GCC(1)
👑 GCC(1) — GNU project C and C++ compiler 🌈
okay so this is GCC(1), the GNU manual page, section 1. Yes bestie, the big one. The one that compiles, like, everything. Buckle up 🎀
✨ NAME ✨
gcc — GNU project C and C++ compiler 🫶
🦋 SYNOPSIS 🦋
gcc [-c|-S|-E] [-std=standard]
[-g] [-pg] [-Olevel]
[-Wwarn...] [-Wpedantic]
[-Idir...] [-Ldir...]
[-Dmacro[=defn]...] [-Umacro]
[-foption...] [-mmachine-option...]
[-o outfile] [@file] infile...lowkey important: only the most useful options are listed right here in the synopsis, okay? The rest of them are down below, and there are so many. Also g++ accepts mostly the same options as gcc — they're basically the same girl, different outfit 💅
🌸 DESCRIPTION 🌸
So when you invoke GCC, it normally does the whole pipeline: preprocessing, compilation, assembly and linking. That's four whole stages, main character energy fr.
The "overall options" let you stop this process at an intermediate stage, like hitting pause on a show you're obsessed with. For example, the -c option says do not run the linker. Then the output consists of object files output by the assembler. Periodt.
Other options get passed on to one or more stages of processing. Some options control the preprocessor and others control 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. (ngl, you really don't.)
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. So if a description for a particular option does not mention a source language, you can use that option with all supported languages. Convenient? Iconic. 💅
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 specific version of GCC. And when you compile C++ programs, you should invoke GCC as g++ instead. Do not skip that part, bestie, it's giving necessary.
The gcc program accepts options and file names as operands. Many options have multi-letter names; therefore multiple single-letter options may not be grouped: -dv is very different from -d -v. Like, do not. 🙈
You can mix options and other arguments. For the most part, the order you use doesn't matter. But order does matter when you use several options of the same kind; for example, if you specify -L more than once, the directories are searched in the order specified. Also, the placement of the -l option is significant. So like… vibe check your ordering on those ones ✨
Many options have long names starting with -f or with -W — for example, -fmove-loop-invariants, -Wformat and so on. Most of these have both positive and negative forms; the negative form of -ffoo is -fno-foo. This manual documents only one of these two forms, whichever one is not the default. iykyk 🫶
Some options take one or more arguments typically separated either by a space or by the equals sign (=) from the option name. Unless documented otherwise, an argument can be either numeric or a string. Numeric arguments must typically be small unsigned decimal or hexadecimal integers. Hexadecimal arguments must begin with the 0x prefix. Arguments to options that specify a size threshold of some sort may be arbitrarily large decimal or hexadecimal integers followed by a byte size suffix designating a multiple of bytes such as "kB" and "KiB" for kilobyte and kibibyte, respectively, "MB" and "MiB" for megabyte and mebibyte, "GB" and "GiB" for gigabyte and gigibyte, and so on. Such arguments are designated by byte-size in the following text. Refer to the NIST, IEC, and other relevant national and international standards for the full listing and explanation of the binary and decimal byte size prefixes. (Yes, the standards girlies really did that. Respect. 👑)
🎀 OPTIONS 🎀
💜 Option Summary 💜
Here's a summary of all the options, grouped by type. The explanations are in the following sections — this is just the map, not the territory, bestie 🗺️✨
🌟 Overall Options 🌟
-c -S -E -o file -dumpbase dumpbase -dumpbase-ext auxdropsuf
-dumpdir dumppfx -x language -v -### --help[=class[,...]]
--target-help --version -pass-exit-codes -pipe -wrapper @file
-ffile-prefix-map=old=new -fcanon-prefix-map -fplugin=file
-fplugin-arg-name=arg -fdump-ada-spec[-slim] -fada-spec-parent=unit
-fdump-go-spec=file --assemble --compile --dumpbase dumpbase
--dumpbase-ext auxdropsuf --dumpdir dumppfx --language=language
--output=file --pass-exit-codes --pipe --preprocess --verbose🌟 C Language Options 🌟
-ansi -std=standard -aux-info filename -fno-asm -fno-builtin
-fno-builtin-function -fcond-mismatch -ffreestanding -fgimple
-fgnu-tm -fgnu89-inline -fhosted -flax-vector-conversions
-fms-extensions -fpermitted-flt-eval-methods=standard
-fplan9-extensions -fsigned-bitfields -funsigned-bitfields
-fsigned-char -funsigned-char -fstrict-flex-arrays[=n]
-fsso-struct=endianness --ansi🌟 C++ Language Options 🌟
--compile-std-module -fabi-compat-version=n -fabi-version=n
-fno-access-control -faligned-new=[n]
-fno-assume-sane-operators-new-delete -fchar8_t -fcheck-new
-fconcepts -fconcepts-diagnostics-depth=n -fconstexpr-depth=n
-fconstexpr-cache-depth=n -fconstexpr-loop-limit=n
-fconstexpr-ops-limit=n -fcontracts
-fcontract-evaluation-semantic=[ignore|observe|enforce|quick_enforce]
-fcontracts-conservative-ipa -fcontract-checks-outlined
-fcontract-disable-optimized-checks
-fcontracts-client-check=[none|pre|all]
-fcontracts-definition-check=[on|off] -fcoroutines
-fdiagnostics-all-candidates -fno-elide-constructors
-fno-enforce-eh-specs -fext-numeric-literals -fno-gnu-keywords
-fno-immediate-escalation -fno-implement-inlines
-fimplicit-constexpr -fno-implicit-inline-templates
-fno-implicit-templates -fmodule-header[=kind]
-fmodule-implicit-inline -fno-module-lazy
-fmodule-mapper=specification -fmodule-only -fmodules
-fms-extensions -fnew-inheriting-ctors -fnew-ttp-matching
-fno-nonansi-builtins -fnothrow-opt -fno-operator-names
-fno-optional-diags -fno-pretty-templates -frange-for-ext-temps
-freflection -fno-rtti -fsized-deallocation -fstrict-enums
-fstrong-eval-order[=kind] -ftemplate-backtrace-limit=n
-ftemplate-depth=n -fno-threadsafe-statics -fuse-cxa-atexit
-fno-use-cxa-get-exception-ptr -fno-weak -nostdinc++
-fvisibility-inlines-hidden -fvisibility-ms-compat
-flang-info-include-translate[=header]
-flang-info-include-translate-not -flang-info-module-cmi[=module]
-stdlib=libstdc++,libc++ -Wabbreviated-auto-in-template-arg
-Wabi-tag -Waligned-new[=kind] -Wcatch-value -Wcatch-value=n
-Wno-class-conversion -Wclass-memaccess -Wcomma-subscript
-Wconditionally-supported -Wno-conversion-null
-Wctad-maybe-unsupported -Wctor-dtor-privacy -Wdangling-reference
-Wno-defaulted-function-deleted -Wno-delete-incomplete
-Wdelete-non-virtual-dtor -Wno-deprecated-array-compare
-Wdeprecated-copy -Wdeprecated-copy-dtor
-Wno-deprecated-enum-enum-conversion
-Wno-deprecated-enum-float-conversion
-Wno-deprecated-literal-operator
-Wdeprecated-variadic-comma-omission -Weffc++
-Wno-elaborated-enum-base -Wno-exceptions
-Wno-expose-global-module-tu-local -Wno-external-tu-local
-Wextra-semi -Wno-global-module -Wno-inaccessible-base
-Wno-inherited-variadic-ctor -Wno-init-list-lifetime
-Winvalid-constexpr -Winvalid-imported-macros -Wno-invalid-offsetof
-Wno-literal-suffix -Wmismatched-new-delete -Wmismatched-tags
-Wmultiple-inheritance -Wnamespaces -Wnarrowing -Wnoexcept
-Wnoexcept-type -Wnon-virtual-dtor -Wpessimizing-move
-Wno-placement-new -Wplacement-new=n -Wrange-loop-construct
-Wredundant-move -Wredundant-tags -Wreorder -Wregister
-Wno-sfinae-incomplete -Wstrict-null-sentinel
-Wno-subobject-linkage -Wtemplates -Wno-non-c-typedef-for-linkage
-Wno-non-template-friend -Wold-style-cast -Woverloaded-virtual
-Wno-pmf-conversions -Wself-move -Wsign-promo -Wsized-deallocation
-Wsuggest-final-methods -Wsuggest-final-types -Wsuggest-override
-Wno-template-body -Wno-template-id-cdtor -Wtemplate-names-tu-local
-Wno-terminate -Wno-vexing-parse -Wvirtual-inheritance
-Wno-virtual-move-assign -Wvolatile🌟 Objective-C and Objective-C++ Language Options 🌟
-fconstant-string-class=class-name -fgnu-runtime -fnext-runtime
-fno-nil-receivers -fobjc-abi-version=n -fobjc-call-cxx-cdtors
-fobjc-direct-dispatch -fobjc-exceptions -fobjc-gc -fobjc-nilcheck
-fobjc-std=objc1 -fno-local-ivars
-fivar-visibility=[public|protected|private|package]
-freplace-objc-classes -fzero-link -gen-decls -Wassign-intercept
-Wno-property-assign-default -Wno-protocol -Wobjc-root-class
-Wselector -Wstrict-selector-match -Wundeclared-selector🌟 OpenMP and OpenACC Options 🌟
-foffload=arg -foffload-options=arg -fopenacc -fopenacc-dim=geom
-fopenmp -fopenmp-simd -fopenmp-target-simd-clone[=device-type]🌟 Diagnostic Message Formatting Options 🌟
-fmessage-length=n -fdiagnostics-plain-output
-fdiagnostics-show-location=[once|every-line]
-fdiagnostics-color=[auto|never|always]
-fdiagnostics-urls=[auto|never|always]
-fdiagnostics-format=[text|sarif-stderr|sarif-file]
-fdiagnostics-add-output=DIAGNOSTICS-OUTPUT-SPEC
-fdiagnostics-set-output=DIAGNOSTICS-OUTPUT-SPEC
-fno-diagnostics-json-formatting -fno-diagnostics-show-option
-fno-diagnostics-show-caret -fno-diagnostics-show-event-links
-fno-diagnostics-show-labels -fno-diagnostics-show-line-numbers
-fno-diagnostics-show-cwe -fno-diagnostics-show-rules
-fno-diagnostics-show-highlight-colors -fno-diagnostics-show-nesting
-fno-diagnostics-show-nesting-locations
-fdiagnostics-show-nesting-levels
-fdiagnostics-minimum-margin-width=width
-fdiagnostics-parseable-fixits -fdiagnostics-generate-patch
-fdiagnostics-show-template-tree -fno-elide-type
-fdiagnostics-path-format=[none|separate-events|inline-events]
-fdiagnostics-show-path-depths -fno-show-column
-fdiagnostics-column-unit=[display|byte]
-fdiagnostics-column-origin=origin
-fdiagnostics-escape-format=[unicode|bytes]
-fdiagnostics-text-art-charset=[none|ascii|unicode|emoji]
-fdiagnostics-show-context[=depth]and that's just the summary, bestie — the real explanations are coming in the next parts, so stay tuned 💖🦋
✨ Warning Options ✨
okay bestie, deep breath. 🫶 this is the section. GCC has opinions about literally everything your code does, and every single opinion has a flag, and every single flag has a chaotic little -Wno- twin that says "actually no." it's giving very much group chat where everyone's typing at once 💅
here is the entire roster, in order, no skips, no cap:
-fsyntax-only,-fmax-errors=n,-Wpedantic,-pedantic-errors-fpermissive,-w,-Wextra,-Wall,-Wabi=n,-Waddress-Wno-address-of-packed-member,-Waggregate-return,-Walloc-size-Walloc-size-larger-than=byte-size,-Walloc-zero,-Walloca-Walloca-larger-than=byte-size,-Wauto-profile-Wno-aggressive-loop-optimizations,-Warith-conversion,-Warray-bounds-Warray-bounds=n,-Warray-compare,-Warray-parameter-Warray-parameter=n,-Wno-attributes,-Wattribute-alias=n-Wno-attribute-alias,-Wno-attribute-warning-Wbidi-chars=[none|unpaired|any|ucn],-Wbool-compare-Wbool-operation,-Wno-builtin-declaration-mismatch-Wno-builtin-macro-redefined,-Wc90-c99-compat,-Wc99-c11-compat-Wc11-c23-compat,-Wc23-c2y-compat,-Wc++-compat,-Wc++11-compat-Wc++14-compat,-Wc++17-compat,-Wc++20-compat,-Wc++26-compat-Wno-c++11-extensions,-Wno-c++14-extensions,-Wno-c++17-extensions-Wno-c++20-extensions,-Wno-c++23-extensions-Wcalloc-transposed-args,-Wcannot-profile,-Wcast-align-Wcast-align=strict,-Wcast-function-type,-Wcast-qual-Wchar-subscripts,-Wclobbered,-Wcomment-Wcompare-distinct-pointer-types,-Wno-complain-wrong-lang-Wconversion,-Wno-coverage-mismatch,-Wno-cpp,-Wdangling-else-Wdangling-pointer,-Wdangling-pointer=n,-Wdate-time,-Wno-deprecated-Wno-deprecated-declarations,-Wno-designated-init-Wno-deprecated-openmp,-Wdisabled-optimization-Wno-discarded-array-qualifiers,-Wno-discarded-qualifiers-Wno-div-by-zero,-Wdouble-promotion,-Wduplicated-branches-Wduplicated-cond,-Wempty-body,-Wno-endif-labels,-Wenum-compare-Wenum-conversion,-Wenum-int-mismatch,-Werror,-Werror=*
like. we are NOT done. not even close. 💀
-Wexpansion-to-defined,-Wfatal-errors-Wflex-array-member-not-at-end,-Wfloat-conversion,-Wfloat-equal-Wformat,-Wformat=2,-Wno-format-contains-nul,-Wno-format-diag-Wno-format-extra-args,-Wformat-nonliteral,-Wformat-overflow=n-Wformat-security,-Wformat-signedness,-Wformat-truncation=n-Wformat-y2k,-Wframe-address,-Wframe-larger-than=byte-size-Wno-free-nonheap-object,-Wheader-guard,-Wno-if-not-aligned-Wno-ignored-attributes,-Wignored-qualifiers-Wno-incompatible-pointer-types,-Whardened,-Wimplicit-Wimplicit-fallthrough,-Wimplicit-fallthrough=n-Wno-implicit-function-declaration,-Wno-implicit-int-Winfinite-recursion,-Winit-self,-Winline,-Wno-int-conversion-Wint-in-bool-context,-Wno-int-to-pointer-cast-Wno-invalid-memory-model,-Winvalid-pch,-Winvalid-utf8-Wno-unicode,-Wjump-misses-init,-Wkeyword-macro-Wlarger-than=byte-size,-Wleading-whitespace=kind-Wlogical-not-parentheses,-Wlogical-op,-Wlong-long-Wno-lto-type-mismatch,-Wmain,-Wmaybe-uninitialized-Wmemset-elt-size,-Wmemset-transposed-args,-Wmisleading-indentation-Wmissing-attributes,-Wmissing-braces,-Wmissing-field-initializers-Wmissing-format-attribute,-Wmissing-include-dirs-Wmissing-noreturn,-Wmusttail-local-addr-Wmaybe-musttail-local-addr,-Wno-missing-profile,-Wno-multichar-Wmultistatement-macros,-Wnonnull,-Wnonnull-compare-Wnormalized=[none|id|nfc|nfkc],-Wnull-dereference,-Wno-odr-Wopenacc-parallelism,-Wopenmp,-Wopenmp-simd,-Wno-overflow-Woverlength-strings,-Wno-override-init-side-effects,-Wpacked-Wno-packed-bitfield-compat,-Wpacked-not-aligned,-Wpadded-Wparentheses,-Wno-pedantic-ms-format,-Wpointer-arith-Wno-pointer-compare,-Wno-pointer-to-int-cast,-Wno-pragmas-Wno-pragma-once-outside-header,-Wno-prio-ctor-dtor,-Wno-psabi-Wredundant-decls,-Wrestrict,-Wno-return-local-addr,-Wreturn-type-Wno-scalar-storage-order,-Wsequence-point,-Wshadow-Wshadow=global,-Wshadow=local,-Wshadow=compatible-local-Wno-shadow-ivar,-Wno-shift-count-negative-Wno-shift-count-overflow,-Wshift-negative-value-Wno-shift-overflow,-Wshift-overflow=n,-Wsign-compare-Wsign-conversion,-Wno-sizeof-array-argument,-Wsizeof-array-div-Wsizeof-pointer-div,-Wsizeof-pointer-memaccess,-Wstack-protector-Wstack-usage=byte-size,-Wstrict-aliasing,-Wstrict-aliasing=n-Wstrict-overflow,-Wstrict-overflow=n,-Wstring-compare-Wno-stringop-overflow,-Wno-stringop-overread-Wno-stringop-truncation,-Wstrict-flex-arrays-Wsuggest-attribute=attribute-name,-Wswitch,-Wno-switch-bool-Wswitch-default,-Wswitch-enum,-Wno-switch-outside-range-Wno-switch-unreachable,-Wsync-nand,-Wsystem-headers-Wtautological-compare,-Wtrailing-whitespace-Wtrailing-whitespace=kind,-Wtrampolines,-Wtrigraphs-Wtrivial-auto-var-init,-Wno-tsan,-Wtype-limits,-Wundef-Wuninitialized,-Wunknown-pragmas,-Wunsuffixed-float-constants-Wunterminated-string-initialization,-Wunused-Wunused-but-set-parameter,-Wunused-but-set-parameter=n-Wunused-but-set-variable,-Wunused-but-set-variable=n-Wunused-const-variable,-Wunused-const-variable=n,-Wunused-function-Wunused-label,-Wunused-local-typedefs,-Wunused-macros-Wunused-parameter,-Wno-unused-result,-Wunused-value-Wunused-variable,-Wuse-after-free,-Wuse-after-free=n-Wuseless-cast,-Wno-varargs,-Wvariadic-macros-Wvector-operation-performance,-Wvla,-Wvla-larger-than=byte-size-Wno-vla-larger-than,-Wvolatile-register-var,-Wwrite-strings-Wno-xor-used-as-pow,-Wzero-as-null-pointer-constant-Wzero-length-bounds,--all-warnings,--extra-warnings,--no-warnings--pedantic,--pedantic-errors
and THAT is the whole warning wall, bestie. you survived. we ate. 🔥
🔍 Static Analyzer Options 🔍
the static analyzer department, aka the part of GCC that stares at your code like it's reading your diary and going "mm. okay. girl. 😭". these are the flags, the dumps, and the analyzer warnings:
-fanalyzer,-fanalyzer-assume-nothrow,-fanalyzer-call-summaries-fanalyzer-checker=name,-fno-analyzer-feasibility-fanalyzer-show-events-in-system-headers,-fno-analyzer-state-merge-fno-analyzer-state-purge,-fno-analyzer-suppress-followups-fanalyzer-transitivity,-fno-analyzer-undo-inlining-fanalyzer-verbose-edges,-fanalyzer-verbose-state-changes-fanalyzer-verbosity=level,-fdump-analyzer,-fdump-analyzer-callgraph-fdump-analyzer-exploded-graph,-fdump-analyzer-exploded-nodes-fdump-analyzer-exploded-nodes-2,-fdump-analyzer-exploded-nodes-3-fdump-analyzer-exploded-paths,-fdump-analyzer-feasibility-fdump-analyzer-infinite-loop,-fdump-analyzer-json-fdump-analyzer-state-purge,-fdump-analyzer-stderr-fdump-analyzer-supergraph,-fdump-analyzer-untracked-Wno-analyzer-double-fclose,-Wno-analyzer-double-free-Wno-analyzer-exposure-through-output-file-Wno-analyzer-exposure-through-uninit-copy-Wno-analyzer-fd-access-mode-mismatch,-Wno-analyzer-fd-double-close-Wno-analyzer-fd-leak,-Wno-analyzer-fd-phase-mismatch-Wno-analyzer-fd-type-mismatch,-Wno-analyzer-fd-use-after-close-Wno-analyzer-fd-use-without-check,-Wno-analyzer-file-leak-Wno-analyzer-free-of-non-heap,-Wno-analyzer-imprecise-fp-arithmetic-Wno-analyzer-infinite-loop,-Wno-analyzer-infinite-recursion-Wno-analyzer-jump-through-null,-Wno-analyzer-malloc-leak-Wno-analyzer-mismatching-deallocation,-Wno-analyzer-null-argument-Wno-analyzer-null-dereference,-Wno-analyzer-out-of-bounds-Wno-analyzer-overlapping-buffers-Wno-analyzer-possible-null-argument-Wno-analyzer-possible-null-dereference-Wno-analyzer-putenv-of-auto-var,-Wno-analyzer-shift-count-negative-Wno-analyzer-shift-count-overflow,-Wno-analyzer-stale-setjmp-buffer-Wno-analyzer-tainted-allocation-size-Wno-analyzer-tainted-assertion,-Wno-analyzer-tainted-array-index-Wno-analyzer-tainted-divisor,-Wno-analyzer-tainted-offset-Wno-analyzer-tainted-size,-Wno-analyzer-throw-of-unexpected-type-Wanalyzer-symbol-too-complex,-Wanalyzer-too-complex-Wno-analyzer-undefined-behavior-ptrdiff-Wno-analyzer-undefined-behavior-strtok-Wno-analyzer-unsafe-call-within-signal-handler-Wno-analyzer-use-after-free-Wno-analyzer-use-of-pointer-in-stale-stack-frame-Wno-analyzer-use-of-uninitialized-value-Wno-analyzer-va-arg-type-mismatch,-Wno-analyzer-va-list-exhausted-Wno-analyzer-va-list-leak,-Wno-analyzer-va-list-use-after-va-end-Wno-analyzer-write-to-const,-Wno-analyzer-write-to-string-literal
🌸 C and Objective-C-only Warning Options 🌸
these ones are exclusively for the C girlies (and Objective-C girlies). if you're writing C++, bestie, these aren't your section, no offense, ily 🫶
-Wbad-function-cast,-Wdeprecated-non-prototype,-Wfree-labels-Wmissing-declarations,-Wmissing-parameter-name-Wmissing-parameter-type,-Wdeclaration-missing-parameter-type-Wmissing-prototypes,-Wmissing-variable-declarations-Wmultiple-parameter-fwd-decl-lists,-Wnested-externs-Wold-style-declaration,-Wold-style-definition,-Wstrict-prototypes-Wtraditional,-Wtraditional-conversion-Wdeclaration-after-statement,-Wpointer-sign
🐛 Debugging Options 🐛
debug info flags!! for when you want your binary to remember everything that ever happened to it, like a little diary it keeps for the debugger 🎀 these control what debug info gets generated and how:
-g,-glevel,-gdwarf,-gdwarf-version-gbtf,-gctf,-gctflevel-gno-prune-btf,-ggdb,-gno-record-gcc-switches,-gstrict-dwarf-gas-loc-support,-gas-locview-support-gcodeview,-gcolumn-info-gdwarf32,-gdwarf64-gno-statement-frontiers-gno-variable-location-views,-gvariable-location-views=incompat5-ginternal-reset-location-views,-ginline-points-gvms,-gz[=type]-gsplit-dwarf,-gdescribe-dies-fdebug-prefix-map=old=new-fdebug-types-section,-fno-eliminate-unused-debug-types-femit-struct-debug-baseonly,-femit-struct-debug-reduced-femit-struct-debug-detailed[=spec-list]-fno-eliminate-unused-debug-symbols,-femit-class-debug-always-fno-merge-debug-strings,-fno-dwarf2-cfi-asm,-fvar-tracking-fvar-tracking-assignments,-fvar-tracking-uninit,--debug
still going, bestie — the flags keep coming and honestly? iconic of them 💜
✨ Optimization Options ✨
bestie. sit down. hydrate. because this list? this is the main character of the whole entire manual. like, this is where gcc keeps every single little switch that decides how hard it's gonna think about your code. it's giving "a girl with a plan". it's giving "let me cook". 💅
no descriptions down here yet — this is the roll call, the guest list, the vibe check. every name spelled exactly right, in exactly the order they were introduced. do not skip a single one, i stg. 🙈
-faggressive-loop-optimizations -falign-functions[=n[:m:[n2[:m2]]]]
-falign-jumps[=n[:m:[n2[:m2]]]] -falign-labels[=n[:m:[n2[:m2]]]]
-falign-loops[=n[:m:[n2[:m2]]]] -fmin-function-alignment=[n]
-fno-allocation-dce -fallow-store-data-races -fassociative-math
-fauto-profile -fauto-profile[=path] -fauto-profile-inlining
-fauto-inc-dec -fbranch-probabilities -fcaller-saves
-fcombine-stack-adjustments -fconserve-stack -ffold-mem-offsets
-fcompare-elim -fcprop-registers -fcrossjumping -fcse-follow-jumps
-fcse-skip-blocks -fcx-fortran-rules -fcx-limited-range -fcx-method
-fdata-sections -fdce -fdelayed-branch
-fdelete-null-pointer-checks -fdep-fusion -fdevirtualize
-fdevirtualize-speculatively -fdevirtualize-at-ltrans -fdse
-fearly-inlining -fexcess-precision=style -fexpensive-optimizations
-fext-dce -ffast-math -ffat-lto-objects -ffinite-loops
-ffinite-math-only -ffloat-store -fforward-propagate
-ffp-contract=style -ffp-int-builtin-inexact -ffunction-sections
-ffuse-ops-with-volatile-access -fgcse -fgcse-after-reload
-fgcse-las -fgcse-lm -fgraphite-identity -fgcse-sm
-fhoist-adjacent-loads -fif-conversion -fif-conversion2
-findirect-inlining -finline-atomics -finline-functions
-finline-functions-called-once -finline-limit=n
-finline-small-functions -finline-stringops[=fn] -fipa-modref
-fipa-cp -fipa-cp-clone -fipa-bit-cp -fipa-vrp -fipa-pta
-fipa-profile -fipa-pure-const -fipa-reference
-fipa-reference-addressable -fipa-reorder-for-locality -fipa-sra
-fipa-stack-alignment -fipa-icf -fipa-icf-functions
-fipa-icf-variables -fira-algorithm=algorithm
-flate-combine-instructions -flifetime-dse -flive-patching=level
-fira-region=region -fira-hoist-pressure -fira-loop-pressure
-fno-ira-share-save-slots -fno-ira-share-spill-slots
-fisolate-erroneous-paths-dereference
-fisolate-erroneous-paths-attribute -fivopts
-fkeep-inline-functions -fkeep-static-functions
-fkeep-static-consts -flimit-function-alignment
-flive-range-shrinkage -floop-block -floop-interchange
-floop-strip-mine -floop-unroll-and-jam -floop-nest-optimize
-floop-parallelize-all -flra-remat -flto
-flto-compression-level=n -flto-toplevel-asm-heuristics
-flto-partition=alg -flto-incremental=path
-flto-incremental-cache-size=n -fmalloc-dce -fmerge-all-constants
-fmerge-constants -fmodulo-sched -fmodulo-sched-allow-regmoves
-fmove-loop-invariants -fmove-loop-stores -fno-branch-count-reg
-fno-defer-pop -fno-function-cse -fno-guess-branch-probability
-fno-inline -fno-math-errno -fno-peephole -fno-peephole2
-fno-printf-return-value -fno-sched-interblock -fno-sched-spec
-fno-signed-zeros -fno-toplevel-reorder -fno-trapping-math
-fno-zero-initialized-in-bss -fomit-frame-pointer -foptimize-crc
-foptimize-sibling-calls -fpartial-inlining -fpeel-loops
-fpredictive-commoning -fprefetch-loop-arrays -fprofile-correction
-fprofile-use -fprofile-use=path -fprofile-partial-training
-fprofile-values -fprofile-reorder-functions -freciprocal-math
-free -frename-registers -freorder-blocks
-freorder-blocks-algorithm=algorithm -freorder-blocks-and-partition
-freorder-functions -frerun-cse-after-loop
-freschedule-modulo-scheduled-loops -frounding-math
-fsave-optimization-record -fsched2-use-superblocks
-fsched-pressure -fsched-spec-load -fsched-spec-load-dangerous
-fsched-stalled-insns-dep[=n] -fsched-stalled-insns[=n]
-fsched-group-heuristic -fsched-critical-path-heuristic
-fsched-spec-insn-heuristic -fsched-rank-heuristic
-fsched-last-insn-heuristic -fsched-dep-count-heuristic
-fschedule-fusion -fschedule-insns -fschedule-insns2
-fsection-anchors -fselective-scheduling -fselective-scheduling2
-fsel-sched-pipelining -fsel-sched-pipelining-outer-loops
-fsemantic-interposition -fshrink-wrap -fshrink-wrap-separate
-fsignaling-nans -fsingle-precision-constant
-fsplit-ivs-in-unroller -fsplit-loops
-fspeculatively-call-stored-functions -fsplit-paths
-fsplit-wide-types -fsplit-wide-types-early -fssa-backprop
-fssa-phiopt -fstdarg-opt -fstore-merging -fstrict-aliasing
-fipa-strict-aliasing -fthread-jumps -ftracer -ftree-bit-ccp
-ftree-builtin-call-dce -ftree-ccp -ftree-ch -ftree-coalesce-vars
-ftree-copy-prop -ftree-cselim -ftree-dce -ftree-dominator-opts
-ftree-dse -ftree-forwprop -ftree-fre -fcode-hoisting
-ftree-loop-if-convert -ftree-loop-im -ftree-phiprop
-ftree-loop-distribution -ftree-loop-distribute-patterns
-ftree-loop-ivcanon -ftree-loop-linear -ftree-loop-optimize
-ftree-loop-vectorize -ftree-parallelize-loops[=n] -ftree-pre
-ftree-partial-pre -ftree-pta -ftree-reassoc -ftree-scev-cprop
-ftree-sink -ftree-slsr -ftree-sra -ftree-switch-conversion
-ftree-tail-merge -ftree-ter -ftree-vectorize -ftree-vrp
-ftrivial-auto-var-init -funconstrained-commons -funit-at-a-time
-funroll-all-loops -funroll-loops -funsafe-math-optimizations
-funswitch-loops -fipa-ra -fvariable-expansion-in-unroller
-fvect-cost-model -fvpt -fweb -fwhole-program -fwpa
-fuse-linker-plugin -fzero-call-used-regs -O -O0 -O1 -O2 -O3
-Os -Ofast -Og -Oz --optimizeokay a few things:
- the sheer volume. that's the point. that's the assignment. 💀
- the way every single
-ftree-*option rolled up like a girl group? obsessed.-ftree-loop-vectorize,-ftree-pre,-ftree-vrp… the whole lineup showed up. 🎀 - the
-fno-girlies are just their regular counterparts in their "not today" era.-fno-inlinesaid no thanks,-fno-peepholesaid no thanks,-fno-math-errnosaid absolutely not. 🫶 - the
-fipa-*family is huge. like HUGE huge. that's a whole family reunion and everyone brought a dish. ✨ - and then, at the very end, like the queens they are:
-O,-O0,-O1,-O2,-O3,-Os,-Ofast,-Og,-Oz, and--optimize. the way they just casually stroll in last… main character energy fr fr. 👑
and bestie, note that the whole-falign-*crowd takes[=n[:m:[n2[:m2]]]],-fmin-function-alignmenttakes=[n],-fexcess-precision=style,-ffp-contract=style,-finline-limit=n,-finline-stringops[=fn],-fira-algorithm=algorithm,-flive-patching=level,-fira-region=region,-flto-compression-level=n,-flto-partition=alg,-flto-incremental=path,-flto-incremental-cache-size=n,-fauto-profile[=path],-fprofile-use=path,-fsched-stalled-insns-dep[=n],-fsched-stalled-insns[=n],-ftree-parallelize-loops[=n],-freorder-blocks-algorithm=algorithm,-fvect-cost-model,-fzero-call-used-regsand-fvariable-expansion-in-unrollerall come with their own little shapes. copy them exactly. no improvising. iykyk. 💅
🌸 Program Instrumentation Options 🌸
this is the section that's like… watching your program's every move. very "i'm not mad i'm just paying attention". very surveillance-but-make-it-adorable. and every one of them is here, ready:
-p -pg -fprofile-arcs -coverage -ftest-coverage
-fcondition-coverage -fpath-coverage -fprofile -fprofile-abs-path
-fprofile-dir=path -fprofile-generate -fprofile-generate=path
-fprofile-info-section -fprofile-info-section=name
-fprofile-note=path -fprofile-prefix-path=path
-fprofile-update=method -fprofile-filter-files=regex
-fprofile-exclude-files=regex
-fprofile-reproducible=[multithreaded|parallel-runs|serial]
-fsanitize=style -fsanitize-recover -fsanitize-recover=style
-fsanitize-trap -fsanitize-trap=style -fasan-shadow-offset=number
-fsanitize-sections=s1,s2,... -fsanitize-undefined-trap-on-error
-fcf-protection -fcf-protection=[full|branch|return|none|check]
-fharden-compares -fharden-conditional-branches -fhardened
-fharden-control-flow-redundancy -fhardcfr-skip-leaf
-fhardcfr-check-exceptions -fhardcfr-check-returning-calls
-fhardcfr-check-noreturn-calls=[always|no-xthrow|nothrow|never]
-fstack-protector -fstack-protector-all -fstack-protector-strong
-fstack-protector-explicit -fstack-check -fstack-limit-register=reg
-fstack-limit-symbol=sym -fno-stack-limit -fsplit-stack
-fstrub=disable -fstrub=strict -fstrub=relaxed -fstrub=all
-fstrub=at-calls -fstrub=internal
-fvtable-verify=[std|preinit|none] -fvtv-counts -fvtv-debug
-finstrument-functions -finstrument-functions-once
-finstrument-functions-exclude-function-list=sym,sym,...
-finstrument-functions-exclude-file-list=file,file,...
-fprofile-prefix-map=old=new -fpatchable-function-entry=N[,M]
--coverage --profile- the
-fprofile-*girlies really said "let me take notes on you" 📝 and honestly? we love a girl who's thorough. -fstack-protectorcame with a whole squad:-fstack-protector-all,-fstack-protector-strong,-fstack-protector-explicit. protective friendship goals. 🛡️-fhardenedand its-fharden-*and-fhardcfr-*besties are out here choosing violence (in the good way). 🔥- the
-fstrub=girlies have six whole moods:disable,strict,relaxed,all,at-calls,internal. she contains multitudes. 💖 - and
<lavender>--coverage</lavender>+<lavender>--profile</lavender>closing the set like a proper finale. slay.
careful with the bracketed values here —-fprofile-reproducible=[multithreaded|parallel-runs|serial],-fcf-protection=[full|branch|return|none|check],-fhardcfr-check-noreturn-calls=[always|no-xthrow|nothrow|never],-fvtable-verify=[std|preinit|none]— those brackets and pipes are literal. do not touch. 🙈
🦋 Preprocessor Options 🦋
the preprocessor is the girl who reads your text before anyone else does and decides what actually makes it to the group chat. she's got a LOT of opinions. here's every single one:
-C -CC -Dmacro[=defn] -dD -dI -dM -dN -dU -fdebug-cpp
-fdirectives-only -fdollars-in-identifiers -fexec-charset=charset
-fextended-identifiers -finput-charset=charset
-fmacro-prefix-map=old=new -fmax-include-depth=depth
-fno-canonical-system-headers -fpch-deps -fpch-preprocess
-fpreprocessed -ftabstop=width -ftrack-macro-expansion
-fwide-exec-charset=charset -fworking-directory -H -imacros file
-include file -M -MD -MF -MG -MM -MMD -MP -MQ -MT
-Mno-modules -no-integrated-cpp -P -pthread -remap -traditional
-traditional-cpp -trigraphs -Umacro -undef -Wp,option
-Xpreprocessor option --comments --comments-in-macros
--define-macro=macro[=defn] --dependencies --dump=letters
--imacros=file --include=file --no-integrated-cpp
--no-line-commands --print-missing-file-dependencies --traditional
--traditional-cpp --trigraphs --trace-includes
--undefine-macro=macro --user-dependencies --write-dependencies
--write-user-dependencies-Dmacro[=defn]and-Umacroare the original define/undefine duo. icons. they've been here since forever and they still ate. 👑- the capital-
Mgirlies (-M,-MD,-MF,-MG,-MM,-MMD,-MP,-MQ,-MT, and don't forget-Mno-modules) are a whole clique that only hangs out together. the way they're all just single letters and vibes? iconic. 💅 <cyan>--dump=letters</cyan>is literally letters. plural. she's mysterious. she's giving nothing and everything. ✨-traditionaland-traditional-cppare the vintage girlies. they remember when things were different. respect your elders. 🎀
🧚 Assembler Options 🧚
three options. three. that's it. short and sweet and she knows it. 💖
-Wa,option -Xassembler option --for-assembler=option-Wa,option,-Xassembler option,--for-assembler=option. a tiny little trio handing things straight over. no notes. perfect. 🌸
⭐ Linker Options ⭐
okay the linker is the girl who actually puts it all together at the end and she has OPINIONS about libraries and startup and what's static and what's shared. look at this roster:
object-file-name -flink-libatomic -fuse-ld=linker -llibrary
-nostartfiles -nodefaultlibs -nolibc -nostdlib -nostdlib++ -e
entry -pie -pthread -r -rdynamic -s -static -static-pie
-static-libgcc -static-libstdc++ -static-libasan -static-libhwasan
-static-liblsan -static-libtsan -static-libubsan -shared
-shared-libgcc -symbolic -T script -Wl,option -Xlinker option -u
symbol -Tbss=addr -Tdata=addr -Ttext=addr -N -n -t -Z -z
keyword --entry=entry --for-linker=option --force-link=symbol
--no-standard-library --pie --static --static-pie --symbolic-staticvs-shared— the eternal debate. both valid. both eating. 🫶- the
-static-lib*girlies showed up as a full lineup:-static-libgcc,-static-libstdc++,-static-libasan,-static-libhwasan,-static-liblsan,-static-libtsan,-static-libubsan. a whole album. 💿 -pie,--pie,-static-pie,--static-pie— yes, all four. they are NOT the same. do not merge them in your head bestie i'm begging. 😭-nostartfiles,-nodefaultlibs,-nolibc,-nostdlib,-nostdlib++— the "no ❤️" girlies. they're allowed to be like that. 💅
object-file-nameand-llibraryare placeholders for real things you actually type — don't go typing those exact words into your terminal and then @ me about it. 🙈
🌈 Directory Options 🌈
this is where gcc goes looking for stuff. include paths, library paths, plugin paths, the whole treasure hunt. every single stop on the map:
-Bprefix -Idir -I- -idirafter dir -imacros file -imultilib dir
-imultiarch dir -iplugindir=dir -iprefix file -iquote dir
-isysroot dir -isystem dir -iwithprefix dir -iwithprefixbefore dir
-Ldir -no-canonical-prefixes --no-sysroot-suffix -nostdinc
-nostdinc++ --embed-dir=dir --embed-directory=dir --include-barrier
--include-directory=dir --include-directory-after=dir
--include-prefix=prefix --include-with-prefix=prefix
--include-with-prefix-after=prefix
--include-with-prefix-before=prefix --no-canonical-prefixes
--no-standard-includes --prefix=prefix --sysroot=dir-Idirand-Ldir— one letter, one capital, two completely different jobs. classic. 💀-I-is literally anIand a dash. that's it. that's the option. she's minimal and she's fine with it. ✨--no-canonical-prefixesis in here twice. yes, twice. don't ask. just list it twice. 🎀<mint>--include-directory=dir</mint>,<mint>--include-directory-after=dir</mint>,<mint>--include-prefix=prefix</mint>,<mint>--include-with-prefix=prefix</mint>,<mint>--include-with-prefix-after=prefix</mint>,<mint>--include-with-prefix-before=prefix</mint>— okay the long forms went OFF. thorough queens. 👑
💜 Code Generation Options 💜
last little bundle in this part and it's the one that decides what actually comes out the other side. registers, exceptions, enums, structs, visibility, all of it:
-fcall-saved-reg -fcall-used-reg -ffixed-reg -fexceptions
-fnon-call-exceptions -fdelete-dead-exceptions -funwind-tables
-fasynchronous-unwind-tables -fno-gnu-unique
-finhibit-size-directive -fcommon -fno-ident -fpcc-struct-return
-fpic -fPIC -fpie -fPIE -fno-plt -fno-jump-tables
-fno-bit-tests -frecord-gcc-switches -freg-struct-return
-fshort-enums -fshort-wchar -fverbose-asm -fpack-struct[=n]
-fleading-underscore -ftls-model=model -fstack-reuse=reuse_level
-ftrampolines -ftrampoline-impl=[stack|heap] -ftrapv -fwrapv
-fwrapv-pointer -fvisibility=[default|internal|hidden|protected]
-fstrict-volatile-bitfields -fsync-libcalls
-fzero-init-padding-bits=value -Qy -Qn-fpic,-fPIC,-fpie,-fPIE— the case-sensitivity girlies said "we are FOUR different options and we WILL be respected." and honestly? correct. 💅-ftrapvand-fwrapvand-fwrapv-pointerare all sitting right next to each other. awkward? no. iconic? yes. 👑-ftrampoline-impl=[stack|heap]</peach>— those brackets and the|are literal, bestie. copy them exactly. 🙈-fvisibility=[default|internal|hidden|protected]</yellow>— four whole moods in one bracket. she's complex. we love her. ✨- and then
-Qyand-Qnjust… end the list. no-f. no lowercase. just vibes. 🎀
and that's not the end of the page, bestie — there's so much more coming and i am NOT done yapping. stay hydrated. 💖
🛠️ Developer Options 🛠️
okay bestie, these are the dev-only flags — the behind-the-scenes lore for compiler girlies who like to peek under the hood. every single one is here, no cap. 💅
-dletters-dumpspecs-dumpmachine-dumpversion-dumpfullversion-fcallgraph-info[=su,da]-fchecking-fchecking=n-fdbg-cnt-list-fdbg-cnt=counter-value-list-fdisable-ipa-pass_name-fdisable-rtl-pass_name-fdisable-rtl-pass-name=range-list-fdisable-tree-pass_name-fdisable-tree-pass-name=range-list-fdump-debug-fdump-earlydebug-fdump-noaddr-fdump-unnumbered-fdump-unnumbered-links-fdump-final-insns[=file]-fdump-internal-locations-fdump-ipa-all-fdump-ipa-cgraph-fdump-ipa-inline-fdump-lang-all-fdump-lang-switch-fdump-lang-switch-options-fdump-lang-switch-options=filename-fdump-passes-fdump-rtl-pass-fdump-rtl-pass=filename-fdump-statistics-fdump-tree-all-fdump-tree-switch-fdump-tree-switch-options-fdump-tree-switch-options=filename-fcompare-debug[=opts]-fcompare-debug-second-fenable-kind-pass-fenable-kind-pass=range-list-fira-verbose=n-flto-report-flto-report-wpa-fmem-report-wpa-fmem-report-fpre-ipa-mem-report-fpost-ipa-mem-report-fopt-info-fopt-info-options[=file]-fmultiflags-fprofile-report-frandom-seed=string-fsched-verbose=n-fsel-sched-verbose-fsel-sched-dump-cfg-fsel-sched-pipelining-verbose-fstats-fstack-usage-ftime-report-ftime-report-details-fvar-tracking-assignments-toggle-gtoggle-print-autofdo-gcov-version-print-file-name=library-print-libgcc-file-name-print-multi-directory-print-multi-lib-print-multi-os-directory-print-multiarch-print-prog-name=program-print-search-dirs-Q-print-sysroot-print-sysroot-headers-suffix-save-temps-save-temps=cwd-save-temps=obj-specs=file-time[=file]--dump=letters--print-autofdo-gcov-version--print-file-name=library--print-libgcc-file-file-name--print-multi-directory--print-multi-lib--print-multi-os-directory--print-multiarch--print-prog-name=program--print-search-dirs--print-sysroot--print-sysroot-headers-suffix--save-temps--specs=file--param name=value
🌍 Target-Specific Options 🌍
and now the target-specific menu — every architecture gets its own lil flag family. iykyk. 🦋
🌟 AArch64 Options (AArch64 Options) 🌟
-mabi=name-mbig-endian-mlittle-endian-menable-sysreg-checking-mgeneral-regs-only-mcmodel=tiny-mcmodel=small-mcmodel=large-mstrict-align-momit-leaf-frame-pointer-mtls-dialect=desc-mtls-dialect=traditional-mtls-size=size-mtp=name-mfix-cortex-a53-835769-mfix-cortex-a53-843419-mlow-precision-recip-sqrt-mlow-precision-sqrt-mlow-precision-div-mmax-vectorization-mautovec-preference=name-mpc-relative-literal-loads-msign-return-address=scope-mbranch-protection=features-mharden-sls=opts-march=name-mcpu=name-mtune=name-moverride=string-mstack-protector-guard=guard-mstack-protector-guard-reg=sysreg-mstack-protector-guard-offset=offset-mtrack-speculation-moutline-atomics-mearly-ra-mearly-ldp-fusion-mlate-ldp-fusion-mnarrow-gp-writes-msve-vector-bits=bits
🌸 Adapteva Epiphany Options (Adapteva Epiphany Options) 🌸
-mhalf-reg-file-mprefer-short-insn-regs-mbranch-cost=num-mcmove-mnops=num-msoft-cmpsf-msplit-lohi-mpost-inc-mpost-modify-mstack-offset=num-mround-nearest-mlong-calls-mshort-calls-msmall16-mfp-mode=mode-mmay-round-for-trunc-mfp-iarith-mvect-double-max-vect-align=num-msplit-vecmove-early-m1reg-reg
💜 AMD GCN Options (AMD GCN Options) 💜
-march=gpu-mtune=gpu-mgang-private-size=bytes-msram-ecc=[on|off|any]-mxnack=[on|off|any]-Wopenacc-dims
🎀 ARC Options (ARC Options) 🎀
-mbarrel-shifter-mjli-always-mcpu=cpu-mA6-mARC600-mA7-mARC700-mdpfp-mdpfp-compact-mdpfp-fast-mno-dpfp-lrsr-mea-mmul32x16-mmul64-matomic-mnorm-mspfp-mspfp-compact-mspfp-fast-msimd-msoft-float-mswap-mlock-mswape-mxy-misize-marclinux-marclinux_prof-mlong-calls-mmedium-calls-msdata-mirq-ctrl-saved-mrgf-banked-regs-mlpc-width=width-G num-mvolatile-cache-mtp-regno=regno-mbitops-mcmem-munaligned-access-mauto-modify-reg-mno-brcc-mcase-vector-pcrel-mno-cond-exec-mearly-cbranchsi-mindexed-loads-mlra-priority-none-mlra-priority-compact-mlra-priority-noncompact-mmillicode-msize-level=level-mtune=cpu-mmultcost=num-mcode-density-frame-mmpy-option=multo-mdiv-rem-mcode-density-mll64-mfpu=fpu-mrf16-mbranch-index
🦋 ARM Options (ARM Options) 🦋
-mapcs-frame-mapcs-mabi=name-mgeneral-regs-only-mno-sched-prolog-mlittle-endian-mbig-endian-mbe8-mbe32-mfloat-abi=name-mfp16-format=name-mthumb-interwork-mcpu=name-march=name-mfpu=name-mtune=name-mstructure-size-boundary=n-mabort-on-noreturn-mlong-calls-msingle-pic-base-mpic-register=reg-mpic-data-is-text-relative-mnop-fun-dllimport-mpoke-function-name-mthumb-marm-mtpcs-frame-mtpcs-leaf-frame-mcaller-super-interworking-mcallee-super-interworking-mtp=name-mtls-dialect=dialect-mword-relocations-mfix-cortex-m3-ldrd-mfix-cortex-a57-aes-1742098-mfix-cortex-a72-aes-1655431-munaligned-access-mslow-flash-data-masm-syntax-unified-mrestrict-it-mpure-code-mcmse-mfix-cmse-cve-2021-35465-mstack-protector-guard=guard-mstack-protector-guard-offset=offset-mfdpic-mbranch-protection=features-mvectorize-with-neon-quad-mvectorize-with-neon-double
✨ AVR Options (AVR Options) ✨
-mmcu=mcu-mabsdata-maccumulate-args-mcvt-mbranch-cost=cost-mfuse-add=level-mfuse-move=level-mfuse-move2-mcall-prologues-mgas-isr-prologues-mint8-mflmap-mdouble=bits-mlong-double=bits-mno-call-main-mn_flash=size-mfract-convert-truncate-mno-interrupts-mmain-is-OS_task-mrelax-mpmem-wrap-around-mrmw-mstrict-X-mtiny-stack-mrodata-in-ram-msplit-bit-shift-msplit-ldst-mshort-calls-mskip-bug-muse-nonzero-bits-nodevicelib-nodevicespecs-Waddr-space-convert-Wmisspelled-isr
🍓 Blackfin Options (Blackfin Options) 🍓
-mcpu=cpu[-sirevision]-msim-momit-leaf-frame-pointer-mspecld-anomaly-mcsync-anomaly-mlow-64k-mstack-check-l1-mid-shared-library-mleaf-id-shared-library-mshared-library-id=n-msep-data-mlong-calls-mfast-fp-minline-plt-mmulticore-mcorea-mcoreb-msdram-micplb
🌈 C6X Options (C6X Options) 🌈
-mbig-endian-mlittle-endian-march=cpu-msim-msdata=sdata-type-mdsbt-mlong-calls
⭐ CRIS Options (CRIS Options) ⭐
-mcpu=cpu-march=cpu-mtune=cpu-mmax-stackframe=n-metrax4-metrax100-mpdebug-mcc-init-mno-side-effects-mstack-align-mdata-align-mconst-align-m32-bit-m16-bit-m8-bit-mno-prologue-epilogue-mbest-lib-options-moverride-best-lib-options-mtrap-using-break8-mtrap-unaligned-atomic-munaligned-atomic-may-use-library-sim-sim2-mmul-bug-workaround
🧚 C-SKY Options (C-SKY Options) 🧚
-march=arch-mcpu=cpu-mbig-endian-mlittle-endian-mfpu=fpu-mdouble-float-mfdivdu-mfloat-abi=name-melrw-mistack-mmp-mcp-mcache-msecurity-mtrust-mdsp-medsp-mvdsp-mdiv-msmart-mhigh-registers-manchor-mpushpop-mmultiple-stld-mconstpool-mstack-size-mccrt-mbranch-cost=n-msched-prolog-msim
👑 Cygwin and MinGW Options (Cygwin and MinGW Options) 👑
-mconsole-mcrtdll=library-mdll-mnop-fun-dllimport-mthreads-municode-mwin32-mwindows-fno-set-stack-executable-fwritable-relocated-rdata-mpe-aligned-commons-muse-libstdc-wrappers
🎀 Darwin Options (Darwin Options) 🎀
-all_load-allowable_client-arch name-arch_errors_fatal-asm_macosx_version_min=version-bind_at_load-bundle-bundle_loader-client_name-compatibility_version-current_version-dead_strip-dependency-file-dylib_file-dylinker-dylinker_install_name-dynamic-dynamiclib-exported_symbols_list-fapple-kext-fconstant-cfstrings-ffix-and-continue-filelist-findirect-data-flat_namespace-force_cpusubtype_ALL-force_flat_namespace-framework name-gfull-gused-headerpad_max_install_names-iframework-image_base-init symbol-name-install_name-keep_private_externs-matt-stubs-mconstant-cfstrings-mdynamic-no-pic-mfix-and-continue-mkernel-mmacosx-version-min=version-mone-byte-bool-msymbol-stubs-mtarget-linker[=]version-nodefaultexport-nodefaultrpaths-pagezero_size-preload-read_only_relocs-sectalign-sectcreate-seg_addr_table-seg1addr-segaddr-segprot-segs_read_only_addr-segs_read_write_addr-sub_library-sub_umbrella-twolevel_namespace-twolevel_namespace_hints-umbrella-undefined-unexported_symbols_list-weak_framework name-weak_reference_mismatches-whatsloaded-whyload-F-ObjC-ObjC++-Wnonportable-cfstrings
💀 DEC Alpha Options (DEC Alpha Options) 💀
-mno-fp-regs-msoft-float-mieee-mieee-with-inexact-mieee-conformant-mfp-trap-mode=mode-mfp-rounding-mode=mode-mtrap-precision=mode-mbuild-constants-mcpu=cpu-type-mtune=cpu-type-mbwx-mmax-mfix-mcix-msafe-bwa-msafe-partial-mfloat-vax-mfloat-ieee-mexplicit-relocs-msmall-data-mlarge-data-msmall-text-mlarge-text-mmemory-latency=time-mtls-kernel-mtls-size=bitsize-mlong-double-128-mlong-double-64
🔥 eBPF Options (eBPF Options) 🔥
-mbig-endian-mlittle-endian-mframe-limit=bytes-mxbpf-mco-re-mjmpext-mjmp32-malu32-mv3-atomics-mbswap-msdiv-msmov-mcpu=version-masm=dialect-minline-memops-threshold=bytes-Wco-re
🫶 FR30 Options (FR30 Options) 🫶
-msmall-model-mno-lsim
🎀 FRV Options (FRV Options) 🎀
-mgpr-32-mgpr-64-mfpr-32-mfpr-64-mhard-float-msoft-float-malloc-cc-mfixed-cc-mdword-mdouble-mmedia-mmuladd-mfdpic-minline-plt-mgprel-ro-multilib-library-pic-mlinked-fp-mlong-calls-malign-labels-mlibrary-pic-macc-4-macc-8-mpack-mno-eflags-mno-cond-move-mno-optimize-membar-mno-scc-mno-cond-exec-mno-vliw-branch-mno-multi-cond-exec-mno-nested-cond-exec-mtomcat-stats-mTLS-mtls-mcpu=cpu
🌸 FT32 Options (FT32 Options) 🌸
-msim-mnodiv-mft32b-mcompress-mnopm
🐧 GNU/Linux Options (GNU/Linux Options) 🐧
-mglibc-muclibc-mmusl-mbionic-mandroid-tno-android-cc-tno-android-ld
🌟 H8/300 Options (H8/300 Options) 🌟
-mrelax-mh-ms-mn-msx-ms2600-mquickcall-mslowbyte-mexr-mint32-malign-300
okay so the machine-dependent option tour is still going and honestly? she's thriving. grab a snack bestie, we're doing like fifteen architectures in a row 💖
HPPA Options 👑
HPPA said "here's my whole personality" and honestly, respect:
-march=architecture-type-mno-atomic-libcalls-mcaller-copies-mdisable-fpregs-mdisable-indexing-mordered-mfast-indirect-calls-mgas-mgnu-ld-mhp-ld-mfixed-range=register-range-mcoherent-ldcw-mlinker-opt-mlong-calls-mlong-load-store-mno-space-regs-msoft-float-mpa-risc-1-0-mpa-risc-1-1-mpa-risc-2-0-mportable-runtime-mschedule=cpu-type-msoft-mult-msio-mwsio-munix=unix-std-nolibdld-static-threads
IA-64 Options 🌈
Itanium walked so your faves could run. lowkey a whole era:
-mbig-endian-mlittle-endian-mgnu-as-mgnu-ld-mno-pic-mvolatile-asm-stop-mregister-names-msdata-mconstant-gp-mauto-pic-minline-float-divide-min-latency-minline-float-divide-max-throughput-mno-inline-float-divide-minline-int-divide-min-latency-minline-int-divide-max-throughput-mno-inline-int-divide-minline-sqrt-min-latency-minline-sqrt-max-throughput-mno-inline-sqrt-mdwarf2-asm-mearly-stop-bits-mfixed-range=register-range-mtls-size=tls-size-mtune=cpu-type-milp32-mlp64-msched-br-data-spec-msched-ar-data-spec-msched-control-spec-msched-br-in-data-spec-msched-ar-in-data-spec-msched-in-control-spec-msched-spec-ldc-msched-spec-control-ldc-msched-stop-bits-after-every-cycle-msched-count-spec-in-critical-path-msel-sched-dont-check-control-spec-msched-fp-mem-deps-zero-cost-msched-max-memory-insns-hard-limit-msched-max-memory-insns=max-insns
LM32 Options 🦋
short, sweet, five flags, no notes:
-mbarrel-shift-enabled-mdivide-enabled-mmultiply-enabled-msign-extend-enabled-muser-enabled
LoongArch Options 🐉
she's new-ish and she came with RECEIPTS:
-march=arch-type-mtune=tune-type-mabi=base-abi-type-mfpu=fpu-type-msimd=simd-type-msoft-float-msingle-float-mdouble-float-mlsx-mlasx-mbranch-cost=n-maddr-reg-reg-cost=n-mcheck-zero-division-mbreak-code=code-mcond-move-int-mcond-move-float-memcpy-mstrict-align-G num-mmax-inline-memcpy-size=n-mexplicit-relocs=style-mexplicit-relocs-mno-explicit-relocs-mdirect-extern-access-mcmodel=code-model-mrelax-mpass-mrelax-to-as-mrecip-mrecip=opt-mfrecipe-mdiv32-mlam-bh-mlamcas-mld-seq-sa-mscq-mtls-dialect=opt-mannotate-tablejump
LynxOS Options 🎀
three options and a dream:
-mshared-mthreads-mlegacy-threads
M32C Options ✨
-mcpu=cpu-msim-memregs=number
M32R/D Options 🌸
-m32r2-m32rx-m32r-mdebug-malign-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
M680x0 Options 💜
the 68k family reunion, and everyone showed up:
-march=arch-mcpu=cpu-mtune=tune-m68000-m68020-m68020-40-m68020-60-m68030-m68040-m68060-m68302-m68332-m68851-mcpu32-mfidoa-m5200-m5206e-m528x-m5307-m5407-mcfv4e-mbitfield-mc68000-mc68020-mrtd-mdiv-mshort-mhard-float-m68881-msoft-float-mpcrel-malign-int-mstrict-align-msep-data-mshared-library-id=n-mid-shared-library-mxgot-mxtls-mlong-jump-table-offsets
MCore Options 🫶
-mhardlit-mdiv-mrelax-immediates-mwide-bitfields-m4byte-functions-mcallgraph-data-mslow-bytes-mno-lsim-mlittle-endian-mbig-endian-m210-m340-mstack-increment
MicroBlaze Options ⭐
the -mxl- girlies really understood the assignment:
-msoft-float-mhard-float-msmall-divides-mcpu=cpu-mmemcpy-mxl-soft-mul-mxl-soft-div-mxl-barrel-shift-mxl-pattern-compare-mxl-gp-opt-mxl-multiply-high-mxl-float-convert-mxl-float-sqrt-mbig-endian-mlittle-endian-mxl-reorder-mxl-mode-app-model-mxl-prefetch-mpic-data-is-text-relative
MIPS Options 👑
okay this is THE section. MIPS did not come to play, she came to serve. deep breath, bestie, we're going in:
-EL-EB-mel-meb-march=arch-mtune=arch-mips1-mips2-mips3-mips4-mips32-mips32r2-mips32r3-mips32r5-mips32r6-mips64-mips64r2-mips64r3-mips64r5-mips64r6-mips16-mmips16e2-mflip-mips16-minterlink-compressed-minterlink-mips16-mabi=abi-mabicalls-mshared-mplt-mxgot-mgp32-mgp64-mfp32-mfpxx-mfp64-mhard-float-msoft-float-mno-float-msingle-float-mdouble-float-modd-spreg-mabs=mode-mnan=encoding-mdsp-mdspr2-mmcu-meva-mvirt-mxpa-mcrc-mginv-mmicromips-mmsa-mloongson-mmi-mloongson-ext-mloongson-ext2-mfpu=fpu-type-msmartmips-mpaired-single-mdmx-mips3d-mmt-mllsc-mlong64-mlong32-msym32-Gnum-mno-local-sdata-mno-extern-sdata-mno-gopt-membedded-data-muninit-const-in-rodata-mcode-readable=setting-mno-data-in-code-mcode-xonly-msplit-addresses-mexplicit-relocs-mexplicit-relocs=release-mno-check-zero-division-mdivide-traps-mdivide-breaks-mno-load-store-pairs-mstrict-align-mno-unaligned-access-mmemcpy-mlong-calls-mmad-mimadd-mno-fused-madd-nocpp-mfix-24k-mfix-r4000-mfix-r4400-mfix-r5900-mfix-r10000-mfix-rm7000-mfix-vr4120-mfix-vr4130-mfix-sb1-mfix4300-mr10k-cache-barrier=setting-mflush-func=func-mno-flush-func-mbranch-cost=num-mbranch-likely-mcompact-branches=policy-mno-fp-exceptions-mvr4130-align-msynci-mno-lxc1-sxc1-mno-madd4-mno-relax-pic-calls-mmcount-ra-address-mframe-header-opt
ate. no crumbs. periodt 💅
MMIX Options 🌈
-mlibfuncs-mepsilon-mabi=gnu-mabi=mmixware-mzero-extend-mknuthdiv-mtoplevel-symbols-melf-mbranch-predict-mbase-addresses-msingle-exit-mset-data-start=address-mset-program-start=address-mno-set-program-start
MN10300 Options 🧚
-mmult-bug-mno-mult-bug-mam33-mam33-2-mam34-mtune=cpu-type-mno-return-pointer-on-d0-mno-crt0-mrelax-mno-liw-mno-setlb
Moxie Options 🍓
tiny queen, four flags, slaying:
-meb-mel-mmul.x-mno-crt0
MSP430 Options 🎀
-msim-masm-hex-mmcu=name-mlarge-msmall-mrelax-mwarn-mcu-mwarn-devices-csv-mcode-region=where-mdata-region=where-muse-lower-region-prefix-msilicon-errata=name[,name...]-msilicon-errata-warn=name[,name...]-mhwmult=type-minrt-mtiny-printf-mmax-inline-shift=n
NDS32 Options 💜
-mbig-endian-mlittle-endian-EB-EL-mabi=name-mfloat-abi=name-mreduced-regs-mfull-regs-malways-align-malign-functions-mfp-as-gp-mcmov-mhw-abs-mext-perf-mext-perf2-mext-string-mext-dsp-mext-fpu-fma-mext-fpu-sp-mext-fpu-dp-mv3push-m16-bit-mvh-misr-vector-size=num-misr-secure=num-mcache-block-size=num-march=arch-mcpu=cpu-mconfig-fpu=num-mconfig-mul=type-mconfig-register-ports=kind-mcmodel=code-model-mctor-dtor-mrelax-mrelax-hint-msched-prolog-epilog-mno-ret-in-naked-func-malways-save-lp-munaligned-access-minline-asm-r15
Nvidia PTX Options ⭐
GPU girlies rise:
-m64-march=arch-misa=arch-march-map=arch-mptx=version-mmainkernel-moptimize-msoft-stack-msoft-stack-reserve-local=size-muniform-simt-mgomp
OpenRISC Options 🦋
-mboard=name-mhard-mul-mhard-div-msoft-mul-msoft-div-msoft-float-mhard-float-mdouble-float-munordered-float-mcmov-mror-mrori-msext-msfimm-mshftimm-mcmodel=code-model
PDP-11 Options 🌸
retro icon, no notes:
-mfpu-msoft-float-mac0-m40-m45-m10-mint32-mint16-msplit-munix-asm-mdec-asm-mgnu-asm-mlra
Picolibc Options ✨
these ones use double dashes, so you KNOW they're fancy:
--oslib=library--crt0=[none|minimal|hosted|semihost]--printf=[d|f|l|i|m]--scanf=[d|f|l|i|m]
PowerPC Options
bestie this one is just a redirect, no content, we love a cross-reference: See RS/6000 and PowerPC Options.
PRU Options 🫶
-mmcu=mcu-minrt-mno-relax-mloop-mmul-mfillzero-mabi=variant
RISC-V Options 🔥
RISC-V said "I'm open source AND I have forty-two options", and honestly? iconic:
-mbranch-cost=N-instruction-mabi=ABI-string-mfdiv-mdiv-mno-fence-tso-misa-spec=ISA-spec-string-march=[ISA|Profile|Profile_ISA|processor-string]-mcpu=processor-string-mtune=processor-string-mpreferred-stack-boundary=num-msmall-data-limit=N-bytes-msave-restore-mno-shorten-memrefs-mstrict-align-mscalar-strict-align-mno-vector-strict-align-mcmodel=medlow-mcmodel=medany-mcmodel=large-mexplicit-relocs-mrelax-mriscv-attribute-malign-data=type-mbig-endian-mlittle-endian-mstack-protector-guard=guard-mstack-protector-guard-reg=reg-mstack-protector-guard-offset=offset-mcsr-check-momit-leaf-frame-pointer-mmovcc-mno-inline-atomics-mno-inline-strlen-mno-inline-strcmp-mno-inline-strncmp-mstringop-strategy=strategy-mtls-dialect=desc-mtls-dialect=trad-mrvv-vector-bits=value-mrvv-max-lmul=value-madjust-lmul-cost-mmax-vectorization-mno-autovec-segment
RL78 Options 🧚
closing this batch out with the microcontroller cutie:
-msim-mallregs-mrelax-mes0-mmul=none-mmul=g13-mmul=g14-mmul=rl78-mcpu=g10-mcpu=g13-mcpu=g14-mcpu=rl78-mg10-mg13-mg14-mrl78-msave-mduc-in-interrupts
✨ RS/6000 and PowerPC Options (RS/6000 and PowerPC Options) ✨
okay bestie, if you're on RS/6000 or PowerPC, these are your flags — no cap:
-mcpu=cpu-type -mtune=cpu-type -mcmodel=code-model -mprofile-kernel
-mpowerpc64 -maltivec -mpowerpc-gpopt -mpowerpc-gfxopt -mmfcrf
-mpopcntb -mpopcntd -mfprnd -mcmpb -mhard-dfp -mfull-toc
-mminimal-toc -mno-fp-in-toc -mno-sum-in-toc -maix64 -maix32
-m64 -m32 -mxl-compat -mpe -malign-power -malign-natural
-msoft-float -mhard-float -mmultiple -mupdate
-mavoid-indexed-addresses -mfused-madd -mbit-align -mbit-word
-mstrict-align -mrelocatable -mrelocatable-lib -mlittle
-mlittle-endian -mbig -mbig-endian -mdynamic-no-pic
-msingle-pic-base -mprioritize-restricted-insns=priority
-msched-costly-dep=dependence_type -minsert-sched-nops=scheme
-mcall-aixdesc -mcall-eabi -mcall-freebsd -mcall-linux
-mcall-netbsd -mcall-openbsd -mcall-sysv -mcall-sysv-eabi
-mcall-sysv-noeabi -mtraceback=traceback_type -maix-struct-return
-msvr4-struct-return -mabi=abi-type -msecure-plt -mbss-plt
-msplit-patch-nops -mregnames -mlongcall -mpltseq -mtls-markers
-mblock-move-inline-limit=num -mblock-compare-inline-limit=num
-mblock-compare-inline-loop-limit=num -mblock-ops-unaligned-vsx
-mstring-compare-inline-limit=num -misel -mvsx -mvrsave -mmulhw
-mdlmzb -mprototype -msim -mmvme -mads -myellowknife -memb
-meabi -msdata -msdata=opt -mreadonly-in-sdata -mvxworks -G num
-mrecip -mrecip=opt -mno-recip -mrecip-precision -mveclibabi=type
-mfriz -mpointers-to-nested-functions -msave-toc-indirect
-mpower8-fusion -mcrypto -mhtm -mquad-memory -mquad-memory-atomic
-mcompat-align-parm -mfloat128 -mfloat128-hardware -mgnu-attribute
-mstack-protector-guard=guard -mstack-protector-guard-reg=reg
-mstack-protector-guard-offset=offset -mprefixed -mpcrel -mmma
-mdense-math -mrop-protect -mprivileged -mno-splat-word-constant
-mno-splat-float-constant -mno-ieee128-constant
-mno-warn-altivec-long🦋 RX Options (RX Options) 🦋
RX energy only, bestie — here's the lineup:
-m64bit-doubles -m32bit-doubles -fpu
-nofpu -mcpu=name -mbig-endian-data -mlittle-endian-data
-msmall-data-limit=N -msim -mas100-syntax -mrelax
-mmax-constant-size=N -mint-register=N -mpid
-mno-allow-string-insns -mjsr -mno-warn-multiple-fast-interrupts
-msave-acc-in-interrupts -mlra💖 S/390 and zSeries Options (S/390 and zSeries Options) 💖
mainframe moment, periodt:
-mtune=cpu-type -march=cpu-type -mhard-float -msoft-float
-mhard-dfp -mlong-double-64 -mlong-double-128 -mbackchain
-mpacked-stack -msmall-exec -mmvcle -m64 -m31 -mdebug -mesa
-mzarch -mhtm -mvx -mzvector -mtpf-trace -mtpf-trace-skip -mmain
-mfused-madd -mno-fused-madd -mwarn-framesize=framesize
-mwarn-dynamicstack -mstack-size=stack-size -mno-stack-size
-mstack-guard=stack-guard -mno-stack-guard
-mstack-protector-guard=guard -mstack-protector-guard-record
-mhotpatch=pre-halfwords,post-halfwords
-mno-pic-data-is-text-relative -mindirect-branch=choice
-mindirect-branch-jump=choice -mindirect-branch-call=choice
-mfunction-return=choice -mfunction-return-mem=choice
-mfunction-return-reg=choice -mindirect-branch-table -mfentry
-mrecord-mcount -mnop-mcount -mpreserve-args -munaliged-symbols🌸 SH Options (SH Options) 🌸
SH gang, this one's for you:
-m1 -m2 -m2e -m2a -m2a-nofpu -m2a-single
-m2a-single-only -m3 -m3e -m4 -m4-nofpu -m4-single
-m4-single-only -m4-100 -m4-100-nofpu -m4-100-single
-m4-100-single-only -m4-200 -m4-200-nofpu -m4-200-single
-m4-200-single-only -m4-300 -m4-300-nofpu -m4-300-single
-m4-300-single-only -m4-340 -m4-400 -m4-500 -m4a -m4al
-m4a-nofpu -m4a-single -m4a-single-only -mb -ml -mdalign
-mrelax -mbigtable -mbitops -mfmovd -mrenesas -mnomacsave -mieee
-misize -minline-ic_invalidate -mprefergot -musermode
-multcost=number -mdiv=strategy -mdivsi3_libfunc=name
-mfixed-range=register-range -maccumulate-outgoing-args
-matomic-model=atomic-model -mbranch-cost=num -mzdcbranch
-mcbranch-force-delay-slot -mfsca -mfsrra -mpretend-cmove -mfdpic
-mtas -mlra☀️ Solaris 2 Options (Solaris 2 Options) ☀️
Solaris 2 said "let me be clear":
-mclear-hwcap
-mno-clear-hwcap -mimpure-text -mno-impure-text -gsctf🔥 SPARC Options (SPARC Options) 🔥
SPARC it up, bestie:
-mcpu=cpu-type -mtune=cpu-type
-mcmodel=code-model -mmemory-model=mem-model -m32 -m64 -mptr32
-mptr64 -mapp-regs -mfaster-structs -mflat -mfpu -mhard-float
-msoft-float -mhard-quad-float -msoft-quad-float -mstack-bias
-mstd-struct-return -munaligned-doubles -muser-mode -mv8plus -mvis
-mvis2 -mvis3 -mvis3b -mvis4 -mvis4b -mcbcond -mfmaf -mfsmuld
-mpopc -msubxc -mfix-at697f -mfix-ut699 -mfix-ut700
-mfix-gr712rc💀 System V Options (System V Options) 💀
old school but still iconic:
-YP,paths -Ym,dir💙 V850 Options (V850 Options) 💙
V850 vibes:
-mlong-calls -mep -mprolog-function
-mspace -mtda=n -msda=n -mzda=n -mv850 -mv850e3v5 -m850e2v4
-mv850e2v3 -mv850e2 -mv850e1 -mv850es -mv850e -mdisable-callt
-mrelax -mlong-jumps -msoft-float -mhard-float -mloop -mrh850-abi
-mghs -mgcc-abi -m8byte-align -mbig-switch -mapp-regs
-msmall-sld -mno-strict-align -mjump-tables-in-data-section🍓 VAX Options (VAX Options) 🍓
VAX, the retro queen:
-munix -mgnu -md -md-float -mg
-mg-float -mlra -mvaxc-alignment -mqmath🧚 Visium Options (Visium Options) 🧚
Visium says hi:
-mdebug -msim -mfpu -mhard-float
-msoft-float -mcpu=cpu-type -mtune=cpu-type -msv-mode -muser-mode🫶 VMS Options (VMS Options) 🫶
VMS, we see you:
-mvms-return-codes -mdebug-main=prefix
-mmalloc64 -mpointer-size=size🌈 VxWorks Options (VxWorks Options) 🌈
VxWorks bestie:
-mrtp -msmp -mvthreads
-non-static -Bstatic -Bdynamic -Xbind-lazy -Xbind-now👑 x86 Options (x86 Options) 👑
the x86 options are literally a whole buffet, bestie. go off:
-mtune=cpu-type -march=cpu-type
-mtune-ctrl=feature-list -mdump-tune-features -mno-default
-mfpmath=unit -masm=dialect -mno-fancy-math-387 -mno-fp-ret-in-387
-m80387 -mhard-float -msoft-float -mieee-fp -mrtd -malign-double
-mpreferred-stack-boundary=num -mincoming-stack-boundary=num -mcld
-mcx16 -msahf -mmovbe -mcrc32 -mmwait -mrecip -mrecip=opt
-mvzeroupper -mstv -mprefer-avx128 -mprefer-vector-width=opt
-mpartial-vector-fp-math -mmove-max=bits -mstore-max=bits
-mnoreturn-no-callee-saved-registers -mmmx -msse -msse2 -msse3
-mssse3 -msse4.1 -msse4.2 -msse4 -mavx -mavx2 -mavx512f
-mavx512cd -mavx512vl -mavx512bw -mavx512dq -mavx512ifma
-mavx512vbmi -msha -maes -mpclmul -mfsgsbase -mrdrnd -mf16c
-mfma -mpconfig -mwbnoinvd -mptwrite -mclflushopt -mclwb
-mxsavec -mxsaves -msse4a -m3dnow -m3dnowa -mpopcnt -mabm
-mbmi -mtbm -mfma4 -mxop -madx -mlzcnt -mbmi2 -mfxsr -mxsave
-mxsaveopt -mrtm -mhle -mlwp -mmwaitx -mclzero -mpku -mgfni
-mvaes -mwaitpkg -mshstk -mmanual-endbr -mcet-switch
-mforce-indirect-call -mavx512vbmi2 -mavx512bf16 -menqcmd
-mvpclmulqdq -mavx512bitalg -mmovdiri -mmovdir64b
-mavx512vpopcntdq -mavx512vnni -mprfchw -mrdpid -mrdseed -msgx
-mavx512vp2intersect -mserialize -mtsxldtrk -mamx-tile -mamx-int8
-mamx-bf16 -muintr -mhreset -mavxvnni -mamx-fp8 -mavx512fp16
-mavxifma -mavxvnniint8 -mavxneconvert -mcmpccxadd -mamx-fp16
-mprefetchi -mraoint -mamx-complex -mavxvnniint16 -msm3 -msha512
-msm4 -mapxf -musermsr -mavx10.1 -mavx10.2 -mamx-avx512
-mamx-tf32 -mmovrs -mamx-movrs -mavx512bmm -mcldemote
-mms-bitfields -mno-align-stringops -minline-all-stringops
-minline-stringops-dynamically -mstringop-strategy=alg -mkl
-mwidekl -mmemcpy-strategy=strategy -mmemset-strategy=strategy
-mpush-args -maccumulate-outgoing-args -m128bit-long-double
-m96bit-long-double -mlong-double-64 -mlong-double-80
-mlong-double-128 -mregparm=num -msseregparm -mveclibabi=type
-mvect8-ret-in-mem -mpc32 -mpc64 -mpc80 -mdaz-ftz -mstackrealign
-mstack-arg-probe -momit-leaf-frame-pointer -mno-red-zone
-mno-tls-direct-seg-refs -mcmodel=code-model -mabi=name
-maddress-mode=mode -m32 -m64 -mx32 -m16 -miamcu
-mlarge-data-threshold=num -msse2avx -mfentry -mrecord-mcount
-mnop-mcount -m8bit-idiv -minstrument-return=type -mrecord-return
-mfentry-name=name -mfentry-section=name -mskip-rax-setup
-mavx256-split-unaligned-load -mavx256-split-unaligned-store
-malign-data=type -mstack-protector-guard=guard
-mstack-protector-guard-reg=reg
-mstack-protector-guard-offset=offset
-mstack-protector-guard-symbol=symbol -mgeneral-regs-only
-mcall-ms2sysv-xlogues -mtls-dialect=type -mrelax-cmpxchg-loop
-mindirect-branch=choice -mfunction-return=choice
-mindirect-branch-register -mharden-sls=choice
-mindirect-branch-cs-prefix -mapx-inline-asm-use-gpr32 -mgather
-mscatter -mneeded -mno-direct-extern-access
-munroll-only-small-loops -mdispatch-scheduler -mlam=choice💜 x86 Windows Options 💜
x86 Windows Options? See Cygwin and MinGW Options.
⭐ Xstormy16 Options (Xstormy16 Options) ⭐
Xstormy16 keeps it simple:
-msim🎀 Xtensa Options (Xtensa Options) 🎀
Xtensa, you know the drill:
-mconst16 -mforce-no-pic
-mno-serialize-volatile -mtext-section-literals -mauto-litpools
-mno-target-align -mlongcalls -mabi=abi-type
-mextra-l32r-costs=cycles -mstrict-align💜 zSeries Options 💜
zSeries Options? See S/390 and zSeries Options.
🎀 Options Controlling the Kind of Output 🎀
okay so compiling is a whole journey, bestie. it can go through up to four stages: preprocessing, compilation proper, assembly, and linking — and they always happen in that exact order, no cutting the line. GCC can preprocess and compile a bunch of files either into a bunch of assembler input files, or into just one assembler input file. then each assembler input file spits out an object file, and linking smooshes all those object files together — both the freshly compiled ones and any you passed in as input — into one executable file. periodt.
and for any given input file, the file name suffix is the vibe check that tells GCC what kind of compilation to do:
file.c— C source code that must be preprocessed.file.i— C source code that should not be preprocessed.file.ii— C++ source code that should not be preprocessed.file.m— Objective-C source code. note: you must link with thelibobjclibrary to make an Objective-C program work.file.mi— Objective-C source code that should not be preprocessed.file.mm,file.M— Objective-C++ source code. note: you must link with thelibobjclibrary to make an Objective-C++ program work. also note that.Mrefers to a literal capital M.file.mii— Objective-C++ source code that should not be preprocessed.file.h— C, C++, Objective-C or Objective-C++ header file to be turned into a precompiled header (default), or C, C++ header file to be turned into an Ada spec (via the-fdump-ada-specswitch).file.cc,file.cp,file.cxx,file.cpp,file.CPP,file.c++,file.C— C++ source code that must be preprocessed. Bestie, note that in.cxxthe last two letters must both literally bex. And.Cmeans a literal capitalC, no substitutions, periodt. ✨file.mm,file.M— Objective-C++ source code that must be preprocessed. 💖file.mii— Objective-C++ source code that should not be preprocessed. 🫶file.hh,file.H,file.hp,file.hxx,file.hpp,file.HPP,file.h++,file.tcc— C++ header file to be turned into a precompiled header or Ada spec. 🌸file.f,file.for,file.ftn,file.fi— Fixed form Fortran source code that should not be preprocessed. 🦋file.F,file.FOR,file.fpp,file.FPP,file.FTN— Fixed form Fortran source code that must be preprocessed (with the traditional preprocessor). 🔥file.f90,file.f95,file.f03,file.f08,file.fii— Free form Fortran source code that should not be preprocessed. 💜file.F90,file.F95,file.F03,file.F08— Free form Fortran source code that must be preprocessed (with the traditional preprocessor). ✨file.cob,file.COB,file.cbl,file.CBL— COBOL source code. 👑file.go— Go source code. 🐹file.d— D source code.file.di— D interface file.file.dd— D documentation code (Ddoc).file.ads— Ada source code file that contains a library unit declaration (a declaration of a package, subprogram, or generic, or a generic instantiation), or a library unit renaming declaration (a package, generic, or subprogram renaming declaration). These files are also called specs. 💅file.adb— Ada source code file containing a library unit body (a subprogram or package body). These files are also called bodies.file.s— Assembler code.file.S,file.sx— Assembler code that must be preprocessed. 🌈other— An object file to be fed straight into linking. Any file name with no recognized suffix is treated this way. fr fr, no suffix = straight to the linker, iykyk.
Okay so you can also just tell gcc what language you're feeding it instead of letting it guess from the name — that's the -x option: 💖
-xlanguage,--language=language,--languagelanguage — 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-xoption. Possible values for language are:
c c-header cpp-output
c++ c++-header c++-system-header c++-user-header c++-cpp-output
c++-system-module
objective-c objective-c-header objective-c-cpp-output objc-cpp-output
objective-c++ objective-c++-header objective-c++-cpp-output
objc++-cpp-output
assembler assembler-with-cpp
ada adascil adawhy
cobol
d
f77 f77-cpp-input f95 f95-cpp-input
go
modula-2 modula-2-cpp-output
rust
algol68
ltoLowkey important:-xdoes not imply a particular language standard. For example-x f77may also require-std=legacyfor some older source codes. Don't come for me, that's just how it is. 😭
-x none— Turn off any specification of a language, so that subsequent files are handled according to their file name suffixes (as if-xhas not been used at all). Back to vibes-based suffix detection, we love a reset. 🎀
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. 💀 Genuinely nothing. Iconic behaviour honestly.
-c,--compile— 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,--assemble— 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,--preprocess— Stop after the preprocessing stage; do not run the compiler proper. The output is in the form of preprocessed source code, which is sent to the standard output. 🌸
Input files that don't require preprocessing are ignored.
-ofile,--output=file,--outputfile — Place the primary output in file file. This applies to whatever sort of output is being produced, whether it be an executable file, an object file, an assembler file or preprocessed C code. 💅
If -o is not specified, the default is to put an executable file in a.out, the object file for source.suffix in source.o, its assembler file in source.s, a precompiled header file in source.suffix.gch, and all preprocessed C source on standard output.
Though -o names only the primary output, it also affects the naming of auxiliary and dump outputs. See the examples below. Unless overridden, both auxiliary outputs and dump outputs are placed in the same directory as the primary output. In auxiliary outputs, the suffix of the input file is replaced with that of the auxiliary output file type; in dump outputs, the suffix of the dump file is appended to the input file suffix. In compilation commands, the base name of both auxiliary and dump outputs is that of the primary output; in compile and link commands, the primary output name, minus the executable suffix, is combined with the input file name. If both share the same base name, disregarding the suffix, the result of the combination is that base name, otherwise, they are concatenated, separated by a dash. 🫶
gcc -c foo.c ...will use foo.o as the primary output, and place aux outputs and dumps next to it, e.g., aux file foo.dwo for -gsplit-dwarf, and dump file foo.c.???r.final for -fdump-rtl-final.
If a non-linker output file is explicitly specified, aux and dump files by default take the same base name:
gcc -c foo.c -o dir/foobar.o ...will name aux outputs dir/foobar.* and dump outputs dir/foobar.c.*.
A linker output will instead prefix aux and dump outputs:
gcc foo.c bar.c -o dir/foobar ...will generally name aux outputs dir/foobar-foo.* and dir/foobar-bar.*, and dump outputs dir/foobar-foo.c.* and dir/foobar-bar.c.*.
The one exception to the above is when the executable shares the base name with the single input:
gcc foo.c -o dir/foo ...in which case aux outputs are named dir/foo.* and dump outputs named dir/foo.c.*.
The location and the names of auxiliary and dump outputs can be adjusted by the options -dumpbase, -dumpbase-ext, -dumpdir, -save-temps=cwd, and -save-temps=obj. ⭐
-dumpbasedumpbase,--dumpbasedumpbase — This option sets the base name for auxiliary and dump output files. It does not affect the name of the primary output file. Intermediate outputs, when preserved, are not regarded as primary outputs, but as auxiliary outputs: 💜
gcc -save-temps -S foo.csaves the (no longer) temporary preprocessed file in foo.i, and then compiles to the (implied) output file foo.s, whereas:
gcc -save-temps -dumpbase save-foo -c foo.cpreprocesses to in save-foo.i, compiles to save-foo.s (now an intermediate, thus auxiliary output), and then assembles to the (implied) output file foo.o.
Absent this option, dump and aux files take their names from the input file, or from the (non-linker) output file, if one is explicitly specified: dump output files (e.g. those requested by -fdump-* options) with the input name suffix, and aux output files (those requested by other non-dump options, e.g. "-save-temps", "-gsplit-dwarf", "-fcallgraph-info") without it. 🦋
Similar suffix differentiation of dump and aux outputs can be attained for explicitly-given -dumpbase basename.suf by also specifying -dumpbase-ext .suf.
If dumpbase is explicitly specified with any directory component, any dumppfx specification (e.g. -dumpdir or -save-temps=*) is ignored, and instead of appending to it, dumpbase fully overrides it: 🔥
gcc foo.c -c -o dir/foo.o -dumpbase alt/foo \
-dumpdir pfx- -save-temps=cwd ...creates auxiliary and dump outputs named alt/foo.*, disregarding dir/ in -o, the ./ prefix implied by -save-temps=cwd, and pfx- in -dumpdir.
When -dumpbase is specified in a command that compiles multiple inputs, or that compiles and then links, it may be combined with dumppfx, as specified under -dumpdir. Then, each input file is compiled using the combined dumppfx, and default values for dumpbase and auxdropsuf are computed for each input file: ✨
gcc foo.c bar.c -c -dumpbase main ...creates foo.o and bar.o as primary outputs, and avoids overwriting the auxiliary and dump outputs by using the dumpbase as a prefix, creating auxiliary and dump outputs named main-foo.* and main-bar.*.
An empty string specified as dumpbase avoids the influence of the output basename in the naming of auxiliary and dump outputs during compilation, computing default values: 💖
gcc -c foo.c -o dir/foobar.o -dumpbase " ...will name aux outputs dir/foo.* and dump outputs dir/foo.c.*. Note how their basenames are taken from the input name, but the directory still defaults to that of the output.
The empty-string dumpbase does not prevent the use of the output basename for outputs during linking: 🎀
gcc foo.c bar.c -o dir/foobar -dumpbase " -flto ...The compilation of the source files will name auxiliary outputs dir/foo.* and dir/bar.*, and dump outputs dir/foo.c.* and dir/bar.c.*. LTO recompilation during linking will use dir/foobar. as the prefix for dumps and auxiliary files. 🌈
-dumpbase-ext auxdropsuf,--dumpbase-ext auxdropsuf— okay so when you're forming the name of an auxiliary output file (and not a dump file, bestie, keep that distinction 💅), you drop the trailingauxdropsuffromdumpbasebefore appending any suffixes. If you don't specify this, it defaults to the suffix of a defaultdumpbase— i.e., the suffix of the input file when-dumpbaseisn't present on the command line, or whendumpbaseis combined withdumppfx.
Example, look:
gcc foo.c -c -o dir/foo.o -dumpbase x-foo.c -dumpbase-ext .c ...This creates dir/foo.o as the main output, and generates auxiliary outputs in dir/x-foo.*, taking the location of the primary output and dropping the .c suffix from the dumpbase. Dump outputs retain the suffix: dir/x-foo.c.*. periodt ✨
This option is disregarded if it doesn't match the suffix of a specified dumpbase, except as an alternative to the executable suffix when appending the linker output base name to dumppfx, as specified below:
gcc foo.c bar.c -o main.out -dumpbase-ext .out ...This creates main.out as the primary output, and avoids overwriting the auxiliary and dump outputs by using the executable name minus auxdropsuf as a prefix, creating auxiliary outputs named main-foo.* and main-bar.* and dump outputs named main-foo.c.* and main-bar.c.*.
-dumpdir dumppfx,--dumpdir dumppfx— bestie when you're forming the name of an auxiliary or dump output file, usedumppfxas a prefix:
gcc -dumpdir pfx- -c foo.c ...This creates foo.o as the primary output, and auxiliary outputs named pfx-foo.*, combining the given dumppfx with the default dumpbase derived from the default primary output, derived in turn from the input name. Dump outputs also take the input name suffix: pfx-foo.c.*.
If dumppfx is to be used as a directory name, it must end with a directory separator:
gcc -dumpdir dir/ -c foo.c -o obj/bar.o ...This creates obj/bar.o as the primary output, and auxiliary outputs named dir/bar.*, combining the given dumppfx with the default dumpbase derived from the primary output name. Dump outputs also take the input name suffix: dir/bar.c.*.
It defaults to the location of the output file, unless the output file is a special file like "/dev/null". Options -save-temps=cwd and -save-temps=obj override this default, just like an explicit -dumpdir option. In case multiple such options are given, the last one prevails:
gcc -dumpdir pfx- -c foo.c -save-temps=obj ...This outputs foo.o, with auxiliary outputs named foo.* because -save-temps=* overrides the dumppfx given by the earlier -dumpdir option. It does not matter that =obj is the default for -save-temps, nor that the output directory is implicitly the current directory. Dump outputs are named foo.c.*.
When compiling from multiple input files, if -dumpbase is specified, dumpbase, minus a auxdropsuf suffix, and a dash are appended to (or override, if containing any directory components) an explicit or defaulted dumppfx, so that each of the multiple compilations gets differently-named aux and dump outputs.
gcc foo.c bar.c -c -dumpdir dir/pfx- -dumpbase main ...This outputs auxiliary dumps to dir/pfx-main-foo.* and dir/pfx-main-bar.*, appending dumpbase- to dumppfx. Dump outputs retain the input file suffix: dir/pfx-main-foo.c.* and dir/pfx-main-bar.c.*, respectively. Contrast with the single-input compilation:
gcc foo.c -c -dumpdir dir/pfx- -dumpbase main ...That, applying -dumpbase to a single source, does not compute and append a separate dumpbase per input file. Its auxiliary and dump outputs go in dir/pfx-main.*.
When compiling and then linking from multiple input files, a defaulted or explicitly specified dumppfx also undergoes the dumpbase- transformation above (e.g. the compilation of foo.c and bar.c above, but without -c). If neither -dumpdir nor -dumpbase are given, the linker output base name, minus auxdropsuf, if specified, or the executable suffix otherwise, plus a dash is appended to the default dumppfx instead. Note, however, that unlike earlier cases of linking:
gcc foo.c bar.c -dumpdir dir/pfx- -o main ...This does not append the output name main to dumppfx, because -dumpdir is explicitly specified. The goal is that the explicitly-specified dumppfx may contain the specified output name as part of the prefix, if desired; only an explicitly-specified -dumpbase would be combined with it, in order to avoid simply discarding a meaningful option.
When compiling and then linking from a single input file, the linker output base name will only be appended to the default dumppfx as above if it does not share the base name with the single input file name. This has been covered in single-input linking cases above, but not with an explicit -dumpdir that inhibits the combination, even if overridden by -save-temps=*:
gcc foo.c -dumpdir alt/pfx- -o dir/main.exe -save-temps=cwd ...Auxiliary outputs are named foo.*, and dump outputs foo.c.*, in the current working directory as ultimately requested by -save-temps=cwd.
Summing it all up for an intuitive though slightly imprecise data flow: the primary output name is broken into a directory part and a basename part; dumppfx is set to the former, unless overridden by -dumpdir or -save-temps=*, and dumpbase is set to the latter, unless overridden by -dumpbase. If there are multiple inputs or linking, this dumpbase may be combined with dumppfx and taken from each input file. Auxiliary output names for each input are formed by combining dumppfx, dumpbase minus suffix, and the auxiliary output suffix; dump output names are only different in that the suffix from dumpbase is retained.
When it comes to auxiliary and dump outputs created during LTO recompilation, a combination of dumppfx and dumpbase, as given or as derived from the linker output name but not from inputs, even in cases in which this combination would not otherwise be used as such, is passed down with a trailing period replacing the compiler-added dash, if any, as a -dumpdir option to lto-wrapper; being involved in linking, this program does not normally get any -dumpbase and -dumpbase-ext, and it ignores them.
When running sub-compilers, lto-wrapper appends LTO stage names to the received dumppfx, ensures it contains a directory component so that it overrides any -dumpdir, and passes that as -dumpbase to sub-compilers.
-v,--verbose— 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. basically it's the yapper mode, but make it useful 💅-###— Like-vexcept the commands are not executed and arguments are quoted unless they contain only alphanumeric characters or./-_. This is useful for shell scripts to capture the driver-generated command lines. no cap, it's for your scripts bestie 🫶--help— Print (on the standard output) a description of the command-line options understood bygcc. If the-voption is also specified then--helpis also passed on to the various processes invoked bygcc, so that they can display the command-line options they accept. If the-Wextraoption has also been specified (prior to the--helpoption), then command-line options that have no documentation associated with them are also displayed. so helpful, literally ✨--target-help— Print (on the standard output) a description of target-specific command-line options for each tool. For some targets extra target-specific information may also be printed.--help={class|[^]qualifier}[,...]— Print (on the standard output) a description of the command-line options understood by the compiler that fit into all specified classes and qualifiers. These are the supported classes:optimizers— Display all of the optimization options supported by the compiler.warnings— Display all of the options controlling warning messages produced by the compiler.target— Display target-specific options. Unlike the--target-helpoption however, target-specific options of the linker and assembler are not displayed. This is because those tools do not currently support the extended--help=syntax.params— Display the values recognized by the--paramoption.language— Display the options supported for language, where language is the name of one of the languages supported in this version of GCC. If an option is supported by all languages, one needs to selectcommonclass.common— Display the options that are common to all languages.
These are the supported qualifiers:
undocumented— Display only those options that are undocumented.joined— Display options taking an argument that appears after an equal sign in the same continuous piece of text, such as:--help=target.separate— Display options taking an argument that appears as a separate word following the original option, such as:-o output-file.
Thus for example to display all the undocumented target-specific switches supported by the compiler, use:
--help=target,undocumentedThe sense of a qualifier can be inverted by prefixing it with the ^ character, so for example to display all binary warning options (i.e., ones that are either on or off and that do not take an argument) that have a description, use:
--help=warnings,^joined,^undocumentedThe argument to --help= should not consist solely of inverted qualifiers.
Combining several classes is possible, although this usually restricts the output so much that there is nothing to display. One case where it does work, however, is when one of the classes is target. For example, to display all the target-specific optimization options, use:
--help=target,optimizersThe --help= option can be repeated on the command line. Each successive use displays its requested class of options, skipping those that have already been displayed. If --help is also specified anywhere on the command line then this takes precedence over any --help= option. iconic behavior 👑
Heads up bestie: the-Qoption is sneaky and it changes how--help=behaves, so listen up 💅
If the -Q option shows up on the command line before the --help= option, then the descriptive text that --help= shows you gets swapped out. Instead of describing the options it's displaying, it just tells you whether the option is enabled, disabled, or set to a specific value — assuming the compiler knows that at the point where the --help= option is used, obviously 🙃
Here's a truncated example straight from the ARM port of gcc:
% gcc -Q -mabi=2 --help=target -c
The following options are target specific:
-mabi= 2
-mabort-on-noreturn [disabled]
-mapcs [disabled]The output is sensitive to the effects of previous command-line options, so like, you can literally find out which optimizations are enabled at -O2 by using:
-Q -O2 --help=optimizersOr, alternatively, you can discover which binary optimizations are enabled by -O3 by using:
gcc -c -Q -O3 --help=optimizers > /tmp/O3-opts
gcc -c -Q -O2 --help=optimizers > /tmp/O2-opts
diff /tmp/O2-opts /tmp/O3-opts | grep enabled--version— shows the version number and the copyrights of the invoked GCC. Slay, receipts included 💖-pass-exit-codes,--pass-exit-codes— okay so normally thegccprogram exits with the code1if any phase of the compiler returns a non-success return code. But if you specify-pass-exit-codes, thegccprogram instead returns with the numerically highest error produced by any phase returning an error indication. The C, C++, and Fortran front ends return4if an internal compiler error is encountered.-pipe,--pipe— uses pipes instead of temporary files for communication between the various stages of compilation. Ngl this fails to work on some systems where the assembler is unable to read from a pipe — but the GNU assembler has no trouble at all, bestie 🫶-wrapper— invokes all subcommands under a wrapper program. The name of the wrapper program and its parameters are passed as a comma separated list.
gcc -c t.c -wrapper gdb,--argsThis invokes all subprograms of gcc under gdb --args, so the invocation of cc1 is gdb --args cc1 ....
-ffile-prefix-map=old=new— when compiling files residing in directory old, it records any references to them in the result of the compilation as if the files resided in directory new instead. Specifying this option is equivalent to specifying all the individual-f*-prefix-mapoptions. This can be used to make reproducible builds that are location independent. Directories referenced by directives are not affected by these options. See also-fmacro-prefix-map,-fdebug-prefix-map,-fprofile-prefix-mapand-fcanon-prefix-map.-fcanon-prefix-map— for the-f*-prefix-mapoptions, normally comparison of old prefix against the filename that would be normally referenced in the result of the compilation is done using textual comparison of the prefixes, or ignoring character case for case insensitive filesystems and considering slashes and backslashes as equal on DOS based filesystems. The-fcanon-prefix-mapcauses such comparisons to be done on canonicalized paths of old and the referenced filename.-fplugin=name.so— loads the plugin code in file name.so, assumed to be a shared object to bedlopen'd by the compiler. The base name of the shared object file is used to identify the plugin for the purposes of argument parsing (see-fplugin-arg-name-key=valuebelow). Each plugin should define the callback functions specified in the Plugins API.-fplugin-arg-name-key=value— defines an argument called key with a value of value for the plugin called name.-fdump-ada-spec[-slim]— for C and C++ source and include files, generates corresponding Ada specs ✨-fada-spec-parent=unit— in conjunction with-fdump-ada-spec[-slim]above, generates Ada specs as child units of parent unit.-fdump-go-spec=file— for input files in any language, generates corresponding Go declarations in file. This generates Goconst,type,var, andfuncdeclarations which may be a useful way to start writing a Go interface to code written in some other language 🐹@file— reads command-line options from file. The options read are inserted in place of the original@fileoption. If file does not exist, or cannot be read, then the option will be treated literally, and not removed.
Options in file are separated by whitespace. A whitespace character may be included in an option by surrounding the entire option in either single or double quotes. Any character (including a backslash) may be included by prefixing the character to be included with a backslash. The file may itself contain additional @file options; any such options will be processed recursively 🔁
✨ Compiling C++ Programs ✨
C++ source files conventionally use one of the suffixes .C, .cc, .cpp, .CPP, .c++, .cp, or .cxx; C++ header files often use .hh, .hpp, .H, or (for shared template code) .tcc; preprocessed C++ files use the suffix .ii; and C++20 module interface units sometimes use .ixx, .cppm, .cxxm, .c++m, or .ccm 👑
GCC recognizes files with these names and compiles them as C++ programs even if you call the compiler the same way as for compiling C programs (usually with the name gcc).
However, the use of gcc does not add the C++ library. g++ is a program that calls GCC and automatically specifies linking against the C++ library. It treats .c, .h and .i files as C++ source files instead of C source files unless -x is used. This program is also useful when precompiling a C header file with a .h extension for use in C++ compilations. On many systems, g++ is also installed with the name c++.
When you compile C++ programs, you may specify many of the same command-line options that you use for compiling programs in any language; or command-line options meaningful for C and related languages; or options that are meaningful only for C++ programs 💅
💖 Options Controlling C Dialect 💖
The following options control the dialect of C (or languages derived from C, such as C++, Objective-C and Objective-C++) that the compiler accepts:
-ansi,--ansi— in C mode, this is equivalent to-std=c90. In C++ mode, it is equivalent to-std=c++98.-std=— determines the language standard. This option is currently only supported when compiling C or C++.
The compiler can accept several base standards, such as c90 or c++98, and GNU dialects of those standards, such as gnu90 or gnu++98. When a base standard is specified, the compiler accepts all programs following that standard plus those using GNU extensions that do not contradict it. For example, -std=c90 turns off certain features of GCC that are incompatible with ISO C90, such as the asm and typeof keywords, but not other GNU extensions that do not have a meaning in ISO C90, such as omitting the middle term of a ?: expression. On the other hand, when a GNU dialect of a standard is specified, all features supported by the compiler are enabled, even when those features change the meaning of the base standard. As a result, some strict-conforming programs may be rejected. The particular standard is used by -Wpedantic to identify which features are GNU extensions given that version of the standard. For example -std=gnu90 -Wpedantic warns about C++ style // comments, while -std=gnu99 -Wpedantic does not.
A value for this option must be provided; possible values are:
c90c89iso9899:1990— support all ISO C90 programs (certain GNU extensions that conflict with ISO C90 are disabled). Same as-ansifor C code.iso9899:199409— ISO C90 as modified in amendment 1.c99c9xiso9899:1999iso9899:199x— ISO C99. This standard is substantially completely supported, modulo bugs and floating-point issues (mainly but not entirely relating to optional C99 features from Annexes F and G). See <https://gcc.gnu.org/projects/c-status.html> for more information. The namesc9xandiso9899:199xare deprecated.c11c1xiso9899:2011— ISO C11, the 2011 revision of the ISO C standard. This standard is substantially completely supported, modulo bugs, floating-point issues (mainly but not entirely relating to optional C11 features from Annexes F and G) and the optional Annexes K (Bounds-checking interfaces) and L (Analyzability). The namec1xis deprecated.c17c18iso9899:2017iso9899:2018— ISO C17, the 2017 revision of the ISO C standard (published in 2018). This standard is same as C11 except for corrections of defects (all of which are also applied with-std=c11) and a new value of__STDC_VERSION__, and so is supported to the same extent as C11.c23c2xiso9899:2024— ISO C23, the 2023 revision of the ISO C standard (published in 2024). The namec2xis deprecated.c2y— the next version of the ISO C standard, still under development. The support for this version is experimental and incomplete 😭gnu90gnu89— GNU dialect of ISO C90 (including some C99 features).gnu99gnu9x— GNU dialect of ISO C99. The namegnu9xis deprecated.gnu11gnu1x— GNU dialect of ISO C11. The namegnu1xis deprecated.gnu17gnu18— GNU dialect of ISO C17.gnu23gnu2x— GNU dialect of ISO C23. This is the default for C code. The namegnu2xis deprecated.gnu2y— the next version of the ISO C standard, still under development, plus GNU extensions. The support for this version is experimental and incomplete. The namegnu2xis deprecated.c++98c++03— the 1998 ISO C++ standard plus the 2003 technical corrigendum and some additional defect reports. Same as-ansifor C++ code.gnu++98gnu++03— GNU dialect of-std=c++98.c++11c++0x— the 2011 ISO C++ standard plus amendments. The namec++0xis deprecated.gnu++11gnu++0x— GNU dialect of-std=c++11. The namegnu++0xis deprecated.
c++14,c++1y— the 2014 ISO C++ standard plus amendments. Ngl, the namec++1yis deprecated bestie, it's giving retro.gnu++14,gnu++1y— GNU dialect of-std=c++14. The namegnu++1yis deprecated.c++17,c++1z— the 2017 ISO C++ standard plus amendments. And yes,c++1zis deprecated too.gnu++17,gnu++1z— GNU dialect of-std=c++17.gnu++1z? deprecated fr.c++20,c++2a— the 2020 ISO C++ standard plus amendments. Heads up: C++20 modules support is still experimental and needs to be turned on with-fmodules. The namec++2ais deprecated.gnu++20,gnu++2a— GNU dialect of-std=c++20. 💅 This one is the default for C++ code, so if you never picked a-std, congrats, this is what you've been living in. C++20 modules support is still experimental and needs to be enabled with-fmodules. The namegnu++2ais deprecated.c++23,c++2b— the 2023 ISO C++ standard plus amendments (published in 2024, late to the party but slay). Support is experimental, and could change in incompatible ways in future releases. The namec++2bis deprecated.gnu++23,gnu++2b— GNU dialect of-std=c++23. Support is experimental, and could change in incompatible ways in future releases. The namegnu++2bis deprecated.c++2c,c++26— the next revision of the ISO C++ standard, planned for 2026. 🦋 Support is highly experimental, and will almost certainly change in incompatible ways in future releases. Not the vibe for production, bestie.gnu++2c,gnu++26— GNU dialect of-std=c++2c. Support is highly experimental, and will almost certainly change in incompatible ways in future releases.
-aux-infofilename — spits out prototyped declarations to the given filename for every function declared and/or defined in a translation unit, header-file ones included. In any language that isn't C, this option is silently ignored — no error, no drama, just nothing.
Besides the declarations themselves, the file drops comments telling you the origin of each declaration (source file and line), whether it was implicit, prototyped or unprototyped (I, or N for new / O for old, 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). For function definitions, you also get a K&R-style list of arguments followed by their declarations, all tucked inside comments, after the declaration. 😌
-fno-asm— do not recognize"asm","inline"or"typeof"as keywords, so your code can use these words as identifiers instead. You can reach for the keywords"__asm__","__inline__"and"__typeof__"instead. In C,-ansiimplies-fno-asm.
In C++, "inline" is a standard keyword and is not affected by this switch. You might want -fno-gnu-keywords instead, which disables "typeof" but not "asm" and "inline". 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. In C23 mode (-std=c23 or -std=gnu23), this switch only affects the "asm" keyword, since "typeof" is a standard keyword in ISO C23.
-fno-builtin,-fno-builtin-function— don't recognize built-in functions that don't begin with__builtin_as prefix. 🚫
Normally GCC generates special code to handle certain built-in functions more efficiently — for instance, calls to "alloca" may become single instructions which adjust the stack directly, and calls to "memcpy" may become inline copy loops. The resulting code is often both smaller and faster, but since the function calls no longer appear as such, you can't set a breakpoint on those calls, nor can you change the behavior of the functions by linking with a different library. Plus, when a function is recognized as a built-in function, GCC may use information about that function to warn about problems with calls to that function, or to generate more efficient code, even if the resulting code still contains calls to that function. For example, warnings are given with -Wformat for bad calls to "printf" when "printf" is built in and "strlen" is known not to modify global memory.
With the -fno-builtin-function option only the built-in function function is disabled. function must not begin with __builtin_. If a function is named that is not built-in in this version of GCC, this option is ignored. There is no corresponding -fbuiltin-function option — so 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))-fcond-mismatch— allow conditional expressions with mismatched types in the second and third arguments. The value of such an expression isvoid. This option is not supported for C++. 🙈-ffreestanding— assert that compilation targets a freestanding environment. This implies-fno-builtin. A freestanding environment is one in which the standard library may not exist, and program startup may not necessarily be at"main". The most obvious example is an OS kernel. This is equivalent to-fno-hosted.-fgimple— enable parsing of function definitions marked with"__GIMPLE". This is an experimental feature that allows unit testing of GIMPLE passes.-fgnu-tm— when the option-fgnu-tmis specified, the compiler generates code for the Linux variant of Intel's current Transactional Memory ABI specification document (Revision 1.1, May 6 2009). This is an experimental feature whose interface may change in future versions of GCC, as the official specification changes. Please note that not all architectures are supported for this feature. 💀
For more information on GCC's support for transactional memory,
Note that the transactional memory feature is not supported with non-call exceptions (-fnon-call-exceptions).
-fgnu89-inline— tells GCC to use the traditional GNU semantics for"inline"functions when in C99 mode.
Using this option is roughly equivalent to adding the "gnu_inline" function attribute to all inline functions.
The option -fno-gnu89-inline explicitly tells GCC to use the C99 semantics for "inline" when in C99 or gnu99 mode (i.e., it specifies the default behavior). This option is not supported in -std=c90 or -std=gnu90 mode.
The preprocessor macros "__GNUC_GNU_INLINE__" and "__GNUC_STDC_INLINE__" may be used to check which semantics are in effect for "inline" functions. ✨
-fhosted— assert that compilation targets a hosted environment. This implies-fbuiltin. A hosted environment is one in which the entire standard library is available, and in which"main"has a return type of"int". Examples are nearly everything except a kernel. This is equivalent to-fno-freestanding.-flax-vector-conversions— allow implicit conversions between vectors with differing numbers of elements and/or incompatible element types. This option should not be used for new code. Bestie, leave it alone.-fms-extensions— accept some non-standard constructs used in Microsoft header files.
In C++ code, this allows member names in structures to be similar to previous types declarations.
typedef int UOW;
struct ABC {
UOW UOW;
};Some cases of unnamed fields in structures and unions are only accepted with this option.
Note that this option is off for all targets except for x86 targets using ms-abi.
-fpermitted-flt-eval-methods=style — ISO/IEC TS 18661-3 defines new permissible values for"FLT_EVAL_METHOD"that indicate that operations and constants with a semantic type that is an interchange or extended format should be evaluated to the precision and range of that type. These new values are a superset of those permitted under C99/C11, which does not specify the meaning of other positive values of"FLT_EVAL_METHOD". As such, code conforming to C11 may not have been written expecting the possibility of the new values.
-fpermitted-flt-eval-methods specifies whether the compiler should allow only the values of "FLT_EVAL_METHOD" specified in C99/C11, or the extended set of values specified in ISO/IEC TS 18661-3.
style is either "c11" or "ts-18661-3" as appropriate.
The default when in a standards compliant mode (-std=c11 or similar) is -fpermitted-flt-eval-methods=c11. The default when in a GNU dialect (-std=gnu11 or similar) is -fpermitted-flt-eval-methods=ts-18661-3.
The -fdeps-* options are used to extract structured dependency information for a source. This involves determining what resources provided by other source files will be required to compile the source as well as what resources are provided by the source. This information can be used to add required dependencies between compilation rules of dependent sources based on their contents rather than requiring such information be reflected within the build tools as well. 🧚
-fdeps-file=file — where to write structured dependency information.-fdeps-format=format — the format to use for structured dependency information.p1689r5is the only supported format right now. Note that when this argument is specified, the output of-MFis stripped of some information (namely C++ modules) so that it does not use extended makefile syntax not understood by most tools. 👑-fdeps-target=file — analogous to-MTbut for structured dependency information. This indicates the target which will ultimately need any required resources and provide any resources extracted from the source that may be required by other sources.-fplan9-extensions— accept some non-standard constructs used in Plan 9 code.
This enables -fms-extensions, permits passing pointers to structures with anonymous fields to functions that expect pointers to elements of the type of the field, and permits referring to anonymous fields declared using a typedef. This is only supported for C, not C++. 🌈
-fsigned-bitfields,-funsigned-bitfields,-fno-signed-bitfields,-fno-unsigned-bitfields— these four are the bit-field signedness police 💅. They decide whether a bit-field counts as signed or unsigned when the declaration is like... rude and doesn't saysignedorunsignedat all. And the default? signed, bestie, because that's the consistent thing — the basic integer types likeintare signed types, so the vibe stays matching. ✨-fsigned-char,-funsigned-char—-fsigned-charlets the typecharbe signed, likesigned char, while-funsigned-charletscharbe unsigned, likeunsigned char. Literally just flipping the personality ofchar. 🦋
ngl the identity crisis runs deep here: -fsigned-char is equivalent to -fno-unsigned-char, which is the negative form of -funsigned-char. And likewise, -fno-signed-char is equivalent to -funsigned-char. Yes it's giving mirror-of-a-mirror. 😭
Every target GCC supports has a default for what char should be, usually written down in the processor-specific ABI document for that target. So it's either like unsigned char by default, or like signed char by default. No in-between, no chaos.
Ideally a portable program should always use signed char or unsigned char when it depends on the signedness of an object. But 💀 so many programs were written using plain char and expecting it signed, or expecting it unsigned, depending on the machines they were written for. These options let you make such a program work with the opposite default. Slay.
And one thing that never changes: 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. Distinct type, matching behavior. Main character with a lookalike. 👑
-fstrict-flex-arrays(C and C++ only),-fstrict-flex-arrays=level(C and C++ only) — controls when the trailing array of a structure gets treated as a flexible array member for the purpose of accessing elements of such an array. Thelevelvalue controls how strict we're being. 🌈
-fstrict-flex-arrays is equivalent to -fstrict-flex-arrays=3, which is the strictest — a trailing array only counts as a flexible array member when it's declared as a flexible array member per C99 standard onwards. Maximum standards-girl energy. ✨
The negative form -fno-strict-flex-arrays is equivalent to -fstrict-flex-arrays=0, which is the least strict — in that case all trailing arrays of structures are treated as flexible array members. Chaos mode, but make it legal.
There are two more levels between 0 and 3, and they exist to support older code that uses the GCC zero-length array extension ([0]) or one-element array as flexible array members ([1]). When level is 1, the trailing array is treated as a flexible array member when it is declared as either [], [0], or [1]. When level is 2, the trailing array is treated as a flexible array member when it is declared as either [], or [0]. So like... a little ladder of strictness. 🪜
You can control this behavior for a specific trailing array field of a structure by using the variable attribute strict_flex_array attribute. Per-field vibes, we love to see it. 🎀
Also, heads up: the -fstrict_flex_arrays option interacts with the -Wstrict-flex-arrays option. They're in the group chat together.
-fsso-struct=endianness— sets the default scalar storage order of structures and unions to the specifiedendianness. The accepted values are big-endian, little-endian and native for the native endianness of the target (which is the default). This option is not supported for C++. Sorry C++ girlies. 🙈
⚠️ Warning: the -fsso-struct switch causes GCC to generate code that is not binary compatible with code generated without it if the specified endianness is not the native endianness of the target. Not a drill, bestie — that's an ABI break in a trench coat.
✨ Options Controlling C++ Dialect ✨
Okay so this section is the C++-only zone. It describes the command-line options that are only meaningful for C++ programs. And the good news? You can also use most of the GNU compiler options regardless of what language your program is in. Multilingual king. 🌍
For example, you might compile a file firstClass.C like this:
g++ -g -fstrict-enums -O -c firstClass.CIn this example, only -fstrict-enums is an option meant only for C++ programs; you can use the other options with any language supported by GCC. See, not everything is C++-exclusive. 💅
Some options for compiling C programs, such as -std, are also relevant for C++ programs. Sharing is caring.
Here is a list of options that are only for compiling C++ programs:
--compile-std-module— builds the compiled module interfaces for the<bits/stdc++.h>header unit and thestdandstd.compatmodules before compiling any source files explicitly specified on the command line. This is intended to be useful for building simple programs that useimport std;with a single command. One command, whole module, iconic. 🫶-fabi-version=n— use versionnof the C++ ABI. The default is version 0. And here's the whole drama, version by version, because I am not skipping any of it: 📜- Version 0 refers to the version conforming most closely to the C++ ABI specification. Therefore, the ABI obtained using version 0 will change in different versions of G++ as ABI bugs are fixed. (Yes, version 0 is a moving target. That's the point. 💀)
- Version 1 is the version of the C++ ABI that first appeared in G++ 3.2.
- Version 2 is the version of the C++ ABI that first appeared in G++ 3.4, and was the default through G++ 4.9.
- Version 3 corrects an error in mangling a constant address as a template argument.
- Version 4, which first appeared in G++ 4.5, implements a standard mangling for vector types.
- Version 5, which first appeared in G++ 4.6, corrects the mangling of attribute const/volatile on function pointer types, decltype of a plain decl, and use of a function parameter in the declaration of another parameter.
- Version 6, which first appeared in G++ 4.7, corrects the promotion behavior of C++11 scoped enums and the mangling of template argument packs, const/static_cast, prefix ++ and --, and a class scope function used as a template argument.
- Version 7, which first appeared in G++ 4.8, that treats nullptr_t as a builtin type and corrects the mangling of lambdas in default argument scope.
- Version 8, which first appeared in G++ 4.9, corrects the substitution behavior of function types with function-cv-qualifiers.
- Version 9, which first appeared in G++ 5.2, corrects the alignment of
nullptr_t. - Version 10, which first appeared in G++ 6.1, adds mangling of attributes that affect type identity, such as ia32 calling convention attributes (e.g. stdcall).
- Version 11, which first appeared in G++ 7, corrects the mangling of sizeof... expressions and operator names. For multiple entities with the same name within a function, that are declared in different scopes, the mangling now changes starting with the twelfth occurrence. It also implies
-fnew-inheriting-ctors. - Version 12, which first appeared in G++ 8, corrects the calling conventions for empty classes on the x86_64 target and for classes with only deleted copy/move constructors. It accidentally changes the calling convention for classes with a deleted copy constructor and a trivial move constructor. Oopsie. 😭
- Version 13, which first appeared in G++ 8.2, fixes the accidental change in version 12. Redemption arc. 🦋
- Version 14, which first appeared in G++ 10, corrects the mangling of the nullptr expression.
- Version 15, which first appeared in G++ 10.3, corrects G++ 10 ABI tag regression.
- Version 16, which first appeared in G++ 11, changes the mangling of
__alignof__to be distinct from that ofalignof, and dependent operator names. - Version 17, which first appeared in G++ 12, fixes layout of classes that inherit from aggregate classes with default member initializers in C++14 and up.
- Version 18, which first appeared in G++ 13, fixes manglings of lambdas that have additional context.
- Version 19, which first appeared in G++ 14, fixes manglings of structured bindings to include ABI tags, handling of cv-qualified
[[no_unique_address]]members, and adds mangling of C++20 constraints on function templates. - Version 20, which first appeared in G++ 15, fixes manglings of lambdas in static data member initializers.
- Version 21, which first appeared in G++ 16, fixes unnecessary captures in noexcept lambdas (c++/119764), layout of a base class with all explicitly defaulted constructors (c++/120012), and mangling of class and array objects with implicitly zero-initialized non-trailing subobjects (c++/121231).
See also -Wabi. 👀
-fabi-compat-version=n— on targets that support strong aliases, G++ works around mangling changes by creating an alias with the correct mangled name when defining a symbol with an incorrect mangled name. This switch specifies which ABI version to use for the alias. Glow-up with a receipt. 💅
With -fabi-version=0 (the default), this defaults to 13 (GCC 8.2 compatibility). If another ABI version is explicitly selected, this defaults to 0. For compatibility with GCC versions 3.2 through 4.9, use -fabi-compat-version=2. Time travel, but make it a flag. ⭐
If this option is not provided but -Wabi=n is, that version is used for compatibility aliases. If this option is provided along with -Wabi (without the version), the version from this option is used for the warning. Communication! We love a coordinated duo. 🫶
-fno-access-control— turns off all access checking. This switch is mainly useful for working around bugs in the access control code.private? Never heard of her. 🙈-faligned-new,-faligned-new=alignment— enables support for C++17newof types that require more alignment thanvoid* ::operator new(std::size_t)provides. A numeric argument such as-faligned-new=32can be used to specify how much alignment (in bytes) is provided by that function, but few users will need to override the default ofalignof(std::max_align_t). Big alignment energy, and honestly? Most of y'all can leave the default alone. 😌
This flag is enabled by default for -std=c++17. Automatic slay. ✨
-fno-assume-sane-operators-new— okay so bestie, this one's about the globalnew,new[],deleteanddelete[]operators 💅 the C++ standard lets you replace them, but a LOT of C++ programs just don't and vibe with whatever the implementation provides. ALSO, the standard says the compiler can straight up omit those calls when they come from new or delete expressions (and by extension it assumes the same thing when__builtin_operator_newor__builtin_operator_deleteget used). This option gives you control over some optimizations around calls to those operators.
With -fassume-sane-operators-new-delete, GCC may assume that calls to the replaceable global operators — whether from new or delete expressions, or from __builtin_operator_new / __builtin_operator_delete calls — don't read or modify any global variables, or variables whose address could escape to the operators (i.e. global state; except for errno for the new and new[] operators). This unlocks most optimizations across those calls ✨ and it's something the implementation-provided operators satisfy unless malloc implementation details are observable in the code or malloc hooks are used — but it might not be satisfied if a program replaces those operators. This behavior is on by default.
With -fno-assume-sane-operators-new-delete, GCC must assume all these calls (whether from new or delete expressions or called directly) may read and write global state unless proven otherwise (e.g. when GCC compiles their implementation). Use this option if those operators are or may be replaced and your code needs to expect that behavior 🫶
-fchar8_t-fno-char8_t— turns on support forchar8_tas adopted for C++20 🌈 That includes: a whole newchar8_tfundamental type, changes to the types of UTF-8 string and character literals, new signatures for user-defined literals, the associated standard library updates, and new__cpp_char8_tand__cpp_lib_char8_tfeature test macros. Iconic.
This option lets functions be overloaded for ordinary and UTF-8 strings:
int f(const char *); // #1
int f(const char8_t *); // #2
int v1 = f("text"); // Calls #1
int v2 = f(u8"text"); // Calls #2and it introduces new signatures for user-defined literals:
int operator""_udl1(char8_t);
int v3 = u8'x'_udl1;
int operator""_udl2(const char8_t*, std::size_t);
int v4 = u8"text"_udl2;
template<typename T, T...> int operator""_udl3();
int v5 = u8"text"_udl3;The change to the types of UTF-8 string and character literals brings incompatibilities with ISO C++11 and later standards 😭 For example, the following code is well-formed under ISO C++11, but is ill-formed when -fchar8_t is specified.
const char *cp = u8"xx";// error: invalid conversion from
// `const char8_t*' to `const char*'
int f(const char*);
auto v = f(u8"xx"); // error: invalid conversion from
// `const char8_t*' to `const char*'
std::string s{u8"xx"}; // error: no matching function for call to
// `std::basic_string<char>::basic_string()'
using namespace std::literals;
s = u8"xx"s; // error: conversion from
// `basic_string<char8_t>' to non-scalar
// type `basic_string<char>' requested-fcheck-new— checks that the pointer returned byoperator newis non-null before it tries to modify the storage allocated 🎀 This check is normally unnecessary because the C++ standard saysoperator newonly returns 0 if it's declared throw(), and in that case the compiler always checks the return value even without this option. In all other cases, whenoperator newhas a non-empty exception specification, memory exhaustion is signalled by throwingstd::bad_alloc. See also new (nothrow).-fconcepts— turns on support for the C++ Concepts feature for constraining template arguments 💜 With-std=c++20and above, Concepts are part of the language standard, so-fconceptsdefaults to on.
Some constructs that were allowed by the earlier C++ Extensions for Concepts Technical Specification, ISO 19217 (2015), but didn't make it into the standard, could additionally be enabled by -fconcepts-ts. The option -fconcepts-ts was deprecated in GCC 14 and removed in GCC 15; users are expected to convert their code to C++20 concepts.
- *
-fconcepts-diagnostics-depth=n* — specify maximum error replay depth during recursive diagnosis of a constraint satisfaction failure. The default is 1. - *
-fconstexpr-depth=n* — sets the maximum nested evaluation depth for C++11 constexpr functions to n. A limit is needed to detect endless recursion during constant expression evaluation. The minimum specified by the standard is 512. - *
-fconstexpr-cache-depth=n* — sets the maximum level of nested evaluation depth for C++11 constexpr functions that will be cached to n. It's a heuristic that trades off compilation speed (when the cache avoids repeated calculations) against memory consumption (when the cache grows very large from highly recursive evaluations). The default is 8. Very few users are likely to want to adjust it, but if your code does heavy constexpr calculations you might want to experiment to find which value works best for you 👑 -fconstexpr-fp-except— Annex F of the C standard specifies that IEC559 floating point exceptions encountered at compile time should not stop compilation. C++ compilers have historically not followed this guidance, instead treating floating point division by zero as non-constant even though it has a well defined value. This flag tells the compiler to give Annex F priority over other rules saying that a particular operation is undefined 🔥
constexpr float inf = 1./0.; // OK with -fconstexpr-fp-except- *
-fconstexpr-loop-limit=n* — sets the maximum number of iterations for a loop in C++14 constexpr functions to n. A limit is needed to detect infinite loops during constant expression evaluation. The default is 262144 (1<<18). - *
-fconstexpr-ops-limit=n* — sets the maximum number of operations during a single constexpr evaluation. Even when the number of iterations of a single loop is limited with the above limit, if there are several nested loops and each of them has many iterations but still smaller than the above limit, or if in the body of some loop or even outside of a loop too many expressions need to be evaluated, the resulting constexpr evaluation might take too long 🥲 The default is 33554432 (1<<25). -fcontracts— enables support for the C++ Contracts feature, as specified in the C++26 working draft (N5003) 📜
On violation of an enforced or observed contract (see -fcontract-evaluation-semantic below), a violation handler is called; the standard library provides a default handler that emits information about the contract that failed.
Users can replace the default violation handler by defining
void
handle_contract_violation (const std::contracts::contract_violation&);- *
-fcontract-evaluation-semantic=semantic* — sets the semantic with which contracts will be evaluated to semantic.
Available values for the evaluation mode are:
"ignored"— the contract checks will be elided, with no checking added at either compile or runtime."observed"— the contract checks are performed (at runtime) and, if one fails, a handler is called that allows actions such as logging or emitting run-time warnings. When the handler returns, the execution of the code will continue. At compile-time the checks are performed forconstexprevaluations, in this case a diagnostic is emitted if the check fails."enforce"— the contract checks are performed and the handler is called if one fails. When the handler returns the execution of the code is terminated, preventing it from continuing past the failed case. At compile-time the checks are applied toconstexprand, if one fails, the program is ill-formed; an error will be emitted."quick_enforce"— the contract checks are performed (at runtime) and, if one fails, the execution is terminated immediately (without calling any handler). At compile-time there is no difference in behaviour between this option and the"enforce"case, since no handler is invoked at compile time.
-fcontracts-conservative-ipa— this prevents inter-procedural analysis from taking action when the body of an inline function is visible in a given TU. This allows for the case that contract evaluation conditions are permitted to differ between TUs, which means that such actions would be potentially incorrect 🚫-fcontract-checks-outlined— by default, contract checks forpreandpostcondition checks are expanded in-line into function bodies. This option causes a small function to be created instead (containing the checks) that is called in the relevant position. This permits different criteria to be applied to the compilation of the checking and the remainder of the function body ✨-fcontract-disable-optimized-checks— whenpreandpostcondition checks are outlined (using-fcontract-checks-outlined), this option disables the optimisation steps for those functions which can avoid unwanted elision of checking steps 💅- *
-fcontracts-client-check=mode* — this option enables insertion of checks at call sites (caller-side checking).
The value of mode determines which contract checks are made:
"none"— no caller/client side checking (default)"pre"— preconditions are checked at the caller/client side."all"— both pre and post-conditions are checked at the caller/client side.
- *
-fcontracts-definition-check=mode* — this option introduces the ability to disable callee/definition-side checks, which are otherwise enabled by default when the contracts feature is active.
The variable mode has the values "on" and "off" 🦋
-fcoroutines— turns on support for the C++ coroutines extension ✨ and yes bestie, with-std=c++20and up, coroutines are literally part of the language standard, so-fcoroutinesjust defaults to on. no extra work required 💅-fdiagnostics-all-candidates— lets the C++ front end spill ALL the candidates when overload resolution fails, even the case where the thing it picked was a deleted function 👀 so you get the full receipts.-fdump-lang-,-fdump-lang-switch,-fdump-lang-switch-options,-fdump-lang-switch-options=filename— these control the dumping of C++-specific info 🎀 the options and filename parts behave exactly like they do in the-fdump-treeoption. here's the tea on which switch values you can pass:all— enable every single one of the below. main character move 👑class— dump class hierarchy information. virtual table info gets emitted too, unless you specifyslim.module— dump module information. the optionslineno(locations),graph(reachability),blocks(clusters),uid(serialization),alias(mergeable),asmname(Elrond),eh(mapper) &vops(macros) may hand you extra info 🦋raw— dump the raw internal tree data. unfiltered. no makeup.tinst— dump the sequence of template instantiations, indented so you can literally see how deep the recursion goes 😭 thelinenooption adds the source location where the instantiation was triggered, and thedetailsoption also dumps pre-instantiation substitutions, like the ones performed during template argument deduction.
lines in the.tinstdump start withIfor an instantiation,Sfor another substitution, andR[IS]for the reopened context of a deferred instantiation. memorize it, iykyk.
-fno-elide-constructors— okay so the C++ standard allows an implementation to omit creating a temporary that's only used to initialize another object of the same type (very slay of it). specifying this option disables that optimization and forces G++ to call the copy constructor in all cases 💀 this option also causes G++ to call trivial member functions which otherwise would be expanded inline.
in C++17, the compiler is required to omit these temporaries, but this option still affects trivial member functions. so yeah, still relevant.
-fno-enforce-eh-specs— don't generate code to check for violation of exception specifications at run time. this option violates the C++ standard 😳 but it may be useful for reducing code size in production builds, much like definingNDEBUG. it does not give user code permission to throw exceptions in violation of the exception specifications — the compiler still optimizes based on the specifications, so throwing an unexpected exception results in undefined behavior at run time. periodt, be careful.-fextern-tls-init,-fno-extern-tls-init— the C++11 and OpenMP standards allowthread_localandthreadprivatevariables to have dynamic (runtime) initialization 🌈 to support that, any use of such a variable goes through a wrapper function that performs any necessary initialization. when the use and the definition of the variable are in the same translation unit, that overhead can be optimized away — but when the use is in a different translation unit there's significant overhead even if the variable doesn't actually need dynamic initialization 😮💨 if the programmer can be sure that no use of the variable in a non-defining TU needs to trigger dynamic initialization (either because the variable is statically initialized, or a use of the variable in the defining TU will be executed before any uses in another TU), they can avoid this overhead with the-fno-extern-tls-initoption.
on targets that support symbol aliases, the default is-fextern-tls-init. on targets that do not support symbol aliases, the default is-fno-extern-tls-init.
-ffold-simple-inlines,-fno-fold-simple-inlines— lets the C++ frontend fold calls tostd::move,std::forward,std::addressof,std::to_underlyingandstd::as_const💖 in contrast to inlining, this means no debug information will be generated for such calls. since these functions are rarely interesting to debug, this flag is enabled by default unless-fno-inlineis active. obsessed with a sensible default tbh.-fno-gnu-keywords— do not recognizetypeofas a keyword, so that code can use this word as an identifier 🧚 you can use the keyword__typeof__instead. this option is implied by the strict ISO C++ dialects:-ansi,-std=c++98,-std=c++11, etc.-fno-immediate-escalation— do not enable immediate function escalation, whereby certain functions can be promoted toconsteval, as specified in P2564R3. for example:
consteval int id(int i) { return i; }
constexpr int f(auto t)
{
return t + id(t); // id causes f<int> to be promoted to consteval
}
void g(int i)
{
f (3);
}this compiles in C++20: f is an immediate-escalating function (due to the auto it is a function template and is declared constexpr) and id(t) is an immediate-escalating expression, so f is promoted to consteval ✨ consequently, the call to id(t) is in an immediate context, so it doesn't have to produce a constant (that is the mechanism allowing consteval function composition). however, with -fno-immediate-escalation, f is not promoted to consteval, and since the call to consteval function id(t) is not a constant expression, the compiler rejects the code 🙈
this option is turned on by default; it is only effective in C++20 mode or later.
-fimplicit-constexpr— make inline functions implicitlyconstexpr, if they satisfy the requirements for a constexpr function 🌸 this option can be used in C++14 mode or later. this can result in initialization changing from dynamic to static, plus other optimizations. glow up for your init order honestly.-fno-implicit-templates— never emit code for non-inline templates that are instantiated implicitly (i.e. by use) — only emit code for explicit instantiations 💅 if you use this option, you must take care to structure your code to include all the necessary explicit instantiations to avoid getting undefined symbols at link time. no cap, linker errors are not the vibe.-fno-implicit-inline-templates— don't emit code for implicit instantiations of inline templates either. the default is to handle inlines differently so that compiles with and without optimization need the same set of explicit instantiations 🎀-fno-implement-inlines— to save space, do not emit out-of-line copies of inline functions controlled by#pragma implementation. this causes linker errors if these functions are not inlined everywhere they are called 😭-fmodules,-fno-modules— enable support for C++20 modules 🦋 the-fno-modulesis usually not needed, as that is the default. even though this is a C++20 feature, it is not currently implicitly enabled by selecting that standard version. rude of it honestly.-fmodule-header,-fmodule-header=user,-fmodule-header=system— compile a header file to create an importable header unit ⭐-fmodule-implicit-inline— member functions defined in their class definitions are not implicitly inline for modular code. this is different to traditional C++ behavior, for good reasons 💅 however, it may result in a difficulty during code porting. this option makes such function definitions implicitly inline. it does however generate an ABI incompatibility, so you must use it everywhere or nowhere 👑 (such definitions outside of a named module remain implicitly inline, regardless.)-fno-module-lazy— disable lazy module importing and module mapper creation. no shortcuts, we doing it the long way.-fmodule-mapper=[hostname]:port[?ident],-fmodule-mapper=|program[?ident] args...,-fmodule-mapper==socket[?ident],-fmodule-mapper=<>[inout][?ident],-fmodule-mapper=<in>out[?ident],-fmodule-mapper=file[?ident]— an oracle to query for module name to filename mappings 🔮 if unspecified, theCXX_MODULE_MAPPERenvironment variable is used, and if that is unset, an in-process default is provided.-fmodule-only— only emit the Compiled Module Interface, inhibiting any object file 🌸 just the CMI, nothing else.-fms-extensions— disableWpedanticwarnings about constructs used in MFC, such as implicit int and getting a pointer to member function via non-standard syntax 🙈 living your truth, quietly.-fnew-inheriting-ctors— enable the P0136 adjustment to the semantics of C++11 constructor inheritance ✨ this is part of C++17 but also considered to be a Defect Report against C++11 and C++14. this flag is enabled by default unless-fabi-version=10or lower is specified.-fnew-ttp-matching— enable the P0522 resolution to Core issue 150, template template parameters and default arguments: this allows a template with default template arguments as an argument for a template template parameter with fewer template parameters 💖 this flag is enabled by default for-std=c++17.-fno-nonansi-builtins— disable built-in declarations of functions that are not mandated by ANSI/ISO C. these includeffs,alloca,_exit,index,bzero,conjf, and other related functions 🧚-fnothrow-opt— treat athrow()exception specification as if it were anoexceptspecification, to reduce or eliminate the text size overhead relative to a function with no exception specification ✨ if the function has local variables of types with non-trivial destructors, the exception specification actually makes the function smaller, because the EH cleanups for those variables can be optimized away. the semantic effect is that an exception thrown out of a function with such an exception specification results in a call toterminaterather thanunexpected💀 noted.-fno-operator-names— do not treat the operator name keywordsand,bitand,bitor,compl,not,orandxoras 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.-fno-pretty-templates— okay so bestie, when an error message points at a specialization of a function template, the compiler normally prints the whole template signature followed by the template arguments and any typedefs or typenames sitting in that signature (likevoid f(T) [with T = int]instead of justvoid f(int)) — that's so you can tell which template is being dramatic. And when an error message points at a specialization of a class template, the compiler omits any template arguments that match the default template arguments for that template. If either of these behaviors is making the error message harder to read instead of easier, hit-fno-pretty-templatesto turn them both off. 💅-frange-for-ext-temps— turns on lifetime extension for C++ range-basedfortemporaries. With-std=c++23and above this is literally part of the language standard, so the lifetime of the temporaries gets extended until the end of the loop by default. This option lets you have that behavior in earlier versions of the standard too. ✨-freflection— enables experimental C++26 Reflection. yes bestie we're reflecting 🪞-fno-rtti— stops the compiler from generating info about every class with virtual functions that the C++ run-time type identification features need (dynamic_castandtypeid). If you don't use those parts of the language, this flag saves you some space. Note that exception handling uses the same information, but G++ generates it as needed. Thedynamic_castoperator can still be used for casts that don't require run-time type information, i.e. casts tovoid *or to unambiguous base classes.
