GCC(1)

📦 version 26.09, in nixos 26.05, nixpkgs unstable

💖 GCC(1) — GNU project C and C++ compiler 💖

okay bestie, welcome to GCC(1) — the GNU project C and C++ compiler manual page. She's iconic, she's busy, and she's about to walk you through literally everything. 🫶

🌸 NAME 🌸

gcc - GNU project C and C++ compiler

That's the name, no cap: she's the GNU compiler for C and C++. 💅

✨ SYNOPSIS ✨

gcc [-c|-S|-E] [-std=standard]
    [-g] [-pg] [-Olevel]
    [-Wwarn...] [-Wpedantic]
    [-Idir...] [-Ldir...]
    [-Dmacro[=defn]...] [-Umacro]
    [-foption...] [-mmachine-option...]
    [-o outfile] [@file] infile...

Only the most useful options are listed here; see below for the remainder. g++ accepts mostly the same options as gcc. 🎀

🦋 DESCRIPTION 🦋

When you invoke GCC, it normally does preprocessing, compilation, assembly and linking. The "overall options" allow you to stop this process at an intermediate stage. For example, the -c option says not to run the linker. Then the output consists of object files output by the assembler. 💅

Other options are passed on to one or more stages of processing. Some options control the preprocessor and others the compiler itself. Yet other options control the assembler and linker; most of these are not documented here, since you rarely need to use any of them. ngl, that's kind of a relief.

Most of the command-line options that you can use with GCC are useful for C programs; when an option is only useful with another language (usually C++), the explanation says so explicitly. If the description for a particular option does not mention a source language, you can use that option with all supported languages. 🌈

The 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. When you compile C++ programs, you should invoke GCC as g++ instead. 🧚

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. Literally do not group them, bestie. 💀

You can mix options and other arguments. For the most part, the order you use doesn't matter. Order does matter when you use several options of the same kind; for example, if you specify -L more than once, the directories are searched in the order specified. Also, the placement of the -l option is significant. 👑

Many options have long names starting with -f or with -W---for example, -fmove-loop-invariants, -Wformat and so on. Most of these have both positive and negative forms; the negative form of -ffoo is -fno-foo. This manual documents only one of these two forms, whichever one is not the default. ✨

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 gibibyte, 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. 📏

💅 OPTIONS 💅

🌟 Option Summary 🌟

Okay so here's the whole option-summary roster, grouped by type — the explanations are in the following sections. 📚

🎀 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  -specs=file
-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

🍓 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

💜 C++ Language Options

-fabi-version=n  -fno-access-control -faligned-new=n
-fno-assume-sane-operators-new-delete -fchar8_t  -fcheck-new
-fconcepts  -fconstexpr-depth=n  -fconstexpr-cache-depth=n
-fconstexpr-loop-limit=n  -fconstexpr-ops-limit=n
-fno-elide-constructors -fno-enforce-eh-specs -fno-gnu-keywords
-fno-immediate-escalation -fno-implicit-templates
-fno-implicit-inline-templates -fno-implement-inlines
-fmodule-header[=kind] -fmodule-only -fmodules
-fmodule-implicit-inline -fno-module-lazy
-fmodule-mapper=specification -fmodule-version-ignore
-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
-fno-rtti  -fsized-deallocation -fstrong-eval-order[=kind]
-ftemplate-backtrace-limit=n -ftemplate-depth=n
-fno-threadsafe-statics  -fuse-cxa-atexit -fno-weak  -nostdinc++
-fvisibility-inlines-hidden -fvisibility-ms-compat
-fext-numeric-literals -flang-info-include-translate[=header]
-flang-info-include-translate-not -flang-info-module-cmi[=module]
-stdlib=libstdc++,libc++ -Wabi-tag  -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 -Weffc++
-Wno-elaborated-enum-base -Wno-exceptions -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
-Wstrict-null-sentinel  -Wno-subobject-linkage  -Wtemplates
-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|json|json-stderr|json-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
-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]

⚠️ Warning Options ⚠️

Okay bestie, this is the warning-flag motherload. I’m keeping every -Wno-..., every =n, every =byte-size, every [...], every *, and every lowercase -w exactly where it belongs. I’m clustering them the way the page does so the wall stays readable—but nothing is getting dropped. Periodt. 💅

  • -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, -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, -Wno-c++11-extensions
  • -Wno-c++14-extensions, -Wno-c++17-extensions, -Wno-c++20-extensions
  • -Wno-c++23-extensions, -Wcalloc-transposed-args, -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
  • -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=*, -Wexpansion-to-defined
  • -Wfatal-errors, -Wflex-array-member-not-at-end, -Wfloat-conversion
  • -Wfloat-equal, -Wformat, -Wformat=2, -Wno-format-contains-nul
  • -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, -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-variable
  • -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

🔍 Static Analyzer Options 🔍

This is the analyzer’s whole little kingdom, and it has so many flags. I’m listing every single one exactly, including the -fdump-analyzer-* family and all the -Wno-analyzer-* chill pills. No cap. 🫶

  • -fanalyzer, -fanalyzer-call-summaries, -fanalyzer-checker=name
  • -fno-analyzer-feasibility, -fanalyzer-fine-grained
  • -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, -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 are the C and Objective-C-only warning options. If you’re living in C++ land, this isn’t your chapter, but it’s still on the page and it’s still getting its full moment. 🌈

  • -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
  • -Wnested-externs, -Wold-style-declaration, -Wold-style-definition
  • -Wstrict-prototypes, -Wtraditional, -Wtraditional-conversion
  • -Wdeclaration-after-statement, -Wpointer-sign

🐛 Debugging Options 🐛

Debug-info flags, my beloved. Every -g cousin is here, plus the -fdebug-* and -fvar-tracking-* friends. Exact spelling, exact brackets, exact [=type] and [=spec-list] energy. 🎀

  • -g, -glevel, -gdwarf, -gdwarf-version, -gbtf, -gctf, -gctflevel
  • -gprune-btf, -gno-prune-btf, -ggdb, -grecord-gcc-switches
  • -gno-record-gcc-switches, -gstrict-dwarf, -gno-strict-dwarf
  • -gas-loc-support, -gno-as-loc-support, -gas-locview-support
  • -gno-as-locview-support, -gcodeview, -gcolumn-info, -gno-column-info
  • -gdwarf32, -gdwarf64, -gstatement-frontiers, -gno-statement-frontiers
  • -gvariable-location-views, -gno-variable-location-views
  • -ginternal-reset-location-views, -gno-internal-reset-location-views
  • -ginline-points, -gno-inline-points, -gvms, -gz[=type], -gsplit-dwarf
  • -gdescribe-dies, -gno-describe-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

✨ Optimization Options ✨

okay bestie sit down, hydrate, and stretch your thumbs, because this is the mother of all flag dumps 💅 this is the roll call of every optimization-ish switch, listed exactly as the page gives them. Every single one has its own full write-up further down, so think of this as the seating chart, not the gossip.

the first wave (yes there are waves)

  • -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-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
  • -fdata-sections
  • -fdce
  • -fdelayed-branch
  • -fdelete-null-pointer-checks
  • -fdevirtualize
  • -fdevirtualize-speculatively
  • -fdevirtualize-at-ltrans
  • -fdse
  • -fearly-inlining
  • -fipa-sra
  • -fexpensive-optimizations
  • -ffat-lto-objects
  • -ffast-math
  • -ffinite-math-only
  • -ffloat-store
  • -fexcess-precision=style
  • -ffinite-loops
  • -fforward-propagate
  • -ffp-contract=style
  • -ffunction-sections

wave two, she's still going 🏃‍♀️

  • -fgcse
  • -fgcse-after-reload
  • -fgcse-las
  • -fgcse-lm
  • -fgraphite-identity
  • -fgcse-sm
  • -fhoist-adjacent-loads
  • -fif-conversion
  • -fif-conversion2
  • -findirect-inlining
  • -finline-stringops[=fn]
  • -finline-functions
  • -finline-functions-called-once
  • -finline-limit=n
  • -finline-small-functions
  • -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-stack-alignment
  • -fipa-icf
  • -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

wave three — the loops era 🌀

  • -floop-block
  • -floop-interchange
  • -floop-strip-mine
  • -floop-unroll-and-jam
  • -floop-nest-optimize
  • -floop-parallelize-all
  • -flra-remat
  • -flto
  • -flto-compression-level
  • -flto-partition=alg
  • -flto-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-fp-int-builtin-inexact
  • -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

wave four: reorder blocks harder 🔁

  • -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
  • -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

wave five: tree territory 🌳 (we're so close bestie)

  • -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

wave six: the finale 🎀

  • -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
  • --param name=value

and then, sitting at the very bottom of this whole parade like they own the place:

  • -O
  • -O0
  • -O1
  • -O2
  • -O3
  • -Os
  • -Ofast
  • -Og
  • -Oz
yes the actual -O levels you use every single day are listed last, after like 200 hyper-specific -f flags. it's giving "saving the main character for the final scene" 👑 no cap.

🫶 Program Instrumentation Options 🫶

this batch is the "watch me do it" crew — profiling, coverage, sanitizers, stack protection, all of it. same deal: every flag exactly as written, deets in the full docs.

  • -p
  • -pg
  • -fprofile-arcs
  • --coverage
  • -ftest-coverage
  • -fcondition-coverage
  • -fpath-coverage
  • -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
  • -fbounds-check
  • -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]

🦋 Preprocessor Options 🦋

the preprocessor girlies rise 💜 macros, includes, charsets — all of it lives here.

  • -Aquestion=answer
  • -A-question[=answer]
  • -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

🌸 Assembler Options 🌸

tiny section, iconic energy — just two ways to whisper straight to the assembler.

  • -Wa,option
  • -Xassembler option

🌈 Linker Options 🌈

the "will it link tho" chapter 😭 full list, exactly as printed:

  • object-file-name
  • -fuse-ld=linker
  • -llibrary
  • -nostartfiles
  • -nodefaultlibs
  • -nolibc
  • -nostdlib
  • -nostdlib++
  • -e entry
  • --entry=entry
  • -pie
  • -pthread
  • -r
  • -rdynamic
  • -s
  • -static
  • -static-pie
  • -static-libgcc
  • -static-libstdc++
  • -static-libasan
  • -static-libtsan
  • -static-liblsan
  • -static-libubsan
  • -shared
  • -shared-libgcc
  • -symbolic
  • -T script
  • -Wl,option
  • -Xlinker option
  • -u symbol
  • -z keyword

🎀 Directory Options 🎀

where to go looking for stuff — include paths, sysroots, plugin dirs, the whole scavenger hunt.

  • -Bprefix
  • -Idir
  • -I-
  • -idirafter dir
  • -imacros file
  • -imultilib dir
  • -iplugindir=dir
  • -iprefix file
  • -iquote dir
  • -isysroot dir
  • -isystem dir
  • -iwithprefix dir
  • -iwithprefixbefore dir
  • --embed-dir=dir
  • -Ldir
  • -no-canonical-prefixes
  • --no-sysroot-suffix
  • -nostdinc
  • -nostdinc++
  • --sysroot=dir

✨ Code Generation Options ✨

the "what does the actual output look like" crew — registers, structs, PIC vs PIE, visibility, all of it. not a single one skipped, promise 🫶

  • -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
  • -fvisibility=[default|internal|hidden|protected]
  • -fstrict-volatile-bitfields
  • -fsync-libcalls
  • -fzero-init-padding-bits=value

💀 Developer Options 💀

the deepest lore 🔥 dumps, debug counters, pass disabling, timing reports, memory reports — this is the "i'm not like other compiler flags" section and honestly? main character energy.

  • -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-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-file-name=library
  • -print-libgcc-file-name
  • -print-multi-directory
  • -print-multi-lib
  • -print-multi-os-directory
  • -print-prog-name=program
  • -print-search-dirs
  • -Q
  • -print-sysroot
  • -print-sysroot-headers-suffix
  • -save-temps
  • -save-temps=cwd
  • -save-temps=obj
  • -time[=file]

🛠️ Machine-Dependent Options 🛠️

okay bestie, buckle in, because this is the part where GCC stops being one compiler and becomes like forty compilers wearing a trench coat 💅 every architecture gets its own lil flag family, and yes, we're going through all of them. no skipping. i'm not letting you scroll past the weird ones.

