gcc(1)

NAME

gcc - GNU project C and C++ compiler

SYNOPSIS

gcc [-c|-S|-E] [-std=standard]
    [-g] [-pg] [-Olevel]
    [-Wwarn...] [-pedantic]
    [-Idir...] [-Ldir...]
    [-Dmacro[=defn]...] [-Umacro]
    [-foption...] [-mmachine-option...]
    [-o outfile] 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: -dr is very different from -d -r.

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.

Many options have long names starting with -f or with -W---for example, -fstrength-reduce, -Wformat and so on. Most of these have both positive and negative forms; the negative form of -ffoo would be -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 -combine -pipe -pass-exit-codes -x language -v -### --help --target-help --version
C Language Options: -ansi -std=standard -fgnu89-inline -aux-info filename -fno-asm -fno-builtin -fno-builtin-function -fhosted -ffreestanding -fms-extensions -trigraphs -no-integrated-cpp -traditional -traditional-cpp -fallow-single-precision -fcond-mismatch -fsigned-bitfields -fsigned-char -funsigned-bitfields -funsigned-char
C++ Language Options: -fabi-version=n -fno-access-control -fcheck-new -fconserve-space -ffriend-injection -fno-const-strings -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 -fno-operator-names -fno-optional-diags -fpermissive -frepo -fno-rtti -fstats -ftemplate-depth-n -fno-threadsafe-statics -fuse-cxa-atexit -fno-weak -nostdinc++ -fno-default-inline -fvisibility-inlines-hidden -Wabi -Wctor-dtor-privacy -Wnon-virtual-dtor -Wreorder -Weffc++ -Wno-deprecated -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-call-cxx-cdtors -fobjc-direct-dispatch -fobjc-exceptions -fobjc-gc -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-show-options
Warning Options: -fsyntax-only -pedantic -pedantic-errors -w -Wextra -Wall -Waggregate-return -Wno-attributes -Wc++-compat -Wcast-align -Wcast-qual -Wchar-subscripts -Wcomment -Wconversion -Wno-deprecated-declarations -Wdisabled-optimization -Wno-div-by-zero -Wno-endif-labels -Werror -Werror-implicit-function-declaration -Wfatal-errors -Wfloat-equal -Wformat -Wformat=2 -Wno-format-extra-args -Wformat-nonliteral -Wformat-security -Wformat-y2k -Wimplicit -Wimplicit-function-declaration -Wimplicit-int -Wimport -Wno-import -Winit-self -Winline -Wno-int-to-pointer-cast -Wno-invalid-offsetof -Winvalid-pch -Wlarger-than-len -Wunsafe-loop-optimizations -Wlong-long -Wmain -Wmissing-braces -Wmissing-field-initializers -Wmissing-format-attribute -Wmissing-include-dirs -Wmissing-noreturn -Wno-multichar -Wnonnull -Wpacked -Wpadded -Wparentheses -Wpointer-arith -Wno-pointer-to-int-cast -Wredundant-decls -Wreturn-type -Wsequence-point -Wshadow -Wsign-compare -Wstack-protector -Wstrict-aliasing -Wstrict-aliasing=2 -Wswitch -Wswitch-default -Wswitch-enum -Wsystem-headers -Wtrigraphs -Wundef -Wuninitialized -Wunknown-pragmas -Wno-pragmas -Wunreachable-code -Wunused -Wunused-function -Wunused-label -Wunused-parameter -Wunused-value -Wunused-variable -Wvariadic-macros -Wvolatile-register-var -Wwrite-strings
C-only Warning Options: -Wbad-function-cast -Wmissing-declarations -Wmissing-prototypes -Wnested-externs -Wold-style-definition -Wstrict-prototypes -Wtraditional -Wdeclaration-after-statement -Wpointer-sign
Debugging Options: -dletters -dumpspecs -dumpmachine -dumpversion -fdump-unnumbered -fdump-translation-unit[-n] -fdump-class-hierarchy[-n] -fdump-ipa-all -fdump-ipa-cgraph -fdump-tree-all -fdump-tree-original[-n] -fdump-tree-optimized[-n] -fdump-tree-inlined[-n] -fdump-tree-cfg -fdump-tree-vcg -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-mudflap[-n] -fdump-tree-dom[-n] -fdump-tree-dse[-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-salias -fdump-tree-fre[-n] -fdump-tree-vrp[-n] -ftree-vectorizer-verbose=n -fdump-tree-storeccp[-n] -feliminate-dwarf2-dups -feliminate-unused-debug-types -feliminate-unused-debug-symbols -fmem-report -fprofile-arcs -frandom-seed=string -fsched-verbose=n -ftest-coverage -ftime-report -fvar-tracking -g -glevel -gcoff -gdwarf-2 -ggdb -gstabs -gstabs+ -gvms -gxcoff -gxcoff+ -p -pg -print-file-name=library -print-libgcc-file-name -print-multi-directory -print-multi-lib -print-prog-name=program -print-search-dirs -Q -save-temps -time
Optimization Options: -falign-functions[=n] -falign-jumps[=n] -falign-labels[=n] -falign-loops[=n] -fmudflap -fmudflapth -fmudflapir -fbranch-probabilities -fprofile-values -fvpt -fbranch-target-load-optimize -fbranch-target-load-optimize2 -fbtr-bb-exclusive -fcaller-saves -fcprop-registers -fcse-follow-jumps -fcse-skip-blocks -fcx-limited-range -fdata-sections -fdelayed-branch -fdelete-null-pointer-checks -fearly-inlining -fexpensive-optimizations -ffast-math -ffloat-store -fforce-addr -ffunction-sections -fgcse -fgcse-lm -fgcse-sm -fgcse-las -fgcse-after-reload -floop-optimize -fcrossjumping -fif-conversion -fif-conversion2 -finline-functions -finline-functions-called-once -finline-limit=n -fkeep-inline-functions -fkeep-static-consts -fmerge-constants -fmerge-all-constants -fmodulo-sched -fno-branch-count-reg -fno-default-inline -fno-defer-pop -floop-optimize2 -fmove-loop-invariants -fno-function-cse -fno-guess-branch-probability -fno-inline -fno-math-errno -fno-peephole -fno-peephole2 -funsafe-math-optimizations -funsafe-loop-optimizations -ffinite-math-only -fno-trapping-math -fno-zero-initialized-in-bss -fomit-frame-pointer -foptimize-register-move -foptimize-sibling-calls -fprefetch-loop-arrays -fprofile-generate -fprofile-use -fregmove -frename-registers -freorder-blocks -freorder-blocks-and-partition -freorder-functions -frerun-cse-after-loop -frerun-loop-opt -frounding-math -fschedule-insns -fschedule-insns2 -fno-sched-interblock -fno-sched-spec -fsched-spec-load -fsched-spec-load-dangerous -fsched-stalled-insns[=n] -fsched-stalled-insns-dep[=n] -fsched2-use-superblocks -fsched2-use-traces -freschedule-modulo-scheduled-loops -fsignaling-nans -fsingle-precision-constant -fstack-protector -fstack-protector-all -fstrength-reduce -fstrict-aliasing -ftracer -fthread-jumps -funroll-all-loops -funroll-loops -fpeel-loops -fsplit-ivs-in-unroller -funswitch-loops -fvariable-expansion-in-unroller -ftree-pre -ftree-ccp -ftree-dce -ftree-loop-optimize -ftree-loop-linear -ftree-loop-im -ftree-loop-ivcanon -fivopts -ftree-dominator-opts -ftree-dse -ftree-copyrename -ftree-sink -ftree-ch -ftree-sra -ftree-ter -ftree-lrs -ftree-fre -ftree-vectorize -ftree-vect-loop-version -ftree-salias -fweb -ftree-copy-prop -ftree-store-ccp -ftree-store-copy-prop -ftree-vrp -funit-at-a-time -fwhole-program --param name=value -O -O0 -O1 -O2 -O3 -Os
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 -isysroot dir -M -MM -MF -MG -MP -MQ -MT -nostdinc -P -fworking-directory -remap -trigraphs -undef -Umacro -Wp,option -Xpreprocessor option
Assembler Option: -Wa,option -Xassembler option
Linker Options: object-file-name -llibrary -nostartfiles -nodefaultlibs -nostdlib -pie -rdynamic -s -static -static-libgcc -shared -shared-libgcc -symbolic -Wl,option -Xlinker option -u symbol
Directory Options: -Bprefix -Idir -iquotedir -Ldir -specs=file -I- --sysroot=dir
Target Options: -V version -b machine
Machine Dependent Options: ARC Options -EB -EL -mmangle-cpu -mcpu=cpu -mtext=text-section -mdata=data-section -mrodata=readonly-data-section; 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 -mwords-little-endian -mfloat-abi=name -msoft-float -mhard-float -mfpe -mthumb-interwork -mno-thumb-interwork -mcpu=name -march=name -mfpu=name -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 -mcirrus-fix-invalid-insns -mno-cirrus-fix-invalid-insns -mpoke-function-name -mthumb -marm -mtpcs-frame -mtpcs-leaf-frame -mcaller-super-interworking -mcallee-super-interworking -mtp=name; AVR Options -mmcu=mcu -msize -minit-stack=n -mno-interrupts -mcall-prologues -mno-tablejump -mtiny-stack -mint8; Blackfin Options -momit-leaf-frame-pointer -mno-omit-leaf-frame-pointer -mspecld-anomaly -mno-specld-anomaly -mcsync-anomaly -mno-csync-anomaly -mlow-64k -mno-low64k -mid-shared-library -mno-id-shared-library -mshared-library-id=n -mlong-calls -mno-long-calls; 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; CRX Options -mmac -mpush-args; 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 -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 -mone-byte-bool; DEC Alpha Options -mno-fp-regs -msoft-float -malpha-as -mgas -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; DEC Alpha/VMS Options -mvms-return-codes; 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; H8/300 Options -mrelax -mh -ms -mn -mint32 -malign-300; HPPA Options -march=architecture-type -mbig-switch -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-big-switch -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; i386 and x86-64 Options -mtune=cpu-type -march=cpu-type -mfpmath=unit -masm=dialect -mno-fancy-math-387 -mno-fp-ret-in-387 -msoft-float -msvr3-shlib -mno-wide-multiply -mrtd -malign-double -mpreferred-stack-boundary=num -mmmx -msse -msse2 -msse3 -mssse3 -msse4a -msse5 -m3dnow -mpopcnt -mabm -mthreads -mno-align-stringops -minline-all-stringops -mpush-args -maccumulate-outgoing-args -m128bit-long-double -m96bit-long-double -mregparm=num -msseregparm -momit-leaf-frame-pointer -mno-red-zone -mno-tls-direct-seg-refs -mcmodel=code-model -m32 -m64 -mlarge-data-threshold=num -mfused-madd -mno-fused-madd; IA-64 Options -mbig-endian -mlittle-endian -mgnu-as -mgnu-ld -mno-pic -mvolatile-asm-stop -mregister-names -mno-sdata -mconstant-gp -mauto-pic -minline-float-divide-min-latency -minline-float-divide-max-throughput -minline-int-divide-min-latency -minline-int-divide-max-throughput -minline-sqrt-min-latency -minline-sqrt-max-throughput -mno-dwarf2-asm -mearly-stop-bits -mfixed-range=register-range -mtls-size=tls-size -mtune=cpu-type -mt -pthread -milp32 -mlp64; 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 -m68000 -m68020 -m68020-40 -m68020-60 -m68030 -m68040 -m68060 -mcpu32 -m5200 -m68881 -mbitfield -mc68000 -mc68020 -mnobitfield -mrtd -mshort -msoft-float -mpcrel -malign-int -mstrict-align -msep-data -mno-sep-data -mshared-library-id=n -mid-shared-library -mno-id-shared-library; M68hc1x Options -m6811 -m6812 -m68hc11 -m68hc12 -m68hcs12 -mauto-incdec -minmax -mlong-calls -mshort -msoft-reg-count=count; 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; MIPS Options -EL -EB -march=arch -mtune=arch -mips1 -mips2 -mips3 -mips4 -mips32 -mips32r2 -mips64 -mips16 -mno-mips16 -mabi=abi -mabicalls -mno-abicalls -mxgot -mno-xgot -mgp32 -mgp64 -mfp32 -mfp64 -mhard-float -msoft-float -msingle-float -mdouble-float -mdsp -mpaired-single -mips3d -mlong64 -mlong32 -msym32 -mno-sym32 -Gnum -membedded-data -mno-embedded-data -muninit-const-in-rodata -mno-uninit-const-in-rodata -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 -mfused-madd -mno-fused-madd -nocpp -mfix-r4000 -mno-fix-r4000 -mfix-r4400 -mno-fix-r4400 -mfix-vr4120 -mno-fix-vr4120 -mfix-vr4130 -mfix-sb1 -mno-fix-sb1 -mflush-func=func -mno-flush-func -mbranch-likely -mno-branch-likely -mfp-exceptions -mno-fp-exceptions -mvr4130-align -mno-vr4130-align; 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 -mam33 -mno-am33 -mam33-2 -mno-am33-2 -mreturn-pointer-on-d0 -mno-crt0 -mrelax; MT Options -mno-crt0 -mbacc -msim -march=cpu-type; 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 -msplit -mno-split -munix-asm -mdec-asm; PowerPC Options See RS/6000 and PowerPC Options.; RS/6000 and PowerPC Options -mcpu=cpu-type -mtune=cpu-type -mpower -mno-power -mpower2 -mno-power2 -mpowerpc -mpowerpc64 -mno-powerpc -maltivec -mno-altivec -mpowerpc-gpopt -mno-powerpc-gpopt -mpowerpc-gfxopt -mno-powerpc-gfxopt -mmfcrf -mno-mfcrf -mpopcntb -mno-popcntb -mfprnd -mno-fprnd -mmfpgpr -mno-mfpgpr -mnew-mnemonics -mold-mnemonics -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 -mstring -mno-string -mupdate -mno-update -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 -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 -misel -mno-isel -misel=yes -misel=no -mspe -mno-spe -mspe=yes -mspe=no -mvrsave -mno-vrsave -mfloat-gprs=yes -mfloat-gprs=no -mfloat-gprs=single -mfloat-gprs=double -mprototype -mno-prototype -msim -mmvme -mads -myellowknife -memb -msdata -msdata=opt -mvxworks -mwindiss -G num -pthread; S/390 and zSeries Options -mtune=cpu-type -march=cpu-type -mhard-float -msoft-float -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; SH Options -m1 -m2 -m2e -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 -misize -mpadstruct -mspace -mprefergot -musermode -multcost=number -mdiv=strategy -mdivsi3_libfunc=name -madjust-unroll -mindexed-addressing -mgettrcost=number -mpt-fixed
-minvalid-symbols; SPARC Options -mcpu=cpu-type -mtune=cpu-type -mcmodel=code-model -m32 -m64 -mapp-regs -mno-app-regs -mfaster-structs -mno-faster-structs -mfpu -mno-fpu -mhard-float -msoft-float -mhard-quad-float -msoft-quad-float -mimpure-text -mno-impure-text -mlittle-endian -mstack-bias -mno-stack-bias -munaligned-doubles -mno-unaligned-doubles -mv8plus -mno-v8plus -mvis -mno-vis -threads -pthreads -pthread; System V Options -Qy -Qn -YP,paths -Ym,dir; TMS320C3x/C4x Options -mcpu=cpu -mbig -msmall -mregparm -mmemparm -mfast-fix -mmpyi -mbk -mti -mdp-isr-reload -mrpts=count -mrptb -mdb -mloop-unsigned -mparallel-insns -mparallel-mpy -mpreserve-float; 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 -mv850e1 -mv850e -mv850 -mbig-switch; VAX Options -mg -mgnu -munix; x86-64 Options See i386 and x86-64 Options.; Xstormy16 Options -msim; Xtensa Options -mconst16 -mno-const16 -mfused-madd -mno-fused-madd -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 -funwind-tables -fasynchronous-unwind-tables -finhibit-size-directive -finstrument-functions -fno-common -fno-ident -fpcc-struct-return -fpic -fPIC -fpie -fPIE -fno-jump-tables -freg-struct-return -fshared-data -fshort-enums -fshort-double -fshort-wchar -fverbose-asm -fpack-struct[=n] -fstack-check -fstack-limit-register=reg -fstack-limit-symbol=sym -fno-stack-limit -fargument-alias -fargument-noalias -fargument-noalias-global -fleading-underscore -ftls-model=model -ftrapv -fwrapv -fbounds-check -fvisibility -fopenmp
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 which must be preprocessed.
file.i: C source code which should not be preprocessed.
file.ii: C++ source code which 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 which 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 which should not be preprocessed.
file.h: C, C++, Objective-C or Objective-C++ header file to be turned into a precompiled header.
file.cc
file.cp
file.cxx
file.cpp
file.CPP
file.c++
file.C: C++ source code which 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 which must be preprocessed.
file.mii: Objective-C++ source code which should not be preprocessed.
file.hh
file.H: C++ header file to be turned into a precompiled header.
file.f
file.for
file.FOR: Fixed form Fortran source code which should not be preprocessed.
file.F
file.fpp
file.FPP: Fixed form Fortran source code which must be preprocessed (with the traditional preprocessor).
file.f90
file.f95: Free form Fortran source code which should not be preprocessed.
file.F90
file.F95: Free form Fortran source code which must be preprocessed (with the traditional preprocessor).
file.ads: Ada source code file which 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: Assembler code which 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 c-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
f95 f95-cpp-input
java
treelang
-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 will exit 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 will instead return with numerically highest error produced by any phase that returned an error
indication.
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 which don't require preprocessing are ignored.
-o file: Place output in file file. This applies regardless 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 all command arguments are quoted. 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.
-combine: If you are compiling multiple source files, this option tells the
driver to pass all the source files to the compiler at once (for
those languages for which the compiler can handle this). This will allow intermodule analysis (IMA) to be performed by the compiler.
Currently the only language for which this is supported is C. If
you pass source files for multiple languages to the driver, using
this option, the driver will invoke the compiler(s) that support
IMA once each, passing each compiler all the source files appropriate for it. For those languages that do not support IMA this
option will be ignored, and the compiler will be invoked once for
each source file in that language. If you use this option in conjunction with -save-temps, the compiler will generate multiple preprocessed files (one for each source file), but only one (combined) .o or .s file.
--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 will also be passed on to the various processes invoked by gcc, so that they can display the command line options they accept. If the -Wextra option is also specified then command line options which have no documentation associated with them will also be displayed.
--target-help: Print (on the standard output) a description of target specific
command line options for each tool.
--version: Display the version number and copyrights of the invoked GCC.
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 or .H; 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, C++ programs often require class libraries as well as a compiler that understands the C++ language---and under some circumstances, you might want to compile programs or header files from standard input, or otherwise without a suffix that flags them as C++ programs. You
might also like to precompile a C header file with a .h extension to be used in C++ compilations. g++ is a program that calls GCC with the default language set to C++, and automatically specifies linking
against the C++ library. 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, support all ISO C90 programs. In C++ mode, remove GNU
extensions that conflict with ISO C++.; 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, -pedantic 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 which would normally be built in but do not have semantics defined by ISO C (such as "alloca" and "ffs") are not built-in functions with -ansi is used.
-std=: Determine the language standard. This option is currently only
supported when compiling C or C++. A value for this option must be provided; possible values are; c89
iso9899:1990
ISO C90 (same as -ansi).; iso9899:199409
ISO C90 as modified in amendment 1.; c99
c9x
iso9899:1999
iso9899:199x
ISO C99. Note that this standard is not yet fully supported;
see <http://gcc.gnu.org/gcc-4.1/c99status.html> for more information. The names c9x and iso9899:199x are deprecated.; gnu89
Default, ISO C90 plus GNU extensions (including some C99 features).; gnu99
gnu9x
ISO C99 plus GNU extensions. When ISO C99 is fully implemented in GCC, this will become the default. The name gnu9x is deprecated.; 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.; Even when this option is not specified, you can still use some of
the features of newer standards in so far as they do not conflict
with previous C standards. For example, you may use "__restrict__" even when -std=c99 is not specified.; The -std options specifying some version of ISO C have the same effects as -ansi, except that features that were not in ISO C90 but are in the specified version (for example, // comments and the "inline" keyword in ISO C99) are not disabled.
-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.; This option is accepted by GCC versions 4.1.3 and up. In GCC versions prior to 4.3, C99 inline semantics are not supported, and
thus this option is effectively assumed to be present regardless of whether or not it is specified; the only effect of specifying it
explicitly is to disable warnings about using inline functions in
C99 mode. Likewise, the option -fno-gnu89-inline is not supported in versions of GCC before 4.3. It will be supported only in C99 or gnu99 mode, not in C89 or gnu89 mode.; The preprocesor 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.
-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 that 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 this 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 takes place in 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 takes place in 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.
-fms-extensions: Accept some non-standard constructs used in Microsoft header files.; Some cases of unnamed fields in structures and unions are only
accepted with this option.
-trigraphs: Support ISO C trigraphs. The -ansi option (and -std options for strict ISO C conformance) implies -trigraphs.
-no-integrated-cpp: Performs a compilation in two passes: preprocessing and compiling. This option allows a user supplied "cc1", "cc1plus", or "cc1obj"
via the -B option. The user supplied compilation step can then add in an additional preprocessing step after normal preprocessing but before compiling. The default is to use the integrated cpp (internal cpp); The semantics of this option will change if "cc1", "cc1plus", and
"cc1obj" are merged.
-traditional
-traditional-cpp: Formerly, these options caused GCC to attempt to emulate a prestandard 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++.
-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; but 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. Version 2 is the version of the C++ ABI that first appeared in G++ 3.4. Version 1 is the version of
the C++ ABI that first appeared in G++ 3.2. Version 0 will always be the version that conforms most closely to the C++ ABI specification. Therefore, the ABI obtained using version 0 will change as
ABI bugs are fixed.; The default is version 2.
-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" will only return 0 if it is declared tthhrrooww(()), in which case the compiler will always check the return value even without
this option. In all other cases, when "operator new" has a nonempty exception specification, memory exhaustion is signalled by
throwing "std::bad_alloc". See also new (nothrow).
-fconserve-space: Put uninitialized or runtime-initialized global variables into the common segment, as C does. This saves space in the executable at
the cost of not diagnosing duplicate definitions. If you compile
with this flag and your program mysteriously crashes after "main()" has completed, you may have an object that is being destroyed twice because two definitions were merged.; This option is no longer useful on most targets, now that support
has been added for putting variables into BSS without making them
common.
-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, and versions of G++ before 4.1 always worked that way. However, in ISO C++ a friend function
which is not declared in an enclosing scope can only be found using argument dependent lookup. This option causes friends to be
injected as they were in earlier releases.; This option is for compatibility, and may be removed in a future
release of G++.
-fno-const-strings: Give string constants type "char *" instead of type "const char *". By default, G++ uses type "const char *" as required by the standard. Even if you use -fno-const-strings, you cannot actually modify the value of a string constant.; This option might be removed in a future release of G++. For maximum portability, you should structure your code so that it works
with string constants that have type "const char *".
-fno-elide-constructors: The C++ standard allows an implementation to omit creating a temporary which 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 runtime. 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 will still optimize based on the specifications, so throwing an unexpected exception will result in undefined behavior.
-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++.; The default if neither flag is given 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 which 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 will 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 will cause linker errors if these functions are not inlined everywhere they are
called.
-fms-extensions: Disable pedantic 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.
-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 will allow some nonconforming code to compile.
-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++ runtime 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 it will generate it as needed.
-fstats: Emit statistics about front-end processing at the end of the compilation. This information is generally only useful to the G++
development team.
-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.
-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 will only work if your C library supports
"__cxa_atexit".
-fvisibility-inlines-hidden: Causes all inlined methods to be marked 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.; 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, or the
method has local static data, you might mark it as having default
visibility.
-fno-weak: Do not use weak symbol support, even if it is provided by the
linker. By default, G++ will use weak symbols if they are available. This option exists only for testing, and should not be used by end-users; it will result 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:
-fno-default-inline: Do not assume inline for functions defined inside a class scope.
Note that these functions will have linkage like inline func; tions; they just won't be inlined by default.
-Wabi (C++ only): Warn when G++ generates code that is probably not compatible with
the vendor-neutral C++ ABI. 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 will be 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.; The known incompatibilities at this point include:; * Incorrect handling of tail-padding for bit-fields. G++ may
attempt to pack data into the same byte as a base class. For
example:

struct A { virtual void f(); int f1 : 1; };
struct B : public A { int f2 : 1; };
In this case, G++ will place "B::f2" into the same byte as"A::f1"; other compilers will not. You can avoid this problem by explicitly padding "A" so that its size is a multiple of the byte size on your platform; that will cause G++ and other compilers to layout "B" identically.
* Incorrect handling of tail-padding for virtual bases. G++ does: not use tail padding when laying out virtual bases. For example:

struct A { virtual void f(); char c1; };
struct B { B(); char c2; };
struct C : public A, public virtual B {};
In this case, G++ will not place "B" into the tail-padding for "A"; other compilers will. You can avoid this problem by explicitly padding "A" so that its size is a multiple of its alignment (ignoring virtual base classes); that will cause G++ and other compilers to layout "C" identically.
* Incorrect handling of bit-fields with declared widths greater: than that of their underlying types, when the bit-fields appear in a union. For example:

union U { int i : 4096; };
Assuming that an "int" does not have 4096 bits, G++ will make the union too small by the number of bits in an "int".
* Empty classes can be placed at incorrect offsets. For example:: struct A {};; struct B {
A a;
virtual void f ();; };; struct C : public B, public A {};
G++ will place the "A" base class of "C" at a nonzero offset; it should be placed at offset zero. G++ mistakenly believes that the "A" data member of "B" is already at offset zero.
* Names of template functions whose types involve "typename" or: template template parameters can be mangled incorrectly.

template <typename Q>
void f(typename Q::X) {}

template <template <typename> class Q>
void f(typename Q<int>::X) {}
Instantiations of these templates may be mangled incorrectly.
-Wctor-dtor-privacy (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.
-Wnon-virtual-dtor (C++ only): Warn when a class appears to be polymorphic, thereby requiring a
virtual destructor, yet it declares a non-virtual one. This warning is enabled by -Wall.
-Wreorder (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 will rearrange 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.
The following -W... options are not affected by -Wall.
-Weffc++ (C++ only): Warn about violations of the following style guidelines from Scott Meyers' Effective C++ book:; * Item 11: Define a copy constructor and an assignment operator
for classes with dynamically allocated memory.; * Item 12: Prefer initialization to assignment in constructors.; * Item 14: Make destructors virtual in base classes.; * Item 15: Have "operator=" return a reference to *this.; * Item 23: Don't try to return a reference when you must return
an object.; Also warn about violations of the following style guidelines from
Scott Meyers' More Effective C++ book:; * Item 6: Distinguish between prefix and postfix forms of incre
ment and decrement operators.; * Item 7: Never overload "&&", "||", or ",".; 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.
-Wno-deprecated (C++ only): Do not warn about usage of deprecated features.
-Wstrict-null-sentinel (C++ only): Warn also 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 not a null pointer, it is guaranteed to of the same size as a pointer.
But this use is not portable across different compilers.
-Wno-non-template-friend (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 unqualifiedid (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++ 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++ 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();
will fail to compile.
-Wno-pmf-conversions (C++ only): Disable the diagnostic for converting a bound pointer to member
function to a plain pointer.
-Wsign-promo (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++ would try to preserve unsignedness, but the standard mandates the current behavior.

struct A {
operator int ();
A& operator = (int);

};

main ()
{

A a,b;
a = b;

}
In this example, G++ will synthesize a default A& operator = (const A&);, while cfront will use the user-defined operator =.
Options Controlling Objective-C and Objective-C++ Dialects
(NOTE: This manual does not describe the Objective-C and Objective-C++ languages themselves. See
This section describes the command-line options that are only meaningful for Objective-C and Objective-C++ programs, but 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, will override 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 (e.g., "[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. Currently, this option is only available in conjunction with the NeXT runtime on Mac OS X 10.3 and later.
-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 that
will run 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 that will run all such default destructors, in reverse
order.; The "- (id) .cxx_construct" and/or "- (void) .cxx_destruct" methods thusly generated will 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 will be invoked by the runtime immediately after a new object instance is allocated; the "- (void) .cxx_destruct" methods will be 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. Currently, this option is only available in conjunction with the NeXT runtime on Mac OS X 10.3 and later.

@try {
...
@throw expr;

...

}
@catch (AnObjCClass *exc) {

...

@throw expr;

...

@throw;

...

}
@catch (AnotherClass *exc) {

...

}
@catch (id allOthers) {

...

}
@finally {

...

@throw expr;

...

}
The @throw statement may appear anywhere in an Objective-C or Objective-C++ program; when used inside of a @catch block, the @throw may appear without an argument (as shown above), in which case the object caught by the @catch will be rethrown.
Note that only (pointers to) Objective-C objects may be thrown and caught using this scheme. When an object is thrown, it will be caught by the nearest @catch clause capable of handling objects of that type, analogously to how "catch" blocks work in C++ and Java. A "@catch(id ...)" clause (as shown above) may also be provided to catch any and all Objective-C exceptions not caught by previous @catch clauses (if any).
The @finally clause, if present, will be executed upon exit from the immediately preceding "@try ... @catch" section. This will happen regardless of whether any exceptions are thrown, caught or rethrown inside the "@try ... @catch" section, analogously to the behavior of the "finally" clause in Java.
There are several caveats to using the new exception mechanism:
* Although currently designed to be binary compatible with: "NS_HANDLER"-style idioms provided by the "NSException" class, the new exceptions can only be used on Mac OS X 10.3 (Panther) and later systems, due to additional functionality needed in
the (NeXT) Objective-C runtime.
* As mentioned above, the new exceptions do not support handling: types other than Objective-C objects. Furthermore, when used from Objective-C++, the Objective-C exception model does not
interoperate with C++ exceptions at this time. This means you cannot @throw an exception from Objective-C and "catch" it in
C++, or vice versa (i.e., "throw ... @catch").
The -fobjc-exceptions switch also enables the use of synchronization blocks for thread-safe execution:: @synchronized (ObjCClass *guard) {
...; }
Upon entering the @synchronized block, a thread of execution shall first check whether a lock has been placed on the corresponding "guard" object by another thread. If it has, the current thread shall wait until the other thread relinquishes its lock. Once "guard" becomes available, the current thread will place its own lock on it, execute the code contained in the @synchronized block, and finally relinquish the lock (thereby making "guard" available to other threads).
Unlike Java, Objective-C does not allow for entire methods to be marked @synchronized. Note that throwing exceptions out of @synchronized blocks is allowed, and will cause the guarding object to be unlocked properly.
-fobjc-gc: Enable garbage collection (GC) in Objective-C and Objective-C++
programs.
-freplace-objc-classes: Emit a special marker instructing lldd(1) not to statically link in the resulting object file, and allow ddyylldd(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.
-gen-decls: Dump interface declarations for all classes seen in the source file to a file named sourcename.decl.
-Wassign-intercept: Warn whenever an Objective-C assignment is being intercepted by the garbage collector.
-Wno-protocol: 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: 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: 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 will omit such warnings if any differences found are confined to
types which share the same size and alignment.
-Wundeclared-selector: 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, ...). The options described below can be used to control the diagnostic messages formatting algorithm, e.g. how many characters per line, how often source location information should be reported. Right now, only the C++ front end can honor these options. However it is expected, in the near future, that the remaining front ends would be able to digest them correctly.
-fmessage-length=n: Try to format error messages so that they fit on lines of about n characters. The default is 72 characters for g++ and 0 for the rest of the front ends supported by GCC. If n is zero, then no
line-wrapping will be done; each error message will appear on a
single line.
-fdiagnostics-show-location=once: Only meaningful in line-wrapping mode. Instructs the diagnostic
messages reporter to emit once source location information; 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-show-options: This option instructs the diagnostic machinery to add text to each diagnostic emitted, which indicates which command line option
directly controls that diagnostic, when such an option is known to the diagnostic machinery.
Options to Request or Suppress Warnings
Warnings are diagnostic messages that report constructions which are not inherently erroneous but which are risky or suggest there may have been an error.
You can request many specific warnings with options beginning -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.
The following options control the amount and kinds of warnings produced by GCC; for further, language-specific options also refer to C++ Dialect Options and Objective-C and Objective-C++ Dialect Options.
-fsyntax-only: Check the code for syntax errors, but don't do anything beyond
that.
-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 will 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.; -pedantic 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 -pedantic 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 -pedantic. 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 gnu89 or gnu99, there is a corresponding base standard, the version of ISO C on which the GNU extended dialect is based. Warnings from -pedantic are given where they are required by the base standard. (It would 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: Like -pedantic, except that errors are produced rather than warnings.
-w Inhibit all warning messages.
-Wno-import: Inhibit warning messages about the use of #import.
-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.
-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.
-Wformat: 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
-pedantic is used with -Wformat, warnings will be 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).; Since -Wformat also checks for null format arguments for several functions, -Wformat also implies -Wnonnull.; -Wformat is included in -Wall. For more control over some aspects of format checking, the options -Wformat-y2k, -Wno-format-extra-args, -Wno-format-zero-length, -Wformat-nonliteral, -Wformat-security, and -Wformat=2 are available, but are not included in -Wall.
-Wformat-y2k: If -Wformat is specified, also warn about "strftime" formats which may yield only a two-digit year.
-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 will suppress 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-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=2: Enable -Wformat plus format checks not included in -Wformat. Currently equivalent to -Wformat -Wformat-nonliteral -Wformat-security -Wformat-y2k.
-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 which are initialized with themselves. Note this option can only be used with the -Wuninitialized option, which in turn only works with -O1 and above.; For example, GCC will warn about "i" being uninitialized in the
following snippet only when -Winit-self has been specified:

int f()
{
int i = i;
return i;