Mixing code compiled with-frttitogether with code compiled with-fno-rttimay not work. For example, programs may fail to link if a class compiled with-fno-rttiis used as a base for a class compiled with-frtti. 😭
-fsized-deallocation— enables the built-in global declarations
void operator delete (void *, std::size_t) noexcept;
void operator delete[] (void *, std::size_t) noexcept;as introduced in C++14. This is useful for user-defined replacement deallocation functions that, for example, use the size of the object to make deallocation faster. Enabled by default under -std=c++14 and above. The flag -Wsized-deallocation warns about places that might want to add a definition.
-fstrict-enums— lets the compiler optimize using the assumption that a value of enumerated type can only be one of the values of the enumeration (as defined in the C++ standard; basically, a value that can be represented in the minimum number of bits needed to represent all the enumerators). This assumption may not be valid if the program uses a cast to convert an arbitrary integer value to the enumerated type. This option has no effect for an enumeration type with a fixed underlying type.-fstrong-eval-order-fstrong-eval-order=kind— evaluates member access, array subscripting, and shift expressions in left-to-right order, and evaluates assignment in right-to-left order, as adopted for C++17.-fstrong-eval-orderis equivalent to-fstrong-eval-order=all, and is enabled by default with-std=c++17or later.-fstrong-eval-order=someenables just the ordering of member access and shift expressions, and is the default for C++ dialects prior to C++17.-fstrong-eval-order=noneis equivalent to-fno-strong-eval-order.
-ftemplate-backtrace-limit=n— sets the max number of template instantiation notes for a single warning or error to n. The default value is10.-ftemplate-depth=n— sets 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 than17(changed to1024in C++11). The default value is900, since the compiler can run out of stack space before hitting1024in some situations. 😮💨-fno-threadsafe-statics— do not emit the extra code that uses the routines specified in the C++ ABI for thread-safe initialization of local statics. You can use this to shave a lil bit of code size in code that doesn't need to be thread-safe.-fuse-cxa-atexit— registers destructors for objects with static storage duration with the__cxa_atexitfunction instead of theatexitfunction. This option is required for fully standards-compliant handling of static destructors, but only works if your C library supports__cxa_atexit.-fno-use-cxa-get-exception-ptr— don't use the__cxa_get_exception_ptrruntime routine. This causesstd::uncaught_exceptionto be incorrect, but it's necessary if the runtime routine isn't available.-fvisibility-inlines-hidden— this switch declares that the user does not attempt to compare pointers to inline functions or methods where the addresses of the two functions are taken in different shared objects.
The effect is that GCC may, effectively, mark inline methods with __attribute__ ((visibility ("hidden"))) so that they don't show up in the export table of a DSO and don't need a PLT indirection when used within the DSO. Turning this on can have a dramatic effect on load and link times of a DSO because it massively reduces the size of the dynamic export table when the library makes heavy use of templates. ✨
The behavior of this switch is not quite the same as marking the methods as hidden directly, because it does not affect static variables local to the function or cause the compiler to deduce that the function is defined in only one shared object.
You may mark a method as having a visibility explicitly to negate the effect of the switch for that method. For example, if you do want to compare pointers to a particular inline method, you might mark it as having default visibility. Marking the enclosing class with explicit visibility has no effect.
Explicitly instantiated inline methods are unaffected by this option as their linkage might otherwise cross a shared library boundary.
-fvisibility-ms-compat— this flag tries to use visibility settings to make GCC's C++ linkage model compatible with that of Microsoft Visual Studio.
The flag makes these changes to GCC's linkage model:
- It sets the default visibility to
hidden, like-fvisibility=hidden. - Types, but not their members, are not hidden by default.
- The One Definition Rule is relaxed for types without explicit visibility specifications that are defined in more than one shared object: those declarations are permitted if they are permitted when this option is not used.
In new code it's better to use -fvisibility=hidden and export those classes that are intended to be externally visible. Unfortunately it's possible for code to rely, perhaps accidentally, on the Visual Studio behavior. 🙈
Among the consequences of these changes are that static data members of the same type with the same name but defined in different shared objects are different, so changing one does not change the other; and that pointers to function members defined in different shared objects may not compare equal. When this flag is given, it is a violation of the ODR to define types with the same name differently.
-fno-weak— do not use weak symbol support, even if the linker provides it. By default, G++ uses weak symbols if they're available. This option exists only for testing, and should not be used by end-users; it results in inferior code and has no benefits. This option may be removed in a future release of G++.
bestie do not use this one fr fr 💀
-fext-numeric-literals(C++ and Objective-C++ only) — accept imaginary, fixed-point, or machine-defined literal number suffixes as GNU extensions. When this option is turned off these suffixes are treated as C++11 user-defined literal numeric suffixes. This is on by default for all pre-C++11 dialects and all GNU dialects:-std=c++98,-std=gnu++98,-std=gnu++11,-std=gnu++14. This option is off by default for ISO C++11 onwards (-std=c++11, ...).-nostdinc++— don't 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.)-flang-info-include-translate-flang-info-include-translate-not-flang-info-include-translate=header— inform of include translation events. The first will note accepted include translations, the second will note declined include translations. The header form will inform of include translations relating to that specific header. If header is of the formuseror<system>it will be resolved to a specific user or system header using the include path.-flang-info-module-cmi-flang-info-module-cmi=module— inform of Compiled Module Interface pathnames. The first will note all read CMI pathnames. The module form will note reading a specific module's CMI. module may be a named module or a header-unit (the latter indicated by either being a pathname containing directory separators or enclosed in<>or"").-stdlib=libstdc++,libc++— when G++ is configured to support this option, it allows specification of alternate C++ runtime libraries. Two options are available:libstdc++(the default, native C++ runtime for G++) andlibc++which is the C++ runtime installed on some operating systems (e.g. Darwin versions from Darwin11 onwards). The option switches G++ to use the headers from the specified library and to emit-lstdc++or-lc++respectively, when a C++ runtime is required for linking. 🫶
In addition, these warning options have meanings only for C++ programs:
-Wabi-tag(C++ and Objective-C++ only) — warn when a type with an ABI tag is used in a context that does not have that ABI tag. See C++ Attributes for more information about ABI tags.-Wno-abbreviated-auto-in-template-arg— disable the error for anautoplaceholder type used within a template argument list to declare a C++20 abbreviated function template, e.g.
void f(S<auto>);-fpermissiveor-Wno-error=abbreviated-auto-in-template-argcan downgrade that error to a warning, so if you're fighting for your life against it, bestie, those are your knobs.-Wcomma-subscript(C++ and Objective-C++ only) — warns you when you use a comma expression inside a subscripting expression. This usage was deprecated in C++20 and it's getting removed in C++23, so like… the clock is ticking, honey. 💀 But note: a comma expression wrapped in( )is not deprecated. Example:
void f(int *a, int b, int c) {
a[b,c]; // deprecated in C++20, invalid in C++23
a[(b,c)]; // OK
}In C++23 it is valid to have comma separated expressions in a subscript when an overloaded subscript operator is found and supports the right number and types of arguments. G++ will accept the formerly valid syntax for code that is not valid in C++23 but used to be valid but deprecated in C++20 — with a pedantic warning you can turn off with -Wno-comma-subscript.
Enabled by default with -std=c++20 unless -Wno-deprecated, and after -std=c++23 regardless of -Wno-deprecated. Before -std=c++20, it's enabled with explicit -Wdeprecated.
Heads up: this warning gets upgraded to an error by -pedantic-errors in C++23 mode or later. 🚨
-Wctad-maybe-unsupported(C++ and Objective-C++ only) — warns when you do class template argument deduction (CTAD) on a type with no explicitly written deduction guides. It's basically pointing at you like 👀 "bestie, this only worked because the compiler synthesized the implicit deduction guides, and that might not be the vibe you intended." Some style guides only allow CTAD on types that specifically "opt-in" — i.e., types designed to support CTAD. You can silence it with this pattern:
struct allow_ctad_t; // any name works
template <typename T> struct S {
S(T) { }
};
// Guide with incomplete parameter type will never be considered.
S(allow_ctad_t) -> S<void>;-Wctor-dtor-privacy(C++ and Objective-C++ only) — warns when a class looks unusable because all its constructors or destructors are private, and it has neither friends nor public static member functions. Also yells if there are no non-private methods and there's at least one private member function that isn't a constructor or destructor. Like, girl, nobody can even touch this class. 🙈-Wdangling-reference(C++ and Objective-C++ only) — warns when a reference is bound to a temporary whose lifetime has ended. For example:
int n = 1;
const int& r = std::max(n - 1, n + 1); // r is danglingIn that example, two temporaries get created (one per argument), and a reference to one of them gets returned. But both temporaries are destroyed at the end of the full expression, so the reference r is dangling. 😭 This warning also catches dangling references in member initializer lists:
const int& f(const int& i) { return i; }
struct S {
const int &r; // r is dangling
S() : r(f(10)) { }
};Member functions get checked too, but only their object argument:
struct S {
const S& self () { return *this; }
};
const S& s = S().self(); // s is danglingCertain functions are safe in this respect, like std::use_facet: they take and return a reference, but they don't return one of their arguments, which can fool the warning. You can exclude such functions by wrapping them in a #pragma:
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wdangling-reference"
const T& foo (const T&) { ... }
#pragma GCC diagnostic popThat #pragma can also surround the whole class — in which case the warning is disabled for all the member functions.
-Wdangling-reference also warns about code like
auto p = std::minmax(1, 2);where std::minmax returns std::pair<const int&, const int&>, and both references dangle after the end of the full expression that contains the call to std::minmax.
The warning does not warn for std::span-like classes. We consider classes of the form:
template<typename T>
struct Span {
T* data_;
std::size len_;
};as "std::span-like" — meaning the class is a non-union class that has a pointer data member and a trivial destructor.
The warning can be disabled by using the gnu::no_dangling attribute. 🧚
This warning is enabled by -Wextra.
-Wdelete-non-virtual-dtor(C++ and Objective-C++ only) — warns whendeleteis used to destroy an instance of a class that has virtual functions and a non-virtual destructor. It's unsafe to delete an instance of a derived class through a pointer to a base class if the base class doesn't have a virtual destructor. This warning is enabled by-Wall.-Wdeprecated-copy(C++ and Objective-C++ only) — warns that the implicit declaration of a copy constructor or copy assignment operator is deprecated if the class has a user-provided copy constructor or copy assignment operator, in C++11 and up. This warning is enabled by-Wextra.-Wdeprecated-copy-dtor(C++ and Objective-C++ only) — like-Wdeprecated-copy, but it also deprecates if the class has a user-provided destructor.-Wno-deprecated-enum-enum-conversion(C++ and Objective-C++ only) — disables the warning for when the usual arithmetic conversions are applied on operands where one is of enumeration type and the other is of a different enumeration type. This conversion was deprecated in C++20. For example:
enum E1 { e };
enum E2 { f };
int k = f - e;-Wdeprecated-enum-enum-conversion is enabled by default with -std=c++20. In pre-C++20 dialects, this warning can be enabled by -Wenum-conversion or -Wdeprecated.
-Wno-deprecated-enum-float-conversion(C++ and Objective-C++ only) — disables the warning for when the usual arithmetic conversions are applied on operands where one is of enumeration type and the other is of a floating-point type. This conversion was deprecated in C++20. For example:
enum E1 { e };
enum E2 { f };
bool b = e <= 3.7;-Wdeprecated-enum-float-conversion is enabled by default with -std=c++20. In pre-C++20 dialects, this warning can be enabled by -Wenum-conversion or -Wdeprecated.
-Wdeprecated-literal-operator(C++ and Objective-C++ only) — warns that declaring a user-defined literal operator with a space before the suffix is deprecated. This warning is enabled by default in C++23, or with explicit-Wdeprecated.
string operator "" _i18n(const char*, std::size_t); // deprecated
string operator ""_i18n(const char*, std::size_t); // preferred-Wdeprecated-variadic-comma-omission(C++ and Objective-C++ only) — warns that omitting a comma before the varargs...at the end of a function parameter list is deprecated. This warning is enabled by default in C++26, or with explicit-Wdeprecated.
void f1(int...); // deprecated
void f1(int, ...); // preferred
template <typename ...T>
void f2(T...); // ok
template <typename ...T>
void f3(T......); // deprecated-Wno-elaborated-enum-base— for C++11 and above, warns if an (invalid) additional enum-base is used in an elaborated-type-specifier. That means: if an enum with a given underlying type and no enumerator list is used in a declaration other than just a standalone declaration of the enum. Enabled by default. This warning is upgraded to an error with-pedantic-errors.-Wno-init-list-lifetime(C++ and Objective-C++ only) — stops the warning about uses ofstd::initializer_listthat are likely to result in dangling pointers. Since the underlying array for aninitializer_listis handled like a normal C++ temporary object, it's easy to accidentally keep a pointer to the array past the end of the array's lifetime. For example:- If a function returns a temporary
initializer_list, or a localinitializer_listvariable, the array's lifetime ends at the end of the return statement, so the value returned has a dangling pointer. - If a new-expression creates an
initializer_list, the array only lives until the end of the enclosing full-expression, so theinitializer_listin the heap has a dangling pointer. - When an
initializer_listvariable is assigned from a brace-enclosed initializer list, the temporary array created for the right side of the assignment only lives until the end of the full-expression, so at the next statement theinitializer_listvariable has a dangling pointer.
- If a function returns a temporary
// li's initial underlying array lives as long as li
std::initializer_list<int> li = { 1,2,3 };
// assignment changes li to point to a temporary array
li = { 4, 5 };
// now the temporary is gone and li has a dangling pointer
int i = li.begin()[0] // undefined behavior- When a list constructor stores the
beginpointer from theinitializer_listargument, this doesn't extend the lifetime of the array, so if a class variable is constructed from a temporaryinitializer_list, the pointer is left dangling by the end of the variable declaration statement.
-Winvalid-constexpr— warns when a function never produces a constant expression. In C++20 and earlier, for everyconstexprfunction and function template, there must be at least one set of function arguments in at least one instantiation such that an invocation of the function or constructor could be an evaluated subexpression of a core constant expression. C++23 removed this restriction, so it's possible to have a function or a function template markedconstexprfor which no invocation satisfies the requirements of a core constant expression. 💅
This warning is on by default as a pedantic warning in C++20 and earlier, bestie. In C++23 tho, -Winvalid-constexpr can be turned on, and then it's just a regular ordinary warning. Like:
void f (int& i);
constexpr void
g (int& i)
{
// Warns by default in C++20, in C++23 only with -Winvalid-constexpr.
f(i);
}-Winvalid-imported-macros— checks that every single imported macro definition is valid by the time compilation wraps up. It's not on by default because, ngl, it needs extra processing to figure out. Super useful when you're prepping sets of header-units and want to make sure the macros stay consistent. 🎀-Wno-literal-suffix(C++ and Objective-C++ only) — turns off the warning you'd normally get when a string or character literal is followed by a ud-suffix that doesn't start with an underscore. As a conforming extension, GCC treats those suffixes as separate preprocessing tokens, so it can keep backwards compatibility with code that uses formatting macros from"<inttypes.h>". Example time:
#define __STDC_FORMAT_MACROS
#include <inttypes.h>
#include <stdio.h>
int main() {
int64_t i64 = 123;
printf("My int64: %" PRId64"\n", i64);
}Here, "PRId64" gets treated as a separate preprocessing token. ✨
This option also controls warnings when a user-defined literal operator is declared with a literal suffix identifier that doesn't begin with an underscore — and heads up, literal suffix identifiers that don't begin with an underscore are reserved for future standardization. These warnings are enabled by default. 👑
-Wno-narrowing(C++ and Objective-C++ only) — for C++11 and later standards, narrowing conversions get diagnosed by default because the standard requires it, no cap. A narrowing conversion from a constant produces an error, and one from a non-constant produces a warning — but-Wno-narrowingsuppresses the diagnostic entirely. Important tho: this does not change the meaning of well-formed code, because narrowing conversions are still considered ill-formed in SFINAE contexts. 💅
With -Wnarrowing in C++98, you get a warning when a narrowing conversion prohibited by C++11 happens inside { }, like:
int i = { 2.2 }; // error: narrowing from double to intThis flag is included in -Wall and -Wc++11-compat. 🦋
-Wnoexcept(C++ and Objective-C++ only) — warns when a noexcept-expression evaluates tofalsebecause of a call to a function that doesn't have a non-throwing exception specification (i.e.throw()or"noexcept") but is known by the compiler to never throw an exception. Like, the compiler knows it won't throw, but the signature isn't saying so. 😭-Wnoexcept-type(C++ and Objective-C++ only) — warns if the C++17 feature that makes"noexcept"part of a function type changes the mangled name of a symbol relative to C++14. It's enabled by-Wabiand-Wc++17-compat. 🌸
Example:
template <class T> void f(T t) { t(); };
void g() noexcept;
void h() { f(g); }In C++14, "f" calls "f<void(*)()>", but in C++17 it calls "f<void(*)()noexcept>". Two totally different symbols bestie. 💀
-Wclass-memaccess(C++ and Objective-C++ only) — warns when the destination of a call to a raw memory function like"memset"or"memcpy"is an object of class type, and when writing into such an object might bypass the class's non-trivial or deleted constructor or copy assignment, violate const-correctness or encapsulation, or corrupt virtual table pointers. Modifying the representation of those objects may violate invariants maintained by member functions of the class. For example, the"memset"call below is undefined behaviour because it modifies a non-trivial class object — so it gets diagnosed. The safe way to initialize or clear the storage of objects like this is by using the appropriate constructor or assignment operator, if one is available. 🫶
std::string str = "abc";
memset (&str, 0, sizeof str);The -Wclass-memaccess option is enabled by -Wall. If you explicitly cast the pointer to the class object to "void *" or to a type that can be safely accessed by the raw memory function, that suppresses the warning. ✨
-Wnon-virtual-dtor(C++ and Objective-C++ only) — warns when a class has virtual functions and an accessible non-virtual destructor, either on itself or in an accessible polymorphic base class — because then it's possible (but unsafe) to delete an instance of a derived class through a pointer to the class itself or the base class. This warning gets automatically enabled if-Weffc++is specified. But ngl, the-Wdelete-non-virtual-dtoroption (enabled by-Wall) should be preferred, because it warns about the unsafe cases without the false positives. 💅-Wregister(C++ and Objective-C++ only) — warns on uses of the"register"storage class specifier, except when it's part of the GNU Explicit Register Variables extension. Using the"register"keyword as a storage class specifier was deprecated in C++11 and removed in C++17. Enabled by default with-std=c++17. 🎀-Wreorder(C++ and Objective-C++ only) — warns when the order of member initializers in your code doesn't match the order they actually have to be executed in. Like:
struct A {
int i;
int j;
A(): j (0), i (1) { }
};The compiler rearranges the member initializers for "i" and "j" to match the declaration order of the members, and emits a warning about it. This warning is enabled by -Wall. 🌈
-Wno-pessimizing-move(C++ and Objective-C++ only) — warns when a call to"std::move"is actually preventing copy elision. 😭 A classic scenario where copy elision can happen is when you return in a function with a class return type, the expression being returned is the name of a non-volatile automatic object, it's not a function parameter, and it has the same type as the function return type.
struct T {
...
};
T fn()
{
T t;
...
return std::move (t);
}But in this example, that "std::move" call is literally blocking copy elision. This warning is enabled by -Wall. 🔥
-Wno-redundant-move(C++ and Objective-C++ only) — warns about redundant calls to"std::move"— like, when a move operation would've happened anyway even without the"std::move"call. This happens because the compiler is forced to treat the object as if it were an rvalue in certain situations, like returning a local variable, where copy elision isn't applicable. Consider:
struct T {
...
};
T fn(T t)
{
...
return std::move (t);
}Here, the "std::move" call is redundant. And because G++ implements Core Issue 1579, here's another example:
struct T { // convertible to U
...
};
struct U {
...
};
U fn()
{
T t;
...
return std::move (t);
}In this example, copy elision isn't applicable because the type of the expression being returned and the function return type differ — yet G++ treats the return value as if it were designated by an rvalue. This warning is enabled by -Wextra. 🦋
-Wrange-loop-construct(C++ and Objective-C++ only) — warns when a C++ range-based for-loop is creating an unnecessary copy. This can happen when the range declaration isn't a reference but probably should be. For example:
struct S { char arr[128]; };
void fn () {
S arr[5];
for (const auto x : arr) { ... }
}It does not warn when the type being copied is a trivially-copyable type whose size is less than 64 bytes. ✨
This warning also fires when a loop variable in a range-based for-loop is initialized with a value of a different type, resulting in a copy. For example:
void fn() {
int arr[10];
for (const double &x : arr) { ... }
}In the example above, in every iteration of the loop a temporary value of type "double" is created and destroyed, and the reference "const double &" is bound to it. 💀 This warning is enabled by -Wall.
-Wredundant-tags(C++ and Objective-C++ only) — warns about redundant class-key and enum-key in references to class types and enumerated types in contexts where the key can be eliminated without causing an ambiguity. For example:
struct foo;
struct foo *p; // warn that keyword struct can be eliminatedOn the other hand, in this example there's no warning:
struct foo;
void foo (); // "hides" struct foo
void bar (struct foo&); // no warning, keyword struct is necessarySee how the keyword is doing actual work there? 👑
-Wno-subobject-linkage(C++ and Objective-C++ only) — turns off the warning you'd get if a class type has a base or a field whose type uses the anonymous namespace or depends on a type with no linkage. Like, if a type A depends on a type B with no or internal linkage, defining it in multiple translation units would be an ODR violation, because the meaning of B is different in each translation unit. If A only appears in a single translation unit, the best way to silence the warning is to give it internal linkage by putting it in an anonymous namespace too. The compiler doesn't give this warning for types defined in the main.Cfile, since those are unlikely to have multiple definitions.-Wsubobject-linkageis enabled by default. 🎀-Weffc++(C++ and Objective-C++ only) — warns about violations of these style guidelines from Scott Meyers' Effective C++ series of books: 📚- Define a copy constructor and an assignment operator for classes with dynamically-allocated memory.
- Prefer initialization to assignment in constructors.
- Have
"operator="return a reference to*this. - Don't try to return a reference when you must return an object.
- Distinguish between prefix and postfix forms of increment and decrement operators.
- Never overload
"&&","||", or",".
This option also enables -Wnon-virtual-dtor, which is also one of the effective C++ recommendations. However, the check is extended to warn about the lack of virtual destructor in accessible non-polymorphic bases classes too. bestie the compiler is really out here checking your whole inheritance tree 💅
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. no cap, the STL did NOT get the memo 🙈
✨ MORE C++ WARNINGS, WE'RE STILL GOING ✨
-Wno-exceptions(C++ and Objective-C++ only) — Disable the warning about the case when an exception handler is shadowed by another handler, which can point out a wrong ordering of exception handlers. like girl, yourcatchblocks are fighting for the spotlight and one of them is losing 😭-Wsfinae-incomplete(C++ and Objective-C++ only) — Warn about a class that is found to be incomplete, or a function withautoreturn type that has not yet been deduced, in a context where that causes substitution failure rather than an error, and then the class or function is defined later in the translation unit. this is problematic because template instantiations or concept checks could have different results if they first occur either before or after the definition.
This warning is enabled by default. -Wsfinae-incomplete=2 adds a warning at the point of substitution failure, to make it easier to track down problems flagged by the default mode. level 2 is giving detective energy 🔍
-Wstrict-null-sentinel(C++ and Objective-C++ only) — Warn about the use of an uncasted"NULL"as sentinel. When compiling only with GCC this is a valid sentinel, as"NULL"is defined to"__null". Although it is a null pointer constant rather than a null pointer, it is guaranteed to be of the same size as a pointer. But this use is not portable across different compilers. so it works here and absolutely nowhere else, iconic behavior honestly 💀-Wno-non-c-typedef-for-linkage(C++ and Objective-C++ only) — Disable pedwarn for unnamed classes with a typedef name for linkage purposes containing C++ specific members, base classes, default member initializers or lambda expressions, including those on nested member classes.
typedef struct {
int a; // non-static data members are ok
struct T { int b; }; // member classes too
enum E { E1, E2, E3 }; // member enumerations as well
int c = 42; // default member initializers are not ok
struct U : A { int c; }; // classes with base classes are not ok
typedef int V; // typedef is not ok
using W = int; // using declaration is not ok
decltype([](){}) x; // lambda expressions not ok
} S;In all these cases, the tag name S should be added after the struct keyword. read the comments, they're literally telling you which vibes are valid 🎀
-Wno-non-template-friend(C++ and Objective-C++ only) — Disable warnings when non-template friend functions are declared within a template. In very old versions of GCC that predate implementation of the ISO standard, declarations such asfriend int foo(int), where the name of the friend is an unqualified-id, could be interpreted as a particular specialization of a template function; the warning exists to diagnose compatibility problems, and is enabled by default. ancient GCC lore fr fr 👑-Wold-style-cast(C++ and Objective-C++ only) — Warn if an old-style (C-style) cast to a non-void type is used within a C++ program. The new-style casts ("dynamic_cast","static_cast","reinterpret_cast", and"const_cast") are less vulnerable to unintended effects and much easier to search for. glow-up your casts bestie, the C-style ones are NOT passing the vibe check 💅-Woverloaded-virtual(C++ and Objective-C++ only)- *
-Woverloaded-virtual=n* ** — 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); // does not override
};the "A" class version of "f" is hidden in "B", and code like:
B* b;
b->f();fails to compile. it just does NOT compile, periodt 😭
In cases where the different signatures are not an accident, the simplest solution is to add a using-declaration to the derived class to un-hide the base function, e.g. add "using A::f;" to "B".
The optional level suffix controls the behavior when all the declarations in the derived class override virtual functions in the base class, even if not all of the base functions are overridden:
struct C {
virtual void f();
virtual void f(int);
};
struct D: public C {
void f(int); // does override
}This pattern is less likely to be a mistake; if D is only used virtually, the user might have decided that the base class semantics for some of the overloads are fine.
At level 1, this case does not warn; at level 2, it does. -Woverloaded-virtual by itself selects level 2. Level 1 is included in -Wall. so default = level 2, and -Wall sneaks in level 1, iykyk ✨
-Wno-pmf-conversions(C++ and Objective-C++ only) — Disable the diagnostic for converting a bound pointer to member function to a plain pointer.-Wsign-promo(C++ and Objective-C++ only) — Warn when overload resolution chooses a promotion from unsigned or enumerated type to a signed type, over a conversion to an unsigned type of the same size. Previous versions of G++ tried to preserve unsignedness, but the standard mandates the current behavior. the standard said "no ❤️" and that was that-Wtemplates(C++ and Objective-C++ only) — Warn when a primary template declaration is encountered. Some coding rules disallow templates, and this may be used to enforce that rule. The warning is inactive inside a system header file, such as the STL, so one can still use the STL. One may also instantiate or specialize templates. anti-template coding rules exist and this is how you enforce them, wild but ok 🌸-Wmismatched-new-delete(C++ and Objective-C++ only) — Warn for mismatches between calls to"operator new"or"operator delete"and the corresponding call to the allocation or deallocation function. This includes invocations of C++"operator delete"with pointers returned from either mismatched forms of"operator new", or from other functions that allocate objects for which the"operator delete"isn't a suitable deallocator, as well as calls to other deallocation functions with pointers returned from"operator new"for which the deallocation function isn't suitable.
For example, the "delete" expression in the function below is diagnosed because it doesn't match the array form of the "new" expression the pointer argument was returned from. Similarly, the call to "free" is also diagnosed.
void f ()
{
int *a = new int[n];
delete a; // warning: mismatch in array forms of expressions
char *p = new char[n];
free (p); // warning: mismatch between new and free
}The related option -Wmismatched-dealloc diagnoses mismatches involving allocation and deallocation functions other than "operator new" and "operator delete".
-Wmismatched-new-delete is included in -Wall. you allocated with new[] bestie, you MUST free with delete[], no mixing and matching 💀
-Wmismatched-tags(C++ and Objective-C++ only) — Warn for declarations of structs, classes, and class templates and their specializations with a class-key that does not match either the definition or the first declaration if no definition is provided.
For example, the declaration of "struct Object" in the argument list of "draw" triggers the warning. To avoid it, either remove the redundant class-key "struct" or replace it with "class" to match its definition.
class Object {
public:
virtual ~Object () = 0;
};
void draw (struct Object*);It is not wrong to declare a class with the class-key "struct" as the example above shows. The -Wmismatched-tags option is intended to help achieve a consistent style of class declarations. In code that is intended to be portable to Windows-based compilers the warning helps prevent unresolved references due to the difference in the mangling of symbols declared with different class-keys. The option can be used either on its own or in conjunction with -Wredundant-tags. consistency is the whole point, keep your class-keys matching or Windows will simply not find your symbols 🙈
-Wmultiple-inheritance(C++ and Objective-C++ only) — Warn when a class is defined with multiple direct base classes. Some coding rules disallow multiple inheritance, and this may be used to enforce that rule. The warning is inactive inside a system header file, such as the STL, so one can still use the STL. One may also define classes that indirectly use multiple inheritance. indirect multiple inheritance is fine, direct is the one getting called out 👀-Wvirtual-inheritance— Warn when a class is defined with a virtual direct base class. Some coding rules disallow multiple inheritance, and this may be used to enforce that rule. The warning is inactive inside a system header file, such as the STL, so one can still use the STL. One may also define classes that indirectly use virtual inheritance.-Wno-virtual-move-assign— Suppress warnings about inheriting from a virtual base with a non-trivial C++11 move assignment operator. This is dangerous because if the virtual base is reachable along more than one path, it is moved multiple times, which can mean both objects end up in the moved-from state. If the move assignment operator is written to avoid moving from a moved-from object, this warning can be disabled. DANGEROUS bestie, the virtual base gets moved MULTIPLE times and both objects can end up moved-from 😭-Wnamespaces— Warn when a namespace definition is opened. Some coding rules disallow namespaces, and this may be used to enforce that rule. The warning is inactive inside a system header file, such as the STL, so one can still use the STL. One may also use using directives and qualified names. yes there are coding rules that ban namespaces. we don't make the rules, we just warn about them 💅-Wno-template-body(C++ and Objective-C++ only) — Disable diagnosing errors when parsing a template, and instead issue an error only upon instantiation of the template. This flag can also be used to downgrade such errors into warnings with Wno-error= or-fpermissive. lazy checking hours, errors wait until instantiation ⭐-Wno-template-id-cdtor(C++ and Objective-C++ only) — Disable the warning about the use of simple-template-id as the declarator-id of a constructor or destructor, which became invalid in C++20 via DR 2237. For example:
template<typename T> struct S {
S<T>(); // should be S();
~S<T>(); // should be ~S();
};-Wtemplate-id-cdtor is enabled by default with -std=c++20; it is also enabled by -Wc++20-compat. C++20 said "no more <T> on your constructors" and honestly it's giving glow-up 🎀
-Wtemplate-names-tu-local— bestie, this one yells at you when a template body is hiding an exposure of a translation-unit-local entity. In most cases, referring to a translation-unit-local entity (like an internal linkage declaration) inside an entity that gets emitted into a module's CMI is straight-up an error. BUT — and this is the plot twist — inside the initializer of a variable, or in the body of a non-inline function, that's not an exposure and no error pops off. ✨
The drama? This can make variable or function templates accidentally become unusable if they reference such an entity, because other translation units that import the template will never be able to instantiate it. 😭 This warning is out here trying to catch cases where that might happen. The presence of an explicit instantiation silences the warning, so that's your escape hatch. 🫶
This flag is enabled by -Wextra.
-Wno-expose-global-module-tu-local— okay so an exposure of a translation-unit-local entity from a module interface is invalid, because it may cause ODR violations and manifest in link errors or other unexpected behaviour. Not the vibe. 🙈 BUT, many existing libraries declare TU-local entities in their interface, and avoiding exposures of these entities may be difficult in some cases, so GCC is being nice.
As an extension, GCC allows exposures of internal variables and functions that were declared in the global module fragment. This warning indicates when such an invalid exposure has occurred, and can be silenced using diagnostic pragmas either at the site of the exposure, or at the point of declaration of the internal declaration. This also applies to "export using" declarations naming such entities. 👑
When combined with -Wtemplate-names-tu-local, GCC will also warn about non-exposure references to TU-local entities in template bodies. Such templates can still be instantiated in other TUs but the above risks regarding exposures of translation-unit-local entities apply.
This warning is enabled by default, and is upgraded to an error by -pedantic-errors.
-Wno-external-tu-local— warns when naming a TU-local entity outside of the translation unit it was declared in. Such declarations will be ignored during name lookup. This can occur when performing ADL from a template declared in the same TU as the internal function:
export module M;
template <typename T> void foo(T t) {
bar(t);
}
struct S {} s;
static void bar(S) {} // internal linkage
// instantiating foo(s) from outside this TU can see ::bar,
// but naming it there is ill-formed.This can be worked around by making "bar" attached to the global module, using "extern "C++"". 🎀
This warning is enabled by default, and is upgraded to an error by -pedantic-errors.
-Wno-terminate(C++ and Objective-C++ only) — disables the warning about a throw-expression that will immediately result in a call to"terminate". Like, immediate. 💀-Wno-vexing-parse(C++ and Objective-C++ only) — warns about the most vexing parse syntactic ambiguity. This warns about the cases when a declaration looks like a variable definition, but the C++ language requires it to be interpreted as a function declaration. For instance:
void f(double a) {
int i(); // extern int i (void);
int n(int(a)); // extern int n (int);
}Another example:
struct S { S(int); };
void f(double a) {
S x(int(a)); // extern struct S x (int);
S y(int()); // extern struct S y (int (*) (void));
S z(); // extern struct S z (void);
}The warning will suggest options how to deal with such an ambiguity; e.g., it can suggest removing the parentheses or using braces instead. So helpful, we love. ✨
This warning is enabled by default.
-Wno-class-conversion(C++ and Objective-C++ only) — do not warn when a conversion function converts an object to the same type, to a base class of that type, or to void; such a conversion function will never be called. It's giving pointless. 💅-Wvolatile(C++ and Objective-C++ only) — warns about deprecated uses of the"volatile"qualifier. This includes postfix and prefix"++"and"--"expressions of"volatile"-qualified types, using simple assignments where the left operand is a"volatile"-qualified non-class type for their value, compound assignments where the left operand is a"volatile"-qualified non-class type,"volatile"-qualified function return type,"volatile"-qualified parameter type, and structured bindings of a"volatile"-qualified type. This usage was deprecated in C++20, so it's lowkey on its way out. 🦋
Enabled by default with -std=c++20. Before -std=c++20, enabled with explicit -Wdeprecated.
-Waligned-new-Waligned-new=[none|global|all]— warns about a new-expression of a type that requires greater alignment than thealignof(std::max_align_t)but uses an allocation function without an explicit alignment parameter. This option is enabled by-Wall.
Normally this only warns about global allocation functions, but -Waligned-new=all also warns about class member allocation functions. 🌈
-Wno-placement-new-Wplacement-new=n— warns about placement new expressions with undefined behavior, such as constructing an object in a buffer that is smaller than the type of the object. For example, the placement new expression below is diagnosed because it attempts to construct an array of 64 integers in a buffer only 64 bytes large. 😭
char buf [64];
new (buf) int[64];This warning is enabled by default.
-Wplacement-new=1— this is the default warning level of-Wplacement-new. At this level the warning is not issued for some strictly undefined constructs that GCC allows as extensions for compatibility with legacy code. For example, the following"new"expression is not diagnosed at this level even though it has undefined behavior according to the C++ standard because it writes past the end of the one-element array.
struct S { int n, a[1]; };
S *s = (S *)malloc (sizeof *s + 31 * sizeof s->a[0]);
new (s->a)int [32]();-Wplacement-new=2— at this level, in addition to diagnosing all the same constructs as at level 1, a diagnostic is also issued for placement new expressions that construct an object in the last member of structure whose type is an array of a single element and whose size is less than the size of the object being constructed. While the previous example would be diagnosed, the following construct makes use of the flexible member array extension to avoid the warning at level 2.
struct S { int n, a[]; };
S *s = (S *)malloc (sizeof *s + 32 * sizeof s->a[0]);
new (s->a)int [32]();-Wcatch-value-Wcatch-value=n(C++ and Objective-C++ only) — warns about catch handlers that do not catch via reference. With-Wcatch-value=1(or-Wcatch-valuefor short) warn about polymorphic class types that are caught by value. With-Wcatch-value=2warn about all class types that are caught by value. With-Wcatch-value=3warn about all types that are not caught by reference.-Wcatch-valueis enabled by-Wall. 🔥-Wconditionally-supported(C++ and Objective-C++ only) — warns for conditionally-supported (C++11 [intro.defs]) constructs.-Wno-defaulted-function-deleted(C++ and Objective-C++ only) — warns when an explicitly defaulted function is deleted by the compiler. That can occur when the function's declared type does not match the type of the function that would have been implicitly declared. This warning is enabled by default. 🙈-Wno-delete-incomplete(C++ and Objective-C++ only) — do not warn when deleting a pointer to incomplete type, which may cause undefined behavior at runtime. This warning is enabled by default. So like, it's on unless you turn it off. 💀-Wextra-semi(C++, Objective-C++ only) — warns about redundant semicolons. There are various contexts in which an extra semicolon can occur. One is a semicolon after in-class function definitions, which is valid in all C++ dialects (and is never a pedwarn):
struct S {
void foo () {};
};Another is an extra semicolon at namespace scope, which has been allowed since C++11 (therefore is a pedwarn in C++98):
struct S {
};
;And yet another is an extra semicolon in class definitions, which has been allowed since C++11 (therefore is a pedwarn in C++98):
struct S {
int a;
;
};-Wno-global-module(C++ and Objective-C++ only) — disables the diagnostic for when the global module fragment of a module unit does not consist only of preprocessor directives.-Wno-inaccessible-base(C++, Objective-C++ only) — this option controls warnings when a base class is inaccessible in a class derived from it due to ambiguity. The warning is enabled by default. Note that the warning for ambiguous virtual bases is enabled by the-Wextraoption. 👑
struct A { int a; };
struct B : A { };
struct C : B, A { };-Wno-inherited-variadic-ctor— suppresses warnings about use of C++11 inheriting constructors when the base class inherited from has a C variadic constructor; the warning is on by default because the ellipsis is not inherited. 🎀-Wno-invalid-offsetof(C++ and Objective-C++ only) — suppresses warnings from applying the"offsetof"macro to a non-POD type. According to the 2014 ISO C++ standard, applying"offsetof"to a non-standard-layout type is undefined. In existing C++ implementations, however,"offsetof"typically gives meaningful results. This flag is for users who are aware that they are writing nonportable code and who have deliberately chosen to ignore the warning about it.
The restrictions on "offsetof" may be relaxed in a future version of the C++ standard. 🦋
-Wsized-deallocation(C++ and Objective-C++ only) — warns about a definition of an unsized deallocation function
void operator delete (void *) noexcept;
void operator delete[] (void *) noexcept;without a definition of the corresponding sized deallocation function
void operator delete (void *, std::size_t) noexcept;
void operator delete[] (void *, std::size_t) noexcept;or the other way around, bestie. This one's enabled by -Wextra along with -fsized-deallocation.
-Wsuggest-final-types— Warns about types with virtual methods where code quality would be improved if the type were declared with the C++11finalspecifier, or, if possible, declared in an anonymous namespace. This lets GCC more aggressively devirtualize the polymorphic calls. This warning is more effective with link-time optimization, where the info about the class hierarchy graph is more complete.-Wsuggest-final-methods— Warns about virtual methods where code quality would be improved if the method were declared with the C++11finalspecifier, or, if possible, its type were declared in an anonymous namespace or with thefinalspecifier. This warning is more effective with link-time optimization, where the info about the class hierarchy graph is more complete. It's recommended to first consider suggestions of-Wsuggest-final-typesand then rebuild with new annotations.-Wsuggest-override— Warns about overriding virtual functions that aren't marked with theoverridekeyword.-Wno-conversion-null(C++ and Objective-C++ only) — Do not warn for conversions betweenNULLand non-pointer types.-Wconversion-nullis enabled by default.
🦋 Options Controlling Objective-C and Objective-C++ Dialects 🦋
(NOTE: This manual does not describe the Objective-C and Objective-C++ languages themselves.
This section is all about the command-line options that are only meaningful for Objective-C and Objective-C++ programs. You can also use most of the language-independent GNU compiler options. For example, you might compile a file some_class.m like this:
gcc -g -fgnu-runtime -O -c some_class.mIn 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's the list of options that are only for compiling Objective-C and Objective-C++ programs:
-fconstant-string-class=class-name— Useclass-nameas the name of the class to instantiate for each literal string specified with the syntax@"...". The default class name isNXConstantStringif the GNU runtime is being used, andNSConstantStringif the NeXT runtime is being used (see below). On Darwin / macOS platforms, the-fconstant-cfstringsoption, if also present, overrides the-fconstant-string-classsetting and causes@"..."literals to be laid out as constant CoreFoundation strings. Note that-fconstant-cfstringsis an alias for the target-specific-mconstant-cfstringsequivalent.-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 / macOS. The macro__NEXT_RUNTIME__is predefined if (and only if) this option is used.-fno-nil-receivers— Assume that all Objective-C message dispatches ([receiver message:arg]) in this translation unit ensure that the receiver is notnil. This allows for more efficient entry points in the runtime to be used. This option is only available in conjunction with the NeXT runtime and ABI version 0 or 1.-fobjc-abi-version=n— Use version n of the Objective-C ABI for the selected runtime. This option is currently supported only for the NeXT runtime. In that case, Version 0 is the traditional (32-bit) ABI without support for properties and other Objective-C 2.0 additions. Version 1 is the traditional (32-bit) ABI with support for properties and other Objective-C 2.0 additions. Version 2 is the modern (64-bit) ABI. If nothing is specified, the default is Version 0 on 32-bit target machines, and Version 2 on 64-bit target machines.-fobjc-call-cxx-cdtors— For each Objective-C class, check if any of its instance variables is a C++ object with a non-trivial default constructor. If so, synthesize a special- (id) .cxx_constructinstance method which runs non-trivial default constructors on any such instance variables, in order, and then returnsself. Similarly, check if any instance variable is a C++ object with a non-trivial destructor, and if so, synthesize a special- (void) .cxx_destructmethod which runs all such default destructors, in reverse order.
The - (id) .cxx_construct and - (void) .cxx_destruct methods thusly generated only operate on instance variables declared in the current Objective-C class, and not those inherited from superclasses. It is the responsibility of the Objective-C runtime to invoke all such methods in an object's inheritance hierarchy. The - (id) .cxx_construct methods are invoked by the runtime immediately after a new object instance is allocated; the - (void) .cxx_destruct methods are invoked immediately before the runtime deallocates an object instance.
As of this writing, only the NeXT runtime on Mac OS X 10.4 and later has support for invoking the - (id) .cxx_construct and - (void) .cxx_destruct methods.
-fobjc-direct-dispatch— Allow fast jumps to the message dispatcher. On Darwin this is accomplished via the comm page.-fobjc-exceptions— Enable syntactic support for structured exception handling in Objective-C, similar to what's offered by C++. This option is required to use the Objective-C keywords@try,@throw,@catch,@finallyand@synchronized. This option is available with both the GNU runtime and the NeXT runtime (but not available in conjunction with the NeXT runtime on Mac OS X 10.2 and earlier).-fobjc-gc— Enable garbage collection (GC) in Objective-C and Objective-C++ programs. This option is only available with the NeXT runtime; the GNU runtime has a different garbage collection implementation that does not require special compiler flags.-fobjc-nilcheck— For the NeXT runtime with version 2 of the ABI, check for a nil receiver in method invocations before doing the actual method call. This is the default and can be disabled using-fno-objc-nilcheck. Class methods and super calls are never checked for nil in this way no matter what this flag is set to. Currently this flag does nothing when the GNU runtime, or an older version of the NeXT runtime ABI, is used.-fobjc-std=objc1— Conform to the language syntax of Objective-C 1.0, the language recognized by GCC 4.0. This only affects the Objective-C additions to the C/C++ language; it does not affect conformance to C/C++ standards, which is controlled by the separate C/C++ dialect option flags. When this option is used with the Objective-C or Objective-C++ compiler, any Objective-C syntax that is not recognized by GCC 4.0 is rejected. This is useful if you need to make sure that your Objective-C code can be compiled with older versions of GCC.-freplace-objc-classes— Emit a special marker instructingld(1)not to statically link in the resulting object file, and allowdyld(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 toobjc_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-linkflag suppresses this behavior and causes calls toobjc_getClass("...")to be retained. This is useful in Zero-Link debugging mode, since it allows for individual class implementations to be modified during program execution. The GNU runtime currently always retains calls toobjc_get_class("...")regardless of command-line options.-fno-local-ivars— By default instance variables in Objective-C can be accessed as if they were local variables from within the methods of the class they're declared in. This can lead to shadowing between instance variables and other variables declared either locally inside a class method or globally with the same name. Specifying the-fno-local-ivarsflag disables this behavior thus avoiding variable shadowing issues.-fivar-visibility=[public|protected|private|package]— Set the default instance variable visibility to the specified option so that instance variables declared outside the scope of any access modifier directives default to the specified visibility.-gen-decls— Dump interface declarations for all classes seen in the source file to a file namedsourcename.decl.-Wassign-intercept(Objective-C and Objective-C++ only) — Warn whenever an Objective-C assignment is being intercepted by the garbage collector.-Wno-property-assign-default(Objective-C and Objective-C++ only) — Do not warn if a property for an Objective-C object has no assign semantics specified.-Wno-protocol(Objective-C and Objective-C++ only) — If a class is declared to implement a protocol, a warning is issued for every method in the protocol that is not implemented by the class. The default behavior is to issue a warning for every method not explicitly implemented in the class, even if a method implementation is inherited from the superclass. If you use the-Wno-protocoloption, then methods inherited from the superclass are considered to be implemented, and no warning is issued for them.-Wobjc-root-class(Objective-C and Objective-C++ only) — bestie, this one yells at you if a class interface has no superclass. Like, most classes are gonna inherit fromNSObject(orObject), right? So if you're intentionally making a root class, just slap__attribute__((objc_root_class))on the interface and the warning will shut up 💅.-Wselector(Objective-C and Objective-C++ only) — warns you when the compiler finds multiple methods of different types for the same selector during compilation. The check runs on the method list at the final stage of compilation. On top of that, there's a check for every selector that shows up in a@selector(...)expression, and a corresponding method for that selector was found during compilation. And here's the tea: because these checks only scan the method table at the very end, these warnings won't be produced if the final stage of compilation is never reached — for example because an error got found during compilation, or because you're using the-fsyntax-onlyoption. So like, no final stage, no warning, periodt.-Wstrict-selector-match(Objective-C and Objective-C++ only) — warns when multiple methods with differing argument and/or return types exist for a given selector, and you're trying to send a message using that selector to a receiver of typeidorClass. When this flag is off (which is the default behavior), the compiler omits those warnings if any differences it found are confined to types that share the same size and alignment. Lowkey forgiving, highkey vibes.-Wundeclared-selector(Objective-C and Objective-C++ only) — warns if a@selector(...)expression referring to an undeclared selector is found. A selector counts as undeclared if no method with that name has been declared before the@selector(...)expression — either explicitly in an@interfaceor@protocoldeclaration, or implicitly in an@implementationsection. This option always does its checks the moment a@selector(...)expression is found, while-Wselectoronly checks in the final stage of compilation. It also enforces that coding style convention where methods and selectors have to be declared before being used. ✨-print-objc-runtime-info— generates a C header describing the largest structure that is passed by value, if any. Cute and useful 🎀.
✨ Options Controlling OpenMP and OpenACC ✨
Okay so GCC supports OpenMP extensions to the C, C++, and Fortran languages with the -fopenmp option. Similarly, OpenACC extensions are supported in all three languages with -fopenacc. Two whole parallel universes, bestie 🌈.
-foffload=disable-foffload=default-foffload=target-list
These specify for which OpenMP and OpenACC offload targets code should be generated. The default behavior, equivalent to -foffload=default, is to generate code for all supported offload targets. The -foffload=disable form generates code only for the host fallback, while -foffload=target-list generates code only for the specified comma-separated list of offload targets.
Offload targets are specified in GCC's internal target-triplet format. You can run the compiler with -v to show the list of configured offload targets under OFFLOAD_TARGET_NAMES. 👑
-foffload-options=options-foffload-options=target-triplet-list=options
With -foffload-options=options, GCC passes the specified options to the compilers for all enabled offloading targets. You can specify options that apply only to a specific target or targets by using the -foffload-options=target-list=options form. The target-list is a comma-separated list in the same format as for the -foffload= option.
Typical command lines are
-foffload-options='-fno-math-errno -ffinite-math-only' \
-foffload-options=nvptx-none=-latomic
-foffload-options=amdgcn-amdhsa=-march=gfx906-fopenacc— enables handling of OpenACC directives#pragma accin C/C++ and!$accin free-form Fortran, plus!$acc,c$accand*$accin fixed-form Fortran. This option implies-pthread, and thus is only supported on targets that have support for-pthread. 🧵-fopenacc-dim=geom— specifies default compute dimensions for parallel offload regions that don't explicitly specify them. The geom value is a triple of:-separated sizes, in order gang, worker, and vector. A size can be omitted, to use a target-specific default value. Slay 💅.-fopenmp— enables handling of OpenMP directives#pragma omp,[[omp::decl(...)]],[[omp::directive(...)]], and[[omp::sequence(...)]]in C/C++. In Fortran, it enables!$ompand the conditional compilation sentinel!$. In fixed source form Fortran, the sentinels can also start withcor*.
This option implies -pthread, and thus is only supported on targets that have support for -pthread. -fopenmp implies -fopenmp-simd. Not me stacking implications like accessories 🔥.
-fopenmp-simd— enables handling of OpenMP'ssimd,declare simd,declare reduction,assume,ordered,scanandloopdirective, and of combined or composite directives withsimdas constituent with#pragma omp,[[omp::directive(...)]],[[omp::sequence(...)]]and[[omp::decl(...)]]in C/C++ and!$ompin Fortran. It additionally enables the conditional compilation sentinel!$in Fortran. In fixed source form Fortran, the sentinels can also start withcor*. Other OpenMP directives are ignored. Unless-fopenmpis additionally specified, theloopregion binds to the current task region, independent of the specifiedbindclause. 🦋-fopenmp-target-simd-clone-fopenmp-target-simd-clone=device-type
In addition to generating SIMD clones for functions marked with the declare simd directive, GCC also generates clones for functions marked with the OpenMP declare target directive that are suitable for vectorization when this option is in effect. The device-type may be one of none, host, nohost, and any, which correspond to keywords for the device_type clause of the declare target directive; clones are generated for the intersection of devices specified. -fopenmp-target-simd-clone is equivalent to -fopenmp-target-simd-clone=any and -fno-openmp-target-simd-clone is equivalent to -fopenmp-target-simd-clone=none.
At -O2 and higher (but not -Os or -Og) this optimization defaults to -fopenmp-target-simd-clone=nohost; otherwise it is disabled by default. iykyk 💖
💅 Options to Control Diagnostic Messages Formatting 💅
Traditionally, diagnostic messages have been formatted irrespective of the output device's aspect (e.g. its width, ...). You can use the options described below to control the formatting algorithm for diagnostic messages — like how many characters per line, how often source location information should be reported. Note that some language front ends may not honor these options. Rude, but okay 🙈.
-fmessage-length=n— try to format error messages so that they fit on lines of about n characters. If n is zero, then no line-wrapping is done; each error message appears on a single line. This is the default for all front ends.
Note — this option also affects the display of the #error and #warning pre-processor directives, and the deprecated function/type/variable attribute. It does not however affect the pragma GCC warning and pragma GCC error pragmas. 👑
-fdiagnostics-plain-output— this option requests that diagnostic output look as plain as possible, which may be useful when running dejagnu or other utilities that need to parse diagnostics output and prefer that it remain more stable over time.-fdiagnostics-plain-outputis currently equivalent to the following options:
-fno-diagnostics-show-caret -fno-diagnostics-show-line-numbers
-fdiagnostics-color=never -fdiagnostics-urls=never
-fdiagnostics-path-format=separate-events
-fdiagnostics-text-art-charset=none
-fno-diagnostics-show-event-links -fno-diagnostics-show-nestingIn the future, if GCC changes the default appearance of its diagnostics, the corresponding option to disable the new behavior will be added to this list. Future-proofing, we love to see it ✨.
-fdiagnostics-show-location=once— only meaningful in line-wrapping mode. Instructs the diagnostic messages reporter to emit source location information once; that is, in case the message is too long to fit on a single physical line and has to be wrapped, the source location won't be emitted (as prefix) again, over and over, in subsequent continuation lines. This is the default behavior. Thank god 😭.-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. Like a little echo, bestie 🌸.-fdiagnostics-color[=WHEN]-fno-diagnostics-color
Use color in diagnostics. WHEN is never, always, or auto. The default depends on how the compiler has been configured — it can be any of the above WHEN options, or also never if the GCC_COLORS environment variable isn't present in the environment, and auto otherwise. auto makes GCC use color only when the standard error is a terminal, and when not executing in an emacs shell. The forms -fdiagnostics-color and -fno-diagnostics-color are aliases for -fdiagnostics-color=always and -fdiagnostics-color=never, respectively. 🎀
The colors are defined by the environment variable GCC_COLORS. Its value is a colon-separated list of capabilities and Select Graphic Rendition (SGR) substrings. SGR commands are interpreted by the terminal or terminal emulator. (See the section in the documentation of your text terminal for permitted values and their meanings as character attributes.) These substring values are integers in decimal representation and can be concatenated with semicolons. Common values to concatenate include:
1for bold4for underline5for blink7for inverse39for default foreground color30to37for foreground colors90to97for 16-color mode foreground colors38;5;0to38;5;255for 88-color and 256-color modes foreground colors49for default background color40to47for background colors100to107for 16-color mode background colors48;5;0to48;5;255for 88-color and 256-color modes background colors
The default GCC_COLORS is
error=01;31:warning=01;35:note=01;36:range1=32:range2=34:locus=01:\
quote=01:path=01;36:fixit-insert=32:fixit-delete=31:\
diff-filename=01:diff-hunk=32:diff-delete=31:diff-insert=32:\
type-diff=01;32:fnname=01;32:targs=35:valid=01;31:invalid=01;32\
highlight-a=01;32:highlight-b=01;34so for the record bestie: 01;31 is bold red, 01;35 is bold magenta, 01;36 is bold cyan, 32 is green, 34 is blue, 01 is bold, and 31 is red ✨
and if you set GCC_COLORS to the empty string? colors are just off. gone. periodt 💅
here's the full lineup of supported capabilities, one by one:
error=— the SGR substring forerror:markers 🙈warning=— the SGR substring forwarning:markersnote=— the SGR substring fornote:markerspath=— the SGR substring that colorizes paths of control-flow events as printed via-fdiagnostics-path-format=, like the identifiers of individual events and the lines showing interprocedural calls and returns 🛤️range1=— the SGR substring for the first additional rangerange2=— the SGR substring for the second additional rangelocus=— the SGR substring for location info, i.e.file:lineorfile:line:columnetc. 📍quote=— the SGR substring for information printed inside quotesfnname=— the SGR substring for names of C++ functionstargs=— the SGR substring for C++ function template parameter bindingsfixit-insert=— the SGR substring for fix-it hints suggesting text to be inserted or replacedfixit-delete=— the SGR substring for fix-it hints suggesting text to be deleteddiff-filename=— the SGR substring for filename headers within generated patchesdiff-hunk=— the SGR substring for the starts of hunks within generated patchesdiff-delete=— the SGR substring for deleted lines within generated patchesdiff-insert=— the SGR substring for inserted lines within generated patchestype-diff=— the SGR substring for highlighting mismatching types within template arguments in the C++ frontendvalid=— the SGR substring for highlighting valid elements within text art diagramsinvalid=— the SGR substring for highlighting invalid elements within text art diagramshighlight-a=,highlight-b=— the SGR substrings for contrasting two different things within diagnostics, like a pair of mismatching types. see-fdiagnostics-show-highlight-colors🎨
✨ -fdiagnostics-urls[=WHEN] ✨
okay so this one embeds URLs into your diagnostics using escape sequences 🔗 like, when -fdiagnostics-show-option prints text showing the command-line option that controls a diagnostic, it'll embed a URL pointing at the docs for that option. cute right?
WHEN is never, always, or auto. and auto? that makes GCC use URL escape sequences only when standard error is a terminal, and only when it's not running inside an emacs shell or any graphical terminal that's known to be incompatible with this feature — see below bestie, i got you 👇
the default depends on how the compiler was configured. it can be any of the WHEN options above, no cap.
GCC can also be configured (via the --with-diagnostics-urls=auto-if-env configure-time option) so the default is affected by environment variables 🌱 under that config, GCC defaults to using auto if either GCC_URLS or TERM_URLS environment variables are present and non-empty in the compiler's environment, or never if neither are.
but — and this is the plot twist 😭 — even with -fdiagnostics-urls=always the behavior depends on those env vars:
- if
GCC_URLSis set to empty orno, do not embed URLs in diagnostics 🚫 - if it's set to
st, URLs use ST escape sequences - if it's set to
bel(the default), URLs use BEL escape sequences - any other non-empty value enables the feature
- if
GCC_URLSis not set, useTERM_URLSas a fallback
note: ST is an ANSI escape sequence, string terminator ESC , and BEL is an ASCII character, CTRL-G, that usually sounds like a beep 🛎️
rn GCC also tries to detect a few terminals that are known to not implement the URL feature, and have bugs — or at least had bugs in some versions that are still in use — where the URL escapes are likely to misbehave, i.e. print garbage all over your screen 💀 that list is currently xfce4-terminal, certain known-to-be-buggy gnome-terminal versions, the linux console, and mingw. you can skip this check with -fdiagnostics-urls=always.
💜 -fno-diagnostics-show-option 💜
by default, every diagnostic that gets emitted includes text showing the command-line option that directly controls it (if such an option is known to the diagnostic machinery). passing the -fno-diagnostics-show-option flag suppresses that behavior. poof, gone ✨
💜 -fno-diagnostics-show-caret 💜
by default, each diagnostic emitted includes the original source line and a caret ^ pointing at the column 🎯 this option suppresses that info. the source line gets truncated to n characters if the -fmessage-length=n option is given. and when the output goes to the terminal, the width is limited to the width given by the COLUMNS environment variable, or — if that's not set — to the terminal width. iykyk 📏
💜 -fno-diagnostics-show-labels 💜
by default, when printing source code (via -fdiagnostics-show-caret), diagnostics can label ranges of source code with pertinent information, like the types of expressions:
printf ("foo %s bar", long_i + long_j);
~^ ~~~~~~~~~~~~~~~
| |
char * long intthis option suppresses the printing of those labels — so in the example above, the vertical bars and the char * and long int text just don't show up 🫥
💜 -fno-diagnostics-show-event-links 💜
by default, when printing execution paths (via -fdiagnostics-path-format=inline-events), GCC prints lines connecting related events, like the line connecting events 1 and 2 in:
3 | if (p)
| ^
| |
| (1) following `false' branch (when `p' is NULL)... ->-+
| |
| |
|+------------------------------------------------------------+
4 || return 0;
5 || return *p;
|| ~
|| |
|+-------->(2) ...to here
| (3) dereference of NULL `p'this option suppresses the printing of those connector lines 🧵
💜 -fno-diagnostics-show-cwe 💜
diagnostic messages can optionally have an associated CWE (https://cwe.mitre.org/index.html) identifier 🪪 GCC itself only provides that metadata for some of the -fanalyzer diagnostics. GCC plugins may also provide diagnostics with such metadata. by default, if this info is present, it gets printed with the diagnostic. this option suppresses the printing of that metadata.
💜 -fno-diagnostics-show-rules 💜
diagnostic messages can optionally have rules associated with them, like from a coding standard or a specification 📜 GCC itself does not do this for any of its diagnostics, but plugins may. by default, if this info is present, it gets printed with the diagnostic. this option suppresses the printing of that metadata.
💜 -fno-diagnostics-show-highlight-colors 💜
GCC can use color for emphasis and contrast when printing diagnostic messages and quoting your source 🎨
for example, in
demo.c: In function `test_bad_format_string_args':
../../src/demo.c:25:18: warning: format `%i' expects argument of type `int', but argument 2 has type `const char *' [-Wformat=]
25 | printf("hello %i", msg);
| ~^ ~~~
| | |
| int const char *
| %s- the
%iandintin the message and theintin the quoted source are colored usinghighlight-a(bold green by default) 💚 - and the
const char *in the message and in the quoted source are both colored usinghighlight-b(bold blue by default) 💙
the intent is to draw the reader's eyes to the relationships between the various aspects of the diagnostic message and the source, using color to group related elements and distinguish between mismatching ones 👀✨
this extra colorization is enabled by default if color printing is enabled (as per -fdiagnostics-color=), but it can be separately disabled via -fno-diagnostics-show-highlight-colors.
💜 -fno-diagnostics-show-line-numbers 💜
by default, when printing source code (via -fdiagnostics-show-caret), a left margin is printed showing line numbers 🔢 this option suppresses that left margin. bye bye numbers 👋
💜 -fdiagnostics-minimum-margin-width=width 💜
this option controls the minimum width of the left margin printed by -fdiagnostics-show-line-numbers 📐 it defaults to 6.
💜 -fdiagnostics-show-context[=depth] 💜
💜 -fno-diagnostics-show-context 💜
with this option, the compiler might print the interesting control flow chain that guards the basic block of the statement which has the warning 🕵️ depth is the maximum depth of the control flow chain.
currently, the list of impacted warning options includes: -Warray-bounds, -Wstringop-overflow, -Wstringop-overread, -Wstringop-truncation. and -Wrestrict. more warning options might get added to this list in future releases, so watch this space 👑
the forms -fdiagnostics-show-context and -fno-diagnostics-show-context are aliases for -fdiagnostics-show-context=1 and -fdiagnostics-show-context=0, respectively.
💜 -fdiagnostics-parseable-fixits 💜
emit fix-it hints in a machine-parseable format, suitable for consumption by IDEs 🤖 for each fix-it, a line gets printed after the relevant diagnostic, starting with the string fix-it:. for example:
fix-it:"test.c":{45:3-45:21}:"gtk_widget_show_all"okay so the location is a half-open range, expressed as a count of bytes, and the very first column counts as byte 1 💖. In the example up above, bytes 3 through 20 of line 45 of test.c are the ones getting swapped out for the replacement string:
00000000011111111112222222222
12345678901234567890123456789
gtk_widget_showall (dlg);
^^^^^^^^^^^^^^^^^^
gtk_widget_show_allThe filename and the replacement string escape backslash as \\, tab as \t, newline as \n, double quotes as \", and non-printable characters as octal (e.g. vertical tab as \013) 🌸.
An empty replacement string means that range is just gone, removed, deleted, no more 🫥. And an empty range (like 45:3-45:3) means the string is getting inserted at that position instead.
-fdiagnostics-generate-patch— prints fix-it hints to stderr in unified diff format, after all the diagnostics are done printing. It's giving "here's the patch you should've written" energy 💅. Example:
--- test.c
+++ test.c
@ -42,5 +42,5 @
void show_cb(GtkDialog *dlg)
{
- gtk_widget_showall(dlg);
+ gtk_widget_show_all(dlg);
}That diff may or may not be colorized, following the same rules as for diagnostics (peep -fdiagnostics-color for the whole vibe) 🎨.
-fdiagnostics-show-template-tree— in the C++ frontend, when diagnostics show mismatching template types, like this:
could not convert 'std::map<int, std::vector<double> >()'
from 'map<[...],vector<double>>' to 'map<[...],vector<float>>this flag turns on printing a tree-like structure showing the common parts and the differing parts of the types, like:
map<
[...],
vector<
[double != float]>>The parts that differ get highlighted with color (double and float in this case) 🌈.
-fno-elide-type— by default, when the C++ frontend prints diagnostics showing mismatching template types, the common parts of the types get printed as[...]to keep the error message simple. For example:
could not convert 'std::map<int, std::vector<double> >()'
from 'map<[...],vector<double>>' to 'map<[...],vector<float>>Specifying the -fno-elide-type flag suppresses that behavior. This flag also affects the output of the -fdiagnostics-show-template-tree flag, so they're besties, lowkey 👯.
-fdiagnostics-path-format=KIND— specify how to print paths of control-flow events for diagnostics that have such a path associated with them.
KIND is none, separate-events, or inline-events, which is the default.
none means to not print diagnostic paths at all 🙈.
separate-events means to print a separate note diagnostic for each event within the diagnostic. For example:
test.c:29:5: error: passing NULL as argument 1 to 'PyList_Append' which requires a non-NULL parameter
test.c:25:10: note: (1) when 'PyList_New' fails, returning NULL
test.c:27:3: note: (2) when 'i < count'
test.c:29:5: note: (3) when calling 'PyList_Append', passing NULL from (1) as argument 1inline-events means to print the events "inline" within the source code. This view attempts to consolidate the events into runs of sufficiently-close events, printing them as labelled ranges within the source ✨.
For example, the same events as above might be printed as:
'test': events 1-3
25 | list = PyList_New(0);
| ^~~~~~~~~~~~~
| |
| (1) when 'PyList_New' fails, returning NULL
26 |
27 | for (i = 0; i < count; i++) {
| ~~~
| |
| (2) when 'i < count'
28 | item = PyLong_FromLong(random());
29 | PyList_Append(list, item);
| ~~~~~~~~~~~~~~~~~~~~~~~~~
| |
| (3) when calling 'PyList_Append', passing NULL from (1) as argument 1Interprocedural control flow is shown by grouping the events by stack frame, and using indentation to show how stack frames are nested, pushed, and popped 🫶.
For example:
'test': events 1-2
|
| 133 | {
| | ^
| | |
| | (1) entering 'test'
| 134 | boxed_int *obj = make_boxed_int (i);
| | ~~~~~~~~~~~~~~~~~~
| | |
| | (2) calling 'make_boxed_int'
|
+--> 'make_boxed_int': events 3-4
|
| 120 | {
| | ^
| | |
| | (3) entering 'make_boxed_int'
| 121 | boxed_int *result = (boxed_int *)wrapped_malloc (sizeof (boxed_int));
| | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
| | |
| | (4) calling 'wrapped_malloc'
|
+--> 'wrapped_malloc': events 5-6
|
| 7 | {
| | ^
| | |
| | (5) entering 'wrapped_malloc'
| 8 | return malloc (size);
| | ~~~~~~~~~~~~~
| | |
| | (6) calling 'malloc'
|
<-------------+
|
'test': event 7
|
| 138 | free_boxed_int (obj);
| | ^~~~~~~~~~~~~~~~~~~~
| | |
| | (7) calling 'free_boxed_int'
|
(etc)-fdiagnostics-show-path-depths— this option provides extra information when printing control-flow paths associated with a diagnostic 👑.
If this option is provided, then the stack depth gets printed for each run of events within -fdiagnostics-path-format=inline-events. And if it's provided alongside -fdiagnostics-path-format=separate-events, then the stack depth and the function declaration get appended when printing each event.
This is intended for use by GCC developers and plugin developers when debugging diagnostics that report interprocedural control flow — so like, nerdy dev-only territory, iykyk 🔧.
-fno-show-column— do not print column numbers in diagnostics. This may be necessary if diagnostics are being scanned by a program that doesn't understand column numbers, such as dejagnu 💀.-fdiagnostics-column-unit=UNIT— select the units for the column number. This affects traditional diagnostics (in the absence of-fno-show-column).
The default UNIT, display, considers the number of display columns occupied by each character. That can be larger than the number of bytes required to encode the character, in the case of tab characters, or it can be smaller, in the case of multibyte characters 😵. For example, the character "GREEK SMALL LETTER PI (U+03C0)" occupies one display column, and its UTF-8 encoding requires two bytes; the character "SLIGHTLY SMILING FACE (U+1F642)" occupies two display columns, and its UTF-8 encoding requires four bytes 🍓.
Setting UNIT to byte changes the column number to the raw byte count in all cases, as was traditionally output by GCC prior to version 11.1.0.
-fdiagnostics-column-origin=ORIGIN— select the origin for column numbers, i.e. the column number assigned to the first column. The default value of1corresponds to traditional GCC behavior and to the GNU style guide 📖. Some utilities may perform better with an origin of0; any non-negative value may be specified.-fdiagnostics-escape-format=FORMAT— when GCC prints pertinent source lines for a diagnostic it normally attempts to print the source bytes directly. However, some diagnostics relate to encoding issues in the source file, such as malformed UTF-8, or issues with Unicode normalization. Those diagnostics get flagged so that GCC will escape bytes that are not printable ASCII when printing their pertinent source lines 🧚.
This option controls how such bytes should be escaped.
The default FORMAT, unicode, displays Unicode characters that are not printable ASCII in the form <U+XXXX>, and bytes that do not correspond to a Unicode character validly-encoded in UTF-8-encoded will be displayed as hexadecimal in the form <XX>.
For example, a source line containing the string before followed by the Unicode character U+03C0 ("GREEK SMALL LETTER PI", with UTF-8 encoding 0xCF 0x80) followed by the byte 0xBF (a stray UTF-8 trailing byte), followed by the string after will be printed for such a diagnostic as:
before<U+03C0><BF>afterSetting FORMAT to bytes will display all non-printable-ASCII bytes in the form <XX>, thus showing the underlying encoding of non-ASCII Unicode characters 😭. For the example above, the following will be printed:
before<CF><80><BF>after-fdiagnostics-text-art-charset=CHARSET— some diagnostics can contain "text art" diagrams: visualizations created from text, intended to be viewed in a monospaced font 🎀.
This option selects which characters should be used for printing such diagrams, if any. CHARSET is none, ascii, unicode, or emoji.
The none value suppresses the printing of such diagrams entirely. The ascii value will ensure that such diagrams are pure ASCII ("ASCII art") 💀. The unicode value will allow for conservative use of unicode drawing characters (such as box-drawing characters) ✨. The emoji value further adds the possibility of emoji in the output (such as emitting U+26A0 WARNING SIGN followed by U+FE0F VARIATION SELECTOR-16 to select the emoji variant of the character) 🔥.
The default is emoji, except when the environment variable LANG is set to C, in which case the default is ascii 🌸.
-fno-diagnostics-show-nesting— okay so some GCC diagnostics are literally built like a little tree 🌳 — they've got an internal structure of nested sub-diagnostics, like when C++ templates get instantiated and everything goes sideways. By default GCC shows you that nesting with indentation and bullet points in its text output, moves the location info onto separate lines so the structure is actually legible, and drops any redundant repeated info. Pick this option and you suppress the indentation, the reformatting and the elision, restoring an older`look"for the diagnostics. Retro vibes, bestie 💅-fno-diagnostics-show-nesting-locations— whenfdiagnostics-show-nestingis enabled, file names and line- and column- numbers get displayed on separate lines from the messages. You can switch that location info off entirely with-fno-diagnostics-show-nesting-locations. This one exists for GCC developers, for writing DejaGnu test cases 🧪-fdiagnostics-show-nesting-levels— whenfdiagnostics-show-nestingis enabled, usefdiagnostics-show-nesting-levelsto also display numbers showing how deep the nesting goes. This exists for GCC developers debugging nested diagnostics, but plugin authors might lowkey find it useful too 👑-fdiagnostics-format=FORMAT— pick a whole different format for printing diagnostics.FORMATis one oftext,sarif-stderrorsarif-file.
Using this option replaces any additional "output sinks" added by -fdiagnostics-add-output=, or the one set by -fdiagnostics-set-output=.
The default is text.
The sarif-stderr and sarif-file formats both emit diagnostics in SARIF Version 2.1.0 format — either to stderr, or to a file named source.sarif, respectively. It's giving structured ✨
-fdiagnostics-add-output=DIAGNOSTICS-OUTPUT-SPEC— adds an additional "output sink" for emitting diagnostics. So you can have more than one place your diagnostics go, no cap 🫶
DIAGNOSTICS-OUTPUT-SPEC should specify a scheme, optionally followed by ":" and one or more KEY=VALUE pairs, in this form:
<SCHEME>
<SCHEME>:<KEY>=<VALUE>
<SCHEME>:<KEY>=<VALUE>,<KEY2>=<VALUE2>etc.
Schemes, keys, or values with a name prefixed "experimental" may change or be removed without notice. Heads up, bestie — don't build your whole life on those 💀
Keys can be per-scheme, or related to GCC as a whole.
SCHEME can be:
text— emit diagnostics to stderr using GCC's classic text output format. The supported keys for thetextscheme are:color=[yes|no]— override colorization settings from-fdiagnostics-colorfor this text output. 🌈show-nesting=[yes|no]— enable a mode that emphasizes hierarchical relationships within diagnostics messages, as per-fdiagnostics-show-nesting. Defaults to"yes".show-nesting-locations=[yes|no]— if"show-nesting=yes", then by default locations are shown; set this key to"no"to disable printing such locations. This exists for use by GCC developers, for writing DejaGnu test cases.show-nesting-levels=[yes|no]— this is a debugging option for use with"show-nesting=yes". Set this key to"yes"to print explicit nesting levels in the output. This exists for use by GCC developers.
sarif— emit diagnostics to a file in SARIF format. The supported keys for thesarifscheme are:file=FILENAME— specify the filename to write the SARIF output to, potentially with a leading absolute or relative path. If not specified, it defaults tosource.sarif.serialization=[json]— specify the serialization format to use when writing out the SARIF. Currently this can only be"json", but it's present as an extension point for experimenting with other serializations.version=[2.1|2.2-prerelease]— specify the version of SARIF to use for the output. If not specified, defaults to2.1."2.2-prerelease"uses an unofficial draft of the future SARIF 2.2 specification and should only be used for experimentation in this release. ⚠️
There is also this key intended for use by GCC developers, rather than end-users, and subject to change or removal without notice:
state-graphs=[yes|no]— this is a debugging feature and defaults to"no". If"state-graphs=yes", then attempt to capture detailed state information from-fanalyzerin the generated SARIF.
experimental-html— emit diagnostics to a file in HTML format. This scheme is experimental, and may go away in future GCC releases. The keys and details of the output are also subject to change. 🙈
The supported keys for the experimental-html scheme are:
css=[yes|no]— add an embedded<style>to the generated HTML. Defaults to yes.file=FILENAME— specify the filename to write the HTML output to, potentially with a leading absolute or relative path. If not specified, it defaults tosource.html.javascript=[yes|no]— add an embedded<script>to the generated HTML providing a barebones UI for viewing results. Defaults to yes.
There are also these keys intended for use by GCC developers, rather than end-users, and subject to change or removal without notice:
show-state-diagrams=[yes|no]— this is a debugging feature and defaults to"no". If"show-state-diagrams=yes", then attempt to usedotto generate SVG diagrams in the generated HTML, visualizing the state at each event in a diagnostic path. These are visible by pressing"j"and"k"to single-step forward and backward through events. Enabling this option will slow down HTML generation. 🐌show-graph-dot-src=[yes|no]— this is a debugging feature and defaults to"no". If"show-graph-dot-src=yes"then if"show-state-diagrams=yes", the generated state diagrams will also show the.dotsource input to GraphViz used for the diagram.show-graph-sarif=[yes|no]— this is a debugging feature and defaults to"no". If"show-graph-sarif=yes"then if"show-state-diagrams=yes", the generated state diagrams will also show a SARIF representation of the state.
As well as scheme-specific keys, the following GCC-related key is usable on sinks of any scheme:
cfgs=[yes|no]— if"cfgs=yes"for a sink, then GCC will attempt to send information to that sink about the control flow graphs for the functions it is compiling. Text sinks ignore the information. SARIF sinks will add the graphs within"theRun.graphs". HTML sinks will generate SVG displaying the graphs. The precise form of the information is subject to change without notice.
For example,
-fdiagnostics-add-output=sarif:version=2.1,file=foo.2.1.sarif
-fdiagnostics-add-output=sarif:version=2.2-prerelease,file=foo.2.2.sarifwould add a pair of outputs, each writing to a different file, using versions 2.1 and 2.2 of the SARIF standard respectively.
In EBNF:
<diagnostics-output-specifier> = <diagnostics-output-name>
| <diagnostics-output-name>, ":", <key-value-pairs>;
<diagnostics-output-name> = "text" | "sarif" | "experimental-html";
<key-value-pairs> = <key-value-pair>
| <key-value-pair> "," <key-value-pairs>;
<key-value-pair> = <key> "=" <value>;
<key> = ? string without a '=' ? ;
<value> = ? string without a ',' ? ;-fdiagnostics-set-output=DIAGNOSTICS-OUTPUT-SPEC— works in a similar way to-fdiagnostics-add-output=, except instead of adding an extra "output sink" for diagnostics, it replaces all existing output sinks — like the ones from-fdiagnostics-format=,-fdiagnostics-add-output=, or a prior call to-fdiagnostics-set-output=. Out with the old, in with the new 💅-fno-diagnostics-json-formatting— by default, when JSON is emitted for diagnostics (via-fdiagnostics-format=sarif-stderror-fdiagnostics-format=sarif-file), GCC adds newlines and indentation to visually emphasize the hierarchical structure of the JSON. Use-fno-diagnostics-json-formattingto suppress that whitespace.
It must be passed before the option it is to affect. Order matters, bestie, don't skip that part 🎀
This is intended for compatibility with tools that do not expect the output to contain newlines, such as that emitted by older GCC releases.
🚨 Options to Request or Suppress Warnings 🚨
Warnings are diagnostic messages that report constructions that are not inherently erroneous but that are risky or suggest there may have been an error. They're like, "girl, this compiles but are you sure?" energy 😭
The following language-independent options do not enable specific warnings but control the kinds of diagnostics produced by GCC.
-fsyntax-only— check the code for syntax errors, but don't do anything beyond that. Just a vibe check, nothing more ✨-fmax-errors=n— limits the maximum number of error messages to n, at which point GCC bails out rather than attempting to continue processing the source code. If n is 0 (the default), there is no limit on the number of error messages produced. If-Wfatal-errorsis also specified, then-Wfatal-errorstakes precedence over this option. 👑-w,--no-warnings— inhibit all warning messages. Silence, iconic, no notes 🤫-Werror— turn all warnings into errors. Every little warning is now a whole problem. Periodt 💀-Werror=— turn the specified warning into an error. The specifier for a warning is appended; for example-Werror=switchturns the warnings controlled by-Wswitchinto errors. This switch takes a negative form, to be used to negate-Werrorfor specific warnings; for example-Wno-error=switchmakes-Wswitchwarnings not be errors, even when-Werroris in effect.
The warning message for each controllable warning includes the option that controls the warning. That option can then be used with -Werror= and -Wno-error= as described above. (Printing of the option in the warning message can be disabled using the -fno-diagnostics-show-option flag.)
Note that specifying -Werror=foo automatically implies -Wfoo. However, -Wno-error=foo does not imply anything. Fr fr, that asymmetry is real 🦋
-Wfatal-errors— bestie, the second the compiler hits its first error it just ✨stops✨. No limping along, no drowning you in a million follow-up error messages. First oopsie = done. 💀
okay so you can also ask for specific warnings one by one with options that start with -W, like -Wunused-variable when you wanna get yelled at about variables you declared and then never touched. 💅 And plot twist: every single one of those specific warning options has a negative form starting with -Wno- that turns the warning OFF — so -Wno-unused-variable is the "actually never mind" version. This manual only lists one of the two forms, whichever one isn't the default. For more language-specific stuff, go peek at C++ Dialect Options and Objective-C and Objective-C++ Dialect Options too. Oh and you can unlock even more warnings by turning on the static analyzer, iykyk. 🦋
Some options like -Wall and -Wextra are lowkey gateways — they turn on other options like -Wunused, which then turn on even more options like -Wunused-variable. The way the positive and negative forms stack is: more specific options win over less specific ones, no matter where they sit on the command line. For two options of the same specificity, the last one wins. And options flipped on or off via pragmas act like they were written at the very end of the command line. Periodt. ✨
When you ask for a warning option that doesn't exist (say -Wunknown-warning), GCC throws an error saying it doesn't recognize the option. BUT — and this is the fun part — if you use the -Wno- form, it's a lil different: you get no diagnostic for -Wno-unknown-warning unless other diagnostics are already being produced. That's on purpose, so you can use shiny new -Wno- options with old compilers. If something goes sideways though, the compiler will warn you that an unrecognized option is hanging around. 👀
Also real talk: how well some warnings work depends on optimizations being enabled too. Like -Wsuggest-final-types hits way harder with link-time optimization. Some other warnings won't even show up at all unless optimization is on. Optimization in general makes warnings about control-flow and data-flow problems better, but in some cases it can also cause false positives. 😭
✨ -Wpedantic, -pedantic, --pedantic ✨
This one issues all the warnings that strict ISO C and ISO C++ demand. It diagnoses every program using forbidden extensions, plus some other programs that don't follow ISO C and ISO C++. It follows whichever version of the ISO C or C++ standard you picked with any -std option.
Valid ISO C and ISO C++ programs should compile just fine with or without this option (tho a rare few need -ansi or a -std option specifying the standard version). But here's the tea: without this option, certain GNU extensions and traditional C and C++ features are also supported. With it, those get diagnosed — or straight-up rejected if you're using -pedantic-errors.
-Wpedantic does not warn you about alternate keywords whose names begin and end with __. That alternate format can also be used to disable warnings for non-ISO __intN types, i.e. __intN__. Pedantic warnings are also disabled in the expression that follows "__extension__". BUT — only system header files should use these escape routes, bestie. Application programs should avoid them. 🙈
Some warnings about non-conforming programs are controlled by options other than -Wpedantic; in many cases they're implied by -Wpedantic but can be turned off separately by their own specific option, e.g. -Wpedantic -Wno-pointer-sign.
Where the standard set with -std is a GNU extended dialect of C, like gnu90 or gnu99, there's a corresponding base standard — the version of ISO C that the GNU dialect is based on. -Wpedantic warnings get given where the base standard requires them. (And ngl it wouldn't make sense for those warnings to only fire for features not in the specified GNU C dialect, since by definition the GNU dialects of C include every feature the compiler supports with the given option, so there'd be literally nothing to warn about. 💀)
✨ -pedantic-errors, --pedantic-errors ✨
Gives you an error whenever the base standard (peep -Wpedantic) requires a diagnostic — in some cases where there's undefined behavior at compile-time, and in some other cases that don't stop compilation of programs that are valid per the standard. Heads up: this is not the same as -Werror=pedantic. That latter option is unlikely to be useful, because it only makes errors out of the diagnostics controlled by -Wpedantic, whereas this option also affects required diagnostics that are always enabled or controlled by options other than -Wpedantic. Different vibe entirely. 💅
If you want the required diagnostics that are warnings by default to be errors instead, but you don't wanna enable the -Wpedantic diagnostics, you can say -pedantic-errors -Wno-pedantic (or -pedantic-errors -Wno-error=pedantic to enable them but only as warnings).
Some required diagnostics are errors by default, but can be downgraded to warnings using -fpermissive or their specific warning option, e.g. -Wno-error=narrowing.
Some diagnostics for non-ISO practices are controlled by specific warning options other than -Wpedantic, but they also get turned into errors by -pedantic-errors. For instance:
-Wattributes(for standard attributes)-Wchanges-meaning(C++)-Wcomma-subscript(C++23 or later)-Wdeclaration-after-statement(C90 or earlier)-Welaborated-enum-base(C++11 or later)-Wimplicit-int(C99 or later)-Wimplicit-function-declaration(C99 or later)-Wincompatible-pointer-types-Wint-conversion-Wlong-long(C90 or earlier)-Wmain-Wnarrowing(C++11 or later)-Wpointer-arith-Wpointer-sign-Wincompatible-pointer-types-Wregister(C++17 or later)-Wvla(C90 or earlier)-Wwrite-strings(C++11 or later)
✨ -fpermissive ✨
Downgrades some required diagnostics about nonconformant code from errors to warnings. So yeah, using -fpermissive lets some nonconforming code compile. Some C++ diagnostics are controlled only by this flag, but it also downgrades some C and C++ diagnostics that have their own flag:
-Wabbreviated-auto-in-template-arg(C++ and Objective-C++ only)-Wdeclaration-missing-parameter-type(C and Objective-C only)-Wimplicit-function-declaration(C and Objective-C only)-Wimplicit-int(C and Objective-C only)-Wincompatible-pointer-types(C and Objective-C only)-Wint-conversion(C and Objective-C only)-Wnarrowing(C++ and Objective-C++ only)-Wreturn-mismatch(C and Objective-C only)-Wtemplate-body(C++ and Objective-C++ only)
-fpermissive is the default for historic C language modes (-std=c89, -std=gnu89, -std=c90, -std=gnu90). 🎀
✨ -Wall, --all-warnings ✨
This turns on all the warnings about constructions that some users find questionable, and that are easy to avoid (or tweak so the warning stops), even when macros are involved. It also turns on some language-specific warnings described over in C++ Dialect Options and Objective-C and Objective-C++ Dialect Options.
-Wall turns on these warning flags:
-Waddress-Waligned-new(C++ and Objective-C++ only)-Warray-bounds=1(only with-O2)-Warray-compare-Warray-parameter=2-Wbool-compare-Wbool-operation-Wc++11-compat-Wc++14-compat-Wc++17compat-Wc++20compat-Wcatch-value(C++ and Objective-C++ only)-Wchar-subscripts-Wclass-memaccess(C++ and Objective-C++ only)-Wcomment-Wdangling-else-Wdangling-pointer=2-Wdelete-non-virtual-dtor(C++ and Objective-C++ only)-Wduplicate-decl-specifier(C and Objective-C only)-Wenum-compare(in C/ObjC; this is on by default in C++)-Wenum-int-mismatch(C and Objective-C only)-Wformat=1-Wformat-contains-nul-Wformat-diag-Wformat-extra-args-Wformat-overflow=1-Wformat-truncation=1-Wformat-zero-length-Wframe-address-Wimplicit(C and Objective-C only)-Wimplicit-function-declaration(C and Objective-C only)-Wimplicit-int(C and Objective-C only)-Winfinite-recursion-Winit-self(C++ and Objective-C++ only)-Wint-in-bool-context-Wlogical-not-parentheses-Wmain(only for C/ObjC and unless-ffreestanding)-Wmaybe-uninitialized-Wmemset-elt-size-Wmemset-transposed-args-Wmisleading-indentation(only for C/C++)-Wmismatched-dealloc-Wmismatched-new-delete(C++ and Objective-C++ only)-Wmissing-attributes-Wmissing-braces(only for C/ObjC)-Wmultistatement-macros-Wnarrowing(C++ and Objective-C++ only)-Wnonnull-Wnonnull-compare-Wopenmp-simd(C and C++ only)-Woverloaded-virtual=1(C++ and Objective-C++ only)-Wpacked-not-aligned-Wparentheses-Wpessimizing-move(C++ and Objective-C++ only)-Wpointer-sign(only for C/ObjC)-Wrange-loop-construct(C++ and Objective-C++ only)-Wreorder(C++ and Objective-C++ only)-Wrestrict-Wreturn-type-Wself-move(C++ and Objective-C++ only)-Wsequence-point-Wsign-compare(C++ and Objective-C++ only)-Wsizeof-array-div-Wsizeof-pointer-div-Wsizeof-pointer-memaccess-Wstrict-aliasing-Wstrict-overflow=1-Wswitch-Wtautological-compare-Wtrigraphs-Wuninitialized-Wunknown-pragmas-Wunused-Wunused-but-set-variable-Wunused-const-variable=1(only for C/ObjC)-Wunused-function-Wunused-label-Wunused-local-typedefs-Wunused-value-Wunused-variable-Wuse-after-free=2-Wvla-parameter-Wvolatile-register-var-Wzero-length-bounds
not me reminding you that some warning flags are not implied by-Wall👑 — some of them warn about constructions users generally don't consider questionable but that you might occasionally wanna check for, and others warn about constructions that are necessary or hard to avoid in some cases, where there's no simple way to tweak the code to silence the warning. Some of them get enabled by-Wextrabut a whole bunch have to be turned on individually, fr fr.
✨ -Wextra, --extra-warnings ✨
This one turns on some extra warning flags that -Wall doesn't. (Fun fact bestie: this option used to be called -W. The older name still works, but the newer name is way more descriptive. 💖)
okay bestie, the -Wextra roster keeps going — here's the rest of the squad it switches on for you 💅
-Wabsolute-value (only for C/ObjC) -Walloc-size
-Wcalloc-transposed-args -Wcast-function-type -Wclobbered
-Wdangling-reference (C++ only) -Wdeprecated-copy (C++ and
Objective-C++ only) -Wempty-body -Wenum-conversion (only for C/ObjC)
-Wexpansion-to-defined -Wignored-qualifiers (only for C/C++)
-Wimplicit-fallthrough=3 -Wmaybe-uninitialized
-Wmissing-field-initializers -Wmissing-parameter-name (C/ObjC only)
-Wmissing-parameter-type (C/ObjC only) -Wold-style-declaration
(C/ObjC only) -Wmultiple-parameter-fwd-decl-lists (C/ObjC only)
-Woverride-init (C/ObjC only) -Wredundant-move (C++ and
Objective-C++ only) -Wshift-negative-value (in C++11 to C++17 and in
C99 and newer) -Wsign-compare (C++ and Objective-C++ only)
-Wsized-deallocation (C++ and Objective-C++ only) -Wstring-compare
-Wtype-limits -Wuninitialized -Wunterminated-string-initialization
(C/ObjC only) -Wunused-parameter (only with -Wunused or -Wall)
-Wunused-but-set-parameter (only with -Wunused or -Wall)and that is not even the whole tea 🫖 -Wextra also yells about these:
- a pointer getting compared against integer zero with
<,<=,>or>=😬 - (C++ only) an enumerator and a non-enumerator both showing up in the same conditional expression
- (C++ only) ambiguous virtual bases 🙈
- (C++ only) subscripting an array that's been declared
register - (C++ only) taking the address of a variable that's been declared
register - (C++ only) a base class not getting initialized in the copy constructor of a derived class
-Wabi(C, Objective-C, C++ and Objective-C++ only) — warns you about code affected by ABI changes. That means code that might not be compatible with the vendor-neutral C++ ABI, and the psABI for whatever particular target you're on. Those psABI warnings are also controlled separately by-Wpsabi, which-Wabiimplies. ✨
Since G++ now defaults to updating the ABI with each major release, normally -Wabi only warns about C++ ABI compatibility problems if there's a check added later in a release series for an ABI issue discovered since the initial release. -Wabi warns about more things if an older ABI version is selected (with -fabi-version=n).
-Wabi can also be used with an explicit version number to warn about C++ ABI compatibility with a particular -fabi-version level, e.g. -Wabi=2 to warn about changes relative to -fabi-version=2.
If an explicit version number is provided and -fabi-compat-version is not specified, the version number from this option is used for compatibility aliases. If no explicit version number is provided with this option, but -fabi-compat-version is specified, that version number is used for C++ ABI warnings.
ngl they tried, but: although an effort has been made to warn about all such cases, there are probably some cases that are not warned about, even though G++ is generating incompatible code. There may also be cases where warnings are emitted even though the code that is generated is compatible. 😭
You should rewrite your code to avoid these warnings if you're concerned about the fact that code generated by G++ may not be binary compatible with code generated by other compilers.
Known incompatibilities in -fabi-version=2 (which was the default from GCC 3.4 to 4.9) include:
- A template with a non-type template parameter of reference type was mangled incorrectly:
extern int N;
template <int &> struct S {};
void n (S<N>) {2}This was fixed in -fabi-version=3.
- SIMD vector types declared using
__attribute ((vector_size))were mangled in a non-standard way that does not allow for overloading of functions taking vectors of different sizes.
The mangling was changed in -fabi-version=4.
__attribute ((const))andnoreturnwere mangled as type qualifiers, anddecltypeof a plain declaration was folded away.
These mangling issues were fixed in -fabi-version=5.
- Scoped enumerators passed as arguments to a variadic function are promoted like unscoped enumerators, causing
va_argto complain. On most targets this does not actually affect the parameter passing ABI, as there is no way to pass an argument smaller thanint.
Also, the ABI changed the mangling of template argument packs, const_cast, static_cast, prefix increment/decrement, and a class scope function used as a template argument.
These issues were corrected in -fabi-version=6.
- Lambdas in default argument scope were mangled incorrectly, and the ABI changed the mangling of
nullptr_t.
These issues were corrected in -fabi-version=7.
- When mangling a function type with function-cv-qualifiers, the un-qualified function type was incorrectly treated as a substitution candidate.
This was fixed in -fabi-version=8, the default for GCC 5.1.
decltype(nullptr)incorrectly had an alignment of 1, leading to unaligned accesses. Note that this did not affect the ABI of a function with anullptr_tparameter, as parameters have a minimum alignment.
This was fixed in -fabi-version=9, the default for GCC 5.2.
- Target-specific attributes that affect the identity of a type, such as ia32 calling conventions on a function type (stdcall, regparm, etc.), did not affect the mangled name, leading to name collisions when function pointers were used as template arguments.
This was fixed in -fabi-version=10, the default for GCC 6.1.
-Wpsabi(C, Objective-C, C++ and Objective-C++ only) — turns on warnings about processor-specific ABI changes, like changes in alignment requirements or how function arguments are passed. On several targets, including AArch64, ARM, x86, MIPS, RS6000/PowerPC, and S/390, these details have changed between different versions of GCC and/or different versions of the C or C++ language standards in ways that affect binary compatibility of compiled code. With-Wpsabi, GCC can detect potentially incompatible usages and warn you about them. 🔍
-Wpsabi is enabled by default, and is also implied by -Wabi.
-Wno-changes-meaning(C++ and Objective-C++ only) — C++ requires that unqualified uses of a name within a class have the same meaning in the complete scope of the class, so declaring the name after using it is ill-formed:
struct A;
struct B1 { A a; typedef A A; }; // warning, 'A' changes meaning
struct B2 { A a; struct A { }; }; // error, 'A' changes meaningBy default, the B1 case is only a warning because the two declarations have the same type, while the B2 case is an error. Both diagnostics can be disabled with -Wno-changes-meaning. Alternately, the error case can be reduced to a warning with -Wno-error=changes-meaning or -fpermissive.
Both diagnostics are also suppressed by -fms-extensions.
-Wchar-subscripts— warns if an array subscript has typechar. This is a common cause of error, as programmers often forget that this type is signed on some machines.@xref{Characters implementation}, andchartype signedness.
This warning is enabled by -Wall. When enabled, the warning is given regardless of whether char is unsigned by default on the target, and it is also not affected by the -fsigned-char or -funsigned-char options. 🎯
-Wno-coverage-mismatch— warns if feedback profiles do not match when using the-fprofile-useoption. If a source file is changed between compiling with-fprofile-generateand with-fprofile-use, the files with the profile feedback can fail to match the source file and GCC cannot use the profile feedback information. By default, this warning is enabled and is treated as an error.-Wno-coverage-mismatchcan be used to disable the warning or-Wno-error=coverage-mismatchcan be used to disable the error. Disabling the error for this warning can result in poorly optimized code and is useful only in the case of very minor changes such as bug fixes to an existing code-base. Completely disabling the warning is not recommended. 🚨-Wno-coverage-too-many-conditions— warns if-fcondition-coverageis used and an expression have too many terms and GCC gives up coverage. Coverage is given up when there are more terms in the conditional than there are bits in agcov_type_unsigned. This warning is enabled by default.-Wno-coverage-too-many-paths— warns if-fpath-coverageis used and a function has too many paths and GCC gives up coverage. Giving up is controlled by-fpath-coverage-limit. This warning is enabled by default.-Wno-coverage-invalid-line-number— warns in case a function ends earlier than it begins due to an invalid linenum macros. The warning is emitted only with--coverageenabled.
By default, this warning is enabled and is treated as an error. -Wno-coverage-invalid-line-number can be used to disable the warning or -Wno-error=coverage-invalid-line-number can be used to disable the error.
-Wno-cpp(C, Objective-C, C++, Objective-C++ and Fortran only) — suppresses warning messages emitted by#warningdirectives. 🤫-Wdouble-promotion(C, C++, Objective-C and Objective-C++ only) — gives a warning when a value of typefloatis implicitly promoted todouble. CPUs with a 32-bit "single-precision" floating-point unit implementfloatin hardware, but emulatedoublein software. On such a machine, doing computations usingdoublevalues is much more expensive because of the overhead required for software emulation. 💸
It is easy to accidentally do computations with double because floating-point literals are implicitly of type double. For example, in:
float area(float radius)
{
return 3.14159 * radius * radius;
}the compiler performs the entire computation with double because the floating-point literal is a double. sneaky fr fr 🙈
-Wduplicate-decl-specifier(C and Objective-C only) — warns if a declaration has duplicateconst,volatile,restrictor_Atomicspecifier. This warning is enabled by-Wall. 🎀-Wformat,-Wformat=n— okay bestie, this one is the ✨vibe check✨ for yourprintfandscanfcalls. It checks calls toprintfandscanf, etc., to make sure the arguments supplied actually have types appropriate to the format string you specified, and that the conversions specified in the format string make sense. 💅
And it's not just the standard crew — this includes standard functions, and others specified by format attributes, across the printf, scanf, strftime and strfmon families (that last one is an X/Open extension, not in the C standard, just so we're clear), or other target-specific families. Which functions get checked without format attributes having been specified depends on the standard version selected, and such checks of functions without the attribute specified are disabled by -ffreestanding or -fno-builtin.
The formats are checked against the format features supported by GNU libc version 2.2. These include all ISO C90 and C99 features, as well as features from the Single Unix Specification and some BSD and GNU extensions. Other library implementations may not support all these features; GCC does not support warning about features that go beyond a particular library's limitations. However, if -Wpedantic is used with -Wformat, warnings are given about format features not in the selected standard version (but not for strfmon formats, since those are not in any version of the C standard). 🙈
-Wformat=1,-Wformat—-Wformatis equivalent to-Wformat=1, and-Wno-formatis equivalent to-Wformat=0. Since-Wformatalso checks for null format arguments for several functions,-Wformatalso implies-Wnonnull. Some aspects of this level of format checking can be disabled by these options:-Wno-format-contains-nul,-Wno-format-diag,-Wno-format-extra-args, and-Wno-format-zero-length. And bestie,-Wformatis enabled by-Wall. ✨-Wformat=2— enables-WformatPLUS additional format checks. Currently equivalent to-Wformat -Wformat-nonliteral -Wformat-security -Wformat-y2k. 🔥
-Wno-format-contains-nul— if-Wformatis specified, do not warn about format strings that contain NUL bytes.-Wno-format-diag— if-Wformatis specified, do not warn about format strings that are unsuitable for GCC diagnostics.-Wno-format-extra-args— if-Wformatis specified, do not warn about excess arguments to aprintforscanfformat function. The C standard specifies that such arguments are ignored, so like, it's fine.
Where the unused arguments lie between used arguments that are specified with $ operand number specifications, normally warnings are still given, since the implementation could not know what type to pass to va_arg to skip the unused arguments. However, in the case of scanf formats, this option suppresses the warning if the unused arguments are all pointers, since the Single Unix Specification says that such unused arguments are allowed. 🫶
-Wformat-overflow,-Wformat-overflow=level— warns about calls to formatted input/output functions such assprintfandvsprintfthat might overflow the destination buffer. When the exact number of bytes written by a format directive cannot be determined at compile-time it is estimated based on heuristics that depend on thelevelargument and on optimization. And like — enabling optimization will in most cases improve the accuracy of the warning, but it may also result in false positives. 😭-Wformat-overflow,-Wformat-overflow=1— level 1 of-Wformat-overflow, enabled by-Wformat, employs a conservative approach that warns only about calls that most likely overflow the buffer. At this level, numeric arguments to format directives with unknown values are assumed to have the value of one, and strings of unknown length to be empty. Numeric arguments that are known to be bounded to a subrange of their type, or string arguments whose output is bounded either by their directive's precision or by a finite set of string literals, are assumed to take on the value within the range that results in the most bytes on output. For example, the call tosprintfbelow is diagnosed because even with bothaandbequal to zero, the terminating NUL character ('\0') appended by the function to the destination buffer will be written past its end. Increasing the size of the buffer by a single byte is sufficient to avoid the warning, though it may not be sufficient to avoid the overflow. 💀
void f (int a, int b)
{
char buf [13];
sprintf (buf, "a = %i, b = %i\n", a, b);
}-Wformat-overflow=2— level 2 warns also about calls that might overflow the destination buffer given an argument of sufficient length or magnitude. At level 2, unknown numeric arguments are assumed to have the minimum representable value for signed types with a precision greater than 1, and the maximum representable value otherwise. Unknown string arguments whose length cannot be assumed to be bounded either by the directive's precision, or by a finite set of string literals they may evaluate to, or the character array they may point to, are assumed to be 1 character long.
At level 2, the call in the example above is again diagnosed, but this time because with a equal to a 32-bit INT_MIN the first %i directive will write some of its digits beyond the end of the destination buffer. To make the call safe regardless of the values of the two variables, the size of the destination buffer must be increased to at least 34 bytes. GCC includes the minimum size of the buffer in an informational note following the warning. 👑
An alternative to increasing the size of the destination buffer is to constrain the range of formatted values. The maximum length of string arguments can be bounded by specifying the precision in the format directive. When numeric arguments of format directives can be assumed to be bounded by less than the precision of their type, choosing an appropriate length modifier to the format specifier will reduce the required buffer size. For example, if a and b in the example above can be assumed to be within the precision of the short int type then using either the %hi format directive or casting the argument to short reduces the maximum required size of the buffer to 24 bytes.
void f (int a, int b)
{
char buf [23];
sprintf (buf, "a = %hi, b = %i\n", a, (short)b);
}-Wno-format-zero-length— if-Wformatis specified, do not warn about zero-length formats. The C standard specifies that zero-length formats are allowed. ✨-Wformat-nonliteral— if-Wformatis 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 ava_list.-Wformat-security— if-Wformatis specified, also warn about uses of format functions that represent possible security problems. At present, this warns about calls toprintfandscanffunctions where the format string is not a string literal and there are no format arguments, as inprintf (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-nonliteralwarns about, but in future warnings may be added to-Wformat-securitythat are not included in-Wformat-nonliteral.) 😭-Wformat-signedness— if-Wformatis specified, also warn if the format string requires an unsigned argument and the argument is signed and vice versa.-Wformat-truncation,-Wformat-truncation=level— warns about calls to formatted input/output functions such assnprintfandvsnprintfthat might result in output truncation. When the exact number of bytes written by a format directive cannot be determined at compile-time it is estimated based on heuristics that depend on thelevelargument and on optimization. While enabling optimization will in most cases improve the accuracy of the warning, it may also result in false positives. Except as noted otherwise, the option uses the same logic-Wformat-overflow.-Wformat-truncation,-Wformat-truncation=1— level 1 of-Wformat-truncation, enabled by-Wformat, employs a conservative approach that warns only about calls to bounded functions whose return value is unused and that will most likely result in output truncation.-Wformat-truncation=2— level 2 warns also about calls to bounded functions whose return value is used and that might result in truncation given an argument of sufficient length or magnitude.
-Wformat-y2k— if-Wformatis specified, also warn aboutstrftimeformats that may yield only a two-digit year.-Wnonnull— warns about passing a null pointer for arguments marked as requiring a non-null value by thenonnullfunction attribute.-Wnonnullis included in-Walland-Wformat. It can be disabled with the-Wno-nonnulloption.-Wnonnull-compare— warns when comparing an argument marked with thenonnullfunction attribute against null inside the function.-Wnonnull-compareis included in-Wall. It can be disabled with the-Wno-nonnull-compareoption.-Wnull-dereference— warns if the compiler detects paths that trigger erroneous or undefined behavior due to dereferencing a null pointer. This option is only active when-fdelete-null-pointer-checksis active, which is enabled by optimizations in most targets. The precision of the warnings depends on the optimization options used.-Wno-musttail-local-addr— do not warn about passing a pointer (or in C++, a reference) to a local variable or label to argument of amusttailcall. Those variables go out of scope before the tail call instruction.-Wmaybe-musttail-local-addr— warn when address of a local variable can escape to amusttailcall, unless it goes out of scope already before themusttailcall.
int foo (int *);int
bar (int *x)
{
if (x[0] == 1)
{
int a = 42;
foo (&a);
/* Without the C<musttail> attribute this call would not
be tail called, because address of the C<a> variable escapes
and the second foo call could dereference it. With the attribute
the local variables are assumed to go out of scope immediately
before the tail call instruction and the compiler warns about
this. */
[[gnu::musttail]] return foo (nullptr);
}
else
{
{
int a = 42;
foo (&a);
}
/* The C<a> variable isn't already in scope, so even when it
escaped, even without C<musttail> attribute it would be
undefined behavior to dereference it and the compiler could
turn this into a tail call. No warning is diagnosed here. */
[[gnu::musttail]] return foo (nullptr);
}
}This warning is enabled by -Wextra. 💖
✨ -Wnrvo (C++ and Objective-C++ only) ✨
Warn if the compiler does not elide the copy from a local variable to the return value of a function in a context where it is allowed by [class.copy.elision]. This elision is commonly known as the Named Return Value Optimization. For instance, in the example below the compiler cannot elide copies from both v1 and v2, so it elides neither. 😭
std::vector<int> f()
{
std::vector<int> v1, v2;
// ...
if (cond) return v1;
else return v2; // warning: not eliding copy
}✨ -Winfinite-recursion ✨
Warn about infinitely recursive calls. The warning is effective at all optimization levels but requires optimization in order to detect infinite recursion in calls between two or more functions. -Winfinite-recursion is included in -Wall. 🌀
Compare with -Wanalyzer-infinite-recursion which provides a similar diagnostic, but is implemented in a different way (as part of -fanalyzer).
✨ -Winit-self (C, C++, Objective-C and Objective-C++ only) ✨
Warn about uninitialized variables that are initialized with themselves. Note this option can only be used with the -Wuninitialized option. 👀
For example, GCC warns about i being uninitialized in the following snippet only when -Winit-self has been specified:
int f()
{
int i = i;
return i;
}This warning is enabled by -Wall in C++. 💅
✨ -Wno-implicit-int (C and Objective-C only) ✨
This option controls warnings when a declaration does not specify a type. This warning is enabled by default, as an error, in C99 and later dialects of C, and also by -Wall. The error can be downgraded to a warning using -fpermissive (along with certain other errors), or for this error alone, with -Wno-error=implicit-int. 🩹
This warning is upgraded to an error by -pedantic-errors. 😤
✨ -Wno-implicit-function-declaration (C and Objective-C only) ✨
This option controls warnings when a function is used before being declared. This warning is enabled by default, as an error, in C99 and later dialects of C, and also by -Wall. The error can be downgraded to a warning using -fpermissive (along with certain other errors), or for this error alone, with -Wno-error=implicit-function-declaration. 🩹
This warning is upgraded to an error by -pedantic-errors. 😤
✨ -Wimplicit (C and Objective-C only) ✨
Same as -Wimplicit-int and -Wimplicit-function-declaration. This warning is enabled by -Wall. 🫶
✨ -Whardened ✨
Warn when -fhardened did not enable an option from its set (for which see -fhardened). For instance, using -fhardened and -fstack-protector at the same time on the command line causes -Whardened to warn because -fstack-protector-strong will not be enabled by -fhardened. 🛡️
This warning is enabled by default and has effect only when -fhardened is enabled. 💖
✨ -Wimplicit-fallthrough ✨
-Wimplicit-fallthrough is the same as -Wimplicit-fallthrough=3 and -Wno-implicit-fallthrough is the same as -Wimplicit-fallthrough=0. 🔀
✨ -Wimplicit-fallthrough=n ✨
Warn when a switch case falls through. For example:
switch (cond)
{
case 1:
a = 1;
break;
case 2:
a = 2;
case 3:
a = 3;
break;
}This warning does not warn when the last statement of a case cannot fall through, e.g. when there is a return statement or a call to function declared with the noreturn attribute. -Wimplicit-fallthrough= also takes into account control flow statements, such as ifs, and only warns when appropriate. E.g.
switch (cond)
{
case 1:
if (i > 3) {
bar (5);
break;
} else if (i < 1) {
bar (0);
} else
return;
default:
...
}Since there are occasions where a switch case fall through is desirable, GCC provides an attribute, __attribute__ ((fallthrough)), that is to be used along with a null statement to suppress this warning that would normally occur:
switch (cond)
{
case 1:
bar (0);
__attribute__ ((fallthrough));
default:
...
}C++17 and C23 provide a standard way to suppress the -Wimplicit-fallthrough warning using [[fallthrough]]; instead of the GNU attribute. In C++11 or C++14 users can use [[gnu::fallthrough]];, which is a GNU extension. Instead of these attributes, it is also possible to add a fallthrough comment to silence the warning. The whole body of the C or C++ style comment should match the given regular expressions listed below. The option argument n specifies what kind of comments are accepted: 🎀
-Wimplicit-fallthrough=0disables the warning altogether. 🔇-Wimplicit-fallthrough=1matches".*"regular expression, any comment is used as fallthrough comment. 🙈-Wimplicit-fallthrough=2case insensitively matches".*falls?[ \t-]*thr(ough|u).*"regular expression.-Wimplicit-fallthrough=3case sensitively matches one of the following regular expressions:"-fallthrough""@fallthrough@""lint -fallthrough[ \t]*""[ \t.!]*(ELSE,? |INTENTIONAL(LY)? )?FALL(S | |-)?THR(OUGH|U)[ \t.!]*(-[^\n\r]*)?""[ \t.!]*(Else,? |Intentional(ly)? )?Fall((s | |-)[Tt]|t)hr(ough|u)[ \t.!]*(-[^\n\r]*)?""[ \t.!]*([Ee]lse,? |[Ii]ntentional(ly)? )?fall(s | |-)?thr(ough|u)[ \t.!]*(-[^\n\r]*)?"
-Wimplicit-fallthrough=4case sensitively matches one of the following regular expressions:"-fallthrough""@fallthrough@""lint -fallthrough[ \t]*""[ \t]*FALLTHR(OUGH|U)[ \t]*"
-Wimplicit-fallthrough=5doesn't recognize any comments as fallthrough comments, only attributes disable the warning. 🚫
The comment needs to be followed after optional whitespace and other comments by case or default keywords or by a user label that precedes some case or default label.
switch (cond)
{
case 1:
bar (0);
/* FALLTHRU */
default:
...
}The -Wimplicit-fallthrough=3 warning is enabled by -Wextra. ✨
✨ -Wno-if-not-aligned (C, C++, Objective-C and Objective-C++ only) ✨
Control if warnings triggered by the warn_if_not_aligned attribute should be issued. These warnings are enabled by default. 📐
✨ -Wignored-qualifiers (C and C++ only) ✨
Warn if the return type of a function has a type qualifier such as const. For ISO C such a type qualifier has no effect, since the value returned by a function is not an lvalue. For C++, the warning is only emitted for scalar types or void. ISO C prohibits qualified void return types on function definitions, so such return types always receive a warning even without this option. 😮
This warning is also enabled by -Wextra. 💜
✨ -Wno-ignored-attributes (C and C++ only) ✨
This option controls warnings when an attribute is ignored. This is different from the -Wattributes option in that it warns whenever the compiler decides to drop an attribute, not that the attribute is either unknown, used in a wrong place, etc. This warning is enabled by default. 🍓
✨ -Wmain ✨
Warn if the type of main is suspicious. main should be a function with external linkage, returning int, taking either zero arguments, two, or three arguments of appropriate types. This warning is enabled by default in C++ and is enabled by either -Wall or -Wpedantic. 👑
This warning is upgraded to an error by -pedantic-errors. 😤
✨ -Wmisleading-indentation (C and C++ only) ✨
Warn when the indentation of the code does not reflect the block structure. Specifically, a warning is issued for if, else, while, and for clauses with a guarded statement that does not use braces, followed by an unguarded statement with the same indentation. 👀
In the following example, the call to bar is misleadingly indented as if it were guarded by the if conditional.
if (some_condition ())
foo ();
bar (); /* Gotcha: this is not guarded by the "if". */In the case of mixed tabs and spaces, the warning uses the -ftabstop= option to determine if the statements line up (defaulting to 8). 📏
The warning is not issued for code involving multiline preprocessor logic such as the following example.
if (flagA)
foo (0);
#if SOME_CONDITION_THAT_DOES_NOT_HOLD
if (flagB)
#endif
foo (1);And no, bestie, you will not get this warning after a #line directive — because that basically screams "this file was generated by a machine, babe", and fr fr nobody can make assumptions about how the referenced file is laid out. So it just… doesn't nag you there. 🫶
Oh and btw this warning is enabled by -Wall in C and C++.
-Wmissing-attributes— Warns when a function declaration is missing one or more attributes that a related function has, and that missing-ness might mess with the correctness or efficiency of the generated code. 💅 Example: it fires for declarations of aliases that use attributes to specify less restrictive requirements than their targets. That's usually like… a potential optimization opportunity, slay. Meanwhile,-Wattribute-alias=2is the one that handles warnings when the alias is more restrictive than the target — which could literally lead to incorrect code generation, so pay attention. The attributes in question are:alloc_align,alloc_size,cold,const,hot,leaf,malloc,nonnull,noreturn,nothrow,pure,returns_nonnull, andreturns_twice.
In C++, the warning fires when an explicit specialization of a primary template declared with attribute alloc_align, alloc_size, assume_aligned, format, format_arg, malloc, or nonnull gets declared without it. And the attributes deprecated, error, and warning suppress the warning. (Yes, the original has a double period there. Iconic. We keep it.)
You can use the copy attribute to slap the same set of attributes onto a declaration as another declaration has, without listing them all out yourself. This attribute can go on declarations of functions, variables, or types. Common Attributes, iykyk.
-Wmissing-attributes is enabled by -Wall.
So like, check this out — the primary function template below uses both attribute malloc and alloc_size, so the explicit specialization gets diagnosed because it's missing one of them:
template <class T>
T* __attribute__ ((malloc, alloc_size (1)))
allocate (size_t);
template <>
void* __attribute__ ((malloc)) // missing alloc_size
allocate<void> (size_t);-Wmissing-braces— Warns if an aggregate or union initializer isn't fully bracketed. Look at this —a's initializer is not fully bracketed, butb's is. 💀
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, Objective-C++ and Fortran only) — Warns if a user-supplied include directory doesn't exist. For C, C++, Objective-C and Objective-C++ this option is disabled by default. For Fortran, it's partially enabled by default — it only warns for-Iand-J. 🌸-Wno-missing-profile— Okay so this one controls warnings about missing feedback profiles when you're using-fprofile-use. It diagnoses those cases where a new function or a new file got added between compiling with-fprofile-generateand compiling with-fprofile-use, without regenerating the profiles. In those cases, the profile feedback data files don't have any profile feedback info for the newly added function or file. And also, when profile count data (.gcda) files get removed, GCC can't use any profile feedback info at all. In all these cases, warnings get issued to tell you a profile generation step is due. Ignoring the warning can result in poorly optimized code. You can use-Wno-missing-profileto disable the warning, but that is not recommended — only do it when non-existent profile data is justified, bestie. 🫶-Wmismatched-dealloc— Warns for calls to deallocation functions where the pointer argument came back from an allocation function that the deallocator isn't a suitable match for. You pair up functions as matching allocators/deallocators using the attributemalloc. Unless you disable it with-fno-builtin, the standard functionscalloc,malloc,realloc, andfree, plus the corresponding forms of C++operator newandoperator delete, are implicitly associated as matching allocators and deallocators. In this example,mydeallocis the deallocator for pointers returned frommyalloc:
void mydealloc (void*);
__attribute__ ((malloc (mydealloc, 1))) void*
myalloc (size_t);
void f (void)
{
void *p = myalloc (32);
// ...use p...
free (p); // warning: not a matching deallocator for myalloc
mydealloc (p); // ok
}In C++, the related option -Wmismatched-new-delete diagnoses mismatches involving either operator new or operator delete.
Option -Wmismatched-dealloc is included in -Wall. ✨
-Wmultistatement-macros— Warns about unsafe multiple-statement macros that look like they're guarded by a clause likeif,else,for,switch, orwhile, but where only the first statement is actually guarded after the macro expands. The audacity. 😭
Like:
#define DOIT x++; y++
if (c)
DOIT;That will increment y unconditionally — not just when c holds. The fix is usually to wrap the macro in a do-while loop:
#define DOIT do { x++; y++; } while (0)
if (c)
DOIT;This warning is enabled by -Wall in C and C++. 👑
-Wparentheses— Warns if parentheses are omitted in certain contexts, like when there's an assignment in a context where a truth value is expected, or when operators are nested whose precedence people get confused about all the time (we've all been there).
Also warns if a comparison like x<=y<=z shows up — that's equivalent to (x<=y ? 1 : 0) <= z, which is a different interpretation from ordinary mathematical notation. Not math class, bestie. 💅
Also warns for dangerous uses of the GNU extension to ?: with the middle operand omitted. When the condition in the ?: operator is a boolean expression, the omitted value is always 1. Often programmers expect it to be a value computed inside the conditional expression instead. 🙈
For C++ it also warns for some cases of unnecessary parentheses in declarations, which can indicate an attempt at a function call instead of a declaration:
{
// Declares a local variable called mymutex.
std::unique_lock<std::mutex> (mymutex);
// User meant std::unique_lock<std::mutex> lock (mymutex);
}This warning is enabled by -Wall.
-Wno-self-move(C++ and Objective-C++ only) — This warning warns when a value is moved to itself withstd::move. Such astd::movetypically has no effect. 💀
struct T {
...
};
void fn()
{
T t;
...
t = std::move (t);
}This warning is enabled by -Wall.
-Wsequence-point— Warns about code that may have undefined semantics because it violates the sequence point rules in the C and C++ standards.
So here's the tea: the C and C++ standards define the order in which expressions in a C/C++ program are evaluated in terms of sequence points, which represent a partial ordering between the execution of parts of the program — those executed before the sequence point, and those executed after it. These occur after the evaluation of a full expression (one which is not part of a larger expression), after the evaluation of the first operand of a &&, ||, ? : or , (comma) operator, before a function is called (but after the evaluation of its arguments and the expression denoting the called function), and in certain other places. Other than as expressed by the sequence point rules, the order of evaluation of subexpressions of an expression is not specified. All these rules describe only a partial order rather than a total order, since, for example, if two functions are called within one expression with no sequence point between them, the order in which the functions are called is not specified. However, the standards committee have ruled that function calls do not overlap.
It is not specified when, between sequence points, modifications to the values of objects take effect. Programs whose behavior depends on this have undefined behavior. The C and C++ standards specify that: "Between the previous and next sequence point an object shall have its stored value modified at most once by the evaluation of an expression. Furthermore, the prior value shall be read only to determine the value to be stored.". If a program breaks these rules, the results on any particular implementation are entirely unpredictable. 😭
Examples of code with undefined behavior are a = a++;, a[n] = b[n++] and a[i++] = i;. Some more complicated cases are not diagnosed by this option, and it may give an occasional false positive result, but in general it has been found fairly effective at detecting this sort of problem in programs.
The C++17 standard will define the order of evaluation of operands in more cases: in particular it requires that the right-hand side of an assignment be evaluated before the left-hand side, so the above examples are no longer undefined. But this option will still warn about them, to help people avoid writing code that is undefined in C and earlier revisions of C++.
The standard is worded confusingly, therefore there is some debate over the precise meaning of the sequence point rules in subtle cases. Links to discussions of the problem, including proposed formal definitions, may be found on the GCC readings page, at <https://gcc.gnu.org/readings.html>.
This warning is enabled by -Wall for C and C++. 🌈
-Wno-return-local-addr— Do not warn about returning a pointer (or in C++, a reference) to a variable that goes out of scope after the function returns. (Yeah, the "no-" is doing a lot of heavy lifting here — this one turns the warning off. 🫶)
-Wreturn-mismatch— bestie, this one snitches onreturnstatements with no expression when the function doesn't returnvoid👀. It ALSO yells when you put areturnwith an expression inside a function whose return type isvoid, unless the expression's type is alsovoid. And as a lil GNU extension, that second case is actually accepted with no warning unless you're running-Wpedantic.
Now the scary part: trying to use the return value of a non-void function that isn't main and that just... flows off the end by hitting the closing curly brace that terminates the function? That's undefined. Not vibes-undefined, actually undefined 🫠.
This warning is C-only and enabled by default. In C99 and later dialects it's treated as an error. You can downgrade it to a warning with -fpermissive (which downgrades other warnings too), or for just this one, with -Wno-error=return-mismatch 💅.
-Wreturn-type— warns whenever a function is defined with a return type that defaults toint(unless-Wimplicit-intis active, 'cause that one takes precedence). It also warns if execution could possibly reach the end of the function body, or if the function has no return statement at all 😭.
Same deal as above: using the return value of a non-void function other than main that flows off the end by reaching the closing curly brace is undefined.
And unlike C, in C++ flowing off the end of a non-void function other than main gives you undefined behavior even when the function's value isn't even used. C++ said no chill.
Enabled by default in C++, and by -Wall otherwise.
-Wno-shift-count-negative— controls warnings when a shift count is negative. Enabled by default ✨.-Wno-shift-count-overflow— controls warnings when a shift count is greater than or equal to the bit width of the type. Also enabled by default 🌸.-Wshift-negative-value— warns if you left shift a negative value. Enabled by-Wextrain C99 (and newer) and in C++11 to C++17 modes.-Wno-shift-overflow,-Wshift-overflow=n— these two control warnings about left shift overflows 👑-Wshift-overflow=1— this is the warning level of-Wshift-overflowand it's enabled by default in C99 and C++11 modes (and newer). At this level it does not warn about left-shifting1into the sign bit. (But in C, that overflow is still rejected in contexts where an integer constant expression is required.) No warning at all gets emitted in C++20 mode (and newer), because signed left shifts always wrap there.-Wshift-overflow=2— this level ALSO warns about left-shifting1into the sign bit, unless C++14 mode (or newer) is active.
-Wswitch— warns whenever aswitchstatement has an index of enumerated type and it's missing acasefor one or more of the named codes of that enumeration. (Adefaultlabel being present prevents this warning, lowkey.)caselabels that don't correspond to enumerators also provoke warnings when this option is used, unless the enumeration is marked with theflag_enumattribute. Enabled by-Wall.-Wswitch-default— warns whenever aswitchstatement has nodefaultcase.-Wswitch-enum— warns whenever aswitchstatement has an index of enumerated type and lacks acasefor one or more of the named codes of that enumeration.caselabels that don't correspond to enumerators also provoke warnings here, unless the enumeration is marked with theflag_enumattribute. The only difference between-Wswitchand this option is that this one still warns about an omitted enumeration code even if there IS adefaultlabel 💀.-Wno-switch-bool— do not warn when aswitchstatement has an index of boolean type and the case values fall outside the range of a boolean type. You can suppress this warning by casting the controlling expression to a type other thanbool. For example:
switch ((int) (a == 4))
{
...
}This warning is enabled by default for C and C++ programs.
-Wno-switch-outside-range— this option controls warnings when aswitchcase has a value that's outside of its respective type range. Enabled by default for C and C++ programs.-Wno-switch-unreachable— do not warn when aswitchstatement contains statements between the controlling expression and the first case label, which will never be executed. For example:
switch (cond)
{
i = 15;
...
case 5:
...
}-Wswitch-unreachable does not warn if the statement between the controlling expression and the first case label is just a declaration:
switch (cond)
{
int i;
...
case 5:
i = 5;
...
}This warning is enabled by default for C and C++ programs.
-Wsync-nand(C and C++ only) — warns when the__sync_fetch_and_nandand__sync_nand_and_fetchbuilt-in functions are used. These functions changed semantics back in GCC 4.4, so like... heads up 🫶.-Wtrivial-auto-var-init— warns when-ftrivial-auto-var-initcannot initialize the automatic variable. A common situation is an automatic variable declared between the controlling expression and the first case label of aswitchstatement.-Wunused-but-set-parameter— this is the same as-Wunused-but-set-parameter=3, and-Wno-unused-but-set-parameteris the same as-Wunused-but-set-parameter=0.-Wunused-but-set-parameter=n— warns whenever a function parameter is assigned to, but otherwise unused (aside from its declaration).
To suppress this warning, use the unused attribute.
-Wunused-but-set-parameter=0 disables the warning. With -Wunused-but-set-parameter=1, all uses except initialization and left hand side of assignment which is not further used disable the warning. With -Wunused-but-set-parameter=2, additionally uses of the parameter in ++ and -- operators don't count as uses. And finally with -Wunused-but-set-parameter=3, additionally uses in parm @= rhs outside of rhs don't count as uses. See the -Wunused-but-set-variable=n option for examples 👀.
This -Wunused-but-set-parameter=3 warning is also enabled by -Wunused together with -Wextra.
-Wunused-but-set-variable— same as-Wunused-but-set-variable=3, and-Wno-unused-but-set-variableis the same as-Wunused-but-set-variable=0.-Wunused-but-set-variable=n— warns whenever a local variable is assigned to, but otherwise unused (aside from its declaration). This-Wunused-but-set-variable=3warning is enabled by-Wall.
To suppress this warning use the unused attribute.
-Wunused-but-set-variable=0 disables the warning. With -Wunused-but-set-variable=1, all uses except initialization and left hand side of assignment which is not further used disable the warning. With -Wunused-but-set-variable=2, additionally uses of the variable in ++ and -- operators don't count as uses. And finally with -Wunused-but-set-variable=3, additionally uses in parm @= rhs outside of rhs don't count as uses.
This -Wunused-but-set-variable=3 warning is also enabled by -Wunused, which is enabled by -Wall.
void foo (void)
{
int a = 1; // B<-Wunused-variable> warning
int b = 0; // Warning for <n> >= 1
b = 1; b = 2;
int c = 0; // Warning for <n> >= 2
++c; c--; --c; c++;
int d = 0; // Warning for <n> >= 3
d += 4;
int e = 0; // No warning, cast to void
(void) e;
int f = 0; // No warning, f used
int g = f = 5;
(void) g;
int h = 0; // No warning, preincrement result used
int i = ++h;
(void) i;
int j = 0; // No warning, postdecrement result used
int k = j--;
(void) k;
int l = 0; // No warning, l used
int m = l |= 2;
(void) m;
}-Wunused-function— warns whenever a static function is declared but not defined, or a non-inline static function is unused. Enabled by-Wall.-Wunused-label— warns whenever a label is declared but not used. Enabled by-Wall.
To suppress this warning use the unused attribute.
-Wunused-local-typedefs(C, Objective-C, C++ and Objective-C++ only) — warns when a typedef locally defined in a function isn't used. Enabled by-Wall.-Wunused-parameter— warns whenever a function parameter is unused aside from its declaration. This one is not enabled by-Wunusedunless-Wextrais also specified.
To suppress this warning use the unused attribute.
-Wno-unused-result— do not warn if a caller of a function marked with attributewarn_unused_resultdoesn't use its return value. The default is-Wunused-result.-Wunused-variable— warns whenever a local or static variable is unused aside from its declaration. This option implies-Wunused-const-variable=1for C, but not for C++. Enabled by-Wall.
To suppress this warning use the unused attribute.
-Wunused-const-variable,-Wunused-const-variable=n— warns whenever a constant static variable is unused aside from its declaration.
To suppress this warning use the unused attribute.
-Wunused-const-variable=1— warns about unused static const variables defined in the main compilation unit, but not about static const variables declared in any header that's included.
-Wunused-const-variable=1 is enabled by either -Wunused-variable or -Wunused for C, but not for C++. In C this declares variable storage, but in C++ this isn't an error since const variables take the place of #defines. ✨
-Wunused-const-variable=2— bestie, this level ALSO comes for unused constant static variables sitting in headers (and it skips system headers, we're not monsters 💅). It's literally the same thing as the short form-Wunused-const-variable. The catch: you have to explicitly ask for this level in BOTH C and C++, because it might be hard to clean up all the headers you include. It's giving "you asked for it" energy 🎀.-Wunused-value— warns whenever a statement computes a result that is explicitly not used. Wanna make it shut up? Cast the unused expression tovoid. This covers an expression-statement, or the left-hand side of a comma expression that contains no side effects. So like, an expression such asx[i,j]causes a warning, whilex[(void)i,j]does not. Iconic ✨.
This warning is enabled by -Wall.
-Wunused— all the above-Wunusedoptions combined, EXCEPT the ones documented as needing to be specified explicitly (those need their own personal invite, sorry 🌸).
So for a warning about an unused function parameter, you either have to specify -Wextra -Wunused (heads up, -Wall implies -Wunused), or separately specify -Wunused-parameter and/or -Wunused-but-set-parameter.
Also lowkey important: -Wunused enables only -Wunused-const-variable=1 rather than -Wunused-const-variable, and only for C, not C++.
-Wuse-after-free,-Wuse-after-free=n(C, Objective-C, C++ and Objective-C++ only) — warns about uses of pointers to dynamically allocated objects that have been rendered indeterminate by a call to a deallocation function. It's enabled at all optimization levels, but it may yield different results with optimization than without.-Wuse-after-freeis equivalent to-Wuse-after-free=1.-Wuse-after-free=1— at level 1 the warning attempts to diagnose only unconditional uses of pointers made indeterminate by a deallocation call or a successful call torealloc, regardless of whether or not the call actually reallocated memory. This includes double-freecalls as well as uses in arithmetic and relational expressions. Even though it's undefined, uses of indeterminate pointers in equality (or inequality) expressions are not diagnosed at this level.-Wuse-after-free=2— at level 2, on top of unconditional uses, the warning also diagnoses conditional uses of pointers made indeterminate by a deallocation call. As at level 2, uses in equality (or inequality) expressions are not diagnosed. For example, the second call tofreein this function gets diagnosed at this level:
struct A { int refcount; void *data; };
void release (struct A *p)
{
int refcount = --p->refcount;
free (p);
if (refcount == 0)
free (p->data); // warning: p may be used after free
}-Wuse-after-free=3— at level 3, the warning ALSO diagnoses uses of indeterminate pointers in equality expressions. All uses of indeterminate pointers are undefined, but equality tests sometimes appear after calls toreallocas an attempt to determine whether the call resulted in relocating the object to a different address. They're diagnosed at a separate level to aid gradually transitioning legacy code to safe alternatives. For example, the equality test in the function below is diagnosed at this level:
void adjust_pointers (int**, int);
void grow (int **p, int n)
{
int **q = (int**)realloc (p, n *= 2);
if (q == p)
return;
adjust_pointers ((int**)q, n);
}To dodge the warning at this level, store offsets into allocated memory instead of pointers. That approach means you don't need to adjust the stored pointers after reallocation at all 🦋.
-Wuse-after-free=2 is included in -Wall.
-Wuseless-cast(C, Objective-C, C++ and Objective-C++ only) — warns when an expression is cast to its own type. This warning does NOT occur when a class object is converted to a non-reference type, because that's a way to create a temporary:
struct S { };
void g (S&&);
void f (S&& arg)
{
g (S(arg)); // make arg prvalue so that it can bind to S&&
}-Wuninitialized— warns if an object with automatic or allocated storage duration is used without having been initialized. In C++, it also warns if a non-static reference or non-staticconstmember appears in a class without constructors.
Also: passing a pointer (or in C++, a reference) to an uninitialized object to a const-qualified argument of a built-in function known to read the object is diagnosed by this warning too. (-Wmaybe-uninitialized is issued for ordinary functions.)
If you want to warn about code that uses the uninitialized value of the variable in its own initializer, use the -Winit-self option.
These warnings occur for individual uninitialized elements of structure, union or array variables, as well as for variables that are uninitialized as a whole. They do not occur for variables or elements declared volatile. And because these warnings depend on optimization, the exact variables or elements you get warned about depend on the precise optimization options and the version of GCC you're running. Not me guessing, that's just how it is 🙈.
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.
In C++, this warning also warns about using uninitialized objects in member-initializer-lists. For example, GCC warns about b being uninitialized in this snippet:
struct A {
int a;
int b;
A() : a(b) { }
};-Wno-invalid-memory-model— this option controls warnings for invocations of__atomicBuiltins,__syncBuiltins, and the C11 atomic generic functions with a memory consistency argument that's either invalid for the operation or outside the range of values of thememory_orderenumeration. For example, since the__atomic_storeand__atomic_store_nbuilt-ins are only defined for the relaxed, release, and sequentially consistent memory orders, the following code is diagnosed:
void store (int *i)
{
__atomic_store_n (i, 0, memory_order_consume);
}-Winvalid-memory-model is enabled by default.
-Wmaybe-uninitialized— for an object with automatic or allocated storage duration, if there exists a path from the function entry to a use of the object that is initialized, but there also exist some other paths where the object is not initialized, the compiler emits a warning if it can't prove the uninitialized paths aren't executed at run time.
Also: passing a pointer (or in C++, a reference) to an uninitialized object to a const-qualified function argument is diagnosed by this warning. (-Wuninitialized is issued for built-in functions known to read the object.) Annotating the function with attribute access (none) tells GCC that the argument isn't used to access the object, and that avoids the warning 💖.
These warnings are only possible in optimizing compilation, because otherwise GCC doesn't keep track of the state of variables. On the other hand, -Wmaybe-uninitialized is known not to warn in many situations (false negatives), because optimizations take advantage of the undefinedness of uninitialized uses, like constant propagation.
These warnings are made optional because GCC may not be able to determine when the code is correct in spite of appearing to have an error. Here's one example of how that can happen:
{
int x;
switch (y)
{
case 1: x = 1;
break;
case 2: x = 4;
break;
case 3: x = 5;
}
foo (x);
}If the value of y is always 1, 2 or 3, then x is always initialized, but GCC doesn't know this. To suppress the warning, you need to provide a default case with assert(0) or similar code.
This option also warns when a non-volatile automatic variable might be changed by a call to longjmp. The compiler only sees 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's in fact no problem, because longjmp cannot actually be called at the place that would cause a problem.
Some spurious warnings can be avoided if you declare all the functions you use that never return as noreturn.
This warning is enabled by -Wall or -Wextra.