🌸 AArch64 Options 🌸

the ARM64 girlies are thriving, and they have a lot to say:

  • -mabi=name — pick your ABI by name 💫
  • -mbig-endian — big-endian output, no cap
  • -mlittle-endian — little-endian output, the usual vibe
  • -mgeneral-regs-only — general registers only, nothing extra
  • -mcmodel=tiny — code model: tiny
  • -mcmodel=small — code model: small
  • -mcmodel=large — code model: large, she's grown
  • -mstrict-align — strict alignment ON
  • -mno-strict-align — strict alignment OFF
  • -momit-leaf-frame-pointer — omit the leaf frame pointer
  • -mtls-dialect=desc — TLS dialect: desc
  • -mtls-dialect=traditional — TLS dialect: traditional, she's old-school 🎀
  • -mtls-size=size — TLS size, you pick the size
  • -mfix-cortex-a53-835769 — the Cortex-A53 erratum 835769 fix, exactly as numbered
  • -mfix-cortex-a53-843419 — and erratum 843419 too
  • -mlow-precision-recip-sqrt — low-precision reciprocal square root
  • -mlow-precision-sqrt — low-precision square root
  • -mlow-precision-div — low-precision divide
  • -mpc-relative-literal-loads — PC-relative literal loads
  • -msign-return-address=scope — return address signing, scoped by scope
  • -mbranch-protection=none|standard|pac-ret[+leaf +b-key]|bti|gcs — branch protection, and the value grammar is literally that: none, standard, pac-ret[+leaf +b-key], bti, or gcs 👑
  • -mharden-sls=opts — harden SLS, with whatever opts you hand her
  • -march=name — target architecture by name
  • -mcpu=name — target CPU by name
  • -mtune=name — tune for name
  • -moverride=string — override with a string
  • -mverbose-cost-dump — verbose cost dump, she's chatty 🗣️
  • -mstack-protector-guard=guard — stack protector guard flavour
  • -mstack-protector-guard-reg=sysreg — the guard lives in sysreg
  • -mstack-protector-guard-offset=offset — guard offset, at offset
  • -mtrack-speculation — track speculation
  • -moutline-atomics — outline those atomics
  • -mearly-ldp-fusion — LDP fusion, early edition
  • -mlate-ldp-fusion — LDP fusion, late edition (both. she wants both.)
  • -Wexperimental-fmv-target — a warning flag, experimental fmv target, handle with care 😌

🦋 Adapteva Epiphany Options 🦋

niche? yes. iconic? also yes. the Epiphany flag set is giving "small but mighty":

  • -mhalf-reg-file — half register file
  • -mprefer-short-insn-regs — prefer short-insn regs
  • -mbranch-cost=num — branch cost of num
  • -mcmove — conditional moves
  • -mnops=num — num nops
  • -msoft-cmpsf — soft compare-single-float
  • -msplit-lohi — split lo/hi
  • -mpost-inc — post increment
  • -mpost-modify — post modify
  • -mstack-offset=num — stack offset of num
  • -mround-nearest — round to nearest
  • -mlong-calls — long calls
  • -mshort-calls — short calls
  • -msmall16 — small16
  • -mfp-mode=mode — floating-point mode, pick a mode
  • -mvect-double — vector double
  • -max-vect-align=num — max vector alignment of num
  • -msplit-vecmove-early — split vecmove early
  • -m1reg-reg — the -m1reg-reg flag, spelling preserved, don't @ me 🫶

💜 AMD GCN Options 💜

GPU time, three flags, immaculate energy:

  • -march=gpu — architecture is the gpu
  • -mtune=gpu — tune for the gpu
  • -mstack-size=bytes — stack size in bytes

🌈 ARC Options 🌈

the ARC list is long and she is not sorry:

  • -mbarrel-shifter — barrel shifter
  • -mjli-always — always JLI
  • -mcpu=cpu — CPU is cpu
  • -mA6 — A6
  • -mARC600 — ARC600
  • -mA7 — A7
  • -mARC700 — ARC700
  • -mdpfp — double-precision FP
  • -mdpfp-compact — DPFP, compact
  • -mdpfp-fast — DPFP, fast
  • -mno-dpfp-lrsr — no DPFP LRSR
  • -mea — EA
  • -mno-mpy — no MPY
  • -mmul32x16 — 32x16 multiply
  • -mmul64 — 64-bit multiply
  • -matomic — atomics
  • -mnorm — norm
  • -mspfp — single-precision FP
  • -mspfp-compact — SPFP, compact
  • -mspfp-fast — SPFP, fast
  • -msimd — SIMD
  • -msoft-float — soft float
  • -mswap — swap
  • -mcrc — CRC
  • -mdsp-packa — DSP packa
  • -mdvbf — DVBF
  • -mlock — lock
  • -mmac-d16 — MAC d16
  • -mmac-24 — MAC 24
  • -mrtsc — RTSC
  • -mswape — swape (yes, with the e)
  • -mtelephony — telephony 📞
  • -mxy — xy
  • -misize — isize
  • -mannotate-align — annotate align
  • -marclinux — ARC Linux
  • -marclinux_prof — ARC Linux prof (underscore stays)
  • -mlong-calls — long calls
  • -mmedium-calls — medium calls
  • -msdata — small data
  • -mirq-ctrl-saved — IRQ ctrl saved
  • -mrgf-banked-regs — RGF banked regs
  • -mlpc-width=width — LPC width of width
  • -G num — capital G, then num
  • -mvolatile-cache — volatile cache
  • -mtp-regno=regno — TP regno regno
  • -malign-call — align call
  • -mauto-modify-reg — auto-modify reg
  • -mbbit-peephole — bbit peephole
  • -mno-brcc — no brcc
  • -mcase-vector-pcrel — case vector PC-relative
  • -mcompact-casesi — compact casesi
  • -mno-cond-exec — no conditional execution
  • -mearly-cbranchsi — early cbranchsi
  • -mexpand-adddi — expand adddi
  • -mindexed-loads — indexed loads
  • -mlra — LRA
  • -mlra-priority-none — LRA priority: none
  • -mlra-priority-compact — LRA priority: compact
  • -mlra-priority-noncompact — LRA priority: noncompact
  • -mmillicode — millicode
  • -mmixed-code — mixed code
  • -mq-class — q class
  • -mRcq — Rcq (capital R, lowercase cq, respect it)
  • -mRcw — Rcw
  • -msize-level=level — size level, level
  • -mtune=cpu — tune for cpu
  • -mmultcost=num — mult cost of num
  • -mcode-density-frame — code density frame
  • -munalign-prob-threshold=probability — unalign prob threshold, a whole probability of it
  • -mmpy-option=multo — MPY option multo
  • -mdiv-rem — divide and remainder
  • -mcode-density — code density
  • -mll64 — LL64
  • -mfpu=fpu — FPU is fpu
  • -mrf16 — RF16
  • -mbranch-index — branch index

🎀 ARM Options 🎀

32-bit ARM said "hold my tea" and produced this absolute unit of a flag list:

  • -mapcs-frame — APCS frame
  • -mno-apcs-frame — no APCS frame
  • -mabi=name — ABI by name
  • -mapcs-stack-check — APCS stack check
  • -mno-apcs-stack-check — no APCS stack check
  • -mapcs-reentrant — APCS reentrant
  • -mno-apcs-reentrant — no APCS reentrant
  • -mgeneral-regs-only — general registers only
  • -msched-prolog — scheduled prologue
  • -mno-sched-prolog — no scheduled prologue
  • -mlittle-endian — little endian
  • -mbig-endian — big endian
  • -mbe8 — BE8
  • -mbe32 — BE32
  • -mfloat-abi=name — float ABI by name
  • -mfp16-format=name — FP16 format by name
  • -mthumb-interwork — Thumb interworking
  • -mno-thumb-interwork — no Thumb interworking
  • -mcpu=name — CPU by name
  • -march=name — arch by name
  • -mfpu=name — FPU by name
  • -mtune=name — tune for name
  • -mprint-tune-info — print the tuning info 📋
  • -mstructure-size-boundary=n — structure size boundary n
  • -mabort-on-noreturn — abort on noreturn
  • -mlong-calls — long calls
  • -mno-long-calls — no long calls
  • -msingle-pic-base — single PIC base
  • -mno-single-pic-base — no single PIC base
  • -mpic-register=reg — PIC register reg
  • -mnop-fun-dllimport — nop fun dllimport
  • -mpoke-function-name — poke the function name 👉
  • -mthumb — Thumb
  • -marm — ARM
  • -mflip-thumb — flip Thumb
  • -mtpcs-frame — TPCS frame
  • -mtpcs-leaf-frame — TPCS leaf frame
  • -mcaller-super-interworking — caller super interworking
  • -mcallee-super-interworking — callee super interworking
  • -mtp=name — TP by name
  • -mtls-dialect=dialect — TLS dialect dialect
  • -mword-relocations — word relocations
  • -mfix-cortex-m3-ldrd — Cortex-M3 LDRD fix
  • -mfix-cortex-a57-aes-1742098 — A57 AES erratum 1742098, exactly that number
  • -mfix-cortex-a72-aes-1655431 — A72 AES erratum 1655431, also exactly
  • -munaligned-access — unaligned access
  • -mneon-for-64bits — NEON for 64-bit
  • -mslow-flash-data — slow flash data
  • -masm-syntax-unified — unified asm syntax
  • -mrestrict-it — restrict IT
  • -mverbose-cost-dump — verbose cost dump
  • -mpure-code — pure code
  • -mcmse — CMSE
  • -mfix-cmse-cve-2021-35465 — the CMSE CVE-2021-35465 fix 🛡️
  • -mstack-protector-guard=guard — stack protector guard flavour
  • -mstack-protector-guard-offset=offset — guard offset offset
  • -mfdpic — FD PIC
  • -mbranch-protection=none|standard|pac-ret[+leaf] [+bti]|bti[+pac-ret[+leaf]] — branch protection with that exact value grammar, brackets and all 💀

🍓 AVR Options 🍓

microcontroller girlies, this one's for you:

  • -mmcu=mcu — the MCU is mcu
  • -mabsdata — absdata
  • -maccumulate-args — accumulate args
  • -mcvt — CVT
  • -mbranch-cost=cost — branch cost cost
  • -mfuse-add=level — fuse add, level
  • -mfuse-move=level — fuse move, level
  • -mfuse-move2 — fuse move 2 (the sequel)
  • -mcall-prologues — call prologues
  • -mgas-isr-prologues — gas ISR prologues
  • -mint8 — int8
  • -mflmap — flmap
  • -mdouble=bits — double is bits bits
  • -mlong-double=bits — long double is bits bits
  • -mno-call-main — no call main
  • -mn_flash=size — n_flash of size (underscore and all)
  • -mfract-convert-truncate — fract convert truncate
  • -mno-interrupts — no interrupts
  • -mmain-is-OS_task — main is OS_task, capitals exactly as written
  • -mrelax — relax
  • -mrmw — RMW
  • -mstrict-X — strict X, capital X, she means it
  • -mtiny-stack — tiny stack
  • -mrodata-in-ram — rodata in RAM
  • -msplit-bit-shift — split bit shift
  • -msplit-ldst — split ld/st
  • -mshort-calls — short calls
  • -mskip-bug — skip the bug 🐛
  • -muse-nonzero-bits — use nonzero bits
  • -nodevicelib — no devicelib
  • -nodevicespecs — no devicespecs
  • -Waddr-space-convert — warn on address-space conversion
  • -Wmisspelled-isr — warn on misspelled ISR (iconic, honestly)

🧚 Blackfin Options 🧚

  • -mcpu=cpu[-sirevision] — CPU cpu, optionally with [-sirevision]
  • -msim — simulator
  • -momit-leaf-frame-pointer — omit leaf frame pointer
  • -mno-omit-leaf-frame-pointer — don't
  • -mspecld-anomaly — speculative load anomaly
  • -mno-specld-anomaly — no speculative load anomaly
  • -mcsync-anomaly — CSYNC anomaly
  • -mno-csync-anomaly — no CSYNC anomaly
  • -mlow-64k — low 64k
  • -mno-low64k — not low 64k
  • -mstack-check-l1 — stack check L1
  • -mid-shared-library — ID shared library
  • -mno-id-shared-library — no ID shared library
  • -mshared-library-id=n — shared library ID n
  • -mleaf-id-shared-library — leaf ID shared library
  • -mno-leaf-id-shared-library — no leaf ID shared library
  • -msep-data — separate data
  • -mno-sep-data — no separate data
  • -mlong-calls — long calls
  • -mno-long-calls — no long calls
  • -mfast-fp — fast FP
  • -minline-plt — inline PLT
  • -mmulticore — multicore
  • -mcorea — core A
  • -mcoreb — core B
  • -msdram — SDRAM
  • -micplb — ICPLB

⭐ C6X Options ⭐

short and sweet:

  • -mbig-endian — big endian
  • -mlittle-endian — little endian
  • -march=cpu — arch cpu
  • -msim — simulator
  • -msdata=sdata-type — small data of sdata-type

🫶 CRIS Options 🫶

  • -mcpu=cpu — CPU cpu
  • -march=cpu — arch cpu
  • -mtune=cpu — tune for cpu
  • -mmax-stack-frame=n — max stack frame n
  • -metrax4 — ETRAX 4
  • -metrax100 — ETRAX 100
  • -mpdebug — pdebug
  • -mcc-init — CC init
  • -mno-side-effects — no side effects (if only 😭)
  • -mstack-align — stack align
  • -mdata-align — data align
  • -mconst-align — const align
  • -m32-bit — 32-bit
  • -m16-bit — 16-bit
  • -m8-bit — 8-bit
  • -mno-prologue-epilogue — no prologue/epilogue
  • -melf — ELF
  • -maout — a.out
  • -sim — -sim, one dash, exactly as the page spells it
  • -sim2 — -sim2, same deal 👀
  • -mmul-bug-workaround — multiply bug workaround
  • -mno-mul-bug-workaround — no multiply bug workaround

