-goto-duplication-insns are unfactored.
The default value is 8.
- max-delay-slot-insn-search
- The maximum number of instructions to consider when looking
for an instruction to fill a delay slot. If more than this arbitrary
number of instructions are searched, the time savings from filling the
delay slot are minimal, so stop searching. Increasing values mean more
aggressive optimization, making the compilation time increase with
probably small improvement in execution time.
- max-delay-slot-live-search
- When trying to fill delay slots, the maximum number of
instructions to consider when searching for a block with valid live
register information. Increasing this arbitrarily chosen value means more
aggressive optimization, increasing the compilation time. This parameter
should be removed when the delay slot code is rewritten to maintain the
control-flow graph.
- max-gcse-memory
- The approximate maximum amount of memory that can be
allocated in order to perform the global common subexpression elimination
optimization. If more memory than specified is required, the optimization
is not done.
- max-gcse-insertion-ratio
- If the ratio of expression insertions to deletions is
larger than this value for any expression, then RTL PRE inserts or removes
the expression and thus leaves partially redundant computations in the
instruction stream. The default value is 20.
- max-pending-list-length
- The maximum number of pending dependencies scheduling
allows before flushing the current state and starting over. Large
functions with few branches or calls can create excessively large lists
which needlessly consume memory and resources.
- max-modulo-backtrack-attempts
- The maximum number of backtrack attempts the scheduler
should make when modulo scheduling a loop. Larger values can exponentially
increase compilation time.
- max-inline-insns-single
- Several parameters control the tree inliner used in GCC.
This number sets the maximum number of instructions (counted in GCC's
internal representation) in a single function that the tree inliner
considers for inlining. This only affects functions declared inline and
methods implemented in a class declaration (C++). The default value is
400.
- max-inline-insns-auto
- When you use -finline-functions (included in
-O3), a lot of functions that would otherwise not be considered for
inlining by the compiler are investigated. To those functions, a different
(more restrictive) limit compared to functions declared inline can be
applied. The default value is 40.
- inline-min-speedup
- When estimated performance improvement of caller + callee
runtime exceeds this threshold (in precent), the function can be inlined
regardless the limit on --param max-inline-insns-single and
--param max-inline-insns-auto.
- large-function-insns
- The limit specifying really large functions. For functions
larger than this limit after inlining, inlining is constrained by
--param large-function-growth. This parameter is useful primarily
to avoid extreme compilation time caused by non-linear algorithms used by
the back end. The default value is 2700.
- large-function-growth
- Specifies maximal growth of large function caused by
inlining in percents. The default value is 100 which limits large function
growth to 2.0 times the original size.
- large-unit-insns
- The limit specifying large translation unit. Growth caused
by inlining of units larger than this limit is limited by --param
inline-unit-growth. For small units this might be too tight. For
example, consider a unit consisting of function A that is inline and B
that just calls A three times. If B is small relative to A, the growth of
unit is 300\% and yet such inlining is very sane. For very large units
consisting of small inlineable functions, however, the overall unit growth
limit is needed to avoid exponential explosion of code size. Thus for
smaller units, the size is increased to --param large-unit-insns
before applying --param inline-unit-growth. The default is
10000.
- inline-unit-growth
- Specifies maximal overall growth of the compilation unit
caused by inlining. The default value is 20 which limits unit growth to
1.2 times the original size. Cold functions (either marked cold via an
attribute or by profile feedback) are not accounted into the unit
size.
- ipcp-unit-growth
- Specifies maximal overall growth of the compilation unit
caused by interprocedural constant propagation. The default value is 10
which limits unit growth to 1.1 times the original size.
- large-stack-frame
- The limit specifying large stack frames. While inlining the
algorithm is trying to not grow past this limit too much. The default
value is 256 bytes.
- large-stack-frame-growth
- Specifies maximal growth of large stack frames caused by
inlining in percents. The default value is 1000 which limits large stack
frame growth to 11 times the original size.
- max-inline-insns-recursive
- max-inline-insns-recursive-auto
- Specifies the maximum number of instructions an out-of-line
copy of a self-recursive inline function can grow into by performing
recursive inlining.
--param max-inline-insns-recursive applies to functions declared
inline. For functions not declared inline, recursive inlining happens only
when -finline-functions (included in -O3) is enabled;
--param max-inline-insns-recursive-auto applies instead. The
default value is 450.
- max-inline-recursive-depth
- max-inline-recursive-depth-auto
- Specifies the maximum recursion depth used for recursive
inlining.
--param max-inline-recursive-depth applies to functions declared
inline. For functions not declared inline, recursive inlining happens only
when -finline-functions (included in -O3) is enabled;
--param max-inline-recursive-depth-auto applies instead. The
default value is 8.
- min-inline-recursive-probability
- Recursive inlining is profitable only for function having
deep recursion in average and can hurt for function having little
recursion depth by increasing the prologue size or complexity of function
body to other optimizers.
When profile feedback is available (see -fprofile-generate) the
actual recursion depth can be guessed from probability that function
recurses via a given call expression. This parameter limits inlining only
to call expressions whose probability exceeds the given threshold (in
percents). The default value is 10.
- early-inlining-insns
- Specify growth that the early inliner can make. In effect
it increases the amount of inlining for code having a large abstraction
penalty. The default value is 14.
- max-early-inliner-iterations
- Limit of iterations of the early inliner. This basically
bounds the number of nested indirect calls the early inliner can resolve.
Deeper chains are still handled by late inlining.
- comdat-sharing-probability
- Probability (in percent) that C++ inline function with
comdat visibility are shared across multiple compilation units. The
default value is 20.
- profile-func-internal-id
- A parameter to control whether to use function internal id
in profile database lookup. If the value is 0, the compiler uses an id
that is based on function assembler name and filename, which makes old
profile data more tolerant to source changes such as function reordering
etc. The default value is 0.
- min-vect-loop-bound
- The minimum number of iterations under which loops are not
vectorized when -ftree-vectorize is used. The number of iterations
after vectorization needs to be greater than the value specified by this
option to allow vectorization. The default value is 0.
- gcse-cost-distance-ratio
- Scaling factor in calculation of maximum distance an
expression can be moved by GCSE optimizations. This is currently supported
only in the code hoisting pass. The bigger the ratio, the more aggressive
code hoisting is with simple expressions, i.e., the expressions that have
cost less than gcse-unrestricted-cost. Specifying 0 disables
hoisting of simple expressions. The default value is 10.
- gcse-unrestricted-cost
- Cost, roughly measured as the cost of a single typical
machine instruction, at which GCSE optimizations do not constrain the
distance an expression can travel. This is currently supported only in the
code hoisting pass. The lesser the cost, the more aggressive code hoisting
is. Specifying 0 allows all expressions to travel unrestricted distances.
The default value is 3.
- max-hoist-depth
- The depth of search in the dominator tree for expressions
to hoist. This is used to avoid quadratic behavior in hoisting algorithm.
The value of 0 does not limit on the search, but may slow down compilation
of huge functions. The default value is 30.
- max-tail-merge-comparisons
- The maximum amount of similar bbs to compare a bb with.
This is used to avoid quadratic behavior in tree tail merging. The default
value is 10.
- max-tail-merge-iterations
- The maximum amount of iterations of the pass over the
function. This is used to limit compilation time in tree tail merging. The
default value is 2.
- max-unrolled-insns
- The maximum number of instructions that a loop may have to
be unrolled. If a loop is unrolled, this parameter also determines how
many times the loop code is unrolled.
- max-average-unrolled-insns
- The maximum number of instructions biased by probabilities
of their execution that a loop may have to be unrolled. If a loop is
unrolled, this parameter also determines how many times the loop code is
unrolled.
- max-unroll-times
- The maximum number of unrollings of a single loop.
- max-peeled-insns
- The maximum number of instructions that a loop may have to
be peeled. If a loop is peeled, this parameter also determines how many
times the loop code is peeled.
- max-peel-times
- The maximum number of peelings of a single loop.
- max-peel-branches
- The maximum number of branches on the hot path through the
peeled sequence.
- max-completely-peeled-insns
- The maximum number of insns of a completely peeled
loop.
- max-completely-peel-times
- The maximum number of iterations of a loop to be suitable
for complete peeling.
- max-completely-peel-loop-nest-depth
- The maximum depth of a loop nest suitable for complete
peeling.
- max-unswitch-insns
- The maximum number of insns of an unswitched loop.
- max-unswitch-level
- The maximum number of branches unswitched in a single
loop.
- lim-expensive
- The minimum cost of an expensive expression in the loop
invariant motion.
- iv-consider-all-candidates-bound
- Bound on number of candidates for induction variables,
below which all candidates are considered for each use in induction
variable optimizations. If there are more candidates than this, only the
most relevant ones are considered to avoid quadratic time complexity.
- iv-max-considered-uses
- The induction variable optimizations give up on loops that
contain more induction variable uses.
- iv-always-prune-cand-set-bound
- If the number of candidates in the set is smaller than this
value, always try to remove unnecessary ivs from the set when adding a new
one.
- scev-max-expr-size
- Bound on size of expressions used in the scalar evolutions
analyzer. Large expressions slow the analyzer.
- scev-max-expr-complexity
- Bound on the complexity of the expressions in the scalar
evolutions analyzer. Complex expressions slow the analyzer.
- omega-max-vars
- The maximum number of variables in an Omega constraint
system. The default value is 128.
- omega-max-geqs
- The maximum number of inequalities in an Omega constraint
system. The default value is 256.
- omega-max-eqs
- The maximum number of equalities in an Omega constraint
system. The default value is 128.
- omega-max-wild-cards
- The maximum number of wildcard variables that the Omega
solver is able to insert. The default value is 18.
- omega-hash-table-size
- The size of the hash table in the Omega solver. The default
value is 550.
- omega-max-keys
- The maximal number of keys used by the Omega solver. The
default value is 500.
- omega-eliminate-redundant-constraints
- When set to 1, use expensive methods to eliminate all
redundant constraints. The default value is 0.
- vect-max-version-for-alignment-checks
- The maximum number of run-time checks that can be performed
when doing loop versioning for alignment in the vectorizer.
- vect-max-version-for-alias-checks
- The maximum number of run-time checks that can be performed
when doing loop versioning for alias in the vectorizer.
- vect-max-peeling-for-alignment
- The maximum number of loop peels to enhance access
alignment for vectorizer. Value -1 means 'no limit'.
- max-iterations-to-track
- The maximum number of iterations of a loop the brute-force
algorithm for analysis of the number of iterations of the loop tries to
evaluate.
- hot-bb-count-ws-permille
- A basic block profile count is considered hot if it
contributes to the given permillage (i.e. 0...1000) of the entire profiled
execution.
- hot-bb-frequency-fraction
- Select fraction of the entry block frequency of executions
of basic block in function given basic block needs to have to be
considered hot.
- max-predicted-iterations
- The maximum number of loop iterations we predict
statically. This is useful in cases where a function contains a single
loop with known bound and another loop with unknown bound. The known
number of iterations is predicted correctly, while the unknown number of
iterations average to roughly 10. This means that the loop without bounds
appears artificially cold relative to the other one.
- builtin-expect-probability
- Control the probability of the expression having the
specified value. This parameter takes a percentage (i.e. 0 ... 100) as
input. The default probability of 90 is obtained empirically.
- align-threshold
- Select fraction of the maximal frequency of executions of a
basic block in a function to align the basic block.
- align-loop-iterations
- A loop expected to iterate at least the selected number of
iterations is aligned.
- tracer-dynamic-coverage
- tracer-dynamic-coverage-feedback
- This value is used to limit superblock formation once the
given percentage of executed instructions is covered. This limits
unnecessary code size expansion.
The tracer-dynamic-coverage-feedback parameter is used only when
profile feedback is available. The real profiles (as opposed to statically
estimated ones) are much less balanced allowing the threshold to be larger
value.
- tracer-max-code-growth
- Stop tail duplication once code growth has reached given
percentage. This is a rather artificial limit, as most of the duplicates
are eliminated later in cross jumping, so it may be set to much higher
values than is the desired code growth.
- tracer-min-branch-ratio
- Stop reverse growth when the reverse probability of best
edge is less than this threshold (in percent).
- tracer-min-branch-ratio
- tracer-min-branch-ratio-feedback
- Stop forward growth if the best edge has probability lower
than this threshold.
Similarly to tracer-dynamic-coverage two values are present, one for
compilation for profile feedback and one for compilation without. The
value for compilation with profile feedback needs to be more conservative
(higher) in order to make tracer effective.
- max-cse-path-length
- The maximum number of basic blocks on path that CSE
considers. The default is 10.
- max-cse-insns
- The maximum number of instructions CSE processes before
flushing. The default is 1000.
- ggc-min-expand
- GCC uses a garbage collector to manage its own memory
allocation. This parameter specifies the minimum percentage by which the
garbage collector's heap should be allowed to expand between collections.
Tuning this may improve compilation speed; it has no effect on code
generation.
The default is 30% + 70% * (RAM/1GB) with an upper bound of 100% when RAM
>= 1GB. If "getrlimit" is available, the notion of
"RAM" is the smallest of actual RAM and "RLIMIT_DATA"
or "RLIMIT_AS". If GCC is not able to calculate RAM on a
particular platform, the lower bound of 30% is used. Setting this
parameter and ggc-min-heapsize to zero causes a full collection to
occur at every opportunity. This is extremely slow, but can be useful for
debugging.
- ggc-min-heapsize
- Minimum size of the garbage collector's heap before it
begins bothering to collect garbage. The first collection occurs after the
heap expands by ggc-min-expand% beyond ggc-min-heapsize.
Again, tuning this may improve compilation speed, and has no effect on
code generation.
The default is the smaller of RAM/8, RLIMIT_RSS, or a limit that tries to
ensure that RLIMIT_DATA or RLIMIT_AS are not exceeded, but with a lower
bound of 4096 (four megabytes) and an upper bound of 131072 (128
megabytes). If GCC is not able to calculate RAM on a particular platform,
the lower bound is used. Setting this parameter very large effectively
disables garbage collection. Setting this parameter and
ggc-min-expand to zero causes a full collection to occur at every
opportunity.
- max-reload-search-insns
- The maximum number of instruction reload should look
backward for equivalent register. Increasing values mean more aggressive
optimization, making the compilation time increase with probably slightly
better performance. The default value is 100.
- max-cselib-memory-locations
- The maximum number of memory locations cselib should take
into account. Increasing values mean more aggressive optimization, making
the compilation time increase with probably slightly better performance.
The default value is 500.
- reorder-blocks-duplicate
- reorder-blocks-duplicate-feedback
- Used by the basic block reordering pass to decide whether
to use unconditional branch or duplicate the code on its destination. Code
is duplicated when its estimated size is smaller than this value
multiplied by the estimated size of unconditional jump in the hot spots of
the program.
The reorder-block-duplicate-feedback parameter is used only when
profile feedback is available. It may be set to higher values than
reorder-block-duplicate since information about the hot spots is
more accurate.
- max-sched-ready-insns
- The maximum number of instructions ready to be issued the
scheduler should consider at any given time during the first scheduling
pass. Increasing values mean more thorough searches, making the
compilation time increase with probably little benefit. The default value
is 100.
- max-sched-region-blocks
- The maximum number of blocks in a region to be considered
for interblock scheduling. The default value is 10.
- max-pipeline-region-blocks
- The maximum number of blocks in a region to be considered
for pipelining in the selective scheduler. The default value is 15.
- max-sched-region-insns
- The maximum number of insns in a region to be considered
for interblock scheduling. The default value is 100.
- max-pipeline-region-insns
- The maximum number of insns in a region to be considered
for pipelining in the selective scheduler. The default value is 200.
- min-spec-prob
- The minimum probability (in percents) of reaching a source
block for interblock speculative scheduling. The default value is 40.
- max-sched-extend-regions-iters
- The maximum number of iterations through CFG to extend
regions. A value of 0 (the default) disables region extensions.
- max-sched-insn-conflict-delay
- The maximum conflict delay for an insn to be considered for
speculative motion. The default value is 3.
- sched-spec-prob-cutoff
- The minimal probability of speculation success (in
percents), so that speculative insns are scheduled. The default value is
40.
- sched-spec-state-edge-prob-cutoff
- The minimum probability an edge must have for the scheduler
to save its state across it. The default value is 10.
- sched-mem-true-dep-cost
- Minimal distance (in CPU cycles) between store and load
targeting same memory locations. The default value is 1.
- selsched-max-lookahead
- The maximum size of the lookahead window of selective
scheduling. It is a depth of search for available instructions. The
default value is 50.
- selsched-max-sched-times
- The maximum number of times that an instruction is
scheduled during selective scheduling. This is the limit on the number of
iterations through which the instruction may be pipelined. The default
value is 2.
- selsched-max-insns-to-rename
- The maximum number of best instructions in the ready list
that are considered for renaming in the selective scheduler. The default
value is 2.
- sms-min-sc
- The minimum value of stage count that swing modulo
scheduler generates. The default value is 2.
- max-last-value-rtl
- The maximum size measured as number of RTLs that can be
recorded in an expression in combiner for a pseudo register as last known
value of that register. The default is 10000.
- max-combine-insns
- The maximum number of instructions the RTL combiner tries
to combine. The default value is 2 at -Og and 4 otherwise.
- integer-share-limit
- Small integer constants can use a shared data structure,
reducing the compiler's memory usage and increasing its speed. This sets
the maximum value of a shared integer constant. The default value is
256.
- ssp-buffer-size
- The minimum size of buffers (i.e. arrays) that receive
stack smashing protection when -fstack-protection is used.
- min-size-for-stack-sharing
- The minimum size of variables taking part in stack slot
sharing when not optimizing. The default value is 32.
- max-jump-thread-duplication-stmts
- Maximum number of statements allowed in a block that needs
to be duplicated when threading jumps.
- max-fields-for-field-sensitive
- Maximum number of fields in a structure treated in a field
sensitive manner during pointer analysis. The default is zero for
-O0 and -O1, and 100 for -Os, -O2, and
-O3.
- prefetch-latency
- Estimate on average number of instructions that are
executed before prefetch finishes. The distance prefetched ahead is
proportional to this constant. Increasing this number may also lead to
less streams being prefetched (see simultaneous-prefetches).
- simultaneous-prefetches
- Maximum number of prefetches that can run at the same
time.
- l1-cache-line-size
- The size of cache line in L1 cache, in bytes.
- l1-cache-size
- The size of L1 cache, in kilobytes.
- l2-cache-size
- The size of L2 cache, in kilobytes.
- min-insn-to-prefetch-ratio
- The minimum ratio between the number of instructions and
the number of prefetches to enable prefetching in a loop.
- prefetch-min-insn-to-mem-ratio
- The minimum ratio between the number of instructions and
the number of memory references to enable prefetching in a loop.
- use-canonical-types
- Whether the compiler should use the "canonical"
type system. By default, this should always be 1, which uses a more
efficient internal mechanism for comparing types in C++ and Objective-C++.
However, if bugs in the canonical type system are causing compilation
failures, set this value to 0 to disable canonical types.
- switch-conversion-max-branch-ratio
- Switch initialization conversion refuses to create arrays
that are bigger than switch-conversion-max-branch-ratio times the
number of branches in the switch.
- max-partial-antic-length
- Maximum length of the partial antic set computed during the
tree partial redundancy elimination optimization ( -ftree-pre) when
optimizing at -O3 and above. For some sorts of source code the
enhanced partial redundancy elimination optimization can run away,
consuming all of the memory available on the host machine. This parameter
sets a limit on the length of the sets that are computed, which prevents
the runaway behavior. Setting a value of 0 for this parameter allows an
unlimited set length.
- sccvn-max-scc-size
- Maximum size of a strongly connected component (SCC) during
SCCVN processing. If this limit is hit, SCCVN processing for the whole
function is not done and optimizations depending on it are disabled. The
default maximum SCC size is 10000.
- sccvn-max-alias-queries-per-access
- Maximum number of alias-oracle queries we perform when
looking for redundancies for loads and stores. If this limit is hit the
search is aborted and the load or store is not considered redundant. The
number of queries is algorithmically limited to the number of stores on
all paths from the load to the function entry. The default maxmimum number
of queries is 1000.
- ira-max-loops-num
- IRA uses regional register allocation by default. If a
function contains more loops than the number given by this parameter, only
at most the given number of the most frequently-executed loops form
regions for regional register allocation. The default value of the
parameter is 100.
- ira-max-conflict-table-size
- Although IRA uses a sophisticated algorithm to compress the
conflict table, the table can still require excessive amounts of memory
for huge functions. If the conflict table for a function could be more
than the size in MB given by this parameter, the register allocator
instead uses a faster, simpler, and lower-quality algorithm that does not
require building a pseudo-register conflict table. The default value of
the parameter is 2000.
- ira-loop-reserved-regs
- IRA can be used to evaluate more accurate register pressure
in loops for decisions to move loop invariants (see -O3). The
number of available registers reserved for some other purposes is given by
this parameter. The default value of the parameter is 2, which is the
minimal number of registers needed by typical instructions. This value is
the best found from numerous experiments.
- lra-inheritance-ebb-probability-cutoff
- LRA tries to reuse values reloaded in registers in
subsequent insns. This optimization is called inheritance. EBB is used as
a region to do this optimization. The parameter defines a minimal
fall-through edge probability in percentage used to add BB to inheritance
EBB in LRA. The default value of the parameter is 40. The value was chosen
from numerous runs of SPEC2000 on x86-64.
- loop-invariant-max-bbs-in-loop
- Loop invariant motion can be very expensive, both in
compilation time and in amount of needed compile-time memory, with very
large loops. Loops with more basic blocks than this parameter won't have
loop invariant motion optimization performed on them. The default value of
the parameter is 1000 for -O1 and 10000 for -O2 and
above.
- loop-max-datarefs-for-datadeps
- Building data dapendencies is expensive for very large
loops. This parameter limits the number of data references in loops that
are considered for data dependence analysis. These large loops are no
handled by the optimizations using loop data dependencies. The default
value is 1000.
- max-vartrack-size
- Sets a maximum number of hash table slots to use during
variable tracking dataflow analysis of any function. If this limit is
exceeded with variable tracking at assignments enabled, analysis for that
function is retried without it, after removing all debug insns from the
function. If the limit is exceeded even without debug insns, var tracking
analysis is completely disabled for the function. Setting the parameter to
zero makes it unlimited.
- max-vartrack-expr-depth
- Sets a maximum number of recursion levels when attempting
to map variable names or debug temporaries to value expressions. This
trades compilation time for more complete debug information. If this is
set too low, value expressions that are available and could be represented
in debug information may end up not being used; setting this higher may
enable the compiler to find more complex debug expressions, but compile
time and memory use may grow. The default is 12.
- min-nondebug-insn-uid
- Use uids starting at this parameter for nondebug insns. The
range below the parameter is reserved exclusively for debug insns created
by -fvar-tracking-assignments, but debug insns may get
(non-overlapping) uids above it if the reserved range is exhausted.
- ipa-sra-ptr-growth-factor
- IPA-SRA replaces a pointer to an aggregate with one or more
new parameters only when their cumulative size is less or equal to
ipa-sra-ptr-growth-factor times the size of the original pointer
parameter.
- sra-max-scalarization-size-Ospeed
- sra-max-scalarization-size-Osize
- The two Scalar Reduction of Aggregates passes (SRA and
IPA-SRA) aim to replace scalar parts of aggregates with uses of
independent scalar variables. These parameters control the maximum size,
in storage units, of aggregate which is considered for replacement when
compiling for speed ( sra-max-scalarization-size-Ospeed) or size (
sra-max-scalarization-size-Osize) respectively.
- tm-max-aggregate-size
- When making copies of thread-local variables in a
transaction, this parameter specifies the size in bytes after which
variables are saved with the logging functions as opposed to save/restore
code sequence pairs. This option only applies when using
-fgnu-tm.
- graphite-max-nb-scop-params
- To avoid exponential effects in the Graphite loop
transforms, the number of parameters in a Static Control Part (SCoP) is
bounded. The default value is 10 parameters. A variable whose value is
unknown at compilation time and defined outside a SCoP is a parameter of
the SCoP.
- graphite-max-bbs-per-function
- To avoid exponential effects in the detection of SCoPs, the
size of the functions analyzed by Graphite is bounded. The default value
is 100 basic blocks.
- loop-block-tile-size
- Loop blocking or strip mining transforms, enabled with
-floop-block or -floop-strip-mine, strip mine each loop in
the loop nest by a given number of iterations. The strip length can be
changed using the loop-block-tile-size parameter. The default value
is 51 iterations.
- loop-unroll-jam-size
- Specify the unroll factor for the
-floop-unroll-and-jam option. The default value is 4.
- loop-unroll-jam-depth
- Specify the dimension to be unrolled (counting from the
most inner loop) for the -floop-unroll-and-jam. The default value
is 2.
- ipa-cp-value-list-size
- IPA-CP attempts to track all possible values and types
passed to a function's parameter in order to propagate them and perform
devirtualization. ipa-cp-value-list-size is the maximum number of
values and types it stores per one formal parameter of a function.
- ipa-cp-eval-threshold
- IPA-CP calculates its own score of cloning profitability
heuristics and performs those cloning opportunities with scores that
exceed ipa-cp-eval-threshold.
- ipa-cp-recursion-penalty
- Percentage penalty the recursive functions will receive
when they are evaluated for cloning.
- ipa-cp-single-call-penalty
- Percentage penalty functions containg a single call to
another function will receive when they are evaluated for cloning.
- ipa-max-agg-items
- IPA-CP is also capable to propagate a number of scalar
values passed in an aggregate. ipa-max-agg-items controls the
maximum number of such values per one parameter.
- ipa-cp-loop-hint-bonus
- When IPA-CP determines that a cloning candidate would make
the number of iterations of a loop known, it adds a bonus of
ipa-cp-loop-hint-bonus to the profitability score of the
candidate.
- ipa-cp-array-index-hint-bonus
- When IPA-CP determines that a cloning candidate would make
the index of an array access known, it adds a bonus of
ipa-cp-array-index-hint-bonus to the profitability score of the
candidate.
- ipa-max-aa-steps
- During its analysis of function bodies, IPA-CP employs
alias analysis in order to track values pointed to by function parameters.
In order not spend too much time analyzing huge functions, it gives up and
consider all memory clobbered after examining ipa-max-aa-steps
statements modifying memory.
- lto-partitions
- Specify desired number of partitions produced during WHOPR
compilation. The number of partitions should exceed the number of CPUs
used for compilation. The default value is 32.
- lto-minpartition
- Size of minimal partition for WHOPR (in estimated
instructions). This prevents expenses of splitting very small programs
into too many partitions.
- cxx-max-namespaces-for-diagnostic-help
- The maximum number of namespaces to consult for suggestions
when C++ name lookup fails for an identifier. The default is 1000.
- sink-frequency-threshold
- The maximum relative execution frequency (in percents) of
the target block relative to a statement's original block to allow
statement sinking of a statement. Larger numbers result in more aggressive
statement sinking. The default value is 75. A small positive adjustment is
applied for statements with memory operands as those are even more
profitable so sink.
- max-stores-to-sink
- The maximum number of conditional stores paires that can be
sunk. Set to 0 if either vectorization ( -ftree-vectorize) or
if-conversion ( -ftree-loop-if-convert) is disabled. The default is
2.
- allow-store-data-races
- Allow optimizers to introduce new data races on stores. Set
to 1 to allow, otherwise to 0. This option is enabled by default at
optimization level -Ofast.
- case-values-threshold
- The smallest number of different values for which it is
best to use a jump-table instead of a tree of conditional branches. If the
value is 0, use the default for the machine. The default is 0.
- tree-reassoc-width
- Set the maximum number of instructions executed in parallel
in reassociated tree. This parameter overrides target dependent heuristics
used by default if has non zero value.
- sched-pressure-algorithm
- Choose between the two available implementations of
-fsched-pressure. Algorithm 1 is the original implementation and is
the more likely to prevent instructions from being reordered. Algorithm 2
was designed to be a compromise between the relatively conservative
approach taken by algorithm 1 and the rather aggressive approach taken by
the default scheduler. It relies more heavily on having a regular register
file and accurate register pressure classes. See haifa-sched.c in
the GCC sources for more details.
The default choice depends on the target.
- max-slsr-cand-scan
- Set the maximum number of existing candidates that are
considered when seeking a basis for a new straight-line strength reduction
candidate.
- asan-globals
- Enable buffer overflow detection for global objects. This
kind of protection is enabled by default if you are using
-fsanitize=address option. To disable global objects protection use
--param asan-globals=0.
- asan-stack
- Enable buffer overflow detection for stack objects. This
kind of protection is enabled by default when using
-fsanitize=address. To disable stack protection use --param
asan-stack=0 option.
- asan-instrument-reads
- Enable buffer overflow detection for memory reads. This
kind of protection is enabled by default when using
-fsanitize=address. To disable memory reads protection use
--param asan-instrument-reads=0.
- asan-instrument-writes
- Enable buffer overflow detection for memory writes. This
kind of protection is enabled by default when using
-fsanitize=address. To disable memory writes protection use
--param asan-instrument-writes=0 option.
- asan-memintrin
- Enable detection for built-in functions. This kind of
protection is enabled by default when using -fsanitize=address. To
disable built-in functions protection use --param
asan-memintrin=0.
- asan-use-after-return
- Enable detection of use-after-return. This kind of
protection is enabled by default when using -fsanitize=address
option. To disable use-after-return detection use --param
asan-use-after-return=0.
- asan-instrumentation-with-call-threshold
- If number of memory accesses in function being instrumented
is greater or equal to this number, use callbacks instead of inline
checks. E.g. to disable inline code use --param
asan-instrumentation-with-call-threshold=0.
- chkp-max-ctor-size
- Static constructors generated by Pointer Bounds Checker may
become very large and significantly increase compile time at optimization
level -O1 and higher. This parameter is a maximum nubmer of
statements in a single generated constructor. Default value is 5000.
- max-fsm-thread-path-insns
- Maximum number of instructions to copy when duplicating
blocks on a finite state automaton jump thread path. The default is
100.
- max-fsm-thread-length
- Maximum number of basic blocks on a finite state automaton
jump thread path. The default is 10.
- max-fsm-thread-paths
- Maximum number of new jump thread paths to create for a
finite state automaton. The default is 50.
Options Controlling the Preprocessor
These options control the C preprocessor, which is run on each C source file
before actual compilation.
If you use the
-E option, nothing is done except preprocessing. Some of
these options make sense only together with
-E because they cause the
preprocessor output to be unsuitable for actual compilation.
- -Wp,option
- You can use -Wp,option to bypass the compiler
driver and pass option directly through to the preprocessor. If
option contains commas, it is split into multiple options at the
commas. However, many options are modified, translated or interpreted by
the compiler driver before being passed to the preprocessor, and
-Wp forcibly bypasses this phase. The preprocessor's direct
interface is undocumented and subject to change, so whenever possible you
should avoid using -Wp and let the driver handle the options
instead.
- -Xpreprocessor option
- Pass option as an option to the preprocessor. You
can use this to supply system-specific preprocessor options that GCC does
not recognize.
If you want to pass an option that takes an argument, you must use
-Xpreprocessor twice, once for the option and once for the
argument.
- -no-integrated-cpp
- Perform preprocessing as a separate pass before
compilation. By default, GCC performs preprocessing as an integrated part
of input tokenization and parsing. If this option is provided, the
appropriate language front end ( cc1, cc1plus, or
cc1obj for C, C++, and Objective-C, respectively) is instead
invoked twice, once for preprocessing only and once for actual compilation
of the preprocessed input. This option may be useful in conjunction with
the -B or -wrapper options to specify an alternate
preprocessor or perform additional processing of the program source
between normal preprocessing and compilation.
- -D name
- Predefine name as a macro, with definition 1.
- -D name=definition
- The contents of definition are tokenized and
processed as if they appeared during translation phase three in a
#define directive. In particular, the definition will be truncated
by embedded newline characters.
If you are invoking the preprocessor from a shell or shell-like program you
may need to use the shell's quoting syntax to protect characters such as
spaces that have a meaning in the shell syntax.
If you wish to define a function-like macro on the command line, write its
argument list with surrounding parentheses before the equals sign (if
any). Parentheses are meaningful to most shells, so you will need to quote
the option. With sh and csh,
-D'name(args...)=definition'
works.
-D and -U options are processed in the order they are given
on the command line. All -imacros file and -include
file options are processed after all -D and -U
options.
- -U name
- Cancel any previous definition of name, either built
in or provided with a -D option.
- -undef
- Do not predefine any system-specific or GCC-specific
macros. The standard predefined macros remain defined.
- -I dir
- Add the directory dir to the list of directories to
be searched for header files. Directories named by -I are searched
before the standard system include directories. If the directory
dir is a standard system include directory, the option is ignored
to ensure that the default search order for system directories and the
special treatment of system headers are not defeated . If dir
begins with "=", then the "=" will be replaced by the
sysroot prefix; see --sysroot and -isysroot.
- -o file
- Write output to file. This is the same as specifying
file as the second non-option argument to cpp. gcc
has a different interpretation of a second non-option argument, so you
must use -o to specify the output file.
- -Wall
- Turns on all optional warnings which are desirable for
normal code. At present this is -Wcomment, -Wtrigraphs,
-Wmultichar and a warning about integer promotion causing a change
of sign in "#if" expressions. Note that many of the
preprocessor's warnings are on by default and have no options to control
them.
- -Wcomment
- -Wcomments
- Warn whenever a comment-start sequence /* appears in
a /* comment, or whenever a backslash-newline appears in a
// comment. (Both forms have the same effect.)
- -Wtrigraphs
- Most trigraphs in comments cannot affect the meaning of the
program. However, a trigraph that would form an escaped newline (
??/ at the end of a line) can, by changing where the comment begins
or ends. Therefore, only trigraphs that would form escaped newlines
produce warnings inside a comment.
This option is implied by -Wall. If -Wall is not given, this
option is still enabled unless trigraphs are enabled. To get trigraph
conversion without warnings, but get the other -Wall warnings, use
-trigraphs -Wall -Wno-trigraphs.
- -Wtraditional
- Warn about certain constructs that behave differently in
traditional and ISO C. Also warn about ISO C constructs that have no
traditional C equivalent, and problematic constructs which should be
avoided.
- -Wundef
- Warn whenever an identifier which is not a macro is
encountered in an #if directive, outside of defined. Such
identifiers are replaced with zero.
- -Wunused-macros
- Warn about macros defined in the main file that are unused.
A macro is used if it is expanded or tested for existence at least
once. The preprocessor will also warn if the macro has not been used at
the time it is redefined or undefined.
Built-in macros, macros defined on the command line, and macros defined in
include files are not warned about.
Note: If a macro is actually used, but only used in skipped
conditional blocks, then CPP will report it as unused. To avoid the
warning in such a case, you might improve the scope of the macro's
definition by, for example, moving it into the first skipped block.
Alternatively, you could provide a dummy use with something like:
#if defined the_macro_causing_the_warning
#endif
- -Wendif-labels
- Warn whenever an #else or an #endif are
followed by text. This usually happens in code of the form
#if FOO
...
#else FOO
...
#endif FOO
The second and third "FOO" should be in comments, but often are
not in older programs. This warning is on by default.
- -Werror
- Make all warnings into hard errors. Source code which
triggers warnings will be rejected.
- -Wsystem-headers
- Issue warnings for code in system headers. These are
normally unhelpful in finding bugs in your own code, therefore suppressed.
If you are responsible for the system library, you may want to see
them.
- -w
- Suppress all warnings, including those which GNU CPP issues
by default.
- -pedantic
- Issue all the mandatory diagnostics listed in the C
standard. Some of them are left out by default, since they trigger
frequently on harmless code.
- -pedantic-errors
- Issue all the mandatory diagnostics, and make all mandatory
diagnostics into errors. This includes mandatory diagnostics that GCC
issues without -pedantic but treats as warnings.
- -M
- Instead of outputting the result of preprocessing, output a
rule suitable for make describing the dependencies of the main
source file. The preprocessor outputs one make rule containing the
object file name for that source file, a colon, and the names of all the
included files, including those coming from -include or
-imacros command-line options.
Unless specified explicitly (with -MT or -MQ), the object file
name consists of the name of the source file with any suffix replaced with
object file suffix and with any leading directory parts removed. If there
are many included files then the rule is split into several lines using
\-newline. The rule has no commands.
This option does not suppress the preprocessor's debug output, such as
-dM. To avoid mixing such debug output with the dependency rules
you should explicitly specify the dependency output file with -MF,
or use an environment variable like DEPENDENCIES_OUTPUT. Debug
output will still be sent to the regular output stream as normal.
Passing -M to the driver implies -E, and suppresses warnings
with an implicit -w.
- -MM
- Like -M but do not mention header files that are
found in system header directories, nor header files that are included,
directly or indirectly, from such a header.
This implies that the choice of angle brackets or double quotes in an
#include directive does not in itself determine whether that header
will appear in -MM dependency output. This is a slight change in
semantics from GCC versions 3.0 and earlier.
- -MF file
- When used with -M or -MM, specifies a file to
write the dependencies to. If no -MF switch is given the
preprocessor sends the rules to the same place it would have sent
preprocessed output.
When used with the driver options -MD or -MMD, -MF
overrides the default dependency output file.
- -MG
- In conjunction with an option such as -M requesting
dependency generation, -MG assumes missing header files are
generated files and adds them to the dependency list without raising an
error. The dependency filename is taken directly from the
"#include" directive without prepending any path. -MG
also suppresses preprocessed output, as a missing header file renders this
useless.
This feature is used in automatic updating of makefiles.
- -MP
- This option instructs CPP to add a phony target for each
dependency other than the main file, causing each to depend on nothing.
These dummy rules work around errors make gives if you remove
header files without updating the Makefile to match.
This is typical output:
test.o: test.c test.h
test.h:
- -MT target
- Change the target of the rule emitted by dependency
generation. By default CPP takes the name of the main input file, deletes
any directory components and any file suffix such as .c, and
appends the platform's usual object suffix. The result is the target.
An -MT option will set the target to be exactly the string you
specify. If you want multiple targets, you can specify them as a single
argument to -MT, or use multiple -MT options.
For example, -MT '$(objpfx)foo.o' might give
$(objpfx)foo.o: foo.c
- -MQ target
- Same as -MT, but it quotes any characters which are
special to Make. -MQ '$(objpfx)foo.o' gives
$$(objpfx)foo.o: foo.c
The default target is automatically quoted, as if it were given with
-MQ.
- -MD
- -MD is equivalent to -M -MF file,
except that -E is not implied. The driver determines file
based on whether an -o option is given. If it is, the driver uses
its argument but with a suffix of .d, otherwise it takes the name
of the input file, removes any directory components and suffix, and
applies a .d suffix.
If -MD is used in conjunction with -E, any -o switch is
understood to specify the dependency output file, but if used without
-E, each -o is understood to specify a target object file.
Since -E is not implied, -MD can be used to generate a
dependency output file as a side-effect of the compilation process.
- -MMD
- Like -MD except mention only user header files, not
system header files.
- -fpch-deps
- When using precompiled headers, this flag will cause the
dependency-output flags to also list the files from the precompiled
header's dependencies. If not specified only the precompiled header would
be listed and not the files that were used to create it because those
files are not consulted when a precompiled header is used.
- -fpch-preprocess
- This option allows use of a precompiled header together
with -E. It inserts a special "#pragma", "#pragma
GCC pch_preprocess " filename"" in the output to
mark the place where the precompiled header was found, and its
filename. When -fpreprocessed is in use, GCC recognizes this
"#pragma" and loads the PCH.
This option is off by default, because the resulting preprocessed output is
only really suitable as input to GCC. It is switched on by
-save-temps.
You should not write this "#pragma" in your own code, but it is
safe to edit the filename if the PCH file is available in a different
location. The filename may be absolute or it may be relative to GCC's
current directory.
- -x c
- -x c++
- -x objective-c
- -x assembler-with-cpp
- Specify the source language: C, C++, Objective-C, or
assembly. This has nothing to do with standards conformance or extensions;
it merely selects which base syntax to expect. If you give none of these
options, cpp will deduce the language from the extension of the source
file: .c, .cc, .m, or .S. Some other common
extensions for C++ and assembly are also recognized. If cpp does not
recognize the extension, it will treat the file as C; this is the most
generic mode.
Note: Previous versions of cpp accepted a -lang option which
selected both the language and the standards conformance level. This
option has been removed, because it conflicts with the -l
option.
- -std=standard
- -ansi
- Specify the standard to which the code should conform.
Currently CPP knows about C and C++ standards; others may be added in the
future.
standard may be one of:
- "c90"
- "c89"
- "iso9899:1990"
- The ISO C standard from 1990. c90 is the customary
shorthand for this version of the standard.
The -ansi option is equivalent to -std=c90.
- "iso9899:199409"
- The 1990 C standard, as amended in 1994.
- "iso9899:1999"
- "c99"
- "iso9899:199x"
- "c9x"
- The revised ISO C standard, published in December 1999.
Before publication, this was known as C9X.
- "iso9899:2011"
- "c11"
- "c1x"
- The revised ISO C standard, published in December 2011.
Before publication, this was known as C1X.
- "gnu90"
- "gnu89"
- The 1990 C standard plus GNU extensions. This is the
default.
- "gnu99"
- "gnu9x"
- The 1999 C standard plus GNU extensions.
- "gnu11"
- "gnu1x"
- The 2011 C standard plus GNU extensions.
- "c++98"
- The 1998 ISO C++ standard plus amendments.
- "gnu++98"
- The same as -std=c++98 plus GNU extensions. This is
the default for C++ code.
- -I-
- Split the include path. Any directories specified with
-I options before -I- are searched only for headers
requested with "#include " file""; they
are not searched for "#include < file>". If
additional directories are specified with -I options after the
-I-, those directories are searched for all #include
directives.
In addition, -I- inhibits the use of the directory of the current
file directory as the first search directory for
"#include " file"". This option has been
deprecated.
- -nostdinc
- Do not search the standard system directories for header
files. Only the directories you have specified with -I options (and
the directory of the current file, if appropriate) are searched.
- -nostdinc++
- Do not search for header files in the C++-specific standard
directories, but do still search the other standard directories. (This
option is used when building the C++ library.)
- -include file
- Process file as if "#include
"file"" appeared as the first line of the primary source
file. However, the first directory searched for file is the
preprocessor's working directory instead of the directory
containing the main source file. If not found there, it is searched for in
the remainder of the "#include "..."" search chain as
normal.
If multiple -include options are given, the files are included in the
order they appear on the command line.
- -imacros file
- Exactly like -include, except that any output
produced by scanning file is thrown away. Macros it defines remain
defined. This allows you to acquire all the macros from a header without
also processing its declarations.
All files specified by -imacros are processed before all files
specified by -include.
- -idirafter dir
- Search dir for header files, but do it after
all directories specified with -I and the standard system
directories have been exhausted. dir is treated as a system include
directory. If dir begins with "=", then the "="
will be replaced by the sysroot prefix; see --sysroot and
-isysroot.
- -iprefix prefix
- Specify prefix as the prefix for subsequent
-iwithprefix options. If the prefix represents a directory, you
should include the final /.
- -iwithprefix dir
- -iwithprefixbefore dir
- Append dir to the prefix specified previously with
-iprefix, and add the resulting directory to the include search
path. -iwithprefixbefore puts it in the same place -I would;
-iwithprefix puts it where -idirafter would.
- -isysroot dir
- This option is like the --sysroot option, but
applies only to header files (except for Darwin targets, where it applies
to both header files and libraries). See the --sysroot option for
more information.
- -imultilib dir
- Use dir as a subdirectory of the directory
containing target-specific C++ headers.
- -isystem dir
- Search dir for header files, after all directories
specified by -I but before the standard system directories. Mark it
as a system directory, so that it gets the same special treatment as is
applied to the standard system directories. If dir begins with
"=", then the "=" will be replaced by the sysroot
prefix; see --sysroot and -isysroot.
- -cxx-isystem dir
- Search dir for C++ header files, after all
directories specified by -I but before the standard system
directories. Mark it as a system directory, so that it gets the same
special treatment as is applied to the standard system directories.
- -iquote dir
- Search dir only for header files requested with
"#include " file""; they are not searched
for "#include < file>", before all directories
specified by -I and before the standard system directories. If
dir begins with "=", then the "=" will be
replaced by the sysroot prefix; see --sysroot and
-isysroot.
- -fdirectives-only
- When preprocessing, handle directives, but do not expand
macros.
The option's behavior depends on the -E and -fpreprocessed
options.
With -E, preprocessing is limited to the handling of directives such
as "#define", "#ifdef", and "#error". Other
preprocessor operations, such as macro expansion and trigraph conversion
are not performed. In addition, the -dD option is implicitly
enabled.
With -fpreprocessed, predefinition of command line and most builtin
macros is disabled. Macros such as "__LINE__", which are
contextually dependent, are handled normally. This enables compilation of
files previously preprocessed with "-E -fdirectives-only".
With both -E and -fpreprocessed, the rules for
-fpreprocessed take precedence. This enables full preprocessing of
files previously preprocessed with "-E -fdirectives-only".
- -iremap src:dst
- Replace the prefix src in __FILE__ with dst
at expansion time. This option can be specified more than once. Processing
stops at the first match.
- -fdollars-in-identifiers
- Accept $ in identifiers.
- -fextended-identifiers
- Accept universal character names in identifiers. This
option is enabled by default for C99 (and later C standard versions) and
C++.
- -fno-canonical-system-headers
- When preprocessing, do not shorten system header paths with
canonicalization.
- -fpreprocessed
- Indicate to the preprocessor that the input file has
already been preprocessed. This suppresses things like macro expansion,
trigraph conversion, escaped newline splicing, and processing of most
directives. The preprocessor still recognizes and removes comments, so
that you can pass a file preprocessed with -C to the compiler
without problems. In this mode the integrated preprocessor is little more
than a tokenizer for the front ends.
-fpreprocessed is implicit if the input file has one of the
extensions .i, .ii or .mi. These are the extensions
that GCC uses for preprocessed files created by -save-temps.
- -ftabstop=width
- Set the distance between tab stops. This helps the
preprocessor report correct column numbers in warnings or errors, even if
tabs appear on the line. If the value is less than 1 or greater than 100,
the option is ignored. The default is 8.
- -fdebug-cpp
- This option is only useful for debugging GCC. When used
with -E, dumps debugging information about location maps. Every
token in the output is preceded by the dump of the map its location
belongs to. The dump of the map holding the location of a token would be:
{"P":F</file/path>;"F":F</includer/path>;"L":<line_num>;"C":<col_num>;"S":<system_header_p>;"M":<map_address>;"E":<macro_expansion_p>,"loc":<location>}
When used without -E, this option has no effect.
- -ftrack-macro-expansion[=level]
- Track locations of tokens across macro expansions. This
allows the compiler to emit diagnostic about the current macro expansion
stack when a compilation error occurs in a macro expansion. Using this
option makes the preprocessor and the compiler consume more memory. The
level parameter can be used to choose the level of precision of
token location tracking thus decreasing the memory consumption if
necessary. Value 0 of level de-activates this option just as
if no -ftrack-macro-expansion was present on the command line.
Value 1 tracks tokens locations in a degraded mode for the sake of
minimal memory overhead. In this mode all tokens resulting from the
expansion of an argument of a function-like macro have the same location.
Value 2 tracks tokens locations completely. This value is the most
memory hungry. When this option is given no argument, the default
parameter value is 2.
Note that "-ftrack-macro-expansion=2" is activated by
default.
- -fexec-charset=charset
- Set the execution character set, used for string and
character constants. The default is UTF-8. charset can be any
encoding supported by the system's "iconv" library routine.
- -fwide-exec-charset=charset
- Set the wide execution character set, used for wide string
and character constants. The default is UTF-32 or UTF-16, whichever
corresponds to the width of "wchar_t". As with
-fexec-charset, charset can be any encoding supported by the
system's "iconv" library routine; however, you will have
problems with encodings that do not fit exactly in
"wchar_t".
- -finput-charset=charset
- Set the input character set, used for translation from the
character set of the input file to the source character set used by GCC.
If the locale does not specify, or GCC cannot get this information from
the locale, the default is UTF-8. This can be overridden by either the
locale or this command-line option. Currently the command-line option
takes precedence if there's a conflict. charset can be any encoding
supported by the system's "iconv" library routine.
- -fworking-directory
- Enable generation of linemarkers in the preprocessor output
that will let the compiler know the current working directory at the time
of preprocessing. When this option is enabled, the preprocessor will emit,
after the initial linemarker, a second linemarker with the current working
directory followed by two slashes. GCC will use this directory, when it's
present in the preprocessed input, as the directory emitted as the current
working directory in some debugging information formats. This option is
implicitly enabled if debugging information is enabled, but this can be
inhibited with the negated form -fno-working-directory. If the
-P flag is present in the command line, this option has no effect,
since no "#line" directives are emitted whatsoever.
- -fno-show-column
- Do not print column numbers in diagnostics. This may be
necessary if diagnostics are being scanned by a program that does not
understand the column numbers, such as dejagnu.
- -A predicate=answer
- Make an assertion with the predicate predicate and
answer answer. This form is preferred to the older form -A
predicate(answer), which is still supported,
because it does not use shell special characters.
- -A -predicate=answer
- Cancel an assertion with the predicate predicate and
answer answer.
- -dCHARS
- CHARS is a sequence of one or more of the following
characters, and must not be preceded by a space. Other characters are
interpreted by the compiler proper, or reserved for future versions of
GCC, and so are silently ignored. If you specify characters whose behavior
conflicts, the result is undefined.