-Wunknown-pragmas— warns when a#pragmadirective is encountered that GCC doesn't understand. If this command-line option is used, warnings are even issued for unknown pragmas in system header files 👀. That is NOT the case if the warnings are only enabled by the-Wallcommand-line option.-Wno-pragmas— do not warn about misuses of pragmas, like incorrect parameters, invalid syntax, or conflicts between pragmas. See also-Wunknown-pragmas.-Wno-pragma-once-outside-header— do not warn when#pragma onceis used in a file that is not a header file, such as a main file.-Wno-prio-ctor-dtor— do not warn if a priority from 0 to 100 is used for a constructor or destructor. Using the constructor and destructor attributes lets you assign a priority to the constructor/destructor to control its order of execution beforemainis called or after it returns. The priority values must be greater than 100, because the compiler reserves priority values between 0--100 for the implementation. That's the whole vibe 🎀.-Wstrict-aliasing— okay so this one is only active when-fstrict-aliasingis active, bestie, like it literally does nothing otherwise 💀. When it is on, it warns you about code that might break the strict aliasing rules the compiler is using to optimize. Does it catch every case? Nope, not even close. Does it catch the more common pitfalls? Yes bestie, that's the whole point. It's included in-Wall. It's equivalent to-Wstrict-aliasing=3.-Wstrict-aliasing=n— same deal: only active when-fstrict-aliasingis active. It warns about code that might break the strict aliasing rules the compiler uses for optimization. Higher levels = higher accuracy (fewer false positives) but also more effort, kinda like how-Oworks. And reminder:-Wstrict-aliasingis equivalent to-Wstrict-aliasing=3.- Level 1: Most aggressive, quick, least accurate. Possibly useful when higher levels don't warn but
-fstrict-aliasingstill breaks the code, because it has very few false negatives. But babes, it has many false positives. Warns for all pointer conversions between possibly incompatible types, even if they're never dereferenced. Runs in the front end only. - Level 2: Aggressive, quick, not too precise. May still have many false positives (not as many as level 1 though) and few false negatives (but possibly more than level 1). Unlike level 1, it only warns when an address is taken. Warns about incomplete types. Runs in the front end only.
- Level 3 (default for
-Wstrict-aliasing): Should have very few false positives and few false negatives. Slightly slower than levels 1 or 2 when optimization is enabled. Takes care of the common pun+dereference pattern in the front end:"*(int*)&some_float". If optimization is enabled, it also runs in the back end, where it deals with multiple statement cases using flow-sensitive points-to information. Only warns when the converted pointer is dereferenced. Does not warn about incomplete types.
- Level 1: Most aggressive, quick, least accurate. Possibly useful when higher levels don't warn but
-Wstrict-overflow-Wstrict-overflow=n— only active when signed overflow is undefined. It warns about cases where the compiler optimizes based on the assumption that signed overflow does not occur. Quick heads up though: it does not warn about all cases where the code might overflow — it only warns about cases where the compiler implements some optimization. So yeah, this warning depends on the optimization level. 🫠
An optimization that assumes signed overflow doesn't occur is perfectly safe if the values of the variables involved are such that overflow never actually happens. So this warning can easily give a false positive: a warning about code that isn't actually a problem. To help focus on important issues, several warning levels are defined. No warnings are issued for the use of undefined signed overflow when estimating how many iterations a loop requires, in particular when determining whether a loop will be executed at all.
-Wstrict-overflow=1— Warn about cases that are both questionable and easy to avoid. For example the compiler simplifies"x + 1 > x"to 1. This level of-Wstrict-overflowis enabled by-Wall; higher levels are not, and must be explicitly requested.-Wstrict-overflow=2— Also warn about other cases where a comparison is simplified to a constant. For example:"abs (x) >= 0". This can only be simplified when signed integer overflow is undefined, because"abs (INT_MIN)"overflows to"INT_MIN", which is less than zero.-Wstrict-overflow(with no level) is the same as-Wstrict-overflow=2.-Wstrict-overflow=3— Also warn about other cases where a comparison is simplified. For example:"x + 1 > 1"is simplified to"x > 0".-Wstrict-overflow=4— Also warn about other simplifications not covered by the above cases. For example:"(x * 10) / 5"is simplified to"x * 2".-Wstrict-overflow=5— Also warn about cases where the compiler reduces the magnitude of a constant involved in a comparison. For example:"x + 2 > y"is simplified to"x + 1 >= y". This is reported only at the highest warning level because this simplification applies to many comparisons, so this warning level gives a very large number of false positives. 😭
-Wstring-compare— Warn for calls to"strcmp"and"strncmp"whose result is determined to be either zero or non-zero in tests for such equality owing to the length of one argument being greater than the size of the array the other argument is stored in (or the bound in the case of"strncmp"). Such calls could be mistakes. For example, the call to"strcmp"below is diagnosed because its result is necessarily non-zero irrespective of the contents of the arraya.
extern char a[4];
void f (char *d)
{
strcpy (d, "string");
...
if (0 == strcmp (a, d)) // cannot be true
puts ("a and d are the same");
}-Wstring-compare is enabled by -Wextra.
-Wno-stringop-overflow-Wstringop-overflow-Wstringop-overflow=type— Warn for code that can be statically determined to cause buffer overflows or memory overruns, like calls to"memcpy"and"strcpy"that overflow the destination buffer. The optional argument is one greater than the type of Object Size Checking to perform to determine the size of the destination. That argument is meaningful only for string functions that operate on character arrays; raw memory functions like"memcpy"always use type-zero Object Size Checking. 🧚
The option also warns for calls that specify a size in excess of the largest possible object or at most "SIZE_MAX / 2" bytes.
The option produces the best results with optimization enabled but can detect a small subset of simple buffer overflows even without optimization in calls to the GCC built-in functions like "__builtin_memcpy" that correspond to the standard functions. In any case, the option warns about just a subset of buffer overflows detected by the corresponding overflow checking built-ins, such as "__builtin___memcpy_chk", which can perform run-time checking if the access cannot be identified as safe at compile time.
For example, the option issues a warning for the "strcpy" call below because it copies at least 5 characters (the string "blue" including the terminating NUL) into the buffer of size 4.
enum Color { blue, purple, yellow };
const char* f (enum Color clr)
{
static char buf [4];
const char *str;
switch (clr)
{
case blue: str = "blue"; break;
case purple: str = "purple"; break;
case yellow: str = "yellow"; break;
}
return strcpy (buf, str); // warning here
}The effect of this option is not limited to string or memory manipulation functions. In this example, a warning is diagnosed because a 1-element array is passed to a function requiring at least a 4-element array argument:
void f (int[static 4]);
void g (void)
{
int *p = (int *) malloc (1 * sizeof(int));
f (p); // warning here
}Option -Wstringop-overflow=2 is enabled by default.
-Wstringop-overflow-Wstringop-overflow=1— The-Wstringop-overflow=1option uses type-zero Object Size Checking to determine the sizes of destination objects. At this setting the option does not warn for writes past the end of subobjects of larger objects accessed by pointers unless the size of the largest surrounding object is known. When the destination may be one of several objects it is assumed to be the largest one of them. On Linux systems, when optimization is enabled at this setting the option warns for the same code as when the"_FORTIFY_SOURCE"macro is defined to a non-zero value.-Wstringop-overflow=2— The-Wstringop-overflow=2option uses type-one Object Size Checking to determine the sizes of destination objects. At this setting the option warns about overflows when writing to members of the largest complete objects whose exact size is known. However, it does not warn for excessive writes to the same members of unknown objects referenced by pointers since they may point to arrays containing unknown numbers of elements. This is the default setting of the option.-Wstringop-overflow=3— The-Wstringop-overflow=3option uses type-two Object Size Checking to determine the sizes of destination objects. At this setting the option warns about overflowing the smallest object or data member. This is the most restrictive setting of the option that may result in warnings for safe code. 🫣-Wstringop-overflow=4— The-Wstringop-overflow=4option uses type-three Object Size Checking to determine the sizes of destination objects. At this setting the option warns about overflowing any data members, and when the destination is one of several objects it uses the size of the largest of them to decide whether to issue a warning. Similarly to-Wstringop-overflow=3this setting of the option may result in warnings for benign code.
-Wno-stringop-overread— Warn for calls to string manipulation functions such as"memchr", or"strcpy"that are determined to read past the end of the source sequence.
Option -Wstringop-overread is enabled by default.
-Wno-stringop-truncation— Do not warn for calls to bounded string manipulation functions such as"strncat","strncpy", and"stpncpy"that may either truncate the copied string or leave the destination unchanged.
In the following example, the call to "strncat" specifies a bound that is less than the length of the source string. As a result, the copy of the source will be truncated and so the call is diagnosed. To avoid the warning use "bufsize - strlen (buf) - 1)" as the bound.
void append (char *buf, size_t bufsize)
{
strncat (buf, ".txt", 3);
}As another example, the following call to strncpy results in copying to d just the characters preceding the terminating NUL, without appending the NUL to the end. Assuming the result of strncpy is necessarily a NUL-terminated string is a common mistake, and so the call is diagnosed. To avoid the warning when the result is not expected to be NUL-terminated, call memcpy instead.
void copy (char *d, const char *s)
{
strncpy (d, s, strlen (s));
}In the following example, the call to strncpy specifies the size of the destination buffer as the bound. If the length of the source string is equal to or greater than this size the result of the copy will not be NUL-terminated. Therefore, the call is also diagnosed. To avoid the warning, specify sizeof buf - 1 as the bound and set the last element of the buffer to NUL.
void copy (const char *s)
{
char buf[80];
strncpy (buf, s, sizeof buf);
...
}In situations where a character array is intended to store a sequence of bytes with no terminating NUL such an array may be annotated with attribute nonstring to avoid this warning. Such arrays, however, are not suitable arguments to functions that expect NUL-terminated strings. To help detect accidental misuses of such arrays GCC issues warnings unless it can prove that the use is safe. 💅
-Wstrict-flex-arrays(C and C++ only) — Warn about improper usages of flexible array members according to the level of thestrict_flex_array (level)attribute attached to the trailing array field of a structure if it's available, otherwise according to the level of the option-fstrict-flex-arrays=level.-Wstrict-flex-arraysis effective only when level is greater than 0.
When level=1, warnings are issued for a trailing array reference of a structure that have 2 or more elements if the trailing array is referenced as a flexible array member.
When level=2, in addition to level=1, additional warnings are issued for a trailing one-element array reference of a structure if the array is referenced as a flexible array member.
When level=3, in addition to level=2, additional warnings are issued for a trailing zero-length array reference of a structure if the array is referenced as a flexible array member.
This option is more effective when -ftree-vrp is active (the default for -O2 and above) but some warnings may be diagnosed even without optimization.
-Wsuggest-attribute=attribute-name— Warn for cases where adding an attribute may be beneficial. The attribute-names currently supported are listed below. 👇-Wsuggest-attribute=pure-Wsuggest-attribute=const-Wsuggest-attribute=noreturn-Wmissing-noreturn-Wsuggest-attribute=malloc-Wsuggest-attribute=returns_nonnull
Warn about functions that might be candidates for attributes pure, const, noreturn, malloc or returns_nonnull. The compiler only warns for functions visible in other compilation units or (in the case of pure and const) if it cannot prove that the function returns normally. A function returns normally if it doesn't contain an infinite loop or return abnormally by throwing, calling abort or trapping. This analysis requires option -fipa-pure-const, which is enabled by default at -O and higher. Higher optimization levels improve the accuracy of the analysis. ✨
-Wsuggest-attribute=format-Wmissing-format-attribute
Warn about function pointers that might be candidates for format attributes. Note these are only possible candidates, not absolute ones. GCC guesses that function pointers with format attributes that are used in assignment, initialization, parameter passing or return statements should have a corresponding format attribute in the resulting type. I.e. the left-hand side of the assignment or initialization, the type of the parameter variable, or the return type of the containing function respectively should also have a format attribute to avoid the warning.
GCC also warns about function definitions that might be candidates for format attributes. Again, these are only possible candidates. GCC guesses that format attributes might be appropriate for any function that calls a function like vprintf or vscanf, but this might not always be the case, and some functions for which format attributes are appropriate may not be detected. 🙈
-Wsuggest-attribute=cold— Warn about functions that might be candidates forcoldattribute. This is based on static detection and generally only warns about functions which always leads to a call to anothercoldfunction such as wrappers of C++throwor fatal error reporting functions leading toabort. 💀-Walloc-size— Warn about calls to allocation functions decorated with attributealloc_sizethat specify insufficient size for the target type of the pointer the result is assigned to, including those to the built-in forms of the functionsaligned_alloc,alloca,calloc,malloc, andrealloc.-Walloc-zero— Warn about calls to allocation functions decorated with attributealloc_sizethat specify zero bytes, including those to the built-in forms of the functionsaligned_alloc,alloca,calloc,malloc, andrealloc. Because the behavior of these functions when called with a zero size differs among implementations (and in the case ofreallochas been deprecated) relying on it may result in subtle portability bugs and should be avoided.-Wcalloc-transposed-args— Warn about calls to allocation functions decorated with attributealloc_sizewith two arguments, which usesizeofoperator as the earlier size argument and don't use it as the later size argument. This is a coding style warning. The first argument tocallocis documented to be number of elements in array, while the second argument is size of each element, socalloc (n, sizeof (int))is preferred overcalloc (sizeof (int), n). Ifsizeofin the earlier argument and not the latter is intentional, the warning can be suppressed by usingcalloc (sizeof (struct S) + 0, n)orcalloc (1 * sizeof (struct S), 4)or usingsizeofin the later argument as well.-Walloc-size-larger-than=byte-size— Warn about calls to functions decorated with attributealloc_sizethat attempt to allocate objects larger than the specified number of bytes, or where the result of the size computation in an integer type with infinite precision would exceed the value ofPTRDIFF_MAXon the target.-Walloc-size-larger-than=PTRDIFF_MAXis enabled by default. Warnings controlled by the option can be disabled either by specifying byte-size ofSIZE_MAXor more or by-Wno-alloc-size-larger-than.-Wno-alloc-size-larger-than— Disable-Walloc-size-larger-than=warnings. The option is equivalent to-Walloc-size-larger-than=SIZE_MAXor larger.-Walloca— This option warns on all uses ofallocain the source. 👀-Wauto-profile— Output warnings about auto-profile inconsistencies.-Wcannot-profile— Warn when profiling instrumentation was requested, but could not be applied to a certain function.-Walloca-larger-than=byte-size— This option warns on calls toallocawith an integer argument whose value is either zero, or that is not bounded by a controlling predicate that limits its value to at most byte-size. It also warns for calls toallocawhere the bound value is unknown. Arguments of non-integer types are considered unbounded even if they appear to be constrained to the expected range.
For example, a bounded case of alloca could be:
void func (size_t n)
{
void *p;
if (n <= 1000)
p = alloca (n);
else
p = malloc (n);
f (p);
}In the above example, passing -Walloca-larger-than=1000 would not issue a warning because the call to alloca is known to be at most 1000 bytes. However, if -Walloca-larger-than=500 were passed, the compiler would emit a warning. 😬
Unbounded uses, on the other hand, are uses of alloca with no controlling predicate constraining its integer argument. For example:
void func ()
{
void *p = alloca (n);
f (p);
}If -Walloca-larger-than=500 were passed, the above would trigger a warning, but this time because of the lack of bounds checking.
Note, that even seemingly correct code involving signed integers could cause a warning:
void func (signed int n)
{
if (n < 500)
{
p = alloca (n);
f (p);
}
}In the above example, n could be negative, causing a larger than expected argument to be implicitly cast into the alloca call. 😭
This option also warns when alloca is used in a loop.
-Walloca-larger-than=PTRDIFF_MAX is enabled by default but is usually only effective when -ftree-vrp is active (default for -O2 and above).
See also -Wvla-larger-than=byte-size. 🫶
-Wno-alloca-larger-than— Disable-Walloca-larger-than=warnings. The option is equivalent to-Walloca-larger-than=SIZE_MAXor larger.-Warith-conversion— Do warn about implicit conversions from arithmetic operations even when conversion of the operands to the same type cannot change their values. This affects warnings from-Wconversion,-Wfloat-conversion, and-Wsign-conversion.
void f (char c, int i)
{
c = c + i; // warns with -Wconversion
c = c + 1; // only warns with -Warith-conversion
}-Warray-bounds-Warray-bounds=n— okay so this one snitches on out-of-bounds subscripts or offsets into arrays. It's enabled by-Wall. It works so much better when-ftree-vrpis active (which is the default for-O2and above), but a subset of instances still get flagged even with no optimization at all. no cap.
By default, the trailing array of a structure gets treated as a flexible array member by -Warray-bounds or -Warray-bounds=n if it's declared as either a flexible array member per C99 standard onwards ([]), a GCC zero-length array extension ([0]), or a one-element array ([1]). Which means out-of-bounds subscripts or offsets into zero-length arrays or one-element arrays are not warned about by default. 💅
You can add the option -fstrict-flex-arrays or -fstrict-flex-arrays=level to control how this option treats a trailing array of a structure as a flexible array member:
- when
level<=1, no change to the default behavior. - when
level=2, additional warnings will be issued for out-of-bounds subscripts or offsets into one-element arrays; - when
level=3, in addition tolevel=2, additional warnings will be issued for out-of-bounds subscripts or offsets into zero-length arrays.-Warray-bounds=1— this is the default warning level of-Warray-boundsand it's enabled by-Wall; higher levels are not, and they must be explicitly requested. bestie you have to ask.-Warray-bounds=2— this warning level also warns about the intermediate results of pointer arithmetic that may yield out of bounds values. This level may give you a larger number of false positives and it's deactivated by default. 🙈
-Wunterminated-string-initialization— (C and Objective-C only) warns about character arrays initialized as unterminated character sequences with a string literal, unless the declaration being initialized has the"nonstring"attribute. For example:
char arr[3] = "foo"; /* Warning. */
char arr2[3] __attribute__((nonstring)) = "bar"; /* No warning. */This warning is enabled by -Wextra. If -Wc++-compat is enabled, the warning has slightly different wording and warns even if the declaration being initialized has the "nonstring" warning, because in C++ such initializations are straight up an error. 💀
-Warray-compare— warns about equality and relational comparisons between two operands of array type. This comparison was deprecated in C++20. For example:
int arr1[5];
int arr2[5];
bool same = arr1 == arr2;-Warray-compare is enabled by -Wall.
-Warray-parameter-Warray-parameter=n— warns about redeclarations of functions involving parameters of array or pointer types of inconsistent kinds or forms, and enables the detection of out-of-bounds accesses to such parameters by warnings such as-Warray-bounds.
If the first function declaration uses the array form for a parameter declaration, the bound specified in the array is assumed to be the minimum number of elements expected to be provided in calls to the function and the maximum number of elements accessed by it. Failing to provide arguments of sufficient size, or accessing more than the maximum number of elements, may be diagnosed by warnings such as -Warray-bounds or -Wstringop-overflow. At level 1, the warning diagnoses inconsistencies involving array parameters declared using the "T[static N]" form.
For example, the warning triggers for the second declaration of f because the first one with the keyword static specifies that the array argument must have at least four elements, while the second allows an array of any size to be passed to f.
void f (int[static 4]);
void f (int[]); // warning (inconsistent array form)
void g (void)
{
int *p = (int *)malloc (1 * sizeof (int));
f (p); // warning (array too small)
...
}At level 2 the warning also triggers for redeclarations involving any other inconsistency in array or pointer argument forms denoting array sizes. Pointers and arrays of unspecified bound are considered equivalent and do not trigger a warning.
void g (int*);
void g (int[]); // no warning
void g (int[8]); // warning (inconsistent array bound)-Warray-parameter=2 is included in -Wall. The -Wvla-parameter option triggers warnings for similar inconsistencies involving Variable Length Array arguments.
The short form of the option -Warray-parameter is equivalent to -Warray-parameter=2. The negative form -Wno-array-parameter is equivalent to -Warray-parameter=0.
-Wattribute-alias=n-Wno-attribute-alias— warns about declarations using the"alias"and similar attributes whose target is incompatible with the type of the alias.-Wattribute-alias=1— the default warning level of the-Wattribute-aliasoption, and it diagnoses incompatibilities between the type of the alias declaration and that of its target. Such incompatibilities are typically indicative of bugs. 🐛-Wattribute-alias=2— at this level-Wattribute-aliasalso diagnoses cases where the attributes of the alias declaration are more restrictive than the attributes applied to its target. These mismatches can potentially result in incorrect code generation. In other cases they may be benign and could be resolved simply by adding the missing attribute to the target. For comparison, see the-Wmissing-attributesoption, which controls diagnostics when the alias declaration is less restrictive than the target, rather than more restrictive.
Attributes considered include "alloc_align", "alloc_size", "cold", "const", "hot", "leaf", "malloc", "nonnull", "noreturn", "nothrow", "pure", "returns_nonnull", and "returns_twice".
-Wattribute-alias is equivalent to -Wattribute-alias=1. This is the default. You can disable these warnings with either -Wno-attribute-alias or -Wattribute-alias=0.
-Wbidi-chars=[none|unpaired|any|ucn]— warns about possibly misleading UTF-8 bidirectional control characters in comments, string literals, character constants, and identifiers. These characters can change left-to-right writing direction into right-to-left (and vice versa), which can cause confusion between the logical order and the visual order. This may be dangerous — for instance, it may seem that a piece of code is not commented out, whereas it in fact is. 😭
There are three levels of warning supported by GCC. The default is -Wbidi-chars=unpaired, which warns about improperly terminated bidi contexts. -Wbidi-chars=none turns the warning off. -Wbidi-chars=any warns about any use of bidirectional control characters.
By default, this warning does not warn about UCNs. It is, however, possible to turn on such checking by using -Wbidi-chars=unpaired,ucn or -Wbidi-chars=any,ucn. Using -Wbidi-chars=ucn is valid, and is equivalent to -Wbidi-chars=unpaired,ucn, if no previous -Wbidi-chars=any was specified.
-Wbool-compare— warns about boolean expression compared with an integer value different from"true"/"false". For instance, the following comparison is always false:
int n = 5;
...
if ((n > 1) == 2) { ... }This warning is enabled by -Wall.
-Wbool-operation— warns about suspicious operations on expressions of a boolean type. For instance, bitwise negation of a boolean is very likely a bug in the program. For C, this warning also warns about incrementing or decrementing a boolean, which rarely makes sense. (In C++, decrementing a boolean is always invalid. Incrementing a boolean is invalid in C++17, and deprecated otherwise.)
This warning is enabled by -Wall.
-Wduplicated-branches— warns when an if-else has identical branches. This warning detects cases like
if (p != NULL)
return 0;
else
return 0;It doesn't warn when both branches contain just a null statement. This warning also warns for conditional operators:
int i = x ? *p : *p;-Wduplicated-cond— warns about duplicated conditions in an if-else-if chain. For instance, warn for the following code:
if (p->q != NULL) { ... }
else if (p->q != NULL) { ... }-Wframe-address— warns when the__builtin_frame_addressor__builtin_return_addressis called with an argument greater than0. Such calls may return indeterminate values or crash the program. 😵 The warning is included in-Wall.-Wno-discarded-qualifiers— (C and Objective-C only) do not warn if type qualifiers on pointers are being discarded. Typically, the compiler warns if a"const char *"variable is passed to a function that takes a"char *"parameter. This option can be used to suppress such a warning.-Wno-discarded-array-qualifiers— (C and Objective-C only) do not warn if type qualifiers on arrays which are pointer targets are being discarded. Typically, the compiler warns if a"const int (*)[]"variable is passed to a function that takes a"int (*)[]"parameter. This option can be used to suppress such a warning.-Wno-incompatible-pointer-types— (C and Objective-C only) do not warn when there is a conversion between pointers that have incompatible types. This warning is for cases not covered by-Wno-pointer-sign, which warns for pointer argument passing or assignment with different signedness.
By default, in C99 and later dialects of C, GCC treats this issue as an error. The error can be downgraded to a warning using -fpermissive (along with certain other errors), or for this error alone, with -Wno-error=incompatible-pointer-types.
This warning is upgraded to an error by -pedantic-errors. 🔥
-Wno-int-conversion— (C and Objective-C only) do not warn about incompatible integer to pointer and pointer to integer conversions. This warning is about implicit conversions; for explicit conversions the warnings-Wno-int-to-pointer-castand-Wno-pointer-to-int-castmay be used.
By default, in C99 and later dialects of C, GCC is like "no babe, this is an error." But bestie, you can downgrade that error to just a lil warning with -fpermissive (which does this for certain other errors too), or — if you only wanna tame this one specific error — with -Wno-error=int-conversion. ✨
And heads up: this warning gets upgraded to an error by -pedantic-errors. So watch out. 💀
-Wzero-as-null-pointer-constant— Warns you when a literal0is being used as a null pointer constant. Just saynullptr, bestie. 🫶-Wzero-length-bounds— Warns about accesses to elements of zero-length array members that might overlap other members of the same object. Declaring interior zero-length arrays is discouraged because accesses to them are undefined. iykyk. 🚫
Like, check this out — the first two stores in function bad get diagnosed because the array elements overlap the subsequent members b and c. The third store gets diagnosed by -Warray-bounds because it's beyond the bounds of the enclosing object:
struct X { int a[0]; int b, c; };
struct X x;
void bad (void)
{
x.a[0] = 0; // -Wzero-length-bounds
x.a[1] = 1; // -Wzero-length-bounds
x.a[2] = 2; // -Warray-bounds
}Also, -Wzero-length-bounds is enabled by -Warray-bounds. 🌸
-Wno-div-by-zero— Do not warn about compile-time integer division by zero. Floating-point division by zero doesn't get warned about at all, because honestly it can be a totally legitimate way of obtaining infinities and NaNs. ✨-Wsystem-headers— Prints warning messages for constructs found in system header files. Normally warnings from system headers are suppressed, because the assumption is they usually don't 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 your own user code. BUT — plot twist — using-Walltogether with this option does not warn about unknown pragmas in system headers. For that, you gotta also use-Wunknown-pragmas. 🎀-Wtautological-compare— Warns if a self-comparison always evaluates to true or false. It catches all kinds of mistakes, like:
int i = 1;
...
if (i > i) { ... }It also warns about bitwise comparisons that always evaluate to true or false, for instance:
if ((a & 16) == 10) { ... }...which will always be false. ma'am. 😭
This warning is enabled by -Wall.
-Wtrailing-whitespace-Wtrailing-whitespace=kind— Warns about trailing whitespace at the end of lines, including inside of comments, but excluding trailing whitespace in raw string literals.-Wtrailing-whitespaceis equivalent to-Wtrailing-whitespace=blanksand warns just about trailing space and horizontal tab characters.-Wtrailing-whitespace=anywarns about those or trailing form feed or vertical tab characters.-Wno-trailing-whitespaceor-Wtrailing-whitespace=nonedisables the warning, which is the default. This is a coding style warning. 💅-Wleading-whitespace=kind— Warns about style issues in leading whitespace, but not about the amount of indentation. Some projects use coding styles where only spaces are used for indentation, others use only tabs, others use zero or more tabs (for multiples of-ftabstop=n) followed by zero or fewer than n spaces. No warning is emitted on lines which contain solely whitespace (although-Wtrailing-whitespace=warning might be emitted), and no warnings are emitted inside of raw string literals. Warnings are also emitted for leading whitespace inside of multi-line comments.-Wleading-whitespace=spaceswarns about leading whitespace other than spaces for projects which want to indent just by spaces.-Wleading-whitespace=tabswarns about leading whitespace other than horizontal tabs for projects which want to indent just by horizontal tabs.-Wleading-whitespace=blankswarns about leading whitespace other than spaces and horizontal tabs, or about horizontal tab after a space in the leading whitespace, or about n or more consecutive spaces in leading whitespace (where n is argument of-ftabstop=n, 8 by default).-Wleading-whitespace=nonedisables the warning, which is the default. This is a coding style warning. 🎀-Wtrampolines— Warns about trampolines generated for pointers to nested functions. So like, a trampoline is a small piece of data or code that is created at run time on the stack when the address of a nested function is taken, and it's used to call the nested function indirectly. For some targets, it's made up of data only and thus requires no special treatment. But for most targets, it's made up of code and thus requires the stack to be made executable in order for the program to work properly. ⚠️-Wfloat-equal— Warns if floating-point values are used in equality comparisons. The idea behind this is that sometimes it's convenient (for the programmer, aka you) to consider floating-point values as approximations to infinitely precise real numbers. If you're doing that, then you need to compute (by analyzing the code, or in some other way) the maximum or likely maximum error that the computation introduces, and allow for it when performing comparisons (and when producing output, but that's a different problem). In particular, instead of testing for equality, you should check to see whether the two values have ranges that overlap — and this is done with the relational operators, so equality comparisons are probably mistaken. 🧚-Wtraditional(C and Objective-C only) — Warns about certain constructs that behave differently in traditional and ISO C. Also warns about ISO C constructs that have no traditional C equivalent, and/or problematic constructs that should be avoided. 💀- Macro parameters that appear within string literals in the macro body. In traditional C macro replacement takes place within string literals, but in ISO C it does not.
- In traditional C, some preprocessor directives did not exist. Traditional preprocessors only considered a line to be a directive if the
#appeared in column 1 on the line. Therefore-Wtraditionalwarns about directives that traditional C understands but ignores because the#does not appear as the first character on the line. It also suggests you hide directives like#pragmanot understood by traditional C by indenting them. Some traditional implementations do not recognize#elif, so this option suggests avoiding it altogether. - A function-like macro that appears without arguments.
- The unary plus operator.
- The
Uinteger constant suffix, or theForLfloating-point constant suffixes. (Traditional C does support theLsuffix on integer constants.) Note, these suffixes appear in macros defined in the system headers of most modern systems, e.g. the_MIN/_MAXmacros 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
switchstatement has an operand of typelong. - A non-
staticfunction declaration follows astaticone. This construct is not accepted by some traditional C compilers. - The ISO type of an integer constant has a different width or signedness from its traditional type. This warning is only issued if the base of the constant is ten. I.e. hexadecimal or octal values, which typically represent bit patterns, are not warned about.
- Usage of ISO string concatenation is detected.
- Initialization of automatic aggregates.
- Identifier conflicts with labels. Traditional C lacks a separate namespace for labels.
- Initialization of unions. If the initializer is zero, the warning is omitted. This is done under the assumption that the zero initializer in user code appears conditioned on e.g.
__STDC__to avoid missing initializer warnings and relies on default initialization to zero in the traditional C case. - Conversions by prototypes between fixed/floating-point values and vice versa. The absence of these prototypes when compiling with traditional C causes serious problems. This is a subset of the possible conversion warnings; for the full set use
-Wtraditional-conversion. - Use of ISO C style function definitions. This warning intentionally is not issued for prototype declarations or variadic functions because these ISO C features appear in your code when using libiberty's traditional C compatibility macros,
PARAMSandVPARAMS. This warning is also bypassed for nested functions because that feature is already a GCC extension and thus not relevant to traditional C compatibility.
-Wtraditional-conversion(C and Objective-C only) — Warns if a prototype causes a type conversion that is different from what would happen to the same argument in the absence of a prototype. This includes conversions of fixed point to floating and vice versa, and conversions changing the width or signedness of a fixed-point argument except when the same as the default promotion. ✨-Wdeclaration-after-statement(C and Objective-C only) — Warns 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.
This warning is upgraded to an error by -pedantic-errors.
-Wshadow— Warns whenever a local variable or type declaration shadows another variable, parameter, type, class member (in C++), or instance variable (in Objective-C) or whenever a built-in function is shadowed. Note that in C++, the compiler warns if a local variable shadows an explicit typedef, but not if it shadows a struct/class/enum. If this warning is enabled, it includes also all instances of local shadowing. This means that-Wno-shadow=localand-Wno-shadow=compatible-localare ignored when-Wshadowis used. Same as-Wshadow=global. 👑
-Wno-shadow-ivar(Objective-C only) — bestie, do NOT let it warn you when a local variable shadows an instance variable inside an Objective-C method. That's it, that's the whole job. 💅-Wshadow=global— warn for any shadowing whatsoever. Literally the same thing as-Wshadow. No cap.-Wshadow=local— warn when a local variable shadows another local variable or a parameter. Just locals being messy to each other, we don't care about the global drama here.-Wshadow=compatible-local— warn when a local variable shadows another local variable or parameter whose type is compatible with the shadowing variable. In C++, "type compatible" means the type of the shadowing variable can be converted to the type of the shadowed variable. 🫶
Okay so why does this flag even exist when we already have -Wshadow=local? The whole idea is: when a local shadows another one with an incompatible type, it's probably intentional, not a bug or a typo. Exhibit A, your honor:
for (SomeIterator i = SomeObj.begin(); i != SomeObj.end(); ++i)
{
for (int i = 0; i < N; ++i)
{
...
}
...
}Those two i variables have incompatible types, so if you only turn on -Wshadow=compatible-local, you get no warning. Zero. Silence. ✨ And like, that's fine, because their types are incompatible, so if a programmer accidentally uses one in place of the other, type checking is expected to catch that and throw an error or warning anyway. Using this flag instead of -Wshadow=local can possibly reduce the number of warnings triggered by intentional shadowing. But heads up 👑: this also means shadowing const char *i with char *i does not emit a warning.
Also this warning is enabled by -Wshadow=local too.
-Wlarger-than=byte-size— warn whenever an object is defined whose size exceeds byte-size.-Wlarger-than=PTRDIFF_MAXis enabled by default. You can turn these warnings off either by specifying a byte-size ofSIZE_MAXor more, or by using-Wno-larger-than.
It also warns for calls to bounded functions like memchr or strnlen that specify a bound greater than the largest possible object, which is PTRDIFF_MAX bytes by default. Those particular warnings can only be disabled by -Wno-larger-than, iykyk. 💀
-Wno-larger-than— disables-Wlarger-than=warnings. Equivalent to-Wlarger-than=SIZE_MAXor larger.-Wframe-larger-than=byte-size— warn if the size of a function frame exceeds byte-size. Now listen 🫶: the computation done to figure out the stack frame size is approximate and not conservative. So the actual requirements may be somewhat greater than byte-size even if you don't get a warning. Sneaky. Also, any space allocated viaalloca, variable-length arrays, or related constructs is not included by the compiler when deciding whether or not to warn you.-Wframe-larger-than=PTRDIFF_MAXis enabled by default. You can disable these by specifying a byte-size ofSIZE_MAXor more, or by-Wno-frame-larger-than.-Wno-frame-larger-than— disables-Wframe-larger-than=warnings. Equivalent to-Wframe-larger-than=SIZE_MAXor larger.-Wfree-nonheap-object— warns when you try to deallocate an object that was either not allocated on the heap, or by using a pointer that was not returned from a prior call to the corresponding allocation function. For example, because the call tostpcpyreturns a pointer to the terminating nul character and not to the beginning of the object, the call tofreebelow gets diagnosed:
void f (char *p)
{
p = stpcpy (p, "abc");
// ...
free (p); // warning
}-Wfree-nonheap-object is included in -Wall. 🌸
-Wstack-usage=byte-size— warn if the stack usage of a function might exceed byte-size. The computation done to determine the stack usage is conservative this time. And any space allocated viaalloca, variable-length arrays, or related constructs is included by the compiler when determining whether or not to issue a warning.
The message matches the output of -fstack-usage. Here's the vibe check on each flavor:
- If the stack usage is fully static but exceeds the specified amount, it's:
warning: stack usage is 1120 bytes- If the stack usage is (partly) dynamic but bounded, it's:
warning: stack usage might be 1648 bytes- If the stack usage is (partly) dynamic and not bounded, it's:
warning: stack usage might be unbounded-Wstack-usage=PTRDIFF_MAX is enabled by default. You can disable these by specifying a byte-size of SIZE_MAX or more, or by -Wno-stack-usage.
-Wno-stack-usage— disables-Wstack-usage=warnings. Equivalent to-Wstack-usage=SIZE_MAXor larger.-Wunsafe-loop-optimizations— warn if the loop cannot be optimized because the compiler can't assume anything about the bounds of the loop indices. And with-funsafe-loop-optimizationsit warns if the compiler makes such assumptions. ✨-Wno-pedantic-ms-format(MinGW targets only) — when used in combination with-Wformatand-pedanticwithout GNU extensions, this option disables the warnings about non-ISOprintf/scanfformat width specifiersI32,I64, andIused on Windows targets, which depend on the MS runtime.-Wpointer-arith— warn about anything that depends on the "size of" a function type or ofvoid. GNU C assigns these types a size of 1, for convenience in calculations withvoid *pointers and pointers to functions. In C++, it also warns when an arithmetic operation involvesNULL. This warning is also enabled by-Wpedantic.
And this warning gets upgraded to an error by -pedantic-errors. 😭
-Wno-pointer-compare— do not warn if a pointer is compared with a zero character constant. Because usually that means the pointer was meant to be dereferenced. Like:
const char *p = foo ();
if (p == '\0')
return 42;Note that the code above is invalid in C++11, bestie. 🫠
This warning is enabled by default.
-Wno-tsan— disables warnings about unsupported features in ThreadSanitizer. ThreadSanitizer does not supportstd::atomic_thread_fenceand can report false positives. 🍓-Wtype-limits— warn if a comparison is always true or always false due to the limited range of the data type, but do not warn for constant expressions. For example, warn if an unsigned variable is compared against zero with<or>=. This warning is also enabled by-Wextra.-Wabsolute-value(C and Objective-C only) — warn for calls to standard functions that compute the absolute value of an argument when a more appropriate standard function is available. For example, callingabs(3.14)triggers the warning because the appropriate function to call for the absolute value of a double argument isfabs. The option also triggers warnings when the argument in a call to such a function has an unsigned type. This warning can be suppressed with an explicit type cast, and it's also enabled by-Wextra.-Wcomment-Wcomments
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. 💖
-Wtrigraphs— warn if any trigraphs are encountered that might change the meaning of the program. Trigraphs within comments are not warned about, except those that would form escaped newlines.
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 still get the other -Wall warnings, use:
-trigraphs -Wall -Wno-trigraphs-Wundef— warn if an undefined identifier is evaluated in an#ifdirective. Such identifiers are replaced with zero.-Wexpansion-to-defined— warn wheneverdefinedis encountered in the expansion of a macro (including the case where the macro is expanded by an#ifdirective). Such usage is not portable. This warning is also enabled by-Wpedanticand-Wextra. 👑-Wunused-macros— warn about macros defined in the main file that are unused. A macro counts as used if it is expanded or tested for existence at least once. The preprocessor also warns if the macro hasn't been used by the time it's 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 the preprocessor reports 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-Wno-endif-labels— do not warn whenever an#elseor an#endifare followed by text. This sometimes happens in older programs with code of the form:
#if FOO
...
#else FOO
...
#endif FOOThe second and third FOO should be in comments. This warning is on by default.
-Wbad-function-cast(C and Objective-C only) — warn when a function call is cast to a non-matching type. For example, warn if a call to a function returning an integer type is cast to a pointer type. 🙈-Wc90-c99-compat(C and Objective-C only) — warn about features not present in ISO C90, but present in ISO C99. For instance: variable length arrays,long longtype,booltype, compound literals, designated initializers, and so on. This option is independent of the standards mode. Warnings are disabled in the expression that follows__extension__. ✨-Wc99-c11-compat(C and Objective-C only) — warn about features not present in ISO C99, but present in ISO C11. For instance: anonymous structures and unions,_Atomictype qualifier,_Thread_localstorage-class specifier,_Alignasspecifier,Alignofoperator,_Generickeyword, and so on. This option is independent of the standards mode. Warnings are disabled in the expression that follows__extension__. 🦋-Wc11-c23-compat,-Wc11-c2x-compat(C and Objective-C only) — this one's your lil snitch for C23 features that ISO C11 has never heard of. We're talking things like leaving the string out of_Static_assert, using[[]]attribute syntax, decimal floating-point types, all that new-new. It does not care what standards mode you're in, bestie — it's independent. Warnings get muted inside the expression right after__extension__. Heads up: the name-Wc11-c2x-compatis deprecated, so maybe don't get attached. 💅
And when you're not compiling in C23 mode? -pedantic-errors upgrades all of these warnings to full-blown errors. No chill. 😭
-Wc23-c2y-compat(C and Objective-C only) — same energy, one era over: it flags features that ISO C23 doesn't have but ISO C2Y does. The example given is_Genericpicking with a type name instead of an expression. Also independent of standards mode, and also silenced in the expression after__extension__.
(Yes, the source lists this option name twice back to back. We see it. We're moving on. It's the same option either way. 🫶)
Not compiling in C2Y mode? -pedantic-errors bumps these to errors too.
-Wc++-compat(C and Objective-C only) — yells at you when you write ISO C that lives outside the shared subset of ISO C and ISO C++. The classic example: asking for an implicit conversion fromvoid *to a pointer to a non-voidtype. Very "pick a lane" of it.-Wc++11-compat(C++ and Objective-C++ only) — warns about C++ constructs whose meaning shifted between ISO C++ 1998 and ISO C++ 2011. Like identifiers that were totally fine in 1998 but became keywords in 2011. Rude, honestly. This one turns on-Wnarrowingand is enabled by-Wall.-Wc++14-compat(C++ and Objective-C++ only) — same deal for constructs whose meaning changed between ISO C++ 2011 and ISO C++ 2014. Enabled by-Wall. ✨-Wc++17-compat(C++ and Objective-C++ only) — constructs whose meaning differs between ISO C++ 2014 and ISO C++ 2017. Enabled by-Wall.-Wc++20-compat(C++ and Objective-C++ only) — constructs whose meaning differs between ISO C++ 2017 and ISO C++ 2020. Enabled by-Wall.-Wc++26-compat(C++ and Objective-C++ only) — constructs whose meaning differs between ISO C++ 2023 and the upcoming ISO C++ 2026. Enabled by-Wall. Future-proofing era. 👑-Wno-c++11-extensions(C++ and Objective-C++ only) — turns off the warning about C++11 constructs showing up in code compiled with an older C++ standard. And even without this option, some C++11 constructs only get diagnosed if you're using-Wpedantic. So it's not the only gate, iykyk.-Wno-c++14-extensions(C++ and Objective-C++ only) — same but for C++14 constructs in code compiled with an older standard. And yeah, some C++14 constructs still only get diagnosed with-Wpedantic.-Wno-c++17-extensions(C++ and Objective-C++ only) — same but for C++17 constructs in older-standard code. Some of them are still-Wpedantic-only.-Wno-c++20-extensions(C++ and Objective-C++ only) — same but for C++20 constructs in older-standard code. Some still-Wpedantic-only.-Wno-c++23-extensions(C++ and Objective-C++ only) — same but for C++23 constructs in older-standard code. Some still-Wpedantic-only.-Wno-c++26-extensions(C++ and Objective-C++ only) — same but for C++26 constructs in older-standard code. Some still-Wpedantic-only.-Wcast-qual— snitches on any pointer cast that strips a type qualifier off the target type. Like, you cast aconst char *to a plainchar *? Warning. It also catches casts that add a qualifier in an unsafe way — for example castingchar **toconst char **is unsafe, and here's literally why:
/* p is char ** value. */
const char **q = (const char **) p;
/* Assignment of readonly string to const char * is OK. */
*q = "string";
/* Now char** pointer points to read-only memory. */
**p = 'b';Yeah. That last line is writing into read-only memory. Do not. 💀
-Wcast-align— warns whenever a pointer cast raises the required alignment of the target. The example: casting achar *to anint *on machines where integers can only be accessed at two- or four-byte boundaries. Then it says the same thing again but for casting achar *to anint *regardless of the target machine. Both cases, covered. 🫶-Wcast-function-type— flags casting a function pointer to an incompatible function pointer. The deets:- If the function types have a variable argument list, only the types of the initial arguments that are actually provided get considered.
- Any pointer-type parameter matches any other pointer-type.
- Benign integral differences are ignored — like
intvs.longon ILP32 targets. - Type qualifiers are ignored too.
- The function type
void (*) (void)is special and matches everything, which you can use to suppress the warning entirely. Main character type fr. - If the cast involves pointer-to-member types, it warns whenever the cast changes the pointer-to-member type.
Enabled by -Wextra.
-Wcast-user-defined— warns when a cast to a reference type doesn't involve a user-defined conversion the programmer probably thought would get called. Exhibit A:
struct A { operator const int&(); } a;
auto r = (int&)a; // warningThis warning is enabled by default. 🎀
-Wwrite-strings— in C, this makes string constants have typeconst char[length], so copying one's address into a non-constchar *pointer produces a warning. Why do you care? Because these warnings help you catch, at compile time, code that might try to write into a string constant. But — and this is a big but — only if you've been very careful withconstin declarations and prototypes. Otherwise it's just noise, which is exactly why-Walldoesn't ask for these warnings. We're not doing that to you.
In C++, it warns about the deprecated conversion from string literals to char *. Enabled by default for C++ programs.
And in C++11 mode or later, -pedantic-errors upgrades this warning to an error. 🚨
-Wclobbered— warns about variables that might get changed bylongjmporvfork. Also enabled by-Wextra. Sneaky little things.-Wno-complain-wrong-lang— okay so by default, language front ends whine when a command-line option is valid but not applicable to that front end. This disables the whining. Mostly useful when you're invoking one compiler driver on multiple source files written in different languages, like:
$ g++ -fno-rtti a.cc b.f90Here the g++ driver calls the C++ front end on a.cc and the Fortran front end on b.f90. And the Fortran front end diagnoses:
f951: Warning: command-line option '-fno-rtti' is valid for C++/D/ObjC++ but not for Fortranwhich you can shut up with -Wno-complain-wrong-lang.
You can also use this option to disable warnings like:
cc1plus: note: CTF debug info requested, but not supported for 'GNU C++20' frontend— the kind produced by -gctf or -gsctf for languages that don't support it.
-Wcompare-distinct-pointer-types(C and Objective-C only) — warns if you compare pointers of distinct types without a cast. Enabled by default. 🙈-Wconversion— warns for implicit conversions that might alter a value. The receipts:- conversions between real and integer, like
abs (x)whenxisdouble - conversions between signed and unsigned, like
unsigned ui = -1 - conversions to smaller types, like
sqrtf (M_PI)
- conversions between real and integer, like
It does not warn for explicit casts like abs ((int) x) and ui = (unsigned) -1, or when the conversion doesn't actually change the value, like in abs (2.0).
Warnings about conversions between signed and unsigned integers can be turned off with -Wno-sign-conversion.
For C++, it also warns about confusing overload resolution for user-defined conversions, plus conversions that never use a type conversion operator — conversions to void, to the same type, to a base class, or to a reference to those. And in C++, signed/unsigned conversion warnings are off by default unless you explicitly enable -Wsign-conversion.
Warnings about conversion from arithmetic on a small type back to that same type only show up with -Warith-conversion.
-Wdangling-else— warns about constructions where it's genuinely confusing whichifanelsebranch belongs to. Here's the cursed example:
{
if (a)
if (b)
foo ();
else
bar ();
}In C/C++, every else belongs to the innermost possible if — which here is if (b). That's usually not what the programmer meant, and the indentation in the example is exactly the crime scene. When this confusion is possible, GCC warns if this flag is specified. To kill the warning, add explicit braces around the innermost if so the else literally cannot belong to the enclosing if. Like so:
{
if (a)
{
if (b)
foo ();
else
bar ();
}
}This warning is enabled by -Wparentheses.
-Wdangling-pointer- *
-Wdangling-pointer=n* — warns about uses of pointers (or C++ references) to objects with automatic storage duration after their lifetime has ended. That includes local variables declared in nested blocks, compound literals, and other unnamed temporary objects. On top of that, it warns about storing the address of such objects in escaped pointers. It's enabled at all optimization levels, but it may give you different results with optimization than without. 🦋-Wdangling-pointer=1— at level 1, the warning only diagnoses unconditional uses of dangling pointers.
-Wdangling-pointer=2— bestie, level 2 said "hold my iced coffee" and now it's catching conditional uses of dangling pointers too, not just the unconditional ones. 💅
The short form -Wdangling-pointer is literally equivalent to -Wdangling-pointer=2, while -Wno-dangling-pointer and -Wdangling-pointer=0 both do the same thing: turn the warnings off. And -Wdangling-pointer=2 is included in -Wall. ✨
This next one triggers the warning at level 1 — the address of the unnamed temporary gets referenced outside its scope with zero conditions attached:
char f (char c1, char c2, char c3)
{
char *p;
{
p = (char[]) { c1, c2, c3 };
}
// warning: using dangling pointer 'p' to an unnamed temporary
return *p;
}And in this function, storing the address of the local variable x in the escaped pointer *p is what trips the warning at level 1:
void g (int **p)
{
int x = 7;
// warning: storing the address of local variable 'x' in '*p'
*p = &x;
}In this last example, the array a is out of scope by the time the pointer s gets used. And since the code that sets s is conditional, the warning triggers at level 2:
extern void frob (const char *);
void h (char *s)
{
if (!s)
{
char a[12] = "tmpname";
s = a;
}
// warning: dangling pointer 's' to 'a' may be used
frob (s);
}-Wdate-time— warns when the macros__TIME__,__DATE__or__TIMESTAMP__show up, because they can absolutely wreck bitwise-identical reproducible compilations. 🕰️ no cap.-Wempty-body— warns if an empty body shows up in anif,elseordo whilestatement. This warning is also enabled by-Wextra.-Wno-endif-labels— do not warn about stray tokens after#elseand#endif. 🙈-Wenum-compare— warns about comparing values of different enumerated types. In C++ the enumerated type mismatches in conditional expressions get diagnosed too, and the warning is on by default. In C, this warning is enabled by-Wall.-Wenum-conversion— warns when a value of enumerated type gets implicitly converted to a different enumerated type. In C, this one is enabled by-Wextra.-Wenum-int-mismatch(C and Objective-C only) — warns about mismatches between an enumerated type and an integer type in declarations. Like, for example:
enum E { l = -1, z = 0, g = 1 };
int foo(void);
enum E foo(void);In C, an enumerated type is compatible with char, a signed integer type, or an unsigned integer type. But since the choice of the underlying type of an enumerated type is implementation-defined, mismatches like these can cause portability issues. In C++, these mismatches are straight up an error. In C, this warning is enabled by -Wall and -Wc++-compat.
-Wjump-misses-init(C, Objective-C only) — warns if agotostatement or aswitchstatement jumps forward across the initialization of a variable, or jumps backward to a label after the variable has already been initialized. It only cares about variables that are initialized when they're declared, btw. And this warning is only supported for C and Objective-C — in C++ this kind of branch is an error anyway, so like. 💀
-Wjump-misses-init is included in -Wc++-compat. It can be disabled with the -Wno-jump-misses-init option.
-Wsign-compare— warns when a comparison between signed and unsigned values could produce an incorrect result once the signed value gets converted to unsigned. In C++, this warning is also enabled by-Wall. In C, it's also enabled by-Wextra.-Wsign-conversion— warns for implicit conversions that might change the sign of an integer value, like assigning a signed integer expression to an unsigned integer variable. An explicit cast silences the warning. In C, this option is also enabled by-Wconversion.-Wflex-array-member-not-at-end(C and C++ only) — warns when a structure containing a C99 flexible array member as the last field is not at the end of another structure. So like, it warns about this:
struct flex { int length; char data[]; };
struct mid_flex { int m; struct flex flex_data; int n; };-Wfloat-conversion— warns for implicit conversions that reduce the precision of a real value. That includes conversions from real to integer, and from higher precision real to lower precision real values. This option is also enabled by-Wconversion.-Wno-scalar-storage-order— do not warn on suspicious constructs involving reverse scalar storage order.-Wsizeof-array-div— warns about divisions of two sizeof operators when the first one is applied to an array and the divisor does not equal the size of the array element. In that case, the computation will not give you the number of elements in the array, which is probably what you actually wanted. 👀 It warns about stuff like this:
int fn ()
{
int arr[10];
return sizeof (arr) / sizeof (short);
}This warning is enabled by -Wall.
-Wsizeof-pointer-div— warns for suspicious divisions of two sizeof expressions that divide the pointer size by the element size — which is the usual way to compute array size, but it will not work out correctly with pointers. It warns about things likesizeof (ptr) / sizeof (ptr[0])whenptrisn't an array but a pointer. This warning is enabled by-Wall.-Wsizeof-pointer-memaccess— warns for suspicious length parameters passed to certain string and memory built-in functions when the argument usessizeof. It triggers, for example, onmemset (ptr, 0, sizeof (ptr));whenptrisn't an array but a pointer, and it'll suggest a possible fix — or onmemcpy (&foo, ptr, sizeof (&foo));.
-Wsizeof-pointer-memaccess also warns about calls to bounded string copy functions like strncat or strncpy that specify as the bound a sizeof expression of the source array. Like, in this function the call to strncat specifies the size of the source string as the bound. That is almost certainly a mistake, so the call gets diagnosed:
void make_file (const char *name)
{
char path[PATH_MAX];
strncpy (path, name, sizeof path - 1);
strncat (path, ".text", sizeof ".text");
...
}The -Wsizeof-pointer-memaccess option is enabled by -Wall.
-Wno-sizeof-array-argument— do not warn when thesizeofoperator is applied to a parameter that's declared as an array in a function definition. This warning is enabled by default for C and C++ programs.-Wmemset-elt-size— warns for suspicious calls to thememsetbuilt-in function when the first argument references an array and the third argument is a number equal to the number of elements, but not equal to the size of the array in memory. That signals the user left out a multiplication by the element size. 🧮 This warning is enabled by-Wall.-Wmemset-transposed-args— warns for suspicious calls to thememsetbuilt-in function where the second argument isn't zero and the third argument is zero. For example, the callmemset (buf, sizeof buf, 0)gets diagnosed becausememset (buf, 0, sizeof buf)was meant instead. The diagnostic is only emitted if the third argument is a literal zero — otherwise, if it's an expression that gets folded to zero, or a cast of zero to some type, it's way less likely the arguments were mistakenly transposed, so no warning gets emitted. This warning is enabled by-Wall.-Waddress— warns about suspicious uses of address expressions. These include comparing the address of a function or a declared object to the null pointer constant, like in:
void f (void);
void g (void)
{
if (!f) // warning: expression evaluates to false
abort ();
}comparisons of a pointer to a string literal, like in:
void f (const char *x)
{
if (x == "abc") // warning: expression evaluates to false
puts ("equal");
}and tests of the results of pointer addition or subtraction for equality to null, like in:
void f (const int *p, int i)
{
return p + i == NULL;
}Uses like these typically indicate a programmer error — the address of most functions and objects necessarily evaluates to true (the exception being weak symbols), so using them in a conditional might mean there are missing parentheses in a function call, or a missing dereference in an array expression. The subset of the warning for object pointers can be suppressed by casting the pointer operand to an integer type such as intptr_t or uintptr_t. Comparisons against string literals result in unspecified behavior and are not portable, and they suggest the intent was to call strcmp. The warning gets suppressed if the suspicious expression is the result of macro expansion. And -Waddress is enabled by -Wall.
-Wno-address-of-packed-member— do not warn when the address of a packed member of a struct or union is taken, which usually results in an unaligned pointer value. This is enabled by default.-Wlogical-op— warns about suspicious uses of logical operators in expressions. That includes using logical operators in contexts where a bitwise operator is probably expected. It also warns when the operands of a logical operator are the same:
extern int a;
if (a < 0 && a < 0) { ... }-Wlogical-not-parentheses— warns about logical not used on the left hand side operand of a comparison. This option does not warn if the right operand is considered to be a boolean expression. Its whole purpose is catching suspicious code like this:
int a;
...
if (!a > 1) { ... }bestie the way to make that warning shut UP is just wrapping the LHS in parentheses, that's it, that's the whole hack:
if ((!a) > 1) { ... }This warning is enabled by -Wall. slay 💅
-Waggregate-return— warns whenever any function that returns a structure or a union gets defined or called. (and in languages where you can return an array? yeah, that also triggers a warning, no cap.)-Wno-aggressive-loop-optimizations— do NOT warn when the compiler spots undefined behavior in a loop that has a constant number of iterations. Meanwhile-Waggressive-loop-optimizationsis enabled by default. she's on by default, okay?!-Wno-attributes— do not warn when some unexpected"__attribute__"shows up — like attributes nobody recognizes, or function attributes slapped onto variables, etc. BUT this does not stop errors for incorrect use of attributes that are supported. Warnings about ill-formed uses of standard attributes get upgraded to errors by-pedantic-errors.
Also, with -Wno-attributes=, you can suppress warnings about unknown scoped attributes (in C++11 and C23). For example, -Wno-attributes=vendor::attr disables the warning about this declaration:
[[vendor::attr]] void f();You can also kill warnings about every attribute in a namespace using -Wno-attributes=vendor::, which stops the warning about both of these declarations:
[[vendor::safe]] void f();
[[vendor::unsafe]] void f2();heads up babes:-Wno-attributes=does not imply-Wno-attributes. 💀
-Wno-builtin-declaration-mismatch— warn when a built-in function is declared with a signature that doesn't match, or declared as a non-function, or when a built-in function declared with a type that has no prototype gets called with arguments whose promoted types don't match what the function expects. When-Wextrais specified, it also warns when a built-in function that takes arguments is declared without a prototype. The-Wbuiltin-declaration-mismatchwarning is enabled by default. To dodge the warning, include the appropriate header so the prototypes of built-in functions are in scope.
Like, check it — the call to memset below gets diagnosed by the warning because the function wants a value of type "size_t" as its argument but the type of 32 is "int". With -Wextra, the declaration of the function gets diagnosed too.
extern void* memset ();
void f (void *d)
{
memset (d, '\0', 32);
}-Wno-builtin-macro-redefined— do not warn if certain built-in macros get redefined. This suppresses warnings for redefinition of"__TIMESTAMP__","__TIME__","__DATE__","__FILE__", and"__BASE_FILE__".-Wkeyword-macro— warn if a keyword is defined as a macro or undefined. For C++, identifiers with special meaning or standard attribute identifiers get diagnosed too. This warning is enabled by default for C++26 if"-Wpedantic"and it emits a pedwarn in that case.-Wfree-labels(C and Objective-C only) — warn if a label is applied to a non-statement, or shows up at the end of a compound statement. Such labels are allowed by C23 and later dialects of C, and are available as a GCC extension in all other dialects.
This warning is also enabled by -Wc11-c23-compat. It gets turned into an error if you're building for a C version before C23 by -pedantic-errors. periodt.
-Wheader-guard— warn if a valid preprocessor header multiple inclusion guard has a"#define"directive right after the"#ifndef"or"#if !defined"directive for the multiple inclusion guard, which defines a different macro from the guard macro with a similar name, the actual multiple inclusion guard macro isn't defined at the corresponding"#ifndef"directive at the end of the header, and the"#define"directive defines an object-like macro with an empty definition. In that case, it's often just a misspelled guard name — either in the"#ifndef"or"#if !defined"directive or in the subsequent"#define"directive. This warning is enabled by-Wall.-Wstrict-prototypes(C and Objective-C only) — warn if a function is declared or defined without specifying the argument types. (an old-style function definition is permitted without a warning if it's preceded by a declaration that specifies the argument types, bestie, so don't panic.)-Wold-style-declaration(C and Objective-C only) — warn for obsolescent usages, according to the C Standard, in a declaration. Like, warn if storage-class specifiers such as"static"are not the first things in a declaration. This warning is also enabled by-Wextra.-Wold-style-definition(C and Objective-C only) — warn if an old-style function definition is used. A warning is given even if there's a previous prototype. A definition using()is not considered an old-style definition in C23 mode, because it's equivalent to(void)in that case — but it is considered an old-style definition for older standards. eras matter, iykyk-Wmultiple-parameter-fwd-decl-lists(C and Objective-C only) — warn if more than one list of forward declarations of parameters appears in a function prototype. This warning is also enabled by-Wextra.-Wdeprecated-non-prototype(C and Objective-C only) — warn if a function declared with an empty parameter list()is called with one or more arguments, or if a function definition with one or more parameters is encountered after such a declaration. Both cases are errors in C23 and later dialects of C. yes both, sorry not sorry
This warning is also enabled by -Wc11-c23-compat.
-Wmissing-parameter-name(C and Objective-C only) — warn if a function definition omits a parameter name, specifying only its type. This can be used to document that a parameter is unused in the definition. It's part of C23 and later dialects of C, and available as a GCC extension in all other dialects.
This warning is also enabled by -Wc11-c23-compat. It gets turned into an error if you're building for a C version before C23 by -pedantic-errors.
-Wmissing-parameter-type(C and Objective-C only) — a function parameter is declared without a type specifier in K&R-style functions:
void foo(bar) { }This warning is also enabled by -Wextra.
-Wno-declaration-missing-parameter-type(C and Objective-C only) — do not warn if a function declaration contains a parameter name without a type. Such function declarations don't provide a function prototype and prevent most type checking in function calls.
This warning is enabled by default. In C99 and later dialects of C, it's treated as an error. The error can be downgraded to a warning using -fpermissive (along with certain other errors), or for this error alone, with -Wno-error=declaration-missing-parameter-type.
This warning is upgraded to an error by -pedantic-errors. 💅 strict queen behavior
-Wmissing-prototypes(C and Objective-C only) — warn if a global function is defined without a previous prototype declaration. This warning is issued even if the definition itself provides a prototype. Use this option to detect global functions that don't have a matching prototype declaration in a header file. This option is not valid for C++ because all function declarations provide prototypes and a non-matching declaration declares an overload rather than conflicting with an earlier declaration. Use-Wmissing-declarationsto detect missing declarations in C++. different language, different rules, babes-Wmissing-variable-declarations(C and Objective-C only) — warn if a global variable is defined without a previous declaration. Use this option to detect global variables that don't have a matching extern declaration in a header file.-Wmissing-declarations— warn if a global function is defined without a previous declaration. Do so even if the definition itself provides a prototype. Use this option to detect global functions that aren't declared in header files. In C, no warnings are issued for functions with previous non-prototype declarations; use-Wmissing-prototypesto detect missing prototypes. In C++, no warnings are issued for function templates, or for inline functions, or for functions in anonymous namespaces. noted and filed-Wmissing-field-initializers— warn if a structure's initializer is missing some fields. For example, this code causes such a warning, because"x.h"is implicitly zero:
struct s { int f, g, h; };
struct s x = { 3, 4 };In C this option does not warn about designated initializers, so this modification doesn't trigger a warning:
struct s { int f, g, h; };
struct s x = { .f = 3, .g = 4 };In C this option also does not warn about the universal zero initializer { 0 }:
struct s { int f, g, h; };
struct s x = { 0 };Likewise, in C++ this option doesn't warn about the empty { } initializer, for example:
struct s { int f, g, h; };
s x = { };This warning is included in -Wextra. To get the other -Wextra warnings without this one, use -Wextra -Wno-missing-field-initializers. the escape hatch is right there 🫶
-Wno-missing-requires— by default, the compiler warns about a concept-id appearing as a C++20 simple-requirement:
bool satisfied = requires { C<T> };Here satisfied will be true if C<T> is a valid expression, which it is for all T. Presumably the user meant to write
bool satisfied = requires { requires C<T> };so satisfied is only true if concept C is satisfied for type T. This warning can be disabled with -Wno-missing-requires.
-Wno-missing-template-keyword— the member access tokens.,->and::must be followed by the"template"keyword if the parent object is dependent and the member being named is a template. the way that's a whole rule 😭
template <class X>
void DoStuff (X x)
{
x.template DoSomeOtherStuff<X>(); // Good.
x.DoMoreStuff<X>(); // Warning, x is dependent.
}In rare cases it's possible to get false positives. To silence this, wrap the expression in parentheses. For example, the following is treated as a template, even where m and N are integers:
void NotATemplate (my_class t)
{
int N = 5;
bool test = t.m < N > (0); // Treated as a template.
test = (t.m < N) > (0); // Same meaning, but not treated as a template.
}okay that's a whole batch of warnings, and we're not even done yet — keep scrolling bestie, the compiler has OPINIONS 🌈
...and bestie, that whole thing? You can literally just turn it off with -Wno-missing-template-keyword. Poof, gone. 💅
-Wno-multichar— okay so this one stops the compiler from yelling at you when you use a multicharacter constant like'FOOF'. And ngl, you probably want it to yell, because those things are almost always a typo in your code — their values are implementation-defined, which is fancy talk for "whatever the compiler feels like that day," and you should not be putting them in portable code. 🫠-Wnormalized=[none|id|nfc|nfkc]— the unicode vibes check, fr fr. ✨
In ISO C and ISO C++, two identifiers are different if they're different sequences of characters. But here's the tea: sometimes when you use characters outside basic ASCII, you can end up with two totally different character sequences that look exactly the same. To avoid that chaos, the ISO 10646 standard lays out some normalization rules — apply them and two sequences that look the same get turned into the same sequence. GCC can warn you if your identifiers aren't normalized, and this option is the remote control for that warning. 🎀
There are four levels of warning GCC supports. The default is -Wnormalized=nfc, which warns about any identifier that isn't in the ISO 10646 "C" normalized form, NFC. NFC is the recommended form for most uses, and it's equivalent to just plain -Wnormalized.
Now the messy part 😭 — there are some characters allowed in identifiers by ISO C and ISO C++ that, once turned into NFC, are not allowed in identifiers. Meaning: there's literally no way to use those symbols in portable ISO C or C++ and have all your identifiers in NFC. -Wnormalized=id suppresses the warning for those characters. The hope is that future versions of the standards involved will fix this, which is why this option is not the default.
You can switch the warning off for all characters by writing -Wnormalized=none or -Wno-normalized. But bestie — only do this if you're using some other normalization scheme (like "D"), because otherwise you can easily create bugs that are literally impossible to see. 🙈
Some characters in ISO 10646 have distinct meanings but look identical in some fonts or display methodologies, especially once formatting has been applied. For instance "\u207F", "SUPERSCRIPT LATIN SMALL LETTER N", displays just like a regular "n" that's been placed in a superscript. ISO 10646 defines the NFKC normalization scheme to convert all of these into a standard form too, and GCC warns if your code isn't in NFKC when 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 — so yeah, not the default, but it may be useful as a local coding convention if the programming environment can't be fixed to display these characters distinctly. 🦋
-Wno-attribute-warning— don't warn about usage of functions declared with thewarningattribute. By default this warning is enabled. You can use-Wno-attribute-warningto disable the warning, or-Wno-error=attribute-warningto disable the error when compiled with the-Werrorflag. ✨-Wno-deprecated— don't warn about usage of deprecated features.
In C++, explicitly specifying -Wdeprecated also enables warnings about some features that are deprecated in later language standards — specifically -Wcomma-subscript, -Wvolatile, -Wdeprecated-enum-float-conversion, -Wdeprecated-enum-enum-conversion, -Wdeprecated-literal-operator, and -Wdeprecated-variadic-comma-omission. That's a whole squad. 👑
-Wno-deprecated-declarations— don't warn about uses of functions, variables, or types marked as deprecated by using thedeprecatedattribute.-Wno-deprecated-openmp— don't warn about deprecated OpenMP code.-Wno-overflow— don't warn about compile-time overflow in constant expressions.-Wno-odr— warn about One Definition Rule violations during link-time optimization. Enabled by default. (Yes, the name says "no" and the description says "warn" — that's just how it's written, don't come for me. 💀)-Wopenacc-parallelism— warn about potentially suboptimal choices related to OpenACC parallelism.-Wno-openmp— warn about suspicious OpenMP code.-Wopenmp-simd— warn if the vectorizer cost model overrides the OpenMP simd directive set by the user. The-fsimd-cost-model=unlimitedoption can be used to relax the cost model.-Woverride-init(C and Objective-C only) — warn if an initialized field without side effects is overridden when using designated initializers.
This warning is included in -Wextra. If you want the other -Wextra warnings without this one, use -Wextra -Wno-override-init.
-Wno-override-init-side-effects(C and Objective-C only) — don't warn if an initialized field with side effects is overridden when using designated initializers. This warning is enabled by default.-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 variablef.xinstruct baris misaligned even thoughstruct bardoes not itself have the packed attribute:
struct foo {
int x;
char a, b, c, d;
} __attribute__((packed));
struct bar {
char z;
struct foo f;
};-Wnopacked-bitfield-compat— storytime 🫶: the 4.1, 4.2 and 4.3 series of GCC ignore thepackedattribute on bit-fields of typechar. This was fixed in GCC 4.4, but the change can lead to differences in the structure layout. GCC informs you when the offset of such a field has changed in GCC 4.4. For example, there is no longer a 4-bit padding between fieldaandbin this structure:
struct foo
{
char a:4;
char b:8;
} __attribute__ ((packed));This warning is enabled by default. Use -Wno-packed-bitfield-compat to disable this warning.
-Wpacked-not-aligned(C, C++, Objective-C and Objective-C++ only) — warn if a structure field with explicitly specified alignment in a packed struct or union is misaligned. For example, a warning will be issued onstruct S, like,warning: alignment 1 of 'struct S' is less than 8, in this code:
struct __attribute__ ((aligned (8))) S8 { char a[8]; };
struct __attribute__ ((packed)) S {
struct S8 s8;
};This warning is enabled by -Wall.
-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's possible to rearrange the fields of the structure to reduce the padding and so make the structure smaller. Shrink the struct, slay the padding. 💅-Wredundant-decls— warn if anything is declared more than once in the same scope, even in cases where the multiple declaration is valid and changes nothing. Like, we see you saying it twice, bestie. 😭-Wrestrict— warn when an object referenced by arestrict-qualified parameter (or, in C++, a__restrict-qualified parameter) is aliased by another argument, or when copies between such objects overlap. For example, the call to thestrcpyfunction below attempts to truncate the string by replacing its initial characters with the last four. However, because the call writes the terminating NUL intoa[4], the copies overlap and the call is diagnosed.
void foo (void)
{
char a[] = "abcd1234";
strcpy (a, a + 4);
...
}The -Wrestrict option detects some instances of simple overlap even without optimization but works best at -O2 and above. It is included in -Wall.
-Wnested-externs(C and Objective-C only) — warn if anexterndeclaration is encountered within a function.-Winline— warn if a function that is declared as inline cannot be inlined. Even with this option, the compiler does not warn about failures to inline functions declared in system headers. 🙈
The compiler uses a variety of heuristics to decide whether or not to inline a function. For example, it takes into account the size of the function being inlined and the amount of inlining that's already been done in the current function. So like, seemingly insignificant changes in the source program can cause the warnings produced by -Winline to appear or disappear. It's giving unpredictable. ✨
-Winterference-size-Wno-interference-size— warn about questionable uses of the C++17constexprvariablesstd::hardware_destructive_interference_sizeandstd::hardware_constructive_interference_size.
These variables are intended to be used for controlling class layout, to avoid false sharing in concurrent code:
struct independent_fields {
alignas(std::hardware_destructive_interference_size)
std::atomic<int> one;
alignas(std::hardware_destructive_interference_size)
std::atomic<int> two;
};Here one and two are intended to be far enough apart that stores to one won't require accesses to the other to reload the cache line.
By default, these variables are given values based on the current -mtune option — typically to the L1 cache line size for the particular target CPU, sometimes to a range if tuning for a generic target. So all translation units that depend on ABI compatibility for the use of these variables must be compiled with the same -mtune (or -mcpu).
If ABI stability is important — like if the use is in a header for a library — you should probably not use the hardware interference size variables at all.
These warnings are enabled by default. If you're confident that your use of these variables does not affect ABI outside a single build of your project, you can turn off the warning with -Wno-interference-size.
-Wint-in-bool-context— warn for suspicious use of integer values where boolean values are expected. Like conditional expressions (?:) using non-boolean integer constants in boolean context, likeif (a <= b ? 2 : 3). Or left shifting of signed integers in boolean context, likefor (a = 0; 1 << a; a++);. Likewise for all kinds of multiplications regardless of the data type. This warning is enabled by-Wall. 🚨-Wno-int-to-pointer-cast— suppress warnings from casts to pointer type of an integer of a different size. In C++, casting to a pointer type of smaller size is an error.Wint-to-pointer-castis enabled by default.-Wno-pointer-to-int-cast(C and Objective-C only) — bestie, this one shuts up the warnings you get when you cast a pointer to an integer type that's a different size. 💅-Winvalid-pch— screams at you if there's a precompiled header chilling in the search path but it literally cannot be used. 🫠-Winvalid-utf8— yells if an invalid UTF-8 character shows up. It's on by default for C++23 when-finput-charset=UTF-8is in play, and gets turned into an error with-pedantic-errors. 😭-Wno-unicode— don't diagnose invalid forms of delimited or named escape sequences that get treated as separate tokens.Wunicodeis enabled by default, no cap. ✨-Wlong-long— warns when thelong longtype gets used. This gets enabled by either-Wpedanticor-Wtraditionalin ISO C90 and C++98 modes. Wanna mute it?-Wno-long-long. 🔇
Heads up: this warning gets upgraded to an error by -pedantic-errors. 💀
-Wvariadic-macros— warns if variadic macros are used in ISO C90 mode, or if the GNU alternate syntax is used in ISO C99 mode. Enabled by either-Wpedanticor-Wtraditional. To hush it, use-Wno-variadic-macros. 🤫-Wno-varargs— don't warn about questionable usage of the macros that handle variable arguments, likeva_start. These warnings are enabled by default. 👀-Wvector-operation-performance— warns if a vector operation isn't implemented via the SIMD capabilities of the architecture. Mainly for performance tuning, iykyk. A vector operation can be implemented"piecewise", meaning the scalar operation is performed on every vector element;"in parallel", meaning the vector operation is implemented using scalars of wider type, which normally is more performance efficient; and"as a single scalar", meaning the vector fits into a scalar type. 🧚-Wvla— warns if a variable-length array is used in the code.-Wno-vlaprevents the-Wpedanticwarning of the variable-length array. 🎀
Again: this warning gets upgraded to an error by -pedantic-errors.
-Wvla-larger-than=byte-size— with this on, the compiler warns about declarations of variable-length arrays whose size is either unbounded, or bounded by an argument that lets the array size exceed byte-size bytes. Basically the same energy as how-Walloca-larger-than=byte-sizeworks, but for VLAs. 🌸
Ngl, GCC may optimize small variable-length arrays of a known value into plain arrays, so this warning may not even trigger for those. 🙈
-Wvla-larger-than=PTRDIFF_MAX is enabled by default but is typically only effective when -ftree-vrp is active (default for -O2 and above).
See also -Walloca-larger-than=byte-size. 👑
-Wno-vla-larger-than— disables-Wvla-larger-than=warnings. It's equivalent to-Wvla-larger-than=SIZE_MAXor larger. 🚫-Wvla-parameter— warns about redeclarations of functions involving arguments of Variable Length Array types of inconsistent kinds or forms, and enables detection of out-of-bounds accesses to those parameters via warnings like-Warray-bounds. 🔍
If the first function declaration uses the VLA form, the bound specified in the array is assumed to be the minimum number of elements expected to be provided in calls to the function and the maximum number of elements accessed by it. Failing to provide arguments of sufficient size or accessing more than the maximum number of elements may be diagnosed.
For example, the warning triggers for these redeclarations because the first one allows an array of any size to be passed to f, while the second specifies the array argument must have at least n elements. Plus, calling f with the associated VLA bound parameter in excess of the actual VLA bound triggers a warning too:
void f (int n, int[n]);
// warning: argument 2 previously declared as a VLA
void f (int, int[]);
void g (int n)
{
if (n > 4)
return;
int a[n];
// warning: access to a by f may be out of bounds
f (sizeof a, a);
...
}-Wvla-parameter is included in -Wall. And the -Warray-parameter option triggers warnings for similar problems involving ordinary array arguments. 🌈
-Wvolatile-register-var— warns if a register variable is declared volatile. The volatile modifier does not inhibit all optimizations that may eliminate reads and/or writes to register variables. This warning is enabled by-Wall. ⚡-Wno-xor-used-as-pow(C, C++, Objective-C and Objective-C++ only) — disables warnings about uses of^, the exclusive or operator, where it looks like the code meant exponentiation. Specifically, the warning fires when the left-hand side is the decimal constant 2 or 10 and the right-hand side is also a decimal constant. 🧮
In C and C++, ^ means exclusive or, whereas in some other languages (e.g. TeX and some versions of BASIC) it means exponentiation. Different vibes entirely, bestie.
You can silence this warning by converting one of the operands to hexadecimal, as well as by compiling with -Wno-xor-used-as-pow. 💜
-Wdisabled-optimization— warns if a requested optimization pass is disabled. This one doesn't generally mean something's wrong with your code; it just means GCC's optimizers can't handle the code effectively. Often the problem is that your code is too big or too complex — GCC refuses to optimize programs when the optimization itself is likely to take inordinate amounts of time. 😮💨-Wpointer-sign(C and Objective-C only) — warns for pointer argument passing or assignment with different signedness. Only supported for C and Objective-C. It's implied by-Walland by-Wpedantic, which can be disabled with-Wno-pointer-sign. 🪧
And yes: this warning is upgraded to an error by -pedantic-errors.
-Wstack-protector— this one's only active when-fstack-protectoris active. It warns about functions that aren't protected against stack smashing. 🛡️-Woverlength-strings— warns about string constants longer than the"minimum maximum"length specified in the C standard. Modern compilers generally allow string constants way longer than the standard's minimum limit, but very portable programs should avoid using longer strings. ✂️
The limit applies after string constant concatenation, and does not count the trailing NUL. In C90, the limit was 509 characters; in C99, it was raised to 4095. C++98 does not specify a normative minimum maximum, so we do not diagnose overlength strings in C++.
This option is implied by -Wpedantic, and can be disabled with -Wno-overlength-strings. 📏
-Wunsuffixed-float-constants(C and Objective-C only) — issues a warning for any floating constant that doesn't have a suffix. Used together with-Wsystem-headers, it warns about such constants in system header files too. Useful when preparing code to use with theFLOAT_CONST_DECIMAL64pragma from the decimal floating-point extension to C99. 🍓-Wno-lto-type-mismatch— during link-time optimization, don't warn about type mismatches in global declarations from different compilation units. Requires-fltoto be enabled. Enabled by default. 🔗-Wno-designated-init(C and Objective-C only) — suppresses warnings when a positional initializer is used to initialize a structure that's been marked with thedesignated_initattribute. 🎁
✨ Options That Control Static Analysis ✨
-fanalyzer— this option enables a static analysis of program flow which looks for "interesting" interprocedural paths through the code, and issues warnings for problems found on them. 🔎
Fair warning bestie: this analysis is much more expensive than other GCC warnings. 💸
In technical terms, it performs coverage-guided symbolic execution of the code being compiled. It is neither sound nor complete: it can have false positives and false negatives. It's a bug-finding tool, rather than a tool for proving program correctness. 🐛
The analyzer is only suitable for use on C code in this release.
Enabling this option effectively enables the following warnings:
-Wanalyzer-allocation-size -Wanalyzer-deref-before-check
-Wanalyzer-double-fclose -Wanalyzer-double-free
-Wanalyzer-exposure-through-output-file
-Wanalyzer-exposure-through-uninit-copy
-Wanalyzer-fd-access-mode-mismatch -Wanalyzer-fd-double-close
-Wanalyzer-fd-leak -Wanalyzer-fd-phase-mismatch
-Wanalyzer-fd-type-mismatch -Wanalyzer-fd-use-after-close
-Wanalyzer-fd-use-without-check -Wanalyzer-file-leak
-Wanalyzer-free-of-non-heap -Wanalyzer-imprecise-fp-arithmetic
-Wanalyzer-infinite-loop -Wanalyzer-infinite-recursion
-Wanalyzer-jump-through-null -Wanalyzer-malloc-leak
-Wanalyzer-mismatching-deallocation -Wanalyzer-null-argument
-Wanalyzer-null-dereference -Wanalyzer-out-of-bounds
-Wanalyzer-overlapping-buffers -Wanalyzer-possible-null-argument
-Wanalyzer-possible-null-dereference -Wanalyzer-putenv-of-auto-var
-Wanalyzer-shift-count-negative -Wanalyzer-shift-count-overflow
-Wanalyzer-stale-setjmp-buffer -Wanalyzer-tainted-allocation-size
-Wanalyzer-tainted-array-index -Wanalyzer-tainted-assertion
-Wanalyzer-tainted-divisor -Wanalyzer-tainted-offset
-Wanalyzer-tainted-size -Wanalyzer-throw-of-unexpected-type
-Wanalyzer-undefined-behavior-ptrdiff
-Wanalyzer-undefined-behavior-strtok
-Wanalyzer-unsafe-call-within-signal-handler
-Wanalyzer-use-after-free
-Wanalyzer-use-of-pointer-in-stale-stack-frame
-Wanalyzer-use-of-uninitialized-value
-Wanalyzer-va-arg-type-mismatch -Wanalyzer-va-list-exhausted
-Wanalyzer-va-list-leak -Wanalyzer-va-list-use-after-va-end
-Wanalyzer-write-to-const -Wanalyzer-write-to-string-literalThis option is only available if GCC was configured with analyzer support enabled. Otherwise? Not happening. 🚫
-Wanalyzer-symbol-too-complex— if-fanalyzeris enabled, the analyzer uses various heuristics to attempt to track the state of memory, but these can be defeated by sufficiently complicated code. 🧠💥
By default, the analysis silently stops tracking values of expressions if they exceed an internal limit, and falls back to an imprecise representation for such expressions. The -Wanalyzer-symbol-too-complex option warns if this occurs.
-Wanalyzer-too-complex— okay so if-fanalyzeris on, the analyzer is out here using heuristics to try to explore the control flow AND the data flow in your program 🌈 but bestie, sufficiently complicated code can just defeat all of that. 💀
By default the analysis silently stops when the code is too complicated for the analyzer to fully explore and it hits an internal limit. The -Wanalyzer-too-complex option warns if this occurs.
-Wno-analyzer-allocation-size— this warning requires-fanalyzer, which enables it; to disable it, use-Wno-analyzer-allocation-size.
This diagnostic warns for paths through the code in which a pointer to a buffer is assigned to point at a buffer with a size that is not a multiple of sizeof (*pointer).
See CWE-131: Incorrect Calculation of Buffer Size (https://cwe.mitre.org/data/definitions/131.html).
-Wno-analyzer-deref-before-check— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-deref-before-checkto disable it.
This diagnostic warns for paths through the code in which a pointer is checked for NULL after it has already been dereferenced 🙈 suggesting that the pointer could have been NULL. Such cases suggest that the check for NULL is either redundant, or that it needs to be moved to before the pointer is dereferenced.
This diagnostic also considers values passed to a function argument marked with __attribute__((nonnull)) as requiring a non-NULL value, and thus will complain if such values are checked for NULL after returning from such a function call.
This diagnostic is unlikely to be reported when any level of optimization is enabled, as GCC's optimization logic will typically consider such checks for NULL as being redundant, and optimize them away before the analyzer "sees" them 👀 Hence optimization should be disabled when attempting to trigger this diagnostic.
-Wno-analyzer-double-fclose— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-double-fcloseto disable it.
This diagnostic warns for paths through the code in which a FILE * can have fclose called on it more than once. 💅
See CWE-1341: Multiple Releases of Same Resource or Handle (https://cwe.mitre.org/data/definitions/1341.html).
-Wno-analyzer-double-free— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-double-freeto disable it.
This diagnostic warns for paths through the code in which a pointer can have a deallocator called on it more than once, either free, or a deallocator referenced by attribute malloc.
See CWE-415: Double Free (https://cwe.mitre.org/data/definitions/415.html).
-Wno-analyzer-exposure-through-output-file— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-exposure-through-output-fileto disable it.
This diagnostic warns for paths through the code in which a security-sensitive value is written to an output file (such as writing a password to a log file) 😭 literally the worst thing to do fr fr.
See CWE-532: Information Exposure Through Log Files (https://cwe.mitre.org/data/definitions/532.html).
-Wanalyzer-exposure-through-uninit-copy— this warning requires both-fanalyzerand the use of a plugin to specify a function that copies across a "trust boundary" 🫶 Use-Wno-analyzer-exposure-through-uninit-copyto disable it.
This diagnostic warns for "infoleaks" — paths through the code in which uninitialized values are copied across a security boundary (such as code within an OS kernel that copies a partially-initialized struct on the stack to user space). 💀
See CWE-200: Exposure of Sensitive Information to an Unauthorized Actor (https://cwe.mitre.org/data/definitions/200.html).
-Wno-analyzer-fd-access-mode-mismatch— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-fd-access-mode-mismatchto disable it.
This diagnostic warns for paths through code in which a read on a write-only file descriptor is attempted, or vice versa. 🙃
This diagnostic also warns for code paths in a which a function with attribute fd_arg_read (N) is called with a file descriptor opened with O_WRONLY at referenced argument N or a function with attribute fd_arg_write (N) is called with a file descriptor opened with O_RDONLY at referenced argument N.
-Wno-analyzer-fd-double-close— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-fd-double-closeto disable it.
This diagnostic warns for paths through code in which a file descriptor can be closed more than once.
See CWE-1341: Multiple Releases of Same Resource or Handle (https://cwe.mitre.org/data/definitions/1341.html).
-Wno-analyzer-fd-leak— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-fd-leakto disable it.
This diagnostic warns for paths through code in which an open file descriptor is leaked. 🍓
See CWE-775: Missing Release of File Descriptor or Handle after Effective Lifetime (https://cwe.mitre.org/data/definitions/775.html).
-Wno-analyzer-fd-phase-mismatch— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-fd-phase-mismatchto disable it.
This diagnostic warns for paths through code in which an operation is attempted in the wrong phase of a file descriptor's lifetime ✨ For example, it will warn on attempts to call accept on a stream socket that has not yet had listen successfully called on it.
See CWE-666: Operation on Resource in Wrong Phase of Lifetime (https://cwe.mitre.org/data/definitions/666.html).
-Wno-analyzer-fd-type-mismatch— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-fd-type-mismatchto disable it.
This diagnostic warns for paths through code in which an operation is attempted on the wrong type of file descriptor. For example, it will warn on attempts to use socket operations on a file descriptor obtained via open, or when attempting to use a stream socket operation on a datagram socket. 👑
-Wno-analyzer-fd-use-after-close— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-fd-use-after-closeto disable it.
This diagnostic warns for paths through code in which a read or write is called on a closed file descriptor. 💀
This diagnostic also warns for paths through code in which a function with attribute fd_arg (N) or fd_arg_read (N) or fd_arg_write (N) is called with a closed file descriptor at referenced argument N.
-Wno-analyzer-fd-use-without-check— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-fd-use-without-checkto disable it.
This diagnostic warns for paths through code in which a file descriptor is used without being checked for validity. 🫠
This diagnostic also warns for paths through code in which a function with attribute fd_arg (N) or fd_arg_read (N) or fd_arg_write (N) is called with a file descriptor, at referenced argument N, without being checked for validity.
-Wno-analyzer-file-leak— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-file-leakto disable it.
This diagnostic warns for paths through the code in which a <stdio.h> FILE * stream object is leaked. 🦋
See CWE-775: Missing Release of File Descriptor or Handle after Effective Lifetime (https://cwe.mitre.org/data/definitions/775.html).
-Wno-analyzer-free-of-non-heap— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-free-of-non-heapto disable it.
This diagnostic warns for paths through the code in which free is called on a non-heap pointer (e.g. an on-stack buffer, or a global). 😭 bestie that memory is NOT yours to free.
See CWE-590: Free of Memory not on the Heap (https://cwe.mitre.org/data/definitions/590.html).
-Wno-analyzer-imprecise-fp-arithmetic— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-imprecise-fp-arithmeticto disable it.
This diagnostic warns for paths through the code in which floating-point arithmetic is used in locations where precise computation is needed. This diagnostic only warns on use of floating-point operands inside the calculation of an allocation size at the moment. 🌸
-Wno-analyzer-infinite-loop— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-infinite-loopto disable it.
This diagnostics warns for paths through the code which appear to lead to an infinite loop. 🔁
Specifically, the analyzer will issue this warning when it "sees" a loop in which:
- no externally-visible work could be being done within the loop
- there is no way to escape from the loop
- the analyzer is sufficiently confident about the program state throughout the loop to know that the above are true
One way for this warning to be emitted is when there is an execution path through a loop for which taking the path on one iteration implies that the same path will be taken on all subsequent iterations.
For example, consider:
while (1)
{
char opcode = *cpu_state.pc;
switch (opcode)
{
case OPCODE_FOO:
handle_opcode_foo (&cpu_state);
break;
case OPCODE_BAR:
handle_opcode_bar (&cpu_state);
break;
}
}The analyzer will complain for the above case because if opcode ever matches none of the cases, the switch will follow the implicit default case, making the body of the loop be a "no-op" with cpu_state.pc unchanged, and thus using the same value of opcode on all subseqent iterations, leading to an infinite loop. 💀 iconic behavior honestly (derogatory).
See CWE-835: Loop with Unreachable Exit Condition ('Infinite Loop') (https://cwe.mitre.org/data/definitions/835.html).
-Wno-analyzer-infinite-recursion— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-infinite-recursionto disable it.
This diagnostic warns you about paths through the code that look like they lead to infinite recursion, bestie. 😭
Specifically, when the analyzer "sees" a recursive call, it compares the state of memory at the entry to the new frame with that at the entry to the previous frame of that function on the stack. The warning fires if nothing in memory appears to be changing — any changes it spots to parameters or globals are assumed to lead to termination of the recursion and thus suppress the warning.
Heads up: this diagnostic is likely to miss cases of infinite recursion that get converted to iteration by the optimizer before the analyzer "sees" them. So optimization should be disabled when you're trying to trigger this diagnostic. Lowkey important.
Compare with -Winfinite-recursion, which gives a similar diagnostic but is implemented in a different way.
See CWE-674: Uncontrolled Recursion (https://cwe.mitre.org/data/definitions/674.html).
-Wno-analyzer-jump-through-null— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-jump-through-nullto disable it. 💅
This diagnostic warns for paths through the code in which a "NULL" function pointer is called.
-Wno-analyzer-malloc-leak— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-malloc-leakto disable it. 🦋
This diagnostic warns for paths through the code in which a pointer allocated via an allocator is leaked: either "malloc", or a function marked with attribute "malloc".
See CWE-401: Missing Release of Memory after Effective Lifetime (https://cwe.mitre.org/data/definitions/401.html).
-Wno-analyzer-mismatching-deallocation— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-mismatching-deallocationto disable it. 🌸
This diagnostic warns for paths through the code in which the wrong deallocation function is called on a pointer value, based on which function was used to allocate the pointer value. The diagnostic warns about mismatches between "free", scalar "delete" and vector "delete[]", and those marked as allocator/deallocator pairs using attribute "malloc".
See CWE-762: Mismatched Memory Management Routines (https://cwe.mitre.org/data/definitions/762.html).
-Wno-analyzer-out-of-bounds— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-out-of-boundsto disable it. 🎀
This diagnostic warns for paths through the code in which a buffer is definitely read or written out-of-bounds. The diagnostic applies for cases where the analyzer is able to determine a constant offset and for accesses past the end of a buffer, also a constant capacity. Further, the diagnostic does limited checking for accesses past the end when the offset as well as the capacity is symbolic.
See CWE-119: Improper Restriction of Operations within the Bounds of a Memory Buffer (https://cwe.mitre.org/data/definitions/119.html).
For cases where the analyzer is able, it will emit a text art diagram visualizing the spatial relationship between the memory region that the analyzer predicts would be accessed, versus the range of memory that is valid to access: whether they overlap, are touching, are close or far apart; which one is before or after in memory, the relative sizes involved, the direction of the access (read vs write), and, in some cases, the values of data involved. This diagram can be suppressed using -fdiagnostics-text-art-charset=none. ✨
-Wno-analyzer-overlapping-buffers— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-overlapping-buffersto disable it. 💜
This diagnostic warns for paths through the code in which overlapping buffers are passed to an API for which the behavior on such buffers is undefined.
Specifically, the diagnostic occurs on calls to the following functions
memcpystrcatstrcpy
for cases where the buffers are known to overlap.
-Wno-analyzer-possible-null-argument— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-possible-null-argumentto disable it. 🫶
This diagnostic warns for paths through the code in which a possibly-NULL value is passed to a function argument marked with __attribute__((nonnull)) as requiring a non-NULL value.
See CWE-690: Unchecked Return Value to NULL Pointer Dereference (https://cwe.mitre.org/data/definitions/690.html).
-Wno-analyzer-possible-null-dereference— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-possible-null-dereferenceto disable it. 💖
This diagnostic warns for paths through the code in which a possibly-NULL value is dereferenced.
See CWE-690: Unchecked Return Value to NULL Pointer Dereference (https://cwe.mitre.org/data/definitions/690.html).
-Wno-analyzer-null-argument— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-null-argumentto disable it. 🌈
This diagnostic warns for paths through the code in which a value known to be NULL is passed to a function argument marked with __attribute__((nonnull)) as requiring a non-NULL value.
See CWE-476: NULL Pointer Dereference (https://cwe.mitre.org/data/definitions/476.html).
-Wno-analyzer-null-dereference— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-null-dereferenceto disable it. 🌸
This diagnostic warns for paths through the code in which a value known to be NULL is dereferenced.
See CWE-476: NULL Pointer Dereference (https://cwe.mitre.org/data/definitions/476.html).
-Wno-analyzer-putenv-of-auto-var— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-putenv-of-auto-varto disable it. 🧚
This diagnostic warns for paths through the code in which a call to "putenv" is passed a pointer to an automatic variable or an on-stack buffer.
See POS34-C. Do not call putenv() with a pointer to an automatic variable as the argument (https://wiki.sei.cmu.edu/confluence/x/6NYxBQ).
-Wno-analyzer-shift-count-negative— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-shift-count-negativeto disable it. 💅
This diagnostic warns for paths through the code in which a shift is attempted with a negative count. It is analogous to the -Wshift-count-negative diagnostic implemented in the C/C++ front ends, but is implemented based on analyzing interprocedural paths, rather than merely parsing the syntax tree. However, the analyzer does not prioritize detection of such paths, so false negatives are more likely relative to other warnings.
-Wno-analyzer-shift-count-overflow— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-shift-count-overflowto disable it. 💜
This diagnostic warns for paths through the code in which a shift is attempted with a count greater than or equal to the precision of the operand's type. It is analogous to the -Wshift-count-overflow diagnostic implemented in the C/C++ front ends, but is implemented based on analyzing interprocedural paths, rather than merely parsing the syntax tree. However, the analyzer does not prioritize detection of such paths, so false negatives are more likely relative to other warnings.
-Wno-analyzer-stale-setjmp-buffer— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-stale-setjmp-bufferto disable it. 😭
This diagnostic warns for paths through the code in which "longjmp" is called to rewind to a "jmp_buf" relating to a "setjmp" call in a function that has returned.
When "setjmp" is called on a "jmp_buf" to record a rewind location, it records the stack frame. The stack frame becomes invalid when the function containing the "setjmp" call returns. Attempting to rewind to it via "longjmp" would reference a stack frame that no longer exists, and likely lead to a crash (or worse). 💀
-Wno-analyzer-tainted-allocation-size— this warning requires-fanalyzerwhich enables it; use-Wno-analyzer-tainted-allocation-sizeto disable it. 🔥
This diagnostic warns for paths through the code in which a value that could be under an attacker's control is used as the size of an allocation without being sanitized, so that an attacker could inject an excessively large allocation and potentially cause a denial of service attack.
See CWE-789: Memory Allocation with Excessive Size Value (https://cwe.mitre.org/data/definitions/789.html).
-Wno-analyzer-tainted-assertion— this warning requires-fanalyzerwhich enables it; use-Wno-analyzer-tainted-assertionto disable it. 🎀
This diagnostic warns for paths through the code in which a value that could be under an attacker's control is used as part of a condition without being first sanitized, and that condition guards a call to a function marked with attribute "noreturn" (such as the function __builtin_unreachable). Such functions typically indicate abnormal termination of the program, such as for assertion failure handlers. For example:
assert (some_tainted_value < SOME_LIMIT);In such cases:
- when assertion-checking is enabled: an attacker could trigger a denial of service by injecting an assertion failure
- when assertion-checking is disabled, such as by defining "NDEBUG", an attacker could inject data that subverts the process, since it presumably violates a precondition that is being assumed by the code.
Note that when assertion-checking is disabled, the assertions are typically removed by the preprocessor before the analyzer has a chance to "see" them, so this diagnostic can only generate warnings on builds in which assertion-checking is enabled. 🙈
For the purpose of this warning, any function marked with attribute "noreturn" is considered as a possible assertion failure handler, including __builtin_unreachable. Note that these functions are sometimes removed by the optimizer before the analyzer "sees" them. Hence optimization should be disabled when attempting to trigger this diagnostic.
See CWE-617: Reachable Assertion (https://cwe.mitre.org/data/definitions/617.html).
The warning can also report problematic constructions such as
switch (some_tainted_value) {
case 0:
/* [...etc; various valid cases omitted...] */
break;
default:
__builtin_unreachable (); /* BUG: attacker can trigger this */
}okay so technically the above isn't an assertion failure, strictly speaking 👀 but the analyzer is still side-eyeing it and you should care 💅
-Wno-analyzer-tainted-array-index— bestie this one needs-fanalyzer(which is what turns it on in the first place), and you switch it off with-Wno-analyzer-tainted-array-index✨
It's gonna warn you about paths through your code where a value that could be under an attacker's control gets used as the index of an array access without being sanitized first — meaning an attacker could inject an out-of-bounds access 😭 yikes
See CWE-129: Improper Validation of Array Index ("https://cwe.mitre.org/data/definitions/129.html").
-Wno-analyzer-tainted-divisor— needs-fanalyzerto be on; disable it with-Wno-analyzer-tainted-divisor💖
This one warns for paths where a value that could be attacker-controlled is used as the divisor in a division or modulus operation without being sanitized, so an attacker could inject a division-by-zero 💀
See CWE-369: Divide By Zero ("https://cwe.mitre.org/data/definitions/369.html").
-Wno-analyzer-tainted-offset— requires-fanalyzer; turn it off with-Wno-analyzer-tainted-offset🎀
It flags paths where a value that could be under an attacker's control is used as a pointer offset without being sanitized, so an attacker could inject an out-of-bounds access 😵
See CWE-823: Use of Out-of-range Pointer Offset ("https://cwe.mitre.org/data/definitions/823.html").
-Wno-analyzer-tainted-size— requires-fanalyzer; disable with-Wno-analyzer-tainted-size🦋
This diagnostic warns for paths in which a value that could be under an attacker's control is used as the size of an operation such as memset without being sanitized, so an attacker could inject an out-of-bounds access 😭
See CWE-129: Improper Validation of Array Index ("https://cwe.mitre.org/data/definitions/129.html").
-Wno-analyzer-throw-of-unexpected-type— requires-fanalyzer; use-Wno-analyzer-throw-of-unexpected-typeto disable it 🌸 heads up: dynamic exception specifications are only available in C++14 and earlier ⏳
It warns for paths where an exception is thrown from a function with a dynamic exception specification, and that exception doesn't comply with the spec 🙈
-Wno-analyzer-undefined-behavior-ptrdiff— requires-fanalyzer, which enables it; use-Wno-analyzer-undefined-behavior-ptrdiffto disable it 💜
It warns for paths where a pointer subtraction happens but the pointers refer to different chunks of memory 😬 that kind of code is relying on undefined behavior, because pointer subtraction is only defined when both pointers point within (or just after) the same array 🚨
See CWE-469: Use of Pointer Subtraction to Determine Size ("https://cwe.mitre.org/data/definitions/469.html").
-Wno-analyzer-undefined-behavior-strtok— requires-fanalyzer, which enables it; use-Wno-analyzer-undefined-behavior-strtokto disable it 🎀
It warns for paths where a call is made to strtok with undefined behavior 😵
Specifically: passing NULL as the first parameter for the initial call to strtok within a process has undefined behavior 👑
-Wno-analyzer-unsafe-call-within-signal-handler— requires-fanalyzer, which enables it; use-Wno-analyzer-unsafe-call-within-signal-handlerto disable it 🔥
It warns for paths where a function known to be async-signal-unsafe (like fprintf) gets called from a signal handler 💀
See CWE-479: Signal Handler Use of a Non-reentrant Function ("https://cwe.mitre.org/data/definitions/479.html").
-Wno-analyzer-use-after-free— requires-fanalyzer, which enables it; use-Wno-analyzer-use-after-freeto disable it 🫶
It warns for paths where a pointer is used after a deallocator has been called on it: either free, or a deallocator referenced by attribute malloc 😭
See CWE-416: Use After Free ("https://cwe.mitre.org/data/definitions/416.html").
-Wno-analyzer-use-of-pointer-in-stale-stack-frame— requires-fanalyzer, which enables it; use-Wno-analyzer-use-of-pointer-in-stale-stack-frameto disable it 🧚
It warns for paths where a pointer is dereferenced that points to a variable in a stale stack frame 😳
-Wno-analyzer-va-arg-type-mismatch— requires-fanalyzer, which enables it; use-Wno-analyzer-va-arg-type-mismatchto disable it 💅
It warns for interprocedural paths where the analyzer detects an attempt to use va_arg to extract a value passed to a variadic call, but with a type that doesn't match the type of the expression that was passed to the call 🙈
See CWE-686: Function Call With Incorrect Argument Type ("https://cwe.mitre.org/data/definitions/686.html").
-Wno-analyzer-va-list-exhausted— requires-fanalyzer, which enables it; use-Wno-analyzer-va-list-exhaustedto disable it 🍓
It warns for interprocedural paths where the analyzer catches an attempt to use va_arg to access the next value passed to a variadic call, but all of the values in the va_list have already been consumed 😭
See CWE-685: Function Call With Incorrect Number of Arguments ("https://cwe.mitre.org/data/definitions/685.html").
-Wno-analyzer-va-list-leak— requires-fanalyzer, which enables it; use-Wno-analyzer-va-list-leakto disable it 💜
It warns for interprocedural paths where the analyzer detects that va_start or va_copy was called on a va_list without a corresponding call to va_end 🚨
-Wno-analyzer-va-list-use-after-va-end— requires-fanalyzer, which enables it; use-Wno-analyzer-va-list-use-after-va-endto disable it 🌈
It warns for interprocedural paths where the analyzer detects an attempt to use a va_list after va_end has been called on it. va_list.
-Wno-analyzer-write-to-const— requires-fanalyzer, which enables it; use-Wno-analyzer-write-to-constto disable it 🎀
It warns for paths where the analyzer detects an attempt to write through a pointer to a const object ✍️ but listen: the analyzer doesn't prioritize detection of these paths, so false negatives are more likely relative to other warnings 👀
-Wno-analyzer-write-to-string-literal— requires-fanalyzer, which enables it; use-Wno-analyzer-write-to-string-literalto disable it 🌸
It warns for paths where the analyzer detects an attempt to write through a pointer to a string literal ✍️ same deal: the analyzer doesn't prioritize detecting these paths, so false negatives are more likely relative to other warnings 😬
-Wno-analyzer-use-of-uninitialized-value— requires-fanalyzer, which enables it; use-Wno-analyzer-use-of-uninitialized-valueto disable it 💖
It warns for paths where an uninitialized value is used 😭
See CWE-457: Use of Uninitialized Variable ("https://cwe.mitre.org/data/definitions/457.html").
okay so the analyzer literally has hardcoded knowledge about the behavior of these memory-management functions, bestie 🧠✨
alloca- The built-in functions
__builtin_alloc,__builtin_alloc_with_align,__builtin_calloc,__builtin_free,__builtin_malloc,__builtin_memcpy,__builtin_memcpy_chk,__builtin_memset,__builtin_memset_chk,__builtin_realloc,__builtin_stack_restore, and__builtin_stack_save callocfreemallocmemsetoperator deleteoperator delete []operator newoperator new []reallocstrdupstrndup
of the following functions for working with file descriptors 📂
openclosecreatdup,dup2anddup3isattypipe, andpipe2readwritesocket,bind,listen,accept, andconnect
of the following functions for working with <stdio.h> streams 🌊
- The built-in functions
__builtin_fprintf,__builtin_fprintf_unlocked,__builtin_fputc,__builtin_fputc_unlocked,__builtin_fputs,__builtin_fputs_unlocked,__builtin_fwrite,__builtin_fwrite_unlocked,__builtin_printf,__builtin_printf_unlocked,__builtin_putc,__builtin_putchar,__builtin_putchar_unlocked,__builtin_putc_unlocked,__builtin_puts,__builtin_puts_unlocked,__builtin_vfprintf, and__builtin_vprintf fopenfcloseferrorfgetsfgets_unlockedfilenofreadgetcgetcharfprintfprintffwrite
and of the following functions 💅
- The built-in functions
__builtin_expect,__builtin_expect_with_probability,__builtin_strchr,__builtin_strcpy,__builtin_strcpy_chk,__builtin_strlen,__builtin_va_copy, and__builtin_va_start - The GNU extensions
erroranderror_at_line getpasslongjmpputenvsetjmpsiglongjmpsignalsigsetjmpstrcatstrchrstrlen
plus — various functions with an __analyzer_ prefix have special meaning to the analyzer, and those are described in the GCC Internals manual 📖 Various internal parameters, which you can set via --param, are used to control the exploration; those are also documented in the GCC Internals manual ✨
The following options control the analyzer. 👑
-fanalyzer-assume-nothrow— so by default, if-fexceptionsis on, the analyzer is like "hmm, any function call without thenothrowattribute could throw an exception 💭". That's actually useful — it catches execution paths that leak because exceptions yeeted past the clean-up code. But it can also give you false positives when you're using headers where the author never bothered addingnothrow.
If you turn on -fanalyzer-assume-nothrow, then the analyzer just assumes external functions don't throw exceptions at all. Handy for cutting down the "noise" when you've enabled -fexceptions on C code — but it could hide real issues if something your C code calls actually can raise an exception. So like, trade-off bestie 🫶
-fanalyzer-call-summaries— simplifies interprocedural analysis by computing the effect of certain calls instead of exploring literally every path through the function from callsite to each possible return. If it's enabled, call summaries are only used for functions with more than one call site, and that are sufficiently complicated.-fanalyzer-checker=name— restricts the analyzer to run just the named checker, and enables it. One checker, no side quests 💅-fanalyzer-debug-text-art-headings— this one is for analyzer developers. If enabled, the analyzer adds extra annotations to any diagrams it generates. Little art labels, cute ✨-fno-analyzer-feasibility— this option is intended for analyzer developers.
By default the analyzer verifies that there's a feasible control flow path for each diagnostic it emits — like, that the conditions that hold aren't mutually exclusive. Diagnostics where no feasible path can be found get rejected. You can suppress this filtering with -fno-analyzer-feasibility, for debugging issues in this code.
-fanalyzer-fine-grained— does nothing. Preserved for backward compatibility. It's giving legacy icon that just vibes there 💀-fanalyzer-show-duplicate-count— this one is for analyzer developers: if multiple diagnostics have been detected as being duplicates of each other, it emits a note when reporting the best diagnostic, giving the number of additional diagnostics that got suppressed by the deduplication logic.-fanalyzer-show-events-in-system-headers— by default the analyzer emits simplified diagnostic paths by hiding events fully located within a system header. With-fanalyzer-show-events-in-system-headers, those events are no longer suppressed. Nothing hidden, all cards on the table 🌸-fno-analyzer-simplify-supergraph— this option is intended for analyzer developers.
By default, the analyzer performs various simplifications to the program supergraph before analyzing it. With -fno-analyzer-simplify-supergraph this simplification can be suppressed, for debugging issues with it.
-fno-analyzer-state-merge— this option is intended for analyzer developers.
By default the analyzer attempts to simplify analysis by merging sufficiently similar states at each program point as it builds its "exploded graph". With -fno-analyzer-state-merge this merging can be suppressed, for debugging state-handling issues.
-fno-analyzer-state-purge— this option is intended for analyzer developers.
By default the analyzer attempts to simplify analysis by purging aspects of state at a program point that appear to no longer be relevant — e.g. the values of locals that aren't accessed later in the function and which aren't relevant to leak analysis.
With -fno-analyzer-state-purge this purging of state can be suppressed, for debugging state-handling issues.
-fno-analyzer-suppress-followups— this option is intended for analyzer developers.
By default the analyzer stops exploring an execution path after encountering certain diagnostics, so it doesn't potentially issue a whole cascade of follow-up diagnostics.
The diagnostics that terminate analysis along a path are:
<-Wanalyzer-null-argument><-Wanalyzer-null-dereference><-Wanalyzer-use-after-free><-Wanalyzer-use-of-pointer-in-stale-stack-frame><-Wanalyzer-use-of-uninitialized-value>
With -fno-analyzer-suppress-followups the analyzer keeps exploring such paths even after those diagnostics, which may be helpful for debugging issues in the analyzer, or for microbenchmarks for detecting undefined behavior.
-fanalyzer-transitivity— this option enables transitivity of constraints within the analyzer.-fno-analyzer-undo-inlining— this option is intended for analyzer developers.
-fanalyzer runs relatively late compared to other code analysis tools, and some optimizations have already been applied to the code. In particular function inlining may have occurred, leading to the interprocedural execution paths emitted by the analyzer containing function frames that don't correspond to those in the original source code. 😭
By default the analyzer attempts to reconstruct the original function frames, and to emit events showing the inlined calls.
With -fno-analyzer-undo-inlining this attempt to reconstruct the original frame information can be disabled, which may be of help when debugging issues in the analyzer.
-fanalyzer-verbose-edges— this option is intended for analyzer developers. It enables more verbose, lower-level detail in the descriptions of control flow within diagnostic paths.-fanalyzer-verbose-state-changes— this option is intended for analyzer developers. It enables more verbose, lower-level detail in the descriptions of events relating to state machines within diagnostic paths.-fanalyzer-verbosity=level— this option controls the complexity of the control flow paths that get emitted for analyzer diagnostics.
The level can be one of:
0— at this level, interprocedural call and return events are displayed, along with the most pertinent state-change events relating to a diagnostic. For example, for a double-freediagnostic, both calls tofreewill be shown.1— as per the previous level, but also show events for the entry to each function.2— as per the previous level, but also show events relating to control flow that are significant to triggering the issue (e.g. "true path taken" at a conditional).
This level is the default. ⭐
3— as per the previous level, but show all control flow events, not just significant ones.4— this level is intended for analyzer developers; it adds various other events intended for debugging the analyzer.
-fdump-analyzer— dumps internal details about what the analyzer is doing tofile.analyzer.txt.-fdump-analyzer-stderroverrides this option.-fdump-analyzer-stderr— dumps internal details about what the analyzer is doing tostderr. This option overrides-fdump-analyzer.-fdump-analyzer-callgraph— dumps a representation of the call graph suitable for viewing with GraphViz tofile.callgraph.dot.-fdump-analyzer-exploded-graph— dumps a representation of the "exploded graph" suitable for viewing with GraphViz tofile.eg.dot. Nodes are color-coded based on state-machine states to emphasize state changes. Aesthetic and informative 🎀-fdump-analyzer-exploded-nodes— emits diagnostics showing where nodes in the "exploded graph" are in relation to the program source.-fdump-analyzer-exploded-nodes-2— dumps a textual representation of the "exploded graph" tofile.eg.txt.-fdump-analyzer-exploded-nodes-3— dumps a textual representation of the "exploded graph" to one dump file per node, tofile.eg-id.txt. This is typically a large number of dump files. Like, a lot 🙈-fdump-analyzer-exploded-paths— dumps a textual representation of the "exploded path" for each diagnostic tofile.idx.kind.epath.txt.-fdump-analyzer-feasibility— dumps internal details about the analyzer's search for feasible paths. The details are written in a form suitable for viewing with GraphViz to filenames of the formfile.*.fg.dot,file.*.tg.dot, andfile.*.fpath.txt.-fdump-analyzer-infinite-loop— dumps internal details about the analyzer's search for infinite loops. The details are written in a form suitable for viewing with GraphViz to filenames of the formfile.*.infinite-loop.dot.-fdump-analyzer-json— dumps a compressed JSON representation of analyzer internals tofile.analyzer.json.gz. The precise format is subject to change.-fdump-analyzer-state-purge— as per-fdump-analyzer-supergraph, dump a representation of the "supergraph" suitable for viewing with GraphViz, but annotate the graph with information on what state will be purged at each node. The graph is written tofile.state-purge.dot.-fdump-analyzer-supergraph— dumps representations of the "supergraph" suitable for viewing with GraphViz tofile.supergraph.index.kind.dot. These show all of the control flow graphs in the program, at various stages of the analysis. The precise set of dumps and what they show is subject to change.-fdump-analyzer-untracked— emits custom warnings with internal details intended for analyzer developers.
🐛 Options for Debugging Your Program 🐛
To tell GCC to emit extra information for use by a debugger, in almost all cases you need only to add -g to your other options. Some debug formats can co-exist (like DWARF with CTF) when each of them is enabled explicitly by adding the respective command line option to your other options.
GCC lets you use -g with -O. The shortcuts taken by optimized code may occasionally be surprising: some variables you declared may not exist at all; flow of control may briefly move where you did not expect it; some statements may not be executed because they compute constant results or their values are already at hand; some statements may execute in different places because they have been moved out of loops. Nevertheless it is possible to debug optimized output. This makes it reasonable to use the optimizer for programs that might have bugs. Slay, honestly 🔥
If you are not using some other optimization option, consider using -Og with -g. With no -O option at all, some compiler passes that collect information useful for debugging do not run at all, so that -Og may result in a better debugging experience. Bestie, take the -Og 💖
-g--debug
Produce debugging information in the operating system's native format (stabs, COFF, XCOFF, or DWARF). GDB can work with this debugging information. 👑
On most systems that use stabs format, -g turns on extra debugging info that only GDB can use — and like, it makes debugging in GDB better but probably makes other debuggers crash or flat-out refuse to read the program 😭. If you want to control for certain whether that extra info gets generated, use -gvms (scroll down bestie, it's right there).
-ggdb— makes debugging info specifically for GDB 💅. Meaning: it uses the most expressive format available — DWARF, stabs, or the native format if neither of those is supported — including GDB extensions if that's at all possible.-gdwarf,-gdwarf-version— produces debugging info in DWARF format (if that's supported).versioncan be2,3,4or5; the default version for most targets is5— the exceptions being VxWorks, TPF and Darwin / macOS, which default to version2, and AIX, which defaults to version4.
Heads up: with DWARF Version 2, some ports require and always use some non-conflicting DWARF 3 extensions in the unwind tables. Not optional, it's just how they roll.
Version 4 may require GDB 7.0 and -fvar-tracking-assignments for maximum benefit. Version 5 requires GDB 8.0 or higher.
And no, GCC does not support DWARF Version 1 anymore — it's substantially different from Version 2 and later, periodt. For historical reasons some other DWARF-related options such as -fno-dwarf2-cfi-asm still keep a reference to DWARF Version 2 in their names, but they apply to all currently-supported versions of DWARF. Iconic naming legacy moment 🎀.
-gbtf— requests BTF debug info ✨. BTF is the default debugging format for the eBPF target. On other targets, like x86, BTF debug info can be generated along with DWARF debug info when both of the debug formats are enabled explicitly via their respective command line options.-gprune-btf,-gno-prune-btf— prunes BTF information before emission 🧹. When pruning, only type information for types used by global variables and file-scope functions will be emitted. If you're compiling for the BPF target with BPF CO-RE enabled, type info will also be emitted for types used in BPF CO-RE relocations. On top of that, struct and union types which are only referred to via pointers from members of other struct or union types shall be pruned and replaced withBTF_KIND_FWD, as though those types were only present in the input as forward declarations.
This option substantially reduces the size of produced BTF information, but at a significant loss in the amount of detailed type information. It is primarily useful when compiling for the BPF target, to minimize the size of the resulting object, and to eliminate BTF information which is not immediately relevant to the BPF program loading process.
This option is enabled by default for the BPF target when generating BTF information.
-gctf,-gctflevel— requests CTF debug info and useslevelto specify how much CTF debug info should be produced 🌸. If-gctfis specified without a value forlevel, the default level of CTF debug information is2.
CTF debug info can be generated along with DWARF debug info when both of the debug formats are enabled explicitly via their respective command line options.
- Level
0produces no CTF debug information at all. So-gctf0negates-gctf. - Level
1produces CTF information for tracebacks only. This includes callsite information, but does not include type information. - Level
2produces type information for entities (functions, data objects etc.) at file-scope or global-scope only.
-gvms— produces debugging info in Alpha/VMS debug format (if that's supported). This is the format used byDEBUGon Alpha/VMS systems 🦋.-gcodeview— produces debugging info in CodeView debug format (if that's supported). This is the format used by Microsoft Visual C++ on Windows 💀.-glevel,-ggdblevel,-gvmslevel— requests debugging info and also useslevelto specify how much info. The default level is2.- Level
0produces no debug information at all. Thus-g0negates-g. - Level
1produces minimal information — enough for making backtraces in parts of the program that you don't plan to debug. This includes descriptions of functions and external variables, and line number tables, but no information about local variables. - Level
2is the normal setting to produce debuggable code. The debug output includes all the info from level 1, plus info about local variables and typedefs to allow stepping through code and examining its state. For C++ code, additional information is also emitted to assist in debugging namespace, class, and template functions. - Level
3includes extra information, such as all the macro definitions present in the program. Some debuggers support macro expansion when you use-g3.
- Level
If you use multiple -g options, with or without level numbers, the last such option is the one that's effective. Last one wins, no cap 👑
-gdwarfdoes not accept a concatenated debug level, to avoid confusion with-gdwarf-level. Instead use an additional-gleveloption to change the debug level for DWARF.
-fno-eliminate-unused-debug-symbols— by default, no debug info is produced for symbols that aren't actually used. Use this option if you want debug info for all symbols 🙈.-femit-class-debug-always— instead of emitting debugging info for a C++ class in only one object file, emit it in all object files using the class. This option should be used only with debuggers that are unable to handle the way GCC normally emits debugging info for classes — because using this option increases the size of debugging info by as much as a factor of two 😭.-fno-merge-debug-strings— directs the linker to not merge together strings in the debugging info that are identical in different object files. Merging is not supported by all assemblers or linkers. Merging decreases the size of the debug info in the output file, at the cost of increasing link processing time. Merging is enabled by default.-fdebug-prefix-map=old=new— when compiling files residing in directoryold, record debugging info describing them as if the files resided in directorynewinstead ✨. This can be used to replace a build-time path with an install-time path in the debug info. It can also be used to change an absolute path to a relative path by using.fornew. This can give more reproducible builds, which are location independent, but may require an extra command to tell GDB where to find the source files. See also-ffile-prefix-mapand-fcanon-prefix-map.-fvar-tracking— runs the variable tracking pass. It computes where variables are stored at each position in code. Better debugging info is then generated (if the debugging information format supports this information) 💖.
It is enabled by default when compiling with optimization (-Os, -O, -O2, ...), debugging information (-g) and the debug info format supports it.
-fvar-tracking-assignments— annotates assignments to user variables early in the compilation and attempts to carry the annotations over throughout the compilation all the way to the end, in an attempt to improve debug info while optimizing 🌈. Use of-gdwarf-4is recommended along with it.
It can be enabled even if var-tracking is disabled — in which case annotations are created and maintained, but discarded at the end. By default, this flag is enabled together with -fvar-tracking, except when selective scheduling is enabled.
-fvar-tracking-uninit,-fno-var-tracking-uninit— performs variable tracking and also marks uninitialized variables in the debug info 🎀. This flag is enabled by default by-fvar-tracking; it also implies-fvar-tracking. To do variable tracking without marking uninitialized variables, use-fvar-tracking -fno-var-tracking-uninit.-gsplit-dwarf— if DWARF debugging info is enabled, separate as much debugging info as possible into a separate output file with the extension.dwo✂️. This option allows the build system to avoid linking files with debug info. To be useful, this option requires a debugger capable of reading.dwofiles.-gdwarf32,-gdwarf64— if DWARF debugging info is enabled,-gdwarf32selects the 32-bit DWARF format and-gdwarf64selects the 64-bit DWARF format. The default is target specific — on most targets it's-gdwarf32though. The 32-bit DWARF format is smaller, but can't support more than 2GiB of debug info in any of the DWARF debug information sections. The 64-bit DWARF format allows larger debug info and might not be well supported by all consumers yet 💀.-gdescribe-dies— adds description attributes to some DWARF DIEs that have no name attribute, such as artificial variables, external references and call site parameter DIEs 👑.-gpubnames— generates DWARF.debug_pubnamesand.debug_pubtypessections 🌟.-ggnu-pubnames— generates.debug_pubnamesand.debug_pubtypessections in a format suitable for conversion into a GDB index. This option is only useful with a linker that can produce GDB index version 7.-gno-pubnames— don't generate DWARF.debug_pubnamesand.debug_pubtypessections 🙅.-fdebug-types-section— when using DWARF Version 4 or higher, type DIEs can be put into their own.debug_typessection instead of making them part of the.debug_infosection. It's more efficient to put them in a separate comdat section since the linker can then remove duplicates 🧚. But not all DWARF consumers support.debug_typessections yet, and on some objects.debug_typesproduces larger instead of smaller debugging information. Rude.-grecord-gcc-switches,-gno-record-gcc-switches— this switch causes the command-line options used to invoke the compiler that may affect code generation to be appended to theDW_AT_producerattribute in DWARF debugging info 💅. The options are concatenated with spaces separating them from each other and from the compiler version. It is enabled by default. See also-frecord-gcc-switchesfor another way of storing compiler options into the object file.-gstrict-dwarf— bestie, this says no to using extensions from a later DWARF standard version than the one you picked with-gdwarf-version. On most targets, using non-conflicting DWARF extensions from later standard versions is allowed, but not with this flag on. 🙅♀️-gno-strict-dwarf— the chill version: allow using extensions of later DWARF standard version than the one selected with-gdwarf-version. ✨-gas-loc-support— tells the compiler that the assembler supports.locdirectives, so it can then let the assembler generate DWARF2+ line number tables. This is generally desirable, because assembler-generated line-number tables are a lot more compact than those the compiler can generate itself. 💅 This option is enabled by default if, at GCC configure time, the assembler is found to support such directives.-gno-as-loc-support— forces GCC to generate DWARF2+ line number tables internally, if DWARF2+ line number tables are to be generated. 😤-gas-locview-support— informs the compiler that the assembler supports "view" assignment and reset assertion checking in.locdirectives. Enabled by default if, at GCC configure time, the assembler is found to support them. 👑-gno-as-locview-support— forces GCC to assign view numbers internally, if-gvariable-location-viewsare explicitly requested. 🫡-gcolumn-info,-gno-column-info— emit location column information into DWARF debugging information, rather than just file and line. This option is enabled by default. 📍-gstatement-frontiers,-gno-statement-frontiers— this one makes GCC drop markers in the internal representation at the beginning of statements, and keep them roughly in place throughout compilation, using them to guide the output ofis_stmtmarkers in the line number table. Enabled by default when compiling with optimization (-Os,-O1,-O2, ...), and outputting DWARF 2 debug information at the normal level. 🗺️-gvariable-location-views,-gvariable-location-views=incompat5,-gno-variable-location-views— augment variable location lists with progressive view numbers implied from the line number table. This lets debug information consumers inspect state at certain points of the program, even if no instructions associated with the corresponding source locations are present at that point. 👀
If the assembler lacks support for view numbers in line number tables, this causes the compiler to emit the line number table, which generally makes them somewhat less compact. The augmented line number tables and location lists are fully backward-compatible, so they can be consumed by debug information consumers that are not aware of these augmentations, but they won't derive any benefit from them either. 🙃
This is enabled by default when outputting DWARF 2 debug information at the normal level, as long as there is assembler support, -fvar-tracking-assignments is enabled and -gstrict-dwarf is not. When assembler support is not available, this may still be enabled, but it forces GCC to output internal line number tables, and if -ginternal-reset-location-views is not enabled, that most certainly leads to silently mismatching location views. 💀
There is a proposed representation for view numbers that is not backward compatible with the location list format introduced in DWARF 5, that can be enabled with -gvariable-location-views=incompat5. This option may be removed in the future, is only provided as a reference implementation of the proposed representation. Debug information consumers are not expected to support this extended format, and they would be rendered unable to decode location lists using it. 🚫
-ginternal-reset-location-views,-gno-internal-reset-location-views— attempt to determine location views that can be omitted from location view lists. This requires the compiler to have very accurate instruction length estimates, which isn't always the case, and it may cause incorrect view lists to be generated silently when using an assembler that does not support location view lists. The GNU assembler flags any such error as a "view number mismatch". This is only enabled on ports that define a reliable estimation function. 🧮-ginline-points,-gno-inline-points— generate extended debug information for inlined functions. Location view tracking markers are inserted at inlined entry points, so that address and view numbers can be computed and output in debug information. This can be enabled independently of location views, in which case the view numbers won't be output, but it can only be enabled along with statement frontiers, and it is only enabled by default if location views are enabled. 🎯-gz[=type]— produce compressed debug sections in DWARF format, if that is supported. If type is not given, the default type depends on the capabilities of the assembler and linker used. type may be one ofnone(don't compress debug sections),zlib(use zlib compression in ELF gABI format), orzstd(use zstd compression in ELF gABI format). If the linker doesn't support writing compressed debug sections, the option is rejected. Otherwise, if the assembler does not support them,-gzis silently ignored when producing object files. 🗜️-femit-struct-debug-baseonly— emit debug information for struct-like types only when the base name of the compilation source file matches the base name of file in which the struct is defined.
This option substantially reduces the size of debugging information, but at significant potential loss in type information to the debugger. See -femit-struct-debug-reduced for a less aggressive option. See -femit-struct-debug-detailed for more detailed control.
This option works only with DWARF debug output. 🧱
-femit-struct-debug-reduced— emit debug information for struct-like types only when the base name of the compilation source file matches the base name of file in which the type is defined, unless the struct is a template or defined in a system header.
This option significantly reduces the size of debugging information, with some potential loss in type information to the debugger. See -femit-struct-debug-baseonly for a more aggressive option. See -femit-struct-debug-detailed for more detailed control.
This option works only with DWARF debug output. 🌸
-femit-struct-debug-detailed[=spec-list]— specify the struct-like types for which the compiler generates debug information. The intent is to reduce duplicate struct debug information between different object files within the same program.
This option is a detailed version of -femit-struct-debug-reduced and -femit-struct-debug-baseonly, which serves for most needs.
A specification has the syntax
[dir:|ind:][ord:|gen:](any|sys|base|none)The optional first word limits the specification to structs that are used directly (dir:) or used indirectly (ind:). A struct type is used directly when it is the type of a variable, member. Indirect uses arise through pointers to structs. That is, when use of an incomplete struct is valid, the use is indirect. An example is struct one direct; struct two * indirect;. 🔍
The optional second word limits the specification to ordinary structs (ord:) or generic structs (gen:). Generic structs are a bit complicated to explain. For C++, these are non-explicit specializations of template classes, or non-template classes within the above. Other programming languages have generics, but -femit-struct-debug-detailed does not yet implement them. 🤷♀️
The third word specifies the source files for those structs for which the compiler should emit debug information. The values none and any have the normal meaning. The value base means that the base of name of the file in which the type declaration appears must match the base of the name of the main compilation file. In practice, this means that when compiling foo.c, debug information is generated for types declared in that file and foo.h, but not other header files. The value sys means those types satisfying base or declared in system or compiler headers. 🗂️
You may need to experiment to determine the best settings for your application.
The default is -femit-struct-debug-detailed=all.
This option works only with DWARF debug output. 💖
-fno-dwarf2-cfi-asm— emit DWARF unwind info as compiler generated.eh_framesection instead of using GAS.cfi_*directives. 🔄-fno-eliminate-unused-debug-types— normally, when producing DWARF output, GCC avoids producing debug symbol output for types that are nowhere used in the source file being compiled. Sometimes it is useful to have GCC emit debugging information for all types declared in a compilation unit, regardless of whether or not they are actually used in that compilation unit, for example if, in the debugger, you want to cast a value to a type that is not actually used in your program (but is declared). More often, however, this results in a significant amount of wasted space. 🗑️
🔥 Options That Control Optimization 🔥
These options control various sorts of optimizations. ⚡
Without any optimization option, the compiler's goal is to reduce the cost of compilation and to make debugging produce the expected results. Statements are independent: if you stop the program with a breakpoint between statements, you can then assign a new value to any variable or change the program counter to any other statement in the function and get exactly the results you expect from the source code. 🎯
Turning on optimization flags makes the compiler attempt to improve the performance and/or code size at the expense of compilation time and possibly the ability to debug the program. 🏎️💨
The compiler performs optimization based on the knowledge it has of the program. Compiling multiple files at once to a single output file mode allows the compiler to use information gained from all of the files when compiling each of them. 🧠
Not all optimizations are controlled directly by a flag. Only optimizations that have a flag are listed in this section. 📋
Most optimizations are completely disabled at -O0 or if an -O level is not set on the command line, even if individual optimization flags are specified. Similarly, -Og suppresses many optimization passes. 🛑
Okay so real talk before we dive in: depending on what you're targeting and how your GCC got configured, the exact set of optimizations that get turned on at each -O level might be slightly different from what's listed here. Wanna know the actual truth for your build? Run GCC with -Q --help=optimizers and it will literally spill the whole list of what's enabled at each level. Iconic. ✨
-O,-O1,--optimize— bestie. Optimize. This makes compilation take a bit more time and a lot more memory for a big function, but it's so worth it.
With -O, the compiler tries to shrink your code size and cut execution time, without doing any of the fancy optimizations that eat up a ton of compilation time.
-O is the recommended level for big machine-generated code — it's the sensible balance between compile time and memory use, because higher levels do optimizations with way more algorithmic complexity than -O does.
-O turns on these optimization flags:
-fauto-inc-dec -fbranch-count-reg -fcombine-stack-adjustments
-fcompare-elim -fcprop-registers -fdce -fdefer-pop -fdelayed-branch
-fdse -fforward-propagate -fguess-branch-probability -fif-conversion
-fif-conversion2 -finline-functions-called-once -fipa-modref
-fipa-profile -fipa-pure-const -fipa-reference
-fipa-reference-addressable -fivopts -fmerge-constants
-fmove-loop-invariants -fmove-loop-stores -fomit-frame-pointer
-freorder-blocks -fshrink-wrap -fshrink-wrap-separate
-fsplit-wide-types -fssa-backprop -fssa-phiopt -ftree-bit-ccp
-ftree-ccp -ftree-ch -ftree-coalesce-vars -ftree-copy-prop
-ftree-dce -ftree-dominator-opts -ftree-dse -ftree-forwprop
-ftree-fre -ftree-phiprop -ftree-pta -ftree-scev-cprop -ftree-sink
-ftree-slsr -ftree-sra -ftree-ter -funit-at-a-time-O2— optimize even more, slay. GCC does basically every supported optimization that isn't a space-versus-speed tradeoff. Compared to-O, this bumps up both your compilation time and the performance of the generated code.
-O2 turns on every optimization flag that -O1 specifies, plus these:
-falign-functions -falign-jumps -falign-labels -falign-loops
-fcaller-saves -fcode-hoisting -fcrossjumping -fcse-follow-jumps
-fcse-skip-blocks -fdelete-null-pointer-checks -fdep-fusion
-fdevirtualize -fdevirtualize-speculatively
-fexpensive-optimizations -ffinite-loops -fgcse -fgcse-lm
-fhoist-adjacent-loads -finline-functions -finline-small-functions
-findirect-inlining -fipa-bit-cp -fipa-cp -fipa-icf -fipa-ra
-fipa-sra -fipa-vrp -fisolate-erroneous-paths-dereference
-flra-remat -foptimize-crc -foptimize-sibling-calls
-foptimize-strlen -fpartial-inlining -fpeephole2
-freorder-blocks-algorithm=stc -freorder-blocks-and-partition
-freorder-functions -frerun-cse-after-loop -fschedule-insns
-fschedule-insns2 -fsched-interblock -fsched-spec
-fspeculatively-call-stored-functions -fstore-merging
-fstrict-aliasing -fthread-jumps -ftree-builtin-call-dce
-ftree-loop-vectorize -ftree-pre -ftree-slp-vectorize
-ftree-switch-conversion -ftree-tail-merge -ftree-vrp
-fvect-cost-model=very-cheapNot me letting you miss this: please note the warning under-fgcseabout invoking-O2on programs that use computed gotos. 👀
-O3— optimize even more than that.-O3turns on everything-O2specifies and also flips on these:
-fgcse-after-reload -fipa-cp-clone -floop-interchange
-floop-unroll-and-jam -fpeel-loops -fpredictive-commoning
-fsplit-loops -fsplit-paths -ftree-loop-distribution
-ftree-partial-pre -funswitch-loops -fvect-cost-model=dynamic
-fversion-loops-for-strides-O0— cut down compilation time and make debugging actually produce the results you expect. This is the default, bestie.
At -O0, GCC completely disables most optimization passes. They don't run even if you explicitly enable them on the command line, or even if -Q --help=optimizers says they're enabled by default. Because so many optimizations GCC does depend on code analysis or canonicalization passes that -O turns on, and running individual optimization passes in isolation just wouldn't be useful. It's giving "no thoughts, head empty" but on purpose. 🙈
-Os— optimize for size.-Osenables all the-O2optimizations except the ones that often blow up your code size:
-falign-functions -falign-jumps -falign-labels -falign-loops
-fprefetch-loop-arrays -freorder-blocks-algorithm=stcIt also enables -finline-functions, makes the compiler tune for code size instead of execution speed, and does further optimizations that are designed to shrink your code.
-Ofast— disregard strict standards compliance, periodt.-Ofastenables all-O3optimizations, plus optimizations that are not valid for all standard-compliant programs. It turns on-ffast-math,-fallow-store-data-racesand the Fortran-specific-fstack-arrays(unless-fmax-stack-var-sizeis specified), and-fno-protect-parens. It turns off-fsemantic-interposition.-Og— optimize while keeping the debugging experience in mind.-Ogshould be your go-to optimization level for the standard edit-compile-debug cycle — it offers a reasonable blend of optimization, fast compilation and debugging experience, especially for code with a high abstraction penalty. Unlike-O0, this enables-fvar-tracking-assignmentsand-fvar-tracking, which handle debug info in the prologue and epilogue of functions better than-O0does.
Like -O0, -Og completely skips a bunch of optimization passes, so individual options controlling them have no effect. Otherwise, -Og enables all -O1 optimization flags except the ones known to greatly interfere with debugging:
-fbranch-count-reg -fdelayed-branch -fdse -fif-conversion
-fif-conversion2 -finline-functions-called-once
-fmove-loop-invariants -fmove-loop-stores -fssa-phiopt
-ftree-bit-ccp -ftree-dse -ftree-pta -ftree-sra-Oz— optimize aggressively for size rather than speed. This might increase the number of instructions that get executed, if those instructions need fewer bytes to encode.-Ozbehaves similarly to-Os, including enabling most-O2optimizations. Tiny but mighty. 🦋
If you use multiple -O options, with or without level numbers, the last one wins — that's the one that's effective.
Options of the form -fflag specify machine-independent flags. Most flags have both a positive and a negative form; the negative form of -ffoo is -fno-foo. In the table below, only one of the forms gets listed — the one you'd typically use. You can figure out the other form by either removing the no- or adding it. iykyk 💅
The following options control specific optimizations. They're either activated by -O options or related to ones that are. You can use these flags in the rare cases when you want to "fine-tune" which optimizations actually get performed.
-fno-defer-pop— for machines that have to pop arguments after a function call, always pop the arguments as soon as each function returns. At levels-O1and higher,-fdefer-popis the default; this lets the compiler let arguments accumulate on the stack for several function calls and pop them all at once. Efficiency queen. 👑-fforward-propagate— perform a forward propagation pass on RTL. The pass tries to combine two instructions and checks whether the result can be simplified. If loop unrolling is active, two passes are performed and the second is scheduled after loop unrolling.
This option is enabled by default at optimization levels -O1, -O2, -O3, -Os.
-favoid-store-forwarding,-fno-avoid-store-forwarding— many CPUs will stall for many cycles when a load partially depends on previous smaller stores. This pass tries to detect those cases and dodge the penalty by changing the order of the load and the store, then fixing up the loaded value.
Disabled by default.