🌸 C-SKY Options 🌸

  • -march=arch — arch arch
  • -mcpu=cpu — CPU cpu
  • -mbig-endian — big endian
  • -EB — -EB, big endian in two letters
  • -mlittle-endian — little endian
  • -EL — -EL, little endian in two letters
  • -mhard-float — hard float
  • -msoft-float — soft float
  • -mfpu=fpu — FPU fpu
  • -mdouble-float — double float
  • -mfdivdu — FDIVDU
  • -mfloat-abi=name — float ABI name
  • -melrw — ELRW
  • -mistack — ISTACK
  • -mmp — MP
  • -mcp — CP
  • -mcache — cache
  • -msecurity — security
  • -mtrust — trust
  • -mdsp — DSP
  • -medsp — EDSP
  • -mvdsp — VDSP
  • -mdiv — div
  • -msmart — smart
  • -mhigh-registers — high registers
  • -manchor — anchor
  • -mpushpop — pushpop
  • -mmultiple-stld — multiple st/ld
  • -mconstpool — const pool
  • -mstack-size — stack size
  • -mccrt — CCRT
  • -mbranch-cost=n — branch cost n
  • -mcse-cc — CSE CC
  • -msched-prolog — scheduled prologue
  • -msim — simulator

🎀 Cygwin and MinGW Options 🎀

windows besties, your moment:

  • -mconsole — console app
  • -mcrtdll=library — CRT DLL is library
  • -mdll — DLL
  • -mnop-fun-dllimport — nop fun dllimport
  • -mthread — thread
  • -municode — unicode
  • -mwin32 — win32
  • -mwindows — windows app
  • -fno-set-stack-executable — don't set the stack executable, very much the safe-vibe flag 🔥

🦋 Darwin Options 🦋

okay this one is a wall, and i'm giving you every single link in the chain, bestie. also heads up: the page literally lists a couple of these twice — -sectobjectsymbols, -segs_read_only_addr and -segs_read_write_addr each show up two times — so they're here two times too. faithful to the source, always 💅

  • -all_load — all load
  • -allowable_client — allowable client
  • -arch — arch
  • -arch_errors_fatal — arch errors fatal
  • -arch_only — arch only
  • -bind_at_load — bind at load
  • -bundle — bundle
  • -bundle_loader — bundle loader
  • -client_name — client name
  • -compatibility_version — compatibility version
  • -current_version — current version
  • -dead_strip — dead strip
  • -dependency-file — dependency file
  • -dylib_file — dylib file
  • -dylinker_install_name — dylinker install name
  • -dynamic — dynamic
  • -dynamiclib — dynamiclib
  • -exported_symbols_list — exported symbols list
  • -filelist — filelist
  • -flat_namespace — flat namespace
  • -force_cpusubtype_ALL — force cpusubtype ALL
  • -force_flat_namespace — force flat namespace
  • -headerpad_max_install_names — headerpad max install names
  • -iframework — iframework
  • -image_base — image base
  • -init — init
  • -install_name — install name
  • -keep_private_externs — keep private externs
  • -multi_module — multi module
  • -multiply_defined — multiply defined
  • -multiply_defined_unused — multiply defined unused
  • -noall_load — no all_load
  • -no_dead_strip_inits_and_terms — no dead strip inits and terms
  • -nodefaultrpaths — no default rpaths
  • -nofixprebinding — no fixprebinding
  • -nomultidefs — no multidefs
  • -noprebind — no prebind
  • -noseglinkedit — no seglinkedit
  • -pagezero_size — pagezero size
  • -prebind — prebind
  • -prebind_all_twolevel_modules — prebind all twolevel modules
  • -private_bundle — private bundle
  • -read_only_relocs — read only relocs
  • -sectalign — sectalign
  • -sectobjectsymbols — sectobjectsymbols (first appearance ✨)
  • -whyload — whyload (she's asking why. valid.)
  • -seg1addr — seg1addr
  • -sectcreate — sectcreate
  • -sectobjectsymbols — sectobjectsymbols (second appearance, still here 💀)
  • -sectorder — sectorder
  • -segaddr — segaddr
  • -segs_read_only_addr — segs read only addr (appearance one)
  • -segs_read_write_addr — segs read write addr (appearance one)
  • -seg_addr_table — seg addr table
  • -seg_addr_table_filename — seg addr table filename
  • -seglinkedit — seglinkedit
  • -segprot — segprot
  • -segs_read_only_addr — segs read only addr (appearance two)
  • -segs_read_write_addr — segs read write addr (appearance two)
  • -single_module — single module
  • -static — static
  • -sub_library — sub library
  • -sub_umbrella — sub umbrella ☂️
  • -twolevel_namespace — twolevel namespace
  • -umbrella — umbrella ☂️
  • -undefined — undefined
  • -unexported_symbols_list — unexported symbols list
  • -weak_reference_mismatches — weak reference mismatches
  • -whatsloaded — whatsloaded
  • -F — capital F
  • -gused — gused
  • -gfull — gfull
  • -mmacosx-version-min=version — min macOS X version version
  • -mkernel — kernel
  • -mone-byte-bool — one byte bool

💜 DEC Alpha Options 💜

  • -mno-fp-regs — no FP regs
  • -msoft-float — soft float
  • -mieee — IEEE
  • -mieee-with-inexact — IEEE with inexact
  • -mieee-conformant — IEEE conformant
  • -mfp-trap-mode=mode — FP trap mode mode
  • -mfp-rounding-mode=mode — FP rounding mode mode
  • -mtrap-precision=mode — trap precision mode
  • -mbuild-constants — build constants
  • -mcpu=cpu-type — CPU cpu-type
  • -mtune=cpu-type — tune for cpu-type
  • -mbwx — BWX
  • -mmax — MAX
  • -mfix — FIX
  • -mcix — CIX
  • -msafe-bwa — safe BWA
  • -msafe-partial — safe partial
  • -mfloat-vax — float VAX
  • -mfloat-ieee — float IEEE
  • -mexplicit-relocs — explicit relocs
  • -msmall-data — small data
  • -mlarge-data — large data
  • -msmall-text — small text
  • -mlarge-text — large text
  • -mmemory-latency=time — memory latency time ⏳

🔥 eBPF Options 🔥

kernel vibes only:

  • -mbig-endian — big endian
  • -mlittle-endian — little endian
  • -mframe-limit=bytes — frame limit in bytes
  • -mxbpf — xBPF
  • -mco-re — CO-RE
  • -mno-co-re — no CO-RE
  • -mjmpext — jump extensions
  • -mjmp32 — jump32
  • -malu32 — ALU32
  • -mv3-atomics — v3 atomics
  • -mbswap — byte swap
  • -msdiv — signed div
  • -msmov — signed mov
  • -mcpu=version — CPU version
  • -masm=dialect — asm dialect
  • -minline-memops-threshold=bytes — inline memops threshold in bytes

🌈 FR30 Options 🌈

  • -msmall-model — small model
  • -mno-lsim — no lsim

🧚 FT32 Options 🧚

  • -msim — simulator
  • -mlra — LRA
  • -mnodiv — no div
  • -mft32b — FT32B
  • -mcompress — compress
  • -mnopm — no PM

👑 FRV Options 👑

  • -mgpr-32 — 32 GPRs
  • -mgpr-64 — 64 GPRs
  • -mfpr-32 — 32 FPRs
  • -mfpr-64 — 64 FPRs
  • -mhard-float — hard float
  • -msoft-float — soft float
  • -malloc-cc — allocate CC
  • -mfixed-cc — fixed CC
  • -mdword — dword
  • -mno-dword — no dword
  • -mdouble — double
  • -mno-double — no double
  • -mmedia — media
  • -mno-media — no media
  • -mmuladd — muladd
  • -mno-muladd — no muladd
  • -mfdpic — FD PIC
  • -minline-plt — inline PLT
  • -mgprel-ro — GP-relative read-only
  • -multilib-library-pic — multilib library PIC
  • -mlinked-fp — linked FP
  • -mlong-calls — long calls
  • -malign-labels — align labels
  • -mlibrary-pic — library PIC
  • -macc-4 — acc 4
  • -macc-8 — acc 8
  • -mpack — pack
  • -mno-pack — no pack
  • -mno-eflags — no eflags
  • -mcond-move — conditional move
  • -mno-cond-move — no conditional move
  • -moptimize-membar — optimize membar
  • -mno-optimize-membar — no optimize membar
  • -mscc — SCC
  • -mno-scc — no SCC
  • -mcond-exec — conditional execution
  • -mno-cond-exec — no conditional execution
  • -mvliw-branch — VLIW branch
  • -mno-vliw-branch — no VLIW branch
  • -mmulti-cond-exec — multi conditional execution
  • -mno-multi-cond-exec — no multi conditional execution
  • -mnested-cond-exec — nested conditional execution
  • -mno-nested-cond-exec — no nested conditional execution
  • -mtomcat-stats — tomcat stats 🐱
  • -mTLS — TLS, capital
  • -mtls — tls, lowercase
  • -mcpu=cpu — CPU cpu

🎀 GNU/Linux Options 🎀

the libc picker:

  • -mglibc — glibc
  • -muclibc — uclibc
  • -mmusl — musl
  • -mbionic — bionic
  • -mandroid — android 🤖
  • -tno-android-cc — no android cc
  • -tno-android-ld — no android ld

🍓 H8/300 Options 🍓

  • -mrelax — relax
  • -mh — H
  • -ms — S
  • -mn — N
  • -mexr — EXR
  • -mno-exr — no EXR
  • -mint32 — int32
  • -malign-300 — align 300

💅 HPPA Options 💅

  • -march=architecture-type — arch of architecture-type
  • -matomic-libcalls — atomic libcalls
  • -mbig-switch — big switch
  • -mcaller-copies — caller copies
  • -mdisable-fpregs — disable FP regs
  • -mdisable-indexing — disable indexing
  • -mordered — ordered
  • -mfast-indirect-calls — fast indirect calls
  • -mgas — gas
  • -mgnu-ld — GNU ld
  • -mhp-ld — HP ld
  • -mfixed-range=register-range — fixed range register-range
  • -mcoherent-ldcw — coherent LDCW
  • -mjump-in-delay — jump in delay slot
  • -mlinker-opt — linker opt
  • -mlong-calls — long calls
  • -mlong-load-store — long load/store
  • -mno-atomic-libcalls — no atomic libcalls
  • -mno-disable-fpregs — don't disable FP regs
  • -mno-disable-indexing — don't disable indexing
  • -mno-fast-indirect-calls — no fast indirect calls
  • -mno-gas — no gas
  • -mno-jump-in-delay — no jump in delay slot
  • -mno-long-load-store — no long load/store
  • -mno-portable-runtime — no portable runtime
  • -mno-soft-float — no soft float
  • -mno-space-regs — no space regs
  • -msoft-float — soft float
  • -mpa-risc-1-0 — PA-RISC 1.0
  • -mpa-risc-1-1 — PA-RISC 1.1
  • -mpa-risc-2-0 — PA-RISC 2.0
  • -mportable-runtime — portable runtime
  • -mschedule=cpu-type — schedule for cpu-type
  • -mspace-regs — space regs
  • -msoft-mult — soft multiply
  • -msio — SIO
  • -mwsio — WSIO
  • -munix=unix-std — unix standard unix-std
  • -nolibdld — no libdld
  • -static — static
  • -threads — threads 🧵

⭐ IA-64 Options ⭐

Itanium walked so the rest of you could run:

  • -mbig-endian — big endian
  • -mlittle-endian — little endian
  • -mgnu-as — GNU as
  • -mgnu-ld — GNU ld
  • -mno-pic — no PIC
  • -mvolatile-asm-stop — volatile asm stop
  • -mregister-names — register names
  • -msdata — small data
  • -mno-sdata — no small data
  • -mconstant-gp — constant GP
  • -mauto-pic — auto PIC
  • -mfused-madd — fused multiply-add
  • -minline-float-divide-min-latency — inline float divide, min latency
  • -minline-float-divide-max-throughput — inline float divide, max throughput
  • -mno-inline-float-divide — no inline float divide
  • -minline-int-divide-min-latency — inline int divide, min latency
  • -minline-int-divide-max-throughput — inline int divide, max throughput
  • -mno-inline-int-divide — no inline int divide
  • -minline-sqrt-min-latency — inline sqrt, min latency
  • -minline-sqrt-max-throughput — inline sqrt, max throughput
  • -mno-inline-sqrt — no inline sqrt
  • -mdwarf2-asm — DWARF2 asm
  • -mearly-stop-bits — early stop bits
  • -mfixed-range=register-range — fixed range register-range
  • -mtls-size=tls-size — TLS size tls-size
  • -mtune=cpu-type — tune for cpu-type
  • -milp32 — ILP32
  • -mlp64 — LP64
  • -msched-br-data-spec — sched branch data speculation
  • -msched-ar-data-spec — sched ar data speculation
  • -msched-control-spec — sched control speculation
  • -msched-br-in-data-spec — sched br in data spec
  • -msched-ar-in-data-spec — sched ar in data spec
  • -msched-in-control-spec — sched in control spec
  • -msched-spec-ldc — sched spec LDC
  • -msched-spec-control-ldc — sched spec control LDC
  • -msched-prefer-non-data-spec-insns — prefer non-data-spec insns
  • -msched-prefer-non-control-spec-insns — prefer non-control-spec insns
  • -msched-stop-bits-after-every-cycle — stop bits after every cycle
  • -msched-count-spec-in-critical-path — count spec in critical path
  • -msel-sched-dont-check-control-spec — sel sched, don't check control spec
  • -msched-fp-mem-deps-zero-cost — FP mem deps, zero cost
  • -msched-max-memory-insns-hard-limit — max memory insns, hard limit
  • -msched-max-memory-insns=max-insns — max memory insns max-insns

🦋 LM32 Options 🦋

last but not least, five little flags:

  • -mbarrel-shift-enabled — barrel shift enabled
  • -mdivide-enabled — divide enabled
  • -mmultiply-enabled — multiply enabled
  • -msign-extend-enabled — sign extend enabled
  • -muser-enabled — user enabled

🦋 LoongArch Options

ok so LoongArch bestie, you literally have a whole menu of knobs:

  • the big ones — -march=arch-type, -mtune=tune-type, -mabi=base-abi-type, -mfpu=fpu-type, -msimd=simd-type
  • float situation — -msoft-float, -msingle-float, -mdouble-float 💅
  • SIMD extensions, yes or no — -mlsx, -mno-lsx, -mlasx, -mno-lasx
  • cost tuning — -mbranch-cost=n, -maddr-reg-reg-cost=n
  • division checking — -mcheck-zero-division, -mno-check-zero-division (pick your drama level)
  • conditional moves, int + float — -mcond-move-int, -mno-cond-move-int, -mcond-move-float, -mno-cond-move-float
  • memcpy shenanigans — -memcpy, -mno-memcpy
  • alignment rules — -mstrict-align, -mno-strict-align
  • the classic G knob — -G num
  • inline memcpy ceiling — -mmax-inline-memcpy-size=n
  • relocation spice — -mexplicit-relocs=style, -mexplicit-relocs, -mno-explicit-relocs
  • extern access, direct or not — -mdirect-extern-access, -mno-direct-extern-access
  • code model + relax — -mcmodel=code-model, -mrelax, -mpass-mrelax-to-as
  • reciprocal math era 🌸 — -mrecip, -mrecip=opt, -mfrecipe, -mno-frecipe
  • div32 — -mdiv32, -mno-div32
  • LAM things — -mlam-bh, -mno-lam-bh, -mlamcas, -mno-lamcas, -mld-seq-sa, -mno-ld-seq-sa
  • TLS flavour — -mtls-dialect=opt
  • tablejump annotations — -mannotate-tablejump, -mno-annotate-tablejump

🎀 M32C Options

  • -mcpu=cpu — pick your CPU, icon
  • -msim — sim life
  • -memregs=number — how many emregs, by number

🍓 M32R/D Options

  • core pick — -m32r2, -m32rx, -m32r
  • -mdebug — debug vibes
  • loop alignment — -malign-loops, -mno-align-loops
  • -missue-rate=number — issue rate, with a number
  • -mbranch-cost=number
  • -mmodel=code-size-model-type
  • sdata — -msdata=sdata-type
  • flush function stuff — -mno-flush-func, -mflush-func=name
  • flush trap stuff — -mno-flush-trap, -mflush-trap=number
  • -G num — the G again, always the G

✨ M680x0 Options ✨

  • arch/cpu/tune trio — -march=arch, -mcpu=cpu, -mtune=tune
  • the chip lineup, literally a whole family tree 👑 — -m68000, -m68020, -m68020-40, -m68020-60, -m68030, -m68040, -m68060, -mcpu32, -m5200, -m5206e, -m528x, -m5307, -m5407, -mcfv4e
  • bitfields on/off — -mbitfield, -mno-bitfield, and the older spelling -mnobitfield
  • c68000 / c68020 alias-ish tags — -mc68000, -mc68020
  • rtd — -mrtd, -mno-rtd
  • div — -mdiv, -mno-div
  • short — -mshort, -mno-short
  • float — -mhard-float, -m68881, -msoft-float
  • -mpcrel — PC relative era
  • alignment — -malign-int, -mstrict-align
  • sep data — -msep-data, -mno-sep-data
  • shared library id — -mshared-library-id=n, -mid-shared-library, -mno-id-shared-library
  • xgot — -mxgot, -mno-xgot
  • -mlong-jump-table-offsets — long jump tables, slay

💜 MCore Options

  • hardlit — -mhardlit, -mno-hardlit
  • div — -mdiv, -mno-div
  • relax immediates — -mrelax-immediates, -mno-relax-immediates
  • wide bitfields — -mwide-bitfields, -mno-wide-bitfields
  • 4byte functions — -m4byte-functions, -mno-4byte-functions
  • callgraph data — -mcallgraph-data, -mno-callgraph-data
  • slow bytes — -mslow-bytes, -mno-slow-bytes
  • -mno-lsim
  • endianness — -mlittle-endian, -mbig-endian
  • -m210, -m340
  • -mstack-increment

🌈 MicroBlaze Options

  • float — -msoft-float, -mhard-float
  • -msmall-divides
  • -mcpu=cpu
  • -mmemcpy
  • the xl-* collection, obsessed 🫶 — -mxl-soft-mul, -mxl-soft-div, -mxl-barrel-shift, -mxl-pattern-compare, -mxl-stack-check, -mxl-gp-opt, -mxl-multiply-high, -mxl-float-convert, -mxl-float-sqrt, -mxl-reorder, -mxl-mode-app-model
  • -mno-clearbss
  • endianness — -mbig-endian, -mlittle-endian
  • -mpic-data-is-text-relative

🧚 MIPS Options 🧚

ok this one is a whole thing, buckle up bestie:

  • endianness, the classic — -EL, -EB
  • arch/tune — -march=arch, -mtune=arch
  • the ISA ladder, climb it 🔥 — -mips1, -mips2, -mips3, -mips4, -mips32, -mips32r2, -mips32r3, -mips32r5, -mips32r6, -mips64, -mips64r2, -mips64r3, -mips64r5, -mips64r6
  • mips16 mode — -mips16, -mno-mips16, -mflip-mips16
  • interlink compressed — -minterlink-compressed, -mno-interlink-compressed
  • interlink mips16 — -minterlink-mips16, -mno-interlink-mips16
  • ABI + abicalls — -mabi=abi, -mabicalls, -mno-abicalls
  • shared / plt — -mshared, -mno-shared, -mplt, -mno-plt
  • xgot — -mxgot, -mno-xgot
  • GPR width — -mgp32, -mgp64
  • FPR width — -mfp32, -mfpxx, -mfp64
  • float mode — -mhard-float, -msoft-float, -mno-float, -msingle-float, -mdouble-float
  • odd spreg — -modd-spreg, -mno-odd-spreg
  • -mabs=mode, -mnan=encoding
  • DSP — -mdsp, -mno-dsp, -mdspr2, -mno-dspr2
  • MCU — -mmcu, -mmno-mcu (yes, that's exactly how it's spelled, no I'm not fixing it 💀)
  • EVA — -meva, -mno-eva
  • virt — -mvirt, -mno-virt
  • xpa — -mxpa, -mno-xpa
  • crc — -mcrc, -mno-crc
  • ginv — -mginv, -mno-ginv
  • micromips — -mmicromips, -mno-micromips
  • MSA — -mmsa, -mno-msa
  • Loongson extensions — -mloongson-mmi, -mno-loongson-mmi, -mloongson-ext, -mno-loongson-ext, -mloongson-ext2, -mno-loongson-ext2
  • -mfpu=fpu-type
  • smartmips — -msmartmips, -mno-smartmips
  • paired single — -mpaired-single, -mno-paired-single
  • dmx — -mdmx, -mno-mdmx
  • mips3d — -mips3d, -mno-mips3d
  • mt — -mmt, -mno-mt
  • llsc — -mllsc, -mno-llsc
  • longness — -mlong64, -mlong32
  • sym32 — -msym32, -mno-sym32
  • -Gnum
  • local sdata — -mlocal-sdata, -mno-local-sdata
  • extern sdata — -mextern-sdata, -mno-extern-sdata
  • gpopt — -mgpopt, -mno-gopt (spelled like that, fr fr)
  • embedded data — -membedded-data, -mno-embedded-data
  • uninit const in rodata — -muninit-const-in-rodata, -mno-uninit-const-in-rodata
  • -mcode-readable=setting
  • split addresses — -msplit-addresses, -mno-split-addresses
  • explicit relocs — -mexplicit-relocs, -mno-explicit-relocs, -mexplicit-relocs=release
  • zero division check — -mcheck-zero-division, -mno-check-zero-division
  • division style — -mdivide-traps, -mdivide-breaks
  • load store pairs — -mload-store-pairs, -mno-load-store-pairs
  • strict align — -mstrict-align, -mno-strict-align
  • unaligned access — -mno-unaligned-access, -munaligned-access
  • memcpy — -mmemcpy, -mno-memcpy
  • long calls — -mlong-calls, -mno-long-calls
  • mad / imadd / fused madd — -mmad, -mno-mad, -mimadd, -mno-imadd, -mfused-madd, -mno-fused-madd
  • -nocpp — no preprocessor era
  • the fix-* family, silicon erratas but make it fashion 💀 — -mfix-24k, -mno-fix-24k, -mfix-r4000, -mno-fix-r4000, -mfix-r4400, -mno-fix-r4400, -mfix-r5900, -mno-fix-r5900, -mfix-r10000, -mno-fix-r10000, -mfix-rm7000, -mno-fix-rm7000, -mfix-vr4120, -mno-fix-vr4120, -mfix-vr4130, -mno-fix-vr4130, -mfix-sb1, -mno-fix-sb1
  • flush func — -mflush-func=func, -mno-flush-func
  • branch cost — -mbranch-cost=num
  • branch likely — -mbranch-likely, -mno-branch-likely
  • -mcompact-branches=policy
  • fp exceptions — -mfp-exceptions, -mno-fp-exceptions
  • vr4130 align — -mvr4130-align, -mno-vr4130-align
  • synci — -msynci, -mno-synci
  • lxc1/sxc1 — -mlxc1-sxc1, -mno-lxc1-sxc1
  • madd4 — -mmadd4, -mno-madd4
  • relax pic calls — -mrelax-pic-calls, -mno-relax-pic-calls
  • -mmcount-ra-address
  • frame header opt — -mframe-header-opt, -mno-frame-header-opt

⭐ MMIX Options

  • libfuncs — -mlibfuncs, -mno-libfuncs
  • epsilon — -mepsilon, -mno-epsilon
  • ABI choice — -mabi=gnu, -mabi=mmixware
  • -mzero-extend
  • -mknuthdiv
  • -mtoplevel-symbols
  • -melf
  • branch predict — -mbranch-predict, -mno-branch-predict
  • base addresses — -mbase-addresses, -mno-base-addresses
  • single exit — -msingle-exit, -mno-single-exit

🌸 MN10300 Options

  • mult bug — -mmult-bug, -mno-mult-bug
  • am33 family — -mno-am33, -mam33, -mam33-2, -mam34
  • -mtune=cpu-type
  • -mreturn-pointer-on-d0
  • -mno-crt0
  • -mrelax
  • -mliw
  • -msetlb

🎀 Moxie Options

  • -meb, -mel — endianness, little Moxie drama
  • -mmul.x — yes, with the dot
  • -mno-crt0

🦋 MSP430 Options

  • -msim
  • -masm-hex
  • -mmcu= and -mcpu= — note the empty-ish looking =, that's how it's written
  • size — -mlarge, -msmall
  • -mrelax
  • -mwarn-mcu
  • -mcode-region=, -mdata-region=
  • silicon errata, the saga — -msilicon-errata=, -msilicon-errata-warn=
  • -mhwmult=
  • -minrt
  • -mtiny-printf
  • -mmax-inline-shift=

💜 NDS32 Options

  • endianness — -mbig-endian, -mlittle-endian
  • registers — -mreduced-regs, -mfull-regs
  • cmov — -mcmov, -mno-cmov
  • perf ext — -mext-perf, -mno-ext-perf
  • perf2 ext — -mext-perf2, -mno-ext-perf2
  • string ext — -mext-string, -mno-ext-string
  • v3push — -mv3push, -mno-v3push
  • 16bit — -m16bit, -mno-16bit
  • sizes — -misr-vector-size=num, -mcache-block-size=num
  • -march=arch
  • -mcmodel=code-model
  • -mctor-dtor
  • -mrelax

✨ Nvidia PTX Options ✨

  • -m64
  • -mmainkernel
  • -moptimize

that's literally it, short and iconic 🫶

🌈 OpenRISC Options

  • -mboard=name
  • -mnewlib
  • mul — -mhard-mul, -msoft-mul
  • div — -mhard-div, -msoft-div
  • float — -msoft-float, -mhard-float, -mdouble-float, -munordered-float
  • -mcmov
  • rotate — -mror, -mrori
  • -msext
  • immediates — -msfimm, -mshftimm
  • -mcmodel=code-model

🍓 PDP-11 Options

  • float — -mfpu, -msoft-float
  • ac0 — -mac0, -mno-ac0
  • models — -m40, -m45, -m10
  • int size — -mint32, -mno-int16, -mint16, -mno-int32
  • -msplit
  • asm dialect — -munix-asm, -mdec-asm, -mgnu-asm
  • -mlra

🔥 PowerPC Options

literally just a redirect: See RS/6000 and PowerPC Options. — they're the same party, bestie, keep scrolling 💅