- M
- Instead of the normal output, generate a list of
#define directives for all the macros defined during the execution
of the preprocessor, including predefined macros. This gives you a way of
finding out what is predefined in your version of the preprocessor.
Assuming you have no file foo.h, the command
touch foo.h; cpp -dM foo.h
will show all the predefined macros.
If you use -dM without the -E option, -dM is
interpreted as a synonym for -fdump-rtl-mach.
- D
- Like M except in two respects: it does not
include the predefined macros, and it outputs both the
#define directives and the result of preprocessing. Both kinds of
output go to the standard output file.
- N
- Like D, but emit only the macro names, not their
expansions.
- I
- Output #include directives in addition to the result
of preprocessing.
- U
- Like D except that only macros that are expanded, or
whose definedness is tested in preprocessor directives, are output; the
output is delayed until the use or test of the macro; and #undef
directives are also output for macros tested but undefined at the
time.
- -P
- Inhibit generation of linemarkers in the output from the
preprocessor. This might be useful when running the preprocessor on
something that is not C code, and will be sent to a program which might be
confused by the linemarkers.
- -C
- Do not discard comments. All comments are passed through to
the output file, except for comments in processed directives, which are
deleted along with the directive.
You should be prepared for side effects when using -C; it causes the
preprocessor to treat comments as tokens in their own right. For example,
comments appearing at the start of what would be a directive line have the
effect of turning that line into an ordinary source line, since the first
token on the line is no longer a #.
- -CC
- Do not discard comments, including during macro expansion.
This is like -C, except that comments contained within macros are
also passed through to the output file where the macro is expanded.
In addition to the side-effects of the -C option, the -CC
option causes all C++-style comments inside a macro to be converted to
C-style comments. This is to prevent later use of that macro from
inadvertently commenting out the remainder of the source line.
The -CC option is generally used to support lint comments.
- -traditional-cpp
- Try to imitate the behavior of old-fashioned C
preprocessors, as opposed to ISO C preprocessors.
- -trigraphs
- Process trigraph sequences. These are three-character
sequences, all starting with ??, that are defined by ISO C to stand
for single characters. For example, ??/ stands for \, so
'??/n' is a character constant for a newline. By default, GCC
ignores trigraphs, but in standard-conforming modes it converts them. See
the -std and -ansi options.
The nine trigraphs and their replacements are
Trigraph: ??( ??) ??< ??> ??= ??/ ??' ??! ??-
Replacement: [ ] { } # \ ^ | ~
- -remap
- Enable special code to work around file systems which only
permit very short file names, such as MS-DOS.
- --help
- --target-help
- Print text describing all the command-line options instead
of preprocessing anything.
- -v
- Verbose mode. Print out GNU CPP's version number at the
beginning of execution, and report the final form of the include
path.
- -H
- Print the name of each header file used, in addition to
other normal activities. Each name is indented to show how deep in the
#include stack it is. Precompiled header files are also printed,
even if they are found to be invalid; an invalid precompiled header file
is printed with ...x and a valid one with ...! .
- -version
- --version
- Print out GNU CPP's version number. With one dash, proceed
to preprocess as normal. With two dashes, exit immediately.
Passing Options to the Assembler
You can pass options to the assembler.
- -Wa,option
- Pass option as an option to the assembler. If
option contains commas, it is split into multiple options at the
commas.
- -Xassembler option
- Pass option as an option to the assembler. You can
use this to supply system-specific assembler options that GCC does not
recognize.
If you want to pass an option that takes an argument, you must use
-Xassembler twice, once for the option and once for the
argument.
Options for Linking
These options come into play when the compiler links object files into an
executable output file. They are meaningless if the compiler is not doing a
link step.
- object-file-name
- A file name that does not end in a special recognized
suffix is considered to name an object file or library. (Object files are
distinguished from libraries by the linker according to the file
contents.) If linking is done, these object files are used as input to the
linker.
- -c
- -S
- -E
- If any of these options is used, then the linker is not
run, and object file names should not be used as arguments.
- -fuse-ld=bfd
- Use the bfd linker instead of the default
linker.
- -fuse-ld=gold
- Use the gold linker instead of the default
linker.
- -llibrary
- -l library
- Search the library named library when linking. (The
second alternative with the library as a separate argument is only for
POSIX compliance and is not recommended.)
It makes a difference where in the command you write this option; the linker
searches and processes libraries and object files in the order they are
specified. Thus, foo.o -lz bar.o searches library z after
file foo.o but before bar.o. If bar.o refers to
functions in z, those functions may not be loaded.
The linker searches a standard list of directories for the library, which is
actually a file named liblibrary.a. The linker then
uses this file as if it had been specified precisely by name.
The directories searched include several standard system directories plus
any that you specify with -L.
Normally the files found this way are library files---archive files whose
members are object files. The linker handles an archive file by scanning
through it for members which define symbols that have so far been
referenced but not defined. But if the file that is found is an ordinary
object file, it is linked in the usual fashion. The only difference
between using an -l option and specifying a file name is that
-l surrounds library with lib and .a and
searches several directories.
- -lobjc
- You need this special case of the -l option in order
to link an Objective-C or Objective-C++ program.
- -nostartfiles
- Do not use the standard system startup files when linking.
The standard system libraries are used normally, unless -nostdlib
or -nodefaultlibs is used.
- -nodefaultlibs
- Do not use the standard system libraries when linking. Only
the libraries you specify are passed to the linker, and options specifying
linkage of the system libraries, such as -static-libgcc or
-shared-libgcc, are ignored. The standard startup files are used
normally, unless -nostartfiles is used.
The compiler may generate calls to "memcmp", "memset",
"memcpy" and "memmove". These entries are usually
resolved by entries in libc. These entry points should be supplied through
some other mechanism when this option is specified.
- -nostdlib
- Do not use the standard system startup files or libraries
when linking. No startup files and only the libraries you specify are
passed to the linker, and options specifying linkage of the system
libraries, such as -static-libgcc or -shared-libgcc, are
ignored.
The compiler may generate calls to "memcmp", "memset",
"memcpy" and "memmove". These entries are usually
resolved by entries in libc. These entry points should be supplied through
some other mechanism when this option is specified.
One of the standard libraries bypassed by -nostdlib and
-nodefaultlibs is libgcc.a, a library of internal
subroutines which GCC uses to overcome shortcomings of particular
machines, or special needs for some languages.
In most cases, you need libgcc.a even when you want to avoid other
standard libraries. In other words, when you specify -nostdlib or
-nodefaultlibs you should usually specify -lgcc as well.
This ensures that you have no unresolved references to internal GCC
library subroutines. (An example of such an internal subroutine is
"__main", used to ensure C++ constructors are called.)
- -pie
- Produce a position independent executable on targets that
support it. For predictable results, you must also specify the same set of
options used for compilation ( -fpie, -fPIE, or model
suboptions) when you specify this linker option.
- -rdynamic
- Pass the flag -export-dynamic to the ELF linker, on
targets that support it. This instructs the linker to add all symbols, not
only used ones, to the dynamic symbol table. This option is needed for
some uses of "dlopen" or to allow obtaining backtraces from
within a program.
- -s
- Remove all symbol table and relocation information from the
executable.
- -static
- On systems that support dynamic linking, this prevents
linking with the shared libraries. On other systems, this option has no
effect.
- -shared
- Produce a shared object which can then be linked with other
objects to form an executable. Not all systems support this option. For
predictable results, you must also specify the same set of options used
for compilation ( -fpic, -fPIC, or model suboptions) when
you specify this linker option.[1]
- -shared-libgcc
- -static-libgcc
- On systems that provide libgcc as a shared library,
these options force the use of either the shared or static version,
respectively. If no shared version of libgcc was built when the
compiler was configured, these options have no effect.
There are several situations in which an application should use the shared
libgcc instead of the static version. The most common of these is
when the application wishes to throw and catch exceptions across different
shared libraries. In that case, each of the libraries as well as the
application itself should use the shared libgcc.
Therefore, the G++ and GCJ drivers automatically add -shared-libgcc
whenever you build a shared library or a main executable, because C++ and
Java programs typically use exceptions, so this is the right thing to do.
If, instead, you use the GCC driver to create shared libraries, you may find
that they are not always linked with the shared libgcc. If GCC
finds, at its configuration time, that you have a non-GNU linker or a GNU
linker that does not support option --eh-frame-hdr, it links the
shared version of libgcc into shared libraries by default.
Otherwise, it takes advantage of the linker and optimizes away the linking
with the shared version of libgcc, linking with the static version
of libgcc by default. This allows exceptions to propagate through such
shared libraries, without incurring relocation costs at library load time.
However, if a library or main executable is supposed to throw or catch
exceptions, you must link it using the G++ or GCJ driver, as appropriate
for the languages used in the program, or using the option
-shared-libgcc, such that it is linked with the shared
libgcc.
- -static-libasan
- When the -fsanitize=address option is used to link a
program, the GCC driver automatically links against libasan. If
libasan is available as a shared library, and the -static
option is not used, then this links against the shared version of
libasan. The -static-libasan option directs the GCC driver
to link libasan statically, without necessarily linking other
libraries statically.
- -static-libtsan
- When the -fsanitize=thread option is used to link a
program, the GCC driver automatically links against libtsan. If
libtsan is available as a shared library, and the -static
option is not used, then this links against the shared version of
libtsan. The -static-libtsan option directs the GCC driver
to link libtsan statically, without necessarily linking other
libraries statically.
- -static-liblsan
- When the -fsanitize=leak option is used to link a
program, the GCC driver automatically links against liblsan. If
liblsan is available as a shared library, and the -static
option is not used, then this links against the shared version of
liblsan. The -static-liblsan option directs the GCC driver
to link liblsan statically, without necessarily linking other
libraries statically.
- -static-libubsan
- When the -fsanitize=undefined option is used to link
a program, the GCC driver automatically links against libubsan. If
libubsan is available as a shared library, and the -static
option is not used, then this links against the shared version of
libubsan. The -static-libubsan option directs the GCC driver
to link libubsan statically, without necessarily linking other
libraries statically.
- -static-libmpx
- When the -fcheck-pointer bounds and -mmpx
options are used to link a program, the GCC driver automatically links
against libmpx. If libmpx is available as a shared library,
and the -static option is not used, then this links against the
shared version of libmpx. The -static-libmpx option directs
the GCC driver to link libmpx statically, without necessarily
linking other libraries statically.
- -static-libmpxwrappers
- When the -fcheck-pointer bounds and -mmpx
options are used to link a program without also using
-fno-chkp-use-wrappers, the GCC driver automatically links against
libmpxwrappers. If libmpxwrappers is available as a shared
library, and the -static option is not used, then this links
against the shared version of libmpxwrappers. The
-static-libmpxwrappers option directs the GCC driver to link
libmpxwrappers statically, without necessarily linking other
libraries statically.
- -static-libstdc++
- When the g++ program is used to link a C++ program,
it normally automatically links against libstdc++. If
libstdc++ is available as a shared library, and the -static
option is not used, then this links against the shared version of
libstdc++. That is normally fine. However, it is sometimes useful
to freeze the version of libstdc++ used by the program without
going all the way to a fully static link. The -static-libstdc++
option directs the g++ driver to link libstdc++ statically,
without necessarily linking other libraries statically.
- -symbolic
- Bind references to global symbols when building a shared
object. Warn about any unresolved references (unless overridden by the
link editor option -Xlinker -z -Xlinker defs). Only a few systems
support this option.
- -T script
- Use script as the linker script. This option is
supported by most systems using the GNU linker. On some targets, such as
bare-board targets without an operating system, the -T option may
be required when linking to avoid references to undefined symbols.
- -Xlinker option
- Pass option as an option to the linker. You can use
this to supply system-specific linker options that GCC does not recognize.
If you want to pass an option that takes a separate argument, you must use
-Xlinker twice, once for the option and once for the argument. For
example, to pass -assert definitions, you must write -Xlinker
-assert -Xlinker definitions. It does not work to write -Xlinker
"-assert definitions", because this passes the entire string
as a single argument, which is not what the linker expects.
When using the GNU linker, it is usually more convenient to pass arguments
to linker options using the option=value syntax than
as separate arguments. For example, you can specify -Xlinker
-Map=output.map rather than -Xlinker -Map -Xlinker output.map.
Other linkers may not support this syntax for command-line options.
- -Wl,option
- Pass option as an option to the linker. If
option contains commas, it is split into multiple options at the
commas. You can use this syntax to pass an argument to the option. For
example, -Wl,-Map,output.map passes -Map output.map to the
linker. When using the GNU linker, you can also get the same effect with
-Wl,-Map=output.map.
- -u symbol
- Pretend the symbol symbol is undefined, to force
linking of library modules to define it. You can use -u multiple
times with different symbols to force loading of additional library
modules.
- -z keyword
- -z is passed directly on to the linker along with
the keyword keyword. See the section in the documentation of your
linker for permitted values and their meanings.
Options for Directory Search
These options specify directories to search for header files, for libraries and
for parts of the compiler:
- -Idir
- Add the directory dir to the head of the list of
directories to be searched for header files. This can be used to override
a system header file, substituting your own version, since these
directories are searched before the system header file directories.
However, you should not use this option to add directories that contain
vendor-supplied system header files (use -isystem for that). If you
use more than one -I option, the directories are scanned in
left-to-right order; the standard system directories come after.
If a standard system include directory, or a directory specified with
-isystem, is also specified with -I, the -I option is
ignored. The directory is still searched but as a system directory at its
normal position in the system include chain. This is to ensure that GCC's
procedure to fix buggy system headers and the ordering for the
"include_next" directive are not inadvertently changed. If you
really need to change the search order for system directories, use the
-nostdinc and/or -isystem options.
- -iplugindir=dir
- Set the directory to search for plugins that are passed by
-fplugin=name instead of
-fplugin=path/name.so. This option is
not meant to be used by the user, but only passed by the driver.
- -iquotedir
- Add the directory dir to the head of the list of
directories to be searched for header files only for the case of
"#include " file""; they are not searched for
"#include < file>", otherwise just like
-I.
- -iremap src:dst
- Replace the prefix src in __FILE__ with dst
at expansion time. This option can be specified more than once. Processing
stops at the first match.
- -Ldir
- Add directory dir to the list of directories to be
searched for -l.
- -Bprefix
- This option specifies where to find the executables,
libraries, include files, and data files of the compiler itself.
The compiler driver program runs one or more of the subprograms cpp,
cc1, as and ld. It tries prefix as a prefix
for each program it tries to run, both with and without
machine/ version/.
For each subprogram to be run, the compiler driver first tries the -B
prefix, if any. If that name is not found, or if -B is not
specified, the driver tries two standard prefixes, /usr/lib/gcc/
and /usr/local/lib/gcc/. If neither of those results in a file name
that is found, the unmodified program name is searched for using the
directories specified in your PATH environment variable.
The compiler checks to see if the path provided by -B refers to a
directory, and if necessary it adds a directory separator character at the
end of the path.
-B prefixes that effectively specify directory names also apply to
libraries in the linker, because the compiler translates these options
into -L options for the linker. They also apply to include files in
the preprocessor, because the compiler translates these options into
-isystem options for the preprocessor. In this case, the compiler
appends include to the prefix.
The runtime support file libgcc.a can also be searched for using the
-B prefix, if needed. If it is not found there, the two standard
prefixes above are tried, and that is all. The file is left out of the
link if it is not found by those means.
Another way to specify a prefix much like the -B prefix is to use the
environment variable GCC_EXEC_PREFIX.
As a special kludge, if the path provided by -B is
[dir/]stageN/, where N is a number in the
range 0 to 9, then it is replaced by [dir/]include. This is to help
with boot-strapping the compiler.
- -specs=file
- Process file after the compiler reads in the
standard specs file, in order to override the defaults which the
gcc driver program uses when determining what switches to pass to
cc1, cc1plus, as, ld, etc. More than one
-specs= file can be specified on the command line, and they
are processed in order, from left to right.
- --sysroot=dir
- Use dir as the logical root directory for headers
and libraries. For example, if the compiler normally searches for headers
in /usr/include and libraries in /usr/lib, it instead
searches dir/usr/include and
dir/usr/lib.
If you use both this option and the -isysroot option, then the
--sysroot option applies to libraries, but the -isysroot
option applies to header files.
The GNU linker (beginning with version 2.16) has the necessary support for
this option. If your linker does not support this option, the header file
aspect of --sysroot still works, but the library aspect does
not.
- --no-sysroot-suffix
- For some targets, a suffix is added to the root directory
specified with --sysroot, depending on the other options used, so
that headers may for example be found in
dir/suffix /usr/include instead of
dir/usr/include. This option disables the addition
of such a suffix.
- -I-
- This option has been deprecated. Please use -iquote
instead for -I directories before the -I- and remove the
-I- option. Any directories you specify with -I options
before the -I- option are searched only for the case of
"#include " file""; they are not searched for
"#include < file>".
If additional directories are specified with -I options after the
-I- option, these directories are searched for all
"#include" directives. (Ordinarily all -I
directories are used this way.)
In addition, the -I- option inhibits the use of the current directory
(where the current input file came from) as the first search directory for
"#include " file"". There is no way to override
this effect of -I-. With -I. you can specify searching the
directory that is current when the compiler is invoked. That is not
exactly the same as what the preprocessor does by default, but it is often
satisfactory.
-I- does not inhibit the use of the standard system directories for
header files. Thus, -I- and -nostdinc are independent.
Specifying Target Machine and Compiler Version
The usual way to run GCC is to run the executable called
gcc, or
machine -gcc when cross-compiling, or
machine-gcc-version to run a version other than the one
that was installed last.
Hardware Models and Configurations
Each target machine types can have its own special options, starting with
-m, to choose among various hardware models or configurations---for
example, 68010 vs 68020, floating coprocessor or none. A single installed
version of the compiler can compile for any model or configuration, according
to the options specified.
Some configurations of the compiler also support additional special options,
usually for compatibility with other compilers on the same platform.
AArch64 Options
These options are defined for AArch64 implementations:
- -mabi=name
- Generate code for the specified data model. Permissible
values are ilp32 for SysV-like data model where int, long int and
pointer are 32-bit, and lp64 for SysV-like data model where int is
32-bit, but long int and pointer are 64-bit.
The default depends on the specific target configuration. Note that the LP64
and ILP32 ABIs are not link-compatible; you must compile your entire
program with the same ABI, and link with a compatible set of
libraries.
- -mbig-endian
- Generate big-endian code. This is the default when GCC is
configured for an aarch64_be-*-* target.
- -mgeneral-regs-only
- Generate code which uses only the general registers.
- -mlittle-endian
- Generate little-endian code. This is the default when GCC
is configured for an aarch64-*-* but not an aarch64_be-*-*
target.
- -mcmodel=tiny
- Generate code for the tiny code model. The program and its
statically defined symbols must be within 1GB of each other. Pointers are
64 bits. Programs can be statically or dynamically linked. This model is
not fully implemented and mostly treated as small.
- -mcmodel=small
- Generate code for the small code model. The program and its
statically defined symbols must be within 4GB of each other. Pointers are
64 bits. Programs can be statically or dynamically linked. This is the
default code model.
- -mcmodel=large
- Generate code for the large code model. This makes no
assumptions about addresses and sizes of sections. Pointers are 64 bits.
Programs can be statically linked only.
- -mstrict-align
- Do not assume that unaligned memory references are handled
by the system.
- -momit-leaf-frame-pointer
- -mno-omit-leaf-frame-pointer
- Omit or keep the frame pointer in leaf functions. The
former behaviour is the default.
- -mtls-dialect=desc
- Use TLS descriptors as the thread-local storage mechanism
for dynamic accesses of TLS variables. This is the default.
- -mtls-dialect=traditional
- Use traditional TLS as the thread-local storage mechanism
for dynamic accesses of TLS variables.
- -mfix-cortex-a53-835769
- -mno-fix-cortex-a53-835769
- Enable or disable the workaround for the ARM Cortex-A53
erratum number 835769. This involves inserting a NOP instruction between
memory instructions and 64-bit integer multiply-accumulate
instructions.
- -mfix-cortex-a53-843419
- -mno-fix-cortex-a53-843419
- Enable or disable the workaround for the ARM Cortex-A53
erratum number 843419. This erratum workaround is made at link time and
this will only pass the corresponding flag to the linker.
- -march=name
- Specify the name of the target architecture, optionally
suffixed by one or more feature modifiers. This option has the form
-march= arch{+[no]feature}*, where the
only permissible value for arch is armv8-a. The permissible
values for feature are documented in the sub-section below.
Where conflicting feature modifiers are specified, the right-most feature is
used.
GCC uses this name to determine what kind of instructions it can emit when
generating assembly code.
Where -march is specified without either of -mtune or
-mcpu also being specified, the code is tuned to perform well
across a range of target processors implementing the target
architecture.
- -mtune=name
- Specify the name of the target processor for which GCC
should tune the performance of the code. Permissible values for this
option are: generic, cortex-a53, cortex-a57,
cortex-a72, exynos-m1, thunderx, xgene1.
Additionally, this option can specify that GCC should tune the performance
of the code for a big.LITTLE system. Permissible values for this option
are: cortex-a57.cortex-a53, cortex-a72.cortex-a53.
Where none of -mtune=, -mcpu= or -march= are specified,
the code is tuned to perform well across a range of target processors.
This option cannot be suffixed by feature modifiers.
- -mcpu=name
- Specify the name of the target processor, optionally
suffixed by one or more feature modifiers. This option has the form
-mcpu= cpu{+[no]feature}*, where the
permissible values for cpu are the same as those available for
-mtune.
The permissible values for feature are documented in the sub-section
below.
Where conflicting feature modifiers are specified, the right-most feature is
used.
GCC uses this name to determine what kind of instructions it can emit when
generating assembly code (as if by -march) and to determine the
target processor for which to tune for performance (as if by
-mtune). Where this option is used in conjunction with
-march or -mtune, those options take precedence over the
appropriate part of this option.
-march and
-mcpu Feature Modifiers
Feature modifiers used with
-march and
-mcpu can be one the
following:
- crc
- Enable CRC extension.
- crypto
- Enable Crypto extension. This implies Advanced SIMD is
enabled.
- fp
- Enable floating-point instructions.
- simd
- Enable Advanced SIMD instructions. This implies
floating-point instructions are enabled. This is the default for all
current possible values for options -march and -mcpu=.
Adapteva Epiphany Options
These
-m options are defined for Adapteva Epiphany:
- -mhalf-reg-file
- Don't allocate any register in the range
"r32"..."r63". That allows code to run on hardware
variants that lack these registers.
- -mprefer-short-insn-regs
- Preferrentially allocate registers that allow short
instruction generation. This can result in increased instruction count, so
this may either reduce or increase overall code size.
- -mbranch-cost=num
- Set the cost of branches to roughly num
"simple" instructions. This cost is only a heuristic and is not
guaranteed to produce consistent results across releases.
- -mcmove
- Enable the generation of conditional moves.
- -mnops=num
- Emit num NOPs before every other generated
instruction.
- -mno-soft-cmpsf
- For single-precision floating-point comparisons, emit an
"fsub" instruction and test the flags. This is faster than a
software comparison, but can get incorrect results in the presence of
NaNs, or when two different small numbers are compared such that their
difference is calculated as zero. The default is -msoft-cmpsf,
which uses slower, but IEEE-compliant, software comparisons.
- -mstack-offset=num
- Set the offset between the top of the stack and the stack
pointer. E.g., a value of 8 means that the eight bytes in the range
"sp+0...sp+7" can be used by leaf functions without stack
allocation. Values other than 8 or 16 are untested and
unlikely to work. Note also that this option changes the ABI; compiling a
program with a different stack offset than the libraries have been
compiled with generally does not work. This option can be useful if you
want to evaluate if a different stack offset would give you better code,
but to actually use a different stack offset to build working programs, it
is recommended to configure the toolchain with the appropriate
--with-stack-offset= num option.
- -mno-round-nearest
- Make the scheduler assume that the rounding mode has been
set to truncating. The default is -mround-nearest.
- -mlong-calls
- If not otherwise specified by an attribute, assume all
calls might be beyond the offset range of the "b" /
"bl" instructions, and therefore load the function address into
a register before performing a (otherwise direct) call. This is the
default.
- -mshort-calls
- If not otherwise specified by an attribute, assume all
direct calls are in the range of the "b" / "bl"
instructions, so use these instructions for direct calls. The default is
-mlong-calls.
- -msmall16
- Assume addresses can be loaded as 16-bit unsigned values.
This does not apply to function addresses for which -mlong-calls
semantics are in effect.
- -mfp-mode=mode
- Set the prevailing mode of the floating-point unit. This
determines the floating-point mode that is provided and expected at
function call and return time. Making this mode match the mode you
predominantly need at function start can make your programs smaller and
faster by avoiding unnecessary mode switches.
mode can be set to one the following values:
- caller
- Any mode at function entry is valid, and retained or
restored when the function returns, and when it calls other functions.
This mode is useful for compiling libraries or other compilation units you
might want to incorporate into different programs with different
prevailing FPU modes, and the convenience of being able to use a single
object file outweighs the size and speed overhead for any extra mode
switching that might be needed, compared with what would be needed with a
more specific choice of prevailing FPU mode.
- truncate
- This is the mode used for floating-point calculations with
truncating (i.e. round towards zero) rounding mode. That includes
conversion from floating point to integer.
- round-nearest
- This is the mode used for floating-point calculations with
round-to-nearest-or-even rounding mode.
- int
- This is the mode used to perform integer calculations in
the FPU, e.g. integer multiply, or integer multiply-and-accumulate.
The default is
-mfp-mode=caller
- -mnosplit-lohi
- -mno-postinc
- -mno-postmodify
- Code generation tweaks that disable, respectively,
splitting of 32-bit loads, generation of post-increment addresses, and
generation of post-modify addresses. The defaults are msplit-lohi,
-mpost-inc, and -mpost-modify.
- -mnovect-double
- Change the preferred SIMD mode to SImode. The default is
-mvect-double, which uses DImode as preferred SIMD mode.
- -max-vect-align=num
- The maximum alignment for SIMD vector mode types.
num may be 4 or 8. The default is 8. Note that this is an ABI
change, even though many library function interfaces are unaffected if
they don't use SIMD vector modes in places that affect size and/or
alignment of relevant types.
- -msplit-vecmove-early
- Split vector moves into single word moves before reload. In
theory this can give better register allocation, but so far the reverse
seems to be generally the case.
- -m1reg-reg
- Specify a register to hold the constant -1, which makes
loading small negative constants and certain bitmasks faster. Allowable
values for reg are r43 and r63, which specify use of
that register as a fixed register, and none, which means that no
register is used for this purpose. The default is -m1reg-none.
ARC Options
The following options control the architecture variant for which code is being
compiled:
- -mbarrel-shifter
- Generate instructions supported by barrel shifter. This is
the default unless -mcpu=ARC601 is in effect.
- -mcpu=cpu
- Set architecture type, register usage, and instruction
scheduling parameters for cpu. There are also shortcut alias
options available for backward compatibility and convenience. Supported
values for cpu are
- ARC600
- Compile for ARC600. Aliases: -mA6,
-mARC600.
- ARC601
- Compile for ARC601. Alias: -mARC601.
- ARC700
- Compile for ARC700. Aliases: -mA7, -mARC700.
This is the default when configured with --with-cpu=arc700.
- -mdpfp
- -mdpfp-compact
- FPX: Generate Double Precision FPX instructions, tuned for
the compact implementation.
- -mdpfp-fast
- FPX: Generate Double Precision FPX instructions, tuned for
the fast implementation.
- -mno-dpfp-lrsr
- Disable LR and SR instructions from using FPX extension aux
registers.
- -mea
- Generate Extended arithmetic instructions. Currently only
"divaw", "adds", "subs", and
"sat16" are supported. This is always enabled for
-mcpu=ARC700.
- -mno-mpy
- Do not generate mpy instructions for ARC700.
- -mmul32x16
- Generate 32x16 bit multiply and mac instructions.
- -mmul64
- Generate mul64 and mulu64 instructions. Only valid for
-mcpu=ARC600.
- -mnorm
- Generate norm instruction. This is the default if
-mcpu=ARC700 is in effect.
- -mspfp
- -mspfp-compact
- FPX: Generate Single Precision FPX instructions, tuned for
the compact implementation.
- -mspfp-fast
- FPX: Generate Single Precision FPX instructions, tuned for
the fast implementation.
- -msimd
- Enable generation of ARC SIMD instructions via
target-specific builtins. Only valid for -mcpu=ARC700.
- -msoft-float
- This option ignored; it is provided for compatibility
purposes only. Software floating point code is emitted by default, and
this default can overridden by FPX options; mspfp,
mspfp-compact, or mspfp-fast for single precision, and
mdpfp, mdpfp-compact, or mdpfp-fast for double
precision.
- -mswap
- Generate swap instructions.
The following options are passed through to the assembler, and also define
preprocessor macro symbols.
- -mdsp-packa
- Passed down to the assembler to enable the DSP Pack A
extensions. Also sets the preprocessor symbol
"__Xdsp_packa".
- -mdvbf
- Passed down to the assembler to enable the dual viterbi
butterfly extension. Also sets the preprocessor symbol
"__Xdvbf".
- -mlock
- Passed down to the assembler to enable the Locked
Load/Store Conditional extension. Also sets the preprocessor symbol
"__Xlock".
- -mmac-d16
- Passed down to the assembler. Also sets the preprocessor
symbol "__Xxmac_d16".
- -mmac-24
- Passed down to the assembler. Also sets the preprocessor
symbol "__Xxmac_24".
- -mrtsc
- Passed down to the assembler to enable the 64-bit
Time-Stamp Counter extension instruction. Also sets the preprocessor
symbol "__Xrtsc".
- -mswape
- Passed down to the assembler to enable the swap byte
ordering extension instruction. Also sets the preprocessor symbol
"__Xswape".
- -mtelephony
- Passed down to the assembler to enable dual and single
operand instructions for telephony. Also sets the preprocessor symbol
"__Xtelephony".
- -mxy
- Passed down to the assembler to enable the XY Memory
extension. Also sets the preprocessor symbol "__Xxy".
The following options control how the assembly code is annotated:
- -misize
- Annotate assembler instructions with estimated
addresses.
- -mannotate-align
- Explain what alignment considerations lead to the decision
to make an instruction short or long.
The following options are passed through to the linker:
- -marclinux
- Passed through to the linker, to specify use of the
"arclinux" emulation. This option is enabled by default in tool
chains built for "arc-linux-uclibc" and
"arceb-linux-uclibc" targets when profiling is not
requested.
- -marclinux_prof
- Passed through to the linker, to specify use of the
"arclinux_prof" emulation. This option is enabled by default in
tool chains built for "arc-linux-uclibc" and
"arceb-linux-uclibc" targets when profiling is requested.
The following options control the semantics of generated code:
- -mepilogue-cfi
- Enable generation of call frame information for
epilogues.
- -mno-epilogue-cfi
- Disable generation of call frame information for
epilogues.
- -mlong-calls
- Generate call insns as register indirect calls, thus
providing access to the full 32-bit address range.
- -mmedium-calls
- Don't use less than 25 bit addressing range for calls,
which is the offset available for an unconditional branch-and-link
instruction. Conditional execution of function calls is suppressed, to
allow use of the 25-bit range, rather than the 21-bit range with
conditional branch-and-link. This is the default for tool chains built for
"arc-linux-uclibc" and "arceb-linux-uclibc"
targets.
- -mno-sdata
- Do not generate sdata references. This is the default for
tool chains built for "arc-linux-uclibc" and
"arceb-linux-uclibc" targets.
- -mucb-mcount
- Instrument with mcount calls as used in UCB code. I.e. do
the counting in the callee, not the caller. By default ARC instrumentation
counts in the caller.
- -mvolatile-cache
- Use ordinarily cached memory accesses for volatile
references. This is the default.
- -mno-volatile-cache
- Enable cache bypass for volatile references.
The following options fine tune code generation:
- -malign-call
- Do alignment optimizations for call instructions.
- -mauto-modify-reg
- Enable the use of pre/post modify with register
displacement.
- -mbbit-peephole
- Enable bbit peephole2.
- -mno-brcc
- This option disables a target-specific pass in
arc_reorg to generate "BRcc" instructions. It has no
effect on "BRcc" generation driven by the combiner pass.
- -mcase-vector-pcrel
- Use pc-relative switch case tables - this enables case
table shortening. This is the default for -Os.
- -mcompact-casesi
- Enable compact casesi pattern. This is the default for
-Os.
- -mno-cond-exec
- Disable ARCompact specific pass to generate conditional
execution instructions. Due to delay slot scheduling and interactions
between operand numbers, literal sizes, instruction lengths, and the
support for conditional execution, the target-independent pass to generate
conditional execution is often lacking, so the ARC port has kept a special
pass around that tries to find more conditional execution generating
opportunities after register allocation, branch shortening, and delay slot
scheduling have been done. This pass generally, but not always, improves
performance and code size, at the cost of extra compilation time, which is
why there is an option to switch it off. If you have a problem with call
instructions exceeding their allowable offset range because they are
conditionalized, you should consider using -mmedium-calls
instead.
- -mearly-cbranchsi
- Enable pre-reload use of the cbranchsi pattern.
- -mexpand-adddi
- Expand "adddi3" and "subdi3" at rtl
generation time into "add.f", "adc" etc.
- -mindexed-loads
- Enable the use of indexed loads. This can be problematic
because some optimizers then assume that indexed stores exist, which is
not the case.
- -mlra
- Enable Local Register Allocation. This is still
experimental for ARC, so by default the compiler uses standard reload
(i.e. -mno-lra).
- -mlra-priority-none
- Don't indicate any priority for target registers.
- -mlra-priority-compact
- Indicate target register priority for r0..r3 /
r12..r15.
- -mlra-priority-noncompact
- Reduce target regsiter priority for r0..r3 / r12..r15.
- -mno-millicode
- When optimizing for size (using -Os), prologues and
epilogues that have to save or restore a large number of registers are
often shortened by using call to a special function in libgcc; this is
referred to as a millicode call. As these calls can pose
performance issues, and/or cause linking issues when linking in a
nonstandard way, this option is provided to turn off millicode call
generation.
- -mmixed-code
- Tweak register allocation to help 16-bit instruction
generation. This generally has the effect of decreasing the average
instruction size while increasing the instruction count.
- -mq-class
- Enable 'q' instruction alternatives. This is the default
for -Os.
- -mRcq
- Enable Rcq constraint handling - most short code generation
depends on this. This is the default.
- -mRcw
- Enable Rcw constraint handling - ccfsm condexec mostly
depends on this. This is the default.
- -msize-level=level
- Fine-tune size optimization with regards to instruction
lengths and alignment. The recognized values for level are:
- 0
- No size optimization. This level is deprecated and treated
like 1.
- 1
- Short instructions are used opportunistically.
- 2
- In addition, alignment of loops and of code after barriers
are dropped.
- 3
- In addition, optional data alignment is dropped, and the
option Os is enabled.
This defaults to
3 when
-Os is in effect. Otherwise, the behavior
when this is not set is equivalent to level
1.
- -mtune=cpu
- Set instruction scheduling parameters for cpu,
overriding any implied by -mcpu=.
Supported values for cpu are
- ARC600
- Tune for ARC600 cpu.
- ARC601
- Tune for ARC601 cpu.
- ARC700
- Tune for ARC700 cpu with standard multiplier block.
- ARC700-xmac
- Tune for ARC700 cpu with XMAC block.
- ARC725D
- Tune for ARC725D cpu.
- ARC750D
- Tune for ARC750D cpu.
- -mmultcost=num
- Cost to assume for a multiply instruction, with 4
being equal to a normal instruction.
- -munalign-prob-threshold=probability
- Set probability threshold for unaligning branches. When
tuning for ARC700 and optimizing for speed, branches without filled
delay slot are preferably emitted unaligned and long, unless profiling
indicates that the probability for the branch to be taken is below
probability. The default is (REG_BR_PROB_BASE/2), i.e. 5000.
The following options are maintained for backward compatibility, but are now
deprecated and will be removed in a future release:
- -margonaut
- Obsolete FPX.
- -mbig-endian
- -EB
- Compile code for big endian targets. Use of these options
is now deprecated. Users wanting big-endian code, should use the
"arceb-elf32" and "arceb-linux-uclibc" targets when
building the tool chain, for which big-endian is the default.
- -mlittle-endian
- -EL
- Compile code for little endian targets. Use of these
options is now deprecated. Users wanting little-endian code should use the
"arc-elf32" and "arc-linux-uclibc" targets when
building the tool chain, for which little-endian is the default.
- -mbarrel_shifter
- Replaced by -mbarrel-shifter.
- -mdpfp_compact
- Replaced by -mdpfp-compact.
- -mdpfp_fast
- Replaced by -mdpfp-fast.
- -mdsp_packa
- Replaced by -mdsp-packa.
- -mEA
- Replaced by -mea.
- -mmac_24
- Replaced by -mmac-24.
- -mmac_d16
- Replaced by -mmac-d16.
- -mspfp_compact
- Replaced by -mspfp-compact.
- -mspfp_fast
- Replaced by -mspfp-fast.
- -mtune=cpu
- Values arc600, arc601, arc700 and
arc700-xmac for cpu are replaced by ARC600,
ARC601, ARC700 and ARC700-xmac respectively
- -multcost=num
- Replaced by -mmultcost.
ARM Options
These
-m options are defined for the ARM port:
- -mabi=name
- Generate code for the specified ABI. Permissible values
are: apcs-gnu, atpcs, aapcs, aapcs-linux and
iwmmxt.
- -mapcs-frame
- Generate a stack frame that is compliant with the ARM
Procedure Call Standard for all functions, even if this is not strictly
necessary for correct execution of the code. Specifying
-fomit-frame-pointer with this option causes the stack frames not
to be generated for leaf functions. The default is -mno-apcs-frame.
This option is deprecated.
- -mapcs
- This is a synonym for -mapcs-frame and is
deprecated.
- -mthumb-interwork
- Generate code that supports calling between the ARM and
Thumb instruction sets. Without this option, on pre-v5 architectures, the
two instruction sets cannot be reliably used inside one program. The
default is -mno-thumb-interwork, since slightly larger code is
generated when -mthumb-interwork is specified. In AAPCS
configurations this option is meaningless.
- -mno-sched-prolog
- Prevent the reordering of instructions in the function
prologue, or the merging of those instruction with the instructions in the
function's body. This means that all functions start with a recognizable
set of instructions (or in fact one of a choice from a small set of
different function prologues), and this information can be used to locate
the start of functions inside an executable piece of code. The default is
-msched-prolog.
- -mfloat-abi=name
- Specifies which floating-point ABI to use. Permissible
values are: soft, softfp and hard.
Specifying soft causes GCC to generate output containing library
calls for floating-point operations. softfp allows the generation
of code using hardware floating-point instructions, but still uses the
soft-float calling conventions. hard allows generation of
floating-point instructions and uses FPU-specific calling conventions.
The default depends on the specific target configuration. Note that the
hard-float and soft-float ABIs are not link-compatible; you must compile
your entire program with the same ABI, and link with a compatible set of
libraries.
- -mlittle-endian
- Generate code for a processor running in little-endian
mode. This is the default for all standard configurations.
- -mbig-endian
- Generate code for a processor running in big-endian mode;
the default is to compile code for a little-endian processor.
- -march=name
- This specifies the name of the target ARM architecture. GCC
uses this name to determine what kind of instructions it can emit when
generating assembly code. This option can be used in conjunction with or
instead of the -mcpu= option. Permissible names are: armv2,
armv2a, armv3, armv3m, armv4, armv4t,
armv5, armv5t, armv5e, armv5te, armv6,
armv6j, armv6t2, armv6z, armv6zk,
armv6-m, armv7, armv7-a, armv7-r,
armv7-m, armv7e-m, armv7ve, armv8-a,
armv8-a+crc, iwmmxt, iwmmxt2, ep9312.
-march=armv7ve is the armv7-a architecture with virtualization
extensions.
-march=armv8-a+crc enables code generation for the ARMv8-A
architecture together with the optional CRC32 extensions.
-march=native causes the compiler to auto-detect the architecture of
the build computer. At present, this feature is only supported on
GNU/Linux, and not all architectures are recognized. If the auto-detect is
unsuccessful the option has no effect.
- -mtune=name
- This option specifies the name of the target ARM processor
for which GCC should tune the performance of the code. For some ARM
implementations better performance can be obtained by using this option.
Permissible names are: arm2, arm250, arm3,
arm6, arm60, arm600, arm610, arm620,
arm7, arm7m, arm7d, arm7dm, arm7di,
arm7dmi, arm70, arm700, arm700i,
arm710, arm710c, arm7100, arm720,
arm7500, arm7500fe, arm7tdmi, arm7tdmi-s,
arm710t, arm720t, arm740t, strongarm,
strongarm110, strongarm1100, strongarm1110,
arm8, arm810, arm9, arm9e, arm920,
arm920t, arm922t, arm946e-s, arm966e-s,
arm968e-s, arm926ej-s, arm940t, arm9tdmi,
arm10tdmi, arm1020t, arm1026ej-s, arm10e,
arm1020e, arm1022e, arm1136j-s, arm1136jf-s,
mpcore, mpcorenovfp, arm1156t2-s,
arm1156t2f-s, arm1176jz-s, arm1176jzf-s,
cortex-a5, cortex-a7, cortex-a8, cortex-a9,
cortex-a12, cortex-a15, cortex-a53,
cortex-a57, cortex-a72, cortex-r4, cortex-r4f,
cortex-r5, cortex-r7, cortex-m7, cortex-m4,
cortex-m3, cortex-m1, cortex-m0,
cortex-m0plus, cortex-m1.small-multiply,
cortex-m0.small-multiply, cortex-m0plus.small-multiply,
exynos-m1, marvell-pj4, xscale, iwmmxt,
iwmmxt2, ep9312, fa526, fa626, fa606te,
fa626te, fmp626, fa726te, xgene1.
Additionally, this option can specify that GCC should tune the performance
of the code for a big.LITTLE system. Permissible names are:
cortex-a15.cortex-a7, cortex-a57.cortex-a53,
cortex-a72.cortex-a53.
-mtune=generic-arch specifies that GCC should tune the
performance for a blend of processors within architecture arch. The
aim is to generate code that run well on the current most popular
processors, balancing between optimizations that benefit some CPUs in the
range, and avoiding performance pitfalls of other CPUs. The effects of
this option may change in future GCC versions as CPU models come and go.
-mtune=native causes the compiler to auto-detect the CPU of the
build computer. At present, this feature is only supported on GNU/Linux,
and not all architectures are recognized. If the auto-detect is
unsuccessful the option has no effect.
- -mcpu=name
- This specifies the name of the target ARM processor. GCC
uses this name to derive the name of the target ARM architecture (as if
specified by -march) and the ARM processor type for which to tune
for performance (as if specified by -mtune). Where this option is
used in conjunction with -march or -mtune, those options
take precedence over the appropriate part of this option.
Permissible names for this option are the same as those for -mtune.
-mcpu=generic-arch is also permissible, and is equivalent to
-march=arch -mtune=generic-arch. See
-mtune for more information.
-mcpu=native causes the compiler to auto-detect the CPU of the build
computer. At present, this feature is only supported on GNU/Linux, and not
all architectures are recognized. If the auto-detect is unsuccessful the
option has no effect.
- -mfpu=name
- This specifies what floating-point hardware (or hardware
emulation) is available on the target. Permissible names are: vfp,
vfpv3, vfpv3-fp16, vfpv3-d16, vfpv3-d16-fp16,
vfpv3xd, vfpv3xd-fp16, neon, neon-fp16,
vfpv4, vfpv4-d16, fpv4-sp-d16, neon-vfpv4,
fpv5-d16, fpv5-sp-d16, fp-armv8,
neon-fp-armv8, and crypto-neon-fp-armv8.
If -msoft-float is specified this specifies the format of
floating-point values.
If the selected floating-point hardware includes the NEON extension (e.g.
-mfpu=neon), note that floating-point operations are not
generated by GCC's auto-vectorization pass unless
-funsafe-math-optimizations is also specified. This is because NEON
hardware does not fully implement the IEEE 754 standard for floating-point
arithmetic (in particular denormal values are treated as zero), so the use
of NEON instructions may lead to a loss of precision.
- -mfp16-format=name
- Specify the format of the "__fp16" half-precision
floating-point type. Permissible names are none, ieee, and
alternative; the default is none, in which case the
"__fp16" type is not defined.
- -mstructure-size-boundary=n
- The sizes of all structures and unions are rounded up to a
multiple of the number of bits set by this option. Permissible values are
8, 32 and 64. The default value varies for different toolchains. For the
COFF targeted toolchain the default value is 8. A value of 64 is only
allowed if the underlying ABI supports it.
Specifying a larger number can produce faster, more efficient code, but can
also increase the size of the program. Different values are potentially
incompatible. Code compiled with one value cannot necessarily expect to
work with code or libraries compiled with another value, if they exchange
information using structures or unions.
- -mabort-on-noreturn
- Generate a call to the function "abort" at the
end of a "noreturn" function. It is executed if the function
tries to return.
- -mlong-calls
- -mno-long-calls
- Tells the compiler to perform function calls by first
loading the address of the function into a register and then performing a
subroutine call on this register. This switch is needed if the target
function lies outside of the 64-megabyte addressing range of the
offset-based version of subroutine call instruction.
Even if this switch is enabled, not all function calls are turned into long
calls. The heuristic is that static functions, functions that have the
"short_call" attribute, functions that are inside the scope of a
"#pragma no_long_calls" directive, and functions whose
definitions have already been compiled within the current compilation unit
are not turned into long calls. The exceptions to this rule are that weak
function definitions, functions with the "long_call" attribute
or the "section" attribute, and functions that are within the
scope of a "#pragma long_calls" directive are always turned into
long calls.
This feature is not enabled by default. Specifying -mno-long-calls
restores the default behavior, as does placing the function calls within
the scope of a "#pragma long_calls_off" directive. Note these
switches have no effect on how the compiler generates code to handle
function calls via function pointers.
- -msingle-pic-base
- Treat the register used for PIC addressing as read-only,
rather than loading it in the prologue for each function. The runtime
system is responsible for initializing this register with an appropriate
value before execution begins.
- -mpic-register=reg
- Specify the register to be used for PIC addressing. For
standard PIC base case, the default is any suitable register determined by
compiler. For single PIC base case, the default is R9 if target is
EABI based or stack-checking is enabled, otherwise the default is
R10.
- -mpic-data-is-text-relative
- Assume that each data segments are relative to text segment
at load time. Therefore, it permits addressing data using PC-relative
operations. This option is on by default for targets other than VxWorks
RTP.
- -mpoke-function-name
- Write the name of each function into the text section,
directly preceding the function prologue. The generated code is similar to
this:
t0
.ascii "arm_poke_function_name", 0
.align
t1
.word 0xff000000 + (t1 - t0)
arm_poke_function_name
mov ip, sp
stmfd sp!, {fp, ip, lr, pc}
sub fp, ip, #4
When performing a stack backtrace, code can inspect the value of
"pc" stored at "fp + 0". If the trace function then
looks at location "pc - 12" and the top 8 bits are set, then we
know that there is a function name embedded immediately preceding this
location and has length "((pc[-3]) & 0xff000000)".
- -mthumb
- -marm
- Select between generating code that executes in ARM and
Thumb states. The default for most configurations is to generate code that
executes in ARM state, but the default can be changed by configuring GCC
with the --with-mode=state configure option.
- -mtpcs-frame
- Generate a stack frame that is compliant with the Thumb
Procedure Call Standard for all non-leaf functions. (A leaf function is
one that does not call any other functions.) The default is
-mno-tpcs-frame.
- -mtpcs-leaf-frame
- Generate a stack frame that is compliant with the Thumb
Procedure Call Standard for all leaf functions. (A leaf function is one
that does not call any other functions.) The default is
-mno-apcs-leaf-frame.
- -mcallee-super-interworking
- Gives all externally visible functions in the file being
compiled an ARM instruction set header which switches to Thumb mode before
executing the rest of the function. This allows these functions to be
called from non-interworking code. This option is not valid in AAPCS
configurations because interworking is enabled by default.
- -mcaller-super-interworking
- Allows calls via function pointers (including virtual
functions) to execute correctly regardless of whether the target code has
been compiled for interworking or not. There is a small overhead in the
cost of executing a function pointer if this option is enabled. This
option is not valid in AAPCS configurations because interworking is
enabled by default.
- -mtp=name
- Specify the access model for the thread local storage
pointer. The valid models are soft, which generates calls to
"__aeabi_read_tp", cp15, which fetches the thread pointer
from "cp15" directly (supported in the arm6k architecture), and
auto, which uses the best available method for the selected
processor. The default setting is auto.
- -mtls-dialect=dialect
- Specify the dialect to use for accessing thread local
storage. Two dialects are supported---gnu and gnu2.
The gnu dialect selects the original GNU scheme for supporting
local and global dynamic TLS models. The gnu2 dialect selects the
GNU descriptor scheme, which provides better performance for shared
libraries. The GNU descriptor scheme is compatible with the original
scheme, but does require new assembler, linker and library support.
Initial and local exec TLS models are unaffected by this option and always
use the original scheme.
- -mword-relocations
- Only generate absolute relocations on word-sized values
(i.e. R_ARM_ABS32). This is enabled by default on targets (uClinux,
SymbianOS) where the runtime loader imposes this restriction, and when
-fpic or -fPIC is specified.