}
-Wimplicit-int: Warn when a declaration does not specify a type. This warning is
enabled by -Wall.
-Wimplicit-function-declaration -Werror-implicit-function-declaration: Give a warning (or error) whenever a function is used before being declared. The form -Wno-error-implicit-function-declaration is not supported. This warning is enabled by -Wall (as a warning, not an error).
-Wimplicit: Same as -Wimplicit-int and -Wimplicit-function-declaration. This warning is enabled by -Wall.
-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 -Wall.
-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.

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. Only the warning for an assignment used as a truth value is supported when compiling C++; the other warnings are only supported when compiling C.; 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, 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 will issue 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" could belong to the enclosing "if". The resulting code would look like this:: {
if (a)
{
if (b)
foo ();

else

bar ();

}

}
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 standard.; The C standard defines the order in which expressions in a 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 standard specifies 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 present implementation of this option only works for C programs. A future implementation may also work for C++ programs.; The C 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.
-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".; For C, also warn if the return type of a function has a type qualifier such as "const". Such a type qualifier has no effect, since
the value returned by a function is not an lvalue. ISO C prohibits qualified "void" return types on function definitions, so such
return types always receive a warning even without this option.; 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.
-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. 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.
-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-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-parameter: Warn whenever a function parameter is unused aside from its declaration.; To suppress this warning use the unused attribute.
-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. This warning is enabled by -Wall.; To suppress this warning cast the expression to void.
-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.; These warnings are possible only in optimizing compilation, because they require data flow information that is computed only when optimizing. If you don't specify -O, you simply won't get these warnings.; If you want to warn about code which 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 which 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 will depend 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.; These warnings are made optional because GCC is not smart enough to see all the reasons why the code might be correct despite 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. Here is another common case:: {
int save_y;
if (change_y) save_y = y, y = new_y;
...
if (change_y) y = save_y;; }
This has no bug because "save_y" is used only if it is set.
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 which 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.
-Wunknown-pragmas: Warn when a #pragma directive is encountered which is not understood by GCC. If this command line option is used, warnings will
even be issued for unknown pragmas in system header files. This is not the case if the warnings were 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 which 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.
-Wstrict-aliasing=2: This option is only active when -fstrict-aliasing is active. It warns about code which might break the strict aliasing rules that
the compiler is using for optimization. This warning catches more cases than -Wstrict-aliasing, but it will also give a warning for some ambiguous cases that are safe.
-Wall: All of the above -W options combined. 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.
The following -W... options 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.
-Wextra: (This option used to be called -W. The older name is still supported, but the newer name is more descriptive.) Print extra warning messages for these events:; * A function can return either with or without a value. (Falling
off the end of the function body is considered returning without a value.) For example, this function would evoke such a
warning:

foo (a)
{
if (a > 0)
return a;

}
* An expression-statement or the left-hand side of a comma: expression contains no side effects. To suppress the warning, cast the unused expression to void. For example, an expression such as x[i,j] will cause a warning, but x[(void)i,j] will not.
* An unsigned value is compared against zero with < or >=.
* Storage-class specifiers like "static" are not the first things: in a declaration. According to the C Standard, this usage is
obsolescent.
* If -Wall or -Wunused is also specified, warn about unused argu: ments.
* A comparison between signed and unsigned values could produce: an incorrect result when the signed value is converted to
unsigned. (But don't warn if -Wno-sign-compare is also specified.)
* An aggregate has an initializer which does not initialize all: members. This warning can be independently controlled by
-Wmissing-field-initializers.
* A function parameter is declared without a type specifier in: K&R-style functions:

void foo(bar) { }
* An empty body occurs in an if or else statement.
* A pointer is compared against integer zero with <, <=, >, or: >=.
* A variable might be changed by longjmp or vfork.
* Any of several floating-point events that often indicate: errors, such as overflow, underflow, loss of precision, etc.
*<(C++ only)>: An enumerator and a non-enumerator both appear in a conditional expression.
*<(C++ only)>: A non-static reference or non-static const member appears in a class without constructors.
*<(C++ only)>: Ambiguous virtual bases.
*<(C++ only)>: Subscripting an array which has been declared register.
*<(C++ only)>: Taking the address of a variable which has been declared register.
*<(C++ only)>: A base class is not initialized in a derived class' copy constructor.
-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 will not warn about unknown pragmas in system headers---for that, -Wunknown-pragmas must also be used.
-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 would 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 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 which
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 does not in ISO C.; * In traditional C, some preprocessor directives did not exist.
Traditional preprocessors would only consider 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 would ignore 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 would not recognize
#elif, so it 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 sepa
rate 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 would cause serious problems. This is a
subset of the possible conversion warnings, for the full set
use -Wconversion.; * Use of ISO C style function definitions. This warning inten
tionally is not issued for prototype declarations or variadic functions because these ISO C features will 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.
-Wdeclaration-after-statement (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 and was not supported by GCC versions before GCC 3.0.
-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 shadows another local variable,
parameter or global variable or whenever a built-in function is
shadowed.
-Wlarger-than-len: Warn whenever an object of larger than len bytes is defined.
-Wunsafe-loop-optimizations: Warn if the loop cannot be optimized because the compiler could not assume anything on the bounds of the loop indices. With -funsafe-loop-optimizations warn if the compiler made such assumptions.
-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.
-Wbad-function-cast (C only): Warn whenever a function call is cast to a non-matching type. For example, warn if "int malloc()" is cast to "anything *".
-Wc++-compat: 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.
-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 *".
-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 will get a warning; when compiling C++, warn about the deprecated conversion from string constants to "char *". These warnings will 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 will just be a nuisance; this is why we did not make -Wall request these warnings.
-Wconversion: 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.; Also, warn if a negative integer constant expression is implicitly converted to an unsigned type. For example, warn about the assignment "x = -1" if "x" is unsigned. But do not warn about explicit
casts like "(unsigned) -1".
-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.
-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-attributes: Do not warn if an unexpected "__attribute__" is used, such as
unrecognized attributes, function attributes applied to variables, etc. This will not stop errors for incorrect use of supported
attributes.
-Wstrict-prototypes (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 which specifies the argument types.)
-Wold-style-definition (C only): Warn if an old-style function definition is used. A warning is
given even if there is a previous prototype.
-Wmissing-prototypes (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. The aim is to detect global functions that
fail to be declared in header files.
-Wmissing-declarations (C only): 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.
-Wmissing-field-initializers: Warn if a structure's initializer has some fields missing. For
example, the following code would cause 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 would not trigger a warning:: struct s { int f, g, h; };
struct s x = { .f = 3, .g = 4 };
This warning is included in -Wextra. To get other -Wextra warnings without this one, use -Wextra -Wno-missing-field-initializers.
-Wmissing-noreturn: Warn about functions which might be candidates for attribute "noreturn". Note these are only possible candidates, not absolute ones. Care should be taken to manually verify functions actually do not
ever return before adding the "noreturn" attribute, otherwise subtle code generation bugs could be introduced. You will not get a
warning for "main" in hosted C environments.
-Wmissing-format-attribute: Warn about function pointers which might be candidates for "format" attributes. Note these are only possible candidates, not absolute ones. GCC will guess 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 will also warn about function definitions which might be candidates for "format" attributes. Again, these are only possible candidates. GCC will guess 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.
-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 which have not been normalized; this option controls
that warning.; There are four levels of warning that GCC supports. The default is -Wnormalized=nfc, which warns about any identifier which is not in the ISO 10646 "C" normalized form, NFC. NFC is the recommended form for most uses.; Unfortunately, there are some characters which ISO C and ISO C++
allow in identifiers that when turned into NFC aren't allowable as 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. You would only want to do this if you were 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", will display just like a regular "n" which
has been placed in a superscript. ISO 10646 defines the NFKC normalisation scheme to convert all these into a standard form as
well, and GCC will warn 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 is unable to be fixed to display these characters distinctly.
-Wno-deprecated-declarations: Do not warn about uses of functions, variables, and types marked as deprecated by using the "deprecated" attribute. (@pxref{Function
Attributes}, @pxref{Variable Attributes}, @pxref{Type Attributes}.)
-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" will be 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;