🌸 PRU Options

  • -mmcu=mcu
  • -minrt
  • -mno-relax
  • -mloop
  • -mabi=variant

🧚 RISC-V Options 🧚

  • -mbranch-cost=N-instruction
  • plt — -mplt, -mno-plt
  • -mabi=ABI-string
  • fdiv — -mfdiv, -mno-fdiv
  • fence-tso — -mfence-tso, -mno-fence-tso
  • div — -mdiv, -mno-div
  • spec + march + mtune — -misa-spec=ISA-spec-string, -march=ISA-string, -mtune=processor-string
  • -mpreferred-stack-boundary=num
  • -msmall-data-limit=N-bytes
  • save restore — -msave-restore, -mno-save-restore
  • shorten memrefs — -mshorten-memrefs, -mno-shorten-memrefs
  • strict align — -mstrict-align, -mno-strict-align
  • code model, pick your size — -mcmodel=medlow, -mcmodel=medany, -mcmodel=large
  • explicit relocs — -mexplicit-relocs, -mno-explicit-relocs
  • relax — -mrelax, -mno-relax
  • riscv attribute — -mriscv-attribute, -mno-riscv-attribute
  • -malign-data=type
  • endianness — -mbig-endian, -mlittle-endian
  • stack protector guard trio 👑 — -mstack-protector-guard=guard, -mstack-protector-guard-reg=reg, -mstack-protector-guard-offset=offset
  • csr check — -mcsr-check, -mno-csr-check
  • movcc — -mmovcc, -mno-movcc
  • inline atomics — -minline-atomics, -mno-inline-atomics
  • inline strlen — -minline-strlen, -mno-inline-strlen
  • inline strcmp — -minline-strcmp, -mno-inline-strcmp
  • inline strncmp — -minline-strncmp, -mno-inline-strncmp
  • TLS dialect — -mtls-dialect=desc, -mtls-dialect=trad

🎀 RL78 Options

  • -msim
  • mul, pick a flavour — -mmul=none, -mmul=g13, -mmul=g14
  • -mallregs
  • cpus — -mcpu=g10, -mcpu=g13, -mcpu=g14, -mg10, -mg13, -mg14
  • double size — -m64bit-doubles, -m32bit-doubles
  • -msave-mduc-in-interrupts

👑 RS/6000 and PowerPC Options 👑

alright bestie, this is the big one, take a breath 🫶

  • the trio — -mcpu=cpu-type, -mtune=cpu-type, -mcmodel=code-model
  • -mpowerpc64
  • altivec — -maltivec, -mno-altivec
  • gpopt — -mpowerpc-gpopt, -mno-powerpc-gpopt
  • gfxopt — -mpowerpc-gfxopt, -mno-powerpc-gfxopt
  • mfcrf — -mmfcrf, -mno-mfcrf
  • popcntb — -mpopcntb, -mno-popcntb
  • popcntd — -mpopcntd, -mno-popcntd
  • fprnd — -mfprnd, -mno-fprnd
  • cmpb — -mcmpb, -mno-cmpb
  • hard dfp — -mhard-dfp, -mno-hard-dfp
  • toc situation — -mfull-toc, -mminimal-toc, -mno-fp-in-toc, -mno-sum-in-toc
  • bitness — -m64, -m32
  • xl compat — -mxl-compat, -mno-xl-compat
  • -mpe
  • alignment — -malign-power, -malign-natural
  • float — -msoft-float, -mhard-float
  • multiple — -mmultiple, -mno-multiple
  • update — -mupdate, -mno-update
  • avoid indexed addresses — -mavoid-indexed-addresses, -mno-avoid-indexed-addresses
  • fused madd — -mfused-madd, -mno-fused-madd
  • bit align — -mbit-align, -mno-bit-align
  • strict align — -mstrict-align, -mno-strict-align
  • relocatable — -mrelocatable, -mno-relocatable, -mrelocatable-lib, -mno-relocatable-lib
  • toc — -mtoc, -mno-toc
  • endian, short + long spellings — -mlittle, -mlittle-endian, -mbig, -mbig-endian
  • -mdynamic-no-pic
  • -mswdiv
  • -msingle-pic-base
  • scheduling knobs — -mprioritize-restricted-insns=priority, -msched-costly-dep=dependence_type, -minsert-sched-nops=scheme
  • the call-* lineup — -mcall-aixdesc, -mcall-eabi, -mcall-freebsd, -mcall-linux, -mcall-netbsd, -mcall-openbsd, -mcall-sysv, -mcall-sysv-eabi, -mcall-sysv-noeabi
  • -mtraceback=traceback_type
  • struct return — -maix-struct-return, -msvr4-struct-return
  • -mabi=abi-type
  • plt style — -msecure-plt, -mbss-plt
  • -msplit-patch-nops
  • longcall — -mlongcall, -mno-longcall
  • pltseq — -mpltseq, -mno-pltseq
  • inline limits, block ops edition — -mblock-move-inline-limit=num, -mblock-compare-inline-limit=num, -mblock-compare-inline-loop-limit=num, -mno-block-ops-unaligned-vsx, -mstring-compare-inline-limit=num
  • isel — -misel, -mno-isel
  • vrsave — -mvrsave, -mno-vrsave
  • mulhw — -mmulhw, -mno-mulhw
  • dlmzb — -mdlmzb, -mno-dlmzb
  • prototype — -mprototype, -mno-prototype
  • the OS/board-ish party 🎉 — -msim, -mmvme, -mads, -myellowknife, -memb
  • sdata — -msdata, -msdata=opt, -mreadonly-in-sdata
  • -mvxworks
  • -G num
  • reciprocal stuff — -mrecip, -mrecip=opt, -mno-recip, -mrecip-precision, -mno-recip-precision
  • -mveclibabi=type
  • friz — -mfriz, -mno-friz
  • pointers to nested functions — -mpointers-to-nested-functions, -mno-pointers-to-nested-functions
  • save toc indirect — -msave-toc-indirect, -mno-save-toc-indirect
  • power8 fusion — -mpower8-fusion, -mno-mpower8-fusion (that's the exact spelling, don't come for me 💀)
  • crypto — -mcrypto, -mno-crypto
  • htm — -mhtm, -mno-htm
  • quad memory — -mquad-memory, -mno-quad-memory, -mquad-memory-atomic, -mno-quad-memory-atomic
  • compat align parm — -mcompat-align-parm, -mno-compat-align-parm
  • float128 — -mfloat128, -mno-float128, -mfloat128-hardware, -mno-float128-hardware
  • gnu attribute — -mgnu-attribute, -mno-gnu-attribute
  • stack protector guard trio again — -mstack-protector-guard=guard, -mstack-protector-guard-reg=reg, -mstack-protector-guard-offset=offset
  • prefixed — -mprefixed, -mno-prefixed
  • pcrel — -mpcrel, -mno-pcrel
  • mma — -mmma, -mno-mmma
  • rop protect — -mrop-protect, -mno-rop-protect
  • privileged — -mprivileged, -mno-privileged

🌸 RX Options 🌸

okay so the RX girlies get their own little buffet of flags, here's the whole menu, no cap:

  • -m64bit-doubles
  • -m32bit-doubles
  • -fpu
  • -nofpu
  • -mcpu=
  • -mbig-endian-data
  • -mlittle-endian-data
  • -msmall-data
  • -msim
  • -mno-sim
  • -mas100-syntax
  • -mno-as100-syntax
  • -mrelax
  • -mmax-constant-size=
  • -mint-register=
  • -mpid
  • -mallow-string-insns
  • -mno-allow-string-insns
  • -mjsr
  • -mno-warn-multiple-fast-interrupts
  • -msave-acc-in-interrupts

👑 S/390 and zSeries Options 👑

big iron besties, this one's for you:

  • -mtune=cpu-type
  • -march=cpu-type
  • -mhard-float
  • -msoft-float
  • -mhard-dfp
  • -mno-hard-dfp
  • -mlong-double-64
  • -mlong-double-128
  • -mbackchain
  • -mno-backchain
  • -mpacked-stack
  • -mno-packed-stack
  • -msmall-exec
  • -mno-small-exec
  • -mmvcle
  • -mno-mvcle
  • -m64
  • -m31
  • -mdebug
  • -mno-debug
  • -mesa
  • -mzarch
  • -mhtm
  • -mvx
  • -mzvector
  • -mtpf-trace
  • -mno-tpf-trace
  • -mtpf-trace-skip
  • -mno-tpf-trace-skip
  • -mfused-madd
  • -mno-fused-madd
  • -mwarn-framesize
  • -mwarn-dynamicstack
  • -mstack-size
  • -mstack-guard
  • -mhotpatch=halfwords,halfwords

🦋 SH Options 🦋

the SH lineup is literally so long, look at all these variants:

  • -m1
  • -m2
  • -m2e
  • -m2a-nofpu
  • -m2a-single-only
  • -m2a-single
  • -m2a
  • -m3
  • -m3e
  • -m4-nofpu
  • -m4-single-only
  • -m4-single
  • -m4
  • -m4a-nofpu
  • -m4a-single-only
  • -m4a-single
  • -m4a
  • -m4al
  • -mb
  • -ml
  • -mdalign
  • -mrelax
  • -mbigtable
  • -mfmovd
  • -mrenesas
  • -mno-renesas
  • -mnomacsave
  • -mieee
  • -mno-ieee
  • -mbitops
  • -misize
  • -minline-ic_invalidate
  • -mpadstruct
  • -mprefergot
  • -musermode
  • -multcost=number
  • -mdiv=strategy
  • -mdivsi3_libfunc=name
  • -mfixed-range=register-range
  • -maccumulate-outgoing-args
  • -matomic-model=atomic-model
  • -mbranch-cost=num
  • -mzdcbranch
  • -mno-zdcbranch
  • -mcbranch-force-delay-slot
  • -mfused-madd
  • -mno-fused-madd
  • -mfsca
  • -mno-fsca
  • -mfsrra
  • -mno-fsrra
  • -mpretend-cmove
  • -mtas

🌈 Solaris 2 Options 🌈

short and sweet, we love a concise queen:

  • -mclear-hwcap
  • -mno-clear-hwcap
  • -mimpure-text
  • -mno-impure-text
  • -pthreads

⭐ SPARC Options ⭐

SPARC stans rise, this is your whole vibe check:

  • -mcpu=cpu-type
  • -mtune=cpu-type
  • -mcmodel=code-model
  • -mmemory-model=mem-model
  • -m32
  • -m64
  • -mapp-regs
  • -mno-app-regs
  • -mfaster-structs
  • -mno-faster-structs
  • -mflat
  • -mno-flat
  • -mfpu
  • -mno-fpu
  • -mhard-float
  • -msoft-float
  • -mhard-quad-float
  • -msoft-quad-float
  • -mstack-bias
  • -mno-stack-bias
  • -mstd-struct-return
  • -mno-std-struct-return
  • -munaligned-doubles
  • -mno-unaligned-doubles
  • -muser-mode
  • -mno-user-mode
  • -mv8plus
  • -mno-v8plus
  • -mvis
  • -mno-vis
  • -mvis2
  • -mno-vis2
  • -mvis3
  • -mno-vis3
  • -mvis3b
  • -mno-vis3b
  • -mvis4
  • -mno-vis4
  • -mvis4b
  • -mno-vis4b
  • -mcbcond
  • -mno-cbcond
  • -mfmaf
  • -mno-fmaf
  • -mfsmuld
  • -mno-fsmuld
  • -mpopc
  • -mno-popc
  • -msubxc
  • -mno-subxc
  • -mfix-at697f
  • -mfix-ut699
  • -mfix-ut700
  • -mfix-gr712rc

💜 System V Options 💜

only four, a minimalist icon:

  • -Qy
  • -Qn
  • -YP,paths
  • -Ym,dir

🎀 V850 Options 🎀

look at all these little guys, obsessed:

  • -mlong-calls
  • -mno-long-calls
  • -mep
  • -mno-ep
  • -mprolog-function
  • -mno-prolog-function
  • -mspace
  • -mtda=n
  • -msda=n
  • -mzda=n
  • -mapp-regs
  • -mno-app-regs
  • -mdisable-callt
  • -mno-disable-callt
  • -mv850e2v3
  • -mv850e2
  • -mv850e1
  • -mv850es
  • -mv850e
  • -mv850
  • -mv850e3v5
  • -mloop
  • -mrelax
  • -mlong-jumps
  • -msoft-float
  • -mhard-float
  • -mgcc-abi
  • -mrh850-abi
  • -mbig-switch

🌸 VAX Options 🌸

vintage core, honestly:

  • -munix
  • -mgnu
  • -md
  • -md-float
  • -mg
  • -mg-float
  • -mlra

🫶 Visium Options 🫶

a cute lil handful for you:

  • -mdebug
  • -msim
  • -mfpu
  • -mno-fpu
  • -mhard-float
  • -msoft-float
  • -mcpu=cpu-type
  • -mtune=cpu-type
  • -msv-mode
  • -muser-mode

🧚 VMS Options 🧚

four flags, we love efficiency:

  • -mvms-return-codes
  • -mdebug-main=prefix
  • -mmalloc64
  • -mpointer-size=size

🔥 VxWorks Options 🔥

the whole set, bestie:

  • -mrtp
  • -msmp
  • -non-static
  • -Bstatic
  • -Bdynamic
  • -Xbind-lazy
  • -Xbind-now

💅 x86 Options 💅

okay, brace yourself, this is the big one — the x86 list ate and left no crumbs, it just goes on and on and on. every single one, in order:

  • -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
  • -mno-wide-multiply
  • -mrtd
  • -malign-double
  • -mpreferred-stack-boundary=num
  • -mincoming-stack-boundary=num
  • -mcld
  • -mcx16
  • -msahf
  • -mmovbe
  • -mcrc32
  • -mmwait
  • -mrecip
  • -mrecip=opt
  • -mvzeroupper
  • -mprefer-avx128
  • -mprefer-vector-width=opt
  • -mpartial-vector-fp-math
  • -mmove-max=bits
  • -mstore-max=bits
  • -mnoreturn-no-callee-saved-registers

then the SIMD fam starts showing up, and it does NOT stop:

  • -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
  • -mthreads
  • -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
  • -mavx10.1-256
  • -mavx10.1-512
  • -mevex512
  • -mavx10.2
  • -mamx-avx512
  • -mamx-tf32
  • -mamx-transpose
  • -mmovrs
  • -mamx-movrs
  • -mcldemote

and the rest of the x86 roster, still going:

  • -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
  • -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
  • -mfentry-name=name
  • -mfentry-section=name
  • -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
  • -mrelax-cmpxchg-loop
  • -mindirect-branch=choice
  • -mfunction-return=choice
  • -mindirect-branch-register
  • -mharden-sls=choice
  • -mindirect-branch-cs-prefix
  • -mneeded
  • -mno-direct-extern-access
  • -munroll-only-small-loops
  • -mlam=choice

x86 Windows Options

not its own list, bestie — it just points you at Cygwin and MinGW Options. go look there 💖

Xstormy16 Options

  • -msim

periodt, that's the whole thing.