- -mfix-cortex-m3-ldrd
- Some Cortex-M3 cores can cause data corruption when
"ldrd" instructions with overlapping destination and base
registers are used. This option avoids generating these instructions. This
option is enabled by default when -mcpu=cortex-m3 is
specified.
- -munaligned-access
- -mno-unaligned-access
- Enables (or disables) reading and writing of 16- and 32-
bit values from addresses that are not 16- or 32- bit aligned. By default
unaligned access is disabled for all pre-ARMv6 and all ARMv6-M
architectures, and enabled for all other architectures. If unaligned
access is not enabled then words in packed data structures are accessed a
byte at a time.
The ARM attribute "Tag_CPU_unaligned_access" is set in the
generated object file to either true or false, depending upon the setting
of this option. If unaligned access is enabled then the preprocessor
symbol "__ARM_FEATURE_UNALIGNED" is also defined.
- -mneon-for-64bits
- Enables using Neon to handle scalar 64-bits operations.
This is disabled by default since the cost of moving data from core
registers to Neon is high.
- -mslow-flash-data
- Assume loading data from flash is slower than fetching
instruction. Therefore literal load is minimized for better performance.
This option is only supported when compiling for ARMv7 M-profile and off
by default.
- -masm-syntax-unified
- Assume inline assembler is using unified asm syntax. The
default is currently off which implies divided syntax. Currently this
option is available only for Thumb1 and has no effect on ARM state and
Thumb2. However, this may change in future releases of GCC. Divided syntax
should be considered deprecated.
- -mrestrict-it
- Restricts generation of IT blocks to conform to the rules
of ARMv8. IT blocks can only contain a single 16-bit instruction from a
select set of instructions. This option is on by default for ARMv8 Thumb
mode.
- -mprint-tune-info
- Print CPU tuning information as comment in assembler file.
This is an option used only for regression testing of the compiler and not
intended for ordinary use in compiling code. This option is disabled by
default.
AVR Options
These options are defined for AVR implementations:
- -mmcu=mcu
- Specify Atmel AVR instruction set architectures (ISA) or
MCU type.
The default for this option is@tie{} avr2.
GCC supports the following AVR devices and ISAs:
- "avr2"
- "Classic" devices with up to 8@tie{}KiB of
program memory. mcu@tie{}= "attiny22",
"attiny26", "at90c8534", "at90s2313",
"at90s2323", "at90s2333", "at90s2343",
"at90s4414", "at90s4433", "at90s4434",
"at90s8515", "at90s8535".
- "avr25"
- "Classic" devices with up to 8@tie{}KiB of
program memory and with the "MOVW" instruction.
mcu@tie{}= "ata5272", "ata6616c",
"attiny13", "attiny13a", "attiny2313",
"attiny2313a", "attiny24", "attiny24a",
"attiny25", "attiny261", "attiny261a",
"attiny43u", "attiny4313", "attiny44",
"attiny44a", "attiny441", "attiny45",
"attiny461", "attiny461a", "attiny48",
"attiny828", "attiny84", "attiny84a",
"attiny841", "attiny85", "attiny861",
"attiny861a", "attiny87", "attiny88",
"at86rf401".
- "avr3"
- "Classic" devices with 16@tie{}KiB up to
64@tie{}KiB of program memory. mcu@tie{}= "at43usb355",
"at76c711".
- "avr31"
- "Classic" devices with 128@tie{}KiB of program
memory. mcu@tie{}= "atmega103",
"at43usb320".
- "avr35"
- "Classic" devices with 16@tie{}KiB up to
64@tie{}KiB of program memory and with the "MOVW" instruction.
mcu@tie{}= "ata5505", "ata6617c",
"ata664251", "atmega16u2", "atmega32u2",
"atmega8u2", "attiny1634", "attiny167",
"at90usb162", "at90usb82".
- "avr4"
- "Enhanced" devices with up to 8@tie{}KiB of
program memory. mcu@tie{}= "ata6285",
"ata6286", "ata6289", "ata6612c",
"atmega48", "atmega48a", "atmega48p",
"atmega48pa", "atmega8", "atmega8a",
"atmega8hva", "atmega8515", "atmega8535",
"atmega88", "atmega88a", "atmega88p",
"atmega88pa", "at90pwm1", "at90pwm2",
"at90pwm2b", "at90pwm3", "at90pwm3b",
"at90pwm81".
- "avr5"
- "Enhanced" devices with 16@tie{}KiB up to
64@tie{}KiB of program memory. mcu@tie{}= "ata5702m322",
"ata5782", "ata5790", "ata5790n",
"ata5795", "ata5831", "ata6613c",
"ata6614q", "atmega16", "atmega16a",
"atmega16hva", "atmega16hva2",
"atmega16hvb", "atmega16hvbrevb",
"atmega16m1", "atmega16u4", "atmega161",
"atmega162", "atmega163", "atmega164a",
"atmega164p", "atmega164pa", "atmega165",
"atmega165a", "atmega165p", "atmega165pa",
"atmega168", "atmega168a", "atmega168p",
"atmega168pa", "atmega169", "atmega169a",
"atmega169p", "atmega169pa", "atmega32",
"atmega32a", "atmega32c1", "atmega32hvb",
"atmega32hvbrevb", "atmega32m1",
"atmega32u4", "atmega32u6", "atmega323",
"atmega324a", "atmega324p", "atmega324pa",
"atmega325", "atmega325a", "atmega325p",
"atmega325pa", "atmega3250", "atmega3250a",
"atmega3250p", "atmega3250pa", "atmega328",
"atmega328p", "atmega329", "atmega329a",
"atmega329p", "atmega329pa", "atmega3290",
"atmega3290a", "atmega3290p",
"atmega3290pa", "atmega406", "atmega64",
"atmega64a", "atmega64c1", "atmega64hve",
"atmega64hve2", "atmega64m1",
"atmega64rfr2", "atmega640", "atmega644",
"atmega644a", "atmega644p", "atmega644pa",
"atmega644rfr2", "atmega645", "atmega645a",
"atmega645p", "atmega6450", "atmega6450a",
"atmega6450p", "atmega649", "atmega649a",
"atmega649p", "atmega6490", "atmega6490a",
"atmega6490p", "at90can32", "at90can64",
"at90pwm161", "at90pwm216", "at90pwm316",
"at90scr100", "at90usb646", "at90usb647",
"at94k", "m3000".
- "avr51"
- "Enhanced" devices with 128@tie{}KiB of program
memory. mcu@tie{}= "atmega128", "atmega128a",
"atmega128rfa1", "atmega128rfr2",
"atmega1280", "atmega1281", "atmega1284",
"atmega1284p", "atmega1284rfr2",
"at90can128", "at90usb1286",
"at90usb1287".
- "avr6"
- "Enhanced" devices with 3-byte PC, i.e. with more
than 128@tie{}KiB of program memory. mcu@tie{}=
"atmega256rfr2", "atmega2560", "atmega2561",
"atmega2564rfr2".
- "avrxmega2"
- "XMEGA" devices with more than 8@tie{}KiB and up
to 64@tie{}KiB of program memory. mcu@tie{}=
"atxmega16a4", "atxmega16a4u",
"atxmega16c4", "atxmega16d4", "atxmega16e5",
"atxmega32a4", "atxmega32a4u",
"atxmega32c3", "atxmega32c4", "atxmega32d3",
"atxmega32d4", "atxmega32e5",
"atxmega8e5".
- "avrxmega4"
- "XMEGA" devices with more than 64@tie{}KiB and up
to 128@tie{}KiB of program memory. mcu@tie{}=
"atxmega64a3", "atxmega64a3u",
"atxmega64a4u", "atxmega64b1",
"atxmega64b3", "atxmega64c3", "atxmega64d3",
"atxmega64d4".
- "avrxmega5"
- "XMEGA" devices with more than 64@tie{}KiB and up
to 128@tie{}KiB of program memory and more than 64@tie{}KiB of RAM.
mcu@tie{}= "atxmega64a1", "atxmega64a1u".
- "avrxmega6"
- "XMEGA" devices with more than 128@tie{}KiB of
program memory. mcu@tie{}= "atxmega128a3",
"atxmega128a3u", "atxmega128b1",
"atxmega128b3", "atxmega128c3",
"atxmega128d3", "atxmega128d4",
"atxmega192a3", "atxmega192a3u",
"atxmega192c3", "atxmega192d3",
"atxmega256a3", "atxmega256a3b",
"atxmega256a3bu", "atxmega256a3u",
"atxmega256c3", "atxmega256d3",
"atxmega384c3", "atxmega384d3".
- "avrxmega7"
- "XMEGA" devices with more than 128@tie{}KiB of
program memory and more than 64@tie{}KiB of RAM. mcu@tie{}=
"atxmega128a1", "atxmega128a1u",
"atxmega128a4u".
- "avrtiny"
- "TINY" Tiny core devices with 512@tie{}B up to
4@tie{}KiB of program memory. mcu@tie{}= "attiny10",
"attiny20", "attiny4", "attiny40",
"attiny5", "attiny9".
- "avr1"
- This ISA is implemented by the minimal AVR core and
supported for assembler only. mcu@tie{}= "attiny11",
"attiny12", "attiny15", "attiny28",
"at90s1200".
- -maccumulate-args
- Accumulate outgoing function arguments and acquire/release
the needed stack space for outgoing function arguments once in function
prologue/epilogue. Without this option, outgoing arguments are pushed
before calling a function and popped afterwards.
Popping the arguments after the function call can be expensive on AVR so
that accumulating the stack space might lead to smaller executables
because arguments need not to be removed from the stack after such a
function call.
This option can lead to reduced code size for functions that perform several
calls to functions that get their arguments on the stack like calls to
printf-like functions.
- -mbranch-cost=cost
- Set the branch costs for conditional branch instructions to
cost. Reasonable values for cost are small, non-negative
integers. The default branch cost is 0.
- -mcall-prologues
- Functions prologues/epilogues are expanded as calls to
appropriate subroutines. Code size is smaller.
- -mint8
- Assume "int" to be 8-bit integer. This affects
the sizes of all types: a "char" is 1 byte, an "int"
is 1 byte, a "long" is 2 bytes, and "long long" is 4
bytes. Please note that this option does not conform to the C standards,
but it results in smaller code size.
- -mn-flash=num
- Assume that the flash memory has a size of num times
64@tie{}KiB.
- -mno-interrupts
- Generated code is not compatible with hardware interrupts.
Code size is smaller.
- -mrelax
- Try to replace "CALL" resp. "JMP"
instruction by the shorter "RCALL" resp. "RJMP"
instruction if applicable. Setting -mrelax just adds the
--mlink-relax option to the assembler's command line and the
--relax option to the linker's command line.
Jump relaxing is performed by the linker because jump offsets are not known
before code is located. Therefore, the assembler code generated by the
compiler is the same, but the instructions in the executable may differ
from instructions in the assembler code.
Relaxing must be turned on if linker stubs are needed, see the section on
"EIND" and linker stubs below.
- -mrmw
- Assume that the device supports the Read-Modify-Write
instructions "XCH", "LAC", "LAS" and
"LAT".
- -msp8
- Treat the stack pointer register as an 8-bit register, i.e.
assume the high byte of the stack pointer is zero. In general, you don't
need to set this option by hand.
This option is used internally by the compiler to select and build multilibs
for architectures "avr2" and "avr25". These
architectures mix devices with and without "SPH". For any
setting other than -mmcu=avr2 or -mmcu=avr25 the compiler
driver adds or removes this option from the compiler proper's command
line, because the compiler then knows if the device or architecture has an
8-bit stack pointer and thus no "SPH" register or not.
- -mstrict-X
- Use address register "X" in a way proposed by the
hardware. This means that "X" is only used in indirect,
post-increment or pre-decrement addressing.
Without this option, the "X" register may be used in the same way
as "Y" or "Z" which then is emulated by additional
instructions. For example, loading a value with "X+const"
addressing with a small non-negative "const < 64" to a
register Rn is performed as
adiw r26, const ; X += const
ld <Rn>, X ; <Rn> = *X
sbiw r26, const ; X -= const
- -mtiny-stack
- Only change the lower 8@tie{}bits of the stack
pointer.
- -nodevicelib
- Don't link against AVR-LibC's device specific library
"libdev.a".
- -Waddr-space-convert
- Warn about conversions between address spaces in the case
where the resulting address space is not contained in the incoming address
space.
"EIND" and Devices with More Than 128 Ki Bytes of Flash
Pointers in the implementation are 16@tie{}bits wide. The address of a function
or label is represented as word address so that indirect jumps and calls can
target any code address in the range of 64@tie{}Ki words.
In order to facilitate indirect jump on devices with more than 128@tie{}Ki bytes
of program memory space, there is a special function register called
"EIND" that serves as most significant part of the target address
when "EICALL" or "EIJMP" instructions are used.
Indirect jumps and calls on these devices are handled as follows by the compiler
and are subject to some limitations:
- *
- The compiler never sets "EIND".
- *
- The compiler uses "EIND" implicitely in
"EICALL"/"EIJMP" instructions or might read
"EIND" directly in order to emulate an indirect call/jump by
means of a "RET" instruction.
- *
- The compiler assumes that "EIND" never changes
during the startup code or during the application. In particular,
"EIND" is not saved/restored in function or interrupt service
routine prologue/epilogue.
- *
- For indirect calls to functions and computed goto, the
linker generates stubs. Stubs are jump pads sometimes also called
trampolines. Thus, the indirect call/jump jumps to such a stub. The
stub contains a direct jump to the desired address.
- *
- Linker relaxation must be turned on so that the linker
generates the stubs correctly in all situations. See the compiler option
-mrelax and the linker option --relax. There are corner
cases where the linker is supposed to generate stubs but aborts without
relaxation and without a helpful error message.
- *
- The default linker script is arranged for code with
"EIND = 0". If code is supposed to work for a setup with
"EIND != 0", a custom linker script has to be used in order to
place the sections whose name start with ".trampolines" into the
segment where "EIND" points to.
- *
- The startup code from libgcc never sets "EIND".
Notice that startup code is a blend of code from libgcc and AVR-LibC. For
the impact of AVR-LibC on "EIND", see the
AVR-LibC user manual
("http://nongnu.org/avr-libc/user-manual/").
- *
- It is legitimate for user-specific startup code to set up
"EIND" early, for example by means of initialization code
located in section ".init3". Such code runs prior to general
startup code that initializes RAM and calls constructors, but after the
bit of startup code from AVR-LibC that sets "EIND" to the
segment where the vector table is located.
#include <avr/io.h>
static void
__attribute__((section(".init3"),naked,used,no_instrument_function))
init3_set_eind (void)
{
__asm volatile ("ldi r24,pm_hh8(__trampolines_start)\n\t"
"out %i0,r24" :: "n" (&EIND) : "r24","memory");
}
The "__trampolines_start" symbol is defined in the linker
script.
- *
- Stubs are generated automatically by the linker if the
following two conditions are met:
- -<The address of a label is taken by means of the
"gs" modifier>
- (short for generate stubs) like so:
LDI r24, lo8(gs(<func>))
LDI r25, hi8(gs(<func>))
- -<The final location of that label is in a code
segment>
- outside the segment where the stubs are
located.
- *
- The compiler emits such "gs" modifiers for code
labels in the following situations:
- -<Taking address of a function or code label.>
- -<Computed goto.>
- -<If prologue-save function is used, see
-mcall-prologues>
- command-line option.
- -<Switch/case dispatch tables. If you do not want such
dispatch>
- tables you can specify the -fno-jump-tables
command-line option.
- -<C and C++ constructors/destructors called during
startup/shutdown.>
- -<If the tools hit a "gs()" modifier explained
above.>
- *
- Jumping to non-symbolic addresses like so is not
supported:
int main (void)
{
/* Call function at word address 0x2 */
return ((int(*)(void)) 0x2)();
}
Instead, a stub has to be set up, i.e. the function has to be called through
a symbol ("func_4" in the example):
int main (void)
{
extern int func_4 (void);
/* Call function at byte address 0x4 */
return func_4();
}
and the application be linked with -Wl,--defsym,func_4=0x4.
Alternatively, "func_4" can be defined in the linker
script.
Handling of the "RAMPD", "RAMPX", "RAMPY" and
"RAMPZ" Special Function Registers
Some AVR devices support memories larger than the 64@tie{}KiB range that can be
accessed with 16-bit pointers. To access memory locations outside this
64@tie{}KiB range, the contentent of a "RAMP" register is used as
high part of the address: The "X", "Y", "Z"
address register is concatenated with the "RAMPX",
"RAMPY", "RAMPZ" special function register, respectively,
to get a wide address. Similarly, "RAMPD" is used together with
direct addressing.
- *
- The startup code initializes the "RAMP" special
function registers with zero.
- *
- If a AVR Named Address Spaces,named address space
other than generic or "__flash" is used, then "RAMPZ"
is set as needed before the operation.
- *
- If the device supports RAM larger than 64@tie{}KiB and the
compiler needs to change "RAMPZ" to accomplish an operation,
"RAMPZ" is reset to zero after the operation.
- *
- If the device comes with a specific "RAMP"
register, the ISR prologue/epilogue saves/restores that SFR and
initializes it with zero in case the ISR code might (implicitly) use
it.
- *
- RAM larger than 64@tie{}KiB is not supported by GCC for AVR
targets. If you use inline assembler to read from locations outside the
16-bit address range and change one of the "RAMP" registers, you
must reset it to zero after the access.
AVR Built-in Macros
GCC defines several built-in macros so that the user code can test for the
presence or absence of features. Almost any of the following built-in macros
are deduced from device capabilities and thus triggered by the
-mmcu=
command-line option.
For even more AVR-specific built-in macros see
AVR Named Address Spaces
and
AVR Built-in Functions.
- "__AVR_ARCH__"
- Build-in macro that resolves to a decimal number that
identifies the architecture and depends on the -mmcu=mcu
option. Possible values are:
2, 25, 3, 31, 35, 4, 5, 51, 6
for mcu="avr2", "avr25", "avr3",
"avr31", "avr35", "avr4", "avr5",
"avr51", "avr6",
respectively and
100, 102, 104, 105, 106, 107
for mcu="avrtiny", "avrxmega2",
"avrxmega4", "avrxmega5", "avrxmega6",
"avrxmega7", respectively. If mcu specifies a device,
this built-in macro is set accordingly. For example, with
-mmcu=atmega8 the macro is defined to 4.
- "__AVR_Device__"
- Setting -mmcu=device defines this built-in
macro which reflects the device's name. For example, -mmcu=atmega8
defines the built-in macro "__AVR_ATmega8__",
-mmcu=attiny261a defines "__AVR_ATtiny261A__", etc.
The built-in macros' names follow the scheme "__AVR_
Device__" where Device is the device name as from the
AVR user manual. The difference between Device in the built-in
macro and device in -mmcu=device is that the latter
is always lowercase.
If device is not a device but only a core architecture like
avr51, this macro is not defined.
- "__AVR_DEVICE_NAME__"
- Setting -mmcu=device defines this built-in
macro to the device's name. For example, with -mmcu=atmega8 the
macro is defined to "atmega8".
If device is not a device but only a core architecture like
avr51, this macro is not defined.
- "__AVR_XMEGA__"
- The device / architecture belongs to the XMEGA family of
devices.
- "__AVR_HAVE_ELPM__"
- The device has the the "ELPM" instruction.
- "__AVR_HAVE_ELPMX__"
- The device has the "ELPM Rn,Z" and
"ELPM R n,Z+" instructions.
- "__AVR_HAVE_MOVW__"
- The device has the "MOVW" instruction to perform
16-bit register-register moves.
- "__AVR_HAVE_LPMX__"
- The device has the "LPM Rn,Z" and
"LPM R n,Z+" instructions.
- "__AVR_HAVE_MUL__"
- The device has a hardware multiplier.
- "__AVR_HAVE_JMP_CALL__"
- The device has the "JMP" and "CALL"
instructions. This is the case for devices with at least 16@tie{}KiB of
program memory.
- "__AVR_HAVE_EIJMP_EICALL__"
- "__AVR_3_BYTE_PC__"
- The device has the "EIJMP" and "EICALL"
instructions. This is the case for devices with more than 128@tie{}KiB of
program memory. This also means that the program counter (PC) is
3@tie{}bytes wide.
- "__AVR_2_BYTE_PC__"
- The program counter (PC) is 2@tie{}bytes wide. This is the
case for devices with up to 128@tie{}KiB of program memory.
- "__AVR_HAVE_8BIT_SP__"
- "__AVR_HAVE_16BIT_SP__"
- The stack pointer (SP) register is treated as 8-bit
respectively 16-bit register by the compiler. The definition of these
macros is affected by -mtiny-stack.
- "__AVR_HAVE_SPH__"
- "__AVR_SP8__"
- The device has the SPH (high part of stack pointer) special
function register or has an 8-bit stack pointer, respectively. The
definition of these macros is affected by -mmcu= and in the cases
of -mmcu=avr2 and -mmcu=avr25 also by -msp8.
- "__AVR_HAVE_RAMPD__"
- "__AVR_HAVE_RAMPX__"
- "__AVR_HAVE_RAMPY__"
- "__AVR_HAVE_RAMPZ__"
- The device has the "RAMPD", "RAMPX",
"RAMPY", "RAMPZ" special function register,
respectively.
- "__NO_INTERRUPTS__"
- This macro reflects the -mno-interrupts command-line
option.
- "__AVR_ERRATA_SKIP__"
- "__AVR_ERRATA_SKIP_JMP_CALL__"
- Some AVR devices (AT90S8515, ATmega103) must not skip
32-bit instructions because of a hardware erratum. Skip instructions are
"SBRS", "SBRC", "SBIS", "SBIC" and
"CPSE". The second macro is only defined if
"__AVR_HAVE_JMP_CALL__" is also set.
- "__AVR_ISA_RMW__"
- The device has Read-Modify-Write instructions (XCH, LAC,
LAS and LAT).
- "__AVR_SFR_OFFSET__=offset"
- Instructions that can address I/O special function
registers directly like "IN", "OUT", "SBI",
etc. may use a different address as if addressed by an instruction to
access RAM like "LD" or "STS". This offset depends on
the device architecture and has to be subtracted from the RAM address in
order to get the respective I/O@tie{}address.
- "__WITH_AVRLIBC__"
- The compiler is configured to be used together with
AVR-Libc. See the --with-avrlibc configure option.
Blackfin Options
- -mcpu=cpu[-sirevision]
- Specifies the name of the target Blackfin processor.
Currently, cpu can be one of bf512, bf514,
bf516, bf518, bf522, bf523, bf524,
bf525, bf526, bf527, bf531, bf532,
bf533, bf534, bf536, bf537, bf538,
bf539, bf542, bf544, bf547, bf548,
bf549, bf542m, bf544m, bf547m, bf548m,
bf549m, bf561, bf592.
The optional sirevision specifies the silicon revision of the target
Blackfin processor. Any workarounds available for the targeted silicon
revision are enabled. If sirevision is none, no workarounds
are enabled. If sirevision is any, all workarounds for the
targeted processor are enabled. The "__SILICON_REVISION__" macro
is defined to two hexadecimal digits representing the major and minor
numbers in the silicon revision. If sirevision is none, the
"__SILICON_REVISION__" is not defined. If sirevision is
any, the "__SILICON_REVISION__" is defined to be 0xffff.
If this optional sirevision is not used, GCC assumes the latest
known silicon revision of the targeted Blackfin processor.
GCC defines a preprocessor macro for the specified cpu. For the
bfin-elf toolchain, this option causes the hardware BSP provided by
libgloss to be linked in if -msim is not given.
Without this option, bf532 is used as the processor by default.
Note that support for bf561 is incomplete. For bf561, only the
preprocessor macro is defined.
- -msim
- Specifies that the program will be run on the simulator.
This causes the simulator BSP provided by libgloss to be linked in. This
option has effect only for bfin-elf toolchain. Certain other
options, such as -mid-shared-library and -mfdpic, imply
-msim.
- -momit-leaf-frame-pointer
- Don't keep the frame pointer in a register for leaf
functions. This avoids the instructions to save, set up and restore frame
pointers and makes an extra register available in leaf functions. The
option -fomit-frame-pointer removes the frame pointer for all
functions, which might make debugging harder.
- -mspecld-anomaly
- When enabled, the compiler ensures that the generated code
does not contain speculative loads after jump instructions. If this option
is used, "__WORKAROUND_SPECULATIVE_LOADS" is defined.
- -mno-specld-anomaly
- Don't generate extra code to prevent speculative loads from
occurring.
- -mcsync-anomaly
- When enabled, the compiler ensures that the generated code
does not contain CSYNC or SSYNC instructions too soon after conditional
branches. If this option is used,
"__WORKAROUND_SPECULATIVE_SYNCS" is defined.
- -mno-csync-anomaly
- Don't generate extra code to prevent CSYNC or SSYNC
instructions from occurring too soon after a conditional branch.
- -mlow-64k
- When enabled, the compiler is free to take advantage of the
knowledge that the entire program fits into the low 64k of memory.
- -mno-low-64k
- Assume that the program is arbitrarily large. This is the
default.
- -mstack-check-l1
- Do stack checking using information placed into L1
scratchpad memory by the uClinux kernel.
- -mid-shared-library
- Generate code that supports shared libraries via the
library ID method. This allows for execute in place and shared libraries
in an environment without virtual memory management. This option implies
-fPIC. With a bfin-elf target, this option implies
-msim.
- -mno-id-shared-library
- Generate code that doesn't assume ID-based shared libraries
are being used. This is the default.
- -mleaf-id-shared-library
- Generate code that supports shared libraries via the
library ID method, but assumes that this library or executable won't link
against any other ID shared libraries. That allows the compiler to use
faster code for jumps and calls.
- -mno-leaf-id-shared-library
- Do not assume that the code being compiled won't link
against any ID shared libraries. Slower code is generated for jump and
call insns.
- -mshared-library-id=n
- Specifies the identification number of the ID-based shared
library being compiled. Specifying a value of 0 generates more compact
code; specifying other values forces the allocation of that number to the
current library but is no more space- or time-efficient than omitting this
option.
- -msep-data
- Generate code that allows the data segment to be located in
a different area of memory from the text segment. This allows for execute
in place in an environment without virtual memory management by
eliminating relocations against the text section.
- -mno-sep-data
- Generate code that assumes that the data segment follows
the text segment. This is the default.
- -mlong-calls
- -mno-long-calls
- Tells the compiler to perform function calls by first
loading the address of the function into a register and then performing a
subroutine call on this register. This switch is needed if the target
function lies outside of the 24-bit addressing range of the offset-based
version of subroutine call instruction.
This feature is not enabled by default. Specifying -mno-long-calls
restores the default behavior. Note these switches have no effect on how
the compiler generates code to handle function calls via function
pointers.
- -mfast-fp
- Link with the fast floating-point library. This library
relaxes some of the IEEE floating-point standard's rules for checking
inputs against Not-a-Number (NAN), in the interest of performance.
- -minline-plt
- Enable inlining of PLT entries in function calls to
functions that are not known to bind locally. It has no effect without
-mfdpic.
- -mmulticore
- Build a standalone application for multicore Blackfin
processors. This option causes proper start files and link scripts
supporting multicore to be used, and defines the macro
"__BFIN_MULTICORE". It can only be used with
-mcpu=bf561[-sirevision].
This option can be used with -mcorea or -mcoreb, which selects
the one-application-per-core programming model. Without -mcorea or
-mcoreb, the single-application/dual-core programming model is
used. In this model, the main function of Core B should be named as
"coreb_main".
If this option is not used, the single-core application programming model is
used.
- -mcorea
- Build a standalone application for Core A of BF561 when
using the one-application-per-core programming model. Proper start files
and link scripts are used to support Core A, and the macro
"__BFIN_COREA" is defined. This option can only be used in
conjunction with -mmulticore.
- -mcoreb
- Build a standalone application for Core B of BF561 when
using the one-application-per-core programming model. Proper start files
and link scripts are used to support Core B, and the macro
"__BFIN_COREB" is defined. When this option is used,
"coreb_main" should be used instead of "main". This
option can only be used in conjunction with -mmulticore.
- -msdram
- Build a standalone application for SDRAM. Proper start
files and link scripts are used to put the application into SDRAM, and the
macro "__BFIN_SDRAM" is defined. The loader should initialize
SDRAM before loading the application.
- -micplb
- Assume that ICPLBs are enabled at run time. This has an
effect on certain anomaly workarounds. For Linux targets, the default is
to assume ICPLBs are enabled; for standalone applications the default is
off.
C6X Options
- -march=name
- This specifies the name of the target architecture. GCC
uses this name to determine what kind of instructions it can emit when
generating assembly code. Permissible names are: c62x, c64x,
c64x+, c67x, c67x+, c674x.
- -mbig-endian
- Generate code for a big-endian target.
- -mlittle-endian
- Generate code for a little-endian target. This is the
default.
- -msim
- Choose startup files and linker script suitable for the
simulator.
- -msdata=default
- Put small global and static data in the
".neardata" section, which is pointed to by register
"B14". Put small uninitialized global and static data in the
".bss" section, which is adjacent to the ".neardata"
section. Put small read-only data into the ".rodata" section.
The corresponding sections used for large pieces of data are
".fardata", ".far" and ".const".
- -msdata=all
- Put all data, not just small objects, into the sections
reserved for small data, and use addressing relative to the
"B14" register to access them.
- -msdata=none
- Make no use of the sections reserved for small data, and
use absolute addresses to access all data. Put all initialized global and
static data in the ".fardata" section, and all uninitialized
data in the ".far" section. Put all constant data into the
".const" section.
CRIS Options
These options are defined specifically for the CRIS ports.
- -march=architecture-type
- -mcpu=architecture-type
- Generate code for the specified architecture. The choices
for architecture-type are v3, v8 and v10 for
respectively ETRAX 4, ETRAX 100, and ETRAX 100 LX.
Default is v0 except for cris-axis-linux-gnu, where the default is
v10.
- -mtune=architecture-type
- Tune to architecture-type everything applicable
about the generated code, except for the ABI and the set of available
instructions. The choices for architecture-type are the same as for
-march=architecture-type.
- -mmax-stack-frame=n
- Warn when the stack frame of a function exceeds n
bytes.
- -metrax4
- -metrax100
- The options -metrax4 and -metrax100 are
synonyms for -march=v3 and -march=v8 respectively.
- -mmul-bug-workaround
- -mno-mul-bug-workaround
- Work around a bug in the "muls" and
"mulu" instructions for CPU models where it applies. This option
is active by default.
- -mpdebug
- Enable CRIS-specific verbose debug-related information in
the assembly code. This option also has the effect of turning off the
#NO_APP formatted-code indicator to the assembler at the beginning
of the assembly file.
- -mcc-init
- Do not use condition-code results from previous
instruction; always emit compare and test instructions before use of
condition codes.
- -mno-side-effects
- Do not emit instructions with side effects in addressing
modes other than post-increment.
- -mstack-align
- -mno-stack-align
- -mdata-align
- -mno-data-align
- -mconst-align
- -mno-const-align
- These options (no- options) arrange (eliminate
arrangements) for the stack frame, individual data and constants to be
aligned for the maximum single data access size for the chosen CPU model.
The default is to arrange for 32-bit alignment. ABI details such as
structure layout are not affected by these options.
- -m32-bit
- -m16-bit
- -m8-bit
- Similar to the stack- data- and const-align options above,
these options arrange for stack frame, writable data and constants to all
be 32-bit, 16-bit or 8-bit aligned. The default is 32-bit alignment.
- -mno-prologue-epilogue
- -mprologue-epilogue
- With -mno-prologue-epilogue, the normal function
prologue and epilogue which set up the stack frame are omitted and no
return instructions or return sequences are generated in the code. Use
this option only together with visual inspection of the compiled code: no
warnings or errors are generated when call-saved registers must be saved,
or storage for local variables needs to be allocated.
- -mno-gotplt
- -mgotplt
- With -fpic and -fPIC, don't generate (do
generate) instruction sequences that load addresses for functions from the
PLT part of the GOT rather than (traditional on other architectures) calls
to the PLT. The default is -mgotplt.
- -melf
- Legacy no-op option only recognized with the cris-axis-elf
and cris-axis-linux-gnu targets.
- -mlinux
- Legacy no-op option only recognized with the
cris-axis-linux-gnu target.
- -sim
- This option, recognized for the cris-axis-elf, arranges to
link with input-output functions from a simulator library. Code,
initialized data and zero-initialized data are allocated
consecutively.
- -sim2
- Like -sim, but pass linker options to locate
initialized data at 0x40000000 and zero-initialized data at
0x80000000.
CR16 Options
These options are defined specifically for the CR16 ports.
- -mmac
- Enable the use of multiply-accumulate instructions.
Disabled by default.
- -mcr16cplus
- -mcr16c
- Generate code for CR16C or CR16C+ architecture. CR16C+
architecture is default.
- -msim
- Links the library libsim.a which is in compatible with
simulator. Applicable to ELF compiler only.
- -mint32
- Choose integer type as 32-bit wide.
- -mbit-ops
- Generates "sbit"/"cbit" instructions
for bit manipulations.
- -mdata-model=model
- Choose a data model. The choices for model are
near, far or medium. medium is default.
However, far is not valid with -mcr16c, as the CR16C
architecture does not support the far data model.
Darwin Options
These options are defined for all architectures running the Darwin operating
system.
FSF GCC on Darwin does not create "fat" object files; it creates an
object file for the single architecture that GCC was built to target. Apple's
GCC on Darwin does create "fat" files if multiple
-arch
options are used; it does so by running the compiler or linker multiple times
and joining the results together with
lipo.
The subtype of the file created (like
ppc7400 or
ppc970 or
i686) is determined by the flags that specify the ISA that GCC is
targeting, like
-mcpu or
-march. The
-force_cpusubtype_ALL option can be used to override this.
The Darwin tools vary in their behavior when presented with an ISA mismatch. The
assembler,
as, only permits instructions to be used that are valid for
the subtype of the file it is generating, so you cannot put 64-bit
instructions in a
ppc750 object file. The linker for shared libraries,
/usr/bin/libtool, fails and prints an error if asked to create a shared
library with a less restrictive subtype than its input files (for instance,
trying to put a
ppc970 object file in a
ppc7400 library). The
linker for executables,
ld, quietly gives the executable the most
restrictive subtype of any of its input files.
- -Fdir
- Add the framework directory dir to the head of the
list of directories to be searched for header files. These directories are
interleaved with those specified by -I options and are scanned in a
left-to-right order.
A framework directory is a directory with frameworks in it. A framework is a
directory with a Headers and/or PrivateHeaders directory
contained directly in it that ends in .framework. The name of a
framework is the name of this directory excluding the .framework.
Headers associated with the framework are found in one of those two
directories, with Headers being searched first. A subframework is a
framework directory that is in a framework's Frameworks directory.
Includes of subframework headers can only appear in a header of a
framework that contains the subframework, or in a sibling subframework
header. Two subframeworks are siblings if they occur in the same
framework. A subframework should not have the same name as a framework; a
warning is issued if this is violated. Currently a subframework cannot
have subframeworks; in the future, the mechanism may be extended to
support this. The standard frameworks can be found in
/System/Library/Frameworks and /Library/Frameworks. An
example include looks like "#include
<Framework/header.h>", where Framework denotes the name
of the framework and header.h is found in the PrivateHeaders
or Headers directory.
- -iframeworkdir
- Like -F except the directory is a treated as a
system directory. The main difference between this -iframework and
-F is that with -iframework the compiler does not warn about
constructs contained within header files found via dir. This option
is valid only for the C family of languages.
- -gused
- Emit debugging information for symbols that are used. For
stabs debugging format, this enables
-feliminate-unused-debug-symbols. This is by default ON.
- -gfull
- Emit debugging information for all symbols and types.
- -mmacosx-version-min=version
- The earliest version of MacOS X that this executable will
run on is version. Typical values of version include 10.1,
10.2, and 10.3.9.
If the compiler was built to use the system's headers by default, then the
default for this option is the system version on which the compiler is
running, otherwise the default is to make choices that are compatible with
as many systems and code bases as possible.
- -mkernel
- Enable kernel development mode. The -mkernel option
sets -static, -fno-common, -fno-use-cxa-atexit,
-fno-exceptions, -fno-non-call-exceptions,
-fapple-kext, -fno-weak and -fno-rtti where
applicable. This mode also sets -mno-altivec, -msoft-float,
-fno-builtin and -mlong-branch for PowerPC targets.
- -mone-byte-bool
- Override the defaults for "bool" so that
"sizeof(bool)==1". By default "sizeof(bool)" is 4 when
compiling for Darwin/PowerPC and 1 when compiling for Darwin/x86, so this
option has no effect on x86.
Warning: The -mone-byte-bool switch causes GCC to generate
code that is not binary compatible with code generated without that
switch. Using this switch may require recompiling all other modules in a
program, including system libraries. Use this switch to conform to a
non-default data model.
- -mfix-and-continue
- -ffix-and-continue
- -findirect-data
- Generate code suitable for fast turnaround development,
such as to allow GDB to dynamically load .o files into
already-running programs. -findirect-data and
-ffix-and-continue are provided for backwards compatibility.
- -all_load
- Loads all members of static archive libraries. See man
ld(1) for more information.
- -arch_errors_fatal
- Cause the errors having to do with files that have the
wrong architecture to be fatal.
- -bind_at_load
- Causes the output file to be marked such that the dynamic
linker will bind all undefined references when the file is loaded or
launched.
- -bundle
- Produce a Mach-o bundle format file. See man ld(1)
for more information.
- -bundle_loader executable
- This option specifies the executable that will load
the build output file being linked. See man ld(1) for more
information.
- -dynamiclib
- When passed this option, GCC produces a dynamic library
instead of an executable when linking, using the Darwin libtool
command.
- -force_cpusubtype_ALL
- This causes GCC's output file to have the ALL
subtype, instead of one controlled by the -mcpu or -march
option.
- -allowable_client client_name
- -client_name
- -compatibility_version
- -current_version
- -dead_strip
- -dependency-file
- -dylib_file
- -dylinker_install_name
- -dynamic
- -exported_symbols_list
- -filelist
- -flat_namespace
- -force_flat_namespace
- -headerpad_max_install_names
- -image_base
- -init
- -install_name
- -keep_private_externs
- -multi_module
- -multiply_defined
- -multiply_defined_unused
- -noall_load
- -no_dead_strip_inits_and_terms
- -nofixprebinding
- -nomultidefs
- -noprebind
- -noseglinkedit
- -pagezero_size
- -prebind
- -prebind_all_twolevel_modules
- -private_bundle
- -read_only_relocs
- -sectalign
- -sectobjectsymbols
- -whyload
- -seg1addr
- -sectcreate
- -sectobjectsymbols
- -sectorder
- -segaddr
- -segs_read_only_addr
- -segs_read_write_addr
- -seg_addr_table
- -seg_addr_table_filename
- -seglinkedit
- -segprot
- -segs_read_only_addr
- -segs_read_write_addr
- -single_module
- -static
- -sub_library
- -sub_umbrella
- -twolevel_namespace
- -umbrella
- -undefined
- -unexported_symbols_list
- -weak_reference_mismatches
- -whatsloaded
- These options are passed to the Darwin linker. The Darwin
linker man page describes them in detail.
DEC Alpha Options
These
-m options are defined for the DEC Alpha implementations:
- -mno-soft-float
- -msoft-float
- Use (do not use) the hardware floating-point instructions
for floating-point operations. When -msoft-float is specified,
functions in libgcc.a are used to perform floating-point
operations. Unless they are replaced by routines that emulate the
floating-point operations, or compiled in such a way as to call such
emulations routines, these routines issue floating-point operations. If
you are compiling for an Alpha without floating-point operations, you must
ensure that the library is built so as not to call them.
Note that Alpha implementations without floating-point operations are
required to have floating-point registers.
- -mfp-reg
- -mno-fp-regs
- Generate code that uses (does not use) the floating-point
register set. -mno-fp-regs implies -msoft-float. If the
floating-point register set is not used, floating-point operands are
passed in integer registers as if they were integers and floating-point
results are passed in $0 instead of $f0. This is a non-standard calling
sequence, so any function with a floating-point argument or return value
called by code compiled with -mno-fp-regs must also be compiled
with that option.
A typical use of this option is building a kernel that does not use, and
hence need not save and restore, any floating-point registers.
- -mieee
- The Alpha architecture implements floating-point hardware
optimized for maximum performance. It is mostly compliant with the IEEE
floating-point standard. However, for full compliance, software assistance
is required. This option generates code fully IEEE-compliant code
except that the inexact-flag is not maintained (see below).
If this option is turned on, the preprocessor macro "_IEEE_FP"
is defined during compilation. The resulting code is less efficient but is
able to correctly support denormalized numbers and exceptional IEEE values
such as not-a-number and plus/minus infinity. Other Alpha compilers call
this option -ieee_with_no_inexact.
- -mieee-with-inexact
- This is like -mieee except the generated code also
maintains the IEEE inexact-flag. Turning on this option causes the
generated code to implement fully-compliant IEEE math. In addition to
"_IEEE_FP", "_IEEE_FP_EXACT" is defined as a
preprocessor macro. On some Alpha implementations the resulting code may
execute significantly slower than the code generated by default. Since
there is very little code that depends on the inexact-flag, you
should normally not specify this option. Other Alpha compilers call this
option -ieee_with_inexact.
- -mfp-trap-mode=trap-mode
- This option controls what floating-point related traps are
enabled. Other Alpha compilers call this option -fptm
trap-mode. The trap mode can be set to one of four values:
- n
- This is the default (normal) setting. The only traps that
are enabled are the ones that cannot be disabled in software (e.g.,
division by zero trap).
- u
- In addition to the traps enabled by n, underflow
traps are enabled as well.
- su
- Like u, but the instructions are marked to be safe
for software completion (see Alpha architecture manual for details).
- sui
- Like su, but inexact traps are enabled as well.
- -mfp-rounding-mode=rounding-mode
- Selects the IEEE rounding mode. Other Alpha compilers call
this option -fprm rounding-mode. The rounding-mode
can be one of:
- n
- Normal IEEE rounding mode. Floating-point numbers are
rounded towards the nearest machine number or towards the even machine
number in case of a tie.
- m
- Round towards minus infinity.
- c
- Chopped rounding mode. Floating-point numbers are rounded
towards zero.
- d
- Dynamic rounding mode. A field in the floating-point
control register ( fpcr, see Alpha architecture reference manual)
controls the rounding mode in effect. The C library initializes this
register for rounding towards plus infinity. Thus, unless your program
modifies the fpcr, d corresponds to round towards plus
infinity.
- -mtrap-precision=trap-precision
- In the Alpha architecture, floating-point traps are
imprecise. This means without software assistance it is impossible to
recover from a floating trap and program execution normally needs to be
terminated. GCC can generate code that can assist operating system trap
handlers in determining the exact location that caused a floating-point
trap. Depending on the requirements of an application, different levels of
precisions can be selected:
- p
- Program precision. This option is the default and means a
trap handler can only identify which program caused a floating-point
exception.
- f
- Function precision. The trap handler can determine the
function that caused a floating-point exception.
- i
- Instruction precision. The trap handler can determine the
exact instruction that caused a floating-point exception.
Other Alpha compilers provide the equivalent options called
-scope_safe
and
-resumption_safe.
- -mieee-conformant
- This option marks the generated code as IEEE conformant.
You must not use this option unless you also specify
-mtrap-precision=i and either -mfp-trap-mode=su or
-mfp-trap-mode=sui. Its only effect is to emit the line .eflag
48 in the function prologue of the generated assembly file.
- -mbuild-constants
- Normally GCC examines a 32- or 64-bit integer constant to
see if it can construct it from smaller constants in two or three
instructions. If it cannot, it outputs the constant as a literal and
generates code to load it from the data segment at run time.
Use this option to require GCC to construct all integer constants
using code, even if it takes more instructions (the maximum is six).
You typically use this option to build a shared library dynamic loader.
Itself a shared library, it must relocate itself in memory before it can
find the variables and constants in its own data segment.
- -mbwx
- -mno-bwx
- -mcix
- -mno-cix
- -mfix
- -mno-fix
- -mmax
- -mno-max
- Indicate whether GCC should generate code to use the
optional BWX, CIX, FIX and MAX instruction sets. The default is to use the
instruction sets supported by the CPU type specified via -mcpu=
option or that of the CPU on which GCC was built if none is
specified.
- -mfloat-vax
- -mfloat-ieee
- Generate code that uses (does not use) VAX F and G
floating-point arithmetic instead of IEEE single and double
precision.
- -mexplicit-relocs
- -mno-explicit-relocs
- Older Alpha assemblers provided no way to generate symbol
relocations except via assembler macros. Use of these macros does not
allow optimal instruction scheduling. GNU binutils as of version 2.12
supports a new syntax that allows the compiler to explicitly mark which
relocations should apply to which instructions. This option is mostly
useful for debugging, as GCC detects the capabilities of the assembler
when it is built and sets the default accordingly.
- -msmall-data
- -mlarge-data
- When -mexplicit-relocs is in effect, static data is
accessed via gp-relative relocations. When -msmall-data is
used, objects 8 bytes long or smaller are placed in a small data
area (the ".sdata" and ".sbss" sections) and are
accessed via 16-bit relocations off of the $gp register. This limits the
size of the small data area to 64KB, but allows the variables to be
directly accessed via a single instruction.
The default is -mlarge-data. With this option the data area is
limited to just below 2GB. Programs that require more than 2GB of data
must use "malloc" or "mmap" to allocate the data in
the heap instead of in the program's data segment.
When generating code for shared libraries, -fpic implies
-msmall-data and -fPIC implies -mlarge-data.
- -msmall-text
- -mlarge-text
- When -msmall-text is used, the compiler assumes that
the code of the entire program (or shared library) fits in 4MB, and is
thus reachable with a branch instruction. When -msmall-data is
used, the compiler can assume that all local symbols share the same $gp
value, and thus reduce the number of instructions required for a function
call from 4 to 1.
The default is -mlarge-text.
- -mcpu=cpu_type
- Set the instruction set and instruction scheduling
parameters for machine type cpu_type. You can specify either the
EV style name or the corresponding chip number. GCC supports
scheduling parameters for the EV4, EV5 and EV6 family of processors and
chooses the default values for the instruction set from the processor you
specify. If you do not specify a processor type, GCC defaults to the
processor on which the compiler was built.
Supported values for cpu_type are
- ev4
- ev45
- 21064
- Schedules as an EV4 and has no instruction set
extensions.
- ev5
- 21164
- Schedules as an EV5 and has no instruction set
extensions.
- ev56
- 21164a
- Schedules as an EV5 and supports the BWX extension.
- pca56
- 21164pc
- 21164PC
- Schedules as an EV5 and supports the BWX and MAX
extensions.
- ev6
- 21264
- Schedules as an EV6 and supports the BWX, FIX, and MAX
extensions.
- ev67
- 21264a
- Schedules as an EV6 and supports the BWX, CIX, FIX, and MAX
extensions.
Native toolchains also support the value
native, which selects the best
architecture option for the host processor.
-mcpu=native has no effect
if GCC does not recognize the processor.
- -mtune=cpu_type
- Set only the instruction scheduling parameters for machine
type cpu_type. The instruction set is not changed.
Native toolchains also support the value native, which selects the
best architecture option for the host processor. -mtune=native has
no effect if GCC does not recognize the processor.
- -mmemory-latency=time
- Sets the latency the scheduler should assume for typical
memory references as seen by the application. This number is highly
dependent on the memory access patterns used by the application and the
size of the external cache on the machine.
Valid options for time are
- number
- A decimal number representing clock cycles.
- L1
- L2
- L3
- main
- The compiler contains estimates of the number of clock
cycles for "typical" EV4 & EV5 hardware for the Level 1, 2
& 3 caches (also called Dcache, Scache, and Bcache), as well as to
main memory. Note that L3 is only valid for EV5.
FR30 Options
These options are defined specifically for the FR30 port.
- -msmall-model
- Use the small address space model. This can produce smaller
code, but it does assume that all symbolic values and addresses fit into a
20-bit range.
- -mno-lsim
- Assume that runtime support has been provided and so there
is no need to include the simulator library ( libsim.a) on the
linker command line.
FRV Options
- -mgpr-32
- Only use the first 32 general-purpose registers.
- -mgpr-64
- Use all 64 general-purpose registers.
- -mfpr-32
- Use only the first 32 floating-point registers.
- -mfpr-64
- Use all 64 floating-point registers.
- -mhard-float
- Use hardware instructions for floating-point
operations.
- -msoft-float
- Use library routines for floating-point operations.
- -malloc-cc
- Dynamically allocate condition code registers.
- -mfixed-cc
- Do not try to dynamically allocate condition code
registers, only use "icc0" and "fcc0".
- -mdword
- Change ABI to use double word insns.
- -mno-dword
- Do not use double word instructions.
- -mdouble
- Use floating-point double instructions.
- -mno-double
- Do not use floating-point double instructions.
- -mmedia
- Use media instructions.
- -mno-media
- Do not use media instructions.
- -mmuladd
- Use multiply and add/subtract instructions.
- -mno-muladd
- Do not use multiply and add/subtract instructions.
- -mfdpic
- Select the FDPIC ABI, which uses function descriptors to
represent pointers to functions. Without any PIC/PIE-related options, it
implies -fPIE. With -fpic or -fpie, it assumes GOT
entries and small data are within a 12-bit range from the GOT base
address; with -fPIC or -fPIE, GOT offsets are computed with
32 bits. With a bfin-elf target, this option implies
-msim.