};
-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 only): Warn if an "extern" declaration is encountered within a function.
-Wunreachable-code: Warn if the compiler detects that code will never be executed.; This option is intended to warn when the compiler detects that at
least a whole line of source code will never be executed, because
some condition is never satisfied or because it is after a procedure that never returns.; It is possible for this option to produce a warning even though
there are circumstances under which part of the affected line can
be executed, so care should be taken when removing apparentlyunreachable code.; For instance, when a function is inlined, a warning may mean that
the line is unreachable in only one inlined copy of the function.; This option is not made part of -Wall because in a debugging version of a program there is often substantial code which checks correct functioning of the program and is, hopefully, unreachable
because the program does work. Another common use of unreachable
code is to provide behavior which is selectable at compile-time.
-Winline: Warn if a function can not be inlined and it was declared as
inline. Even with this option, the compiler will 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++ only): Suppress warnings from applying the offsetof macro to a non-POD type. According to the 1998 ISO C++ standard, applying offsetof to a non-POD type is undefined. In existing C++ implementations, however, offsetof typically gives meaningful results even when applied to certain kinds of non-POD types. (Such as a simple struct that fails to be a POD type only by virtue of having a constructor.)
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 (C only): Suppress warnings from casts to pointer type of an integer of a
different size.
-Wno-pointer-to-int-cast (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 default. To inhibit the warning messages, use -Wno-long-long. Flags -Wlong-long and -Wno-long-long are taken into account only when -pedantic flag is used.
-Wvariadic-macros: Warn if variadic macros are used in pedantic ISO C90 mode, or the
GNU alternate syntax when in pedantic ISO C99 mode. This is
default. To inhibit the warning messages, use -Wno-variadic-macros.
-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.
-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 were unable to handle the code effectively. Often, the problem is that your code is too big or too complex; GCC will refuse to optimize programs when
the optimization itself is likely to take inordinate amounts of
time.
-Wpointer-sign: 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 -pedantic, which can be disabled with -Wno-pointer-sign.
-Werror: Make all warnings into errors.
-Wstack-protector: This option is only active when -fstack-protector is active. It warns about functions that will not be protected against stack
smashing.
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 for: mat (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 will probably make 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 were already at hand; some statements may execute in different places because
they were 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.
-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.
-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 which 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.
-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-2: Produce debugging information in DWARF version 2 format (if that is supported). This is the format used by DBX on IRIX 6. With this
option, GCC uses features of DWARF version 3 when they are useful;
version 3 is upward compatible with version 2, but may still cause problems for older debuggers.
-gvms: Produce debugging information in VMS debug format (if that is supported). This is the format used by DEBUG on 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 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, but no
information about local variables and no line numbers.; 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 DWARF2.
-feliminate-dwarf2-dups: Compress DWARF2 debugging information by eliminating duplicated
information about each symbol. This option only makes sense when
generating DWARF2 debugging information with -gdwarf-2.
-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.
-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 profiledirected 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.; @bullet
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.; @cvmmfu
Link your object files with -lgcov or -fprofile-arcs (the latter implies the former).; @dwnngv
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).; @exoohw
For profile-directed optimizations, compile the source files
again with the same optimization and code generation options
plus -fbranch-probabilities.; @fyppix
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 will match the source files more closely, if
you do not optimize.
-dletters
-fdump-rtl-pass: 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. 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.; Most debug dumps can be enabled either passing a letter to the -d option, or with a long -fdump-rtl switch; here are the possible letters for use in letters and pass, and their meanings:; -dA Annotate the assembler output with miscellaneous debugging
information.; -db
-fdump-rtl-bp
Dump after computing branch probabilities, to file.09.bp.; -dB
-fdump-rtl-bbro
Dump after block reordering, to file.30.bbro.; -dc
-fdump-rtl-combine
Dump after instruction combination, to the file file.17.combine.; -dC
-fdump-rtl-ce1
-fdump-rtl-ce2
-dC and -fdump-rtl-ce1 enable dumping after the first if conversion, to the file file.11.ce1. -dC and -fdump-rtl-ce2 enable dumping after the second if conversion, to the file
file.18.ce2.; -dd
-fdump-rtl-btl
-fdump-rtl-dbr
-dd and -fdump-rtl-btl enable dumping after branch target load optimization, to file.31.btl. -dd and -fdump-rtl-dbr enable dumping after delayed branch scheduling, to file.36.dbr.; -dD Dump all macro definitions, at the end of preprocessing, in
addition to normal output.; -dE
-fdump-rtl-ce3
Dump after the third if conversion, to file.28.ce3.; -df
-fdump-rtl-cfg
-fdump-rtl-life
-df and -fdump-rtl-cfg enable dumping after control and data flow analysis, to file.08.cfg. -df and -fdump-rtl-cfg enable dumping dump after life analysis, to file.16.life.; -dg
-fdump-rtl-greg
Dump after global register allocation, to file.23.greg.; -dG
-fdump-rtl-gcse
-fdump-rtl-bypass
-dG and -fdump-rtl-gcse enable dumping after GCSE, to file.05.gcse. -dG and -fdump-rtl-bypass enable dumping after jump bypassing and control flow optimizations, to
file.07.bypass.; -dh
-fdump-rtl-eh
Dump after finalization of EH handling code, to file.02.eh.; -di
-fdump-rtl-sibling
Dump after sibling call optimizations, to file.01.sibling.; -dj
-fdump-rtl-jump
Dump after the first jump optimization, to file.03.jump.; -dk
-fdump-rtl-stack
Dump after conversion from registers to stack, to
file.33.stack.; -dl
-fdump-rtl-lreg
Dump after local register allocation, to file.22.lreg.; -dL
-fdump-rtl-loop
-fdump-rtl-loop2
-dL and -fdump-rtl-loop enable dumping after the first loop optimization pass, to file.06.loop. -dL and -fdump-rtl-loop2 enable dumping after the second pass, to file.13.loop2.; -dm
-fdump-rtl-sms
Dump after modulo scheduling, to file.20.sms.; -dM
-fdump-rtl-mach
Dump after performing the machine dependent reorganization
pass, to file.35.mach.; -dn
-fdump-rtl-rnreg
Dump after register renumbering, to file.29.rnreg.; -dN
-fdump-rtl-regmove
Dump after the register move pass, to file.19.regmove.; -do
-fdump-rtl-postreload
Dump after post-reload optimizations, to file.24.postreload.; -dr
-fdump-rtl-expand
Dump after RTL generation, to file.00.expand.; -dR
-fdump-rtl-sched2
Dump after the second scheduling pass, to file.32.sched2.; -ds
-fdump-rtl-cse
Dump after CSE (including the jump optimization that sometimes follows CSE), to file.04.cse.; -dS
-fdump-rtl-sched
Dump after the first scheduling pass, to file.21.sched.; -dt
-fdump-rtl-cse2
Dump after the second CSE pass (including the jump optimization that sometimes follows CSE), to file.15.cse2.; -dT
-fdump-rtl-tracer
Dump after running tracer, to file.12.tracer.; -dV
-fdump-rtl-vpt
-fdump-rtl-vartrack
-dV and -fdump-rtl-vpt enable dumping after the value profile transformations, to file.10.vpt. -dV and -fdump-rtl-vartrack enable dumping after variable tracking, to file.34.vartrack.; -dw
-fdump-rtl-flow2
Dump after the second flow pass, to file.26.flow2.; -dz
-fdump-rtl-peephole2
Dump after the peephole pass, to file.27.peephole2.; -dZ
-fdump-rtl-web
Dump after live range splitting, to file.14.web.; -da
-fdump-rtl-all
Produce all the dumps listed above.; -dH Produce a core dump whenever an error occurs.; -dm Print statistics on memory usage, at the end of the run, to
standard error.; -dp Annotate the assembler output with a comment indicating which
pattern and alternative was 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.; -dv For each of the other indicated dump files (either with -d or
-fdump-rtl-pass), dump a representation of the control flow graph suitable for viewing with VCG to file.pass.vcg.; -dx Just generate RTL for a function instead of compiling it. Usu
ally used with r (-fdump-rtl-expand).; -dy Dump debugging information during parsing, to standard error.
-fdump-unnumbered: When doing debugging dumps (see -d option above), suppress instruction numbers and line number note 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-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. 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. 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. The following
dumps are possible:; all Enables all inter-procedural analysis dumps; currently the only
produced dump is the cgraph dump.; cgraph
Dumps information about call-graph optimization, unused function removal, and inlining decisions.
-fdump-tree-switch
-fdump-tree-switch-options: 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. If the
-options form is used, options is a list of - separated options that control the details of the dump. Not all options are applicable to all dumps, those which are not meaningful will be 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.; slim
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.; 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).; stats
Enable dumping various statistics about the pass (not honored
by every dump option).; blocks
Enable showing basic block boundaries (disabled in raw dumps).; 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.; all Turn on all options, except raw, slim and lineno.; 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.; inlined
Dump after function inlining, to file.inlined.; 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.; vcg Dump the control flow graph of each function to a file in VCG
format. The file name is made by appending .vcg to the source file name. Note that if the file contains more than one function, the generated file cannot be used directly by VCG. You
will need to cut and paste each function's graph into its own
separate file first.; 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.; salias
Dump structure aliasing variable information to a file. This
file name is made by appending .salias 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 append
ing .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.; mudflap
Dump each function after adding mudflap instrumentation. The
file name is made by appending .mudflap 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 opti
mization 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.; 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.
-ftree-vectorizer-verbose=n: This option controls the amount of debugging output the vectorizer prints. This information is written to standard error, unless
-fdump-tree-all or -fdump-tree-vect is specified, in which case it is output to the usual dump listing file, .vect.
-frandom-seed=string: This option provides a seed that GCC uses when it would otherwise
use random numbers. It is used to generate 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 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 -dS or -dR is specified, in which case it is output to the usual dump listing file, .sched 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 -dRS. 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, controlflow and regions info. And for n over four, -fsched-verbose also includes dependence info.
-save-temps: 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 would produce 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.
-time: 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). 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.
-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.
-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 @samp{-, without spaces between multiple switches. This is supposed to ease shell-processing.
-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 will search---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 /.
-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.)
-feliminate-unused-debug-types: Normally, when producing DWARF2 output, GCC will emit debugging
information for all types declared in a compilation unit, regardless of whether or not they are actually used in that compilation
unit. Sometimes this is useful, such as 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. With this option, GCC will
avoid producing debug symbol output for types that are nowhere used in the source file being compiled.
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 would 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. Optimization levels -O and above, in particular, enable unitat-a-time mode, which allows the compiler to consider information gained from later functions in the file when compiling a function. Compiling multiple files at once to a single output file in unit-at-atime 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.
-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: -fdefer-pop -fdelayed-branch -fguess-branch-probability -fcprop-registers -floop-optimize -fif-conversion -fif-conversion2 -ftree-ccp -ftree-dce -ftree-dominator-opts -ftree-dse -ftree-ter -ftree-lrs -ftree-sra -ftree-copyrename -ftree-fre -ftree-ch -funit-at-a-time -fmerge-constants; -O also turns on -fomit-frame-pointer on machines where doing so does not interfere with debugging.; -O doesn't turn on -ftree-sra for the Ada compiler. This option must be explicitly specified on the command line to be enabled for the Ada compiler.
-O2 Optimize even more. GCC performs nearly all supported optimiza: tions that do not involve a space-speed tradeoff. The compiler
does not perform loop unrolling or function inlining when you specify -O2. 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 -fcrossjumping -foptimize-sibling-calls -fcse-follow-jumps -fcse-skip-blocks -fgcse -fgcse-lm -fexpensive-optimizations -fstrength-reduce -frerun-cse-after-loop -frerun-loop-opt -fcaller-saves -fpeephole2 -fschedule-insns -fschedule-insns2 -fsched-interblock -fsched-spec -fregmove -fstrict-aliasing -fdelete-null-pointer-checks -freorder-blocks -freorder-functions -falign-functions -falign-jumps -falign-loops -falign-labels -ftree-vrp -ftree-pre; 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 and -fgcse-after-reload options.
-O0 Do not optimize. 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 -ftree-vect-loop-version; 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 would be -fno-foo. In the table below, only one of the forms is listed---the one you typically will 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-default-inline: Do not make member functions inline by default merely because they are defined inside the class scope (C++ only). Otherwise, when you specify -O, member functions defined inside class scope are compiled inline by default; i.e., you don't need to add inline in front of the member function name.
-fno-defer-pop: Always pop the arguments to each function call as soon as that
function returns. For machines which 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.
-fforce-mem: Force memory operands to be copied into registers before doing
arithmetic on them. This produces better code by making all memory references potential common subexpressions. When they are not common subexpressions, instruction combination should eliminate the
separate register-load. This option is now a nop and will be
removed in 4.2.
-fforce-addr: Force memory address constants to be copied into registers before
doing arithmetic on them.
-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.; Enabled at levels -O, -O2, -O3, -Os.
-foptimize-sibling-calls: Optimize sibling and tail recursive calls.; Enabled at levels -O2, -O3, -Os.
-fno-inline: Don't pay attention to the "inline" keyword. Normally this option is used to keep the compiler from expanding any functions inline.
Note that if you are not optimizing, no functions can be expanded
inline.
-finline-functions: Integrate all simple functions into their callers. The compiler
heuristically decides which functions are simple enough to be 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 if -funit-at-a-time is enabled.
-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.
-finline-limit=n: By default, GCC limits the size of functions that can be inlined.
This flag allows the control of this limit for functions that are
explicitly marked as inline (i.e., marked with the inline keyword
or defined within the class definition in c++). n is the size of functions that can be inlined in number of pseudo instructions (not counting parameter handling). The default value of n is 600.
Increasing this value can result in more inlined code at the cost
of compilation time and memory consumption. Decreasing usually
makes the compilation faster and less code will be inlined (which
presumably means slower programs). This option is particularly
useful for programs that use inlining heavily such as those based
on recursive templates with C++.; 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 I<n>/2.; max-inline-insns-auto
is set to I<n>/2.; min-inline-insns
is set to 130 or I<n>/4, whichever is smaller.; max-inline-insns-rtl
is set to I<n>.; See below for a documentation of the individual parameters controlling inlining.; 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.
-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 C. 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 the variable was 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 non-automatic variable to have distinct location, so using this option will result 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.
-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.; The default is -fbranch-count-reg, enabled when -fstrength-reduce is enabled.
-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.
-fmudflap -fmudflapth -fmudflapir: For front-ends that support it (C and C++), instrument all risky
pointer/array dereferencing operations, some standard library
string/heap functions, and some other associated constructs with
range/validity tests. Modules so instrumented should be immune to buffer overflows, invalid heap use, and some other classes of C/C++ programming errors. The instrumentation relies on a separate runtime library (libmudflap), which will be linked into a program if -fmudflap is given at link time. Run-time behavior of the instrumented program is controlled by the MUDFLAP_OPTIONS environment variable. See "env MUDFLAP_OPTIONS=-help a.out" for its options.; Use -fmudflapth instead of -fmudflap to compile and to link if your program is multi-threaded. Use -fmudflapir, in addition to -fmudflap or -fmudflapth, if instrumentation should ignore pointer reads. This produces less instrumentation (and therefore faster
execution) and still provides some protection against outright memory corrupting writes, but allows erroneously read data to propagate within a program.
-fstrength-reduce: Perform the optimizations of loop strength reduction and elimination of iteration variables.; Enabled at levels -O2, -O3, -Os.
-fthread-jumps: Perform optimizations where we 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.
-fcse-follow-jumps: In common subexpression elimination, 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 will follow 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 which 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
has been performed.; Enabled at levels -O2, -O3, -Os.
-frerun-loop-opt: Run the loop optimizer twice.; 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 runtime 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 will attempt to move loads which 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 gcse is enabled.
-fgcse-sm: When -fgcse-sm is enabled, a store motion pass is run after global common subexpression elimination. This pass will attempt 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
cleanup redundant spilling.
-floop-optimize: Perform loop optimizations: move constant expressions out of loops, simplify exit test conditions and optionally do strength-reduction as well.; Enabled at levels -O, -O2, -O3, -Os.
-floop-optimize2: Perform loop optimizations using the new loop optimizer. The optimizations (loop unrolling, peeling and unswitching, loop invariant motion) are enabled by separate flags.
-funsafe-loop-optimizations: If given, the loop optimizer will assume that loop indices do not
overflow, and that the 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. Using -Wunsafe-loop-optimizations, the compiler will warn you if it finds this kind of loop.
-fcrossjumping: Perform cross-jumping transformation. This transformation unifies equivalent code and save code size. The resulting code may or may not perform better than without cross-jumping.; Enabled at levels -O2, -O3, -Os.
-fif-conversion: Attempt to transform conditional jumps into branch-less equivalents. This include 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 "if-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.
-fdelete-null-pointer-checks: Use global dataflow analysis to identify and eliminate useless
checks for null pointers. The compiler assumes that dereferencing a null pointer would have halted the program. If a pointer is
checked after it has already been dereferenced, it cannot be null.; In some environments, this assumption is not true, and programs can safely dereference null pointers. Use
-fno-delete-null-pointer-checks to disable this optimization for programs which depend on that behavior.; Enabled at levels -O2, -O3, -Os.
-fexpensive-optimizations: Perform a number of minor optimizations that are relatively expensive.; Enabled at levels -O2, -O3, -Os.
-foptimize-register-move -fregmove: Attempt to reassign register numbers in move instructions and as
operands of other simple instructions in order to maximize the
amount of register tying. This is especially helpful on machines
with two-operand instructions.; Note -fregmove and -foptimize-register-move are the same optimization.; 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, -Os.
-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-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-fsched-stalled-insns and -fsched-stalled-insns=0 are equivalent and mean that no insns will be moved prematurely. If n is unspecified then there is no limit on how many queued insns can be moved prematurely.
-fsched-stalled-insns-dep -fsched-stalled-insns-dep=n: Define how many insn groups (cycles) will be examined for a dependency on a stalled insn that is 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 and its value is not zero. +-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, do use superblock scheduling algorithm. Superblock scheduling allows motion across basic block boundaries resulting on 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.
-fsched2-use-traces: Use -fsched2-use-superblocks algorithm when scheduling after register allocation and additionally perform code duplication in order
to increase the size of superblocks using tracer pass. See
-ftracer for details on trace formation.; This mode should produce faster but significantly longer programs. Also without -fbranch-probabilities the traces constructed may not match the reality and hurt the performance. This only makes sense when scheduling after register allocation, i.e. with -fschedule-insns2 or at -O2 or higher.
-freschedule-modulo-scheduled-loops: The modulo scheduling comes before the traditional scheduling, if a loop was modulo scheduled we may want to prevent the later scheduling passes from changing its schedule, we use this option to control that.
-fcaller-saves: Enable values to be allocated in registers that will be 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 than would otherwise be produced.; 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.
-ftree-pre: Perform Partial Redundancy Elimination (PRE) on trees. This flag
is enabled by default at -O2 and -O3.
-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 faster than PRE, though it exposes fewer redundancies.
This flag is enabled by default at -O 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.
-ftree-store-copy-prop: Perform copy propagation of memory loads and stores. This pass
eliminates unnecessary copy operations in memory references (structures, global variables, arrays, etc). This flag is enabled by
default at -O2 and higher.
-ftree-salias: Perform structural alias analysis on trees. This flag is enabled
by default at -O and higher.
-ftree-sink: Perform forward store motion on trees. This flag is enabled by
default at -O and higher.
-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.
-ftree-store-ccp: Perform sparse conditional constant propagation (CCP) on trees.
This pass operates on both local scalar variables and memory stores and loads (global variables, structures, arrays, etc). This flag
is enabled by default at -O2 and higher.
-ftree-dce: Perform dead code elimination (DCE) on trees. This flag is enabled by default at -O and higher.
-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-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 linear loop transformations on tree. This flag can improve cache performance and allow further loop optimizations to take
place.
-ftree-loop-im: Perform loop invariant motion on trees. This pass moves only
invariants that would be 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 the loop for that 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-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-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-lrs: Perform live range splitting during the SSA->normal phase. Distinct live ranges of a variable are split into unique variables,
allowing for better optimization later. This is enabled by default at -O and higher.
-ftree-vectorize: Perform loop vectorization on trees.
-ftree-vect-loop-version: Perform loop versioning when doing loop vectorization on trees.
When a loop appears to be vectorizable except that data alignment
or data dependence cannot be determined at compile time then vectorized and non-vectorized versions of the loop are generated along with runtime checks for alignment or dependence to control which
version is executed. This option is enabled by default except at
level -Os where it is disabled.
-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.
-ftracer: Perform tail duplication to enlarge superblock size. This transformation simplifies the control flow of the function allowing
other optimizations to do better job.
-funroll-loops: Unroll loops whose number of iterations can be determined at compile time or upon entry to the loop. -funroll-loops implies both -fstrength-reduce and -frerun-cse-after-loop. This option makes code larger, and may or may not make it run faster.
-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,
-fsplit-ivs-in-unroller: Enables expressing 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.; Combination of -fweb and CSE is often sufficient to obtain the same effect. However in cases the loop body is more complicated than a single basic block, this is not reliable. It also does not work at all on some of the architectures due to restrictions in the CSE
pass.; This optimization is enabled by default.
-fvariable-expansion-in-unroller: With this option, the compiler will create multiple copies of some local variables when unrolling a loop which can result in superior code.
-fprefetch-loop-arrays: If supported by the target machine, generate instructions to
prefetch memory to improve the performance of loops that access
large arrays.; These options may generate better or worse code; results are highly dependent on the structure of loops within the source code.
-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 will use 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 will be 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.
-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 in to make this option
effective. See -fprofile-arcs for details.; Enabled at levels -O2, -O3, -Os.
-fstrict-aliasing: Allows 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() {