🍓 Xtensa Options 🍓

here's the Xtensa set:

  • -mconst16
  • -mno-const16
  • -mfused-madd
  • -mno-fused-madd
  • -mforce-no-pic
  • -mserialize-volatile
  • -mno-serialize-volatile
  • -mtext-section-literals
  • -mno-text-section-literals
  • -mauto-litpools
  • -mno-auto-litpools
  • -mtarget-align
  • -mno-target-align
  • -mlongcalls
  • -mno-longcalls
  • -mabi=abi-type
  • -mextra-l32r-costs=cycles
  • -mstrict-align
  • -mno-strict-align

zSeries Options

also not a separate list — it just says see S/390 and zSeries Options. we already covered those, scroll up queen 💅

✨ Options Controlling the Kind of Output ✨

okay so this is the part where you find out that a "compile" is actually up to four stages, and they happen in this exact order, no skipping, no shuffling:

  1. preprocessing
  2. compilation proper
  3. assembly
  4. linking

literally always in that order. GCC can preprocess and compile several files either into several assembler input files, or into one assembler input file — your call. then each assembler input file produces an object file, and linking combines all the object files (the ones that were just newly compiled, plus the ones you specified as input) into an executable file. it's giving assembly line but make it compiler 💅

and here's the thing — for any given input file, the file name suffix is what decides which kind of compilation gets done. so the suffix is basically the vibe check for your file. here's the whole chart:

  • file.c — C source code, and it 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. heads up: you must link with the libobjc library to make an Objective-C program work.
  • file.mi — Objective-C source code that should not be preprocessed.
  • file.mm, file.M — Objective-C++ source code. and yes, you must link with the libobjc library to make an Objective-C++ program work. also note .M refers to a literal capital M, capital letters matter, don't get cute with it.
  • file.mii — Objective-C++ source code that should not be preprocessed.
  • file.h — a C, C++, Objective-C or Objective-C++ header file to be turned into a precompiled header (that's the default), or a C, C++ header file to be turned into an Ada spec (via the -fdump-ada-spec switch).
  • file.cc, file.cp, file.cxx, file.cpp, file.CPP, file.c++, file.C — C++ source code that must be preprocessed. careful: in .cxx the last two letters must both be literally x. same deal, .C refers to a literal capital C.
  • file.mm, file.M — Objective-C++ source code that must be preprocessed. (yes it appears again, that's not a typo, both exist 🫶)
  • file.mii — Objective-C++ source code that should not be preprocessed. (also a repeat, also not a typo, we keep it 💖)
  • 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. slay, COBOL gets a seat at the table.
  • file.go — Go source code.
  • file.d — D source code.
  • file.di — D interface file.
  • file.dd — D documentation code (Ddoc).
  • file.ads — an Ada source code file that contains a library unit declaration (that's 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 — an 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 gets treated exactly this way. no suffix, no problem, straight to link 💅
  • -x language — this is how you tell the compiler what language it's looking at instead of letting it guess from the file suffix (honestly, judging by file extension alone? kinda valid but also lowkey shady). It applies to every following input file until the next -x comes along, so it's a "from here on out" situation. Possible values for language, and yes, all of them:
c  c-header  cpp-output
c++  c++-header  c++-system-header c++-user-header c++-cpp-output
objective-c  objective-c-header  objective-c-cpp-output
objective-c++ objective-c++-header objective-c++-cpp-output
assembler  assembler-with-cpp
ada
cobol
d
f77  f77-cpp-input f95  f95-cpp-input
go
⚠️ heads up bestie: -x does not imply a particular language standard. Like, it tells gcc the language but not the dialect — for example -x f77 may also require -std=legacy for some older source codes. Two separate vibes, two separate flags.
  • -x none — cancels any language specification you set earlier, so subsequent files go back to being handled according to their file name suffixes (exactly as if -x had never been used at all). The "never mind, forget I said anything" of flags 🫶

Okay so here's the tea on stopping the pipeline early: if you only want some of the stages of compilation, you use -x (or filename suffixes) to tell gcc where to start, and then one of -c, -S, or -E to say where it should stop. And ngl, some combinations are just a no-op — for example -x cpp-output -E literally instructs gcc to do nothing at all. Iconic of it, honestly 💀

  • -c — compile or assemble the source files, but do not link. The linking stage simply is not done, periodt. The ultimate output is 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 that don't require compilation or assembly are just ignored. Not your problem, not gcc's problem 🙈

  • -S — stop after the stage of compilation proper; do not assemble. The output is an assembler code file for each non-assembler input file you specified.

By default, the assembler file name for a source file is made by replacing the suffix .c, .i, etc., with .s.

Input files that don't require compilation are ignored.

  • -E — stop after the preprocessing stage; do not run the compiler proper. The output is preprocessed source code, and it gets sent to the standard output. Slay, preprocessor-only run ✨

Input files that don't require preprocessing are ignored.

  • -o file — puts the primary output in file. This applies to whatever sort of output is being produced — executable file, object file, assembler file, or preprocessed C code. One flag to name them all 💅

If -o is not specified, the defaults are: 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.

So even though -o names only the primary output, it also affects the naming of auxiliary and dump outputs — see the examples below. Unless overridden, both aux outputs and dump outputs land in the same directory as the primary output. For auxiliary outputs, the suffix of the input file is replaced with that of the auxiliary output file type; for 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 just that base name; otherwise they get concatenated, separated by a dash. Fr fr, it's a whole naming ceremony 🎀

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 instead prefixes 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 all of 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 are named dir/foo.c.*. Special case, but she's valid ✨

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.

  • -dumpbase dumpbase — sets the base name for auxiliary and dump output files. It does not affect the name of the primary output file, okay? Stay in your lane, -dumpbase. Intermediate outputs, when preserved, are not regarded as primary outputs but as auxiliary outputs:
gcc -save-temps -S foo.c

saves 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.c

preprocesses to save-foo.i, compiles to save-foo.s (now an intermediate, thus an 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) get the input name suffix; aux output files (those requested by other non-dump options, e.g. -save-temps, -gsplit-dwarf, -fcallgraph-info) go without it.

You can get similar suffix differentiation of dump and aux outputs for an 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. It said "I'm the main character now" and honestly? It was 💅

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.*. Notice how their basenames are taken from the input name, but the directory still defaults to that of the output. Sneaky but correct 🧚

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.*. Then LTO recompilation during linking will use dir/foobar. as the prefix for dumps and auxiliary files.

  • -dumpbase-ext auxdropsuf — when forming the name of an auxiliary (but not a dump) output file, drop the trailing auxdropsuf from dumpbase before appending any suffixes. If not specified, this option defaults to the suffix of a default dumpbase — i.e. the suffix of the input file when -dumpbase isn't present in the command line, or dumpbase is combined with dumppfx.
gcc foo.c -c -o dir/foo.o -dumpbase x-foo.c -dumpbase-ext .c ...

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, however, retain the suffix: dir/x-foo.c.*. Dumps said "I'm keeping it" 💀

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 ...

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 — when forming the name of an auxiliary or dump output file, use dumppfx as a prefix:
gcc -dumpdir pfx- -c foo.c ...

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 ...

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. And in case multiple such options are given, the last one prevails — last one wins, no cap 🏆

gcc -dumpdir pfx- -c foo.c -save-temps=obj ...

...outputs foo.o, with auxiliary outputs named foo.* because -save-temps=* overrides the dumppfx given by the earlier -dumpdir option. And like — 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 you're compiling from multiple input files, if -dumpbase is specified, then dumpbase (minus an auxdropsuf suffix) and a dash get appended to — or override, if it contains any directory components — an explicit or defaulted dumppfx, so that each of the multiple compilations gets differently-named aux and dump outputs. Like so:

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. Now contrast that with the single-input compilation:

gcc foo.c -c -dumpdir dir/pfx- -dumpbase main ...

which, 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 ...

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 overriden 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, bestie 🧚

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 — 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. Iconic lil' "what is actually happening" button 💖
  • -### — Like -v except the commands are not executed and arguments are quoted unless they contain only alphanumeric characters or ./-_. This is useful for shell scripts to capture the driver-generated command lines. Lowkey the safe snoop mode 👀
  • --help — Print (on the standard output) a description of the command-line options understood by gcc. If the -v option is also specified then --help is also passed on to the various processes invoked by gcc, so that they can display the command-line options they accept. If the -Wextra option has also been specified (prior to the --help option), then command-line options that have no documentation associated with them are also displayed. The "tell me everything" button fr 🎀
  • --target-help — Print (on the standard output) a description of target-specific command-line options for each tool. For some targets extra target-specific information may also be printed ⭐

💅 --help={class|[^]qualifier}[,...]

Print (on the standard output) a description of the command-line options understood by the compiler that fit into all specified classes and qualifiers. These are the supported classes:

  • optimizers — Display all of the optimization options supported by the compiler.
  • warnings — Display all of the options controlling warning messages produced by the compiler.
  • target — Display target-specific options. Unlike the --target-help option however, target-specific options of the linker and assembler are not displayed. This is because those tools do not currently support the extended --help= syntax.
  • params — Display the values recognized by the --param option.
  • language — Display the options supported for language, where language is the name of one of the languages supported in this version of GCC. If an option is supported by all languages, one needs to select common class.
  • 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.

So like, for example — to display all the undocumented target-specific switches supported by the compiler, use:

--help=target,undocumented

The sense of a qualifier can be inverted by prefixing it with the ^ character, so for example to display all binary warning options (i.e., ones that are either on or off and that do not take an argument) that have a description, use:

--help=warnings,^joined,^undocumented
⚠️ The argument to --help= should not consist solely of inverted qualifiers. Don't do it bestie 😭

Combining several classes is possible, although this usually restricts the output so much that there is nothing to display. One case where it does work, however, is when one of the classes is target. For example, to display all the target-specific optimization options, use:

--help=target,optimizers

The --help= option can be repeated on the command line. Each successive use displays its requested class of options, skipping those that have already been displayed. If --help is also specified anywhere on the command line then this takes precedence over any --help= option 👑

If the -Q option appears on the command line before the --help= option, then the descriptive text displayed by --help= is changed. Instead of describing the displayed options, an indication is given as to whether the option is enabled, disabled or set to a specific value (assuming that the compiler knows this at the point where the --help= option is used) 🙈

Here is a truncated example from the ARM port of gcc:

% gcc -Q -mabi=2 --help=target -c
The following options are target specific:
-mabi=                                2
-mabort-on-noreturn                   [disabled]
-mapcs                                [disabled]

The output is sensitive to the effects of previous command-line options, so for example it is possible to find out which optimizations are enabled at -O2 by using:

-Q -O2 --help=optimizers

Alternatively you can discover which binary optimizations are enabled by -O3 by using:

gcc -c -Q -O3 --help=optimizers > /tmp/O3-opts
gcc -c -Q -O2 --help=optimizers > /tmp/O2-opts
diff /tmp/O2-opts /tmp/O3-opts | grep enabled
  • --version — Display the version number and copyrights of the invoked GCC. The classic "who am I" check 🌈
  • -pass-exit-codes — Normally the gcc program exits with the code of 1 if any phase of the compiler returns a non-success return code. If you specify -pass-exit-codes, the gcc program instead returns with the numerically highest error produced by any phase returning an error indication. The C, C++, and Fortran front ends return 4 if an internal compiler error is encountered 💀
  • -pipe — Use pipes rather than temporary files for communication between the various stages of compilation. This fails to work on some systems where the assembler is unable to read from a pipe; but the GNU assembler has no trouble. Faster vibes on GNU, periodt 🦋
  • -specs=file — Process file after the compiler reads in the standard specs file, in order to override the defaults which the gcc driver program uses when determining what switches to pass to cc1, cc1plus, as, ld, etc. More than one -specs=file can be specified on the command line, and they are processed in order, from left to right 🍓
  • -wrapper — Invoke all subcommands under a wrapper program. The name of the wrapper program and its parameters are passed as a comma separated list.
gcc -c t.c -wrapper gdb,--args

This invokes all subprograms of gcc under gdb --args, thus the invocation of cc1 is gdb --args cc1 .... Main character energy for your whole compile 💜

  • -ffile-prefix-map=old=new — okay so when you're compiling files that live in directory old, this makes the compiler record any references to them as if they lived in directory new instead. Specifying this one option is literally equivalent to specifying all the individual -f*-prefix-map options, no cap. Super useful for making reproducible builds that are location independent, which is a whole vibe. Heads up though: directories referenced by directives are not affected by these options. See also -fmacro-prefix-map, -fdebug-prefix-map, -fprofile-prefix-map and -fcanon-prefix-map.
  • -fcanon-prefix-map — so for the -f*-prefix-map options, normally the comparison of the old prefix against the filename that'd be referenced in the compilation result is done with a plain textual comparison of the prefixes. Or, on case insensitive filesystems, ignoring character case, and on DOS based filesystems treating slashes and backslashes as equal. This option makes those comparisons happen on canonicalized paths of old and the referenced filename instead. Glow up for your paths ✨
  • -fplugin=name.so — loads the plugin code in file name.so, which is assumed to be a shared object that the compiler will dlopen. The base name of the shared object file is what identifies the plugin for argument parsing purposes (peep -fplugin-arg-name-key=value below 👇). Every plugin should define the callback functions specified in the Plugins API. Iconic behavior.
  • -fplugin-arg-name-key=value — defines an argument called key with a value of value for the plugin called name. That's it, that's the whole thing 💅
  • -fdump-ada-spec[-slim] — for C and C++ source and include files, generates corresponding Ada specs. Ada stans rise 🫶
  • -fada-spec-parent=unit — used together with -fdump-ada-spec[-slim] above, this generates Ada specs as child units of parent unit. Family tree moment.
  • -fdump-go-spec=file — for input files in any language, generates corresponding Go declarations in file. It generates Go const, type, var, and func declarations, which honestly may be a useful way to start writing a Go interface to code written in some other language. Crossover episode 🔥
  • @file — reads command-line options from file. The options that get read are inserted in place of the original @file option. And if file doesn't exist, or can't be read? Then the option is treated literally, and not removed. No silent disappearing acts here.