- -minline-plt
- Enable inlining of PLT entries in function calls to
functions that are not known to bind locally. It has no effect without
-mfdpic. It's enabled by default if optimizing for speed and
compiling for shared libraries (i.e., -fPIC or -fpic), or
when an optimization option such as -O3 or above is present in the
command line.
- -mTLS
- Assume a large TLS segment when generating thread-local
code.
- -mtls
- Do not assume a large TLS segment when generating
thread-local code.
- -mgprel-ro
- Enable the use of "GPREL" relocations in the
FDPIC ABI for data that is known to be in read-only sections. It's enabled
by default, except for -fpic or -fpie: even though it may
help make the global offset table smaller, it trades 1 instruction for 4.
With -fPIC or -fPIE, it trades 3 instructions for 4, one of
which may be shared by multiple symbols, and it avoids the need for a GOT
entry for the referenced symbol, so it's more likely to be a win. If it is
not, -mno-gprel-ro can be used to disable it.
- -multilib-library-pic
- Link with the (library, not FD) pic libraries. It's implied
by -mlibrary-pic, as well as by -fPIC and -fpic
without -mfdpic. You should never have to use it explicitly.
- -mlinked-fp
- Follow the EABI requirement of always creating a frame
pointer whenever a stack frame is allocated. This option is enabled by
default and can be disabled with -mno-linked-fp.
- -mlong-calls
- Use indirect addressing to call functions outside the
current compilation unit. This allows the functions to be placed anywhere
within the 32-bit address space.
- -malign-labels
- Try to align labels to an 8-byte boundary by inserting NOPs
into the previous packet. This option only has an effect when VLIW packing
is enabled. It doesn't create new packets; it merely adds NOPs to existing
ones.
- -mlibrary-pic
- Generate position-independent EABI code.
- -macc-4
- Use only the first four media accumulator registers.
- -macc-8
- Use all eight media accumulator registers.
- -mpack
- Pack VLIW instructions.
- -mno-pack
- Do not pack VLIW instructions.
- -mno-eflags
- Do not mark ABI switches in e_flags.
- -mcond-move
- Enable the use of conditional-move instructions (default).
This switch is mainly for debugging the compiler and will likely be removed
in a future version.
- -mno-cond-move
- Disable the use of conditional-move instructions.
This switch is mainly for debugging the compiler and will likely be removed
in a future version.
- -mscc
- Enable the use of conditional set instructions (default).
This switch is mainly for debugging the compiler and will likely be removed
in a future version.
- -mno-scc
- Disable the use of conditional set instructions.
This switch is mainly for debugging the compiler and will likely be removed
in a future version.
- -mcond-exec
- Enable the use of conditional execution (default).
This switch is mainly for debugging the compiler and will likely be removed
in a future version.
- -mno-cond-exec
- Disable the use of conditional execution.
This switch is mainly for debugging the compiler and will likely be removed
in a future version.
- -mvliw-branch
- Run a pass to pack branches into VLIW instructions
(default).
This switch is mainly for debugging the compiler and will likely be removed
in a future version.
- -mno-vliw-branch
- Do not run a pass to pack branches into VLIW instructions.
This switch is mainly for debugging the compiler and will likely be removed
in a future version.
- -mmulti-cond-exec
- Enable optimization of "&&" and
"||" in conditional execution (default).
This switch is mainly for debugging the compiler and will likely be removed
in a future version.
- -mno-multi-cond-exec
- Disable optimization of "&&" and
"||" in conditional execution.
This switch is mainly for debugging the compiler and will likely be removed
in a future version.
- -mnested-cond-exec
- Enable nested conditional execution optimizations
(default).
This switch is mainly for debugging the compiler and will likely be removed
in a future version.
- -mno-nested-cond-exec
- Disable nested conditional execution optimizations.
This switch is mainly for debugging the compiler and will likely be removed
in a future version.
- -moptimize-membar
- This switch removes redundant "membar"
instructions from the compiler-generated code. It is enabled by
default.
- -mno-optimize-membar
- This switch disables the automatic removal of redundant
"membar" instructions from the generated code.
- -mtomcat-stats
- Cause gas to print out tomcat statistics.
- -mcpu=cpu
- Select the processor type for which to generate code.
Possible values are frv, fr550, tomcat, fr500,
fr450, fr405, fr400, fr300 and
simple.
GNU/Linux Options
These
-m options are defined for GNU/Linux targets:
- -mglibc
- Use the GNU C library. This is the default except on
*-*-linux-*uclibc* and *-*-linux-*android* targets.
- -muclibc
- Use uClibc C library. This is the default on
*-*-linux-*uclibc* targets.
- -mbionic
- Use Bionic C library. This is the default on
*-*-linux-*android* targets.
- -mandroid
- Compile code compatible with Android platform. This is the
default on *-*-linux-*android* targets.
When compiling, this option enables -mbionic, -fPIC,
-fno-exceptions and -fno-rtti by default. When linking, this
option makes the GCC driver pass Android-specific options to the linker.
Finally, this option causes the preprocessor macro "__ANDROID__"
to be defined.
- -tno-android-cc
- Disable compilation effects of -mandroid, i.e., do
not enable -mbionic, -fPIC, -fno-exceptions and
-fno-rtti by default.
- -tno-android-ld
- Disable linking effects of -mandroid, i.e., pass
standard Linux linking options to the linker.
H8/300 Options
These
-m options are defined for the H8/300 implementations:
- -mrelax
- Shorten some address references at link time, when
possible; uses the linker option -relax.
- -mh
- Generate code for the H8/300H.
- -ms
- Generate code for the H8S.
- -mn
- Generate code for the H8S and H8/300H in the normal mode.
This switch must be used either with -mh or -ms.
- -ms2600
- Generate code for the H8S/2600. This switch must be used
with -ms.
- -mexr
- Extended registers are stored on stack before execution of
function with monitor attribute. Default option is -mexr. This
option is valid only for H8S targets.
- -mno-exr
- Extended registers are not stored on stack before execution
of function with monitor attribute. Default option is -mno-exr.
This option is valid only for H8S targets.
- -mint32
- Make "int" data 32 bits by default.
- -malign-300
- On the H8/300H and H8S, use the same alignment rules as for
the H8/300. The default for the H8/300H and H8S is to align longs and
floats on 4-byte boundaries. -malign-300 causes them to be aligned
on 2-byte boundaries. This option has no effect on the H8/300.
HPPA Options
These
-m options are defined for the HPPA family of computers:
- -march=architecture-type
- Generate code for the specified architecture. The choices
for architecture-type are 1.0 for PA 1.0, 1.1 for PA
1.1, and 2.0 for PA 2.0 processors. Refer to
/usr/lib/sched.models on an HP-UX system to determine the proper
architecture option for your machine. Code compiled for lower numbered
architectures runs on higher numbered architectures, but not the other way
around.
- -mpa-risc-1-0
- -mpa-risc-1-1
- -mpa-risc-2-0
- Synonyms for -march=1.0, -march=1.1, and
-march=2.0 respectively.
- -mjump-in-delay
- This option is ignored and provided for compatibility
purposes only.
- -mdisable-fpregs
- Prevent floating-point registers from being used in any
manner. This is necessary for compiling kernels that perform lazy context
switching of floating-point registers. If you use this option and attempt
to perform floating-point operations, the compiler aborts.
- -mdisable-indexing
- Prevent the compiler from using indexing address modes.
This avoids some rather obscure problems when compiling MIG generated code
under MACH.
- -mno-space-regs
- Generate code that assumes the target has no space
registers. This allows GCC to generate faster indirect calls and use
unscaled index address modes.
Such code is suitable for level 0 PA systems and kernels.
- -mfast-indirect-calls
- Generate code that assumes calls never cross space
boundaries. This allows GCC to emit code that performs faster indirect
calls.
This option does not work in the presence of shared libraries or nested
functions.
- -mfixed-range=register-range
- Generate code treating the given register range as fixed
registers. A fixed register is one that the register allocator cannot use.
This is useful when compiling kernel code. A register range is specified
as two registers separated by a dash. Multiple register ranges can be
specified separated by a comma.
- -mlong-load-store
- Generate 3-instruction load and store sequences as
sometimes required by the HP-UX 10 linker. This is equivalent to the
+k option to the HP compilers.
- -mportable-runtime
- Use the portable calling conventions proposed by HP for ELF
systems.
- -mgas
- Enable the use of assembler directives only GAS
understands.
- -mschedule=cpu-type
- Schedule code according to the constraints for the machine
type cpu-type. The choices for cpu-type are 700
7100, 7100LC, 7200, 7300 and 8000.
Refer to /usr/lib/sched.models on an HP-UX system to determine the
proper scheduling option for your machine. The default scheduling is
8000.
- -mlinker-opt
- Enable the optimization pass in the HP-UX linker. Note this
makes symbolic debugging impossible. It also triggers a bug in the HP-UX 8
and HP-UX 9 linkers in which they give bogus error messages when linking
some programs.
- -msoft-float
- Generate output containing library calls for floating
point. Warning: the requisite libraries are not available for all
HPPA targets. Normally the facilities of the machine's usual C compiler
are used, but this cannot be done directly in cross-compilation. You must
make your own arrangements to provide suitable library functions for
cross-compilation.
-msoft-float changes the calling convention in the output file;
therefore, it is only useful if you compile all of a program with
this option. In particular, you need to compile libgcc.a, the
library that comes with GCC, with -msoft-float in order for this to
work.
- -msio
- Generate the predefine, "_SIO", for server IO.
The default is -mwsio. This generates the predefines,
"__hp9000s700", "__hp9000s700__" and
"_WSIO", for workstation IO. These options are available under
HP-UX and HI-UX.
- -mgnu-ld
- Use options specific to GNU ld. This passes
-shared to ld when building a shared library. It is the
default when GCC is configured, explicitly or implicitly, with the GNU
linker. This option does not affect which ld is called; it only
changes what parameters are passed to that ld. The ld that
is called is determined by the --with-ld configure option, GCC's
program search path, and finally by the user's PATH. The linker
used by GCC can be printed using which `gcc -print-prog-name=ld`.
This option is only available on the 64-bit HP-UX GCC, i.e. configured
with hppa*64*-*-hpux*.
- -mhp-ld
- Use options specific to HP ld. This passes -b
to ld when building a shared library and passes +Accept
TypeMismatch to ld on all links. It is the default when GCC is
configured, explicitly or implicitly, with the HP linker. This option does
not affect which ld is called; it only changes what parameters are
passed to that ld. The ld that is called is determined by
the --with-ld configure option, GCC's program search path, and
finally by the user's PATH. The linker used by GCC can be printed
using which `gcc -print-prog-name=ld`. This option is only
available on the 64-bit HP-UX GCC, i.e. configured with
hppa*64*-*-hpux*.
- -mlong-calls
- Generate code that uses long call sequences. This ensures
that a call is always able to reach linker generated stubs. The default is
to generate long calls only when the distance from the call site to the
beginning of the function or translation unit, as the case may be, exceeds
a predefined limit set by the branch type being used. The limits for
normal calls are 7,600,000 and 240,000 bytes, respectively for the PA 2.0
and PA 1.X architectures. Sibcalls are always limited at 240,000 bytes.
Distances are measured from the beginning of functions when using the
-ffunction-sections option, or when using the -mgas and
-mno-portable-runtime options together under HP-UX with the SOM
linker.
It is normally not desirable to use this option as it degrades performance.
However, it may be useful in large applications, particularly when partial
linking is used to build the application.
The types of long calls used depends on the capabilities of the assembler
and linker, and the type of code being generated. The impact on systems
that support long absolute calls, and long pic symbol-difference or
pc-relative calls should be relatively small. However, an indirect call is
used on 32-bit ELF systems in pic code and it is quite long.
- -munix=unix-std
- Generate compiler predefines and select a startfile for the
specified UNIX standard. The choices for unix-std are 93,
95 and 98. 93 is supported on all HP-UX versions.
95 is available on HP-UX 10.10 and later. 98 is available on
HP-UX 11.11 and later. The default values are 93 for HP-UX 10.00,
95 for HP-UX 10.10 though to 11.00, and 98 for HP-UX 11.11
and later.
-munix=93 provides the same predefines as GCC 3.3 and 3.4.
-munix=95 provides additional predefines for "XOPEN_UNIX"
and "_XOPEN_SOURCE_EXTENDED", and the startfile unix95.o.
-munix=98 provides additional predefines for
"_XOPEN_UNIX", "_XOPEN_SOURCE_EXTENDED",
"_INCLUDE__STDC_A1_SOURCE" and
"_INCLUDE_XOPEN_SOURCE_500", and the startfile unix98.o.
It is important to note that this option changes the interfaces for
various library routines. It also affects the operational behavior of the
C library. Thus, extreme care is needed in using this option.
Library code that is intended to operate with more than one UNIX standard
must test, set and restore the variable "__xpg4_extended_mask"
as appropriate. Most GNU software doesn't provide this capability.
- -nolibdld
- Suppress the generation of link options to search libdld.sl
when the -static option is specified on HP-UX 10 and later.
- -static
- The HP-UX implementation of setlocale in libc has a
dependency on libdld.sl. There isn't an archive version of libdld.sl.
Thus, when the -static option is specified, special link options
are needed to resolve this dependency.
On HP-UX 10 and later, the GCC driver adds the necessary options to link
with libdld.sl when the -static option is specified. This causes
the resulting binary to be dynamic. On the 64-bit port, the linkers
generate dynamic binaries by default in any case. The -nolibdld
option can be used to prevent the GCC driver from adding these link
options.
- -threads
- Add support for multithreading with the dce thread
library under HP-UX. This option sets flags for both the preprocessor and
linker.
IA-64 Options
These are the
-m options defined for the Intel IA-64 architecture.
- -mbig-endian
- Generate code for a big-endian target. This is the default
for HP-UX.
- -mlittle-endian
- Generate code for a little-endian target. This is the
default for AIX5 and GNU/Linux.
- -mgnu-as
- -mno-gnu-as
- Generate (or don't) code for the GNU assembler. This is the
default.
- -mgnu-ld
- -mno-gnu-ld
- Generate (or don't) code for the GNU linker. This is the
default.
- -mno-pic
- Generate code that does not use a global pointer register.
The result is not position independent code, and violates the IA-64
ABI.
- -mvolatile-asm-stop
- -mno-volatile-asm-stop
- Generate (or don't) a stop bit immediately before and after
volatile asm statements.
- -mregister-names
- -mno-register-names
- Generate (or don't) in, loc, and out
register names for the stacked registers. This may make assembler output
more readable.
- -mno-sdata
- -msdata
- Disable (or enable) optimizations that use the small data
section. This may be useful for working around optimizer bugs.
- -mconstant-gp
- Generate code that uses a single constant global pointer
value. This is useful when compiling kernel code.
- -mauto-pic
- Generate code that is self-relocatable. This implies
-mconstant-gp. This is useful when compiling firmware code.
- -minline-float-divide-min-latency
- Generate code for inline divides of floating-point values
using the minimum latency algorithm.
- -minline-float-divide-max-throughput
- Generate code for inline divides of floating-point values
using the maximum throughput algorithm.
- -mno-inline-float-divide
- Do not generate inline code for divides of floating-point
values.
- -minline-int-divide-min-latency
- Generate code for inline divides of integer values using
the minimum latency algorithm.
- -minline-int-divide-max-throughput
- Generate code for inline divides of integer values using
the maximum throughput algorithm.
- -mno-inline-int-divide
- Do not generate inline code for divides of integer
values.
- -minline-sqrt-min-latency
- Generate code for inline square roots using the minimum
latency algorithm.
- -minline-sqrt-max-throughput
- Generate code for inline square roots using the maximum
throughput algorithm.
- -mno-inline-sqrt
- Do not generate inline code for "sqrt".
- -mfused-madd
- -mno-fused-madd
- Do (don't) generate code that uses the fused multiply/add
or multiply/subtract instructions. The default is to use these
instructions.
- -mno-dwarf2-asm
- -mdwarf2-asm
- Don't (or do) generate assembler code for the DWARF 2 line
number debugging info. This may be useful when not using the GNU
assembler.
- -mearly-stop-bits
- -mno-early-stop-bits
- Allow stop bits to be placed earlier than immediately
preceding the instruction that triggered the stop bit. This can improve
instruction scheduling, but does not always do so.
- -mfixed-range=register-range
- Generate code treating the given register range as fixed
registers. A fixed register is one that the register allocator cannot use.
This is useful when compiling kernel code. A register range is specified
as two registers separated by a dash. Multiple register ranges can be
specified separated by a comma.
- -mtls-size=tls-size
- Specify bit size of immediate TLS offsets. Valid values are
14, 22, and 64.
- -mtune=cpu-type
- Tune the instruction scheduling for a particular CPU, Valid
values are itanium, itanium1, merced,
itanium2, and mckinley.
- -milp32
- -mlp64
- Generate code for a 32-bit or 64-bit environment. The
32-bit environment sets int, long and pointer to 32 bits. The 64-bit
environment sets int to 32 bits and long and pointer to 64 bits. These are
HP-UX specific flags.
- -mno-sched-br-data-spec
- -msched-br-data-spec
- (Dis/En)able data speculative scheduling before reload.
This results in generation of "ld.a" instructions and the
corresponding check instructions ("ld.c" / "chk.a").
The default is 'disable'.
- -msched-ar-data-spec
- -mno-sched-ar-data-spec
- (En/Dis)able data speculative scheduling after reload. This
results in generation of "ld.a" instructions and the
corresponding check instructions ("ld.c" / "chk.a").
The default is 'enable'.
- -mno-sched-control-spec
- -msched-control-spec
- (Dis/En)able control speculative scheduling. This feature
is available only during region scheduling (i.e. before reload). This
results in generation of the "ld.s" instructions and the
corresponding check instructions "chk.s". The default is
'disable'.
- -msched-br-in-data-spec
- -mno-sched-br-in-data-spec
- (En/Dis)able speculative scheduling of the instructions
that are dependent on the data speculative loads before reload. This is
effective only with -msched-br-data-spec enabled. The default is
'enable'.
- -msched-ar-in-data-spec
- -mno-sched-ar-in-data-spec
- (En/Dis)able speculative scheduling of the instructions
that are dependent on the data speculative loads after reload. This is
effective only with -msched-ar-data-spec enabled. The default is
'enable'.
- -msched-in-control-spec
- -mno-sched-in-control-spec
- (En/Dis)able speculative scheduling of the instructions
that are dependent on the control speculative loads. This is effective
only with -msched-control-spec enabled. The default is
'enable'.
- -mno-sched-prefer-non-data-spec-insns
- -msched-prefer-non-data-spec-insns
- If enabled, data-speculative instructions are chosen for
schedule only if there are no other choices at the moment. This makes the
use of the data speculation much more conservative. The default is
'disable'.
- -mno-sched-prefer-non-control-spec-insns
- -msched-prefer-non-control-spec-insns
- If enabled, control-speculative instructions are chosen for
schedule only if there are no other choices at the moment. This makes the
use of the control speculation much more conservative. The default is
'disable'.
- -mno-sched-count-spec-in-critical-path
- -msched-count-spec-in-critical-path
- If enabled, speculative dependencies are considered during
computation of the instructions priorities. This makes the use of the
speculation a bit more conservative. The default is 'disable'.
- -msched-spec-ldc
- Use a simple data speculation check. This option is on by
default.
- -msched-control-spec-ldc
- Use a simple check for control speculation. This option is
on by default.
- -msched-stop-bits-after-every-cycle
- Place a stop bit after every cycle when scheduling. This
option is on by default.
- -msched-fp-mem-deps-zero-cost
- Assume that floating-point stores and loads are not likely
to cause a conflict when placed into the same instruction group. This
option is disabled by default.
- -msel-sched-dont-check-control-spec
- Generate checks for control speculation in selective
scheduling. This flag is disabled by default.
- -msched-max-memory-insns=max-insns
- Limit on the number of memory insns per instruction group,
giving lower priority to subsequent memory insns attempting to schedule in
the same instruction group. Frequently useful to prevent cache bank
conflicts. The default value is 1.
- -msched-max-memory-insns-hard-limit
- Makes the limit specified by msched-max-memory-insns
a hard limit, disallowing more than that number in an instruction group.
Otherwise, the limit is "soft", meaning that non-memory
operations are preferred when the limit is reached, but memory operations
may still be scheduled.
LM32 Options
These
-m options are defined for the LatticeMico32 architecture:
- -mbarrel-shift-enabled
- Enable barrel-shift instructions.
- -mdivide-enabled
- Enable divide and modulus instructions.
- -mmultiply-enabled
- Enable multiply instructions.
- -msign-extend-enabled
- Enable sign extend instructions.
- -muser-enabled
- Enable user-defined instructions.
M32C Options
- -mcpu=name
- Select the CPU for which code is generated. name may
be one of r8c for the R8C/Tiny series, m16c for the M16C (up
to /60) series, m32cm for the M16C/80 series, or m32c for
the M32C/80 series.
- -msim
- Specifies that the program will be run on the simulator.
This causes an alternate runtime library to be linked in which supports,
for example, file I/O. You must not use this option when generating
programs that will run on real hardware; you must provide your own runtime
library for whatever I/O functions are needed.
- -memregs=number
- Specifies the number of memory-based pseudo-registers GCC
uses during code generation. These pseudo-registers are used like real
registers, so there is a tradeoff between GCC's ability to fit the code
into available registers, and the performance penalty of using memory
instead of registers. Note that all modules in a program must be compiled
with the same value for this option. Because of that, you must not use
this option with GCC's default runtime libraries.
M32R/D Options
These
-m options are defined for Renesas M32R/D architectures:
- -m32r2
- Generate code for the M32R/2.
- -m32rx
- Generate code for the M32R/X.
- -m32r
- Generate code for the M32R. This is the default.
- -mmodel=small
- Assume all objects live in the lower 16MB of memory (so
that their addresses can be loaded with the "ld24" instruction),
and assume all subroutines are reachable with the "bl"
instruction. This is the default.
The addressability of a particular object can be set with the
"model" attribute.
- -mmodel=medium
- Assume objects may be anywhere in the 32-bit address space
(the compiler generates "seth/add3" instructions to load their
addresses), and assume all subroutines are reachable with the
"bl" instruction.
- -mmodel=large
- Assume objects may be anywhere in the 32-bit address space
(the compiler generates "seth/add3" instructions to load their
addresses), and assume subroutines may not be reachable with the
"bl" instruction (the compiler generates the much slower
"seth/add3/jl" instruction sequence).
- -msdata=none
- Disable use of the small data area. Variables are put into
one of ".data", ".bss", or ".rodata" (unless
the "section" attribute has been specified). This is the
default.
The small data area consists of sections ".sdata" and
".sbss". Objects may be explicitly put in the small data area
with the "section" attribute using one of these sections.
- -msdata=sdata
- Put small global and static data in the small data area,
but do not generate special code to reference them.
- -msdata=use
- Put small global and static data in the small data area,
and generate special instructions to reference them.
- -G num
- Put global and static objects less than or equal to
num bytes into the small data or BSS sections instead of the normal
data or BSS sections. The default value of num is 8. The
-msdata option must be set to one of sdata or use for
this option to have any effect.
All modules should be compiled with the same -G num value.
Compiling with different values of num may or may not work; if it
doesn't the linker gives an error message---incorrect code is not
generated.
- -mdebug
- Makes the M32R-specific code in the compiler display some
statistics that might help in debugging programs.
- -malign-loops
- Align all loops to a 32-byte boundary.
- -mno-align-loops
- Do not enforce a 32-byte alignment for loops. This is the
default.
- -missue-rate=number
- Issue number instructions per cycle. number
can only be 1 or 2.
- -mbranch-cost=number
- number can only be 1 or 2. If it is 1 then branches
are preferred over conditional code, if it is 2, then the opposite
applies.
- -mflush-trap=number
- Specifies the trap number to use to flush the cache. The
default is 12. Valid numbers are between 0 and 15 inclusive.
- -mno-flush-trap
- Specifies that the cache cannot be flushed by using a
trap.
- -mflush-func=name
- Specifies the name of the operating system function to call
to flush the cache. The default is _flush_cache, but a function
call is only used if a trap is not available.
- -mno-flush-func
- Indicates that there is no OS function for flushing the
cache.
M680x0 Options
These are the
-m options defined for M680x0 and ColdFire processors. The
default settings depend on which architecture was selected when the compiler
was configured; the defaults for the most common choices are given below.
- -march=arch
- Generate code for a specific M680x0 or ColdFire instruction
set architecture. Permissible values of arch for M680x0
architectures are: 68000, 68010, 68020, 68030,
68040, 68060 and cpu32. ColdFire architectures are
selected according to Freescale's ISA classification and the permissible
values are: isaa, isaaplus, isab and isac.
GCC defines a macro "__mcf arch__" whenever it is
generating code for a ColdFire target. The arch in this macro is
one of the -march arguments given above.
When used together, -march and -mtune select code that runs on
a family of similar processors but that is optimized for a particular
microarchitecture.
- -mcpu=cpu
- Generate code for a specific M680x0 or ColdFire processor.
The M680x0 cpus are: 68000, 68010, 68020,
68030, 68040, 68060, 68302, 68332 and
cpu32. The ColdFire cpus are given by the table below, which
also classifies the CPUs into families:
- Family : -mcpu arguments
- 51 : 51 51ac 51ag 51cn
51em 51je 51jf 51jg 51jm 51mm
51qe 51qm
- 5206 : 5202 5204 5206
- 5206e : 5206e
- 5208 : 5207 5208
- 5211a : 5210a 5211a
- 5213 : 5211 5212 5213
- 5216 : 5214 5216
- 52235 : 52230 52231 52232
52233 52234 52235
- 5225 : 5224 5225
- 52259 : 52252 52254 52255
52256 52258 52259
- 5235 : 5232 5233 5234
5235 523x
- 5249 : 5249
- 5250 : 5250
- 5271 : 5270 5271
- 5272 : 5272
- 5275 : 5274 5275
- 5282 : 5280 5281 5282
528x
- 53017 : 53011 53012 53013
53014 53015 53016 53017
- 5307 : 5307
- 5329 : 5327 5328 5329
532x
- 5373 : 5372 5373 537x
- 5407 : 5407
- 5475 : 5470 5471 5472
5473 5474 5475 547x 5480 5481
5482 5483 5484 5485
-mcpu=cpu overrides
-march=arch if
arch is
compatible with
cpu. Other combinations of
-mcpu and
-march are rejected.
GCC defines the macro "__mcf_cpu_
cpu" when ColdFire target
cpu is selected. It also defines "__mcf_family_
family", where the value of
family is given by the table
above.
- -mtune=tune
- Tune the code for a particular microarchitecture within the
constraints set by -march and -mcpu. The M680x0
microarchitectures are: 68000, 68010, 68020,
68030, 68040, 68060 and cpu32. The ColdFire
microarchitectures are: cfv1, cfv2, cfv3, cfv4
and cfv4e.
You can also use -mtune=68020-40 for code that needs to run
relatively well on 68020, 68030 and 68040 targets. -mtune=68020-60
is similar but includes 68060 targets as well. These two options select
the same tuning decisions as -m68020-40 and -m68020-60
respectively.
GCC defines the macros "__mc arch" and
"__mcarch__" when tuning for 680x0 architecture
arch. It also defines "mc arch" unless either
-ansi or a non-GNU -std option is used. If GCC is tuning for
a range of architectures, as selected by -mtune=68020-40 or
-mtune=68020-60, it defines the macros for every architecture in
the range.
GCC also defines the macro "__m uarch__" when tuning for
ColdFire microarchitecture uarch, where uarch is one of the
arguments given above.
- -m68000
- -mc68000
- Generate output for a 68000. This is the default when the
compiler is configured for 68000-based systems. It is equivalent to
-march=68000.
Use this option for microcontrollers with a 68000 or EC000 core, including
the 68008, 68302, 68306, 68307, 68322, 68328 and 68356.
- -m68010
- Generate output for a 68010. This is the default when the
compiler is configured for 68010-based systems. It is equivalent to
-march=68010.
- -m68020
- -mc68020
- Generate output for a 68020. This is the default when the
compiler is configured for 68020-based systems. It is equivalent to
-march=68020.
- -m68030
- Generate output for a 68030. This is the default when the
compiler is configured for 68030-based systems. It is equivalent to
-march=68030.
- -m68040
- Generate output for a 68040. This is the default when the
compiler is configured for 68040-based systems. It is equivalent to
-march=68040.
This option inhibits the use of 68881/68882 instructions that have to be
emulated by software on the 68040. Use this option if your 68040 does not
have code to emulate those instructions.
- -m68060
- Generate output for a 68060. This is the default when the
compiler is configured for 68060-based systems. It is equivalent to
-march=68060.
This option inhibits the use of 68020 and 68881/68882 instructions that have
to be emulated by software on the 68060. Use this option if your 68060
does not have code to emulate those instructions.
- -mcpu32
- Generate output for a CPU32. This is the default when the
compiler is configured for CPU32-based systems. It is equivalent to
-march=cpu32.
Use this option for microcontrollers with a CPU32 or CPU32+ core, including
the 68330, 68331, 68332, 68333, 68334, 68336, 68340, 68341, 68349 and
68360.
- -m5200
- Generate output for a 520X ColdFire CPU. This is the
default when the compiler is configured for 520X-based systems. It is
equivalent to -mcpu=5206, and is now deprecated in favor of that
option.
Use this option for microcontroller with a 5200 core, including the MCF5202,
MCF5203, MCF5204 and MCF5206.
- -m5206e
- Generate output for a 5206e ColdFire CPU. The option is now
deprecated in favor of the equivalent -mcpu=5206e.
- -m528x
- Generate output for a member of the ColdFire 528X family.
The option is now deprecated in favor of the equivalent
-mcpu=528x.
- -m5307
- Generate output for a ColdFire 5307 CPU. The option is now
deprecated in favor of the equivalent -mcpu=5307.
- -m5407
- Generate output for a ColdFire 5407 CPU. The option is now
deprecated in favor of the equivalent -mcpu=5407.
- -mcfv4e
- Generate output for a ColdFire V4e family CPU (e.g.
547x/548x). This includes use of hardware floating-point instructions. The
option is equivalent to -mcpu=547x, and is now deprecated in favor
of that option.
- -m68020-40
- Generate output for a 68040, without using any of the new
instructions. This results in code that can run relatively efficiently on
either a 68020/68881 or a 68030 or a 68040. The generated code does use
the 68881 instructions that are emulated on the 68040.
The option is equivalent to -march=68020 -mtune=68020-40.
- -m68020-60
- Generate output for a 68060, without using any of the new
instructions. This results in code that can run relatively efficiently on
either a 68020/68881 or a 68030 or a 68040. The generated code does use
the 68881 instructions that are emulated on the 68060.
The option is equivalent to -march=68020 -mtune=68020-60.
- -mhard-float
- -m68881
- Generate floating-point instructions. This is the default
for 68020 and above, and for ColdFire devices that have an FPU. It defines
the macro "__HAVE_68881__" on M680x0 targets and
"__mcffpu__" on ColdFire targets.
- -msoft-float
- Do not generate floating-point instructions; use library
calls instead. This is the default for 68000, 68010, and 68832 targets. It
is also the default for ColdFire devices that have no FPU.
- -mdiv
- -mno-div
- Generate (do not generate) ColdFire hardware divide and
remainder instructions. If -march is used without -mcpu, the
default is "on" for ColdFire architectures and "off"
for M680x0 architectures. Otherwise, the default is taken from the target
CPU (either the default CPU, or the one specified by -mcpu). For
example, the default is "off" for -mcpu=5206 and
"on" for -mcpu=5206e.
GCC defines the macro "__mcfhwdiv__" when this option is
enabled.
- -mshort
- Consider type "int" to be 16 bits wide, like
"short int". Additionally, parameters passed on the stack are
also aligned to a 16-bit boundary even on targets whose API mandates
promotion to 32-bit.
- -mno-short
- Do not consider type "int" to be 16 bits wide.
This is the default.
- -mnobitfield
- -mno-bitfield
- Do not use the bit-field instructions. The -m68000,
-mcpu32 and -m5200 options imply -mnobitfield.
- -mbitfield
- Do use the bit-field instructions. The -m68020
option implies -mbitfield. This is the default if you use a
configuration designed for a 68020.
- -mrtd
- Use a different function-calling convention, in which
functions that take a fixed number of arguments return with the
"rtd" instruction, which pops their arguments while returning.
This saves one instruction in the caller since there is no need to pop the
arguments there.
This calling convention is incompatible with the one normally used on Unix,
so you cannot use it if you need to call libraries compiled with the Unix
compiler.
Also, you must provide function prototypes for all functions that take
variable numbers of arguments (including "printf"); otherwise
incorrect code is generated for calls to those functions.
In addition, seriously incorrect code results if you call a function with
too many arguments. (Normally, extra arguments are harmlessly ignored.)
The "rtd" instruction is supported by the 68010, 68020, 68030,
68040, 68060 and CPU32 processors, but not by the 68000 or 5200.
- -mno-rtd
- Do not use the calling conventions selected by
-mrtd. This is the default.
- -malign-int
- -mno-align-int
- Control whether GCC aligns "int",
"long", "long long", "float",
"double", and "long double" variables on a 32-bit
boundary ( -malign-int) or a 16-bit boundary
(-mno-align-int). Aligning variables on 32-bit boundaries produces
code that runs somewhat faster on processors with 32-bit busses at the
expense of more memory.
Warning: if you use the -malign-int switch, GCC aligns
structures containing the above types differently than most published
application binary interface specifications for the m68k.
- -mpcrel
- Use the pc-relative addressing mode of the 68000 directly,
instead of using a global offset table. At present, this option implies
-fpic, allowing at most a 16-bit offset for pc-relative addressing.
-fPIC is not presently supported with -mpcrel, though this
could be supported for 68020 and higher processors.
- -mno-strict-align
- -mstrict-align
- Do not (do) assume that unaligned memory references are
handled by the system.
- -msep-data
- Generate code that allows the data segment to be located in
a different area of memory from the text segment. This allows for
execute-in-place in an environment without virtual memory management. This
option implies -fPIC.
- -mno-sep-data
- Generate code that assumes that the data segment follows
the text segment. This is the default.
- -mid-shared-library
- Generate code that supports shared libraries via the
library ID method. This allows for execute-in-place and shared libraries
in an environment without virtual memory management. This option implies
-fPIC.
- -mno-id-shared-library
- Generate code that doesn't assume ID-based shared libraries
are being used. This is the default.
- -mshared-library-id=n
- Specifies the identification number of the ID-based shared
library being compiled. Specifying a value of 0 generates more compact
code; specifying other values forces the allocation of that number to the
current library, but is no more space- or time-efficient than omitting
this option.
- -mxgot
- -mno-xgot
- When generating position-independent code for ColdFire,
generate code that works if the GOT has more than 8192 entries. This code
is larger and slower than code generated without this option. On M680x0
processors, this option is not needed; -fPIC suffices.
GCC normally uses a single instruction to load values from the GOT. While
this is relatively efficient, it only works if the GOT is smaller than
about 64k. Anything larger causes the linker to report an error such as:
relocation truncated to fit: R_68K_GOT16O foobar
If this happens, you should recompile your code with -mxgot. It
should then work with very large GOTs. However, code generated with
-mxgot is less efficient, since it takes 4 instructions to fetch
the value of a global symbol.
Note that some linkers, including newer versions of the GNU linker, can
create multiple GOTs and sort GOT entries. If you have such a linker, you
should only need to use -mxgot when compiling a single object file
that accesses more than 8192 GOT entries. Very few do.
These options have no effect unless GCC is generating position-independent
code.
MCore Options
These are the
-m options defined for the Motorola M*Core processors.
- -mhardlit
- -mno-hardlit
- Inline constants into the code stream if it can be done in
two instructions or less.
- -mdiv
- -mno-div
- Use the divide instruction. (Enabled by default).
- -mrelax-immediate
- -mno-relax-immediate
- Allow arbitrary-sized immediates in bit operations.
- -mwide-bitfields
- -mno-wide-bitfields
- Always treat bit-fields as "int"-sized.
- -m4byte-functions
- -mno-4byte-functions
- Force all functions to be aligned to a 4-byte
boundary.
- -mcallgraph-data
- -mno-callgraph-data
- Emit callgraph information.
- -mslow-bytes
- -mno-slow-bytes
- Prefer word access when reading byte quantities.
- -mlittle-endian
- -mbig-endian
- Generate code for a little-endian target.
- -m210
- -m340
- Generate code for the 210 processor.
- -mno-lsim
- Assume that runtime support has been provided and so omit
the simulator library ( libsim.a) from the linker command
line.
- -mstack-increment=size
- Set the maximum amount for a single stack increment
operation. Large values can increase the speed of programs that contain
functions that need a large amount of stack space, but they can also
trigger a segmentation fault if the stack is extended too much. The
default value is 0x1000.
MeP Options
- -mabsdiff
- Enables the "abs" instruction, which is the
absolute difference between two registers.
- -mall-opts
- Enables all the optional instructions---average, multiply,
divide, bit operations, leading zero, absolute difference, min/max, clip,
and saturation.
- -maverage
- Enables the "ave" instruction, which computes the
average of two registers.
- -mbased=n
- Variables of size n bytes or smaller are placed in
the ".based" section by default. Based variables use the $tp
register as a base register, and there is a 128-byte limit to the
".based" section.
- -mbitops
- Enables the bit operation instructions---bit test
("btstm"), set ("bsetm"), clear ("bclrm"),
invert ("bnotm"), and test-and-set ("tas").
- -mc=name
- Selects which section constant data is placed in.
name may be tiny, near, or far.
- -mclip
- Enables the "clip" instruction. Note that
-mclip is not useful unless you also provide -mminmax.
- -mconfig=name
- Selects one of the built-in core configurations. Each MeP
chip has one or more modules in it; each module has a core CPU and a
variety of coprocessors, optional instructions, and peripherals. The
"MeP-Integrator" tool, not part of GCC, provides these
configurations through this option; using this option is the same as using
all the corresponding command-line options. The default configuration is
default.
- -mcop
- Enables the coprocessor instructions. By default, this is a
32-bit coprocessor. Note that the coprocessor is normally enabled via the
-mconfig= option.
- -mcop32
- Enables the 32-bit coprocessor's instructions.
- -mcop64
- Enables the 64-bit coprocessor's instructions.
- -mivc2
- Enables IVC2 scheduling. IVC2 is a 64-bit VLIW
coprocessor.
- -mdc
- Causes constant variables to be placed in the
".near" section.
- -mdiv
- Enables the "div" and "divu"
instructions.
- -meb
- Generate big-endian code.
- -mel
- Generate little-endian code.
- -mio-volatile
- Tells the compiler that any variable marked with the
"io" attribute is to be considered volatile.
- -ml
- Causes variables to be assigned to the ".far"
section by default.
- -mleadz
- Enables the "leadz" (leading zero)
instruction.
- -mm
- Causes variables to be assigned to the ".near"
section by default.
- -mminmax
- Enables the "min" and "max"
instructions.
- -mmult
- Enables the multiplication and multiply-accumulate
instructions.
- -mno-opts
- Disables all the optional instructions enabled by
-mall-opts.
- -mrepeat
- Enables the "repeat" and "erepeat"
instructions, used for low-overhead looping.
- -ms
- Causes all variables to default to the ".tiny"
section. Note that there is a 65536-byte limit to this section. Accesses
to these variables use the %gp base register.
- -msatur
- Enables the saturation instructions. Note that the compiler
does not currently generate these itself, but this option is included for
compatibility with other tools, like "as".
- -msdram
- Link the SDRAM-based runtime instead of the default
ROM-based runtime.
- -msim
- Link the simulator run-time libraries.
- -msimnovec
- Link the simulator runtime libraries, excluding built-in
support for reset and exception vectors and tables.
- -mtf
- Causes all functions to default to the ".far"
section. Without this option, functions default to the ".near"
section.
- -mtiny=n
- Variables that are n bytes or smaller are allocated
to the ".tiny" section. These variables use the $gp base
register. The default for this option is 4, but note that there's a
65536-byte limit to the ".tiny" section.
MicroBlaze Options
- -msoft-float
- Use software emulation for floating point (default).
- -mhard-float
- Use hardware floating-point instructions.
- -mmemcpy
- Do not optimize block moves, use "memcpy".
- -mno-clearbss
- This option is deprecated. Use
-fno-zero-initialized-in-bss instead.
- -mcpu=cpu-type
- Use features of, and schedule code for, the given CPU.
Supported values are in the format
vX.YY. Z, where X is a
major version, YY is the minor version, and Z is
compatibility code. Example values are v3.00.a, v4.00.b,
v5.00.a, v5.00.b, v5.00.b, v6.00.a.
- -mxl-soft-mul
- Use software multiply emulation (default).
- -mxl-soft-div
- Use software emulation for divides (default).
- -mxl-barrel-shift
- Use the hardware barrel shifter.
- -mxl-pattern-compare
- Use pattern compare instructions.
- -msmall-divides
- Use table lookup optimization for small signed integer
divisions.
- -mxl-stack-check
- This option is deprecated. Use -fstack-check
instead.
- -mxl-gp-opt
- Use GP-relative ".sdata"/".sbss"
sections.
- -mxl-multiply-high
- Use multiply high instructions for high part of 32x32
multiply.
- -mxl-float-convert
- Use hardware floating-point conversion instructions.
- -mxl-float-sqrt
- Use hardware floating-point square root instruction.
- -mbig-endian
- Generate code for a big-endian target.
- -mlittle-endian
- Generate code for a little-endian target.
- -mxl-reorder
- Use reorder instructions (swap and byte reversed
load/store).
- -mxl-mode-app-model
- Select application model app-model. Valid models
are
- executable
- normal executable (default), uses startup code
crt0.o.
- xmdstub
- for use with Xilinx Microprocessor Debugger (XMD) based
software intrusive debug agent called xmdstub. This uses startup file
crt1.o and sets the start address of the program to 0x800.
- bootstrap
- for applications that are loaded using a bootloader. This
model uses startup file crt2.o which does not contain a processor
reset vector handler. This is suitable for transferring control on a
processor reset to the bootloader rather than the application.
- novectors
- for applications that do not require any of the MicroBlaze
vectors. This option may be useful for applications running within a
monitoring application. This model uses crt3.o as a startup
file.
Option
-xl-mode-app-model is a deprecated alias for
-mxl-mode- app-model.
MIPS Options
- -EB
- Generate big-endian code.
- -EL
- Generate little-endian code. This is the default for
mips*el-*-* configurations.
- -march=arch
- Generate code that runs on arch, which can be the
name of a generic MIPS ISA, or the name of a particular processor. The ISA
names are: mips1, mips2, mips3, mips4,
mips32, mips32r2, mips32r3, mips32r5,
mips32r6, mips64, mips64r2, mips64r3,
mips64r5 and mips64r6. The processor names are: 4kc,
4km, 4kp, 4ksc, 4kec, 4kem,
4kep, 4ksd, 5kc, 5kf, 20kc,
24kc, 24kf2_1, 24kf1_1, 24kec,
24kef2_1, 24kef1_1, 34kc, 34kf2_1,
34kf1_1, 34kn, 74kc, 74kf2_1, 74kf1_1,
74kf3_2, 1004kc, 1004kf2_1, 1004kf1_1,
loongson2e, loongson2f, loongson3a, m4k,
m14k, m14kc, m14ke, m14kec, octeon,
octeon+, octeon2, octeon3, orion,
p5600, r2000, r3000, r3900, r4000,
r4400, r4600, r4650, r4700, r6000,
r8000, rm7000, rm9000, r10000, r12000,
r14000, r16000, sb1, sr71000, vr4100,
vr4111, vr4120, vr4130, vr4300, vr5000,
vr5400, vr5500, xlr and xlp. The special value
from-abi selects the most compatible architecture for the selected
ABI (that is, mips1 for 32-bit ABIs and mips3 for 64-bit
ABIs).
The native Linux/GNU toolchain also supports the value native, which
selects the best architecture option for the host processor.
-march=native has no effect if GCC does not recognize the
processor.
In processor names, a final 000 can be abbreviated as k (for
example, -march=r2k). Prefixes are optional, and vr may be
written r.
Names of the form nf2_1 refer to processors with FPUs clocked
at half the rate of the core, names of the form nf1_1 refer
to processors with FPUs clocked at the same rate as the core, and names of
the form nf3_2 refer to processors with FPUs clocked a ratio
of 3:2 with respect to the core. For compatibility reasons,
nf is accepted as a synonym for nf2_1 while
nx and bfx are accepted as synonyms for
n f1_1.
GCC defines two macros based on the value of this option. The first is
"_MIPS_ARCH", which gives the name of target architecture, as a
string. The second has the form "_MIPS_ARCH_ foo", where
foo is the capitalized value of "_MIPS_ARCH". For
example, -march=r2000 sets "_MIPS_ARCH" to
"r2000" and defines the macro "_MIPS_ARCH_R2000".
Note that the "_MIPS_ARCH" macro uses the processor names given
above. In other words, it has the full prefix and does not abbreviate
000 as k. In the case of from-abi, the macro names
the resolved architecture (either "mips1" or "mips3").
It names the default architecture when no -march option is
given.
- -mtune=arch
- Optimize for arch. Among other things, this option
controls the way instructions are scheduled, and the perceived cost of
arithmetic operations. The list of arch values is the same as for
-march.
When this option is not used, GCC optimizes for the processor specified by
-march. By using -march and -mtune together, it is
possible to generate code that runs on a family of processors, but
optimize the code for one particular member of that family.
-mtune defines the macros "_MIPS_TUNE" and
"_MIPS_TUNE_ foo", which work in the same way as the
-march ones described above.
- -mips1
- Equivalent to -march=mips1.
- -mips2
- Equivalent to -march=mips2.
- -mips3
- Equivalent to -march=mips3.
- -mips4
- Equivalent to -march=mips4.
- -mips32
- Equivalent to -march=mips32.
- -mips32r3
- Equivalent to -march=mips32r3.
- -mips32r5
- Equivalent to -march=mips32r5.
- -mips32r6
- Equivalent to -march=mips32r6.
- -mips64
- Equivalent to -march=mips64.
- -mips64r2
- Equivalent to -march=mips64r2.
- -mips64r3
- Equivalent to -march=mips64r3.
- -mips64r5
- Equivalent to -march=mips64r5.
- -mips64r6
- Equivalent to -march=mips64r6.
- -mips16
- -mno-mips16
- Generate (do not generate) MIPS16 code. If GCC is targeting
a MIPS32 or MIPS64 architecture, it makes use of the MIPS16e ASE.
MIPS16 code generation can also be controlled on a per-function basis by
means of "mips16" and "nomips16" attributes.
- -mflip-mips16
- Generate MIPS16 code on alternating functions. This option
is provided for regression testing of mixed MIPS16/non-MIPS16 code
generation, and is not intended for ordinary use in compiling user
code.
- -minterlink-compressed
- -mno-interlink-compressed
- Require (do not require) that code using the standard
(uncompressed) MIPS ISA be link-compatible with MIPS16 and microMIPS code,
and vice versa.
For example, code using the standard ISA encoding cannot jump directly to
MIPS16 or microMIPS code; it must either use a call or an indirect jump.
-minterlink-compressed therefore disables direct jumps unless GCC
knows that the target of the jump is not compressed.
- -minterlink-mips16
- -mno-interlink-mips16
- Aliases of -minterlink-compressed and
-mno-interlink-compressed. These options predate the microMIPS ASE
and are retained for backwards compatibility.
- -mabi=32
- -mabi=o64
- -mabi=n32
- -mabi=64
- -mabi=eabi
- Generate code for the given ABI.
Note that the EABI has a 32-bit and a 64-bit variant. GCC normally generates
64-bit code when you select a 64-bit architecture, but you can use
-mgp32 to get 32-bit code instead.
For information about the O64 ABI, see <
http://gcc.gnu.org/projects/mipso64-abi.html>.
GCC supports a variant of the o32 ABI in which floating-point registers are
64 rather than 32 bits wide. You can select this combination with
-mabi=32 -mfp64. This ABI relies on the "mthc1"
and "mfhc1" instructions and is therefore only supported for
MIPS32R2, MIPS32R3 and MIPS32R5 processors.
The register assignments for arguments and return values remain the same,
but each scalar value is passed in a single 64-bit register rather than a
pair of 32-bit registers. For example, scalar floating-point values are
returned in $f0 only, not a
$f0/$f1 pair. The set of
call-saved registers also remains the same in that the even-numbered
double-precision registers are saved.
Two additional variants of the o32 ABI are supported to enable a transition
from 32-bit to 64-bit registers. These are FPXX ( -mfpxx) and FP64A
( -mfp64 -mno-odd-spreg). The FPXX extension mandates that
all code must execute correctly when run using 32-bit or 64-bit registers.
The code can be interlinked with either FP32 or FP64, but not both. The
FP64A extension is similar to the FP64 extension but forbids the use of
odd-numbered single-precision registers. This can be used in conjunction
with the "FRE" mode of FPUs in MIPS32R5 processors and allows
both FP32 and FP64A code to interlink and run in the same process without
changing FPU modes.
- -mabicalls
- -mno-abicalls
- Generate (do not generate) code that is suitable for
SVR4-style dynamic objects. -mabicalls is the default for
SVR4-based systems.
- -mshared
- -mno-shared
- Generate (do not generate) code that is fully
position-independent, and that can therefore be linked into shared
libraries. This option only affects -mabicalls.