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 will
work as expected. However, this code might not:: int f() {
a_union t;
int* ip;
t.d = 3.0;
ip = &t.i;
return *ip;; }
Every language that wishes to perform language-specific alias analysis should define a function that computes, given an "tree" node, an alias set for the node. Nodes in different alias sets are not allowed to alias. For an example, see the C front-end function "c_get_alias_set".
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 would align 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 will not be 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 will not be 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. The hope is that the loop will be executed many times, which will make up for any execution of the dummy operations.; -fno-align-loops and -falign-loops=1 are equivalent and mean that loops will not be 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 will not be aligned.; If n is not specified or is zero, use a machine-dependent default.; Enabled at levels -O2, -O3.
-funit-at-a-time: Parse the whole compilation unit before starting to produce code.
This allows some extra optimizations to take place but consumes
more memory (in general). There are some compatibility issues with unit-at-at-time mode:; * enabling unit-at-a-time mode may change the order in which
functions, variables, and top-level "asm" statements are emitted, and will likely break code relying on some particular
ordering. The majority of such top-level "asm" statements,
though, can be replaced by "section" attributes.; * unit-at-a-time mode removes unreferenced static variables and
functions. This may result in undefined references when an
"asm" statement refers directly to variables or functions that are otherwise unused. In that case either the variable/function shall be listed as an operand of the "asm" statement operand or, in the case of top-level "asm" statements the attribute "used" shall be used on the declaration.; * Static functions now can use non-standard passing conventions
that may break "asm" statements calling functions directly.
Again, attribute "used" will prevent this behavior.; As a temporary workaround, -fno-unit-at-a-time can be used, but this scheme may not be supported by future releases of GCC.; Enabled at levels -O, -O2, -O3, -Os.
-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 will no longer stay in a
"home register".; Enabled by default with -funroll-loops.
-fwhole-program: Assume that the current compilation unit represents 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 a affect gets more aggressively
optimized by interprocedural optimizers. While this option is
equivalent to proper use of "static" keyword for programs consisting of single file, in combination with option --combine this flag can be used to compile most of smaller scale C programs since the
functions and variables become local for the whole combined compilation unit, not for the single source file itself.
-fcprop-registers: After register allocation and post-register allocation instruction splitting, we perform a copy-propagation pass to try to reduce
scheduling dependencies and occasionally eliminate the copy.; Enabled at levels -O, -O2, -O3, -Os.
-fprofile-generate: 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".
-fprofile-use: Enable profile feedback directed optimizations, and optimizations
generally profitable only with profile feedback available.; The following options are enabled: "-fbranch-probabilities",
"-fvpt", "-funroll-loops", "-fpeel-loops", "-ftracer",
"-fno-loop-optimize".
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.
-ffast-math: Sets -fno-math-errno, -funsafe-math-optimizations, -fno-trapping-math, -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 should never be turned on by any -O option since it can result in incorrect output for programs which depend on an exact
implementation of IEEE or ISO rules/specifications for math functions.
-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 should never be turned on by any -O option since it can result in incorrect output for programs which depend on an exact
implementation of IEEE or ISO rules/specifications for math functions.; 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 should never be turned on by any -O option since it can result in incorrect output for programs which depend on an exact
implementation of IEEE or ISO rules/specifications for math functions.; The default is -fno-unsafe-math-optimizations.
-ffinite-math-only: Allow optimizations for floating-point arithmetic that assume that arguments and results are not NaNs or +-Infs.; This option should never be turned on by any -O option since it can result in incorrect output for programs which depend on an exact
implementation of IEEE or ISO rules/specifications.; The default is -fno-finite-math-only.
-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 implies -fno-signaling-nans. 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 which 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 uservisible 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 constant as single precision constant instead of implicitly converting it to double precision constant.
-fcx-limited-range: When enabled, this option states that a range reduction step is not needed when performing complex division. 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.
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 the 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 mostly 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 and adds REG_VALUE_PROFILE notes to instructions for their later usage in optimizations.; Enabled with -fprofile-generate and -fprofile-use.
-fvpt: If combined with -fprofile-arcs, it instructs the compiler to add a 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 operation 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 will most benefit processors with lots of registers. Depending on the debug information format adopted by the target, however, it can make debugging impossible, since variables will no longer
stay in a "home register".; Enabled by default with -funroll-loops.
-ftracer: Perform tail duplication to enlarge superblock size. This transformation simplifies the control flow of the function allowing
other optimizations to do 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 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 the loops for that 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 new 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).
-fprefetch-loop-arrays: If supported by the target machine, generate instructions to
prefetch memory to improve the performance of loops that access
large arrays.; Disabled at level -Os.
-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 will create larger object and executable files and will also be
slower. You will not be able to 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 in 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.
--param name=value: In some places, GCC uses various constants to control the amount of optimization that is done. For example, GCC will not inline functions that contain more that 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 given in the following table:; salias-max-implicit-fields
The maximum number of fields in a variable without direct
structure accesses for which structure aliasing will consider
trying to track each field. The default is 5; sra-max-structure-size
The maximum structure size, in bytes, at which the scalar
replacement of aggregates (SRA) optimization will perform block copies. The default value, 0, implies that GCC will select the most appropriate size itself.; sra-field-structure-ratio
The threshold ratio (as a percentage) between instantiated
fields and the complete structure size. We say that if the
ratio of the number of bytes in instantiated fields to the number of bytes in the complete structure exceeds this parameter, then block copies are not used. The default is 75.; max-crossjump-edges
The maximum number of incoming edges to consider for crossjumping. 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 compile time increase with probably small improvement in executable size.; min-crossjump-insns
The minimum number of instructions which must be matched at the end of two blocks before crossjumping will be performed on
them. This value is ignored in the case where all instructions in the block being crossjumped 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-gotoduplication-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 is searched, the time savings from filling the delay slot will be minimal so stop searching.
Increasing values mean more aggressive optimization, making the compile time increase with probably small improvement in executable run 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 compile
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 will be allocated in order to perform the global common subexpression elimination optimization. If more memory than specified is required, the
optimization will not be done.; max-gcse-passes
The maximum number of passes of GCSE to run. The default is 1.; max-pending-list-length
The maximum number of pending dependencies scheduling will
allow 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-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 will consider for inlining. This only affects
functions declared inline and methods implemented in a class
declaration (C++). The default value is 450.; 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 will be investigated. To those functions, a
different (more restrictive) limit compared to functions
declared inline can be applied. The default value is 90.; 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 backend. This parameter is ignored when -funit-at-a-time is not used. The default value is 2700.; large-function-growth
Specifies maximal growth of large function caused by inlining
in percents. This parameter is ignored when -funit-at-a-time is not used. 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 (consider unit consisting of function A that is inline and B
that just calls A three time. 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 inlininable 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 aplying --param inline-unit-growth. The default is 10000; inline-unit-growth
Specifies maximal overall growth of the compilation unit caused by inlining. This parameter is ignored when -funit-at-a-time is not used. The default value is 50 which limits unit growth to 1.5 times the original size.; max-inline-insns-recursive max-inline-insns-recursive-auto
Specifies maximum number of instructions out-of-line copy of
self recursive inline function can grow into by performing
recursive inlining.

For functions declared inline --param max-inline-insns-recursive is taken into account. For function not declared inline, recursive inlining happens only when -finline-functions (included in -O3) is enabled and --param max-inline-insnsrecursive-auto is used. The default value is 450.; max-inline-recursive-depth max-inline-recursive-depth-auto
Specifies maximum recursion depth used by the recursive inlining.

For functions declared inline --param max-inline-recursivedepth is taken into account. For function not declared inline, recursive inlining happens only when -finline-functions (included in -O3) is enabled and --param max-inline-recursivedepth-auto is used. The default value is 450.; 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 will recurse via given call expression. This parameter limits inlining only to call expression whose probability
exceeds given threshold (in percents). The default value is
10.; inline-call-cost
Specify cost of call instruction relative to simple arithmetics operations (having cost of 1). Increasing this cost disqualifies inlining of non-leaf functions and at the same time
increases size of leaf function that is believed to reduce
function size by being inlined. In effect it increases amount of inlining for code having large abstraction penalty (many
functions that just pass the arguments to other functions) and decrease inlining for code with low abstraction penalty. The
default value is 16.; max-unrolled-insns
The maximum number of instructions that a loop should have if
that loop is unrolled, and if the loop is unrolled, it 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 should have if that loop is
unrolled, and if the loop is unrolled, it 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 should have if
that loop is peeled, and if the loop is peeled, it determines
how many times the loop code is peeled.; max-peel-times
The maximum number of peelings of a single loop.; 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-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
that all candidates are considered for each use in induction
variable optimizations. Only the most relevant candidates are considered if there are more candidates, 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 number of candidates in the set is smaller than this value, we always try to remove unnecessary ivs from the set during its optimization when a new iv is added to the set.; scev-max-expr-size
Bound on size of expressions used in the scalar evolutions analyzer. Large expressions slow the analyzer.; vect-max-version-checks
The maximum number of runtime checks that can be performed when doing loop versioning in the vectorizer. See option ftreevect-loop-version for more information.; max-iterations-to-track
The maximum number of iterations of a loop the brute force
algorithm for analysis of # of iterations of the loop tries to evaluate.; hot-bb-count-fraction
Select fraction of the maximal count of repetitions of basic
block in program given basic block needs to have to be considered hot.; hot-bb-frequency-fraction
Select fraction of the maximal 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 function contain single loop with known bound and other loop with unknown. We predict the known number of iterations correctly, while the unknown number of
iterations average to roughly 10. This means that the loop
without bounds would appear artificially cold relative to the
other one.; 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 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 rather hokey argument, as most of the duplicates will be 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 do have 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
Maximum number of basic blocks on path that cse considers. The default is 10.; max-cse-insns
The maximum instructions CSE process before flushing. The
default is 1000.; global-var-threshold
Counts the number of function calls (n) and the number of callclobbered variables (v). If nxv is larger than this limit, a single artificial variable will be created to represent all the call-clobbered variables at function call sites. This artificial variable will then be made to alias every call-clobbered
variable. (done as "int * size_t" on the host machine; beware overflow).; max-aliased-vops
Maximum number of virtual operands allowed to represent aliases before triggering the alias grouping heuristic. Alias grouping reduces compile times and memory consumption needed for aliasing at the expense of precision loss in alias information.; 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
which 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 compile time increase with probably slightly better performance. The default value is 100.; max-cselib-memory-location
The maximum number of memory locations cselib should take into account. Increasing values mean more aggressive optimization, making the compile time increase with probably slightly better performance. The default value is 500.; max-flow-memory-location
Similar as max-cselib-memory-location but for dataflow liveness. The default value is 100.; reorder-blocks-duplicate reorder-blocks-duplicate-feedback
Used by 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 is used only when profile feedback is available and 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-sched-region-insns
The maximum number of insns in a region to be considered for
interblock scheduling. The default value is 100.; min-sched-prob
The minimum probability of reaching a source block for
interblock speculative scheduling. The default value is 40.; 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.; 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's. The
default value is 256.; min-virtual-mappings
Specifies the minimum number of virtual mappings in the incremental SSA updater that should be registered to trigger the
virtual mappings heuristic defined by virtual-mappings-ratio.
The default value is 100.; virtual-mappings-ratio
If the number of virtual mappings is virtual-mappings-ratio
bigger than the number of virtual symbols to be updated, then
the incremental SSA updater switches to a full update for those symbols. The default ratio is 3.; ssp-buffer-size
The minimum size of buffers (i.e. arrays) that will receive
stack smashing protection when -fstack-protection is used.; 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 we will treat in a
field sensitive manner during pointer analysis.
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.: 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 which GCC does not know how to 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.
-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 .
-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.
-Wimport: Warn the first time #import is used.
-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 basename of the source file with any suffix
replaced with object file suffix. 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 gen: eration, -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, including any
path, deletes 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 take the basename of the input file 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 @pxref{dashMF,,-MF}),
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:; "iso9899:1990"
"c89"
The ISO C standard from 1990. c89 is the customary shorthand for this version of the standard.