-ffp-contract=style —-ffp-contract=offdisables floating-point expression contraction.-ffp-contract=fastenables floating-point expression contraction, like forming fused multiply-add operations if the target has native support for them.-ffp-contract=onenables floating-point expression contraction if the language standard allows it. This is implemented for C and C++, where it enables contraction within one expression, but not across different statements.
The default is -ffp-contract=off for C in a standards compliant mode (-std=c11 or similar), and -ffp-contract=fast otherwise.
-ffp-int-builtin-inexact— allow the built-in functionsceil,floor,roundandtrunc, plus theirfloatandlong doublevariants, to generate code that raises theinexactfloating-point exception for noninteger arguments. ISO C99 and C11 allow these functions to raise theinexactexception, but ISO/IEC TS 18661-1:2014 — the C bindings to IEEE 754-2008, as integrated into ISO C23 — does not allow these functions to do so.
The default is -fno-fp-int-builtin-inexact, disallowing the exception to be raised, unless C17 or an earlier C standard is selected. This option does nothing unless -ftrapping-math is in effect.
Even if -fno-fp-int-builtin-inexact is used, if the functions generate a call to a library function, then the inexact exception may still be raised if the library implementation doesn't follow TS 18661.
-fomit-frame-pointer— omit the frame pointer in functions that don't need one. This avoids the instructions to save, set up and restore the frame pointer; on many targets it also makes an extra register available.
On some targets this flag has no effect, because the standard calling sequence always uses a frame pointer, so it can't be omitted.
Note that -fno-omit-frame-pointer doesn't guarantee the frame pointer is used in all functions. Several targets always omit the frame pointer in leaf functions.
Enabled by default at -O1 and higher.
-foptimize-crc— detect loops calculating CRC (performing polynomial long division) and swap them out for a faster implementation. It detects 8, 16, 32, and 64 bit CRC, with a constant polynomial without the leading 1 bit, for both bit-forward and bit-reversed cases. If the target supports a CRC instruction and the polynomial used in the source code matches the polynomial used in the CRC instruction, it generates that CRC instruction. Otherwise, if the target supports a carry-less-multiplication instruction, it generates CRC using that; otherwise it generates table-based CRC.
Enabled by default at -O2 and higher.
-foptimize-sibling-calls— optimize sibling and tail recursive calls.
...and that last one? Enabled at levels -O2, -O3, -Os. bestie we're picking up mid-sentence and honestly? iconic.
-foptimize-strlen— okay so this one takes all your classic C string girlies — likestrlen,strchrorstrcpy— plus their_FORTIFY_SOURCEcounterparts, and swaps them out for faster alternatives. glow-up behavior. ✨
Enabled at levels -O2, -O3.
-finline-atomics,-fno-inline-atomics— inlines__atomicoperations whenever a lock-free instruction sequence is available. This optimization is enabled by default 💅-finline-stringops[=fn]— expands memory and string operations inline (for now, onlymemsetcounts), even when the length is variable or big enough that it'd normally require looping. Most useful when you're hanging out with-ffreestandingand-fno-builtin. 🫶
Sometimes it lets the compiler generate code that flexes known alignment and length multipliers, but lowkey — even then it may be less efficient than optimized runtime implementations, and it can grow code size so much that a less performant but shared implementation actually runs faster thanks to better use of code caches. This option is disabled by default.
-fno-inline— do not expand any functions inline, apart from the ones marked with thealways_inlineattribute. This is the default when you're not optimizing. 🙈
Single functions can be exempted from inlining by marking them with the noinline attribute.
-finline-small-functions— integrates functions into their callers when the function body is smaller than the expected function call code (so the overall size of the program gets smaller, slay). The compiler heuristically decides which functions are simple enough to be worth integrating this way. And this inlining applies to all functions, even the ones not declared inline.
Enabled at levels -O2, -O3, -Os.
-findirect-inlining— inlines indirect calls too, the ones that get discovered as known-at-compile-time thanks to previous inlining. Heads up: this only does anything when inlining itself is turned on by the-finline-functionsor-finline-small-functionsoptions.
Enabled at levels -O2, -O3, -Os.
-finline-functions— considers all functions for inlining, even if they're not declared inline. The compiler heuristically decides which functions are worth integrating 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 levels -O2, -O3, -Os. Also enabled by -fprofile-use and -fauto-profile.
-finline-functions-called-once— considers allstaticfunctions called once for inlining into their caller, even if they're not markedinline. If a call to a given function gets integrated, then the function is not output as assembler code in its own right.
Enabled at levels -O1, -O2, -O3 and -Os, but not -Og.
-fearly-inlining— inlines functions marked byalways_inline, plus functions whose body seems smaller than the function call overhead, early — before doing-fprofile-generateinstrumentation and the real inlining pass. Doing it this way makes profiling significantly cheaper and usually makes inlining faster on programs with large chains of nested wrapper functions.
Enabled by default. 🌸
-fipa-sra— performs interprocedural scalar replacement of aggregates, removal of unused parameters, and replacement of parameters passed by reference with parameters passed by value. fr fr a whole makeover.
Enabled at levels -O2, -O3 and -Os.
-finline-limit=n— by default, GCC limits the size of functions that can be inlined. This flag lets you coarse-control that limit.nis the size of functions that can be inlined, measured in number of pseudo instructions.
Inlining is actually controlled by a number of internal parameters, which are documented in the GCC Internals manual and should not normally be set directly. 👀
Note: there may be no value to -finline-limit that results in default behavior.
Note: "pseudo instruction" represents, in this particular context, an abstract measurement of a 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 another.
-fno-keep-inline-dllexport— this is a more fine-grained version of-fkeep-inline-functions, which applies only to functions that are declared using thedllexportattribute or declspec.-fkeep-inline-functions— in C, emitsstaticfunctions that are declaredinlineinto the object file, even if the function has been inlined into all of its callers. This switch does not affect functions using theextern inlineextension in GNU C90. In C++, it emits any and all inline functions into the object file.-fkeep-static-functions— emitsstaticfunctions into the object file, even if the function is never used.-fkeep-static-consts— emits variables declaredstatic constwhen 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 whether a variable is referenced, regardless of whether or not optimization is turned on, use the -fno-keep-static-consts option.
-fmerge-constants— attempts 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 -O1, -O2, -O3, -Os.
-fmerge-all-constants— attempts to merge identical constants and identical variables.
This option implies -fmerge-constants. On top of what -fmerge-constants does, it also considers e.g. even constant initialized arrays or initialized constant variables with integral or floating-point types. But listen — languages like C or C++ require each variable, including multiple instances of the same variable in recursive calls, to have distinct locations, so using this option results in non-conforming behavior. 🚨 not a vibe for standard-compliance girlies.
-fmodulo-sched— performs swing modulo scheduling immediately before the first scheduling pass. This pass looks at innermost loops and reorders their instructions by overlapping different iterations. 🔄-fmodulo-sched-allow-regmoves— performs more aggressive SMS-based modulo scheduling with register moves allowed. By setting this flag, certain anti-dependences edges are deleted, which triggers the generation of reg-moves based on the life-range analysis. This option is effective only with-fmodulo-schedenabled.-fno-branch-count-reg— disables the optimization pass that scans for opportunities to use "decrement and branch" instructions on a count register, instead of instruction sequences that decrement a register, compare it against zero, and then branch based on the result. This option is only meaningful on architectures that support such instructions, which include x86, PowerPC, IA-64 and S/390. 📝 Note that the-fno-branch-count-regoption doesn't remove the decrement and branch instructions from the generated instruction stream introduced by other optimization passes.
The default is -fbranch-count-reg at -O1 and higher, except for -Og.
-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
-ffuse-ops-with-volatile-access— allows limited optimization of operations with volatile memory access when doing so does not change the semantics outlined in
The default is -ffuse-ops-with-volatile-access
-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 that 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 except in Ada.
-fthread-jumps— performs optimizations that check to see if a jump branches to a location where another comparison subsumed by the first is found. If so, the first branch is redirected to either the destination of the second branch or a point immediately following it, depending on whether the condition is known to be true or false. 🔀
Enabled at levels -O1, -O2, -O3, -Os.
-fsplit-wide-types— when you're using a type that occupies multiple registers, such aslong longon a 32-bit system, this splits the registers apart and allocates them independently. This normally generates better code for those types, but it may make debugging more difficult. 🐛
Enabled at levels -O1, -O2, -O3, -Os.
-fsplit-wide-types-early— fully splits wide types early, instead of very late. This option has no effect unless-fsplit-wide-typesis turned on.
This is the default on some targets.
-fcse-follow-jumps— in common subexpression elimination (CSE), this scans through jump instructions when the target of the jump is not reached by any other path. For example, when CSE encounters anifstatement with anelseclause, CSE follows the jump when the condition tested is false. 🫣
Enabled at levels -O2, -O3, -Os.
-fcse-skip-blocks— this is similar to-fcse-follow-jumps, but it causes CSE to follow jumps that conditionally skip over blocks. When CSE encounters a simpleifstatement with no else clause,-fcse-skip-blockscauses CSE to follow the jump around the body of theif.
Enabled at levels -O2, -O3, -Os.
-frerun-cse-after-loop— re-runs common subexpression elimination after loop optimizations are performed. running it back, as she should. 💫
Enabled at levels -O2, -O3, -Os.
-fgcse— performs a global common subexpression elimination pass. This pass also performs global constant and copy propagation.
Note: when compiling a program using computed gotos, a GCC extension, you may get better run-time performance if you disable the global common subexpression elimination pass by adding -fno-gcse to the command line.
Enabled at levels -O2, -O3, -Os.
-fgcse-lm— okay so when this one's on, global common subexpression elimination tries to move loads that are only killed by stores into themselves. The payoff? A loop with a load/store sequence can get rewritten so the load lives outside the loop and you just get a copy/store inside it instead. 💫 Enabled by default whenever-fgcseis enabled.-fgcse-sm— turns on a store motion pass that runs after global common subexpression elimination, and it tries to move stores out of loops. Pair it with-fgcse-lmand loops with a load/store sequence can become a load before the loop plus a store after the loop. 🦋 Not enabled at any optimization level, bestie — you gotta ask for it.-fgcse-las— with this on, the global common subexpression elimination pass yeets 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 enabled, a redundant load elimination pass runs after reload, and its whole job is cleaning up redundant spilling. ✨ Enabled by-O3,-fprofile-useand-fauto-profile.-faggressive-loop-optimizations— this tells the loop optimizer to use language constraints to derive bounds for how many times a loop iterates. It assumes your loop code isn't invoking undefined behavior — like, no signed integer overflows, no out-of-bound array accesses, none of that 💀. Those iteration bounds then guide loop unrolling and peeling and loop exit test optimizations. Enabled by default.-funconstrained-commons— tells the compiler that variables declared in common blocks (think Fortran) might later get overridden with longer trailing arrays. This prevents certain optimizations that depend on knowing the array bounds. 🙈-fcrossjumping— performs the cross-jumping transformation, which unifies equivalent code and saves code size. Heads up though: the resulting code may or may not actually perform better than without cross-jumping. Enabled at levels-O2,-O3,-Os.-fauto-inc-dec— combines increments or decrements of addresses with memory accesses. This pass is always skipped on architectures that don't have instructions to support it. Enabled by default at-O1and higher on architectures that do support it. 🔥-fdce— performs dead code elimination (DCE) on RTL. Enabled by default at-O1and higher.-fdse— performs dead store elimination (DSE) on RTL. Enabled by default at-O1and higher.-fif-conversion— attempts to transform conditional jumps into branch-less equivalents. That includes conditional moves, min, max, set flags and abs instructions, plus some tricks that are doable with standard arithmetics. 🎀 The use of conditional execution on chips where it's available is controlled by-fif-conversion2. Enabled at levels-O1,-O2,-O3,-Os, but not with-Og.-fif-conversion2— uses conditional execution (where available) to transform conditional jumps into branch-less equivalents. Enabled at levels-O1,-O2,-O3,-Os, but not with-Og.-fdeclone-ctor-dtor— okay so the C++ ABI requires multiple entry points for constructors and destructors: one for a base subobject, one for a complete object, and one for a virtual destructor that callsoperator deleteafterwards. For a hierarchy with virtual bases, the base and complete variants are clones — meaning two whole copies of the function 😭. With this option, the base and complete variants get changed into thunks that call a common implementation. Enabled by-Os.-fdelete-null-pointer-checks— assumes that programs cannot safely dereference null pointers, and that no code or data element lives at address zero. This enables simple constant folding optimizations at all optimization levels. On top of that, other optimization passes in GCC use this flag to control global dataflow analyses that eliminate useless checks for null pointers — they assume a memory access to address zero always results in a trap, so if a pointer is checked after it's already been dereferenced, it can't be null.
But fr fr, in some environments this assumption is not true. Use -fno-delete-null-pointer-checks to disable this optimization for programs that depend on that behavior.
This option is enabled by default on most targets. On AVR and MSP430, it's completely disabled. Passes that use the dataflow information are enabled independently at different optimization levels. 💅
-fdevirtualize— attempts to convert calls to virtual functions into direct calls. This happens both within a procedure and interprocedurally, as part of indirect inlining (-findirect-inlining) and interprocedural constant propagation (-fipa-cp). Enabled at levels-O2,-O3,-Os. 👑-fdevirtualize-speculatively— attempts to convert calls to virtual functions into speculative direct calls. Based on analysis of the type inheritance graph, it figures out for a given call the set of likely targets. If that set is small — preferably of size 1 — the call gets changed into a conditional deciding between direct and indirect calls. These speculative calls enable more optimizations, like inlining. And when they seem useless after further optimization, they get converted back into their original form. 🌈-fdevirtualize-at-ltrans— streams extra information needed for aggressive devirtualization when running the link-time optimizer in local transformation mode. This option enables more devirtualization but significantly increases the size of streamed data. For this reason it is disabled by default. 💀-fexpensive-optimizations— performs a number of minor optimizations that are relatively expensive. Enabled at levels-O2,-O3,-Os.-fext-dce— performs dead code elimination on zero and sign extensions, with special dataflow analysis. 🧚-fno-ext-dce— turns that off. (Yep, they're two sides of the same coin.)-free— attempts to remove redundant extension instructions. This is especially helpful for the x86-64 architecture, which implicitly zero-extends in 64-bit registers after writing to their lower 32-bit half. ✨ Enabled for Alpha, AArch64, LoongArch, PowerPC, RISC-V, SPARC, h83000 and x86 at levels-O2,-O3,-Os.-fno-lifetime-dse— in C++ the value of an object is only affected by changes within its lifetime: when the constructor begins, the object has an indeterminate value, and any changes during the object's lifetime are dead when the object is destroyed. Normally dead store elimination takes advantage of this. But if your code relies on the value of the object storage persisting beyond the lifetime of the object, you can use this flag to disable the optimization. Want to preserve stores before the constructor starts (like, because youroperator newclears the object storage) but still treat the object as dead after the destructor? Use-flifetime-dse=1. The default behavior can be explicitly selected with-flifetime-dse=2, and-flifetime-dse=0is equivalent to-fno-lifetime-dse. 🫶-flive-range-shrinkage— attempts to decrease register pressure through register live range shrinkage. This is helpful for fast processors with small or moderate size register sets. 🍓- *
-fira-algorithm=algorithm* — uses the specified coloring algorithm for the integrated register allocator. Thealgorithmargument can bepriority, which specifies Chow's priority coloring, orCB, which specifies Chaitin-Briggs coloring. Chaitin-Briggs coloring is not implemented for all architectures, but for those targets that do support it, it's the default because it generates better code. 💖 - *
-fira-region=region* — uses specified regions for the integrated register allocator. Theregionargument should be one of the following:all— use all loops as register allocation regions. This can give the best results for machines with a small and/or irregular register set.mixed— use all loops except for loops with small register pressure as the regions. This value usually gives the best results in most cases and for most architectures, and is enabled by default when compiling with optimization for speed (-O,-O2, ...).one— use all functions as a single region. This typically results in the smallest code size, and is enabled by default for-Osor-O0.
-fira-hoist-pressure— uses IRA to evaluate register pressure in the code hoisting pass for decisions to hoist expressions. This option usually results in smaller code, but it can slow the compiler down. 🐌 Enabled at level-Osfor all targets.-fira-loop-pressure— uses IRA to evaluate register pressure in loops for decisions to move loop invariants. This usually generates faster and smaller code on machines with large register files (>= 32registers), but it can slow the compiler down. Enabled at level-O3for some targets. 🌸-fno-ira-share-save-slots— disables sharing of stack slots used for saving call-used hard registers living through a call. Each hard register gets a separate stack slot, and as a result function stack frames are larger. 😬-fno-ira-share-spill-slots— disables sharing of stack slots allocated for pseudo-registers. Each pseudo-register that does not get a hard register gets a separate stack slot, and as a result function stack frames are larger. 😭-flra-remat— enables CFG-sensitive rematerialization in LRA. Instead of loading values of spilled pseudos, LRA tries to rematerialize (recalculate) values if it's profitable. Enabled at levels-O2,-O3,-Os. 🔥-fdelayed-branch— if supported for the target machine, attempts to reorder instructions to exploit instruction slots available after delayed branch instructions. Enabled at levels-O1,-O2,-O3,-Os, but not at-Og. ⭐
-fschedule-insns— okay so if your target machine supports it, this tries to reorder instructions so you're not just sitting there waiting on data that hasn't shown up yet. It's giving "fill the dead air" energy, and it specifically helps machines with slow floating-point or memory load instructions, because it lets other instructions get issued while the load or floaty instruction is still cooking. 🍳
Conventionally it's on at -O2 and -O3, but bestie, many targets override that. Like on x86 it's disabled at all levels, and on AArch64 it's only enabled at -O3. Targets stay doing their own thing. 💅
-fschedule-insns2— similar vibe to-fschedule-insns, but this one asks for an extra pass of instruction scheduling after register allocation is done. This is especially slay on machines with a small number of registers and where memory load instructions take more than one cycle.
Enabled at levels -O2, -O3, -Os.
-fno-sched-interblock— disables instruction scheduling across basic blocks. That's normally enabled when you're scheduling before register allocation, i.e. with-fschedule-insnsor at-O2or higher.-fno-sched-spec— disables speculative motion of non-load instructions, which is normally enabled when scheduling before register allocation, i.e. with-fschedule-insnsor at-O2or higher.-fsched-pressure— enables register pressure sensitive insn scheduling before register allocation. This only makes sense when scheduling before register allocation is on, i.e. with-fschedule-insnsor at-O2or higher. Using this can improve the generated code and shrink it, by stopping register pressure from climbing above the number of available hard registers and the spills that would follow in register allocation. 🫶-fsched-spec-load— allows speculative motion of some load instructions. Again, only makes sense when scheduling before register allocation, i.e. with-fschedule-insnsor at-O2or higher.-fsched-spec-load-dangerous— allows speculative motion of more load instructions. (yes bestie, it literally has "dangerous" in the name, it said what it said 😭). Only makes sense when scheduling before register allocation, i.e. with-fschedule-insnsor at-O2or higher.-fsched-stalled-insns,-fsched-stalled-insns=n— defines how many insns (if any) can be moved prematurely from the queue of stalled insns into the ready list during the second scheduling pass. So:-fno-sched-stalled-insnsmeans no insns get moved prematurely, and-fsched-stalled-insns=0means there's no limit on how many queued insns can be moved prematurely. And-fsched-stalled-insnswith no value is the same as-fsched-stalled-insns=1.-fsched-stalled-insns-dep,-fsched-stalled-insns-dep=n— defines how many insn groups (cycles) get examined for a dependency on a stalled insn that's a candidate for premature removal from the queue of stalled insns. This only does anything during the second scheduling pass, and only if-fsched-stalled-insnsis being used.-fno-sched-stalled-insns-depis the same as-fsched-stalled-insns-dep=0, and-fsched-stalled-insns-depwith no value is the same as-fsched-stalled-insns-dep=1.-fsched2-use-superblocks— when scheduling after register allocation, use superblock scheduling. This allows motion across basic block boundaries, which gives you faster schedules. ✨ Heads up though: this option is experimental, because not all machine descriptions used by GCC model the CPU closely enough to avoid unreliable results from the algorithm. Not every target is ready for this, bestie.
This only makes sense when scheduling after register allocation, i.e. with -fschedule-insns2 or at -O2 or higher.
-fsched-group-heuristic— enables the group heuristic in the scheduler. This one favors the instruction that belongs to a schedule group. On by default when scheduling is enabled, i.e. with-fschedule-insnsor-fschedule-insns2or at-O2or higher.-fsched-critical-path-heuristic— enables the critical-path heuristic in the scheduler. It favors instructions on the critical path. On by default when scheduling is enabled, i.e. with-fschedule-insnsor-fschedule-insns2or at-O2or higher.-fsched-spec-insn-heuristic— enables the speculative instruction heuristic in the scheduler. It favors speculative instructions with greater dependency weakness. On by default when scheduling is enabled, i.e. with-fschedule-insnsor-fschedule-insns2or at-O2or higher.-fsched-rank-heuristic— enables the rank heuristic in the scheduler. It favors the instruction belonging to a basic block with greater size or frequency. On by default when scheduling is enabled, i.e. with-fschedule-insnsor-fschedule-insns2or at-O2or higher.-fsched-last-insn-heuristic— enables the last-instruction heuristic in the scheduler. It favors the instruction that's less dependent on the last instruction scheduled. On by default when scheduling is enabled, i.e. with-fschedule-insnsor-fschedule-insns2or at-O2or higher.-fsched-dep-count-heuristic— enables the dependent-count heuristic in the scheduler. It favors the instruction that has more instructions depending on it. On by default when scheduling is enabled, i.e. with-fschedule-insnsor-fschedule-insns2or at-O2or higher.-fspeculatively-call-stored-functions— attempts to convert indirect calls of function pointers into pointers loaded from a structure field, if all the visible stores to that field store just a single candidate. When it does that, it turns the call into a conditional that decides between the direct call and the original indirect one. These speculative calls often unlock more optimizations, like inlining. And if they turn out to be useless after further optimization, they get converted back into their original form. 🔁-freschedule-modulo-scheduled-loops— so modulo scheduling happens before traditional scheduling. If a loop is modulo scheduled, later scheduling passes might change its schedule. Use this option to control that behavior.-fselective-scheduling— schedule instructions using the selective scheduling algorithm. Selective scheduling runs instead of the first scheduler pass.-fselective-scheduling2— schedule instructions using the selective scheduling algorithm. Selective scheduling runs instead of the second scheduler pass.-fsel-sched-pipelining— enables software pipelining of innermost loops during selective scheduling. This option does nothing unless one of-fselective-schedulingor-fselective-scheduling2is turned on.-fsel-sched-pipelining-outer-loops— when pipelining loops during selective scheduling, also pipeline outer loops. This option does nothing unless-fsel-sched-pipeliningis turned on.-fsemantic-interposition— some object formats, like ELF, let the dynamic linker interpose symbols. What that means is: for symbols exported from the DSO, the compiler can't do interprocedural propagation, inlining, and other optimizations, because it has to assume the function or variable in question might change. And yeah, this feature is useful — like, say, for rewriting memory allocation functions with a debugging implementation — but it is expensive in terms of code quality. 😭
With -fno-semantic-interposition, the compiler assumes that if interposition happens for functions, the overwriting function will have precisely the same semantics (and side effects). Same deal for variables — if interposition happens for variables, the constructor of the variable will be the same.
This flag has no effect for functions explicitly declared inline (where it's never allowed for interposition to change semantics) and for symbols explicitly declared weak.
-fshrink-wrap— emit function prologues only before the parts of the function that actually need them, instead of dumping them at the top of the function. Enabled by default at-Oand higher.-fshrink-wrap-separate— shrink-wrap separate parts of the prologue and epilogue separately, so those parts only get executed when they're needed. On by default, but it has no effect unless-fshrink-wrapis also turned on and the target supports this.-fcaller-saves— enables allocation of values to registers that get clobbered by function calls, by emitting extra instructions to save and restore the registers around those calls. That allocation only happens when it looks like it'll produce better code.
This option is always enabled by default on certain machines, usually the ones that have no call-preserved registers to use instead.
Enabled at levels -O2, -O3, -Os.
-fcombine-stack-adjustments— tracks stack adjustments (pushes and pops) and stack memory references, then tries to find ways to combine them. Enabled by default at-O1and higher.-fipa-ra— use caller save registers for allocation if those registers aren't used by any called function. In that case there's no need to save and restore them around calls. This is only possible if the called functions are part of the same compilation unit as the current function and they're compiled before it.
Enabled at levels -O2, -O3, -Os — however, the option is disabled if generated code is going to be instrumented for profiling (-p, or -pg) or if the callee's register usage can't be known exactly (this happens on targets that don't expose prologues and epilogues in RTL).
-fconserve-stack— attempt to minimize stack usage. The compiler tries to use less stack space, even if that makes the program slower. This option implies setting thelarge-stack-frameparameter to100and thelarge-stack-frame-growthparameter to400.-ftree-reassoc— perform reassociation on trees. Enabled by default at-O1and higher.-fcode-hoisting— perform code hoisting. Code hoisting tries to move the evaluation of expressions that get executed on all paths to the function exit as early as possible. This is especially useful as a code size optimization, but it often helps with code speed too. Enabled by default at-O2and higher. 🎀-ftree-pre— perform partial redundancy elimination (PRE) on trees. Enabled by default at-O2and-O3.-ftree-partial-pre— make partial redundancy elimination (PRE) more aggressive. Enabled by default at-O3.-ftree-forwprop— does forward propagation on trees ✨ like it's passing the tea along the tree. On by default at-O1and higher, periodt.-ftree-fre— full redundancy elimination (FRE) on trees, bestie 💅. The difference between FRE and PRE? FRE only looks at expressions computed on all paths leading to the redundant computation. It's faster than PRE, but lowkey exposes fewer redundancies. On by default at-O1and higher.-ftree-phiprop— hoists loads from conditional pointers on trees 🦋. This pass is on by default at-O1and higher.-fhoist-adjacent-loads— speculatively hoists loads from both branches of an if-then-else when the loads come from adjacent locations in the same structure and the target architecture has a conditional move instruction 👑. On by default at-O2and higher.-ftree-copy-prop— copy propagation on trees, so those unnecessary copy operations get eliminated, no cap 🧚. On by default at-O1and higher.-fipa-pure-const— discovers which functions are pure or constant 🔥. Enabled by default at-O1and higher.-fipa-reference— discovers which static variables do not escape the compilation unit 🌸. Enabled by default at-O1and higher.-fipa-reference-addressable— discovers read-only, write-only and non-addressable static variables 💖. Enabled by default at-O1and higher.-fipa-reorder-for-locality— groups call chains close together in the binary layout so code locality is better and jump distances between frequently called functions get minimized ⭐. Unlike-freorder-functions, this pass considers the call chains between functions and groups them together, instead of grouping all hot/normal/cold/never-executed functions into separate sections. Unlike-fprofile-reorder-functionsit aims to improve code locality throughout the whole runtime of the program, not just program startup. This option is incompatible with an explicit-flto-partition=option since it enforces a custom partitioning scheme. If you use this option it's recommended to also use profile feedback, but this option is not enabled by default otherwise 🙈.-fipa-stack-alignment— reduces stack alignment on call sites if possible 🎀. Enabled by default.-fipa-pta— interprocedural pointer analysis plus interprocedural modification and reference analysis ✨. Heads up bestie: this one can cause excessive memory and compile-time usage on large compilation units 😭. It is not enabled by default at any optimization level.-fipa-profile— interprocedural profile propagation 💜. Functions called only from cold functions get marked as cold. Also functions executed once (likecold,noreturn, static constructors or destructors) get identified. Cold functions and loopless parts of functions executed once are then optimized for size. Enabled by default at-O1and higher.-fipa-modref— interprocedural mod/ref analysis 🦋. This optimization analyzes the side effects of functions (memory locations that are modified or referenced) and enables better optimization across the function call boundary. Enabled by default at-O1and higher.-fipa-cp— interprocedural constant propagation 🔥. This optimization analyzes the program to figure out when values passed to functions are constants and then optimizes accordingly. It can substantially increase performance if the application has constants passed to functions. Enabled by default at-O2,-Osand-O3. It's also enabled by-fprofile-useand-fauto-profile.-fipa-cp-clone— function cloning to make interprocedural constant propagation stronger 💅. When enabled, interprocedural constant propagation performs function cloning when an externally visible function can be called with constant arguments. Because this optimization can create multiple copies of functions, it may significantly increase code size 😳. Enabled by default at-O3. It's also enabled by-fprofile-useand-fauto-profile.-fipa-bit-cp— when enabled, performs interprocedural bitwise constant propagation ⭐. Enabled by default at-O2and by-fprofile-useand-fauto-profile. It requires that-fipa-cpis enabled.-fipa-vrp— when enabled, performs interprocedural propagation of value ranges 🌈. Enabled by default at-O2. It requires that-fipa-cpis enabled.-fipa-icf-functions-fipa-icf-variables-fipa-icf— performs Identical Code Folding for functions (-fipa-icf-functions), read-only variables (-fipa-icf-variables), or both (-fipa-icf) 🎀. The optimization reduces code size and may disturb unwind stacks by replacing a function with an equivalent one that has a different name. It works more effectively with link-time optimization enabled.
Okay so real talk: even though the behavior is similar to the Gold Linker's ICF optimization, GCC ICF works on different levels and thus the optimizations are not the same — there are equivalences found only by GCC and equivalences found only by Gold 👀.
-fipa-icf is enabled by default at -O2 and -Os.
-flate-combine-instructions— enables two instruction combination passes that run relatively late in the compilation process 🌸. One of the passes runs before register allocation and the other after register allocation. The main aim of the passes is to substitute definitions into all uses.
Most targets enable this flag by default at -O2 and -Os.
-flive-patching=level— controls GCC's optimizations so the output is suitable for live-patching 💀.
Here's the lore: if the compiler's optimization uses a function's body or information extracted from its body to optimize/change another function, that latter one is called an impacted function of the former. So if a function gets patched, its impacted functions should be patched too 👑
The impacted functions are determined by the compiler's interprocedural optimizations. For example, a caller is impacted when inlining a function into its caller, cloning a function and changing its caller to call this new clone, or extracting a function's pureness/constness information to optimize its direct or indirect callers, etc. 🧚
Usually, the more IPA optimizations enabled, the larger the number of impacted functions for each function. In order to control the number of impacted functions and more easily compute the list of impacted functions, IPA optimizations can be partially enabled at two different levels.
The level argument should be one of the following:
inline-clone— only enables inlining and cloning optimizations, which includes inlining, cloning, interprocedural scalar replacement of aggregates and partial inlining 🫶. As a result, when patching a function, all its callers and its clones' callers are impacted, therefore they need to be patched as well.
-flive-patching=inline-clone disables the following optimization flags: -fwhole-program -fipa-pta -fipa-reference -fipa-ra -fipa-icf -fipa-icf-functions -fipa-icf-variables -fipa-bit-cp -fipa-vrp -fipa-pure-const -fipa-reference-addressable -fipa-stack-alignment -fipa-modref
inline-only-static— only enables inlining of static functions 🌟. As a result, when patching a static function, all its callers are impacted and so need to be patched as well.
In addition to all the flags that -flive-patching=inline-clone disables, -flive-patching=inline-only-static disables these additional optimization flags: -fipa-cp-clone -fipa-sra -fpartial-inlining -fipa-cp
When -flive-patching is specified without any value, the default value is inline-clone ✨.
This flag is disabled by default.
Note that-flive-patchingis not supported with link-time optimization (-flto) 😭.
-fisolate-erroneous-paths-dereference— detects paths that trigger erroneous or undefined behavior due to dereferencing a null pointer (with-fdelete-null-pointer-checksenabled) or a division by zero 🚨. It isolates those paths from the main control flow and turns the statement with erroneous or undefined behavior into a trap. Enabled by default at-O2and higher.-fisolate-erroneous-paths-attribute— detects paths that trigger erroneous or undefined behavior due to a null value being used in a way forbidden by areturns_nonnullornonnullattribute 👀. It isolates those paths from the main control flow and turns the statement with erroneous or undefined behavior into a trap. This is not currently enabled, but may be enabled by-O2in the future.-ftree-sink— performs forward store motion on trees 💅. Enabled by default at-O1and higher.-ftree-bit-ccp— performs sparse conditional bit constant propagation on trees and propagates pointer alignment information ⭐. This pass only operates on local scalar variables and is enabled by default at-O1and higher, except for-Og. It requires that-ftree-ccpis enabled.-ftree-ccp— performs sparse conditional constant propagation (CCP) on trees 🌸. This pass only operates on local scalar variables and is enabled by default at-O1and higher.-fssa-backprop— propagates information about uses of a value up the definition chain in order to simplify the definitions 🦋. For example, this pass strips sign operations if the sign of a value never matters. Enabled by default at-O1and higher.-fssa-phiopt— performs pattern matching on SSA PHI nodes to optimize conditional code 🔥. This pass is enabled by default at-O1and higher, except for-Og.-ftree-switch-conversion— converts simple initializations in a switch into initializations from a scalar array ✨. Enabled by default at-O2and higher.-ftree-tail-merge— looks for identical code sequences 👯. When it finds them, it replaces one with a jump to the other. This optimization is known as tail merging or cross jumping. Enabled by default at-O2and higher. The compilation time in this pass can be limited usingmax-tail-merge-comparisonsparameter andmax-tail-merge-iterationsparameter.-ftree-cselim— does conditional store elimination on trees ✨ it's on by default at-O1and up, but only on targets that actually have conditional move instructions 💅-ftree-dce— dead code elimination (DCE) on trees, bestie 🧹 enabled by default at-O1and higher, no cap-ftree-builtin-call-dce— conditional dead code elimination (DCE) for calls to built-in functions that might seterrnobut are otherwise completely side-effect free 🫶 on by default at-O2and higher, as long as-Osisn't also specified-ffinite-loops— assume that a loop with an exit will eventually take that exit and not loop forever 🔥 this lets the compiler delete loops that otherwise have no side-effects, because it stops counting "eventual endless looping" as a side effect
Enabled by default at -O2 for C++ with -std=c++11 or higher 👑
-ftree-dominator-opts— does a whole lil spa day of simple scalar cleanups (constant/copy propagation, redundancy elimination, range propagation and expression simplification) based on a dominator tree traversal 💖 it also does jump threading (reducing jumps to jumps). On by default at-O1and higher-ftree-dse— dead store elimination (DSE) on trees. A dead store is a store into a memory location that gets overwritten by another store later with no loads in between 🙈 so the earlier store can just be deleted. Enabled by default at-O1and higher-ftree-ch— loop header copying on trees 🌸 it's a win because it makes code motion optimizations more effective, and it also saves one jump. On by default at-O1and higher. Not enabled for-Os, because it usually makes the code bigger 😭-ftree-loop-optimize— loop optimizations on trees, enabled by default at-O1and higher ✨-ftree-loop-linear-floop-strip-mine-floop-block— loop nest optimizations. Same as-floop-nest-optimize💫 to use this code transformation, GCC has to be configured with--with-islto enable the Graphite loop transformation infrastructure-fgraphite-identity— enables the identity transformation for graphite. For every SCoP we generate the polyhedral representation and transform it back to gimple 🦋 with-fgraphite-identitywe can check the costs or benefits of the GIMPLE -> GRAPHITE -> GIMPLE transformation. The isl code generator also does some minimal optimizations, like index splitting and dead code elimination in loops-floop-nest-optimize— enables the isl based loop nest optimizer 🎀 it's a generic loop nest optimizer based on the Pluto optimization algorithms. It calculates a loop structure optimized for data-locality and parallelism. This option is experimental ⚠️-floop-parallelize-all— use the Graphite data dependence analysis to find loops that can be parallelized. Parallelize all the loops that can be analyzed to not contain loop carried dependences, without even checking whether parallelizing them is profitable 💀-ftree-coalesce-vars— while transforming the program out of the SSA representation, try to reduce copying by coalescing versions of different user-defined variables instead of just compiler temporaries 🫶 this can severely limit your ability to debug an optimized program compiled with-fno-var-tracking-assignments😭 in the negated form, this flag prevents SSA coalescing of user variables. Enabled by default if optimization is enabled, and it does very little otherwise-ftree-loop-if-convert— tries to transform conditional jumps in the innermost loops into branch-less equivalents ✨ the goal is to remove control-flow from the innermost loops so the vectorization pass has an easier time handling them. Enabled by default if vectorization is enabled-ftree-loop-distribution— performs loop distribution 🌈 this flag can improve cache performance on big loop bodies and let further loop optimizations (like parallelization or vectorization) happen. For example, the loop
DO I = 1, N
A(I) = B(I) + C
D(I) = E(I) * F
ENDDOgets transformed to
DO I = 1, N
A(I) = B(I) + C
ENDDO
DO I = 1, N
D(I) = E(I) * F
ENDDOEnabled by default at -O3 👑 it's also enabled by -fprofile-use and -fauto-profile
-ftree-loop-distribute-patterns— does loop distribution of patterns that can be code generated with calls to a library 💅 enabled by default at-O2and higher, and by-fprofile-useand-fauto-profile
This pass distributes the initialization loops and generates a call to memset zero 🧚 for example, the loop
DO I = 1, N
A(I) = 0
B(I) = A(I) + I
ENDDOis transformed to
DO I = 1, N
A(I) = 0
ENDDO
DO I = 1, N
B(I) = A(I) + I
ENDDOand the initialization loop becomes a call to memset zero ✨
-floop-interchange— performs loop interchange outside of graphite 💫 this flag can improve cache performance on loop nests and let further loop optimizations (like vectorization) happen. For example, the loop
for (int i = 0; i < N; i++)
for (int j = 0; j < N; j++)
for (int k = 0; k < N; k++)
c[i][j] = c[i][j] + a[i][k]*b[k][j];is transformed to
for (int i = 0; i < N; i++)
for (int k = 0; k < N; k++)
for (int j = 0; j < N; j++)
c[i][j] = c[i][j] + a[i][k]*b[k][j];Enabled by default at -O3 🔥 and also by -fprofile-use and -fauto-profile
-floop-unroll-and-jam— applies unroll and jam transformations on feasible loops 🎀 in a loop nest this unrolls the outer loop by some factor and fuses the resulting multiple inner loops. Enabled by default at-O3, and also by-fprofile-useand-fauto-profile-ftree-loop-im— performs loop invariant motion on trees. This pass only moves invariants that are hard to handle at RTL level (function calls, operations that expand to nontrivial sequences of insns) 💅 with-funswitch-loopsit also moves operands of conditions that are invariant out of the loop, so we can use just trivial invariantness analysis in loop unswitching. The pass also includes store motion ✨-ftree-loop-ivcanon— creates a canonical counter for the number of iterations in loops where figuring out the iteration count requires complicated analysis 🦋 later optimizations can then determine the number easily. Especially useful in connection with unrolling-ftree-scev-cprop— performs final value replacement 🌸 if a variable is modified in a loop in such a way that its value on exiting the loop can be determined using only its initial value and the number of loop iterations, then uses of the final value get replaced by that computation — as long as it's sufficiently cheap. This reduces data dependencies and may allow further simplifications. Enabled by default at-O1and higher-fivopts— performs induction variable optimizations (strength reduction, induction variable merging and induction variable elimination) on trees 💖 enabled by default at-O1and higher-ftree-parallelize-loops-ftree-parallelize-loops=n— parallelize loops, i.e. split their iteration space to run in multiple threads 🧵 this is only possible for loops whose iterations are independent and can be arbitrarily reordered. The optimization is only profitable on multiprocessor machines, for loops that are CPU-intensive rather than constrained by e.g. memory bandwidth. This option implies-pthread, and so it's only supported on targets that support-pthread⚠️
When a positive value n is specified, the number of threads is fixed at compile time and can't be changed after compilation. The compiler generates #pragma omp parallel num_threads(n) 👑
When used without =n (i.e., -ftree-parallelize-loops), the number of threads is determined at program execution time via the OMP_NUM_THREADS environment variable 🌈 if OMP_NUM_THREADS isn't set, the OpenMP runtime automatically detects the number of available processors and uses that value. This lets you create binaries that adapt to different hardware configurations without recompiling ✨
-ftree-pta— performs function-local points-to analysis on trees 💅 enabled by default at-O1and higher, except for-Og-ftree-sra— performs scalar replacement of aggregates. This pass replaces structure references with scalars to stop structures from getting committed to memory too early 🎀 enabled by default at-O1and higher, except for-Og-fstore-merging— merges narrow stores to consecutive memory addresses 🧚 this pass merges contiguous stores of immediate values narrower than a word into fewer wider stores to reduce the number of instructions. Enabled by default at-O2and higher as well as-Os-ftree-ter— performs temporary expression replacement during the SSA->normal phase 🦋 single use/single def temporaries get 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. Enabled by default at-O1and higher-ftree-slsr— performs straight-line strength reduction on trees 💖 this recognizes related expressions involving multiplications and replaces them with less expensive calculations when possible. Enabled by default at-O1and higher-ftree-vectorize— performs vectorization on trees 🔥 this flag enables-ftree-loop-vectorizeand-ftree-slp-vectorizeif they're not explicitly specified-ftree-loop-vectorize— performs loop vectorization on trees ✨ enabled by default at-O2and by-ftree-vectorize,-fprofile-use, and-fauto-profile-ftree-slp-vectorize— does basic block vectorization on trees 🦋. It's on by default at-O2, and it also gets switched on by-ftree-vectorize,-fprofile-use, and-fauto-profile. Slay.-ftrivial-auto-var-init=choice— bestie this one is so good. It initializes automatic variables or temporary objects with either a pattern or with zeroes, which bumps up the security and predictability of a program by preventing uninitialized memory disclosure and use 💅. BUT — important — GCC still considers an automatic variable that doesn't have an explicit initializer as uninitialized.-Wuninitializedand-Wanalyzer-use-of-uninitialized-valuewill still report warning messages on such automatic variables or temporary objects, and the compiler will perform optimization as if the variable were uninitialized. With this option, GCC will also initialize any padding of automatic variables or temporary objects that have structure or union types to zeroes. However, the current implementation cannot initialize automatic variables whose initialization is bypassed throughswitchorgotostatement. Use-Wtrivial-auto-var-initto report all such cases.
The three values of choice are:
uninitialized— doesn't initialize any automatic variables.pattern— initialize automatic variables with values which will likely transform logic bugs into crashes down the line, are easily recognized in a crash dump, and without being values that programmers can rely on for useful program semantics. The current value is byte-repeatable pattern with byte"0xFE". The values used for pattern initialization might be changed in the future 👀.zero— initialize automatic variables with zeroes.
The default is uninitialized except for C++26, in which case if -ftrivial-auto-var-init= is not specified at all, automatic variables or temporary objects are zero initialized, but zero initialization of padding bits does not happen.
Note that the initializer values, whether zero or pattern, refer to data representation (in memory or machine registers), rather than to their interpretation as numerical values. This distinction may be important in languages that support types with biases or implicit multipliers, and with such extensions as hardbool. For example, a variable that uses 8 bits to represent (biased) quantities in the range 160..400 will be initialized with the bit patterns 0x00 or 0xFE, depending on choice, whether or not these representations stand for values in that range, and even if they do, the interpretation of the value held by the variable will depend on the bias. A hardbool variable that uses say 0x5A and 0xA5 for "false" and "true", respectively, will trap with either choice of trivial initializer, i.e., zero initialization will not convert to the representation for "false", even if it would for a "static" variable of the same type. This means the initializer pattern doesn't generally depend on the type of the initialized variable. One notable exception is that (non-hardened) boolean variables that fit in registers are initialized with "false" (zero), even when pattern is requested. 🙈
You can control this behavior for a specific variable by using the variable attribute "uninitialized" standard attribute or the C++26 "[[indeterminate]]". ✨
-fvect-cost-model=model— changes up the cost model used for vectorization 💫. The model argument should be one ofunlimited,dynamic,cheaporvery-cheap. With theunlimitedmodel the vectorized code-path is assumed to be profitable, while with thedynamicmodel a runtime check guards the vectorized code-path to enable it only for iteration counts that will likely execute faster than when executing the original scalar loop. Thecheapmodel disables vectorization of loops where doing so would be cost prohibitive — for example due to required runtime checks for data dependence or alignment — but otherwise is equal to thedynamicmodel. Thevery-cheapmodel disables vectorization of loops when any runtime check for data dependence or alignment is required, and it also disables vectorization of epilogue loops, but otherwise is equal to thecheapmodel.
The default cost model depends on other optimization flags and is either dynamic or cheap. iykyk 🎀
-fsimd-cost-model=model— changes the cost model used for vectorization of loops marked with the OpenMPsimddirective 🌈. The model argument should be one ofunlimited,dynamic,cheap. All values of model have the same meaning as described in-fvect-cost-model, and by default a cost model defined with-fvect-cost-modelis used.-ftree-vrp— performs 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. It's enabled by default at-O2and higher. Null pointer check elimination is only done if-fdelete-null-pointer-checksis enabled. no cap 🫶-fsplit-paths— splits paths leading to loop backedges 🔀. This can improve dead code elimination and common subexpression elimination. It's enabled by default at-O3and above.-fsplit-ivs-in-unroller— enables expression of values of induction variables in later iterations of the unrolled loop using the value in the first iteration 🌀. This breaks long dependency chains, thus improving efficiency of the scheduling passes. literally iconic.
A combination of -fweb and CSE is often sufficient to obtain the same effect. However, that is not reliable in cases where the loop body is more complicated than a single basic block. It also does not work at all on some architectures due to restrictions in the CSE pass.
This optimization is enabled by default.
-fvariable-expansion-in-unroller— with this option, the compiler creates multiple copies of some local variables when unrolling a loop, which can result in superior code 👑.
This optimization is enabled by default for PowerPC targets, but disabled by default otherwise.
-fpartial-inlining— inline parts of functions ✂️. This option has any effect only when inlining itself is turned on by the-finline-functionsor-finline-small-functionsoptions.
Enabled at levels -O2, -O3, -Os.
-fpredictive-commoning— performs predictive commoning optimization, i.e., reusing computations (especially memory loads and stores) performed in previous iterations of loops 🔁.
This option is enabled at level -O3. It's also enabled by -fprofile-use and -fauto-profile.
-fprefetch-loop-arrays— if supported by the target machine, generates instructions to prefetch memory to improve the performance of loops that access large arrays 🚀.
This option may generate better or worse code; results are highly dependent on the structure of loops within the source code. 🙃
Disabled at level -Os.
-fno-printf-return-value— do not substitute constants for known return value of formatted output functions such assprintf,snprintf,vsprintf, andvsnprintf(but notprintfoffprintf) 🙅♀️. This transformation allows GCC to optimize or even eliminate branches based on the known return value of these functions called with arguments that are either constant, or whose values are known to be in a range that makes determining the exact return value possible. For example, when-fprintf-return-valueis in effect, both the branch and the body of theifstatement (but not the call tosnprint) can be optimized away wheniis a 32-bit or smaller integer because the return value is guaranteed to be at most 8.
char buf[9];
if (snprintf (buf, "%08x", i) >= sizeof buf)
...The -fprintf-return-value option relies on other optimizations and yields best results with -O2 and above. It works in tandem with the -Wformat-overflow and -Wformat-truncation options. The -fprintf-return-value option is enabled by default.
-fno-peephole,-fno-peephole2— disable any machine-specific peephole optimizations 👀. The difference between-fno-peepholeand-fno-peephole2is in how they are implemented in the compiler; some targets use one, some use the other, a few use both.
-fpeephole is enabled by default. -fpeephole2 enabled at levels -O2, -O3, -Os.
-fno-guess-branch-probability— do not guess branch probabilities using heuristics 🎲.
GCC uses heuristics to guess branch probabilities if they are not provided by profiling feedback (-fprofile-arcs). These heuristics are based on the control flow graph. If some branch probabilities are specified by "__builtin_expect", then the heuristics are used to guess branch probabilities for the rest of the control flow graph, taking the "__builtin_expect" info into account. The interactions between the heuristics and "__builtin_expect" can be complex, and in some cases, it may be useful to disable the heuristics so that the effects of "__builtin_expect" are easier to understand.
It's also possible to specify expected probability of the expression with "__builtin_expect_with_probability" built-in function.
The default is -fguess-branch-probability at levels -O, -O2, -O3, -Os.
-freorder-blocks— reorders basic blocks in the compiled function in order to reduce number of taken branches and improve code locality 🧚.
Enabled at levels -O1, -O2, -O3, -Os.
-freorder-blocks-algorithm=algorithm— use the specified algorithm for basic block reordering 🧩. The algorithm argument can besimple, which does not increase code size (except sometimes due to secondary effects like alignment), orstc, the "software trace cache" algorithm, which tries to put all often executed code together, minimizing the number of branches executed by making extra copies of code.
The default is simple at levels -O1, -Os, and stc 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.ofiles, to improve paging and cache locality performance 💖.
okay so this optimization straight-up ghosts you the moment any of these show up: exception handling or unwind tables (on targets that use setjump/longjump or some target-specific scheme), linkonce sections, functions with a user-defined section attribute, and honestly any architecture that doesn't do named sections. And when -fsplit-stack is in the mix it's not on by default (so you don't get linker errors, bestie), but you can totally turn it on explicitly — as long as your linker actually works. 💅
Enabled for x86 at levels -O2, -O3, -Os.
-freorder-functions— shuffles functions around in the object file so the code lives closer together and your CPU's little heart is happy (code locality, we love her). Unlike-fipa-reorder-for-locality, this one's whole vibe is grouping all the functions in a category — hot / normal / cold / never-executed — together. It pulls this off with special subsections:.text.hotfor the functions that run the most, and.text.unlikelyfor the ones that basically never get executed. Plot twist: the actual reordering is done by the linker, so your object file format needs named sections and your linker has to place them in a reasonable way. 🙈
This option is giving nothing unless you either hand over profile feedback (peep -fprofile-arcs for the deets) or manually annotate your functions with "hot" or "cold" attributes. No cap.
Enabled at levels -O2, -O3, -Os.
-fstrict-aliasing— lets the compiler assume the strictest aliasing rules that apply to whatever language you're compiling. For C (and C++), this switches on optimizations based on the type of expressions. Specifically: accessing an object of one type through an expression of a different type is not allowed, unless the types are compatible, or differ only in signedness or qualifiers, or the expression has a character type. Accessing scalar objects via a corresponding vector type is also allowed, periodt.
For example, an unsigned int can alias an int, but not a void* or a double. A character type may alias any other type. ✨
Pay special attention to code like this:
union a_union {
int i;
double d;
};
int f() {
union a_union t;
t.d = 3.0;
return t.i;
}Reading from a different union member than the one you most recently wrote to — aka "type-punning" — is super common. And even with -fstrict-aliasing, type-punning is allowed in C, as long as the memory is accessed through the union type. In ISO C++, type-punning through a union type is undefined behavior, but GCC supports it as an extension. So the code above works as expected. 🫶
However, this code might not:
int f() {
union a_union t;
int* ip;
t.d = 3.0;
ip = &t.i;
return *ip;
}Similarly, accessing by taking the address, casting the resulting pointer and then dereferencing the result has undefined behavior — even if the cast uses a union type, like:
int f() {
double d = 3.0;
return ((union a_union *) &d)->i;
}The -fstrict-aliasing option is enabled at levels -O2, -O3, -Os.
-fipa-strict-aliasing— controls whether the rules of-fstrict-aliasingget applied across function boundaries. Heads up tho: if multiple functions get inlined into a single function, those memory accesses are no longer considered to be crossing a function boundary. 🦋
The -fipa-strict-aliasing option is enabled by default and is effective only in combination with -fstrict-aliasing.
-falign-functions-falign-functions=n-falign-functions=n:m-falign-functions=n:m:n2-falign-functions=n:m:n2:m2— aligns the start of functions to the next power-of-two greater than or equal to n, skipping up to m-1 bytes. This makes sure at least the first m bytes of the function can be fetched by the CPU without crossing an n-byte alignment boundary. It's an optimization of code performance, and alignment is ignored for functions considered cold. If you need alignment for all functions, use-fmin-function-alignmentinstead. 💖
If m isn't specified, it defaults to n.
Examples, bestie: -falign-functions=32 aligns functions to the next 32-byte boundary; -falign-functions=24 aligns to the next 32-byte boundary only if it can do it by skipping 23 bytes or less; -falign-functions=32:7 aligns to the next 32-byte boundary only if it can be done by skipping 6 bytes or less.
The second pair of n2:m2 values lets you specify a secondary alignment: -falign-functions=64:7:32:3 aligns to the next 64-byte boundary if that can be done by skipping 6 bytes or less, otherwise it aligns to the next 32-byte boundary if that can be done by skipping 2 bytes or less. If m2 isn't specified, it defaults to n2.
Some assemblers only support this flag when n is a power of two — in that case, it gets rounded up.
-fno-align-functions and -falign-functions=1 are equivalent and mean functions are not aligned.
If n isn't specified or is zero, a machine-dependent default is used. The maximum allowed n option value is 65536.
Enabled at levels -O2, -O3.
-flimit-function-alignment— with this one on, the compiler tries to avoid unnecessarily overaligning functions. It attempts to instruct the assembler to align by the amount specified by-falign-functions, but not to skip more bytes than the size of the function. 🎀-falign-labels-falign-labels=n-falign-labels=n:m-falign-labels=n:m:n2-falign-labels=n:m:n2:m2— aligns all branch targets to a power-of-two boundary.
The parameters of this option are analogous to the -falign-functions option. -fno-align-labels and -falign-labels=1 are equivalent and mean labels are not aligned.
If -falign-loops or -falign-jumps are applicable and are greater than this value, then their values are used instead.
If n isn't specified or is zero, a machine-dependent default is used — which is very likely to be 1, meaning no alignment. The maximum allowed n option value is 65536.
Enabled at levels -O2, -O3.
-falign-loops-falign-loops=n-falign-loops=n:m-falign-loops=n:m:n2-falign-loops=n:m:n2:m2— aligns loops to a power-of-two boundary. If the loops are executed many times, this makes up for any execution of the dummy padding instructions. It's an optimization of code performance, and alignment is ignored for loops considered cold. 🌸
If -falign-labels is greater than this value, then its value is used instead.
The parameters of this option are analogous to the -falign-functions option. -fno-align-loops and -falign-loops=1 are equivalent and mean loops are not aligned. The maximum allowed n option value is 65536.
If n isn't specified or is zero, a machine-dependent default is used.
Enabled at levels -O2, -O3.
-falign-jumps-falign-jumps=n-falign-jumps=n:m-falign-jumps=n:m:n2-falign-jumps=n:m:n2:m2— aligns branch targets to a power-of-two boundary, for branch targets where the targets can only be reached by jumping. In this case, no dummy operations need be executed. It's an optimization of code performance, and alignment is ignored for jumps considered cold. 🧚
If -falign-labels is greater than this value, then its value is used instead.
The parameters of this option are analogous to the -falign-functions option. -fno-align-jumps and -falign-jumps=1 are equivalent and mean that loops are not aligned.
If n isn't specified or is zero, a machine-dependent default is used. The maximum allowed n option value is 65536.
Enabled at levels -O2, -O3.
-fmin-function-alignment— specifies the minimal alignment of functions to the next power-of-two greater than or equal to n. Unlike-falign-functions, this alignment gets applied to all functions (yes, even the ones considered cold 👑). The alignment is also not affected by-flimit-function-alignment.-fno-allocation-dce— do not remove unused C++ allocations (usingoperator "new"andoperator "delete") in dead code elimination.
See also -fmalloc-dce.
-fallow-store-data-races— allows the compiler to perform optimizations that may introduce new data races on stores, without proving that the variable can't be concurrently accessed by other threads. Doesn't affect optimization of local data. It's safe to use this option if you know global data won't be accessed by multiple threads. 💀
Examples of optimizations enabled by -fallow-store-data-races include hoisting or if-conversions that may cause a value that was already in memory to be re-written with that same value. That re-writing is safe in a single-threaded context but may be unsafe in a multi-threaded context. Note that on some processors, if-conversions may be required in order to enable vectorization.
Enabled at level -Ofast.
-funit-at-a-time— this option is left for compatibility reasons.-funit-at-a-timehas no effect, while-fno-unit-at-a-timeimplies-fno-toplevel-reorderand-fno-section-anchors.
Enabled by default.
-fno-toplevel-reorder— do not reorder top-level functions, variables, andasmstatements. Output them in the same order that they appear in the input file. When this option is used, unreferenced static variables are not removed. This option is intended to support existing code that relies on a particular ordering. For new code, it is better to use attributes when possible. 🌈
-ftoplevel-reorder is the default at -O1 and higher, and also at -O0 if -fsection-anchors is explicitly requested. Additionally -fno-toplevel-reorder implies -fno-section-anchors.
-funreachable-traps— with this option, the compiler turns calls to__builtin_unreachableinto traps, instead of using them for optimization. This also affects any such calls implicitly generated by the compiler. ⭐
This option has the same effect as -fsanitize=unreachable -fsanitize-trap=unreachable, but does not affect the values of those options. If -fsanitize=unreachable is enabled, that option takes priority over this one.
This option is enabled by default at -O0 and -Og.
-fweb— builds "webs" like register allocation does, and gives each web its own lil pseudo register 💅. That means the register allocation pass can work on pseudos directly, and it also boosts a bunch of other optimization passes like CSE, the loop optimizer, and the trivial dead code remover. Downside bestie: it can make debugging literally impossible, because variables stop living in a "home register" 😭.
Enabled by default with -funroll-loops.
-fwhole-program— acts like this one compilation unit is the entire program, no cap. Every public function and variable — exceptmainand anything merged by the attributeexternally_visible— becomes static, which means the interprocedural optimizers can go absolutely feral on it 🔥.
With -flto this option has limited use. In most cases the precise list of symbols used or exported from the binary is known from the resolution info the linker plugin hands to the link-time optimizer. It's still useful if no linker plugin is used, or during an incremental link step when final code is produced (with -flto -flinker-output=nolto-rel).
✨ and now… the LTO section, aka the main character ✨
-flto[=n]— okay so this runs the standard link-time optimizer 👑. When you invoke it with source code, it generates GIMPLE (one of GCC's internal representations) and writes it into special ELF sections inside the object file. Then when those object files get linked together, all the function bodies are read back out of those ELF sections and instantiated as if they'd all been part of the same translation unit. iconic.
To use the link-time optimizer, -flto and your optimization options should be specified at compile time and during the final link. It's recommended that you compile all the files participating in the same link with the same options, and also specify those options at link time. Like, for example:
gcc -c -O2 -flto foo.c
gcc -c -O2 -flto bar.c
gcc -o myprog -flto -O2 foo.o bar.oThe first two invocations to GCC save a bytecode representation of GIMPLE into special ELF sections inside foo.o and bar.o. The final invocation reads the GIMPLE bytecode from foo.o and bar.o, merges the two files into a single internal image, and compiles the result as usual. Since both foo.o and bar.o are merged into a single image, all the interprocedural analyses and optimizations in GCC now work across the two files as if they were one 🫶. So like, the inliner can inline functions in bar.o into functions in foo.o and vice-versa. besties working together fr fr.
Another (simpler) way to enable link-time optimization is:
gcc -o myprog -flto -O2 foo.c bar.cThat generates bytecode for foo.c and bar.c, merges them into a single GIMPLE representation and optimizes them as usual to produce myprog.
The important thing to keep in mind — and bestie this is the important thing — is that to enable link-time optimizations you need to use the GCC driver to perform the link step. GCC automatically performs link-time optimization if any of the objects involved were compiled with the -flto command-line option. You can always override that automatic decision by passing -fno-lto to the link command.
To make whole-program optimization effective, it's necessary to make certain assumptions. The compiler needs to know what functions and variables can be accessed by libraries and runtime outside of the link-time optimized unit. When the linker supports it, the linker plugin (see -fuse-linker-plugin) passes information to the compiler about used and externally visible symbols. When the linker plugin isn't available, -fwhole-program should be used to let the compiler make these assumptions, which leads to more aggressive optimization decisions.
When a file is compiled with -flto without -fuse-linker-plugin, the generated object file is bigger than a regular object file, because it contains GIMPLE bytecodes and the usual final code (see -ffat-lto-objects). So object files with LTO information can be linked as normal object files; if -fno-lto is passed to the linker, no interprocedural optimizations get applied. Note that when -fno-fat-lto-objects is enabled the compile stage is faster, but you cannot perform a regular, non-LTO link on them 😬.
When producing the final binary, GCC only applies link-time optimizations to those files that contain bytecode. So you can absolutely mix and match object files and libraries with GIMPLE bytecodes and final object code 💅. GCC automatically picks which files to optimize in LTO mode and which files to link without further processing.
Generally, options specified at link time override those specified at compile time, although in some cases GCC tries to infer link-time options from the settings used to compile the input files.
If you don't specify an optimization level option -O at link time, then GCC uses the highest optimization level used when compiling the object files. But note: it's generally ineffective to specify an optimization level option only at link time and not at compile time, for two reasons. First, compiling without optimization suppresses compiler passes that gather information needed for effective optimization at link time. Second, some early optimization passes can only be performed at compile time and not at link time. so yeah, do both, ok? 🙈
There are some code generation flags preserved by GCC when generating bytecodes, because they need to be used during the final link. Currently, the following options and their settings are taken from the first object file that explicitly specifies them: -fcommon, -fexceptions, -fnon-call-exceptions, -fgnu-tm and all the -m target flags.
The following options -fPIC, -fpic, -fpie and -fPIE are combined based on the following scheme:
-fPIC + -fpic = -fpic
-fPIC + -fno-pic = -fno-pic
-fpic/-fPIC + (no option) = (no option)
-fPIC + -fPIE = -fPIE
-fpic + -fPIE = -fpie
-fPIC/-fpic + -fpie = -fpieCertain ABI-changing flags are required to match in all compilation units, and trying to override this at link time with a conflicting value is just ignored, sorry 💀. This includes options such as -freg-struct-return and -fpcc-struct-return.
Other options such as -ffp-contract, -fno-strict-overflow, -fwrapv, -fno-trapv or -fno-strict-aliasing are passed through to the link stage and merged conservatively for conflicting translation units. Specifically -fno-strict-overflow, -fwrapv and -fno-trapv take precedence; and for example -ffp-contract=off takes precedence over -ffp-contract=fast. You can override them at link time.
Diagnostic options such as -Wstringop-overflow are passed through to the link stage and their setting matches that of the compile-step at function granularity. Note that this matters only for diagnostics emitted during optimization. Also note that code transforms such as inlining can lead to warnings being enabled or disabled for regions of code that aren't consistent with the setting at compile time.
When you need to pass options to the assembler via -Wa or -Xassembler, make sure to either compile such translation units with -fno-lto or consistently use the same assembler options on all translation units. You can alternatively also specify assembler options at LTO link time ✨.
To enable debug info generation you need to supply -g at compile time. If any of the input files at link time were built with debug info generation enabled, the link will enable debug info generation as well. Any elaborate debug info settings like the dwarf level -gdwarf-5 need to be explicitly repeated at the linker command line, and mixing different settings in different translation units is discouraged 🙅.
If LTO encounters objects with C linkage declared with incompatible types in separate translation units to be linked together (undefined behavior according to ISO C99 6.2.7), a non-fatal diagnostic may be issued. The behavior is still undefined at run time bestie, the diagnostic doesn't save you 😭. Similar diagnostics may be raised for other languages.
Another feature of LTO is that it's possible to apply interprocedural optimizations on files written in different languages:
gcc -c -flto foo.c
g++ -c -flto bar.cc
gfortran -c -flto baz.f90
g++ -o myprog -flto -O3 foo.o bar.o baz.o -lgfortranNotice that the final link is done with g++ to get the C++ runtime libraries, and -lgfortran is added to get the Fortran runtime libraries. In general, when mixing languages in LTO mode, you should use the same link command options as when mixing languages in a regular (non-LTO) compilation.
If object files containing GIMPLE bytecode are stored in a library archive, say libfoo.a, it's possible to extract and use them in an LTO link if you're using a linker with plugin support. To create static libraries suitable for LTO, use gcc-ar and gcc-ranlib instead of ar and ranlib; to show the symbols of object files with GIMPLE bytecode, use gcc-nm. Those commands require that ar, ranlib and nm have been compiled with plugin support. At link time, use the flag -fuse-linker-plugin to make sure the library participates in the LTO optimization process:
gcc -o myprog -O2 -flto -fuse-linker-plugin a.o b.o -lfooWith the linker plugin enabled, the linker extracts the needed GIMPLE files from libfoo.a and passes them on to the running GCC to make them part of the aggregated GIMPLE image to be optimized 🧚.
If you're not using a linker with plugin support and/or don't enable the linker plugin, then the objects inside libfoo.a are extracted and linked as usual, but they do not participate in the LTO optimization process. In order to make a static library suitable for both LTO optimization and usual linkage, compile its object files with -flto -ffat-lto-objects.
Link-time optimizations do not need the whole program to be sitting there in one piece to do their thing, bestie. So if your program doesn't need to export any symbols, you can literally combine -flto and -fwhole-program and let the interprocedural optimizers make way more aggressive assumptions, which may lead to improved optimization opportunities. ✨
Heads up tho: you do not need-fwhole-programwhen the linker plugin is active (peep-fuse-linker-plugin).
The current implementation of LTO makes no attempt to generate bytecode that's portable between different types of hosts. The bytecode files are versioned and there's a strict version check, so bytecode files generated in one version of GCC do not work with an older or newer version of GCC. 💀 Fr fr, keep your versions matched.
Also, link-time optimization does not work well with generation of debugging information on systems other than those using a combination of ELF and DWARF. It's giving "we tried".
If you specify the optional n, the optimization and code generation done at link time is executed in parallel using n parallel jobs by utilizing an installed make program. The environment variable MAKE may be used to override the program used. 👑
You can also specify -flto=jobserver to use GNU make's job server mode to determine the number of parallel jobs. This is useful when the Makefile calling GCC is already executing in parallel. You must prepend a + to the command recipe in the parent Makefile for this to work. This option likely only works if MAKE is GNU make. Even without the option value, GCC tries to automatically detect a running GNU make's job server. 🌈
Use -flto=auto to use GNU make's job server, if available, or otherwise fall back to autodetection of the number of CPU threads present in your system. Main character energy, honestly.
✨ -flto-partition=alg ✨
Specify the partitioning algorithm used by the link-time optimizer. The value is either 1to1 to specify a partitioning mirroring the original source files, or balanced to specify partitioning into equally sized chunks (whenever possible), or max to create new partition for every symbol where possible, or cache to balance chunk sizes while keeping related symbols together for better caching in incremental LTO. 🧚
Specifying none as an algorithm disables partitioning and streaming completely. The default value is balanced.
While 1to1 can be used as an workaround for various code ordering issues, the max partitioning is intended for internal testing only. 🙈
The value one specifies that exactly one partition should be used, while the value none bypasses partitioning and executes the link-time optimization step directly from the WPA phase. Periodt.
🌸 -flto-incremental=path 🌸
Enable incremental LTO, with its cache in given existing directory. Can significantly shorten edit-compile cycles with LTO. 🔥
When used with LTO (-flto), the output of translation units inside LTO is cached. Cached translation units are likely to be encountered again when recompiling with small code changes, leading to recompile time reduction.
Multiple GCC instances can use the same cache in parallel. Bestie behavior, honestly. 🫶
💖 -flto-incremental-cache-size=n 💖
Specifies number of cache entries in incremental LTO after which to prune old entries. This is a soft limit — temporarily there may be more entries. Ngl, it's flexible like that.
🎀 -flto-compression-level=n 🎀
This option specifies the level of compression used for intermediate language written to LTO object files, and is only meaningful in conjunction with LTO mode (-flto).
GCC currently supports two LTO compression algorithms. For zstd, valid values are 0 (no compression) to 19 (maximum compression), while zlib supports values from 0 to 9. Values outside this range are clamped to either minimum or maximum of the supported values. If the option is not given, a default balanced compression setting is used. ✨
💅 -flto-toplevel-asm-heuristics 💅
Enables heuristics to find symbols used in top-level basic "asm". This will restrict link-time optimizations that could cause renaming or deletion of such symbols, which would result in missing symbol errors by linker. 😭
This flag is intended for projects that have not converted to using top-level extended "asm", which specify the usage directly without any false positives.
The heuristics are simple and do not parse the assembly. The heuristics scan through top-level assembly for all possible identifiers; if an identifier is found among declared symbols, the symbol will be marked to restrict link-time optimizations. Static symbols disable more optimizations. Identifiers followed by ':' disable more optimizations as well, because they might be a locally defined symbol in assembly, even when the declaration is marked 'extern'. 💀
⭐ -fuse-linker-plugin ⭐
Enables the use of a linker plugin during link-time optimization. This option relies on plugin support in the linker, which is available in gold or in GNU ld 2.21 or newer.
This option enables the extraction of object files with GIMPLE bytecode out of library archives. This improves the quality of optimization by exposing more code to the link-time optimizer. This information specifies what symbols can be accessed externally (by non-LTO object or during dynamic linking). Resulting code quality improvements on binaries (and shared libraries that use hidden visibility) are similar to -fwhole-program. See -flto for a description of the effect of this flag and how to use it. 👑
This option is enabled by default when LTO support in GCC is enabled and GCC was configured for use with a linker supporting plugins (GNU ld 2.21 or newer or gold). Slay.
🦋 -ffat-lto-objects 🦋
Fat LTO objects are object files that contain both the intermediate language and the object code. This makes them usable for both LTO linking and normal linking. This option is effective only when compiling with -flto and is ignored at link time.
-fno-fat-lto-objects improves compilation time over plain LTO, but requires the complete toolchain to be aware of LTO. It requires a linker with linker plugin support for basic functionality. Additionally, nm, ar and ranlib need to support linker plugins to allow a full-featured build environment (capable of building static libraries etc). GCC provides the gcc-ar, gcc-nm, gcc-ranlib wrappers to pass the right options to these tools. With non fat LTO makefiles need to be modified to use them. 😭
Note that modern binutils provide plugin auto-load mechanism. Installing the linker plugin into$libdir/bfd-pluginshas the same effect as usage of the command wrappers (gcc-ar,gcc-nmandgcc-ranlib).
The default is -fno-fat-lto-objects on targets with linker plugin support. Iykyk.
🌈 -fcompare-elim 🌈
After register allocation and post-register allocation instruction splitting, identify arithmetic instructions that compute processor flags similar to a comparison operation based on that arithmetic. If possible, eliminate the explicit comparison operation. ✨
This pass only applies to certain targets that cannot explicitly represent the comparison operation before register allocation is complete.
Enabled at levels -O1, -O2, -O3, -Os.
💜 -ffold-mem-offsets 💜
💜 -fno-fold-mem-offsets 💜
Try to eliminate add instructions by folding them in memory loads/stores.
Enabled at levels -O2, -O3.
🍓 -fcprop-registers 🍓
After register allocation and post-register allocation instruction splitting, perform a copy-propagation pass to try to reduce scheduling dependencies and occasionally eliminate the copy. 💅
Enabled at levels -O1, -O2, -O3, -Os.
🧚 -fprofile-correction 🧚
Profiles collected using an instrumented binary for multi-threaded programs may be inconsistent due to missed counter updates. When this option is specified, GCC uses heuristics to correct or smooth out such inconsistencies. By default, GCC emits an error message when an inconsistent profile is detected. 😭
This option is enabled by -fauto-profile.
🎀 -fprofile-partial-training 🎀
With "-fprofile-use" all portions of programs not executed during training runs are optimized aggressively for size rather than speed. In some cases it is not practical to train all possible hot paths in the program. (For example, it may contain functions specific to a given hardware and training may not cover all hardware configurations the program later runs on.)
With "-fprofile-partial-training" profile feedback is ignored for all functions not executed during the training runs, causing them to be optimized as if they were compiled without profile feedback. This leads to better performance when the training is not representative, at the cost of significantly bigger code. Trade-offs, bestie. 💀
👑 -fprofile-use 👑
👑 -fprofile-use=path 👑
Enable profile feedback-directed optimizations, and the following optimizations, many of which are generally profitable only with profile feedback available:
-fbranch-probabilities -fprofile-values -funroll-loops
-fpeel-loops -ftracer -fvpt -finline-functions -fipa-cp
-fipa-cp-clone -fipa-bit-cp -fpredictive-commoning -fsplit-loops
-funswitch-loops -fgcse-after-reload -ftree-loop-vectorize
-ftree-slp-vectorize -fvect-cost-model=dynamic
-ftree-loop-distribute-patterns -fprofile-reorder-functionsBefore you can use this option, you must first generate profiling information. No skipping steps, bestie. 🙈
By default, GCC emits an error message if the feedback profiles do not match the source code. This error can be turned into a warning by using -Wno-error=coverage-mismatch. Note this may result in poorly optimized code. 😭 Additionally, by default, GCC also emits a warning message if the feedback profiles do not exist (see -Wmissing-profile).
If path is specified, GCC looks at the path to find the profile feedback data files. See -fprofile-dir.
🌸 -fauto-profile 🌸
🌸 -fauto-profile=path 🌸
Enable sampling-based feedback-directed optimizations, and the following optimizations, many of which are generally profitable only with profile feedback available:
-fbranch-probabilities -fprofile-values -funroll-loops
-fpeel-loops -ftracer -fvpt -finline-functions -fipa-cp
-fipa-cp-clone -fipa-bit-cp -fpredictive-commoning -fsplit-loops
-funswitch-loops -fgcse-after-reload -ftree-loop-vectorize
-ftree-slp-vectorize -fvect-cost-model=dynamic
-ftree-loop-distribute-patterns -fprofile-correctionNotice that the last one is-fprofile-correctionhere, not-fprofile-reorder-functionslike in-fprofile-use. Not me pointing out the one-line difference — but yes, that's literally the difference. 💅
path is the name of a file containing AutoFDO profile information, bestie. If you just… don't give one? It defaults to fbdata.afdo in the current directory. She's got a default, she's low maintenance like that. 💅
Okay so to actually make an AutoFDO profile data file you have to run your program with the perf utility on a supported GNU/Linux target system. That's the whole prerequisite, no cap. More deets here: https://perfwiki.github.io/main/.
Like, for example:
perf record -e br_inst_retired:near_taken -b -o perf.data \
-- your_programThen — and this is the part everyone forgets 🙈 — you use the create_gcov tool to convert that raw profile data into a format GCC can actually read. You also have to hand this tool the unstripped binary for your program. Yes, unstripped. Don't strip it. See https://github.com/google/autofdo for the lore.
Like:
create_gcov --binary=your_program.unstripped --profile=perf.data \
--gcov=profile.afdo-fauto-profile-inlining— when auto-profile is available, inline all relevant functions which was inlined in the tran run before reading the profile feedback. This makes the profile way more context sensitive ✨ and it's enabled by default, so it's already doing its thing whether you noticed or not. 👑
The following options control compiler behavior regarding floating-point arithmetic. These options trade off between speed and correctness. All must be specifically enabled. Every single one. No sneaking. 🚨
-fexcess-precision=style— this option lets you control excess precision on machines where floating-point operations happen in a format with more precision or range than the IEEE standard and interchange floating-point types. A classic example of such a target is x87 floating point on x86 processors, which uses an 80-bit representation internally instead of the 64-bit IEEE format. For most programs the excess precision is harmless — genuinely, it's giving harmless — but some programs may rely on the requirements of the C or C++ language standards for handling IEEE values. 😭
By default, -fexcess-precision=fast is in effect. That means operations may be carried out in a wider precision than the types specified in the source if that would result in faster code, and it is unpredictable when rounding to the types specified in the source code takes place. When compiling C or C++, if -fexcess-precision=standard is specified then excess precision follows the rules specified in ISO C99 or C++; in particular, both casts and assignments cause values to be rounded to their semantic types (whereas -ffloat-store only affects assignments). This option is enabled by default for C or C++ if a strict conformance option such as -std=c99 or -std=c++17 is used. -ffast-math enables -fexcess-precision=fast by default regardless of whether a strict conformance option is used. If -fexcess-precision=16 is specified, constants and the results of expressions with types _Float16 and __bf16 are computed without excess precision. 🧚
Heads up: -fexcess-precision=standard is not implemented for languages other than C or C++. And on the x86, it has no effect if -mfpmath=sse or -mfpmath=sse+387 is specified; in the former case, IEEE semantics apply without excess precision, and in the latter, rounding is unpredictable. 💀
-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 has generally been subsumed by-fexcess-precision=standard, which is more general. If you do use-ffloat-store, you may need to modify your program to explicitly store intermediate computations in temporary variables, since-ffloat-storehandles rounding to IEEE format only on assignments and not casts as-fexcess-precision=standarddoes. So like… know what you're signing up for. 🫶-ffast-math— sets the options-fno-math-errno,-funsafe-math-optimizations,-ffinite-math-only,-fno-rounding-math,-fno-signaling-nans,-fcx-limited-rangeand-fexcess-precision=fast. A whole bundle, one flag, icon behavior. ✨
This option causes the preprocessor macro __FAST_MATH__ to be defined.
This option is not turned on by any-Ooption besides-Ofastsince it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that do not require the guarantees of these specifications.
-fno-math-errno— do not seterrnoafter calling math functions that are executed with a single instruction, e.g.,sqrt. A program that relies on IEEE exceptions for math error handling may want to use this flag for speed while maintaining IEEE arithmetic compatibility.
This option is not turned on by any-Ooption besides-Ofastsince it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that do not require the guarantees of these specifications.
The default is -fmath-errno.
On Darwin systems, the math library never sets errno. There is therefore no reason for the compiler to consider the possibility that it might, and -fno-math-errno is the default there. 🍓
-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. She's called unsafe for a reason, babe. 💀
This option is not turned on by any-Ooption besides-Ofastsince it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that do not require the guarantees of these specifications. Enables-fno-signed-zeros,-fno-trapping-math,-fassociative-mathand-freciprocal-math.
The default is -fno-unsafe-math-optimizations.
-fassociative-math— allow re-association of operands in series of floating-point operations. This violates the ISO C and C++ language standard by possibly changing computation result. NOTE: re-ordering may change the sign of zero as well as ignore NaNs and inhibit or create underflow or overflow (and thus cannot be used on code that relies on rounding behavior like(x + 2**52) - 2**52). May also reorder floating-point comparisons and thus may not be used when ordered comparisons are required. This option requires that both-fno-signed-zerosand-fno-trapping-mathbe in effect. Moreover, it doesn't make much sense with-frounding-math. For Fortran the option is automatically enabled when both-fno-signed-zerosand-fno-trapping-mathare in effect.
The default is -fno-associative-math.
-freciprocal-math— allow the reciprocal of a value to be used instead of dividing by the value if this enables optimizations. For examplex / ycan be replaced withx * (1/y), which is useful if(1/y)is subject to common subexpression elimination. Note that this loses precision and increases the number of flops operating on the value. Trade-offs, bestie, trade-offs. ⚖️
The default is -fno-reciprocal-math.
-ffinite-math-only— allow optimizations for floating-point arithmetic that assume that arguments and results are not NaNs or +-Infs. She simply refuses to acknowledge the concept of NaN. 💅
This option is not turned on by any-Ooption besides-Ofastsince it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that do not require the guarantees of these specifications.
The default is -fno-finite-math-only.
-fno-signed-zeros— allow optimizations for floating-point arithmetic that ignore the signedness of zero. IEEE arithmetic specifies the behavior of distinct+0.0and-0.0values, which then prohibits simplification of expressions such asx+0.0or0.0*x(even with-ffinite-math-only). This option implies that the sign of a zero result isn't significant. Zero is zero, okay?? 😤
The default is -fsigned-zeros.
-fno-trapping-math— compile code assuming that floating-point operations cannot generate user-visible traps. These traps include division by zero, overflow, underflow, inexact result and invalid operation. This option requires that-fno-signaling-nansbe in effect. Setting this option may allow faster code if one relies on "non-stop" IEEE arithmetic, for example.
This option is not turned on by any-Ooption besides-Ofastsince it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions.
The default is -ftrapping-math.
Future versions of GCC may provide finer control of this setting using C99's FENV_ACCESS pragma. This command-line option will be used along with -frounding-math to specify the default state for FENV_ACCESS. 🔮
-frounding-math— disable transformations and optimizations that assume default floating-point rounding behavior (round-to-nearest). 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'sFENV_ACCESSpragma. This command-line option will be used along with-ftrapping-mathto specify the default state forFENV_ACCESS.
-fsignaling-nans— compile code assuming that IEEE signaling NaNs may generate user-visible traps during floating-point operations. Setting this option disables optimizations that may change the number of exceptions visible with signaling NaNs. This option implies-ftrapping-math. 🚨
This option causes the preprocessor macro __SUPPORT_SNAN__ to be defined.
The default is -fno-signaling-nans.
This option is experimental and does not currently guarantee to disable all GCC optimizations that affect signaling NaN behavior.
-fsingle-precision-constant— okay so floating-point constants? This tells GCC to just treat them as single precision, instead of doing the sneaky implicit glow-up into double-precision constants. no automatic promotion bestie, we keep it single. 💅-fcx-limited-range— when you turn this on, you're basically telling the compiler "bestie, complex division does NOT need a range reduction step, we're fine". Also it skips checking whether the result of a complex multiplication or division is"NaN + I*NaN"with a rescue attempt in that case. This one is switched on by-ffast-math🫶
It controls the default setting of the ISO C99 "CX_LIMITED_RANGE" pragma. But like — plot twist — the option applies to all languages. iconic.
-fcx-fortran-rules— complex multiplication and division follow Fortran rules. Range reduction is done as part of complex division here, but nope, still no checking whether the result of a complex multiplication or division is"NaN + I*NaN"with an attempt to rescue it. it's giving "we move, no safety net" 🦋- *
-fcx-method=method* — complex multiplication and division follow whatever method you name. The method argument should be one oflimited-range,fortranorstdc.
The default is to honor language specific constraints, which means fortran for Fortran and stdc otherwise. lowkey sensible ngl ✨
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. 😭 not me warning you twice, fr fr be careful.
-fbranch-probabilities— so you ran a program compiled with-fprofile-arcs, right? Now compile it a second time using-fbranch-probabilitiesto improve optimizations based on how many times each branch was taken. When a program compiled with-fprofile-arcsexits, it saves arc execution counts to a file calledsourcename.gcdafor each source file. The info in that 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. See details about the file naming in-fprofile-arcs. 🎀
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.cc, instead of guessing which path a branch is most likely to take, the REG_BR_PROB values are used to exactly determine which path is taken more often. 💅
Enabled by -fprofile-use and -fauto-profile.
-fprofile-values— if you combine it with-fprofile-arcs, it adds code so that some data about values of expressions in the program gets gathered. 🍓
With -fbranch-probabilities, it reads back the data gathered from profiling values of expressions for usage in optimizations.
Enabled by -fprofile-generate, -fprofile-use, and -fauto-profile.
-fprofile-reorder-functions— function reordering based on profile instrumentation collects the first time of execution of a function and orders these functions in ascending order, aiming to optimize program startup through more efficient loading of text segments. it's giving "organized queen" 👑
Enabled with -fprofile-use.
-fvpt— if combined with-fprofile-arcs, this option instructs the compiler to add code to gather information about values of expressions. 🌸
With -fbranch-probabilities, it reads back the data gathered and actually performs the optimizations based on them. Currently the optimizations include specialization of division operations using the knowledge about the value of the denominator.
Enabled with -fprofile-use and -fauto-profile.
-frename-registers— attempt to avoid false dependencies in scheduled code by making use of registers left over after register allocation. This optimization most benefits processors with lots of registers. Depending on the debug information format adopted by the target, however, it can make debugging impossible, since variables no longer stay in a"home register". 💀
Enabled by default with -funroll-loops.
-fschedule-fusion— performs a target dependent pass over the instruction stream to schedule instructions of the same type together, because the target machine can execute them more efficiently if they're adjacent to each other in the instruction flow. adjacent besties perform better, no cap 🫶
Enabled at levels -O2, -O3, -Os.
-fdep-fusion— detect macro-op fusible pairs consisting of single-use instructions and their uses, and place such pairs together in the instruction stream to increase fusion opportunities in hardware. This pass is executed once before register allocation, and another time before register renaming. twice the pass, twice the slay ✨
Enabled at levels -O2, -O3, -Os.
-ftracer— perform tail duplication to enlarge superblock size. This transformation simplifies the control flow of the function, allowing other optimizations to do a better job.
Enabled by -fprofile-use and -fauto-profile.
-funroll-loops— unroll loops whose number of iterations can be determined at compile time or upon entry to the loop.-funroll-loopsimplies-frerun-cse-after-loop,-fweband-frename-registers. It also turns on complete loop peeling (i.e. complete removal of loops with a small constant number of iterations). This option makes code larger, and may or may not make it run faster. 🙈 the tradeoff is real.
Enabled by -fprofile-use and -fauto-profile.
-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-loopsimplies the same options as-funroll-loops.-fpeel-loops— peels loops for which there's enough information that they do not roll much (from profile feedback or static analysis). It also turns on complete loop peeling (i.e. complete removal of loops with small constant number of iterations). 🧚
Enabled by -O3, -fprofile-use, and -fauto-profile.
-fmalloc-dce— controls whether"malloc"(and its variants such as"calloc"or"strdup") can be optimized away, provided its return value is only used as a parameter of a"free"call or compared with"NULL". If-fmalloc-dce=1is used, only calls to"free"are allowed, while with-fmalloc-dce=2comparisons with"NULL"pointer are also considered safe to remove. 💅
The default is -fmalloc-dce=2. See also -fallocation-dce.
-fmove-loop-invariants— enables the loop invariant motion pass in the RTL loop optimizer. Enabled at level-O1and higher, except for-Og. 🌈-fmove-loop-stores— enables the loop store motion pass in the GIMPLE loop optimizer. This moves invariant stores to after the end of the loop in exchange for carrying the stored value in a register across the iteration. Note: for this option to have an effect,-ftree-loop-imhas to be enabled as well. Enabled at level-O1and higher, except for-Og.-fsplit-loops— split a loop into two if it contains a condition that's always true for one side of the iteration space and false for the other. one loop said "not me splitting myself" and honestly? valid 💜
Enabled by -fprofile-use and -fauto-profile.
-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).
Enabled by -fprofile-use and -fauto-profile.
-fversion-loops-for-strides— if a loop iterates over an array with a variable stride, create another version of the loop that assumes the stride is always one. For example:
for (int i = 0; i < n; ++i)
x[i * stride] = ...;becomes:
if (stride == 1)
for (int i = 0; i < n; ++i)
x[i] = ...;
else
for (int i = 0; i < n; ++i)
x[i * stride] = ...;This is particularly useful for assumed-shape arrays in Fortran where (for example) it allows better vectorization assuming contiguous accesses. This flag is enabled by default at -O3. It is also enabled by -fprofile-use and -fauto-profile. 🔥
-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 have linkers with such optimizations. On AIX, the linker rearranges sections (CSECTs) based on the call graph. The performance impact varies.
Together with a linker garbage collection (linker --gc-sections option) these options may lead to smaller statically-linked executables (after stripping).
On ELF/DWARF systems these options do not degenerate the quality of the debug information. There could be issues with other object files/debug info formats.
Only use these options when there are significant benefits from doing so. When you specify these options, the assembler and linker create larger object and executable files and are also slower. These options affect code generation. They prevent optimizations by the compiler and assembler using relative locations inside a translation unit, since the locations are unknown until link time. An example of such an optimization is relaxing calls to short call instructions. 💀 so like, know what you're doing bestie.
-fstdarg-opt— optimize the prologue of variadic argument functions with respect to usage of those arguments. 🌸-fsection-anchors— try to reduce the number of symbolic address calculations by using shared"anchor"symbols to address nearby objects. This transformation can help to reduce the number of GOT entries and GOT accesses on some targets. ✨
For example, the implementation of the following function "foo":
static int a, b, c;
int foo (void) { return a + b + c; }usually calculates the addresses of all three variables, but if you compile it with -fsection-anchors, it accesses the variables from a common anchor point instead. The effect is similar to the following pseudocode (which isn't valid C):
int foo (void)
{
register int *xr = &x;
return xr[&a - &x] + xr[&b - &x] + xr[&c - &x];
}Not all targets support this option. 🙈 iykyk.
-fzero-call-used-regs=choice— bestie, this one zeroes out the call-used registers when a function returns, and the whole point is security: it either makes Return-Oriented Programming (ROP) attacks harder or stops info from leaking out through registers. The possible values of choice are exactly the same ones as for thezero_call_used_regsattribute. Default?skip. And if you wanna lock this in for one specific function only, use thezero_call_used_regsfunction attribute. 💅
✨ Program Instrumentation Options ✨
Okay so GCC has a whole buffet of command-line options that control adding run-time instrumentation to the code it normally generates. Like, imagine your program is a main character and you're giving it a little camera crew. 🎥 One reason you'd do this: collecting profiling statistics so you can find your program's hot spots, do code coverage analysis, or do profile-guided optimizations. Another flavor of instrumentation is adding run-time checking to catch programming errors like invalid pointer dereferences or out-of-bounds array accesses — plus straight-up hostile attacks like stack smashing or C++ vtable hijacking. 😤 And there's also a general hook you can use to implement other forms of tracing or function-level instrumentation for debug or program analysis purposes. So many options, we love to see it. 🫶
-p,--profile,-fprofile,-pg— generate extra code that writes profile information suitable for the analysis programprof(that's for-p,--profile, and-fprofile) orgprof(that's for-pg). You must use this option when compiling the source files you want data about, and you also must use it when linking. Miss one and it's giving nothing, fr fr. You can use the function attributeno_instrument_functionto suppress profiling of individual functions when compiling with these options.-fprofile-arcs— add code so that program flow arcs get instrumented. During execution the program records how many times each branch and call is executed, and how many times it is taken or returns. On targets that support constructors with priority support, profiling properly handles constructors, destructors and C++ constructors (and destructors) of classes which are used as a type of a global variable. ✨
When the compiled program exits it saves this data to a file called auxname.gcda for each source file. That data may be used for profile-directed optimizations (-fbranch-probabilities), or for test coverage analysis (-ftest-coverage). Each object file's auxname is generated from the name of the output file, if explicitly specified and it is not the final executable, otherwise it is the basename of the source file. In both cases any suffix is removed (e.g. foo.gcda for input file dir/foo.c, or dir/foo.gcda for output file specified as -o dir/foo.o).
Note that if a command line directly links source files, the corresponding .gcda files will be prefixed with the unsuffixed name of the output file. E.g. gcc a.c b.c -o binary would generate binary-a.gcda and binary-b.gcda files. iykyk 💖
-fcondition-coverage— add code so that program conditions are instrumented. During execution the program records what terms in a conditional contributes to a decision, which can be used to verify that all terms in a Boolean function are tested and have an independent effect on the outcome of a decision. The result can be read withgcov --conditions. Obsessed. 🎀-fpath-coverage— add code so that the paths taken are tracked. During execution the program records the prime paths taken. Heads up though: the number of paths grows very fast with complexity, and to avoid exploding compile times GCC will give up instrumentation if the approximate number of paths exceeds the limit controlled by-fpath-coverage-limit. The result can be read withgcov --prime-paths --prime-paths-lines --prime-paths-source, 😭-fpath-coverage-limit=limit— the threshold at which point-fpath-coveragegives up on instrumenting a function. This limit is approximate and conservative, because GCC uses a pessimistic heuristic which slightly overcounts the running number of paths, and gives up if the threshold is reached before finding all the paths. This option is not for fine grained control over which functions to instrument — rather it is intended to limit the effect of path explosion and keep compile times reasonable. The default is250000. 💀--coverage,-coverage— this option is used to compile and link code instrumented for coverage analysis. The options-coverageand--coverageare equivalent; both are a synonym for-fprofile-arcs -ftest-coverage(when compiling) and-lgcov(when linking). See the documentation for those options for more details. ✨
Here's the whole glow-up routine, step by step:
- Compile the source files with
-fprofile-arcsplus optimization and code generation options. For test coverage analysis, use the additional-ftest-coverageoption. You do not need to profile every source file in a program. - Compile the source files additionally with
-fprofile-abs-pathto create absolute path names in the.gcnofiles. This allowsgcovto find the correct sources in projects where compilations occur with different working directories. - Link your object files with
-lgcovor-fprofile-arcs(the latter implies the former). - Run the program on a representative workload to generate the arc profile information. This may be repeated any number of times. You can run concurrent instances of your program, and provided that the file system supports locking, the data files will be correctly updated. Unless a strict ISO C dialect option is in effect,
forkcalls are detected and correctly handled without double counting.
Moreover, an object file can be recompiled multiple times and the corresponding .gcda file merges as long as the source file and the compiler options are unchanged.
- For profile-directed optimizations, compile the source files again with the same optimization and code generation options plus
-fbranch-probabilities. - For test coverage analysis, use
gcovto produce human readable information from the.gcnoand.gcdafiles. Refer to thegcovdocumentation 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.
With -fcondition-coverage, for each conditional in your program GCC creates a bitset and records the exercised boolean values that have an independent effect on the outcome of that expression.
With -fpath-coverage, GCC finds and enumerates and records the taken prime paths of each function, unless the number of paths would exceed the limit controlled by -fpath-coverage-limit. If the limit is exceeded the function is not instrumented as if -fpath-coverage was not used. A prime path is the longest sequence of unique blocks, except possibly the first and last, which is not a subpath of any other path. 🌈
-ftest-coverage— produce a notes file that thegcovcode-coverage utility can use to show program coverage. Each source file's note file is calledauxname.gcno. Refer to the-fprofile-arcsoption above for a description ofauxnameand instructions on how to generate test coverage data. Coverage data matches the source files more closely if you do not optimize. ✨-fprofile-abs-path— automatically convert relative source file names to absolute path names in the.gcnofiles. This allowsgcovto find the correct sources in projects where compilations occur with different working directories. 🦋-fprofile-dir=path— set the directory to search for the profile data files in to path. This option affects only the profile data generated by-fprofile-generate,-ftest-coverage,-fprofile-arcsand used by-fprofile-useand-fbranch-probabilitiesand its related options. Both absolute and relative paths can be used. By default, GCC uses the current directory as path, thus the profile data file appears in the same directory as the object file. In order to prevent the file name clashing, if the object file name is not an absolute path, we mangle the absolute path of thesourcename.gcdafile and use it as the file name of a.gcdafile. See details about the file naming in-fprofile-arcs. See similar option-fprofile-note.
When an executable is run in a massive parallel environment, it is recommended to save profile to different folders. That can be done with variables in path that are exported during run-time:
%p— process ID.%q{VAR}— value of environment variable VAR
-fprofile-generate,-fprofile-generate=path— enable options usually used for instrumenting application to produce profile useful for later recompilation with profile feedback based optimization. You must use-fprofile-generateboth when compiling and when linking your program. 👑
The following options are enabled: -fprofile-arcs, -fprofile-values, -finline-functions, and -fipa-bit-cp.
If path is specified, GCC looks at the path to find the profile feedback data files. See -fprofile-dir.
To optimize the program based on the collected profile information, use -fprofile-use. 💅
-fprofile-info-section,-fprofile-info-section=name— register the profile information in the specified section instead of using a constructor/destructor. The section name is name if it is specified, otherwise the section name defaults to".gcov_info". A pointer to the profile information generated by-fprofile-arcsis placed in the specified section for each translation unit. This option disables the profile information registration through a constructor and it disables the profile information processing through a destructor. This option is not intended to be used in hosted environments such as GNU/Linux. It targets freestanding environments (for example embedded systems) with limited resources which do not support constructors/destructors or the C library file I/O. 🌸
okay so the linker can literally collect all the input sections into one continuous memory block and define start and end symbols for you. Here's a GNU linker script example that defines a linker output section, bestie:
.gcov_info :
{
PROVIDE (__gcov_info_start = .);
KEEP (*(.gcov_info))
PROVIDE (__gcov_info_end = .);
}And then your program can dump the profiling information registered in this linker set — for example, like this 🫶:
#include <gcov.h>
#include <stdio.h>
#include <stdlib.h>
extern const struct gcov_info *const __gcov_info_start[];
extern const struct gcov_info *const __gcov_info_end[];
static void
dump (const void *d, unsigned n, void *arg)
{
const unsigned char *c = d;
for (unsigned i = 0; i < n; ++i)
printf ("%02x", c[i]);
}
static void
filename (const char *f, void *arg)
{
__gcov_filename_to_gcfn (f, dump, arg );
}
static void *
allocate (unsigned length, void *arg)
{
return malloc (length);
}
static void
dump_gcov_info (void)
{
const struct gcov_info *const *info = __gcov_info_start;
const struct gcov_info *const *end = __gcov_info_end;
/* Obfuscate variable to prevent compiler optimizations. */
__asm__ ("" : "+r" (info));
while (info != end)
{
void *arg = NULL;
__gcov_info_to_gcda (*info, filename, dump, allocate, arg);
putchar ('\n');
++info;
}
}
int
main (void)
{
dump_gcov_info ();
return 0;
}The merge-stream subcommand of gcov-tool may be used to deserialize the data stream generated by the __gcov_filename_to_gcfn and __gcov_info_to_gcda functions and merge the profile information into .gcda files on the host filesystem. slay ✨
-fprofile-note=path— ifpathis specified, GCC saves the.gcnofile into thepathlocation. heads up tho: if you combine this option with multiple source files, the.gcnofile will get overwritten. 💀-fprofile-prefix-path=path— this one can be used together withprofile-generate=profile_dirandprofile-use=profile_dirto tell GCC where the base directory of the built source tree is. By defaultprofile_dirwill contain files with mangled absolute paths of all object files in the built project. That's not desirable when the directory used to build the instrumented binary differs from the directory used to build the binary optimized with profile feedback, because then the profile data won't be found during the optimized build 😭. In such setups,-fprofile-prefix-path=pathwithpathpointing to the base directory of the build can be used to strip the irrelevant part of the path and keep all file names relative to the main build directory. clean.-fprofile-prefix-map=old=new— when compiling files residing in directoryold, record profiling information (with--coverage) describing them as if the files resided in directorynewinstead. See also-ffile-prefix-mapand-fcanon-prefix-map👀.-fprofile-update=method— changes the update method for an application instrumented for profile feedback based optimization. Themethodargument should be one ofsingle,atomicorprefer-atomic. The first one is useful for single-threaded applications, while the second one prevents profile corruption by emitting thread-safe code. 🧵
Warning: when an application does not properly join all threads (or creates an detached thread), a profile file can be still corrupted.
Using prefer-atomic would be transformed either to atomic, when supported by a target, or to single otherwise. The GCC driver automatically selects prefer-atomic when -pthread is present in the command line, otherwise the default method is single.
If atomic is selected, then the profile information is updated using atomic operations on a best-effort basis. Ideally, the profile information is updated through atomic operations in hardware. If the target platform does not support the required atomic operations in hardware, however, libatomic is available, then the profile information is updated through calls to libatomic. If the target platform neither supports the required atomic operations in hardware nor libatomic, then the profile information is not atomically updated and a warning is issued. In this case, the obtained profiling information may be corrupt for multi-threaded applications. 💀
For performance reasons, if 64-bit counters are used for the profiling information and the target platform only supports 32-bit atomic operations in hardware, then the performance critical profiling updates are done using two 32-bit atomic operations for each counter update. If a signal interrupts these two operations updating a counter, then the profiling information may be in an inconsistent state. 😭
-fprofile-filter-files=regex— instrument only functions from files whose name matches any of the regular expressions (separated by semi-colons). For example,-fprofile-filter-files=main\.c;module.*\.cwill instrument onlymain.cand all C files starting with'module'. ✨-fprofile-exclude-files=regex— instrument only functions from files whose name does not match any of the regular expressions (separated by semi-colons). For example,-fprofile-exclude-files=/usr/.*will prevent instrumentation of all files that are located in the/usr/folder. 🚫-fprofile-reproducible=[multithreaded|parallel-runs|serial]— controls the level of reproducibility of the profile gathered by-fprofile-generate. This makes it possible to rebuild a program with the same outcome, which is useful, for example, for distribution packages. 📦
With -fprofile-reproducible=serial the profile gathered by -fprofile-generate is reproducible provided the trained program behaves the same at each invocation of the train run, it is not multi-threaded and profile data streaming is always done in the same order. Note that profile streaming happens at the end of program run but also before fork function is invoked.
Note that it is quite common that execution counts of some part of programs depends, for example, on length of temporary file names or memory space randomization (that may affect hash-table collision rate). Such non-reproducible part of programs may be annotated by no_instrument_function function attribute. gcov-dump with -l can be used to dump gathered data and verify that they are indeed reproducible. 🔍
With -fprofile-reproducible=parallel-runs collected profile stays reproducible regardless the order of streaming of the data into gcda files. This setting makes it possible to run multiple instances of instrumented program in parallel (such as with make -j). This reduces quality of gathered data, in particular of indirect call profiling. trade-offs, we love to see it.
-fsanitize=address— enable AddressSanitizer, a fast memory error detector. 🕵️♀️ Memory access instructions are instrumented to detect out-of-bounds and use-after-free bugs. The option enables-fsanitize-address-use-after-scope. Seehttps://github.com/google/sanitizers/wiki/AddressSanitizerfor more details. The run-time behavior can be influenced using theASAN_OPTIONSenvironment variable. When set to"help=1", the available options are shown at startup of the instrumented program. Seehttps://github.com/google/sanitizers/wiki/AddressSanitizerFlags#run-time-flagsfor a list of supported options. The option cannot be combined with-fsanitize=threador-fsanitize=hwaddress. Note that the only targets-fsanitize=hwaddressis currently supported on are x86-64 (only with-mlam=u48or-mlam=u57options) and AArch64, in both cases only in ABIs with 64-bit pointers. Similarly,-fsanitize=memtag-stackis currently only supported on AArch64 ABIs with 64-bit pointers.
When compiling with -fsanitize=address, you should also use -g to produce more meaningful output. To get more accurate stack traces, it is possible to use options such as -O0, -O1, or -Og (which, for instance, prevent most function inlining), -fno-optimize-sibling-calls (which prevents optimizing sibling and tail recursive calls; this option is implicit for -O0, -O1, or -Og), or -fno-ipa-icf (which disables Identical Code Folding for functions). Using -fno-omit-frame-pointer also improves stack traces. Since multiple runs of the program may yield backtraces with different addresses due to ASLR (Address Space Layout Randomization), it may be desirable to turn ASLR off. On Linux, this can be achieved with setarch \uname -m` -R ./prog`. 🌈
-fsanitize=kernel-address— enable AddressSanitizer for Linux kernel. Seehttps://github.com/google/kernel-sanitizersfor more details. 🐧-fsanitize=hwaddress— enable Hardware-assisted AddressSanitizer, which uses a hardware ability to ignore the top byte of a pointer to allow the detection of memory errors with a low memory overhead. Memory access instructions are instrumented to detect out-of-bounds and use-after-free bugs. The option enables-fsanitize-address-use-after-scope. Seehttps://clang.llvm.org/docs/HardwareAssistedAddressSanitizerDesign.htmlfor more details. The run-time behavior can be influenced using theHWASAN_OPTIONSenvironment variable. When set to"help=1", the available options are shown at startup of the instrumented program. The option cannot be combined with-fsanitize=threador-fsanitize=address, and is currently only available on AArch64. 💅-fsanitize=kernel-hwaddress— bestie this one turns on Hardware-assisted AddressSanitizer so you can compile the Linux kernel with it 💖 It's like-fsanitize=kernel-addressbut it uses a whole alternate instrumentation method, and it's also like-fsanitize=hwaddressexcept with the instrumentation differences you need to compile the Linux kernel. Those differences exist so we avoid hwasan library initialization calls, and so we can account for the stack pointer having a different value in its top byte ✨
Heads up: this option has different defaults than-fsanitize=hwaddress. Instrumenting the stack andallocacalls are not on by default. And using a random frame tag is not implemented for kernel instrumentation. No cap 🫶
-fsanitize=memtag-stack— this one uses Memory Tagging Extension instructions instead of instrumentation so you can detect memory errors 🦋 Just like HWASAN, it's also a probabilistic method, so it's giving vibes, not guarantees. This option is only available on those AArch64 architectures that support Memory Tagging Extensions 👑-fsanitize=pointer-compare— instruments the comparison operation (<,<=,>,>=) when it's got pointer operands 🔍 This option must be combined with either-fsanitize=kernel-addressor-fsanitize=address. It cannot be combined with-fsanitize=thread.
Note: by default the check is disabled at run time. To enable it, add"detect_invalid_pointer_pairs=2"to the environment variableASAN_OPTIONS. Using"detect_invalid_pointer_pairs=1"detects an invalid operation only when both pointers are non-null 💅
-fsanitize=pointer-subtract— instruments subtraction with pointer operands 🧚 Same rules as its sibling: it must be combined with either-fsanitize=kernel-addressor-fsanitize=address, and it cannot be combined with-fsanitize=thread.
Note: by default the check is disabled at run time. To enable it, add"detect_invalid_pointer_pairs=2"to the environment variableASAN_OPTIONS. Using"detect_invalid_pointer_pairs=1"detects an invalid operation only when both pointers are non-null 💅
-fsanitize=shadow-call-stack— enables ShadowCallStack, a security enhancement mechanism used to protect programs against return address overwrites (think stack buffer overflows, eek 😭). The way it works: it saves a function's return address to a separately allocated shadow call stack in the function prologue, and restores the return address from the shadow call stack in the function epilogue. Instrumentation only happens in functions that actually need to save the return address to the stack ✨
Currently it only supports the aarch64 platform. It is specifically designed for linux kernels that enable the CONFIG_SHADOW_CALL_STACK option. For user space programs, runtime support is not currently provided in libc and libgcc. Users who want to use this feature in user space need to provide their own support for the runtime. And it should be noted that this may cause the ABI rules to be broken 💀
On aarch64, the instrumentation makes use of the platform register x18. This generally means that any code that may run on the same thread as code compiled with ShadowCallStack must be compiled with the flag -ffixed-x18, otherwise functions compiled without -ffixed-x18 might clobber x18 and so corrupt the shadow stack pointer 🫠
Also, because there is no userspace runtime support, code compiled with ShadowCallStack cannot use exception handling. Use -fno-exceptions to turn off exceptions.
See <https://clang.llvm.org/docs/ShadowCallStack.html> for more details 👑
-fsanitize=thread— enables ThreadSanitizer, a fast data race detector 🔥 Memory access instructions are instrumented to detect data race bugs. See <https://github.com/google/sanitizers/wiki#threadsanitizer> for more details. The run-time behavior can be influenced using theTSAN_OPTIONSenvironment variable; see <https://github.com/google/sanitizers/wiki/ThreadSanitizerFlags> for a list of supported options. The option cannot be combined with-fsanitize=addressor-fsanitize=leak.
When compiling with -fsanitize=thread, you should also use -g so you get more meaningful output 🎀
Note that sanitized atomic builtins cannot throw exceptions when operating on invalid memory addresses with non-call exceptions (-fnon-call-exceptions).
-fsanitize=leak— enables LeakSanitizer, a memory leak detector 🍓 This option only matters for linking of executables. The executable is linked against a library that overridesmallocand other allocator functions. See <https://github.com/google/sanitizers/wiki/AddressSanitizerLeakSanitizer> for more details. The run-time behavior can be influenced using theLSAN_OPTIONSenvironment variable. The option cannot be combined with-fsanitize=thread.-fsanitize=undefined— enables UndefinedBehaviorSanitizer, a fast undefined behavior detector 🌈 Various computations are instrumented to detect undefined behavior at runtime. See <https://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html> for more details. The run-time behavior can be influenced using theUBSAN_OPTIONSenvironment variable. Current suboptions are, and buckle up bestie, there's a whole list 💅-fsanitize=shift— this option enables checking that the result of a shift operation is not undefined. Note that what exactly is considered undefined differs slightly between C and C++, as well as between ISO C90 and C99, etc. This option has two suboptions,-fsanitize=shift-baseand-fsanitize=shift-exponent.-fsanitize=shift-exponent— this option enables checking that the second argument of a shift operation is not negative and is smaller than the precision of the promoted first argument 🔍-fsanitize=shift-base— if the second argument of a shift operation is within range, check that the result of a shift operation is not undefined. Note that what exactly is considered undefined differs slightly between C and C++, as well as between ISO C90 and C99, etc.-fsanitize=integer-divide-by-zero— detect integer division by zero 💀-fsanitize=unreachable— with this option, the compiler turns the__builtin_unreachablecall into a diagnostics message call instead. When reaching the__builtin_unreachablecall, the behavior is undefined 🫠-fsanitize=vla-bound— this option instructs the compiler to check that the size of a variable length array is positive ✨-fsanitize=null— this option enables pointer checking. Particularly, the application built with this option turned on will issue an error message when it tries to dereference aNULLpointer, or if a reference (possibly an rvalue reference) is bound to aNULLpointer, or if a method is invoked on an object pointed by aNULLpointer 👑-fsanitize=return— this option enables return statement checking. Programs built with this option turned on will issue an error message when the end of a non-void function is reached without actually returning a value. This option works in C++ only 🎀-fsanitize=signed-integer-overflow— this option enables signed integer overflow checking. We check that the result of+,*, and both unary and binary-does not overflow in the signed arithmetics. This also detectsINT_MIN / -1signed division. Note, integer promotion rules must be taken into account. That is, the following is not an overflow:
signed char a = SCHAR_MAX;
a++;-fsanitize=bounds— this option enables instrumentation of array bounds. Various out of bounds accesses are detected. Flexible array members, flexible array member-like arrays, and initializers of variables with static storage are not instrumented, with the exception of flexible array member-like arrays for which-fstrict-flex-arraysor-fstrict-flex-arrays=options orstrict_flex_arrayattributes say they shouldn't be treated like flexible array member-like arrays 🦋-fsanitize=bounds-strict— this option enables strict instrumentation of array bounds. Most out of bounds accesses are detected, including flexible array member-like arrays. Initializers of variables with static storage are not instrumented.-fsanitize=alignment— this option enables checking of alignment of pointers when they are dereferenced, or when a reference is bound to an insufficiently aligned target, or when a method or constructor is invoked on an insufficiently aligned object 🔍-fsanitize=object-size— this option enables instrumentation of memory references using the__builtin_dynamic_object_sizefunction. Various out of bounds pointer accesses are detected.-fsanitize=float-divide-by-zero— detect floating-point division by zero 🍓 Unlike other similar options,-fsanitize=float-divide-by-zerois not enabled by-fsanitize=undefined, since floating-point division by zero can be a legitimate way of obtaining infinities and NaNs. So it's literally not always a crime, periodt.-fsanitize=float-cast-overflow— this option enables floating-point type to integer conversion checking. We check that the result of the conversion does not overflow. Unlike other similar options,-fsanitize=float-cast-overflowis not enabled by-fsanitize=undefined. This option does not work well withFE_INVALIDexceptions enabled 😭-fsanitize=nonnull-attribute— this option enables instrumentation of calls, checking whether null values are not passed to arguments marked as requiring a non-null value by thenonnullfunction attribute 🫶-fsanitize=returns-nonnull-attribute— this option enables instrumentation of return statements in functions marked with thereturns_nonnullfunction attribute, to detect returning of null values from such functions ✨-fsanitize=bool— this option enables instrumentation of loads frombool. If a value other than0/1is loaded, a run-time error is issued 💀-fsanitize=enum— this option enables instrumentation of loads from an enum type. If a value outside the range of values for the enum type is loaded, a run-time error is issued 👑
-fsanitize=vptr— bestie this one instruments your C++ member function calls, member accesses, and some conversions between pointers to base and derived classes. The whole point? To verify the object you're referencing actually has the correct dynamic type. Vibe check on your pointers fr. 💅-fsanitize=pointer-overflow— instruments pointer arithmetic. If the pointer arithmetic overflows, you get a run-time error. No silent chaos allowed. ✨-fsanitize=builtin— instruments the arguments passed to selected builtin functions. If an invalid value gets passed to one of those arguments, boom, run-time error. Example: passing0as the argument to__builtin_ctzor__builtin_clzinvokes undefined behavior, and this option diagnoses it. 🫶
Heads up bestie 😭 sanitizers tend to increase the rate of false positive warnings, most notably the ones around-Wmaybe-uninitialized. We recommend against combining-Werrorand [the use of] sanitizers. Do not do it. Please.
While -ftrapv makes traps get emitted for signed overflows, -fsanitize=undefined gives you a diagnostic message instead. This currently works only for the C family of languages.
-fno-sanitize=all— disables all previously enabled sanitizers. And no,-fsanitize=allis not allowed, because some sanitizers simply cannot be used together. It's giving incompatible friend groups. 🙈-fasan-shadow-offset=number— forces GCC to use a custom shadow offset in AddressSanitizer checks. Super useful for experimenting with different shadow memory layouts in Kernel AddressSanitizer. 🧚-fsanitize-sections=s1,s2,...— sanitize global variables in selected user-defined sections. Andsimay contain wildcards, so go off. ✨-fsanitize-recover[=opts]—-fsanitize-recover=controls error recovery mode for the sanitizers named in the comma-separated list of opts. Turning this on for a sanitizer component makes it attempt to keep running the program as if no error happened. Translation: multiple runtime errors can be reported in a single program run, and the exit code of the program may indicate success even when errors have been reported. The-fno-sanitize-recover=option alters this behavior: only the first detected error is reported and then the program exits with a non-zero exit code.
Currently this feature only works for -fsanitize=undefined (and its suboptions except for -fsanitize=unreachable and -fsanitize=return), -fsanitize=float-cast-overflow, -fsanitize=float-divide-by-zero, -fsanitize=bounds-strict, -fsanitize=kernel-address and -fsanitize=address. For these sanitizers error recovery is turned on by default, except -fsanitize=address, for which this feature is experimental. -fsanitize-recover=all and -fno-sanitize-recover=all are also accepted — the former enables recovery for all sanitizers that support it, the latter disables recovery for all sanitizers that support it.
Even if recovery mode is turned on on the compiler side, it also needs to be enabled on the runtime library side, otherwise the failures are still fatal. The runtime library defaults to halt_on_error=0 for ThreadSanitizer and UndefinedBehaviorSanitizer, while the default value for AddressSanitizer is halt_on_error=1. You can override this through setting the halt_on_error flag in the corresponding environment variable.
Syntax without an explicit opts parameter is deprecated 💀 It is equivalent to specifying an opts list of:
undefined,float-cast-overflow,float-divide-by-zero,bounds-strict-fsanitize-address-use-after-scope— enables sanitization of local variables to detect use-after-scope bugs. And it sets-fstack-reusetonone. 👑-fsanitize-trap[=opts]— this one instructs the compiler to report undefined behavior for the sanitizers named in the comma-separated list of opts using__builtin_traprather than alibubsanlibrary routine. If this option is enabled for a certain sanitizer, it takes precedence over-fsanitizer-recover=for that sanitizer —__builtin_trapwill be emitted and be fatal regardless of whether recovery is enabled or disabled using-fsanitize-recover=.
The advantage? The libubsan library is not needed and is not linked in, so this is usable even in freestanding environments. Iconic honestly. 🌈
Currently this feature works with -fsanitize=undefined (and its suboptions except for -fsanitize=vptr), -fsanitize=float-cast-overflow, -fsanitize=float-divide-by-zero and -fsanitize=bounds-strict. -fsanitize-trap=all can also be specified, which enables it for undefined suboptions, -fsanitize=float-cast-overflow, -fsanitize=float-divide-by-zero and -fsanitize=bounds-strict. If -fsanitize-trap=undefined or -fsanitize-trap=all is used and -fsanitize=vptr is enabled on the command line, the instrumentation is silently ignored as the instrumentation always needs libubsan support, and -fsanitize-trap=vptr is not allowed.
-fsanitize-undefined-trap-on-error— the deprecated equivalent of-fsanitize-trap=all. Old news bestie, it's giving legacy. 💀-fsanitize-coverage=trace-pc— enables coverage-guided fuzzing code instrumentation. Inserts a call to__sanitizer_cov_trace_pcinto every basic block. Every. Single. One. 🔥-fsanitize-coverage=trace-cmp— enables dataflow guided fuzzing code instrumentation. It inserts a call to__sanitizer_cov_trace_cmp1,__sanitizer_cov_trace_cmp2,__sanitizer_cov_trace_cmp4or__sanitizer_cov_trace_cmp8for integral comparison with both operands variable, or__sanitizer_cov_trace_const_cmp1,__sanitizer_cov_trace_const_cmp2,__sanitizer_cov_trace_const_cmp4or__sanitizer_cov_trace_const_cmp8for integral comparison with one operand constant,__sanitizer_cov_trace_cmpfor__sanitizer_cov_trace_cmpdfor float or double comparisons, and__sanitizer_cov_trace_switchfor switch statements. A whole alphabet of trace calls, we love to see it. ✨-fcf-protection=[full|branch|return|none|check]/-fcf-protection— enables code instrumentation to increase program security by checking that target addresses of control-flow transfer instructions (such as indirect function call, function return, indirect jump) are valid. This prevents diverting the flow of control to an unexpected target. It's intended to protect against threats like Return-oriented Programming (ROP), and similarly call/jmp-oriented programming (COP/JOP). Not the girls messing around 💅
The -fcf-protection= keywords are interpreted like this:
- The value
branchtells the compiler to implement checking of validity of control-flow transfer at the point of indirect branch instructions, i.e. call/jmp instructions. - The value
returnimplements checking of validity at the point of returning from a function. - The value
fullis an alias for specifying bothbranchandreturn. - The value
checkis used for the final link with link-time optimization (LTO). An error is issued if LTO object files are compiled with different-fcf-protectionvalues. The valuecheckis ignored at the compile time. - The value
noneturns off instrumentation.
-fcf-protection is an alias for -fcf-protection=full. To override a previous -fcf-protection option on the command line, add -fcf-protection=none and then -fcf-protection=kind.
The macro __CET__ is defined when -fcf-protection is used. The first bit of __CET__ is set to 1 for the value branch and the second bit of __CET__ is set to 1 for the return.
You can also use the nocf_check attribute to identify which functions and calls should be skipped from instrumentation.
Currently the x86 GNU/Linux target provides an implementation based on Intel Control-flow Enforcement Technology (CET) which works for i686 processor or newer. 🎀
-fharden-compares— for every logical test that survives gimple optimizations and is not the condition in a conditional branch (for example, conditions tested for conditional moves, or to store in boolean variables), emit extra code to compute and verify the reversed condition, and to call__builtin_trapif the results do not match. Use it with-fharden-conditional-branchesto cover all conditionals. Double-checking your double-checks, no cap. 🫶-fharden-conditional-branches— for every non-vectorized conditional branch that survives gimple optimizations, emit extra code to compute and verify the reversed condition, and to call__builtin_trapif the result is unexpected. Use it with-fharden-comparesto cover all conditionals. The way these two complete each other 😭💖-fharden-control-flow-redundancy— emits extra code to set booleans when entering basic blocks, and to verify and trap, at function exits, when the booleans do not form an execution path that is compatible with the control flow graph.
Verification takes place before returns, before mandatory tail calls (see below) and, optionally, before escaping exceptions with -fhardcfr-check-exceptions, before returning calls with -fhardcfr-check-returning-calls, and before noreturn calls with -fhardcfr-check-noreturn-calls.
Tail call optimization takes place too late to affect control flow redundancy, but calls annotated as mandatory tail calls by language front-ends, and any calls marked early enough as potential tail calls, would also have verification issued before the call — but these possibilities are merely theoretical, as these conditions can only be met when using custom compiler plugins. iykyk 🌸
-fhardcfr-skip-leaf— disables-fharden-control-flow-redundancyin leaf functions. Let the leaves live bestie 🍃-fhardcfr-check-exceptions— when-fharden-control-flow-redundancyis active, check the recorded execution path against the control flow graph at exception escape points, as if the function body was wrapped with a cleanup handler that performed the check and reraised. This option is enabled by default; use-fno-hardcfr-check-exceptionsto disable it.-fhardcfr-check-returning-calls— when-fharden-control-flow-redundancyis active, check the recorded execution path against the control flow graph before any function call immediately followed by a return of its result, if any, so as to not prevent tail-call optimization, whether or not it is ultimately optimized to a tail call.
This option is enabled by default whenever sibling call optimizations are enabled (see -foptimize-sibling-calls), but it can be enabled (or disabled, using its negated form) explicitly, regardless of the optimizations. ⭐
-fhardcfr-check-noreturn-calls=[always|no-xthrow|nothrow|never]— so when-fharden-control-flow-redundancyis doing its thing, this decides whether we vibe-check the recorded execution path against the control flow graph right beforenoreturncalls 💅. Your choices are: all of them (always), the ones that aren't expected to hand control back to the caller through an exception (no-xthrow, and yes bestie that's the default ✨), the ones that may not return control to the caller through an exception either (nothrow), or literally none of them (never). Pick your fighter.
Lowkey important: if you check before a noreturn function that might return control to the caller through an exception, the check can happen more than once — like, if the exception gets caught in the caller, whether that's by a handler or a cleanup 🙈. And when -fhardcfr-check-exceptions is also on, the compiler will dodge associating a noreturn call with the implicitly-added cleanup handler, since that'd be redundant with the check that already happened before the call. But other handlers or cleanups in the function, if they get activated, will mess with the recorded execution path and check it again when another checkpoint is hit. The checkpoint might even be another noreturn call, so yeah — checking may end up happening multiple times. It's giving overachiever 😭.
Also, various optimizers may cause calls to be marked as noreturn and/or nothrow, even when the corresponding attributes aren't there — which can affect where checks get placed before calls, plus the addition of implicit cleanup handlers for them. This unpredictability, and the fact that raising and reraising exceptions frequently amounts to implicitly calling noreturn functions, is exactly why no-xthrow is the default here: it only excludes from the noreturn treatment the internal functions used to (re)raise exceptions, which aren't affected by these optimizations. No cap.
-fhardened— turns on a whole lil squad of flags for C and C++ that level up the security of the generated code without touching its ABI 🛡️✨. The exact flags enabled might change between major releases of GCC, but rn they are:
-D_FORTIFY_SOURCE=3 -D_GLIBCXX_ASSERTIONS
-ftrivial-auto-var-init=zero -fPIE -pie -Wl,-z,relro,-z,now
-fstack-protector-strong -fstack-clash-protection
-fcf-protection=full (x86 GNU/Linux only)You can get the list of options -fhardened enables by using the --help=hardened option. Very handy, very demure.
If your system glibc is older than 2.35, then -D_FORTIFY_SOURCE=2 gets used instead. FYI.
This option is meant for production builds, not just for debug builds. Like, wear it out, don't save it for the group chat.
Currently, -fhardened is only supported on GNU/Linux targets.
And here's the cute part: -fhardened only enables a particular option if it wasn't already specified anywhere on the command line. So for example, -fhardened -fstack-protector will only enable -fstack-protector, but not -fstack-protector-strong. Respect the user's choices 💅.
-fstack-protector— emits extra code to check for buffer overflows, like those stack smashing attacks 💀. It does this by adding a guard variable to functions with vulnerable objects. That includes functions that callalloca, and functions with buffers larger than or equal to 8 bytes. The guards get 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. Periodt. Only variables that are actually allocated on the stack count, btw — optimized-away variables or variables allocated in registers don't count. Sorry bestie, rules are rules.-fstack-protector-all— same energy as-fstack-protector, except all functions are protected 👑. Everyone eats.-fstack-protector-strong— like-fstack-protectorbut it pulls in additional functions to protect — those that have local array definitions, or have references to local frame addresses. Again, only variables actually allocated on the stack are considered; optimized away variables or variables allocated in registers don't count 🙅♀️.-fstack-protector-explicit— like-fstack-protector, but it only protects those functions that have thestack_protectattribute. Selective. Exclusive. Iconic.-fstack-check— generates code to verify you don't go beyond the boundary of the stack 🚧. You should specify this flag if you're running in a multi-threaded environment, but you only rarely need it in a single-threaded environment, since stack overflow is automatically detected on nearly all systems when there's only one stack.
Heads up: this switch doesn't actually cause the checking to be done — the operating system or the language runtime has to do that. The switch just generates code to make sure they see the stack being extended.
You can additionally specify a string parameter: no means no checking, generic means force the use of old-style checking, specific means use the best checking method and is equivalent to bare -fstack-check.
Old-style checking is a generic mechanism that needs no specific target support in the compiler, but bestie, it comes with these drawbacks:
- Modified allocation strategy for large objects: they are always allocated dynamically if their size exceeds a fixed threshold. Note this may change the semantics of some code. 😬
- Fixed limit on the size of the static frame of functions: when it's topped by a particular function, stack checking is not reliable and a warning is issued by the compiler.
- Inefficiency: because of both the modified allocation strategy and the generic implementation, code performance is hampered. 💀
Note that old-style stack checking is also the fallback method for specific if no target support has been added in the compiler.
-fstack-check= is designed for Ada's needs to detect infinite recursion and stack overflows. specific is an excellent choice when compiling Ada code ✨. It's not generally sufficient to protect against stack-clash attacks though — to protect against those you want -fstack-clash-protection.
-fstack-clash-protection— generates code to prevent stack clash style attacks 🛡️. When this option is on, the compiler will only allocate one page of stack space at a time, and each page is accessed immediately after allocation. So it prevents allocations from jumping over any stack guard page provided by the operating system. Immaculate.
Most targets don't fully support stack clash protection. However, on those targets -fstack-clash-protection will still protect dynamic stack allocations. And -fstack-clash-protection may also provide limited protection for static stack allocations if the target supports -fstack-check=specific.
-fstack-limit-register=reg,-fstack-limit-symbol=sym,-fno-stack-limit— generates code to make sure the stack doesn't grow beyond a certain value, either the value of a register or the address of a symbol. If a larger stack is required, a signal is raised at run time 🚨. For most targets, the signal is raised before the stack overruns the boundary, so it's possible to catch the signal without taking special precautions. Nice.
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.
You can locally override stack limit checking by using the no_stack_limit function attribute.
-fsplit-stack— generates code to automatically split the stack before it overflows ✂️. The resulting program has a discontiguous stack which can only overflow if the program is unable to allocate any more memory. This is most useful when running threaded programs, since it's no longer necessary to calculate a good stack size to use for each thread. Currently this is only implemented for the x86 targets running GNU/Linux.
When code compiled with -fsplit-stack calls code compiled without -fsplit-stack, there may not be much stack space available for the latter code to run 😭. If compiling all code — including library code — with -fsplit-stack isn't an option, then the linker can fix up these calls so that the code compiled without -fsplit-stack always has a large stack. Support for this is implemented in the gold linker in GNU Binutils release 2.21 and later.
-fstrub=disable— disables stack scrubbing entirely, ignoring anystrubattributes. Bye 👋.-fstrub=strict— functions default tostrubmodedisabled, and it strictly applies the restriction that only functions associated withstrub-callablemodes (at-calls,callableandalways_inlineinternal) arecallableby functions withstrub-enabled modes (at-callsandinternal). Very strict. Very serious. 💅-fstrub=relaxed— restores the default stack scrub (strub) setting, meaningstrubis only enabled as required bystrubattributes associated with function and data types. "Relaxed" means strub contexts are only prevented from calling functions explicitly associated withstrubmodedisabled. This option is only useful to override other-fstrub=*options that come before it in the command line. Chill vibes only 🌸.-fstrub=at-calls— enablesat-callsstrubmode where viable. The primary use of this option is for testing. It exercises thestrubmachinery in scenarios strictly local to a translation unit. Thisstrubmode modifies function interfaces, so any function that's visible to other translation units, or that has its address taken, won't be affected by this option. Optimization options may also affect viability. See thestrubattribute documentation for details on viability and eligibility requirements 📖.-fstrub=internal— enablesinternalstrubmode where viable. The primary use of this option is for testing. This option is intended to thoroughly exercise parts of thestrubmachinery that implement the less efficient, but interface-preservingstrubmode. Functions that wouldn't be affected by this option are quite uncommon 👀.-fstrub=all— enables somestrubmode where viable. When both strub modes are viable,at-callsis preferred.-fdump-ipa-strubmadds function attributes that tell which mode was selected for each function. The primary use of this option is for testing, to thoroughly exercise thestrubmachinery. Go off 🔥.-fvtable-verify=[std|preinit|none]— C++ only bestie, sorry C girlies 💅 this one flips on (or off, if you pass-fvtable-verify=none) the security feature that checks at run time, on every single virtual call, that the vtable pointer you're calling through is actually valid for the type of the object and hasn't been corrupted or overwritten. If it catches a bad vtable pointer at run time, it reports an error and immediately halts the program. No cap. 🛑
This option makes run-time data structures get built at program startup, and those are what verifies the vtable pointers. The std and preinit values control when those data structures get built. Either way they're built before execution reaches main. With -fvtable-verify=std the data structures are built after shared libraries have been loaded and initialized. With -fvtable-verify=preinit they get built before shared libraries have been loaded and initialized.
If this option shows up multiple times on the command line with different values, none takes highest priority over both std and preinit, and preinit takes priority over std. Ranking queen behavior fr.
-fvtv-debug— when you pair this with-fvtable-verify=stdor-fvtable-verify=preinit, it makes the debug versions of the runtime functions for the vtable verification feature get called. This flag also makes the compiler log info about which vtable pointers it finds for each class. That info gets written to a file namedvtv_set_ptr_data.login the directory named by the environment variableVTV_LOGS_DIRif that's defined, or the current working directory otherwise. 🗂️
Note: this feature appends data to the log file. If you want a fresh log file, make sure you delete any existing one first, bestie. Don't be out here confused about why your log is 900 lines long. 💀
-fvtv-counts— this is a debugging flag. When used together with-fvtable-verify=stdor-fvtable-verify=preinit, it makes the compiler keep track of the total number of virtual calls it encounters and the number of verifications it inserts. It also counts the number of calls to certain run-time library functions that it inserts and logs that info for each compilation unit. The compiler writes all this to a file namedvtv_count_data.login the directory named by the environment variableVTV_LOGS_DIRif that's defined, or the current working directory otherwise. It also counts the size of the vtable pointer sets for each class and writes that tovtv_class_set_sizes.login the same directory. 📊
Note: this feature appends data to the log files. To get fresh log files, delete any existing ones. Same energy as above, okay? 🙈
-finstrument-functions— generates instrumentation calls for entry and exit to functions. Right after a function is entered and right before it exits, these profiling functions get called with the address of the current function and its call site. (On some platforms__builtin_return_addressdoesn't work beyond the current function, so the call site info might 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, and you can look that up exactly in the symbol table. ✨
This instrumentation also happens for functions that get expanded inline inside other functions. The profiling calls indicate where, conceptually, the inline function is entered and exited. Which means addressable versions of those functions have to be available. If every single use you have of a function gets expanded inline, that might mean extra code size. If you use extern inline in your C code, an addressable version of those functions must be provided. (Normally that's the case anyway, but if you get lucky and the optimizer always expands the functions inline, you might have gotten away without providing static copies. Living dangerously, love that for you. 🍀)
A function can be given the attribute no_instrument_function, and then this instrumentation doesn't happen. That's useful for stuff like the profiling functions listed above, high-priority interrupt routines, and any functions the profiling functions can't safely be called from (maybe signal handlers, if the profiling routines generate output or allocate memory).
-finstrument-functions-once— similar to-finstrument-functions, but the profiling functions only get called once per instrumented function. So the first profiling function is called after the first entry into the instrumented function, and the second profiling function is called before the exit corresponding to that first entry.
The definition of "once" for this option is a lil vague because the implementation is not protected against data races. So the implementation only guarantees that the profiling functions are called at least once per process and at most once per thread — but the calls are always paired. Meaning: if a thread calls the first function, then it will call the second function, unless it never reaches the exit of the instrumented function. 🧵
-finstrument-functions-exclude-file-list=file,file,...— sets the list of functions excluded from instrumentation (peep the description of-finstrument-functionsfor the tea). If the file containing a function definition matches one of file, then that function doesn't get instrumented. The match is done on substrings: if the file parameter is a substring of the file name, it counts as a match.
For example:
-finstrument-functions-exclude-file-list=/bits/stl,include/systhat excludes any inline function defined in files whose pathnames contain /bits/stl or include/sys. 🚫
If, for some reason, you want to include letter , in one of sym, write ,. For example, -finstrument-functions-exclude-file-list=',,tmp' (note the single quote surrounding the option, bestie, the quotes are doing WORK here).
-finstrument-functions-exclude-function-list=sym,sym,...— same vibe as-finstrument-functions-exclude-file-list, except this one sets the list of function names excluded from instrumentation. The function name matched is its user-visible name, likevector<int> blah(const vector<int> &), not the internal mangled name (e.g.,_Z4blahRSt6vectorIiSaIiEE). The match is on substrings: if the sym parameter is a substring of the function name, it counts as a match. For C99 and C++ extended identifiers, the function name must be given in UTF-8, not using universal character names. 🌈-fpatchable-function-entry=N[,M]— generates N NOPs right at the beginning of each function, with the function entry point before the Mth NOP. If M is omitted it defaults to0, so the function entry points to the address just at the first NOP. Those NOP instructions reserve extra space you can use to patch in whatever instrumentation you want at run time, as long as the code segment is writable. The amount of space is controllable indirectly through the number of NOPs; the NOP instruction used corresponds to the instruction emitted by the internal GCC back-end interfacegen_nop. This behavior is target-specific and may also depend on the architecture variant and/or other compilation options. 🔧
For run-time identification, the starting addresses of these areas — which correspond to their respective function entries minus M — are additionally collected in the __patchable_function_entries section of the resulting binary.
Heads up: the value of__attribute__ ((patchable_function_entry (N,M)))takes precedence over the command-line option-fpatchable-function-entry=N,M. You can use it to increase the area size or to remove it completely on a single function. IfN=0, no pad location is recorded.
The NOP instructions are inserted at — and maybe before, depending on M — the function entry address, even before the prologue. On PowerPC with the ELFv2 ABI, for a function with dual entry points, the local entry point is this function entry address by default. See the -msplit-patch-nops option to change that.
The maximum value of N and M is 65535. On PowerPC with the ELFv2 ABI, for a function with dual entry points, the supported values for M are 0, 2, 6 and 14 when you're not using -msplit-patch-nops. 👑
🌸 Options Controlling the Preprocessor 🌸
These options control the C preprocessor, which runs on each C source file before the actual compilation happens. It's giving opening act, but honestly it's doing a lot of the heavy lifting. 💅
If you use the -E option, nothing happens except preprocessing. Some of these options only make sense together with -E, because they make the preprocessor output unsuitable for actual compilation.
On top of the options listed right here, there are a bunch of options controlling search paths for include files documented in Directory Options. Options that control preprocessor diagnostics are listed in Warning Options. Go read those too, bestie, they're right there. 📚
-D name,--define-macro=name,--define-macro name— predefine name as a macro, with definition1. Simple as that. ✨-D name=definition,--define-macro=name=definition,--define-macro name=definition— the contents of definition get tokenized and processed as if they appeared during translation phase three in a#definedirective. In particular, the definition is truncated by embedded newline characters.
If you're invoking the preprocessor from a shell or a shell-like program, you may need to use the shell's quoting syntax to protect characters like spaces that mean something in the shell syntax. Shells are dramatic like that. 🐚
If you want to define a function-like macro on the command line, write its argument list with surrounding parentheses before the equals sign (if there is one). Parentheses are meaningful to most shells, so you should quote the option. With sh and csh:
-D'name(args...)=definition'works. 🫶
-D and -U options are processed in the order they're given on the command line. All -imacros file and -include file options are processed after all -D and -U options. Order matters, always has.