All -mabicalls code has traditionally been position-independent,
regardless of options like -fPIC and -fpic. However, as an
extension, the GNU toolchain allows executables to use absolute accesses
for locally-binding symbols. It can also use shorter GP initialization
sequences and generate direct calls to locally-defined functions. This
mode is selected by -mno-shared.
-mno-shared depends on binutils 2.16 or higher and generates objects
that can only be linked by the GNU linker. However, the option does not
affect the ABI of the final executable; it only affects the ABI of
relocatable objects. Using -mno-shared generally makes executables
both smaller and quicker.
-mshared is the default.
- -mplt
- -mno-plt
- Assume (do not assume) that the static and dynamic linkers
support PLTs and copy relocations. This option only affects -mno-shared
-mabicalls. For the n64 ABI, this option has no effect without
-msym32.
You can make -mplt the default by configuring GCC with
--with-mips-plt. The default is -mno-plt otherwise.
- -mxgot
- -mno-xgot
- Lift (do not lift) the usual restrictions on the size of
the global offset table.
GCC normally uses a single instruction to load values from the GOT. While
this is relatively efficient, it only works if the GOT is smaller than
about 64k. Anything larger causes the linker to report an error such as:
relocation truncated to fit: R_MIPS_GOT16 foobar
If this happens, you should recompile your code with -mxgot. This
works with very large GOTs, although the code is also less efficient,
since it takes three instructions to fetch the value of a global symbol.
Note that some linkers can create multiple GOTs. If you have such a linker,
you should only need to use -mxgot when a single object file
accesses more than 64k's worth of GOT entries. Very few do.
These options have no effect unless GCC is generating position independent
code.
- -mgp32
- Assume that general-purpose registers are 32 bits
wide.
- -mgp64
- Assume that general-purpose registers are 64 bits
wide.
- -mfp32
- Assume that floating-point registers are 32 bits wide.
- -mfp64
- Assume that floating-point registers are 64 bits wide.
- -mfpxx
- Do not assume the width of floating-point registers.
- -mhard-float
- Use floating-point coprocessor instructions.
- -msoft-float
- Do not use floating-point coprocessor instructions.
Implement floating-point calculations using library calls instead.
- -mno-float
- Equivalent to -msoft-float, but additionally asserts
that the program being compiled does not perform any floating-point
operations. This option is presently supported only by some bare-metal
MIPS configurations, where it may select a special set of libraries that
lack all floating-point support (including, for example, the
floating-point "printf" formats). If code compiled with
-mno-float accidentally contains floating-point operations, it is
likely to suffer a link-time or run-time failure.
- -msingle-float
- Assume that the floating-point coprocessor only supports
single-precision operations.
- -mdouble-float
- Assume that the floating-point coprocessor supports
double-precision operations. This is the default.
- -modd-spreg
- -mno-odd-spreg
- Enable the use of odd-numbered single-precision
floating-point registers for the o32 ABI. This is the default for
processors that are known to support these registers. When using the o32
FPXX ABI, -mno-odd-spreg is set by default.
- -mabs=2008
- -mabs=legacy
- These options control the treatment of the special
not-a-number (NaN) IEEE 754 floating-point data with the "abs.
fmt" and "neg. fmt" machine instructions.
By default or when -mabs=legacy is used the legacy treatment is
selected. In this case these instructions are considered arithmetic and
avoided where correct operation is required and the input operand might be
a NaN. A longer sequence of instructions that manipulate the sign bit of
floating-point datum manually is used instead unless the
-ffinite-math-only option has also been specified.
The -mabs=2008 option selects the IEEE 754-2008 treatment. In this
case these instructions are considered non-arithmetic and therefore
operating correctly in all cases, including in particular where the input
operand is a NaN. These instructions are therefore always used for the
respective operations.
- -mnan=2008
- -mnan=legacy
- These options control the encoding of the special
not-a-number (NaN) IEEE 754 floating-point data.
The -mnan=legacy option selects the legacy encoding. In this case
quiet NaNs (qNaNs) are denoted by the first bit of their trailing
significand field being 0, whereas signalling NaNs (sNaNs) are denoted by
the first bit of their trailing significand field being 1.
The -mnan=2008 option selects the IEEE 754-2008 encoding. In this
case qNaNs are denoted by the first bit of their trailing significand
field being 1, whereas sNaNs are denoted by the first bit of their
trailing significand field being 0.
The default is -mnan=legacy unless GCC has been configured with
--with-nan=2008.
- -mllsc
- -mno-llsc
- Use (do not use) ll, sc, and sync
instructions to implement atomic memory built-in functions. When neither
option is specified, GCC uses the instructions if the target architecture
supports them.
-mllsc is useful if the runtime environment can emulate the
instructions and -mno-llsc can be useful when compiling for
nonstandard ISAs. You can make either option the default by configuring
GCC with --with-llsc and --without-llsc respectively.
--with-llsc is the default for some configurations; see the
installation documentation for details.
- -mdsp
- -mno-dsp
- Use (do not use) revision 1 of the MIPS DSP ASE.
This option defines the preprocessor macro "__mips_dsp". It also
defines "__mips_dsp_rev" to 1.
- -mdspr2
- -mno-dspr2
- Use (do not use) revision 2 of the MIPS DSP ASE.
This option defines the preprocessor macros "__mips_dsp" and
"__mips_dspr2". It also defines "__mips_dsp_rev" to
2.
- -msmartmips
- -mno-smartmips
- Use (do not use) the MIPS SmartMIPS ASE.
- -mpaired-single
- -mno-paired-single
- Use (do not use) paired-single floating-point instructions.
This option requires hardware floating-point support to be enabled.
- -mdmx
- -mno-mdmx
- Use (do not use) MIPS Digital Media Extension instructions.
This option can only be used when generating 64-bit code and requires
hardware floating-point support to be enabled.
- -mips3d
- -mno-mips3d
- Use (do not use) the MIPS-3D ASE. The option -mips3d
implies -mpaired-single.
- -mmicromips
- -mno-micromips
- Generate (do not generate) microMIPS code.
MicroMIPS code generation can also be controlled on a per-function basis by
means of "micromips" and "nomicromips"
attributes.
- -mmt
- -mno-mt
- Use (do not use) MT Multithreading instructions.
- -mmcu
- -mno-mcu
- Use (do not use) the MIPS MCU ASE instructions.
- -meva
- -mno-eva
- Use (do not use) the MIPS Enhanced Virtual Addressing
instructions.
- -mvirt
- -mno-virt
- Use (do not use) the MIPS Virtualization Application
Specific instructions.
- -mxpa
- -mno-xpa
- Use (do not use) the MIPS eXtended Physical Address (XPA)
instructions.
- -mlong64
- Force "long" types to be 64 bits wide. See
-mlong32 for an explanation of the default and the way that the
pointer size is determined.
- -mlong32
- Force "long", "int", and pointer types
to be 32 bits wide.
The default size of "int"s, "long"s and pointers depends
on the ABI. All the supported ABIs use 32-bit "int"s. The n64
ABI uses 64-bit "long"s, as does the 64-bit EABI; the others use
32-bit "long"s. Pointers are the same size as "long"s,
or the same size as integer registers, whichever is smaller.
- -msym32
- -mno-sym32
- Assume (do not assume) that all symbols have 32-bit values,
regardless of the selected ABI. This option is useful in combination with
-mabi=64 and -mno-abicalls because it allows GCC to generate
shorter and faster references to symbolic addresses.
- -G num
- Put definitions of externally-visible data in a small data
section if that data is no bigger than num bytes. GCC can then
generate more efficient accesses to the data; see -mgpopt for
details.
The default -G option depends on the configuration.
- -mlocal-sdata
- -mno-local-sdata
- Extend (do not extend) the -G behavior to local data
too, such as to static variables in C. -mlocal-sdata is the default
for all configurations.
If the linker complains that an application is using too much small data,
you might want to try rebuilding the less performance-critical parts with
-mno-local-sdata. You might also want to build large libraries with
-mno-local-sdata, so that the libraries leave more room for the
main program.
- -mextern-sdata
- -mno-extern-sdata
- Assume (do not assume) that externally-defined data is in a
small data section if the size of that data is within the -G limit.
-mextern-sdata is the default for all configurations.
If you compile a module Mod with -mextern-sdata -G
num -mgpopt, and Mod references a variable Var
that is no bigger than num bytes, you must make sure that
Var is placed in a small data section. If Var is defined by
another module, you must either compile that module with a high-enough
-G setting or attach a "section" attribute to
Var's definition. If Var is common, you must link the
application with a high-enough -G setting.
The easiest way of satisfying these restrictions is to compile and link
every module with the same -G option. However, you may wish to
build a library that supports several different small data limits. You can
do this by compiling the library with the highest supported -G
setting and additionally using -mno-extern-sdata to stop the
library from making assumptions about externally-defined data.
- -mgpopt
- -mno-gpopt
- Use (do not use) GP-relative accesses for symbols that are
known to be in a small data section; see -G, -mlocal-sdata
and -mextern-sdata. -mgpopt is the default for all
configurations.
-mno-gpopt is useful for cases where the $gp register might not hold
the value of "_gp". For example, if the code is part of a
library that might be used in a boot monitor, programs that call boot
monitor routines pass an unknown value in $gp. (In such situations, the
boot monitor itself is usually compiled with -G0.)
-mno-gpopt implies -mno-local-sdata and
-mno-extern-sdata.
- -membedded-data
- -mno-embedded-data
- Allocate variables to the read-only data section first if
possible, then next in the small data section if possible, otherwise in
data. This gives slightly slower code than the default, but reduces the
amount of RAM required when executing, and thus may be preferred for some
embedded systems.
- -muninit-const-in-rodata
- -mno-uninit-const-in-rodata
- Put uninitialized "const" variables in the
read-only data section. This option is only meaningful in conjunction with
-membedded-data.
- -mcode-readable=setting
- Specify whether GCC may generate code that reads from
executable sections. There are three possible settings:
- -mcode-readable=yes
- Instructions may freely access executable sections. This is
the default setting.
- -mcode-readable=pcrel
- MIPS16 PC-relative load instructions can access executable
sections, but other instructions must not do so. This option is useful on
4KSc and 4KSd processors when the code TLBs have the Read Inhibit bit set.
It is also useful on processors that can be configured to have a dual
instruction/data SRAM interface and that, like the M4K, automatically
redirect PC-relative loads to the instruction RAM.
- -mcode-readable=no
- Instructions must not access executable sections. This
option can be useful on targets that are configured to have a dual
instruction/data SRAM interface but that (unlike the M4K) do not
automatically redirect PC-relative loads to the instruction RAM.
- -msplit-addresses
- -mno-split-addresses
- Enable (disable) use of the "%hi()" and
"%lo()" assembler relocation operators. This option has been
superseded by -mexplicit-relocs but is retained for backwards
compatibility.
- -mexplicit-relocs
- -mno-explicit-relocs
- Use (do not use) assembler relocation operators when
dealing with symbolic addresses. The alternative, selected by
-mno-explicit-relocs, is to use assembler macros instead.
-mexplicit-relocs is the default if GCC was configured to use an
assembler that supports relocation operators.
- -mcheck-zero-division
- -mno-check-zero-division
- Trap (do not trap) on integer division by zero.
The default is -mcheck-zero-division.
- -mdivide-traps
- -mdivide-breaks
- MIPS systems check for division by zero by generating
either a conditional trap or a break instruction. Using traps results in
smaller code, but is only supported on MIPS II and later. Also, some
versions of the Linux kernel have a bug that prevents trap from generating
the proper signal ("SIGFPE"). Use -mdivide-traps to allow
conditional traps on architectures that support them and
-mdivide-breaks to force the use of breaks.
The default is usually -mdivide-traps, but this can be overridden at
configure time using --with-divide=breaks. Divide-by-zero checks
can be completely disabled using -mno-check-zero-division.
- -mmemcpy
- -mno-memcpy
- Force (do not force) the use of "memcpy" for
non-trivial block moves. The default is -mno-memcpy, which allows
GCC to inline most constant-sized copies.
- -mlong-calls
- -mno-long-calls
- Disable (do not disable) use of the "jal"
instruction. Calling functions using "jal" is more efficient but
requires the caller and callee to be in the same 256 megabyte segment.
This option has no effect on abicalls code. The default is
-mno-long-calls.
- -mmad
- -mno-mad
- Enable (disable) use of the "mad",
"madu" and "mul" instructions, as provided by the
R4650 ISA.
- -mimadd
- -mno-imadd
- Enable (disable) use of the "madd" and
"msub" integer instructions. The default is -mimadd on
architectures that support "madd" and "msub" except
for the 74k architecture where it was found to generate slower code.
- -mfused-madd
- -mno-fused-madd
- Enable (disable) use of the floating-point
multiply-accumulate instructions, when they are available. The default is
-mfused-madd.
On the R8000 CPU when multiply-accumulate instructions are used, the
intermediate product is calculated to infinite precision and is not
subject to the FCSR Flush to Zero bit. This may be undesirable in some
circumstances. On other processors the result is numerically identical to
the equivalent computation using separate multiply, add, subtract and
negate instructions.
- -nocpp
- Tell the MIPS assembler to not run its preprocessor over
user assembler files (with a .s suffix) when assembling them.
- -mfix-24k
- -mno-fix-24k
- Work around the 24K E48 (lost data on stores during refill)
errata. The workarounds are implemented by the assembler rather than by
GCC.
- -mfix-r4000
- -mno-fix-r4000
- Work around certain R4000 CPU errata:
- -
- A double-word or a variable shift may give an incorrect
result if executed immediately after starting an integer division.
- -
- A double-word or a variable shift may give an incorrect
result if executed while an integer multiplication is in progress.
- -
- An integer division may give an incorrect result if started
in a delay slot of a taken branch or a jump.
- -mfix-r4400
- -mno-fix-r4400
- Work around certain R4400 CPU errata:
- -
- A double-word or a variable shift may give an incorrect
result if executed immediately after starting an integer division.
- -mfix-r10000
- -mno-fix-r10000
- Work around certain R10000 errata:
- -
- "ll"/"sc" sequences may not behave
atomically on revisions prior to 3.0. They may deadlock on revisions 2.6
and earlier.
This option can only be used if the target architecture supports branch-likely
instructions.
-mfix-r10000 is the default when
-march=r10000 is
used;
-mno-fix-r10000 is the default otherwise.
- -mfix-rm7000
- -mno-fix-rm7000
- Work around the RM7000 "dmult"/"dmultu"
errata. The workarounds are implemented by the assembler rather than by
GCC.
- -mfix-vr4120
- -mno-fix-vr4120
- Work around certain VR4120 errata:
- -
- "dmultu" does not always produce the correct
result.
- -
- "div" and "ddiv" do not always produce
the correct result if one of the operands is negative.
The workarounds for the division errata rely on special functions in
libgcc.a. At present, these functions are only provided by the
"mips64vr*-elf" configurations.
Other VR4120 errata require a NOP to be inserted between certain pairs of
instructions. These errata are handled by the assembler, not by GCC
itself.
- -mfix-vr4130
- Work around the VR4130 "mflo"/"mfhi"
errata. The workarounds are implemented by the assembler rather than by
GCC, although GCC avoids using "mflo" and "mfhi" if
the VR4130 "macc", "macchi", "dmacc" and
"dmacchi" instructions are available instead.
- -mfix-sb1
- -mno-fix-sb1
- Work around certain SB-1 CPU core errata. (This flag
currently works around the SB-1 revision 2 "F1" and
"F2" floating-point errata.)
- -mr10k-cache-barrier=setting
- Specify whether GCC should insert cache barriers to avoid
the side-effects of speculation on R10K processors.
In common with many processors, the R10K tries to predict the outcome of a
conditional branch and speculatively executes instructions from the
"taken" branch. It later aborts these instructions if the
predicted outcome is wrong. However, on the R10K, even aborted
instructions can have side effects.
This problem only affects kernel stores and, depending on the system, kernel
loads. As an example, a speculatively-executed store may load the target
memory into cache and mark the cache line as dirty, even if the store
itself is later aborted. If a DMA operation writes to the same area of
memory before the "dirty" line is flushed, the cached data
overwrites the DMA-ed data. See the R10K processor manual for a full
description, including other potential problems.
One workaround is to insert cache barrier instructions before every memory
access that might be speculatively executed and that might have side
effects even if aborted. -mr10k-cache-barrier=setting
controls GCC's implementation of this workaround. It assumes that aborted
accesses to any byte in the following regions does not have side
effects:
- 1.
- the memory occupied by the current function's stack
frame;
- 2.
- the memory occupied by an incoming stack argument;
- 3.
- the memory occupied by an object with a link-time-constant
address.
It is the kernel's responsibility to ensure that speculative accesses to these
regions are indeed safe.
If the input program contains a function declaration such as:
void foo (void);
then the implementation of "foo" must allow "j foo" and
"jal foo" to be executed speculatively. GCC honors this restriction
for functions it compiles itself. It expects non-GCC functions (such as
hand-written assembly code) to do the same.
The option has three forms:
- -mr10k-cache-barrier=load-store
- Insert a cache barrier before a load or store that might be
speculatively executed and that might have side effects even if
aborted.
- -mr10k-cache-barrier=store
- Insert a cache barrier before a store that might be
speculatively executed and that might have side effects even if
aborted.
- -mr10k-cache-barrier=none
- Disable the insertion of cache barriers. This is the
default setting.
- -mflush-func=func
- -mno-flush-func
- Specifies the function to call to flush the I and D caches,
or to not call any such function. If called, the function must take the
same arguments as the common "_flush_func", that is, the address
of the memory range for which the cache is being flushed, the size of the
memory range, and the number 3 (to flush both caches). The default depends
on the target GCC was configured for, but commonly is either
"_flush_func" or "__cpu_flush".
- mbranch-cost=num
- Set the cost of branches to roughly num
"simple" instructions. This cost is only a heuristic and is not
guaranteed to produce consistent results across releases. A zero cost
redundantly selects the default, which is based on the -mtune
setting.
- -mbranch-likely
- -mno-branch-likely
- Enable or disable use of Branch Likely instructions,
regardless of the default for the selected architecture. By default,
Branch Likely instructions may be generated if they are supported by the
selected architecture. An exception is for the MIPS32 and MIPS64
architectures and processors that implement those architectures; for
those, Branch Likely instructions are not be generated by default because
the MIPS32 and MIPS64 architectures specifically deprecate their use.
- -mfp-exceptions
- -mno-fp-exceptions
- Specifies whether FP exceptions are enabled. This affects
how FP instructions are scheduled for some processors. The default is that
FP exceptions are enabled.
For instance, on the SB-1, if FP exceptions are disabled, and we are
emitting 64-bit code, then we can use both FP pipes. Otherwise, we can
only use one FP pipe.
- -mvr4130-align
- -mno-vr4130-align
- The VR4130 pipeline is two-way superscalar, but can only
issue two instructions together if the first one is 8-byte aligned. When
this option is enabled, GCC aligns pairs of instructions that it thinks
should execute in parallel.
This option only has an effect when optimizing for the VR4130. It normally
makes code faster, but at the expense of making it bigger. It is enabled
by default at optimization level -O3.
- -msynci
- -mno-synci
- Enable (disable) generation of "synci"
instructions on architectures that support it. The "synci"
instructions (if enabled) are generated when
"__builtin___clear_cache" is compiled.
This option defaults to -mno-synci, but the default can be overridden
by configuring GCC with --with-synci.
When compiling code for single processor systems, it is generally safe to
use "synci". However, on many multi-core (SMP) systems, it does
not invalidate the instruction caches on all cores and may lead to
undefined behavior.
- -mrelax-pic-calls
- -mno-relax-pic-calls
- Try to turn PIC calls that are normally dispatched via
register $25 into direct calls. This is only possible if the linker can
resolve the destination at link-time and if the destination is within
range for a direct call.
-mrelax-pic-calls is the default if GCC was configured to use an
assembler and a linker that support the ".reloc" assembly
directive and -mexplicit-relocs is in effect. With
-mno-explicit-relocs, this optimization can be performed by the
assembler and the linker alone without help from the compiler.
- -mmcount-ra-address
- -mno-mcount-ra-address
- Emit (do not emit) code that allows "_mcount" to
modify the calling function's return address. When enabled, this option
extends the usual "_mcount" interface with a new
ra-address parameter, which has type "intptr_t *" and is
passed in register $12. "_mcount" can then modify the return
address by doing both of the following:
- *
- Returning the new address in register $31.
- *
- Storing the new address in "*ra-address",
if ra-address is nonnull.
The default is
-mno-mcount-ra-address.
MMIX Options
These options are defined for the MMIX:
- -mlibfuncs
- -mno-libfuncs
- Specify that intrinsic library functions are being
compiled, passing all values in registers, no matter the size.
- -mepsilon
- -mno-epsilon
- Generate floating-point comparison instructions that
compare with respect to the "rE" epsilon register.
- -mabi=mmixware
- -mabi=gnu
- Generate code that passes function parameters and return
values that (in the called function) are seen as registers $0 and up, as
opposed to the GNU ABI which uses global registers $231 and up.
- -mzero-extend
- -mno-zero-extend
- When reading data from memory in sizes shorter than 64
bits, use (do not use) zero-extending load instructions by default, rather
than sign-extending ones.
- -mknuthdiv
- -mno-knuthdiv
- Make the result of a division yielding a remainder have the
same sign as the divisor. With the default, -mno-knuthdiv, the sign
of the remainder follows the sign of the dividend. Both methods are
arithmetically valid, the latter being almost exclusively used.
- -mtoplevel-symbols
- -mno-toplevel-symbols
- Prepend (do not prepend) a : to all global symbols,
so the assembly code can be used with the "PREFIX" assembly
directive.
- -melf
- Generate an executable in the ELF format, rather than the
default mmo format used by the mmix simulator.
- -mbranch-predict
- -mno-branch-predict
- Use (do not use) the probable-branch instructions, when
static branch prediction indicates a probable branch.
- -mbase-addresses
- -mno-base-addresses
- Generate (do not generate) code that uses base
addresses. Using a base address automatically generates a request
(handled by the assembler and the linker) for a constant to be set up in a
global register. The register is used for one or more base address
requests within the range 0 to 255 from the value held in the register.
The generally leads to short and fast code, but the number of different
data items that can be addressed is limited. This means that a program
that uses lots of static data may require -mno-base-addresses.
- -msingle-exit
- -mno-single-exit
- Force (do not force) generated code to have a single exit
point in each function.
MN10300 Options
These
-m options are defined for Matsushita MN10300 architectures:
- -mmult-bug
- Generate code to avoid bugs in the multiply instructions
for the MN10300 processors. This is the default.
- -mno-mult-bug
- Do not generate code to avoid bugs in the multiply
instructions for the MN10300 processors.
- -mam33
- Generate code using features specific to the AM33
processor.
- -mno-am33
- Do not generate code using features specific to the AM33
processor. This is the default.
- -mam33-2
- Generate code using features specific to the AM33/2.0
processor.
- -mam34
- Generate code using features specific to the AM34
processor.
- -mtune=cpu-type
- Use the timing characteristics of the indicated CPU type
when scheduling instructions. This does not change the targeted processor
type. The CPU type must be one of mn10300, am33,
am33-2 or am34.
- -mreturn-pointer-on-d0
- When generating a function that returns a pointer, return
the pointer in both "a0" and "d0". Otherwise, the
pointer is returned only in "a0", and attempts to call such
functions without a prototype result in errors. Note that this option is
on by default; use -mno-return-pointer-on-d0 to disable it.
- -mno-crt0
- Do not link in the C run-time initialization object
file.
- -mrelax
- Indicate to the linker that it should perform a relaxation
optimization pass to shorten branches, calls and absolute memory
addresses. This option only has an effect when used on the command line
for the final link step.
This option makes symbolic debugging impossible.
- -mliw
- Allow the compiler to generate Long Instruction Word
instructions if the target is the AM33 or later. This is the
default. This option defines the preprocessor macro
"__LIW__".
- -mnoliw
- Do not allow the compiler to generate Long Instruction
Word instructions. This option defines the preprocessor macro
"__NO_LIW__".
- -msetlb
- Allow the compiler to generate the SETLB and
Lcc instructions if the target is the AM33 or later. This is
the default. This option defines the preprocessor macro
"__SETLB__".
- -mnosetlb
- Do not allow the compiler to generate SETLB or
Lcc instructions. This option defines the preprocessor macro
"__NO_SETLB__".
Moxie Options
- -meb
- Generate big-endian code. This is the default for
moxie-*-* configurations.
- -mel
- Generate little-endian code.
- -mmul.x
- Generate mul.x and umul.x instructions. This is the default
for moxiebox-*-* configurations.
- -mno-crt0
- Do not link in the C run-time initialization object
file.
MSP430 Options
These options are defined for the MSP430:
- -masm-hex
- Force assembly output to always use hex constants. Normally
such constants are signed decimals, but this option is available for
testsuite and/or aesthetic purposes.
- -mmcu=
- Select the MCU to target. This is used to create a C
preprocessor symbol based upon the MCU name, converted to upper case and
pre- and post-fixed with __. This in turn is used by the
msp430.h header file to select an MCU-specific supplementary header
file.
The option also sets the ISA to use. If the MCU name is one that is known to
only support the 430 ISA then that is selected, otherwise the 430X ISA is
selected. A generic MCU name of msp430 can also be used to select
the 430 ISA. Similarly the generic msp430x MCU name selects the
430X ISA.
In addition an MCU-specific linker script is added to the linker command
line. The script's name is the name of the MCU with .ld appended.
Thus specifying -mmcu=xxx on the gcc command line defines
the C preprocessor symbol "__XXX__" and cause the linker to
search for a script called xxx.ld.
This option is also passed on to the assembler.
- -mcpu=
- Specifies the ISA to use. Accepted values are
msp430, msp430x and msp430xv2. This option is
deprecated. The -mmcu= option should be used to select the
ISA.
- -msim
- Link to the simulator runtime libraries and linker script.
Overrides any scripts that would be selected by the -mmcu=
option.
- -mlarge
- Use large-model addressing (20-bit pointers, 32-bit
"size_t").
- -msmall
- Use small-model addressing (16-bit pointers, 16-bit
"size_t").
- -mrelax
- This option is passed to the assembler and linker, and
allows the linker to perform certain optimizations that cannot be done
until the final link.
- mhwmult=
- Describes the type of hardware multiply supported by the
target. Accepted values are none for no hardware multiply,
16bit for the original 16-bit-only multiply supported by early
MCUs. 32bit for the 16/32-bit multiply supported by later MCUs and
f5series for the 16/32-bit multiply supported by F5-series MCUs. A
value of auto can also be given. This tells GCC to deduce the
hardware multiply support based upon the MCU name provided by the
-mmcu option. If no -mmcu option is specified then
32bit hardware multiply support is assumed. auto is the
default setting.
Hardware multiplies are normally performed by calling a library routine.
This saves space in the generated code. When compiling at -O3 or
higher however the hardware multiplier is invoked inline. This makes for
bigger, but faster code.
The hardware multiply routines disable interrupts whilst running and restore
the previous interrupt state when they finish. This makes them safe to use
inside interrupt handlers as well as in normal code.
- -minrt
- Enable the use of a minimum runtime environment - no static
initializers or constructors. This is intended for memory-constrained
devices. The compiler includes special symbols in some objects that tell
the linker and runtime which code fragments are required.
NDS32 Options
These options are defined for NDS32 implementations:
- -mbig-endian
- Generate code in big-endian mode.
- -mlittle-endian
- Generate code in little-endian mode.
- -mreduced-regs
- Use reduced-set registers for register allocation.
- -mfull-regs
- Use full-set registers for register allocation.
- -mcmov
- Generate conditional move instructions.
- -mno-cmov
- Do not generate conditional move instructions.
- -mperf-ext
- Generate performance extension instructions.
- -mno-perf-ext
- Do not generate performance extension instructions.
- -mv3push
- Generate v3 push25/pop25 instructions.
- -mno-v3push
- Do not generate v3 push25/pop25 instructions.
- -m16-bit
- Generate 16-bit instructions.
- -mno-16-bit
- Do not generate 16-bit instructions.
- -misr-vector-size=num
- Specify the size of each interrupt vector, which must be 4
or 16.
- -mcache-block-size=num
- Specify the size of each cache block, which must be a power
of 2 between 4 and 512.
- -march=arch
- Specify the name of the target architecture.
- -mcmodel=code-model
- Set the code model to one of
- small
- All the data and read-only data segments must be within
512KB addressing space. The text segment must be within 16MB addressing
space.
- medium
- The data segment must be within 512KB while the read-only
data segment can be within 4GB addressing space. The text segment should
be still within 16MB addressing space.
- large
- All the text and data segments can be within 4GB addressing
space.
- -mctor-dtor
- Enable constructor/destructor feature.
- -mrelax
- Guide linker to relax instructions.
Nios II Options
These are the options defined for the Altera Nios II processor.
- -G num
- Put global and static objects less than or equal to
num bytes into the small data or BSS sections instead of the normal
data or BSS sections. The default value of num is 8.
- -mgpopt=option
- -mgpopt
- -mno-gpopt
- Generate (do not generate) GP-relative accesses. The
following option names are recognized:
- none
- Do not generate GP-relative accesses.
- local
- Generate GP-relative accesses for small data objects that
are not external or weak. Also use GP-relative addressing for objects that
have been explicitly placed in a small data section via a
"section" attribute.
- global
- As for local, but also generate GP-relative accesses
for small data objects that are external or weak. If you use this option,
you must ensure that all parts of your program (including libraries) are
compiled with the same -G setting.
- data
- Generate GP-relative accesses for all data objects in the
program. If you use this option, the entire data and BSS segments of your
program must fit in 64K of memory and you must use an appropriate linker
script to allocate them within the addressible range of the global
pointer.
- all
- Generate GP-relative addresses for function pointers as
well as data pointers. If you use this option, the entire text, data, and
BSS segments of your program must fit in 64K of memory and you must use an
appropriate linker script to allocate them within the addressible range of
the global pointer.
-mgpopt is equivalent to
-mgpopt=local, and
-mno-gpopt is
equivalent to
-mgpopt=none.
The default is
-mgpopt except when
-fpic or
-fPIC is
specified to generate position-independent code. Note that the Nios II ABI
does not permit GP-relative accesses from shared libraries.
You may need to specify
-mno-gpopt explicitly when building programs that
include large amounts of small data, including large GOT data sections. In
this case, the 16-bit offset for GP-relative addressing may not be large
enough to allow access to the entire small data section.
- -mel
- -meb
- Generate little-endian (default) or big-endian
(experimental) code, respectively.
- -mbypass-cache
- -mno-bypass-cache
- Force all load and store instructions to always bypass
cache by using I/O variants of the instructions. The default is not to
bypass the cache.
- -mno-cache-volatile
- -mcache-volatile
- Volatile memory access bypass the cache using the I/O
variants of the load and store instructions. The default is not to bypass
the cache.
- -mno-fast-sw-div
- -mfast-sw-div
- Do not use table-based fast divide for small numbers. The
default is to use the fast divide at -O3 and above.
- -mno-hw-mul
- -mhw-mul
- -mno-hw-mulx
- -mhw-mulx
- -mno-hw-div
- -mhw-div
- Enable or disable emitting "mul",
"mulx" and "div" family of instructions by the
compiler. The default is to emit "mul" and not emit
"div" and "mulx".
- -mcustom-insn=N
- -mno-custom-insn
- Each -mcustom-insn=N option
enables use of a custom instruction with encoding N when generating
code that uses insn. For example, -mcustom-fadds=253
generates custom instruction 253 for single-precision floating-point add
operations instead of the default behavior of using a library call.
The following values of insn are supported. Except as otherwise
noted, floating-point operations are expected to be implemented with
normal IEEE 754 semantics and correspond directly to the C operators or
the equivalent GCC built-in functions.
Single-precision floating point:
- fadds, fsubs, fdivs, fmuls
- Binary arithmetic operations.
- fnegs
- Unary negation.
- fabss
- Unary absolute value.
- fcmpeqs, fcmpges, fcmpgts,
fcmples, fcmplts, fcmpnes
- Comparison operations.
- fmins, fmaxs
- Floating-point minimum and maximum. These instructions are
only generated if -ffinite-math-only is specified.
- fsqrts
- Unary square root operation.
- fcoss, fsins, ftans, fatans,
fexps, flogs
- Floating-point trigonometric and exponential functions.
These instructions are only generated if
-funsafe-math-optimizations is also specified.
Double-precision floating point:
- faddd, fsubd, fdivd, fmuld
- Binary arithmetic operations.
- fnegd
- Unary negation.
- fabsd
- Unary absolute value.
- fcmpeqd, fcmpged, fcmpgtd,
fcmpled, fcmpltd, fcmpned
- Comparison operations.
- fmind, fmaxd
- Double-precision minimum and maximum. These instructions
are only generated if -ffinite-math-only is specified.
- fsqrtd
- Unary square root operation.
- fcosd, fsind, ftand, fatand,
fexpd, flogd
- Double-precision trigonometric and exponential functions.
These instructions are only generated if
-funsafe-math-optimizations is also specified.
Conversions:
- fextsd
- Conversion from single precision to double precision.
- ftruncds
- Conversion from double precision to single precision.
- fixsi, fixsu, fixdi, fixdu
- Conversion from floating point to signed or unsigned
integer types, with truncation towards zero.
- round
- Conversion from single-precision floating point to signed
integer, rounding to the nearest integer and ties away from zero. This
corresponds to the "__builtin_lroundf" function when
-fno-math-errno is used.
- floatis, floatus, floatid,
floatud
- Conversion from signed or unsigned integer types to
floating-point types.
In addition, all of the following transfer instructions for internal registers X
and Y must be provided to use any of the double-precision floating-point
instructions. Custom instructions taking two double-precision source operands
expect the first operand in the 64-bit register X. The other operand (or only
operand of a unary operation) is given to the custom arithmetic instruction
with the least significant half in source register
src1 and the most
significant half in
src2. A custom instruction that returns a
double-precision result returns the most significant 32 bits in the
destination register and the other half in 32-bit register Y. GCC
automatically generates the necessary code sequences to write register X
and/or read register Y when double-precision floating-point instructions are
used.
- fwrx
- Write src1 into the least significant half of X and
src2 into the most significant half of X.
- fwry
- Write src1 into Y.
- frdxhi, frdxlo
- Read the most or least (respectively) significant half of X
and store it in dest.
- frdy
- Read the value of Y and store it into dest.
Note that you can gain more local control over generation of Nios II custom
instructions by using the "target("custom-
insn=
N")" and "target("no-custom-
insn")" function attributes or pragmas.
- -mcustom-fpu-cfg=name
- This option enables a predefined, named set of custom
instruction encodings (see -mcustom-insn above). Currently,
the following sets are defined:
-mcustom-fpu-cfg=60-1 is equivalent to: -mcustom-fmuls=252
-mcustom-fadds=253 -mcustom-fsubs=254
-fsingle-precision-constant
-mcustom-fpu-cfg=60-2 is equivalent to: -mcustom-fmuls=252
-mcustom-fadds=253 -mcustom-fsubs=254
-mcustom-fdivs=255 -fsingle-precision-constant
-mcustom-fpu-cfg=72-3 is equivalent to: -mcustom-floatus=243
-mcustom-fixsi=244 -mcustom-floatis=245
-mcustom-fcmpgts=246 -mcustom-fcmples=249
-mcustom-fcmpeqs=250 -mcustom-fcmpnes=251
-mcustom-fmuls=252 -mcustom-fadds=253
-mcustom-fsubs=254 -mcustom-fdivs=255
-fsingle-precision-constant
Custom instruction assignments given by individual
-mcustom-insn = options override those given by
-mcustom-fpu-cfg=, regardless of the order of the options on the
command line.
Note that you can gain more local control over selection of a FPU
configuration by using the "target("custom-fpu-cfg=
name")" function attribute or pragma.
These additional
-m options are available for the Altera Nios II ELF
(bare-metal) target:
- -mhal
- Link with HAL BSP. This suppresses linking with the
GCC-provided C runtime startup and termination code, and is typically used
in conjunction with -msys-crt0= to specify the location of the
alternate startup code provided by the HAL BSP.
- -msmallc
- Link with a limited version of the C library,
-lsmallc, rather than Newlib.
- -msys-crt0=startfile
- startfile is the file name of the startfile (crt0)
to use when linking. This option is only useful in conjunction with
-mhal.
- -msys-lib=systemlib
- systemlib is the library name of the library that
provides low-level system calls required by the C library, e.g.
"read" and "write". This option is typically used to
link with a library provided by a HAL BSP.
Nvidia PTX Options
These options are defined for Nvidia PTX:
- -m32
- -m64
- Generate code for 32-bit or 64-bit ABI.
- -mmainkernel
- Link in code for a __main kernel. This is for stand-alone
instead of offloading execution.
PDP-11 Options
These options are defined for the PDP-11:
- -mfpu
- Use hardware FPP floating point. This is the default. (FIS
floating point on the PDP-11/40 is not supported.)
- -msoft-float
- Do not use hardware floating point.
- -mac0
- Return floating-point results in ac0 (fr0 in Unix assembler
syntax).
- -mno-ac0
- Return floating-point results in memory. This is the
default.
- -m40
- Generate code for a PDP-11/40.
- -m45
- Generate code for a PDP-11/45. This is the default.
- -m10
- Generate code for a PDP-11/10.
- -mbcopy-builtin
- Use inline "movmemhi" patterns for copying
memory. This is the default.
- -mbcopy
- Do not use inline "movmemhi" patterns for copying
memory.
- -mint16
- -mno-int32
- Use 16-bit "int". This is the default.
- -mint32
- -mno-int16
- Use 32-bit "int".
- -mfloat64
- -mno-float32
- Use 64-bit "float". This is the default.
- -mfloat32
- -mno-float64
- Use 32-bit "float".
- -mabshi
- Use "abshi2" pattern. This is the default.
- -mno-abshi
- Do not use "abshi2" pattern.
- -mbranch-expensive
- Pretend that branches are expensive. This is for
experimenting with code generation only.
- -mbranch-cheap
- Do not pretend that branches are expensive. This is the
default.
- -munix-asm
- Use Unix assembler syntax. This is the default when
configured for pdp11-*-bsd.
- -mdec-asm
- Use DEC assembler syntax. This is the default when
configured for any PDP-11 target other than pdp11-*-bsd.
picoChip Options
These
-m options are defined for picoChip implementations:
- -mae=ae_type
- Set the instruction set, register set, and instruction
scheduling parameters for array element type ae_type. Supported
values for ae_type are ANY, MUL, and MAC.
-mae=ANY selects a completely generic AE type. Code generated with
this option runs on any of the other AE types. The code is not as
efficient as it would be if compiled for a specific AE type, and some
types of operation (e.g., multiplication) do not work properly on all
types of AE.
-mae=MUL selects a MUL AE type. This is the most useful AE type for
compiled code, and is the default.
-mae=MAC selects a DSP-style MAC AE. Code compiled with this option
may suffer from poor performance of byte (char) manipulation, since the
DSP AE does not provide hardware support for byte load/stores.
- -msymbol-as-address
- Enable the compiler to directly use a symbol name as an
address in a load/store instruction, without first loading it into a
register. Typically, the use of this option generates larger programs,
which run faster than when the option isn't used. However, the results
vary from program to program, so it is left as a user option, rather than
being permanently enabled.
- -mno-inefficient-warnings
- Disables warnings about the generation of inefficient code.
These warnings can be generated, for example, when compiling code that
performs byte-level memory operations on the MAC AE type. The MAC AE has
no hardware support for byte-level memory operations, so all byte
load/stores must be synthesized from word load/store operations. This is
inefficient and a warning is generated to indicate that you should rewrite
the code to avoid byte operations, or to target an AE type that has the
necessary hardware support. This option disables these warnings.
PowerPC Options
These are listed under
RL78 Options
- -msim
- Links in additional target libraries to support operation
within a simulator.
- -mmul=none
- -mmul=g13
- -mmul=rl78
- Specifies the type of hardware multiplication support to be
used. The default is none, which uses software multiplication
functions. The g13 option is for the hardware multiply/divide
peripheral only on the RL78/G13 targets. The rl78 option is for the
standard hardware multiplication defined in the RL78 software manual.
- -m64bit-doubles
- -m32bit-doubles
- Make the "double" data type be 64 bits
(-m64bit-doubles) or 32 bits ( -m32bit-doubles) in size. The
default is -m32bit-doubles.
IBM RS/6000 and PowerPC Options
These
-m options are defined for the IBM RS/6000 and PowerPC:
- -mpowerpc-gpopt
- -mno-powerpc-gpopt
- -mpowerpc-gfxopt
- -mno-powerpc-gfxopt
- -mpowerpc64
- -mno-powerpc64
- -mmfcrf
- -mno-mfcrf
- -mpopcntb
- -mno-popcntb
- -mpopcntd
- -mno-popcntd
- -mfprnd
- -mno-fprnd
- -mcmpb
- -mno-cmpb
- -mmfpgpr
- -mno-mfpgpr
- -mhard-dfp
- -mno-hard-dfp
- You use these options to specify which instructions are
available on the processor you are using. The default value of these
options is determined when configuring GCC. Specifying the
-mcpu=cpu_type overrides the specification of these options.
We recommend you use the -mcpu=cpu_type option rather than
the options listed above.
Specifying -mpowerpc-gpopt allows GCC to use the optional PowerPC
architecture instructions in the General Purpose group, including
floating-point square root. Specifying -mpowerpc-gfxopt allows GCC
to use the optional PowerPC architecture instructions in the Graphics
group, including floating-point select.
The -mmfcrf option allows GCC to generate the move from condition
register field instruction implemented on the POWER4 processor and other
processors that support the PowerPC V2.01 architecture. The
-mpopcntb option allows GCC to generate the popcount and
double-precision FP reciprocal estimate instruction implemented on the
POWER5 processor and other processors that support the PowerPC V2.02
architecture. The -mpopcntd option allows GCC to generate the
popcount instruction implemented on the POWER7 processor and other
processors that support the PowerPC V2.06 architecture. The -mfprnd
option allows GCC to generate the FP round to integer instructions
implemented on the POWER5+ processor and other processors that support the
PowerPC V2.03 architecture. The -mcmpb option allows GCC to
generate the compare bytes instruction implemented on the POWER6 processor
and other processors that support the PowerPC V2.05 architecture. The
-mmfpgpr option allows GCC to generate the FP move to/from
general-purpose register instructions implemented on the POWER6X processor
and other processors that support the extended PowerPC V2.05 architecture.
The -mhard-dfp option allows GCC to generate the decimal
floating-point instructions implemented on some POWER processors.
The -mpowerpc64 option allows GCC to generate the additional 64-bit
instructions that are found in the full PowerPC64 architecture and to
treat GPRs as 64-bit, doubleword quantities. GCC defaults to
-mno-powerpc64.
- -mcpu=cpu_type
- Set architecture type, register usage, and instruction
scheduling parameters for machine type cpu_type. Supported values
for cpu_type are 401, 403, 405, 405fp,
440, 440fp, 464, 464fp, 476,
476fp, 505, 601, 602, 603, 603e,
604, 604e, 620, 630, 740, 7400,
7450, 750, 801, 821, 823, 860,
970, 8540, a2, e300c2, e300c3,
e500mc, e500mc64, e5500, e6500, ec603e,
G3, G4, G5, titan, power3,
power4, power5, power5+, power6,
power6x, power7, power8, powerpc,
powerpc64, powerpc64le, and rs64.
-mcpu=powerpc, -mcpu=powerpc64, and -mcpu=powerpc64le
specify pure 32-bit PowerPC (either endian), 64-bit big endian PowerPC and
64-bit little endian PowerPC architecture machine types, with an
appropriate, generic processor model assumed for scheduling purposes.
The other options specify a specific processor. Code generated under those
options runs best on that processor, and may not run at all on others.
The -mcpu options automatically enable or disable the following
options:
-maltivec -mfprnd -mhard-float -mmfcrf -mmultiple -mpopcntb
-mpopcntd -mpowerpc64 -mpowerpc-gpopt -mpowerpc-gfxopt
-msingle-float -mdouble-float -msimple-fpu -mstring -mmulhw -mdlmzb
-mmfpgpr -mvsx -mcrypto -mdirect-move -mpower8-fusion
-mpower8-vector -mquad-memory -mquad-memory-atomic
The particular options set for any particular CPU varies between compiler
versions, depending on what setting seems to produce optimal code for that
CPU; it doesn't necessarily reflect the actual hardware's capabilities. If
you wish to set an individual option to a particular value, you may
specify it after the -mcpu option, like -mcpu=970
-mno-altivec.
On AIX, the -maltivec and -mpowerpc64 options are not enabled
or disabled by the -mcpu option at present because AIX does not
have full support for these options. You may still enable or disable them
individually if you're sure it'll work in your environment.
- -mtune=cpu_type
- Set the instruction scheduling parameters for machine type
cpu_type, but do not set the architecture type or register usage,
as -mcpu=cpu_type does. The same values for cpu_type
are used for -mtune as for -mcpu. If both are specified, the
code generated uses the architecture and registers set by -mcpu,
but the scheduling parameters set by -mtune.
- -mcmodel=small
- Generate PowerPC64 code for the small model: The TOC is
limited to 64k.
- -mcmodel=medium
- Generate PowerPC64 code for the medium model: The TOC and
other static data may be up to a total of 4G in size.
- -mcmodel=large
- Generate PowerPC64 code for the large model: The TOC may be
up to 4G in size. Other data and code is only limited by the 64-bit
address space.
- -maltivec
- -mno-altivec
- Generate code that uses (does not use) AltiVec
instructions, and also enable the use of built-in functions that allow
more direct access to the AltiVec instruction set. You may also need to
set -mabi=altivec to adjust the current ABI with AltiVec ABI
enhancements.
When -maltivec is used, rather than -maltivec=le or
-maltivec=be, the element order for Altivec intrinsics such as
"vec_splat", "vec_extract", and "vec_insert"
match array element order corresponding to the endianness of the target.
That is, element zero identifies the leftmost element in a vector register
when targeting a big-endian platform, and identifies the rightmost element
in a vector register when targeting a little-endian platform.
- -maltivec=be
- Generate Altivec instructions using big-endian element
order, regardless of whether the target is big- or little-endian. This is
the default when targeting a big-endian platform.
The element order is used to interpret element numbers in Altivec intrinsics
such as "vec_splat", "vec_extract", and
"vec_insert". By default, these match array element order
corresponding to the endianness for the target.
- -maltivec=le
- Generate Altivec instructions using little-endian element
order, regardless of whether the target is big- or little-endian. This is
the default when targeting a little-endian platform. This option is
currently ignored when targeting a big-endian platform.
The element order is used to interpret element numbers in Altivec intrinsics
such as "vec_splat", "vec_extract", and
"vec_insert". By default, these match array element order
corresponding to the endianness for the target.
- -mvrsave
- -mno-vrsave
- Generate VRSAVE instructions when generating AltiVec
code.
- -mgen-cell-microcode
- Generate Cell microcode instructions.
- -mwarn-cell-microcode
- Warn when a Cell microcode instruction is emitted. An
example of a Cell microcode instruction is a variable shift.
- -msecure-plt
- Generate code that allows ld and ld.so to
build executables and shared libraries with non-executable
".plt" and ".got" sections. This is a PowerPC 32-bit
SYSV ABI option.
- -mbss-plt
- Generate code that uses a BSS ".plt" section that
ld.so fills in, and requires ".plt" and ".got"
sections that are both writable and executable. This is a PowerPC 32-bit
SYSV ABI option.
- -misel
- -mno-isel
- This switch enables or disables the generation of ISEL
instructions.
- -misel=yes/no
- This switch has been deprecated. Use -misel and
-mno-isel instead.
- -mspe
- -mno-spe
- This switch enables or disables the generation of SPE simd
instructions.
- -mpaired
- -mno-paired
- This switch enables or disables the generation of PAIRED
simd instructions.
- -mspe=yes/no
- This option has been deprecated. Use -mspe and
-mno-spe instead.
- -mvsx
- -mno-vsx
- Generate code that uses (does not use) vector/scalar (VSX)
instructions, and also enable the use of built-in functions that allow
more direct access to the VSX instruction set.
- -mcrypto
- -mno-crypto
- Enable the use (disable) of the built-in functions that
allow direct access to the cryptographic instructions that were added in
version 2.07 of the PowerPC ISA.
- -mdirect-move
- -mno-direct-move
- Generate code that uses (does not use) the instructions to
move data between the general purpose registers and the vector/scalar
(VSX) registers that were added in version 2.07 of the PowerPC ISA.
- -mpower8-fusion
- -mno-power8-fusion
- Generate code that keeps (does not keeps) some integer
operations adjacent so that the instructions can be fused together on
power8 and later processors.
- -mpower8-vector
- -mno-power8-vector
- Generate code that uses (does not use) the vector and
scalar instructions that were added in version 2.07 of the PowerPC ISA.
Also enable the use of built-in functions that allow more direct access to
the vector instructions.
- -mquad-memory
- -mno-quad-memory
- Generate code that uses (does not use) the non-atomic quad
word memory instructions. The -mquad-memory option requires use of
64-bit mode.
- -mquad-memory-atomic
- -mno-quad-memory-atomic
- Generate code that uses (does not use) the atomic quad word
memory instructions. The -mquad-memory-atomic option requires use
of 64-bit mode.
- -mupper-regs-df
- -mno-upper-regs-df
- Generate code that uses (does not use) the scalar double
precision instructions that target all 64 registers in the vector/scalar
floating point register set that were added in version 2.06 of the PowerPC
ISA. -mupper-regs-df is turned on by default if you use any of the
-mcpu=power7, -mcpu=power8, or -mvsx options.