The -ansi option is equivalent to -std=c89.; "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.; "gnu89"
The 1990 C standard plus GNU extensions. This is the default.; "gnu99"
"gnu9x"
The 1999 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.
-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. See the --sysroot option for more information.
-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.
-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.
-fdollars-in-identifiers: Accept $ in identifiers.
-fextended-identifiers: Accept universal character names in identifiers. This option is
experimental; in a future version of GCC, it will be enabled by
default for C99 and C++.
-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.
-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 direc
tives 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.
D Like M except in two respects: it does not include the prede: fined 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 prepro: cessing.
-P Inhibit generation of linemarkers in the output from the preproces: sor. 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 nor: mal 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 which GCC does not know
how to 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.
-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 will be passed to the linker. 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 will be passed to the linker. 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 that 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. (For example, __main, used to ensure C++ constructors will be called.)
-pie: Produce a position independent executable on targets which support it. For predictable results, you must also specify the same set of options that were used to generate code (-fpie, -fPIE, or model suboptions) when you specify this 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 exe: cutable.
-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 that were used to generate code (-fpic, -fPIC, or model suboptions) when you specify this 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 will not always be 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 will link the shared version of libgcc into shared libraries by default. Otherwise, it will take advantage of the linker and optimize 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.
-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.
-Xlinker option: Pass option as an option to the linker. You can use this to supply system-specific linker options which GCC does not know how to recognize.; If you want to pass an option that takes an 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.
-Wl,option: Pass option as an option to the linker. If option contains commas, it is split into multiple options at the commas.
-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.
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 leftto-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 will be ignored. The directory will still be 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.
-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.
-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 was not specified, the driver tries two standard prefixes, which are
/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 will check to see if the path provided by the -B
refers to a directory, and if necessary it will add 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
includes 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 run-time 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 will be 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 that 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 would normally search for headers in
/usr/include and libraries in /usr/lib, it will instead search dir/usr/include and dir/usr/lib.; If you use both this option and the -isysroot option, then the --sysroot option will apply to libraries, but the -isysroot option will apply 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 will still work, but the library aspect will not.
-I- This option has been deprecated. Please use -iquote instead for -I: directories before the -I- and remove the -I-. 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-, 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 which was current when the compiler was 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. Sometimes this is inconvenient, so GCC provides options that will switch to another cross-compiler or version.
-b machine: The argument machine specifies the target machine for compilation.; The value to use for machine is the same as was specified as the machine type when configuring GCC as a cross-compiler. For example, if a cross-compiler was configured with configure arm-elf, meaning to compile for an arm processor with elf binaries, then you would specify -b arm-elf to run that cross compiler. Because there are other options beginning with -b, the configuration must contain a hyphen.
-V version: The argument version specifies which version of GCC to run. This is useful when multiple versions are installed. For example, version might be 4.0, meaning to run GCC version 4.0.
The -V and -b options work by running the <machine>-gcc-<version> executable, so there's no real reason to use them if you can just run that directly.
Hardware Models and Configurations
Earlier we discussed the standard option -b which chooses among different installed compilers for completely different target machines, such as VAX vs. 68000 vs. 80386.
In addition, each of these 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.
ARC Options
These options are defined for ARC implementations:
-EL Compile code for little endian mode. This is the default.
-EB Compile code for big endian mode.
-mmangle-cpu: Prepend the name of the cpu to all public symbol names. In multiple-processor systems, there are many ARC variants with different
instruction and register set characteristics. This flag prevents
code compiled for one cpu to be linked with code compiled for
another. No facility exists for handling variants that are "almost identical". This is an all or nothing option.
-mcpu=cpu: Compile code for ARC variant cpu. Which variants are supported depend on the configuration. All variants support -mcpu=base, this is the default.
-mtext=text-section
-mdata=data-section
-mrodata=readonly-data-section: Put functions, data, and readonly data in text-section, data-section, and readonly-data-section respectively by default. This can be overridden with the "section" attribute.
ARM Options
These -m options are defined for Advanced RISC Machines (ARM) architectures:
-mabi=name: Generate code for the specified ABI. Permissible values are: apcsgnu, 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 will cause the stack frames not to be generated for leaf functions. The default is
-mno-apcs-frame.
-mapcs: This is a synonym for -mapcs-frame.
-mthumb-interwork: Generate code which supports calling between the ARM and Thumb
instruction sets. Without this option 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.
-mno-sched-prolog: Prevent the reordering of instructions in the function prolog, or
the merging of those instruction with the instructions in the function's body. This means that all functions will 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 if functions inside an executable
piece of code. The default is -msched-prolog.
-mhard-float: Generate output containing floating point instructions. This is
the default.
-msoft-float: Generate output containing library calls for floating point. Warning: the requisite libraries are not available for all ARM 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.
-mfloat-abi=name: Specifies which ABI to use for floating point values. Permissible values are: soft, softfp and hard.; soft and hard are equivalent to -msoft-float and -mhard-float respectively. softfp allows the generation of floating point instructions, but still uses the soft-float calling conventions.
-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.
-mwords-little-endian: This option only applies when generating code for big-endian processors. Generate code for a little-endian word order but a bigendian byte order. That is, a byte order of the form 32107654. Note: this option should only be used if you require compatibility with code for big-endian ARM processors generated by versions of
the compiler prior to 2.8.
-mcpu=name: This specifies the name of the target ARM processor. GCC uses this name to determine what kind of instructions it can emit when generating assembly code. Permissible names are: arm2, arm250, arm3, arm6, arm60, arm600, arm610, arm620, arm7, arm7m, arm7d, arm7dm, arm7di, arm7dmi, arm70, arm700, arm700i, arm710, arm710c, arm7100, arm7500, arm7500fe, arm7tdmi, arm7tdmi-s, arm8, strongarm, strongarm110, strongarm1100, 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, arm1176jz-s, arm1176jzf-s, xscale, iwmmxt, ep9312.
-mtune=name: This option is very similar to the -mcpu= option, except that instead of specifying the actual target processor type, and hence
restricting which instructions can be used, it specifies that GCC
should tune the performance of the code as if the target were of
the type specified in this option, but still choosing the instructions that it will generate based on the cpu specified by a -mcpu= option. For some ARM implementations better performance can be
obtained by using this option.
-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, armv5te, armv6, armv6j, iwmmxt, ep9312.
-mfpu=name
-mfpe=number
-mfp=number: This specifies what floating point hardware (or hardware emulation) is available on the target. Permissible names are: fpa, fpe2, fpe3, maverick, vfp. -mfp and -mfpe are synonyms for -mfpu=fpenumber, for compatibility with older versions of GCC.; If -msoft-float is specified this specifies the format of floating point values.
-mstructure-size-boundary=n: The size of all structures and unions will be 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 the 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 will be 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 will lie 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 will be
turned into long calls. The heuristic is that static functions,
functions which 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, will not be turned into long calls. The exception to this rule is 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, will always be turned into long calls.
This feature is not enabled by default. Specifying -mno-long-calls will restore the default behavior, as will 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.
-mnop-fun-dllimport
Disable support for the "dllimport" attribute.
-msingle-pic-base
Treat the register used for PIC addressing as read-only, rather
than loading it in the prologue for each function. The run-time
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. The default is R10 unless stack-checking is enabled, when R9 is used.
-mcirrus-fix-invalid-insns Insert NOPs into the instruction stream to in order to work around problems with invalid Maverick instruction combinations. This
option is only valid if the -mcpu=ep9312 option has been used to enable generation of instructions for the Cirrus Maverick floating point co-processor. This option is not enabled by default, since
the problem is only present in older Maverick implementations. The default can be re-enabled by use of the -mno-cirrus-fix-invalid-insns switch.
-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
Generate code for the 16-bit Thumb instruction set. The default is to use the 32-bit ARM instruction set.
-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.
-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.
-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.
AVR Options
These options are defined for AVR implementations:
-mmcu=mcu
Specify ATMEL AVR instruction set or MCU type.
Instruction set avr1 is for the minimal AVR core, not supported by the C compiler, only for assembler programs (MCU types: at90s1200, attiny10, attiny11, attiny12, attiny15, attiny28).
Instruction set avr2 (default) is for the classic AVR core with up to 8K program memory space (MCU types: at90s2313, at90s2323, attiny22, at90s2333, at90s2343, at90s4414, at90s4433, at90s4434, at90s8515, at90c8534, at90s8535).
Instruction set avr3 is for the classic AVR core with up to 128K program memory space (MCU types: atmega103, atmega603, at43usb320, at76c711).
Instruction set avr4 is for the enhanced AVR core with up to 8K program memory space (MCU types: atmega8, atmega83, atmega85).
Instruction set avr5 is for the enhanced AVR core with up to 128K program memory space (MCU types: atmega16, atmega161, atmega163, atmega32, atmega323, atmega64, atmega128, at43usb355, at94k).
-msize
Output instruction sizes to the asm file.
-minit-stack=N
Specify the initial stack address, which may be a symbol or numeric value, __stack is the default.
-mno-interrupts
Generated code is not compatible with hardware interrupts. Code
size will be smaller.
-mcall-prologuesFunctions prologues/epilogues expanded as call to appropriate subroutines. Code size will be smaller.
-mno-tablejump
Do not generate tablejump insns which sometimes increase code size.
-mtiny-stack
Change only the low 8 bits of the stack pointer.
-mint8
Assume int to be 8 bit integer. This affects the sizes of all
types: A char will be 1 byte, an int will be 1 byte, an long will
be 2 bytes and long long will be 4 bytes. Please note that this
option does not comply to the C standards, but it will provide you with smaller code size.
Blackfin Options
-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 will ensure that the generated code does not contain speculative loads after jump instructions. This option is enabled by default.
-mno-specld-anomalyDon't generate extra code to prevent speculative loads from occurring.
-mcsync-anomaly
When enabled, the compiler will ensure that the generated code does not contain CSYNC or SSYNC instructions too soon after conditional branches. This option is enabled by default.
-mno-csync-anomaly
Don't generate extra code to prevent CSYNC or SSYNC instructions
from occurring too soon after a conditional branch.
-mlow-64kWhen 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.
-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
Specified the identification number of the ID based shared library being compiled. Specifying a value of 0 will generate more compact code, specifying other values will force the allocation of that
number to the current library but is no more space or time efficient than omitting this option.
-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 will lie 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 will restore the default behavior. Note these switches have no
effect on how the compiler generates code to handle function calls via function pointers.
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.
-melinux-stacksize=nOnly available with the cris-axis-aout target. Arranges for indications in the program to the kernel loader that the stack of the
program should be set to 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 to turn 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) arranges (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-epilogueWith -mno-prologue-epilogue, the normal function prologue and epilogue that sets 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 variable 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.
-maout
Legacy no-op option only recognized with the cris-axis-aout target.
-melf
Legacy no-op option only recognized with the cris-axis-elf and
cris-axis-linux-gnu targets.
-melinux
Only recognized with the cris-axis-aout target, where it selects a GNU/linux-like multilib, include files and instruction set for
-march=v8.
-mlinux
Legacy no-op option only recognized with the cris-axis-linux-gnu
target.
-sim
This option, recognized for the cris-axis-aout and 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.
CRX Options
These options are defined specifically for the CRX ports.
-mmac
Enable the use of multiply-accumulate instructions. Disabled by
default.
-mpush-args
Push instructions will be used to pass outgoing arguments when
functions are called. Enabled by default.
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 will create an object file for the single architecture that it 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 targetting,
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, will only permit instructions to be used that are valid for the subtype of the file it is generating, so you
cannot put 64-bit instructions in an ppc750 object file. The linker for shared libraries, /usr/bin/libtool, will fail and print 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, will quietly give the executable the most restrictive subtype of any of its input files.
-FdirAdd 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 will be 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.
-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.
The default for this option is to make choices that seem to be most useful.
-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 turn around development. Needed to enable 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_fatalCause 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.
-bundleProduce a Mach-o bundle format file. See man ld(1) for more information.
-bundle_loader executable This option specifies the executable that will be loading the build output file being linked. See man ld(1) for more information.
-dynamiclib
When passed this option, GCC will produce 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 will be 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 will issue floating-point operations. If you are compiling for an Alpha without floatingpoint 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 su, but the instructions are marked to be safe for soft ware 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 controlregister (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 func tion 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. Under DEC Unix, this has the effect that IEEEconformant math library routines will be linked in.
-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 will output the constant as a literal and generate code to load it from the data segment at runtime.
Use this option to require GCC to construct all integer constants using code, even if it takes more instructions (the maximum is six).
You would 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.
-malpha-as
-mgasSelect whether to generate code to be assembled by the vendor-supplied assembler (-malpha-as) or by the GNU assembler -mgas.
-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 was 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-relocsOlder 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 will choose the
default values for the instruction set from the processor you specify. If you do not specify a processor type, GCC will default 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
21264Schedules 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.
-mtune=cpu_type
Set only the instruction scheduling parameters for machine type
cpu_type. The instruction set is not changed.
-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.
DEC Alpha/VMS Options
These -m options are defined for the DEC Alpha/VMS implementations:
-mvms-return-codes
Return VMS condition codes from main. The default is to return
POSIX style condition (e.g. error) codes.
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, that 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.
-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-callsUse 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-membarThis 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.
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.
-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 will run 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.
-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.
-mjump-in-delayFill delay slots of function calls with unconditional jump instructions by modifying the return pointer for the function call to be
the target of the conditional jump.
-mdisable-fpregs
Prevent floating point registers from being used in any manner.
This is necessary for compiling kernels which perform lazy context switching of floating point registers. If you use this option and attempt to perform floating point operations, the compiler will
abort.
-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 which performs faster indirect calls.
This option will 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 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.
-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-runtimeUse 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-floatGenerate 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. The embedded target hppa1.1-*-pro does 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.
-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-ldUse GNU ld specific options. 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 have any affect on 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 HP ld specific options. 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 have any affect on 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 will degrade 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-stdGenerate 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.
Intel 386 and AMD x86-64 Options
These -m options are defined for the i386 and x86-64 family of computers:
-mtune=cpu-type
Tune to cpu-type everything applicable about the generated code, except for the ABI and the set of available instructions. The
choices for cpu-type are:
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 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, the code generated option will change to reflect the processors that were most common when that version of GCC was 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.
i386 Original Intel's i386 CPU.
i486 Intel's i486 CPU. (No scheduling is implemented for this chip.)
i586, pentium Intel Pentium CPU with no MMX support.
pentium-mmx Intel PentiumMMX CPU based on Pentium core with MMX instruction set support.
pentiumpro Intel PentiumPro CPU.
i686 Same as "generic", but when used as "march" option, PentiumPro instruction set will be used, so the code will run on all i686 familly chips.
pentium2Intel Pentium2 CPU based on PentiumPro core with MMX instruction set support.
pentium3, pentium3m Intel Pentium3 CPU based on PentiumPro core with MMX and SSE instruction set support.
pentium-m Low power version of Intel Pentium3 CPU with MMX, SSE and SSE2 instruction set support. Used by Centrino notebooks.
pentium4, pentium4mIntel Pentium4 CPU with MMX, SSE and SSE2 instruction set support.
prescott Improved version of Intel Pentium4 CPU with MMX, SSE, SSE2 and SSE3 instruction set support.
nocona Improved version of Intel Pentium4 CPU with 64-bit extensions, MMX, SSE, SSE2 and SSE3 instruction set support.
core2 Intel Core2 CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3 and SSSE3 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 AMD K8 core based CPUs with x86-64 instruction set support. (This supersets MMX, SSE, SSE2, 3dNOW!, enhanced 3dNOW! and 64-bit instruction set extensions.)
amdfam10AMD Family 10 core based CPUs with x86-64 instruction set support. (This supersets MMX, SSE, SSE2, SSE3, SSE4A, SSE5, 3dNOW!, enhanced 3dNOW!, ABM 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 Winchip2 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 CPU with MMX and SSE instruction set support. (No scheduling is implemented for this chip.)
While picking a specific cpu-type will schedule things appropriately for that particular chip, the compiler will not generate any code that does not run on the i386 without the -march=cpu-type option being used.
-march=cpu-type
Generate instructions for the machine type cpu-type. The choices for cpu-type are the same as for -mtune. Moreover, specifying -march=cpu-type implies -mtune=cpu-type.
-mcpu=cpu-type
A deprecated synonym for -mtune.
-m386
-m486
-mpentium
-mpentiumpro
These options are synonyms for -mtune=i386, -mtune=i486, -mtune=pentium, and -mtune=pentiumpro respectively. These synonyms are deprecated.
-mfpmath=unit
Generate floating point arithmetics for selected unit unit. The choices for unit are:
387 Use the standard 387 floating point coprocessor present major ity of chips and emulated otherwise. Code compiled with this
option will run almost everywhere. The temporary results are
computed in 80bit precision instead of 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 i386 compiler.