-U name,--undefine-macro=name,--undefine-macro name— cancels any previous definition of name, whether it was built in or provided with a-Doption. Undefined. Gone. Not your problem anymore. 💀-includefile,--include=file,--includefile — okay so this one is like, it processes file as if#include "file"was literally the very first line of your primary source file. Cute right? But bestie, listen: the first directory it searches for file is the preprocessor's working directory, NOT the directory containing the main source file. If it's not there, then yeah, it gets searched for in the rest of the#include "..."search chain like normal. 🫶
And if you give multiple -include options? They're included in the order they appear on the command line. Order matters, no cap.
-imacrosfile,--imacros=file,--imacrosfile — exactly like-include, EXCEPT that any output produced by scanning file is just... thrown away. 🗑️ But the macros it defines stay defined. So it's giving "give me all the macros from a header without the declarations, tyvm." This lets you acquire all the macros from a header without also processing its declarations.
Oh and heads up: all files specified by -imacros are processed before all files specified by -include. So don't come for me about ordering. 💅
-undef— do NOT predefine any system-specific or GCC-specific macros. The standard predefined macros stay defined though, those are safe. ✨-pthread— defines the additional macros you need for using the POSIX threads library. You should use this option consistently for both compilation AND linking, bestie, don't half-do it. 🙈 This option is supported on GNU/Linux targets, most other Unix derivatives, and also on x86 Cygwin and MinGW targets.-M,--dependencies— instead of outputting the result of preprocessing, output a rule suitable formakedescribing the dependencies of the main source file. The preprocessor outputs onemakerule containing the object file name for that source file, a colon, and the names of all the included files — including those coming from-includeor-imacroscommand-line options. 👑
Unless specified explicitly (with -MT or -MQ), the object file name consists of the name of the source file with any suffix replaced with object file suffix and with any leading directory parts removed. If there are many included files then the rule is split into several lines using \-newline. The rule has no commands.
This option does not suppress the preprocessor's debug output, such as -dM. To avoid mixing such debug output with the dependency rules you should explicitly specify the dependency output file with -MF, or use an environment variable like DEPENDENCIES_OUTPUT. Debug output is still sent to the regular output stream as normal.
Passing -M to the driver implies -E, and suppresses warnings with an implicit -w. 💀
-MM,--user-dependencies— like-Mbut 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 appears in -MM dependency output.
-MFfile — when used with-Mor-MM, specifies a file to write the dependencies to. If no-MFswitch is given the preprocessor sends the rules to the same place it would send preprocessed output.
When used with the driver options -MD or -MMD, -MF overrides the default dependency output file.
If file is -, then the dependencies are written to stdout. 🖨️
-MG,--print-missing-file-dependencies— in conjunction with an option such as-Mrequesting dependency generation,-MGassumes 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#includedirective without prepending any path.-MGalso suppresses preprocessed output, since a missing header file renders this useless.
This feature is used in automatic updating of makefiles. 🛠️
-Mno-modules— disable dependency generation for compiled module interfaces.-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 errorsmakegives if you remove header files without updating the Makefile to match. So iconic of it honestly.
This is typical output:
test.o: test.c test.h
test.h:-MTtarget — change the target of the rule emitted by dependency generation. By default CPP takes the name of the main input file, deletes any directory components and any file suffix such as.c, and appends the platform's usual object suffix. The result is the target. 🎯
An -MT option sets 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
$(objpfx)foo.o: foo.c-MQtarget — same as-MT, but it quotes any characters which are special to Make.-MQ '$(objpfx)foo.o'gives
$$(objpfx)foo.o: foo.cThe default target is automatically quoted, as if it were given with -MQ. ✨
-MD,--write-dependencies—-MDis equivalent to-M -MFfile, except that-Eis not implied. The driver determines file based on whether an-ooption is given. If it is, the driver uses its argument but with a suffix of.d, otherwise it takes the name of the input file, removes any directory components and suffix, and applies a.dsuffix.
If -MD is used in conjunction with -E, any -o switch is understood to specify the dependency output file, but if used without -E, each -o is understood to specify a target object file.
Since -E is not implied, -MD can be used to generate a dependency output file as a side effect of the compilation process. Sneaky, but make it fashion. 💅
-MMD,--write-user-dependencies— like-MDexcept mention only user header files, not system header files.-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-Cto 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.
-fdirectives-only— when preprocessing, handle directives, but do not expand macros.
The option's behavior depends on the -E and -fpreprocessed options. 👀
With -E, preprocessing is limited to the handling of directives such as #define, #ifdef, and #error. Other preprocessor operations, such as macro expansion and trigraph conversion are not performed. In addition, the -dD option is implicitly enabled.
With -fpreprocessed, predefinition of command line and most builtin macros is disabled. Macros such as __LINE__, which are contextually dependent, are handled normally. This enables compilation of files previously preprocessed with -E -fdirectives-only.
With both -E and -fpreprocessed, the rules for -fpreprocessed take precedence. This enables full preprocessing of files previously preprocessed with -E -fdirectives-only.
-fdollars-in-identifiers— accept$in identifiers. 💰-fextended-identifiers— accept universal character names and extended characters in identifiers. This option is enabled by default for C99 (and later C standard versions) and C++.-fno-canonical-system-headers— when preprocessing, do not shorten system header paths with canonicalization.-fmax-include-depth=depth — set the maximum depth of the nested#include. The default is200. 🌀-fsearch-include-path[=kind]— look for input files on the#includepath, not just the current directory. This is particularly useful with C++20 modules, for which both header units and module interface units need to be compiled directly:
g++ -c -std=c++20 -fmodules -fsearch-include-path bits/stdc++.h bits/std.cckind defaults to user, which looks on the #include "..." search path; you can also explicitly specify system for the #include <...> search path.
-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 than1or greater than100, the option is ignored. The default is8. 📏-ftrack-macro-expansion[=level]— track locations of tokens across macro expansions. This allows the compiler to emit diagnostic about the current macro expansion stack when a compilation error occurs in a macro expansion. Using this option makes the preprocessor and the compiler consume more memory. 😭 The level parameter can be used to choose the level of precision of token location tracking thus decreasing the memory consumption if necessary. Value0of level de-activates this option. Value1tracks tokens locations in a degraded mode for the sake of minimal memory overhead. In this mode all tokens resulting from the expansion of an argument of a function-like macro have the same location. Value2tracks tokens locations completely. This value is the most memory hungry. When this option is given no argument, the default parameter value is2.
Note that -ftrack-macro-expansion=2 is activated by default. 🎀
-fmacro-prefix-map=old=new — when preprocessing files residing in directory old, expand the__FILE__and__BASE_FILE__macros as if the files resided in directory new instead. This can be used to change an absolute path to a relative path by using.for new which can result in more reproducible builds that are location independent. This option also affects__builtin_FILE()during compilation. See also-ffile-prefix-mapand-fcanon-prefix-map. 🗺️-fexec-charset=charset — set the execution character set, used for string and character constants. The default isUTF-8. charset can be any encoding supported by the system'siconvlibrary routine. 🌈-fwide-exec-charset=charset— sets the wide execution character set, the one used for wide string and character constants. The default is one ofUTF-32BE,UTF-32LE,UTF-16BE, orUTF-16LE— whichever matches the width ofwchar_tplus the big-endian or little-endian byte order your codegen is using. Same tea as-fexec-charset: charset can be literally any encoding the system's"iconv"library routine supports, but bestie, you will have problems with encodings that don't fit exactly inwchar_t. It's giving square peg, round hole. 💅-finput-charset=charset— sets the input character set, used to translate from the character set of the input file over to the source character set GCC actually works with. Default isUTF-8. Again, charset can be any encoding the system's"iconv"library routine supports. ✨
If the input character set is UTF-8, warnings about ill-formed code unit sequences get issued if -Winvalid-utf8 is enabled. Otherwise, no diagnostics at all when the input character set matches the execution character set. But if they're different? Ill-formed code unit sequences become an error during transcoding to the execution character set. No cap. 😭
-fpch-deps— when you're using precompiled headers, this flag makes the dependency-output flags also list the files from the precompiled header's dependencies. If you don't specify it, only the precompiled header gets listed, not the files that were used to create it — because those files are literally not consulted when a precompiled header is used. It's giving "why would I cite my sources, I'm not even reading them." 🙈-fpch-preprocess— lets you use a precompiled header together with-E. It inserts a special"#pragma", specifically#pragma GCC pch_preprocess "filename", into the output to mark where the precompiled header was found, plus its filename. When-fpreprocessedis 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 gets 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 lives somewhere else. The filename may be absolute, or it may be relative to GCC's current directory. 🦋
-fworking-directory— enables generation of linemarkers in the preprocessor output that let the compiler know the current working directory at the time of preprocessing. When this is on, the preprocessor emits, after the initial linemarker, a second linemarker with the current working directory followed by two slashes. GCC uses that 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 you can inhibit it with the negated form-fno-working-directory. And if the-Pflag is on the command line, this option has no effect, since no"#line"directives are emitted whatsoever. Periodt. 💜-C,--comments— do not discard comments. All comments get passed through to the output file, except for comments in processed directives, which are deleted along with the directive. 🍓
Be prepared for side effects when you use -C; it makes the preprocessor 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 #. The way that a comment can just demote a directive like that… iconic chaos. 🌸
-CC,--comments-in-macros— do not discard comments, including during macro expansion. This is like-C, except that comments contained within macros also get passed through to the output file wherever the macro is expanded. 🔥
On top of the side effects of -C, the -CC option causes all C++-style comments inside a macro to be converted to C-style comments. Why? To prevent later use of that macro from inadvertently commenting out the rest of the source line. Foresight. Queen behavior. 👑
The -CC option is generally used to support lint comments.
-P,--no-line-commands— inhibit generation of linemarkers in the output from the preprocessor. This might be useful when you're running the preprocessor on something that isn't C code, and it'll be sent to a program that might get confused by the linemarkers. Understandable, honestly. 🙈-traditional,--traditional,-traditional-cpp,--traditional-cpp— try to imitate the behavior of pre-standard C preprocessors, as opposed to ISO C preprocessors. See the GNU CPP manual for details. 📖
Heads up bestie: GCC does not otherwise attempt to emulate a pre-standard C compiler, and these options are only supported with the -E switch, or when invoking CPP explicitly.
-trigraphs,--trigraphs— support ISO C trigraphs. These are three-character sequences, all starting with??, that ISO C defines to stand for single characters. For example,??/stands for\, so'??/n'is a character constant for a newline. ✨
The nine trigraphs and their replacements are:
Trigraph: ??( ??) ??< ??> ??= ??/ ??' ??! ??-
Replacement: [ ] { } # \ ^ | ~By default, GCC ignores trigraphs, but in standard-conforming modes it converts them. See the -std and -ansi options. 🌈
-remap— enable special code to work around file systems that only permit very short file names, such as MS-DOS. It's giving 8.3 energy. 💀-H,--trace-includes— print the name of each header file used, in addition to other normal activities. Each name is indented to show how deep in the#includestack it is. Precompiled header files are also printed, even if they're found to be invalid; an invalid precompiled header file is printed with...xand a valid one with...!. 🫶-dletters,--dump=letters,--dump letters— says to make debugging dumps during compilation as specified by letters. The flags documented here are the ones relevant to the preprocessor. Other letters get interpreted by the compiler proper, or reserved for future versions of GCC, and so are silently ignored. If you specify letters whose behavior conflicts, the result is undefined. 🚨-dM,--dump=M— instead of the normal output, generate a list of#definedirectives 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 filefoo.h, the command
touch foo.h; cpp -dM foo.hshows all the predefined macros. 🌟
If you use -dM without the -E option, -dM is interpreted as a synonym for -fdump-rtl-mach. Sneaky little plot twist.
-dD,--dump=D— like-dMexcept that it outputs both the#definedirectives and the result of preprocessing. Both kinds of output go to the standard output file. Double the slay. 💅-dN,--dump=N— like-dD, but emit only the macro names, not their expansions. Minimalist. 🎀-dI,--dump=I— output#includedirectives in addition to the result of preprocessing.-dU,--dump=U— like-dDexcept that only macros that are expanded, or whose definedness is tested in preprocessor directives, are output; the output is delayed until the use or test of the macro; and#undefdirectives are also output for macros tested but undefined at the time. It's giving "only show up when it's relevant." 💜
-fdebug-cpp— this option is only useful for debugging GCC. When used from CPP or with-E, it dumps debugging information about location maps. Every token in the output is preceded by the dump of the map its location belongs to. 🗺️
When used from GCC without -E, this option has no effect. Nothing. Zilch. 🌸
-Wp,option— you can use-Wp,optionto bypass the compiler driver and pass option directly through to the preprocessor. If option contains commas, it gets split into multiple options at the commas. However — and this is a big however — many options are modified, translated or interpreted by the compiler driver before being passed to the preprocessor, and-Wpforcibly bypasses this phase. The preprocessor's direct interface is undocumented and subject to change, so whenever possible you should avoid using-Wpand let the driver handle the options instead. 🙈-Xpreprocessor option— pass option as an option to the preprocessor. You can use this to supply system-specific preprocessor options that GCC does not recognize. 🔧
If you want to pass an option that takes an argument, you must use -Xpreprocessor twice — once for the option and once for the argument. Yes, twice. I know. 😭
-no-integrated-cpp,--no-integrated-cpp— perform preprocessing as a separate pass before compilation. By default, GCC performs preprocessing as an integrated part of input tokenization and parsing. If this option is provided, the appropriate language front end (cc1,cc1plus, orcc1objfor C, C++, and Objective-C, respectively) is instead invoked twice — once for preprocessing only, and once for actual compilation of the preprocessed input. This option may be useful in conjunction with the-Bor-wrapperoptions to specify an alternate preprocessor or perform additional processing of the program source between normal preprocessing and compilation. Two passes, no cap. ✨
🌈 Passing Options to the Assembler 🌈
You can pass options to the assembler. Bestie, the assembler deserves love too. 🫶
-Wa,option— pass option as an option to the assembler. If option contains commas, it gets split into multiple options at the commas.-Xassembler option,--for-assembler=option,--for-assembler option— pass option as an option to the assembler. You can use this to supply system-specific assembler options that GCC does not recognize. 🔧
If you want to pass an option that takes an argument, you must use -Xassembler twice — once for the option and once for the argument. Same rule as before, iconic consistency. 💅
🌸 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. Like, literally pointless. Zero notes. 💀
object-file-name— so if a file name does NOT end in one of the special recognized suffixes, GCC is like "okay bestie, this is an object file or a library." Iconic. (The linker is the one that tells object files apart from libraries, by peeking at the file contents, nosy queen.) If linking is happening, those object files get fed to the linker as input. ✨-c,-S,-E— if you use any of these, the linker does not run at all, and you should not be passing object file names as arguments. Periodt. 💅-flink-libatomic— turns on linking of libatomic if it's supported by the target, and lowkey it's enabled by default. The negative form-fno-link-libatomicexplicitly disables linking of libatomic if you're feeling like "not today." 🦋- *
-flinker-output=type* — this one controls code generation of the link-time optimizer. By default the linker output is automatically determined by the linker plugin (she decides 💁). But for debugging the compiler, and if you want incremental linking with a non-LTO object file, manually controlling the type can be useful. Here are the flavors:- If type is
exec, code generation produces a static binary. In this case-fpicand-fpieare both disabled. ✋ - If type is
dyn, code generation produces a shared library. In this case-fpicor-fPICis preserved, but not enabled automatically. This lets you build shared libraries without position-independent code on architectures where that's possible, i.e. on x86. iykyk - If type is
pie, code generation produces an-fpieexecutable. This results in similar optimizations asexecexcept that-fpieis not disabled if specified at compilation time. 🥧 - If type is
rel, the compiler assumes incremental linking is being done. The sections containing intermediate code for link-time optimization are merged, pre-optimized, and output to the resulting object file. Also, if-ffat-lto-objectsis specified, binary code is produced for future non-LTO linking. The object file produced by incremental linking is smaller than a static library produced from the same object files. And at link time, the result of incremental linking also loads faster than a static library, assuming the majority of objects in the library are used. ✨ - Finally,
nolto-relconfigures the compiler for incremental linking where code generation is forced, a final binary is produced, and the intermediate code for later link-time optimization is stripped. When multiple object files are linked together, the resulting code is better optimized than with link-time optimizations disabled (for example, cross-module inlining happens), but most of the benefits of whole-program optimizations are lost. 😭
- If type is
During the incremental link (by -r) the linker plugin defaults to rel. GNU Binutils 2.44 or later is needed to incrementally link LTO objects and non-LTO objects into a single mixed object file. If any of the object files in an incremental link cannot be used for link-time optimization, the linker plugin issues a warning and uses nolto-rel. To maintain whole-program optimization, link such objects into a static library instead. 🫶
-fuse-ld=bfd— use the bfd linker instead of the default linker. 🎀-fuse-ld=gold— use the gold linker instead of the default linker. 👑-fuse-ld=lld— use the LLVM lld linker instead of the default linker. 💜-fuse-ld=mold— use the Modern Linker (mold) instead of the default linker. 🍓-fuse-ld=wild— use the Wild linker (wild) instead of the default linker. 🌸-llibrary,-l library— search the library named library when linking. (That second form, with the library as a separate argument, exists only for POSIX compliance and is not recommended, bestie.) The-loption gets passed directly to the linker by GCC, so go read your linker documentation for the exact details — the general description below applies to the GNU linker. 💅
The linker searches a standard list of directories for the library. Those directories include several standard system directories plus any that you specify with -L. Static libraries are archives of object files and have file names like liblibrary.a. Some targets also support shared libraries, which typically have names like liblibrary.so. If both static and shared libraries are found, the linker gives preference to linking with the shared library — unless -static is used. 🌈
Where you write this option in the command actually matters, no cap: the linker searches and processes libraries and object files in the order they're specified. So 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. 😭
-lobjc— you need this special case of the-loption in order to link an Objective-C or Objective-C++ program. That's it, that's the vibe. 🎀-nostartfiles— do not use the standard system startup files when linking. The standard system libraries are used normally, unless-nostdlib,-nolibc, or-nodefaultlibsis used. 🦋-nodefaultlibs— do not use the standard system libraries when linking. Only the libraries you specify get passed to the linker, and options specifying linkage of the system libraries — like-static-libgccor-shared-libgcc— are ignored. The standard startup files are used normally, unless-nostartfilesis 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. 👀
-nolibc— do not use the C library, or system libraries tightly coupled with it, when linking. You still link with the startup files, libgcc, or toolchain-provided language support libraries such as libgnat, libgfortran or libstdc++ — unless options preventing their inclusion are used as well. This typically removes-lcfrom the link command line, as well as system libraries that normally go with it and become meaningless when the absence of a C library is assumed, for example-lpthreador-lmin some configurations. This is intended for bare-board targets when there is indeed no C library available. 🫶-nostdlib,--no-standard-libraries— do not use the standard system startup files or libraries when linking. No startup files, and only the libraries you specify get passed to the linker, and options specifying linkage of the system libraries — such as-static-libgccor-shared-libgcc— are ignored. ✨
The compiler may generate calls to memcmp, memset, memcpy and memmove. These entries are usually resolved by entries in libc. These entry points should be supplied through some other mechanism when this option is specified. 👑
One of the standard libraries bypassed by -nostdlib and -nodefaultlibs is libgcc.a, a library of internal subroutines which GCC uses to overcome shortcomings of particular machines, or special needs for some languages. 💀
In most cases, you need libgcc.a even when you want to avoid other standard libraries. In other words, when you specify -nostdlib or -nodefaultlibs you should usually specify -lgcc as well. This ensures that you have no unresolved references to internal GCC library subroutines. (An example of such an internal subroutine is __main, used to ensure C++ constructors are called.) 🎀
-nostdlib++— do not implicitly link with standard C++ libraries. Bye bye, C++ runtime. 💜-eentry,--entry=entry, *--entry*entry — specify that the program entry point is entry. The argument is interpreted by the linker; the GNU linker accepts either a symbol name or an address. 🚪✨-pie,--pie— produce a dynamically linked position independent executable on targets that support it. For predictable results, you must also specify the same set of options used for compilation (-fpie,-fPIE, or model suboptions) when you specify this linker option. Matching sets only, bestie — don't mix and match. 🥧-no-pie— don't produce a dynamically linked position independent executable. 🚫-static-pie,--static-pie— produce a static position independent executable on targets that support it. A static position independent executable is similar to a static executable, but it can be loaded at any address without a dynamic linker. For predictable results, you must also specify the same set of options used for compilation (-fpie,-fPIE, or model suboptions) when you specify this linker option. 🌈-pthread— link with the POSIX threads library. This option is supported on GNU/Linux targets, most other Unix derivatives, and also on x86 Cygwin and MinGW targets. On some targets this option also sets flags for the preprocessor, so it should be used consistently for both compilation and linking. 🧵💅-r— produce a relocatable object as output. This is also known as partial linking. 🎀-rdynamic— pass the flag-export-dynamicto 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 ofdlopenor to allow obtaining backtraces from within a program. 🔥-s— remove all symbol table and relocation information from the executable. Stripped and thriving. ✂️-static,--static— on systems that support dynamic linking, this overrides-pieand prevents linking with the shared libraries. On other systems, this option has no effect. 🗿-shared,--shared— produce a shared object which can then be linked with other objects to form an executable. Not all systems support this option. For predictable results, you must also specify the same set of options used for compilation (-fpic,-fPIC, or model suboptions) when you specify this linker option.[1] 🦋-shared-libgcc,-static-libgcc— on systems that provide libgcc as a shared library, these options force the use of either the shared or static version, respectively. If no shared version of libgcc was built when the compiler was configured, these options have no effect. 🙈
There are several situations in which an application should use the shared libgcc instead of the static version. The most common of these is when the application wishes to throw and catch exceptions across different shared libraries. In that case, each of the libraries as well as the application itself should use the shared libgcc. 🫶
Therefore, the g++ driver is out here automatically adding -shared-libgcc whenever you build a shared library <em>or</em> a main executable 💖 because C++ programs typically use exceptions, so this is literally the right thing to do. no notes.
If, instead, you use the GCC driver to create shared libraries, you may find that they are not always linked with the shared libgcc 😭 If GCC finds, at its configuration time, that you have a non-GNU linker or a GNU linker that does not support option --eh-frame-hdr, it links the shared version of libgcc into shared libraries by default. Otherwise, it takes advantage of the linker and optimizes away the linking with the shared version of libgcc, linking with the static version of libgcc by default. This allows exceptions to propagate through such shared libraries, without incurring relocation costs at library load time ✨
However — if a library or main executable is supposed to throw or catch exceptions, you must link it using theg++driver, or using the option-shared-libgcc, such that it is linked with the sharedlibgcc. no cap, this one matters 💅
-static-libasan— when the-fsanitize=addressoption is used to link a program, the GCC driver automatically links againstlibasan🦋 Iflibasanis available as a shared library, and the-staticoption is not used, then this links against the shared version oflibasan. The-static-libasanoption directs the GCC driver to linklibasanstatically, without necessarily linking other libraries statically.-static-libhwasan— when the-fsanitize=hwaddressoption is used to link a program, the GCC driver automatically links againstlibhwasan🌸 Iflibhwasanis available as a shared library, and the-staticoption is not used, then this links against the shared version oflibhwasan. The-static-libhwasanoption directs the GCC driver to linklibhwasanstatically, without necessarily linking other libraries statically.-static-liblsan— when the-fsanitize=leakoption is used to link a program, the GCC driver automatically links againstliblsan💜 Ifliblsanis available as a shared library, and the-staticoption is not used, then this links against the shared version ofliblsan. The-static-liblsanoption directs the GCC driver to linkliblsanstatically, without necessarily linking other libraries statically.-static-libtsan— when the-fsanitize=threadoption is used to link a program, the GCC driver automatically links againstlibtsan🧚 Iflibtsanis available as a shared library, and the-staticoption is not used, then this links against the shared version oflibtsan. The-static-libtsanoption directs the GCC driver to linklibtsanstatically, without necessarily linking other libraries statically.-static-libubsan— when the-fsanitize=undefinedoption is used to link a program, the GCC driver automatically links againstlibubsan🎀 Iflibubsanis available as a shared library, and the-staticoption is not used, then this links against the shared version oflibubsan. The-static-libubsanoption directs the GCC driver to linklibubsanstatically, without necessarily linking other libraries statically.-static-libstdc++— when theg++program is used to link a C++ program, it normally automatically links againstlibstdc++⭐ Iflibstdc++is available as a shared library, and the-staticoption is not used, then this links against the shared version oflibstdc++. That is normally fine. However, it is sometimes useful to freeze the version oflibstdc++used by the program without going all the way to a fully static link. The-static-libstdc++option directs theg++driver to linklibstdc++statically, without necessarily linking other libraries statically.-symbolic,--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, so like… vibe check your platform first 🙈-Tscript — use script as the linker script 📜 This option is supported by most systems using the GNU linker. On some targets, such as bare-board targets without an operating system, the-Toption may be required when linking to avoid references to undefined symbols.-Xlinkeroption — pass option as an option to the linker 💫 You can use this to supply system-specific linker options that GCC does not recognize.
If you want to pass an option that takes a separate argument, you must use -Xlinker twice, once for the option and once for the argument 🫶 For example, to pass -assert definitions, you must write -Xlinker -assert -Xlinker definitions. It does not work to write -Xlinker "-assert definitions", because this passes the entire string as a single argument, which is not what the linker expects. bestie that's a whole trap, don't fall in it 😭
When using the GNU linker, it is usually more convenient to pass arguments to linker options using the option=value syntax than as separate arguments ✨ For example, you can specify -Xlinker -Map=output.map rather than -Xlinker -Map -Xlinker output.map. Other linkers may not support this syntax for command-line options.
-Wl,option,--for-linker=option,--for-linkeroption — pass option as an option to the linker 🌈 If option contains commas, it is split into multiple options at the commas. You can use this syntax to pass an argument to the option. For example,-Wl,-Map,output.mappasses-Map output.mapto the linker. When using the GNU linker, you can also get the same effect with-Wl,-Map=output.map.-usymbol,--force-link=symbol,--force-linksymbol — pretend the symbol symbol is undefined, to force linking of library modules to define it 👑 You can use-umultiple times with different symbols to force loading of additional library modules.-Tbss=addr,-Tdata=addr,-Ttext=addr,-N,-n,-t,-Z,-zkeyword — these options are passed through to the linker without interpretation by GCC 💀 Refer to your linker documentation for the meanings of these options. GCC is literally just handing them over, no thoughts, head empty, go read the linker's own docs fr fr.
✨ Options for Directory Search ✨ 🦋
These options specify directories to search for header files, for libraries and for parts of the compiler 💖
-Idir,-iquotedir,-isystemdir,-idirafterdir,--include-directory-after=dir,--include-directory-afterdir,--include-directory=dir,--include-directorydir — add the directory dir to the list of directories to be searched for header files during preprocessing 🎀--include-directoryis an alias for-I, while--include-directory-afteris an alias for-idirafter. If dir begins with=or$SYSROOT, then the=or$SYSROOTis replaced by the sysroot prefix; see--sysrootand-isysroot.
Directories specified with -iquote apply only to the quote form of the directive, #include "file" 🌸 Directories specified with -I, -isystem, or -idirafter apply to lookup for both the #include "file" and #include <file> directives.
You can specify any number or combination of these options on the command line to search for header files in several directories 💅 The lookup order is as follows:
- For the quote form of the include directive, the directory of the current file is searched first.
- For the quote form of the include directive, the directories specified by
-iquoteoptions are searched in left-to-right order, as they appear on the command line. - Directories specified with
-Ioptions are scanned in left-to-right order. - Directories specified with
-isystemoptions are scanned in left-to-right order. - Standard system directories are scanned.
- Directories specified with
-idirafteroptions are scanned in left-to-right order.
You can use -I to override a system header file, substituting your own version, since these directories are searched before the standard 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. iykyk 🙈
The -isystem and -idirafter options also mark the directory as a system directory, so that it gets the same special treatment that is applied to the standard system directories 🧚
If a standard system include directory, or a directory specified with -isystem, is also specified with -I, the -I option is ignored 😱 The directory is still searched but as a system directory at its normal position in the system include chain. This is to ensure that GCC's procedure to fix buggy system headers and the ordering for the #include_next directive are not inadvertently changed. If you really need to change the search order for system directories, use the -nostdinc and/or -isystem options.
-I-,--include-barrier— split the include path 🚧 This option has been deprecated. Please use-iquoteinstead for-Idirectories before the-I-and remove the-I-option.
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" 💀 There is no way to override this effect of -I-. periodt.
-iprefixprefix,--include-prefix=prefix,--include-prefixprefix — specify prefix as the prefix for subsequent-iwithprefixoptions 🎀 If the prefix represents a directory, you should include the final/.-iwithprefixdir,-iwithprefixbeforedir,--include-with-prefix=prefix,--include-with-prefixprefix,--include-with-prefix-after=prefix,--include-with-prefix-afterprefix,--include-with-prefix-before=prefix,--include-with-prefix-beforeprefix — append dir to the prefix specified previously with-iprefix, and add the resulting directory to the include search path 🌈-iwithprefixbeforeputs it in the same place-Iwould;-iwithprefixputs it where-idirafterwould.
--include-with-prefix and --include-with-prefix-after are both aliases for -iwithprefix, while --include-with-prefix-before is an alias for -iwithprefixbefore ✨
-isysroot dir— it's giving--sysrootbut make it ✨headers only✨ bestie. except on Darwin targets, where it applies to both header files and libraries, no cap. peep--sysrootfor the full tea. 💖-imultilib dir— usediras a subdirectory of the directory that's holding the target-specific C++ headers. slay.-imultiarch dir— usediras a subdirectory of the directory containing architecture-specific C++ headers. 💅-nostdinc,--no-standard-includes— do NOT go searching the standard system directories for header files. only the directories you explicitly said with-I,-iquote,-isystem, and/or-idirafterget searched (plus the directory of the current file, if appropriate). periodt.-nostdinc++— skip the C++-specific standard directories for header files, but lowkey still search the other standard directories. (this one's used when building the C++ library, iykyk.)--embed-dir=dir,--embed-directory=dir,--embed-directory dir— append thedirdirectory to the list of searched directories for the"#embed"preprocessing directive or the"__has_embed"macro. there are literally no default directories for"#embed", bestie.
if dir begins with = or $SYSROOT, then that = or $SYSROOT gets swapped out for the sysroot prefix; peep --sysroot and -isysroot. 🦋
-iplugindir=dir— set the directory to search for plugins that get passed by-fplugin=nameinstead of-fplugin=path/name.so. this is not meant to be used by the user, it's only passed by the driver. 👑-Ldir,--library-directory=dir,--library-directory dir— add directorydirto the list of directories to be searched for-l. 💖-Bprefix,--prefix=prefix,--prefix prefix— this one specifies where to find the executables, libraries, include files, and data files of the compiler itself. main character energy fr fr.
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 the corresponding target machine and compiler version.
for each subprogram to be run, the compiler driver first tries the -B prefix, if any. if that name is not found, or if -B is not specified, the driver tries two standard prefixes, /usr/lib/gcc/ and /usr/local/lib/gcc/. if neither of those results in a file name that is found, the unmodified program name is searched for using the directories specified in your PATH environment variable.
the compiler checks to see if the path provided by -B refers to a directory, and if necessary it adds a directory separator character at the end of the path.
-B prefixes that effectively specify directory names also apply to libraries in the linker, because the compiler translates these options into -L options for the linker. they also apply to include files in the preprocessor, because the compiler translates these options into -isystem options for the preprocessor. in this case, the compiler appends include to the prefix.
the runtime support file libgcc.a can also be searched for using the -B prefix, if needed. if it is not found there, the two standard prefixes above are tried, and that is all. the file is left out of the link if it is not found by those means.
another way to specify a prefix much like the -B prefix is to use the environment variable GCC_EXEC_PREFIX.
as a special kludge, if the path provided by -B is [dir/]stageN/, where N is a number in the range 0 to 9, then it is replaced by [dir/]include. this is to help with boot-strapping the compiler. 🎀
-no-canonical-prefixes,--no-canonical-prefixes— do not expand any symbolic links, resolve references to/../or/./, or make the path absolute when generating a relative prefix. periodt.--sysroot=dir,--sysroot dir— usediras the logical root directory for headers and libraries. so like, if the compiler normally searches for headers in/usr/includeand libraries in/usr/lib, it instead searchesdir/usr/includeanddir/usr/lib. iconic.
if you use both this option and the -isysroot option, then the --sysroot option applies to libraries, but the -isysroot option applies to header files.
the GNU linker (beginning with version 2.16) has the necessary support for this option. if your linker does not support this option, the header file aspect of --sysroot still works, but the library aspect does not. 😭
--no-sysroot-suffix— for some targets, a suffix is added to the root directory specified with--sysroot, depending on the other options used, so that headers may for example be found indir/suffix/usr/includeinstead ofdir/usr/include. this option disables the addition of such a suffix. 🙈
✨ Options for Code Generation Conventions ✨
these machine-independent options control the interface conventions used in code generation. 💅
most of them have both positive and negative forms; the negative form of -ffoo is -fno-foo. in the table below, only one of the forms is listed — the one that is not the default. you can figure out the other form by either removing no- or adding it.
-fstack-reuse=reuse-level— this option controls stack space reuse for user declared local/auto variables and compiler generated temporaries.reuse_levelcan beall,named_vars, ornone.allenables stack reuse for all local variables and temporaries,named_varsenables the reuse only for user defined local variables with names, andnonedisables stack reuse completely. the default value isall. the option is needed when the program extends the lifetime of a scoped local variable or a compiler generated temporary beyond the end point defined by the language. when a lifetime of a variable ends, and if the variable lives in memory, the optimizing compiler has the freedom to reuse its stack space with other temporaries or scoped local variables whose live range does not overlap with it. legacy code extending local lifetime is likely to break with the stack reuse optimization. 💀
for example,
int *p;
{
int local1;
p = &local1;
local1 = 10;
....
}
{
int local2;
local2 = 20;
...
}
if (*p == 10) // out of scope use of local1
{
}another example:
struct A
{
A(int k) : i(k), j(k) { }
int i;
int j;
};
A *ap;
void foo(const A& ar)
{
ap = &ar;
}
void bar()
{
foo(A(10)); // temp object's lifetime ends when foo returns
{
A a(20);
....
}
ap->i+= 10; // ap references out of scope temp whose space
// is reused with a. What is the value of ap->i?
}the lifetime of a compiler generated temporary is well defined by the C++ standard. when a lifetime of a temporary ends, and if the temporary lives in memory, the optimizing compiler has the freedom to reuse its stack space with other temporaries or scoped local variables whose live range does not overlap with it. however some of the legacy code relies on the behavior of older compilers in which temporaries' stack space is not reused, the aggressive stack reuse can lead to runtime errors. this option is used to control the temporary stack reuse optimization.
-ftrapv— generates traps for signed overflow on addition, subtraction, multiplication operations. the options-ftrapvand-fwrapvoverride each other, so using-ftrapv -fwrapvon the command-line results in-fwrapvbeing effective. note that only active options override, so using-ftrapv -fwrapv -fno-wrapvon the command-line results in-ftrapvbeing effective. 🔥-fwrapv— 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. the options-ftrapvand-fwrapvoverride each other, so using-ftrapv -fwrapvon the command-line results in-fwrapvbeing effective. note that only active options override, so using-ftrapv -fwrapv -fno-wrapvon the command-line results in-ftrapvbeing effective. ✨-fwrapv-pointer— instructs the compiler to assume that pointer arithmetic overflow on addition and subtraction wraps around using twos-complement representation. this flag disables some optimizations which assume pointer overflow is invalid. 💅-fstrict-overflow— implies-fno-wrapv -fno-wrapv-pointerand when negated implies-fwrapv -fwrapv-pointer. 🌈-fexceptions— enable exception handling. generates extra code needed to propagate exceptions. for some targets, this implies GCC generates frame unwind information for all functions, which can produce significant data size overhead, although it does not affect execution. if you do not specify this option, GCC enables it by default for languages like C++ that normally require exception handling, and disables it for languages like C that do not normally require it. however, you may need to enable this option when compiling C code that needs to interoperate properly with exception handlers written in C++. you may also wish to disable this option if you are compiling older C++ programs that don't use exception handling. 🫶-fnon-call-exceptions— generate code that allows trapping instructions to throw exceptions. note that this requires platform-specific runtime support that does not exist everywhere. moreover, it only allows trapping instructions to throw exceptions, i.e. memory references or floating-point instructions. it does not allow exceptions to be thrown from arbitrary signal handlers such as"SIGALRM". this enables-fexceptions. ⭐-fdelete-dead-exceptions— bestie, this one lets the compiler treat instructions that might throw exceptions but like... don't actually do anything else for the program's execution as totally deletable. Optimization food. ✨ Heads up tho: it does not touch calls to functions — except the ones tagged with thepureorconstattributes, those are fair game. This is on by default for the Ada and C++ compilers, because the language specs literally permit it, periodt. And the optimization passes that actually remove dead exceptions? They get enabled independently at different optimization levels. 💅-funwind-tables— kinda like-fexceptions, except it only generates whatever static data is needed and doesn't mess with the generated code any other way. You normally don't need to turn this on yourself — instead, a language processor that needs this handling enables it on your behalf. Let the language processor cook. 🍓-fasynchronous-unwind-tables— generates an unwind table in DWARF format, if the target machine supports it. The table is exact at each instruction boundary, so it can be used for stack unwinding from asynchronous events — think debuggers or garbage collectors swooping in at any moment. 🔥-fno-gnu-unique— on systems with a recent GNU assembler and C library, the C++ compiler uses theSTB_GNU_UNIQUEbinding to make sure that definitions of template static data members and static local variables in inline functions stay unique even whenRTLD_LOCALis in the picture. This is necessary so you don't get problems when a library is used by two differentRTLD_LOCALplugins that both depend on a definition in one of them and therefore start disagreeing about the binding of the symbol. 😭 But — and this is the catch — it makesdlcloseget ignored for affected DSOs. So if your program relies on reinitializing a DSO viadlcloseanddlopen, you can use-fno-gnu-unique. iykyk 💀-fpcc-struct-return— returnsshortstructandunionvalues in memory like the longer ones, instead of in registers. This convention is less efficient, ngl, but the glow-up is that it allows intercallability between GCC-compiled files and files compiled with other compilers — particularly the Portable C Compiler (pcc). 🫶 The precise convention for returning structures in memory depends on the target configuration macros. And "short" structures and unions are those whose size and alignment match that of some integer type. Oh and — Warning: code compiled with the-fpcc-struct-returnswitch is not binary compatible with code compiled with the-freg-struct-returnswitch. Use it to conform to a non-default application binary interface. ⚠️-freg-struct-return— returnsstructandunionvalues in registers when possible. For small structures this is more efficient than-fpcc-struct-return. 💅 If you specify neither-fpcc-struct-returnnor-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. main character decision fr. Warning: code compiled with the-freg-struct-returnswitch is not binary compatible with code compiled with the-fpcc-struct-returnswitch. Use it to conform to a non-default application binary interface. ⚠️-fshort-enums— allocates to anenumtype only as many bytes as it actually needs for the declared range of possible values. Specifically, theenumtype is equivalent to the smallest integer type that has enough room. ✨ This option has no effect for an enumeration type with a fixed underlying type. Warning: the-fshort-enumsswitch 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— overrides the underlying type forwchar_tto beshort unsigned intinstead of the default for the target. This one is useful for building programs to run under WINE. 🍷 Warning: the-fshort-wcharswitch 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. ⚠️-fcommon— in C code, this option controls the placement of global variables defined without an initializer — known as tentative definitions in the C standard. Tentative definitions are distinct from declarations of a variable with theexternkeyword, which do not allocate storage. 🧚 The default is-fno-common, which specifies that the compiler places uninitialized global variables in the BSS section of the object file. This inhibits the merging of tentative definitions by the linker, so you get a multiple-definition error if the same variable is accidentally defined in more than one compilation unit. 😭 The-fcommonplaces uninitialized global variables in a common block. This allows the linker to resolve all tentative definitions of the same variable in different compilation units to the same object, or to a non-tentative definition. This behavior is inconsistent with C++, and on many targets implies a speed and code size penalty on global variable references. It is mainly useful to enable legacy code to link without errors. 🫡-fno-ident,-Qy,-Qn—-fno-identsuppresses emission of.identassembler directives and causes the#identpreprocessor directive to be ignored. And-Qyand-Qnare obsolete synonyms for-fidentand-fno-ident, respectively. yes bestie they're old news, but they still live here. 💜-finhibit-size-directive— don't output a.sizeassembler directive, or anything else that would cause trouble if the function gets split in the middle and the two halves end up placed at locations far apart in memory. This option is used when compilingcrtstuff.c; you should not need to use it for anything else. 🙈-fverbose-asm— puts 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. The added comments include:- information on the compiler version and command-line options,
- the source code lines associated with the assembly instructions, in the form
FILENAME:LINENUMBER:CONTENT OF LINE, - hints on which high-level expressions correspond to the various assembly instruction operands.
For example, given this C source file:
int test (int n)
{
int i;
int total = 0;
for (i = 0; i < n; i++)
total += i * i;
return total;
}compiling to (x86_64) assembly via -S and emitting the result direct to stdout via -o -
gcc -S test.c -fverbose-asm -Os -o -gives output similar to this:
.file "test.c"
# GNU C11 (GCC) version 7.0.0 20160809 (experimental) (x86_64-pc-linux-gnu)
[...snip...]
# options passed:
[...snip...]
.text
.globl test
.type test, @function
test:
.LFB0:
.cfi_startproc
# test.c:4: int total = 0;
xorl %eax, %eax # <retval>
# test.c:6: for (i = 0; i < n; i++)
xorl %edx, %edx # i
.L2:
# test.c:6: for (i = 0; i < n; i++)
cmpl %edi, %edx # n, i
jge .L5 #,
# test.c:7: total += i * i;
movl %edx, %ecx # i, tmp92
imull %edx, %ecx # i, tmp92
# test.c:6: for (i = 0; i < n; i++)
incl %edx # i
# test.c:7: total += i * i;
addl %ecx, %eax # tmp92, <retval>
jmp .L2 #
.L5:
# test.c:10: }
ret
.cfi_endproc
.LFE0:
.size test, .-test
.ident "GCC: (GNU) 7.0.0 20160809 (experimental)"
.section .note.GNU-stack,"",@progbitsThe comments are intended for humans rather than machines, and hence the precise format of the comments is subject to change. 👀
-frecord-gcc-switches— this switch causes the command line used to invoke the compiler to be recorded into the object file that is being created. It's only implemented on some targets, and the exact format of the recording is target and binary file format dependent — but it usually takes the form of a section containing ASCII text. This switch is related to the-fverbose-asmswitch, but that one only records information in the assembler output file as comments, so it never reaches the object file. See also-grecord-gcc-switchesfor another way of storing compiler options into the object file. 🎀-fpic— generates 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 — and btw the dynamic loader is not part of GCC, it's 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-fpicdoes not work; in that case, recompile with-fPICinstead. (These maximums are 8k on the SPARC, 28k on AArch64 and 32k on the m68k and RS/6000. The x86 has no such limit. 👑) Position-independent code requires special support, and therefore works only on certain machines. For the x86, GCC supports PIC for System V but not for the Sun 386i. Code generated for the IBM RS/6000 is always position-independent. 💅 And when this flag is set, the macros__pic__and__PIC__are defined to 1.-fPIC— bestie, if the target machine supports it, this emits position-independent code, the kind that's suitable for dynamic linking and that dodges any limit on the size of the global offset table. It actually makes a difference on AArch64, m68k, PowerPC and SPARC.
Position-independent code needs special support, so it only works on certain machines. No cap.
When this flag is set, the macros __pic__ and __PIC__ are defined to 2. 💅
-fpie,-fPIE— these are like-fpicand-fPICbut the generated position-independent code can only be linked into executables. Usually you use these to compile code that's gonna be linked using the-pieGCC option.
Both -fpie and -fPIE define the macros __pie__ and __PIE__. The macros have the value 1 for -fpie and 2 for -fPIE. Periodt.
-fno-plt— do not use the PLT for external function calls in position-independent code. Instead, load the callee address at call sites from the GOT and branch to it. This gives you more efficient code by eliminating PLT stubs and exposing GOT loads to optimizations. On architectures such as 32-bit x86 where PLT stubs expect the GOT pointer in a specific register, this gives more register allocation freedom to the compiler.
Lazy binding requires use of the PLT; with -fno-plt all external symbols are resolved at load time.
Alternatively, the function attribute noplt can be used to avoid calls through the PLT for specific external functions.
In position-dependent code, a few targets also convert calls to functions that are marked to not use the PLT to use the GOT instead.
-fno-jump-tables— don't use jump tables forswitchstatements even where it would be more efficient than other code generation strategies. This one's your girl in conjunction with-fpicor-fPICfor building code that forms part of a dynamic linker and cannot reference the address of a jump table. On some targets, jump tables don't require a GOT and this option isn't needed.-fno-bit-tests— don't use bit tests forswitchstatements even where it would be more efficient than other code generation strategies. That's it, that's the whole vibe.-ffixed-reg— treat the register named reg as a fixed register; generated code should never refer to it (except perhaps as a stack pointer, frame pointer or in some other fixed role).
reg must be the name of a register. The register names accepted are machine-specific and are defined in the REGISTER_NAMES macro in the machine description macro file.
This flag does not have a negative form, because it specifies a three-way choice. 💀
-fcall-used-reg— treat the register named reg as an allocable register that is clobbered by function calls. It may be allocated for temporaries or variables that do not live across a call. Functions compiled this way do not save and restore the register reg.
It is an error to use this flag with the frame pointer or stack pointer. Use of this flag for other registers that have fixed pervasive roles in the machine's execution model produces disastrous results. 😭
This flag does not have a negative form, because it specifies a three-way choice.
-fcall-saved-reg— treat the register named reg as an allocable register saved by functions. It may be allocated even for temporaries or variables that live across a call. Functions compiled this way save and restore the register reg if they use it.
It is an error to use this flag with the frame pointer or stack pointer. Use of this flag for other registers that have fixed pervasive roles in the machine's execution model produces disastrous results.
A different sort of disaster results from the use of this flag for a register in which function values may be returned. 🙈
This flag does not have a negative form, because it specifies a three-way choice.
-fpack-struct[=n]— without a value specified, pack all structure members together without holes. When a value is specified (which must be a small power of two), pack structure members according to this value, representing the maximum alignment (that is, objects with default alignment requirements larger than this are output potentially unaligned at the next fitting location).
⚠️ Warning: the -fpack-struct switch causes GCC to generate code that is not binary compatible with code generated without that switch. Additionally, it makes the code suboptimal. Use it to conform to a non-default application binary interface.
-fleading-underscore— this option and its counterpart,-fno-leading-underscore, forcibly change the way C symbols are represented in the object file. One use is to help link with legacy assembly code.
⚠️ Warning: the -fleading-underscore switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface. Not all targets provide complete support for this switch. 🫶
-ftls-model=model— alter the thread-local storage model to be used. The model argument should be one ofglobal-dynamic,local-dynamic,initial-execorlocal-exec. Heads up, the choice is subject to optimization: the compiler may use a more efficient model for symbols not visible outside of the translation unit, or if-fpicis not given on the command line.
The default without -fpic is initial-exec; with -fpic the default is global-dynamic.
-ftrampolines— for targets that normally need trampolines for nested functions, always generate them instead of using descriptors. Otherwise, for targets that don't need them — like for example HP-PA or IA-64 — do nothing.
A trampoline is a small piece of code that is created at run time on the stack when the address of a nested function is taken, and is used to call the nested function indirectly. Therefore, it requires the stack to be made executable in order for the program to work properly. 👑
-fno-trampolines is enabled by default on a language by language basis to let the compiler avoid generating them, if it computes that this is safe, and replace them with descriptors. Descriptors are made up of data only, but the generated code must be prepared to deal with them. As of this writing, -fno-trampolines is enabled by default only for Ada.
Moreover, code compiled with-ftrampolinesand code compiled with-fno-trampolinesare not binary compatible if nested functions are present. This option must therefore be used on a program-wide basis and be manipulated with extreme care. 🚨
For languages other than Ada, the -ftrampolines and -fno-trampolines options currently have no effect, and trampolines are always generated on platforms that need them for nested functions.
-ftrampoline-impl=[stack|heap]— by default, trampolines are generated on stack. However, certain platforms (such as the Apple M1) do not permit an executable stack. Compiling with-ftrampoline-impl=heapgenerates calls to__gcc_nested_func_ptr_createdand__gcc_nested_func_ptr_deletedin order to allocate and deallocate trampoline space on the executable heap. These functions are implemented in libgcc, and will only be provided on specific targets: x86_64 Darwin, x86_64 and aarch64 Linux.
🚨 PLEASE NOTE: Heap trampolines are not guaranteed to be correctly deallocated if yousetjmp, instantiate nested functions, and thenlongjmpback to a state prior to having allocated those nested functions. Not me finding a new way to leak memory. 💀
-fvisibility=[default|internal|hidden|protected]— set the default ELF image symbol visibility to the specified option — all symbols are marked with this unless overridden within the code. Using this feature can very substantially improve linking and load times of shared object libraries, produce more optimized code, provide near-perfect API export and prevent symbol clashes. It is strongly recommended that you use this in any shared objects you distribute. Slay.
Despite the nomenclature, default always means public; i.e., available to be linked against from outside the shared object. protected and internal are pretty useless in real-world usage so the only other commonly used option is hidden. The default if -fvisibility isn't specified is default, i.e., make every symbol public.
A good explanation of the benefits offered by ensuring ELF symbols have the correct visibility is given by How To Write Shared Libraries by Ulrich Drepper (which can be found at <https://www.akkadia.org/drepper/>) — however a superior solution made possible by this option to marking things hidden when the default is public is to make the default hidden and mark things public. This is the norm with DLLs on Windows and with -fvisibility=hidden and __attribute__ ((visibility("default"))) instead of __declspec(dllexport) you get almost identical semantics with identical syntax. This is a great boon to those working with cross-platform projects. 🌈
For those adding visibility support to existing code, you may find #pragma GCC visibility of use. This works by you enclosing the declarations you wish to set visibility for with (for example) #pragma GCC visibility push(hidden) and #pragma GCC visibility pop. Bear in mind that symbol visibility should be viewed as part of the API interface contract and thus all new code should always specify visibility when it is not the default; i.e., declarations only for use within the local DSO should always be marked explicitly as hidden as so to avoid PLT indirection overheads — making this abundantly clear also aids readability and self-documentation of the code. Note that due to ISO C++ specification requirements, operator new and operator delete must always be of default visibility.
Be aware that headers from outside your project, in particular system headers and headers from any other library you use, may not be expecting to be compiled with visibility other than the default. You may need to explicitly say #pragma GCC visibility push(default) before including any such headers. 🎀
"extern" declarations are not affected by <code>-fvisibility</code>, so a whole lot of code can get recompiled with <code>-fvisibility=hidden</code> and literally nothing needs editing. BUT. That also means calls to <code>"extern"</code> functions with no explicit visibility go through the PLT, so bestie, it's way more effective to use <code>__attribute ((visibility))</code> and/or <code>#pragma GCC visibility</code> to tell the compiler which <code>"extern"</code> declarations should be treated as hidden. 🫶
Also important: <code>-fvisibility</code> does affect C++ vague linkage entities. Which means — say it with me — an exception class that gets thrown between DSOs must be explicitly marked with default visibility so the type_info nodes get unified between the DSOs. No cap. 💅
Want the whole overview of these techniques, their benefits, and how to actually use them? It lives at <code>https://gcc.gnu.org/wiki/Visibility</code>. Go read it, iykyk. 📖✨
-fstrict-volatile-bitfields— use this if accesses to volatile bit-fields (or other structure fields, though the compiler usually honors those types anyway) should be a single access of the width of the field's type, aligned to a natural alignment if possible. For example, targets with memory-mapped peripheral registers might require all such accesses to be 16 bits wide; with this flag you can declare all peripheral bit-fields as"unsigned short"(assuming short is 16 bits on these targets) to force GCC to use 16-bit accesses instead of, perhaps, a more efficient 32-bit access. 🌸
If this option is disabled, the compiler uses the most efficient instruction. In the previous example, that might be a 32-bit load instruction, even though that accesses bytes that do not contain any portion of the bit-field, or memory-mapped registers unrelated to the one being updated. 😭
In some cases, such as when the "packed" attribute is applied to a structure field, it may not be possible to access the field with a single read or write that is correctly aligned for the target machine. In this case GCC falls back to generating multiple accesses rather than code that will fault or truncate the result at run time. 🙈
Note: due to restrictions of the C/C++11 memory model, write accesses are not allowed to touch non bit-field members. So it's recommended to define all bits of the field's type as bit-field members. Do it. 💅
The default value of this option is determined by the application binary interface for the target processor.
-fsync-libcalls— this one controls whether any out-of-line instance of the"__sync"family of functions may be used to implement the C++11"__atomic"family of functions. 🦋
The default value of this option is enabled, so the only useful form of the option is -fno-sync-libcalls. This option is used in the implementation of the libatomic runtime library.
-fzero-init-padding-bits=value— guarantee zero initialization of padding bits in automatic variable initializers. Certain languages guarantee zero initialization of padding bits in certain cases, e.g. C23 when using empty initializers ("{}"), or C++ when using zero-initialization, or C guarantees that fields not specified in an initializer have their padding bits zero initialized. This option lets you change when padding bits in initializers are guaranteed to be zero initialized. The default is-fzero-init-padding-bits=standard, which makes no further guarantees than the corresponding standard. E.g. 👇
struct A { char a; unsigned long long b; char c; };
union B { char a; unsigned long long b; };
struct A a = {}; // C23 guarantees padding bits are zero.
struct A b = { 1, 2, 3 }; // No guarantees.
union B c = {}; // C23 guarantees padding bits are zero.
union B d = { 1 }; // No guarantees.-fzero-init-padding-bits=unions guarantees zero initialization of padding bits in unions on top of what the standards guarantee, if the initializer of an union is empty (then all bits of the union are zero initialized) or if the initialized member of the union is smaller than the size of the union (in that case guarantees padding bits outside of the initialized member to be zero initialized). This was the GCC behavior before GCC 15 and in the above example guarantees zero initialization of last sizeof (unsigned long long) - 1 bytes in the union. ⭐
-fzero-init-padding-bits=all guarantees additionally zero initialization of padding bits of other aggregates, so the padding in between b.a and b.b (if any) and tail padding in the structure (if any). 👑
🛠️ GCC Developer Options 🛠️
This section is about command-line options that are primarily of interest to GCC developers — think options to support compiler testing, and investigation of compiler bugs and compile-time performance problems. That includes options that produce debug dumps at various points in the compilation; that print statistics such as memory use and execution time; and that print information about GCC's configuration, such as where it searches for libraries. For ordinary compilation and linking tasks you should rarely need any of these, bestie. Like, almost never. 🧚
Many developer options that cause GCC to dump output to a file take an optional =filename suffix. You can specify stdout or - to dump to standard output, and stderr for standard error. 💖
If =filename is omitted, a default dump file name is constructed by concatenating the base dump file name, a pass number, phase letter, and pass name. The base dump file name is the name of output file produced by the compiler if explicitly specified and not an executable; otherwise it is the source file name. The pass number is determined by the order passes are registered with the compiler's pass manager. This is generally the same as the order of execution, but passes registered by plugins, target-specific passes, or passes that are otherwise registered late are numbered higher than the pass named final, even if they are executed earlier. 😭 The phase letter is one of i (inter-procedural analysis), l (language-specific), r (RTL), or t (tree). The files are created in the directory of the output file. ✨
-fcallgraph-info-fcallgraph-info=MARKERS— makes the compiler output callgraph information for the program, on a per-object-file basis. The information is generated in the common VCG format. It can be decorated with additional, per-node and/or per-edge information, if a list of comma-separated markers is additionally specified. When the"su"marker is specified, the callgraph is decorated with stack usage information; it is equivalent to-fstack-usage. When the"da"marker is specified, the callgraph is decorated with information about dynamically allocated objects. 🍓
When compiling with -flto, no callgraph information is output along with the object file. At LTO link time, -fcallgraph-info may generate multiple callgraph information files next to intermediate LTO output files.
-dletters--dump=letters--dump letters-fdump-rtl-pass-fdump-rtl-pass-options-fdump-rtl-pass-options=filename— says to make debugging dumps during compilation at times specified by letters when using-dor by pass when using-fdump-rtl. This is used for debugging the RTL-based passes of the compiler. 💅
Some -dletters switches have different meaning when -E is used for preprocessing.
The -options form allows greater control over the details of the dump. See -fdump-tree.
Here are actual instances of command-line options following these patterns and their meanings, bestie — buckle up, there's a lot: 🌈
-fdump-rtl-alignments— dump after branch alignments have been computed.-fdump-rtl-asmcons— dump after fixing rtl statements that have unsatisfied in/out constraints.-fdump-rtl-auto_inc_dec— dump after auto-inc-dec discovery. This pass is only run on architectures that have auto inc or auto dec instructions.-fdump-rtl-barriers— dump after cleaning up the barrier instructions.-fdump-rtl-bbpart— dump after partitioning hot and cold basic blocks.-fdump-rtl-bbro— dump after block reordering.-fdump-rtl-btl1-fdump-rtl-btl2—-fdump-rtl-btl1and-fdump-rtl-btl2enable dumping after the two branch target load optimization passes.-fdump-rtl-bypass— dump after jump bypassing and control flow optimizations.-fdump-rtl-combine— dump after the RTL instruction combination pass.-fdump-rtl-compgotos— dump after duplicating the computed gotos.-fdump-rtl-ce1-fdump-rtl-ce2-fdump-rtl-ce3—-fdump-rtl-ce1,-fdump-rtl-ce2, and-fdump-rtl-ce3enable dumping after the three if conversion passes.-fdump-rtl-cprop_hardreg— dump after hard register copy propagation.-fdump-rtl-csa— dump after combining stack adjustments.-fdump-rtl-cse1-fdump-rtl-cse2—-fdump-rtl-cse1and-fdump-rtl-cse2enable dumping after the two common subexpression elimination passes.-fdump-rtl-dce— dump after the standalone dead code elimination passes.-fdump-rtl-dbr— dump after delayed branch scheduling.-fdump-rtl-dce1-fdump-rtl-dce2—-fdump-rtl-dce1and-fdump-rtl-dce2enable dumping after the two dead store elimination passes.-fdump-rtl-eh— dump after finalization of EH handling code.-fdump-rtl-eh_ranges— dump after conversion of EH handling range regions.-fdump-rtl-expand— dump after RTL generation.-fdump-rtl-fwprop1-fdump-rtl-fwprop2—-fdump-rtl-fwprop1and-fdump-rtl-fwprop2enable dumping after the two forward propagation passes.-fdump-rtl-gcse1-fdump-rtl-gcse2—-fdump-rtl-gcse1and-fdump-rtl-gcse2enable dumping after global common subexpression elimination.-fdump-rtl-init-regs— dump after the initialization of the registers.-fdump-rtl-initvals— dump after the computation of the initial value sets.-fdump-rtl-into_cfglayout— dump after converting to cfglayout mode.-fdump-rtl-ira— dump after iterated register allocation.-fdump-rtl-jump— dump after the second jump optimization.-fdump-rtl-loop2—-fdump-rtl-loop2enables dumping after the rtl loop optimization passes.
-fdump-rtl-mach— dump after the machine dependent reorganization pass runs, but only if that pass even exists for your target. 🙈-fdump-rtl-mode_sw— dump after all the redundant mode switches get yeeted. ✨-fdump-rtl-rnreg— dump after register renumbering. 🔢-fdump-rtl-outof_cfglayout— dump after converting out of cfglayout mode. 🦋-fdump-rtl-peephole2— dump after the peephole pass. 👀-fdump-rtl-postreload— dump after post-reload optimizations. 🔥-fdump-rtl-pro_and_epilogue— dump after generating the function prologues and epilogues. 🎀-fdump-rtl-sched1,-fdump-rtl-sched2— bestie, these two enable dumping after the basic block scheduling passes. ⭐-fdump-rtl-ree— dump after sign/zero extension elimination. 💅-fdump-rtl-seqabstr— dump after common sequence discovery. 🔍-fdump-rtl-shorten— dump after shortening branches. ✂️-fdump-rtl-split1,-fdump-rtl-split2,-fdump-rtl-split3,-fdump-rtl-split4,-fdump-rtl-split5— these five options enable dumping after five rounds of instruction splitting. Yes, FIVE. Round by round. It's giving dedication. 💀-fdump-rtl-sms— dump after modulo scheduling. Heads up tho: this pass only runs on some architectures, so don't come for it if your target skips it. 🙈-fdump-rtl-stack— dump after converting from GCC's "flat register file" registers to the x87's stack-like registers. And again, this pass is only run on x86 variants. 🏗️-fdump-rtl-subreg1,-fdump-rtl-subreg2— these two enable dumping after the two subreg expansion passes. 🫶-fdump-rtl-vartrack— dump after variable tracking. 📍-fdump-rtl-vregs— dump after converting virtual registers to hard registers. 🔄-fdump-rtl-web— dump after live range splitting. 🕸️-fdump-rtl-regclass,-fdump-rtl-subregs_of_mode_init,-fdump-rtl-subregs_of_mode_finish,-fdump-rtl-dfinit,-fdump-rtl-dfinish— okay so these dumps are defined, they exist, they're real… but they always produce empty files. Not a bug bestie, that's just literally what the manual says. 💀-da,--dump=a,-fdump-rtl-all— produce all the dumps listed above. All of them. Every single one. 🌈-dA,--dump=A— annotate the assembler output with miscellaneous debugging information. 🎀-dD,--dump=D— dump all macro definitions at the end of preprocessing, in addition to normal output. 📝-dH,--dump=H— produce a core dump whenever an error occurs. Yeah. On purpose. That's the feature. 😭-dp,--dump=p— annotate the assembler output with a comment indicating which pattern and alternative is used. The length and cost of each instruction are also printed. 💅-dP,--dump=P— dump the RTL in the assembler output as a comment before each instruction. Also turns on-dpannotation. Two-for-one, slay. ✨-dx,--dump=x— just generate RTL for a function instead of compiling it. Usually used with-fdump-rtl-expand. 👑-fdump-debug— dump debugging information generated during the debug generation phase. 🐛-fdump-earlydebug— dump debugging information generated during the early debug generation phase. 🌱-fdump-noaddr— when doing debugging dumps, suppress address output. This makes it more feasible to usediffon debugging dumps for compiler invocations with different compiler binaries and/or different text / bss / data / heap / stack / dso start locations. 🧚-freport-bug— collect and dump debug information into a temporary file if an internal compiler error (ICE) occurs. Your little crash souvenir. 🧊-fdump-unnumbered— when doing debugging dumps, suppress instruction numbers and address output. This makes it more feasible to usediffon debugging dumps for compiler invocations with different options, in particular with and without-g. 🌸-fdump-unnumbered-links— when doing debugging dumps (see the-doption above), suppress instruction numbers for the links to the previous and next instructions in a sequence. 🔗-fdump-internal-locations— dump detailed information about GCC's internal representation of source code locations. 📌-fdump-ipa-switch,-fdump-ipa-switch-options— control the dumping at various stages of inter-procedural analysis language tree to a file. The file name is generated by appending a switch-specific suffix to the source file name, and the file is created in the same directory as the output file. The following dumps are possible:all— enables all inter-procedural analysis dumps. 🌈cgraph— dumps information about call-graph optimization, unused function removal, and inlining decisions. 🌳inline— dump after function inlining. 🫂strubm— dump after selecting "strub" modes, and recording the selections as function attributes. 🎀strub— dump "strub" transformations: interface changes, function wrapping, and insertion of builtin calls for stack scrubbing and watermarking. 🧼
Additionally, the options -optimized, -missed, -note, and -all can be provided, with the same meaning as for -fopt-info, defaulting to -optimized.
For example, -fdump-ipa-inline-optimized-missed will emit information on callsites that were inlined, along with callsites that were not inlined. 💅
By default, the dump will contain messages about successful optimizations (equivalent to -optimized) together with low-level details about the analysis. 👑
-fdump-ipa-clones— create a dump file containing information about creation of call graph node clones and removals of call graph nodes during inter-procedural optimizations and transformations. Its main intended use is that tools that create live-patches can determine the set of functions that need to be live-patched to completely replace a particular function (see-flive-patching). The file name is generated by appending suffixipa-clonesto the source file name, and the file is created in the same directory as the output file. Each entry in the file is on a separate line containing semicolon separated fields. 📄
In the case of call graph clone creation, the individual fields are:
- String
"Callgraph clone". - Name of the function being cloned as it is presented to the assembler.
- A number that uniquely represents the function being cloned in the call graph. Note that the number is unique only within a compilation unit or within whole-program analysis but is likely to be different in the two phases.
- The file name of the source file where the function is defined.
- The line on which the function definition is located.
- The column where the function definition is located.
- Name of the new function clone as it is presented to the assembler.
- A number that uniquely represents the new function clone in the call graph. Note that the number is unique only within a compilation unit or within whole-program analysis but is likely to be different in the two phases.
- The file name of the source file where the source code location of the new clone points to.
- The line to which the source code location of the new clone points to.
- The column to which the source code location of the new clone points to.
- A string that determines the reason for cloning.
In the case of call graph clone removal, the individual fields are:
- String
"Callgraph removal". - Name of the function being removed as it would be presented to the assembler.
- A number that uniquely represents the function being cloned in the call graph. Note that the number is unique only within a compilation unit or within whole-program analysis but is likely to be different in the two phases.
- The file name of the source file where the function is defined.
- The line on which the function definition is located.
- The column where the function definition is located.
-fdump-lang— dump language-specific information. The file name is made by appending.langto the source file name. 🗣️-fdump-lang-all,-fdump-lang-switch,-fdump-lang-switch-options,-fdump-lang-switch-options=filename— control the dumping of language-specific information. The options and filename portions behave as described in the-fdump-treeoption.-fdump-tree-allenables all language-specific dumps; other options vary with the language. For instance, see 🌸-fdump-passes— print onstderrthe list of optimization passes that are turned on and off by the current command-line options. 🎢-fdump-statistics-option— enable and control dumping of pass statistics in a separate file. The file name is generated by appending a suffix ending in.statisticsto the source file name, and the file is created in the same directory as the output file. If the-optionform is used,-statscauses counters to be summed over the whole compilation unit while-detailsdumps every event as the passes generate them. The default with no option is to sum counters for each function compiled. 📊-fdump-tree-all,-fdump-tree-switch,-fdump-tree-switch-options,-fdump-tree-switch-options=filename— control the dumping at various stages of processing the intermediate language tree to a file. If the-optionsform is used, options is a list of-separated options which control the details of the dump. Not all options are applicable to all dumps; those that are not meaningful are ignored. The following options are available:address— print the address of each node. Usually this is not meaningful as it changes according to the environment and source file. Its primary use is for tying up a dump file with a debug environment. 📍asmname— if"DECL_ASSEMBLER_NAME"has been set for a given decl, use that in the dump instead of"DECL_NAME". Its primary use is ease of use working backward from mangled names in the assembly file. 🔤
slim— okay so when you're dumping front-end intermediate representations, this one says: don't dump members of a scope or a function body just because that scope got reached. Only dump those items when they're directly reachable by some other path. And when you're dumping pretty-printed trees, this option stops the bodies of control structures from getting dumped. When dumping RTL? It prints the RTL in slim (condensed) form instead of the default LISP-like representation. 💅raw— prints a raw representation of the tree. By default trees get pretty-printed into a C-like representation, so this is the "no makeup, no filter" version.details— enables more detailed dumps (not honored by every dump option). It also includes information from the optimization passes.stats— enables dumping various statistics about the pass (not honored by every dump option, bestie, don't come for me).blocks— enables showing basic block boundaries (disabled in raw dumps).graph— for each of the other indicated dump files (-fdump-rtl-pass), it dumps a representation of the control flow graph suitable for viewing with GraphViz tofile.passid.pass.dot. Every function in the file is pretty-printed as a subgraph so GraphViz can render them all in a single plot. Lowkey iconic.
RTL is always dumped in slim form here.
vops— enables showing virtual operands for every statement.lineno— enables showing line numbers for statements.uid— enables showing the unique ID (DECL_UID) for each variable.verbose— enables showing the tree dump for each statement.eh— enables showing the EH region number holding each statement.scev— enables showing scalar evolution analysis details.optimized— enables showing optimization information (only available in certain passes).missed— enables showing missed optimization information (only available in certain passes).note— enables other detailed optimization information (only available in certain passes).folding— enables dumping information about match-and-simplify (match.pd) patterns, when they are applied.all— turns on all the options, exceptraw,slim,verboseandlineno. (Not me pointing out the four exceptions again, they're literally right there.)optall— turns on all optimization options, i.e.,optimized,missed, andnote.
So to figure out what tree dumps are even available, or to find the dump for a pass you actually care about, follow these steps, bestie. 🫶
- Invoke GCC with
-fdump-passesand in the stderr output look for a code that corresponds to the pass you're interested in. For example, the codestree-evrp,tree-vrp1, andtree-vrp2correspond to the three Value Range Propagation passes. The number at the end distinguishes distinct invocations of the same pass. - To enable the creation of the dump file, append the pass code to the
-fdump-option prefix and invoke GCC with it. For example, to enable the dump from the Early Value Range Propagation pass, invoke GCC with the-fdump-tree-evrpoption. Optionally, you may specify the name of the dump file. If you don't specify one, GCC creates it as described below. - Find the pass dump in a file whose name is composed of three components separated by a period: the name of the source file GCC was invoked to compile, a numeric suffix indicating the pass number followed by the letter
tfor tree passes (and the letterrfor RTL passes), and finally the pass code. For example, the Early VRP pass dump might be in a file namedmyfile.c.038t.evrpin the current working directory. Note that the numeric codes are not stable and may change from one version of GCC to another. 💀
🌈 -fopt-info / -fopt-info-options / -fopt-info-options=filename 🌈
Controls optimization dumps from various optimization passes. If the -options form is used, options is a list of - separated option keywords to select the dump details and optimizations.
The options can be divided into three groups:
- options describing what kinds of messages should be emitted,
- options describing the verbosity of the dump, and
- options describing which optimizations should be included.
The options from each group can be freely mixed since they're non-overlapping. However, if there are any conflicts, the later options override the earlier options on the command line. (Yes, order matters, keep up ✨.)
The following options control which kinds of messages should be emitted:
optimized— print information when an optimization is successfully applied. It's up to a pass to decide which information is relevant. For example, the vectorizer passes print the source location of loops which are successfully vectorized.missed— print information about missed optimizations. Individual passes control which information to include in the output.note— print verbose information about optimizations, such as certain transformations, more detailed messages about decisions, etc.all— print detailed optimization information. This includesoptimized,missed, andnote.
The following option controls the dump verbosity:
internals— by default, only "high-level" messages are emitted. This option enables additional, more detailed, messages, which are likely to only be of interest to GCC developers. (iykyk 🧚)
One or more of the following option keywords can be used to describe a group of optimizations:
ipa— enable dumps from all interprocedural optimizations.loop— enable dumps from all loop optimizations.inline— enable dumps from all inlining optimizations.omp— enable dumps from all OMP (Offloading and Multi Processing) optimizations.vec— enable dumps from all vectorization optimizations.optall— enable dumps from all optimizations. This is a superset of the optimization groups listed above.
If options is omitted, it defaults to optimized-optall, which means to dump messages about successful optimizations from all the passes, omitting messages that are treated as "internals".
If the filename is provided, then the dumps from all the applicable optimizations are concatenated into the filename. Otherwise the dump is output onto stderr. Though multiple -fopt-info options are accepted, only one of them can include a filename. If other filenames are provided then all but the first such option are ignored.
Note that the output filename is overwritten in case of multiple translation units. If a combined output from multiple translation units is desired, stderr should be used instead. 🙈
In the following example, the optimization info is output to stderr:
gcc -O3 -fopt-infoThis example:
gcc -O3 -fopt-info-missed=missed.alloutputs missed optimization report from all the passes into missed.all, and this one:
gcc -O2 -ftree-vectorize -fopt-info-vec-missedprints information about missed optimization opportunities from vectorization passes on stderr. Note that -fopt-info-vec-missed is equivalent to -fopt-info-missed-vec. The order of the optimization group names and message types listed after -fopt-info does not matter. (Slay, flexible queen. 💖)
As another example,
gcc -O3 -fopt-info-inline-optimized-missed=inline.txtoutputs information about missed optimizations as well as optimized locations from all the inlining passes into inline.txt.
Finally, consider:
gcc -fopt-info-vec-missed=vec.miss -fopt-info-loop-optimized=loop.optHere the two output filenames vec.miss and loop.opt are in conflict since only one output file is allowed. In this case, only the first option takes effect and the subsequent options are ignored. Thus only vec.miss is produced, which contains dumps from the vectorizer about missed opportunities. 😭
🌸 -fsave-optimization-record 🌸
Write a SRCFILE.opt-record.json.gz file detailing what optimizations were performed, for those optimizations that support -fopt-info.
This option is experimental and the format of the data within the compressed JSON file is subject to change. No cap, they said it, not me.
It's roughly equivalent to a machine-readable version of -fopt-info-all, as a collection of messages with source file, line number and column number, with the following additional data for each message:
- the execution count of the code being optimized, along with metadata about whether this was from actual profile data, or just an estimate, allowing consumers to prioritize messages by code hotness,
- the function name of the code being optimized, where applicable,
- the "inlining chain" for the code being optimized, so that when a function is inlined into several different places (which might themselves be inlined), the reader can distinguish between the copies,
- objects identifying those parts of the message that refer to expressions, statements or symbol-table nodes, which of these categories they are, and, when available, their source code location,
- the GCC pass that emitted the message, and
- the location in GCC's own code from which the message was emitted
Additionally, some messages are logically nested within other messages, reflecting implementation details of the optimization passes. 🦋
✨ -fsched-verbose=n ✨
On targets that use instruction scheduling, this option controls the amount of debugging output the scheduler prints to the dump files.
For n greater than zero, -fsched-verbose outputs the same information as -fdump-rtl-sched1 and -fdump-rtl-sched2. For n greater than one, it also outputs basic block probabilities, detailed ready list information and unit/insn info. For n greater than two, it includes RTL at abort point, control-flow and regions info. And for n over four, -fsched-verbose also includes dependence info. (The way it just keeps escalating... 💅)
🎀 -fenable-kind-pass / -fdisable-kind-pass=range-list 🎀
This is a set of options that are used to explicitly disable/enable optimization passes. These options are intended for use for debugging GCC. Compiler users should use regular options for enabling/disabling passes instead. Periodt. 👑
-fdisable-ipa-pass— turn OFF the IPA pass pass. pass is literally the pass name, bestie. And if that same pass gets statically invoked in the compiler more than once? 💅 then you gotta tack on a sequential number starting from1. iykyk.-fdisable-rtl-pass,-fdisable-rtl-pass=range-list— turn OFF the RTL pass pass. pass = the pass name. Same deal as above: if the same pass is statically invoked multiple times, append a sequential number starting from1. Now range-list is a comma-separated list of function ranges or assembler names. Each range is a number pair separated by a colon, and the range is inclusive on both ends. If a range is trivial, you can just write it as a single number instead of a pair. If the function's call graph node's uid falls inside one of your specified ranges, the pass gets disabled for that function. The uid shows up in the function header of a dump file, and you can dump the pass names with-fdump-passes. 🎀-fdisable-tree-pass,-fdisable-tree-pass=range-list— turn OFF the tree pass pass. For the option argument description, go peek at-fdisable-rtl. 🦋-fenable-ipa-pass— turn ON the IPA pass pass. pass is the pass name, and yep, if it's statically invoked in the compiler multiple times, append a sequential number starting from1. ⭐-fenable-rtl-pass,-fenable-rtl-pass=range-list— turn ON the RTL pass pass. For the argument description and examples, see-fdisable-rtl. ✨-fenable-tree-pass,-fenable-tree-pass=range-list— turn ON the tree pass pass. Again,-fdisable-rtlhas the description of the option arguments. 💜
Here are some examples showing uses of these options, slay:
# disable ccp1 for all functions
-fdisable-tree-ccp1
# disable complete unroll for function whose cgraph node uid is 1
-fenable-tree-cunroll=1
# disable gcse2 for functions at the following ranges [1,1],
# [300,400], and [400,1000]
# disable gcse2 for functions foo and foo2
-fdisable-rtl-gcse2=foo,foo2
# disable early inlining
-fdisable-tree-einline
# disable ipa inlining
-fdisable-ipa-inline
# enable tree full unroll
-fenable-tree-unroll-fchecking,-fchecking=n— enable internal consistency checking. The default depends on how the compiler was configured, fr fr. And-fchecking=2enables further internal consistency checking that might affect code generation. 👀-frandom-seed=string— this gives GCC a seed to use in place of random numbers when it generates certain symbol names that have to be different in every compiled file. It's also used to place unique stamps in coverage data files and the object files that produce them. You can use-frandom-seedto produce reproducibly identical object files. 🌈
The string can be either a number (decimal, octal or hex) or an arbitrary string (in which case it gets converted to a number by computing CRC32).
The string should be different for every file you compile. no cap.
-save-temps,--save-temps— store the usual "temporary" intermediate files permanently. Name them as auxiliary output files, as specified described under-dumpbaseand-dumpdir. 💾
When used together with the -x command-line option, -save-temps is sensible enough to avoid overwriting an input source file with the same extension as an intermediate file. You can grab that corresponding intermediate file by renaming the source file before using -save-temps.
-save-temps=cwd— equivalent to-save-temps -dumpdir ./. 🫶-save-temps=obj— equivalent to-save-temps -dumpdir outdir/, whereoutdir/is the directory of the output file specified after the-ooption, including any directory separators. If the-ooption isn't used, then-save-temps=objbehaves like-save-temps=cwd. 🎀-specs=file,--specs=file,--specs file— process file after the compiler reads in the standard specs file, so you can override the defaults that thegccdriver program uses when figuring out what switches to pass tocc1,cc1plus,as,ld, etc. You can specify more than one-specs=fileon the command line, and they get processed in order, from left to right. 📋-time[=file]— report the CPU time taken by each subprocess in the compilation sequence. For C source files, this is the compiler proper and assembler (plus the linker if linking is done). ⏱️
Without an output file specified, the output looks like this:
# cc1 0.12 0.01
# as 0.00 0.01The first number on each line is the "user time" — that's 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, bestie.
With an output file specified, the output gets appended to the named file, and it looks like this:
0.12 0.01 cc1 <options>
0.00 0.01 as <options>The "user time" and the "system time" move before the program name, and the options passed to the program are displayed too, so later you can tell what file was being compiled and with which options. 💅
-fdump-final-insns[=file]— dump the final internal representation (RTL) to file. If the optional argument is omitted (or if file is.), the name of the dump file is determined by appending.gkdto the dump base name — see-dumpbase. 🗂️-fcompare-debug[=opts]— if no error occurs during compilation, run the compiler a second time, adding opts and-fcompare-debug-secondto the arguments passed to the second compilation. Dump the final internal representation in both compilations, and print an error if they differ. 👀
If the equal sign is omitted, the default -gtoggle is used.
The environment variable GCC_COMPARE_DEBUG, if defined, non-empty and nonzero, implicitly enables -fcompare-debug. If GCC_COMPARE_DEBUG is defined to a string starting with a dash, then it's used for opts, otherwise the default -gtoggle is used.
-fcompare-debug=, with the equal sign but without opts, is equivalent to -fno-compare-debug, which disables the dumping of the final representation and the second compilation, preventing even GCC_COMPARE_DEBUG from taking effect. 💀
To verify full coverage during -fcompare-debug testing, set GCC_COMPARE_DEBUG to say -fcompare-debug-not-overridden, which GCC rejects as an invalid option in any actual compilation (rather than preprocessing, assembly or linking). To get just a warning, setting GCC_COMPARE_DEBUG to -w%n-fcompare-debug not overridden will do. ⭐
-fcompare-debug-second— this option is implicitly passed to the compiler for the second compilation requested by-fcompare-debug, along with options to silence warnings, and omitting other options that would cause the compiler to produce output to files or to standard output as a side effect. Dump files and preserved temporary files are renamed so as to contain the.gkadditional extension during the second compilation, to avoid overwriting those generated by the first. 🌸
When this option is passed to the compiler driver, it causes the first compilation to be skipped, which makes it useful for little other than debugging the compiler proper. 😭
-gtoggle— turn off generation of debug info, if leaving out this option generates it, or turn it on at level 2 otherwise. The position of this argument in the command line does not matter; it takes effect after all other options are processed, and it does so only once, no matter how many times it's given. This is mainly intended to be used with-fcompare-debug. 🔁-fvar-tracking-assignments-toggle— toggle-fvar-tracking-assignments, in the same way that-gtoggletoggles-g. 🎚️-Q— when used on the command line prior to--help=,-Qacts as a modifier to the help output. 👑
Otherwise, this option makes the compiler print out each function name as it's compiled, and print some statistics about each pass when it finishes.
-ftime-report— makes the compiler print some statistics tostderrabout the time consumed by each pass when it finishes. ⏱️
If SARIF output of diagnostics was requested via -fdiagnostics-format=sarif-file or -fdiagnostics-format=sarif-stderr then the -ftime-report information is instead emitted in JSON form as part of SARIF output. The precise format of this JSON data is subject to change, and the values may not exactly match those emitted to stderr due to being written out at a slightly different place within the compiler. 🫠
-ftime-report-details— record the time consumed by infrastructure parts separately for each pass. 🧮-fira-verbose=n— control the verbosity of the dump file for the integrated register allocator. The default value is5. If the value n is greater or equal to10, the dump output is sent tostderrusing the same format as n minus 10. 💅-flto-report— prints a report with internal details on the workings of the link-time optimizer. The contents of this report vary from version to version. It's meant to be useful to GCC developers when processing object files in LTO mode (via-flto). 🧚
Disabled by default.
-flto-report-wpa— like-flto-report, but only print for the WPA phase of link-time optimization. ✨-fmem-report— makes the compiler print some statistics about permanent memory allocation when it finishes. 🧠-fmem-report-wpa— makes the compiler print some statistics about permanent memory allocation for the WPA phase only. 🍓-fpre-ipa-mem-report,-fpost-ipa-mem-report— makes the compiler print some statistics about permanent memory allocation before or after interprocedural optimization. 🎀-fmultiflags— this option enables multilib-aware "TFLAGS" to be used to build target libraries with options different from those the compiler is configured to use by default, through the use of specs set up by compiler internals, by the target, or by builders at configure time. 🌈
Like TFLAGS, this whole thing is giving "build the target libraries for portable baseline environments while the compiler itself defaults to the more demanding ones" 💅 and honestly? iconic. Why? Because users can easily override the defaults the compiler is configured to use when building their own programs if those defaults aren't it for their target environment — whereas rebuilding the runtime libraries is usually not as easy or desirable. rebuilds are giving pain, bestie 😭
Unlike TFLAGS tho, using specs means you can pick different flags for different multilibs ✨ The way to do that? Build with make TFLAGS=-fmultiflags, after configuring --with-specs=%{fmultiflags:...}.
Heads up: this option is discarded by the driver once it's done processing the driver self spec. gone. poof. 🫥
It's also useful for checking that TFLAGS are actually being used to build all target libraries — configure a non-bootstrap compiler with --with-specs='%{!fmultiflags:%emissing TFLAGS}' and build the compiler and target libraries. vibe check passed ✅
-fprofile-report— makes the compiler print some stats about how consistent the (estimated) profile is, plus the effect of each individual pass. receipts 📊-fstack-usage— makes the compiler spit out stack usage info for the program, per-function 🥞 The dump filename is made by appending.suto the auxname. The auxname is generated from the name of the output file (if it was explicitly specified and it's not an executable), otherwise it's the basename of the source file. An entry is made up of three fields:- The name of the function.
- A number of bytes.
- One or more qualifiers:
static,dynamic,bounded.
The qualifier static means the function manipulates the stack statically: a fixed number of bytes are allocated for the frame on function entry and released on function exit, and no other stack adjustments happen in the function. The second field is that fixed number of bytes.
The qualifier dynamic means the function manipulates the stack dynamically: on top of the static allocation described above, stack adjustments happen in the body of the function, like push/pop-ing arguments around function calls. If bounded is also there, the amount of these adjustments is bounded at compile time and the second field is an upper bound of the total stack used by the function. If bounded isn't there, the amount of those adjustments is not bounded at compile time and the second field only represents the bounded part. no cap 🧢
-fstats— emit statistics about front-end processing at the end of compilation. This one is only supported by the C++ front end, and ngl the info is generally only useful to the G++ development team. it's giving insider info 🕵️-fdbg-cnt-list— print the name and the counter upper bound for all debug counters.- *
-fdbg-cnt=counter-value-list* — set the internal debug counter lower and upper bound. counter-value-list is a comma-separated list ofname:lower_bound1-upper_bound1 [:lower_bound2-upper_bound2...]tuples, which sets the name of the counter and the list of closed intervals. The lower_bound is optional and is zero initialized if not set. For example, with-fdbg-cnt=dce:2-4:10-11,tail_call:10,dbg_cnt(dce)returns true only for the second, third, fourth, tenth and eleventh invocation. Fordbg_cnt(tail_call), true is returned for the first 10 invocations. math queen 🧮✨ -print-autofdo-gcov-version,--print-autofdo-gcov-version— print the current version of GCOV being used by the AutoFDO infrastructure.-print-file-name=library, *--print-file-name=library*, *--print-file-name*library — print the full absolute name of the library file library that would be used when linking — and do nothing else. With this option, GCC does not compile or link anything; it just prints the file name. one job, done slay 💅-print-multi-directory,--print-multi-directory— print the directory name corresponding to the multilib selected by any other switches present on the command line. This directory is supposed to exist inGCC_EXEC_PREFIX.-print-multi-lib,--print-multi-lib— print the mapping from multilib directory names to the compiler switches that enable them. The directory name is separated from the switches by;, and each switch starts with an@instead of the-, with no spaces between multiple switches. This is supposed to make shell processing easier 🐚-print-multi-os-directory,--print-multi-os-directory— print the path to OS libraries for the selected multilib, relative to somelibsubdirectory. If OS libraries are present in thelibsubdirectory and no multilibs are used, this is usually just.; if OS libraries are present inlibsuffix sibling directories, this prints e.g.../lib64,../libor../lib32; or if OS libraries are present inlib/subdir subdirectories it prints e.g.amd64,sparcv9orev6.-print-multiarch,--print-multiarch— print the path to OS libraries for the selected multiarch, relative to somelibsubdirectory.-print-prog-name=program, *--print-prog-name=program*, *--print-prog-name*program — like-print-file-name, but searches for a program such ascpp.-print-libgcc-file-name,--print-libgcc-file-name— same as-print-file-name=libgcc.a.
This one 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-search-dirs— print the name of the configured installation directory and a list of the program and library directories gcc searches — and don't do anything else.
This is useful when gcc prints the error message installation problem, cannot exec cpp0: No such file or directory. To fix that, you either need to put cpp0 and the other compiler components where gcc expects to find them, or you can set the environment variable GCC_EXEC_PREFIX to the directory where you installed them. Don't forget the trailing / 🙈
-print-sysroot,--print-sysroot— print the target sysroot directory that's used during compilation. This is the target sysroot specified either at configure time or via the--sysrootoption, possibly with an extra suffix that depends on compilation options. If no target sysroot is specified, the option prints nothing. silence 💀-print-sysroot-headers-suffix,--print-sysroot-headers-suffix— print the suffix added to the target sysroot when searching for headers, or give an error if the compiler is not configured with such a suffix — and don't do anything else.-dumpmachine— print the compiler's target machine (for example,i686-pc-linux-gnu) — and don't do anything else. main character energy fr 🎀-dumpversion— print the compiler version (for example,3.0,6.3.0or7) — and don't do anything else. This is the compiler version used in filesystem paths and specs. Depending on how the compiler was configured, it can be just a single number (major version), two numbers separated by a dot (major and minor version) or three numbers separated by dots (major, minor and patchlevel version).-dumpfullversion— print the full compiler version — and don't do anything else. The output is always three numbers separated by dots: major, minor and patchlevel version.-dumpspecs— print the compiler's built-in specs — and don't do anything else. (This one is used when GCC itself is being built 👑)
--paramname=value,--param=name=value — GCC by convention uses parameters that can be specified on the command line instead of hard-wired constants to represent arbitrary compiler limits or heuristics. Many parameters are related to optimization; for example, GCC does not inline functions that contain more than a certain number of instructions. The static analyzer similarly uses parameters to limit complexity, link-time optimization uses parameters to control partitioning, and so on. Other parameters control aspects of GCC that are completely internal, such as its memory allocation and garbage collection strategy. Still others control target-specific behavior.
The --param option gives you a uniform interface for specifying values for these compiler parameters. But — 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. You should not depend on parameter settings for correct compilation of your program. They're exposed via the command line for the convenience of developers debugging compilation problems, or in some cases to provide workarounds for compiler bugs. iykyk 🫶
🎯 Target-Specific Options 🎯
Every target machine supported by GCC can have its own options — for example, to let you compile for a particular processor variant or ABI, or to control optimizations specific to that machine. Similarly, GCC also has options specific to particular operating systems or runtime environments on the target. so many flavours, we love variety 🌈
By convention, the names of machine-specific options start with -m. Some configurations of the compiler also support additional target-specific options, usually for compatibility with other compilers on the same platform.
AArch64 Options
These options are defined for AArch64 implementations:
-mabi=name — generate code for the specified data model. Permissible values areilp32for a SysV-like data model whereint,long intand pointers are 32 bits, andlp64for a SysV-like data model whereintis 32 bits butlong intand pointers are 64 bits.
The default depends on the specific target configuration. Note that the LP64 and ILP32 ABIs are not link-compatible; you must compile your entire program with the same ABI, and link with a compatible set of libraries. don't mix and match bestie, that's a crime 🚨
The ilp32 model is deprecated. 💀
-mbig-endian— generate big-endian code. This is the default when GCC is configured for anaarch64_be-*-*target.-mlittle-endian— generate little-endian code. This is the default when GCC is configured for anaarch64-*-*target but not anaarch64_be-*-*target.-menable-sysreg-checking— generates an error message if an attempt is made to access a system register which is not available on the target architecture. caught in 4k 📸-mgeneral-regs-only— bestie, this one makes the compiler live that minimalist life 💅 it generates code that only touches the general-purpose registers, and absolutely refuses to let floating-point and Advanced SIMD registers into the group chat. It does not put any restrictions on the assembler though, so the assembler can still do its own thing. 🙈-mcmodel=tiny— code model for the smol era 🎀 the program and its statically defined symbols all have to live within 1MB of each other. Programs can be statically or dynamically linked, no drama.-mcmodel=small— the classic ✨ program and its statically defined symbols must be within 4GB of each other. Statically or dynamically linked, whatever you want. And this is the default code model, so if you didn't ask for anything, this is what you got. 👑-mcmodel=large— no assumptions, no limits, no cap 🦋 this one makes no assumptions about addresses and sizes of sections. Programs can be statically linked only. And heads up:-mcmodel=largeis incompatible with-mabi=ilp32,-fpicand-fPIC. Do not try to make them be friends, it will not work. 💀-mtp=name— picks which system register gets to be the thread pointer. The valid values aretpidr_el0,tpidrro_el0,tpidr_el1,tpidr_el2,tpidr_el3. For backwards compatibility the aliasesel0,el1,el2,el3are also accepted (aw, legacy support 🫶). The default setting istpidr_el0. And the official rec: it is recommended to compile all code intended to interoperate with the same value of this option, so you don't end up accessing a different thread pointer from the wrong exception level. Chaos prevention, literally. 😭-mstrict-align,-mno-strict-align— avoid or allow generating memory accesses that may not be aligned on a natural object boundary, as described in the architecture specification. So one of these is the responsible friend and the other one is the "it'll probably be fine" friend. 🌸-momit-leaf-frame-pointer,-mno-omit-leaf-frame-pointer— omit or keep the frame pointer in leaf functions. The former behavior (omitting it) is the default. 🍓-mstack-protector-guard=guard-mstack-protector-guard-reg=reg-mstack-protector-guard-offset=offset
These three together generate stack protection code using a canary at guard. The supported locations are global for a global canary, or sysreg for a canary in an appropriate system register.
With that latter sysreg choice, the options -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset furthermore specify which system register to use as base register for reading the canary, and from what offset from that base register. And ngl — there is no default register or offset, because this is entirely for use within the Linux kernel. iykyk 🧚
-mtls-dialect=desc— uses TLS descriptors as the thread-local storage mechanism for dynamic accesses of TLS variables. This is the default. ✨-mtls-dialect=traditional— uses traditional TLS as the thread-local storage mechanism for dynamic accesses of TLS variables. The vintage option, if you will. 🎀-mtls-size=size— specifies the bit size of immediate TLS offsets. Valid values are12,24,32,48. Heads up though: this option requires binutils 2.26 or newer, so no dusty toolchains allowed. 💅-mfix-cortex-a53-835769,-mno-fix-cortex-a53-835769— enable or disable the workaround for the ARM Cortex-A53 erratum number 835769. The fix involves inserting aNOPinstruction between memory instructions and 64-bit integer multiply-accumulate instructions. And important: this flag will be ignored if an architecture or cpu is specified on the command line which does not need the workaround. So it knows when it's not wanted. 🙈-mfix-cortex-a53-843419,-mno-fix-cortex-a53-843419— enable or disable the workaround for the ARM Cortex-A53 erratum number 843419. This one is different bestie, because this erratum workaround is made at link time and this will only pass the corresponding flag to the linker. Same rule applies: this flag will be ignored if an architecture or cpu is specified on the command line which does not need the workaround. 🦋-mlow-precision-recip-sqrt,-mno-low-precision-recip-sqrt— enable or disable the reciprocal square root approximation. This option only has an effect if-ffast-mathor-funsafe-math-optimizationsis used as well. Enabling this reduces precision of reciprocal square root results to about 16 bits for single precision and to 32 bits for double precision. Speed over precision, main character energy, but like, know what you're doing. 🔥-mlow-precision-sqrt,-mno-low-precision-sqrt— enable or disable the square root approximation. Again, this option only has an effect if-ffast-mathor-funsafe-math-optimizationsis used as well. Enabling this reduces precision of square root results to about 16 bits for single precision and to 32 bits for double precision. And plot twist: if enabled, it implies-mlow-precision-recip-sqrt. 😭-mlow-precision-div,-mno-low-precision-div— enable or disable the division approximation. Yep, same deal: this option only has an effect if-ffast-mathor-funsafe-math-optimizationsis used as well. Enabling this reduces precision of division results to about 16 bits for single precision and to 32 bits for double precision. ⭐-mtrack-speculation,-mno-track-speculation— enable or disable generation of additional code to track speculative execution through conditional branches. The tracking state can then be used by the compiler when expanding calls to__builtin_speculation_safe_valueto permit a more efficient code sequence to be generated. Obsessed with the security girlies who plan ahead. 👑-moutline-atomics,-mno-outline-atomics— enable or disable calls to out-of-line helpers to implement atomic operations. These helpers will, at runtime, determine if the LSE instructions from ARMv8.1-A can be used; if not, they will use the load/store-exclusive instructions that are present in the base ARMv8.0 ISA. Very "we'll figure it out at runtime" of them. 🫶
This option is only applicable when compiling for the base ARMv8.0 instruction set. If you're using a later revision, e.g. -march=armv8.1-a or -march=armv8-a+lse, the ARMv8.1-Atomics instructions will be used directly. The same applies when using -mcpu= when the selected cpu supports the lse feature. This option is on by default. ✨
-mmax-vectorization,-mno-max-vectorization— enable or disable an override to the vectorizer cost model making vectorization always appear profitable. This option can be combined with-mautovec-preferenceallowing precise control over which ISA will be used for auto-vectorization. And unlike-fno-vect-cost-modelor-fvect-cost-model=unlimited, this option does not turn off cost comparison between different vector modes. Subtle but huge. 💅-mautovec-preference=name— forces an ISA selection strategy for auto-vectorization. The possible values of name are:default— use the default heuristics.asimd-only— use only Advanced SIMD for auto-vectorization.sve-only— use only SVE for auto-vectorization.prefer-asimd— use both Advanced SIMD and SVE. Prefer Advanced SIMD when the costs are deemed equal.prefer-sve— use both Advanced SIMD and SVE. Prefer SVE when the costs are deemed equal.
For best performance it is highly recommended to use-mcpuor-mtuneinstead. This parameter should only be used for code exploration. Don't ship it, bestie, I'm begging. 😭
-march=name— specifies the name of the target architecture and, optionally, one or more feature modifiers. This option has the form:
-march=arch{+[no]feature}*The table below summarizes the permissible values for arch and the features that they enable by default. The columns are: arch value : Architecture : Includes by default — and the right-most conflicting feature modifier wins, but more on that in a sec. 🌈
armv8-a: Armv8-A :+fp,+simdarmv8.1-a: Armv8.1-A :armv8-a,+crc,+lse,+rdmaarmv8.2-a: Armv8.2-A :armv8.1-aarmv8.3-a: Armv8.3-A :armv8.2-a,+pauth,+fcma,+jscvtarmv8.4-a: Armv8.4-A :armv8.3-a,+flagm,+fp16fml,+dotprod,+rcpc2armv8.5-a: Armv8.5-A :armv8.4-a,+sb,+ssbs,+predres,+frintts,+flagm2armv8.6-a: Armv8.6-A :armv8.5-a,+bf16,+i8mmarmv8.7-a: Armv8.7-A :armv8.6-a,+wfxt,+xsarmv8.8-a: Armv8.8-a :armv8.7-a,+mopsarmv8.9-a: Armv8.9-a :armv8.8-aarmv9-a: Armv9-A :armv8.5-a,+sve,+sve2armv9.1-a: Armv9.1-A :armv9-a,+bf16,+i8mmarmv9.2-a: Armv9.2-A :armv9.1-a,+wfxt,+xsarmv9.3-a: Armv9.3-A :armv9.2-a,+mopsarmv9.4-a: Armv9.4-A :armv9.3-a,+sve2p1armv9.5-a: Armv9.5-A :armv9.4-a,cpa,+faminmax,+lutarmv8-r: Armv8-R :armv8-r
The value native is available on native AArch64 GNU/Linux and causes the compiler to pick the architecture of the host system. This option has no effect if the compiler is unable to recognize the architecture of the host system. And when -march=native is given and no other -mcpu or -mtune is given, then GCC will pick the host CPU as the CPU to tune for as well as select the architecture features from. That is, -march=native is treated as -mcpu=native. It's giving "just read the room" energy. 🦋
The permissible values for feature are listed in the sub-section on aarch64-feature-modifiers,,-march and -mcpu Feature Modifiers. Where conflicting feature modifiers are specified, the right-most feature is used. Periodt. 💅
GCC uses name to determine what kind of instructions it can emit when generating assembly code. If -march is specified without either of -mtune or -mcpu also being specified, the code is tuned to perform well across a range of target processors implementing the target architecture. Generalist queen. ✨
-mtune=name— specifies the name of the target processor for which GCC should tune the performance of the code. The permissible values for this option are:
generic, cortex-a35, cortex-a53, cortex-a55, cortex-a57,
cortex-a72, cortex-a73, cortex-a75, cortex-a76, cortex-a76ae,
cortex-a77, cortex-a65, cortex-a65ae, cortex-a34, cortex-a78,
cortex-a78ae, cortex-a78c, ares, exynos-m1, emag, falkor, oryon-1,
neoverse-512tvb, neoverse-e1, neoverse-n1, neoverse-n2, neoverse-v1,
neoverse-v2, grace, neoverse-v3, neoverse-v3ae, armagicpu,
neoverse-n3, olympus, rigel, cortex-a725, cortex-x925, qdf24xx,
saphira, phecda, xgene1, vulcan, octeontx, octeontx81, octeontx83,
octeontx2, octeontx2t98, octeontx2t96 octeontx2t93, octeontx2f95,
octeontx2f95n, octeontx2f95mm, a64fx, fujitsu-monaka, thunderx,
thunderxt88, thunderxt88p1, thunderxt81, tsv110, hip12, thunderxt83,
thunderx2t99, thunderx3t110, zeus, cortex-a57.cortex-a53,
cortex-a72.cortex-a53, cortex-a73.cortex-a35, cortex-a73.cortex-a53,
cortex-a75.cortex-a55, cortex-a76.cortex-a55, cortex-r82,
cortex-r82ae, cortex-x1, cortex-x1c, cortex-x2, cortex-x3,
cortex-x4, cortex-a510, cortex-a520, cortex-a520ae, cortex-a710,
cortex-a715, cortex-a720, cortex-a720ae, ampere1, ampere1a,
ampere1b, ampere1c, cobalt-100, apple-m1, apple-m2, apple-m3,
apple-m4, apple-m5, c1-nano, c1-pro, c1-premium c1-ultra and native.That's the whole roster, bestie — every single one of them, no cuts, no "etc." 🌸
The values cortex-a57.cortex-a53, cortex-a72.cortex-a53, cortex-a73.cortex-a35, cortex-a73.cortex-a53, cortex-a75.cortex-a55, cortex-a76.cortex-a55, apple-m1, apple-m2, apple-m3, gb10 are like, "bestie we're doing a big.LITTLE situation" — GCC should tune for a big.LITTLE system. Two brains, one chip, iconic. 💅
The value neoverse-512tvb says tune for Neoverse cores that (a) implement SVE and (b) have a total vector bandwidth of 512 bits per cycle. Translation for the girlies: GCC should tune for Neoverse cores that can execute 4 128-bit Advanced SIMD arithmetic instructions a cycle, and can execute an equivalent number of SVE arithmetic instructions per cycle (2 for 256-bit SVE, 4 for 128-bit SVE). It's more general than tuning for one specific core like Neoverse V1, but more specific than the default tuning described below. Range queen. 🎀
Also, on native AArch64 GNU/Linux systems, the value native tunes performance to the host system. No cap. But heads up bestie — this option does nothing if the compiler can't recognize the processor of the host system. 😭
Where none of -mtune=, -mcpu= or -march= are specified, the code is tuned to perform well across a range of target processors. ✨
Real talk: this option <strong>cannot</strong> be suffixed by feature modifiers. Don't try it. She said what she said. 💀
-mcpu=name— Name the target processor, optionally with one or more feature modifiers tacked on the end. The form is-mcpu=cpu{+[no]feature}*, where the allowedcpuvalues are the same ones-mtunetakes. What canfeaturebe? That's documented in the sub-section aarch64-feature-modifiers,,march and -mcpu Feature Modifiers. And if you hand it conflicting feature modifiers, the <strong>right-most</strong> feature wins — last one standing gets the crown. 👑
GCC uses name for two things: deciding what kind of instructions it can emit when generating assembly code (as if by -march), and deciding which target processor to tune for performance (as if by -mtune). If you also pass -march or -mtune, those take precedence over the matching part of this option. Teamwork, but with a hierarchy. 🫶
-mcpu=neoverse-512tvb is the special one — she doesn't point at a specific core, she points at <strong>all</strong> Neoverse cores that (a) implement SVE and (b) have a total vector bandwidth of 512 bits a cycle. Unless -march overrides it, -mcpu=neoverse-512tvb generates code that can run on a Neoverse V1 core, because Neoverse V1 is the first Neoverse core with those properties. And unless -mtune overrides it, -mcpu=neoverse-512tvb tunes code exactly like -mtune=neoverse-512tvb. She's the blueprint, fr fr. 🌸
-moverride=string— Override the tuning decisions the back-end makes in response to a-mtune=switch. The syntax, semantics, and accepted values forstringhere are <strong>not</strong> guaranteed to stay consistent across releases. Wild card energy, lowkey unstable.
This option is only intended to be useful when developing GCC. If you're not hacking on GCC itself, this isn't your moment. 🙈
-mpc-relative-literal-loads,-mno-pc-relative-literal-loads— Turn PC-relative literal loads on or off. With this on, literal pools get accessed using a <strong>single instruction</strong> and are emitted after each function. The catch? It caps the maximum size of functions at 1MB. It's enabled by default for-mcmodel=tiny. 💖-msign-return-address=scope— Pick which function scope gets return address signing. The permissible values:none— disables return address signing.non-leaf— enables pointer signing for functions that are <strong>not</strong> leaf functions.all— enables pointer signing for <strong>all</strong> functions.
Default is none.
Deprecated alert: this option has been deprecated by -mbranch-protection. She's retiring, wish her well. 😭
-mbranch-protection=features— Choose which branch protection features to use.featurescan take one of these forms:none— the default, turns off all types of branch protection.standard— turns on all types of branch protection features. If a feature has extra tuning options,standardsets it to its standard level.pac-ret— turns on return address signing to its standard level: signing functions that save the return address to memory (non-leaf functions practically always do this) using the <strong>A-key</strong>.pac-ret+leaf— extends thepac-retsigning to also include leaf functions.pac-ret+b-keyorpac-ret+leaf+b-key— sign the functions with the <strong>B-key</strong> instead of the A-key. Fancy.bti— turns on the branch target identification mechanism.gcs— turns on guarded control stack compatible code generation. 🛡️
-mharden-sls=opts— Enable compiler hardening against straight line speculation (SLS).optsis a comma-separated list of these:retbrblr
Also, -mharden-sls=all enables <strong>all</strong> SLS hardening, while -mharden-sls=none disables <strong>all</strong> SLS hardening. All or nothing, no in-between, periodt. ✨
-mearly-ra=scope— Decide when to run an early register allocation pass. This pass runs <strong>before</strong> instruction scheduling and tries to find a spill-free allocation of floating-point and vector code. It also tries to make use of strided multi-register instructions, like SME2's stridedLD1andST1. The possible values ofscope:all— runs the pass on all functions.strided— runs the pass on functions that have access to strided multi-register instructions.none— disables the pass.
-mearly-ra=all is the default for -O2 and above, and for -Os. -mearly-ra=none is the default otherwise. ✨
-mearly-ldp-fusion,-mno-early-ldp-fusion— Enable the copy of the AArch64 load/store pair fusion pass that runs <strong>before</strong> register allocation. Enabled by default at-Oand above. 🌈-mlate-ldp-fusion,-mno-late-ldp-fusion— Enable the copy of the AArch64 load/store pair fusion pass that runs <strong>after</strong> register allocation. Enabled by default at-Oand above. Early girlie and late girlie, both slaying. 👑-mnarrow-gp-writes— Enable conversion of 64-bit general purpose register writes into equivalent 32-bit operations when the upper 32 bits are known to be zero. This pass is controllable with-mnarrow-gp-writesand is active at-O2and above, but it's <strong>not</strong> enabled by default — except for-mcpu=olympus, which gets special treatment because she's the favorite. 💅-msve-vector-bits=bits— Set the number of bits in an SVE vector register. This option only does anything when SVE is enabled. GCC supports two flavors of SVE code generation:- <strong>"vector-length agnostic"</strong> output — works with any size of vector register.
- <strong>"vector-length specific"</strong> output — lets GCC make assumptions about the vector length when that's useful for optimization reasons.
The possible values of bits are: scalable, 128, 256, 512, 1024 and 2048. Picking scalable selects vector-length agnostic output. At present, -msve-vector-bits=128 also generates vector-length agnostic output for big-endian targets. Every other value generates vector-length specific code.
Heads up: the behavior of these values may change in future releases, and no value <strong>except</strong>scalableshould be relied on for producing code that's portable across different hardware SVE vector lengths. Don't build your whole personality on512, bestie. 😭
The default is -msve-vector-bits=scalable, which produces vector-length agnostic code. ✨
✨ -march and -mcpu Feature Modifiers ✨
Feature modifiers used with -march and -mcpu can be any of the following — and also their inverses, prefixed with no (nofeature). The full roster, alphabetically, because we keep it organized:
crc— Enable CRC extension. This is on by default for-march=armv8.1-a. 🍓crypto— Enable Crypto extension. This also enables Advanced SIMD and floating-point instructions. She brings her friends. 💖fp— Enable floating-point instructions. This is on by default for <strong>all</strong> possible values for options-marchand-mcpu. Universal queen.simd— Enable Advanced SIMD instructions. This also enables floating-point instructions. This is on by default for <strong>all</strong> possible values for options-marchand-mcpu. Also universal. 👑sve— Enable Scalable Vector Extension instructions. This also enables Advanced SIMD and floating-point instructions.lse— Enable Large System Extension instructions. This is on by default for-march=armv8.1-a.rdma— Enable Round Double Multiply Accumulate instructions. This is on by default for-march=armv8.1-a. 🌀fp16— Enable FP16 extension. This also enables floating-point instructions.fp16fml— Enable FP16 fmla extension. This also enables FP16 extensions and floating-point instructions. This option is enabled by default for-march=armv8.4-a.
Use of this option with architectures prior to Armv8.2-A is <strong>not supported</strong>. Time traveler behavior is not on the menu. 💀
rcpc— Enable the RCpc extension. This enables the use of theLDAPRinstructions for load-acquire atomic semantics, and passes it on to the assembler, enabling inline asm statements to use instructions from the RCpc extension.dotprod— Enable the Dot Product extension. This also enables Advanced SIMD instructions. ⭐aes— Enable the Armv8-a aes and pmull crypto extension. This also enables Advanced SIMD instructions. 🔐sha2— Enable the Armv8-a sha2 crypto extension. This also enables Advanced SIMD instructions.sha3— Enable the sha512 and sha3 crypto extension. This also enables Advanced SIMD instructions.
Use of this option with architectures prior to Armv8.2-A is <strong>not supported</strong>.
sm4— Enable the sm3 and sm4 crypto extension. This also enables Advanced SIMD instructions.
Use of this option with architectures prior to Armv8.2-A is <strong>not supported</strong>.
profile— Enable the Statistical Profiling extension. This option is only there to enable the extension at the <strong>assembler level</strong> and does not affect code generation. She's here for the vibe, not the output. 💅rng— Enable the Armv8.5-a Random Number instructions. This option is only to enable the extension at the <strong>assembler level</strong> and does not affect code generation. Same energy asprofile. 🎲memtag— Enable the Armv8.5-a Memory Tagging Extensions.
Use of this option with architectures prior to Armv8.5-A is <strong>not supported</strong>.
sb— Enable the Armv8-a Speculation Barrier instruction. This option is only to enable the extension at the <strong>assembler level</strong> and does not affect code generation. This option is enabled by default for-march=armv8.5-a. 🛡️ssbs— Enable the Armv8-a Speculative Store Bypass Safe instruction. This option is only to enable the extension at the <strong>assembler level</strong> and does not affect code generation. This option is enabled by default for-march=armv8.5-a.predres— turns on the Armv8-a Execution and Data Prediction Restriction instructions ✨. Heads up tho: this one is only for enabling the extension at the assembler level and does not affect code generation. It's on by default for-march=armv8.5-a.sve2— enables the Armv8-a Scalable Vector Extension 2 💜. This also enables SVE instructions.sve2-bitperm— enables SVE2 bitperm instructions. This also enables SVE2 instructions.sve2-sm4— enables SVE2 sm4 instructions. This also enables SVE2 instructions.sve2-aes— enables SVE2 aes instructions. This also enables SVE2 instructions.sve2-sha3— enables SVE2 sha3 instructions. This also enables SVE2 instructions.sve2p1— enables SVE2.1 instructions. This also enables SVE2 instructions.tme— enable the Transactional Memory Extension 🌈.i8mm— enable 8-bit Integer Matrix Multiply instructions 🔥. This also enables Advanced SIMD and floating-point instructions. It's enabled by default for-march=armv8.6-a. Ngl, using this option with architectures prior to Armv8.2-A is not supported.f32mm— enable 32-bit Floating point Matrix Multiply instructions. This also enables SVE instructions. Use of this option with architectures prior to Armv8.2-A is not supported.f64mm— enable 64-bit Floating point Matrix Multiply instructions. This also enables SVE instructions. Use of this option with architectures prior to Armv8.2-A is not supported.bf16— enable brain half-precision floating-point instructions 🧠. This also enables Advanced SIMD and floating-point instructions. It's enabled by default for-march=armv8.6-a. Use of this option with architectures prior to Armv8.2-A is not supported.ls64— enable the 64-byte atomic load and store instructions for accelerators.mops— enable the instructions to accelerate memory operations likememcpy,memmove,memset💅. This option is enabled by default for-march=armv8.8-aflagm— enable the Flag Manipulation instructions Extension.flagm2— enable the FlagM2 flag conversion instructions.pauth— enable the Pointer Authentication Extension.cssc— enable the Common Short Sequence Compression instructions.cmpbr— enable the shorter compare and branch instructions,cbb,cbhandcb.sme— enable the Scalable Matrix Extension.sme-i16i64— enable the FEAT_SME_I16I64 extension to SME. This also enables SME instructions.sme-f64f64— enable the FEAT_SME_F64F64 extension to SME. This also enables SME instructions.sme2— enable the Scalable Matrix Extension 2. This also enables SME instructions.sme-f8f16— enable the FEAT_SME_F8F16 extension to SME. This also enables SME2 and FP8 instructions.sme-f8f32— enable the FEAT_SME_F8F32 extension to SME. This also enables SME2 and FP8 instructions.sme-b16b16— enable the FEAT_SME_B16B16 extension to SME. This also enables SME2 and SVE_B16B16 instructions.sme-f16f16— enable the FEAT_SME_F16F16 extension to SME. This also enables SME2 instructions.sme2p1— enable the Scalable Matrix Extension version 2.1. This also enables SME2 instructions.fcma— enable the complex number SIMD extensions.jscvt— enable thefjcvtzsJavaScript conversion instruction 🍓.frintts— enable floating-point round to integral value instructions.wfxt— enablewfetandwfitinstructions.xs— enable the XS memory attribute extension.lse128— enable the LSE128 128-bit atomic instructions extension. This also enables LSE instructions.d128— enable support for 128-bit system register read/write instructions. This also enables the LSE128 extension.gcs— enable support for Armv9.4-a Guarded Control Stack extension 🛡️.the— enable support for Armv8.9-a/9.4-a translation hardening extension.rcpc2— enable the RCpc2 extension.rcpc3— enable the RCpc3 (Release Consistency) extension.fp8— enable the fp8 (8-bit floating point) extension.fp8fma— enable the fp8 (8-bit floating point) multiply accumulate extension.ssve-fp8fma— enable the fp8 (8-bit floating point) multiply accumulate extension in streaming mode 🌊.fp8dot4— enable the fp8 (8-bit floating point) to single-precision 4-way dot product extension.ssve-fp8dot4— enable the fp8 (8-bit floating point) to single-precision 4-way dot product extension in streaming mode 🌊.fp8dot2— enable the fp8 (8-bit floating point) to half-precision 2-way dot product extension.ssve-fp8dot2— enable the fp8 (8-bit floating point) to half-precision 2-way dot product extension in streaming mode 🌊.faminmax— enable the Floating Point Absolute Maximum/Minimum extension.lut— enable the Lookup Table extension.sme-lutv2— enable the SME Lookup Table v2 (LUTv2) extension.cpa— enable the Checked Pointer Arithmetic instructions.sve-b16b16— enable the SVE non-widening brain floating-point (bf16) extension. This only has an effect whensve2orsme2are also enabled 👀.sve-bfscale— enable the SVE_BFSCALE extension.poe2— enable the Permission Overlays Extension 2.tev— enable the TIndex Exception-like Vector Extension.tlbid— enable the TLBI Domains Extension.gcie— enable the GICv5 (Generic Interrupt Controller) CPU Interface Extension.lscp— enable the load acquire and store release pair extension.mops-go— enable tag only variants of MOPS instructions. This also enables the instructions to accelerate memory operations and Armv8.5-a Memory Tagging Extensions.sve2p3— enable SVE2.3. This also enables SVE2.2 instructions.sme2p3— enable SME2.3. This also enables SME2.2 instructions.f16f32dot— enable Armv9.7-a f16f32dot instructions. This also enables Advanced SIMD, floating-point instructions and Armv8.2-a FP16 instructions.sve-b16mm— enable the SVE B16MM Extension. This also enables SVE instructions, Advanced SIMD and floating-point instructions.mtetc— enable Data cache tag block operations. This also enables Armv8.5-a Memory Tagging Extensions.f16f32mm— enable Armv9.7-a f16f32mm instructions. This also enables Advanced SIMD, floating-point instructions and Armv8.2-a FP16 instructions.f16mm— enable f16mm instructions. This also enables Advanced SIMD, floating-point instructions and Armv8.2-a FP16 instructions.
Bestie, remember the chain rule here: Feature crypto implies aes, sha2, and simd, which implies fp. Conversely, nofp implies nosimd, which implies nocrypto, noaes and nosha2. It's giving domino effect 💀.
✨ Adapteva Epiphany Options ✨
These -m options are defined for Adapteva Epiphany, bestie 🫶:
-mhalf-reg-file,-mno-half-reg-file— don't allocate any register in the ranger32...r63. That allows code to run on hardware variants that lack these registers.-mprefer-short-insn-regs,-mno-prefer-short-insn-regs— preferentially allocate registers that allow short instruction generation. This can result in increased instruction count, so this may either reduce or increase overall code size 🤷.- *
-mbranch-cost=num* — set the cost of branches to roughly num "simple" instructions. This cost is only a heuristic and is not guaranteed to produce consistent results across releases. -mcmove,-mno-cmove— enable the generation of conditional moves.- *
-mnops=num* — emit num NOPs before every other generated instruction. -mno-soft-cmpsf,-msoft-cmpsf— for single-precision floating-point comparisons, emit anfsubinstruction and test the flags. This is faster than a software comparison, but can get incorrect results in the presence of NaNs, or when two different small numbers are compared such that their difference is calculated as zero 😭. The default is-msoft-cmpsf, which uses slower, but IEEE-compliant, software comparisons.-mstack-offset=num — set the offset between the top of the stack and the stack pointer. E.g., a value of 8 means that the eight bytes in the rangesp+0...sp+7can be used by leaf functions without stack allocation. Values other than 8 or 16 are untested and unlikely to work 🚨. Note also that this option *changes the ABI; compiling a program with a different stack offset than the libraries have been compiled with generally does not work. This option can be useful if you want to evaluate if a different stack offset would give you better code, but to actually use a different stack offset to build working programs, it is recommended to configure the toolchain with the appropriate--with-stack-offset=num* option.-mno-round-nearest,-mround-nearest—-mno-round-nearestmakes the scheduler assume that the rounding mode has been set to truncating. The default is-mround-nearest.-mlong-calls— if not otherwise specified by an attribute, assume all calls might be beyond the offset range of theb/blinstructions, and therefore load the function address into a register before performing a (otherwise direct) call. This is the default.-mshort-calls— if not otherwise specified by an attribute, assume all direct calls are in the range of theb/blinstructions, so use these instructions for direct calls.
The default is -mlong-calls. Note that -mlong-calls is equivalent to -mno-short-calls, and similarly -mno-long-calls is equivalent to -mshort-calls.
-msmall16,-mno-small16— assume addresses can be loaded as 16-bit unsigned values. This does not apply to function addresses for which-mlong-callssemantics are in effect.- *
-mfp-mode=mode* — set the prevailing mode of the floating-point unit. This determines the floating-point mode that is provided and expected at function call and return time. Making this mode match the mode you predominantly need at function start can make your programs smaller and faster by avoiding unnecessary mode switches 🔥.
mode can be set to one the following values:
caller— any mode is valid when you enter a function, and it gets kept or put back exactly how it was when the function returns, and also when it calls other functions 💖. This one is so useful for compiling libraries or other compilation units you might wanna drop into different programs that have different prevailing FPU modes — and the convenience of being able to use a single object file lowkey outweighs the size and speed overhead of any extra mode switching that might be needed, compared with what you'd need if you picked a more specific prevailing FPU mode. iykyk.truncate— the mode for floating-point calculations with truncating (i.e. round towards zero) rounding mode 🎀. That includes conversion from floating point to integer.round-nearest— the mode for floating-point calculations with round-to-nearest-or-even rounding mode ✨.int— the mode used to perform integer calculations in the FPU, e.g. integer multiply, or integer multiply-and-accumulate 🦋.
The default is -mfp-mode=caller — no cap.
-mmay-round-for-trunc,-mno-may-round-for-trunc— bestie, this one lets floating point to integer truncation get replaced with rounding to save mode switching 💅. It's disabled by default.-mfp-iarith,-mno-fp-iarith— enables use of the floating-point unit for integer add and subtract 🔥. Also disabled by default.-msplit-lohi,-mno-split-lohi— code generation tweak that controls splitting of 32-bit loads ✨.-mpost-inc,-mno-post-inc— code generation tweak that controls generation of post-increment addresses ✨.-mpost-modify,-mno-post-modify— code generation tweak that controls generation of post-modify addresses ✨.
The defaults are msplit-lohi, -mpost-inc, and -mpost-modify.
-mno-vect-double— changes the preferred SIMD mode toSImode. The default is-mvect-double, which usesDImodeas the preferred SIMD mode 🌈.-max-vect-align=num— the maximum alignment for SIMD vector mode types 💜. num may be4or8. The default is8. Heads up: this is an ABI change, even though many library function interfaces are unaffected if they don't use SIMD vector modes in places that affect size and/or alignment of relevant types 👀.-msplit-vecmove-early,-mno-split-vecmove-early— split vector moves into single word moves before reload 🎀. In theory this can give better register allocation, but so far the reverse seems to be generally the case 💀.-m1reg-reg— specify a register to hold the constant-1, which makes loading small negative constants and certain bitmasks faster ⚡. Allowable values for reg arer43andr63, which specify use of that register as a fixed register, andnone, which means that no register is used for this purpose. The default is-m1reg-none.
💖 AMD GCN Options 💖
These options are defined specifically for the AMD GCN port 🫶.
-march=gpu— set architecture type for gpu 🌸.-mtune=gpu— set tuning for gpu 🌸.
Supported values for gpu are:
gfx900— compile for GCN5 Vega 10 devices (gfx900).gfx902— compile for GCN5 Vegagfx902devices. (Experimental)gfx904— compile for GCN5 Vegagfx904devices. (Experimental)gfx906— compile for GCN5 Vega 20 devices (gfx906).gfx908— compile for CDNA1 Instinct MI100 series devices (gfx908).gfx909— compile for GCN5 Vegagfx909devices. (Experimental)gfx90a— compile for CDNA2 Instinct MI200 series devices (gfx90a).gfx90c— compile for GCN5 Vega 7 devices (gfx90c).gfx942— compile for CDNA3 Instinct MI300 series devices (gfx942). (Experimental)gfx950— compile for the CDNA3gfx950devices. (Experimental)gfx9-generic— compile generic code for Vega devices, executable on the following subset of GFX9 devices:gfx900,gfx902,gfx904,gfx906,gfx909andgfx90c✨.gfx9-4-generic— compile generic code for CDNA3 devices, executable on the following subset of GFX9 devices:gfx942andgfx950. (Experimental)gfx1030— compile for RDNA2gfx1030devices (GFX10 series).gfx1031— compile for RDNA2gfx1031devices (GFX10 series). (Experimental)gfx1032— compile for RDNA2gfx1032devices (GFX10 series). (Experimental)gfx1033— compile for RDNA2gfx1033devices (GFX10 series). (Experimental)gfx1034— compile for RDNA2gfx1034devices (GFX10 series). (Experimental)gfx1035— compile for RDNA2gfx1035devices (GFX10 series). (Experimental)gfx1036— compile for RDNA2gfx1036devices (GFX10 series).gfx10-3-generic— compile generic code for GFX10-3 devices, executable ongfx1030,gfx1031,gfx1032,gfx1033,gfx1034,gfx1035, andgfx1036🌈.gfx1100— compile for RDNA3gfx1100devices (GFX11 series).gfx1101— compile for RDNA3gfx1101devices (GFX11 series). (Experimental)gfx1102— compile for RDNA3gfx1102devices (GFX11 series). (Experimental)gfx1103— compile for RDNA3gfx1103devices (GFX11 series).gfx1150— compile for RDNA3gfx1150devices (GFX11 series). (Experimental)gfx1151— compile for RDNA3gfx1151devices (GFX11 series). (Experimental)gfx1152— compile for RDNA3gfx1152devices (GFX11 series). (Experimental)gfx1153— compile for RDNA3gfx1153devices (GFX11 series). (Experimental)gfx11-generic— compile generic code for GFX11 devices, executable ongfx1100,gfx1101,gfx1102,gfx1103,gfx1150,gfx1151,gfx1152, andgfx1153🌈.
-mgang-private-size=bytes— set the amount of local data-share (LDS) memory to reserve for gang-private variables 🎀. The default is512.-msram-ecc=on,-msram-ecc=off,-msram-ecc=any— compile binaries suitable for devices with the SRAM-ECC feature enabled, disabled, or either mode 💅. This feature can be enabled per-process on some devices. The compiled code must match the device mode. The default isany, for devices that support it.-mxnack=on,-mxnack=off,-mxnack=any— compile binaries suitable for devices with the XNACK feature enabled, disabled, or either mode 🔥. Some devices always require XNACK and some allow the user to configure XNACK. The compiled code must match the device mode. The default is-mxnack=anyon devices that support Unified Shared Memory, and-mxnack=nootherwise.-Wopenacc-dims,-Wno-openacc-dims— control warnings about invalid OpenACC dimensions 👀.
🦋 ARC Options 🦋
The following options control the architecture variant for which code is being compiled:
-mbarrel-shifter— generate instructions supported by barrel shifter ✨. This is the default unless-mcpu=ARC601or-mcpu=ARCEMis in effect.-mjli-always— force to call a function usingjli_sinstruction 💅. This option is valid only for ARCv2 architecture.-mcpu=cpu— set architecture type, register usage, and instruction scheduling parameters for cpu 🎀. There are also shortcut alias options available for backward compatibility and convenience. Supported values for cpu are:arc600— compile for ARC600. Aliases:-mA6,-mARC600.arc601— compile for ARC601. Alias:-mARC601.arc700— compile for ARC700. Aliases:-mA7,-mARC700. This is the default when configured with--with-cpu=arc700.arcem— compile for ARC EM.archs— compile for ARC HS.em— compile for ARC EM CPU with no hardware extensions.em4— compile for ARC EM4 CPU.em4_dmips— compile for ARC EM4 DMIPS CPU.em4_fpus— compile for ARC EM4 DMIPS CPU with the single-precision floating-point extension.em4_fpuda— compile for ARC EM4 DMIPS CPU with single-precision floating-point and double assist instructions.hs— compile for ARC HS CPU with no hardware extensions except the atomic instructions.hs34— compile for ARC HS34 CPU.hs38— compile for ARC HS38 CPU.hs38_linux— compile for ARC HS38 CPU with all hardware extensions on.hs4x— compile for ARC HS4x CPU.hs4xd— compile for ARC HS4xD CPU.hs4x_rel31— compile for ARC HS4x CPU release 3.10a.arc600_norm— compile for ARC 600 CPU with "norm" instructions enabled.arc600_mul32x16— compile for ARC 600 CPU with "norm" and 32x16-bit multiply instructions enabled.arc600_mul64— compile for ARC 600 CPU with "norm" and "mul64"-family instructions enabled.arc601_norm— compile for ARC 601 CPU with "norm" instructions enabled.arc601_mul32x16— compile for ARC 601 CPU with "norm" and 32x16-bit multiply instructions enabled.arc601_mul64— compile for ARC 601 CPU with "norm" and "mul64"-family instructions enabled.nps400— compile for ARC 700 on NPS400 chip.em_mini— compile for ARC EM minimalist configuration featuring reduced register set.
-mdpfp,-mdpfp-compact— generate double-precision FPX instructions, tuned for the compact implementation 💜.-mdpfp-fast— generate double-precision FPX instructions, tuned for the fast implementation ⚡.-mno-dpfp-lrsr,-mdpfp-lrsr— control whetherlrandsrinstructions use FPX extension aux registers 🌸. This is enabled by default.-mea— generate extended arithmetic instructions 🎀. Currently onlydivaw,adds,subs, andsat16are supported. Only valid for-mcpu=ARC700.-mmul32x16— generate 32x16-bit multiply and multiply-accumulate instructions ✨.-mmul64— generatemul64andmulu64instructions 💅. Only valid for-mcpu=ARC600.-mnorm— generatenorminstructions 🌈. This is the default if-mcpu=ARC700is in effect.-mspfp,-mspfp-compact— generate single-precision FPX instructions, tuned for the compact implementation 💖.-mspfp-fast— generate single-precision FPX instructions, tuned for the fast implementation ⚡.-msimd— turns on generation of ARC SIMD instructions through target-specific builtins. Only valid for-mcpu=ARC700and nothing else, she's got a very specific type. 💅-msoft-float— bestie. This option is ignored. It is provided for compatibility purposes only. Software floating-point code is emitted by default, and that default can be overridden by the FPX options:-mspfp,-mspfp-compact, or-mspfp-fastfor single precision, and-mdpfp,-mdpfp-compact, or-mdpfp-fastfor double precision. It literally does nothing, it's just here to be polite. 💀-mswap— generate"swap"instructions. Swapping, but make it ✨architecture✨.-matomic— enables use of the locked load/store conditional extension to implement atomic memory built-in functions. Not available for ARC 6xx or ARC EM cores, sorry babes.-mdiv-rem— enable"div"and"rem"instructions for ARCv2 cores. Dividing and remainder-ing, iconic behavior. 🌸-mcode-density,-mno-code-density— enable code density instructions for ARC EM. This one is on by default for ARC HS. 👑-mll64— enable double load/store operations for ARC HS cores. Loading and storing, but two at a time, main character energy. 🦋- *
-mtp-regno=regno* — specify the thread pointer register number. That's it, that's the whole job. 🎀 -mbitops— enable use of NPS400 bit operations.-mcmem— enable use of the NPS400xld/xstextension.-mmpy-option=multo — compile ARCv2 code with a multiplier design option. You can pass multo as either a string or a numeric value, whatever feels right to you.wlh1is the **default. The recognized values are, and yes we're doing all of them:0,none— no multiplier available. There is no multiplier. It's not there. 🙈1,w— 16x16 multiplier, fully pipelined. The following instructions are enabled:"mpyw"and"mpyuw".2,wlh1— 32x32 multiplier, fully pipelined (1 stage). The following instructions are additionally enabled:"mpy","mpyu","mpym","mpymu", and"mpy_s".3,wlh2— 32x32 multiplier, fully pipelined (2 stages). The following instructions are additionally enabled:"mpy","mpyu","mpym","mpymu", and"mpy_s".4,wlh3— two 16x16 multipliers, blocking, sequential. The following instructions are additionally enabled:"mpy","mpyu","mpym","mpymu", and"mpy_s".5,wlh4— one 16x16 multiplier, blocking, sequential. The following instructions are additionally enabled:"mpy","mpyu","mpym","mpymu", and"mpy_s".6,wlh5— one 32x4 multiplier, blocking, sequential. The following instructions are additionally enabled:"mpy","mpyu","mpym","mpymu", and"mpy_s".7,plus_dmpy— ARC HS SIMD support.8,plus_macd— ARC HS SIMD support.9,plus_qmacw— ARC HS SIMD support.
This option is only available for ARCv2 cores. Periodt.
- *
-mfpu=fpu* — enables support for specific floating-point hardware extensions for ARCv2 cores. Supported values for fpu are, and there are a lot of them, so buckle up 🎀:fpus— enables support for single-precision floating-point hardware extensions.fpud— enables support for double-precision floating-point hardware extensions. The single-precision floating-point extension is also enabled. Not available for ARC EM.fpuda— enables support for double-precision floating-point hardware extensions using double-precision assist instructions. The single-precision floating-point extension is also enabled. This option is only available for ARC EM.fpuda_div— enables support for double-precision floating-point hardware extensions using double-precision assist instructions. The single-precision floating-point, square-root, and divide extensions are also enabled. This option is only available for ARC EM.fpuda_fma— enables support for double-precision floating-point hardware extensions using double-precision assist instructions. The single-precision floating-point and fused multiply and add hardware extensions are also enabled. This option is only available for ARC EM.fpuda_all— enables support for double-precision floating-point hardware extensions using double-precision assist instructions. All single-precision floating-point hardware extensions are also enabled. This option is only available for ARC EM.fpus_div— enables support for single-precision floating-point, square-root and divide hardware extensions.fpud_div— enables support for double-precision floating-point, square-root and divide hardware extensions. This option includes optionfpus_div. Not available for ARC EM.fpus_fma— enables support for single-precision floating-point and fused multiply and add hardware extensions.fpud_fma— enables support for double-precision floating-point and fused multiply and add hardware extensions. This option includes optionfpus_fma. Not available for ARC EM.fpus_all— enables support for all single-precision floating-point hardware extensions. She ate. 💅fpud_all— enables support for all single- and double-precision floating-point hardware extensions. Not available for ARC EM.
- *
-mirq-ctrl-saved=register-range*, blink, lp_count — specifies general-purposes registers that the processor automatically saves/restores on interrupt entry and exit. register-range is specified as two registers separated by a dash. The register range always starts with"r0", and the upper limit is the"fp"register.blinkandlp_countare optional. This option is only valid for ARC EM and ARC HS cores. - *
-mrgf-banked-regs=number* — specifies the number of registers replicated in the second register bank on entry to a fast interrupt. Fast interrupts are interrupts with the highest priority level P0. These interrupts save onlyPCandSTATUS32registers to avoid memory transactions during interrupt entry and exit sequences. Use this option when you are using fast interrupts in an ARC V2 family processor. Permitted values are4,8,16, and32. 🧚 -mlpc-width=width — specify the width of the"lp_count"register. Valid values for width are8,16,20,24,28and32bits. The default width is fixed to *32 bits. If the width is less than 32, the compiler does not attempt to transform loops in your program to use the zero-delay loop mechanism unless it is known that the"lp_count"register can hold the required loop-counter value. Depending on the width specified, the compiler and run-time library might continue to use the loop mechanism for various needs. This option defines the macro"__ARC_LPC_WIDTH__"with the value of *width. 🫶-mrf16— this option instructs the compiler to generate code for a 16-entry register file. This option defines the"__ARC_RF16__"preprocessor macro. ✨-mbranch-index— enable use of"bi"or"bih"instructions to implement jump tables.
The following options are passed through to the assembler, and also define preprocessor macro symbols. (double duty, we love a multitasker 💖)
-mlock— passed down to the assembler to enable the locked load/store conditional extension. Also sets the preprocessor symbol"__Xlock".-mswape— passed down to the assembler to enable the swap byte ordering extension instruction. Also sets the preprocessor symbol"__Xswape".-mxy— passed down to the assembler to enable the XY memory extension. Also sets the preprocessor symbol"__Xxy".
The following options control how the assembly code is annotated:
-misize— annotate assembler instructions with estimated addresses. Little notes in the margins, very ✨annotated✨.
The following options are passed through to the linker:
-marclinux,-mno-arclinux— passed through to the linker, to specify use of the"arclinux"emulation. This option is enabled by default in tool chains built for"arc-linux-uclibc"and"arceb-linux-uclibc"targets when profiling is not requested.-marclinux_prof,-mno-arclinux_prof— passed through to the linker, to specify use of the"arclinux_prof"emulation. This option is enabled by default in tool chains built for"arc-linux-uclibc"and"arceb-linux-uclibc"targets when profiling is requested.