Options in file are separated by whitespace. A whitespace character can be included in an option by surrounding the entire option in either single or double quotes. Any character (including a backslash) can be included by prefixing the character you want with a backslash. And plot twist: the file may itself contain additional @file options — any such options get processed recursively. It's giving inception 😭

🧚 Compiling C++ Programs 🧚

C++ source files conventionally use one of these 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. So many little name tags, honestly adorable 🎀

GCC recognizes files with these names and compiles them as C++ programs — even if you call the compiler the exact same way you'd call it for compiling C programs (usually with the name gcc). Sneaky but slay.

However — and this is the important part bestie — using 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++. Main character energy 👑

When you compile C++ programs, you may specify many of the same command-line options 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. Pick your fighter 🌈

✨ 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. Dialect check 💅

  • -ansi — in C mode, this is equivalent to -std=c90. In C++ mode, it's equivalent to -std=c++98. Simple as.
  • -std= — determines the language standard. This option is currently only supported when compiling C or C++. So like, don't come here with your other languages, bestie.

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 don't 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 don't 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 what -Wpedantic uses to identify which features are GNU extensions for that version of the standard. For example, -std=gnu90 -Wpedantic warns about C++ style // comments, while -std=gnu99 -Wpedantic does not. The more you know ⭐

A value for this option must be provided — it's not optional, it's giving commitment issues. Possible values are:

  • c90, c89, iso9899:1990 — supports all ISO C90 programs (certain GNU extensions that conflict with ISO C90 are disabled). Same as -ansi for C code.
  • iso9899:199409 — ISO C90 as modified in amendment 1. Amendments, we love to see it.
  • c99, c9x, iso9899:1999, iso9899:199x — ISO C99. This standard is substantially completely supported, modulo bugs and floating-point issues (mainly but not entirely relating to optional C99 features from Annexes F and G). See <https://gcc.gnu.org/c99status.html> for more info. The names c9x and iso9899:199x are deprecated. We don't stan deprecated names, lowkey.
  • c11, c1x, iso9899:2011 — ISO C11, the 2011 revision of the ISO C standard. Substantially completely supported, modulo bugs, floating-point issues (mainly but not entirely relating to optional C11 features from Annexes F and G) and the optional Annexes K (Bounds-checking interfaces) and L (Analyzability). The name c1x is deprecated.
  • c17, c18, iso9899:2017, iso9899:2018 — ISO C17, the 2017 revision of the ISO C standard (published in 2018). This standard is the 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 it's supported to the same extent as C11. Basically C11's sibling.
  • c23, c2x, iso9899:2024 — ISO C23, the 2023 revision of the ISO C standard (published in 2024). The name c2x is deprecated.
  • c2y — the next version of the ISO C standard, still under development. Support for this version is experimental and incomplete. Careful bestie 🚧
  • gnu90, gnu89 — GNU dialect of ISO C90 (including some C99 features). Lil mashup.
  • gnu99, gnu9x — GNU dialect of ISO C99. The name gnu9x is deprecated.
  • gnu11, gnu1x — GNU dialect of ISO C11. The name gnu1x is deprecated.
  • gnu17, gnu18 — GNU dialect of ISO C17.
  • gnu23, gnu2x — GNU dialect of ISO C23. This is the default for C code. The name gnu2x is deprecated.
  • gnu2y — the next version of the ISO C standard, still under development, plus GNU extensions. Support for this version is experimental and incomplete. Also, heads up, the manpage says the name gnu2x is deprecated here (yes, again, it's a whole thing).
  • c++98, c++03 — the 1998 ISO C++ standard plus the 2003 technical corrigendum and some additional defect reports. Same as -ansi for C++ code.
  • gnu++98, gnu++03 — GNU dialect of -std=c++98.
  • c++11, c++0x — the 2011 ISO C++ standard plus amendments. The name c++0x is deprecated.
  • gnu++11, gnu++0x — GNU dialect of -std=c++11. The name gnu++0x is deprecated.
  • c++14, c++1y — the 2014 ISO C++ standard plus amendments. The name c++1y is deprecated.
  • gnu++14, gnu++1y — GNU dialect of -std=c++14. The name gnu++1y is deprecated.
  • c++17, c++1z — the 2017 ISO C++ standard plus amendments. The name c++1z is deprecated.
  • gnu++17, gnu++1z — GNU dialect of -std=c++17. This is the default for C++ code. The name gnu++1z is deprecated.
  • c++20, c++2a — the 2020 ISO C++ standard plus amendments. Support is experimental, and could change in incompatible ways in future releases. The name c++2a is deprecated. ⚠️
  • gnu++20, gnu++2a — GNU dialect of -std=c++20. Support is experimental, and could change in incompatible ways in future releases. The name gnu++2a is deprecated.
  • c++23, c++2b — the 2023 ISO C++ standard plus amendments (published in 2024). Support is experimental, and could change in incompatible ways in future releases. The name c++2b is deprecated.
  • gnu++23, gnu++2b — GNU dialect of -std=c++23. Support is experimental, and could change in incompatible ways in future releases. The name gnu++2b is 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. Bestie, do not ship this to prod 💀
  • 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. Same warning, same energy 🙈
  • -aux-info filename — outputs to the given filename prototyped declarations for all functions declared and/or defined in a translation unit, including those in header files. This option is silently ignored in any language other than C. Not a peep out of it 🍓

...so like, besides the actual declarations, the file also tells you — in comments — where each declaration came from (source file and line, bestie, receipts included 💅), whether it was implicit, prototyped, or unprototyped (that's I, N for new or O for old, respectively, in the very 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 character right after that). And for function definitions? You also get a K&R-style list of arguments followed by their declarations, tucked inside comments, right after the declaration. So organized ✨

🚫 -fno-asm

Do not recognize asm, inline or typeof as keywords, so your code can use these words as identifiers (living its truth as variable names, iconic). Instead, you can use the keywords __asm__, __inline__ and __typeof__. In C, -ansi implies -fno-asm.

In C++, inline is a standard keyword and is not affected by this switch — it does not care about your flag. You may want to use the -fno-gnu-keywords flag 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. The keyword glow-up is real 🦋

🧠 -fno-builtin

🧠 -fno-builtin-function

Don't recognize built-in functions that do not begin with __builtin_ as prefix. No cap.

GCC normally generates special code to handle certain built-in functions more efficiently — for instance, calls to alloca may become single instructions which adjust the stack directly, and calls to memcpy may become inline copy loops. The resulting code is often both smaller and faster, but since the function calls no longer appear as such, you cannot set a breakpoint on those calls, nor can you change the behavior of the functions by linking with a different library. So like, slay for performance, but debugging gets a little 😭. In addition, when a function is recognized as a built-in function, GCC may use information about that function to warn about problems with calls to that function, or to generate more efficient code, even if the resulting code still contains calls to that function. For example, warnings are given with -Wformat for bad calls to printf when printf is built in and strlen is known not to modify global memory. It's giving "your compiler is watching you" 👀

With the -fno-builtin-function option, only the built-in function function is disabled. function must not begin with __builtin_. If a function is named that is not built-in in this version of GCC, this option is ignored. There is no corresponding -fbuiltin-function option; if you wish to enable built-in functions selectively when using -fno-builtin or -ffreestanding, you may define macros such as:

#define abs(n)          __builtin_abs ((n))
#define strcpy(d, s)    __builtin_strcpy ((d), (s))

🙃 -fcond-mismatch

Allow conditional expressions with mismatched types in the second and third arguments. The value of such an expression is void. This option is not supported for C++ — C++ said no thanks bestie 💀

🏝️ -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. Like, you're out here raw-dogging bare metal, no libc safety net 🌈

🧪 -fgimple

Enable parsing of function definitions marked with __GIMPLE. This is an experimental feature that allows unit testing of GIMPLE passes. Experimental! ⚠️ Cute little lab experiment energy.

🧵 -fgnu-tm

When the option -fgnu-tm is specified, the compiler generates code for the Linux variant of Intel's current Transactional Memory ABI specification document (Revision 1.1, May 6 2009). This is an experimental feature whose interface may change in future versions of GCC, as the official specification changes. Please note that not all architectures are supported for this feature. 🙈

For more information on GCC's support for transactional memory,

(bestie the manpage literally trails off there 💀 that's how it's written, I'm not hiding anything from you)

Note that the transactional memory feature is not supported with non-call exceptions (-fnon-call-exceptions).

🩷 -fgnu89-inline

The option -fgnu89-inline tells GCC to use the traditional GNU semantics for inline functions when in C99 mode.

Using this option is roughly equivalent to adding the gnu_inline function attribute to all inline functions.

The option -fno-gnu89-inline explicitly tells GCC to use the C99 semantics for inline when in C99 or gnu99 mode (i.e., it specifies the default behavior). This option is not supported in -std=c90 or -std=gnu90 mode.

The preprocessor macros __GNUC_GNU_INLINE__ and __GNUC_STDC_INLINE__ may be used to check which semantics are in effect for inline functions. Vibe check for your 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. So like, the normal situation, with all the amenities 🛋️

🐍 -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. Red flag! 🚩 It's giving legacy chaos.

🪟 -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. Look at this iconic little situation:

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. Not for everyone, it's exclusive 💅

🔬 -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. Defaults, but make them fashion ✨

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. Basically: your build system gets to know who needs whom, main-character dependency graph energy 👑

📄 -fdeps-file=file

Where to write structured dependency information. As in, where the receipts go 🧾

📐 -fdeps-format=format

The format to use for structured dependency information. p1689r5 is the only supported format right now. Note that when this argument is specified, the output of -MF is stripped of some information (namely C++ modules) so that it does not use extended makefile syntax not understood by most tools. Keeping it simple for the normies 🫶

🎯 -fdeps-target=file

Analogous to -MT but 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. It's the "who's this all for" of the whole operation 💖

🖥️ -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++. C only bestie, C++ can see itself out 🚪

➕➖ -fsigned-bitfields

➕➖ -funsigned-bitfields

➕➖ -fno-signed-bitfields

➕➖ -fno-unsigned-bitfields

These options control whether a bit-field is signed or unsigned, when the declaration does not use either signed or unsigned. By default, such a bit-field is signed, because this is consistent: the basic integer types such as int are signed types. Consistency ate 🍽️

✍️ -fsigned-char

Let the type char be signed, like signed char.

Note that this is equivalent to -fno-unsigned-char, which is the negative form of -funsigned-char. Likewise, the option -fno-signed-char is equivalent to -funsigned-char. Flip flopping but make it documented 💅

🔤 -funsigned-char

Let the type char be unsigned, like unsigned char.

Each kind of machine has a default for what char should be. It is either like unsigned char by default or like signed char by default.

Ideally, a portable program should always use signed char or unsigned char when it depends on the signedness of an object. But many programs have been written to use plain char and expect it to be signed, or expect it to be unsigned, depending on the machines they were written for. This option, and its inverse, let you make such a program work with the opposite default. Legacy code rehabilitation arc 🩹

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. Yes even then. Still distinct. Periodt.

🧱 -fstrict-flex-arrays (C and C++ only)

🧱 -fstrict-flex-arrays=level (C and C++ only)

Control when to treat the trailing array of a structure as a flexible array member for the purpose of accessing the elements of such an array. The value of level controls the level of strictness. Stern teacher energy 📏

-fstrict-flex-arrays is equivalent to -fstrict-flex-arrays=3, which is the strictest; a trailing array is treated as a flexible array member only when it is declared as a flexible array member per C99 standard onwards.