sse Use scalar floating point instructions present in the SSE instruction set. This instruction set is supported by Pentium3 and newer chips, in the AMD line by Athlon-4, Athlon-xp and
Athlon-mp chips. The earlier version of SSE instruction set
supports only single precision arithmetics, thus the double and extended precision arithmetics is still done using 387. Later version, present only in Pentium4 and the future AMD x86-64
chips supports double precision arithmetics too.
For the i386 compiler, you need to 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 80bit.
This is the default choice for the x86-64 compiler.
sse,387Attempt to utilize both instruction sets at once. This effectively double 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 instable performance.
-masm=dialect
Output asm instructions using selected dialect. Supported choices are intel or att (the default one). Darwin does not support intel.
-mieee-fp
-mno-ieee-fpControl whether or not the compiler uses IEEE floating point comparisons. These handle correctly the case where the result of a
comparison is unordered.
-msoft-floatGenerate 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 FreeBSD, OpenBSD and NetBSD. This option is overridden when -march indicates that the target cpu will always have an FPU and so the instruction will not need emulation. As of revision 2.6.1, 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 will produce 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 will be aligned differently than the published application binary interface specifications for the 386 and will
not be 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 i386
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) would prefer "long double" to be aligned to an 8 or 16 byte boundary. In arrays or structures conforming to the ABI, this would not be possible. So specifying a -m128bit-long-double will align "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 to be 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, the structures and arrays containing "long double" variables will
change their size as well as function calling convention for function taking "long double" will be modified. Hence they will not be binary compatible with arrays or structures in code compiled without that switch.
-mmlarge-data-threshold=number When -mcmodel=medium is specified, the data greater than threshold are placed in large data section. This value must be the same
across all object linked into the binary and defaults to 65535.
-msvr3-shlib
-mno-svr3-shlib
Control whether GCC places uninitialized local variables into the
"bss" or "data" segments. -msvr3-shlib places them into "bss". These options are meaningful only on System V Release 3.
-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 will be generated for calls to those functions.
In addition, seriously incorrect code will result 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.
-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).
On Pentium and PentiumPro, "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 will most likely misalign 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
-mno-mmx
-msse
-mno-sse
-msse2
-mno-sse2
-msse3
-mno-sse3
-mssse3
-mno-ssse3
-msse4a
-mno-sse4a
-msse5
-mno-sse5
-m3dnow
-mno-3dnow
-mpopcnt
-mno-popcnt
-mabm
-mno-abm
These switches enable or disable the use of instructions in the
MMX, SSE, SSE2 or 3DNow! extended instruction sets. 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 have SSE/SSE2 instructions generated automatically from floating-point code (as opposed to 387 instructions), see -mfpmath=sse.
These options will enable GCC to use these extended instructions in generated code, even without -mfpmath=sse. Applications which perform runtime 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.
-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-argsIf enabled, the maximum amount of space required for outgoing arguments will be computed in the function prologue. This is faster on most modern CPUs because of reduced dependencies, improved scheduling and reduced stack usage when 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 Mingw32. Code that relies 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 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 destination is
known to be aligned at least to 4 byte boundary. This enables more inlining, increase code size, but may improve performance of code
that depends on fast memcpy, strlen and memset for short lengths.
-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.
-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 legal depends on the operating system, and whether it maps the
segment to cover the entire TLS area.
For systems that use GNU libc, the default is on.
These -m switches are supported in addition to the above on AMD x86-64 processors in 64-bit environments.
-m32
-m64Generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long and pointer to 32 bits and generates code
that runs on any i386 system. The 64-bit environment sets int to
32 bits and long and pointer to 64 bits and generates code for
AMD's x86-64 architecture.
-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 will not be 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 but symbols can be located anywhere in the address space. Programs can be statically or dynamically
linked, but building of shared libraries are not supported with the medium model.
-mcmodel=large
Generate code for the large model: This model makes no assumptions about addresses and sizes of sections. Currently GCC does not
implement this model.
-mfused-madd
-mno-fused-madd
Enable automatic generation of fused floating point multiply-add
instructions if the ISA supports such instructions. The
-mfused-madd option is on by default.
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-picGenerate 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.
-minline-int-divide-min-latencyGenerate code for inline divides of integer values using the minimum latency algorithm.
-minline-int-divide-max-throughputGenerate code for inline divides of integer values using the maximum throughput algorithm.
-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-dwarf2-asm
-mdwarf2-asm
Don't (or do) generate assembler code for the DWARF2 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 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.
-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.
-mt
-pthread
Add support for multithreading using the POSIX threads library.
This option sets flags for both the preprocessor and linker. It
does not affect the thread safety of object code produced by the
compiler or that of libraries supplied with it. These are HP-UX
specific flags.
-milp32
-mlp64Generate 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.
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 will use during code generation. These pseudo-registers will be 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 the default runtime
libraries gcc builds.
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 will generate "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 will generate "seth/add3" instructions to load their
addresses), and assume subroutines may not be reachable with the
"bl" instruction (the compiler will generate the much slower
"seth/add3/jl" instruction sequence).
-msdata=none
Disable use of the small data area. Variables will be 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=usePut 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 will give an error message---incorrect code will not be generated.
-mdebugMakes 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=numbernumber can only be 1 or 2. If it is 1 then branches will be preferred over conditional code, if it is 2, then the opposite will
apply.
-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 will only be 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 the 68000 series. The default values for these options depends on which style of 68000 was selected when the compiler was configured; the defaults for the most common choices are given below.
-m68000
-mc68000
Generate output for a 68000. This is the default when the compiler is configured for 68000-based systems.
Use this option for microcontrollers with a 68000 or EC000 core, including the 68008, 68302, 68306, 68307, 68322, 68328 and 68356.
-m68020
-mc68020
Generate output for a 68020. This is the default when the compiler is configured for 68020-based systems.
-m68881
Generate output containing 68881 instructions for floating point.
This is the default for most 68020 systems unless --nfp was specified when the compiler was configured.
-m68030
Generate output for a 68030. This is the default when the compiler is configured for 68030-based systems.
-m68040
Generate output for a 68040. This is the default when the compiler is configured for 68040-based systems.
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.
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.
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" family cpu. This is the
default when the compiler is configured for 520X-based systems.
Use this option for microcontroller with a 5200 core, including the MCF5202, MCF5203, MCF5204 and MCF5202.
-m68020-40Generate output for a 68040, without using any of the new instructions. This results in code which 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.
-m68020-60Generate output for a 68060, without using any of the new instructions. This results in code which 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.
-msoft-floatGenerate output containing library calls for floating point. Warning: the requisite libraries are not available for all m68k targets. 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. The embedded targets m68k-*-aout and m68k-*-coff do provide software floating point support.
-mshortConsider 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.
-mnobitfield
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 will be generated for calls to those functions.
In addition, seriously incorrect code will result 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.
-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 will align 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 will be handled by the system.
-msep-dataGenerate 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
Specified the identification number of the ID based shared library being compiled. Specifying a value of 0 will generate more compact code, specifying other values will force the allocation of that
number to the current library but is no more space or time efficient than omitting this option.
M68hc1x Options
These are the -m options defined for the 68hc11 and 68hc12 microcontrollers. The default values for these options depends on which style of microcontroller was selected when the compiler was configured; the defaults for the most common choices are given below.
-m6811
-m68hc11Generate output for a 68HC11. This is the default when the compiler is configured for 68HC11-based systems.
-m6812
-m68hc12Generate output for a 68HC12. This is the default when the compiler is configured for 68HC12-based systems.
-m68S12
-m68hcs12
Generate output for a 68HCS12.
-mauto-incdecEnable the use of 68HC12 pre and post auto-increment and autodecrement addressing modes.
-minmax
-nominmax
Enable the use of 68HC12 min and max instructions.
-mlong-calls
-mno-long-calls
Treat all calls as being far away (near). If calls are assumed to be far away, the compiler will use the "call" instruction to call a function and the "rtc" instruction for returning.
-mshort
Consider type "int" to be 16 bits wide, like "short int".
-msoft-reg-count=count
Specify the number of pseudo-soft registers which are used for the code generation. The maximum number is 32. Using more pseudo-soft register may or may not result in better code depending on the program. The default is 4 for 68HC11 and 2 for 68HC12.
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 four 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.
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 will run 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, and mips64. The processor names are: 4kc, 4km, 4kp, 5kc, 5kf, 20kc, 24k, 24kc, 24kf, 24kx, m4k, orion, r2000, r3000, r3900, r4000, r4400, r4600, r4650, r6000, r8000, rm7000, rm9000, sb1, sr71000, vr4100, vr4111, vr4120, vr4130, vr4300, vr5000, vr5400 and vr5500. 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).
In processor names, a final 000 can be abbreviated as k (for example, -march=r2k). Prefixes are optional, and vr may be written r.
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 will set _MIPS_ARCH to "r2000" and define the macro _MIPS_ARCH_R2000.
Note that the _MIPS_ARCH macro uses the processor names given above. In other words, it will have the full prefix and will 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 will optimize for the processor
specified by -march. By using -march and -mtune together, it is possible to generate code that will run 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.
-mips32r2
Equivalent to -march=mips32r2.
-mips64
Equivalent to -march=mips64.
-mips16
-mno-mips16
Generate (do not generate) MIPS16 code. If GCC is targetting a
MIPS32 or MIPS64 architecture, it will make use of the MIPS16e ASE.
-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>.
-mabicalls
-mno-abicalls
Generate (do not generate) SVR4-style position-independent code.
-mabicalls is the default for SVR4-based systems.
-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 will only work if the GOT is smaller than about 64k. Anything larger will cause 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. It should then work with very large GOTs, although it will also be
less efficient, since it will take 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.
-mhard-float
Use floating-point coprocessor instructions.
-msoft-float
Do not use floating-point coprocessor instructions. Implement
floating-point calculations using library calls instead.
-msingle-floatAssume that the floating-point coprocessor only supports singleprecision operations.
-mdouble-floatAssume that the floating-point coprocessor supports double-precision operations. This is the default.
-mdsp
-mno-dsp
Use (do not use) the MIPS DSP ASE.
-mpaired-single
-mno-paired-single
Use (do not use) paired-single floating-point 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.
-mlong64Force "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-sym32Assume (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 global and static items less than or equal to num bytes into the small data or bss section instead of the normal data or bss
section. This allows the data to be accessed using a single
instruction.
All modules should be compiled with the same -G num value.
-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.
-msplit-addresses
-mno-split-addressesEnable (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-breaksMIPS 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.
-mfused-madd
-mno-fused-madd
Enable (disable) use of the floating point multiply-accumulate
instructions, when they are available. The default is
-mfused-madd.
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.
-nocpp
Tell the MIPS assembler to not run its preprocessor over user
assembler files (with a .s suffix) when assembling them.
-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-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 will avoid 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.)
-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-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 which implement those architectures;
for those, Branch Likely instructions will 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 we
schedule FP instructions 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 will align 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.
MMIX Options
These options are defined for the MMIX:
-mlibfuncs
-mno-libfuncsSpecify 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 which uses features specific to the AM33 processor.
-mno-am33Do not generate code which uses features specific to the AM33 processor. This is the default.
-mreturn-pointer-on-d0
When generating a function which 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 would 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.
-mrelaxIndicate 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.
MT Options
These -m options are defined for Morpho MT architectures:
-march=cpu-type
Generate code that will run on cpu-type, which is the name of a system representing a certain processor type. Possible values for cpu-type are ms1-64-001, ms1-16-002, ms1-16-003 and ms2.
When this option is not used, the default is -march=ms1-16-002.
-mbacc
Use byte loads and stores when generating code.
-mno-bacc
Do not use byte loads and stores when generating code.
-msim
Use simulator runtime
-mno-crt0
Do not link in the C run-time initialization object file crti.o. Other run-time initialization and termination files such as
startup.o and exit.o are still included on the linker command line.
PDP-11 Options
These options are defined for the PDP-11:
-mfpuUse 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.
-mac0Return 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.
-msplit
Generate code for a system with split I&D.
-mno-split
Generate code for a system without split I&D. 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.
PowerPC Options
These are listed under
IBM RS/6000 and PowerPC Options
These -m options are defined for the IBM RS/6000 and PowerPC:
-mpower
-mno-power
-mpower2
-mno-power2
-mpowerpc
-mno-powerpc
-mpowerpc-gpopt
-mno-powerpc-gpopt
-mpowerpc-gfxopt
-mno-powerpc-gfxopt
-mpowerpc64
-mno-powerpc64
-mmfcrf
-mno-mfcrf
-mpopcntb
-mno-popcntb
-mfprnd
-mno-fprnd
-mmfpgpr
-mno-mfpgpr
GCC supports two related instruction set architectures for the
RS/6000 and PowerPC. The POWER instruction set are those instructions supported by the rios chip set used in the original RS/6000 systems and the PowerPC instruction set is the architecture of the Freescale MPC5xx, MPC6xx, MPC8xx microprocessors, and the IBM 4xx, 6xx, and follow-on microprocessors.
Neither architecture is a subset of the other. However there is a large common subset of instructions supported by both. An MQ register is included in processors supporting the POWER architecture.
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.
The -mpower option allows GCC to generate instructions that are found only in the POWER architecture and to use the MQ register.
Specifying -mpower2 implies -power and also allows GCC to generate instructions that are present in the POWER2 architecture but not
the original POWER architecture.
The -mpowerpc option allows GCC to generate instructions that are found only in the 32-bit subset of the PowerPC architecture. Specifying -mpowerpc-gpopt implies -mpowerpc and also allows GCC to use the optional PowerPC architecture instructions in the General Purpose group, including floating-point square root. Specifying
-mpowerpc-gfxopt implies -mpowerpc and also 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 -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 -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 -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.
If you specify both -mno-power and -mno-powerpc, GCC will use only the instructions in the common subset of both architectures plus
some special AIX common-mode calls, and will not use the MQ register. Specifying both -mpower and -mpowerpc permits GCC to use any instruction from either architecture and to allow use of the MQ
register; specify this for the Motorola MPC601.
-mnew-mnemonics
-mold-mnemonics
Select which mnemonics to use in the generated assembler code.
With -mnew-mnemonics, GCC uses the assembler mnemonics defined for the PowerPC architecture. With -mold-mnemonics it uses the assembler mnemonics defined for the POWER architecture. Instructions
defined in only one architecture have only one mnemonic; GCC uses
that mnemonic irrespective of which of these options is specified.
GCC defaults to the mnemonics appropriate for the architecture in
use. Specifying -mcpu=cpu_type sometimes overrides the value of these option. Unless you are building a cross-compiler, you should normally not specify either -mnew-mnemonics or -mold-mnemonics, but should instead accept the default.
-mcpu=cpu_type
Set architecture type, register usage, choice of mnemonics, and
instruction scheduling parameters for machine type cpu_type. Supported values for cpu_type are 401, 403, 405, 405fp, 440, 440fp, 505, 601, 602, 603, 603e, 604, 604e, 620, 630, 740, 7400, 7450, 750, 801, 821, 823, 860, 970, 8540, ec603e, G3, G4, G5, power, power2, power3, power4, power5, power5+, power6, power6x, common, powerpc, powerpc64, rios, rios1, rios2, rsc, and rs64.
-mcpu=common selects a completely generic processor. Code generated under this option will run on any POWER or PowerPC processor. GCC will use only the instructions in the common subset of both
architectures, and will not use the MQ register. GCC assumes a
generic processor model for scheduling purposes.
-mcpu=power, -mcpu=power2, -mcpu=powerpc, and -mcpu=powerpc64 specify generic POWER, POWER2, pure 32-bit PowerPC (i.e., not MPC601), and 64-bit 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 will run 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, -mnew-mnemonics, -mpopcntb, -mpower, -mpower2, -mpowerpc64, -mpowerpc-gpopt, -mpowerpc-gfxopt, -mstring, -mmfpgpr. The particular options set for any particular CPU will vary 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, register usage, or choice of mnemonics, as -mcpu=cpu_type would. The same values for cpu_type are used for -mtune as for -mcpu. If both are specified, the code generated will use the architecture, registers, and
mnemonics set by -mcpu, but the scheduling parameters set by -mtune.
-mswdiv
-mno-swdivGenerate code to compute division as reciprocal estimate and iterative refinement, creating opportunities for increased throughput.
This feature requires: optional PowerPC Graphics instruction set
for single precision and FRE instruction for double precision,
assuming divides cannot generate user-visible traps, and the domain values not include Infinities, denormals or zero denominator.
-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.
-mvrsave
-mno-vrsave
Generate VRSAVE instructions when generating AltiVec code.
-msecure-plt
Generate code that allows ld and ld.so to build executables and
shared libraries with non-exec .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-iselThis 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-iselThis switch enables or disables the generation of SPE simd instructions.
-mspe=yes/no
This option has been deprecated. Use -mspe and -mno-spe instead.
-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 will allocate at least one TOC entry
for each unique non-automatic variable reference in your program.
GCC will also place 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 will produce 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 and -mpowerpc, while -maix32 disables the 64-bit ABI and implies -mno-powerpc64. GCC defaults to -maix32.
-mxl-compat
-mno-xl-compatProduce 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 which 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.
-mpeSupport 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.
-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 the instructions usage 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 the
instructions usage in little endian mode.
-mupdate
-mno-updateGenerate 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.
-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 is used.
-mno-bit-align
-mbit-alignOn 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 would be aligned to a 4 byte
boundary and have a size of 4 bytes. By using -mno-bit-align, the structure would be aligned to a 1 byte boundary and be one byte in size.
-mno-strict-align
-mstrict-align
On System V.4 and embedded PowerPC systems do not (do) assume that unaligned memory references will be handled by the system.
-mrelocatable
-mno-relocatable
On embedded PowerPC systems generate code that allows (does not
allow) the program to be relocated to a different address at runtime. If you use -mrelocatable on any module, all objects linked together must be compiled with -mrelocatable or -mrelocatable-lib.
-mrelocatable-lib
-mno-relocatable-lib
On embedded PowerPC systems generate code that allows (does not
allow) the program to be relocated to a different address at runtime. Modules compiled with -mrelocatable-lib can be linked with either modules compiled without -mrelocatable and -mrelocatable-lib or with modules compiled with 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.
-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 0/1/2 to assign no/highest/second-highest 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 which latency >= number is costly.