- -mupper-regs-sf
- -mno-upper-regs-sf
- Generate code that uses (does not use) the scalar single
precision instructions that target all 64 registers in the vector/scalar
floating point register set that were added in version 2.07 of the PowerPC
ISA. -mupper-regs-sf is turned on by default if you use either of
the -mcpu=power8 or -mpower8-vector options.
- -mupper-regs
- -mno-upper-regs
- Generate code that uses (does not use) the scalar
instructions that target all 64 registers in the vector/scalar floating
point register set, depending on the model of the machine.
If the -mno-upper-regs option is used, it turns off both
-mupper-regs-sf and -mupper-regs-df options.
- -mfloat-gprs=yes/single/double/no
- -mfloat-gprs
- This switch enables or disables the generation of
floating-point operations on the general-purpose registers for
architectures that support it.
The argument yes or single enables the use of single-precision
floating-point operations.
The argument double enables the use of single and double-precision
floating-point operations.
The argument no disables floating-point operations on the
general-purpose registers.
This option is currently only available on the MPC854x.
- -m32
- -m64
- Generate code for 32-bit or 64-bit environments of Darwin
and SVR4 targets (including GNU/Linux). The 32-bit environment sets int,
long and pointer to 32 bits and generates code that runs on any PowerPC
variant. The 64-bit environment sets int to 32 bits and long and pointer
to 64 bits, and generates code for PowerPC64, as for
-mpowerpc64.
- -mfull-toc
- -mno-fp-in-toc
- -mno-sum-in-toc
- -mminimal-toc
- Modify generation of the TOC (Table Of Contents), which is
created for every executable file. The -mfull-toc option is
selected by default. In that case, GCC allocates at least one TOC entry
for each unique non-automatic variable reference in your program. GCC also
places floating-point constants in the TOC. However, only 16,384 entries
are available in the TOC.
If you receive a linker error message that saying you have overflowed the
available TOC space, you can reduce the amount of TOC space used with the
-mno-fp-in-toc and -mno-sum-in-toc options.
-mno-fp-in-toc prevents GCC from putting floating-point constants
in the TOC and -mno-sum-in-toc forces GCC to generate code to
calculate the sum of an address and a constant at run time instead of
putting that sum into the TOC. You may specify one or both of these
options. Each causes GCC to produce very slightly slower and larger code
at the expense of conserving TOC space.
If you still run out of space in the TOC even when you specify both of these
options, specify -mminimal-toc instead. This option causes GCC to
make only one TOC entry for every file. When you specify this option, GCC
produces code that is slower and larger but which uses extremely little
TOC space. You may wish to use this option only on files that contain less
frequently-executed code.
- -maix64
- -maix32
- Enable 64-bit AIX ABI and calling convention: 64-bit
pointers, 64-bit "long" type, and the infrastructure needed to
support them. Specifying -maix64 implies -mpowerpc64, while
-maix32 disables the 64-bit ABI and implies -mno-powerpc64.
GCC defaults to -maix32.
- -mxl-compat
- -mno-xl-compat
- Produce code that conforms more closely to IBM XL compiler
semantics when using AIX-compatible ABI. Pass floating-point arguments to
prototyped functions beyond the register save area (RSA) on the stack in
addition to argument FPRs. Do not assume that most significant double in
128-bit long double value is properly rounded when comparing values and
converting to double. Use XL symbol names for long double support
routines.
The AIX calling convention was extended but not initially documented to
handle an obscure K&R C case of calling a function that takes the
address of its arguments with fewer arguments than declared. IBM XL
compilers access floating-point arguments that do not fit in the RSA from
the stack when a subroutine is compiled without optimization. Because
always storing floating-point arguments on the stack is inefficient and
rarely needed, this option is not enabled by default and only is necessary
when calling subroutines compiled by IBM XL compilers without
optimization.
- -mpe
- Support IBM RS/6000 SP Parallel Environment
(PE). Link an application written to use message passing with special
startup code to enable the application to run. The system must have PE
installed in the standard location ( /usr/lpp/ppe.poe/), or the
specs file must be overridden with the -specs= option to
specify the appropriate directory location. The Parallel Environment does
not support threads, so the -mpe option and the -pthread
option are incompatible.
- -malign-natural
- -malign-power
- On AIX, 32-bit Darwin, and 64-bit PowerPC GNU/Linux, the
option -malign-natural overrides the ABI-defined alignment of
larger types, such as floating-point doubles, on their natural size-based
boundary. The option -malign-power instructs GCC to follow the
ABI-specified alignment rules. GCC defaults to the standard alignment
defined in the ABI.
On 64-bit Darwin, natural alignment is the default, and -malign-power
is not supported.
- -msoft-float
- -mhard-float
- Generate code that does not use (uses) the floating-point
register set. Software floating-point emulation is provided if you use the
-msoft-float option, and pass the option to GCC when linking.
- -msingle-float
- -mdouble-float
- Generate code for single- or double-precision
floating-point operations. -mdouble-float implies
-msingle-float.
- -msimple-fpu
- Do not generate "sqrt" and "div"
instructions for hardware floating-point unit.
- -mfpu=name
- Specify type of floating-point unit. Valid values for
name are sp_lite (equivalent to -msingle-float
-msimple-fpu), dp_lite (equivalent to -mdouble-float
-msimple-fpu), sp_full (equivalent to -msingle-float),
and dp_full (equivalent to -mdouble-float).
- -mxilinx-fpu
- Perform optimizations for the floating-point unit on Xilinx
PPC 405/440.
- -mmultiple
- -mno-multiple
- Generate code that uses (does not use) the load multiple
word instructions and the store multiple word instructions. These
instructions are generated by default on POWER systems, and not generated
on PowerPC systems. Do not use -mmultiple on little-endian PowerPC
systems, since those instructions do not work when the processor is in
little-endian mode. The exceptions are PPC740 and PPC750 which permit
these instructions in little-endian mode.
- -mstring
- -mno-string
- Generate code that uses (does not use) the load string
instructions and the store string word instructions to save multiple
registers and do small block moves. These instructions are generated by
default on POWER systems, and not generated on PowerPC systems. Do not use
-mstring on little-endian PowerPC systems, since those instructions
do not work when the processor is in little-endian mode. The exceptions
are PPC740 and PPC750 which permit these instructions in little-endian
mode.
- -mupdate
- -mno-update
- Generate code that uses (does not use) the load or store
instructions that update the base register to the address of the
calculated memory location. These instructions are generated by default.
If you use -mno-update, there is a small window between the time
that the stack pointer is updated and the address of the previous frame is
stored, which means code that walks the stack frame across interrupts or
signals may get corrupted data.
- -mavoid-indexed-addresses
- -mno-avoid-indexed-addresses
- Generate code that tries to avoid (not avoid) the use of
indexed load or store instructions. These instructions can incur a
performance penalty on Power6 processors in certain situations, such as
when stepping through large arrays that cross a 16M boundary. This option
is enabled by default when targeting Power6 and disabled otherwise.
- -mfused-madd
- -mno-fused-madd
- Generate code that uses (does not use) the floating-point
multiply and accumulate instructions. These instructions are generated by
default if hardware floating point is used. The machine-dependent
-mfused-madd option is now mapped to the machine-independent
-ffp-contract=fast option, and -mno-fused-madd is mapped to
-ffp-contract=off.
- -mmulhw
- -mno-mulhw
- Generate code that uses (does not use) the half-word
multiply and multiply-accumulate instructions on the IBM 405, 440, 464 and
476 processors. These instructions are generated by default when targeting
those processors.
- -mdlmzb
- -mno-dlmzb
- Generate code that uses (does not use) the string-search
dlmzb instruction on the IBM 405, 440, 464 and 476 processors. This
instruction is generated by default when targeting those processors.
- -mno-bit-align
- -mbit-align
- On System V.4 and embedded PowerPC systems do not (do)
force structures and unions that contain bit-fields to be aligned to the
base type of the bit-field.
For example, by default a structure containing nothing but 8
"unsigned" bit-fields of length 1 is aligned to a 4-byte
boundary and has a size of 4 bytes. By using -mno-bit-align, the
structure is aligned to a 1-byte boundary and is 1 byte in size.
- -mno-strict-align
- -mstrict-align
- On System V.4 and embedded PowerPC systems do not (do)
assume that unaligned memory references are handled by the system.
- -mrelocatable
- -mno-relocatable
- Generate code that allows (does not allow) a static
executable to be relocated to a different address at run time. A simple
embedded PowerPC system loader should relocate the entire contents of
".got2" and 4-byte locations listed in the ".fixup"
section, a table of 32-bit addresses generated by this option. For this to
work, all objects linked together must be compiled with
-mrelocatable or -mrelocatable-lib. -mrelocatable
code aligns the stack to an 8-byte boundary.
- -mrelocatable-lib
- -mno-relocatable-lib
- Like -mrelocatable, -mrelocatable-lib
generates a ".fixup" section to allow static executables to be
relocated at run time, but -mrelocatable-lib does not use the
smaller stack alignment of -mrelocatable. Objects compiled with
-mrelocatable-lib may be linked with objects compiled with any
combination of the -mrelocatable options.
- -mno-toc
- -mtoc
- On System V.4 and embedded PowerPC systems do not (do)
assume that register 2 contains a pointer to a global area pointing to the
addresses used in the program.
- -mlittle
- -mlittle-endian
- On System V.4 and embedded PowerPC systems compile code for
the processor in little-endian mode. The -mlittle-endian option is
the same as -mlittle.
- -mbig
- -mbig-endian
- On System V.4 and embedded PowerPC systems compile code for
the processor in big-endian mode. The -mbig-endian option is the
same as -mbig.
- -mdynamic-no-pic
- On Darwin and Mac OS X systems, compile code so that it is
not relocatable, but that its external references are relocatable. The
resulting code is suitable for applications, but not shared
libraries.
- -msingle-pic-base
- Treat the register used for PIC addressing as read-only,
rather than loading it in the prologue for each function. The runtime
system is responsible for initializing this register with an appropriate
value before execution begins.
- -mprioritize-restricted-insns=priority
- This option controls the priority that is assigned to
dispatch-slot restricted instructions during the second scheduling pass.
The argument priority takes the value 0, 1, or
2 to assign no, highest, or second-highest (respectively) priority
to dispatch-slot restricted instructions.
- -msched-costly-dep=dependence_type
- This option controls which dependences are considered
costly by the target during instruction scheduling. The argument
dependence_type takes one of the following values:
- no
- No dependence is costly.
- all
- All dependences are costly.
- true_store_to_load
- A true dependence from store to load is costly.
- store_to_load
- Any dependence from store to load is costly.
- number
- Any dependence for which the latency is greater than or
equal to number is costly.
- -minsert-sched-nops=scheme
- This option controls which NOP insertion scheme is used
during the second scheduling pass. The argument scheme takes one of
the following values:
- no
- Don't insert NOPs.
- pad
- Pad with NOPs any dispatch group that has vacant issue
slots, according to the scheduler's grouping.
- regroup_exact
- Insert NOPs to force costly dependent insns into separate
groups. Insert exactly as many NOPs as needed to force an insn to a new
group, according to the estimated processor grouping.
- number
- Insert NOPs to force costly dependent insns into separate
groups. Insert number NOPs to force an insn to a new group.
- -mcall-sysv
- On System V.4 and embedded PowerPC systems compile code
using calling conventions that adhere to the March 1995 draft of the
System V Application Binary Interface, PowerPC processor supplement. This
is the default unless you configured GCC using
powerpc-*-eabiaix.
- -mcall-sysv-eabi
- -mcall-eabi
- Specify both -mcall-sysv and -meabi
options.
- -mcall-sysv-noeabi
- Specify both -mcall-sysv and -mno-eabi
options.
- -mcall-aixdesc
- On System V.4 and embedded PowerPC systems compile code for
the AIX operating system.
- -mcall-linux
- On System V.4 and embedded PowerPC systems compile code for
the Linux-based GNU system.
- -mcall-freebsd
- On System V.4 and embedded PowerPC systems compile code for
the FreeBSD operating system.
- -mcall-netbsd
- On System V.4 and embedded PowerPC systems compile code for
the NetBSD operating system.
- -mcall-openbsd
- On System V.4 and embedded PowerPC systems compile code for
the OpenBSD operating system.
- -maix-struct-return
- Return all structures in memory (as specified by the AIX
ABI).
- -msvr4-struct-return
- Return structures smaller than 8 bytes in registers (as
specified by the SVR4 ABI).
- -mabi=abi-type
- Extend the current ABI with a particular extension, or
remove such extension. Valid values are altivec, no-altivec,
spe, no-spe, ibmlongdouble, ieeelongdouble,
elfv1, elfv2.
- -mabi=spe
- Extend the current ABI with SPE ABI extensions. This does
not change the default ABI, instead it adds the SPE ABI extensions to the
current ABI.
- -mabi=no-spe
- Disable Book-E SPE ABI extensions for the current ABI.
- -mabi=ibmlongdouble
- Change the current ABI to use IBM extended-precision long
double. This is a PowerPC 32-bit SYSV ABI option.
- -mabi=ieeelongdouble
- Change the current ABI to use IEEE extended-precision long
double. This is a PowerPC 32-bit Linux ABI option.
- -mabi=elfv1
- Change the current ABI to use the ELFv1 ABI. This is the
default ABI for big-endian PowerPC 64-bit Linux. Overriding the default
ABI requires special system support and is likely to fail in spectacular
ways.
- -mabi=elfv2
- Change the current ABI to use the ELFv2 ABI. This is the
default ABI for little-endian PowerPC 64-bit Linux. Overriding the default
ABI requires special system support and is likely to fail in spectacular
ways.
- -mprototype
- -mno-prototype
- On System V.4 and embedded PowerPC systems assume that all
calls to variable argument functions are properly prototyped. Otherwise,
the compiler must insert an instruction before every non-prototyped call
to set or clear bit 6 of the condition code register ("CR") to
indicate whether floating-point values are passed in the floating-point
registers in case the function takes variable arguments. With
-mprototype, only calls to prototyped variable argument functions
set or clear the bit.
- -msim
- On embedded PowerPC systems, assume that the startup module
is called sim-crt0.o and that the standard C libraries are
libsim.a and libc.a. This is the default for
powerpc-*-eabisim configurations.
- -mmvme
- On embedded PowerPC systems, assume that the startup module
is called crt0.o and the standard C libraries are libmvme.a
and libc.a.
- -mads
- On embedded PowerPC systems, assume that the startup module
is called crt0.o and the standard C libraries are libads.a
and libc.a.
- -myellowknife
- On embedded PowerPC systems, assume that the startup module
is called crt0.o and the standard C libraries are libyk.a
and libc.a.
- -mvxworks
- On System V.4 and embedded PowerPC systems, specify that
you are compiling for a VxWorks system.
- -memb
- On embedded PowerPC systems, set the "PPC_EMB"
bit in the ELF flags header to indicate that eabi extended
relocations are used.
- -meabi
- -mno-eabi
- On System V.4 and embedded PowerPC systems do (do not)
adhere to the Embedded Applications Binary Interface (EABI), which is a
set of modifications to the System V.4 specifications. Selecting
-meabi means that the stack is aligned to an 8-byte boundary, a
function "__eabi" is called from "main" to set up the
EABI environment, and the -msdata option can use both
"r2" and "r13" to point to two separate small data
areas. Selecting -mno-eabi means that the stack is aligned to a
16-byte boundary, no EABI initialization function is called from
"main", and the -msdata option only uses "r13"
to point to a single small data area. The -meabi option is on by
default if you configured GCC using one of the powerpc*-*-eabi*
options.
- -msdata=eabi
- On System V.4 and embedded PowerPC systems, put small
initialized "const" global and static data in the
".sdata2" section, which is pointed to by register
"r2". Put small initialized non-"const" global and
static data in the ".sdata" section, which is pointed to by
register "r13". Put small uninitialized global and static data
in the ".sbss" section, which is adjacent to the
".sdata" section. The -msdata=eabi option is incompatible
with the -mrelocatable option. The -msdata=eabi option also
sets the -memb option.
- -msdata=sysv
- On System V.4 and embedded PowerPC systems, put small
global and static data in the ".sdata" section, which is pointed
to by register "r13". Put small uninitialized global and static
data in the ".sbss" section, which is adjacent to the
".sdata" section. The -msdata=sysv option is incompatible
with the -mrelocatable option.
- -msdata=default
- -msdata
- On System V.4 and embedded PowerPC systems, if
-meabi is used, compile code the same as -msdata=eabi,
otherwise compile code the same as -msdata=sysv.
- -msdata=data
- On System V.4 and embedded PowerPC systems, put small
global data in the ".sdata" section. Put small uninitialized
global data in the ".sbss" section. Do not use register
"r13" to address small data however. This is the default
behavior unless other -msdata options are used.
- -msdata=none
- -mno-sdata
- On embedded PowerPC systems, put all initialized global and
static data in the ".data" section, and all uninitialized data
in the ".bss" section.
- -mblock-move-inline-limit=num
- Inline all block moves (such as calls to "memcpy"
or structure copies) less than or equal to num bytes. The minimum
value for num is 32 bytes on 32-bit targets and 64 bytes on 64-bit
targets. The default value is target-specific.
- -G num
- On embedded PowerPC systems, put global and static items
less than or equal to num bytes into the small data or BSS sections
instead of the normal data or BSS section. By default, num is 8.
The -G num switch is also passed to the linker. All modules
should be compiled with the same -G num value.
- -mregnames
- -mno-regnames
- On System V.4 and embedded PowerPC systems do (do not) emit
register names in the assembly language output using symbolic forms.
- -mlongcall
- -mno-longcall
- By default assume that all calls are far away so that a
longer and more expensive calling sequence is required. This is required
for calls farther than 32 megabytes (33,554,432 bytes) from the current
location. A short call is generated if the compiler knows the call cannot
be that far away. This setting can be overridden by the
"shortcall" function attribute, or by "#pragma
longcall(0)".
Some linkers are capable of detecting out-of-range calls and generating glue
code on the fly. On these systems, long calls are unnecessary and generate
slower code. As of this writing, the AIX linker can do this, as can the
GNU linker for PowerPC/64. It is planned to add this feature to the GNU
linker for 32-bit PowerPC systems as well.
On Darwin/PPC systems, "#pragma longcall" generates "jbsr
callee, L42", plus a branch island (glue code). The two target
addresses represent the callee and the branch island. The Darwin/PPC
linker prefers the first address and generates a "bl callee" if
the PPC "bl" instruction reaches the callee directly; otherwise,
the linker generates "bl L42" to call the branch island. The
branch island is appended to the body of the calling function; it computes
the full 32-bit address of the callee and jumps to it.
On Mach-O (Darwin) systems, this option directs the compiler emit to the
glue for every direct call, and the Darwin linker decides whether to use
or discard it.
In the future, GCC may ignore all longcall specifications when the linker is
known to generate glue.
- -mtls-markers
- -mno-tls-markers
- Mark (do not mark) calls to "__tls_get_addr" with
a relocation specifying the function argument. The relocation allows the
linker to reliably associate function call with argument setup
instructions for TLS optimization, which in turn allows GCC to better
schedule the sequence.
- -pthread
- Adds support for multithreading with the pthreads
library. This option sets flags for both the preprocessor and linker.
- -mrecip
- -mno-recip
- This option enables use of the reciprocal estimate and
reciprocal square root estimate instructions with additional
Newton-Raphson steps to increase precision instead of doing a divide or
square root and divide for floating-point arguments. You should use the
-ffast-math option when using -mrecip (or at least
-funsafe-math-optimizations, -finite-math-only,
-freciprocal-math and -fno-trapping-math). Note that while
the throughput of the sequence is generally higher than the throughput of
the non-reciprocal instruction, the precision of the sequence can be
decreased by up to 2 ulp (i.e. the inverse of 1.0 equals 0.99999994) for
reciprocal square roots.
- -mrecip=opt
- This option controls which reciprocal estimate instructions
may be used. opt is a comma-separated list of options, which may be
preceded by a "!" to invert the option:
- all
- Enable all estimate instructions.
- default
- Enable the default instructions, equivalent to
-mrecip.
- none
- Disable all estimate instructions, equivalent to
-mno-recip.
- div
- Enable the reciprocal approximation instructions for both
single and double precision.
- divf
- Enable the single-precision reciprocal approximation
instructions.
- divd
- Enable the double-precision reciprocal approximation
instructions.
- rsqrt
- Enable the reciprocal square root approximation
instructions for both single and double precision.
- rsqrtf
- Enable the single-precision reciprocal square root
approximation instructions.
- rsqrtd
- Enable the double-precision reciprocal square root
approximation instructions.
So, for example,
-mrecip=all,!rsqrtd enables all of the reciprocal
estimate instructions, except for the "FRSQRTE",
"XSRSQRTEDP", and "XVRSQRTEDP" instructions which handle
the double-precision reciprocal square root calculations.
- -mrecip-precision
- -mno-recip-precision
- Assume (do not assume) that the reciprocal estimate
instructions provide higher-precision estimates than is mandated by the
PowerPC ABI. Selecting -mcpu=power6, -mcpu=power7 or
-mcpu=power8 automatically selects -mrecip-precision. The
double-precision square root estimate instructions are not generated by
default on low-precision machines, since they do not provide an estimate
that converges after three steps.
- -mveclibabi=type
- Specifies the ABI type to use for vectorizing intrinsics
using an external library. The only type supported at present is
mass, which specifies to use IBM's Mathematical Acceleration
Subsystem (MASS) libraries for vectorizing intrinsics using external
libraries. GCC currently emits calls to "acosd2",
"acosf4", "acoshd2", "acoshf4",
"asind2", "asinf4", "asinhd2",
"asinhf4", "atan2d2", "atan2f4",
"atand2", "atanf4", "atanhd2",
"atanhf4", "cbrtd2", "cbrtf4",
"cosd2", "cosf4", "coshd2",
"coshf4", "erfcd2", "erfcf4",
"erfd2", "erff4", "exp2d2",
"exp2f4", "expd2", "expf4",
"expm1d2", "expm1f4", "hypotd2",
"hypotf4", "lgammad2", "lgammaf4",
"log10d2", "log10f4", "log1pd2",
"log1pf4", "log2d2", "log2f4",
"logd2", "logf4", "powd2",
"powf4", "sind2", "sinf4",
"sinhd2", "sinhf4", "sqrtd2",
"sqrtf4", "tand2", "tanf4",
"tanhd2", and "tanhf4" when generating code for
power7. Both -ftree-vectorize and
-funsafe-math-optimizations must also be enabled. The MASS
libraries must be specified at link time.
- -mfriz
- -mno-friz
- Generate (do not generate) the "friz" instruction
when the -funsafe-math-optimizations option is used to optimize
rounding of floating-point values to 64-bit integer and back to floating
point. The "friz" instruction does not return the same value if
the floating-point number is too large to fit in an integer.
- -mpointers-to-nested-functions
- -mno-pointers-to-nested-functions
- Generate (do not generate) code to load up the static chain
register ("r11") when calling through a pointer on AIX and
64-bit Linux systems where a function pointer points to a 3-word
descriptor giving the function address, TOC value to be loaded in register
"r2", and static chain value to be loaded in register
"r11". The -mpointers-to-nested-functions is on by
default. You cannot call through pointers to nested functions or pointers
to functions compiled in other languages that use the static chain if you
use -mno-pointers-to-nested-functions.
- -msave-toc-indirect
- -mno-save-toc-indirect
- Generate (do not generate) code to save the TOC value in
the reserved stack location in the function prologue if the function calls
through a pointer on AIX and 64-bit Linux systems. If the TOC value is not
saved in the prologue, it is saved just before the call through the
pointer. The -mno-save-toc-indirect option is the default.
- -mcompat-align-parm
- -mno-compat-align-parm
- Generate (do not generate) code to pass structure
parameters with a maximum alignment of 64 bits, for compatibility with
older versions of GCC.
Older versions of GCC (prior to 4.9.0) incorrectly did not align a structure
parameter on a 128-bit boundary when that structure contained a member
requiring 128-bit alignment. This is corrected in more recent versions of
GCC. This option may be used to generate code that is compatible with
functions compiled with older versions of GCC.
The -mno-compat-align-parm option is the default.
RX Options
These command-line options are defined for RX targets:
- -m64bit-doubles
- -m32bit-doubles
- Make the "double" data type be 64 bits
(-m64bit-doubles) or 32 bits ( -m32bit-doubles) in size. The
default is -m32bit-doubles. Note RX floating-point hardware
only works on 32-bit values, which is why the default is
-m32bit-doubles.
- -fpu
- -nofpu
- Enables (-fpu) or disables (-nofpu) the use
of RX floating-point hardware. The default is enabled for the RX600 series
and disabled for the RX200 series.
Floating-point instructions are only generated for 32-bit floating-point
values, however, so the FPU hardware is not used for doubles if the
-m64bit-doubles option is used.
Note If the -fpu option is enabled then
-funsafe-math-optimizations is also enabled automatically. This is
because the RX FPU instructions are themselves unsafe.
- -mcpu=name
- Selects the type of RX CPU to be targeted. Currently three
types are supported, the generic RX600 and RX200 series
hardware and the specific RX610 CPU. The default is RX600.
The only difference between RX600 and RX610 is that the
RX610 does not support the "MVTIPL" instruction.
The RX200 series does not have a hardware floating-point unit and so
-nofpu is enabled by default when this type is selected.
- -mbig-endian-data
- -mlittle-endian-data
- Store data (but not code) in the big-endian format. The
default is -mlittle-endian-data, i.e. to store data in the
little-endian format.
- -msmall-data-limit=N
- Specifies the maximum size in bytes of global and static
variables which can be placed into the small data area. Using the small
data area can lead to smaller and faster code, but the size of area is
limited and it is up to the programmer to ensure that the area does not
overflow. Also when the small data area is used one of the RX's registers
(usually "r13") is reserved for use pointing to this area, so it
is no longer available for use by the compiler. This could result in
slower and/or larger code if variables are pushed onto the stack instead
of being held in this register.
Note, common variables (variables that have not been initialized) and
constants are not placed into the small data area as they are assigned to
other sections in the output executable.
The default value is zero, which disables this feature. Note, this feature
is not enabled by default with higher optimization levels ( -O2
etc) because of the potentially detrimental effects of reserving a
register. It is up to the programmer to experiment and discover whether
this feature is of benefit to their program. See the description of the
-mpid option for a description of how the actual register to hold
the small data area pointer is chosen.
- -msim
- -mno-sim
- Use the simulator runtime. The default is to use the
libgloss board-specific runtime.
- -mas100-syntax
- -mno-as100-syntax
- When generating assembler output use a syntax that is
compatible with Renesas's AS100 assembler. This syntax can also be handled
by the GAS assembler, but it has some restrictions so it is not generated
by default.
- -mmax-constant-size=N
- Specifies the maximum size, in bytes, of a constant that
can be used as an operand in a RX instruction. Although the RX instruction
set does allow constants of up to 4 bytes in length to be used in
instructions, a longer value equates to a longer instruction. Thus in some
circumstances it can be beneficial to restrict the size of constants that
are used in instructions. Constants that are too big are instead placed
into a constant pool and referenced via register indirection.
The value N can be between 0 and 4. A value of 0 (the default) or 4
means that constants of any size are allowed.
- -mrelax
- Enable linker relaxation. Linker relaxation is a process
whereby the linker attempts to reduce the size of a program by finding
shorter versions of various instructions. Disabled by default.
- -mint-register=N
- Specify the number of registers to reserve for fast
interrupt handler functions. The value N can be between 0 and 4. A
value of 1 means that register "r13" is reserved for the
exclusive use of fast interrupt handlers. A value of 2 reserves
"r13" and "r12". A value of 3 reserves
"r13", "r12" and "r11", and a value of 4
reserves "r13" through "r10". A value of 0, the
default, does not reserve any registers.
- -msave-acc-in-interrupts
- Specifies that interrupt handler functions should preserve
the accumulator register. This is only necessary if normal code might use
the accumulator register, for example because it performs 64-bit
multiplications. The default is to ignore the accumulator as this makes
the interrupt handlers faster.
- -mpid
- -mno-pid
- Enables the generation of position independent data. When
enabled any access to constant data is done via an offset from a base
address held in a register. This allows the location of constant data to
be determined at run time without requiring the executable to be
relocated, which is a benefit to embedded applications with tight memory
constraints. Data that can be modified is not affected by this option.
Note, using this feature reserves a register, usually "r13", for
the constant data base address. This can result in slower and/or larger
code, especially in complicated functions.
The actual register chosen to hold the constant data base address depends
upon whether the -msmall-data-limit and/or the
-mint-register command-line options are enabled. Starting with
register "r13" and proceeding downwards, registers are allocated
first to satisfy the requirements of -mint-register, then
-mpid and finally -msmall-data-limit. Thus it is possible
for the small data area register to be "r8" if both
-mint-register=4 and -mpid are specified on the command
line.
By default this feature is not enabled. The default can be restored via the
-mno-pid command-line option.
- -mno-warn-multiple-fast-interrupts
- -mwarn-multiple-fast-interrupts
- Prevents GCC from issuing a warning message if it finds
more than one fast interrupt handler when it is compiling a file. The
default is to issue a warning for each extra fast interrupt handler found,
as the RX only supports one such interrupt.
Note: The generic GCC command-line option
-ffixed-reg has
special significance to the RX port when used with the "interrupt"
function attribute. This attribute indicates a function intended to process
fast interrupts. GCC ensures that it only uses the registers "r10",
"r11", "r12" and/or "r13" and only provided that
the normal use of the corresponding registers have been restricted via the
-ffixed- reg or
-mint-register command-line options.
S/390 and zSeries Options
These are the
-m options defined for the S/390 and zSeries architecture.
- -mhard-float
- -msoft-float
- Use (do not use) the hardware floating-point instructions
and registers for floating-point operations. When -msoft-float is
specified, functions in libgcc.a are used to perform floating-point
operations. When -mhard-float is specified, the compiler generates
IEEE floating-point instructions. This is the default.
- -mhard-dfp
- -mno-hard-dfp
- Use (do not use) the hardware decimal-floating-point
instructions for decimal-floating-point operations. When
-mno-hard-dfp is specified, functions in libgcc.a are used
to perform decimal-floating-point operations. When -mhard-dfp is
specified, the compiler generates decimal-floating-point hardware
instructions. This is the default for -march=z9-ec or higher.
- -mlong-double-64
- -mlong-double-128
- These switches control the size of "long double"
type. A size of 64 bits makes the "long double" type equivalent
to the "double" type. This is the default.
- -mbackchain
- -mno-backchain
- Store (do not store) the address of the caller's frame as
backchain pointer into the callee's stack frame. A backchain may be needed
to allow debugging using tools that do not understand DWARF 2 call frame
information. When -mno-packed-stack is in effect, the backchain
pointer is stored at the bottom of the stack frame; when
-mpacked-stack is in effect, the backchain is placed into the
topmost word of the 96/160 byte register save area.
In general, code compiled with -mbackchain is call-compatible with
code compiled with -mmo-backchain; however, use of the backchain
for debugging purposes usually requires that the whole binary is built
with -mbackchain. Note that the combination of -mbackchain,
-mpacked-stack and -mhard-float is not supported. In order
to build a linux kernel use -msoft-float.
The default is to not maintain the backchain.
- -mpacked-stack
- -mno-packed-stack
- Use (do not use) the packed stack layout. When
-mno-packed-stack is specified, the compiler uses the all fields of
the 96/160 byte register save area only for their default purpose; unused
fields still take up stack space. When -mpacked-stack is specified,
register save slots are densely packed at the top of the register save
area; unused space is reused for other purposes, allowing for more
efficient use of the available stack space. However, when
-mbackchain is also in effect, the topmost word of the save area is
always used to store the backchain, and the return address register is
always saved two words below the backchain.
As long as the stack frame backchain is not used, code generated with
-mpacked-stack is call-compatible with code generated with
-mno-packed-stack. Note that some non-FSF releases of GCC 2.95 for
S/390 or zSeries generated code that uses the stack frame backchain at run
time, not just for debugging purposes. Such code is not call-compatible
with code compiled with -mpacked-stack. Also, note that the
combination of -mbackchain, -mpacked-stack and
-mhard-float is not supported. In order to build a linux kernel use
-msoft-float.
The default is to not use the packed stack layout.
- -msmall-exec
- -mno-small-exec
- Generate (or do not generate) code using the
"bras" instruction to do subroutine calls. This only works
reliably if the total executable size does not exceed 64k. The default is
to use the "basr" instruction instead, which does not have this
limitation.
- -m64
- -m31
- When -m31 is specified, generate code compliant to
the GNU/Linux for S/390 ABI. When -m64 is specified, generate code
compliant to the GNU/Linux for zSeries ABI. This allows GCC in particular
to generate 64-bit instructions. For the s390 targets, the default
is -m31, while the s390x targets default to
-m64.
- -mzarch
- -mesa
- When -mzarch is specified, generate code using the
instructions available on z/Architecture. When -mesa is specified,
generate code using the instructions available on ESA/390. Note that
-mesa is not possible with -m64. When generating code
compliant to the GNU/Linux for S/390 ABI, the default is -mesa.
When generating code compliant to the GNU/Linux for zSeries ABI, the
default is -mzarch.
- -mmvcle
- -mno-mvcle
- Generate (or do not generate) code using the
"mvcle" instruction to perform block moves. When
-mno-mvcle is specified, use a "mvc" loop instead. This
is the default unless optimizing for size.
- -mdebug
- -mno-debug
- Print (or do not print) additional debug information when
compiling. The default is to not print debug information.
- -march=cpu-type
- Generate code that runs on cpu-type, which is the
name of a system representing a certain processor type. Possible values
for cpu-type are g5, g6, z900, z990,
z9-109, z9-ec, z10, z196, zEC12, and
z13. When generating code using the instructions available on
z/Architecture, the default is -march=z900. Otherwise, the default
is -march=g5.
- -mtune=cpu-type
- Tune to cpu-type everything applicable about the
generated code, except for the ABI and the set of available instructions.
The list of cpu-type values is the same as for -march. The
default is the value used for -march.
- -mtpf-trace
- -mno-tpf-trace
- Generate code that adds (does not add) in TPF OS specific
branches to trace routines in the operating system. This option is off by
default, even when compiling for the TPF OS.
- -mfused-madd
- -mno-fused-madd
- Generate code that uses (does not use) the floating-point
multiply and accumulate instructions. These instructions are generated by
default if hardware floating point is used.
- -mwarn-framesize=framesize
- Emit a warning if the current function exceeds the given
frame size. Because this is a compile-time check it doesn't need to be a
real problem when the program runs. It is intended to identify functions
that most probably cause a stack overflow. It is useful to be used in an
environment with limited stack size e.g. the linux kernel.
- -mwarn-dynamicstack
- Emit a warning if the function calls "alloca" or
uses dynamically-sized arrays. This is generally a bad idea with a limited
stack size.
- -mstack-guard=stack-guard
- -mstack-size=stack-size
- If these options are provided the S/390 back end emits
additional instructions in the function prologue that trigger a trap if
the stack size is stack-guard bytes above the stack-size
(remember that the stack on S/390 grows downward). If the
stack-guard option is omitted the smallest power of 2 larger than
the frame size of the compiled function is chosen. These options are
intended to be used to help debugging stack overflow problems. The
additionally emitted code causes only little overhead and hence can also
be used in production-like systems without greater performance
degradation. The given values have to be exact powers of 2 and
stack-size has to be greater than stack-guard without
exceeding 64k. In order to be efficient the extra code makes the
assumption that the stack starts at an address aligned to the value given
by stack-size. The stack-guard option can only be used in
conjunction with stack-size.
- -mhotpatch=pre-halfwords,post-halfwords
- If the hotpatch option is enabled, a
"hot-patching" function prologue is generated for all functions
in the compilation unit. The funtion label is prepended with the given
number of two-byte NOP instructions ( pre-halfwords, maximum
1000000). After the label, 2 * post-halfwords bytes are appended,
using the largest NOP like instructions the architecture allows (maximum
1000000).
If both arguments are zero, hotpatching is disabled.
This option can be overridden for individual functions with the
"hotpatch" attribute.
Score Options
These options are defined for Score implementations:
- -meb
- Compile code for big-endian mode. This is the default.
- -mel
- Compile code for little-endian mode.
- -mnhwloop
- Disable generation of "bcnz" instructions.
- -muls
- Enable generation of unaligned load and store
instructions.
- -mmac
- Enable the use of multiply-accumulate instructions.
Disabled by default.
- -mscore5
- Specify the SCORE5 as the target architecture.
- -mscore5u
- Specify the SCORE5U of the target architecture.
- -mscore7
- Specify the SCORE7 as the target architecture. This is the
default.
- -mscore7d
- Specify the SCORE7D as the target architecture.
SH Options
These
-m options are defined for the SH implementations:
- -m1
- Generate code for the SH1.
- -m2
- Generate code for the SH2.
- -m2e
- Generate code for the SH2e.
- -m2a-nofpu
- Generate code for the SH2a without FPU, or for a SH2a-FPU
in such a way that the floating-point unit is not used.
- -m2a-single-only
- Generate code for the SH2a-FPU, in such a way that no
double-precision floating-point operations are used.
- -m2a-single
- Generate code for the SH2a-FPU assuming the floating-point
unit is in single-precision mode by default.
- -m2a
- Generate code for the SH2a-FPU assuming the floating-point
unit is in double-precision mode by default.
- -m3
- Generate code for the SH3.
- -m3e
- Generate code for the SH3e.
- -m4-nofpu
- Generate code for the SH4 without a floating-point
unit.
- -m4-single-only
- Generate code for the SH4 with a floating-point unit that
only supports single-precision arithmetic.
- -m4-single
- Generate code for the SH4 assuming the floating-point unit
is in single-precision mode by default.
- -m4
- Generate code for the SH4.
- -m4-100
- Generate code for SH4-100.
- -m4-100-nofpu
- Generate code for SH4-100 in such a way that the
floating-point unit is not used.
- -m4-100-single
- Generate code for SH4-100 assuming the floating-point unit
is in single-precision mode by default.
- -m4-100-single-only
- Generate code for SH4-100 in such a way that no
double-precision floating-point operations are used.
- -m4-200
- Generate code for SH4-200.
- -m4-200-nofpu
- Generate code for SH4-200 without in such a way that the
floating-point unit is not used.
- -m4-200-single
- Generate code for SH4-200 assuming the floating-point unit
is in single-precision mode by default.
- -m4-200-single-only
- Generate code for SH4-200 in such a way that no
double-precision floating-point operations are used.
- -m4-300
- Generate code for SH4-300.
- -m4-300-nofpu
- Generate code for SH4-300 without in such a way that the
floating-point unit is not used.
- -m4-300-single
- Generate code for SH4-300 in such a way that no
double-precision floating-point operations are used.
- -m4-300-single-only
- Generate code for SH4-300 in such a way that no
double-precision floating-point operations are used.
- -m4-340
- Generate code for SH4-340 (no MMU, no FPU).
- -m4-500
- Generate code for SH4-500 (no FPU). Passes
-isa=sh4-nofpu to the assembler.
- -m4a-nofpu
- Generate code for the SH4al-dsp, or for a SH4a in such a
way that the floating-point unit is not used.
- -m4a-single-only
- Generate code for the SH4a, in such a way that no
double-precision floating-point operations are used.
- -m4a-single
- Generate code for the SH4a assuming the floating-point unit
is in single-precision mode by default.
- -m4a
- Generate code for the SH4a.
- -m4al
- Same as -m4a-nofpu, except that it implicitly passes
-dsp to the assembler. GCC doesn't generate any DSP instructions at
the moment.
- -m5-32media
- Generate 32-bit code for SHmedia.
- -m5-32media-nofpu
- Generate 32-bit code for SHmedia in such a way that the
floating-point unit is not used.
- -m5-64media
- Generate 64-bit code for SHmedia.
- -m5-64media-nofpu
- Generate 64-bit code for SHmedia in such a way that the
floating-point unit is not used.
- -m5-compact
- Generate code for SHcompact.
- -m5-compact-nofpu
- Generate code for SHcompact in such a way that the
floating-point unit is not used.
- -mb
- Compile code for the processor in big-endian mode.
- -ml
- Compile code for the processor in little-endian mode.
- -mdalign
- Align doubles at 64-bit boundaries. Note that this changes
the calling conventions, and thus some functions from the standard C
library do not work unless you recompile it first with
-mdalign.
- -mrelax
- Shorten some address references at link time, when
possible; uses the linker option -relax.
- -mbigtable
- Use 32-bit offsets in "switch" tables. The
default is to use 16-bit offsets.
- -mbitops
- Enable the use of bit manipulation instructions on
SH2A.
- -mfmovd
- Enable the use of the instruction "fmovd". Check
-mdalign for alignment constraints.
- -mrenesas
- Comply with the calling conventions defined by
Renesas.
- -mno-renesas
- Comply with the calling conventions defined for GCC before
the Renesas conventions were available. This option is the default for all
targets of the SH toolchain.
- -mnomacsave
- Mark the "MAC" register as call-clobbered, even
if -mrenesas is given.
- -mieee
- -mno-ieee
- Control the IEEE compliance of floating-point comparisons,
which affects the handling of cases where the result of a comparison is
unordered. By default -mieee is implicitly enabled. If
-ffinite-math-only is enabled -mno-ieee is implicitly set,
which results in faster floating-point greater-equal and less-equal
comparisons. The implcit settings can be overridden by specifying either
-mieee or -mno-ieee.
- -minline-ic_invalidate
- Inline code to invalidate instruction cache entries after
setting up nested function trampolines. This option has no effect if
-musermode is in effect and the selected code generation option
(e.g. -m4) does not allow the use of the "icbi"
instruction. If the selected code generation option does not allow the use
of the "icbi" instruction, and -musermode is not in
effect, the inlined code manipulates the instruction cache address array
directly with an associative write. This not only requires privileged mode
at run time, but it also fails if the cache line had been mapped via the
TLB and has become unmapped.
- -misize
- Dump instruction size and location in the assembly
code.
- -mpadstruct
- This option is deprecated. It pads structures to multiple
of 4 bytes, which is incompatible with the SH ABI.
- -matomic-model=model
- Sets the model of atomic operations and additional
parameters as a comma separated list. For details on the atomic built-in
functions see __atomic Builtins. The following models and
parameters are supported:
- none
- Disable compiler generated atomic sequences and emit
library calls for atomic operations. This is the default if the target is
not "sh*-*-linux*".
- soft-gusa
- Generate GNU/Linux compatible gUSA software atomic
sequences for the atomic built-in functions. The generated atomic
sequences require additional support from the interrupt/exception handling
code of the system and are only suitable for SH3* and SH4* single-core
systems. This option is enabled by default when the target is
"sh*-*-linux*" and SH3* or SH4*. When the target is SH4A, this
option also partially utilizes the hardware atomic instructions
"movli.l" and "movco.l" to create more efficient code,
unless strict is specified.
- soft-tcb
- Generate software atomic sequences that use a variable in
the thread control block. This is a variation of the gUSA sequences which
can also be used on SH1* and SH2* targets. The generated atomic sequences
require additional support from the interrupt/exception handling code of
the system and are only suitable for single-core systems. When using this
model, the gbr-offset= parameter has to be specified as well.
- soft-imask
- Generate software atomic sequences that temporarily disable
interrupts by setting "SR.IMASK = 1111". This model works only
when the program runs in privileged mode and is only suitable for
single-core systems. Additional support from the interrupt/exception
handling code of the system is not required. This model is enabled by
default when the target is "sh*-*-linux*" and SH1* or SH2*.
- hard-llcs
- Generate hardware atomic sequences using the
"movli.l" and "movco.l" instructions only. This is
only available on SH4A and is suitable for multi-core systems. Since the
hardware instructions support only 32 bit atomic variables access to 8 or
16 bit variables is emulated with 32 bit accesses. Code compiled with this
option is also compatible with other software atomic model
interrupt/exception handling systems if executed on an SH4A system.
Additional support from the interrupt/exception handling code of the
system is not required for this model.
- gbr-offset=
- This parameter specifies the offset in bytes of the
variable in the thread control block structure that should be used by the
generated atomic sequences when the soft-tcb model has been
selected. For other models this parameter is ignored. The specified value
must be an integer multiple of four and in the range 0-1020.
- strict
- This parameter prevents mixed usage of multiple atomic
models, even if they are compatible, and makes the compiler generate
atomic sequences of the specified model only.
- -mtas
- Generate the "tas.b" opcode for
"__atomic_test_and_set". Notice that depending on the particular
hardware and software configuration this can degrade overall performance
due to the operand cache line flushes that are implied by the
"tas.b" instruction. On multi-core SH4A processors the
"tas.b" instruction must be used with caution since it can
result in data corruption for certain cache configurations.
- -mprefergot
- When generating position-independent code, emit function
calls using the Global Offset Table instead of the Procedure Linkage
Table.
- -musermode
- -mno-usermode
- Don't allow (allow) the compiler generating privileged mode
code. Specifying -musermode also implies
-mno-inline-ic_invalidate if the inlined code would not work in
user mode. -musermode is the default when the target is
"sh*-*-linux*". If the target is SH1* or SH2* -musermode
has no effect, since there is no user mode.
- -multcost=number
- Set the cost to assume for a multiply insn.
- -mdiv=strategy
- Set the division strategy to be used for integer division
operations. For SHmedia strategy can be one of:
- fp
- Performs the operation in floating point. This has a very
high latency, but needs only a few instructions, so it might be a good
choice if your code has enough easily-exploitable ILP to allow the
compiler to schedule the floating-point instructions together with other
instructions. Division by zero causes a floating-point exception.
- inv
- Uses integer operations to calculate the inverse of the
divisor, and then multiplies the dividend with the inverse. This strategy
allows CSE and hoisting of the inverse calculation. Division by zero
calculates an unspecified result, but does not trap.
- inv:minlat
- A variant of inv where, if no CSE or hoisting
opportunities have been found, or if the entire operation has been hoisted
to the same place, the last stages of the inverse calculation are
intertwined with the final multiply to reduce the overall latency, at the
expense of using a few more instructions, and thus offering fewer
scheduling opportunities with other code.
- call
- Calls a library function that usually implements the
inv:minlat strategy. This gives high code density for
"m5-*media-nofpu" compilations.
- call2
- Uses a different entry point of the same library function,
where it assumes that a pointer to a lookup table has already been set up,
which exposes the pointer load to CSE and code hoisting
optimizations.
- inv:call
- inv:call2
- inv:fp
- Use the inv algorithm for initial code generation,
but if the code stays unoptimized, revert to the call,
call2, or fp strategies, respectively. Note that the
potentially-trapping side effect of division by zero is carried by a
separate instruction, so it is possible that all the integer instructions
are hoisted out, but the marker for the side effect stays where it is. A
recombination to floating-point operations or a call is not possible in
that case.
- inv20u
- inv20l
- Variants of the inv:minlat strategy. In the case
that the inverse calculation is not separated from the multiply, they
speed up division where the dividend fits into 20 bits (plus sign where
applicable) by inserting a test to skip a number of operations in this
case; this test slows down the case of larger dividends. inv20u
assumes the case of a such a small dividend to be unlikely, and
inv20l assumes it to be likely.
For targets other than SHmedia
strategy can be one of:
- call-div1
- Calls a library function that uses the single-step division
instruction "div1" to perform the operation. Division by zero
calculates an unspecified result and does not trap. This is the default
except for SH4, SH2A and SHcompact.
- call-fp
- Calls a library function that performs the operation in
double precision floating point. Division by zero causes a floating-point
exception. This is the default for SHcompact with FPU. Specifying this for
targets that do not have a double precision FPU defaults to
"call-div1".
- call-table
- Calls a library function that uses a lookup table for small
divisors and the "div1" instruction with case distinction for
larger divisors. Division by zero calculates an unspecified result and
does not trap. This is the default for SH4. Specifying this for targets
that do not have dynamic shift instructions defaults to
"call-div1".
When a division strategy has not been specified the default strategy is selected
based on the current target. For SH2A the default strategy is to use the
"divs" and "divu" instructions instead of library function
calls.
- -maccumulate-outgoing-args
- Reserve space once for outgoing arguments in the function
prologue rather than around each call. Generally beneficial for
performance and size. Also needed for unwinding to avoid changing the
stack frame around conditional code.
- -mdivsi3_libfunc=name
- Set the name of the library function used for 32-bit signed
division to name. This only affects the name used in the
call and inv:call division strategies, and the compiler
still expects the same sets of input/output/clobbered registers as if this
option were not present.
- -mfixed-range=register-range
- Generate code treating the given register range as fixed
registers. A fixed register is one that the register allocator can not
use. This is useful when compiling kernel code. A register range is
specified as two registers separated by a dash. Multiple register ranges
can be specified separated by a comma.
- -mindexed-addressing
- Enable the use of the indexed addressing mode for
SHmedia32/SHcompact. This is only safe if the hardware and/or OS implement
32-bit wrap-around semantics for the indexed addressing mode. The
architecture allows the implementation of processors with 64-bit MMU,
which the OS could use to get 32-bit addressing, but since no current
hardware implementation supports this or any other way to make the indexed
addressing mode safe to use in the 32-bit ABI, the default is
-mno-indexed-addressing.
- -mgettrcost=number
- Set the cost assumed for the "gettr" instruction
to number. The default is 2 if -mpt-fixed is in effect, 100
otherwise.