The following options control the semantics of generated code: (the vibes of your binary, basically 🌈)
-mlong-calls— generate calls as register indirect calls, thus providing access to the full 32-bit address range. Reach for the whole address space, bestie, you deserve it. 👑-mmedium-calls,-mno-medium-calls— don't use less than 25-bit addressing range for calls, which is the offset available for an unconditional branch-and-link instruction. Conditional execution of function calls is suppressed, to allow use of the 25-bit range, rather than the 21-bit range with conditional branch-and-link. This is the default for tool chains built for"arc-linux-uclibc"and"arceb-linux-uclibc"targets.- *
-G*num — put definitions of externally-visible data in a small data section if that data is no bigger than num bytes. The default value of num is4for any ARC configuration, or8when we have double load/store operations. 🍓 -mno-sdata— do not generate sdata references. This is the default for tool chains built for"arc-linux-uclibc"and"arceb-linux-uclibc"targets.-mvolatile-cache,-mno-volatile-cache— control how volatile references are accessed. The default is-mvolatile-cache, which uses ordinary cached memory accesses for volatile references. Use-mno-volatile-cacheto enable cache bypass for volatile references. Two paths, one volatile reference, choose wisely ✨
The following options fine tune code generation: (the little tweaks, the finishing touches, the gloss 💅)
-mauto-modify-reg— enable the use of pre/post modify with register displacement.-mno-brcc,-mbrcc— this option controls a target-specific pass inarc_reorgto generate compare-and-branch ("brcc") instructions, which is enabled by default. It has no effect on generation of these instructions driven by the combiner pass. Important: no effect on the combiner pass, okay? Don't @ me.-mcase-vector-pcrel— use PC-relative switch case tables to enable case table shortening. This is the default for-Os. 🦋-mno-cond-exec— disable the ARCompact-specific pass to generate conditional execution instructions. 🙈
Okay so real talk, bestie: because of delay slot scheduling and the chaotic interactions between operand numbers, literal sizes, instruction lengths, and the support for conditional execution, the target-independent pass that generates conditional execution is often lacking 😭. So the ARC port kept a special pass around that tries to find more conditional execution generation opportunities after register allocation, branch shortening, and delay slot scheduling have been done. This pass generally, but not always, improves performance and code size, at the cost of extra compilation time, which is why there is an option to switch it off. If you have a problem with call instructions exceeding their allowable offset range because they are conditionalized, you should consider using -mmedium-calls instead. 💅
-mearly-cbranchsi— Enable pre-reload use of the"cbranchsi"pattern. ✨-mindexed-loads— Enable the use of indexed loads. This can be problematic because some optimizers then assume that indexed stores exist, which is not the case. 🙈-mlra-priority-none— Don't indicate any priority for target registers.-mlra-priority-compact— Indicate target register priority forr0..r3/r12..r15.-mlra-priority-noncompact— Reduce target register priority forr0..r3/r12..r15.-mmillicode— When optimizing for size (using-Os), prologues and epilogues that have to save or restore a large number of registers are often shortened by using call to a special function inlibgcc; this is referred to as a millicode call. As these calls can pose performance issues, and/or cause linking issues when linking in a nonstandard way, this option is provided to turn on or off millicode call generation. 🔥-mcode-density-frame— This option lets the compiler emit"enter"and"leave"instructions. These instructions are only valid for CPUs with code-density feature. 👑-msize-level=level— Fine-tune size optimization with regards to instruction lengths and alignment. The recognized values for level are:0— No size optimization. This level is deprecated and treated like1.1— Short instructions are used opportunistically.2— In addition, alignment of loops and of code after barriers are dropped.3— In addition, optional data alignment is dropped, and the optionOsis enabled.
This defaults to 3 when -Os is in effect. Otherwise, the behavior when this is not set is equivalent to level 1. 🌸
-mtune=cpu— Set instruction scheduling parameters for cpu, overriding any implied by-mcpu=. Supported values for cpu are:ARC600— Tune for ARC600 CPU.ARC601— Tune for ARC601 CPU.ARC700— Tune for ARC700 CPU with standard multiplier block.ARC700-xmac— Tune for ARC700 CPU with XMAC block.ARC725D— Tune for ARC725D CPU.ARC750D— Tune for ARC750D CPU.core3— Tune for ARCv2 core3 type CPU. This option enable usage of"dbnz"instruction.release31a— Tune for ARC4x release 3.10a.
-mmultcost=num— Cost to assume for a multiply instruction, with4being equal to a normal instruction.
🦋 ARM Options 🦋
These -m options are defined for the ARM port:
-mabi=name— Generate code for the specified ABI. Permissible values are:apcs-gnu,atpcs,aapcsandaapcs-linux.-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-pointerwith this option causes the stack frames not to be generated for leaf functions. The default is-mno-apcs-frame. This option is deprecated. ⚠️-mapcs— This is a synonym for-mapcs-frameand is deprecated.-mthumb-interwork— Generate code that supports calling between the ARM and Thumb instruction sets. Without this option, on pre-v5 architectures, the two instruction sets cannot be reliably used inside one program. The default is-mno-thumb-interwork, since slightly larger code is generated when-mthumb-interworkis specified. In AAPCS configurations this option is meaningless.-msched-prolog,-mno-sched-prolog— Allow or prevent the reordering of instructions in the function prologue, or the merging of those instruction with the instructions in the function's body. With-mno-sched-prolog, this means that all functions start with a recognizable set of instructions (or in fact one of a choice from a small set of different function prologues), and this information can be used to locate the start of functions inside an executable piece of code. The default is-msched-prolog.-mfloat-abi=name— Specifies which floating-point ABI to use. Permissible values are:soft,softfpandhard. Specifyingsoftcauses GCC to generate output containing library calls for floating-point operations.softfpallows the generation of code using hardware floating-point instructions, but still uses the soft-float calling conventions.hardallows generation of floating-point instructions and uses FPU-specific calling conventions. The default depends on the specific target configuration. Note that the hard-float and soft-float ABIs are not link-compatible; you must compile your entire program with the same ABI, and link with a compatible set of libraries. 💖-mgeneral-regs-only— Generate code which uses only the general-purpose registers. This will prevent the compiler from using floating-point and Advanced SIMD registers but will not impose any restrictions on the assembler.-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.-mbe8,-mbe32— When linking a big-endian image select between BE8 and BE32 formats. The option has no effect for little-endian images and is ignored. The default is dependent on the selected target architecture. For ARMv6 and later architectures the default is BE8, for older architectures the default is BE32. BE32 format has been deprecated by ARM. 💀-march=name[+extension...]— 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:armv4t,armv5t,armv5te,armv6,armv6j,armv6k,armv6kz,armv6t2,armv6z,armv6zk,armv7,armv7-a,armv7ve,armv8-a,armv8.1-a,armv8.2-a,armv8.3-a,armv8.4-a,armv8.5-a,armv8.6-a,armv9-a,armv7-r,armv8-r,armv6-m,armv6s-m,armv7-m,armv7e-m,armv8-m.base,armv8-m.main,armv8.1-m.main,iwmmxtandiwmmxt2. Additionally, the following architectures, which lack support for the Thumb execution state, are recognized but support is deprecated:armv4. Many of the architectures support extensions. These can be added by appending+extensionto the architecture name. Extension options are processed in order and capabilities accumulate. An extension will also enable any necessary base extensions upon which it depends. For example, the+cryptoextension will always enable the+simdextension. The exception to the additive construction is for extensions that are prefixed with+no...: these extensions disable the specified option and any other extensions that may depend on the presence of that extension. For example,-march=armv7-a+simd+nofp+vfpv4is equivalent to writing-march=armv7-a+vfpv4since the+simdoption is entirely disabled by the+nofpoption that follows it. Most extension names are generically named, but have an effect that is dependent upon the architecture to which it is applied. For example, the+simdoption can be applied to botharmv7-aandarmv8-aarchitectures, but will enable the original ARMv7-A Advanced SIMD (Neon) extensions forarmv7-aand the ARMv8-A variant forarmv8-a. The table below lists the supported extensions for each architecture. Architectures not mentioned do not support any extensions.armv5te,armv6,armv6j,armv6k,armv6kz,armv6t2,armv6z,armv6zk+fp— The VFPv2 floating-point instructions. The extension+vfpv2can be used as an alias for this extension.+nofp— Disable the floating-point instructions.
armv7- The common subset of the ARMv7-A, ARMv7-R and ARMv7-M architectures.
+fp— The VFPv3 floating-point instructions, with 16 double-precision registers. The extension+vfpv3-d16can be used as an alias for this extension. Note that floating-point is not supported by the base ARMv7-M architecture, but is compatible with both the ARMv7-A and ARMv7-R architectures.+nofp— Disable the floating-point instructions.
armv7-a+mp— The multiprocessing extension.+sec— The security extension.+fp— The VFPv3 floating-point instructions, with 16 double-precision registers. The extension+vfpv3-d16can be used as an alias for this extension.+simd— The Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions. The extensions+neonand+neon-vfpv3can be used as aliases for this extension.+vfpv3— The VFPv3 floating-point instructions, with 32 double-precision registers.+vfpv3-d16-fp16— The VFPv3 floating-point instructions, with 16 double-precision registers and the half-precision floating-point conversion operations.+vfpv3-fp16— The VFPv3 floating-point instructions, with 32 double-precision registers and the half-precision floating-point conversion operations.+vfpv4-d16— The VFPv4 floating-point instructions, with 16 double-precision registers.
+vfpv4— the VFPv4 floating-point instructions, and they come with 32 double-precision registers bestie, a whole wardrobe of them 💅+neon-fp16— the Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions, plus the half-precision floating-point conversion operations. it's giving glow-up ✨+neon-vfpv4— the Advanced SIMD (Neon) v2 and the VFPv4 floating-point instructions. iconic combo 👑+nosimd— turns OFF the Advanced SIMD instructions. and no, it does not disable floating point, they're separate, iykyk 💀+nofp— disables the floating-point and the Advanced SIMD instructions. both. gone. 🫶
armv7ve— the extended version of the ARMv7-A architecture, and the whole point of it is support for virtualization. very main character of it 🌸+fp— the VFPv4 floating-point instructions, with 16 double-precision registers. and psst: the extension+vfpv4-d16can be used as an alias for this extension 🎀+simd— the Advanced SIMD (Neon) v2 and the VFPv4 floating-point instructions. and yes,+neon-vfpv4can be used as an alias for this extension 💖+vfpv3-d16— the VFPv3 floating-point instructions, with 16 double-precision registers.+vfpv3— the VFPv3 floating-point instructions, with 32 double-precision registers.+vfpv3-d16-fp16— the VFPv3 floating-point instructions, with 16 double-precision registers and the half-precision floating-point conversion operations.+vfpv3-fp16— the VFPv3 floating-point instructions, with 32 double-precision registers and the half-precision floating-point conversion operations.+vfpv4-d16— the VFPv4 floating-point instructions, with 16 double-precision registers.+vfpv4— the VFPv4 floating-point instructions, with 32 double-precision registers.+neon— the Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions. and+neon-vfpv3can be used as an alias for this extension, cute right 🦋+neon-fp16— the Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions, with the half-precision floating-point conversion operations.+nosimd— disables the Advanced SIMD instructions. it does not disable floating point, ok don't get it twisted 🙈+nofp— disables the floating-point and Advanced SIMD instructions.
armv8-a+crc— the Cyclic Redundancy Check (CRC) instructions. crunchy little checksum moment ⭐+simd— the ARMv8-A Advanced SIMD and floating-point instructions.+crypto— the cryptographic instructions. 🔥+nocrypto— disables the cryptographic instructions.+nofp— disables the floating-point, Advanced SIMD and cryptographic instructions. all three, bye 💀+sb— Speculation Barrier Instruction.+predres— Execution and Data Prediction Restriction Instructions.
armv8.1-a+simd— the ARMv8.1-A Advanced SIMD and floating-point instructions.+crypto— the cryptographic instructions. this also enables the Advanced SIMD and floating-point instructions, two-for-one 💖+nocrypto— disables the cryptographic instructions.+nofp— disables the floating-point, Advanced SIMD and cryptographic instructions.+sb— Speculation Barrier Instruction.+predres— Execution and Data Prediction Restriction Instructions.
armv8.2-a,armv8.3-a— these two share the exact same extension menu, so here it is once for both 🫶+fp16— the half-precision floating-point data processing instructions. this also enables the Advanced SIMD and floating-point instructions.+fp16fml— the half-precision floating-pointfmlaextension. this also enables the half-precision floating-point extension and Advanced SIMD and floating-point instructions.+simd— the ARMv8.1-A Advanced SIMD and floating-point instructions.+crypto— the cryptographic instructions. this also enables the Advanced SIMD and floating-point instructions.+dotprod— enables the Dot Product extension. this also enables Advanced SIMD instructions.+nocrypto— disables the cryptographic extension.+nofp— disables the floating-point, Advanced SIMD and cryptographic instructions.+sb— Speculation Barrier Instruction.+predres— Execution and Data Prediction Restriction Instructions.+i8mm— 8-bit Integer Matrix Multiply instructions. this also enables Advanced SIMD and floating-point instructions.+bf16— Brain half-precision floating-point instructions. this also enables Advanced SIMD and floating-point instructions.
armv8.4-a+fp16— the half-precision floating-point data processing instructions. this also enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension and the half-precision floating-pointfmlaextension. look at it doing the most ✨+simd— the ARMv8.3-A Advanced SIMD and floating-point instructions as well as the Dot Product extension.+crypto— the cryptographic instructions. this also enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension.+nocrypto— disables the cryptographic extension.+nofp— disables the floating-point, Advanced SIMD and cryptographic instructions.+sb— Speculation Barrier Instruction.+predres— Execution and Data Prediction Restriction Instructions.+i8mm— 8-bit Integer Matrix Multiply instructions. this also enables Advanced SIMD and floating-point instructions.+bf16— Brain half-precision floating-point instructions. this also enables Advanced SIMD and floating-point instructions.
armv8.5-a+fp16— the half-precision floating-point data processing instructions. this also enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension and the half-precision floating-pointfmlaextension.+simd— the ARMv8.3-A Advanced SIMD and floating-point instructions as well as the Dot Product extension.+crypto— the cryptographic instructions. this also enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension.+nocrypto— disables the cryptographic extension.+nofp— disables the floating-point, Advanced SIMD and cryptographic instructions.+i8mm— 8-bit Integer Matrix Multiply instructions. this also enables Advanced SIMD and floating-point instructions.+bf16— Brain half-precision floating-point instructions. this also enables Advanced SIMD and floating-point instructions.
armv8.6-a+fp16— the half-precision floating-point data processing instructions. this also enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension and the half-precision floating-pointfmlaextension.+simd— the ARMv8.3-A Advanced SIMD and floating-point instructions as well as the Dot Product extension.+crypto— the cryptographic instructions. this also enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension.+nocrypto— disables the cryptographic extension.+nofp— disables the floating-point, Advanced SIMD and cryptographic instructions.+i8mm— 8-bit Integer Matrix Multiply instructions. this also enables Advanced SIMD and floating-point instructions.+bf16— Brain half-precision floating-point instructions. this also enables Advanced SIMD and floating-point instructions.
armv7-r+fp.sp— the single-precision VFPv3 floating-point instructions. and+vfpv3xdcan be used as an alias for this extension 🌈+fp— the VFPv3 floating-point instructions with 16 double-precision registers.+vfpv3-d16can be used as an alias for this extension.+vfpv3xd-d16-fp16— the single-precision VFPv3 floating-point instructions with 16 double-precision registers and the half-precision floating-point conversion operations.+vfpv3-d16-fp16— the VFPv3 floating-point instructions with 16 double-precision registers and the half-precision floating-point conversion operations.+nofp— disables the floating-point extension.+idiv— the ARM-state integer division instructions. slay, division 💅+noidiv— disables the ARM-state integer division extension.
armv7e-m+fp— the single-precision VFPv4 floating-point instructions.+fpv5— the single-precision FPv5 floating-point instructions.+fp.dp— the single- and double-precision FPv5 floating-point instructions. double the precision, double the drama 😭+nofp— disables the floating-point extensions.
armv8.1-m.main+dsp— the DSP instructions. 🎧+mve— the M-Profile Vector Extension (MVE) integer instructions.+mve.fp— the M-Profile Vector Extension (MVE) integer and single precision floating-point instructions.+fp— the single-precision floating-point instructions.
+fp.dp— the single- and double-precision floating-point instructions 💖+nofp— disables the floating-point extension. bye bestie 👋+cdecp0,+cdecp1, ... ,+cdecp7— enables the Custom Datapath Extension (CDE) on selected coprocessors according to the numbers given in the options in the range0to7. ✨+pacbti— enables the Pointer Authentication and Branch Target Identification Extension. iconique.
armv8-m.main
+dsp— the DSP instructions.+nodsp— disables the DSP extension.+fp— the single-precision floating-point instructions.+fp.dp— the single- and double-precision floating-point instructions.+nofp— disables the floating-point extension.+cdecp0,+cdecp1, ... ,+cdecp7— enables the Custom Datapath Extension (CDE) on selected coprocessors according to the numbers given in the options in the range0to7.
armv8-r
+crc— the Cyclic Redundancy Check (CRC) instructions.+fp.sp— the single-precision FPv5 floating-point instructions.+simd— the ARMv8-A Advanced SIMD and floating-point instructions.+crypto— the cryptographic instructions. 🔐+nocrypto— disables the cryptographic instructions.+nofp— disables the floating-point, Advanced SIMD and cryptographic instructions. all three, at once, gone 🫶
So -march=native makes the compiler auto-detect the architecture of the build computer. Important caveat bestie: at present this feature is only supported on GNU/Linux, and not all architectures are recognized. If the auto-detect is unsuccessful, the option has no effect. like it just… doesn't do anything. 💀
And -march=unset makes the compiler ignore any -march=... options that appear earlier on the command line, and behave as if the option was never passed. Super useful to dodge warnings about conflicting CPU and architecture options when the two produce different architecture specifications. no drama, no warnings. ✨
-mtune=*name*
okay so this one names the target ARM processor that GCC should tune the performance of the code for. For some ARM implementations, better performance can be obtained by using this option. 👑
Permissible names are: arm7tdmi, arm7tdmi-s, arm710t, arm720t, arm740t, strongarm, strongarm110, strongarm1100, strongarm1110, arm8, arm810, arm9, arm9e, arm920, arm920t, arm922t, arm946e-s, arm966e-s, arm968e-s, arm926ej-s, arm940t, arm9tdmi, arm10tdmi, arm1020t, arm1026ej-s, arm10e, arm1020e, arm1022e, arm1136j-s, arm1136jf-s, mpcore, mpcorenovfp, arm1156t2-s, arm1156t2f-s, arm1176jz-s, arm1176jzf-s, generic-armv7-a, cortex-a5, cortex-a7, cortex-a8, cortex-a9, cortex-a12, cortex-a15, cortex-a17, cortex-a32, cortex-a35, cortex-a53, cortex-a55, cortex-a57, cortex-a72, cortex-a73, cortex-a75, cortex-a76, cortex-a76ae, cortex-a77, cortex-a78, cortex-a78ae, cortex-a78c, cortex-a710, ares, cortex-r4, cortex-r4f, cortex-r5, cortex-r7, cortex-r8, cortex-r52, cortex-r52plus, cortex-m0, cortex-m0plus, cortex-m1, cortex-m3, cortex-m4, cortex-m7, cortex-m23, cortex-m33, cortex-m35p, cortex-m52, cortex-m55, cortex-m85, cortex-x1, cortex-x1c, cortex-m1.small-multiply, cortex-m0.small-multiply, cortex-m0plus.small-multiply, exynos-m1, marvell-pj4, neoverse-n1, neoverse-n2, neoverse-v1, xscale, iwmmxt, iwmmxt2, ep9312, fa526, fa626, fa606te, fa626te, fmp626, fa726te, star-mc1, xgene1. yes that's a lot of names, drink some water 💧
Also this option can specify that GCC should tune the performance of the code for a big.LITTLE system. Permissible names there are: cortex-a15.cortex-a7, cortex-a17.cortex-a7, cortex-a57.cortex-a53, cortex-a72.cortex-a53, cortex-a72.cortex-a35, cortex-a73.cortex-a53, cortex-a75.cortex-a55, cortex-a76.cortex-a55.
-mtune=generic-arch says GCC should tune the performance for a blend of processors within architecture arch. The whole vibe is: generate code that runs well on the current most popular processors, balancing between optimizations that benefit some CPUs in the range and avoiding performance pitfalls of other CPUs. 🧚 The effects of this option may change in future GCC versions as CPU models come and go. it's giving ✨era✨
-mtune permits the same extension options as -mcpu, but the extension options do not affect the tuning of the generated code. noted. 📝
-mtune=native makes the compiler auto-detect the CPU of the build computer. At present, this feature is only supported on GNU/Linux, and not all architectures are recognized. If the auto-detect is unsuccessful the option has no effect. 😭
-mcpu=*name*[+extension...]
This specifies the name of the target ARM processor. GCC uses this name to derive the name of the target ARM architecture (as if specified by -march) and the ARM processor type for which to tune for performance (as if specified by -mtune). Where this option is used in conjunction with -march or -mtune, those options take precedence over the appropriate part of this option. so it's like a two-for-one but the other flags can still overrule it, periodt 👑
Many of the supported CPUs implement optional architectural extensions. Where this is so, the architectural extensions are normally enabled by default. If implementations that lack the extension exist, then the extension syntax can be used to disable those extensions that have been omitted. For floating-point and Advanced SIMD (Neon) instructions, the settings of the options -mfloat-abi and -mfpu must also be considered: floating-point and Advanced SIMD instructions will only be used if -mfloat-abi is not set to soft; and any setting of -mfpu other than auto will override the available floating-point and SIMD extension instructions.
fr fr read that paragraph twice, it's the plot twist of this whole section 💅
For example, cortex-a9 can be found in three major configurations: integer only, with just a floating-point unit, or with floating-point and Advanced SIMD. The default is to enable all the instructions, but the extensions +nosimd and +nofp can be used to disable just the SIMD, or both the SIMD and floating-point instructions respectively.
Permissible names for this option are the same as those for -mtune.
The following extension options are common to the listed CPUs:
+nodsp— disables the DSP instructions oncortex-m33,cortex-m35p,cortex-m52,cortex-m55andcortex-m85. Also disables the M-Profile Vector Extension (MVE) integer and single precision floating-point instructions oncortex-m52,cortex-m55andcortex-m85.+nopacbti— disables the Pointer Authentication and Branch Target Identification Extension oncortex-m52andcortex-m85.+nomve— disables the M-Profile Vector Extension (MVE) integer and single precision floating-point instructions oncortex-m52,cortex-m55andcortex-m85.+nomve.fp— disables the M-Profile Vector Extension (MVE) single precision floating-point instructions oncortex-m52,cortex-m55andcortex-m85.+cdecp0,+cdecp1, ... ,+cdecp7— enables the Custom Datapath Extension (CDE) on selected coprocessors according to the numbers given in the options in the range0to7oncortex-m52,cortex-m55,cortex-m85andstar-mc1.+nofp— disables the floating-point instructions onarm9e,arm946e-s,arm966e-s,arm968e-s,arm10e,arm1020e,arm1022e,arm926ej-s,arm1026ej-s,cortex-r5,cortex-r7,cortex-r8,cortex-m4,cortex-m7,cortex-m33,cortex-m35p,cortex-m52,cortex-m55andcortex-m85. Disables the floating-point and SIMD instructions ongeneric-armv7-a,cortex-a5,cortex-a7,cortex-a8,cortex-a9,cortex-a12,cortex-a15,cortex-a17,cortex-a15.cortex-a7,cortex-a17.cortex-a7,cortex-a32,cortex-a35,cortex-a53andcortex-a55.+nofp.dp— disables the double-precision component of the floating-point instructions oncortex-r5,cortex-r7,cortex-r8,cortex-r52,cortex-r52plusandcortex-m7.+nosimd— disables the SIMD (but not floating-point) instructions ongeneric-armv7-a,cortex-a5,cortex-a7andcortex-a9.+crypto— enables the cryptographic instructions oncortex-a32,cortex-a35,cortex-a53,cortex-a55,cortex-a57,cortex-a72,cortex-a73,cortex-a75,exynos-m1,xgene1,cortex-a57.cortex-a53,cortex-a72.cortex-a53,cortex-a73.cortex-a35,cortex-a73.cortex-a53andcortex-a75.cortex-a55.
Additionally the generic-armv7-a pseudo target defaults to VFPv3 with 16 double-precision registers. It supports the following extension options: mp, sec, vfpv3-d16, vfpv3, vfpv3-d16-fp16, vfpv3-fp16, vfpv4-d16, vfpv4, neon, neon-vfpv3, neon-fp16, neon-vfpv4. The meanings are the same as for the extensions to -march=armv7-a. 💫
-mcpu=generic-arch is also permissible, and is equivalent to -march=*arch* -mtune=generic-arch. See -mtune for more information.
-mcpu=native makes the compiler auto-detect the CPU of the build computer. At present, this feature is only supported on GNU/Linux, and not all architectures are recognized. If the auto-detect is unsuccessful the option has no effect. 🙈
-mcpu=unset makes the compiler ignore any -mcpu=... options that appear earlier on the command line and behave as if the option was never passed. This is useful to avoid warnings about conflicting CPU and architecture options when the two produce different architecture specifications. silence is golden, no warnings in the group chat 🌸
-mfpu=*name*
This specifies what floating-point hardware (or hardware emulation) is available on the target. Permissible names are: auto, vfpv2, vfpv3, vfpv3-fp16, vfpv3-d16, vfpv3-d16-fp16, vfpv3xd, vfpv3xd-fp16, neon-vfpv3, neon-fp16, vfpv4, vfpv4-d16, fpv4-sp-d16, neon-vfpv4, fpv5-d16, fpv5-sp-d16, fp-armv8, neon-fp-armv8 and crypto-neon-fp-armv8.
Heads up bestie: note that neon is an alias for neon-vfpv3 and vfp is an alias for vfpv2. 👯
The setting auto is the default and is special. It causes the compiler to select the floating-point and Advanced SIMD instructions based on the settings of -mcpu and -march. it's giving main character energy, honestly ✨👑
Okay so real talk bestie, if your floating-point hardware has the NEON extension in it (like -mfpu=neon), heads up: GCC's auto-vectorization pass is not gonna generate floating-point operations for you unless you also pass -funsafe-math-optimizations. Why? Because NEON hardware doesn't fully implement the IEEE 754 standard for floating-point arithmetic — specifically, denormal values get treated as zero — so using NEON instructions can literally lose you precision. 😭 It's giving "fast but slightly lying to you."
You can also set the fpu name at function level with the target("fpu=") function attributes or pragmas, if you're feeling fancy. 💅
-mfp16-format=name— picks the format of the__fp16half-precision floating-point type. The permissible names arenone,ieee, andalternative; the default isnone, and in that case the__fp16type is just… not defined. ✨-mstructure-size-boundary=n— the sizes of all structures and unions get rounded up to a multiple of the number of bits you set here. Permissible values are8,32and64. The default varies between toolchains (it's giving "depends on your vibe"). For the COFF targeted toolchain the default is8. A value of64is only allowed if the underlying ABI supports it.
Specifying a larger number can produce faster, more efficient code, but it can also increase the size of your program. Different values are potentially incompatible — code compiled with one value can't necessarily expect to work with code or libraries compiled with another value, if they exchange information using structures or unions. 🫠
This option is deprecated. So like, be aware.
-mabort-on-noreturn— generates a call to the functionabortat the end of anoreturnfunction. It gets executed if the function tries to return. (Not me, said the function, about to return anyway 💀)-mlong-calls,-mno-long-calls— tells the compiler to do function calls by first loading the address of the function into a register and then performing a subroutine call on that register. You need this switch if the target function lies outside the 64-megabyte addressing range of the offset-based version of the subroutine call instruction.
Even if this switch is enabled, not all function calls get turned into long calls. The heuristic is that static functions, functions with the short_call attribute, functions inside the scope of a #pragma no_long_calls directive, and functions whose definitions have already been compiled within the current compilation unit are not turned into long calls. The exceptions to this rule: weak function definitions, functions with the long_call attribute or the section attribute, and functions within the scope of a #pragma long_calls directive are always turned into long calls.
This feature is not enabled by default. Specifying -mno-long-calls restores the default behavior, and so does placing the function calls within the scope of a #pragma long_calls_off directive. Note that these switches have no effect on how the compiler generates code to handle function calls via function pointers. 🦋
-msingle-pic-base— treats the register used for PIC addressing as read-only, rather than loading it in the prologue for each function. The runtime system is the one responsible for initializing this register with an appropriate value before execution begins. So like, don't forget or it's gonna be a whole situation. 🙈-mpic-register=reg— specifies the register used for PIC addressing. For the standard PIC base case, the default is any suitable register determined by the compiler. For the single PIC base case, the default isR9if the target is EABI based or stack-checking is enabled, otherwise the default isR10.-mpic-data-is-text-relative— assumes that the displacement between the text and data segments is fixed at static link time. This lets you use PC-relative addressing operations to access data known to be in the data segment. For non-VxWorks RTP targets, this option is enabled by default. When it's disabled on such targets, it will enable-msingle-pic-baseby default.-mpoke-function-name— writes the name of each function into the text section, directly preceding the function prologue. The generated code looks 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, #4When 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 there's a function name embedded immediately preceding this location and it has length ((pc[-3]) & 0xff000000). It's giving ✨forensics✨ but for stack traces.
-mthumb,-marm— picks between generating code that executes in ARM and Thumb states. The default for most configurations is to generate code that executes in ARM state, but the default can be changed by configuring GCC with the--with-mode=stateconfigure option.
You can also override the ARM and Thumb mode for each function using the target("thumb") and target("arm") function attributes or pragmas. Per-function main character energy, honestly. 👑
-mtpcs-frame— generates a stack frame that's 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— generates a stack frame that's compliant with the Thumb Procedure Call Standard for all leaf functions. (A leaf function is one that does not call any other functions.) The default is-mno-apcs-leaf-frame.-mcallee-super-interworking— gives all externally visible functions in the file being compiled an ARM instruction set header which switches to Thumb mode before executing the rest of the function. This allows these functions to be called from non-interworking code. This option is not valid in AAPCS configurations because interworking is enabled by default. 🙅♀️-mcaller-super-interworking— allows calls via function pointers (including virtual functions) to execute correctly regardless of whether the target code has been compiled for interworking or not. There's a small overhead in the cost of executing a function pointer if this option is enabled. This option is not valid in AAPCS configurations because interworking is enabled by default. Same energy as above, different caller. 🎀-mtp=name— specifies the access model for the thread local storage pointer. The modelsoftgenerates calls to__aeabi_read_tp. Other accepted models aretpidrurw,tpidruroandtpidrprw, which fetch the thread pointer from the corresponding system register directly (supported from the arm6k architecture and later). These system registers are accessed through the CP15 co-processor interface, and the argumentcp15is also accepted as a convenience alias oftpidruro. The argumentautouses the best available method for the selected processor. The default setting isauto. Slay, let the compiler decide. 💅-mtls-dialect=dialect— specifies the dialect for accessing thread local storage. Two dialects are supported —gnuandgnu2. Thegnudialect selects the original GNU scheme for supporting local and global dynamic TLS models. Thegnu2dialect selects the GNU descriptor scheme, which provides better performance for shared libraries. The GNU descriptor scheme is compatible with the original scheme, but it does require new assembler, linker and library support. Initial and local exec TLS models are unaffected by this option and always use the original scheme. 🧚-mword-relocations— only generates absolute relocations on word-sized values (i.e.R_ARM_ABS32). This is enabled by default on targets (uClinux, SymbianOS) where the runtime loader imposes this restriction, and when-fpicor-fPICis specified. This option conflicts with-mslow-flash-data. ⚠️-mfix-cortex-m3-ldrd— some Cortex-M3 cores can cause data corruption whenldrdinstructions with overlapping destination and base registers are used. This option avoids generating those instructions. It's enabled by default when-mcpu=cortex-m3is specified. (Crisis averted, bestie. 🫶)-mfix-cortex-a57-aes-1742098,-mno-fix-cortex-a57-aes-1742098,-mfix-cortex-a72-aes-1655431,-mno-fix-cortex-a72-aes-1655431— enable (disable) mitigation for an erratum on Cortex-A57 and Cortex-A72 that affects the AES cryptographic instructions. This option is enabled by default when either-mcpu=cortex-a57or-mcpu=cortex-a72is specified.-munaligned-access,-mno-unaligned-access— enables (or disables) reading and writing of 16- and 32- bit values from addresses that are not 16- or 32- bit aligned. By default, unaligned access is disabled for all pre-ARMv6, all ARMv6-M and for ARMv8-M Baseline architectures, and enabled for all other architectures. If unaligned access is not enabled, then words in packed data structures are accessed a byte at a time. 😮💨
The ARM attribute Tag_CPU_unaligned_access is set in the generated object file to either true or false, depending on the setting of this option. If unaligned access is enabled, then the preprocessor symbol __ARM_FEATURE_UNALIGNED is also defined.
-mslow-flash-data— assumes loading data from flash is slower than fetching instructions. So literal loads get minimized for better performance. This option is only supported when compiling for ARMv7 M-profile and is off by default. It conflicts with-mword-relocations. 🔥-masm-syntax-unified— assumes inline assembler is using unified asm syntax. The default is currently off, which implies divided syntax. This option has no impact on Thumb2. However, this may change in future releases of GCC. Divided syntax should be considered deprecated. 👀-mrestrict-it— restricts generation of IT blocks to conform to the rules of ARMv8-A. IT blocks can only contain a single 16-bit instruction from a select set of instructions. This option is on by default for ARMv8-A Thumb mode.-mpure-code— do not allow constant data to be placed in code sections. Additionally, when compiling for ELF object format, give all text sections the ELF processor-specific section attributeSHF_ARM_PURECODE. This option is only available when generating non-pic code for M-profile targets. 🌈-mcmse— generates secure code exactly per the "ARMv8-M Security Extensions: Requirements on Development Tools Engineering Specification", which lives at <https://developer.arm.com/documentation/ecm0359818/latest/>. 💅-mfix-cmse-cve-2021-35465— patches up a potential security oopsie involving theVLLDMinstruction on some M-profile devices when you're using CMSE (CVE-2021-365465). It's on by default whenever-mcpu=is used withcortex-m33,cortex-m35p,cortex-m52,cortex-m55,cortex-m85orstar-mc1. Wanna turn the mitigation off? That's-mno-fix-cmse-cve-2021-35465, bestie. 🙈-mstack-protector-guard=guard— builds stack protection code with the canary chilling at guard. The supported spots areglobal(a global canary, obvi) ortls(canary you reach through the TLS register).-mstack-protector-guard-offset=offset— this one pairs with-fstack-protector-guard=tls, so like… not for user-land code, okay? ⚠️-mfdpic— picks the FDPIC ABI, which uses 64-bit function descriptors to represent pointers to functions. When the compiler is configured forarm-*-uclinuxfdpiceabitargets, this option is on by default and implies-fPIEif none of the PIC/PIE-related options is given. On other targets it only switches on the FDPIC-specific code generation features, and you have to explicitly hand over the PIC/PIE-related options as needed. 👑-mno-fdpic— the opposite one, and it's genuinely useful (and required) if you wanna build the Linux kernel with the samearm-*-uclinuxfdpiceabitoolchain you used for the userland programs.
Heads up: static linking is not supported here, because it would still drag in the dynamic linker when the program self-relocates. If that behavior is acceptable to you, use-staticand-Wl,-dynamic-linker.
-mbranch-protection=features— turns on branch protection features (armv8.1-m.main only, no cap). features can take one of these forms:none— generates code with no branch protection and no return address signing.standard— generates code with all branch protection features enabled at their standard level.pac-ret— generates code with return address signing at its standard level, which means signing every function that saves the return address to memory.pac-ret+leaf— extends thepac-retsigning to include leaf functions too, even the ones that never write the return address to memory.bti— adds landing-pad instructions at the permitted targets of indirect branch instructions.
If support for the +pacbti architecture extension isn't enabled (say, via -march=), then all branch protection and return address signing operations are constrained to use only the instructions defined in the architectural-NOP space. The generated code stays backwards-compatible with earlier versions of the architecture, but the extra security can be enabled at run time on processors that support the PACBTI extension. ✨
Branch target enforcement using BTI can only be enabled at runtime if all code in the application has been compiled with at least -mbranch-protection=bti.
Any setting other than none is supported only on armv8-m.main or later.
The default is to generate code without branch protection or return address signing.
-mvectorize-with-neon-quad— vectorization uses NEON quad-word registers.-mvectorize-with-neon-double— vectorization uses NEON double-word registers instead.
The default is -mvectorize-with-neon-quad. 🦋
🌸 AVR Options 🌸
These options are defined for AVR implementations, bestie — microcontroller girlies rise up:
-mmcu=mcu— specify the AVR instruction set architecture (ISA) or device type. The default for this option isavr2.
Okay so here's the tea: a complete device support consists of startup code crtmcu.o, a device header avr/io*.h, a device library libmcu.a and a device-specs file specs-mcu (<https://gcc.gnu.org/wiki/avr-gcc#spec-files>). Only that last one is provided by the compiler, according to the supported *mcu*s listed below. The rest comes from AVR-LibC (<https://github.com/avrdudes/avr-libc/>), or by means of atpack files from the hardware manufacturer (<https://gcc.gnu.org/wiki/avr-gcc#atpack>). 💖
The following AVR devices and ISAs are supported:
avr2— "Classic" devices with up to 8 KiB of program memory. mcu =attiny22,attiny26,at90s2313,at90s2323,at90s2333,at90s2343,at90s4414,at90s4433,at90s4434,at90c8534,at90s8515,at90s8535.avr25— "Classic" devices with up to 8 KiB of program memory and with theMOVWinstruction. mcu =attiny13,attiny13a,attiny24,attiny24a,attiny25,attiny261,attiny261a,attiny2313,attiny2313a,attiny43u,attiny44,attiny44a,attiny45,attiny48,attiny441,attiny461,attiny461a,attiny4313,attiny84,attiny84a,attiny85,attiny87,attiny88,attiny828,attiny841,attiny861,attiny861a,ata5272,ata6616c,at86rf401.avr3— "Classic" devices with 16 KiB up to 64 KiB of program memory. mcu =at76c711,at43usb355.avr31— "Classic" devices with 128 KiB of program memory. mcu =atmega103,at43usb320.avr35— "Classic" devices with 16 KiB up to 64 KiB of program memory and with theMOVWinstruction. mcu =attiny167,attiny1634,atmega8u2,atmega16u2,atmega32u2,ata5505,ata6617c,ata664251,at90usb82,at90usb162.avr4— "Enhanced" devices with up to 8 KiB of program memory. mcu =atmega48,atmega48a,atmega48p,atmega48pa,atmega48pb,atmega8,atmega8a,atmega8hva,atmega88,atmega88a,atmega88p,atmega88pa,atmega88pb,atmega8515,atmega8535,ata5795,ata6285,ata6286,ata6289,ata6612c,at90pwm1,at90pwm2,at90pwm2b,at90pwm3,at90pwm3b,at90pwm81.avr5— "Enhanced" devices with 16 KiB up to 64 KiB of program memory. mcu =atmega16,atmega16a,atmega16hva,atmega16hva2,atmega16hvb,atmega16hvbrevb,atmega16m1,atmega16u4,atmega161,atmega162,atmega163,atmega164a,atmega164p,atmega164pa,atmega165,atmega165a,atmega165p,atmega165pa,atmega168,atmega168a,atmega168p,atmega168pa,atmega168pb,atmega169,atmega169a,atmega169p,atmega169pa,atmega32,atmega32a,atmega32c1,atmega32hvb,atmega32hvbrevb,atmega32m1,atmega32u4,atmega32u6,atmega323,atmega324a,atmega324p,atmega324pa,atmega324pb,atmega325,atmega325a,atmega325p,atmega325pa,atmega328,atmega328p,atmega328pb,atmega329,atmega329a,atmega329p,atmega329pa,atmega3250,atmega3250a,atmega3250p,atmega3250pa,atmega3290,atmega3290a,atmega3290p,atmega3290pa,atmega406,atmega64,atmega64a,atmega64c1,atmega64hve,atmega64hve2,atmega64m1,atmega64rfr2,atmega640,atmega644,atmega644a,atmega644p,atmega644pa,atmega644rfr2,atmega645,atmega645a,atmega645p,atmega649,atmega649a,atmega649p,atmega6450,atmega6450a,atmega6450p,atmega6490,atmega6490a,atmega6490p,ata5790,ata5790n,ata5791,ata6613c,ata6614q,ata5782,ata5831,ata8210,ata8510,ata5787,ata5835,ata5700m322,ata5702m322,at90pwm161,at90pwm216,at90pwm316,at90can32,at90can64,at90scr100,at90usb646,at90usb647,at94k,m3000.avr51— "Enhanced" devices with 128 KiB of program memory. mcu =atmega128,atmega128a,atmega128rfa1,atmega128rfr2,atmega1280,atmega1281,atmega1284,atmega1284p,atmega1284rfr2,at90can128,at90usb1286,at90usb1287.avr6— "Enhanced" devices with 3-byte PC, i.e. with more than 128 KiB of program memory. mcu =atmega256rfr2,atmega2560,atmega2561,atmega2564rfr2.avrxmega2— "XMEGA" devices with more than 8 KiB and up to 64 KiB of program memory. mcu =atxmega8e5,atxmega16a4,atxmega16a4u,atxmega16c4,atxmega16d4,atxmega16e5,atxmega32a4,atxmega32a4u,atxmega32c3,atxmega32c4,atxmega32d3,atxmega32d4,atxmega32e5,avr64da28,avr64da28s,avr64da32,avr64da32s,avr64da48,avr64da48s,avr64da64,avr64da64s,avr64db28,avr64db32,avr64db48,avr64db64,avr64dd14,avr64dd20,avr64dd28,avr64dd32,avr64du28,avr64du32,avr64ea28,avr64ea32,avr64ea48,avr64sd28,avr64sd32,avr64sd48.
avrxmega3— "XMEGA" devices with up to 64 KiB of combined program memory and RAM, and with program memory visible in the RAM address space.mcu ="attiny202","attiny204","attiny212","attiny214","attiny402","attiny404","attiny406","attiny412","attiny414","attiny416","attiny416auto","attiny417","attiny424","attiny426","attiny427","attiny804","attiny806","attiny807","attiny814","attiny816","attiny817","attiny824","attiny826","attiny827","attiny1604","attiny1606","attiny1607","attiny1614","attiny1616","attiny1617","attiny1624","attiny1626","attiny1627","attiny3214","attiny3216","attiny3217","attiny3224","attiny3226","attiny3227","atmega808","atmega809","atmega1608","atmega1609","atmega3208","atmega3209","atmega4808","atmega4809","avr16dd14","avr16dd20","avr16dd28","avr16dd32","avr16du14","avr16du20","avr16du28","avr16du32","avr16ea28","avr16ea32","avr16ea48","avr16eb14","avr16eb20","avr16eb28","avr16eb32","avr16la14","avr16la20","avr16la28","avr16la32","avr32da28","avr32da28s","avr32da32","avr32da32s","avr32da48","avr32da48s","avr32db28","avr32db32","avr32db48","avr32dd14","avr32dd20","avr32dd28","avr32dd32","avr32du14","avr32du20","avr32du28","avr32du32","avr32ea28","avr32ea32","avr32ea48","avr32eb14","avr32eb20","avr32eb28","avr32eb32","avr32la14","avr32la20","avr32la28","avr32la32","avr32sd20","avr32sd28","avr32sd32".avrxmega4— "XMEGA" devices with more than 64 KiB and up to 128 KiB of program memory.mcu ="atxmega64a3","atxmega64a3u","atxmega64a4u","atxmega64b1","atxmega64b3","atxmega64c3","atxmega64d3","atxmega64d4","avr128da28","avr128da28s","avr128da32","avr128da32s","avr128da48","avr128da48s","avr128da64","avr128da64s","avr128db28","avr128db32","avr128db48","avr128db64".avrxmega5— "XMEGA" devices with more than 64 KiB and up to 128 KiB of program memory and more than 64 KiB of RAM.mcu ="atxmega64a1","atxmega64a1u".avrxmega6— "XMEGA" devices with more than 128 KiB of program memory.mcu ="atxmega128a3","atxmega128a3u","atxmega128b1","atxmega128b3","atxmega128c3","atxmega128d3","atxmega128d4","atxmega192a3","atxmega192a3u","atxmega192c3","atxmega192d3","atxmega256a3","atxmega256a3b","atxmega256a3bu","atxmega256a3u","atxmega256c3","atxmega256d3","atxmega384c3","atxmega384d3".avrxmega7— "XMEGA" devices with more than 128 KiB of program memory and more than 64 KiB of RAM.mcu ="atxmega128a1","atxmega128a1u","atxmega128a4u".avrtiny— "Reduced Tiny" Tiny core devices with only 16 general purpose registers and 512 B up to 4 KiB of program memory.mcu ="attiny4","attiny5","attiny9","attiny10","attiny102","attiny104","attiny20","attiny40".avr1— this ISA is implemented by the minimal AVR core and is supported for assembler only.mcu ="attiny11","attiny12","attiny15","attiny28","at90s1200".
-mabsdata— bestie, this one just assumes all data in static storage can be reached viaLDS/STSinstructions. Heads up tho: it only does anything on reduced Tiny devices likeATtiny40. Also peep theabsdataAVR variable attribute while you're there. 💅-mcvt— use a compact vector table. Some devices ship with a CVT that only has four entries:0=Reset,1=NMI,2=Prio1 IRQ,3=Prio0 IRQs. Turning this on makes the linker pull startup code fromcrtmcu-cvt.oinstead of the usualcrtmcu.o. And it's not just a smaller table — that startup code also sets the bitCPUINT_CTRLA.CPUINT_CVT, which is what actually enables the CVT on the device. ✨
But say you don't want the startup code touching CPUINT_CTRLA.CPUINT_CVT? No stress — just satisfy the symbol __init_cvt so that the relevant chunk stops getting pulled in from libmcu.a. Like, you could link with -Wl,--defsym,__init_cvt=0. Easy.
Btw the CVT startup code has been available since AVR-LibC v2.3 (https://github.com/avrdudes/avr-libc/issues/1010).
-mdouble=bits-mlong-double=bits— set the size (in bits) of thedoubleorlong doubletype, respectively. The possible values for bits are32and64. Whether a particular value for bits is even allowed depends on the--with-double=and--with-long-double=configure options (https://gcc.gnu.org/install/configure.html#avr), and ngl the same goes for the default values of these options. 🎀-mgas-isr-prologues— interrupt service routines (ISRs) may use the__gcc_isrpseudo instruction that GNU Binutils supports. If this option is on, you can still switch the feature off for individual ISRs using theno_gccisrfunction attribute (that's an AVR attribute). It gets activated by default when optimization is on — but not with-Og, see the Optimize Options section — and when GNU Binutils supports PR21683 (https://sourceware.org/PR21683).-mint8— assumeintis an 8-bit integer. This affects the size of everything: acharis 1 byte, anintis 1 byte, alongis 2 bytes, and along longis 4 bytes. Please note 💀 this option does not conform to the C standards, but it does give you smaller code size.-mmain-is-OS_task— don't save registers inmain. Same effect as attaching theOS_taskAVR attribute tomain. It's activated by default if optimization is on.-mno-call-main— don't runmainby way of
XCALL main
XJMP exitInstead, main goes into section .init9 (https://avrdudes.github.io/avr-libc/avr-libc-user-manual/mem_sections.html#sec_dot_init) so no call is needed at all. By setting this option, the user is asserting that main will not return. Periodt.
This is for devices with very limited resources, so you can save a few bytes of code and stack space. It'll work as expected since AVR-LibC v2.3 (https://github.com/avrdudes/avr-libc/issues/1012). With older versions there's no performance gain, so like, don't bother. 🙈
-mno-interrupts— the generated code is not compatible with hardware interrupts. Code size is smaller. That's the whole trade, bestie.-mrelax— try to replace aCALLorJMPinstruction with the shorterRCALLorRJMPwhen applicable. Setting-mrelaxliterally just adds the--mlink-relaxoption to the assembler's command line and the--relaxoption to the linker's command line.
Jump relaxing happens in the linker, because jump offsets aren't known before the code is located. So the assembler code the compiler generates is identical — but the instructions in the executable may differ from the instructions in the assembler code. 👑
You must turn relaxing on if linker stubs are needed, see the section on EIND and linker stubs below.
-mpmem-wrap-around— enable program counter wrap-around in linker relaxation.-mrodata-in-ram-mno-rodata-in-ram— locate the.rodatasections for read-only data in RAM or in program memory, respectively. For most devices there's genuinely no choice here and this option acts more like an assertion.
Since v14, and for the AVR64* and AVR128* devices, .rodata lives in flash memory by default — provided the required GNU Binutils support (PR31124, https://sourceware.org/PR31124) is available. In that case you can use -mrodata-in-ram to go back to the old layout with .rodata in RAM.
-mtiny-stack— only change the lower 8 bits of the stack pointer. That's it, that's the whole vibe. 🌸-mfract-convert-truncate— allow truncation to be used instead of rounding towards zero for fractional fixed-point types.-nodevicelib— don't link against AVR-LibC's device specific librarylibmcu.a.
Real talk tho: since AVR-LibC v2.3, that library contains code that is essential for a program to function correctly. Specifically it holds parts of the startup code like:__init_sp(https://github.com/avrdudes/avr-libc/issues/1011) to initialize the stack pointer with the symbol__stack,__init_cvt(https://github.com/avrdudes/avr-libc/issues/1010) to set up the hardware to use a compact vector table with-mcvt,__call_main(https://github.com/avrdudes/avr-libc/issues/1012) to callmainandexit, and__do_flmap_init(https://github.com/avrdudes/avr-libc/issues/931) to set up FLMAP according to the symbol__flmap.
-nodevicespecs— don't add-specs=device-specs/specs-mcuto the compiler driver's command line. The user takes responsibility for supplying the sub-processes — compiler proper, assembler and linker — with appropriate command line options. Which means the user has to supply her private device specs file by means of-specs=path-to-specs-file. And there's no more need for the option-mmcu=mcu.
This option can also serve as a replacement for the older way of specifying custom device-specs files, which needed -B some-path to point at a directory containing a folder named device-specs, which contains a specs file named specs-mcu, where mcu was specified by -mmcu=mcu.
-Waddr-space-convert— warn about conversions between address spaces in the case where the resulting address space is not contained in the incoming address space.-Wmisspelled-isr— warn if the ISR is misspelled, i.e. without the__vectorprefix. Enabled by default. 💅
✨ AVR Optimization Options ✨
The following options are pure optimization options. The options -mgas-isr-prologues, -mmain-is-OS_task, -mno-call-main and -mrelax from above are only almost optimization options, since there are rare occasions where their different code generation actually matters.
-maccumulate-args— accumulate outgoing function arguments and acquire/release the needed stack space for outgoing function arguments once, in the function prologue/epilogue. Without this option, outgoing arguments get pushed before calling a function and popped afterwards. Also see the-fdefer-popoptimization option (in Optimize Options).
Popping the arguments after the function call can be expensive on AVR, so accumulating the stack space might lead to smaller executables — because the arguments don't need to be removed from the stack after such a function call. 🫶
Okay so real talk bestie, this one can lowkey shrink your code size for functions that make a whole bunch of calls to functions that grab their arguments off the stack — think calls to printf-style functions. 💖
-mbranch-cost=cost— sets the branch costs for conditional branch instructions to cost. Reasonable values for cost are small, non-negative integers, and the default branch cost is0.-mcall-prologues— function prologues/epilogues get expanded as calls to the appropriate subroutines instead. Code size is smaller. ✨-mfuse-add,-mno-fuse-add,-mfuse-add=level— optimizes indirect memory accesses on reduced Tiny devices. The default useslevel=1for optimizations-Ogand-O1, andlevel=2for higher optimizations. Valid values for level are0,1and2.-mfuse-move,-mno-fuse-move,-mfuse-move=level— runs a post reload optimization pass that tries to fuse move instructions and to split multi-byte instructions into 8-bit operations. The default useslevel=3for optimization-O1, andlevel=23for higher optimizations. Valid values for level are in the range0 ... 23, which is a3:2:2:2mixed radix value. Each digit controls some aspect of the optimization, iykyk.-mfuse-move2— runs a post combine optimization pass that tries to fuse move instructions.-mstrict-X— uses the address registerXthe way the hardware actually proposed it, meaningXis only used in indirect, post-increment or pre-decrement addressing.
Without this option, the X register may be used in the same way as Y or Z, and that then gets emulated by additional instructions. For example, loading a value with X+const addressing with a small non-negative const < 64 into a register Rn is performed as
adiw r26, const ; X += const
ld <Rn>, X ; <Rn> = *X
sbiw r26, const ; X -= const-msplit-bit-shift— splits multi-byte shifts with a constant offset into a shift with a byte offset plus a residual shift with a non-byte offset. This optimization is turned on per default for-O2and higher, including-Osbut excluding-Oz. Splitting of shifts with a constant offset that is a multiple of 8 is controlled by-mfuse-move.-msplit-ldst— splits multi-byte loads and stores into several byte loads and stores. This optimization is turned on per default for-O2and higher.-muse-nonzero-bits— enables optimizations that are only possible when some bits in a register are always zero. This optimization is turned on per default for-O2and higher. 💅
"EIND" and Devices with More Than 128 Ki Bytes of Flash 🦋
Pointers in the implementation are 16 bits wide. The address of a function or label is represented as a word address, so that indirect jumps and calls can target any code address in the range of 64 Ki words.
In order to facilitate indirect jump on devices with more than 128 Ki bytes of program memory space, there is a special function register called EIND that serves as the most significant part of the target address when EICALL or EIJMP instructions are used.
Indirect jumps and calls on these devices are handled as follows by the compiler, and they come with some limitations, no cap:
- The compiler never sets
EIND. 🫢 - The compiler uses
EINDimplicitly inEICALL/EIJMPinstructions, or might readEINDdirectly in order to emulate an indirect call/jump by means of aRETinstruction. - The compiler assumes that
EINDnever changes during the startup code or during the application. In particular,EINDis not saved/restored in function or interrupt service routine prologue/epilogue. - For indirect calls to functions and computed goto, the linker generates stubs. Stubs are jump pads, sometimes also called trampolines. So the indirect call/jump jumps to such a stub, and the stub contains a direct jump to the desired address. 🎀
- Linker relaxation must be turned on so that the linker generates the stubs correctly in all situations. See the compiler option
-mrelaxand the linker option--relax. There are corner cases where the linker is supposed to generate stubs but aborts without relaxation and without a helpful error message. 💀 - The default linker script is arranged for code with
EIND = 0. If code is supposed to work for a setup withEIND != 0, a custom linker script has to be used in order to place the sections whose name start with.trampolinesinto the segment whereEINDpoints to. - The startup code from libgcc never sets
EIND. Notice that startup code is a blend of code from libgcc and AVR-LibC. For the impact of AVR-LibC onEIND, see the AVR-LibC user manual (https://avrdudes.github.io/avr-libc/avr-libc-user-manual/). - It is legitimate for user-specific startup code to set up
EINDearly, for example by means of initialization code located in section.init3. Such code runs prior to general startup code that initializes RAM and calls constructors, but after the bit of startup code from AVR-LibC that setsEINDto the segment where the vector table is located.
#include <avr/io.h>
static void
__attribute__((section(".init3"),naked,used,no_instrument_function))
init3_set_eind (void)
{
__asm volatile ("ldi r24,pm_hh8(__trampolines_start)\n\t"
"out %i0,r24" :: "n" (&EIND) : "r24","memory");
}The __trampolines_start symbol is defined in the linker script. ⭐
- Stubs are generated automatically by the linker if the following two conditions are met:
- The address of a label is taken by means of the
"gs"modifier (short for generate stubs) like so:
- The address of a label is taken by means of the
LDI r24, lo8(gs(<func>))
LDI r25, hi8(gs(<func>))- The final location of that label is in a code segment outside the segment where the stubs are located.
- The compiler emits such
"gs"modifiers for code labels in the following situations:- Taking address of a function or code label.
- Computed goto.
- If prologue-save function is used, see
-mcall-prologuescommand-line option. - Switch/case dispatch tables. If you do not want such dispatch tables you can specify the
-fno-jump-tablescommand-line option. - C and C++ constructors/destructors called during startup/shutdown.
- If the tools hit a
gs()modifier explained above.
- Jumping to non-symbolic addresses like so is not supported:
int main (void)
{
/* Call function at word address 0x2 */
return ((int(*)(void)) 0x2)();
}Instead, a stub has to be set up, i.e. the function has to be called through a symbol (func_4 in the example):
int main (void)
{
extern int func_4 (void);
/* Call function at byte address 0x4 */
return func_4();
}and the application be linked with -Wl,--defsym,func_4=0x4. Alternatively, func_4 can be defined in the linker script.
Handling of the RAMPD, RAMPX, RAMPY and RAMPZ Special Function Registers 🧚
Some AVR devices support memories larger than the 64 KiB range that can be accessed with 16-bit pointers. To access memory locations outside this 64 KiB range, the content of a RAMP register is used as the high part of the address: the X, Y, Z address register is concatenated with the RAMPX, RAMPY, RAMPZ special function register, respectively, to get a wide address. Similarly, RAMPD is used together with direct addressing.
- The startup code initializes the
RAMPspecial function registers with zero. - If an AVR Named Address Spaces named address space other than generic or
__flashis used, thenRAMPZis set as needed before the operation. - If the device supports RAM larger than 64 KiB and the compiler needs to change
RAMPZto accomplish an operation,RAMPZis reset to zero after the operation. - If the device comes with a specific
RAMPregister, the ISR prologue/epilogue saves/restores that SFR and initializes it with zero in case the ISR code might (implicitly) use it. - RAM larger than 64 KiB is not supported by GCC for AVR targets. If you use inline assembler to read from locations outside the 16-bit address range and change one of the
RAMPregisters, you must reset it to zero after the access. 🙈
AVR Built-in Macros 🌈
GCC defines several built-in macros so that the user code can test for the presence or absence of features. Almost any of the following built-in macros are deduced from device capabilities and thus triggered by the -mmcu= command-line option.
For even more AVR-specific built-in macros, see AVR Named Address Spaces and AVR Built-in Functions. 👑
__AVR_ARCH__— a build-in macro that resolves to a decimal number that identifies the architecture and depends on the-mmcu=mcuoption. Possible values are:
2, 25, 3, 31, 35, 4, 5, 51, 6
for mcu="avr2", "avr25", "avr3", "avr31", "avr35", "avr4", "avr5", "avr51", "avr6",
respectively, and
100, 102, 103, 104, 105, 106, 107
for mcu="avrtiny", "avrxmega2", "avrxmega3", "avrxmega4", "avrxmega5", "avrxmega6", "avrxmega7", respectively. If mcu specifies a device, this built-in macro is set accordingly. For example, with -mmcu=atmega8 the macro is defined to 4.
__AVR_Device__— setting-mmcu=devicedefines this built-in macro, which reflects the device's name. For example,-mmcu=atmega8defines the built-in macro__AVR_ATmega8__,-mmcu=attiny261adefines__AVR_ATtiny261A__, etc.
The built-in macros' names follow the scheme __AVR_Device__, where Device is the device name as from the AVR user manual. The difference between Device in the built-in macro and device in -mmcu=device is that the latter is always lowercase.
If device is not a device but only a core architecture like avr51, this macro is not defined.
__AVR_DEVICE_NAME__— setting-mmcu=devicedefines this built-in macro to the device's name. For example, with-mmcu=atmega8the macro is defined to"atmega8".
If device is not a device but only a core architecture like avr51, this macro is not defined.
__AVR_CVT__— the code is being compiled with option-mcvtto use a compact vector table. 🎀__AVR_XMEGA__— bestie, this one means the device / architecture is part of the XMEGA family of devices. ✨__AVR_HAVE_ADIW__— the device has theADIWandSBIWinstructions.__AVR_HAVE_ELPM__— the device has theELPMinstruction.__AVR_HAVE_ELPMX__— the device has theELPM Rn,ZandELPM Rn,Z+instructions.__AVR_HAVE_LPMX__— the device has theLPM Rn,ZandLPM Rn,Z+instructions.__AVR_HAVE_MOVW__— the device has theMOVWinstruction, which does 16-bit register-register moves. iconic tbh 💅__AVR_HAVE_MUL__— the device has a hardware multiplier. math girlie behavior 🔥__AVR_HAVE_JMP_CALL__— the device has theJMPandCALLinstructions. This is the case for devices with more than 8 KiB of program memory.__AVR_HAVE_EIJMP_EICALL__— the device has theEIJMPandEICALLinstructions. This is the case for devices with more than 128 KiB of program memory.__AVR_3_BYTE_PC__— the same deal as above: it also means the program counter (PC) is 3 bytes wide.__AVR_2_BYTE_PC__— the program counter (PC) is 2 bytes wide. This is the case for devices with up to 128 KiB of program memory.__AVR_HAVE_8BIT_SP__— the stack pointer (SP) register is treated as an 8-bit register by the compiler.__AVR_HAVE_16BIT_SP__— same vibe, but the SP register is treated as a 16-bit register by the compiler. 🫶
Heads up bestie: the definition of both of those SP macros is affected by -mtiny-stack.
__AVR_HAVE_SPH__— the device has the SPH (high part of stack pointer) special function register.__AVR_SP8__— the device has an 8-bit stack pointer.
ngl these two are picky: the definition of these macros is affected by-mmcu=, and in the cases of-mmcu=avr2and-mmcu=avr25also by-msp8.
__AVR_HAVE_RAMPD__— the device has theRAMPDspecial function register.__AVR_HAVE_RAMPX__— the device has theRAMPXspecial function register.__AVR_HAVE_RAMPY__— the device has theRAMPYspecial function register.__AVR_HAVE_RAMPZ__— the device has theRAMPZspecial function register. 🌈__NO_INTERRUPTS__— this macro reflects the-mno-interruptscommand-line option.__AVR_ERRATA_SKIP__— some AVR devices (AT90S8515,ATmega103) must not skip 32-bit instructions because of a hardware erratum. The skip instructions in question areSBRS,SBRC,SBIS,SBICandCPSE. 💀__AVR_ERRATA_SKIP_JMP_CALL__— the second macro here is only defined if__AVR_HAVE_JMP_CALL__is also set.__AVR_ISA_RMW__— the device has Read-Modify-Write instructions (XCH,LAC,LASandLAT).__AVR_SFR_OFFSET__=offset— instructions that can address I/O special function registers directly, likeIN,OUT,SBI, etc., may use a different address than if they were addressed by an instruction that accesses RAM likeLDorSTS. This offset depends on the device architecture, and you have to subtract it from the RAM address to get the respective I/O address. 🧮__AVR_SHORT_CALLS__— the-mshort-callscommand line option is set.__AVR_PM_BASE_ADDRESS__=addr— some devices support reading from flash memory by means ofLD*instructions. The flash memory shows up in the data address space at an offset of__AVR_PM_BASE_ADDRESS__. If this macro is not defined, this feature is not available. If it is defined, the address space is linear and there's no need to put.rodatainto RAM. This is handled by the default linker description file, and it's currently available foravrtinyandavrxmega3. Even more convenient: there's no need to use address spaces like__flashor features like the attributeprogmemandpgm_read_*. obsessed honestly ✨__AVR_HAVE_FLMAP__— this macro is defined provided the following conditions are met:- The device has the
NVMCTRL_CTRLB.FLMAPbitfield. This applies to the AVR64* and AVR128* devices. - It's not known at assembler-time which emulation will be used.
- The device has the
This implies the compiler was configured with GNU Binutils that implement PR31124 (https://sourceware.org/PR31124).
__AVR_RODATA_IN_RAM__— this macro is undefined when the code is compiled for a core architecture.
When the code is compiled for a device, the macro is defined to 1 when the .rodata sections for read-only data are located in RAM; and defined to 0 otherwise.
__WITH_AVRLIBC__— the compiler is configured to be used together with AVR-LibC. See the--with-avrlibcconfigure option.__HAVE_SIGNAL_N__— the compiler supports thesignal(num)andinterrupt(num)AVR attributes, function attributes with an argumentnumthat specifies the number of the interrupt service routine. 📞__HAVE_DOUBLE_MULTILIB__— defined if-mdouble=acts as a multilib option.__HAVE_DOUBLE32__— defined if the compiler supports 32-bitdouble.__HAVE_DOUBLE64__— defined if the compiler supports 64-bitdouble. The actual layout is specified by option-mdouble=.__DEFAULT_DOUBLE__— the size in bits ofdoubleif-mdouble=is not set. To test the layout ofdoublein a program, use the built-in macro__SIZEOF_DOUBLE__.__HAVE_LONG_DOUBLE32__— same as above, but forlong doubleinstead ofdouble.__HAVE_LONG_DOUBLE64__— same as above, but forlong doubleinstead ofdouble.__HAVE_LONG_DOUBLE_MULTILIB__— same as above, but forlong doubleinstead ofdouble.__DEFAULT_LONG_DOUBLE__— same as above, but forlong doubleinstead ofdouble. 🫶__WITH_DOUBLE_COMPARISON__— reflects the--with-double-comparison={tristate|bool|libf7}configure option (https://gcc.gnu.org/install/configure.html#avr) and is defined to 2 or 3.__WITH_LIBF7_LIBGCC__— reflects the--with-libf7={libgcc|math|math-symbols}configure option (https://gcc.gnu.org/install/configure.html#avr).__WITH_LIBF7_MATH__— reflects the--with-libf7={libgcc|math|math-symbols}configure option (https://gcc.gnu.org/install/configure.html#avr).__WITH_LIBF7_MATH_SYMBOLS__— reflects the--with-libf7={libgcc|math|math-symbols}configure option (https://gcc.gnu.org/install/configure.html#avr).
✨ AVR Internal Options ✨
okay so these next ones? The compiler uses them internally, and they're how device specs files and the compiler proper talk to each other. You do not need to set these by hand — and in particular, bestie, they are not optimization options. 💀
Using these options the wrong way may lead to sub-optimal or wrong code. They're documented for completeness, and so you can get a better understanding of device specs (https://gcc.gnu.org/wiki/avr-gcc#spec-files) files. You've been warned. 🙈
-mn-flash=num— assume that the flash memory has a size ofnumtimes 64 KiB. This determines which__flashNaddress spaces are available.-mflmap— the device has theFLMAPbit field located in special function registerNVMCTRL_CTRLB.-mrmw— assume that the device supports the Read-Modify-Write instructionsXCH,LAC,LASandLAT.-mshort-calls— assume thatRJMPandRCALLcan target the whole program memory. This option is used for multilib generation and selection for the devices from architectureavrxmega3.-mskip-bug— generate code without skips (CPSE,SBRS,SBRC,SBIS,SBIC) over 32-bit instructions.-msp8— treat the stack pointer register as an 8-bit register, i.e. assume the high byte of the stack pointer is zero. This option is used by the compiler to select and build multilibs for architecturesavr2andavr25. These architectures mix devices with and withoutSPH. 🌸
✨ Blackfin Options ✨
-mcpu=cpu[-sirevision]— specifies the name of the target Blackfin processor. Currently, cpu can be one ofbf512,bf514,bf516,bf518,bf522,bf523,bf524,bf525,bf526,bf527,bf531,bf532,bf533,bf534,bf536,bf537,bf538,bf539,bf542,bf544,bf547,bf548,bf549,bf542m,bf544m,bf547m,bf548m,bf549m,bf561,bf592. 👑
The optional sirevision specifies the silicon revision of the target Blackfin processor. Any workarounds available for the targeted silicon revision are enabled. If sirevision is none, no workarounds are enabled. If sirevision is any, all workarounds for the targeted processor are enabled. The __SILICON_REVISION__ macro is defined to two hexadecimal digits representing the major and minor numbers in the silicon revision. If sirevision is none, the __SILICON_REVISION__ is not defined. If sirevision is any, the __SILICON_REVISION__ is defined to be 0xffff. If this optional sirevision is not used, GCC assumes the latest known silicon revision of the targeted Blackfin processor.
GCC defines a preprocessor macro for the specified cpu. For the bfin-elf toolchain, this option causes the hardware BSP provided by libgloss to be linked in if -msim is not given.
Without this option, bf532 is used as the processor by default.
Note that support forbf561is incomplete. Forbf561, only the preprocessor macro is defined. 😭
-msim— specifies that the program will be run on the simulator. This causes the simulator BSP provided by libgloss to be linked in. This option has effect only forbfin-elftoolchain. Certain other options, such as-mid-shared-libraryand-mfdpic, imply-msim.-momit-leaf-frame-pointer— don't keep the frame pointer in a register for leaf functions. This avoids the instructions to save, set up and restore frame pointers, and makes an extra register available in leaf functions. lowkey a glow-up for your register budget ✨-mspecld-anomaly/-mno-specld-anomaly— when enabled, the compiler ensures that the generated code does not contain speculative loads after jump instructions. If this option is used,__WORKAROUND_SPECULATIVE_LOADSis defined.-mcsync-anomaly/-mno-csync-anomaly— when enabled, the compiler ensures that the generated code does not contain CSYNC or SSYNC instructions too soon after conditional branches. If this option is used,__WORKAROUND_SPECULATIVE_SYNCSis defined.-mlow64k/-mno-low64k— when enabled, the compiler is free to take advantage of the knowledge that the entire program fits into the low 64k of memory. The default behavior is to assume that the program is arbitrarily large (-mno-low64k).-mstack-check-l1— do stack checking using information placed into L1 scratchpad memory by the uClinux kernel.-mid-shared-library/-mno-id-shared-library— generate code that supports shared libraries via the library ID method. This allows for execute in place and shared libraries in an environment without virtual memory management. This option implies-fPIC. With abfin-elftarget, this option implies-msim. The default is-mno-id-shared-library, to generate code that doesn't assume ID-based shared libraries are being used.-mleaf-id-shared-library/-mno-leaf-id-shared-library— generate code that supports shared libraries via the library ID method, but assumes that this library or executable won't link against any other ID shared libraries. That allows the compiler to use faster code for jumps and calls. 🏎️
The default is -mno-leaf-id-shared-library, and what that means is the compiler makes no assumption about whether the code it's building is gonna link against any ID shared libraries. Result? Slower code gets generated for jump and call insns. That's the tradeoff, bestie, that's the vibe. 🐢
-mshared-library-id=n— tells GCC the identification number of the ID-based shared library it's compiling. Give it a value of0and you get more compact code ✨; give it any other value and it forces that number to be allocated to the current library — but ngl, that's no more space-efficient or time-efficient than just not passing the option at all. So like…0or nothing, honestly.-msep-data,-mno-sep-data— generate code that lets the data segment live in a totally different area of memory than the text segment. Why do you care? Because it enables execute in place in an environment with no virtual memory management, by eliminating relocations against the text section. 🦋 The default is-mno-sep-data, which tells GCC to generate code that assumes the data segment just follows the text segment like a loyal little shadow.-mlong-calls,-mno-long-calls— tells the compiler to do function calls the long way: first load the address of the function into a register, then perform a subroutine call on that register. You need this switch if the target function lives outside the 24-bit addressing range of the offset-based version of the subroutine call instruction. 📏
This feature is not enabled by default. Specifying -mno-long-calls restores the default behavior. And heads up — these switches have no effect on how the compiler generates code to handle function calls via function pointers. None. Zero. 💅
-mfast-fp— link with the fast floating-point library. This library relaxes some of the IEEE floating-point standard's rules for checking inputs against Not-a-Number (NAN), all in the interest of performance. Speed over strictness, iconic behavior honestly.-minline-plt— enable inlining of PLT entries in function calls to functions that aren't known to bind locally. It has no effect without-mfdpic. No-mfdpic, no party. 🎀-mmulticore— build a standalone application for multicore Blackfin processors. This option causes proper start files and link scripts supporting multicore to be used, and defines the macro"__BFIN_MULTICORE". It can only be used with-mcpu=bf561[-sirevision].
You can pair this with -mcorea or -mcoreb, which selects the one-application-per-core programming model. Without -mcorea or -mcoreb, you get the single-application/dual-core programming model instead. In that model, the main function of Core B should be named "coreb_main".
If this option isn't used, the single-core application programming model is what you get. Simple as.
-mcorea— build a standalone application for Core A of BF561 when you're using the one-application-per-core programming model. Proper start files and link scripts get used to support Core A, and the macro"__BFIN_COREA"is defined. This option can only be used in conjunction with-mmulticore. 👑-mcoreb— build a standalone application for Core B of BF561 when you're using the one-application-per-core programming model. Proper start files and link scripts get used to support Core B, and the macro"__BFIN_COREB"is defined. When you use this option,"coreb_main"should be used instead of"main". And again: this option can only be used in conjunction with-mmulticore. 🫶-msdram— build a standalone application for SDRAM. Proper start files and link scripts get used to put the application into SDRAM, and the macro"__BFIN_SDRAM"is defined. The loader should initialize SDRAM before loading the application. Don't skip that part, the loader's got a job. 🍓-micplb— assume that ICPLBs are enabled at run time. This has an effect on certain anomaly workarounds. For Linux targets, the default is to assume ICPLBs are enabled; for standalone applications the default is off.
🎀 C6X Options 🎀
-march=name— this specifies the name of the target architecture. GCC uses this name to figure out what kind of instructions it can emit when generating assembly code. The permissible names are:c62x,c64x,c64x+,c67x,c67x+,c674x.-mbig-endian— generate code for a big-endian target.-mlittle-endian— generate code for a little-endian target. This is the default. 🧚-msim— choose startup files and linker script suitable for the simulator.-msdata=default— put small global and static data in the.neardatasection, which is pointed to by registerB14. Put small uninitialized global and static data in the.bsssection, which is adjacent to the.neardatasection. Put small read-only data into the.rodatasection. The corresponding sections used for large pieces of data are.fardata,.farand.const.-msdata=all— put all data, not just small objects, into the sections reserved for small data, and use addressing relative to theB14register to access them.-msdata=none— make no use of the sections reserved for small data, and use absolute addresses to access all data. Put all initialized global and static data in the.fardatasection, and all uninitialized data in the.farsection. Put all constant data into the.constsection.-mdsbt— compile for the DSBT shared library ABI. This option is required to compile with-fpicor-fPIC, and implies-fpic.-mlong-calls— avoid generating PC-relative calls; use indirection instead.
🌈 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 arev3,v8andv10for respectively ETRAX 4, ETRAX 100, and ETRAX 100 LX. Default isv0.-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-stackframe=n— warn when the stack frame of a function exceeds n bytes. 📏-metrax4,-metrax100— these two are synonyms for-march=v3and-march=v8respectively. Cute little nicknames.-mmul-bug-workaround,-mno-mul-bug-workaround— work around a bug in themulsandmuluinstructions for CPU models where it applies. This option is disabled by default. 🐛-mpdebug— enable CRIS-specific verbose debug-related information in the assembly code. This option also has the effect of turning off the#NO_APPformatted-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. No borrowing from the past, we do fresh compares here. 💅-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 (and theirno-forms) arrange (or eliminate arrangements) for the stack frame, individual data and constants to be aligned for the maximum single data access size for the chosen CPU model. The default is to arrange for 32-bit alignment. ABI details such as structure layout are not affected by these options.-m32-bit,-m16-bit,-m8-bit— similar to the stack-, data- and const-align options above, these options arrange for stack frame, writable data and constants to all be 32-bit, 16-bit or 8-bit aligned. The default is 32-bit alignment.-mno-prologue-epilogue,-mprologue-epilogue— with-mno-prologue-epilogue, the normal function prologue and epilogue which set up the stack frame are omitted, and no return instructions or return sequences are generated in the code.
Use this option only together with visual inspection of the compiled code: no warnings or errors are generated when call-saved registers must be saved, or storage for local variables needs to be allocated. 😭 Like, it will not tell you. You have to look.
-mbest-lib-options,-moverride-best-lib-options—-mbest-lib-optionsselects the most feature-enabling options allowed by other options. This option has nono-form, but-moverride-best-lib-optionsdisables it regardless of the relative order of the two options on the command line. Sneaky little override. 🙈-mtrap-using-break8— emit traps asbreak 8. This is the default for CRIS v3 and up. If disabled, calls toabortare used instead.-mtrap-unaligned-atomic— emit checks causingbreak 8instructions to execute when applying atomic builtins on misaligned memory.-munaligned-atomic-may-use-library— handle atomic builtins that may be applied to unaligned data by calling library functions. This option overrides-mtrap-unaligned-atomic.-sim— this option 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 at0x40000000and zero-initialized data at0x80000000. 📍
✨ C-SKY Options ✨
GCC supports these options when compiling for C-SKY V2 processors. 🫶
-march=arch— specify the C-SKY target architecture. Valid values for arch are:ck801,ck802,ck803,ck807, andck810. The default isck810.-mcpu=cpu— specify the C-SKY target processor. Valid values for cpu are:ck801,ck801t,ck802,ck802t,ck802j,ck803,ck803h,ck803t,ck803ht,ck803f,ck803fh,ck803e,ck803eh,ck803et,ck803eht,ck803ef,ck803efh,ck803ft,ck803eft,ck803efht,ck803r1,ck803hr1,ck803tr1,ck803htr1,ck803fr1,ck803fhr1,ck803er1,ck803ehr1,ck803etr1,ck803ehtr1,ck803efr1,ck803efhr1,ck803ftr1,ck803eftr1,ck803efhtr1,ck803s,ck803st,ck803se,ck803sf,ck803sef,ck803seft,ck807e,ck807ef,ck807,ck807f,ck810e,ck810et,ck810ef,ck810eft,ck810,ck810v,ck810f,ck810t,ck810fv,ck810tv,ck810ft, andck810ftv. Yeah, it's a whole alphabet soup, no cap. 🍜-mbig-endian,-mlittle-endian— select big- or little-endian code. The default is little-endian.-mfloat-abi=name— specifies which floating-point ABI to use. The permissible values are:soft,softfpandhard.
Specifying soft causes GCC to generate output containing library calls for floating-point operations. softfp allows the generation of code using hardware floating-point instructions, but still uses the soft-float calling conventions. hard allows generation of floating-point instructions and uses FPU-specific calling conventions. Pick your fighter. 💖
okay so the default here is literally "it depends" — it depends on the specific target configuration, bestie. And this part matters: the hard-float and soft-float ABIs are not link-compatible. You have to compile your entire program with the same ABI, and link with a compatible set of libraries. Whole thing. No mixing. 💀
-mdouble-float,-mno-double-float— when-mhard-floatis in effect, this turns on generation of double-precision float instructions. It's the default except when you're compiling for CK803.-mfdivdu,-mno-fdivdu— when-mhard-floatis in effect, this enables generation of"frecipd","fsqrtd", and"fdivd"instructions. Default except when compiling for CK803.-mfpu=fpu — select the floating-point processor. This one can *only be used with-mhard-float, no cap. The values for *fpu arefpv2_sf(equivalent to-mno-double-float -mno-fdivdu),fpv2(-mdouble-float -mno-divdu), andfpv2_divd(-mdouble-float -mdivdu).-melrw,-mno-elrw— enable the extended"lrw"instruction. Defaults to on for CK801 and off otherwise. 🎀-mistack,-mno-istack— enable interrupt stack instructions; default is off.
The -mistack option is required to handle the "interrupt" and "isr" function attributes. So like, if you need those attributes, you need this. Periodt.
-mmp,-mno-mp— enable multiprocessor instructions; default is off.-mcp,-mno-cp— enable coprocessor instructions; default is off.-mcache,-mno-cache— enable coprocessor instructions; default is off. (yes bestie, that's literally what the page says — I did not write that, I'm just the messenger 💅)-msecurity,-mno-security— enable C-SKY security instructions; default is off.-mtrust,-mno-trust— enable C-SKY trust instructions; default is off.-mdsp,-mno-dsp,-medsp,-mno-edsp,-mvdsp,-mno-vdsp— enable C-SKY DSP, Enhanced DSP, or Vector DSP instructions, respectively. All of these default to off.-mdiv,-mno-div— generate divide instructions. Default is off.-msmart,-mno-smart— generate code for Smart Mode, using only registers numbered 0-7 so you can use 16-bit instructions. This option is ignored for CK801, where this is the required behavior, and it defaults to on for CK802. For other targets, the default is off.-mhigh-registers,-mno-high-registers— generate code using the high registers numbered 16-31. Not supported on CK801, CK802, or CK803, and it's enabled by default for other processors.-manchor,-mno-anchor— generate code using global anchor symbol addresses.-mpushpop,-mno-pushpop— generate code using"push"and"pop"instructions. Defaults to on.-mmultiple-stld,-mno-multiple-stld— generate code using"stm"and"ldm"instructions. Not supported on CK801, but enabled by default on other processors.-mconstpool,-mno-constpool— create constant pools in the compiler instead of deferring it to the assembler. This is the default and it's required for correct code generation on CK801 and CK802, and it's optional on other processors.-mstack-size,-mno-stack-size— emit.stack_sizedirectives for each function in the assembly output. Defaults to off.-mccrt,-mno-ccrt— generate code for the C-SKY compiler runtime instead of libgcc. Defaults to off.- *
-mbranch-cost=n* — set the branch costs to roughly n instructions. The default is 1. -msched-prolog,-mno-sched-prolog— permit scheduling of function prologue and epilogue sequences.
Careful bestie. Using this option can result in code that is not compliant with the C-SKY V2 ABI prologue requirements, and that cannot be debugged or backtraced. It is disabled by default. So like, it's giving chaos for a reason. 🚨
-msim,-mno-sim— links the librarylibsemi.a, which is in compatible with simulator. Applicable to ELF compiler only. (that wording is straight from the page, I'm leaving it exactly as-is 🫶)
🪟 Cygwin and MinGW Options 🪟
These extra options are available for Microsoft Windows targets:
-mconsole— this one says a console application is to be generated, by instructing the linker to set the PE header subsystem type that console applications require. Available for Cygwin and MinGW targets, and enabled by default on those targets. ✨- *
-mcrtdll=library* — preprocess, compile or link with the specified C RunTime DLL library. This option adjusts the predefined macros"__CRTDLL__","__MSVCRT__","_UCRT"and"__MSVCRT_VERSION__"for the specified CRT library, chooses the start file for the CRT library, and links with the CRT library.
Recognized CRT library names for the preprocessor are: "crtdll*", "msvcrt10*", "msvcrt20*", "msvcrt40*", "msvcr40*", "msvcrtd*", "msvcrt-os*", "msvcr70*", "msvcr71*", "msvcr80*", "msvcr90*", "msvcr100*", "msvcr110*", "msvcr120*" and "ucrt*".
If this option is not specified, then the default MinGW import library "msvcrt" is used for linking, and no other adjustment for the preprocessor is done. MinGW import library "msvcrt" is just a symlink to (or a copy of) another MinGW CRT import library chosen during MinGW compilation. MinGW import library "msvcrt-os" is for the Windows system CRT DLL library "msvcrt.dll" and in most cases is the default MinGW import library.
ngl, generally speaking, changing the CRT DLL requires recompiling the entire MinGW CRT. This option is for experimental and testing purposes only. It's available for MinGW targets. 🙈
-mdll— available for Cygwin and MinGW targets. It specifies that a DLL — a dynamic link library — is to be generated, which enables the selection of the required runtime startup object and entry point. 🦋-mnop-fun-dllimport— available for Cygwin and MinGW targets. It specifies that the"dllimport"attribute should be ignored.-mthreads— support thread-safe exception handling on MinGW. Programs that rely on thread-safe exception handling must compile and link all code with the-mthreadsoption. When compiling,-mthreadsdefines-D_MT; when linking, it links in a special thread helper library-lmingwthrd, which cleans up per-thread exception-handling data. 🔥-municode— available for MinGW-w64 targets. It causes the"UNICODE"preprocessor macro to be predefined, and it chooses Unicode-capable runtime startup code.-mwin32— available for Cygwin and MinGW targets. It specifies that the typical Microsoft Windows predefined macros are to be set in the pre-processor, but it does not influence the choice of runtime library/startup code.-mwindows— available for Cygwin and MinGW targets. It specifies that a GUI application is to be generated, by instructing the linker to set the PE header subsystem type appropriately. 👑-fno-set-stack-executable— available for MinGW targets. It specifies that the executable flag for the stack used by nested functions isn't set. This is necessary for binaries running in kernel mode of Microsoft Windows, because there the User32 API — which is used to set executable privileges — isn't available.-fwritable-relocated-rdata— available for MinGW and Cygwin targets. It specifies that relocated-data in a read-only section is put into the".data"section. This is necessary for older runtimes that don't support modification of".rdata"sections for pseudo-relocation.-mpe-aligned-commons— available for Cygwin and MinGW targets. It specifies that the GNU extension to the PE file format that permits the correct alignment of COMMON variables should be used when generating code. It's enabled by default if GCC detects that the target assembler found during configuration supports the feature.-muse-libstdc-wrappers— use Cygwin DLL wrappers to support C++ operators"new"and"delete".
See also under x86 Options for standard options. 💜
🍎 Darwin Options 🍎
These options are defined for all architectures running the Darwin operating system.
Okay so the tea on fat files: FSF GCC on Darwin does not create "fat" object files; it creates an object file for the single architecture that GCC was built to target. Apple's GCC on Darwin does create "fat" files if multiple -arch options are used — it does that by running the compiler or linker multiple times and joining the results together with lipo.
The subtype of the file created (like ppc7400 or ppc970 or i686) is determined by the flags that specify the ISA that GCC is targeting, like -mcpu or -march. The -force_cpusubtype_ALL option can be used to override this. 🌈
The Darwin tools vary in their behavior when presented with an ISA mismatch. The assembler, as, only permits instructions to be used that are valid for the subtype of the file it is generating, so you cannot put 64-bit instructions in a ppc750 object file. The linker for shared libraries, /usr/bin/libtool, fails and prints an error if asked to create a shared library with a less restrictive subtype than its input files (for instance, trying to put a ppc970 object file in a ppc7400 library). The linker for executables, ld, quietly gives the executable the most restrictive subtype of any of its input files. (quietly! the audacity ✨)
-Fdir — add the framework directory dir to the head of the list of directories to be searched for header files. These directories are interleaved with those specified by-Ioptions and are scanned in a left-to-right order. 🎀
okay so a framework directory is literally just a directory that has frameworks in it 💖 and a framework? it's a directory with a Headers and/or PrivateHeaders directory sitting directly inside it, and the directory name ends in .framework. The name of the framework is that directory name minus the .framework part, periodt.
The headers that belong to the framework live in one of those two directories — and Headers gets searched first ✨
A subframework is a framework directory chillin' inside a framework's Frameworks directory. Includes of subframework headers can only show up in a header of a framework that contains the subframework, or in a sibling subframework header. Two subframeworks are siblings if they're in the same framework, no cap. A subframework should not have the same name as a framework — if you violate that, a warning gets issued 😭. And rn, a subframework cannot have subframeworks; in the future the mechanism may be extended to support that. The standard frameworks live in /System/Library/Frameworks and /Library/Frameworks. An example include looks like "#include <Framework/header.h>", where Framework is the framework name and header.h is found in the PrivateHeaders or Headers directory. iconic.
-iframeworkdir— like-Fexcept the directory is treated as a system directory 🫶. The main difference between-iframeworkand-Fis that with-iframeworkthe compiler does not warn about constructs contained within header files found via dir. This option is valid only for the C family of languages.-arch name— generate output for architecture name. As described above, GCC generates output for the architecture it was configured for, using its usual options to select subarchitecture variants. The-archoption is accepted for compatibility, but an error is diagnosed if name is inconsistent with GCC's own idea of the target architecture 💀.-dependency-file filename— alias for the preprocessor option-MF filename.-fapple-kext— generate code for Darwin loadable kernel extensions.-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. everything. the whole vibe.-matt-stubs-mno-att-stubs— enable AT&T-style PIC stubs. This is the default when supported by the target architecture (currently x86 only).-mconstant-cfstrings-fconstant-cfstrings— when the NeXT runtime is being used (the default on these systems), override any-fconstant-string-classsetting and cause"@"...""literals to be laid out as constant CoreFoundation strings. btw-fconstant-cfstringsis an alias for-mconstant-cfstrings💅-mdynamic-no-pic— generate code suitable for executables (not shared libraries). This option is incompatible with-fpic,-fPIC,-fpie, or-fPIE⚠️.-mfix-and-continue-ffix-and-continue-findirect-data— generate code suitable for fast turnaround development, like letting GDB dynamically load.ofiles into already-running programs.-findirect-dataand-ffix-and-continueare provided for backwards compatibility.-mkernel— enable kernel development mode 👑. The-mkerneloption sets-static,-fno-common,-fno-use-cxa-atexit,-fno-exceptions,-fno-non-call-exceptions,-fapple-kext,-fno-weakand-fno-rttiwhere applicable. This mode also sets-mno-altivec,-msoft-float,-fno-builtinand-mlong-branchfor PowerPC targets.-mmacosx-version-min=version-asm_macosx_version_min=version— the-mmacosx-version-minoption specifies that the earliest version of MacOS X that this executable will run on is version. Typical values supported for version include12,10.12, and10.5.8. If the compiler was built to use the system's headers by default, then the default for this option is the system version on which the compiler is running; otherwise the default is to make choices that are compatible with as many systems and code bases as possible.-asm_macosx_version_min=versionis similar, but the GCC driver passes its version information only to the assembler.-mone-byte-bool— override the defaults for"bool"so that"sizeof(bool)==1"🍓. By defaultsizeof(bool)is4when compiling for Darwin/PowerPC and1when 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.
-msymbol-stubs-mno-symbol-stubs— force generation of external symbol indirection stubs for PIC references. By default, this option is enabled automatically if the target linker version (-mtarget-linker) is old enough to require them.-mtarget-linker=version-mtarget-linker version— specify the target ld64 version, overriding any version specified in the GCC configuration. Newer linker versions support improved code generation in some cases, for example for PIC code ✨-ObjC— equivalent to-x objective-c; specifies that the input is Objective-C source code.-ObjC++— equivalent to-x objective-c++; specifies that the input is Objective-C++ source code.-Wnonportable-cfstrings-Wno-nonportable-cfstrings— warn if constant CoreFoundation string objects contain non-portable characters. This warning is enabled by default.-all_load— loads all members of static archive libraries. Seeman ld(1)for more information.-arch_errors_fatal— cause the errors having to do with files that have the wrong architecture to be fatal 💀.-bind_at_load— causes the output file to be marked such that the dynamic linker will bind all undefined references when the file is loaded or launched.-bundle— produce a Mach-o bundle format file. Seeman ld(1)for more information.-bundle_loader executable— this option specifies the executable that will load the build output file being linked. Seeman ld(1)for more information.-dynamiclib— when passed this option, GCC produces a dynamic library instead of an executable when linking, using the Darwinlibtoolcommand.-force_cpusubtype_ALL— this causes GCC's output file to have theALLsubtype, instead of one controlled by the-mcpuor-marchoption.-nodefaultrpaths— do not add default run paths for the compiler library directories to executables, modules or dynamic libraries. On macOS 10.5 and later, the embedded runpath is added by default unless the user adds-nodefaultrpathsto the link line. Run paths are needed (and therefore enforced) to build on macOS version 10.11 or later.-nodefaultexport— do not add default symbol exports to modules or dynamic libraries.-allowable_client client_name-client_name-compatibility_version-current_version-dead_strip-dylib_file-dylinker-dylinker_install_name-dynamic-exported_symbols_list-filelist-flat_namespace-force_flat_namespace-framework-headerpad_max_install_names-image_base-init symbol-name-install_name-keep_private_externs-pagezero_size-preload-read_only_relocs-sectalign-sectcreate-seg_addr_table-seg1addr-segaddr-segprot-segs_read_only_addr-segs_read_write_addr-sub_library-sub_umbrella-twolevel_namespace-twolevel_namespace_hints-umbrella-undefined-unexported_symbols_list-weak_framework-weak_reference_mismatches-whatsloaded-whyload— 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-floatis specified, functions inlibgcc.aare 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 emulation routines, these routines issue floating-point operations. If you are compiling for an Alpha without floating-point operations, you must ensure that the library is built so as not to call them. Note that Alpha implementations without floating-point operations are required to have floating-point registers 👑-mfp-regs-mno-fp-regs— generate code that uses (does not use) the floating-point register set.-mno-fp-regsimplies-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$0instead of$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-regsmust 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 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— okay so this is like-mieeebut the generated code also keeps the IEEE inexact-flag up to date. Turning it on makes the generated code implement fully-compliant IEEE math ✨. On top of_IEEE_FP, it also defines_IEEE_FP_EXACTas a preprocessor macro. Heads up bestie: on some Alpha implementations the resulting code can run significantly slower than what you'd get by default 😭. And since very little code actually depends on the inexact-flag, you should normally not specify this option. Other Alpha compilers call it-ieee_with_inexact.- *
-mfp-trap-mode=trap-mode* — this one controls which floating-point-related traps get enabled. Other Alpha compilers call it-fptmtrap-mode. The trap mode can be set to one of four values:n— the default (normal) setting 💅. The only traps enabled are the ones that can't be disabled in software (like, e.g., the division by zero trap).u— on top of everythingnenables, underflow traps are enabled too.su— likeu, but the instructions are marked as safe for software completion (peep the Alpha architecture manual for the deets).sui— likesu, but inexact traps are enabled as well.
- *
-mfp-rounding-mode=rounding-mode* — picks the IEEE rounding mode 🎀. Other Alpha compilers call it-fprmrounding-mode. The rounding-mode can be one of:n— normal IEEE rounding mode. Floating-point numbers round towards the nearest machine number, or towards the even machine number if there's a tie.m— round towards minus infinity.c— chopped rounding mode. Floating-point numbers round towards zero.d— dynamic rounding mode. A field in the floating-point control register (fpcr, see the Alpha architecture reference manual) controls which rounding mode is actually in effect. The C library initializes this register for rounding towards plus infinity. So unless your program modifies thefpcr,dcorresponds to round towards plus infinity ✨.
- *
-mtrap-precision=trap-precision* — so in the Alpha architecture, floating-point traps are imprecise 💀. Which means without software assistance it is literally impossible to recover from a floating trap, and program execution normally needs to be terminated. GCC can generate code that helps operating system trap handlers figure out the exact location that caused a floating-point trap. Depending on what your app needs, you can pick different levels of precision:p— program precision. This option is the default, and it means a trap handler can only identify which program caused a floating-point exception.f— function precision. The trap handler can determine the function that caused a floating-point exception.i— instruction precision. The trap handler can determine the exact instruction that caused a floating-point exception.
Other Alpha compilers provide the equivalent options called -scope_safe and -resumption_safe.
-mieee-conformant— this marks the generated code as IEEE conformant 👑. But bestie, you must not use this option unless you also specify-mtrap-precision=iand either-mfp-trap-mode=suor-mfp-trap-mode=sui. Its only effect is to emit the line.eflag 48in the function prologue of the generated assembly file.-mbuild-constants— normally GCC looks at a 32- or 64-bit integer constant to see if it can build it from smaller constants in two or three instructions. If it can't, it outputs the constant as a literal and generates code to load it from the data segment at run time. Use this option to require GCC to construct all integer constants using code, even if it takes more instructions (the max is six). You typically use this option to build a shared library dynamic loader 🛠️. Itself a shared library, it has to relocate itself in memory before it can find the variables and constants in its own data segment.-mbwx-mno-bwx-mcix-mno-cix-mfix-mno-fix-mmax-mno-max— these tell GCC whether it should generate code that uses the optional BWX, CIX, FIX and MAX instruction sets. The default is to use the instruction sets supported by the CPU type you specified via-mcpu=, or the one of the CPU GCC was built on if you didn't specify any.-msafe-bwa-mno-safe-bwa— these say whether, in the absence of the optional BWX instruction set, GCC should generate multi-thread and async-signal safe code for byte and aligned word memory accesses.-msafe-partial-mno-safe-partial— these say whether GCC should generate multi-thread and async-signal safe code for partial memory accesses. That includes piecemeal accesses to unaligned data, plus block accesses to leading and trailing parts of aggregate types or other objects in memory that don't respectively start and end on an aligned 64-bit data boundary.-mfloat-vax-mfloat-ieee— generate code that uses (does not use) VAX F and G floating-point arithmetic instead of IEEE single and double precision.-mexplicit-relocs-mno-explicit-relocs— older Alpha assemblers gave you no way to generate symbol relocations except through assembler macros 🙈. And using those macros doesn't allow optimal instruction scheduling. GNU Binutils as of version 2.12 supports a new syntax that lets the compiler explicitly mark which relocations should apply to which instructions. This option is mostly useful for debugging, since GCC detects the capabilities of the assembler when it's built and sets the default accordingly.-msmall-data-mlarge-data— when-mexplicit-relocsis in effect, static data is accessed via gp-relative relocations. When-msmall-datais used, objects 8 bytes long or smaller get placed in a small data area (the.sdataand.sbsssections) and are accessed via 16-bit relocations off of the$gpregister. This limits the size of the small data area to 64KB, but it lets the variables be directly accessed via a single instruction ⚡.
The default is -mlarge-data. With this option the data area is limited to just below 2GB. Programs that need more than 2GB of data must use malloc or mmap to allocate the data in the heap instead of in the program's data segment.
When generating code for shared libraries, -fpic implies -msmall-data and -fPIC implies -mlarge-data.
-msmall-text-mlarge-text— when-msmall-textis used, the compiler assumes that the code of the entire program (or shared library) fits in 4MB, and is thus reachable with a branch instruction. When-msmall-datais used, the compiler can assume that all local symbols share the same$gpvalue, and thus reduce the number of instructions required for a function call from 4 to 1 👑.
The default is -mlarge-text.
- *
-mcpu=cpu_type* — sets the instruction set and instruction scheduling parameters for machine type cpu_type. You can specify either the EV style name or the corresponding chip number. GCC supports scheduling parameters for the EV4, EV5 and EV6 family of processors and chooses the default values for the instruction set from the processor you specify. If you don't specify a processor type, GCC defaults to the processor on which the compiler was built.
Supported values for cpu_type are
ev4ev4521064— schedules as an EV4 and has no instruction set extensions.ev521164— schedules as an EV5 and has no instruction set extensions.ev5621164a— schedules as an EV5 and supports the BWX extension.pca5621164pc21164PC— schedules as an EV5 and supports the BWX and MAX extensions.ev621264— schedules as an EV6 and supports the BWX, FIX, and MAX extensions.ev6721264a— schedules as an EV6 and supports the BWX, CIX, FIX, and MAX extensions.
Native toolchains also support the value native, which selects the best architecture option for the host processor. -mcpu=native has no effect if GCC doesn't recognize the processor.
- *
-mtune=cpu_type* — sets only the instruction scheduling parameters for machine type cpu_type. The instruction set is not changed.
Native toolchains also support the value native, which selects the best architecture option for the host processor. -mtune=native has no effect if GCC doesn't recognize the processor.
- *
-mmemory-latency=time* — sets the latency the scheduler should assume for typical memory references as seen by the application. This number is super dependent on the memory access patterns your application uses and the size of the external cache on the machine.
Valid options for time are
- number — a decimal number representing clock cycles.
L1L2L3main— 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 thatL3is only valid for EV5.
-mtls-kernel— emitrdvalinstead ofrduniqfor thread pointer.- *
-mtls-size=bitsize* — specify the bit size of immediate TLS offsets. Valid values for bitsize are16,32, and64; it defaults to32. -mlong-double-128-mlong-double-64— specify the size of thelong doubletype. Note that-mlong-double-128is incompatible with VAX floating point ⚠️.
✨ eBPF Options ✨
- *
-mframe-limit=bytes* — this specifies the hard limit for frame sizes, in bytes. Currently, the value you can specify should be less than or equal to32767. Defaults to whatever limit is imposed by the version of the Linux kernel targeted. -mbig-endian— generate code for a big-endian target.-mlittle-endian— generate code for a little-endian target. This is the default 💅.-mjmpext-mno-jmpext— enable or disable generation of extra conditional-branch instructions. Enabled for CPU v2 and above.-mjmp32-mno-jmp32— enable or disable generation of 32-bit jump instructions. Enabled for CPU v3 and above.-malu32,-mno-alu32— turn on (or off) generating 32-bit ALU instructions 💅. It's enabled for CPU v3 and above.-mv3-atomics,-mno-v3-atomics— turn on (or off) the instructions for general atomic operations that got introduced back in CPU v3 ✨. Enabled for CPU v3 and above.-mbswap,-mno-bswap— turn on (or off) byte swap instructions 🦋. Enabled for CPU v4 and above.-msdiv,-mno-sdiv— turn on (or off) signed division and modulus instructions 🌸. Enabled for CPU v4 and above.-msmov,-mno-smov— turn on (or off) sign-extending move and memory load instructions 🎀. Enabled for CPU v4 and above.- *
-mcpu=version* — this is the one that says which version of the eBPF ISA you're targeting 👑. Lowkey important: newer versions may not be supported by all kernels. The default is v4.
Here's the whole roster of supported version values:
v1— the first stable eBPF ISA, no special features, no extensions, just vibes. 💜v2— supports the jump extensions, same as in-mjmpext.v3— everything fromv2, plus:- 32-bit jump operations, as in
-mjmp32 - 32-bit ALU operations, as in
-malu32 - general atomic operations, as in
-mv3-atomics
- 32-bit jump operations, as in
v4— everything fromv3, plus:- Byte swap instructions, as in
-mbswap - Signed division and modulus instructions, as in
-msdiv - Sign-extending move and memory load instructions, as in
-msmov
- Byte swap instructions, as in
-mco-re,-mno-co-re— turn on (or off) BPF Compile Once - Run Everywhere (CO-RE) support 🧚. It's enabled by default when you're generating BTF debug information for the BPF target (-gbtf).-mxbpf— generate code for an expanded version of BPF, one that relaxes some of the restrictions the BPF architecture is out here imposing 😤:- Save and restore callee-saved registers at function entry and exit, respectively.
- *
-masm=dialect* — outputs assembly instructions using the selected eBPF dialect ✨. The default ispseudoc.
Supported values for dialect are:
normal— outputs normal assembly dialect.pseudoc— outputs pseudo-c assembly dialect.
-minline-memops-threshold=bytes — this sets a size threshold, in bytes, at or below whichmemmove,memcpyandmemsetshall **always be expanded inline 🫶. Anything bigger than this threshold would have to be done with a library call instead of being expanded inline, but since BPF doesn't allow libcalls, going over this threshold means you get a compile-time error 💀. The default is1024bytes.-Wco-re— turns on warnings for the scenarios where-mco-reis enabled, but it may not be possible to generate CO-RE compatible code 🙈.
🎀 FR30 Options 🎀
These options are defined specifically for the FR30 port, bestie.
-msmall-model— use the small address space model 🌸. This can produce smaller code, but it does assume that all symbolic values and addresses fit into a 20-bit range. No cap.-mno-lsim— assume that runtime support has already been provided, so there's no need to include the simulator library (libsim.a) on the linker command line ✨.
🦋 FRV Options 🦋
-mgpr-32— only use the first 32 general-purpose registers.-mgpr-64— use all 64 general-purpose registers 👑.-mfpr-32— use only the first 32 floating-point registers.-mfpr-64— use all 64 floating-point registers 💅.-mhard-float— use hardware instructions for floating-point operations 🔥.-msoft-float— use library routines for floating-point operations.-malloc-cc— dynamically allocate condition code registers.-mfixed-cc— do not try to dynamically allocate condition code registers, only useicc0andfcc0.-mdword,-mno-dword— control whether the ABI uses double-word instructions 🌈.-mdouble,-mno-double— turn on (or off) use of floating-point double instructions.-mmedia,-mno-media— turn on (or off) use of media instructions 🎀.-mmuladd,-mno-muladd— turn on (or off) use of multiply and add/subtract instructions.-mfdpic— select the FDPIC ABI, which uses function descriptors to represent pointers to functions ✨. With no PIC/PIE-related options at all, it implies-fPIE. With-fpicor-fpie, it assumes GOT entries and small data are within a 12-bit range from the GOT base address; with-fPICor-fPIE, GOT offsets are computed with 32 bits. With abfin-elftarget, this option implies-msim.-minline-plt— enable inlining of PLT entries in function calls to functions that are not known to bind locally 👑. It has no effect without-mfdpic. It's enabled by default if you're optimizing for speed and compiling for shared libraries (i.e.,-fPICor-fpic), or when an optimization option such as-O3or 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 ofGPRELrelocations in the FDPIC ABI for data that's known to be in read-only sections 🌸. It's enabled by default, except for-fpicor-fpie: even though it may help make the global offset table smaller, it trades 1 instruction for 4. With-fPICor-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 isn't,-mno-gprel-rocan 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-fPICand-fpicwithout-mfdpic. You should never have to use it explicitly, fr fr.-mlinked-fp— follow the EABI requirement of always creating a frame pointer whenever a stack frame is allocated 🫶. This option is enabled by default and can be disabled with-mno-linked-fp.-mlong-calls— use indirect addressing to call functions outside the current compilation unit. This allows the functions to be placed anywhere within the 32-bit address space ✨.-malign-labels— try to align labels to an 8-byte boundary by inserting NOPs into the previous packet 🌈. This option only has an effect when VLIW packing is enabled. It doesn't create new packets; it merely adds NOPs to existing ones.-mlibrary-pic— generate position-independent EABI code.-macc-4— use only the first four media accumulator registers.-macc-8— use all eight media accumulator registers 👑.-mpack,-mno-pack— turn on (or off) packing VLIW instructions.-mno-eflags— do not mark ABI switches ine_flags.-mcond-move,-mno-cond-move— turn on (or off) the use of conditional-move instructions; it is enabled by default 💅.
This switch is mainly for debugging the compiler and will likely be removed in a future version. 😭
-mscc,-mno-scc— turn on (or off) the use of conditional set instructions; it is enabled by default.
This switch is mainly for debugging the compiler and will likely be removed in a future version. 😭
-mcond-exec,-mno-cond-exec— turn on (or off) the use of conditional execution; it is enabled by default.
This switch is mainly for debugging the compiler and will likely be removed in a future version. 😭
-mvliw-branch,-mno-vliw-branch— turn on (or off) an optimization pass to pack branches into VLIW instructions; it is enabled by default.
This switch is mainly for debugging the compiler and will likely be removed in a future version. 😭
-mmulti-cond-exec,-mno-multi-cond-exec— turn on (or off) optimization of&&and||in conditional execution; it is enabled by default.
This switch is mainly for debugging the compiler and will likely be removed in a future version. 😭
-mnested-cond-exec,-mno-nested-cond-exec— turn on (or off) nested conditional execution optimizations; it is enabled by default.
This switch is mainly for debugging the compiler and will likely be removed in a future version. 😭
-moptimize-membar,-mno-optimize-membar— this switch removes redundantmembarinstructions from the compiler-generated code 🔥. It is enabled by default.-mtomcat-stats— causegasto print out tomcat statistics 🐱.- *
-mcpu=cpu* — select the processor type you want to generate code for 👑. Possible values arefrv,fr550,tomcat,fr500,fr450,fr405,fr400,fr300andsimple.
🧚 FT32 Options 🧚
These options are defined specifically for the FT32 port.
-msim— says the program is gonna be run on the simulator 🦋. This causes an alternate runtime startup and library to be linked. 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 💀.-mnodiv— do not usedivandmodinstructions.-mft32b— enable use of the extended instructions of the FT32B processor ✨.-mcompress— compress all code using the Ft32B code compression scheme.-mnopm— do not generate code that reads program memory.
🌈 GNU/Linux Options 🌈
These -m options are defined for GNU/Linux targets, bestie:
-mglibc— use the GNU C library 🫶. This is the default except on*-*-linux-*uclibc*,*-*-linux-*musl*and*-*-linux-*android*targets.-muclibc— use uClibc C library. This is the default on*-*-linux-*uclibc*targets.-mmusl— use the musl C library. This is the default on*-*-linux-*musl*targets.-mbionic— use Bionic C library. This is the default on*-*-linux-*android*targets.-mandroid,-mno-android— compile code compatible with the Android platform 🤖. This is the default on*-*-linux-*android*targets.
When compiling, this option enables -mbionic, -fPIC, -fno-exceptions and -fno-rtti by default ✨. When linking, this option makes the GCC driver pass Android-specific options to the linker. Finally, this option causes the preprocessor macro __ANDROID__ to be defined 👑
This option can be disabled completely with -mno-android.
-tno-android-cc— disable compilation effects of-mandroid, i.e., do not enable-mbionic,-fPIC,-fno-exceptionsand-fno-rttiby default.-tno-android-ld— disable linking effects of-mandroid, i.e., pass standard Linux linking options to the linker.
🌸 H8/300 Options 🌸
Ok bestie, these -m options are the whole personality of the H8/300 implementations. Let's go 💖
-mrelax— shortens some address references at link time when it can; it does this by using the linker option-relax. Sneaky little glow-up at the end there ✨-mh— generates code for the H8/300H.-ms— generates code for the H8S.-mn— generates code for the H8S and the H8/300H in the normal mode. Heads up though: this switch must be used with either-mhor-ms. It does not fly solo, no ma'am.-msx— generates H8SX code.-ms2600— generates code for the H8S/2600. And yes, this switch must be used with-ms.-mquickcall— uses registers for argument passing. fast girl behavior 🏃♀️💨-mslowbyte— treats access to byte-sized memory as slow.-mexr,-mno-exr— stores extended registers on the stack before execution of functions with the"monitor"attribute. Default is-mexr. This option is valid only for H8S targets, so don't try it elsewhere, it's not for everyone 💅-mint32— makes"int"data 32 bits by default.-malign-300— on the H8/300H and H8S, uses 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, and-malign-300makes them align on 2-byte boundaries instead. This option has no effect on the H8/300. It simply does not apply there 🙈
🎀 HPPA Options 🎀
And now for the HPPA family of computers — these -m options are the main character energy of this whole section, no cap 💜
-march=architecture-type— generates code for the specified architecture. Your choices for architecture-type are1.0for PA 1.0,1.1for PA 1.1, and2.0for PA 2.0 processors. On an HP-UX system, peek at/usr/lib/sched.modelsto figure out the right architecture option for your machine. Also super important: code compiled for lower-numbered architectures runs on higher-numbered architectures, but not the other way around. It's giving "you can go up but you can't go down" 😭-mpa-risc-1-0,-mpa-risc-1-1,-mpa-risc-2-0— these are just synonyms for-march=1.0,-march=1.1, and-march=2.0, respectively. Same vibe, different outfit 💅-matomic-libcalls,-mno-atomic-libcalls— generates libcalls for atomic loads and stores when sync libcalls are disabled. This option is enabled by default. It only affects the generation of atomic libcalls by the HPPA backend.
Both the sync and libatomic libcall implementations use locking. As a result, processor stores are not atomic with respect to other atomic operations. Processor loads up to DImode are atomic with respect to other atomic operations, provided they are implemented as a single access.
The PA-RISC architecture does not support any atomic operations in hardware except for the "ldcw" instruction. So literally all atomic support is implemented using sync and atomic libcalls. Sync libcall support lives in libgcc.a. Atomic libcall support lives in libatomic.
This option generates "__atomic_exchange" calls for atomic stores. It also provides special handling for atomic DImode accesses on 32-bit targets.
-mcaller-copies— the caller copies function arguments passed by hidden reference. Use this one with care, bestie, because it is not compatible with the default 32-bit runtime 😬 However, only aggregates larger than eight bytes are passed by hidden reference, and the option gives better compatibility with OpenMP.-mcoherent-ldcw— uses the ldcw/ldcd coherent cache-control hint.-mdisable-fpregs— disables floating-point registers. Equivalent to-msoft-float.-mdisable-indexing— stops the compiler from using indexing address modes. This avoids some rather obscure problems when compiling MIG generated code under MACH. niche but iconic ✨-mfast-indirect-calls— generates code that assumes calls never cross space boundaries. This lets GCC emit code that performs faster indirect calls.
Heads up: this option does not work in the presence of shared libraries or nested functions. 💀
-mfixed-range=register-range— generates code treating the given register range as fixed registers. A fixed register is one the register allocator cannot use. Super useful when compiling kernel code. A register range is specified as two registers separated by a dash. You can specify multiple register ranges separated by a comma.-mgas— enables the use of assembler directives only GAS understands.-mgnu-ld— uses options specific to GNU ld. This passes-sharedto ld when building a shared library. It's the default when GCC is configured, explicitly or implicitly, with the GNU linker. This option does not affect which ld is called; it only changes what parameters get passed to that ld. Which ld actually gets called is determined by the--with-ldconfigure option, GCC's program search path, and finally by the user'sPATH. You can print the linker GCC uses withwhich gcc -print-prog-name=ld. This option is only available on the 64-bit HP-UX GCC, i.e. configured withhppa*64*-*-hpux*.-mhp-ld— uses options specific to HP ld. This passes-bto ld when building a shared library, and passes+Accept TypeMismatchto ld on all links. It's the default when GCC is configured, explicitly or implicitly, with the HP linker. Same deal as above: this option does not affect which ld is called; it only changes what parameters get passed to that ld. Which ld is called is determined by the--with-ldconfigure option, GCC's program search path, and finally by the user'sPATH. The linker used by GCC can be printed usingwhich gcc -print-prog-name=ld. This option is only available on the 64-bit HP-UX GCC, i.e. configured withhppa*64*-*-hpux*.-mlinker-opt— enables the optimization pass in the HP-UX linker. Note that this makes symbolic debugging impossible 😭 so pick your struggle.-mlong-calls— generates code that uses long call sequences. This makes sure 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 are7,600,000and240,000bytes, respectively for the PA 2.0 and PA 1.X architectures. Sibling calls are always limited at240,000bytes.
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's normally not desirable to use this option since it degrades performance. However, it may be useful in large applications, particularly when partial linking is used to build the application.
The types of long calls used depend 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. it's giving "long" and it means it 💀
-mlong-load-store— generates 3-instruction load and store sequences as sometimes required by the HP-UX 10 linker. This is equivalent to the+koption to the HP compilers.-mno-space-regs— generates code that assumes the target has no space registers. This lets GCC generate faster indirect calls and use unscaled index address modes.
Such code is suitable for level 0 PA systems and kernels. 🫶
-mordered— assumes memory references are ordered and barriers are not needed.-mportable-runtime— uses the portable calling conventions proposed by HP for ELF systems.-mschedule=cpu-type— schedules code according to the constraints for the machine type cpu-type. The choices for cpu-type are7007100,7100LC,7200,7300and8000. On an HP-UX system, check/usr/lib/sched.modelsto figure out the proper scheduling option for your machine. The default scheduling is8000.-msio,-mwsio—-msiogenerates the predefine"_SIO"for server IO. The default is-mwsio, which generates the predefines"__hp9000s700","__hp9000s700__"and"_WSIO"for workstation IO. These options are available under HP-UX and HI-UX.-msoft-float— generates output containing library calls for floating point.
Warning bestie: the requisite libraries are not available for all HPPA targets. Normally the facilities of the machine's usual C compiler are used, but this cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation. 😬
-msoft-float changes the calling convention in the output file; so it's 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 for this to work.
-msoft-mult— uses software integer multiplication.
This disables the use of the "xmpyu" instruction.
-munix=unix-std— generates compiler predefines and selects a startfile for the specified UNIX standard. The choices for unix-std are93,95and98.93is supported on all HP-UX versions.95is available on HP-UX 10.10 and later.98is available on HP-UX 11.11 and later. The default values are93for HP-UX 10.00,95for HP-UX 10.10 though to 11.00, and98for 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", plus the startfile unix95.o. -munix=98 provides additional predefines for "_XOPEN_UNIX", "_XOPEN_SOURCE_EXTENDED", "_INCLUDE__STDC_A1_SOURCE" and "_INCLUDE_XOPEN_SOURCE_500", plus the startfile unix98.o.
It's important to note that this option changes the interfaces for various library routines. It also affects the operational behavior of the C library. So extreme care is needed when using this option. extreme care. not medium care. 🔥
Library code that's 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— suppresses the generation of link options to searchlibdld.slwhen the-staticoption is specified on HP-UX 10 and later.-static— the HP-UX C library implementation of"setlocale"has a dependency onlibdld.sl. There isn't an archive version oflibdld.sl. So when the-staticoption is specified, special link options are needed to resolve this dependency. 🎀
On HP-UX 10 and later, the GCC driver is out here adding the necessary options to link with libdld.sl whenever the -static option is specified. 💅 And the plot twist? That makes the resulting binary dynamic. On the 64-bit port, the linkers generate dynamic binaries by default in any case, so it's giving that either way. If you want to stop the GCC driver from adding these link options, the -nolibdld option is right there for you. 🫶
-threads— adds support for multithreading with the dce thread library under HP-UX. She sets flags for both the preprocessor and the linker, doing double duty, iconic behavior honestly. ✨
🔥 IA-64 Options 🔥
These are the -m options defined for the Intel IA-64 architecture. Buckle up bestie, there's a lot of them. 💖
-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. Heads up: the result is not position independent code, and it violates the IA-64 ABI. 😭 So like, she's a rule breaker.-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, andoutregister names for the stacked registers. This may make assembler output more readable, which is a whole vibe. ✨-mno-sdata,-msdata— disable (or enable) optimizations that use the small data section. This may be useful for working around optimizer bugs. 🙈-mconstant-gp— generate code that uses a single constant global pointer value. This is useful when compiling kernel code.-mauto-pic— generate code that is self-relocatable. This implies-mconstant-gp. This is useful when compiling firmware code.-minline-float-divide-min-latency— generate code for inline divides of floating-point values using the minimum latency algorithm.-minline-float-divide-max-throughput— generate code for inline divides of floating-point values using the maximum throughput algorithm.-mno-inline-float-divide— do not generate inline code for divides of floating-point values. 🙅-minline-int-divide-min-latency— generate code for inline divides of integer values using the minimum latency algorithm.-minline-int-divide-max-throughput— generate code for inline divides of integer values using the maximum throughput algorithm.-mno-inline-int-divide— do not generate inline code for divides of integer values.-minline-sqrt-min-latency— generate code for inline square roots using the minimum latency algorithm.-minline-sqrt-max-throughput— generate code for inline square roots using the maximum throughput algorithm.-mno-inline-sqrt— do not generate inline code forsqrt.-mno-dwarf2-asm,-mdwarf2-asm— don't (or do) generate assembler code for the DWARF 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. 💀 can, bestie, not will.-mfixed-range=register-range — generate code treating the given register range as fixed registers. A fixed register is one that the register allocator cannot use. This is useful when compiling kernel code. A register range is specified as two registers separated by a dash. Multiple register ranges can be specified separated by a comma. 🎀-mtls-size=tls-size — specify bit size of immediate TLS offsets. Valid values are14,22, and64.-mtune=cpu-type — tune the instruction scheduling for a particular CPU. Valid values areitanium,itanium1,merced,itanium2, andmckinley. 👑-milp32,-mlp64— generate code for a 32-bit or 64-bit environment. The 32-bit environment setsint,longand pointer to 32 bits. The 64-bit environment setsintto 32 bits andlongand pointer to 64 bits. These are HP-UX specific flags.-mno-sched-br-data-spec,-msched-br-data-spec— (dis/en)able data speculative scheduling before reload. This results in generation ofld.ainstructions and the corresponding check instructions (ld.c/chk.a). The default setting is disabled.-msched-ar-data-spec,-mno-sched-ar-data-spec— (en/dis)able data speculative scheduling after reload. This results in generation ofld.ainstructions and the corresponding check instructions (ld.c/chk.a). The default setting is enabled. ✨-mno-sched-control-spec,-msched-control-spec— (dis/en)able control speculative scheduling. This feature is available only during region scheduling (i.e. before reload). This results in generation of theld.sinstructions and the corresponding check instructionschk.s. The default setting is disabled.-msched-br-in-data-spec,-mno-sched-br-in-data-spec— (en/dis)able speculative scheduling of the instructions that are dependent on the data speculative loads before reload. This is effective only with-msched-br-data-specenabled. The default setting is enabled.-msched-ar-in-data-spec,-mno-sched-ar-in-data-spec— (en/dis)able speculative scheduling of the instructions that are dependent on the data speculative loads after reload. This is effective only with-msched-ar-data-specenabled. The default setting is enabled.-msched-in-control-spec,-mno-sched-in-control-spec— (en/dis)able speculative scheduling of the instructions that are dependent on the control speculative loads. This is effective only with-msched-control-specenabled. The default setting is enabled.-mno-sched-count-spec-in-critical-path,-msched-count-spec-in-critical-path— if enabled, speculative dependencies are considered during computation of the instructions priorities. This makes the use of the speculation a bit more conservative. The default setting is disabled.-msched-spec-ldc,-mno-sched-spec-ldc— use a simple data speculation check. This option is on by default.-msched-control-spec-ldc,-mno-sched-control-spec-ldc— use a simple check for control speculation. This option is on by default.-msched-stop-bits-after-every-cycle,-mno-sched-stop-bits-after-every-cycle— place a stop bit after every cycle when scheduling. This option is on by default.-msched-fp-mem-deps-zero-cost,-mno-sched-fp-mem-deps-zero-cost— assume that floating-point stores and loads are not likely to cause a conflict when placed into the same instruction group. This option is disabled by default.-msel-sched-dont-check-control-spec,-mno-sel-sched-dont-check-control-spec— generate checks for control speculation in selective scheduling. This flag is disabled by default.-msched-max-memory-insns=max-insns — limit on the number of memory insns per instruction group, giving lower priority to subsequent memory insns attempting to schedule in the same instruction group. Frequently useful to prevent cache bank conflicts. The default value is1.-msched-max-memory-insns-hard-limit,-mno-sched-max-memory-insns-hard-limit— makes the limit specified bymsched-max-memory-insnsa hard limit, disallowing more than that number in an instruction group. Otherwise, the limit is "soft", meaning that non-memory operations are preferred when the limit is reached, but memory operations may still be scheduled. 🌸
🦋 LM32 Options 🦋
These -m options are defined for the LatticeMico32 architecture:
-mbarrel-shift-enabled— enable barrel-shift instructions.-mdivide-enabled— enable divide and modulus instructions.-mmultiply-enabled— enable multiply instructions.-msign-extend-enabled— enable sign extend instructions.-muser-enabled— enable user-defined instructions.
🌈 LoongArch Options 🌈
These command-line options are defined for LoongArch targets:
-march=arch-type — generate instructions for the machine type arch-type.-march=arch-type allows GCC to generate code that may not run at all on processors other than the one indicated. 😭 Yeah, she said what she said.
The choices for arch-type are:
native— local processor type detected by the native compiler.loongarch64— generic LoongArch 64-bit processor.la464— LoongArch LA464-based processor with LSX, LASX.la664— LoongArch LA664-based processor with LSX, LASX and all LoongArch v1.1 instructions.la64v1.0— LoongArch64 ISA version 1.0.la64v1.1— LoongArch64 ISA version 1.1.la32v1.0— LoongArch32 ISA version 1.0.la32rv1.0— LoongArch32 Reduced ISA version 1.0.
More information about LoongArch ISA versions can be found at https://github.com/loongson/la-toolchain-conventions. 🎀
-mtune=tune-type — optimize the generated code for the given processor target.
The choices for tune-type are:
native— local processor type detected by the native compiler.generic— generic LoongArch processor.loongarch64— generic LoongArch 64-bit processor.la464— LoongArch LA464 core.la664— LoongArch LA664 core.loongarch32— generic LoongArch 32-bit processor.
-mabi=base-abi-type — generate code for the specified calling convention. base-abi-type can be one of:lp64d— uses 64-bit general purpose registers and 32/64-bit floating-point registers for parameter passing. Data model is LP64, whereintis 32 bits, whilelong intand pointers are 64 bits.lp64f— uses 64-bit general purpose registers and 32-bit floating-point registers for parameter passing. Data model is LP64, whereintis 32 bits, whilelong intand pointers are 64 bits.lp64s— uses 64-bit general purpose registers and no floating-point registers for parameter passing. Data model is LP64, whereintis 32 bits, whilelong intand pointers are 64 bits. 💅
-mfpu=fpu-type — generate code for the specified FPU type, which can be one of:64— allow the use of hardware floating-point instructions for 32-bit and 64-bit operations. ✨32— allow the use of hardware floating-point instructions for 32-bit operations.none0— prevent the use of hardware floating-point instructions. nope, not today 💅
-msimd=simd-type— turns on generation of LoongArch SIMD instructions for vectorization and via builtin functions. the value can be one of:lasx— enable generating instructions from the 256-bit LoongArch Advanced SIMD Extension (LASX) and the 128-bit LoongArch SIMD Extension (LSX). both bestie, we're doing both 💖lsx— enable generating instructions from the 128-bit LoongArch SIMD Extension (LSX).none— no LoongArch SIMD instruction may be generated. rest in peace 😭
-msoft-float— forces-mfpu=noneand prevents the use of floating-point registers for parameter passing. heads up: this option may change the target ABI. 🫶-msingle-float— forces-mfpu=32and allows the use of 32-bit floating-point registers for parameter passing. again, this option may change the target ABI.-mdouble-float— forces-mfpu=64and allows the use of 32/64-bit floating-point registers for parameter passing. and yes, this option may change the target ABI too. 🦋-mlasx-mno-lasx-mlsx-mno-lsx— incrementally adjust the scope of the SIMD extensions (none / LSX / LASX) that the compiler can use for code generation. enabling LASX with-mlasxautomatically enables LSX, and disabling LSX with-mno-lsxautomatically disables LASX. these driver-only options act upon the final-msimdconfiguration state and make incremental changes in the order they appear on the GCC driver's command line, deriving the final / canonicalized-msimdoption that gets passed to the compiler proper. lowkey elegant fr fr ✨-mbranch-cost=n— set the cost of branches to roughly n instructions.-maddr-reg-reg-cost=n— set the cost of ADDRESS_REG_REG to the value calculated by n.-mcheck-zero-division-mno-check-zero-divison— trap (do not trap) on integer division by zero. the default is-mcheck-zero-divisionfor-O0or-Og, and-mno-check-zero-divisionfor other optimization levels.-mbreak-code=code— emit a"break"code instruction for irrecoverable traps from"__builtin_trap"or inserted by the compiler (for example an erroneous path isolated with-fisolate-erroneous-paths-dereference), or an"amswap.w $r0, $r1, $r0"instruction which will cause the hardware to trigger an Instruction Not-defined Exception if code is negative or greater than 32767. the default is-1, meaning to use the"amswap.w"instruction. 💀-mcond-move-int-mno-cond-move-int— conditional moves for integral data in general-purpose registers are enabled (disabled). the default is-mcond-move-int.-mcond-move-float-mno-cond-move-float— conditional moves for floating-point registers are enabled (disabled). the default is-mcond-move-float.-mmemcpy-mno-memcpy— force (do not force) the use of"memcpy"for non-trivial block moves. the default is-mno-memcpy, which lets GCC inline most constant-sized copies. setting the optimization level to-Osalso forces the use of"memcpy", but-mno-memcpymay override this behavior if explicitly specified, regardless of the order of these options on the command line.-mstrict-align-mno-strict-align— avoid or allow generating memory accesses that may not be aligned on a natural object boundary as described in the architecture specification. the default is-mno-strict-align.-G num— put global and static data smaller than num bytes into a small data section. the default value is0.-mmax-inline-memcpy-size=n— inline all block moves (such as calls to"memcpy"or structure copies) less than or equal to n bytes. the default value of n is1024.-mcmodel=code-model— set the code model to one of:tiny-static(not implemented yet)tiny(not implemented yet)normal— the text segment must be within 128MB addressing space. the data segment must be within 2GB addressing space.medium— the text segment and data segment must be within 2GB addressing space. this is the default code model unless GCC has been configured with--with-cmodel=specifying a different default code model.large(not implemented yet)extreme— this mode does not limit the size of the code segment and data segment. the-mcmodel=extremeoption is incompatible with-fpltand/or-mexplicit-relocs=none. 🚫
-mexplicit-relocs=style-mexplicit-relocs-mno-explicit-relocs— set when to use assembler relocation operators when dealing with symbolic addresses. the alternative is to use assembler macros instead, which may limit instruction scheduling but allow linker relaxation. with-mexplicit-relocs=none, the assembler macros are always used; with-mexplicit-relocs=always, the assembler relocation operators are always used; and with-mexplicit-relocs=autothe compiler uses the relocation operators where linker relaxation is impossible to improve code quality, and macros elsewhere. 💅
the default value for the option is determined with the assembler capability detected during GCC build-time and the setting of -mrelax: -mexplicit-relocs=none if the assembler does not support relocation operators at all, -mexplicit-relocs=always if the assembler supports relocation operators but -mrelax is not enabled, -mexplicit-relocs=auto if the assembler supports relocation operators and -mrelax is enabled.
for backward compatibility, -mexplicit-relocs is equivalent to -mexplicit-relocs=always, while -mno-explicit-relocs is equivalent to -mexplicit-relocs=none.
-mdirect-extern-access-mno-direct-extern-access— control use of the GOT to access external symbols. the default is-mno-direct-extern-access: the GOT is used for external symbols with default visibility, but not used for other external symbols.
with -mdirect-extern-access, the GOT is not used and all external symbols are PC-relatively addressed. it is only suitable for environments where no dynamic link is performed, like firmwares, OS kernels, executables linked with -static or -static-pie. -mdirect-extern-access is not compatible with -fPIC or -fpic. 😭
-mrelax-mno-relax— take (do not take) advantage of linker relaxations. if-mpass-mrelax-to-asis enabled, this option is also passed to the assembler. the default is determined during GCC build-time by detecting corresponding assembler support:-mrelaxif the assembler supports both the-mrelaxoption and the conditional branch relaxation (it's required or the".align"directives and conditional branch instructions in the assembly code outputted by GCC may be rejected by the assembler because of a relocation overflow),-mno-relaxotherwise. 🦋-mpass-mrelax-to-as-mno-pass-mrelax-to-as— pass (do not pass) the-mrelaxor-mno-relaxoption to the assembler. the default is determined during GCC build-time by detecting corresponding assembler support:-mpass-mrelax-to-asif the assembler supports the-mrelaxoption,-mno-pass-mrelax-to-asotherwise. this option is mostly useful for debugging, or interoperation with assemblers different from the build-time one.-mrecip— this option enables use of the reciprocal estimate and reciprocal square root estimate instructions with additional Newton-Raphson steps to increase precision instead of doing a divide or square root and divide for floating-point arguments. these instructions are generated only when-funsafe-math-optimizationsis enabled together with-ffinite-math-onlyand-fno-trapping-math. this option is off by default. before you can use this option, you must be sure the target CPU supports the"frecipe"and"frsqrte"instructions. note that while the throughput of the sequence is higher than the throughput of the non-reciprocal instruction, the precision of the sequence can be decreased by up to 2 ulp (i.e. the inverse of 1.0 equals 0.99999994). 🤯-mrecip=opt— this option controls which reciprocal estimate instructions may be used. opt is a comma-separated list of options, which may be preceded by a!to invert the option:all— enable all estimate instructions.default— enable the default instructions, equivalent to-mrecip.none— disable all estimate instructions, equivalent to-mno-recip.div— enable the approximation for scalar division.vec-div— enable the approximation for vectorized division.sqrt— enable the approximation for scalar square root.vec-sqrt— enable the approximation for vectorized square root.rsqrt— enable the approximation for scalar reciprocal square root.vec-rsqrt— enable the approximation for vectorized reciprocal square root.
so, for example, -mrecip=all,!sqrt enables all of the reciprocal approximations, except for scalar square root. ✨
-mfrecipe-mno-frecipe— use (do not use)"frecipe.{s/d}"and"frsqrte.{s/d}"instructions. when compiling with-march=la664, it is enabled by default. otherwise the default is-mno-frecipe.-mdiv32-mno-div32— use (do not use)"div.w[u]"and"mod.w[u]"instructions with input not sign-extended. when compiling with-march=la664, it is enabled by default. otherwise the default is-mno-div32.-mlam-bh-mno-lam-bh— use (do not use)"am{swap/add}[_db].{b/h}"instructions. when compiling with-march=la664, it is enabled by default. otherwise the default is-mno-lam-bh.-mlamcas-mno-lamcas— use (do not use)"amcas[_db].{b/h/w/d}"instructions. when compiling with-march=la664, it is enabled by default. otherwise the default is-mno-lamcas.-mld-seq-sa-mno-ld-seq-sa— whether a same-address load-load barrier ("dbar 0x700") is needed. when compiling with-march=la664, it is enabled by default. otherwise the default is-mno-ld-seq-sa, the load-load barrier is needed.-mscq-mno-scq— use (do not use) the 16-byte conditional store instruction"sc.q". the default is-mscqif the machine type specified with-march=supports this instruction,-mno-scqotherwise. 🎀-mtls-dialect=opt— bestie this one picks which TLS dialect is allowed for the general dynamic and local dynamic TLS models. The opt you pass can only be one of:trad— use traditional TLS. This is the default, no cap.desc— use TLS descriptors.
-mannotate-tablejump-mno-annotate-tablejump— this makes an annotation section called.discard.tablejump_annotateso that the"jirl"instruction gets correlated with the jump table whenever a jump table is being used to optimize a"switch"statement. Some external tools literally need this annotation to analyze control flow — likeobjtoolfrom the Linux kernel building system, for example. The default is-mno-annotate-tablejump. 🫶
🌈 LynxOS Options 🌈
These options are here for LynxOS targets, bestie.
-mshared— use shared libraries. The default is to link with static libraries instead. (So if you say nothing, you get static. Iconic behaviour honestly.)-mthreads— support multi-threading. 💅-mlegacy-threads— support legacy multi-threading. Heads up though: this option is incompatible with-mthreads, so don't even try to run both, she will not accept it.
🎀 M32C Options 🎀
-mcpu=name— select the CPU you're generating code for. name can be one ofr8cfor the R8C/Tiny series,m16cfor the M16C (up to /60) series,m32cmfor the M16C/80 series, orm32cfor the M32C/80 series.-msim— says the program is gonna run on the simulator. This links in an alternate runtime library that supports stuff like file I/O. > You must not use this option when generating programs that will run on real hardware — you have to provide your own runtime library for whatever I/O functions you need. Fr fr, don't do it.-memregs=number— specifies how many memory-based pseudo-registers GCC uses while generating code. These pseudo-registers get used just like real registers, so there's a whole tradeoff between how well GCC can fit the code into the available registers and the performance penalty of using memory instead of registers. Lowkey important: all modules in a program must be compiled with the same value for this option. And because of that, you must not use this option with GCC's default runtime libraries. 😭
✨ M32R/D Options ✨
These -m options are defined for Renesas M32R/D architectures, bestie:
-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 their addresses can be loaded with the"ld24"instruction), and assume all subroutines are reachable with the"bl"instruction. This is the default.
The addressability of a particular object can be set with the "model" attribute.