-minsert-sched-nops=scheme This option controls which nop insertion scheme will be 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 which 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-sysvOn System V.4 and embedded PowerPC systems compile code using calling conventions that adheres 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
Specify both -mcall-sysv and -meabi options.
-mcall-sysv-noeabi
Specify both -mcall-sysv and -mno-eabi options.
-mcall-solaris
On System V.4 and embedded PowerPC systems compile code for the
Solaris operating system.
-mcall-linux
On System V.4 and embedded PowerPC systems compile code for the
Linux-based GNU system.
-mcall-gnu
On System V.4 and embedded PowerPC systems compile code for the
Hurd-based GNU system.
-mcall-netbsd
On System V.4 and embedded PowerPC systems compile code for the
NetBSD 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.
-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 Booke 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.
-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 were passed in the floating point registers in case the function takes a variable arguments.
With -mprototype, only calls to prototyped variable argument functions will 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.
-mwindissSpecify that you are compiling for the WindISS simulation environment.
-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 to 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, do not call an initialization function from "main", and the -msdata option will only use "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
-msdataOn 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 and
static data in the .sdata section. Put small uninitialized global and static 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.
-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-regnamesOn System V.4 and embedded PowerPC systems do (do not) emit register names in the assembly language output using symbolic forms.
-mlongcall
-mno-longcall
Default to making all function calls indirectly, using a register, so that functions which reside further than 32 megabytes
(33,554,432 bytes) from the current location can be called. 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" will generate "jbsr callee, L42", plus a "branch island" (glue code). The two target addresses represent the callee and the "branch island". The Darwin/PPC linker will prefer the first address and generate a "bl callee" if the PPC "bl" instruction will reach the callee directly; otherwise, the linker will generate "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, we may cause GCC to ignore all longcall specifications when the linker is known to generate glue.
-pthread
Adds support for multithreading with the pthreads library. This option sets flags for both the preprocessor and linker.
S/390 and zSeries Options
These are the -m options defined for the S/390 and zSeries architecture.
-mhard-float
-msoft-floatUse (do not use) the hardware floating-point instructions and registers for floating-point operations. When -msoft-float is specified, functions in libgcc.a will be used to perform floating-point operations. When -mhard-float is specified, the compiler generates IEEE floating-point instructions. This is the default.
-mlong-double-64
-mlong-double-128
These switches control the size of "long double" type. A size of
64bit 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-debugPrint (or do not print) additional debug information when compiling. The default is to not print debug information.
-march=cpu-type
Generate code that will run on cpu-type, which is the name of a system representing a certain processor type. Possible values for cpu-type are g5, g6, z900, and z990. 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 which 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
These arguments always have to be used in conjunction. If they are present the s390 back end emits additional instructions in the
function prologue which trigger a trap if the stack size is stackguard bytes above the stack-size (remember that the stack on s390 grows downward). 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.
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.
-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-onlyGenerate 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.
-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.
-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 will 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.
-mfmovd
Enable the use of the instruction "fmovd".
-mhitachi
Comply with the calling conventions defined by Renesas.
-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 except for sh-symbianelf.
-mnomacsave
Mark the "MAC" register as call-clobbered, even if -mhitachi is given.
-mieee
Increase IEEE-compliance of floating-point code. At the moment,
this is equivalent to -fno-finite-math-only. When generating 16 bit SH opcodes, getting IEEE-conforming results for comparisons of NANs / infinities incurs extra overhead in every floating point
comparison, therefore the default is set to -ffinite-math-only.
-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.
-mspace
Optimize for space instead of speed. Implied by -Os.
-mprefergot
When generating position-independent code, emit function calls
using the Global Offset Table instead of the Procedure Linkage Table.
-musermode
Generate a library function call to invalidate instruction cache
entries, after fixing up a trampoline. This library function call doesn't assume it can write to the whole memory address space.
This is the default when the target is "sh-*-linux*".
-multcost=number
Set the cost to assume for a multiply insn.
-mdiv=strategy
Set the division strategy to use for SHmedia code. strategy must be one of: call, call2, fp, inv, inv:minlat, inv20u, inv20l,
inv:call, inv:call2, inv:fp . "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" is a variant of "inv" where if no cse / 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 / code hoisting optimizations. "inv:call", "inv:call2" and "inv:fp" all 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 fp operations or a call is not possible in that case. "inv20u" and
"inv20l" are variants of the "inv:minlat" strategy. In the case
that the inverse calculation was nor 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.
-mdivsi3_libfunc=nameSet the name of the library function used for 32 bit signed division to name. This only affect the name used in the call and inv:call division strategies, and the compiler will still expect
the same sets of input/output/clobbered registers as if this option was not present.
-madjust-unroll
Throttle unrolling to avoid thrashing target registers. This
option only has an effect if the gcc code base supports the TARGET_ADJUST_UNROLL_MAX target hook.
-mindexed-addressingEnable 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 will generally generate
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 ptabs / ptrel before a branch, or
hoist it 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 will be done before testing against -1. That means that all the constructors will be run a bit quicker, 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 the user specifies a specific cost with -mgettrcost, -mno-pt-fixed also implies -mgettrcost=100;
this deters register allocation using target registers for storing ordinary integers.
-minvalid-symbolsAssume symbols might be invalid. Ordinary function symbols generated by the compiler will always be 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 will cause
ptabs / ptrel to trap. This option is only meaningful when
-mno-pt-fixed is in effect. It will then prevent cross-basic-block cse, hoisting and most scheduling of symbol loads. The default is -mno-invalid-symbols.
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.
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.
-mfpu
-mhard-float
Generate output containing floating point instructions. This is
the default.
-mno-fpu
-msoft-floatGenerate 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-floatGenerate 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.
-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 will not be directly in line with the
rules of the ABI.
-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 will 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.
This option is only available on SunOS and Solaris.
-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, sparclite, f930, f934, hypersparc, sparclite86x, sparclet, tsc701, v9, ultrasparc, ultrasparc3, and niagara.
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
v8: supersparc, hypersparc
sparclite: f930, f934, sparclite86x
sparclet: tsc701
v9: ultrasparc, ultrasparc3, niagara
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.
-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.
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, f930, f934, sparclite86x, tsc701, ultrasparc, ultrasparc3, and niagara.
-mv8plus
-mno-v8plusWith -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-bit wide. This is enabled by default on Solaris in
32-bit mode for all SPARC-V9 processors.
-mvis
-mno-visWith -mvis, GCC generates code that takes advantage of the UltraSPARC Visual Instruction Set extensions. The default is -mno-vis.
These -m options are supported in addition to the above on SPARC-V9 processors in 64-bit environments:
-mlittle-endian
Generate code for a processor running in little-endian mode. It is only available for a few configurations and most notably not on
Solaris and Linux.
-m32
-m64Generate 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=medlowGenerate code for 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.
-mcmodel=medmid
Generate code for 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.
-mcmodel=medany
Generate code for 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.
-mcmodel=embmedanyGenerate code for 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.
-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.
These switches are supported in addition to the above on Solaris:
-threads
Add support for multithreading using the Solaris 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.
-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.
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,dirLook in the directory dir to find the M4 preprocessor. The assembler uses this option.
TMS320C3x/C4x Options
These -m options are defined for TMS320C3x/C4x implementations:
-mcpu=cpu_type
Set the instruction set, register set, and instruction scheduling
parameters for machine type cpu_type. Supported values for cpu_type are c30, c31, c32, c40, and c44. The default is c40 to generate code for the TMS320C40.
-mbig-memory
-mbig
-msmall-memory
-msmall
Generates code for the big or small memory model. The small memory model assumed that all data fits into one 64K word page. At runtime the data page (DP) register must be set to point to the 64K
page containing the .bss and .data program sections. The big memory model is the default and requires reloading of the DP register for every direct memory access.
-mbk
-mno-bk
Allow (disallow) allocation of general integer operands into the
block count register BK.
-mdb
-mno-db
Enable (disable) generation of code using decrement and branch,
DBcond(D), instructions. This is enabled by default for the C4x.
To be on the safe side, this is disabled for the C3x, since the
maximum iteration count on the C3x is 2^{23 + 1} (but who iterates loops more than 2^{23} times on the C3x?). Note that GCC will try to reverse a loop so that it can utilize the decrement and branch
instruction, but will give up if there is more than one memory reference in the loop. Thus a loop where the loop counter is decremented can generate slightly more efficient code, in cases where
the RPTB instruction cannot be utilized.
-mdp-isr-reload
-mparanoid
Force the DP register to be saved on entry to an interrupt service routine (ISR), reloaded to point to the data section, and restored on exit from the ISR. This should not be required unless someone
has violated the small memory model by modifying the DP register,
say within an object library.
-mmpyi
-mno-mpyi
For the C3x use the 24-bit MPYI instruction for integer multiplies instead of a library call to guarantee 32-bit results. Note that
if one of the operands is a constant, then the multiplication will be performed using shifts and adds. If the -mmpyi option is not specified for the C3x, then squaring operations are performed
inline instead of a library call.
-mfast-fix
-mno-fast-fix
The C3x/C4x FIX instruction to convert a floating point value to an integer value chooses the nearest integer less than or equal to the floating point value rather than to the nearest integer. Thus if
the floating point number is negative, the result will be incorrectly truncated an additional code is necessary to detect and correct this case. This option can be used to disable generation of
the additional code required to correct the result.
-mrptb
-mno-rptb
Enable (disable) generation of repeat block sequences using the
RPTB instruction for zero overhead looping. The RPTB construct is only used for innermost loops that do not call functions or jump
across the loop boundaries. There is no advantage having nested
RPTB loops due to the overhead required to save and restore the RC, RS, and RE registers. This is enabled by default with -O2.
-mrpts=count
-mno-rptsEnable (disable) the use of the single instruction repeat instruction RPTS. If a repeat block contains a single instruction, and
the loop count can be guaranteed to be less than the value count, GCC will emit a RPTS instruction instead of a RPTB. If no value is specified, then a RPTS will be emitted even if the loop count cannot be determined at compile time. Note that the repeated instruction following RPTS does not have to be reloaded from memory each
iteration, thus freeing up the CPU buses for operands. However,
since interrupts are blocked by this instruction, it is disabled by default.
-mloop-unsigned
-mno-loop-unsigned
The maximum iteration count when using RPTS and RPTB (and DB on the C40) is 2^{31 + 1} since these instructions test if the iteration
count is negative to terminate the loop. If the iteration count is unsigned there is a possibility than the 2^{31 + 1} maximum iteration count may be exceeded. This switch allows an unsigned iteration count.
-mti
Try to emit an assembler syntax that the TI assembler (asm30) is
happy with. This also enforces compatibility with the API employed by the TI C3x C compiler. For example, long doubles are passed as structures rather than in floating point registers.
-mregparm
-mmemparm
Generate code that uses registers (stack) for passing arguments to functions. By default, arguments are passed in registers where
possible rather than by pushing arguments on to the stack.
-mparallel-insns
-mno-parallel-insns
Allow the generation of parallel instructions. This is enabled by default with -O2.
-mparallel-mpy
-mno-parallel-mpyAllow the generation of MPY||ADD and MPY||SUB parallel instructions, provided -mparallel-insns is also specified. These instructions have tight register constraints which can pessimize the code generation of large functions.
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 will always load the functions address up into a register, and call 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-functionDo 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.
-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 will cause r2 and r5 to be used in the code generated
by the compiler. This setting is the default.
-mno-app-regs
This option will cause r2 and r5 to be treated as fixed registers.
-mv850e1
Specify that the target processor is the V850E1. The preprocessor constants __v850e1__ and __v850e__ will be defined if this option is used.
-mv850e
Specify that the target processor is the V850E. The preprocessor
constant __v850e__ will be defined if this option is used.
If neither -mv850 nor -mv850e nor -mv850e1 are defined then a default target processor will be chosen and the relevant __v850*__ preprocessor constant will be defined.
The preprocessor constants __v850 and __v851__ are always defined, regardless of which processor variant is the target.
-mdisable-callt
This option will suppress generation of the CALLT instruction for
the v850e and v850e1 flavors of the v850 architecture. The default is -mno-disable-callt which allows the CALLT instruction to be used.
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 you will assemble with the GNU assembler.
-mg Output code for g-format floating point numbers instead of d-for mat.
x86-64 Options
These are listed under
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-const16Enable 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.
-mtext-section-literals
-mno-text-section-literals 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-alignWhen 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 will be performed. The default is -mtarget-align. These options do not affect the treatment of auto-aligned instructions like "LOOP",
which the assembler will always align, either by widening density
instructions or by inserting no-op instructions.
-mlongcalls
-mno-longcallsWhen 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 will still show
direct call instructions---look at the disassembled object code to see the actual instructions. Note that the assembler will use an
indirect call for every cross-file call, not just those that really will be 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 would be -fno-foo. In the table below, only one of the forms is listed---the one which 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.
-ftrapvThis 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.
-fexceptionsEnable exception handling. Generates extra code needed to propagate exceptions. For some targets, this implies GCC will generate 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 will enable it by default
for languages like C++ which normally require exception handling,
and disable 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-exceptionsGenerate 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".
-funwind-tables
Similar to -fexceptions, except that it will just generate any needed static data, but will not affect the generated code in any
other way. You will normally not enable this option; instead, a
language processor that needs this handling would enable it on your behalf.
-fasynchronous-unwind-tables Generate unwind table in dwarf2 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).
-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 GCCcompiled 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
will be equivalent to the smallest integer type which 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.
-fshared-dataRequests that the data and non-"const" variables of this compilation be shared data rather than private data. The distinction
makes sense only on certain operating systems, where shared data is shared between processes running the same program, while private
data exists in one copy per process.
-fno-commonIn C, allocate even 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 will get an
error when you link them. The only reason this might be useful is if you wish to verify that the program will work on other systems
which always work 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.
-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 386 has no such limit.)
Position-independent code requires special support, and therefore works only on certain machines. For the 386, GCC supports PIC for System V but not for the Sun 386i. Code generated for the IBM RS/6000 is always position-independent.
-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.
-fpie
-fPIEThese 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 will be used during linking.
-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 which 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-regTreat 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 will not save and restore the register reg.
It is an error to used 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 will produce 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 will save and restore the register reg if they use it.
It is an error to used 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 will produce disastrous results.
A different sort of disaster will result 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 will be 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 will be 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 will 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 anyways, 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 will not be done. This can be used, for example, for the profiling functions listed above, highpriority 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).
-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 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 must do that. The switch causes generation of code to ensure that the operating system sees the stack being extended.
-fstack-limit-register=reg -fstack-limit-symbol=sym -fno-stack-limitGenerate 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 the stack would grow beyond the value, a signal is raised. 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.
-fargument-alias
-fargument-noalias
-fargument-noalias-global Specify the possible relationships among parameters and between
parameters and global data.
-fargument-alias specifies that arguments (parameters) may alias each other and may alias global storage.-fargument-noalias specifies that arguments do not alias each other, but may alias global
storage.-fargument-noalias-global specifies that arguments do not alias each other and do not alias global storage.
Each language will automatically use whatever option is required by the language standard. You should not need to use these options yourself.
-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=modelAlter the thread-local storage model to be used. The model argument should be one of "global-dynamic", "local-dynamic", "initial-exec" or "local-exec".
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 will be 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 ie; 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 will be "hidden". The default if -fvisibility isn't specified is "default", i.e., make every symbol public---this causes the same behavior as previous versions of GCC.
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://people.redhat.com/~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 DLL's 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 ie; 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.
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 will 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 will be thrown
between DSOs must be explicitly marked with default visibility so
that the type_info nodes will be unified between the DSOs.
An overview of these techniques, their benefits and how to use them is at <http://gcc.gnu.org/wiki/Visibility>.
-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 v2.5 <http://www.openmp.org/>.

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 that allow 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 would otherwise be 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_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 will attempt to figure out an appropriate prefix to use based on the pathname it was 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 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 will search foo/bar where it would normally search /usr/local/lib/bar. These alternate directories are searched first; the standard directories come next.

COMPILER_PATHThe 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 will use mblen and mbtowc as defined by the default locale to recognize and translate multibyte characters.

Some additional environments variables affect the behavior of the preprocessor.

CPATH
C_INCLUDE_PATH
CPLUS_INCLUDE_PATH
OBJC_INCLUDE_PATHEach 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_OUTPUTIf 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.

BUGS

For instructions on reporting bugs, see <http://gcc.gnu.org/bugs.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, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998,
1999, 2000, 2001, 2002, 2003, 2004, 2005 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 1.2 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.

gcc-4.1.2 2010-01-13 GCC(1)

docs.sk

comprehensive documentation repository

See also

gcc(1)

NAME

SYNOPSIS

DESCRIPTION

OPTIONS

ENVIRONMENT

BUGS

FOOTNOTES

SEE ALSO

AUTHOR

COPYRIGHT