- -mpt-fixed
- Assume "pt*" instructions won't trap. This
generally generates better-scheduled code, but is unsafe on current
hardware. The current architecture definition says that "ptabs"
and "ptrel" trap when the target anded with 3 is 3. This has the
unintentional effect of making it unsafe to schedule these instructions
before a branch, or hoist them out of a loop. For example,
"__do_global_ctors", a part of libgcc that runs
constructors at program startup, calls functions in a list which is
delimited by -1. With the -mpt-fixed option, the "ptabs"
is done before testing against -1. That means that all the constructors
run a bit more quickly, but when the loop comes to the end of the list,
the program crashes because "ptabs" loads -1 into a target
register.
Since this option is unsafe for any hardware implementing the current
architecture specification, the default is -mno-pt-fixed. Unless
specified explicitly with -mgettrcost, -mno-pt-fixed also
implies -mgettrcost=100; this deters register allocation from using
target registers for storing ordinary integers.
- -minvalid-symbols
- Assume symbols might be invalid. Ordinary function symbols
generated by the compiler are always valid to load with
"movi"/"shori"/"ptabs" or
"movi"/"shori"/"ptrel", but with assembler
and/or linker tricks it is possible to generate symbols that cause
"ptabs" or "ptrel" to trap. This option is only
meaningful when -mno-pt-fixed is in effect. It prevents
cross-basic-block CSE, hoisting and most scheduling of symbol loads. The
default is -mno-invalid-symbols.
- -mbranch-cost=num
- Assume num to be the cost for a branch instruction.
Higher numbers make the compiler try to generate more branch-free code if
possible. If not specified the value is selected depending on the
processor type that is being compiled for.
- -mzdcbranch
- -mno-zdcbranch
- Assume (do not assume) that zero displacement conditional
branch instructions "bt" and "bf" are fast. If
-mzdcbranch is specified, the compiler prefers zero displacement
branch code sequences. This is enabled by default when generating code for
SH4 and SH4A. It can be explicitly disabled by specifying
-mno-zdcbranch.
- -mcbranch-force-delay-slot
- Force the usage of delay slots for conditional branches,
which stuffs the delay slot with a "nop" if a suitable
instruction can't be found. By default this option is disabled. It can be
enabled to work around hardware bugs as found in the original SH7055.
- -mfused-madd
- -mno-fused-madd
- Generate code that uses (does not use) the floating-point
multiply and accumulate instructions. These instructions are generated by
default if hardware floating point is used. The machine-dependent
-mfused-madd option is now mapped to the machine-independent
-ffp-contract=fast option, and -mno-fused-madd is mapped to
-ffp-contract=off.
- -mfsca
- -mno-fsca
- Allow or disallow the compiler to emit the "fsca"
instruction for sine and cosine approximations. The option -mfsca
must be used in combination with -funsafe-math-optimizations. It is
enabled by default when generating code for SH4A. Using -mno-fsca
disables sine and cosine approximations even if
-funsafe-math-optimizations is in effect.
- -mfsrra
- -mno-fsrra
- Allow or disallow the compiler to emit the
"fsrra" instruction for reciprocal square root approximations.
The option -mfsrra must be used in combination with
-funsafe-math-optimizations and -ffinite-math-only. It is
enabled by default when generating code for SH4A. Using -mno-fsrra
disables reciprocal square root approximations even if
-funsafe-math-optimizations and -ffinite-math-only are in
effect.
- -mpretend-cmove
- Prefer zero-displacement conditional branches for
conditional move instruction patterns. This can result in faster code on
the SH4 processor.
Solaris 2 Options
These
-m options are supported on Solaris 2:
- -mclear-hwcap
- -mclear-hwcap tells the compiler to remove the
hardware capabilities generated by the Solaris assembler. This is only
necessary when object files use ISA extensions not supported by the
current machine, but check at runtime whether or not to use them.
- -mimpure-text
- -mimpure-text, used in addition to -shared,
tells the compiler to not pass -z text to the linker when linking a
shared object. Using this option, you can link position-dependent code
into a shared object.
-mimpure-text suppresses the "relocations remain against
allocatable but non-writable sections" linker error message. However,
the necessary relocations trigger copy-on-write, and the shared object is
not actually shared across processes. Instead of using
-mimpure-text, you should compile all source code with -fpic
or -fPIC.
These switches are supported in addition to the above on Solaris 2:
- -pthreads
- Add support for multithreading using the POSIX threads
library. This option sets flags for both the preprocessor and linker. This
option does not affect the thread safety of object code produced by the
compiler or that of libraries supplied with it.
- -pthread
- This is a synonym for -pthreads.
SPARC Options
These
-m options are supported on the SPARC:
- -mno-app-regs
- -mapp-regs
- Specify -mapp-regs to generate output using the
global registers 2 through 4, which the SPARC SVR4 ABI reserves for
applications. Like the global register 1, each global register 2 through 4
is then treated as an allocable register that is clobbered by function
calls. This is the default.
To be fully SVR4 ABI-compliant at the cost of some performance loss, specify
-mno-app-regs. You should compile libraries and system software
with this option.
- -mflat
- -mno-flat
- With -mflat, the compiler does not generate
save/restore instructions and uses a "flat" or single register
window model. This model is compatible with the regular register window
model. The local registers and the input registers (0--5) are still
treated as "call-saved" registers and are saved on the stack as
needed.
With -mno-flat (the default), the compiler generates save/restore
instructions (except for leaf functions). This is the normal operating
mode.
- -mfpu
- -mhard-float
- Generate output containing floating-point instructions.
This is the default.
- -mno-fpu
- -msoft-float
- Generate output containing library calls for floating
point. Warning: the requisite libraries are not available for all
SPARC targets. Normally the facilities of the machine's usual C compiler
are used, but this cannot be done directly in cross-compilation. You must
make your own arrangements to provide suitable library functions for
cross-compilation. The embedded targets sparc-*-aout and
sparclite-*-* do provide software floating-point support.
-msoft-float changes the calling convention in the output file;
therefore, it is only useful if you compile all of a program with
this option. In particular, you need to compile libgcc.a, the
library that comes with GCC, with -msoft-float in order for this to
work.
- -mhard-quad-float
- Generate output containing quad-word (long double)
floating-point instructions.
- -msoft-quad-float
- Generate output containing library calls for quad-word
(long double) floating-point instructions. The functions called are those
specified in the SPARC ABI. This is the default.
As of this writing, there are no SPARC implementations that have hardware
support for the quad-word floating-point instructions. They all invoke a
trap handler for one of these instructions, and then the trap handler
emulates the effect of the instruction. Because of the trap handler
overhead, this is much slower than calling the ABI library routines. Thus
the -msoft-quad-float option is the default.
- -mno-unaligned-doubles
- -munaligned-doubles
- Assume that doubles have 8-byte alignment. This is the
default.
With -munaligned-doubles, GCC assumes that doubles have 8-byte
alignment only if they are contained in another type, or if they have an
absolute address. Otherwise, it assumes they have 4-byte alignment.
Specifying this option avoids some rare compatibility problems with code
generated by other compilers. It is not the default because it results in
a performance loss, especially for floating-point code.
- -muser-mode
- -mno-user-mode
- Do not generate code that can only run in supervisor mode.
This is relevant only for the "casa" instruction emitted for the
LEON3 processor. This is the default.
- -mno-faster-structs
- -mfaster-structs
- With -mfaster-structs, the compiler assumes that
structures should have 8-byte alignment. This enables the use of pairs of
"ldd" and "std" instructions for copies in structure
assignment, in place of twice as many "ld" and "st"
pairs. However, the use of this changed alignment directly violates the
SPARC ABI. Thus, it's intended only for use on targets where the developer
acknowledges that their resulting code is not directly in line with the
rules of the ABI.
- -mcpu=cpu_type
- Set the instruction set, register set, and instruction
scheduling parameters for machine type cpu_type. Supported values
for cpu_type are v7, cypress, v8,
supersparc, hypersparc, leon, leon3,
leon3v7, sparclite, f930, f934,
sparclite86x, sparclet, tsc701, v9,
ultrasparc, ultrasparc3, niagara, niagara2,
niagara3 and niagara4.
Native Solaris and GNU/Linux toolchains also support the value
native, which selects the best architecture option for the host
processor. -mcpu=native has no effect if GCC does not recognize the
processor.
Default instruction scheduling parameters are used for values that select an
architecture and not an implementation. These are v7, v8,
sparclite, sparclet, v9.
Here is a list of each supported architecture and their supported
implementations.
- v7
- cypress, leon3v7
- v8
- supersparc, hypersparc, leon, leon3
- sparclite
- f930, f934, sparclite86x
- sparclet
- tsc701
- v9
- ultrasparc, ultrasparc3, niagara, niagara2, niagara3,
niagara4
By default (unless configured otherwise), GCC generates code for the V7 variant
of the SPARC architecture. With
-mcpu=cypress, the compiler
additionally optimizes it for the Cypress CY7C602 chip, as used in the
SPARCStation/SPARCServer 3xx series. This is also appropriate for the older
SPARCStation 1, 2, IPX etc.
With
-mcpu=v8, GCC generates code for the V8 variant of the SPARC
architecture. The only difference from V7 code is that the compiler emits the
integer multiply and integer divide instructions which exist in SPARC-V8 but
not in SPARC-V7. With
-mcpu=supersparc, the compiler additionally
optimizes it for the SuperSPARC chip, as used in the SPARCStation 10, 1000 and
2000 series.
With
-mcpu=sparclite, GCC generates code for the SPARClite variant of the
SPARC architecture. This adds the integer multiply, integer divide step and
scan ("ffs") instructions which exist in SPARClite but not in
SPARC-V7. With
-mcpu=f930, the compiler additionally optimizes it for
the Fujitsu MB86930 chip, which is the original SPARClite, with no FPU. With
-mcpu=f934, the compiler additionally optimizes it for the Fujitsu
MB86934 chip, which is the more recent SPARClite with FPU.
With
-mcpu=sparclet, GCC generates code for the SPARClet variant of the
SPARC architecture. This adds the integer multiply, multiply/accumulate,
integer divide step and scan ("ffs") instructions which exist in
SPARClet but not in SPARC-V7. With
-mcpu=tsc701, the compiler
additionally optimizes it for the TEMIC SPARClet chip.
With
-mcpu=v9, GCC generates code for the V9 variant of the SPARC
architecture. This adds 64-bit integer and floating-point move instructions, 3
additional floating-point condition code registers and conditional move
instructions. With
-mcpu=ultrasparc, the compiler additionally
optimizes it for the Sun UltraSPARC I/II/IIi chips. With
-mcpu=ultrasparc3, the compiler additionally optimizes it for the Sun
UltraSPARC III/III+/IIIi/IIIi+/IV/IV+ chips. With
-mcpu=niagara, the
compiler additionally optimizes it for Sun UltraSPARC T1 chips. With
-mcpu=niagara2, the compiler additionally optimizes it for Sun
UltraSPARC T2 chips. With
-mcpu=niagara3, the compiler additionally
optimizes it for Sun UltraSPARC T3 chips. With
-mcpu=niagara4, the
compiler additionally optimizes it for Sun UltraSPARC T4 chips.
- -mtune=cpu_type
- Set the instruction scheduling parameters for machine type
cpu_type, but do not set the instruction set or register set that
the option -mcpu=cpu_type does.
The same values for -mcpu=cpu_type can be used for
-mtune= cpu_type, but the only useful values are those that
select a particular CPU implementation. Those are cypress,
supersparc, hypersparc, leon, leon3,
leon3v7, f930, f934, sparclite86x,
tsc701, ultrasparc, ultrasparc3, niagara,
niagara2, niagara3 and niagara4. With native Solaris
and GNU/Linux toolchains, native can also be used.
- -mv8plus
- -mno-v8plus
- With -mv8plus, GCC generates code for the SPARC-V8+
ABI. The difference from the V8 ABI is that the global and out registers
are considered 64 bits wide. This is enabled by default on Solaris in
32-bit mode for all SPARC-V9 processors.
- -mvis
- -mno-vis
- With -mvis, GCC generates code that takes advantage
of the UltraSPARC Visual Instruction Set extensions. The default is
-mno-vis.
- -mvis2
- -mno-vis2
- With -mvis2, GCC generates code that takes advantage
of version 2.0 of the UltraSPARC Visual Instruction Set extensions. The
default is -mvis2 when targeting a cpu that supports such
instructions, such as UltraSPARC-III and later. Setting -mvis2 also
sets -mvis.
- -mvis3
- -mno-vis3
- With -mvis3, GCC generates code that takes advantage
of version 3.0 of the UltraSPARC Visual Instruction Set extensions. The
default is -mvis3 when targeting a cpu that supports such
instructions, such as niagara-3 and later. Setting -mvis3 also sets
-mvis2 and -mvis.
- -mcbcond
- -mno-cbcond
- With -mcbcond, GCC generates code that takes
advantage of compare-and-branch instructions, as defined in the Sparc
Architecture 2011. The default is -mcbcond when targeting a cpu
that supports such instructions, such as niagara-4 and later.
- -mpopc
- -mno-popc
- With -mpopc, GCC generates code that takes advantage
of the UltraSPARC population count instruction. The default is
-mpopc when targeting a cpu that supports such instructions, such
as Niagara-2 and later.
- -mfmaf
- -mno-fmaf
- With -mfmaf, GCC generates code that takes advantage
of the UltraSPARC Fused Multiply-Add Floating-point extensions. The
default is -mfmaf when targeting a cpu that supports such
instructions, such as Niagara-3 and later.
- -mfix-at697f
- Enable the documented workaround for the single erratum of
the Atmel AT697F processor (which corresponds to erratum #13 of the AT697E
processor).
- -mfix-ut699
- Enable the documented workarounds for the floating-point
errata and the data cache nullify errata of the UT699 processor.
These
-m options are supported in addition to the above on SPARC-V9
processors in 64-bit environments:
- -m32
- -m64
- Generate code for a 32-bit or 64-bit environment. The
32-bit environment sets int, long and pointer to 32 bits. The 64-bit
environment sets int to 32 bits and long and pointer to 64 bits.
- -mcmodel=which
- Set the code model to one of
- medlow
- The Medium/Low code model: 64-bit addresses, programs must
be linked in the low 32 bits of memory. Programs can be statically or
dynamically linked.
- medmid
- The Medium/Middle code model: 64-bit addresses, programs
must be linked in the low 44 bits of memory, the text and data segments
must be less than 2GB in size and the data segment must be located within
2GB of the text segment.
- medany
- The Medium/Anywhere code model: 64-bit addresses, programs
may be linked anywhere in memory, the text and data segments must be less
than 2GB in size and the data segment must be located within 2GB of the
text segment.
- embmedany
- The Medium/Anywhere code model for embedded systems: 64-bit
addresses, the text and data segments must be less than 2GB in size, both
starting anywhere in memory (determined at link time). The global register
%g4 points to the base of the data segment. Programs are statically linked
and PIC is not supported.
- -mmemory-model=mem-model
- Set the memory model in force on the processor to one
of
- default
- The default memory model for the processor and operating
system.
- rmo
- Relaxed Memory Order
- pso
- Partial Store Order
- tso
- Total Store Order
- sc
- Sequential Consistency
These memory models are formally defined in Appendix D of the Sparc V9
architecture manual, as set in the processor's "PSTATE.MM"
field.
- -mstack-bias
- -mno-stack-bias
- With -mstack-bias, GCC assumes that the stack
pointer, and frame pointer if present, are offset by -2047 which must be
added back when making stack frame references. This is the default in
64-bit mode. Otherwise, assume no such offset is present.
SPU Options
These
-m options are supported on the SPU:
- -mwarn-reloc
- -merror-reloc
- The loader for SPU does not handle dynamic relocations. By
default, GCC gives an error when it generates code that requires a dynamic
relocation. -mno-error-reloc disables the error,
-mwarn-reloc generates a warning instead.
- -msafe-dma
- -munsafe-dma
- Instructions that initiate or test completion of DMA must
not be reordered with respect to loads and stores of the memory that is
being accessed. With -munsafe-dma you must use the
"volatile" keyword to protect memory accesses, but that can lead
to inefficient code in places where the memory is known to not change.
Rather than mark the memory as volatile, you can use -msafe-dma to
tell the compiler to treat the DMA instructions as potentially affecting
all memory.
- -mbranch-hints
- By default, GCC generates a branch hint instruction to
avoid pipeline stalls for always-taken or probably-taken branches. A hint
is not generated closer than 8 instructions away from its branch. There is
little reason to disable them, except for debugging purposes, or to make
an object a little bit smaller.
- -msmall-mem
- -mlarge-mem
- By default, GCC generates code assuming that addresses are
never larger than 18 bits. With -mlarge-mem code is generated that
assumes a full 32-bit address.
- -mstdmain
- By default, GCC links against startup code that assumes the
SPU-style main function interface (which has an unconventional parameter
list). With -mstdmain, GCC links your program against startup code
that assumes a C99-style interface to "main", including a local
copy of "argv" strings.
- -mfixed-range=register-range
- Generate code treating the given register range as fixed
registers. A fixed register is one that the register allocator cannot use.
This is useful when compiling kernel code. A register range is specified
as two registers separated by a dash. Multiple register ranges can be
specified separated by a comma.
- -mea32
- -mea64
- Compile code assuming that pointers to the PPU address
space accessed via the "__ea" named address space qualifier are
either 32 or 64 bits wide. The default is 32 bits. As this is an
ABI-changing option, all object code in an executable must be compiled
with the same setting.
- -maddress-space-conversion
- -mno-address-space-conversion
- Allow/disallow treating the "__ea" address space
as superset of the generic address space. This enables explicit type casts
between "__ea" and generic pointer as well as implicit
conversions of generic pointers to "__ea" pointers. The default
is to allow address space pointer conversions.
- -mcache-size=cache-size
- This option controls the version of libgcc that the
compiler links to an executable and selects a software-managed cache for
accessing variables in the "__ea" address space with a
particular cache size. Possible options for cache-size are
8, 16, 32, 64 and 128. The default
cache size is 64KB.
- -matomic-updates
- -mno-atomic-updates
- This option controls the version of libgcc that the
compiler links to an executable and selects whether atomic updates to the
software-managed cache of PPU-side variables are used. If you use atomic
updates, changes to a PPU variable from SPU code using the
"__ea" named address space qualifier do not interfere with
changes to other PPU variables residing in the same cache line from PPU
code. If you do not use atomic updates, such interference may occur;
however, writing back cache lines is more efficient. The default behavior
is to use atomic updates.
- -mdual-nops
- -mdual-nops=n
- By default, GCC inserts nops to increase dual issue when it
expects it to increase performance. n can be a value from 0 to 10.
A smaller n inserts fewer nops. 10 is the default, 0 is the same as
-mno-dual-nops. Disabled with -Os.
- -mhint-max-nops=n
- Maximum number of nops to insert for a branch hint. A
branch hint must be at least 8 instructions away from the branch it is
affecting. GCC inserts up to n nops to enforce this, otherwise it
does not generate the branch hint.
- -mhint-max-distance=n
- The encoding of the branch hint instruction limits the hint
to be within 256 instructions of the branch it is affecting. By default,
GCC makes sure it is within 125.
- -msafe-hints
- Work around a hardware bug that causes the SPU to stall
indefinitely. By default, GCC inserts the "hbrp" instruction to
make sure this stall won't happen.
Options for System V
These additional options are available on System V Release 4 for compatibility
with other compilers on those systems:
- -G
- Create a shared object. It is recommended that
-symbolic or -shared be used instead.
- -Qy
- Identify the versions of each tool used by the compiler, in
a ".ident" assembler directive in the output.
- -Qn
- Refrain from adding ".ident" directives to the
output file (this is the default).
- -YP,dirs
- Search the directories dirs, and no others, for
libraries specified with -l.
- -Ym,dir
- Look in the directory dir to find the M4
preprocessor. The assembler uses this option.
TILE-Gx Options
These
-m options are supported on the TILE-Gx:
- -mcmodel=small
- Generate code for the small model. The distance for direct
calls is limited to 500M in either direction. PC-relative addresses are 32
bits. Absolute addresses support the full address range.
- -mcmodel=large
- Generate code for the large model. There is no limitation
on call distance, pc-relative addresses, or absolute addresses.
- -mcpu=name
- Selects the type of CPU to be targeted. Currently the only
supported type is tilegx.
- -m32
- -m64
- Generate code for a 32-bit or 64-bit environment. The
32-bit environment sets int, long, and pointer to 32 bits. The 64-bit
environment sets int to 32 bits and long and pointer to 64 bits.
- -mbig-endian
- -mlittle-endian
- Generate code in big/little endian mode, respectively.
TILEPro Options
These
-m options are supported on the TILEPro:
- -mcpu=name
- Selects the type of CPU to be targeted. Currently the only
supported type is tilepro.
- -m32
- Generate code for a 32-bit environment, which sets int,
long, and pointer to 32 bits. This is the only supported behavior so the
flag is essentially ignored.
V850 Options
These
-m options are defined for V850 implementations:
- -mlong-calls
- -mno-long-calls
- Treat all calls as being far away (near). If calls are
assumed to be far away, the compiler always loads the function's address
into a register, and calls indirect through the pointer.
- -mno-ep
- -mep
- Do not optimize (do optimize) basic blocks that use the
same index pointer 4 or more times to copy pointer into the "ep"
register, and use the shorter "sld" and "sst"
instructions. The -mep option is on by default if you
optimize.
- -mno-prolog-function
- -mprolog-function
- Do not use (do use) external functions to save and restore
registers at the prologue and epilogue of a function. The external
functions are slower, but use less code space if more than one function
saves the same number of registers. The -mprolog-function option is
on by default if you optimize.
- -mspace
- Try to make the code as small as possible. At present, this
just turns on the -mep and -mprolog-function options.
- -mtda=n
- Put static or global variables whose size is n bytes
or less into the tiny data area that register "ep" points to.
The tiny data area can hold up to 256 bytes in total (128 bytes for byte
references).
- -msda=n
- Put static or global variables whose size is n bytes
or less into the small data area that register "gp" points to.
The small data area can hold up to 64 kilobytes.
- -mzda=n
- Put static or global variables whose size is n bytes
or less into the first 32 kilobytes of memory.
- -mv850
- Specify that the target processor is the V850.
- -mv850e3v5
- Specify that the target processor is the V850E3V5. The
preprocessor constant "__v850e3v5__" is defined if this option
is used.
- -mv850e2v4
- Specify that the target processor is the V850E3V5. This is
an alias for the -mv850e3v5 option.
- -mv850e2v3
- Specify that the target processor is the V850E2V3. The
preprocessor constant "__v850e2v3__" is defined if this option
is used.
- -mv850e2
- Specify that the target processor is the V850E2. The
preprocessor constant "__v850e2__" is defined if this option is
used.
- -mv850e1
- Specify that the target processor is the V850E1. The
preprocessor constants "__v850e1__" and "__v850e__"
are defined if this option is used.
- -mv850es
- Specify that the target processor is the V850ES. This is an
alias for the -mv850e1 option.
- -mv850e
- Specify that the target processor is the V850E. The
preprocessor constant "__v850e__" is defined if this option is
used.
If neither -mv850 nor -mv850e nor -mv850e1 nor
-mv850e2 nor -mv850e2v3 nor -mv850e3v5 are defined
then a default target processor is chosen and the relevant
__v850*__ preprocessor constant is defined.
The preprocessor constants "__v850" and "__v851__" are
always defined, regardless of which processor variant is the target.
- -mdisable-callt
- -mno-disable-callt
- This option suppresses generation of the "CALLT"
instruction for the v850e, v850e1, v850e2, v850e2v3 and v850e3v5 flavors
of the v850 architecture.
This option is enabled by default when the RH850 ABI is in use (see
-mrh850-abi), and disabled by default when the GCC ABI is in use.
If "CALLT" instructions are being generated then the C
preprocessor symbol "__V850_CALLT__" is defined.
- -mrelax
- -mno-relax
- Pass on (or do not pass on) the -mrelax command-line
option to the assembler.
- -mlong-jumps
- -mno-long-jumps
- Disable (or re-enable) the generation of PC-relative jump
instructions.
- -msoft-float
- -mhard-float
- Disable (or re-enable) the generation of hardware floating
point instructions. This option is only significant when the target
architecture is V850E2V3 or higher. If hardware floating point
instructions are being generated then the C preprocessor symbol
"__FPU_OK__" is defined, otherwise the symbol
"__NO_FPU__" is defined.
- -mloop
- Enables the use of the e3v5 LOOP instruction. The use of
this instruction is not enabled by default when the e3v5 architecture is
selected because its use is still experimental.
- -mrh850-abi
- -mghs
- Enables support for the RH850 version of the V850 ABI. This
is the default. With this version of the ABI the following rules
apply:
- *
- Integer sized structures and unions are returned via a
memory pointer rather than a register.
- *
- Large structures and unions (more than 8 bytes in size) are
passed by value.
- *
- Functions are aligned to 16-bit boundaries.
- *
- The -m8byte-align command-line option is
supported.
- *
- The -mdisable-callt command-line option is enabled
by default. The -mno-disable-callt command-line option is not
supported.
When this version of the ABI is enabled the C preprocessor symbol
"__V850_RH850_ABI__" is defined.
- -mgcc-abi
- Enables support for the old GCC version of the V850 ABI.
With this version of the ABI the following rules apply:
- *
- Integer sized structures and unions are returned in
register "r10".
- *
- Large structures and unions (more than 8 bytes in size) are
passed by reference.
- *
- Functions are aligned to 32-bit boundaries, unless
optimizing for size.
- *
- The -m8byte-align command-line option is not
supported.
- *
- The -mdisable-callt command-line option is supported
but not enabled by default.
When this version of the ABI is enabled the C preprocessor symbol
"__V850_GCC_ABI__" is defined.
- -m8byte-align
- -mno-8byte-align
- Enables support for "double" and "long
long" types to be aligned on 8-byte boundaries. The default is to
restrict the alignment of all objects to at most 4-bytes. When
-m8byte-align is in effect the C preprocessor symbol
"__V850_8BYTE_ALIGN__" is defined.
- -mbig-switch
- Generate code suitable for big switch tables. Use this
option only if the assembler/linker complain about out of range branches
within a switch table.
- -mapp-regs
- This option causes r2 and r5 to be used in the code
generated by the compiler. This setting is the default.
- -mno-app-regs
- This option causes r2 and r5 to be treated as fixed
registers.
VAX Options
These
-m options are defined for the VAX:
- -munix
- Do not output certain jump instructions ("aobleq"
and so on) that the Unix assembler for the VAX cannot handle across long
ranges.
- -mgnu
- Do output those jump instructions, on the assumption that
the GNU assembler is being used.
- -mg
- Output code for G-format floating-point numbers instead of
D-format.
Visium Options
- -mdebug
- A program which performs file I/O and is destined to run on
an MCM target should be linked with this option. It causes the libraries
libc.a and libdebug.a to be linked. The program should be run on the
target under the control of the GDB remote debugging stub.
- -msim
- A program which performs file I/O and is destined to run on
the simulator should be linked with option. This causes libraries libc.a
and libsim.a to be linked.
- -mfpu
- -mhard-float
- Generate code containing floating-point instructions. This
is the default.
- -mno-fpu
- -msoft-float
- Generate code containing library calls for floating-point.
-msoft-float changes the calling convention in the output file;
therefore, it is only useful if you compile all of a program with
this option. In particular, you need to compile libgcc.a, the
library that comes with GCC, with -msoft-float in order for this to
work.
- -mcpu=cpu_type
- Set the instruction set, register set, and instruction
scheduling parameters for machine type cpu_type. Supported values
for cpu_type are mcm, gr5 and gr6.
mcm is a synonym of gr5 present for backward compatibility.
By default (unless configured otherwise), GCC generates code for the GR5
variant of the Visium architecture.
With -mcpu=gr6, GCC generates code for the GR6 variant of the Visium
architecture. The only difference from GR5 code is that the compiler will
generate block move instructions.
- -mtune=cpu_type
- Set the instruction scheduling parameters for machine type
cpu_type, but do not set the instruction set or register set that
the option -mcpu=cpu_type would.
- -msv-mode
- Generate code for the supervisor mode, where there are no
restrictions on the access to general registers. This is the default.
- -muser-mode
- Generate code for the user mode, where the access to some
general registers is forbidden: on the GR5, registers r24 to r31 cannot be
accessed in this mode; on the GR6, only registers r29 to r31 are
affected.
VMS Options
These
-m options are defined for the VMS implementations:
- -mvms-return-codes
- Return VMS condition codes from "main". The
default is to return POSIX-style condition (e.g. error) codes.
- -mdebug-main=prefix
- Flag the first routine whose name starts with prefix
as the main routine for the debugger.
- -mmalloc64
- Default to 64-bit memory allocation routines.
- -mpointer-size=size
- Set the default size of pointers. Possible options for
size are 32 or short for 32 bit pointers, 64
or long for 64 bit pointers, and no for supporting only 32
bit pointers. The later option disables "pragma
pointer_size".
VxWorks Options
The options in this section are defined for all VxWorks targets. Options
specific to the target hardware are listed with the other options for that
target.
- -mrtp
- GCC can generate code for both VxWorks kernels and real
time processes (RTPs). This option switches from the former to the latter.
It also defines the preprocessor macro "__RTP__".
- -non-static
- Link an RTP executable against shared libraries rather than
static libraries. The options -static and -shared can also
be used for RTPs; -static is the default.
- -Bstatic
- -Bdynamic
- These options are passed down to the linker. They are
defined for compatibility with Diab.
- -Xbind-lazy
- Enable lazy binding of function calls. This option is
equivalent to -Wl,-z,now and is defined for compatibility with
Diab.
- -Xbind-now
- Disable lazy binding of function calls. This option is the
default and is defined for compatibility with Diab.
x86 Options
These
-m options are defined for the x86 family of computers.
- -march=cpu-type
- Generate instructions for the machine type cpu-type.
In contrast to -mtune=cpu-type, which merely tunes the
generated code for the specified cpu-type,
-march=cpu-type allows GCC to generate code that may not run
at all on processors other than the one indicated. Specifying
-march= cpu-type implies -mtune=cpu-type.
The choices for cpu-type are:
- native
- This selects the CPU to generate code for at compilation
time by determining the processor type of the compiling machine. Using
-march=native enables all instruction subsets supported by the
local machine (hence the result might not run on different machines).
Using -mtune=native produces code optimized for the local machine
under the constraints of the selected instruction set.
- i386
- Original Intel i386 CPU.
- i486
- Intel i486 CPU. (No scheduling is implemented for this
chip.)
- i586
- pentium
- Intel Pentium CPU with no MMX support.
- pentium-mmx
- Intel Pentium MMX CPU, based on Pentium core with MMX
instruction set support.
- pentiumpro
- Intel Pentium Pro CPU.
- i686
- When used with -march, the Pentium Pro instruction
set is used, so the code runs on all i686 family chips. When used with
-mtune, it has the same meaning as generic.
- pentium2
- Intel Pentium II CPU, based on Pentium Pro core with MMX
instruction set support.
- pentium3
- pentium3m
- Intel Pentium III CPU, based on Pentium Pro core with MMX
and SSE instruction set support.
- pentium-m
- Intel Pentium M; low-power version of Intel Pentium III CPU
with MMX, SSE and SSE2 instruction set support. Used by Centrino
notebooks.
- pentium4
- pentium4m
- Intel Pentium 4 CPU with MMX, SSE and SSE2 instruction set
support.
- prescott
- Improved version of Intel Pentium 4 CPU with MMX, SSE, SSE2
and SSE3 instruction set support.
- nocona
- Improved version of Intel Pentium 4 CPU with 64-bit
extensions, MMX, SSE, SSE2 and SSE3 instruction set support.
- core2
- Intel Core 2 CPU with 64-bit extensions, MMX, SSE, SSE2,
SSE3 and SSSE3 instruction set support.
- nehalem
- Intel Nehalem CPU with 64-bit extensions, MMX, SSE, SSE2,
SSE3, SSSE3, SSE4.1, SSE4.2 and POPCNT instruction set support.
- westmere
- Intel Westmere CPU with 64-bit extensions, MMX, SSE, SSE2,
SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES and PCLMUL instruction set
support.
- sandybridge
- Intel Sandy Bridge CPU with 64-bit extensions, MMX, SSE,
SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AES and PCLMUL instruction
set support.
- ivybridge
- Intel Ivy Bridge CPU with 64-bit extensions, MMX, SSE,
SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AES, PCLMUL, FSGSBASE,
RDRND and F16C instruction set support.
- haswell
- Intel Haswell CPU with 64-bit extensions, MOVBE, MMX, SSE,
SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AVX2, AES, PCLMUL,
FSGSBASE, RDRND, FMA, BMI, BMI2 and F16C instruction set support.
- broadwell
- Intel Broadwell CPU with 64-bit extensions, MOVBE, MMX,
SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AVX2, AES, PCLMUL,
FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX and PREFETCHW
instruction set support.
- bonnell
- Intel Bonnell CPU with 64-bit extensions, MOVBE, MMX, SSE,
SSE2, SSE3 and SSSE3 instruction set support.
- silvermont
- Intel Silvermont CPU with 64-bit extensions, MOVBE, MMX,
SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PCLMUL and RDRND
instruction set support.
- knl
- Intel Knight's Landing CPU with 64-bit extensions, MOVBE,
MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AVX2, AES,
PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW,
AVX512F, AVX512PF, AVX512ER and AVX512CD instruction set support.
- k6
- AMD K6 CPU with MMX instruction set support.
- k6-2
- k6-3
- Improved versions of AMD K6 CPU with MMX and 3DNow!
instruction set support.
- athlon
- athlon-tbird
- AMD Athlon CPU with MMX, 3dNOW!, enhanced 3DNow! and SSE
prefetch instructions support.
- athlon-4
- athlon-xp
- athlon-mp
- Improved AMD Athlon CPU with MMX, 3DNow!, enhanced 3DNow!
and full SSE instruction set support.
- k8
- opteron
- athlon64
- athlon-fx
- Processors based on the AMD K8 core with x86-64 instruction
set support, including the AMD Opteron, Athlon 64, and Athlon 64 FX
processors. (This supersets MMX, SSE, SSE2, 3DNow!, enhanced 3DNow! and
64-bit instruction set extensions.)
- k8-sse3
- opteron-sse3
- athlon64-sse3
- Improved versions of AMD K8 cores with SSE3 instruction set
support.
- amdfam10
- barcelona
- CPUs based on AMD Family 10h cores with x86-64 instruction
set support. (This supersets MMX, SSE, SSE2, SSE3, SSE4A, 3DNow!, enhanced
3DNow!, ABM and 64-bit instruction set extensions.)
- bdver1
- CPUs based on AMD Family 15h cores with x86-64 instruction
set support. (This supersets FMA4, AVX, XOP, LWP, AES, PCL_MUL, CX16, MMX,
SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction
set extensions.)
- bdver2
- AMD Family 15h core based CPUs with x86-64 instruction set
support. (This supersets BMI, TBM, F16C, FMA, FMA4, AVX, XOP, LWP, AES,
PCL_MUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and
64-bit instruction set extensions.)
- bdver3
- AMD Family 15h core based CPUs with x86-64 instruction set
support. (This supersets BMI, TBM, F16C, FMA, FMA4, FSGSBASE, AVX, XOP,
LWP, AES, PCL_MUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1,
SSE4.2, ABM and 64-bit instruction set extensions.
- bdver4
- AMD Family 15h core based CPUs with x86-64 instruction set
support. (This supersets BMI, BMI2, TBM, F16C, FMA, FMA4, FSGSBASE, AVX,
AVX2, XOP, LWP, AES, PCL_MUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A,
SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.
- btver1
- CPUs based on AMD Family 14h cores with x86-64 instruction
set support. (This supersets MMX, SSE, SSE2, SSE3, SSSE3, SSE4A, CX16, ABM
and 64-bit instruction set extensions.)
- btver2
- CPUs based on AMD Family 16h cores with x86-64 instruction
set support. This includes MOVBE, F16C, BMI, AVX, PCL_MUL, AES, SSE4.2,
SSE4.1, CX16, ABM, SSE4A, SSSE3, SSE3, SSE2, SSE, MMX and 64-bit
instruction set extensions.
- winchip-c6
- IDT WinChip C6 CPU, dealt in same way as i486 with
additional MMX instruction set support.
- winchip2
- IDT WinChip 2 CPU, dealt in same way as i486 with
additional MMX and 3DNow! instruction set support.
- c3
- VIA C3 CPU with MMX and 3DNow! instruction set support. (No
scheduling is implemented for this chip.)
- c3-2
- VIA C3-2 (Nehemiah/C5XL) CPU with MMX and SSE instruction
set support. (No scheduling is implemented for this chip.)
- geode
- AMD Geode embedded processor with MMX and 3DNow!
instruction set support.
- -mtune=cpu-type
- Tune to cpu-type everything applicable about the
generated code, except for the ABI and the set of available instructions.
While picking a specific cpu-type schedules things appropriately
for that particular chip, the compiler does not generate any code that
cannot run on the default machine type unless you use a
-march=cpu-type option. For example, if GCC is configured
for i686-pc-linux-gnu then -mtune=pentium4 generates code that is
tuned for Pentium 4 but still runs on i686 machines.
The choices for cpu-type are the same as for -march. In
addition, -mtune supports 2 extra choices for cpu-type:
- generic
- Produce code optimized for the most common IA32/AMD64/EM64T
processors. If you know the CPU on which your code will run, then you
should use the corresponding -mtune or -march option instead
of -mtune=generic. But, if you do not know exactly what CPU users
of your application will have, then you should use this option.
As new processors are deployed in the marketplace, the behavior of this
option will change. Therefore, if you upgrade to a newer version of GCC,
code generation controlled by this option will change to reflect the
processors that are most common at the time that version of GCC is
released.
There is no -march=generic option because -march indicates the
instruction set the compiler can use, and there is no generic instruction
set applicable to all processors. In contrast, -mtune indicates the
processor (or, in this case, collection of processors) for which the code
is optimized.
- intel
- Produce code optimized for the most current Intel
processors, which are Haswell and Silvermont for this version of GCC. If
you know the CPU on which your code will run, then you should use the
corresponding -mtune or -march option instead of
-mtune=intel. But, if you want your application performs better on
both Haswell and Silvermont, then you should use this option.
As new Intel processors are deployed in the marketplace, the behavior of
this option will change. Therefore, if you upgrade to a newer version of
GCC, code generation controlled by this option will change to reflect the
most current Intel processors at the time that version of GCC is released.
There is no -march=intel option because -march indicates the
instruction set the compiler can use, and there is no common instruction
set applicable to all processors. In contrast, -mtune indicates the
processor (or, in this case, collection of processors) for which the code
is optimized.
- -mcpu=cpu-type
- A deprecated synonym for -mtune.
- -mfpmath=unit
- Generate floating-point arithmetic for selected unit
unit. The choices for unit are:
- 387
- Use the standard 387 floating-point coprocessor present on
the majority of chips and emulated otherwise. Code compiled with this
option runs almost everywhere. The temporary results are computed in
80-bit precision instead of the precision specified by the type, resulting
in slightly different results compared to most of other chips. See
-ffloat-store for more detailed description.
This is the default choice for x86-32 targets.
- sse
- Use scalar floating-point instructions present in the SSE
instruction set. This instruction set is supported by Pentium III and
newer chips, and in the AMD line by Athlon-4, Athlon XP and Athlon MP
chips. The earlier version of the SSE instruction set supports only
single-precision arithmetic, thus the double and extended-precision
arithmetic are still done using 387. A later version, present only in
Pentium 4 and AMD x86-64 chips, supports double-precision arithmetic too.
For the x86-32 compiler, you must use -march=cpu-type,
-msse or -msse2 switches to enable SSE extensions and make
this option effective. For the x86-64 compiler, these extensions are
enabled by default.
The resulting code should be considerably faster in the majority of cases
and avoid the numerical instability problems of 387 code, but may break
some existing code that expects temporaries to be 80 bits.
This is the default choice for the x86-64 compiler.
- sse,387
- sse+387
- both
- Attempt to utilize both instruction sets at once. This
effectively doubles the amount of available registers, and on chips with
separate execution units for 387 and SSE the execution resources too. Use
this option with care, as it is still experimental, because the GCC
register allocator does not model separate functional units well,
resulting in unstable performance.
- -masm=dialect
- Output assembly instructions using selected dialect.
Also affects which dialect is used for basic "asm" and extended
"asm". Supported choices (in dialect order) are att or
intel. The default is att. Darwin does not support
intel.
- -mieee-fp
- -mno-ieee-fp
- Control whether or not the compiler uses IEEE
floating-point comparisons. These correctly handle the case where the
result of a comparison is unordered.
- -msoft-float
- Generate output containing library calls for floating
point.
Warning: the requisite libraries are not part of GCC. Normally the
facilities of the machine's usual C compiler are used, but this can't be
done directly in cross-compilation. You must make your own arrangements to
provide suitable library functions for cross-compilation.
On machines where a function returns floating-point results in the 80387
register stack, some floating-point opcodes may be emitted even if
-msoft-float is used.
- -mno-fp-ret-in-387
- Do not use the FPU registers for return values of
functions.
The usual calling convention has functions return values of types
"float" and "double" in an FPU register, even if there
is no FPU. The idea is that the operating system should emulate an FPU.
The option -mno-fp-ret-in-387 causes such values to be returned in
ordinary CPU registers instead.
- -mno-fancy-math-387
- Some 387 emulators do not support the "sin",
"cos" and "sqrt" instructions for the 387. Specify
this option to avoid generating those instructions. This option is the
default on OpenBSD and NetBSD. This option is overridden when
-march indicates that the target CPU always has an FPU and so the
instruction does not need emulation. These instructions are not generated
unless you also use the -funsafe-math-optimizations switch.
- -malign-double
- -mno-align-double
- Control whether GCC aligns "double", "long
double", and "long long" variables on a two-word boundary
or a one-word boundary. Aligning "double" variables on a
two-word boundary produces code that runs somewhat faster on a Pentium at
the expense of more memory.
On x86-64, -malign-double is enabled by default.
Warning: if you use the -malign-double switch, structures
containing the above types are aligned differently than the published
application binary interface specifications for the x86-32 and are not
binary compatible with structures in code compiled without that
switch.
- -m96bit-long-double
- -m128bit-long-double
- These switches control the size of "long double"
type. The x86-32 application binary interface specifies the size to be 96
bits, so -m96bit-long-double is the default in 32-bit mode.
Modern architectures (Pentium and newer) prefer "long double" to
be aligned to an 8- or 16-byte boundary. In arrays or structures
conforming to the ABI, this is not possible. So specifying
-m128bit-long-double aligns "long double" to a 16-byte
boundary by padding the "long double" with an additional 32-bit
zero.
In the x86-64 compiler, -m128bit-long-double is the default choice as
its ABI specifies that "long double" is aligned on 16-byte
boundary.
Notice that neither of these options enable any extra precision over the x87
standard of 80 bits for a "long double".
Warning: if you override the default value for your target ABI, this
changes the size of structures and arrays containing "long
double" variables, as well as modifying the function calling
convention for functions taking "long double". Hence they are
not binary-compatible with code compiled without that switch.
- -mlong-double-64
- -mlong-double-80
- -mlong-double-128
- These switches control the size of "long double"
type. A size of 64 bits makes the "long double" type equivalent
to the "double" type. This is the default for 32-bit Bionic C
library. A size of 128 bits makes the "long double" type
equivalent to the "__float128" type. This is the default for
64-bit Bionic C library.
Warning: if you override the default value for your target ABI, this
changes the size of structures and arrays containing "long
double" variables, as well as modifying the function calling
convention for functions taking "long double". Hence they are
not binary-compatible with code compiled without that switch.
- -malign-data=type
- Control how GCC aligns variables. Supported values for
type are compat uses increased alignment value compatible
uses GCC 4.8 and earlier, abi uses alignment value as specified by
the psABI, and cacheline uses increased alignment value to match
the cache line size. compat is the default.
- -mlarge-data-threshold=threshold
- When -mcmodel=medium is specified, data objects
larger than threshold are placed in the large data section. This
value must be the same across all objects linked into the binary, and
defaults to 65535.
- -mrtd
- Use a different function-calling convention, in which
functions that take a fixed number of arguments return with the "ret
num" instruction, which pops their arguments while returning.
This saves one instruction in the caller since there is no need to pop the
arguments there.
You can specify that an individual function is called with this calling
sequence with the function attribute "stdcall". You can also
override the -mrtd option by using the function attribute
"cdecl".
Warning: this calling convention is incompatible with the one
normally used on Unix, so you cannot use it if you need to call libraries
compiled with the Unix compiler.
Also, you must provide function prototypes for all functions that take
variable numbers of arguments (including "printf"); otherwise
incorrect code is generated for calls to those functions.
In addition, seriously incorrect code results if you call a function with
too many arguments. (Normally, extra arguments are harmlessly
ignored.)
- -mregparm=num
- Control how many registers are used to pass integer
arguments. By default, no registers are used to pass arguments, and at
most 3 registers can be used. You can control this behavior for a specific
function by using the function attribute "regparm".
Warning: if you use this switch, and num is nonzero, then you
must build all modules with the same value, including any libraries. This
includes the system libraries and startup modules.
- -msseregparm
- Use SSE register passing conventions for float and double
arguments and return values. You can control this behavior for a specific
function by using the function attribute "sseregparm".
Warning: if you use this switch then you must build all modules with
the same value, including any libraries. This includes the system
libraries and startup modules.
- -mvect8-ret-in-mem
- Return 8-byte vectors in memory instead of MMX registers.
This is the default on Solaris@tie{}8 and 9 and VxWorks to match the ABI
of the Sun Studio compilers until version 12. Later compiler versions
(starting with Studio 12 Update@tie{}1) follow the ABI used by other x86
targets, which is the default on Solaris@tie{}10 and later. Only
use this option if you need to remain compatible with existing code
produced by those previous compiler versions or older versions of
GCC.
- -mpc32
- -mpc64
- -mpc80
- Set 80387 floating-point precision to 32, 64 or 80 bits.
When -mpc32 is specified, the significands of results of
floating-point operations are rounded to 24 bits (single precision);
-mpc64 rounds the significands of results of floating-point
operations to 53 bits (double precision) and -mpc80 rounds the
significands of results of floating-point operations to 64 bits (extended
double precision), which is the default. When this option is used,
floating-point operations in higher precisions are not available to the
programmer without setting the FPU control word explicitly.
Setting the rounding of floating-point operations to less than the default
80 bits can speed some programs by 2% or more. Note that some mathematical
libraries assume that extended-precision (80-bit) floating-point
operations are enabled by default; routines in such libraries could suffer
significant loss of accuracy, typically through so-called
"catastrophic cancellation", when this option is used to set the
precision to less than extended precision.
- -mstackrealign
- Realign the stack at entry. On the x86, the
-mstackrealign option generates an alternate prologue and epilogue
that realigns the run-time stack if necessary. This supports mixing legacy
codes that keep 4-byte stack alignment with modern codes that keep 16-byte
stack alignment for SSE compatibility. See also the attribute
"force_align_arg_pointer", applicable to individual
functions.
- -mpreferred-stack-boundary=num
- Attempt to keep the stack boundary aligned to a 2 raised to
num byte boundary. If -mpreferred-stack-boundary is not
specified, the default is 4 (16 bytes or 128 bits).
Warning: When generating code for the x86-64 architecture with SSE
extensions disabled, -mpreferred-stack-boundary=3 can be used to
keep the stack boundary aligned to 8 byte boundary. Since x86-64 ABI
require 16 byte stack alignment, this is ABI incompatible and intended to
be used in controlled environment where stack space is important
limitation. This option leads to wrong code when functions compiled with
16 byte stack alignment (such as functions from a standard library) are
called with misaligned stack. In this case, SSE instructions may lead to
misaligned memory access traps. In addition, variable arguments are
handled incorrectly for 16 byte aligned objects (including x87 long double
and __int128), leading to wrong results. You must build all modules with
-mpreferred-stack-boundary=3, including any libraries. This
includes the system libraries and startup modules.
- -mincoming-stack-boundary=num
- Assume the incoming stack is aligned to a 2 raised to
num byte boundary. If -mincoming-stack-boundary is not
specified, the one specified by -mpreferred-stack-boundary is used.
On Pentium and Pentium Pro, "double" and "long double"
values should be aligned to an 8-byte boundary (see -malign-double)
or suffer significant run time performance penalties. On Pentium III, the
Streaming SIMD Extension (SSE) data type "__m128" may not work
properly if it is not 16-byte aligned.
To ensure proper alignment of this values on the stack, the stack boundary
must be as aligned as that required by any value stored on the stack.
Further, every function must be generated such that it keeps the stack
aligned. Thus calling a function compiled with a higher preferred stack
boundary from a function compiled with a lower preferred stack boundary
most likely misaligns the stack. It is recommended that libraries that use
callbacks always use the default setting.
This extra alignment does consume extra stack space, and generally increases
code size. Code that is sensitive to stack space usage, such as embedded
systems and operating system kernels, may want to reduce the preferred
alignment to -mpreferred-stack-boundary=2.