-mmodel=medium— assume objects may be anywhere in the 32-bit address space (the compiler generates"seth/add3"instructions to load their addresses), and assume all subroutines are reachable with the"bl"instruction.-mmodel=large— assume objects may be anywhere in the 32-bit address space (the compiler generates"seth/add3"instructions to load their addresses), and assume subroutines may not be reachable with the"bl"instruction — so the compiler generates the much slower"seth/add3/jl"instruction sequence. 💀-msdata=none— disable use of the small data area. Variables get 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 the 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 don't generate special code to reference them.-msdata=use— put small global and static data in the small data area, and generate special instructions to reference them.-G num— put global and static objects that are 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 is8. The-msdataoption must be set to one ofsdataorusefor this option to do literally anything.
All modules should be compiled with the same -G num value. Compiling with different values of num may or may not work; if it doesn't, the linker gives an error message — incorrect code is not generated.
-mdebug— makes the M32R-specific code in the compiler display some statistics that might help in debugging programs. 👀-malign-loops-mno-align-loops— align all loops to a 32-byte boundary. This option is disabled by default.-missue-rate=number— issue number instructions per cycle. number can only be1or2.-mbranch-cost=number— number can only be1or2. If it's1then branches are preferred over conditional code; if it's2, then the opposite applies.-mflush-trap=number— specifies the trap number to use to flush the cache. The default is12. Valid numbers are between0and15inclusive.-mno-flush-trap— specifies that the cache cannot be flushed by using a trap.-mflush-func=name— specifies the name of the operating system function to call to flush the cache. The default is_flush_cache, but a function call is only used if a trap is not available.-mno-flush-func— indicates that there is no OS function for flushing the cache.
👑 M680x0 Options 👑
These are the -m options defined for M680x0 and ColdFire processors. The default settings depend on which architecture was selected when the compiler was configured; the defaults for the most common choices are given below. 🦋
-march=arch— generate code for a specific M680x0 or ColdFire instruction set architecture. Permissible values of arch for M680x0 architectures are:68000,68010,68020,68030,68040,68060andcpu32. ColdFire architectures are selected according to Freescale's ISA classification and the permissible values are:isaa,isaaplus,isabandisac.
GCC defines a macro "__mcfarch__" whenever it is generating code for a ColdFire target. The arch in this macro is one of the -march arguments given above.
When used together, -march and -mtune select code that runs on a family of similar processors but that is optimized for a particular microarchitecture.
-mcpu=cpu— generate code for a specific M680x0 or ColdFire processor. The M680x0 cpus are:68000,68010,68020,68030,68040,68060,68302,68332andcpu32. The ColdFire cpus are given by the table below, which also classifies the CPUs into families:
Family : -mcpu arguments
51 : 51 51ac 51ag 51cn 51em 51je 51jf 51jg 51jm 51mm 51qe 51qm
5206 : 5202 5204 5206
5206e : 5206e
5208 : 5207 5208
5211a : 5210a 5211a
5213 : 5211 5212 5213
5216 : 5214 5216
52235 : 52230 52231 52232 52233 52234 52235
5225 : 5224 5225
52259 : 52252 52254 52255 52256 52258 52259
5235 : 5232 5233 5234 5235 523x
5249 : 5249
5250 : 5250
5271 : 5270 5271
5272 : 5272
5275 : 5274 5275
5282 : 5280 5281 5282 528x
53017 : 53011 53012 53013 53014 53015 53016 53017
5307 : 5307
5329 : 5327 5328 5329 532x
5373 : 5372 5373 537x
5407 : 5407
5475 : 5470 5471 5472 5473 5474 5475 547x 5480 5481 5482 5483 5484 5485-mcpu=cpu overrides -march=arch if arch is compatible with cpu. Other combinations of -mcpu and -march are rejected. 🙈
GCC defines the macro "__mcf_cpu_cpu" when ColdFire target cpu is selected. It also defines "__mcf_family_family", where the value of family is given by the table above.
-mtune=tune— tune the code for a particular microarchitecture within the constraints set by-marchand-mcpu. The M680x0 microarchitectures are:68000,68010,68020,68030,68040,68060andcpu32. The ColdFire microarchitectures are:cfv1,cfv2,cfv3,cfv4andcfv4e.
You can also use -mtune=68020-40 for code that needs to run relatively well on 68020, 68030 and 68040 targets. -mtune=68020-60 is similar but includes 68060 targets as well. These two options select the same tuning decisions as -m68020-40 and -m68020-60 respectively.
GCC defines the macros "__mcarch" and "__mcarch__" when tuning for 680x0 architecture arch. It also defines "mcarch" unless either -ansi or a non-GNU -std option is used. If GCC is tuning for a range of architectures, as selected by -mtune=68020-40 or -mtune=68020-60, it defines the macros for every architecture in the range.
GCC also defines the macro "__muarch__" when tuning for ColdFire microarchitecture uarch, where uarch is one of the arguments given above.
-m68000-mc68000— generate output for a 68000. This is the default when the compiler is configured for 68000-based systems. It is equivalent to-march=68000.
Use this option for microcontrollers with a 68000 or EC000 core, including the 68008, 68302, 68306, 68307, 68322, 68328 and 68356.
-m68010— generate output for a 68010. This is the default when the compiler is configured for 68010-based systems. It is equivalent to-march=68010.-m68020-mc68020— generate output for a 68020. This is the default when the compiler is configured for 68020-based systems. It is equivalent to-march=68020.-m68030— generate output for a 68030. This is the default when the compiler is configured for 68030-based systems. It is equivalent to-march=68030.-m68040— generate output for a 68040. This is the default when the compiler is configured for 68040-based systems. It is equivalent to-march=68040.
This option inhibits the use of 68881/68882 instructions that have to be emulated by software on the 68040. Use this option if your 68040 does not have code to emulate those instructions.
-m68060— generate output for a 68060. This is the default when the compiler is configured for 68060-based systems. It is equivalent to-march=68060.
This option inhibits the use of 68020 and 68881/68882 instructions that have to be emulated by software on the 68060. Use this option if your 68060 does not have code to emulate those instructions.
-m68302— generate code for a 68302.-m68332— generate code for a 68332.-m68851— okay so the 68851 was an external MMU for the 68020 processor. GCC accepts this option, but does not use it. 💅-mcpu32— Generate output for a CPU32. This is the default when the compiler is configured for CPU32-based systems. It is equivalent to-march=cpu32.
Use this option for microcontrollers with a CPU32 or CPU32+ core, including the 68330, 68331, 68332, 68333, 68334, 68336, 68340, 68341, 68349 and 68360. ✨
-mfidoa— Generate code for a Fido A. 🐶-m5200— Generate output for a 520X ColdFire CPU. This is the default when the compiler is configured for 520X-based systems. It is equivalent to-mcpu=5206, and is now deprecated in favor of that option. 😭
Use this option for microcontroller with a 5200 core, including the MCF5202, MCF5203, MCF5204 and MCF5206.
-m5206e— Generate output for a 5206e ColdFire CPU. The option is now deprecated in favor of the equivalent-mcpu=5206e. 💀-m528x— Generate output for a member of the ColdFire 528X family. The option is now deprecated in favor of the equivalent-mcpu=528x.-m5307— Generate output for a ColdFire 5307 CPU. The option is now deprecated in favor of the equivalent-mcpu=5307.-m5407— Generate output for a ColdFire 5407 CPU. The option is now deprecated in favor of the equivalent-mcpu=5407.-mcfv4e— Generate output for a ColdFire V4e family CPU (e.g. 547x/548x). This includes use of hardware floating-point instructions. The option is equivalent to-mcpu=547x, and is now deprecated in favor of that option. 🫠-m68020-40— Generate output for a 68040, without using any of the new instructions. This results in code that can run relatively efficiently on either a 68020/68881 or a 68030 or a 68040. The generated code does use the 68881 instructions that are emulated on the 68040.
The option is equivalent to -march=68020 -mtune=68020-40. 🔥
-m68020-60— Generate output for a 68060, without using any of the new instructions. This results in code that can run relatively efficiently on either a 68020/68881 or a 68030 or a 68040. The generated code does use the 68881 instructions that are emulated on the 68060.
The option is equivalent to -march=68020 -mtune=68020-60.
-mhard-float,-m68881— Generate floating-point instructions. This is the default for 68020 and above, and for ColdFire devices that have an FPU. It defines the macro__HAVE_68881__on M680x0 targets and__mcffpu__on ColdFire targets. 💖-msoft-float— Do not generate floating-point instructions; use library calls instead. This is the default for 68000, 68010, and 68832 targets. It is also the default for ColdFire devices that have no FPU. 🍓-mdiv,-mno-div— Generate (do not generate) ColdFire hardware divide and remainder instructions. If-marchis used without-mcpu, the default is "on" for ColdFire architectures and "off" for M680x0 architectures. Otherwise, the default is taken from the target CPU (either the default CPU, or the one specified by-mcpu). For example, the default is "off" for-mcpu=5206and "on" for-mcpu=5206e.
GCC defines the macro __mcfhwdiv__ when this option is enabled. ✨
-mshort,-mno-short,-mnoshort— Consider typeintto be 16 bits wide, likeshort 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. This option is disabled by default. 🙈-mbitfield,-mno-bitfield,-mnobitfield— Control use of the bit-field instructions. The-m68000,-mcpu32and-m5200options imply-mnobitfield; the-m68020option implies-mbitfield. 💅-mrtd,-mno-rtd,-mnortd— Control use of a different function-calling convention, in which functions that take a fixed number of arguments return with thertdinstruction, which pops their arguments while returning. This saves one instruction in the caller since there is no need to pop the arguments there.
This calling convention is incompatible with the one normally used on Unix, so you cannot use it if you need to call libraries compiled with the Unix compiler. 😭
Also, you must provide function prototypes for all functions that take variable numbers of arguments (including printf); otherwise incorrect code is generated for calls to those functions. 🫠
In addition, seriously incorrect code results if you call a function with too many arguments. (Normally, extra arguments are harmlessly ignored.) 💀
The rtd instruction is supported by the 68010, 68020, 68030, 68040, 68060 and CPU32 processors, but not by the 68000 or 5200.
The default is -mno-rtd.
-malign-int,-mno-align-int— Control whether GCC alignsint,long,long long,float,double, andlong doublevariables on a 32-bit boundary (-malign-int) or a 16-bit boundary (-mno-align-int). Aligning variables on 32-bit boundaries produces code that runs somewhat faster on processors with 32-bit busses at the expense of more memory.
Warning: if you use the -malign-int switch, GCC aligns structures containing the above types differently than most published application binary interface specifications for the m68k. 😱
-mpcrel— Use the pc-relative addressing mode of the 68000 directly, instead of using a global offset table. At present, this option implies-fpic, allowing at most a 16-bit offset for pc-relative addressing.-fPICis not presently supported with-mpcrel, though this could be supported for 68020 and higher processors. 🌸-mno-strict-align,-mstrict-align— Do not (do) assume that unaligned memory references are handled by the system. 💫-msep-data,-mno-sep-data— With-msep-data, generate code that allows the data segment to be located in a different area of memory from the text segment. This allows for execute-in-place in an environment without virtual memory management. This option implies-fPIC.
This option is disabled by default; GCC generates code that assumes that the data segment follows the text segment. 🦋
-mid-shared-library,-mno-id-shared-library— If enabled, 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.
This option is disabled by default. 🎀
-mshared-library-id=n— Specifies the identification number of the ID-based shared library being compiled. Specifying a value of0generates more compact code; specifying other values forces the allocation of that number to the current library, but is no more space- or time-efficient than omitting this option. ✨-mxgot,-mno-xgot— When generating position-independent code for ColdFire, generate code that works if the GOT has more than 8192 entries. This code is larger and slower than code generated without this option. On M680x0 processors, this option is not needed;-fPICsuffices.
GCC normally uses a single instruction to load values from the GOT. While this is relatively efficient, it only works if the GOT is smaller than about 64k. Anything larger causes the linker to report an error such as:
relocation truncated to fit: R_68K_GOT16O foobarIf this happens, you should recompile your code with -mxgot. It should then work with very large GOTs. However, code generated with -mxgot is less efficient, since it takes 4 instructions to fetch the value of a global symbol. 😭
Note that some linkers, including newer versions of the GNU linker, can create multiple GOTs and sort GOT entries. If you have such a linker, you should only need to use -mxgot when compiling a single object file that accesses more than 8192 GOT entries. Very few do.
These options have no effect unless GCC is generating position-independent code. 💅
-mxtls— Support TLS segment larger than 64K. The considerations for using this option are similar to those for-mxgotabove; the default 16-bit offset addressing for TLS is more efficient, but this may be inadequate for some programs. 🌈-mlong-jump-table-offsets— Use 32-bit offsets inswitchtables. The default is to use 16-bit offsets. 🔥
🦋 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-immediates,-mno-relax-immediates— Allow arbitrary-sized immediates in bit operations. 🎀-mwide-bitfields,-mno-wide-bitfields— Always treat bit-fields asint-sized. 💅-m4byte-functions,-mno-4byte-functions— Force all functions to be aligned to a 4-byte boundary. 📏-mcallgraph-data,-mno-callgraph-data— Emit callgraph information. 📊-mslow-bytes,-mno-slow-bytes— Prefer word access when reading byte quantities. 🐢-mlittle-endian,-mbig-endian— Generate code for a little- or big-endian target, respectively. The default is big-endian. Little-endian code is supported only with-m340. 🌍-m210— Generate code for the 210 processor. 🔢-m340— Generate code for the 340 processor. 🔢-mno-lsim— Assume that runtime support has been provided and so omit the simulator library (libsim.a) from the linker command line. 🚫-mstack-increment=size— Set the maximum amount for a single stack increment operation. Large values can increase the speed of programs that contain functions that need a large amount of stack space, but they can also trigger a segmentation fault if the stack is extended too much. The default value is0x1000. 😬
🌈 MicroBlaze Options 🌈
-msoft-float— Use software emulation for floating point (default). 🧚-mhard-float— Use hardware floating-point instructions. ⚡-mmemcpy— Do not optimize block moves, usememcpy. 📦-mcpu=cpu-type— Use features of, and schedule code for, the given CPU. Supported values are in the formatvX.YY.Z, whereXis a major version,YYis the minor version, andZis compatibility code. Example values arev3.00.a,v4.00.b,v5.00.a,v5.00.b,v6.00.a. 💖-mxl-soft-mul,-mno-xl-soft-mul— use software multiply emulation 💫 this one is on by default, bestie, no need to ask.-mxl-soft-div,-mno-xl-soft-div— software emulation for divides. also on by default 🫶-mxl-barrel-shift— use the hardware barrel shifter ⚙️-mxl-pattern-compare— use pattern compare instructions 🔍-msmall-divides— table lookup optimization for small signed integer divisions 🍰-mxl-gp-opt— use GP-relative.sdata/.sbsssections 📦-mxl-multiply-high— use multiply high instructions for the high part of a 32x32 multiply ✖️-mxl-float-convert— use hardware floating-point conversion instructions 🧮-mxl-float-sqrt— use the hardware floating-point square root instruction √-mbig-endian— generate code for a big-endian target 🐘-mlittle-endian— generate code for a little-endian target 🐭-mxl-reorder— use reorder instructions (swap and byte reversed load/store) 🔄-mxl-mode-app-model— select the application model app-model. The valid models are, and yes there's lore here:executable— the normal executable (default), uses startup codecrt0.o✨xmdstub— for use with the Xilinx Microprocessor Debugger (XMD) based software intrusive debug agent called xmdstub. This uses startup filecrt1.oand sets the start address of the program to0x800🪲bootstrap— for applications that are loaded using a bootloader. This model uses startup filecrt2.owhich does not contain a processor reset vector handler. This is suitable for transferring control on a processor reset to the bootloader rather than the application 🥾novectors— for applications that do not require any of the MicroBlaze vectors. This option may be useful for applications running within a monitoring application. This model usescrt3.oas a startup file 👀
-mxl-prefetch— enable insertion of prefetch (wic) instructions at call sites 🔮-mpic-data-is-text-relative— assume that the displacement between the text and data segments is fixed at static link time. This lets data be referenced by offset from the start of the text address instead of the GOT, since PC-relative addressing is not supported 💅
✨ MIPS Options ✨
-EB,-meb— generate big-endian code 🐘-EL,-mel— generate little-endian code 🐭 this is the default formips*el-*-*configurations.-march=arch— generate code that runs on arch, which can be the name of a generic MIPS ISA or the name of a particular processor. The ISA names are:mips1,mips2,mips3,mips4,mips32,mips32r2,mips32r3,mips32r5,mips32r6,mips64,mips64r2,mips64r3,mips64r5andmips64r6. The processor names are:4kc,4km,4kp,4ksc,4kec,4kem,4kep,4ksd,5kc,5kf,20kc,24kc,24kf2_1,24kf1_1,24kec,24kef2_1,24kef1_1,34kc,34kf2_1,34kf1_1,34kn,74kc,74kf2_1,74kf1_1,74kf3_2,1004kc,1004kf2_1,1004kf1_1,i6400,i6500,interaptiv,loongson2e,loongson2f,loongson3a,gs464,gs464e,gs264e,m4k,m14k,m14kc,m14ke,m14kec,m5100,m5101,octeon,octeon+,octeon2,octeon3,orion,p5600,p6600,r2000,r3000,r3900,r4000,r4400,r4600,r4650,r4700,r5900,r6000,r8000,rm7000,rm9000,r10000,r12000,r14000,r16000,sb1,sr71000,vr4100,vr4111,vr4120,vr4130,vr4300,vr5000,vr5400,vr5500,xlrandxlp.allegrex. The special valuefrom-abiselects the most compatible architecture for the selected ABI (that is,mips1for 32-bit ABIs andmips3for 64-bit ABIs).
The native Linux/GNU toolchain also supports the value native, which selects the best architecture option for the host processor. -march=native has no effect if GCC does not recognize the processor. 😬
In processor names, a final 000 can be abbreviated as k (for example, -march=r2k). Prefixes are optional, and vr may be written r. ✂️
Names of the form nf2_1 refer to processors with FPUs clocked at half the rate of the core, names of the form nf1_1 refer to processors with FPUs clocked at the same rate as the core, and names of the form nf3_2 refer to processors with FPUs clocked a ratio of 3:2 with respect to the core. For compatibility reasons, nf is accepted as a synonym for nf2_1 while nx and bfx are accepted as synonyms for *n*f1_1. 🕰️
GCC defines two macros based on the value of this option. The first is _MIPS_ARCH, which gives the name of the target architecture, as a string. The second has the form _MIPS_ARCH_foo, where foo is the capitalized value of _MIPS_ARCH. For example, -march=r2000 sets _MIPS_ARCH to "r2000" and defines the macro _MIPS_ARCH_R2000. 👑
Note that the _MIPS_ARCH macro uses the processor names given above. In other words, it has the full prefix and does not abbreviate 000 as k. In the case of from-abi, the macro names the resolved architecture (either "mips1" or "mips3"). It names the default architecture when no -march option is given. 🧠
-mtune=arch— optimize for arch. Among other things, this option controls the way instructions are scheduled, and the perceived cost of arithmetic operations. The list of arch values is the same as for-march. 🎛️
When this option is not used, GCC optimizes for the processor specified by -march. By using -march and -mtune together, it is possible to generate code that runs on a family of processors, but optimize the code for one particular member of that family. the way that's such a glow-up move ✨
-mtune defines the macros _MIPS_TUNE and _MIPS_TUNE_foo, which work in the same way as the -march ones described above. 💖
-mips1— equivalent to-march=mips1.-mips2— equivalent to-march=mips2.-mips3— equivalent to-march=mips3.-mips4— equivalent to-march=mips4.-mips32— equivalent to-march=mips32.-mips32r3— equivalent to-march=mips32r3.-mips32r5— equivalent to-march=mips32r5.-mips32r6— equivalent to-march=mips32r6.-mips64— equivalent to-march=mips64.-mips64r2— equivalent to-march=mips64r2.-mips64r3— equivalent to-march=mips64r3.-mips64r5— equivalent to-march=mips64r5.-mips64r6— equivalent to-march=mips64r6.-mips16,-mno-mips16— generate (do not generate) MIPS16 code. If GCC is targeting a MIPS32 or MIPS64 architecture, it makes use of the MIPS16e ASE. 🎀
MIPS16 code generation can also be controlled on a per-function basis by means of mips16 and nomips16 attributes. lowkey handy fr.
-mmips16e2,-mno-mips16e2— use (do not use) the MIPS16e2 ASE. This option modifies the behavior of the-mips16option such that it targets the MIPS16e2 ASE. 🦋-mflip-mips16,-mflip-mips16— generate MIPS16 code on alternating functions. This option is provided for regression testing of mixed MIPS16/non-MIPS16 code generation, and is not intended for ordinary use in compiling user code. 🚨 (yes it's listed twice up there, that's just how it is bestie 💀)-minterlink-compressed,-mno-interlink-compressed— require (do not require) that code using the standard (uncompressed) MIPS ISA be link-compatible with MIPS16 and microMIPS code, and vice versa. 🔗
For example, code using the standard ISA encoding cannot jump directly to MIPS16 or microMIPS code; it must either use a call or an indirect jump. -minterlink-compressed therefore disables direct jumps unless GCC knows that the target of the jump is not compressed. 🛑
-minterlink-mips16,-mno-interlink-mips16— aliases of-minterlink-compressedand-mno-interlink-compressed. These options predate the microMIPS ASE and are retained for backwards compatibility. 🕰️💅-mabi=32,-mabi=o64,-mabi=n32,-mabi=64,-mabi=eabi— generate code for the given ABI. 🧬
Note that the EABI has a 32-bit and a 64-bit variant. GCC normally generates 64-bit code when you select a 64-bit architecture, but you can use -mgp32 to get 32-bit code instead.
For information about the O64 ABI, see https://gcc.gnu.org/projects/mipso64-abi.html. 🌐
GCC supports a variant of the o32 ABI in which floating-point registers are 64 rather than 32 bits wide. You can select this combination with -mabi=32 -mfp64. This ABI relies on the mthc1 and mfhc1 instructions and is therefore only supported for MIPS32R2, MIPS32R3 and MIPS32R5 processors. ⚠️
The register assignments for arguments and return values remain the same, but each scalar value is passed in a single 64-bit register rather than a pair of 32-bit registers. For example, scalar floating-point values are returned in $f0 only, not a $f0/$f1 pair. The set of call-saved registers also remains the same in that the even-numbered double-precision registers are saved. 💾
Two additional variants of the o32 ABI are supported to enable a transition from 32-bit to 64-bit registers. These are FPXX (-mfpxx) and FP64A (-mfp64 -mno-odd-spreg). The FPXX extension mandates that all code must execute correctly when run using 32-bit or 64-bit registers. The code can be interlinked with either FP32 or FP64, but not both. The FP64A extension is similar to the FP64 extension but forbids the use of odd-numbered single-precision registers. This can be used in conjunction with the FRE mode of FPUs in MIPS32R5 processors and allows both FP32 and FP64A code to interlink and run in the same process without changing FPU modes. 🤯 iconic tbh.
-mabicalls,-mno-abicalls— generate (do not generate) code that is suitable for SVR4-style dynamic objects.-mabicallsis the default for SVR4-based systems. 🎯-mshared,-mno-shared— generate (do not generate) code that is fully position-independent, and that can therefore be linked into shared libraries. This option only affects-mabicalls. 📚
All -mabicalls code has traditionally been position-independent, regardless of options like -fPIC and -fpic. However, as an extension, the GNU toolchain allows executables to use absolute accesses for locally-binding symbols. It can also use shorter GP initialization sequences and generate direct calls to locally-defined functions. This mode is selected by -mno-shared. 🌀
-mno-shared depends on binutils 2.16 or higher and generates objects that can only be linked by the GNU linker. However, the option does not affect the ABI of the final executable; it only affects the ABI of relocatable objects. Using -mno-shared generally makes executables both smaller and quicker. 🔥
-mshared is the default, bestie. That's just the vibe. ✨
-mplt,-mno-plt— assume (or, like, don't assume) that the static and dynamic linkers out here support PLTs and copy relocations. 🫶 Heads up though: this option only affects-mno-shared -mabicalls. And for the n64 ABI? It literally does nothing without-msym32. So don't come for me.
You can make -mplt the default by configuring GCC with --with-mips-plt. Otherwise the default is -mno-plt. Periodt.
-mxgot,-mno-xgot— lift (or don't lift) the usual restrictions on the size of the global offset table. 👑
So normally GCC uses a single instruction to load values from the GOT. Iconic, efficient, love that for us — but it only works if the GOT is smaller than about 64k. Anything bigger and the linker gets dramatic and reports an error like:
relocation truncated to fit: R_MIPS_GOT16 foobarIf that happens, recompile your code with -mxgot. It works with very large GOTs, but the code is also less efficient — it takes three instructions to fetch the value of a global symbol. Trade-offs, bestie. 😭
Ngl though, note that some linkers can create multiple GOTs. If you have one of those, you should only need -mxgot when a single object file accesses more than 64k's worth of GOT entries. Very few do.
These options have no effect unless GCC is generating position independent code. 🚨
-mgp32— assume that general-purpose registers are 32 bits wide.-mgp64— assume that general-purpose registers are 64 bits wide.-mfp32— assume that floating-point registers are 32 bits wide.-mfp64— assume that floating-point registers are 64 bits wide.-mfpxx— do not assume the width of floating-point registers. Mysterious. Elusive. She contains multitudes. 🦋-mhard-float— use floating-point coprocessor instructions.-msoft-float— do not use floating-point coprocessor instructions. Implement floating-point calculations using library calls instead.-mno-float— equivalent to-msoft-float, but additionally asserts that the program being compiled does not perform any floating-point operations. This option is presently supported only by some bare-metal MIPS configurations, where it may select a special set of libraries that lack all floating-point support (including, for example, the floating-pointprintfformats). ⚠️ If code compiled with-mno-floataccidentally contains floating-point operations, it is likely to suffer a link-time or run-time failure. Like, it will crash out.-msingle-float— assume that the floating-point coprocessor only supports single-precision operations.-mdouble-float— assume that the floating-point coprocessor supports double-precision operations. This is the default. 💅-modd-spreg,-mno-odd-spreg— enable the use of odd-numbered single-precision floating-point registers for the o32 ABI. This is the default for processors that are known to support these registers. When using the o32 FPXX ABI,-mno-odd-spregis set by default.-mabs=2008,-mabs=legacy— these options control the treatment of the special not-a-number (NaN) IEEE 754 floating-point data with theabs.fmtandneg.fmtmachine instructions. 🌸
By default, or when -mabs=legacy is used, the legacy treatment is selected. In this case these instructions are considered arithmetic and avoided where correct operation is required and the input operand might be a NaN. A longer sequence of instructions that manipulate the sign bit of floating-point datum manually is used instead — unless the -ffinite-math-only option has also been specified.
The -mabs=2008 option selects the IEEE 754-2008 treatment. In this case these instructions are considered non-arithmetic and therefore operating correctly in all cases, including in particular where the input operand is a NaN. These instructions are therefore always used for the respective operations. Slay.
-mnan=2008,-mnan=legacy— these options control the encoding of the special not-a-number (NaN) IEEE 754 floating-point data. 🧚
The -mnan=legacy option selects the legacy encoding. In this case quiet NaNs (qNaNs) are denoted by the first bit of their trailing significand field being 0, whereas signaling NaNs (sNaNs) are denoted by the first bit of their trailing significand field being 1.
The -mnan=2008 option selects the IEEE 754-2008 encoding. In this case qNaNs are denoted by the first bit of their trailing significand field being 1, whereas sNaNs are denoted by the first bit of their trailing significand field being 0. (Yes, it's flipped. That's the whole point. 💀)
The default is -mnan=legacy unless GCC has been configured with --with-nan=2008.
-mllsc,-mno-llsc— use (do not use)ll,sc, andsyncinstructions to implement atomic memory built-in functions. When neither option is specified, GCC uses the instructions if the target architecture supports them.
-mllsc is useful if the runtime environment can emulate the instructions, and -mno-llsc can be useful when compiling for nonstandard ISAs. You can make either option the default by configuring GCC with --with-llsc and --without-llsc respectively. --with-llsc is the default for some configurations; see the installation documentation for details. 👑
-mdsp,-mno-dsp— use (do not use) revision 1 of the MIPS DSP ASE. This option defines the preprocessor macro__mips_dsp. It also defines__mips_dsp_revto1.-mdspr2,-mno-dspr2— use (do not use) revision 2 of the MIPS DSP ASE. This option defines the preprocessor macros__mips_dspand__mips_dspr2. It also defines__mips_dsp_revto2.-msmartmips,-mno-smartmips— use (do not use) the MIPS SmartMIPS ASE.-mpaired-single,-mno-paired-single— use (do not use) paired-single floating-point instructions. This option requires hardware floating-point support to be enabled. 🫶-mdmx,-mno-mdmx— use (do not use) MIPS Digital Media Extension instructions. This option can only be used when generating 64-bit code and requires hardware floating-point support to be enabled.-mips3d,-mno-mips3d— use (do not use) the MIPS-3D ASE. The option-mips3dimplies-mpaired-single.-mmicromips,-mno-micromips— generate (do not generate) microMIPS code.
MicroMIPS code generation can also be controlled on a per-function basis by means of micromips and nomicromips attributes.
-mmt,-mno-mt— use (do not use) MT Multithreading instructions.-mmcu,-mno-mcu— use (do not use) the MIPS MCU ASE instructions.-meva,-mno-eva— use (do not use) the MIPS Enhanced Virtual Addressing instructions.-mvirt,-mno-virt— use (do not use) the MIPS Virtualization (VZ) instructions.-mxpa,-mno-xpa— use (do not use) the MIPS eXtended Physical Address (XPA) instructions.-mcrc,-mno-crc— use (do not use) the MIPS Cyclic Redundancy Check (CRC) instructions.-mginv,-mno-ginv— use (do not use) the MIPS Global INValidate (GINV) instructions.-mmsa,-mno-msa— use (do not use) the MIPS MSA extension instructions.-mloongson-mmi,-mno-loongson-mmi— use (do not use) the MIPS Loongson MultiMedia extensions Instructions (MMI).-mloongson-ext,-mno-loongson-ext— use (do not use) the MIPS Loongson EXTensions (EXT) instructions.-mloongson-ext2,-mno-loongson-ext2— use (do not use) the MIPS Loongson EXTensions r2 (EXT2) instructions. 🔥-mlong64— forcelongtypes to be 64 bits wide. See-mlong32for an explanation of the default and the way that the pointer size is determined.-mlong32— forcelong,int, and pointer types to be 32 bits wide.
The default size of ints, longs and pointers depends on the ABI. All the supported ABIs use 32-bit ints. The n64 ABI uses 64-bit longs, as does the 64-bit EABI; the others use 32-bit longs. Pointers are the same size as longs, or the same size as integer registers, whichever is smaller. ✨
-msym32,-mno-sym32— assume (do not assume) that all symbols have 32-bit values, regardless of the selected ABI. This option is useful in combination with-mabi=64and-mno-abicallsbecause it allows GCC to generate shorter and faster references to symbolic addresses. Fast and short? Main character energy fr fr.-G num— put definitions of externally-visible data in a small data section if that data is no bigger than num bytes. GCC can then generate more efficient accesses to the data; see-mgpoptfor details.
The default -G option depends on the configuration. 🤷♀️
-mlocal-sdata,-mno-local-sdata— extend (do not extend) the-Gbehavior to local data too, such as to static variables in C.-mlocal-sdatais the default for all configurations.
If the linker complains that an application is using too much small data, you might want to try rebuilding the less performance-critical parts with -mno-local-sdata. You might also want to build large libraries with -mno-local-sdata, so that the libraries leave more room for the main program. 🎀
-mextern-sdata,-mno-extern-sdata— assume (do not assume) that externally-defined data is in a small data section if the size of that data is within the-Glimit.-mextern-sdatais the default for all configurations.
If you compile a module Mod with -mextern-sdata -G num -mgpopt, and Mod references a variable Var that is no bigger than num bytes, you must make sure that Var is placed in a small data section. If Var is defined by another module, you must either compile that module with a high-enough -G setting or attach a section attribute to Var's definition. If Var is common, you must link the application with a high-enough -G setting. 😤
The easiest way of satisfying these restrictions is to compile and link every module with the same -G option. However, you may wish to build a library that supports several different small data limits. You can do this by compiling the library with the highest supported -G setting and additionally using -mno-extern-sdata to stop the library from making assumptions about externally-defined data.
-mgpopt,-mno-gpopt— use (do not use) GP-relative accesses for symbols that are known to be in a small data section; see-G,-mlocal-sdataand-mextern-sdata.-mgpoptis the default for all configurations. 💅
-mno-gpopt is your bestie for the cases where the $gp register might not be holding the value of "_gp" at all. Like, imagine the code is part of a library that might get used in a boot monitor, and the programs that call boot monitor routines are just yeeting some unknown value into $gp. (When that's the sitch, the boot monitor itself is usually compiled with -G0.)
Heads up bestie:-mno-gpoptimplies-mno-local-sdataand-mno-extern-sdata. It's giving package deal. 💅
-membedded-data-mno-embedded-data— Allocate variables to the read-only data section first if that's possible, then next in the small data section if possible, otherwise in data. This makes the code slightly slower than the default, but it reduces how much RAM you need when executing, so it might be the vibe you want for some embedded systems. 🍓-muninit-const-in-rodata-mno-uninit-const-in-rodata— Put uninitialized"const"variables in the read-only data section. Only meaningful in conjunction with-membedded-data, so don't try it solo, it won't do anything.-mcode-readable=setting — Specify whether GCC may generate code that reads from executable sections. There are three possible settings:-mcode-readable=yes— Instructions may freely access executable sections. This is the default setting.-mcode-readable=pcrel— MIPS16 PC-relative load instructions can access executable sections, but other instructions must not do so. This option is useful on 4KSc and 4KSd processors when the code TLBs have the Read Inhibit bit set. It's also useful on processors that can be configured to have a dual instruction/data SRAM interface and that, like the M4K, automatically redirect PC-relative loads to the instruction RAM.-mcode-readable=no— Instructions must not access executable sections. This option can be useful on targets that are configured to have a dual instruction/data SRAM interface but that (unlike the M4K) do not automatically redirect PC-relative loads to the instruction RAM.
On SDE targets, -mcode-data-in-code is available as a traditional alias for -mcode-readable=no, and -mcode-xonly for -mcode-readable=pcrel. iykyk 🧚
-msplit-addresses-mno-split-addresses— Enable (disable) use of the%hi()and%lo()assembler relocation operators. This option has been superseded by-mexplicit-relocsbut it's retained for backwards compatibility, so it's still hanging around like that one friend who never leaves the group chat.-mexplicit-relocs=none-mexplicit-relocs=base-mexplicit-relocs=pcrel-mexplicit-relocs-mno-explicit-relocs— These options control whether explicit relocs (such as%gp_rel) are used. The default value depends on the version of GAS when GCC itself was built.
The "base" explicit-relocs support was introduced into GAS in 2001. The "pcrel" explicit-relocs support was introduced into GAS in 2014, which supports %pcrel_hi and %pcrel_lo.
-mcheck-zero-division-mno-check-zero-division— Trap (do not trap) on integer division by zero. 😭
The default is -mcheck-zero-division.
-mdivide-traps-mdivide-breaks— MIPS systems check for division by zero by generating either a conditional trap or a break instruction. Using traps results in smaller code, but it's 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-trapsto allow conditional traps on architectures that support them and-mdivide-breaksto 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.
-mload-store-pairs-mno-load-store-pairs— Enable (disable) an optimization that pairs consecutive load or store instructions to enable load/store bonding. This option is enabled by default but it only takes effect when the selected architecture is known to support bonding. ✨-mstrict-align-mno-strict-align-munaligned-access-mno-unaligned-access— Disable (enable) direct unaligned access for MIPS Release 6. MIPSr6 requires load/store unaligned-access support, either by hardware or by trapping and emulation. In the latter case-mstrict-alignmay be needed by the operating system kernel. The options-munaligned-accessand-mno-unaligned-accessare obsolete, and only provided for backward compatibility.-mmemcpy-mno-memcpy— Force (do not force) the use of"memcpy"for non-trivial block moves. The default is-mno-memcpy, which allows GCC to inline most constant-sized copies. 🔥-mlong-calls-mno-long-calls— Disable (do not disable) use of the"jal"instruction. Calling functions using"jal"is more efficient but requires the caller and callee to be in the same 256 megabyte segment.
This option has no effect on abicalls code. The default is -mno-long-calls.
-mmad-mno-mad— Enable (disable) use of the"mad","madu"and"mul"instructions, as provided by the R4650 ISA.-mimadd-mno-imadd— Enable (disable) use of the"madd"and"msub"integer instructions. The default is-mimaddon architectures that support"madd"and"msub"— except for the 74k architecture where it was found to generate slower code. 🙈-mfused-madd-mno-fused-madd— Enable (disable) use of the floating-point multiply-accumulate instructions, when they are available. The default is-mfused-madd.
On the R8000 CPU when multiply-accumulate instructions are used, the intermediate product is calculated to infinite precision and is not subject to the FCSR Flush to Zero bit. This may be undesirable in some circumstances. On other processors the result is numerically identical to the equivalent computation using separate multiply, add, subtract and negate instructions.
-nocpp— Tell the MIPS assembler to not run its preprocessor over user assembler files (with a.ssuffix) when assembling them.-mfix-24k-mno-fix-24k— Work around the 24K E48 (lost data on stores during refill) errata. The workarounds are implemented by the assembler rather than by GCC.-mfix-r4000-mno-fix-r4000— Work around certain R4000 CPU errata:- A double-word or a variable shift may give an incorrect result if executed immediately after starting an integer division.
- A double-word or a variable shift may give an incorrect result if executed while an integer multiplication is in progress.
- An integer division may give an incorrect result if started in a delay slot of a taken branch or a jump.
-mfix-r4400-mno-fix-r4400— Work around certain R4400 CPU errata:- A double-word or a variable shift may give an incorrect result if executed immediately after starting an integer division.
-mfix-r10000-mno-fix-r10000— Work around certain R10000 errata:"ll"/"sc"sequences may not behave atomically on revisions prior to 3.0. They may deadlock on revisions 2.6 and earlier.
This option can only be used if the target architecture supports branch-likely instructions. -mfix-r10000 is the default when -march=r10000 is used; -mno-fix-r10000 is the default otherwise.
-mfix-r5900-mno-fix-r5900— Do not attempt to schedule the preceding instruction into the delay slot of a branch instruction placed at the end of a short loop of six instructions or fewer, and always schedule a"nop"instruction there instead. The short loop bug under certain conditions causes loops to execute only once or twice, due to a hardware bug in the R5900 chip. The workaround is implemented by the assembler rather than by GCC.-mfix-rm7000-mno-fix-rm7000— Work around the RM7000"dmult"/"dmultu"errata. The workarounds are implemented by the assembler rather than by GCC.-mfix-vr4120-mno-fix-vr4120— Work around certain VR4120 errata:"dmultu"does not always produce the correct result."div"and"ddiv"do not always produce the correct result if one of the operands is negative.
The workarounds for the division errata rely on special functions in libgcc.a. At present, these functions are only provided by the "mips64vr*-elf" configurations.
Other VR4120 errata require a NOP to be inserted between certain pairs of instructions. These errata are handled by the assembler, not by GCC itself.
-mfix-vr4130-mno-fix-vr4130— Work around the VR4130"mflo"/"mfhi"errata. The workarounds are implemented by the assembler rather than by GCC, although GCC avoids using"mflo"and"mfhi"if the VR4130"macc","macchi","dmacc"and"dmacchi"instructions are available instead.-mfix-sb1-mno-fix-sb1— Work around certain SB-1 CPU core errata. (This flag currently works around the SB-1 revision 2"F1"and"F2"floating-point errata.)-mfix4300-mno-fix4300— Work around a bug in early VR4300 silicon that causes multiplies with certain operands to corrupt immediately following multiplies. 💀-mr10k-cache-barrier=setting — Specify whether GCC should insert cache barriers to avoid the side effects of speculation on R10K processors.
In common with many processors, the R10K tries to predict the outcome of a conditional branch and speculatively executes instructions from the "taken" branch. It later aborts these instructions if the predicted outcome is wrong. However, on the R10K, even aborted instructions can have side effects. 😭
This problem only affects kernel stores and, depending on the system, kernel loads. As an example, a speculatively-executed store may load the target memory into cache and mark the cache line as dirty, even if the store itself is later aborted. If a DMA operation writes to the same area of memory before the "dirty" line is flushed, the cached data overwrites the DMA-ed data. See the R10K processor manual for a full description, including other potential problems.
One workaround is to insert cache barrier instructions before every memory access that might be speculatively executed and that might have side effects even if aborted. -mr10k-cache-barrier=setting controls GCC's implementation of this workaround. It assumes that aborted accesses to any byte in the following regions does not have side effects:
- the memory occupied by the current function's stack frame;
- the memory occupied by an incoming stack argument;
- the memory occupied by an object with a link-time-constant address.
okay so basically it's the kernel's whole job to make sure speculative accesses to those three regions are actually safe 💖 no cap.
If the input program has a function declaration like this:
void foo (void);then the implementation of foo has to allow j foo and jal foo to be executed speculatively. GCC respects that restriction for functions it compiles itself, and it expects non-GCC functions (like hand-written assembly, bestie, be careful 😭) to do the same.
The option has three forms:
-mr10k-cache-barrier=load-store— insert a cache barrier before a load or store that might be speculatively executed and that might have side effects even if it gets aborted.-mr10k-cache-barrier=store— insert a cache barrier before a store that might be speculatively executed and that might have side effects even if it gets aborted.-mr10k-cache-barrier=none— disable inserting cache barriers entirely. This is the default, periodt.
the cache flush function one 🧚
-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 at all. If it does get called, the function has to take the same arguments as the common _flush_func, and those are: the address of the memory range for which the cache is being flushed, the size of the memory range, and the number 3 (which means flush both caches, iykyk). The default depends on the target GCC was configured for, but it's commonly either _flush_func or __cpu_flush 💅.
-mbranch-cost=num— set the cost of branches to roughlynum"simple" instructions. Heads up though: this cost is only a heuristic and it is not guaranteed to produce consistent results across releases. A zero cost redundantly selects the default, which is based on the-mtunesetting. fr fr, don't count on it being stable.-mbranch-likely-mno-branch-likely
Enable or disable use of Branch Likely instructions, regardless of whatever the default is for the architecture you picked. By default, Branch Likely instructions may be generated if the selected architecture supports them. The exception is MIPS32 and MIPS64 architectures and the processors that implement those architectures — for those, Branch Likely instructions are not generated by default, because MIPS32 and MIPS64 specifically deprecate their use. deprecated! 💀
-mcompact-branches=never-mcompact-branches=optimal-mcompact-branches=always
These control which form of branches get generated. The default is -mcompact-branches=optimal.
-mcompact-branches=nevermakes sure compact branch instructions are never generated.-mcompact-branches=alwaysmakes sure a compact branch instruction is generated if one is available for MIPS Release 6 onwards. If a compact branch instruction isn't available (or you're pre-R6), a delay slot form of the branch is used instead.- If it's used for MIPS16/microMIPS targets, it's just ignored right now. The behavior for MIPS16/microMIPS may change in the future, since they do actually have some compact branch instructions 👀.
-mcompact-branches=optimalcauses a delay slot branch to be used if one is available in the current ISA and the delay slot is successfully filled. If the delay slot is not filled, a compact branch gets chosen if one is available.-mfp-exceptions-mno-fp-exceptions
Specifies whether FP exceptions are enabled. This affects how FP instructions get scheduled for some processors. The default is that FP exceptions are enabled. For instance, on the SB-1, if FP exceptions are disabled and we're emitting 64-bit code, then we can use both FP pipes 🎀. Otherwise? Only one FP pipe, sad.
-mvr4130-align-mno-vr4130-align
The VR4130 pipeline is two-way superscalar, but it can only issue two instructions together if the first one is 8-byte aligned. When this option is enabled, GCC aligns pairs of instructions that it thinks should execute in parallel. This option only does anything when optimizing for the VR4130. It normally makes code faster, but at the expense of making it bigger (the classic tradeoff, bestie 😭). It's enabled by default at optimization level -O3.
-msynci-mno-synci
Enable (or disable) generation of synci instructions on architectures that support it. The synci instructions (if enabled) get generated when __builtin___clear_cache is compiled. This option defaults to -mno-synci, but the default can be overridden by configuring GCC with --with-synci.
When compiling code for single processor systems, it's generally safe to use synci. But on many multi-core (SMP) systems, it does not invalidate the instruction caches on all cores and may lead to undefined behavior. be so careful 💀
-mrelax-pic-calls-mno-relax-pic-calls
Try to turn PIC calls that are normally dispatched via register $25 into direct calls. This is only possible if the linker can resolve the destination at link time and if the destination is within range for a direct call. -mrelax-pic-calls is the default if GCC was configured to use an assembler and a linker that support the .reloc assembly directive and -mexplicit-relocs is in effect. With -mno-explicit-relocs, this optimization can be performed by the assembler and the linker alone, no compiler help needed 💅.
-mmcount-ra-address-mno-mcount-ra-address
Emit (or don't emit) code that allows _mcount to modify the calling function's return address. When enabled, this option extends the usual _mcount interface with a new ra-address parameter, which has type intptr_t * and is passed in register $12. _mcount can then modify the return address by doing both of these:
- Returning the new address in register
$31. - Storing the new address in
*ra-address, if ra-address is nonnull.
The default is -mno-mcount-ra-address.
-mframe-header-opt-mno-frame-header-opt
Enable (or disable) frame header optimization in the o32 ABI. When using the o32 ABI, calling functions allocate 16 bytes on the stack for the called function to write out register arguments. When enabled, this optimization suppresses the allocation of the frame header if it can be determined that it's unused. This optimization is off by default at all optimization levels — yes, even -O3, iconic behavior honestly.
-mlxc1-sxc1-mno-lxc1-sxc1
When applicable, enable (or disable) the generation of lwxc1, swxc1, ldxc1, sdxc1 instructions. Enabled by default ✨.
-mmadd4-mno-madd4
When applicable, enable (or disable) the generation of 4-operand madd.s, madd.d and related instructions. Enabled by default ✨.
🌈 MMIX Options 🌈
These options are defined for the MMIX:
-mlibfuncs-mno-libfuncs
Specify that intrinsic library functions are being compiled, passing all values in registers, no matter the size.
-mepsilon-mno-epsilon
Generate floating-point comparison instructions that compare with respect to the rE epsilon register.
-mabi=mmixware-mabi=gnu
Generate code that passes function parameters and return values that (in the called function) are seen as registers $0 and up, as opposed to the GNU ABI which uses global registers $231 and up.
-mzero-extend-mno-zero-extend
When reading data from memory in sizes shorter than 64 bits, use (or don't 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, fyi 👑
-mtoplevel-symbols-mno-toplevel-symbols
Prepend (or don't 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 (or don't use) the probable-branch instructions, when static branch prediction indicates a probable branch.
-mbase-addresses-mno-base-addresses
Generate (or don't 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. This generally leads to short and fast code, but the number of different data items that can be addressed is limited. Which means a program that uses lots of static data may require -mno-base-addresses.
-msingle-exit-mno-single-exit
Force (or don't force) generated code to have a single exit point in each function.
-mset-data-start=address
When linking, set the start of the data section to address.
-mset-program-start=address-mno-set-program-start
When linking, set the program start address to address. It defaults to 0x100 unless -mno-set-program-start is used to suppress this option entirely.
🦋 MN10300 Options 🦋
These -m options are defined for Matsushita MN10300 architectures:
-mmult-bug-mno-mult-bug
When enabled, generate code to avoid bugs in the multiply instructions for the MN10300 processors. This is the default.
-mam33-mno-am33
Generate code using features specific to the AM33 processor. The default is -mno-am33.
-mam33-2-mno-am33-2
Generate code using features specific to the AM33/2.0 processor. The default is -mno-am33-2.
-mam34-mno-am34
Generate code using features specific to the AM34 processor. The default is -mno-am34.
-mtune=cpu-type
Use the timing characteristics of the indicated CPU type when scheduling instructions. This does not change the targeted processor type, bestie, they're different things 💅. The CPU type must be one of mn10300, am33, am33-2 or am34.
-mreturn-pointer-on-d0,-mno-return-pointer-on-d0— so when you're generating a function that returns a pointer, this makes it hand the pointer back in botha0andd0. Otherwise bestie the pointer only comes back ina0, and then if you try to call a function like that without a prototype you get errors, which is giving chaos ngl. Heads up: this option is on by default, so if you want it off you gotta say-mno-return-pointer-on-d0. 💅-mno-crt0— do not link in the C run-time initialization object file. That's it, that's the whole thing. 🫶-mrelax— tells the linker to do a relaxation optimization pass so it can shorten branches, calls and absolute memory addresses. Lowkey only does anything when you put it on the command line for the final link step, so don't expect magic earlier. ⚠️ And fr fr: this option makes symbolic debugging impossible. Not "hard", impossible. Choose wisely.-mliw,-mno-liw— lets the compiler generate Long Instruction Word instructions if the target is the AM33 or later. Enabled by default, no cap.-mliwdefines the preprocessor macro__LIW__;-mno-liwdefines the preprocessor macro__NO_LIW__. ✨-msetlb,-mno-setlb— allows the compiler to generate theSETLBandLccinstructions if the target is the AM33 or later. Also enabled by default.-msetlbdefines the preprocessor macro__SETLB__;-mno-setlbdefines the preprocessor macro__NO_SETLB__. 🎀
🦋 MOXIE OPTIONS 🦋
-meb— generate big-endian code. This is the default formoxie-*-*configurations. 👑-mel— generate little-endian code.-mmul.x,-mno-mul.x— generatemul.xandumul.xinstructions. This one is enabled by default formoxiebox-*-*configurations. 🍓-mno-crt0— do not link in the C run-time initialization object file. (Yes bestie, you're seeing double, it really does show up again here.) 🌸
🌈 MSP430 OPTIONS 🌈
These options are defined for the MSP430, and there are a lot of them, so buckle up:
-masm-hex— forces assembly output to always use hex constants. Normally those constants are signed decimals, but this option exists for testsuite and/or aesthetic purposes. It's giving "I just like how it looks". 💅-mmcu=name— selects the MCU to target. This is used to create a C preprocessor symbol based upon the MCU name, converted to upper case and pre- and post-fixed with__. That symbol in turn is used by themsp430.hheader file to pick an MCU-specific supplementary header file. ✨
The option also sets the ISA to use. If the MCU name is one that is known to only support the 430 ISA then that is selected, otherwise the 430X ISA is selected. A generic MCU name of msp430 can also be used to select the 430 ISA. Similarly the generic msp430x MCU name selects the 430X ISA. 🧚
In addition an MCU-specific linker script is added to the linker command line. The script's name is the name of the MCU with .ld appended. Thus specifying -mmcu=xxx on the gcc command line defines the C preprocessor symbol __XXX__ and cause the linker to search for a script called xxx.ld. 🎀
The ISA and hardware multiply supported for the different MCUs is hard-coded into GCC. However, an external devices.csv file can be used to extend device support beyond those that have been hard-coded. 👑
GCC searches for the devices.csv file using the following methods in the given precedence order, where the first method takes precedence over the second which takes precedence over the third:
- Include path specified with
-Iand-L—devices.csvis searched for in each of the directories specified by include paths and linker library search paths. - Path specified by the environment variable
MSP430_GCC_INCLUDE_DIR— define the value of the global environment variableMSP430_GCC_INCLUDE_DIRto the full path to the directory containingdevices.csv, and GCC will search this directory fordevices.csv. Ifdevices.csvis found, this directory is also registered as an include path and linker library path. Header files and linker scripts in this directory can therefore be used without manually specifying-Iand-Lon the command line. - The
msp430-elf{,bare}/include/devicesdirectory — finally, GCC examinesmsp430-elf{,bare}/include/devicesfrom the toolchain root directory. This directory does not exist in a default installation, but if you have created it and copieddevices.csvthere, then the MCU data is read. As above, this directory is also registered as an include path and linker library path.
If none of the above search methods find devices.csv, then the hard-coded MCU data is used. 🫶
-mwarn-mcu,-mno-warn-mcu— this option enables or disables warnings about conflicts between the MCU name specified by the-mmcuoption and the ISA set by the-mcpuoption and/or the hardware multiply support set by the-mhwmultoption. It also toggles warnings about unrecognized MCU names. This option is on by default. 🦋-msim— link to the simulator runtime libraries and linker script. Overrides any scripts that would be selected by the-mmcu=option. 💀-mlarge— use large-model addressing (20-bit pointers, 20-bitsize_t).-msmall— use small-model addressing (16-bit pointers, 16-bitsize_t).-mrelax— this option is passed to the assembler and linker, and allows the linker to perform certain optimizations that cannot be done until the final link. ⭐mhwmult=type— describes the type of hardware multiply supported by the target. Accepted values for type arenonefor no hardware multiply,16bitfor the original 16-bit-only multiply supported by early MCUs,32bitfor the 16/32-bit multiply supported by later MCUs andf5seriesfor the 16/32-bit multiply supported by F5-series MCUs. A value ofautocan also be given. This tells GCC to deduce the hardware multiply support based upon the MCU name provided by the-mmcuoption. If no-mmcuoption is specified or if the MCU name is not recognized, then no hardware multiply support is assumed.autois the default setting. 🍓
Hardware multiplies are normally performed by calling a library routine. This saves space in the generated code. When compiling at -O3 or higher however the hardware multiplier is invoked inline. This makes for bigger, but faster code. 🔥
The hardware multiply routines disable interrupts whilst running and restore the previous interrupt state when they finish. This makes them safe to use inside interrupt handlers as well as in normal code. 💖
-minrt— enable the use of a minimum runtime environment without support for static initializers or constructors. This is intended for memory-constrained devices. The compiler includes special symbols in some objects that tell the linker and runtime which code fragments are required. 🌸-mtiny-printf— enable reduced code sizeprintfandputslibrary functions. The tiny implementations of these functions are not reentrant, so must be used with caution in multi-threaded applications. ⚠️
Support for streams has been removed and the string to be printed are always sent to stdout via the write syscall. The string is not buffered before it is sent to write. 👑
This option requires Newlib Nano IO, so GCC must be configured with --enable-newlib-nano-formatted-io. 🎀
-mmax-inline-shift=n— this option takes an integer n between 0 and 64 inclusive, and sets the maximum number of inline shift instructions which should be emitted to perform a shift operation by a constant amount. When this value needs to be exceeded, anmspabihelper function is used instead. The default value is4. ✨
This only affects cases where a shift by multiple positions cannot be completed with a single instruction (e.g. all shifts >1 on the 430 ISA). 🧚
Shifts of a 32-bit value are at least twice as costly, so the value passed for this option is divided by 2 and the resulting value used instead. 💅
-mcode-region=where,-mdata-region=where— these options tell the compiler where to place functions and data that do not have one of thelower,upper,eitherorsectionattributes. Possible values for where arelower,upper,eitherorany. The first three behave like the corresponding attribute. The fourth possible value,any, is the default. It leaves placement entirely up to the linker script and how it assigns the standard sections (.text,.data, etc) to the memory regions. 🫶-muse-lower-region-prefix— add the.lowerprefix to section names when compiling with-mcode-region=loweror-mdata-region=lower. Disabled by default. 🌈-msilicon-errata=name[,name...]— this option passes on a request to assembler to enable the fixes for the named silicon errata. Refer to the assembler documentation for details. 🔥-msilicon-errata-warn=name[,name...]— this option passes on a request to the assembler to enable warning messages when a named silicon errata might need to be applied. Refer to the assembler documentation for details. ⭐-mwarn-devices-csv,-mno-warn-devices-csv— warn ifdevices.csvis not found or there are problems parsing it (default: on). 💖
💜 NDS32 OPTIONS 💜
These options are defined for NDS32 implementations:
-mbig-endian,-EB— generate code in big-endian mode.-mlittle-endian,-EL— generate code in little-endian mode.-mabi=2,-mfloat-abi=soft— use the soft floating-point ABI.-mabi=2fp+,-mfloat-abi=hard— use the hard floating-point ABI. 💅-mreduced-regs— use reduced-set registers for register allocation.-mfull-regs— use full-set registers for register allocation.-malways-align— always align function entry, jump targets, and return address.-malign-functions— align function entry to 4 bytes. ✨-mfp-as-gp,-mno-fp-as-gp— enable/disablefp-as-gpoptimization.-mcmov,-mno-cmov— enable/disable generation of conditional move instructions.-mhw-abs,-mno-hw-abs— enable/disable generation of hardwareabsinstructions.-mext-perf,-mno-ext-perf— enable/disable generation of performance extension instructions.-mext-perf2,-mno-ext-perf2— enable/disable generation of performance extension 2 instructions.-mext-string,-mno-ext-string— enable/disable generation of string extension instructions.-mext-dsp,-mno-ext-dsp— enable/disable generation of DSP extension instructions.-mext-fpu-fma,-mno-ext-fpu-fma— enable/disable generation of floating-point multiply-accumulation instructions. 🦋-mext-fpu-sp,-mno-ext-fpu-sp— turns generation of single-precision floating-point instructions on/off. ✨-mext-fpu-dp,-mno-ext-fpu-dp— same vibe but for double-precision floating-point instructions. 💅-mv3push,-mno-v3push— on/off switch for generating v3push25/pop25instructions.-m16-bit,-mno-16-bit— enables/disables generation of 16-bit instructions. 🎀-mvh,-mno-vh— enables/disables Virtual Hosting support. 🦋-misr-vector-size=num— sets the size of each interrupt vector, and bestie, it must be4or16, no in-between.-misr-secure=num— sets the security level ofc-isrfor the whole file.-mcache-block-size=num— sets the size of each cache block, which has to be a power of 2 between4and512.-march=arch— picks the target architecture. arch can be one ofv2,v3,v3j,v3m,v3f, orv3s.-mcpu=cpu— tells it which CPU to compile for. cpu can be one ofn6,n650,n7,n705,n8,n801,sn8,sn801,s8,s801,e8,e801,n820,s830,e830,n9,n903,n903a,n968,n968a,n10,n1033,n1033a,n1033-fpu,n1033-spu,n1068,n1068a,n1068-fpu,n1068a-fpu,d10,d1088,d1088-fpu,d1088-spu,n15,d15,n15s,d15s,n15f,d15f,n12,n1213,n1233,n1233-fpu,n1233-spu,n13,n1337,n1337-fpu,n1337-spu, orsimple. yes that's a lot, we love options 👑-mconfig-fpu=num— sets an FPU configuration value from0to7;0--3is as FPU spec says, and4--7correspond to0--3.-mconfig-mul=type— configuration for the multiply instruction. type can be one offast,fast1(equivalent tofast),fast2, orslow. The default isfast1. 💖-mconfig-register-ports=kind— sets the numbers of read/write ports forn9/n10cores. The value should be3r2wor2r1w.-mcmodel=code-model— sets the code model to one of these lil guys:small— all the data and read-only data segments must be within512KBaddressing space, and the text segment must be within16MBaddressing space.medium— the data segment must be within512KBwhile the read-only data segment can be within4GBaddressing space. The text segment should still be within16MBaddressing space.large— all the text and data segments can be within4GBaddressing space.
The default is large on GNU/Linux targets and medium on other ELF targets.
-mctor-dtor,-mno-ctor-dtor— enables/disables the constructor/destructor feature. 🎀-mrelax,-mno-relax— enables/disables the linker option to relax instructions.-mrelax-hint,-mno-relax-hint— enables/disables insertion of hints for the linker to do relaxation.-msched-prolog-epilog,-mno-sched-prolog-epilog— enables/disables scheduling of a function's prologue and epilogue sequence.-mret-in-naked-func,-mno-ret-in-naked-func— enables/disables generation of return instructions in naked functions. This one is enabled by default, so it's already slaying. 💅-malways-save-lp,-mno-always-save-lp— enables/disables always saving$lpon the stack.-munaligned-access,-mno-unaligned-access— enables/disables unaligned word and halfword accesses to packed data.-minline-asm-r15,-mno-inline-asm-r15— allows/disallows use ofr15for inline asm.
🎮 Nvidia PTX Options 🎮
These options are defined for Nvidia PTX, bestie:
-m64— ignored, but preserved for backward compatibility. Only 64-bit ABI is supported, so like… it doesn't do anything, it's just here for the vibes. 🫶-march=architecture-string— generates code for the specified PTX ISA target architecture. Valid architecture strings aresm_30,sm_35,sm_37,sm_50,sm_52,sm_53,sm_61,sm_70,sm_75,sm_80, andsm_89. The default depends on how the compiler has been configured, see--with-arch.
This option sets the value of the preprocessor macro __PTX_SM__; for instance, for sm_35, it has the value 350. ✨
-misa=architecture-string— alias of-march=. twin behavior, no cap.-march-map=architecture-string— selects the closest available-march=value that is not more capable. For instance, for-march-map=sm_50select-march=sm_37, and for-march-map=sm_53select-march=sm_53.-mptx=version-string— generates code for the specified PTX ISA version. Valid version strings are3.1,4.0,4.1,4.2,5.0,6.0,6.3,7.0,7.3, and7.8. The default PTX ISA version is the one that added support for the selected PTX ISA target architecture, see-march=, but at least6.3, or7.3for-march=sm_52and higher.
This option sets the values of the preprocessor macros __PTX_ISA_VERSION_MAJOR__ and __PTX_ISA_VERSION_MINOR__; for instance, for 3.1 the macros have the values 3 and 1, respectively. 💜
-mmainkernel— links in code for a__mainkernel. This is for stand-alone instead of offloading execution.-moptimize,-mno-optimize— enables/disables partitioned execution optimizations. This option is enabled by default when any level of optimization is selected. 🔥-msoft-stack,-mno-soft-stack— okay so this one's got layers, stay with me:
For -mno-soft-stack (the default, unless -mgomp has been specified), use PTX "native" stacks, that is, generate code that uses .local memory or PTX "alloca" directly for stack storage. Unless -mptx=7.3 or higher and -march=sm_52 or higher are active, variable-length arrays and dynamically allocating memory on the stack with "alloca" are not supported. 💀
For -msoft-stack (implied by -mgomp), generate code that does not use .local memory or PTX "alloca" directly for stack storage. Instead, a per-warp stack pointer is maintained explicitly. This enables variable-length stack allocation (with variable-length arrays or "alloca"), and when global memory is used for underlying storage, makes it possible to access automatic variables from other threads, or with atomic instructions. This code generation variant is used for OpenMP offloading, but the option is exposed on its own for the purpose of testing the compiler; to generate code suitable for linking into programs using OpenMP offloading, use option -mgomp.
-msoft-stack-reserve-local=size— sets the size of.localmemory used for the stack when the exact amount is not known. It defaults to128. 🌸-muniform-simt,-mno-uniform-simt— enables/disables a code generation variant that allows execution of all threads in each warp, while maintaining memory state and side effects as if only one thread in each warp was active outside of OpenMP SIMD regions. All atomic operations and calls to runtime (malloc,free,vprintf) are conditionally executed (iff current lane index equals the master lane index), and the register being assigned is copied via a shuffle instruction from the master lane. Outside of SIMD regions lane 0 is the master; inside, each thread sees itself as the master. Shared memory arrayint __nvptx_uni[]stores all-zeros or all-ones bitmasks for each warp, indicating current mode (0outside of SIMD regions). Each thread can bitwise-and the bitmask at positiontid.ywith current lane index to compute the master lane index. 😭 (this one is a whole essay fr fr)-mgomp,-mno-gomp— enables/disables generation of code for use in OpenMP offloading.-mgompenables-msoft-stackand-muniform-simtoptions, and selects a corresponding multilib variant.
🌈 OpenRISC Options 🌈
These options are defined for OpenRISC:
-mboard=name— configures a board specific runtime. This is passed to the linker for newlib board library linking. The default isor1ksim.-msoft-div,-mhard-div— selects software or hardware divide (l.div,l.divu) instructions. This default is hardware divide.-msoft-mul,-mhard-mul— selects software or hardware multiply (l.mul,l.muli) instructions. This default is hardware multiply.-msoft-float,-mhard-float— selects software or hardware for floating point operations. The default is software. ✨-mdouble-float— when-mhard-floatis selected, enables generation of double-precision floating point instructions. By default functions fromlibgccare used to perform double-precision floating point operations.-munordered-float— when-mhard-floatis selected, enables generation of unordered floating point compare and set flag (lf.sfun*) instructions. By default functions fromlibgccare used to perform unordered floating point compare and set flag operations.-mcmov— enables generation of conditional move (l.cmov) instructions. By default the equivalents are generated using set and branch.-mror— enables generation of rotate right (l.ror) instructions. By default functions fromlibgccare used to perform rotate right operations.-mrori— enables generation of rotate right with immediate (l.rori) instructions. By default functions fromlibgccare used to perform rotate right with immediate operations.-msext— enables generation of sign extension (l.ext*) instructions. By default memory loads are used to perform sign extension.-msfimm— enables generation of compare and set flag with immediate (l.sf*i) instructions. By default extra instructions are generated to store the immediate to a register first.-mshftimm— enables generation of shift with immediate (l.srai,l.srli,l.slli) instructions. By default extra instructions are generated to store the immediate to a register first.-mcmodel=small— generates OpenRISC code for the small model: the GOT is limited to64kand function call jumps are limited to64Moffsets. This is the default model. 👑-mcmodel=large— generates OpenRISC code for the large model: the GOT may grow up to4Gin size and function call jumps can target the full4Gaddress space.
🦋 PDP-11 Options 🦋
These options are defined for the PDP-11, yes the ancient one, and she's still iconic:
-mfpu— uses hardware FPP floating point. This is the default. (FIS floating point on the PDP-11/40 is not supported.) Implies-m45.-msoft-float— do not use hardware floating point.-mac0,-mno-ac0— with-mac0, return floating-point results inac0(fr0in Unix assembler syntax). The default,-mno-ac0, is to return floating-point results in memory. 💅-m40— generates code for a PDP-11/40. Implies-msoft-float -mno-split.-m45— generates code for a PDP-11/45. This is the default.-m10— generates code for a PDP-11/10. Implies-msoft-float -mno-split.-mint16,-mno-int32— bestie, this makesinta 16-bit lil guy. This is the default. ✨-mint32,-mno-int16— and this one makesinta 32-bit queen. 💅-msplit— the target has split instruction and data space, okay? Implies-m45. 🦋-munix-asm— use Unix assembler syntax. 🎀-mdec-asm— use DEC assembler syntax. 🍓-mgnu-asm— use GNU assembler syntax. This is the default. ✨-mlra— use the new LRA register allocator. By default, the oldreloadallocator is used, no cap. 🫶
✨ Picolibc Options ✨
These options control compilation and linking when using Picolibc, and honestly? they're giving build-system main character energy: 💖
--oslib=library— search the library named library after the C library, which lets symbols the C library left undefined get defined by this library instead. The C library,libgccand this library are all placed between--start-groupand--end-groupflags so that each one can refer to symbols in the others. For many targets, Picolibc provides a semihost variant (you ask for it with--oslib=semihost) which provides enough basic OS functionality to support console and file I/O when it's run in an emulator or when you're using an in-circuit debugger. 🔥--crt0=[none|minimal|hosted|semihost]— swaps the defaultcrt0.oname out forcrt0-variant.o. Let's break it down, bestie:- The
nonevariant provides no startup code at all, which lets you supply your own. Very DIY of it. minimalperforms basic memory setup but does not invoke any constructors.- When no
-crt0option is provided, the default initialization code adds calls to all constructors. hostedadds a call toexitwhenmainreturns. 🎀semihostaccesses a command-line parameter supplied via the semihosting interface and splits that into arguments at whitespace boundaries, passing the resulting array of strings tomaininargcandargv. On some targets, including AArch64, ARC, ARM, LoongArch, M680x0, RISC-V, SH, and x86,semihostalso traps hardware exceptions and prints information to the console. 👑
- The
Heads up:--crt0=semihostdepends upon APIs provided by--oslib=semihost. They're a package deal, fr fr. 💀
--printf=[d|f|l|i|m]— pick yourprintfvariant. Picolibc provides five differentprintfvariants which offer decreasing levels of functionality along with decreasing code size, so it's a whole vibe check:dis the default level, offering full C17 and POSIX.1-2024 conformance. ⭐fprovides the same feature set, but supportsfloatvalues instead ofdoublewhich are passed using theprintf_floatmacro.lelides all floating-point and POSIX positional parameter support.ilimits integers to those no larger thanlong.mremoves support for most formatting options including width and precision. The formats and arguments are still parsed correctly, but output does not respect those parameters. 😭
--scanf=[d|f|l|i|m]— and here's thescanftwin, same five-tier situation, decreasing levels of functionality along with decreasing code size:dis the default level, offering full C17 and POSIX.1-2024 conformance. 💜fremoves support fordoublevalues.lelides all floating point support.ilimits integers to those no larger thanlong.mremoves support for%[conversion specifiers. 🦋
✨ PowerPC Options ✨
These are listed under
✨ PRU Options ✨
These command-line options are defined for PRU target, bestie, and they're kinda niche but lowkey iconic: 🌸
-minrt— link with a minimum runtime environment. This can significantly reduce the size of the final ELF binary, but some standard C runtime features are removed. 🙈
This option disables support for static initializers and constructors. Beware that the compiler could still generate code with static initializers and constructors. It is up to the programmer to ensure that the source program does not use those features. No cap, that's on you. 💅
The minimal startup code does not pass argc and argv arguments to main, so the latter must be declared as int main (void). This is already the norm for most firmware projects, so honestly? slay.
-mmcu=mcu— specify the PRU hardware variant to use. A correspondingly-named spec file is loaded, passing the memory region sizes to the linker and defining hardware-specific C macros. ✨
Newlib provides only the sim spec, intended for running regression tests using a simulator. Specs for real hardware can be obtained by installing the GnuPruMcu (https://github.com/dinuxbg/gnuprumcu/) package. 🎀
-mno-relax— make GCC pass the--no-relaxcommand-line option to the linker instead of the--relaxoption. 🫶-mloop,-mno-loop— allow (or do not allow) GCC to use the LOOP instruction. 👑-mmul,-mno-mul— allow (or do not allow) GCC to use the PRU multiplier unit. ✖️-mfillzero,-mno-fillzero— allow (or do not allow) GCC to use the FILL and ZERO instructions. 🌈-mabi=variant— specify the ABI variant to output code for.-mabi=tiselects the unmodified TI ABI, while-mabi=gnuselects a GNU variant that copes more naturally with certain GCC assumptions. These are the differences, and there are a few, so buckle up:- Function Pointer Size — TI ABI specifies that function (code) pointers are 16-bit, whereas GNU supports only 32-bit data and code pointers. Literally different sizes, bestie.
- Optional Return Value Pointer — function return values larger than 64 bits are passed by using a hidden pointer as the first argument of the function. TI ABI, though, mandates that the pointer can be
NULLin case the caller is not using the returned value. GNU always passes and expects a valid return value pointer. 🦋 - Size Of Struct Containing Bit-fields — TI ABI mandates that struct size is determined by the bit-field type, if it contains any. On the other hand, GNU allocates the smallest amount of bytes which would fit the bit-field. For example, TI ABI reserves 4 bytes for this struct, whereas GNU reserves a single byte:
struct S { int i:1; };- Access Size For Volatile Bit-fields — TI ABI mandates that volatile bit-fields are accessed using their type. In contrast, GNU ABI uses the smallest integer type fitting the bit-field. For example, TI ABI requires a single load of 4 bytes for the following bit-field. GNU generates a load of 1 byte:
struct S { volatile int i:1; };The current -mabi=ti implementation simply raises a compile error when any of the above code constructs is detected. As a consequence the standard C library cannot be built and it is omitted when linking with -mabi=ti. 😭
Relaxation is a GNU feature and for safety reasons is disabled when using -mabi=ti. The TI toolchain does not emit relocations for QBBx instructions, so the GNU linker cannot adjust them when shortening adjacent LDI32 pseudo instructions.
✨ RISC-V Options ✨
These command-line options are defined for RISC-V targets, and they're giving architecture-nerd excellence: 💖
-mbranch-cost=n— set the cost of branches to roughly n instructions. 🔥-mabi=ABI-string— specify integer and floating-point calling convention. ABI-string contains two parts: the size of integer types and the registers used for floating-point types. For example-march=rv64ifd -mabi=lp64dmeans that long and pointers are 64-bit (implicitly defining int to be 32-bit), and that floating-point values up to 64 bits wide are passed in F registers. Contrast this with-march=rv64ifd -mabi=lp64f, which still allows the compiler to generate code that uses the F and D extensions but only allows floating-point values up to 32 bits long to be passed in registers; or-march=rv64ifd -mabi=lp64, in which no floating-point arguments are passed in registers. 🧚
The default for this argument is system dependent; if you want a specific calling convention you should specify one explicitly. The valid calling conventions are: ilp32, ilp32f, ilp32d, lp64, lp64f, and lp64d. Some calling conventions are impossible to implement on some ISAs: for example, -march=rv32if -mabi=ilp32d is invalid because the ABI requires 64-bit values be passed in F registers, but F registers are only 32 bits wide. There are also the ilp32e ABI that can only be used with the rv32e architecture and the lp64e ABI that can only be used with the rv64e. Those ABIs are not well-specified at present, and are subject to change. You've been warned. 💅
-mfdiv,-mno-fdiv— do or don't use hardware floating-point divide and square root instructions. This requires the F or D extensions for floating-point registers. The default is to use them if the specified architecture has these instructions. ✨-mfence-tso,-mno-fence-tso— do or don't use thefence.tsoinstruction, which is unimplemented on some processors (including those from T-Head). If thefence.tsoinstruction is not available then a stronger fence is used instead. 🫶-mdiv,-mno-div— do or don't use hardware instructions for integer division. This requires the M extension. The default is to use them if the specified architecture has these instructions. 🌸-misa-spec=ISA-spec-string— specify the version of the RISC-V Unprivileged (formerly User-Level) ISA specification generated code should conform to. The possibilities for ISA-spec-string are, and yes, one of them is literally just2.2:2.2— produce code conforming to version 2.2.20190608— produce code conforming to version 20190608.20191213— produce code conforming to version 20191213.
The default is -misa-spec=20191213 unless GCC has been configured with --with-isa-spec= specifying a different default version. 👑
-march=[ISA|Profile|Profile_ISA|processor-string]— generate code for given RISC-V ISA or profile or a combination of them (e.g.rv64im rvi20u64 rvi20u64_zbb). The names of ISAs and profiles must be lower case. Examples includerv64i,rv32g,rv32e,rv32imaf,rva22u64andrva23u64. To combine a named profile with optional RISC-V ISA extensions, give the profile first and then append the extension name(s) using an underscore as a delimiter (e.g.rvi20u64_zca_zcb rva23u64_zacas). Additionally, a special value help (-march=help) is accepted to list all supported extensions. 🌈
-march=unset causes the compiler to ignore any -march=... options that appear earlier on the command line, behaving as if the option was never passed. This is useful for ensuring that the architecture is taken from the -mcpu option, and an error results if no -mcpu option is given when -march=unset is used. 🦋
The syntax of the ISA string is defined as follows:
- the string has to start with
rv32orrv64, and then right after that comesi,e, org— that lil trio is what we call the base ISA 💅 - the rest of the string is a whole list of extension names. Extension names come in two flavours: multi-letter (think
zba) and single-letter (thinkv). Single-letter ones can be stacked back-to-back no problem, but multi-letter ones must be separated by underscores, bestie, don't forget 🫶 - an underscore can show up anywhere after the base ISA. It literally does nothing functional, it's just there to make stuff readable and to act as a separator ✨
- extension names can carry an optional version number, written as
<major>p<minor>or just<major>— so likem2p1orm2🎀
Supported extensions are listed below: 👑
g— version:-— General-purpose computing base extension;gexpands toi,m,a,f,d,zicsrandzifencei.e— version:2.0— Reduced base integer extension.i— versions:2.02.1— Base integer extension.m— version:2.0— Integer multiplication and division extension.a— versions:2.02.1— Atomic extension.f— versions:2.02.2— Single-precision floating-point extension.d— versions:2.02.2— Double-precision floating-point extension.c— version:2.0— Compressed extension.b— version:1.0— Standard extension for bit manipulation functions.v— version:1.0— Vector extension.h— version:1.0— Hypervisor extension.zic64b— version:1.0— Cache block size is 64 bytes.zicbom— version:1.0— Cache-block management extension.zicbop— version:1.0— Cache-block prefetch extension.zicboz— version:1.0— Cache-block zero extension.ziccamoa— version:1.0— Main memory supports all atomics in A.ziccif— version:1.0— Main memory supports instruction fetch with atomicity requirement.zicclsm— version:1.0— Main memory supports misaligned loads/stores.ziccrse— version:1.0— Main memory supports forward progress on LR/SC sequences.zicfilp— version:1.0— Control-flow integrity landing pad extension.zicfiss— version:1.0— Control-flow integrity shadow stack extension.zicntr— version:2.0— Standard extension for base counters and timers.zicond— version:1.0— Integer conditional operations extension.zicsr— version:2.0— Control and status register access extension.zifencei— version:2.0— Instruction-fetch fence extension.zihintntl— version:1.0— Non-temporal locality hints extension.zihintpause— version:2.0— Pause hint extension.zihpm— version:2.0— Standard extension for hardware performance counters.zimop— version:1.0— May-be-operations extension.zilsd— version:1.0— Load/store pair instructions extension.zmmul— version:1.0— Integer multiplication extension.za128rs— version:1.0— Reservation set size of 128 bytes.za64rs— version:1.0— Reservation set size of 64 bytes.zaamo— version:1.0— Atomic memory operations extension.zabha— version:1.0— Byte and halfword atomic memory operations extension.zacas— version:1.0— Atomic compare-and-swap instructions extension.zalrsc— version:1.0— Load-reserved/store-conditional subset of the A extension.zawrs— version:1.0— Wait-on-reservation-set extension.zama16b— version:1.0— Misaligned loads, stores, and AMOs that are fully contained within a naturally-aligned 16-byte boundary are atomic.zfa— version:1.0— Additional floating-point extension.zfbfmin— version:1.0— Minimal BF16 support extension.zfh— version:1.0— Half-precision floating-point extension.zfhmin— version:1.0— Minimal half-precision floating-point extension.zfinx— version:1.0— Single-precision floating-point in integer registers extension.zdinx— version:1.0— Double-precision floating-point in integer registers extension.zca— version:1.0— Integer compressed instruction extension.zcb— version:1.0— Simple compressed instruction extension.zcd— version:1.0— Compressed double-precision floating point loads and stores extension.zce— version:1.0— Compressed instruction extensions for embedded processors.zcf— version:1.0— Compressed single-precision floating point loads and stores extension.zcmop— version:1.0— Compressed may-be-operations extension.zcmp— version:1.0— Compressed push pop extension.zcmt— version:1.0— Table jump instruction extension.zclsd— version:1.0— Compressed load/store pair instructions extension.zba— version:1.0— Address calculation extension.zbb— version:1.0— Basic bit manipulation extension.zbc— version:1.0— Carry-less multiplication extension.zbkb— version:1.0— Cryptography bit-manipulation extension.zbkc— version:1.0— Cryptography carry-less multiply extension.zbkx— version:1.0— Cryptography crossbar permutation extension.zbs— version:1.0— Single-bit operation extension.zk— version:1.0— Standard scalar cryptography extension.zkn— version:1.0— NIST algorithm suite extension.zknd— version:1.0— AES Decryption extension.zkne— version:1.0— AES Encryption extension.zknh— version:1.0— Hash function extension.zkr— version:1.0— Entropy source extension.zks— version:1.0— ShangMi algorithm suite extension.zksed— version:1.0— SM4 block cipher extension.zksh— version:1.0— SM3 hash function extension.zkt— version:1.0— Data independent execution latency extension.ztso— version:1.0— Total store ordering extension.zvbb— version:1.0— Vector basic bit-manipulation extension.zvbc— version:1.0— Vector carry-less multiplication extension.zve32f— version:1.0— Vector extensions for embedded processors.zve32x— version:1.0— Vector extensions for embedded processors.zve64d— version:1.0— Vector extensions for embedded processors.zve64f— version:1.0— Vector extensions for embedded processors.zve64x— version:1.0— Vector extensions for embedded processors.zvfbfmin— version:1.0— Vector BF16 converts extension.zvfbfwma— version:1.0— Vector BF16 widening multiply/add extension.zvfh— version:1.0— Vector half-precision floating-point extension.zvfhmin— version:1.0— Vector minimal half-precision floating-point extension.zvkb— version:1.0— Vector cryptography bit-manipulation extension.zvkg— version:1.0— Vector GCM/GMAC extension.zvkn— version:1.0— Vector NIST Algorithm Suite extension,zvknwill expand to.zvknc— version:1.0— Vector NIST Algorithm Suite with carry-less multiply extension,zvknc.zvkned— version:1.0— Vector AES block cipher extension.zvkng— version:1.0— Vector NIST Algorithm Suite with GCM extension,zvkngwill expand.zvknha— version:1.0— Vector SHA-2 secure hash extension.zvknhb— version:1.0— Vector SHA-2 secure hash extension.zvks— version:1.0— Vector ShangMi algorithm suite extension,zvkswill expand.zvksc— version:1.0— Vector ShangMi algorithm suite with carry-less multiplication extension,.zvksed— version:1.0— Vector SM4 block cipher extension.zvksg— version:1.0— Vector ShangMi algorithm suite with GCM extension.zvksh— version:1.0— Vector SM3 secure hash extension.zvkt— version:1.0— Vector data independent execution latency extension.zvl1024b— version:1.0— Minimum vector length standard extensions.zvl128b— version:1.0— Minimum vector length standard extensions.zvl16384b— version:1.0— Minimum vector length standard extension.zvl2048b— version:1.0— Minimum vector length standard extensions.zvl256b— version:1.0— Minimum vector length standard extensions.zvl32768b— version:1.0— Minimum vector length standard extension.zvl32b— version:1.0— Minimum vector length standard extensions.zvl4096b— version:1.0— Minimum vector length standard extensions.zvl512b— version:1.0— Minimum vector length standard extensions.zvl64b— version:1.0— Minimum vector length standard extensions.zvl65536b— version:1.0— Minimum vector length standard extension.zvl8192b— version:1.0— Minimum vector length standard extension.zhinx— version:1.0— Half-precision floating-point in integer registers extension.zhinxmin— version:1.0— Minimal half-precision floating-point in integer registers extension.sdtrig— version:1.0— Debug triggers extension.sha— version:1.0— The augmented hypervisor extension.shcounterenw— version:1.0— Support writeable enables for any supported counter.shgatpa— version:1.0—SvNNx4mode supported for all modes supported bysatp.shlcofideleg— version:1.0— Delegating LCOFI interrupts to VS-mode.shtvala— version:1.0— Thehtvalregister provides all needed values.shvstvala— version:1.0— Thevstvalregister provides all needed values.shvstvecd— version:1.0— Thevstvecregister supports direct mode.shvsatpa— version:1.0— Thevsatpregister supports all modes supported bysatp.smaia— version:1.0— Advanced interrupt architecture extension.smcntrpmf— version:1.0— Cycle and instret privilege mode filtering.smcsrind— version:1.0— Machine-level indirect CSR access.
smepmp(version1.0) — PMP enhancements for memory access and execution prevention on machine modesmmpm(version1.0) — Supervisor-mode pointer masking extensionsmnpm(version1.0) — Supervisor-mode pointer masking extensionsmrnmi(version1.0) — Resumable non-maskable interruptssmstateen(version1.0) — State enable extensionsmdbltrp(version1.0) — Double trap extensionsssaia(version1.0) — Advanced interrupt architecture extension for supervisor modessccptr(version1.0) — Main memory supports page table readssscofpmf(version1.0) — Count overflow and filtering extensionsscounterenw(version1.0) — Support writeable enables for any supported countersscsrind(version1.0) — Supervisor-mode indirect CSR accessssnpm(version1.0) — Supervisor-mode pointer masking extensionsspm(version1.0) — Supervisor-mode pointer masking extensionssstateen(version1.0) — Supervisor-mode state-enable extensionsstc(version1.0) — Supervisor-mode timer interrupts extensionsstvala(version1.0) — Stval provides all needed valuessstvecd(version1.0) — Stvec supports direct modessstrict(version1.0) — Unimplemented reserved encodings raise illegal instruction exceptions and no non-conforming extensions are presentssdbltrp(version1.0) — Double trap extensionsssu64xl(version1.0) — UXLEN=64 must be supportedsupm(version1.0) — User-mode pointer masking extensionsvinval(version1.0) — Fine-grained address-translation cache invalidation extensionsvnapot(version1.0) — NAPOT translation contiguity extensionsvpbmt(version1.0) — Page-based memory types extensionsvvptc(version1.0) — Extension for obviating memory-management instructions after marking PTEs validsvadu(version1.0) — Hardware updating of A/D bits extensionsvade(version1.0) — Cause exception when hardware updating of A/D bits is disabledsvbare(version1.0) — Satp mode bare is supportedxcvalu(version1.0) — Core-V miscellaneous ALU extensionxcvbi(version1.0) — Core-V immediate branch extensionxcvelw(version1.0) — Core-V event load word extensionxcvmac(version1.0) — Core-V multiply-accumulate extensionxcvsimd(version1.0) — Core-V SIMD extensionxsfcease(version1.0) — SiFive CEASE instruction extensionxsfvcp(version1.0) — SiFive VCIX vector coprocessor extensionxsfvfnrclipxfqf(version1.0) — SiFive FP32-to-int8 ranged clip instructionsxsfvqmaccdod(version1.0) — SiFive int8 matrix multiplication extensionxsfvqmaccqoq(version1.0) — SiFive int8 matrix multiplication extensionxtheadba(version1.0) — T-head address calculation extensionxtheadbb(version1.0) — T-head basic bit-manipulation extensionxtheadbs(version1.0) — T-head single-bit instructions extensionxtheadcmo(version1.0) — T-head cache management operations extensionxtheadcondmov(version1.0) — T-head conditional move extensionxtheadfmemidx(version1.0) — T-head indexed memory operations for floating-point registers extensionxtheadfmv(version1.0) — T-head double floating-point high-bit data transmission extensionxtheadint(version1.0) — T-head acceleration interruption extensionxtheadmac(version1.0) — T-head multiply-accumulate extensionxtheadmemidx(version1.0) — T-head indexed memory operation extensionxtheadmempair(version1.0) — T-head two-GPR memory operation extensionxtheadsync(version1.0) — T-head multi-core synchronization extensionxtheadvector(version1.0) — T-head vector extensionxventanacondops(version1.0) — Ventana integer conditional operations extensionxmipscmov(version1.0) — Mips conditional move extensionxmipscbop(version1.0) — Mips prefetch extensionxandesperf(version5.0) — Andes performace extensionxandesbfhcvt(version5.0) — Andes bfloat16 conversion extensionxandesvbfhcvt(version5.0) — Andes vector bfloat16 conversion extensionxandesvsintload(version5.0) — Andes vector INT4 load extensionxandesvpackfph(version5.0) — Andes vector packed FP16 extensionxandesvdot(version5.0) — Andes vector dot product extensionxsmtvdot(version1.0) — SpacemiT vector dot product extension
When -march= is not specified, GCC uses the setting from -mcpu.
If both -march and -mcpu= are not specified, the default for this argument is system dependent; if you want a specific architecture extension, you should specify one explicitly. periodt 💅
When the RISC-V specifications define an extension as depending on other extensions, GCC implicitly adds the dependent extensions to the enabled extension set if they weren't added explicitly. like it's doing the friend-group math for you 🫶
Core Name 👑
-mcpu=processor-string— Use architecture of and optimize the output for the given processor, specified by particular CPU name. Permissible values for this option are:sifive-e20,sifive-e21,sifive-e24,sifive-e31,sifive-e34,sifive-e76,sifive-s21,sifive-s51,sifive-s54,sifive-s76,sifive-u54,sifive-u74,sifive-x280,sifive-p450,sifive-p670,thead-c906,xt-c908,xt-c908v,xt-c910,xt-c910v2,xt-c920,xt-c920v2,tt-ascalon-x,tt-ascalon-d8,xiangshan-nanhu,xiangshan-kunminghu,mips-p8700,andes-n22,andes-n25,andes-a25,andes-nx25,andes-ax25,andes-a27,andes-ax27,andes-n225,andes-d23,andes-n45,andes-nx45,andes-a45,andes-ax45,spacemit-x60.
Note that -mcpu does not override -march or -mtune. no cap, it just doesn't 💀
Tune Name 🎀
-mtune=processor-string— Optimize the output for the given processor, specified by microarchitecture or particular CPU name. Permissible values for this option are:generic,rocket,sifive-3-series,sifive-5-series,sifive-7-series,sifive-p400-series,sifive-p600-series,tt-ascalon-x,tt-ascalon-d8,thead-c906,xt-c908,xt-c908v,xt-c910,xt-c910v2,xt-c920,xt-c920v2,xiangshan-nanhu,xiangshan-kunminghu,spacemit-x60,generic-ooo,size,mips-p8700,andes-25-series,andes-23-series,andes-45-series,- and all valid options for
-mcpu=.
When -mtune= is not specified, GCC uses the setting from -mcpu. The default is generic if neither is specified.
The size choice is not intended for use by end-users. This is used when -Os is specified. It overrides the instruction cost info provided by -mtune=, but does not override the pipeline info. This helps reduce code size while still giving good performance.
-mpreferred-stack-boundary=num— Attempt to keep the stack boundary aligned to a 2 raised to num byte boundary. If-mpreferred-stack-boundaryis not specified, the default is4(16 bytes or 128-bits).
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. bestie do NOT mix and match this one 😭
-msmall-data-limit=n— Put global and static data smaller than n bytes into a special section (on some targets).-msave-restore,-mno-save-restore— Do or don't use smaller but slower prologue and epilogue code that uses library function calls. The default is to use fast inline prologues and epilogues.-mmovcc,-mno-movcc— Do or don't produce branchless conditional-move code sequences even with targets that do not have specific instructions for conditional operations. If enabled, sequences of ALU operations are produced using base integer ISA instructions where profitable.-minline-atomics,-mno-inline-atomics— Do or don't use smaller but slower subword atomic emulation code that uses libatomic function calls. The default is to use fast inline subword atomics that do not require libatomic.-minline-strlen,-mno-inline-strlen— Do or do not attempt to inline"strlen"calls if possible. Inlining can only be done if the string is properly aligned and instructions for accelerated processing are available. The default is to inline"strlen"calls.-minline-strcmp,-mno-inline-strcmp— Do or do not attempt to inline"strcmp"calls if possible. Inlining can only be done if the strings are properly aligned and instructions for accelerated processing are available. The default is to inline"strcmp"calls.-minline-strncmp,-mno-inline-strncmp— Do or do not attempt to inline"strncmp"calls if possible. Inlining can only be done if the strings are properly aligned and instructions for accelerated processing are available. The default is to inline"strncmp"calls.-mstringop-strategy=strategy— Specify a particular strategy for inlining string and memory operations. strategy may be one ofauto,libcall,scalar, orvector. four little vibes, pick your fighter ✨-mshorten-memrefs,-mno-shorten-memrefs— Do or do not attempt to make more use of compressed load/store instructions by replacing a load/store of 'base register + large offset' with a new load/store of 'new base + small offset'. If the new base gets stored in a compressed register, then the new load/store can be compressed. Currently targets 32-bit integer load/stores only. tiny instructions, big slay 🦋-mstrict-align-mno-strict-align— okay so this one is like, do we generate unaligned memory accesses or not? The default depends on whether the processor we're optimizing for supports fast unaligned access. So like, if your chip eats unaligned accesses for breakfast, GCC's gonna let it 💅-mscalar-strict-align-mno-scalar-strict-align— same energy bestie, do not or do generate unaligned memory accesses, default set by whether the processor we're optimizing for supports fast unaligned access or not. And plot twist: this is literally just an alias for-mstrict-align. same thing, different name, iconic ✨-mvector-strict-align-mno-vector-strict-align— this one's about unaligned vector memory accesses specifically. Default is off unless the processor we're optimizing for explicitly supports element-misaligned vector memory access. So most of the time, no unaligned vector vibes 🦋-mmax-vectorization-mno-max-vectorization— turn on or off an override to the vectorizer cost model that makes vectorization always look profitable. Like, "every vectorization is a good idea, no thoughts head empty" energy. But ngl, unlike-fno-vect-cost-modelor-fvect-cost-model=unlimited, this one does not turn off the cost comparison between different vector modes. So the modes still get compared, bestie 👑-mcmodel=medlow— generate code for the medium-low code model. The program and its statically defined symbols have to lie within a single 2 GiB address range and must lie between absolute addresses -2 GiB and +2 GiB. Programs can be statically or dynamically linked. And this is the default code model unless GCC has been configured with--with-cmodel=specifying a different default code model. So like, it's the main character until someone says otherwise 💖-mcmodel=medany— medium-any code model time. The program and its statically defined symbols must be within any single 2 GiB address range. Programs can be statically or dynamically linked.
⚠️ The code generated by the medium-any code model is position-independent, but is not guaranteed to function correctly when linked into position-independent executables or libraries. so like, know that before you ship it 😭
-mcmodel=large— generate code for a large code model, which has no restrictions on size or placement of symbols. free-range symbols, they can go wherever 🕊️-mexplicit-relocs-mno-explicit-relocs— use or don't use assembler relocation operators when dealing with symbolic addresses. The alternative is to use assembler macros instead, which may limit optimization. so like, pick your fighter 💀-mrelax-mno-relax— take advantage of linker relaxations to reduce the number of instructions needed to materialize symbol addresses. The default is to take advantage of linker relaxations. fewer instructions = slay 🔥-mriscv-attribute-mno-riscv-attribute— emit (or don't emit) RISC-V attribute to record extra information into ELF objects. Heads up though: this feature requires at least binutils 2.32. no binutils, no vibes 🌸-mcsr-check-mno-csr-check— enables or disables the CSR checking. simple as that bestie ✨-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. free real estate, we love to see it 💅-malign-data=type— control how GCC aligns variables and constants of array, structure, or union types. Supported values for type arexlen, which uses x register width as the alignment value, andnatural, which uses natural alignment.xlenis the default. ok soxlenis the moment unless you say otherwise 👑-mbig-endian— generate big-endian code. This is the default when GCC is configured for ariscv64be-*-*orriscv32be-*-*target.
🚨 Support for RISC-V big-endian is experimental. The ABI is not yet stable and could change in incompatible ways in future releases. so like, it's giving beta test, proceed with caution fr fr
-mlittle-endian— generate little-endian code. This is the default when GCC is configured for ariscv64-*-*orriscv32-*-*but not ariscv64be-*-*orriscv32be-*-*target. read that carefully bestie, the "not" is doing heavy lifting 💖-mstack-protector-guard=guard-mstack-protector-guard-reg=reg-mstack-protector-guard-offset=offset— generate stack protection code using canary at guard. Supported locations areglobalfor a global canary ortlsfor per-thread canary in the TLS block.
With the latter choice, the options -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset furthermore specify which register to use as base register for reading the canary, and from what offset from that base register. And there's no default register or offset because this is entirely for use within the Linux kernel. like, kernel people only, y'all know what you're doing 🫶
-mtls-dialect=desc— use TLS descriptors as the thread-local storage mechanism for dynamic accesses of TLS variables.-mtls-dialect=trad— use traditional TLS as the thread-local storage mechanism for dynamic accesses of TLS variables. And this one is the default, so it's already the vibe unless you switch it up ✨-mrvv-vector-bits=value— specify how the number of bits for an RVV vector register, as taken from the-march=option, is interpreted. The value parameter is specified as a string keyword and may be one ofscalableorzvl. The default isscalable, which tells GCC to interpret the number as a minimum, whilezvltells GCC to use exactly the number of bits specified. no more, no less, periodt 💅-mrvv-max-lmul=value— this option allows explicit control over the maximum length multiplier (LMUL) used when generating code for the RISC-V Vector Extensions (RVV). The value parameter is specified as a string keyword and may be one ofm1,m2,m4,m8, ordynamic. The default ism1for compatibility with existing hardware that does not support the other options. backwards compatible queen 👑-madjust-lmul-cost-mno-adjust-lmul-cost— this option adjusts the cost model used to schedule vector instructions to multiply the latency of instructions by the RVV length multiplier, LMUL. It is disabled by default. so like, off by default, opt in if you want that math 🧮-mautovec-segment-mno-autovec-segment— enable or disable generation of vector segment load/store instructions. This option is enabled by default. we love a default-on feature 🌈
🎀 RL78 Options 🎀
-msim— links in additional target libraries to support operation within a simulator. sim mode activated 🧚-mmul=none-mmul=g10-mmul=g13-mmul=g14-mmul=rl78— specifies the type of hardware multiplication and division support to be used. The simplest isnone, which uses software for both multiplication and division. This is the default. Theg13value is for the hardware multiply/divide peripheral found on the RL78/G13 (S2 core) targets. Theg14value selects the use of the multiplication and division instructions supported by the RL78/G14 (S3 core) parts. The valuerl78is an alias forg14and the valuemg10is an alias fornone.
In addition, a C preprocessor macro is defined, based upon the setting of this option. Possible values are: __RL78_MUL_NONE__, __RL78_MUL_G13__ or __RL78_MUL_G14__. macros got hands fr fr ✨
-mcpu=g10-mcpu=g13-mcpu=g14-mcpu=rl78— specifies the RL78 core to target. The default is the G14 core, also known as an S3 core or just RL78. The G13 or S2 core does not have multiply or divide instructions, instead it uses a hardware peripheral for these operations. The G10 or S1 core does not have register banks, so it uses a different calling convention. ok so different cores, different whole personalities 💅
If this option is set it also selects the type of hardware multiply support to use, unless this is overridden by an explicit -mmul=none option on the command line. Thus specifying -mcpu=g13 enables the use of the G13 hardware multiply peripheral and specifying -mcpu=g10 disables the use of hardware multiplications altogether.
Note bestie: although the RL78/G14 core is the default target, specifying-mcpu=g14or-mcpu=rl78on the command line does change the behavior of the toolchain since it also enables G14 hardware multiply support. If these options are not specified on the command line, then software multiplication routines are used even though the code targets the RL78 core. This is for backward compatibility with older toolchains which did not have hardware multiply and divide support. so like, the default is not the same as typing the default. chaotic? yes. real? also yes 😭
In addition, a C preprocessor macro is defined, based upon the setting of this option. Possible values are: __RL78_G10__, __RL78_G13__ or __RL78_G14__.
-mg10-mg13-mg14-mrl78— these are aliases for the corresponding-mcpu=option. They are provided for backwards compatibility. legacy support, we respect it 🫶-mallregs— allow the compiler to use all of the available registers. By default registersr24..r31are reserved for use in interrupt handlers. With this option enabled these registers can be used in ordinary functions as well. no more holding back, let them cook 🔥-mrelax-mno-relax— enable/disable assembler and linker relaxation. This is enabled by default at-Os. size optimization stays winning ✨-mes0— assumeESis zero throughout program execution, and use it to address read-only data. just assume it, it's fine 💖-msave-mduc-in-interrupts-mno-save-mduc-in-interrupts— specifies that interrupt handler functions should preserve the MDUC registers. This is only necessary if normal code might use the MDUC registers, for example because it performs multiplication and division operations. The default is to ignore the MDUC registers as this makes the interrupt handlers faster. And heads up: the target option-mg13needs to be passed for this to work, as this feature is only available on the G13 target (S2 core). The MDUC registers are only saved if the interrupt handler performs a multiplication or division operation or it calls another function. so like, conditional saving, only when it matters 💅
👑 IBM RS/6000 and PowerPC Options 👑
These -m options are defined for the IBM RS/6000 and PowerPC:
-mcpu=cpu_type— set architecture type, register usage, and instruction scheduling parameters for machine type cpu_type. Supported values for cpu_type are401,403,405,405fp,440,440fp,464,464fp,476,476fp,505,601,602,603,603e,604,604e,620,630,740,7400,7450,750,801,821,823,860,970,8540,a2,e300c2,e300c3,e500mc,e500mc64,e5500,e6500,ec603e,G3,G4,G5,titan,power3,power4,power5,power5+,power6,power6x,power7,power8,power9,power10,power11,future,powerpc,powerpc64,powerpc64le,rs64, andnative. that is a long list bestie, pick carefully 🎀
-mcpu=powerpc, -mcpu=powerpc64, and -mcpu=powerpc64le specify pure 32-bit PowerPC (either endian), 64-bit big endian PowerPC and 64-bit little endian PowerPC architecture machine types, with an appropriate, generic processor model assumed for scheduling purposes.
Specifying native as cpu type detects and selects the architecture option that corresponds to the host processor of the system performing the compilation. -mcpu=native has no effect if GCC does not recognize the processor. so if GCC's confused, it just. does nothing. relatable honestly 🙈
Okay so real talk — the other options down here are all about picking one specific processor. Code you generate with them runs best on that processor, and lowkey might not run at all on others. 🙈
The -mcpu options auto-enable or auto-disable this whole lil squad:
-maltivec -mfprnd -mhard-float -mmfcrf -mmultiple -mpopcntb
-mpopcntd -mpowerpc64 -mpowerpc-gpopt -mpowerpc-gfxopt -mmulhw
-mdlmzb -mmfpgpr -mvsx -mcrypto -mhtm -mpower8-fusion
-mquad-memory -mquad-memory-atomic -mfloat128 -mfloat128-hardware
-mprefixed -mpcrel -mmma -mdense-math -mrop-protectAnd bestie, the exact set of options a given CPU turns on varies between compiler versions — it just depends on whatever setting seems to make the most optimal code for that CPU. It does not necessarily reflect what the actual hardware can do. 💀 If you wanna force an individual option to a specific value, just drop it after the -mcpu option, like this:
-mcpu=970 -mno-altivecAIX tea though: on AIX, the -maltivec and -mpowerpc64 options are not enabled or disabled by -mcpu right now, because AIX doesn't have full support for them. You can still flip them on or off yourself individually if you're sure it'll work in your environment. 🫶
In all other cases, we recommend you use the -mcpu=cpu_type option instead of the options that control generation of specific instructions.
-mtune=cpu_type— Sets the instruction scheduling parameters for machine type cpu_type, but does not set the architecture type or register usage the way-mcpu=cpu_typedoes. Same cpu_type values as-mcpu. If you specify both, the generated code uses the architecture and registers from-mcpu, but the scheduling parameters from-mtune. ✨-mpowerpc64,-mno-powerpc64—-mpowerpc64lets GCC generate the extra 64-bit instructions found in the full PowerPC64 architecture and treat GPRs as 64-bit, doubleword quantities. GCC defaults to-mno-powerpc64. 🌸-mcmodel=small— Generate PowerPC64 code for the small model: the TOC is limited to64k.-mcmodel=medium— Generate PowerPC64 code for the medium model: the TOC and other static data may be up to a total of4Gin size. This is the default for 64-bit Linux. 💅-mcmodel=large— Generate PowerPC64 code for the large model: the TOC may be up to4Gin size. Other data and code is only limited by the 64-bit address space.-mprofile-kernel— Available on PowerPC64 GNU/Linux targets. When used with-pg, it causes calls tomcountto be inserted before the function prologue instead of after it. 🎀-maltivec,-mno-altivec— Generate code that uses (does not use) AltiVec instructions, and also enables the built-in functions that give you more direct access to the AltiVec instruction set. You may also need to set-mabi=altivecto adjust the current ABI with AltiVec ABI enhancements.
When -maltivec is used, the element order for AltiVec intrinsics like vec_splat, vec_extract, and vec_insert matches array element order corresponding to the endianness of the target. That is, element zero identifies the leftmost element in a vector register when targeting a big-endian platform, and identifies the rightmost element in a vector register when targeting a little-endian platform. 🦋
-mpowerpc-gpopt,-mno-powerpc-gpopt— Specifying-mpowerpc-gpoptallows GCC to use the optional PowerPC architecture instructions in the General Purpose group, including floating-point square root. Specifying-mpowerpc-gfxopt,-mno-powerpc-gfxopt—-mpowerpc-gfxoptallows GCC to use the optional PowerPC architecture instructions in the Graphics group, including floating-point select.-mmfcrf,-mno-mfcrf— The-mmfcrfoption 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.-mpopcntb,-mno-popcntb— The-mpopcntboption 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.-mpopcntd,-mno-popcntd— The-mpopcntdoption allows GCC to generate the popcount instruction implemented on the POWER7 processor and other processors that support the PowerPC V2.06 architecture.-mfprnd,-mno-fprnd— The-mfprndoption 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.-mcmpb,-mno-cmpb— The-mcmpboption allows GCC to generate the compare bytes instruction implemented on the POWER6 processor and other processors that support the PowerPC V2.05 architecture.-mhard-dfp,-mno-hard-dfp— The-mhard-dfpoption allows GCC to generate the decimal floating-point instructions implemented on some POWER processors.-mvrsave,-mno-vrsave— Generate VRSAVE instructions when generating AltiVec code.-msecure-plt— Generate code that allowsldandld.soto build executables and shared libraries with non-executable.pltand.gotsections. This is a PowerPC 32-bit SYSV ABI option. 💜-mbss-plt— Generate code that uses a BSS.pltsection thatld.sofills in, and requires.pltand.gotsections that are both writable and executable. This is a PowerPC 32-bit SYSV ABI option.-msplit-patch-nops— When adding NOPs for a patchable area via the-fpatchable-function-entryoption, emit the "before" NOPs in front of the global entry point and the "after" NOPs after the local entry point. This makes the sequence of NOPs not consecutive when a global entry point is generated. Without this option the NOPs are emitted directly before and after the local entry point, making them consecutive but moving global and local entry point further apart. If only a single entry point is generated, this option has no effect.-misel,-mno-isel— This switch enables or disables the generation of ISEL instructions.-mvsx,-mno-vsx— Generate code that uses (does not use) vector/scalar (VSX) instructions, and also enable the built-in functions that allow more direct access to the VSX instruction set.-mcrypto,-mno-crypto— Enable the use (disable) of the built-in functions that allow direct access to the cryptographic instructions that were added in version 2.07 of the PowerPC ISA. 🔥-mhtm,-mno-htm— Enable (disable) the use of the built-in functions that allow direct access to the Hardware Transactional Memory (HTM) instructions that were added in version 2.07 of the PowerPC ISA.-mpower8-fusion,-mno-power8-fusion— Generate code that keeps (does not keeps) some integer operations adjacent so that the instructions can be fused together on power8 and later processors. 😭 (yes, "does not keeps" — we report, we don't fix)-mquad-memory,-mno-quad-memory— Generate code that uses (does not use) the non-atomic quad word memory instructions. The-mquad-memoryoption requires use of 64-bit mode.-mquad-memory-atomic,-mno-quad-memory-atomic— Generate code that uses (does not use) the atomic quad word memory instructions. The-mquad-memory-atomicoption requires use of 64-bit mode.-mfloat128,-mno-float128— Enable/disable the__float128keyword for IEEE 128-bit floating point, and use either software emulation for IEEE 128-bit floating point or hardware instructions.
The VSX instruction set (-mvsx) must be enabled to use the IEEE 128-bit floating point support. The IEEE 128-bit floating point is only supported on Linux. 🐧
The default for -mfloat128 is enabled on PowerPC Linux systems using the VSX instruction set, and disabled on other systems.
If you use the ISA 3.0 instruction set (-mcpu=power9) on a 64-bit system, the IEEE 128-bit floating point support also enables the generation of ISA 3.0 IEEE 128-bit floating point instructions. Otherwise, if you do not specify generation of ISA 3.0 instructions or you are targeting a 32-bit big-endian system, IEEE 128-bit floating point is handled with software emulation.
-mfloat128-hardware,-mno-float128-hardware— Enable/disable using ISA 3.0 hardware instructions to support the__float128data type.
The default for -mfloat128-hardware is enabled on PowerPC Linux systems using the ISA 3.0 instruction set, and disabled on other systems.
-m32,-m64— Generate code for 32-bit or 64-bit environments of Darwin and SVR4 targets (including GNU/Linux). The 32-bit environment setsint,longand pointer to 32 bits and generates code that runs on any PowerPC variant. The 64-bit environment setsintto 32 bits andlongand 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-tocoption is selected by default. In that case, GCC allocates at least one TOC entry for each unique non-automatic variable reference in your program. GCC also places floating-point constants in the TOC. However, only16,384entries are available in the TOC.
If you receive a linker error message that saying you have overflowed the available TOC space, you can reduce the amount of TOC space used with the -mno-fp-in-toc and -mno-sum-in-toc options. -mno-fp-in-toc prevents GCC from putting floating-point constants in the TOC and -mno-sum-in-toc forces GCC to generate code to calculate the sum of an address and a constant at run time instead of putting that sum into the TOC. You may specify one or both of these options. Each causes GCC to produce very slightly slower and larger code at the expense of conserving TOC space.
If you still run out of space in the TOC even when you specify both of these options, specify -mminimal-toc instead. This option causes GCC to make only one TOC entry for every file. When you specify this option, GCC produces code that is slower and larger but which uses extremely little TOC space. You may wish to use this option only on files that contain less frequently-executed code. 🫠
-maix64,-maix32— okay so bestie, this is the AIX ABI vibe switch 💅-maix64turns ON the 64-bit AIX ABI and calling convention: 64-bit pointers, 64-bitlongtype, and all the infrastructure needed to support them. And the way that it works is-maix64implies-mpowerpc64, while-maix32disables the 64-bit ABI and implies-mno-powerpc64. GCC defaults to-maix32. periodt.-mxl-compat,-mno-xl-compat— produces code that conforms more closely to IBM XL compiler semantics when you're using the AIX-compatible ABI. Concretely: it passes floating-point arguments to prototyped functions beyond the register save area (RSA) on the stack in addition to argument FPRs. It does not assume that the most significant double in a 128-bitlong doublevalue is properly rounded when comparing values and converting todouble. And it uses XL symbol names forlong doublesupport routines. slay ✨
The AIX calling convention was extended but not initially documented to handle an obscure K&R C case of calling a function that takes the address of its arguments with fewer arguments than declared. IBM XL compilers access floating-point arguments that do not fit in the RSA from the stack when a subroutine is compiled without optimization. Because always storing floating-point arguments on the stack is inefficient and rarely needed, this option is not enabled by default and only is necessary when calling subroutines compiled by IBM XL compilers without optimization. iykyk 🫶
-mpe— support for the IBM RS/6000 SP Parallel Environment (PE) 🌀. It links 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-mpeoption and the-pthreadoption are incompatible. not them fighting fr 💀-malign-natural,-malign-power— on AIX, 32-bit Darwin, and 64-bit PowerPC GNU/Linux,-malign-naturaloverrides the ABI-defined alignment of larger types, such as floating-point doubles, on their natural size-based boundary 🌸. The-malign-poweroption 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-floatoption, and pass the option to GCC when linking. don't forget the linking part bestie, it matters 💅-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-mmultipleon little-endian PowerPC systems, since those instructions do not work when the processor is in little-endian mode. The exceptions are PPC740 and PPC750 which permit these instructions in little-endian mode. exceptions ate, honestly 👑-mupdate,-mno-update— generate code that uses (does not use) the load or store instructions that update the base register to the address of the calculated memory location. These instructions are generated by default. If you use-mno-update, there is a small window between the time that the stack pointer is updated and the address of the previous frame is stored, which means code that walks the stack frame across interrupts or signals may get corrupted data. 😭 lowkey scary, be careful.-mavoid-indexed-addresses,-mno-avoid-indexed-addresses— generate code that tries to avoid (not avoid) the use of indexed load or store instructions. These instructions can incur a performance penalty on Power6 processors in certain situations, such as when stepping through large arrays that cross a 16M boundary. This option is enabled by default when targeting Power6 and disabled otherwise. vibe check on the Power6 specifically 🫡-mfused-madd,-mno-fused-madd— generate code that uses (does not use) the floating-point multiply and accumulate instructions. These instructions are generated by default if hardware floating point is used. The machine-dependent-mfused-maddoption is now mapped to the machine-independent-ffp-contract=fastoption, and-mno-fused-maddis mapped to-ffp-contract=off. glow up moment fr ✨-mmulhw,-mno-mulhw— generate code that uses (does not use) the half-word multiply and multiply-accumulate instructions on the IBM 405, 440, 464 and 476 processors. These instructions are generated by default when targeting those processors. iconic little club of four 🎀-mdlmzb,-mno-dlmzb— generate code that uses (does not use) the string-searchdlmzbinstruction on the IBM 405, 440, 464 and 476 processors. This instruction is generated by default when targeting those processors. same little club, different instruction 💜-mno-bit-align,-mbit-align— on System V.4 and embedded PowerPC systems do not (do) force structures and unions that contain bit-fields to be aligned to the base type of the bit-field.
For example, by default a structure containing nothing but 8 unsigned bit-fields of length 1 is aligned to a 4-byte boundary and has a size of 4 bytes. By using -mno-bit-align, the structure is aligned to a 1-byte boundary and is 1 byte in size. shrink era 🦋
-mbit-word,-mno-bit-word— allow bit-fields to cross word boundaries. that's it, that's the whole vibe 🌈-mno-strict-align,-mstrict-align— on System V.4 and embedded PowerPC systems do not (do) assume that unaligned memory references are handled by the system. choose your fighter 🥊-mrelocatable,-mno-relocatable— generate code that allows (does not allow) a static executable to be relocated to a different address at run time. A simple embedded PowerPC system loader should relocate the entire contents of.got2and 4-byte locations listed in the.fixupsection, a table of 32-bit addresses generated by this option. For this to work, all objects linked together must be compiled with-mrelocatableor-mrelocatable-lib.-mrelocatablecode aligns the stack to an 8-byte boundary.-mrelocatable-lib,-mno-relocatable-lib— like-mrelocatable,-mrelocatable-libgenerates a.fixupsection to allow static executables to be relocated at run time, but-mrelocatable-libdoes not use the smaller stack alignment of-mrelocatable. Objects compiled with-mrelocatable-libmay be linked with objects compiled with any combination of the-mrelocatableoptions. mix and match, bestie, it's giving flexible 💅-mlittle,-mlittle-endian— on System V.4 and embedded PowerPC systems compile code for the processor in little-endian mode. The-mlittle-endianoption 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-endianoption is the same as-mbig.-mdynamic-no-pic— on Darwin / macOS 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. applications only, bestie, don't push it 😌-msingle-pic-base— treat the register used for PIC addressing as read-only, rather than loading it in the prologue for each function. The runtime system is responsible for initializing this register with an appropriate value before execution begins. that's on the runtime system, not us 🫶-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 value0,1, or2to assign no, highest, or second-highest (respectively) priority to dispatch-slot restricted instructions. pick your rank 👑-msched-costly-dep=dependence_type — this option controls which dependences are considered costly by the target during instruction scheduling. The argument dependence_type takes one of the following values:no— no dependence is costly.all— all dependences are costly.true_store_to_load— a true dependence from store to load is costly.store_to_load— any dependence from store to load is costly.- number — any dependence for which the latency is greater than or equal to number is costly.
-minsert-sched-nops=scheme — this option controls which NOP insertion scheme is used during the second scheduling pass. The argument scheme takes one of the following values:no— don't insert NOPs.pad— pad with NOPs any dispatch group that has vacant issue slots, according to the scheduler's grouping.regroup_exact— insert NOPs to force costly dependent insns into separate groups. Insert exactly as many NOPs as needed to force an insn to a new group, according to the estimated processor grouping.- number — insert NOPs to force costly dependent insns into separate groups. Insert number NOPs to force an insn to a new group.
-mcall-sysv— on System V.4 and embedded PowerPC systems compile code using calling conventions that adhere to the March 1995 draft of the System V Application Binary Interface, PowerPC processor supplement. This is the default unless you configured GCC usingpowerpc-*-eabiaix.-mcall-sysv-eabi,-mcall-eabi— specify both-mcall-sysvand-meabioptions. combo deal 🎀-mcall-sysv-noeabi— specify both-mcall-sysvand-mno-eabioptions.-mcall-aixdesc— on System V.4 and embedded PowerPC systems compile code for the AIX operating system.-mcall-linux— on System V.4 and embedded PowerPC systems compile code for the Linux-based GNU system.-mcall-freebsd— on System V.4 and embedded PowerPC systems compile code for the FreeBSD operating system.-mcall-netbsd— on System V.4 and embedded PowerPC systems compile code for the NetBSD operating system.-mcall-openbsd— on System V.4 and embedded PowerPC systems compile code for the OpenBSD operating system. the whole OS friend group showed up 🌈-mtraceback=traceback_type — select the type of traceback table. Valid values for traceback_type arefull,part, andno. three options, no notes, we love a low-maintenance flag 💅-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). tiny structs get the VIP register treatment ✨-mabi=abi-type— okay so this one extends the current ABI with a particular extension, or removes such an extension. ✨ The valid values are:altivec,no-altivec,ibmlongdouble,ieeelongdouble,elfv1,elfv2, and for AIX:vec-extabi,vec-default. that's the whole menu bestie 💅-mabi=ibmlongdouble— changes the current ABI to use IBM extended-precisionlong double. ngl this is not likely to work if your system defaults to using IEEE extended-precisionlong double. 😭 And if you change thelong doubletype from IEEE extended-precision, the compiler issues a warning unless you use the-Wno-psabioption. Oh and it requires-mlong-double-128to be enabled. no cap.-mabi=ieeelongdouble— changes the current ABI to use IEEE extended-precisionlong double. same warning energy fr: this is not likely to work if your system defaults to using IBM extended-precisionlong double. If you change thelong doubletype from IBM extended-precision, the compiler issues a warning unless you use the-Wno-psabioption. Requires-mlong-double-128to be enabled. 🫶-mabi=elfv1— changes the current ABI to use the ELFv1 ABI. This is the default ABI for big-endian PowerPC 64-bit Linux. 🐧 Overriding the default ABI requires special system support and is likely to fail in spectacular ways. iconic but terrifying.-mabi=elfv2— changes the current ABI to use the ELFv2 ABI. This is the default ABI for little-endian PowerPC 64-bit Linux. Overriding the default ABI requires special system support and is likely to fail in spectacular ways. same drama, different endianness 💀-mgnu-attribute,-mno-gnu-attribute— emits.gnu_attributeassembly directives that set tag/value pairs in a.gnu.attributessection, specifying ABI variations in function parameters or return values. 🎀-mprototype,-mno-prototype— on System V.4 and embedded PowerPC systems, this assumes that all calls to variable argument functions are properly prototyped. Otherwise, the compiler has to insert an instruction before every non-prototyped call to set or clear bit 6 of the condition code register (CR) to indicate whether floating-point values are passed in the floating-point registers, in case the function takes variable arguments. With-mprototype, only calls to prototyped variable argument functions set or clear that bit. okay periodt ✨-msim— on embedded PowerPC systems, assumes the startup module is calledsim-crt0.oand that the standard C libraries arelibsim.aandlibc.a. This is the default forpowerpc-*-eabisimconfigurations. 🌸-mmvme— on embedded PowerPC systems, assumes the startup module is calledcrt0.oand the standard C libraries arelibmvme.aandlibc.a. 🦋-mads— on embedded PowerPC systems, assumes the startup module is calledcrt0.oand the standard C libraries arelibads.aandlibc.a. ⭐-myellowknife— on embedded PowerPC systems, assumes the startup module is calledcrt0.oand the standard C libraries arelibyk.aandlibc.a. yellowknife is such a name I'm obsessed 💛-mvxworks— on System V.4 and embedded PowerPC systems, this specifies that you're compiling for a VxWorks system. 🌈-memb— on embedded PowerPC systems, sets thePPC_EMBbit 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 (or do not) adhere to the Embedded Applications Binary Interface (EABI), which is a set of modifications to the System V.4 specifications. Selecting-meabimeans the stack is aligned to an 8-byte boundary, a function__eabiis called frommainto set up the EABI environment, and the-msdataoption can use bothr2andr13to point to two separate small data areas. Selecting-mno-eabimeans the stack is aligned to a 16-byte boundary, no EABI initialization function is called frommain, and the-msdataoption only usesr13to point to a single small data area. The-meabioption is on by default if you configured GCC using one of thepowerpc*-*-eabi*options. 👑-msdata=eabi— on System V.4 and embedded PowerPC systems, puts small initializedconstglobal and static data in the.sdata2section, which is pointed to by registerr2. Puts small initialized non-constglobal and static data in the.sdatasection, which is pointed to by registerr13. Puts small uninitialized global and static data in the.sbsssection, which is adjacent to the.sdatasection. Heads up: the-msdata=eabioption is incompatible with the-mrelocatableoption. It also sets the-memboption. 💖-msdata=sysv— on System V.4 and embedded PowerPC systems, puts small global and static data in the.sdatasection, which is pointed to by registerr13. Puts small uninitialized global and static data in the.sbsssection, which is adjacent to the.sdatasection. Also incompatible with the-mrelocatableoption. 🫠-msdata=default,-msdata— on System V.4 and embedded PowerPC systems, if-meabiis used, compiles code the same as-msdata=eabi; otherwise it compiles code the same as-msdata=sysv. the way it just picks for you 🎀-msdata=data— on System V.4 and embedded PowerPC systems, puts small global data in the.sdatasection. Puts small uninitialized global data in the.sbsssection. It does not use registerr13to address small data though. This is the default behavior unless other-msdataoptions are used. 💅-msdata=none,-mno-sdata— on embedded PowerPC systems, puts all initialized global and static data in the.datasection, and all uninitialized data in the.bsssection. simple. clean. iconic. ✨-mreadonly-in-sdata— puts read-only objects in the.sdatasection as well. This is the default. 🌸-mblock-move-inline-limit=num— inlines all block moves (like calls tomemcpyor structure copies) that are less than or equal to num bytes. The minimum value for num is 32 bytes on 32-bit targets and 64 bytes on 64-bit targets. The default value is target-specific. 🦋-mblock-compare-inline-limit=num— generates non-looping inline code for all block compares (like calls tomemcmpor structure compares) that are less than or equal to num bytes. If num is0, all inline expansion (non-loop and loop) of block compare is disabled. The default value is target-specific. 💜-mblock-compare-inline-loop-limit=num— generates an inline expansion using loop code for all block compares that are less than or equal to num bytes, but greater than the limit for non-loop inline block compare expansion. If the block length isn't constant, at most num bytes are compared beforememcmpis called to compare the rest of the block. The default value is target-specific. ⭐-mstring-compare-inline-limit=num— compares at most num string bytes with inline code. If the difference or the end of the string isn't found at the end of the inline compare, a call tostrcmporstrncmptakes care of the rest of the comparison. The default is 64 bytes. 🍓-G num— on embedded PowerPC systems, puts 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 is8. The-G numswitch is also passed to the linker, and all modules should be compiled with the same-G numvalue. don't be messy about it bestie 🙈-mregnames,-mno-regnames— on System V.4 and embedded PowerPC systems, do (or do not) emit register names in the assembly language output using symbolic forms. 💅-mlongcall,-mno-longcall— by default this assumes that all calls are far away, so a longer and more expensive calling sequence is required. This is required for calls farther than 32 megabytes (33,554,432bytes) from the current location. A short call is generated if the compiler knows the call can't be that far away. This setting can be overridden by theshortcallfunction 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's planned to add this feature to the GNU linker for 32-bit PowerPC systems as well. 🛠️
On PowerPC64 ELFv2 and 32-bit PowerPC systems with newer GNU linkers, GCC can generate long calls using an inline PLT call sequence (see -mpltseq). PowerPC with -mbss-plt and PowerPC64 ELFv1 (big-endian) do not support inline PLT calls.
On Darwin/PPC systems, #pragma longcall generates jbsr callee, L42, plus a branch island (glue code). The two target addresses represent the callee and the branch island. The Darwin/PPC linker prefers the first address and generates a bl callee if the PPC bl instruction reaches the callee directly; otherwise, the linker generates bl L42 to call the branch island. The branch island is appended to the body of the calling function; it computes the full 32-bit address of the callee and jumps to it. 🧚
On Mach-O (Darwin) systems, this option directs the compiler to emit glue for every direct call, and the Darwin linker decides whether to use it or discard it.
In the future, GCC may ignore all longcall specifications when the linker is known to generate glue. future vibes, noted 🔮
-mpltseq,-mno-pltseq— implements (or doesn't implement)-fno-pltand long calls using an inline PLT call sequence that supports lazy linking and long calls to functions in dlopen'd shared libraries. Inline PLT calls are only supported on PowerPC64 ELFv2 and 32-bit PowerPC systems with newer GNU linkers, and they're enabled by default if the support is detected when configuring GCC, and — in the case of 32-bit PowerPC — if GCC is configured with--enable-secureplt. Careful:-mpltseqcode and-mbss-plt32-bit PowerPC relocatable objects may not be linked together. 💀-mtls-markers,-mno-tls-markers— marks (or doesn't mark) calls to__tls_get_addrwith a relocation specifying the function argument. The relocation lets the linker reliably associate the function call with the argument setup instructions for TLS optimization, which in turn lets GCC better schedule the sequence. slay scheduling ✨💖-mrecip,-mno-recip— okay so this one tells GCC to use the reciprocal estimate and reciprocal square root estimate instructions 🎀, plus extra Newton-Raphson steps to bump the precision back up, instead of straight-up doing a divide (or a square root and a divide) for floating-point args. Bestie, you really want to be running this with-ffast-mathwhen you turn on-mrecip— or at minimum-funsafe-math-optimizations,-ffinite-math-only,-freciprocal-mathand-fno-trapping-math. Heads up tho: even though the throughput of this sequence is usually higher than the non-reciprocal instruction, the precision can drop by up to 2 ulp 😭 (like, the inverse of1.0equals0.99999994) for reciprocal square roots. It's giving speed for accuracy, periodt.-mrecip=opt— this one picks which reciprocal estimate instructions are allowed to show up. opt is a comma-separated list of options, and any of them can get a!in front to invert it:all— enable every estimate instruction, the whole roster 💅default— enable the default instructions, which is literally the same as-mrecipnone— disable all estimate instructions, equivalent to-mno-recipdiv— turn on the reciprocal approximation instructions for both single and double precisiondivf— turn on the single-precision reciprocal approximation instructionsdivd— turn on the double-precision reciprocal approximation instructionsrsqrt— turn on the reciprocal square root approximation instructions for both single and double precisionrsqrtf— turn on the single-precision reciprocal square root approximation instructionsrsqrtd— turn on the double-precision reciprocal square root approximation instructions
So for example, -mrecip=all,!rsqrtd enables all of the reciprocal estimate instructions, except for the FRSQRTE, XSRSQRTEDP, and XVRSQRTEDP instructions which handle the double-precision reciprocal square root calculations. Iconic little syntax tbh ✨
-mrecip-precision,-mno-recip-precision— assume (or, with theno-version, do not assume) that the reciprocal estimate instructions provide higher-precision estimates than the PowerPC ABI mandates. Picking-mcpu=power6,-mcpu=power7or-mcpu=power8automatically selects-mrecip-precisionfor you 🫶. The double-precision square root estimate instructions are not generated by default on low-precision machines, because they don't provide an estimate that converges after three steps. No cap.-mveclibabi=type— sets the ABI type to use when vectorizing intrinsics through an external library. The only type supported right now ismass, which says "use IBM's Mathematical Acceleration Subsystem (MASS) libraries" for vectorizing intrinsics with external libraries. When generating code for power7, GCC currently emits calls toacosd2,acosf4,acoshd2,acoshf4,asind2,asinf4,asinhd2,asinhf4,atan2d2,atan2f4,atand2,atanf4,atanhd2,atanhf4,cbrtd2,cbrtf4,cosd2,cosf4,coshd2,coshf4,erfcd2,erfcf4,erfd2,erff4,exp2d2,exp2f4,expd2,expf4,expm1d2,expm1f4,hypotd2,hypotf4,lgammad2,lgammaf4,log10d2,log10f4,log1pd2,log1pf4,log2d2,log2f4,logd2,logf4,powd2,powf4,sind2,sinf4,sinhd2,sinhf4,sqrtd2,sqrtf4,tand2,tanf4,tanhd2, andtanhf4🧚. Both-ftree-vectorizeand-funsafe-math-optimizationshave to be enabled too. And the MASS libraries must be specified at link time — don't forget that part bestie 🙈-mfriz,-mno-friz— generate (or don't generate) thefrizinstruction when-funsafe-math-optimizationsis used to optimize rounding of floating-point values to 64-bit integer and back to floating point. Warning tho 💀: thefrizinstruction does not return the same value if the floating-point number is too large to fit in an integer.-mpointers-to-nested-functions,-mno-pointers-to-nested-functions— generate (or don't generate) code to load up the static chain register (r11) when calling through a pointer on AIX and 64-bit Linux systems, where a function pointer points to a 3-word descriptor giving the function address, the TOC value to be loaded in registerr2, and the static chain value to be loaded in registerr11.-mpointers-to-nested-functionsis on by default. And lowkey important: you cannot call through pointers to nested functions, or pointers to functions compiled in other languages that use the static chain, if you use-mno-pointers-to-nested-functions😭-msave-toc-indirect,-mno-save-toc-indirect— generate (or don't generate) code to save the TOC value in the reserved stack location in the function prologue, if the function calls through a pointer on AIX and 64-bit Linux systems. If the TOC value isn't saved in the prologue, it gets saved right before the call through the pointer instead. The default is-mno-save-toc-indirect.-mcompat-align-parm,-mno-compat-align-parm— generate (or don't generate) code that passes structure parameters with a maximum alignment of 64 bits, for compatibility with older versions of GCC.
Older versions of GCC (prior to 4.9.0) incorrectly did not align a structure parameter on a 128-bit boundary when that structure contained a member requiring 128-bit alignment. That's fixed in more recent versions of GCC. This option is your time machine 🕰️ — use it to generate code that's compatible with functions compiled with older versions of GCC.
-mno-compat-align-parm is the default.
-mstack-protector-guard=guard,-mstack-protector-guard-reg=reg,-mstack-protector-guard-offset=offset— generate stack protection code using a canary at guard. The supported locations areglobalfor a global canary, ortlsfor a per-thread canary in the TLS block (which is the default with GNU libc version 2.4 or later).
With that second choice, -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset go even further and specify which register to use as the base register for reading the canary, and from what offset from that base register. The default for those is as specified in the relevant ABI 🎀
-mpcrel,-mno-pcrel— generate (or don't generate) pc-relative addressing. The-mpcreloption requires that the medium code model (-mcmodel=medium) and prefixed addressing (-mprefixed) options are enabled. Yes bestie, it has prerequisites, read the fine print 💅-mprefixed,-mno-prefixed— generate (or don't generate) addressing modes using prefixed load and store instructions. The-mprefixedoption requires that-mcpu=power10(or later) is enabled.-mmma,-mno-mma— generate (or don't generate) the MMA instructions. The-mmaoption requires that the option-mcpu=power10(or later) is enabled.-mdense-math,-mno-dense-math— generate (or don't generate) Dense Math Facility (DMF) instructions. The-mdense-mathoption requires that the option-mcpu=future(or later) is enabled. Future tech energy fr fr 🔥-mrop-protect,-mno-rop-protect— generate (or don't generate) ROP protection instructions when the target processor supports them. Currently this option disables the shrink-wrap optimization (-fshrink-wrap).-mprivileged,-mno-privileged— generate (or don't generate) code that runs in privileged state. Main character privileges, literally 👑-mblock-ops-unaligned-vsx,-mno-block-ops-unaligned-vsx— generate (or don't generate) unaligned vsx loads and stores for inline expansion ofmemcpyandmemmove.-msplat-word-constant,-mno-splat-word-constant— generate (or don't generate) code that uses theXXSPLTIWinstruction. This option is enabled by default ✨-msplat-float-constant,-mno-splat-float-constant— generate (or don't generate) code that uses theXXSPLTIDPinstruction. This option is enabled by default ✨-mieee128-constant,-mno-ieee128-constant— generate (or don't generate) code that uses theLXVKQinstruction. This option is enabled by default ✨-mwarn-altivec-long,-mno-warn-altivec-long— enable or disable warnings about deprecated vector long ... Altivec type usage. This option is enabled by default. Consider this your little warning bell 🔔
🌈 RX Options 🌈
These command-line options are defined for RX targets 🦋:
-m64bit-doubles,-m32bit-doubles— make thedoubledata type 64 bits (-m64bit-doubles) or 32 bits (-m32bit-doubles) in size. The default is-m32bit-doubles. And bestie, note that RX floating-point hardware only works on 32-bit values, which is exactly why the default is-m32bit-doubles💅-fpu,-nofpu— enables (-fpu) or disables (-nofpu) the use of RX floating-point hardware. The default is enabled for the RX600 series and disabled for the RX200 series.
Floating-point instructions are only generated for 32-bit floating-point values, however, so the FPU hardware is not used for doubles if you use the -m64bit-doubles option 😭
Note: if-fpuis enabled, then-funsafe-math-optimizationsis also enabled automatically. That's because the RX FPU instructions are themselves unsafe. Yeah. They said it, not me 💀
-mcpu=name— selects the type of RX CPU to target. Currently three types are supported: the generic RX600 and RX200 series hardware, and the specific RX610 CPU. The default is RX600.
The only difference between RX600 and RX610 is that the RX610 does not support the MVTIPL instruction. Tiny difference, big consequences ✨
The RX200 series has no hardware floating-point unit, so -nofpu is enabled by default when this type is selected.
-mbig-endian-data,-mlittle-endian-data— store data (but not code!) in the big-endian format. The default is-mlittle-endian-data, i.e. storing data in the little-endian format 🎀-msmall-data-limit=N— specifies the maximum size in bytes of global and static variables that can be placed into the small data area. Using the small data area can lead to smaller and faster code, but the size of the area is limited and it is up to the programmer to make sure the area doesn't overflow 😬 Also, when the small data area is used, one of the RX's registers (usuallyr13) gets reserved for pointing to this area, so it's no longer available for use by the compiler. That could end up giving you slower and/or larger code if variables get pushed onto the stack instead of being held in this register. Trade-offs, bestie, trade-offs 🙈
Note: common variables (variables that have not been initialized) and constants are not placed into the small data area, since they are assigned to other sections in the output executable.
The default value is zero, which disables this whole feature btw. And no, it is not turned on for you at higher optimization levels (-O2 etc) — because reserving a register can genuinely do more harm than good, bestie. It's fully on you, the programmer, to experiment and figure out if it actually helps your program. 💅 Wanna know how the actual register holding the small data area pointer gets picked? That's in the -mpid description, go look. 👀
-msim-mno-sim— Use the simulator runtime. The default is to use the libgloss board-specific runtime.-mas100-syntax-mno-as100-syntax— When you're generating assembler output, use a syntax that plays nice with Renesas's AS100 assembler. The GAS assembler can handle this syntax too, but it comes with some restrictions, so it's not what gets generated by default.-mmax-constant-size=N— Sets the max size, in bytes, of a constant that can be used as an operand in an RX instruction. Okay so the RX instruction set does allow constants up to 4 bytes long in instructions, but a longer value = a longer instruction. So sometimes it's lowkey beneficial to restrict how big the constants in your instructions get. Constants that are too big get yeeted into a constant pool instead and referenced via register indirection. 🌸
The value N can be between 0 and 4. A value of 0 (the default) or 4 means constants of any size are allowed.