- -mmmx
- -msse
- -msse2
- -msse3
- -mssse3
- -msse4
- -msse4a
- -msse4.1
- -msse4.2
- -mavx
- -mavx2
- -mavx512f
- -mavx512pf
- -mavx512er
- -mavx512cd
- -msha
- -maes
- -mpclmul
- -mclfushopt
- -mfsgsbase
- -mrdrnd
- -mf16c
- -mfma
- -mfma4
- -mno-fma4
- -mprefetchwt1
- -mxop
- -mlwp
- -m3dnow
- -mpopcnt
- -mabm
- -mbmi
- -mbmi2
- -mlzcnt
- -mfxsr
- -mxsave
- -mxsaveopt
- -mxsavec
- -mxsaves
- -mrtm
- -mtbm
- -mmpx
- -mmwaitx
- These switches enable the use of instructions in the MMX,
SSE, SSE2, SSE3, SSSE3, SSE4.1, AVX, AVX2, AVX512F, AVX512PF, AVX512ER,
AVX512CD, SHA, AES, PCLMUL, FSGSBASE, RDRND, F16C, FMA, SSE4A, FMA4, XOP,
LWP, ABM, BMI, BMI2, FXSR, XSAVE, XSAVEOPT, LZCNT, RTM, MPX, MWAITX or
3DNow! extended instruction sets. Each has a corresponding -mno-
option to disable use of these instructions.
These extensions are also available as built-in functions: see x86
Built-in Functions, for details of the functions enabled and disabled
by these switches.
To generate SSE/SSE2 instructions automatically from floating-point code (as
opposed to 387 instructions), see -mfpmath=sse.
GCC depresses SSEx instructions when -mavx is used. Instead, it
generates new AVX instructions or AVX equivalence for all SSEx
instructions when needed.
These options enable GCC to use these extended instructions in generated
code, even without -mfpmath=sse. Applications that perform run-time
CPU detection must compile separate files for each supported architecture,
using the appropriate flags. In particular, the file containing the CPU
detection code should be compiled without these options.
- -mdump-tune-features
- This option instructs GCC to dump the names of the x86
performance tuning features and default settings. The names can be used in
-mtune-ctrl=feature-list.
- -mtune-ctrl=feature-list
- This option is used to do fine grain control of x86 code
generation features. feature-list is a comma separated list of
feature names. See also -mdump-tune-features. When
specified, the feature is turned on if it is not preceded with
^, otherwise, it is turned off.
-mtune-ctrl=feature-list is intended to be used by GCC
developers. Using it may lead to code paths not covered by testing and can
potentially result in compiler ICEs or runtime errors.
- -mno-default
- This option instructs GCC to turn off all tunable features.
See also -mtune-ctrl=feature-list and
-mdump-tune-features.
- -mcld
- This option instructs GCC to emit a "cld"
instruction in the prologue of functions that use string instructions.
String instructions depend on the DF flag to select between autoincrement
or autodecrement mode. While the ABI specifies the DF flag to be cleared
on function entry, some operating systems violate this specification by
not clearing the DF flag in their exception dispatchers. The exception
handler can be invoked with the DF flag set, which leads to wrong
direction mode when string instructions are used. This option can be
enabled by default on 32-bit x86 targets by configuring GCC with the
--enable-cld configure option. Generation of "cld"
instructions can be suppressed with the -mno-cld compiler option in
this case.
- -mvzeroupper
- This option instructs GCC to emit a "vzeroupper"
instruction before a transfer of control flow out of the function to
minimize the AVX to SSE transition penalty as well as remove unnecessary
"zeroupper" intrinsics.
- -mprefer-avx128
- This option instructs GCC to use 128-bit AVX instructions
instead of 256-bit AVX instructions in the auto-vectorizer.
- -mcx16
- This option enables GCC to generate "CMPXCHG16B"
instructions. "CMPXCHG16B" allows for atomic operations on
128-bit double quadword (or oword) data types. This is useful for
high-resolution counters that can be updated by multiple processors (or
cores). This instruction is generated as part of atomic built-in
functions: see __sync Builtins or __atomic Builtins for
details.
- -msahf
- This option enables generation of "SAHF"
instructions in 64-bit code. Early Intel Pentium 4 CPUs with Intel 64
support, prior to the introduction of Pentium 4 G1 step in December 2005,
lacked the "LAHF" and "SAHF" instructions which are
supported by AMD64. These are load and store instructions, respectively,
for certain status flags. In 64-bit mode, the "SAHF" instruction
is used to optimize "fmod", "drem", and
"remainder" built-in functions; see Other Builtins for
details.
- -mmovbe
- This option enables use of the "movbe"
instruction to implement "__builtin_bswap32" and
"__builtin_bswap64".
- -mcrc32
- This option enables built-in functions
"__builtin_ia32_crc32qi", "__builtin_ia32_crc32hi",
"__builtin_ia32_crc32si" and "__builtin_ia32_crc32di"
to generate the "crc32" machine instruction.
- -mrecip
- This option enables use of "RCPSS" and
"RSQRTSS" instructions (and their vectorized variants
"RCPPS" and "RSQRTPS") with an additional
Newton-Raphson step to increase precision instead of "DIVSS" and
"SQRTSS" (and their vectorized variants) for single-precision
floating-point arguments. These instructions are generated only when
-funsafe-math-optimizations is enabled together with
-finite-math-only and -fno-trapping-math. Note that while
the throughput of the sequence is higher than the throughput of the
non-reciprocal instruction, the precision of the sequence can be decreased
by up to 2 ulp (i.e. the inverse of 1.0 equals 0.99999994).
Note that GCC implements "1.0f/sqrtf( x)" in terms of
"RSQRTSS" (or "RSQRTPS") already with
-ffast-math (or the above option combination), and doesn't need
-mrecip.
Also note that GCC emits the above sequence with additional Newton-Raphson
step for vectorized single-float division and vectorized "sqrtf(
x)" already with -ffast-math (or the above option
combination), and doesn't need -mrecip.
- -mrecip=opt
- This option controls which reciprocal estimate instructions
may be used. opt is a comma-separated list of options, which may be
preceded by a ! to invert the option:
- all
- Enable all estimate instructions.
- default
- Enable the default instructions, equivalent to
-mrecip.
- none
- Disable all estimate instructions, equivalent to
-mno-recip.
- div
- Enable the approximation for scalar division.
- vec-div
- Enable the approximation for vectorized division.
- sqrt
- Enable the approximation for scalar square root.
- vec-sqrt
- Enable the approximation for vectorized square root.
So, for example,
-mrecip=all,!sqrt enables all of the reciprocal
approximations, except for square root.
- -mveclibabi=type
- Specifies the ABI type to use for vectorizing intrinsics
using an external library. Supported values for type are
svml for the Intel short vector math library and acml for
the AMD math core library. To use this option, both
-ftree-vectorize and -funsafe-math-optimizations have to be
enabled, and an SVML or ACML ABI-compatible library must be specified at
link time.
GCC currently emits calls to "vmldExp2", "vmldLn2",
"vmldLog102", "vmldLog102", "vmldPow2",
"vmldTanh2", "vmldTan2", "vmldAtan2",
"vmldAtanh2", "vmldCbrt2", "vmldSinh2",
"vmldSin2", "vmldAsinh2", "vmldAsin2",
"vmldCosh2", "vmldCos2", "vmldAcosh2",
"vmldAcos2", "vmlsExp4", "vmlsLn4",
"vmlsLog104", "vmlsLog104", "vmlsPow4",
"vmlsTanh4", "vmlsTan4", "vmlsAtan4",
"vmlsAtanh4", "vmlsCbrt4", "vmlsSinh4",
"vmlsSin4", "vmlsAsinh4", "vmlsAsin4",
"vmlsCosh4", "vmlsCos4", "vmlsAcosh4" and
"vmlsAcos4" for corresponding function type when
-mveclibabi=svml is used, and "__vrd2_sin",
"__vrd2_cos", "__vrd2_exp", "__vrd2_log",
"__vrd2_log2", "__vrd2_log10",
"__vrs4_sinf", "__vrs4_cosf", "__vrs4_expf",
"__vrs4_logf", "__vrs4_log2f",
"__vrs4_log10f" and "__vrs4_powf" for the
corresponding function type when -mveclibabi=acml is used.
- -mabi=name
- Generate code for the specified calling convention.
Permissible values are sysv for the ABI used on GNU/Linux and other
systems, and ms for the Microsoft ABI. The default is to use the
Microsoft ABI when targeting Microsoft Windows and the SysV ABI on all
other systems. You can control this behavior for specific functions by
using the function attributes "ms_abi" and
"sysv_abi".
- -mtls-dialect=type
- Generate code to access thread-local storage using the
gnu or gnu2 conventions. gnu is the conservative
default; gnu2 is more efficient, but it may add compile- and
run-time requirements that cannot be satisfied on all systems.
- -mpush-args
- -mno-push-args
- Use PUSH operations to store outgoing parameters. This
method is shorter and usually equally fast as method using SUB/MOV
operations and is enabled by default. In some cases disabling it may
improve performance because of improved scheduling and reduced
dependencies.
- -maccumulate-outgoing-args
- If enabled, the maximum amount of space required for
outgoing arguments is computed in the function prologue. This is faster on
most modern CPUs because of reduced dependencies, improved scheduling and
reduced stack usage when the preferred stack boundary is not equal to 2.
The drawback is a notable increase in code size. This switch implies
-mno-push-args.
- -mthreads
- Support thread-safe exception handling on MinGW. Programs
that rely on thread-safe exception handling must compile and link all code
with the -mthreads option. When compiling, -mthreads defines
-D_MT; when linking, it links in a special thread helper library
-lmingwthrd which cleans up per-thread exception-handling
data.
- -mno-align-stringops
- Do not align the destination of inlined string operations.
This switch reduces code size and improves performance in case the
destination is already aligned, but GCC doesn't know about it.
- -minline-all-stringops
- By default GCC inlines string operations only when the
destination is known to be aligned to least a 4-byte boundary. This
enables more inlining and increases code size, but may improve performance
of code that depends on fast "memcpy", "strlen", and
"memset" for short lengths.
- -minline-stringops-dynamically
- For string operations of unknown size, use run-time checks
with inline code for small blocks and a library call for large
blocks.
- -mstringop-strategy=alg
- Override the internal decision heuristic for the particular
algorithm to use for inlining string operations. The allowed values for
alg are:
- rep_byte
- rep_4byte
- rep_8byte
- Expand using i386 "rep" prefix of the specified
size.
- byte_loop
- loop
- unrolled_loop
- Expand into an inline loop.
- libcall
- Always use a library call.
- -mmemcpy-strategy=strategy
- Override the internal decision heuristic to decide if
"__builtin_memcpy" should be inlined and what inline algorithm
to use when the expected size of the copy operation is known.
strategy is a comma-separated list of
alg:max_size:dest_align triplets. alg is
specified in -mstringop-strategy, max_size specifies the max
byte size with which inline algorithm alg is allowed. For the last
triplet, the max_size must be "-1". The max_size
of the triplets in the list must be specified in increasing order. The
minimal byte size for alg is 0 for the first triplet and
"max_size + 1" of the preceding range.
- -mmemset-strategy=strategy
- The option is similar to -mmemcpy-strategy= except
that it is to control "__builtin_memset" expansion.
- -momit-leaf-frame-pointer
- Don't keep the frame pointer in a register for leaf
functions. This avoids the instructions to save, set up, and restore frame
pointers and makes an extra register available in leaf functions. The
option -fomit-leaf-frame-pointer removes the frame pointer for leaf
functions, which might make debugging harder.
- -mtls-direct-seg-refs
- -mno-tls-direct-seg-refs
- Controls whether TLS variables may be accessed with offsets
from the TLS segment register (%gs for 32-bit, %fs for 64-bit), or whether
the thread base pointer must be added. Whether or not this is valid
depends on the operating system, and whether it maps the segment to cover
the entire TLS area.
For systems that use the GNU C Library, the default is on.
- -msse2avx
- -mno-sse2avx
- Specify that the assembler should encode SSE instructions
with VEX prefix. The option -mavx turns this on by default.
- -mfentry
- -mno-fentry
- If profiling is active (-pg), put the profiling
counter call before the prologue. Note: On x86 architectures the attribute
"ms_hook_prologue" isn't possible at the moment for
-mfentry and -pg.
- -mrecord-mcount
- -mno-record-mcount
- If profiling is active (-pg), generate a
__mcount_loc section that contains pointers to each profiling call. This
is useful for automatically patching and out calls.
- -mnop-mcount
- -mno-nop-mcount
- If profiling is active (-pg), generate the calls to
the profiling functions as nops. This is useful when they should be
patched in later dynamically. This is likely only useful together with
-mrecord-mcount.
- -mskip-rax-setup
- -mno-skip-rax-setup
- When generating code for the x86-64 architecture with SSE
extensions disabled, -mskip-rax-setup can be used to skip setting
up RAX register when there are no variable arguments passed in vector
registers.
Warning: Since RAX register is used to avoid unnecessarily saving
vector registers on stack when passing variable arguments, the impacts of
this option are callees may waste some stack space, misbehave or jump to a
random location. GCC 4.4 or newer don't have those issues, regardless the
RAX register value.
- -m8bit-idiv
- -mno-8bit-idiv
- On some processors, like Intel Atom, 8-bit unsigned integer
divide is much faster than 32-bit/64-bit integer divide. This option
generates a run-time check. If both dividend and divisor are within range
of 0 to 255, 8-bit unsigned integer divide is used instead of
32-bit/64-bit integer divide.
- -mavx256-split-unaligned-load
- -mavx256-split-unaligned-store
- Split 32-byte AVX unaligned load and store.
- -mstack-protector-guard=guard
- Generate stack protection code using canary at
guard. Supported locations are global for global canary or
tls for per-thread canary in the TLS block (the default). This
option has effect only when -fstack-protector or
-fstack-protector-all is specified.
These
-m switches are supported in addition to the above on x86-64
processors in 64-bit environments.
- -m32
- -m64
- -mx32
- -m16
- Generate code for a 16-bit, 32-bit or 64-bit environment.
The -m32 option sets "int", "long", and pointer
types to 32 bits, and generates code that runs on any i386 system.
The -m64 option sets "int" to 32 bits and "long"
and pointer types to 64 bits, and generates code for the x86-64
architecture. For Darwin only the -m64 option also turns off the
-fno-pic and -mdynamic-no-pic options.
The -mx32 option sets "int", "long", and pointer
types to 32 bits, and generates code for the x86-64 architecture.
The -m16 option is the same as -m32, except for that it
outputs the ".code16gcc" assembly directive at the beginning of
the assembly output so that the binary can run in 16-bit mode.
- -mno-red-zone
- Do not use a so-called "red zone" for x86-64
code. The red zone is mandated by the x86-64 ABI; it is a 128-byte area
beyond the location of the stack pointer that is not modified by signal or
interrupt handlers and therefore can be used for temporary data without
adjusting the stack pointer. The flag -mno-red-zone disables this
red zone.
- -mcmodel=small
- Generate code for the small code model: the program and its
symbols must be linked in the lower 2 GB of the address space. Pointers
are 64 bits. Programs can be statically or dynamically linked. This is the
default code model.
- -mcmodel=kernel
- Generate code for the kernel code model. The kernel runs in
the negative 2 GB of the address space. This model has to be used for
Linux kernel code.
- -mcmodel=medium
- Generate code for the medium model: the program is linked
in the lower 2 GB of the address space. Small symbols are also placed
there. Symbols with sizes larger than -mlarge-data-threshold are
put into large data or BSS sections and can be located above 2GB. Programs
can be statically or dynamically linked.
- -mcmodel=large
- Generate code for the large model. This model makes no
assumptions about addresses and sizes of sections.
- -maddress-mode=long
- Generate code for long address mode. This is only supported
for 64-bit and x32 environments. It is the default address mode for 64-bit
environments.
- -maddress-mode=short
- Generate code for short address mode. This is only
supported for 32-bit and x32 environments. It is the default address mode
for 32-bit and x32 environments.
x86 Windows Options
These additional options are available for Microsoft Windows targets:
- -mconsole
- This option specifies that a console application is to be
generated, by instructing the linker to set the PE header subsystem type
required for console applications. This option is available for Cygwin and
MinGW targets and is enabled by default on those targets.
- -mdll
- This option is available for Cygwin and MinGW targets. It
specifies that a DLL---a dynamic link library---is to be generated,
enabling the selection of the required runtime startup object and entry
point.
- -mnop-fun-dllimport
- This option is available for Cygwin and MinGW targets. It
specifies that the "dllimport" attribute should be ignored.
- -mthread
- This option is available for MinGW targets. It specifies
that MinGW-specific thread support is to be used.
- -municode
- This option is available for MinGW-w64 targets. It causes
the "UNICODE" preprocessor macro to be predefined, and chooses
Unicode-capable runtime startup code.
- -mwin32
- This option is available for Cygwin and MinGW targets. It
specifies that the typical Microsoft Windows predefined macros are to be
set in the pre-processor, but does not influence the choice of runtime
library/startup code.
- -mwindows
- This option is available for Cygwin and MinGW targets. It
specifies that a GUI application is to be generated by instructing the
linker to set the PE header subsystem type appropriately.
- -fno-set-stack-executable
- This option is available for MinGW targets. It specifies
that the executable flag for the stack used by nested functions isn't set.
This is necessary for binaries running in kernel mode of Microsoft
Windows, as there the User32 API, which is used to set executable
privileges, isn't available.
- -fwritable-relocated-rdata
- This option is available for MinGW and Cygwin targets. It
specifies that relocated-data in read-only section is put into .data
section. This is a necessary for older runtimes not supporting
modification of .rdata sections for pseudo-relocation.
- -mpe-aligned-commons
- This option is available for Cygwin and MinGW targets. It
specifies that the GNU extension to the PE file format that permits the
correct alignment of COMMON variables should be used when generating code.
It is enabled by default if GCC detects that the target assembler found
during configuration supports the feature.
See also under
x86 Options for standard options.
Xstormy16 Options
These options are defined for Xstormy16:
- -msim
- Choose startup files and linker script suitable for the
simulator.
Xtensa Options
These options are supported for Xtensa targets:
- -mconst16
- -mno-const16
- Enable or disable use of "CONST16" instructions
for loading constant values. The "CONST16" instruction is
currently not a standard option from Tensilica. When enabled,
"CONST16" instructions are always used in place of the standard
"L32R" instructions. The use of "CONST16" is enabled
by default only if the "L32R" instruction is not available.
- -mfused-madd
- -mno-fused-madd
- Enable or disable use of fused multiply/add and
multiply/subtract instructions in the floating-point option. This has no
effect if the floating-point option is not also enabled. Disabling fused
multiply/add and multiply/subtract instructions forces the compiler to use
separate instructions for the multiply and add/subtract operations. This
may be desirable in some cases where strict IEEE 754-compliant results are
required: the fused multiply add/subtract instructions do not round the
intermediate result, thereby producing results with more bits of
precision than specified by the IEEE standard. Disabling fused multiply
add/subtract instructions also ensures that the program output is not
sensitive to the compiler's ability to combine multiply and add/subtract
operations.
- -mserialize-volatile
- -mno-serialize-volatile
- When this option is enabled, GCC inserts "MEMW"
instructions before "volatile" memory references to guarantee
sequential consistency. The default is -mserialize-volatile. Use
-mno-serialize-volatile to omit the "MEMW"
instructions.
- -mforce-no-pic
- For targets, like GNU/Linux, where all user-mode Xtensa
code must be position-independent code (PIC), this option disables PIC for
compiling kernel code.
- -mtext-section-literals
- -mno-text-section-literals
- These options control the treatment of literal pools. The
default is -mno-text-section-literals, which places literals in a
separate section in the output file. This allows the literal pool to be
placed in a data RAM/ROM, and it also allows the linker to combine literal
pools from separate object files to remove redundant literals and improve
code size. With -mtext-section-literals, the literals are
interspersed in the text section in order to keep them as close as
possible to their references. This may be necessary for large assembly
files.
- -mtarget-align
- -mno-target-align
- When this option is enabled, GCC instructs the assembler to
automatically align instructions to reduce branch penalties at the expense
of some code density. The assembler attempts to widen density instructions
to align branch targets and the instructions following call instructions.
If there are not enough preceding safe density instructions to align a
target, no widening is performed. The default is -mtarget-align.
These options do not affect the treatment of auto-aligned instructions
like "LOOP", which the assembler always aligns, either by
widening density instructions or by inserting NOP instructions.
- -mlongcalls
- -mno-longcalls
- When this option is enabled, GCC instructs the assembler to
translate direct calls to indirect calls unless it can determine that the
target of a direct call is in the range allowed by the call instruction.
This translation typically occurs for calls to functions in other source
files. Specifically, the assembler translates a direct "CALL"
instruction into an "L32R" followed by a "CALLX"
instruction. The default is -mno-longcalls. This option should be
used in programs where the call target can potentially be out of range.
This option is implemented in the assembler, not the compiler, so the
assembly code generated by GCC still shows direct call instructions---look
at the disassembled object code to see the actual instructions. Note that
the assembler uses an indirect call for every cross-file call, not just
those that really are out of range.
zSeries Options
These are listed under
Options for Code Generation Conventions
These machine-independent options control the interface conventions used in code
generation.
Most of them have both positive and negative forms; the negative form of
-ffoo is
-fno-foo. In the table below, only one of the forms is
listed---the one that is not the default. You can figure out the other form by
either removing
no- or adding it.
- -fbounds-check
- For front ends that support it, generate additional code to
check that indices used to access arrays are within the declared range.
This is currently only supported by the Java and Fortran front ends, where
this option defaults to true and false respectively.
- -fstack-reuse=reuse-level
- This option controls stack space reuse for user declared
local/auto variables and compiler generated temporaries.
reuse_level can be all, named_vars, or none.
all enables stack reuse for all local variables and temporaries,
named_vars enables the reuse only for user defined local variables
with names, and none disables stack reuse completely. The default
value is all. The option is needed when the program extends the
lifetime of a scoped local variable or a compiler generated temporary
beyond the end point defined by the language. When a lifetime of a
variable ends, and if the variable lives in memory, the optimizing
compiler has the freedom to reuse its stack space with other temporaries
or scoped local variables whose live range does not overlap with it.
Legacy code extending local lifetime is likely to break with the stack
reuse optimization.
For example,
int *p;
{
int local1;
p = &local1;
local1 = 10;
....
}
{
int local2;
local2 = 20;
...
}
if (*p == 10) // out of scope use of local1
{
}
Another example:
struct A
{
A(int k) : i(k), j(k) { }
int i;
int j;
};
A *ap;
void foo(const A& ar)
{
ap = &ar;
}
void bar()
{
foo(A(10)); // temp object's lifetime ends when foo returns
{
A a(20);
....
}
ap->i+= 10; // ap references out of scope temp whose space
// is reused with a. What is the value of ap->i?
}
The lifetime of a compiler generated temporary is well defined by the C++
standard. When a lifetime of a temporary ends, and if the temporary lives
in memory, the optimizing compiler has the freedom to reuse its stack
space with other temporaries or scoped local variables whose live range
does not overlap with it. However some of the legacy code relies on the
behavior of older compilers in which temporaries' stack space is not
reused, the aggressive stack reuse can lead to runtime errors. This option
is used to control the temporary stack reuse optimization.
- -ftrapv
- This option generates traps for signed overflow on
addition, subtraction, multiplication operations.
- -fwrapv
- This option instructs the compiler to assume that signed
arithmetic overflow of addition, subtraction and multiplication wraps
around using twos-complement representation. This flag enables some
optimizations and disables others. This option is enabled by default for
the Java front end, as required by the Java language specification.
- -fexceptions
- Enable exception handling. Generates extra code needed to
propagate exceptions. For some targets, this implies GCC generates frame
unwind information for all functions, which can produce significant data
size overhead, although it does not affect execution. If you do not
specify this option, GCC enables it by default for languages like C++ that
normally require exception handling, and disables it for languages like C
that do not normally require it. However, you may need to enable this
option when compiling C code that needs to interoperate properly with
exception handlers written in C++. You may also wish to disable this
option if you are compiling older C++ programs that don't use exception
handling.
- -fnon-call-exceptions
- Generate code that allows trapping instructions to throw
exceptions. Note that this requires platform-specific runtime support that
does not exist everywhere. Moreover, it only allows trapping
instructions to throw exceptions, i.e. memory references or floating-point
instructions. It does not allow exceptions to be thrown from arbitrary
signal handlers such as "SIGALRM".
- -fdelete-dead-exceptions
- Consider that instructions that may throw exceptions but
don't otherwise contribute to the execution of the program can be
optimized away. This option is enabled by default for the Ada front end,
as permitted by the Ada language specification. Optimization passes that
cause dead exceptions to be removed are enabled independently at different
optimization levels.
- -funwind-tables
- Similar to -fexceptions, except that it just
generates any needed static data, but does not affect the generated code
in any other way. You normally do not need to enable this option; instead,
a language processor that needs this handling enables it on your
behalf.
- -fasynchronous-unwind-tables
- Generate unwind table in DWARF 2 format, if supported by
target machine. The table is exact at each instruction boundary, so it can
be used for stack unwinding from asynchronous events (such as debugger or
garbage collector).
- -fno-gnu-unique
- On systems with recent GNU assembler and C library, the C++
compiler uses the "STB_GNU_UNIQUE" binding to make sure that
definitions of template static data members and static local variables in
inline functions are unique even in the presence of
"RTLD_LOCAL"; this is necessary to avoid problems with a library
used by two different "RTLD_LOCAL" plugins depending on a
definition in one of them and therefore disagreeing with the other one
about the binding of the symbol. But this causes "dlclose" to be
ignored for affected DSOs; if your program relies on reinitialization of a
DSO via "dlclose" and "dlopen", you can use
-fno-gnu-unique.
- -fpcc-struct-return
- Return "short" "struct" and
"union" values in memory like longer ones, rather than in
registers. This convention is less efficient, but it has the advantage of
allowing intercallability between GCC-compiled files and files compiled
with other compilers, particularly the Portable C Compiler (pcc).
The precise convention for returning structures in memory depends on the
target configuration macros.
Short structures and unions are those whose size and alignment match that of
some integer type.
Warning: code compiled with the -fpcc-struct-return switch is
not binary compatible with code compiled with the
-freg-struct-return switch. Use it to conform to a non-default
application binary interface.
- -freg-struct-return
- Return "struct" and "union" values in
registers when possible. This is more efficient for small structures than
-fpcc-struct-return.
If you specify neither -fpcc-struct-return nor
-freg-struct-return, GCC defaults to whichever convention is
standard for the target. If there is no standard convention, GCC defaults
to -fpcc-struct-return, except on targets where GCC is the
principal compiler. In those cases, we can choose the standard, and we
chose the more efficient register return alternative.
Warning: code compiled with the -freg-struct-return switch is
not binary compatible with code compiled with the
-fpcc-struct-return switch. Use it to conform to a non-default
application binary interface.
- -fshort-enums
- Allocate to an "enum" type only as many bytes as
it needs for the declared range of possible values. Specifically, the
"enum" type is equivalent to the smallest integer type that has
enough room.
Warning: the -fshort-enums switch causes GCC to generate code
that is not binary compatible with code generated without that switch. Use
it to conform to a non-default application binary interface.
- -fshort-double
- Use the same size for "double" as for
"float".
Warning: the -fshort-double switch causes GCC to generate
code that is not binary compatible with code generated without that
switch. Use it to conform to a non-default application binary
interface.
- -fshort-wchar
- Override the underlying type for "wchar_t" to be
"short unsigned int" instead of the default for the target. This
option is useful for building programs to run under WINE.
Warning: the -fshort-wchar switch causes GCC to generate code
that is not binary compatible with code generated without that switch. Use
it to conform to a non-default application binary interface.
- -fno-common
- In C code, controls the placement of uninitialized global
variables. Unix C compilers have traditionally permitted multiple
definitions of such variables in different compilation units by placing
the variables in a common block. This is the behavior specified by
-fcommon, and is the default for GCC on most targets. On the other
hand, this behavior is not required by ISO C, and on some targets may
carry a speed or code size penalty on variable references. The
-fno-common option specifies that the compiler should place
uninitialized global variables in the data section of the object file,
rather than generating them as common blocks. This has the effect that if
the same variable is declared (without "extern") in two
different compilations, you get a multiple-definition error when you link
them. In this case, you must compile with -fcommon instead.
Compiling with -fno-common is useful on targets for which it
provides better performance, or if you wish to verify that the program
will work on other systems that always treat uninitialized variable
declarations this way.
- -fno-ident
- Ignore the "#ident" directive.
- -finhibit-size-directive
- Don't output a ".size" assembler directive, or
anything else that would cause trouble if the function is split in the
middle, and the two halves are placed at locations far apart in memory.
This option is used when compiling crtstuff.c; you should not need
to use it for anything else.
- -fverbose-asm
- Put extra commentary information in the generated assembly
code to make it more readable. This option is generally only of use to
those who actually need to read the generated assembly code (perhaps while
debugging the compiler itself).
-fno-verbose-asm, the default, causes the extra information to be
omitted and is useful when comparing two assembler files.
- -frecord-gcc-switches
- This switch causes the command line used to invoke the
compiler to be recorded into the object file that is being created. This
switch is only implemented on some targets and the exact format of the
recording is target and binary file format dependent, but it usually takes
the form of a section containing ASCII text. This switch is related to the
-fverbose-asm switch, but that switch only records information in
the assembler output file as comments, so it never reaches the object
file. See also -grecord-gcc-switches for another way of storing
compiler options into the object file.
- -fpic
- Generate position-independent code (PIC) suitable for use
in a shared library, if supported for the target machine. Such code
accesses all constant addresses through a global offset table (GOT). The
dynamic loader resolves the GOT entries when the program starts (the
dynamic loader is not part of GCC; it is part of the operating system). If
the GOT size for the linked executable exceeds a machine-specific maximum
size, you get an error message from the linker indicating that
-fpic does not work; in that case, recompile with -fPIC
instead. (These maximums are 8k on the SPARC and 32k on the m68k and
RS/6000. The x86 has no such limit.)
Position-independent code requires special support, and therefore works only
on certain machines. For the x86, GCC supports PIC for System V but not
for the Sun 386i. Code generated for the IBM RS/6000 is always
position-independent.
When this flag is set, the macros "__pic__" and
"__PIC__" are defined to 1.
- -fPIC
- If supported for the target machine, emit
position-independent code, suitable for dynamic linking and avoiding any
limit on the size of the global offset table. This option makes a
difference on the m68k, PowerPC and SPARC.
Position-independent code requires special support, and therefore works only
on certain machines.
When this flag is set, the macros "__pic__" and
"__PIC__" are defined to 2.
- -fpie
- -fPIE
- These options are similar to -fpic and -fPIC,
but generated position independent code can be only linked into
executables. Usually these options are used when -pie GCC option is
used during linking.
-fpie and -fPIE both define the macros "__pie__"
and "__PIE__". The macros have the value 1 for -fpie and
2 for -fPIE.
- -fno-jump-tables
- Do not use jump tables for switch statements even where it
would be more efficient than other code generation strategies. This option
is of use in conjunction with -fpic or -fPIC for building
code that forms part of a dynamic linker and cannot reference the address
of a jump table. On some targets, jump tables do not require a GOT and
this option is not needed.
- -ffixed-reg
- Treat the register named reg as a fixed register;
generated code should never refer to it (except perhaps as a stack
pointer, frame pointer or in some other fixed role).
reg must be the name of a register. The register names accepted are
machine-specific and are defined in the "REGISTER_NAMES" macro
in the machine description macro file.
This flag does not have a negative form, because it specifies a three-way
choice.
- -fcall-used-reg
- Treat the register named reg as an allocable
register that is clobbered by function calls. It may be allocated for
temporaries or variables that do not live across a call. Functions
compiled this way do not save and restore the register reg.
It is an error to use this flag with the frame pointer or stack pointer. Use
of this flag for other registers that have fixed pervasive roles in the
machine's execution model produces disastrous results.
This flag does not have a negative form, because it specifies a three-way
choice.
- -fcall-saved-reg
- Treat the register named reg as an allocable
register saved by functions. It may be allocated even for temporaries or
variables that live across a call. Functions compiled this way save and
restore the register reg if they use it.
It is an error to use this flag with the frame pointer or stack pointer. Use
of this flag for other registers that have fixed pervasive roles in the
machine's execution model produces disastrous results.
A different sort of disaster results from the use of this flag for a
register in which function values may be returned.
This flag does not have a negative form, because it specifies a three-way
choice.
- -fpack-struct[=n]
- Without a value specified, pack all structure members
together without holes. When a value is specified (which must be a small
power of two), pack structure members according to this value,
representing the maximum alignment (that is, objects with default
alignment requirements larger than this are output potentially unaligned
at the next fitting location.
Warning: the -fpack-struct switch causes GCC to generate code
that is not binary compatible with code generated without that switch.
Additionally, it makes the code suboptimal. Use it to conform to a
non-default application binary interface.
- -finstrument-functions
- Generate instrumentation calls for entry and exit to
functions. Just after function entry and just before function exit, the
following profiling functions are called with the address of the current
function and its call site. (On some platforms,
"__builtin_return_address" does not work beyond the current
function, so the call site information may not be available to the
profiling functions otherwise.)
void __cyg_profile_func_enter (void *this_fn,
void *call_site);
void __cyg_profile_func_exit (void *this_fn,
void *call_site);
The first argument is the address of the start of the current function,
which may be looked up exactly in the symbol table.
This instrumentation is also done for functions expanded inline in other
functions. The profiling calls indicate where, conceptually, the inline
function is entered and exited. This means that addressable versions of
such functions must be available. If all your uses of a function are
expanded inline, this may mean an additional expansion of code size. If
you use "extern inline" in your C code, an addressable version
of such functions must be provided. (This is normally the case anyway, but
if you get lucky and the optimizer always expands the functions inline,
you might have gotten away without providing static copies.)
A function may be given the attribute "no_instrument_function", in
which case this instrumentation is not done. This can be used, for
example, for the profiling functions listed above, high-priority interrupt
routines, and any functions from which the profiling functions cannot
safely be called (perhaps signal handlers, if the profiling routines
generate output or allocate memory).
- -finstrument-functions-exclude-file-list=file,file,...
- Set the list of functions that are excluded from
instrumentation (see the description of -finstrument-functions). If
the file that contains a function definition matches with one of
file, then that function is not instrumented. The match is done on
substrings: if the file parameter is a substring of the file name,
it is considered to be a match.
For example:
-finstrument-functions-exclude-file-list=/bits/stl,include/sys
excludes any inline function defined in files whose pathnames contain
/bits/stl or include/sys.
If, for some reason, you want to include letter , in one of
sym, write ,. For example,
-finstrument-functions-exclude-file-list=',,tmp' (note the single
quote surrounding the option).
- -finstrument-functions-exclude-function-list=sym,sym,...
- This is similar to
-finstrument-functions-exclude-file-list, but this option sets the
list of function names to be excluded from instrumentation. The function
name to be matched is its user-visible name, such as
"vector<int> blah(const vector<int> &)", not the
internal mangled name (e.g., "_Z4blahRSt6vectorIiSaIiEE"). The
match is done on substrings: if the sym parameter is a substring of
the function name, it is considered to be a match. For C99 and C++
extended identifiers, the function name must be given in UTF-8, not using
universal character names.
- -fstack-check
- Generate code to verify that you do not go beyond the
boundary of the stack. You should specify this flag if you are running in
an environment with multiple threads, but you only rarely need to specify
it in a single-threaded environment since stack overflow is automatically
detected on nearly all systems if there is only one stack.
Note that this switch does not actually cause checking to be done; the
operating system or the language runtime must do that. The switch causes
generation of code to ensure that they see the stack being extended.
You can additionally specify a string parameter: no means no
checking, generic means force the use of old-style checking,
specific means use the best checking method and is equivalent to
bare -fstack-check.
Old-style checking is a generic mechanism that requires no specific target
support in the compiler but comes with the following drawbacks:
- 1.
- Modified allocation strategy for large objects: they are
always allocated dynamically if their size exceeds a fixed threshold.
- 2.
- Fixed limit on the size of the static frame of functions:
when it is topped by a particular function, stack checking is not reliable
and a warning is issued by the compiler.
- 3.
- Inefficiency: because of both the modified allocation
strategy and the generic implementation, code performance is
hampered.
Note that old-style stack checking is also the fallback method for
specific if no target support has been added in the compiler.
- -fstack-limit-register=reg
- -fstack-limit-symbol=sym
- -fno-stack-limit
- Generate code to ensure that the stack does not grow beyond
a certain value, either the value of a register or the address of a
symbol. If a larger stack is required, a signal is raised at run time. For
most targets, the signal is raised before the stack overruns the boundary,
so it is possible to catch the signal without taking special precautions.
For instance, if the stack starts at absolute address 0x80000000 and
grows downwards, you can use the flags
-fstack-limit-symbol=__stack_limit and
-Wl,--defsym,__stack_limit=0x7ffe0000 to enforce a stack limit of
128KB. Note that this may only work with the GNU linker.
- -fsplit-stack
- Generate code to automatically split the stack before it
overflows. The resulting program has a discontiguous stack which can only
overflow if the program is unable to allocate any more memory. This is
most useful when running threaded programs, as it is no longer necessary
to calculate a good stack size to use for each thread. This is currently
only implemented for the x86 targets running GNU/Linux.
When code compiled with -fsplit-stack calls code compiled without
-fsplit-stack, there may not be much stack space available for the
latter code to run. If compiling all code, including library code, with
-fsplit-stack is not an option, then the linker can fix up these
calls so that the code compiled without -fsplit-stack always has a
large stack. Support for this is implemented in the gold linker in GNU
binutils release 2.21 and later.
- -fleading-underscore
- This option and its counterpart,
-fno-leading-underscore, forcibly change the way C symbols are
represented in the object file. One use is to help link with legacy
assembly code.
Warning: the -fleading-underscore switch causes GCC to
generate code that is not binary compatible with code generated without
that switch. Use it to conform to a non-default application binary
interface. Not all targets provide complete support for this switch.
- -ftls-model=model
- Alter the thread-local storage model to be used. The
model argument should be one of global-dynamic,
local-dynamic, initial-exec or local-exec. Note that
the choice is subject to optimization: the compiler may use a more
efficient model for symbols not visible outside of the translation unit,
or if -fpic is not given on the command line.
The default without -fpic is initial-exec; with -fpic
the default is global-dynamic.
- -fvisibility=[default|internal|hidden|protected]
- Set the default ELF image symbol visibility to the
specified option---all symbols are marked with this unless overridden
within the code. Using this feature can very substantially improve linking
and load times of shared object libraries, produce more optimized code,
provide near-perfect API export and prevent symbol clashes. It is
strongly recommended that you use this in any shared objects you
distribute.
Despite the nomenclature, default always means public; i.e.,
available to be linked against from outside the shared object.
protected and internal are pretty useless in real-world
usage so the only other commonly used option is hidden. The default
if -fvisibility isn't specified is default, i.e., make every
symbol public.
A good explanation of the benefits offered by ensuring ELF symbols have the
correct visibility is given by "How To Write Shared Libraries"
by Ulrich Drepper (which can be found at <
http://www.akkadia.org/drepper/>)---however a superior solution
made possible by this option to marking things hidden when the default is
public is to make the default hidden and mark things public. This is the
norm with DLLs on Windows and with -fvisibility=hidden and
"__attribute__ ((visibility("default")))" instead of
"__declspec(dllexport)" you get almost identical semantics with
identical syntax. This is a great boon to those working with
cross-platform projects.
For those adding visibility support to existing code, you may find
"#pragma GCC visibility" of use. This works by you enclosing the
declarations you wish to set visibility for with (for example)
"#pragma GCC visibility push(hidden)" and "#pragma GCC
visibility pop". Bear in mind that symbol visibility should be viewed
as part of the API interface contract and thus all new code
should always specify visibility when it is not the default; i.e.,
declarations only for use within the local DSO should always be
marked explicitly as hidden as so to avoid PLT indirection
overheads---making this abundantly clear also aids readability and
self-documentation of the code. Note that due to ISO C++ specification
requirements, "operator new" and "operator delete"
must always be of default visibility.
Be aware that headers from outside your project, in particular system
headers and headers from any other library you use, may not be expecting
to be compiled with visibility other than the default. You may need to
explicitly say "#pragma GCC visibility push(default)" before
including any such headers.
"extern" declarations are not affected by -fvisibility, so
a lot of code can be recompiled with -fvisibility=hidden with no
modifications. However, this means that calls to "extern"
functions with no explicit visibility use the PLT, so it is more effective
to use "__attribute ((visibility))" and/or "#pragma GCC
visibility" to tell the compiler which "extern"
declarations should be treated as hidden.
Note that -fvisibility does affect C++ vague linkage entities. This
means that, for instance, an exception class that is be thrown between
DSOs must be explicitly marked with default visibility so that the
type_info nodes are unified between the DSOs.
An overview of these techniques, their benefits and how to use them is at
< http://gcc.gnu.org/wiki/Visibility>.
- -fstrict-volatile-bitfields
- This option should be used if accesses to volatile
bit-fields (or other structure fields, although the compiler usually
honors those types anyway) should use a single access of the width of the
field's type, aligned to a natural alignment if possible. For example,
targets with memory-mapped peripheral registers might require all such
accesses to be 16 bits wide; with this flag you can declare all peripheral
bit-fields as "unsigned short" (assuming short is 16 bits on
these targets) to force GCC to use 16-bit accesses instead of, perhaps, a
more efficient 32-bit access.
If this option is disabled, the compiler uses the most efficient
instruction. In the previous example, that might be a 32-bit load
instruction, even though that accesses bytes that do not contain any
portion of the bit-field, or memory-mapped registers unrelated to the one
being updated.
In some cases, such as when the "packed" attribute is applied to a
structure field, it may not be possible to access the field with a single
read or write that is correctly aligned for the target machine. In this
case GCC falls back to generating multiple accesses rather than code that
will fault or truncate the result at run time.
Note: Due to restrictions of the C/C++11 memory model, write accesses are
not allowed to touch non bit-field members. It is therefore recommended to
define all bits of the field's type as bit-field members.
The default value of this option is determined by the application binary
interface for the target processor.
- -fsync-libcalls
- This option controls whether any out-of-line instance of
the "__sync" family of functions may be used to implement the
C++11 "__atomic" family of functions.
The default value of this option is enabled, thus the only useful form of
the option is -fno-sync-libcalls. This option is used in the
implementation of the libatomic runtime library.
ENVIRONMENT
This section describes several environment variables that affect how GCC
operates. Some of them work by specifying directories or prefixes to use when
searching for various kinds of files. Some are used to specify other aspects
of the compilation environment.
Note that you can also specify places to search using options such as
-B,
-I and
-L. These take precedence over places specified using
environment variables, which in turn take precedence over those specified by
the configuration of GCC.
- LANG
- LC_CTYPE
- LC_MESSAGES
- LC_ALL
- These environment variables control the way that GCC uses
localization information which allows GCC to work with different national
conventions. GCC inspects the locale categories LC_CTYPE and
LC_MESSAGES if it has been configured to do so. These locale
categories can be set to any value supported by your installation. A
typical value is en_GB.UTF-8 for English in the United Kingdom
encoded in UTF-8.
The LC_CTYPE environment variable specifies character classification.
GCC uses it to determine the character boundaries in a string; this is
needed for some multibyte encodings that contain quote and escape
characters that are otherwise interpreted as a string end or escape.
The LC_MESSAGES environment variable specifies the language to use in
diagnostic messages.
If the LC_ALL environment variable is set, it overrides the value of
LC_CTYPE and LC_MESSAGES; otherwise, LC_CTYPE and
LC_MESSAGES default to the value of the LANG environment
variable. If none of these variables are set, GCC defaults to traditional
C English behavior.
- TMPDIR
- If TMPDIR is set, it specifies the directory to use
for temporary files. GCC uses temporary files to hold the output of one
stage of compilation which is to be used as input to the next stage: for
example, the output of the preprocessor, which is the input to the
compiler proper.
- GCC_COMPARE_DEBUG
- Setting GCC_COMPARE_DEBUG is nearly equivalent to
passing -fcompare-debug to the compiler driver. See the
documentation of this option for more details.
- GCC_EXEC_PREFIX
- If GCC_EXEC_PREFIX is set, it specifies a prefix to
use in the names of the subprograms executed by the compiler. No slash is
added when this prefix is combined with the name of a subprogram, but you
can specify a prefix that ends with a slash if you wish.
If GCC_EXEC_PREFIX is not set, GCC attempts to figure out an
appropriate prefix to use based on the pathname it is invoked with.
If GCC cannot find the subprogram using the specified prefix, it tries
looking in the usual places for the subprogram.
The default value of GCC_EXEC_PREFIX is
prefix/lib/gcc/ where prefix is the prefix to
the installed compiler. In many cases prefix is the value of
"prefix" when you ran the configure script.
Other prefixes specified with -B take precedence over this prefix.
This prefix is also used for finding files such as crt0.o that are
used for linking.
In addition, the prefix is used in an unusual way in finding the directories
to search for header files. For each of the standard directories whose
name normally begins with /usr/local/lib/gcc (more precisely, with
the value of GCC_INCLUDE_DIR), GCC tries replacing that beginning
with the specified prefix to produce an alternate directory name. Thus,
with -Bfoo/, GCC searches foo/bar just before it searches
the standard directory /usr/local/lib/bar. If a standard directory
begins with the configured prefix then the value of prefix
is replaced by GCC_EXEC_PREFIX when looking for header files.
- COMPILER_PATH
- The value of COMPILER_PATH is a colon-separated list
of directories, much like PATH. GCC tries the directories thus
specified when searching for subprograms, if it can't find the subprograms
using GCC_EXEC_PREFIX.
- LIBRARY_PATH
- The value of LIBRARY_PATH is a colon-separated list
of directories, much like PATH. When configured as a native
compiler, GCC tries the directories thus specified when searching for
special linker files, if it can't find them using GCC_EXEC_PREFIX.
Linking using GCC also uses these directories when searching for ordinary
libraries for the -l option (but directories specified with
-L come first).
- LANG
- This variable is used to pass locale information to the
compiler. One way in which this information is used is to determine the
character set to be used when character literals, string literals and
comments are parsed in C and C++. When the compiler is configured to allow
multibyte characters, the following values for LANG are
recognized:
- C-JIS
- Recognize JIS characters.
- C-SJIS
- Recognize SJIS characters.
- C-EUCJP
- Recognize EUCJP characters.
If
LANG is not defined, or if it has some other value, then the compiler
uses "mblen" and "mbtowc" as defined by the default locale
to recognize and translate multibyte characters.
Some additional environment variables affect the behavior of the preprocessor.
- CPATH
- C_INCLUDE_PATH
- CPLUS_INCLUDE_PATH
- OBJC_INCLUDE_PATH
- Each variable's value is a list of directories separated by
a special character, much like PATH, in which to look for header
files. The special character, "PATH_SEPARATOR", is
target-dependent and determined at GCC build time. For Microsoft
Windows-based targets it is a semicolon, and for almost all other targets
it is a colon.
CPATH specifies a list of directories to be searched as if specified
with -I, but after any paths given with -I options on the
command line. This environment variable is used regardless of which
language is being preprocessed.
The remaining environment variables apply only when preprocessing the
particular language indicated. Each specifies a list of directories to be
searched as if specified with -isystem, but after any paths given
with -isystem options on the command line.
In all these variables, an empty element instructs the compiler to search
its current working directory. Empty elements can appear at the beginning
or end of a path. For instance, if the value of CPATH is
":/special/include", that has the same effect as
-I. -I/special/include.
- DEPENDENCIES_OUTPUT
- If this variable is set, its value specifies how to output
dependencies for Make based on the non-system header files processed by
the compiler. System header files are ignored in the dependency output.
The value of DEPENDENCIES_OUTPUT can be just a file name, in which
case the Make rules are written to that file, guessing the target name
from the source file name. Or the value can have the form file
target, in which case the rules are written to file
file using target as the target name.
In other words, this environment variable is equivalent to combining the
options -MM and -MF, with an optional -MT switch
too.
- SUNPRO_DEPENDENCIES
- This variable is the same as DEPENDENCIES_OUTPUT
(see above), except that system header files are not ignored, so it
implies -M rather than -MM. However, the dependence on the
main input file is omitted.
- CPP_RESTRICTED
- If this variable is defined, cpp will skip any include file
which is not a regular file, and will continue searching for the requested
name (this is always done if the found file is a directory).
BUGS
For instructions on reporting bugs, see <
http://www.NetBSD.org/Misc/send-pr.html>.
FOOTNOTES
- 1.
- On some systems, gcc -shared needs to build
supplementary stub code for constructors to work. On multi-libbed systems,
gcc -shared must select the correct support libraries to link
against. Failing to supply the correct flags may lead to subtle defects.
Supplying them in cases where they are not necessary is innocuous.
SEE ALSO
gpl(7),
gfdl(7),
fsf-funding(7),
cpp(1),
gcov(1),
as(1),
ld(1),
gdb(1),
adb(1),
dbx(1),
sdb(1) and the Info entries for
gcc,
cpp,
as,
ld,
binutils and
gdb.
AUTHOR
See the Info entry for
gcc, or <
http://gcc.gnu.org/onlinedocs/gcc/Contributors.html>, for
contributors to GCC.
COPYRIGHT
Copyright (c) 1988-2015 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document under the
terms of the GNU Free Documentation License, Version 1.3 or any later version
published by the Free Software Foundation; with the Invariant Sections being
"GNU General Public License" and "Funding Free Software",
the Front-Cover texts being (a) (see below), and with the Back-Cover Texts
being (b) (see below). A copy of the license is included in the
gfdl(7)
man page.
(a) The FSF's Front-Cover Text is:
A GNU Manual
(b) The FSF's Back-Cover Text is:
You have freedom to copy and modify this GNU Manual, like GNU
software. Copies published by the Free Software Foundation raise
funds for GNU development.