-mrelax— Enable linker relaxation. Linker relaxation is where the linker tries to shrink your program by hunting down shorter versions of various instructions. Disabled by default.-mint-register=N— Specify how many registers to reserve for fast interrupt handler functions. N can be between 0 and 4. A value of 1 means register"r13"is reserved for the exclusive use of fast interrupt handlers. A value of 2 reserves"r13"and"r12". A value of 3 reserves"r13","r12"and"r11", and a value of 4 reserves"r13"through"r10". A value of 0, the default, reserves nothing. 🙅-msave-acc-in-interrupts— Says interrupt handler functions should preserve the accumulator register. This is only necessary if normal code might use the accumulator register, like when it does 64-bit multiplications. The default is to ignore the accumulator because that makes the interrupt handlers faster. ⚡-mpid-mno-pid— Turns on generation of position-independent data. When it's enabled, any access to constant data goes through an offset from a base address held in a register. That means the location of constant data can be figured out at run time without needing the executable to be relocated — which is a whole win for embedded applications with tight memory constraints. Data that can be modified is not affected by this option, ok? 🫶
Heads up: using this feature reserves a register, usually "r13", for the constant data base address. And that can make your code slower and/or bigger, especially in complicated functions. 😭
Which register actually ends up holding the constant data base address depends on whether -msmall-data-limit and/or -mint-register are enabled on the command line. Starting with register "r13" and going downwards, registers get allocated first to satisfy -mint-register, then -mpid, and finally -msmall-data-limit. So yes, it is possible for the small data area register to be "r8" if you pass both -mint-register=4 and -mpid. Wild, I know. 💀
By default this feature is not enabled. You can restore the default with the -mno-pid command-line option.
-mno-warn-multiple-fast-interrupts-mwarn-multiple-fast-interrupts— Stops GCC from throwing a warning message if it finds more than one fast interrupt handler while compiling a file. The default is to issue a warning for each extra fast interrupt handler it finds, because the RX only supports one such interrupt. 🚨-mallow-string-insns-mno-allow-string-insns— Enables or disables the use of the string manipulation instructions"SMOVF","SCMPU","SMOVB","SMOVU","SUNTIL""SWHILE"and also the"RMPA"instruction. These instructions may prefetch data, which is not safe if you're accessing an I/O register. (See section 12.2.7 of the RX62N Group User's Manual for more info.) 📖
The default is to allow these instructions, but GCC cannot reliably detect every circumstance where a string instruction might be used to touch an I/O register, so their use can't be disabled automatically. Nope. Instead it's on the programmer to use -mno-allow-string-insns if their program accesses I/O space.
When the instructions are enabled, GCC defines the C preprocessor symbol "__RX_ALLOW_STRING_INSNS__", otherwise it defines "__RX_DISALLOW_STRING_INSNS__".
-mjsr-mno-jsr— Use only (or not only)"JSR"instructions to access functions. This option comes in handy when code size exceeds the range of"BSR"instructions. Note that-mno-jsrdoes not mean "don't useJSR" — it means any type of branch may be used. 🌀-mlra— Use the new LRA register allocator. By default, the old"reload"allocator is used.
Note: the generic GCC command-line option-ffixed-reghas special significance to the RX port when it's used with the"interrupt"function attribute. This attribute marks a function intended to process fast interrupts. GCC makes sure it only uses the registers"r10","r11","r12"and/or"r13"— and only provided that the normal use of the corresponding registers has been restricted via the-ffixed-regor-mint-registercommand-line options. ✨
💜 S/390 and zSeries Options 💜
These are the -m options defined for the S/390 and zSeries architecture. 🦋
-mhard-float-msoft-float— Use (or don't use) the hardware floating-point instructions and registers for floating-point operations. When-msoft-floatis specified, functions inlibgcc.aare used to do floating-point operations. When-mhard-floatis specified, the compiler generates IEEE floating-point instructions. This is the default. ⭐-mhard-dfp-mno-hard-dfp— Use (or don't use) the hardware decimal-floating-point instructions for decimal-floating-point operations. When-mno-hard-dfpis specified, functions inlibgcc.aare used to do decimal-floating-point operations. When-mhard-dfpis specified, the compiler generates decimal-floating-point hardware instructions. This is the default for-march=z9-ecor higher.-mlong-double-64-mlong-double-128— These switches control the size of thelong doubletype. A size of 64 bits makes thelong doubletype equivalent to thedoubletype. This is the default.-mbackchain-mno-backchain— Store (or don't store) the address of the caller's frame as a backchain pointer into the callee's stack frame. A backchain might be needed so debugging tools that don't understand DWARF call frame information can still work. When-mno-packed-stackis in effect, the backchain pointer is stored at the bottom of the stack frame; when-mpacked-stackis in effect, the backchain goes into the topmost word of the 96/160 byte register save area. 🎀
Generally speaking, code compiled with -mbackchain is call-compatible with code compiled with -mno-backchain — however, using the backchain for debugging usually requires that the whole binary is built with -mbackchain. Note that the combo of -mbackchain, -mpacked-stack and -mhard-float is not supported. And if you wanna build a linux kernel, use -msoft-float. 🐧
The default is to not maintain the backchain.
-mpacked-stack-mno-packed-stack— Use (or don't use) the packed stack layout. When-mno-packed-stackis specified, the compiler uses all fields of the 96/160 byte register save area only for their default purpose; unused fields still take up stack space. When-mpacked-stackis specified, register save slots get densely packed at the top of the register save area, and the unused space gets reused for other stuff — more efficient use of the stack space you've got. But when-mbackchainis 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. No exceptions. 💅
As long as the stack frame backchain isn't used, code generated with -mpacked-stack is call-compatible with code generated with -mno-packed-stack. But note: 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. That code is not call-compatible with code compiled with -mpacked-stack. Also, 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 don't generate) code using the"bras"instruction to do subroutine calls. This only works reliably if the total executable size doesn't exceed 64k. The default is to use the"basr"instruction instead, which doesn't have that limitation. 🔥-m64-m31— When-m31is specified, generate code compliant to the GNU/Linux for S/390 ABI. When-m64is specified, generate code compliant to the GNU/Linux for zSeries ABI. This lets GCC generate 64-bit instructions, in particular. For the s390 targets the default is-m31, while the s390x targets default to-m64.
Careful bestie — -m31 is deprecated and support will be removed. 😭
-mzarch-mesa— When-mzarchis specified, generate code using the instructions available on z/Architecture. When-mesais specified, generate code using the instructions available on ESA/390. Note that-mesais 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. 🌈-mhtm-mno-htm
-mhtm turns on a whole lil' bundle of builtins that use the instructions from the transactional execution facility — the one that showed up with the IBM zEnterprise EC12 machine generation S/390 System z Built-in Functions. And bestie, -mhtm is enabled by default when you're using -march=zEC12 💖
-mvx-mno-vx
When you say -mvx, GCC generates code using the instructions from the vector extension facility, introduced with the IBM z13 machine generation. Heads up tho — this option changes the ABI for some vector type values, specifically around alignment and calling conventions 👀. If vector type values are being used in an ABI-relevant context, a GAS .gnu_attribute command gets added to mark the resulting binary with the ABI used. -mvx is enabled by default when using -march=z13 ✨
-mzvector-mno-zvector
-mzvector enables vector language extensions and builtins using instructions from the vector extension facility introduced with the IBM z13 machine generation. This option adds support for vector to be used as a keyword to define vector type variables and arguments 🎀. vector is only available when GNU extensions are enabled — it does not get expanded when you're requesting strict standard compliance, e.g. with -std=c99. On top of the GCC low-level builtins, -mzvector enables a set of builtins added for compatibility with AltiVec-style implementations like Power and Cell. To make use of these builtins you must include the header file vecintrin.h. And -mzvector is disabled by default, no cap.
-mmvcle-mno-mvcle
Generate (or do not generate) code using the mvcle instruction to perform block moves. When -mno-mvcle is specified, use a mvc loop instead. This is the default unless optimizing for size 💅
-mdebug-mno-debug
Print (or do not print) additional debug information when compiling. The default is to not print debug information.
-march=cpu-type</>
Generate code that runs on cpu-type, which is the name of a system representing a certain processor type. The possible values for cpu-type are z900/arch5, z990/arch6, z9-109, z9-ec/arch7, z10/arch8, z196/arch9, zEC12, z13/arch11, z14/arch12, z15/arch13, z16/arch14, z17/arch15, and native 🌈
The default is -march=z900.
Specifying native as the cpu type can be used to select the best architecture option for the host processor. -march=native has no effect if GCC does not recognize the processor 🙈
-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.
-mtpf-trace-skip-mno-tpf-trace-skip
Generate code that changes (does not change) the default branch targets enabled by -mtpf-trace to point to specialized trace routines providing the ability of selectively skipping function trace entries for the TPF OS. This option is off by default, even when compiling for the TPF OS and specifying -mtpf-trace.
-mmain
For TPF OS target, use this option when linking to add the startup code and other linker options to produce a main object 🫶
-mfused-madd-mno-fused-madd
Generate code that uses (does not use) the floating-point multiply and accumulate instructions. These instructions are generated by default if hardware floating point is used ✨
-mwarn-framesize=framesize</>
Emit a warning if the current function exceeds the given frame size. Because this is a compile-time check it doesn't need to be a real problem when the program runs. It is intended to identify functions that most probably cause a stack overflow 💀. It's useful for environments with limited stack size, e.g. the Linux kernel.
A value of zero disables this warning; that is the default.
-mwarn-dynamicstack-mno-warn-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</>-mno-stack-guard-mno-stack-size
If these options are enabled, the S/390 back end emits additional instructions in the function prologue that trigger a trap if the stack size is stack-guard bytes above the stack-size (remember that the stack on S/390 grows downward ⬇️). If the stack-guard option is omitted, the smallest power of 2 larger than the frame size of the compiled function is chosen.
These options are intended to be used to help debugging stack overflow problems 🐛. The additionally emitted code causes only little overhead and hence can also be used in production-like systems without greater performance degradation 👑
The given values have to be exact powers of 2, and stack-size has to be greater than stack-guard without exceeding 64k. In order to be efficient, the extra code makes the assumption that the stack starts at an address aligned to the value given by stack-size.
The-mstack-guard=option can only be used in conjunction with-mstack-size=. You can override these options on the command line with-mno-stack-guardand-mno-stack-size, respectively.
-mhotpatch=pre-halfwords,post-halfwords</>
If the hotpatch option is enabled, a "hot-patching" function prologue is generated for all functions in the compilation unit 🔥. The function label is prepended with the given number of two-byte NOP instructions (pre-halfwords, maximum 1000000). After the label, 2 * post-halfwords bytes are appended, using the largest NOP-like instructions the architecture allows (maximum 1000000).
If both arguments are zero, hotpatching is disabled.
This option can be overridden for individual functions with the hotpatch attribute.
-mpic-data-is-text-relative-mno-pic-data-is-text-relative
When compiling for S/390 with -fpic or -fPIC, normally GCC emits code using relative addressing between code and data. However, for hotpatching it might be required to introduce new .text parts while using the existing .data and .bss sections. In this case, use -mno-pic-data-is-text-relative to force addressing through the GOT instead 💅
-mstack-protector-guard=guard</>-mstack-protector-guard-record
Generate stack protection code using canary at guard. Supported locations are global for a global canary or tls for a per-thread canary in the TLS block (the default) 🐤
Option -mstack-protector-guard-record results in the generation of section __stack_protector_loc containing pointers to all instructions which load the address of the global guard. Thus, this option has only an effect in conjunction with -mstack-protector-guard=global. The intended use is for the Linux kernel.
-mindirect-branch=choice</>-mindirect-branch-jump=choice</>-mindirect-branch-call=choice</>-mfunction-return=choice</>-mfunction-return-mem=choice</>-mfunction-return-reg=choice</>
This group of options addresses security vulnerabilities related to speculative execution and indirect branch prediction by enabling replacement of indirect branches and function returns with direct branches to a thunk 🔐
Specifying a choice of keep causes generation of the default code. thunk triggers generation of out-of-line thunks and replaces the formerly indirect branch with a direct branch to the thunk. thunk-extern does the branch replacement like thunk but does not emit the thunks. thunk-inline is only available with -mindirect-branch-jump; it should be used in preference to thunk in user-space applications to support correct stack unwinding and exception handling 🧚
-mindirect-branch sets the value of -mindirect-branch-jump and -mindirect-branch-call. -mfunction-return sets the value of -mfunction-return-reg and -mfunction-return-mem.
All of these options can also be set on a per-function basis using function attributes ✨
-mindirect-branch-table-mno-indirect-branch-table
This option is useful in conjunction with the -mindirect-branch and -mfunction-return options. When enabled, it causes generation of tables pointing to the branch locations which have been patched by those options. The tables are emitted in sections named .s390_indirect_jump, .s390_indirect_call, .s390_return_reg, and .s390_return_mem. Each section consists of an array of 32-bit elements; each entry holds the offset from the entry to the patched location 📋
-mfentry-mno-fentry
When used in conjunction with the -pg option, emit profiling code using the __fentry__ hook provided by GLIBC 2.29 or later. This is only available for 64-bit code. If this option is not enabled, profiling uses the less efficient _mcount hook instead 😭
-mrecord-mcount-mno-record-mcount
When enabled, generate a __mcount_loc section with the locations of all generated profiling calls to _mcount or __fentry__ 🌸
mnop-mcountmno-nop-mcount
Instead of generating calls to _mcount or __fentry__ with -pg, insert a sequence no-op instructions of the same length at the locations where the calls would have been inserted. This can be used in conjunction with the -mrecord-mcount option to patch the call sequences into the object file without recompiling it 🔥
-mpreserve-args-mno-preserve-args
When enabled, save all argument registers to the stack, and generate corresponding CFI information. This is useful for applications that want to implement their own stack unwinding and need access to function arguments 🫶
-munaligned-symbols-mno-unaligned-symbols
Assume that external symbols with a natural alignment of 1 are potentially unaligned. By default, all symbols without explicit alignment are assumed to be aligned on a 2-byte boundary as mandated by the IBM Z ABI 👑
🌈 SH Options 🌈
These -m options are defined for the SH implementations, bestie, let's get into it 💖
-m1— Generate code for the SH1.-m2— Generate code for the SH2.-m2e— Generate code for the SH2e.-m2a-nofpu— code for the SH2a with no FPU at all, or for a SH2a-FPU but set up so the floating-point unit never gets touched. 💅-m2a-single-only— code for the SH2a-FPU where double-precision floating-point operations are literally not used, periodt.-m2a-single— code for the SH2a-FPU, assuming the FPU is chilling in single-precision mode by default.-m2a— code for the SH2a-FPU, assuming the FPU is in double-precision mode by default.-m3— code for the SH3. that's it, that's the whole vibe ✨-m3e— code for the SH3e.-m4-nofpu— code for the SH4, but the floating-point unit? not invited.-m4-single-only— code for the SH4 with an FPU that only does single-precision arithmetic. double who? never heard of her 🙈-m4-single— code for the SH4, assuming the FPU defaults to single-precision mode.-m4— code for the SH4. the classic 👑-m4-100— code for the SH4-100.-m4-100-nofpu— code for the SH4-100 in such a way that the floating-point unit is not used.-m4-100-single— code for the SH4-100, assuming the FPU is in single-precision mode by default.-m4-100-single-only— code for the SH4-100 in such a way that no double-precision floating-point operations are used.-m4-200— code for the SH4-200.-m4-200-nofpu— code for the SH4-200 without in such a way that the floating-point unit is not used. (yes bestie, the original really does say "without in such a way", we don't ask questions, we just report the facts 💀)-m4-200-single— code for the SH4-200, assuming the FPU is in single-precision mode by default.-m4-200-single-only— code for the SH4-200 in such a way that no double-precision floating-point operations are used.-m4-300— code for the SH4-300.-m4-300-nofpu— code for the SH4-300 without in such a way that the floating-point unit is not used.-m4-300-single— code for the SH4-300 in such a way that no double-precision floating-point operations are used.-m4-300-single-only— code for the SH4-300 in such a way that no double-precision floating-point operations are used.-m4-340— code for the SH4-340 (no MMU, no FPU).-m4-400— code for the SH4-400 (no MMU, no FPU).-m4-500— code for the SH4-500 (no FPU). It passes-isa=sh4-nofputo the assembler. 🛠️-m4a-nofpu— code for the SH4al-dsp, or for a SH4a in such a way that the floating-point unit is not used.-m4a-single-only— code for the SH4a in such a way that no double-precision floating-point operations are used.-m4a-single— code for the SH4a, assuming the FPU is in single-precision mode by default.-m4a— code for the SH4a.-m4al— same as-m4a-nofpu, except it implicitly passes-dspto the assembler. Heads up: GCC doesn't generate any DSP instructions at the moment. 🫠-mb— compile for the processor in big-endian mode.-ml— compile for the processor in little-endian mode.-mdalign— align doubles at 64-bit boundaries. > Careful bestie: this changes the calling conventions, so 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; it uses the linker option-relax.-mbigtable— use 32-bit offsets inswitchtables. The default is to use 16-bit offsets.-mbitops— turn on the use of bit manipulation instructions on SH2A. 🎛️-mfmovd— enable use of the instructionfmovd. Go check-mdalignfor the alignment constraints. 🔍-mrenesas,-mno-renesas— comply with the calling conventions defined by Renesas. The default for all targets of the SH toolchain is-mno-renesas, which uses the calling conventions defined for GCC before the Renesas conventions existed. ✨-mnomacsave— mark theMACregister as call-clobbered, even if-mrenesasis given.-mieee,-mno-ieee— control the IEEE compliance of floating-point comparisons, which affects how we handle cases where the result of a comparison is unordered. By default-mieeeis implicitly enabled. If-ffinite-math-onlyis enabled,-mno-ieeeis implicitly set, which gives you faster floating-point greater-equal and less-equal comparisons. You can override those implicit settings by specifying either-mieeeor-mno-ieee. 🧠-minline-ic_invalidate,-mno-inline-ic_invalidate— inline code to invalidate instruction cache entries after setting up nested function trampolines. This option has no effect if-musermodeis in effect and the selected code generation option (e.g.-m4) does not allow the use of theicbiinstruction. If the selected code generation option does not allow the use of theicbiinstruction, and-musermodeis not in effect, the inlined code manipulates the instruction cache address array directly with an associative write. > That not only requires privileged mode at run time, but it also fails if the cache line had been mapped via the TLB and has become unmapped. 💀-misize— dump instruction size and location in the assembly code. 📏-matomic-model=model— sets the model of atomic operations, plus any additional parameters, as a comma-separated list. For the deets on the atomic built-in functions see__atomicBuiltins. Here are the supported models and parameters:none— disable compiler generated atomic sequences and emit library calls for atomic operations. This is the default if the target is notsh*-*-linux*.soft-gusa— generate GNU/Linux-compatible gUSA software atomic sequences for the atomic built-in functions. The generated atomic sequences need extra support from the interrupt/exception handling code of the system and are only suitable for SH3* and SH4* single-core systems. This option is enabled by default when the target issh*-*-linux*and SH3* or SH4*. When the target is SH4A, this option also partially utilizes the hardware atomic instructionsmovli.landmovco.lto create more efficient code, unlessstrictis specified.soft-tcb— generate software atomic sequences that use a variable in the thread control block. It's a variation of the gUSA sequences that can also be used on SH1* and SH2* targets. The generated atomic sequences need extra support from the interrupt/exception handling code of the system and are only suitable for single-core systems. When you use this model, thegbr-offset=parameter has to be specified as well.soft-imask— generate software atomic sequences that temporarily disable interrupts by settingSR.IMASK = 1111. This model works only when the program runs in privileged mode and is only suitable for single-core systems. No extra support from the interrupt/exception handling code of the system is required. This model is enabled by default when the target issh*-*-linux*and SH1* or SH2*.hard-llcs— generate hardware atomic sequences using only themovli.landmovco.linstructions. This is only available on SH4A and is suitable for multi-core systems. Since the hardware instructions support only 32-bit atomic variables, access to 8- or 16-bit variables is emulated with 32-bit accesses. Code compiled with this option is also compatible with other software atomic model interrupt/exception handling systems if executed on an SH4A system. No extra support from the interrupt/exception handling code of the system is required for this model.gbr-offset=— this parameter specifies the offset in bytes of the variable in the thread control block structure that the generated atomic sequences should use when thesoft-tcbmodel has been selected. For other models this parameter is ignored. The value you give must be an integer multiple of four and in the range 0-1020.strict— this parameter prevents mixed usage of multiple atomic models, even if they are compatible, and makes the compiler generate atomic sequences of the specified model only.
-mtas— generate thetas.bopcode for__atomic_test_and_set. > Heads up: depending on the particular hardware and software configuration this can degrade overall performance because of the operand cache line flushes that thetas.binstruction implies. On multi-core SH4A processors thetas.binstruction must be used with caution since it can result in data corruption for certain cache configurations. 😭-mprefergot— when generating position-independent code, emit function calls using the Global Offset Table instead of the Procedure Linkage Table.-musermode,-mno-usermode— don't allow (allow) the compiler to generate privileged mode code. Specifying-musermodealso implies-mno-inline-ic_invalidateif the inlined code would not work in user mode.-musermodeis the default when the target issh*-*-linux*. If the target is SH1* or SH2*,-musermodehas no effect, since there is no user mode. 🫶-multcost=number— set the cost to assume for a multiply instruction. 🔢-mdiv=strategy— set the division strategy to be used for integer division operations. strategy can be one of:call-div1— calls a library function that uses the single-step division instructiondiv1to perform the operation. Division by zero calculates an unspecified result and does not trap. This is the default except for SH4, SH2A and SHcompact.call-fp— calls a library function that performs the operation in double precision floating point. Division by zero causes a floating-point exception. This is the default for SHcompact with FPU. Specifying this for targets that do not have a double-precision FPU defaults tocall-div1.
call-table— calls a library function that uses a lookup table for small divisors and thediv1instruction with case distinction for larger divisors. Division by zero calculates an unspecified result and does not trap. This is the default for SH4. If you specify this for targets that do not have dynamic shift instructions, it defaults tocall-div1.
Okay so when you haven't specified a division strategy at all, the default gets picked based on the current target. For SH2A, the default strategy is to use the divs and divu instructions instead of library function calls. 💅
-maccumulate-outgoing-args-mno-accumulate-outgoing-args
Reserve space once for outgoing arguments in the function prologue instead of around each call. This is generally beneficial for performance and size, and it's also needed for unwinding to avoid changing the stack frame around conditional code. -maccumulate-outgoing-args is enabled by default. ✨
-mdivsi3_libfunc=name
Set the name of the library function used for 32-bit signed division to name. This only affects the name used in the call division strategies, and the compiler still expects the same sets of input/output/clobbered registers as if this option were not present. No cap.
-mfixed-range=register-range
Generate code treating the given register range as fixed registers. A fixed register is one that the register allocator cannot use. This is useful when compiling kernel code. A register range is specified as two registers separated by a dash. Multiple register ranges can be specified separated by a comma. 🫶
-mbranch-cost=num
Assume num to be the cost for a branch instruction. Higher numbers make the compiler try to generate more branch-free code if possible. If not specified, the value is selected depending on the processor type that is being compiled for.
-mzdcbranch-mno-zdcbranch
Assume (or do not assume) that zero displacement conditional branch instructions bt and bf are fast. If -mzdcbranch is specified, the compiler prefers zero displacement branch code sequences. This is enabled by default when generating code for SH4 and SH4A. It can be explicitly disabled by specifying -mno-zdcbranch.
-mcbranch-force-delay-slot
Force the usage of delay slots for conditional branches, which stuffs the delay slot with a nop if a suitable instruction cannot be found. By default this option is disabled. It can be enabled to work around hardware bugs as found in the original SH7055. 💀
-mfsca-mno-fsca
Allow or disallow the compiler to emit the fsca instruction for sine and cosine approximations. The option -mfsca must be used in combination with -funsafe-math-optimizations. It is enabled by default when generating code for SH4A. Using -mno-fsca disables sine and cosine approximations even if -funsafe-math-optimizations is in effect.
-mfsrra-mno-fsrra
Allow or disallow the compiler to emit the fsrra instruction for reciprocal square root approximations. The option -mfsrra must be used in combination with -funsafe-math-optimizations and -ffinite-math-only. It is enabled by default when generating code for SH4A. Using -mno-fsrra disables reciprocal square root approximations even if -funsafe-math-optimizations and -ffinite-math-only are in effect.
-mpretend-cmove-mno-pretend-cmove
Prefer or don't prefer zero-displacement conditional branches for conditional move instruction patterns. This can result in faster code on the SH4 processor. 🔥
-mfdpic-mno-fdpic
Generate code using the FDPIC ABI.
-mlra-mno-lra
Use the new LRA register allocator. By default, the old reload allocator is used.
Solaris 2 Options ☀️
These options are supported on Solaris 2:
-mclear-hwcap-mno-clear-hwcap
-mclear-hwcap tells the compiler to remove the hardware capabilities generated by the Solaris assembler. This is only necessary when object files use ISA extensions not supported by the current machine, but check at runtime whether or not to use them.
-mimpure-text-mno-impure-text
-mimpure-text, used in addition to -shared, tells the compiler to not pass -z text to the linker when linking a shared object. Using this option, you can link position-dependent code into a shared object.
-mimpure-text suppresses the relocations remain against allocatable but non-writable sections linker error message. However, the necessary relocations trigger copy-on-write, and the shared object is not actually shared across processes. Instead of using -mimpure-text, you should compile all source code with -fpic or -fPIC.
bestie I'm being so serious rn — that last part is not a suggestion, it's the move. ✋
-gsctf
Generate Solaris CTF. Needs to be used both for compilation and linking. See ctf(7) for more information. This is only supported since Solaris 11.4 SRU 84 where the necessary toolchain support was added.
SPARC Options 🌸
These -m options are supported on the SPARC:
-mno-app-regs-mapp-regs
Specify -mapp-regs to generate output using the global registers 2 through 4, which the SPARC SVR4 ABI reserves for applications. Like the global register 1, each global register 2 through 4 is then treated as an allocable register that is clobbered by function calls. This is the default.
To be fully SVR4 ABI-compliant at the cost of some performance loss, specify -mno-app-regs. You should compile libraries and system software with this option.
-mflat-mno-flat
With -mflat, the compiler does not generate save/restore instructions and uses a "flat" or single register window model. This model is compatible with the regular register window model. The local registers and the input registers (0--5) are still treated as "call-saved" registers and are saved on the stack as needed.
With -mno-flat (the default), the compiler generates save/restore instructions (except for leaf functions). This is the normal operating mode.
-mfpu-mhard-float
Generate output containing floating-point instructions. This is the default. 💖
-mno-fpu-msoft-float
Generate output containing library calls for floating point.
⚠️ Warning: the requisite libraries are not available for all SPARC targets. Normally the facilities of the machine's usual C compiler are used, but this cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation. The embedded targetssparc-*-aoutandsparclite-*-*do provide software floating-point support.
-msoft-float changes the calling convention in the output file; therefore, it is only useful if you compile all of a program with this option. In particular, you need to compile libgcc.a, the library that comes with GCC, with -msoft-float in order for this to work.
-mhard-quad-float
Generate output containing quad-word (long double) floating-point instructions.
-msoft-quad-float
Generate output containing library calls for quad-word (long double) floating-point instructions. The functions called are those specified in the SPARC ABI. This is the default.
As of this writing, there are no SPARC implementations that have hardware support for the quad-word floating-point instructions. They all invoke a trap handler for one of these instructions, and then the trap handler emulates the effect of the instruction. Because of the trap handler overhead, this is much slower than calling the ABI library routines. Thus the -msoft-quad-float option is the default.
-mno-unaligned-doubles-munaligned-doubles
Assume that doubles have 8-byte alignment. This is the default.
With -munaligned-doubles, GCC assumes that doubles have 8-byte alignment only if they are contained in another type, or if they have an absolute address. Otherwise, it assumes they have 4-byte alignment. Specifying this option avoids some rare compatibility problems with code generated by other compilers. It is not the default because it results in a performance loss, especially for floating-point code.
-muser-mode-mno-user-mode
Do not generate code that can only run in supervisor mode. This is relevant only for the casa instruction emitted for the LEON3 processor. This is the default.
-mfaster-structs-mno-faster-structs
With -mfaster-structs, the compiler assumes that structures should have 8-byte alignment. This enables the use of pairs of ldd and std instructions for copies in structure assignment, in place of twice as many ld and st pairs. However, the use of this changed alignment directly violates the SPARC ABI. Thus, it's intended only for use on targets where the developer acknowledges that their resulting code is not directly in line with the rules of the ABI. 🫣
-mstd-struct-return-mno-std-struct-return
With -mstd-struct-return, the compiler generates checking code in functions returning structures or unions to detect size mismatches between the two sides of function calls, as per the 32-bit ABI.
The default is -mno-std-struct-return. This option has no effect in 64-bit mode.
-mcpu=cpu_type
Set the instruction set, register set, and instruction scheduling parameters for machine type cpu_type. Supported values for cpu_type are v7, cypress, v8, supersparc, hypersparc, leon, leon3, leon3v7, leon5, sparclite, f930, f934, sparclite86x, sparclet, tsc701, v9, ultrasparc, ultrasparc3, niagara, niagara2, niagara3, niagara4, niagara7 and m8.
Native Solaris and GNU/Linux toolchains also support the value native, which selects the best architecture option for the host processor. -mcpu=native has no effect if GCC does not recognize the processor.
Default instruction scheduling parameters are used for values that select an architecture and not an implementation. These are v7, v8, sparclite, sparclet, v9.
Here is a list of each supported architecture and their supported implementations. 🦋
v7—cypress,leon3v7v8—supersparc,hypersparc,leon,leon3,leon5sparclitef930,f934,sparclite86x
sparclettsc701
v9—ultrasparc,ultrasparc3,niagara,niagara2,niagara3,niagara4,niagara7,m8
Okay so by default (unless your build was configured to be different, iykyk) GCC spits out code for the V7 variant of the SPARC architecture. Add -mcpu=cypress and the compiler also optimizes for the Cypress CY7C602 chip — the one that lives in the SPARCStation/SPARCServer 3xx series. It's also the move for the older SPARCStation 1, 2, IPX etc. 🫶
With -mcpu=v8, GCC generates code for the V8 variant of the SPARC architecture. The only glow-up 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. And with -mcpu=supersparc, it additionally optimizes 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 for the Fujitsu MB86930 chip — that's the original SPARClite, with no FPU. With -mcpu=f934, it additionally optimizes 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 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. Then it's a whole roster of extra tuning:
-mcpu=ultrasparc— additionally optimizes for the Sun UltraSPARC I/II/IIi chips-mcpu=ultrasparc3— additionally optimizes for the Sun UltraSPARC III/III+/IIIi/IIIi+/IV/IV+ chips-mcpu=niagara— additionally optimizes for Sun UltraSPARC T1 chips-mcpu=niagara2— additionally optimizes for Sun UltraSPARC T2 chips-mcpu=niagara3— additionally optimizes for Sun UltraSPARC T3 chips-mcpu=niagara4— additionally optimizes for Sun UltraSPARC T4 chips-mcpu=niagara7— additionally optimizes for Oracle SPARC M7 chips-mcpu=m8— additionally optimizes for Oracle M8 chips 👑
-mtune=cpu_type— sets the instruction scheduling parameters for machine type cpu_type, but does not set the instruction set or register set that-mcpu=cpu_typedoes. Same values work here as for-mcpu=cpu_type, but the only ones that actually do anything useful are the ones picking a specific CPU implementation:cypress,supersparc,hypersparc,leon,leon3,leon3v7,leon5,f930,f934,sparclite86x,tsc701,ultrasparc,ultrasparc3,niagara,niagara2,niagara3,niagara4,niagara7andm8. And with native Solaris and GNU/Linux toolchains,nativeis also on the menu. 🎀-mv8plus,-mno-v8plus— with-mv8plus, GCC generates code for the SPARC-V8+ ABI. The difference from the V8 ABI? The global and out registers are treated as 64 bits wide. This is on by default on Solaris in 32-bit mode for all SPARC-V9 processors. ✨-mvis,-mno-vis— with-mvis, GCC generates code that takes advantage of the UltraSPARC Visual Instruction Set extensions. Default is-mno-vis.-mvis2,-mno-vis2— with-mvis2, GCC generates code that takes advantage of version 2.0 of the UltraSPARC Visual Instruction Set extensions. Default is-mvis2when targeting a cpu that supports such instructions, like UltraSPARC-III and later. Setting-mvis2also sets-mvis. 🦋-mvis3,-mno-vis3— with-mvis3, GCC generates code that takes advantage of version 3.0 of the UltraSPARC Visual Instruction Set extensions. Default is-mvis3when targeting a cpu that supports such instructions, like niagara-3 and later. Setting-mvis3also sets-mvis2and-mvis. ✨-mvis3b,-mno-vis3b— with-mvis3b, GCC generates code that takes advantage of version 3.0 of the UltraSPARC Visual Instruction Set extensions, plus the additional VIS instructions introduced in the Oracle SPARC Architecture 2011. Default is-mvis3bwhen targeting a cpu that supports such instructions, like niagara-7 and later. Setting-mvis3balso sets-mvis3,-mvis2and-mvis. 💅-mvis4,-mno-vis4— with-mvis4, GCC generates code that takes advantage of version 4.0 of the UltraSPARC Visual Instruction Set extensions. Default is-mvis4when targeting a cpu that supports such instructions, like niagara-7 and later. Setting-mvis4also sets-mvis3b,-mvis3,-mvis2and-mvis. 🌈-mvis4b,-mno-vis4b— with-mvis4b, GCC generates code that takes advantage of version 4.0 of the UltraSPARC Visual Instruction Set extensions, plus the additional VIS instructions introduced in the Oracle SPARC Architecture 2017. Default is-mvis4bwhen targeting a cpu that supports such instructions, like m8 and later. Setting-mvis4balso sets-mvis4,-mvis3b,-mvis3,-mvis2and-mvis. 👑-mcbcond,-mno-cbcond— with-mcbcond, GCC generates code that takes advantage of the UltraSPARC Compare-and-Branch-on-Condition instructions. Default is-mcbcondwhen targeting a CPU that supports such instructions, like Niagara-4 and later.-mfmaf,-mno-fmaf— with-mfmaf, GCC generates code that takes advantage of the UltraSPARC Fused Multiply-Add Floating-point instructions. Default is-mfmafwhen targeting a CPU that supports such instructions, like Niagara-3 and later. 🔥-mfsmuld,-mno-fsmuld— with-mfsmuld, GCC generates code that takes advantage of the Floating-point Multiply Single to Double (FsMULd) instruction. Default is-mfsmuldwhen targeting a CPU supporting the architecture versions V8 or V9 with FPU — except-mcpu=leon. 🙈-mpopc,-mno-popc— with-mpopc, GCC generates code that takes advantage of the UltraSPARC Population Count instruction. Default is-mpopcwhen targeting a CPU that supports such an instruction, like Niagara-2 and later.-msubxc,-mno-subxc— with-msubxc, GCC generates code that takes advantage of the UltraSPARC Subtract-Extended-with-Carry instruction. Default is-msubxcwhen targeting a CPU that supports such an instruction, like Niagara-7 and later. 💜-mfix-at697f— enables the documented workaround for the single erratum of the Atmel AT697F processor (which corresponds to erratum #13 of the AT697E processor).-mfix-ut699— enables the documented workarounds for the floating-point errata and the data cache nullify errata of the UT699 processor.-mfix-ut700— enables the documented workaround for the back-to-back store errata of the UT699E/UT700 processor.-mfix-gr712rc— enables the documented workaround for the back-to-back store errata of the GR712RC processor.
These -m options are supported in addition to the above on SPARC-V9 processors in 64-bit environments: 🧚
-m32,-m64— generate code for a 32-bit or 64-bit environment. The 32-bit environment setsint,longand pointer to 32 bits. The 64-bit environment setsintto 32 bits andlongand pointer to 64 bits.-mptr32,-mptr64— use 32- or 64-bit pointers, respectively. Unlike the-m32and-m64options, this affects only the pointer size and not the ABI.-mcmodel=which— set the code model to one of:medlow— the Medium/Low code model: 64-bit addresses, programs must be linked in the low 32 bits of memory. Programs can be statically or dynamically linked.medmid— the Medium/Middle code model: 64-bit addresses, programs must be linked in the low 44 bits of memory, the text and data segments must be less than 2GB in size and the data segment must be located within 2GB of the text segment.medany— the Medium/Anywhere code model: 64-bit addresses, programs may be linked anywhere in memory, the text and data segments must be less than 2GB in size and the data segment must be located within 2GB of the text segment.embmedany— the Medium/Anywhere code model for embedded systems: 64-bit addresses, the text and data segments must be less than 2GB in size, both starting anywhere in memory (determined at link time). The global register%g4points to the base of the data segment. Programs are statically linked and PIC is not supported.
-mmemory-model=mem-model— set the memory model in force on the processor to one of:default— the default memory model for the processor and operating system.rmo— Relaxed Memory Orderpso— Partial Store Ordertso— Total Store Ordersc— Sequential Consistency
These memory models are formally defined in Appendix D of the SPARC-V9 architecture manual, as set in the processor's "PSTATE.MM" field. 📖
-mstack-bias,-mno-stack-bias— with-mstack-bias, GCC assumes that the stack pointer, and frame pointer if present, are offset by-2047which must be added back when making stack frame references. This is the default in 64-bit mode. Otherwise, assume no such offset is present. 😭
💾 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-symbolicor-sharedbe used instead. 🫶-YP,dirs— search the directories dirs, and no others, for libraries specified with-l.-Ym,dir— look in the directory dir to find the M4 preprocessor. The assembler uses this option.
🌈 V850 Options
These -m options are defined for V850 implementations:
-mlong-calls,-mno-long-calls— treat all calls as being far away (near). If calls are assumed to be far away, the compiler always loads the function's address into a register, and calls indirect through the pointer. 📞-mno-ep,-mep— okay so bestie, these two are a lil yin-yang 💫.-mno-epsays do not optimize basic blocks that use the same index pointer 4 or more times to copy that pointer into the"ep"register and then use the shorter"sld"and"sst"instructions.-mepsays do it. And plot twist:-mepis on by default if you optimize ✨-mno-prolog-function,-mprolog-function— do not use (or do use) external functions to save and restore registers at the prologue and epilogue of a function. The external functions are slower, but they use less code space if more than one function saves the same number of registers.-mprolog-functionis on by default if you optimize 🫶-mspace— try to make the code as small as possible. Right now it's giving minimalism, and it does that by just turning on the-mepand-mprolog-functionoptions 🎀-mtda=n— put static or global variables whose size is n bytes or less into the tiny data area that register"ep"points to. Heads up: the tiny data area can hold up to 256 bytes in total (128 bytes for byte references). Tiny but mighty fr fr 🐜-msda=n— put static or global variables whose size is n bytes or less into the small data area that register"gp"points to. That small data area can hold up to 64 kilobytes ✨-mzda=n— put static or global variables whose size is n bytes or less into the first 32 kilobytes of memory 💅-mv850— specify that the target processor is the V850.-mv850e3v5— specify that the target processor is the V850E3V5. The preprocessor constant"__v850e3v5__"gets defined if you use this option 👑-mv850e2v4— specify that the target processor is the V850E3V5. This is an alias for the-mv850e3v5option. (Yes, that's literally what the page says, we don't make the rules 💀)-mv850e2v3— specify that the target processor is the V850E2V3. The preprocessor constant"__v850e2v3__"is defined if this option is used ✨-mv850e2— specify that the target processor is the V850E2. The preprocessor constant"__v850e2__"is defined if this option is used 🌸-mv850e1— specify that the target processor is the V850E1. The preprocessor constants"__v850e1__"and"__v850e__"are both defined if this option is used 🫶-mv850es— specify that the target processor is the V850ES. This is an alias for the-mv850e1option 💫-mv850e— specify that the target processor is the V850E. The preprocessor constant"__v850e__"is defined if this option is used 🌈
okay so if neither-mv850nor-mv850enor-mv850e1nor-mv850e2nor-mv850e2v3nor-mv850e3v5are defined, then a default target processor is chosen and the relevant__v850*__preprocessor constant gets defined. And the preprocessor constants"__v850"and"__v851__"are always defined, no matter which processor variant is the target. Always. Iconic consistency 💖
-mdisable-callt,-mno-disable-callt— this option suppresses generation of the"CALLT"instruction for the v850e, v850e1, v850e2, v850e2v3 and v850e3v5 flavors of the v850 architecture.
It's enabled by default when the RH850 ABI is in use (peep -mrh850-abi), and disabled by default when the GCC ABI is in use. If "CALLT" instructions are being generated then the C preprocessor symbol "__V850_CALLT__" is defined 🔥
-mrelax,-mno-relax— pass on (or do not pass on) the-mrelaxcommand-line option to the assembler. Passing the vibes along, respectfully 🎀-mlong-jumps,-mno-long-jumps— disable (or re-enable) the generation of PC-relative jump instructions ✨-msoft-float,-mhard-float— disable (or re-enable) the generation of hardware floating point instructions. This option is only significant when the target architecture is V850E2V3 or higher. If hardware floating point instructions are being generated then the C preprocessor symbol"__FPU_OK__"is defined, otherwise the symbol"__NO_FPU__"is defined 💅-mloop— enables the use of the e3v5 LOOP instruction. The use of this instruction is not enabled by default when the e3v5 architecture is selected, because its use is still experimental 🙈-mrh850-abi,-mghs— enables support for the RH850 version of the V850 ABI. This is the default. With this version of the ABI, the following rules apply:- Integer sized structures and unions are returned via a memory pointer rather than a register.
- Large structures and unions (more than 8 bytes in size) are passed by value.
- Functions are aligned to 16-bit boundaries.
- The
-m8byte-aligncommand-line option is supported. - The
-mdisable-calltcommand-line option is enabled by default. The-mno-disable-calltcommand-line option is not supported.
When this version of the ABI is enabled, the C preprocessor symbol "__V850_RH850_ABI__" is defined 👑
-mgcc-abi— enables support for the old GCC version of the V850 ABI. With this version of the ABI, the following rules apply:- Integer sized structures and unions are returned in register
"r10". - Large structures and unions (more than 8 bytes in size) are passed by reference.
- Functions are aligned to 32-bit boundaries, unless optimizing for size.
- The
-m8byte-aligncommand-line option is not supported. - The
-mdisable-calltcommand-line option is supported but not enabled by default.
- Integer sized structures and unions are returned in register
When this version of the ABI is enabled, the C preprocessor symbol "__V850_GCC_ABI__" is defined 🌸
-m8byte-align,-mno-8byte-align— enables support for"double"and"long long"types to be aligned on 8-byte boundaries. The default is to restrict the alignment of all objects to at most 4-bytes. When-m8byte-alignis in effect, the C preprocessor symbol"__V850_8BYTE_ALIGN__"is defined 💫-mbig-switch,-mno-big-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. It's giving emergency button 🚨-mapp-regs,-mno-app-regs—-mapp-regscauses r2 and r5 to be used in the code generated by the compiler. This setting is the default.-mno-app-regscauses r2 and r5 to be treated as fixed registers 🎀-msmall-sld,-mno-small-sld— enable or disable the use of the short load instructions ✨-mno-strict-align— do not enforce strict data alignment 🌈-mjump-tables-in-data-section,-mno-jump-tables-in-data-section— enable or disable placement of jump tables for switch statements in the".data"section rather than the".code"section. The default is-mno-jump-tables-in-data-section💅
🦋 VAX Options 🦋
These -m options are defined for the VAX:
-munix— do not output certain jump instructions ("aobleq"and so on) that the Unix assembler for the VAX cannot handle across long ranges 🙈-mgnu— do output those jump instructions, on the assumption that the GNU assembler is being used 🫶-md,-md-float— use the D_floating data format for double-precision floating-point numbers instead of G_floating.-mg,-mg-float— use the G_floating data format for double-precision floating-point numbers instead of D_floating.-mlra,-mno-lra— enable Local Register Allocation. This is still experimental for the VAX, so by default the compiler uses standard reload ⚠️-mvaxc-alignment— use VAXC conventions for alignment of structure members.-mqmath,-mno-qmath— enable or disable new instruction patterns for 64-bit integer addition and subtraction ✨
🌸 Visium Options 🌸
-mdebug— use this option when linking programs that perform file I/O and are destined to run on an MCM target. It causes the librarieslibc.aandlibdebug.ato be linked. You should run the program on the target under the control of the GDB remote debugging stub 🐛-msim— use this option when linking programs that perform file I/O and are destined to run on the simulator. It causes the librarieslibc.aandlibsim.ato be linked 🎀-mfpu,-mhard-float— generate code containing floating-point instructions. This is the default 💅-mno-fpu,-msoft-float— generate code containing library calls for floating-point.
bestie listen:-msoft-floatchanges the calling convention in the output file, so it is only useful if you compile all of a program with this option. In particular, you need to compilelibgcc.a, the library that comes with GCC, with-msoft-floatin order for this to work. Don't do half the program, that's a mess 😭
-mcpu=cpu_type— set the instruction set, register set, and instruction scheduling parameters for machine type cpu_type. Supported values for cpu_type aremcm,gr5andgr6.
mcm is a synonym of gr5 present for backward compatibility.
By default (unless configured otherwise), GCC generates code for the GR5 variant of the Visium architecture.
With -mcpu=gr6, GCC generates code for the GR6 variant of the Visium architecture. The only difference from GR5 code is that the compiler will generate block move instructions ✨
-mtune=cpu_type— set the instruction scheduling parameters for machine type cpu_type, but do not set the instruction set or register set that the option-mcpu=cpu_typewould 🎀-msv-mode— generate code for the supervisor mode, where there are no restrictions on the access to general registers. This is the default 👑-muser-mode— generate code for the user mode, where the access to some general registers is forbidden: on the GR5, registers r24 to r31 cannot be accessed in this mode; on the GR6, only registers r29 to r31 are affected 🙈
💜 VMS Options 💜
These -m options are defined for the VMS implementations:
-mvms-return-codes— return VMS condition codes from"main". The default is to return POSIX-style condition (e.g. error) codes 🌈-mdebug-main=prefix— flag the first routine whose name starts with prefix as the main routine for the debugger 🐛-mmalloc64— default to 64-bit memory allocation routines.-mpointer-size=size— set the default size of pointers. Possible options for size are32orshortfor 32 bit pointers,64orlongfor 64 bit pointers, andnofor supporting only 32 bit pointers. The latter option disables"pragma pointer_size"⚠️
🧚 VxWorks Options 🧚
The options in this section are defined for all VxWorks targets. Options specific to the target hardware are listed with the other options for that target 💖
-mrtp— okay so GCC can generate code for both VxWorks kernels and real time processes (RTPs). This option switches from the former to the latter, bestie. It also defines the preprocessor macro__RTP__. 💖-msmp— Select SMP runtimes for linking. Not available on architectures other than PowerPC, nor on VxWorks version 7 or later, in which the selection is part of the VxWorks build configuration and the library paths are the same for either choice. 🫶-mvthreads— Link against the VxWorks AE vThread runtime environment. Periodt.-non-static— Link an RTP executable against shared libraries rather than static libraries. The options-staticand-sharedcan also be used for RTPs;-staticis the default. 👑-Bstatic,-Bdynamic— These options are passed down to the linker. They are defined for compatibility with Diab. (iykyk)-Xbind-lazy— Enable lazy binding of function calls. This option is equivalent to-Wl,-z,nowand is defined for compatibility with Diab.-Xbind-now— Disable lazy binding of function calls. This option is the default and is defined for compatibility with Diab.
x86 Options ✨
This section documents -m options available for the x86 family of computers. 🌈
The following group of options allows compilation to target a specific processor. Slay.
-march=cpu-type— Generate instructions for the machine type cpu-type. In contrast to-mtune=cpu-type, which merely tunes the generated code for the specified cpu-type,-march=cpu-typeallows GCC to generate code that may not run at all on processors other than the one indicated. Specifying-march=cpu-typeimplies-mtune=cpu-type, except where noted otherwise. ⚠️
The choices for cpu-type are:
native— This selects the CPU to generate code for at compilation time by determining the processor type of the compiling machine. Using-march=nativeenables all instruction subsets supported by the local machine (hence the result might not run on different machines). Using-mtune=nativeproduces code optimized for the local machine under the constraints of the selected instruction set. 🏠x86-64— A generic CPU with 64-bit extensions, MMX, SSE, SSE2, and FXSR instruction set support.x86-64-v2,x86-64-v3,x86-64-v4— These choices for cpu-type select the corresponding micro-architecture level from the x86-64 psABI. On ABIs other than the x86-64 psABI they select the same CPU features as the x86-64 psABI documents for the particular micro-architecture level.
Since these cpu-type values do not have a corresponding -mtune setting, using -march with these values enables generic tuning. Specific tuning can be enabled using the -mtune=other-cpu-type option with an appropriate other-cpu-type value. 💅
i386— Original Intel i386 CPU.i486— Intel i486 CPU. (No scheduling is implemented for this chip.)i586,pentium— Intel Pentium CPU with no MMX support.lakemont— Intel Lakemont MCU, based on Intel Pentium CPU.pentium-mmx— Intel Pentium MMX CPU, based on Pentium core with MMX instruction set support.pentiumpro— Intel Pentium Pro CPU with no MMX support.i686— When used with-march, the Pentium Pro instruction set is used, so the code runs on all i686 family chips. When used with-mtune, it has the same meaning asgeneric.pentium2— Intel Pentium II CPU, based on Pentium Pro core with MMX and FXSR instruction set support.pentium3,pentium3m— Intel Pentium III CPU, based on Pentium Pro core with MMX, FXSR and SSE instruction set support.pentium-m— Intel Pentium M; low-power version of Intel Pentium III CPU with MMX, SSE, SSE2 and FXSR instruction set support. Used by Centrino notebooks.pentium4,pentium4m— Intel Pentium 4 CPU with MMX, SSE, SSE2 and FXSR instruction set support.prescott— Improved version of Intel Pentium 4 CPU with MMX, SSE, SSE2, SSE3 and FXSR instruction set support.nocona— Improved version of Intel Pentium 4 CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, CX16 and FXSR instruction set support.core2— Intel Core 2 CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, CX16, SAHF and FXSR instruction set support.nehalem,corei7— Intel Nehalem CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF and FXSR instruction set support.westmere— Intel Westmere CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR and PCLMUL instruction set support.sandybridge,corei7-avx— Intel Sandy Bridge CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE and PCLMUL instruction set support.ivybridge,core-avx-i— Intel Ivy Bridge CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND and F16C instruction set support.haswell,core-avx2— Intel Haswell CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE and HLE instruction set support.broadwell— Intel Broadwell CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX and PREFETCHW instruction set support.skylake— Intel Skylake CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES and SGX instruction set support.skylake-avx512— Intel Skylake Server CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, CLWB, AVX512VL, AVX512BW, AVX512DQ and AVX512CD instruction set support.cascadelake— Intel Cascade Lake CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, CLWB, AVX512VL, AVX512BW, AVX512DQ, AVX512CD and AVX512VNNI instruction set support.cannonlake— Intel Cannon Lake Server CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, PKU, AVX512VBMI, AVX512IFMA and SHA instruction set support.cooperlake— Intel Cooper Lake CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, CLWB, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, AVX512VNNI and AVX512BF16 instruction set support.icelake-client— Intel Ice Lake Client CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2 , VPCLMULQDQ, AVX512BITALG, RDPID and AVX512VPOPCNTDQ instruction set support.icelake-server— Intel Ice Lake Server CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2 , VPCLMULQDQ, AVX512BITALG, RDPID, AVX512VPOPCNTDQ, PCONFIG, WBNOINVD and CLWB instruction set support.tigerlake— Intel Tiger Lake CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2, VPCLMULQDQ, AVX512BITALG, RDPID, AVX512VPOPCNTDQ, MOVDIRI, MOVDIR64B, CLWB, AVX512VP2INTERSECT and KEYLOCKER instruction set support.rocketlake— Intel Rocket Lake CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2, VPCLMULQDQ, AVX512BITALG, RDPID and AVX512VPOPCNTDQ instruction set support.
alderlake,raptorlake,meteorlake,gracemont— Intel Alder Lake/Raptor Lake/Meteor Lake/Gracemont CPU with 64-bit extensions, and bestie the feature list is long:MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, KL, WIDEKLandAVX-VNNIinstruction set support. 💅arrowlake— Intel Arrow Lake CPU with 64-bit extensions,MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, KL, WIDEKL, AVX-VNNI, UINTR, AVXIFMA, AVXVNNIINT8, AVXNECONVERTandCMPCCXADDinstruction set support. ✨arrowlake-s,lunarlake— Intel Arrow Lake S/Lunar Lake CPU with 64-bit extensions,MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, KL, WIDEKL, AVX-VNNI, UINTR, AVXIFMA, AVXVNNIINT8, AVXNECONVERT, CMPCCXADD, AVXVNNIINT16, SHA512, SM3andSM4instruction set support. 🦋pantherlake,wildcatlake— Intel Panther Lake/Wildcat Lake CPU with 64-bit extensions,MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, AVX-VNNI, UINTR, AVXIFMA, AVXVNNIINT8, AVXNECONVERT, CMPCCXADD, AVXVNNIINT16, SHA512, SM3andSM4instruction set support. 🐆novalake— Intel Nova Lake CPU with 64-bit extensions,MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, AVX-VNNI, UINTR, AVXIFMA, AVXVNNIINT8, AVXNECONVERT, CMPCCXADD, AVXVNNIINT16, SHA512, SM3, SM4, PREFETCHI, APX_F, AVX10.1, AVX10.2andMOVRSinstruction set support. 🌟sapphirerapids,emeraldrapids— Intel Sapphire Rapids/Emerald Rapids CPU with 64-bit extensions,MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2, VPCLMULQDQ, AVX512BITALG, RDPID, AVX512VPOPCNTDQ, PCONFIG, WBNOINVD, CLWB, MOVDIRI, MOVDIR64B, ENQCMD, CLDEMOTE, PTWRITE, WAITPKG, SERIALIZE, TSXLDTRK, UINTR, AMX-BF16, AMX-TILE, AMX-INT8, AVX-VNNI, AVX512-FP16andAVX512BF16instruction set support. 💎graniterapids— Intel Granite Rapids CPU with 64-bit extensions,MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2, VPCLMULQDQ, AVX512BITALG, RDPID, AVX512VPOPCNTDQ, PCONFIG, WBNOINVD, CLWB, MOVDIRI, MOVDIR64B, ENQCMD, CLDEMOTE, PTWRITE, WAITPKG, SERIALIZE, TSXLDTRK, UINTR, AMX-BF16, AMX-TILE, AMX-INT8, AVX-VNNI, AVX512-FP16, AVX512BF16, AMX-FP16andPREFETCHIinstruction set support. 🪨graniterapids-d— Intel Granite Rapids D CPU with 64-bit extensions,MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2, VPCLMULQDQ, AVX512BITALG, RDPID, AVX512VPOPCNTDQ, PCONFIG, WBNOINVD, CLWB, MOVDIRI, MOVDIR64B, ENQCMD, CLDEMOTE, PTWRITE, WAITPKG, SERIALIZE, TSXLDTRK, UINTR, AMX-BF16, AMX-TILE, AMX-INT8, AVX-VNNI, AVX512FP16, AVX512BF16, AMX-FP16, PREFETCHIandAMX-COMPLEXinstruction set support. 🫶diamondrapids— Intel Diamond Rapids CPU with 64-bit extensions, and okay the list here is giving encyclopedia:MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2, VPCLMULQDQ, AVX512BITALG, RDPID, AVX512VPOPCNTDQ, PCONFIG, WBNOINVD, CLWB, MOVDIRI, MOVDIR64B, ENQCMD, CLDEMOTE, PTWRITE, WAITPKG, SERIALIZE, TSXLDTRK, UINTR, AMX-BF16, AMX-TILE, AMX-INT8, AVX-VNNI, AVX512FP16, AVX512BF16, AMX-FP16, PREFETCHI, AMX-COMPLEX, AVX10.1-512, AVX-IFMA, AVX-NE-CONVERT, AVX-VNNI-INT16, AVX-VNNI-INT8, CMPccXADD, SHA512, SM3, SM4, AVX10.2-512, APX_F, AMX-AVX512, AMX-FP8, MOVRSandAMX-MOVRSinstruction set support. 👑 (and yes,MOVBEreally does show up twice in there — that's not me being chaotic, that's just how it's written. 🙈)bonnell,atom— Intel Bonnell CPU with 64-bit extensions,MOVBE, MMX, SSE, SSE2, SSE3, SSSE3andCX16instruction set support. Tiny list, huge legacy. 🍓silvermont,slm— Intel Silvermont CPU with 64-bit extensions,MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, PCLMUL, PREFETCHWandRDRNDinstruction set support. ⭐goldmont— Intel Goldmont CPU with 64-bit extensions,MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, PCLMUL, PREFETCHW, RDRND, AES, SHA, RDSEED, XSAVE, XSAVEC, XSAVES, XSAVEOPT, CLFLUSHOPTandFSGSBASEinstruction set support. ✨goldmont-plus— Intel Goldmont Plus CPU with 64-bit extensions,MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, PCLMUL, PREFETCHW, RDRND, AES, SHA, RDSEED, XSAVE, XSAVEC, XSAVES, XSAVEOPT, CLFLUSHOPT, FSGSBASE, PTWRITE, RDPIDandSGXinstruction set support. 🎀tremont— Intel Tremont CPU with 64-bit extensions,MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, PCLMUL, PREFETCHW, RDRND, AES, SHA, RDSEED, XSAVE, XSAVEC, XSAVES, XSAVEOPT, CLFLUSHOPT, FSGSBASE, PTWRITE, RDPID, SGX, CLWB, GFNI-SSE, MOVDIRI, MOVDIR64B, CLDEMOTEandWAITPKGinstruction set support. 🌈sierraforest— Intel Sierra Forest CPU with 64-bit extensions,MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, CLDEMOTE, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, KL, WIDEKL, AVX-VNNI, AVXIFMA, AVXVNNIINT8, AVXNECONVERT, CMPCCXADD, ENQCMDandUINTRinstruction set support. 🌲grandridge— Intel Grand Ridge CPU with 64-bit extensions,MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, CLDEMOTE, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, KL, WIDEKL, AVX-VNNI, AVXIFMA, AVXVNNIINT8, AVXNECONVERT, CMPCCXADD, ENQCMDandUINTRinstruction set support. 🏔️clearwaterforest— Intel Clearwater Forest CPU with 64-bit extensions,MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, CLDEMOTE, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, AVX-VNNI, ENQCMD, UINTR, AVXIFMA, AVXVNNIINT8, AVXNECONVERT, CMPCCXADD, AVXVNNIINT16, SHA512, SM3, SM4, USER_MSRandPREFETCHIinstruction set support. 💧k6— AMD K6 CPU withMMXinstruction set support. short king energy. 🧚k6-2,k6-3— Improved versions of AMD K6 CPU withMMXand3DNow!instruction set support. 💜athlon,athlon-tbird— AMD Athlon CPU withMMX,3dNOW!,enhanced 3DNow!andSSEprefetch instructions support. 🔥athlon-4,athlon-xp,athlon-mp— Improved AMD Athlon CPU withMMX,3DNow!,enhanced 3DNow!and fullSSEinstruction set support. ⭐k8,opteron,athlon64,athlon-fx— Processors based on the AMD K8 core with x86-64 instruction set support, including the AMD Opteron, Athlon 64, and Athlon 64 FX processors. (This supersetsMMX,SSE,SSE2,3DNow!,enhanced 3DNow!and 64-bit instruction set extensions.) vintage but iconic, no notes. 🕰️k8-sse3,opteron-sse3,athlon64-sse3— Improved versions of AMD K8 cores withSSE3instruction set support. 🌸amdfam10,barcelona— CPUs based on AMD Family 10h cores with x86-64 instruction set support. (This supersetsCX16,MMX,SSE,SSE2,SSE3,SSE4A,3DNow!,enhanced 3DNow!,ABMand 64-bit instruction set extensions.) 🫶bdver1— CPUs built on AMD Family 15h cores with x86-64 instruction set support. This one supersetsFMA4,AVX,XOP,LWP,AES,PCLMUL,CX16,MMX,SSE,SSE2,SSE3,SSE4A,SSSE3,SSE4.1,SSE4.2,ABMand the 64-bit instruction set extensions. 💅bdver2— AMD Family 15h core based CPUs with x86-64 instruction set support. This one supersetsBMI,TBM,F16C,FMA,FMA4,AVX,XOP,LWP,AES,PCLMUL,CX16,MMX,SSE,SSE2,SSE3,SSE4A,SSSE3,SSE4.1,SSE4.2,ABMand the 64-bit instruction set extensions. ✨bdver3— AMD Family 15h core based CPUs with x86-64 instruction set support. This one supersetsBMI,TBM,F16C,FMA,FMA4,FSGSBASE,AVX,XOP,LWP,AES,PCLMUL,CX16,MMX,SSE,SSE2,SSE3,SSE4A,SSSE3,SSE4.1,SSE4.2,ABMand the 64-bit instruction set extensions. 🦋bdver4— AMD Family 15h core based CPUs with x86-64 instruction set support. This one supersetsBMI,BMI2,TBM,F16C,FMA,FMA4,FSGSBASE,AVX,AVX2,XOP,LWP,AES,PCLMUL,CX16,MOVBE,MMX,SSE,SSE2,SSE3,SSE4A,SSSE3,SSE4.1,SSE4.2,ABMand the 64-bit instruction set extensions. 🌸znver1— AMD Family 17h core based CPUs with x86-64 instruction set support. This one supersetsBMI,BMI2,F16C,FMA,FSGSBASE,AVX,AVX2,ADCX,RDSEED,MWAITX,SHA,CLZERO,AES,PCLMUL,CX16,MOVBE,MMX,SSE,SSE2,SSE3,SSE4A,SSSE3,SSE4.1,SSE4.2,ABM,XSAVEC,XSAVES,CLFLUSHOPT,POPCNT, and the 64-bit instruction set extensions. 👑znver2— AMD Family 17h core based CPUs with x86-64 instruction set support. This one supersetsBMI,BMI2,CLWB,F16C,FMA,FSGSBASE,AVX,AVX2,ADCX,RDSEED,MWAITX,SHA,CLZERO,AES,PCLMUL,CX16,MOVBE,MMX,SSE,SSE2,SSE3,SSE4A,SSSE3,SSE4.1,SSE4.2,ABM,XSAVEC,XSAVES,CLFLUSHOPT,POPCNT,RDPID,WBNOINVD, and the 64-bit instruction set extensions. 💖znver3— AMD Family 19h core based CPUs with x86-64 instruction set support. This one supersetsBMI,BMI2,CLWB,F16C,FMA,FSGSBASE,AVX,AVX2,ADCX,RDSEED,MWAITX,SHA,CLZERO,AES,PCLMUL,CX16,MOVBE,MMX,SSE,SSE2,SSE3,SSE4A,SSSE3,SSE4.1,SSE4.2,ABM,XSAVEC,XSAVES,CLFLUSHOPT,POPCNT,RDPID,WBNOINVD,PKU,VPCLMULQDQ,VAES, and the 64-bit instruction set extensions. 🌈znver4— AMD Family 19h core based CPUs with x86-64 instruction set support. This one supersetsBMI,BMI2,CLWB,F16C,FMA,FSGSBASE,AVX,AVX2,ADCX,RDSEED,MWAITX,SHA,CLZERO,AES,PCLMUL,CX16,MOVBE,MMX,SSE,SSE2,SSE3,SSE4A,SSSE3,SSE4.1,SSE4.2,ABM,XSAVEC,XSAVES,CLFLUSHOPT,POPCNT,RDPID,WBNOINVD,PKU,VPCLMULQDQ,VAES,AVX512F,AVX512DQ,AVX512IFMA,AVX512CD,AVX512BW,AVX512VL,AVX512BF16,AVX512VBMI,AVX512VBMI2,AVX512VNNI,AVX512BITALG,AVX512VPOPCNTDQ,GFNIand the 64-bit instruction set extensions. 🔥znver5— AMD Family 1ah core based CPUs with x86-64 instruction set support. This one supersetsBMI,BMI2,CLWB,F16C,FMA,FSGSBASE,AVX,AVX2,ADCX,RDSEED,MWAITX,SHA,CLZERO,AES,PCLMUL,CX16,MOVBE,MMX,SSE,SSE2,SSE3,SSE4A,SSSE3,SSE4.1,SSE4.2,ABM,XSAVEC,XSAVES,CLFLUSHOPT,POPCNT,RDPID,WBNOINVD,PKU,VPCLMULQDQ,VAES,AVX512F,AVX512DQ,AVX512IFMA,AVX512CD,AVX512BW,AVX512VL,AVX512BF16,AVX512VBMI,AVX512VBMI2,AVX512VNNI,AVX512BITALG,AVX512VPOPCNTDQ,GFNI,AVXVNNI,MOVDIRI,MOVDIR64B,AVX512VP2INTERSECT,PREFETCHIand the 64-bit instruction set extensions. ⭐znver6— AMD Family 1ah core based CPUs with x86-64 instruction set support. This one supersetsBMI,BMI2,CLWB,F16C,FMA,FSGSBASE,AVX,AVX2,ADCX,RDSEED,MWAITX,SHA,CLZERO,AES,PCLMUL,CX16,MOVBE,MMX,SSE,SSE2,SSE3,SSE4A,SSSE3,SSE4.1,SSE4.2,ABM,XSAVEC,XSAVES,CLFLUSHOPT,POPCNT,RDPID,WBNOINVD,PKU,VPCLMULQDQ,VAES,AVX512F,AVX512DQ,AVX512IFMA,AVX512CD,AVX512BW,AVX512VL,AVX512BF16,AVX512VBMI,AVX512VBMI2,AVX512VNNI,AVX512BITALG,AVX512VPOPCNTDQ,GFNI,AVXVNNI,MOVDIRI,MOVDIR64B,AVX512VP2INTERSECT,PREFETCHI,AVXVNNIINT8,AVXIFMA,AVX512FP16,AVXNECONVERT,AVX512BMMand the 64-bit instruction set extensions. 💜btver1— CPUs built on AMD Family 14h cores with x86-64 instruction set support. This one supersetsMMX,SSE,SSE2,SSE3,SSSE3,SSE4A,CX16,ABMand the 64-bit instruction set extensions. 🎀btver2— CPUs built on AMD Family 16h cores with x86-64 instruction set support. This one includesMOVBE,F16C,BMI,AVX,PCLMUL,AES,SSE4.2,SSE4.1,CX16,ABM,SSE4A,SSSE3,SSE3,SSE2,SSE,MMXand the 64-bit instruction set extensions. 🧚winchip-c6— IDT WinChip C6 CPU, dealt in same way asi486with additionalMMXinstruction set support. ✨winchip2— IDT WinChip 2 CPU, dealt in same way asi486with additionalMMXand3DNow!instruction set support. 🌟c3— VIA C3 CPU withMMXand3DNow!instruction set support. (No scheduling is implemented for this chip, bestie, so don't come for me. 🙈)c3-2— VIA C3-2 (Nehemiah/C5XL) CPU withMMXandSSEinstruction set support. (No scheduling is implemented for this chip.) 🫶c7— VIA C7 (Esther) CPU withMMX,SSE,SSE2andSSE3instruction set support. (No scheduling is implemented for this chip.) 💅samuel-2— VIA Eden Samuel 2 CPU withMMXand3DNow!instruction set support. (No scheduling is implemented for this chip.) 🌸nehemiah— VIA Eden Nehemiah CPU withMMXandSSEinstruction set support. (No scheduling is implemented for this chip.) 🦋esther— VIA Eden Esther CPU withMMX,SSE,SSE2andSSE3instruction set support. (No scheduling is implemented for this chip.) 🌈eden-x2— VIA Eden X2 CPU with x86-64,MMX,SSE,SSE2andSSE3instruction set support. (No scheduling is implemented for this chip.) ⭐eden-x4— VIA Eden X4 CPU with x86-64,MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,AVXandAVX2instruction set support. (No scheduling is implemented for this chip.) 🔥nano— Generic VIA Nano CPU with x86-64,MMX,SSE,SSE2,SSE3andSSSE3instruction set support. (No scheduling is implemented for this chip.) 👑nano-1000— VIA Nano 1xxx CPU with x86-64,MMX,SSE,SSE2,SSE3andSSSE3instruction set support. (No scheduling is implemented for this chip.) ✨nano-2000— VIA Nano 2xxx CPU with x86-64,MMX,SSE,SSE2,SSE3andSSSE3instruction set support. (No scheduling is implemented for this chip.) 💖nano-3000— VIA Nano 3xxx CPU with x86-64,MMX,SSE,SSE2,SSE3,SSSE3andSSE4.1instruction set support. (No scheduling is implemented for this chip.) 🌟nano-x2— VIA Nano Dual Core CPU with x86-64,MMX,SSE,SSE2,SSE3,SSSE3andSSE4.1instruction set support. (No scheduling is implemented for this chip.) 🎀nano-x4— VIA Nano Quad Core CPU with x86-64,MMX,SSE,SSE2,SSE3,SSSE3andSSE4.1instruction set support. (No scheduling is implemented for this chip.) 🧚lujiazui— ZHAOXIN lujiazui CPU with x86-64,MOVBE,MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,POPCNT,AES,PCLMUL,RDRND,XSAVE,XSAVEOPT,FSGSBASE,CX16,ABM,BMI,BMI2,FXSR,RDSEEDinstruction set support. And okay, plot twist: while the CPUs do supportAVXandF16C, these aren't enabled by-march=lujiazuifor performance reasons. No cap. 💅yongfeng— ZHAOXIN yongfeng CPU with x86-64,MOVBE,MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,AVX,POPCNT,AES,PCLMUL,RDRND,XSAVE,XSAVEOPT,FSGSBASE,CX16,ABM,BMI,BMI2,F16C,FXSR,RDSEED,AVX2,FMA,SHA,LZCNTinstruction set support. ✨shijidadao— ZHAOXIN shijidadao CPU with x86-64,MOVBE,MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,AVX,POPCNT,AES,PCLMUL,RDRND,XSAVE,XSAVEOPT,FSGSBASE,CX16,ABM,BMI,BMI2,F16C,FXSR,RDSEED,AVX2,FMA,SHA,LZCNTinstruction set support. 🦋geode— AMD Geode embedded processor withMMXand3DNow!instruction set support. 🌸c86-4g-m4— HYGON c86-4g-m4 CPU with x86-64,MMX,SSE,SSE2,SSE3,SSE4A,CX16,ABM,SSSE3,SSE4.1,SSE4.2,AES,PCLMUL,AVX,AVX2,BMI,BMI2,F16C,FMA,PRFCHW,FXSR,SHA,XSAVE,XSAVEOPT,XSAVEC,FSGSBASE,RDRND,MOVBE,MWAITX,ADX,RDSEED,CLZERO,CLFLUSHOPT,XSAVES,LZCNT,POPCNTinstruction set support. 💖c86-4g-m6— HYGON c86-4g-m6 CPU with x86-64,MMX,SSE,SSE2,SSE3,SSE4A,CX16,ABM,SSSE3,SSE4.1,SSE4.2,AES,PCLMUL,AVX,AVX2,BMI,BMI2,F16C,FMA,PRFCHW,FXSR,SHA,XSAVE,XSAVEOPT,XSAVEC,FSGSBASE,RDRND,MOVBE,MWAITX,ADX,RDSEED,CLZERO,CLFLUSHOPT,XSAVES,LZCNT,POPCNTinstruction set support. 🌈c86-4g-m7— HYGON c86-4g-m7 CPU with x86-64,MMX,SSE,SSE2,SSE3,SSE4A,CX16,ABM,SSSE3,SSE4.1,SSE4.2,AES,PCLMUL,AVX,AVX2,BMI,BMI2,F16C,FMA,PRFCHW,FXSR,SHA,XSAVE,XSAVEOPT,XSAVEC,FSGSBASE,RDRND,MOVBE,MWAITX,ADX,RDSEED,CLZERO,CLFLUSHOPT,XSAVES,LZCNT,POPCNT,AVX512F,AVX512DQ,AVX512IFMA,AVX512CD,AVX512BW,AVX512VL,AVX512BF16,AVX512VBMI,AVX512VBMI2,GFNI,AVX512VNNI,VAES,AVX512BITALG,AVX512VPOPCNTDQ,AVX512VP2INTERSECT,AVXVNNI,VPCLMULQDQ,WBNOINVDinstruction set support. 🔥c86-4g-m8— HYGON c86-4g-m8 CPU with x86-64,MMX,SSE,SSE2,SSE3,SSE4A,CX16,ABM,SSSE3,SSE4.1,SSE4.2,AES,PCLMUL,AVX,AVX2,BMI,BMI2,F16C,FMA,PRFCHW,FXSR,SHA,XSAVE,XSAVEOPT,XSAVEC,FSGSBASE,RDRND,MOVBE,MWAITX,ADX,RDSEED,CLZERO,CLFLUSHOPT,XSAVES,LZCNT,POPCNT,AVX512F,AVX512DQ,AVX512IFMA,AVX512CD,AVX512BW,AVX512VL,AVX512BF16,AVX512VBMI,AVX512VBMI2,GFNI,AVX512VNNI,VAES,AVX512BITALG,AVX512VPOPCNTDQ,AVX512VP2INTERSECT,AVXVNNI,VPCLMULQDQ,WBNOINVDinstruction set support. 👑-mtune=cpu-type— tunes literally everything applicable about the code that comes out, except the ABI and the set of available instructions, to cpu-type. ✨ Picking a specific cpu-type schedules stuff all nice for that particular chip, but bestie listen: the compiler still won't spit out any code that can't run on the default machine type — unless you also pass a-march=cpu-typeoption. Like, if GCC is configured fori686-pc-linux-gnuthen-mtune=pentium4generates code tuned for Pentium 4 that still runs on i686 machines. 💅
The choices for cpu-type are the exact same ones as for -march. Plus, -mtune supports 2 extra choices for cpu-type:
generic— produces code optimized for the most common IA32/AMD64/EM64T processors. 🫶 If you know the CPU your code will run on, then you should use the corresponding-mtuneor-marchoption instead of-mtune=generic. But if you have no idea what CPU the users of your app are gonna have? Then this is your girl. 🌸
As new processors get deployed in the marketplace, the behavior of this option will change. So if you upgrade to a newer version of GCC, code generation controlled by this option will change to reflect the processors that are most common at the time that version of GCC is released. (iconic but also 😭)
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. Meanwhile -mtune indicates the processor (or, in this case, a whole collection of processors) for which the code is optimized. Not the same thing, bestie. 💀
intel— produces code optimized for the most current Intel processors, which are Haswell and Silvermont for this version of GCC. ⭐ If you know the CPU your code will run on, use the corresponding-mtuneor-marchoption instead of-mtune=intel. But if you want your application performing better on both Diamond Rapids and Clearwater Forest, then you should use this option. 👑
As new Intel processors are deployed in the marketplace, the behavior of this option will change. Therefore, if you upgrade to a newer version of GCC, code generation controlled by this option will change to reflect the most current Intel processors at the time that version of GCC is released. ✨
There is no -march=intel option because -march indicates the instruction set the compiler can use, and there is no common instruction set applicable to all processors. In contrast, -mtune indicates the processor (or, in this case, collection of processors) for which the code is optimized. 🌈
The following options allow more detailed control over which instruction set extensions are targeted by GCC. Each one has a corresponding -mno- option to disable use of these instructions. 🎀
These extensions are also available as built-in functions: see x86 Built-in Functions, for details of the functions enabled and disabled by these switches. iykyk 🔥
These options enable GCC to use these extended instructions in generated code. Applications that perform run-time CPU detection must compile separate files for each supported architecture, using the appropriate flags. In particular, the file containing the CPU detection code should be compiled without these options. 🙈
To control whether 387 or SSE/AVX instructions are generated automatically for floating-point arithmetic, see -mfpmath=, below. 👇
-mmmx— support MMX built-in functions.-msse— support MMX and SSE built-in functions and code generation.-msse2— support MMX, SSE and SSE2 built-in functions and code generation.-msse3— support MMX, SSE, SSE2 and SSE3 built-in functions and code generation.-mssse3— support MMX, SSE, SSE2, SSE3 and SSSE3 built-in functions and code generation.-msse4.1— support MMX, SSE, SSE2, SSE3, SSSE3 and SSE4.1 built-in functions and code generation.-msse4.2— support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1 and SSE4.2 built-in functions and code generation.-msse4— support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1 and SSE4.2 built-in functions and code generation.
Heads up bestie:-msse4enables both SSE4.1 and SSE4.2 support, while-mno-sse4turns those features off. Neither form of the option touches SSE4A support — that's controlled separately by-msse4a. 💅
-msse4a— support MMX, SSE, SSE2, SSE3 and SSE4A built-in functions and code generation.-mavx— support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2 and AVX built-in functions and code generation.-mavx2— support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX and AVX2 built-in functions and code generation.-mavx512f— support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2 and AVX512F built-in functions and code generation.-mavx512cd— support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX512F and AVX512CD built-in functions and code generation.-mavx512vl— support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX512F and AVX512VL built-in functions and code generation.-mavx512bw— support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX512F and AVX512BW built-in functions and code generation.-mavx512dq— support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX512F and AVX512DQ built-in functions and code generation.-mavx512ifma— support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX512F and AVX512IFMA built-in functions and code generation.-mavx512vbmi— support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX512F and AVX512VBMI built-in functions and code generation.-mavx512vpopcntdq— support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX512F and AVX512VPOPCNTDQ built-in functions and code generation.-mavx512vp2intersect— support AVX512VP2INTERSECT built-in functions and code generation.-mavx512vnni— support AVX512VNNI built-in functions and code generation.-mavx512vbmi2— support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX512F and AVX512VBMI2 built-in functions and code generation.-mavx512bf16— support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX512F and AVX512BF16 built-in functions and code generation.-mavx512fp16— support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX512F and AVX512-FP16 built-in functions and code generation.-mavx512bitalg— support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX512F and AVX512BITALG built-in functions and code generation.-mavx512bmm— support AVX512BMM built-in functions and code generation.-mavxvnni— support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, and AVXVNNI built-in functions and code generation.-mavxifma— support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, and AVXIFMA built-in functions and code generation.-mavxvnniint8— support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2 and AVXVNNIINT8 built-in functions and code generation.-mavxneconvert— support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, and AVXNECONVERT build-in functions and code generation.-mavxvnniint16— support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2 and AVXVNNIINT16 built-in functions and code generation.-mavx10.1— support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, and AVX10.1 built-in functions and code generation.-mavx10.2— support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX10.1 and AVX10.2 built-in functions and code generation.-msha— support SHA1 and SHA256 built-in functions and code generation.-maes— support AES built-in functions and code generation.-mpclmul— support PCLMUL built-in functions and code generation.-mclflushopt— support CLFLUSHOPT instructions.-mclwb— support CLWB instruction.-mfsgsbase— support FSGSBASE built-in functions and code generation.-mptwrite— support PTWRITE built-in functions and code generation.-mrdrnd— support RDRND built-in functions and code generation.-mf16c— support F16C built-in functions and code generation.-mfma— support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX and FMA built-in functions and code generation.-mfma4— support FMA4 built-in functions and code generation.-mpconfig— support PCONFIG built-in functions and code generation.-mwbnoinvd— support WBNOINVD built-in functions and code generation.-mprfchw— support PREFETCHW instruction.-mrdpid— support RDPID built-in functions and code generation.-mrdseed— support RDSEED instruction.-msgx— support SGX built-in functions and code generation.-mxop— support XOP built-in functions and code generation.-mlwp— support LWP built-in functions and code generation.-m3dnow— support 3DNow! built-in functions.-m3dnowa— support Athlon 3Dnow! built-in functions.-mpopcnt— support code generation ofpopcntinstruction.-mabm— support code generation of Advanced Bit Manipulation (ABM) instructions.-madx— support flag-preserving add-carry instructions.-mbmi— support BMI built-in functions and code generation.-mbmi2— support BMI2 built-in functions and code generation.-mlzcnt— support LZCNT built-in function and code generation.-mfxsr— support FXSAVE and FXRSTOR instructions.-mxsave— support XSAVE and XRSTOR instructions.-mxsaveopt— support XSAVEOPT instruction.-mxsavec— support XSAVEC instructions.-mxsaves— support XSAVES and XRSTORS instructions.-mrtm— support RTM built-in functions and code generation.-mhle— support Hardware Lock Elision prefixes.-mtbm— support TBM built-in functions and code generation.-mmwaitx— support MWAITX and MONITORX built-in functions and code generation.-mclzero— support CLZERO built-in functions and code generation.-mpku— support PKU built-in functions and code generation.-mgfni— support GFNI built-in functions and code generation.-mvaes— bestie this one says YES to VAES built-in functions and code generation. ✨-mwaitpkg— turns on WAITPKG built-in functions and code generation, no cap.-mvpclmulqdq— VPCLMULQDQ built-in functions and code generation, come through 💅-mmovdiri— MOVDIRI built-in functions and code generation, we love to see it.-mmovdir64b— MOVDIR64B built-in functions and code generation, iconic behavior.-menqcmd— ENQCMD built-in functions and code generation, periodt.-muintr— UINTR built-in functions and code generation, slay.-mtsxldtrk— TSXLDTRK built-in functions and code generation, obsessed.-mcldemote— CLDEMOTE built-in functions and code generation, it's giving optimized.-mserialize— SERIALIZE built-in functions and code generation, main character energy 🔥-mamx-tile— AMX-TILE built-in functions and code generation, ate.-mamx-int8— AMX-INT8 built-in functions and code generation, we stan.-mamx-bf16— AMX-BF16 built-in functions and code generation, chef's kiss.-mhreset— HRESET built-in functions and code generation, cute.-mkl— KL built-in functions and code generation, lowkey essential.-mwidekl— WIDEKL built-in functions and code generation, highkey needed.-mcmpccxadd— bestie this enables MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, and CMPCCXADD build-in functions and code generation. That's a whole squad. 👑-mamx-fp16— AMX-FP16 built-in functions and code generation, gorgeous.-mprefetchi— PREFETCHI built-in functions and code generation, so ahead of schedule.-mraoint— RAOINT built-in functions and code generation, we love a queen who plans ahead.-mamx-complex— AMX-COMPLEX built-in functions and code generation, literally so much range.-msm3— enables MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX and SM3 built-in functions and code generation. The whole friendship group showed up. 🌸-msm4— same energy: MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX and SM4 built-in functions and code generation.-msha512— MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX and SHA512 built-in functions and code generation. Hashing but make it glam.-mapxf— supports code generation for APX features, including EGPR, PUSH2POP2, NDD, PPX, NF, CCMP and ZU. Yes, all of them, every single one. 💖-musermsr— USER_MSR built-in functions and code generation, adorable.-mamx-avx512— okay so this one is a whole party: MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX10.1, AVX10.2 and AMX-AVX512 built-in functions and code generation. The guest list is unreal. 🎀-mamx-tf32— AMX-TF32 built-in functions and code generation, so real.-mamx-fp8— AMX-FP8 built-in functions and code generation, tiny but mighty.-mmovrs— MOVRS built-in functions and code generation, we see you.-mamx-movrs— AMX-MOVRS built-in functions and code generation, and that's that on that.
These additional options are available for the x86 processor family. Bestie, this next batch is x86-flavored and there's a LOT, so grab a snack 🍓
-mfpmath=unit— generates floating-point arithmetic for whichever unit you picked. The choices for unit are:387— uses the standard 387 floating-point coprocessor that's on most chips and emulated everywhere else. Code compiled with this option runs almost everywhere. The catch: temporary results get computed in 80-bit precision instead of whatever precision the type actually specifies, so the results come out slightly different compared to most other chips. See-ffloat-storefor the full tea. This is the default choice for non-Darwin x86-32 targets.sse— uses the scalar floating-point instructions in the SSE instruction set. Pentium III and newer chips have this, and on the AMD side it's Athlon-4, Athlon XP and Athlon MP. The earlier version of SSE only does single-precision arithmetic, so double and extended-precision arithmetic still go through 387. A later version — only on Pentium 4 and AMD x86-64 chips — supports double-precision arithmetic too.
For the x86-32 compiler you must use -march=cpu-type, -msse or -msse2 switches to enable SSE extensions and actually make this option do anything. For the x86-64 compiler, these extensions are enabled by default. ✨
The resulting code should be considerably faster in the majority of cases and dodges the numerical instability drama of 387 code — but it may break some existing code that expects temporaries to be 80 bits. 😭
This is the default choice for the x86-64 compiler, Darwin x86-32 targets, and the default choice for x86-32 targets with the SSE2 instruction set when -ffast-math is enabled.
Also: GCC depresses SSE instructions when -mavx (or another option enabling AVX extensions) is used. Instead it generates new AVX instructions or AVX equivalents for all SSE instructions when needed.
sse,387— same as below, queensse+387— same as below, queenboth— attempts to utilize both instruction sets at once. This effectively doubles the amount of available registers, and on chips with separate execution units for 387 and SSE it doubles the execution resources too. Use this option with care because it's still experimental — the GCC register allocator doesn't model separate functional units well, so you get unstable performance. 🙈
-mieee-fp,-mno-ieee-fp— controls whether or not the compiler uses IEEE floating-point comparisons. These correctly handle the case where the result of a comparison is unordered.-m80387,-mhard-float— generates output containing 80387 instructions for floating point.-mno-80387,-msoft-float— generates 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 cannot 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 typesfloatanddoublein an FPU register, even if there is no FPU — the idea being that the operating system should emulate an FPU. The option-mno-fp-ret-in-387causes such values to be returned in ordinary CPU registers instead.-mno-fancy-math-387— some 387 emulators don't support thesin,cosandsqrtinstructions for the 387. Specify this option to avoid generating those instructions. This option is overridden when-marchindicates that the target CPU always has an FPU and so the instruction does not need emulation. These instructions are not generated unless you also use the-funsafe-math-optimizationsswitch.-malign-double,-mno-align-double— controls whether GCC alignsdouble,long double, andlong longvariables on a two-word boundary or a one-word boundary. Aligningdoublevariables on a two-word boundary produces code that runs somewhat faster on a Pentium at the expense of more memory. On x86-64,-malign-doubleis enabled by default.
⚠️ Warning: if you use the -malign-double switch, structures containing the above types are aligned differently than the published application binary interface specifications for the x86-32 and are not binary compatible with structures in code compiled without that switch.
-m96bit-long-double,-m128bit-long-double— these switches control the size of thelong doubletype. The x86-32 application binary interface specifies the size to be 96 bits, so-m96bit-long-doubleis the default in 32-bit mode.
Modern architectures (Pentium and newer) prefer long double to be aligned to an 8- or 16-byte boundary. In arrays or structures conforming to the ABI, this is not possible. So specifying -m128bit-long-double aligns long double to a 16-byte boundary by padding the long double with an additional 32-bit zero.
In the x86-64 compiler, -m128bit-long-double is the default choice as its ABI specifies that long double is aligned on a 16-byte boundary.
Notice that neither of these options enable any extra precision over the x87 standard of 80 bits for a long double.
⚠️ Warning: if you override the default value for your target ABI, this changes the size of structures and arrays containinglong doublevariables, as well as modifying the function calling convention for functions takinglong double. Hence they are not binary-compatible with code compiled without that switch.
-mlong-double-64,-mlong-double-80,-mlong-double-128— these switches also control the size of thelong doubletype. A size of 64 bits makes thelong doubletype equivalent to thedoubletype — this is the default for 32-bit Bionic C library. A size of 128 bits makes thelong doubletype equivalent to the__float128type — this is the default for 64-bit Bionic C library.
⚠️ Warning: if you override the default value for your target ABI, this changes the size of structures and arrays containinglong doublevariables, as well as modifying the function calling convention for functions takinglong double. Hence they are not binary-compatible with code compiled without that switch.
-malign-data=type— controls how GCC aligns variables. Supported values for type are:compatuses increased alignment value compatible with GCC 4.8 and earlier,abiuses alignment value as specified by the psABI, andcachelineuses increased alignment value to match the cache line size.compatis the default.-mlarge-data-threshold=threshold— when-mcmodel=mediumor-mcmodel=largeis specified, data objects larger than threshold are placed in large data sections. The default is65535.-mrtd— uses a different function-calling convention, in which functions that take a fixed number of arguments return with theret numinstruction, which pops their arguments while returning. This saves one instruction in the caller since there's 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. 💅
ok bestie, big yikes alert 🚨 this calling convention is incompatible with the one normally used on Unix, so you literally cannot use it if you need to call libraries compiled with the Unix compiler.
Also — you must provide function prototypes for all functions that take variable numbers of arguments (including printf); otherwise incorrect code is generated for calls to those functions.
And like, one more thing: seriously incorrect code results if you call a function with too many arguments. (Normally, extra arguments are harmlessly ignored, so this is new and bad.)
-mregparm=num— controls how many registers get 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 attributeregparm.
Warning: if you use this switch, and num is nonzero, then you must build all modules with the same value, including any libraries. Yes, that means the system libraries and startup modules too. 💀
-msseregparm— uses SSE register passing conventions forfloatanddoublearguments and return values. You can control this behavior for a specific function by using the function attributesseregparm.
Warning: if you use this switch then you must build all modules with the same value, including any libraries. Including the system libraries and startup modules. no exceptions bestie 🙈
-mvect8-ret-in-mem— returns 8-byte vectors in memory instead of MMX registers. This is the default on VxWorks to match the ABI of the Sun Studio compilers until version 12. Only use this option if you need to remain compatible with existing code produced by those previous compiler versions or older versions of GCC.-mpc32,-mpc64,-mpc80— set 80387 floating-point precision to 32, 64 or 80 bits. When-mpc32is specified, the significands of results of floating-point operations are rounded to 24 bits (single precision);-mpc64rounds the significands of results of floating-point operations to 53 bits (double precision) and-mpc80rounds the significands of results of floating-point operations to 64 bits (extended double precision), which is the default. When this option is used, floating-point operations in higher precisions are not available to the programmer without setting the FPU control word explicitly.
Setting the rounding of floating-point operations to less than the default 80 bits can speed some programs by 2% or more. Note that some mathematical libraries assume that extended-precision (80-bit) floating-point operations are enabled by default; routines in such libraries could suffer significant loss of accuracy, typically through so-called "catastrophic cancellation", when this option is used to set the precision to less than extended precision. catastrophic cancellation is giving main character villain energy fr 😭
-mdaz-ftz,-mno-daz-ftz— the flush-to-zero (FTZ) and denormals-are-zero (DAZ) flags in the MXCSR register are used to control floating-point calculations. The-Ofast,-ffast-math, or-funsafe-math-optimizationsoptions normally link in startup code that sets these flags except when building a shared library (-shared). You can use the-mdaz-ftzand-mno-daz-ftzoptions to explicitly enable or disable setting these flags, regardless of other options passed to GCC.-mstackrealign— realign the stack at entry. On the x86, the-mstackrealignoption generates an alternate prologue and epilogue that realigns the run-time stack if necessary. This supports mixing legacy codes that keep 4-byte stack alignment with modern codes that keep 16-byte stack alignment for SSE compatibility. See also the attributeforce_align_arg_pointer, applicable to individual functions. mixing eras? it's giving crossover episode 🦋-mstack-arg-probe— emit stack probing code in the function prologue.-mpreferred-stack-boundary=num— attempt to keep the stack boundary aligned to a 2 raised to num byte boundary. If-mpreferred-stack-boundaryis not specified, the default is 4 (16 bytes or 128 bits).
Warning: When generating code for the x86-64 architecture with SSE extensions disabled,-mpreferred-stack-boundary=3can be used to keep the stack boundary aligned to 8 byte boundary. Since x86-64 ABI require 16 byte stack alignment, this is ABI incompatible and intended to be used in controlled environment where stack space is important limitation. This option leads to wrong code when functions compiled with 16 byte stack alignment (such as functions from a standard library) are called with misaligned stack. In this case, SSE instructions may lead to misaligned memory access traps. In addition, variable arguments are handled incorrectly for 16 byte aligned objects (including x87long doubleand__int128), leading to wrong results. You must build all modules with-mpreferred-stack-boundary=3, including any libraries. This includes the system libraries and startup modules. 💀💀
-mincoming-stack-boundary=num— assume the incoming stack is aligned to a 2 raised to num byte boundary. If-mincoming-stack-boundaryis not specified, the one specified by-mpreferred-stack-boundaryis used.
On Pentium and Pentium Pro, double and long double values should be aligned to an 8-byte boundary (see -malign-double) or suffer significant run time performance penalties. On Pentium III, the Streaming SIMD Extension (SSE) data type __m128 may not work properly if it is not 16-byte aligned.
To ensure proper alignment of this values on the stack, the stack boundary must be as aligned as that required by any value stored on the stack. Further, every function must be generated such that it keeps the stack aligned. Thus calling a function compiled with a higher preferred stack boundary from a function compiled with a lower preferred stack boundary most likely misaligns the stack. It is recommended that libraries that use callbacks always use the default setting.
This extra alignment does consume extra stack space, and generally increases code size. Code that is sensitive to stack space usage, such as embedded systems and operating system kernels, may want to reduce the preferred alignment to -mpreferred-stack-boundary=2. embedded girlies stay winning 🫶
-mdump-tune-features— this option instructs GCC to dump the names of the x86 performance tuning features and default settings. The names can be used in-mtune-ctrl=feature-list. free lore drop ✨-mtune-ctrl=feature-list— this option is used to do fine-grain control of x86 code generation features. feature-list is a comma-separated list of feature names. See also-mdump-tune-features. When specified, the feature is turned on if it is not preceded with^; otherwise, it is turned off.-mtune-ctrl=feature-listis intended to be used by GCC developers. Using it may lead to code paths not covered by testing and can potentially result in compiler ICEs or runtime errors.
bestie this is a dev-only zone, touch at your own risk 🚧
-mno-default— this option instructs GCC to turn off all tunable features. See also-mtune-ctrl=feature-listand-mdump-tune-features. turning everything off is so me on a monday 💅-mcld— this option instructs GCC to emit acldinstruction in the prologue of functions that use string instructions. String instructions depend on the DF flag to select between autoincrement or autodecrement mode. While the ABI specifies the DF flag to be cleared on function entry, some operating systems violate this specification by not clearing the DF flag in their exception dispatchers. The exception handler can be invoked with the DF flag set, which leads to wrong direction mode when string instructions are used. This option can be enabled by default on 32-bit x86 targets by configuring GCC with the--enable-cldconfigure option. Generation ofcldinstructions can be suppressed with the-mno-cldcompiler option in this case.-mvzeroupper— this option instructs GCC to emit avzeroupperinstruction before a transfer of control flow out of the function to minimize the AVX to SSE transition penalty as well as remove unnecessaryzeroupperintrinsics.-mstv,-mno-stv— enable/disable Scalar to Vectorization pass transforming 64-bit integer computation into vector ones. This optimization is restricted to-O2and higher.-mprefer-avx128— this option instructs GCC to use 128-bit AVX instructions instead of 256-bit AVX instructions in the auto-vectorizer.-mprefer-vector-width=opt— this option instructs GCC to use opt-bit vector width in instructions instead of default on the selected platform.-mpartial-vector-fp-math,-mno-partial-vector-fp-math— this option enables GCC to generate floating-point operations that might affect the set of floating-point status flags on partial vectors, where vector elements reside in the low part of the 128-bit SSE register. Unless-fno-trapping-mathis specified, the compiler guarantees correct behavior by sanitizing all input operands to have zeroes in the unused upper part of the vector register. Note that by using built-in functions or inline assembly with partial vector arguments, NaNs, denormal or invalid values can leak into the upper part of the vector, causing possible performance issues when-fno-trapping-mathis in effect. These issues can be mitigated by manually sanitizing the upper part of the partial vector argument register or by using-mdaz-ftzto set denormals-are-zero (DAZ) flag in the MXCSR register.
This option is enabled by default. default-on queen 👑
-mmove-max=bits— this option instructs GCC to set the maximum number of bits can be moved from memory to memory efficiently to bits. The valid bits are 128, 256 and 512.-mstore-max=bits— this option instructs GCC to set the maximum number of bits can be stored to memory efficiently to bits. The valid bits are 128, 256 and 512.none— No extra limitations applied to GCC other than defined by the selected platform.128— Prefer 128-bit vector width for instructions.256— Prefer 256-bit vector width for instructions.512— Prefer 512-bit vector width for instructions.
-mnoreturn-no-callee-saved-registers— okay so this one is lowkey clever ✨ it optimizes functions that have the"noreturn"attribute or the"_Noreturn"specifier by simply not saving the callee-saved registers in the function prologue (except for the"BP"register, bestie stays safe 💅). Why bother saving stuff you'll never return to use? BUT — and this is the tea — it can mess with debugging of the caller of that"noreturn"function, or literally any function further up the call stack 😭 so it is not enabled by default. Drama, honestly.-mcx16— this lets GCC generate"CMPXCHG16B"instructions in 64-bit code so it can do compare-and-exchange operations on 16-byte aligned 128-bit objects 🎀 super useful for atomic updates of data structures that are bigger than one machine word. The compiler uses this instruction to implement __sync Builtins. But heads up: for __atomic Builtins operating on 128-bit integers, a library call is always used, no cap.-msahf— enables generation of"SAHF"instructions in 64-bit code. Story time: early Intel Pentium 4 CPUs with Intel 64 support, before the Pentium 4 G1 step dropped in December 2005, straight up lacked the"LAHF"and"SAHF"instructions that AMD64 supports 🙈 these are load and store instructions respectively for certain status flags. In 64-bit mode, the"SAHF"instruction is used to optimize"fmod","drem", and"remainder"built-in functions — see Other Builtins for the deets 🦋-mmovbe— this one enables use of the"movbe"instruction to optimize byte swapping of four and eight byte entities 🔥 byte-swap but make it fast.-mshstk— the-mshstkoption enables shadow stack built-in functions from x86 Control-flow Enforcement Technology (CET) 👑-mcrc32— enables the built-in functions"__builtin_ia32_crc32qi","__builtin_ia32_crc32hi","__builtin_ia32_crc32si"and"__builtin_ia32_crc32di"so they generate the"crc32"machine instruction 💖-mmwait— enables the built-in functions"__builtin_ia32_monitor"and"__builtin_ia32_mwait"to generate the"monitor"and"mwait"machine instructions 🌸-mrecip— this one enables use of"RCPSS"and"RSQRTSS"instructions (plus their vectorized variants"RCPPS"and"RSQRTPS") with an extra Newton-Raphson step to bump precision, instead of"DIVSS"and"SQRTSS"(and their vectorized variants) for single-precision floating-point arguments. These instructions are only generated when-funsafe-math-optimizationsis enabled together with-ffinite-math-onlyand-fno-trapping-math. Quick warning bestie: while the throughput of the sequence is higher than the throughput of the non-reciprocal instruction, the precision can be decreased by up to 2 ulp — like, the inverse of 1.0 equals0.99999994😭
Also note that GCC implements "1.0f/sqrtf(x)" in terms of "RSQRTSS" (or "RSQRTPS") already with -ffast-math (or the above option combination), and doesn't need -mrecip 💅
And also also — GCC emits the above sequence with an additional Newton-Raphson step for vectorized single-float division and vectorized sqrtf(x) already with -ffast-math (or the above option combination), and doesn't need -mrecip. So yeah, it's giving self-sufficient ✨
-mrecip=opt— this one controls which reciprocal estimate instructions may be used.optis a comma-separated list of options, and any of them can be preceded by a!to invert it 👑all— enable all estimate instructions.default— enable the default instructions, equivalent to-mrecip.none— disable all estimate instructions, equivalent to-mno-recip.div— enable the approximation for scalar division.vec-div— enable the approximation for vectorized division.sqrt— enable the approximation for scalar square root.vec-sqrt— enable the approximation for vectorized square root.
So for example, -mrecip=all,!sqrt enables all of the reciprocal approximations, except for square root 🎀 mix and match, it's your world.
-mveclibabi=type— specifies the ABI type to use for vectorizing intrinsics using an external library. Supported values fortypearesvmlfor the Intel short vector math library,aoclfor the math library (LibM) from AMD Optimizing CPU Libraries (AOCL) andacmlfor the end-of-life AMD core math library (to which AOCL-LibM is the successor). To use this option, both-ftree-vectorizeand-funsafe-math-optimizationshave to be enabled, and an SVML or ACML ABI-compatible library must be specified at link time 🫶
GCC currently emits calls to "vmldExp2", "vmldLn2", "vmldLog102", "vmldPow2", "vmldTanh2", "vmldTan2", "vmldAtan2", "vmldAtanh2", "vmldCbrt2", "vmldSinh2", "vmldSin2", "vmldAsinh2", "vmldAsin2", "vmldCosh2", "vmldCos2", "vmldAcosh2", "vmldAcos2", "vmlsExp4", "vmlsLn4", "vmlsLog104", "vmlsPow4", "vmlsTanh4", "vmlsTan4", "vmlsAtan4", "vmlsAtanh4", "vmlsCbrt4", "vmlsSinh4", "vmlsSin4", "vmlsAsinh4", "vmlsAsin4", "vmlsCosh4", "vmlsCos4", "vmlsAcosh4" and "vmlsAcos4" for corresponding function type when -mveclibabi=svml is used, "amd_vrs4_acosf", "amd_vrs16_acosf", "amd_vrd8_asin", "amd_vrs4_asinf", "amd_vrs8_asinf", "amd_vrs16_asinf", "amd_vrd2_atan", "amd_vrd8_atan", "amd_vrs4_atanf", "amd_vrs8_atanf", "amd_vrs16_atanf", "amd_vrd2_cos", "amd_vrd4_cos", "amd_vrd8_cos", "amd_vrs4_cosf", "amd_vrs8_cosf", "amd_vrs16_cosf", "amd_vrs4_coshf", "amd_vrs8_coshf", "amd_vrd2_erf", "amd_vrd4_erf", "amd_vrd8_erf", "amd_vrs4_erff", "amd_vrs8_erff", "amd_vrs16_erff", "amd_vrd2_exp", "amd_vrd4_exp", "amd_vrd8_exp", "amd_vrs4_expf", "amd_vrs8_expf", "amd_vrs16_expf", "amd_vrd2_exp10", "amd_vrs4_exp10f", "amd_vrd2_exp2", "amd_vrd4_exp2", "amd_vrd8_exp2", "amd_vrs4_exp2f", "amd_vrs8_exp2f", "amd_vrs16_exp2f", "amd_vrs4_expm1f", "amd_vrd2_log", "amd_vrd4_log", "amd_vrd8_log", "amd_vrs4_logf", "amd_vrs8_logf", "amd_vrs16_logf", "amd_vrd2_log10", "amd_vrs4_log10f", "amd_vrs8_log10f", "amd_vrs16_log10f", "amd_vrd2_log1p", "amd_vrs4_log1pf", "amd_vrd2_log2", "amd_vrd4_log2", "amd_vrd8_log2", "amd_vrs4_log2f", "amd_vrs8_log2f", "amd_vrs16_log2f", "amd_vrd2_pow", "amd_vrd4_pow", "amd_vrd8_pow", "amd_vrs4_powf", "amd_vrs8_powf", "amd_vrs16_powf", "amd_vrd2_sin", "amd_vrd4_sin", "amd_vrd8_sin", "amd_vrs4_sinf", "amd_vrs8_sinf", "amd_vrs16_sinf", "amd_vrd2_tan", "amd_vrd4_tan", "amd_vrd8_tan", "amd_vrs16_tanf", "amd_vrs4_tanhf", "amd_vrs8_tanhf", "amd_vrs16_tanhf" for the corresponding function type when -mveclibabi=aocl is used, and "__vrd2_sin", "__vrd2_cos", "__vrd2_exp", "__vrd2_log", "__vrd2_log2", "__vrd2_log10", "__vrs4_sinf", "__vrs4_cosf", "__vrs4_expf", "__vrs4_logf", "__vrs4_log2f", "__vrs4_log10f" and "__vrs4_powf" for the corresponding function type when -mveclibabi=acml is used. Iconic list, no notes 💅
-mabi=name— generate code for the specified calling convention. Permissible values aresysvfor the ABI used on GNU/Linux and other systems, andmsfor the Microsoft ABI. The default is to use the Microsoft ABI when targeting Microsoft Windows and the SysV ABI on all other systems 🌈 you can control this behavior for specific functions by using the function attributes"ms_abi"and"sysv_abi"— per-function ABI vibes, obsessed.-masm=dialect— output assembly instructions using the selected dialect. Also affects which dialect is used for basic"asm"and extended"asm". Supported choices (in dialect order) areattorintel. The default isatt. Darwin does not supportintel🙈 sorry Darwin girlies.-mforce-indirect-call— force all calls to functions to be indirect. Useful when using Intel Processor Trace, where it generates more precise timing information for function calls ⭐-mmanual-endbr— insert ENDBR instruction at function entry only via the"cf_check"function attribute. This is useful when used with the option-fcf-protection=branchto control ENDBR insertion at the function entry 🎀 you're the one in charge now.-mcet-switch— by default, CET instrumentation is turned off on switch statements that use a jump table and indirect branch track is disabled. Since jump tables are stored in read-only memory, this does not result in a direct loss of hardening. BUT — if the jump table index is attacker-controlled, the indirect jump may not be constrained by CET 😭 this option turns on CET instrumentation to enable indirect branch track for switch statements with jump tables, which leads to the jump targets reachable via any indirect jumps. Stay safe out there bestie 🫶-mcall-ms2sysv-xlogues— due to differences in 64-bit ABIs, any Microsoft ABI function that calls a System V ABI function must consider RSI, RDI and XMM6-15 as clobbered. By default, the code for saving and restoring these registers is emitted inline, resulting in fairly lengthy prologues and epilogues 😮💨 using-mcall-ms2sysv-xloguesemits prologues and epilogues that use stubs in the static portion of libgcc to perform these saves and restores, thus reducing function size at the cost of a few extra instructions. Trade-off queen ✨-mtls-dialect=type— generate code to access thread-local storage using thegnuorgnu2conventions.gnuis the conservative default;gnu2is more efficient, but it may add compile- and run-time requirements that cannot be satisfied on all systems 🌸 efficient but demanding, we love a high-maintenance option.-mpush-args/-mno-push-args— use PUSH operations to store outgoing parameters. This method is shorter and usually equally fast as the method using SUB/MOV operations, and is enabled by default 💅 in some cases disabling it may improve performance because of improved scheduling and reduced dependencies.-maccumulate-outgoing-args— if enabled, the maximum amount of space required for outgoing arguments is computed in the function prologue. This is faster on most modern CPUs because of reduced dependencies, improved scheduling and reduced stack usage when the preferred stack boundary is not equal to2. The drawback? A notable increase in code size 📈 this switch implies-mno-push-args.-mms-bitfields/-mno-ms-bitfields— enable/disable bit-field layout compatible with the native Microsoft Windows compiler 👑
If "packed" is used on a structure, or if bit-fields are used, it may be that the Microsoft ABI lays out the structure differently than the way GCC normally does. Particularly when moving packed data between functions compiled with GCC and the native Microsoft compiler (either via function call or as data in a file), it may be necessary to access either format 🦋 so yeah, know which layout you're dealing with.
Okay bestie, this next bit is the tea on how the compiler lays out your structs when you're in Microsoft mode — and honestly it's giving chaos but make it precise. 💅
This option is enabled by default for Microsoft Windows targets. You can also flip this behavior locally by using variable or type attributes. More deets? Go peek at x86 Attributes. ✨
The Microsoft structure layout algorithm is fairly simple, except for the whole bit-field packing situation, which is where the plot thickens. 😭 The padding and alignment of struct members, plus whether a bit-field can straddle a storage-unit boundary, are determined by these rules:
- Structure members are stored sequentially in the order they're declared: the first member gets the lowest memory address and the last member gets the highest. 🫶
- Every data object has an alignment requirement. For all data except structures, unions and arrays, that requirement is either the size of the object or the current packing size (set with either the
"aligned"attribute or the"pack"pragma) — whichever is less. For structures, unions and arrays, the alignment requirement is the largest alignment requirement of its members. Every object gets allocated an offset so that:
offset % alignment_requirement == 0- Adjacent bit-fields get packed into the same 1-, 2- or 4-byte allocation unit if the integral types are the same size and if the next bit-field fits into the current allocation unit without crossing the boundary imposed by the common alignment requirements of the bit-fields. 🎀
🦋 How MSVC treats zero-length bit-fields 🦋
- If a zero-length bit-field is inserted between two bit-fields that are normally coalesced, they are not coalesced anymore.
For example:
struct
{
unsigned long bf_1 : 12;
unsigned long : 0;
unsigned long bf_2 : 12;
} t1;The size of "t1" is 8 bytes with the zero-length bit-field. If you removed that zero-length bit-field, "t1"'s size would be 4 bytes. 💀
- If a zero-length bit-field is inserted after a bit-field,
"foo", and the alignment of the zero-length bit-field is greater than the member that follows it,"bar", then"bar"gets aligned as the type of the zero-length bit-field. 😮
For example:
struct
{
char foo : 4;
short : 0;
char bar;
} t2;
struct
{
char foo : 4;
short : 0;
double bar;
} t3;For "t2", "bar" is placed at offset 2, rather than offset 1. Accordingly, the size of "t2" is 4. For "t3", the zero-length bit-field does not affect the alignment of "bar" or, as a result, the size of the structure. 🌸
Taking all this into account, it's important to note the following:
- If a zero-length bit-field follows a normal bit-field, the type of the zero-length bit-field may affect the alignment of the structure as a whole. For example,
"t2"has a size of 4 bytes, since the zero-length bit-field follows a normal bit-field, and is of typeshort. ✨ - Even if a zero-length bit-field is not followed by a normal bit-field, it may still affect the alignment of the structure:
struct
{
char foo : 6;
long : 0;
} t4;Here, "t4" takes up 4 bytes. 🙈
- Zero-length bit-fields following non-bit-field members are ignored:
struct
{
char foo;
long : 0;
char bar;
} t5;Here, "t5" takes up 2 bytes. 💖
✨ The rest of the x86 options, bestie ✨
-mno-align-stringops— Do not align the destination of inlined string operations. This switch shrinks your code size and boosts performance when the destination is already aligned but GCC doesn't know that. 🤷♀️-minline-all-stringops— By default GCC inlines string operations only when the destination is known to be aligned to at least a 4-byte boundary. This enables more inlining and increases code size, but it may improve performance for code that depends on fast"memcpy"and"memset"for short lengths. The option also enables inline expansion of"strlen"for all pointer alignments. 💅-minline-stringops-dynamically— For string operations of unknown size, use run-time checks: inline code for small blocks, a library call for large blocks. 🎀-mstringop-strategy=alg— Override the internal decision heuristic for which algorithm gets used when inlining string operations. The allowed values for alg are:rep_byterep_4byterep_8byte
Expand using i386 "rep" prefix of the specified size. 🔥
byte_looploopunrolled_loop
Expand into an inline loop. 🔁
libcall— Always use a library call. 📞
-mmemcpy-strategy=strategy— Override the internal decision heuristic for whether"__builtin_memcpy"should be inlined, and which inline algorithm to use when the expected size of the copy is known. strategy is a comma-separated list ofalg:max_size:dest_aligntriplets. alg is specified in-mstringop-strategy, max_size specifies the max byte size with which inline algorithm alg is allowed. For the last triplet, the max_size must be-1. The max_size of the triplets in the list must be specified in increasing order. The minimal byte size for alg is0for the first triplet and"max_size + 1"of the preceding range. 💜-mmemset-strategy=strategy— This one is similar to-mmemcpy-strategy=except it controls the"__builtin_memset"expansion. 🌸-momit-leaf-frame-pointer— Don't keep the frame pointer in a register for leaf functions. This avoids the instructions to save, set up and restore frame pointers, and it frees up an extra register in leaf functions. The option-momit-leaf-frame-pointerremoves the frame pointer for leaf functions, which might make debugging harder. 😭-mtls-direct-seg-refs,-mno-tls-direct-seg-refs— Controls whether TLS variables may be accessed with offsets from the TLS segment register (%gsfor 32-bit,%fsfor 64-bit), or whether the thread base pointer must be added. Whether or not this is valid depends on the operating system, and whether it maps the segment to cover the entire TLS area. 🧚
For systems that use the GNU C Library, the default is on.
-msse2avx,-mno-sse2avx— Specify that the assembler should encode SSE instructions with a VEX prefix. The option-mavxturns this on by default. ✨-mfentry,-mno-fentry— If profiling is active (-pg), put the profiling counter call before the prologue. Note: on x86 architectures the attribute"ms_hook_prologue"isn't possible at the moment for-mfentryand-pg. 🙈-mrecord-mcount,-mno-record-mcount— If profiling is active (-pg), generate a section that contains pointers to each profiling call; this is useful for automatically patching the calls. You can use themfentry-section=option to set the name of the section; it defaults to__mcount_loc. 🎀-mnop-mcount,-mno-nop-mcount— If profiling is active (-pg), generate the calls to the profiling functions as NOPs. This is useful when they should be patched in later dynamically. This is likely only useful together with-mrecord-mcount. 💅-minstrument-return=type— With-pg -mfentry, instrument function exits according to type, which may be one ofnoneto not instrument,callto generate a call to"__return__", ornop5to generate a 5-byte nop sequence. This option only instruments true returns ending with a"ret", not sibling calls via a jump. 🔥-mrecord-return,-mno-record-return— Generate a"__return_loc"section pointing to all return instrumentation code. 🌈-mfentry-name=name— Set the name of the"__fentry__"symbol called at function entry for-pg -mfentryfunctions. 🫶-mfentry-section=name— Set the name of the section used to record-mrecord-mcountcalls. The default is__mcount_loc. ⭐-mskip-rax-setup,-mno-skip-rax-setup— When generating code for the x86-64 architecture with SSE extensions disabled,-mskip-rax-setupcan be used to skip setting up the RAX register when there are no variable arguments passed in vector registers. 😌-m8bit-idiv,-mno-8bit-idiv— On some processors, like Intel Atom, 8-bit unsigned integer divide is way faster than 32-bit/64-bit integer divide. This option generates a run-time check. If both dividend and divisor are within range of0to255, an 8-bit unsigned integer divide is used instead of a 32-bit/64-bit integer divide. 💖-mavx256-split-unaligned-load,-mavx256-split-unaligned-store— Split 32-byte AVX unaligned load and store. ✂️-mstack-protector-guard=guard,-mstack-protector-guard-reg=reg,-mstack-protector-guard-offset=offset,-mstack-protector-guard-symbol=symbol— Generate stack protection code using a canary at guard. Supported locations areglobalfor a global canary ortlsfor a per-thread canary in the TLS block (that's the default). This option only has an effect when-fstack-protectoror-fstack-protector-allis specified. 🛡️
With the latter choice, the options -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset furthermore specify which segment register (%fs or %gs) to use as base register for reading the canary, and from what offset from that base register. The default for those is as specified in the relevant ABI. 💅
-mstack-protector-guard-symbol=symbol overrides the offset with a symbol reference to a canary in the TLS block. 🎀
-mgeneral-regs-only— Generate code that uses only the general-purpose registers. This stops the compiler from using floating-point, vector, mask and bound registers. 🚫-mrelax-cmpxchg-loop— okay so when GCC is spitting out a compare-and-swap loop for the__syncBuiltins and__atomicBuiltins and there's no native instruction to do the job, this flag makes it optimize for the messy highly-contended case 💅. It issues an atomic load before theCMPXCHGinstruction, and it uses thePAUSEinstruction to save CPU power when it's restarting the loop. Being nice to your CPU? Iconic.-mindirect-branch=choice— converts indirect call and jump using choice. The default iskeep, which keeps indirect call and jump unmodified (no touchy 🙅).thunkconverts indirect call and jump to call and return thunk.thunk-inlineconverts indirect call and jump to inlined call and return thunk.thunk-externconverts indirect call and jump to external call and return thunk provided in a separate object file. You can control this behavior for a specific function by using the function attributeindirect_branch.
Note that-mcmodel=largeis incompatible with-mindirect-branch=thunkand-mindirect-branch=thunk-externsince the thunk function may not be reachable in the large code model. 💀
Note that-mindirect-branch=thunk-externis compatible with-fcf-protection=branchsince the external thunk can be made to enable control-flow check. slay.
-mfunction-return=choice— converts function return using choice. The default iskeep, which keeps function return unmodified.thunkconverts function return to call and return thunk.thunk-inlineconverts function return to inlined call and return thunk.thunk-externconverts function return to external call and return thunk provided in a separate object file. You can control this behavior for a specific function by using the function attributefunction_return.
Note that-mindirect-return=thunk-externis compatible with-fcf-protection=branchsince the external thunk can be made to enable control-flow check.
Note that-mcmodel=largeis incompatible with-mfunction-return=thunkand-mfunction-return=thunk-externsince the thunk function may not be reachable in the large code model.
-mindirect-branch-register— forces indirect call and jump via register. that's it, that's the whole vibe ✨-mharden-sls=choice— generates code to mitigate against straight line speculation (SLS) using choice. The default isnonewhich disables all SLS hardening.returnenables SLS hardening for function returns.indirect-jmpenables SLS hardening for indirect jumps.allenables all SLS hardening. pick your fighter 🫶-mindirect-branch-cs-prefix— adds a CS prefix to call and jmp to indirect thunk with branch target in r8-r15 registers, so that the call and jmp instruction length is 6 bytes. This allows them to potentially be replaced withlfence; call *%r8-r15orlfence; jmp *%r8-r15at run time. runtime glow-up fr fr 🔥-mapx-inline-asm-use-gpr32— so by default, EGPR usage is disabled in inline asm constraints because GCC cannot be aware of whether the asm instructions support GP32 or not. If your inline asm can handle EGPR, use-mapx-inline-asm-use-gpr32. trust yourself bestie 💜-mgather-mscatter— enable vectorization for gather and scatter instructions, respectively. get you a compiler that can do both 😌
These -m switches are supported in addition to the above on x86-64 processors in 64-bit environments.
-m32-m64-mx32-m16— generate code for a 16-bit, 32-bit or 64-bit environment. The-m32option setsint,long, and pointer types to 32 bits, and generates code that runs in 32-bit mode.
The -m64 option sets int to 32 bits and long and pointer types to 64 bits, and generates code for the x86-64 architecture. For Darwin only the -m64 option also turns off the -fno-pic and -mdynamic-no-pic options.
The -mx32 option sets int, long, and pointer types to 32 bits, and generates code for the x86-64 architecture.
The -m16 option is the same as -m32, except for that it outputs the .code16gcc assembly directive at the beginning of the assembly output so that the binary can run in 16-bit mode. retro era 💫
-miamcu— the-miamcuoption generates code that conforms to Intel MCU psABI. It requires the-m32option to be turned on. no-m32, no service 🙈-mno-red-zone— do not use a so-called "red zone" for x86-64 code. The red zone is mandated by the x86-64 ABI; it is a 128-byte area beyond the location of the stack pointer that is not modified by signal or interrupt handlers and therefore can be used for temporary data without adjusting the stack pointer. The flag-mno-red-zonedisables this red zone. (kernel girlies know 🎀)-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. the standard-issue main character ✨-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. mandatory, not optional, periodt 👑-mcmodel=medium— generate code for the medium model: the program is linked in the lower 2 GB of the address space. Small symbols are also placed there. Symbols with sizes larger than-mlarge-data-thresholdare put into large data or BSS sections and can be located above 2GB. Programs can be statically or dynamically linked.-mcmodel=large— generate code for the large model. This model makes no assumptions about addresses and sizes of sections. unbothered, unconstrained 🌈-maddress-mode=long— generate code for long address mode. This is only supported for 64-bit and x32 environments. It is the default address mode for 64-bit environments.-maddress-mode=short— generate code for short address mode. This is only supported for 32-bit and x32 environments. It is the default address mode for 32-bit and x32 environments.-mneeded-mno-needed— emitGNU_PROPERTY_X86_ISA_1_NEEDEDGNU property for Linux target to indicate the micro-architecture ISA level required to execute the binary. letting the linker know your standards 💅-mno-direct-extern-access— without-fpicor-fPIC, always use the GOT pointer to access external symbols. With-fpicor-fPIC, treat access to protected symbols as local symbols. The default is-mdirect-extern-access.
Warning: shared libraries compiled with-mno-direct-extern-accessand executable compiled with-mdirect-extern-accessmay not be binary compatible if protected symbols are used in shared libraries and executable. do not mix and match bestie, it's giving heartbreak 💀
-munroll-only-small-loops-mno-unroll-only-small-loops— controls conservative small loop unrolling. It is enabled by default with-O2, and unrolls loops with less than 4 instructions by 1 time. This gives better utilization of the instruction decoding pipeline on modern processors. You can disable this with-mno-unroll-only-small-loops, and it is also disabled if the more general options-funroll-loopsor-funroll-all-loopsare either enabled or explicitly disabled. small loops are the moment 🦋-mdispatch-scheduler— enable instruction scheduling. This is only supported onbdver1,bdver2,bdver3,bdver4, andznver1processors and additionally requires-fschedule-insns-fsched-pressure. very specific guest list ngl-mlam=choice— LAM (linear-address masking) allows special bits in the pointer to be used for metadata. The default isnone. Withu48, pointer bits in positions 62:48 can be used for metadata; Withu57, pointer bits in positions 62:57 can be used for metadata. bits doing double duty, we love a multitasker 💖
x86 Windows Options
Xstormy16 Options
These options are defined for Xstormy16:
-msim— choose startup files and linker script suitable for the simulator. simulator arc, unlocked 🎮
Xtensa Options
These options are supported for Xtensa targets:
-mconst16-mno-const16— enable or disable use ofCONST16instructions for loading constant values. When enabled,CONST16instructions are always used in place of the standardL32Rinstructions. The use ofCONST16is enabled by default only if theL32Rinstruction is not available. if you can't have the classic, you get the upgrade 🧚-mserialize-volatile-mno-serialize-volatile— when this option is enabled, GCC insertsMEMWinstructions beforevolatilememory references to guarantee sequential consistency. The default is-mserialize-volatile. Use-mno-serialize-volatileto omit theMEMWinstructions.-mforce-no-pic-mno-force-no-pic— for targets like GNU/Linux, where all user-mode Xtensa code must be position-independent code (PIC), this option disables PIC for compiling kernel code. kernel code said "not like the other girls" 😌-mtext-section-literals-mno-text-section-literals— these options control the treatment of literal pools. The default is-mno-text-section-literals, which places literals in a separate section in the output file. This allows the literal pool to be placed in a data RAM/ROM, and it also allows the linker to combine literal pools from separate object files to remove redundant literals and improve code size. With-mtext-section-literals, the literals are interspersed in the text section in order to keep them as close as possible to their references. This may be necessary for large assembly files. Literals for each function are placed right before that function. keeping your literals close to home 🫶-mauto-litpools-mno-auto-litpools— these options control the treatment of literal pools. The default is-mno-auto-litpools, which places literals in a separate section in the output file unless-mtext-section-literalsis used. With-mauto-litpoolsthe literals are interspersed in the text section by the assembler. Compiler does not produce explicit.literaldirectives and loads literals into registers withMOVIinstructions instead ofL32Rto let the assembler do relaxation and place literals as necessary. This option allows assembler to create several literal pools per function and assemble very big functions, which may not be possible with-mtext-section-literals. letting the assembler cook 🔥-mtarget-align-mno-target-align— when this option is enabled, GCC instructs the assembler to automatically align instructions to reduce branch penalties at the expense of some code density. The assembler attempts to widen density instructions to align branch targets and the instructions following call instructions. If there are not enough preceding safe density instructions to align a target, no widening is performed. The default is-mtarget-align. These options do not affect the treatment of auto-aligned instructions likeLOOP, which the assembler always aligns, either by widening density instructions or by inserting NOP instructions.LOOPdoesn't take requests, it's always aligned 💅-mlongcalls,-mno-longcalls— okay so when this is on, GCC tells the assembler to turn direct calls into indirect calls, unless it can figure out that the target of a direct call is already inside the range the call instruction can actually reach 💅. This swap usually happens for calls to functions sitting in other source files, bestie. Concretely: the assembler takes a directCALLinstruction and rewrites it into anL32Rfollowed by aCALLX. The default is-mno-longcalls. Reach for this one in programs where the call target could potentially be out of range 🔥. Big important tea though: this option lives in the assembler, not the compiler, so the assembly code GCC spits out still shows direct call instructions — if you wanna see what's actually happening you gotta look at the disassembled object code 👀. And also, the assembler uses an indirect call for every cross-file call, not just the ones that are really out of range. periodt.-mabi=name— generate code for the ABI you name here. Permissible values are:call0,windowed. The default ABI gets chosen by the Xtensa core configuration, no cap 🎀.-mabi=call0— when this is on, function parameters get passed in registersa2througha7, registersa12througha15are caller-saved, and registera15may be used as a frame pointer. With this version of the ABI enabled, the C preprocessor symbol__XTENSA_CALL0_ABI__is defined ✨.-mabi=windowed— when this is on, function parameters get passed in registersa10througha15, and the called function rotates the register window by 8 registers on entry so that its arguments are found in registersa2througha7. Registera7may be used as a frame pointer. The register window gets rotated 8 registers back upon return 🔄. With this version of the ABI enabled, the C preprocessor symbol__XTENSA_WINDOWED_ABI__is defined 💖.-mextra-l32r-costs=n— sets an extra cost, in clock cycles, for instruction RAM/ROM access onL32Rinstructions 🕰️. When optimizing for speed, this affects whether a constant gets loaded from the literal pool usingL32R, or synthesized from a small one with a couple of arithmetic instructions. The default value is0🍓.-mstrict-align,-mno-strict-align— avoid, or allow, generating memory accesses that may not be aligned on a natural object boundary the way the architecture specification describes 🌸. The default is-mno-strict-alignfor cores that support both unaligned loads and stores in hardware, and-mstrict-alignfor all other cores 👑
🌈 zSeries Options 🌈
These are listed under
🌈 ENVIRONMENT 🌈
Okay bestie, this section is about the ~vibes~ GCC reads from your environment — env vars that change how it behaves. Some of them work by handing over directories or prefixes to use when hunting down various kinds of files. Others just tweak ✨other aspects✨ of the compilation environment. Either way, they're the little gremlins behind the scenes 💅
Heads up though: you can also point GCC at places to search with options like-B,-Iand-L. Those win over anything you set in the environment, and the environment wins over whatever the configuration of GCC itself says. Order of power, fr fr 👑
💬 LANG, LC_CTYPE, LC_MESSAGES, LC_ALL
These four control how GCC uses localization info, which is what lets GCC vibe with different national conventions for diagnostic and informational output 🌍 GCC peeks at the locale categories LC_CTYPE and LC_MESSAGES if it was configured to do so. Those categories can be set to literally any value your installation supports. A classic example value is en_GB.UTF-8 — English in the United Kingdom, encoded in UTF-8 🫶
If LC_ALL is set, it overrides whatever LC_CTYPE and LC_MESSAGES say. Otherwise, LC_CTYPE and LC_MESSAGES just default to the value of LANG. And if none of these are set at all? GCC defaults to traditional C English behavior. very demure, very mindful 🎀
Lowkey important: these environment variables do not touch the encodings of input files, or of strings in output files produced by GCC. Those are controlled by the-finput-charsetand-fexec-charsetoptions respectively. Different lane, different rule 💅
🗂️ TMPDIR
If TMPDIR is set, it specifies the directory to use for temporary files. GCC uses temp files to hold the output of one stage of compilation that's about to be fed as input to the next stage — for example, the output of the preprocessor, which is the input to the compiler proper. it's giving relay race 🏃♀️
📝 GCC_DIAGNOSTICS_LOG
If GCC_DIAGNOSTICS_LOG is set, then extra info about the diagnostics subsystem gets emitted. If you set it to an empty value, that info goes to stderr; otherwise GCC will attempt to open that file and write the info there 📂
The precise content and format of this info is subject to change — it's meant for GCC developers, not for end-users. So like, peek if you want, but don't build your whole personality on it 🙈
🔍 GCC_COMPARE_DEBUG
Setting GCC_COMPARE_DEBUG is nearly equivalent to passing -fcompare-debug to the compiler driver. See the documentation of that option for the full tea ☕
🧭 GCC_EXEC_PREFIX
If GCC_EXEC_PREFIX is set, it specifies a prefix to use in the names of the subprograms the compiler executes. No slash gets 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 want to. your call, bestie 💁♀️
If GCC_EXEC_PREFIX is not set, GCC attempts to figure out an appropriate prefix based on the pathname it was invoked with.
If GCC can't find the subprogram using the specified prefix, it tries looking in the usual places for it. no biggie, it just keeps searching 🦋
The default value of GCC_EXEC_PREFIX is prefix/lib/gcc/ where prefix is the prefix to the installed compiler. In many cases prefix is the value of "prefix" when you ran the configure script.
Other prefixes specified with -B take precedence over this prefix ✨
This prefix is also used for finding files such as crt0.o that are used for linking.
And in addition — and this part is unusual, so buckle up 🎢 — the prefix is used in a weird way when 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. So with -Bfoo/, GCC searches foo/bar just before it searches the standard directory /usr/local/lib/bar. And if a standard directory begins with the configured prefix, then the value of prefix is replaced by GCC_EXEC_PREFIX when looking for header files. iconic behavior, honestly 👑
🛣️ COMPILER_PATH
The value of COMPILER_PATH is a colon-separated list of directories, much like PATH. GCC tries the directories you list here when searching for subprograms — but only if it couldn't find the subprograms using GCC_EXEC_PREFIX. backup plan energy 💜
📚 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 listed here when searching for special linker files, if it can't find them using GCC_EXEC_PREFIX. Linking with GCC also uses these directories when searching for ordinary libraries for the -l option — but directories specified with -L come first. line cutters, but make it linkers 😤
🩹 GCC_EXTRA_DIAGNOSTIC_OUTPUT
If GCC_EXTRA_DIAGNOSTIC_OUTPUT is set to one of the following values, then extra text gets emitted to stderr whenever fix-it hints are emitted. Note that -fdiagnostics-parseable-fixits and -fno-diagnostics-parseable-fixits take precedence over this environment variable 👀
fixits-v1— Emit parseable fix-it hints, equivalent to-fdiagnostics-parseable-fixits. In particular, columns are expressed as a count of bytes, starting at byte 1 for the initial column.fixits-v2— As"fixits-v1", but columns are expressed as display columns, as per-fdiagnostics-column-unit=display.
🧪 EXPERIMENTAL_SARIF_SOCKET
If EXPERIMENTAL_SARIF_SOCKET is set in the environment, then the compiler will attempt to connect to a UNIX domain stream socket with that name, and send an "OnSarifResult" JSON-RPC 2.0 notification to it for each diagnostic that occurs, where the value of the notification is a SARIF "result" object 📡
The compiler will fail immediately if EXPERIMENTAL_SARIF_SOCKET is set and it cannot connect to it. no chill, no retry, just vibes over 💀
🚨 This feature is experimental and subject to change or removal without notice. Do not get attached, bestie 😭
Some additional environment variables affect the behavior of the preprocessor. yes, there's a whole second layer, we're still going 🫶
🐣 CPATH, C_INCLUDE_PATH, CPLUS_INCLUDE_PATH, OBJC_INCLUDE_PATH
Each variable's value is a list of directories separated by a special character, much like PATH, in which to look for header files. That special character, "PATH_SEPARATOR", is target-dependent and determined at GCC build time. For Microsoft Windows-based targets it's a semicolon, and for almost all other targets it's a colon. semicolon girlies vs colon girlies, we don't pit 💅
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 one 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 tells 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. sneaky but valid ✨
📦 DEPENDENCIES_OUTPUT
If this variable is set, its value specifies how to output dependencies for Make based on the non-system header files processed by the compiler. System header files are ignored in the dependency output. not them getting left out 💀
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. combo meal 🍔
☀️ 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. almost the same, but not quite 💜
⏰ SOURCE_DATE_EPOCH
If this variable is set, its value specifies a UNIX timestamp to be used in replacement of the current date and time in the "__DATE__" and "__TIME__" macros, so that the embedded timestamps become reproducible 🔁
The value of SOURCE_DATE_EPOCH must be a UNIX timestamp, defined as the number of seconds (excluding leap seconds) since 01 Jan 1970 00:00:00 represented in ASCII; identical to the output of "date +%s" on GNU/Linux and other systems that support the %s extension in the "date" command.
The value should be a known timestamp such as the last modification time of the source or package, and it should be set by the build process. reproducible builds girlies, we love you 🌸
🐛 BUGS 🐛
For instructions on how to report bugs, see <http://bugzilla.redhat.com/bugzilla/>. go tell them everything, they want to hear it 🫶
📎 FOOTNOTES 📎
- On some systems,
gcc -sharedneeds to build supplementary stub code for constructors to work. On multi-libbed systems,gcc -sharedmust 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. And heads up:-sharedsuppresses the addition of startup code to alter the floating-point environment as done with-ffast-math,-Ofastor-funsafe-math-optimizationson some targets.
👀 SEE ALSO 👀
gpl(7), gfdl(7), fsf-funding(7), cpp(1), gcov(1), as(1), ld(1), gdb(1) and the Info entries for gcc, cpp, as, ld, binutils and gdb. the whole friend group fr fr 🫂
💖 AUTHOR 💖
See the Info entry for gcc, or <https://gcc.gnu.org/onlinedocs/gcc/Contributors.html>, for contributors to GCC. it took a village, and the village ate 👑
⚖️ COPYRIGHT ⚖️
Copyright (c) 1988-2026 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with the Invariant Sections being "GNU General Public License" and "Funding Free Software", the Front-Cover texts being (a) (see below), and with the Back-Cover Texts being (b) (see below). A copy of the license is included in the gfdl(7) man page.
(a) The FSF's Front-Cover Text is:
A GNU Manual(b) The FSF's Back-Cover Text is:
You have freedom to copy and modify this GNU Manual, like GNU
software. Copies published by the Free Software Foundation raise
funds for GNU development.and that's a wrap on the whole page, bestie!! 🎀✨ you just read through the manpage for gcc-16, dated 2026-08-19, living under the name GCC(1). go forth and compile something iconic 💅💖
📦 where this page comes from
- fedora 44: cross-gcc-common 16.2.1-1.fc44 · GPL-3.0-or-later AND LGPL-3.0-or-later AND (GPL-3.0-or-later WITH GCC-exception-3.1) AND (GPL-3.0-or-later WITH Texinfo-exception) AND (LGPL-2.1-or-later WITH GCC-exception-2.0) AND (GPL-2.0-or-later WITH GCC-exception-2.0) AND (GPL-2.0-or-later WITH GNU-compiler-exception) AND BSL-1.0 AND GFDL-1.3-or-later AND Linux-man-pages-copyleft-2-para AND SunPro AND BSD-1-Clause AND BSD-2-Clause AND BSD-2-Clause-Views AND BSD-3-Clause AND BSD-4-Clause AND BSD-Source-Code AND Zlib AND MIT AND Apache-2.0 AND (Apache-2.0 WITH LLVM-Exception) AND ZPL-2.1 AND ISC AND LicenseRef-Fedora-Public-Domain AND HP-1986 AND curl
- 💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash on : a language model's rewrite of the original, under its terms