The negative form -fno-strict-flex-arrays is equivalent to -fstrict-flex-arrays=0, which is the least strict. In this case all trailing arrays of structures are treated as flexible array members. Zero strictness, vibes only 🌸

So like, there are two more levels chillin' between 0 and 3 — they exist purely to support older code that uses the GCC zero-length array extension ([0]) or a one-element array as a flexible array member ([1]). When level is 1, the trailing array gets treated as a flexible array member when it's declared as either [], [0], or [1]. When level is 2, the trailing array counts as a flexible array member when it's declared as either [] or [0]. Slay, legacy code support 💅

You can control this behavior for a specific trailing array field of a structure by using the variable attribute "strict_flex_array" attribute. iykyk.

The -fstrict_flex_arrays option interacts with the -Wstrict-flex-arrays option. They're like, in conversation with each other 🫶

  • -fsso-struct= endianness — sets the default scalar storage order of structures and unions to the specified endianness. The accepted values are big-endian, little-endian and native for the native endianness of the target (which is the default, bestie). Heads up tho: this option is not supported for C++.
⚠️ Warning, fr fr: the -fsso-struct switch makes GCC generate code that is not binary compatible with code generated without it, if the specified endianness isn't the native endianness of the target. So like, don't casually mix and match unless you know exactly what you're doing 😭

🦋 Options Controlling C++ Dialect 🦋

Okay so this little section? It describes the command-line options that are only meaningful for C++ programs. But plot twist — you can also use most of the GNU compiler options regardless of what language your program is in. For example, you might compile a file firstClass.C like this:

g++ -g -fstrict-enums -O -c firstClass.C

In 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. Main character behavior honestly 👑

Some options for compiling C programs, such as -std, are also relevant for C++ programs.

Here's the list of options that are only for compiling C++ programs:

  • -fabi-version= n — use version n of the C++ ABI. The default is version 0.

Version 0 refers to the version conforming most closely to the C++ ABI specification. Therefore, the ABI obtained using version 0 will change in different versions of G++ as ABI bugs are fixed. (Iconic, unstable, a little chaotic 🎀)

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. (oop 🙈)

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 of "alignof", 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.

Version 20, which first appeared in G++ 15, fixes manglings of lambdas in static data member initializers.

See also -Wabi. 👀

  • -fabi-compat-version= n — on targets that support strong aliases, G++ works around mangling changes by creating an alias with the correct mangled name when defining a symbol with an incorrect mangled name. This switch specifies which ABI version to use for the alias.

With -fabi-version=0 (the default), this defaults to 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.

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.

  • -fno-access-control — turns off all access checking. This switch is mainly useful for working around bugs in the access control code. Like, hacking around a bug, no shame 💀
  • -faligned-new — enables support for C++17 "new" of types that require more alignment than "void* ::operator new(std::size_t)" provides. A numeric argument such as "-faligned-new=32" can be used to specify how much alignment (in bytes) is provided by that function, but few users will need to override the default of alignof(std::max_align_t).

This flag is enabled by default for -std=c++17. Obsessed with a sensible default 🌸

  • -fno-assume-sane-operators-new — okay so the C++ standard allows replacing the global "new", "new[]", "delete" and "delete[]" operators, though a lot of C++ programs don't replace them and just use the implementation provided version. Furthermore, the C++ standard allows omitting those calls if they are made from new or delete expressions (and by extension the same is assumed if "__builtin_operator_new" or "__builtin_operator_delete" functions are used). This option allows control over some optimizations around calls to those operators.

With "-fassume-sane-operators-new-delete" option GCC may assume that calls to the replaceable global operators from new or delete expressions or from "__builtin_operator_new" or "__builtin_operator_delete" calls don't read or modify any global variables or variables whose address could escape to the operators (global state; except for "errno" for the "new" and "new[]" operators). This allows most optimizations across those calls and is something that the implementation provided operators satisfy unless "malloc" implementation details are observable in the code or unless "malloc" hooks are used, but might not be satisfied if a program replaces those operators. This behavior is enabled by default.

With "-fno-assume-sane-operators-new-delete" option 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 code needs to expect such behavior. It's giving "trust issues but make it compiler flags" 😭

  • -fchar8_t, -fno-char8_t — enable support for "char8_t" as adopted for C++20. This includes the addition of a new "char8_t" fundamental type, changes to the types of UTF-8 string and character literals, new signatures for user-defined literals, associated standard library updates, and new "__cpp_char8_t" and "__cpp_lib_char8_t" feature test macros.

This option enables functions to 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 #2

and 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 introduces 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. Sad but true 🥲

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 — bestie, this makes the compiler double-check that the pointer operator new hands back is non-null before it goes and writes into that storage 💅 Normally you do NOT need this, because the C++ standard says operator new only returns 0 if it's declared throw(), and in that case the compiler is already checking the return value with or without this flag. In every other case — when operator new has a non-empty exception specification — running out of memory is signalled by throwing std::bad_alloc. See also new (nothrow) 👑
  • -fconcepts — turns on support for the C++ Concepts feature, the whole "constrain your template arguments" vibe ✨ With -std=c++20 and above, Concepts are literally part of the language standard, so -fconcepts defaults 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. That option was deprecated in GCC 14 and removed in GCC 15 — so if that's you, convert your code to C++20 concepts, no cap 🙈
  • -fconstexpr-depth=n — sets the maximum nested evaluation depth for C++11 constexpr functions to n. You need a limit, otherwise the compiler would just sit there detecting endless recursion during constant expression evaluation forever 😭 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, and it's trading off compilation speed (when the cache avoids repeated calculations) against memory consumption (when the cache grows absolutely massive from highly recursive evaluations). Default is 8 🎀 Very few users are likely to want to adjust it, but if your code does heavy constexpr calculations you might wanna experiment to find which value works best for you — go off, scientist 🧚
  • -fconstexpr-fp-except — okay so Annex F of the C standard says that IEC559 floating point exceptions encountered at compile time should not stop compilation. But 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. Again, 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. Because listen: even when the number of iterations of a single loop is limited with the above limit, if you have several nested loops and each of them has many iterations but still smaller than that limit, or if in a body of some loop — or even outside of a loop — too many expressions need to be evaluated, the resulting constexpr evaluation might take way too long 😮‍💨 The default is 33554432 (1<<25).
  • -fcontracts — enables experimental support for the C++ Contracts feature, as briefly added to and then removed from the C++20 working paper (N4820). The implementation also includes proposed enhancements from papers P1290, P1332, and P1429. This functionality is intended mostly for those interested in experimentation towards refining the feature to get it into shape for a future C++ standard 🔬 (so like, experimental experimental, you've been warned 🫶)

On violation of a checked contract, the violation handler is called. Users can replace the violation handler by defining:

void
handle_contract_violation (const std::experimental::contract_violation&);

There are different sets of additional flags that can be used together to specify which contracts will be checked and how — for N4820 contracts, P1332 contracts, or P1429 contracts — and these sets cannot be used together 🚫

  • -fcontract-mode=[on|off] — controls whether any contracts have any semantics at all. Defaults to on.
  • -fcontract-assumption-mode=[on|off] — [N4820] controls whether contracts with level axiom should have the assume semantic. Defaults to on.
  • -fcontract-build-level=[off|default|audit] — [N4820] specifies which level of contracts to generate checks for. Defaults to default.
  • -fcontract-continuation-mode=[on|off] — [N4820] controls whether to allow the program to continue executing after a contract violation. That is, do checked contracts have the maybe semantic described below rather than the never semantic. Defaults to off.
  • -fcontract-role=<name>:<default>,<audit>,<axiom> — [P1332] specifies the concrete semantics for each contract level of a particular contract role.
  • -fcontract-semantic=[default|audit|axiom]:<semantic> — [P1429] specifies the concrete semantic for a particular contract level.
  • -fcontract-strict-declarations=[on|off] — controls whether to reject adding contracts to a function after its first declaration. Defaults to off.

The possible concrete semantics that can be specified with -fcontract-role or -fcontract-semantic are:

  • "ignore" — this contract has no effect.
  • "assume" — this contract is treated like C++23 [[assume]].
  • "check_never_continue", "never", "abort" — this contract is checked. If it fails, the violation handler is called. If the handler returns, std::terminate is called 💀
  • "check_maybe_continue", "maybe" — this contract is checked. If it fails, the violation handler is called. If the handler returns, execution continues normally 🦋
  • -fcoroutines — enables support for the C++ coroutines extension (experimental) 🌈
  • -fdiagnostics-all-candidates — permits the C++ front end to note all candidates during overload resolution failure, including when a deleted function is selected 📋 (yes, all of them, brace yourself for the wall of text 😭)
  • -fno-elide-constructors — the C++ standard allows an implementation to omit creating a temporary that is only used to initialize another object of the same type. Specifying this option disables that optimization, and forces G++ to call the copy constructor in all cases. 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.
  • -fno-enforce-eh-specs — don't generate code to check for violation of exception specifications at run time. This option violates the C++ standard, but may be useful for reducing code size in production builds, much like defining NDEBUG 🏭 This does not give user code permission to throw exceptions in violation of the exception specifications — the compiler still optimizes based on the specifications, so throwing an unexpected exception results in undefined behavior at run time ⚠️ like, fr fr, don't do this casually.
  • -fextern-tls-init, -fno-extern-tls-init — the C++11 and OpenMP standards allow thread_local and threadprivate variables to have dynamic (runtime) initialization. To support this, any use of such a variable goes through a wrapper function that performs any necessary initialization 🧵 When the use and definition of the variable are in the same translation unit, this overhead can be optimized away — but when the use is in a different translation unit there is significant overhead even if the variable doesn't actually need dynamic initialization. If the programmer can be sure that no use of the variable in a non-defining TU needs to trigger dynamic initialization (either because the variable is statically initialized, or a use of the variable in the defining TU will be executed before any uses in another TU), they can avoid this overhead with the -fno-extern-tls-init option. On targets that support symbol aliases, the default is -fextern-tls-init. On targets that do not support symbol aliases, the default is -fno-extern-tls-init ✨
  • -ffold-simple-inlines, -fno-fold-simple-inlines — permits the C++ frontend to fold calls to std::move, std::forward, std::addressof and std::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-inline is active.
  • -fno-gnu-keywords — do not recognize typeof as a keyword, so that code can use this word as an identifier. You can use the keyword __typeof__ instead 👑 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 to consteval, 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 doesn't have to produce a constant (that's the mechanism allowing consteval function composition 🧠✨). However, with -fno-immediate-escalation, f is not promoted to consteval, and since the call to the 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 — makes inline functions implicitly constexpr, 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 and other optimizations — lowkey a glow-up for your binary 🌸
  • -fno-implicit-templates — okay so bestie, this one means: never emit code for non-inline templates that get instantiated implicitly (i.e. just because somebody used them). You only emit code for explicit instantiations. 💅 Which means you have to actually structure your code to include every explicit instantiation you need, or you'll get undefined symbols at link time and cry. No cap.
  • -fno-implicit-inline-templates — same energy but for inline templates: don't emit code for implicit instantiations of those either. The default handles inlines differently on purpose, so that compiling with and without optimization needs the same set of explicit instantiations. Consistency queen. ✨
  • -fno-implement-inlines — to save space, don't emit out-of-line copies of inline functions controlled by #pragma implementation. But 🚨 this causes linker errors if those functions aren't inlined everywhere they're called. So like, only if you're sure.
  • -fmodules, -fno-modules — turns on support for C++20 modules. The -fno-modules is usually not needed since that's the default. And even though this is a C++20 feature, it is not currently implicitly enabled by selecting that standard version — you gotta ask for it. 🙈
  • -fmodule-header, -fmodule-header=user, -fmodule-header=system — compile a header file to create an importable header unit. That's it, that's the vibe. 🎀
  • -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. But it may result in a difficulty during code porting 😭, so 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. Lazy era over. 💀
  • -fmodule-mapper=... — an oracle 🌈 to query for module name to filename mappings. The forms are:
-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]

If unspecified, the CXX_MODULE_MAPPER environment variable is used, and if that's unset, an in-process default is provided. Something for everyone fr fr.

  • -fmodule-only — only emit the Compiled Module Interface, inhibiting any object file. Just the CMI, nothing else, main character energy. 👑
  • -fms-extensions — disables Wpedantic warnings about constructs used in MFC, such as implicit int and getting a pointer to member function via non-standard syntax. Letting your Windows-isms breathe. 🦋
  • -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=10 or 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. Say that five times fast 😭. 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 include ffs, alloca, _exit, index, bzero, conjf, and other related functions. The whole crew, evicted. 💅
  • -fnothrow-opt — treat a throw() exception specification as if it were a noexcept specification, to reduce or eliminate the text size overhead relative to a function with no exception specification. If the function has local variables of types with non-trivial destructors, the exception specification actually makes the function smaller because the EH cleanups for those variables can be optimized away. The semantic effect is that an exception thrown out of a function with such an exception specification results in a call to terminate rather than unexpected. Different function, still dramatic. ⭐
  • -fno-operator-names — do not treat the operator name keywords and, bitand, bitor, compl, not, or and xor as synonyms as keywords. (Yes, that's the wording. Ngl it's giving slightly cursed sentence, but it's the original and we respect it.) 🫶
  • -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. Optional drama: declined. 🙅
  • -fno-pretty-templates — okay so normally, when an error message refers to a specialization of a function template, the compiler prints the signature of the template followed by the template arguments and any typedefs or typenames in the signature (e.g. void f(T) [with T = int] rather than void f(int)) so that it's clear which template is involved. And when an error message refers to a specialization of a class template, the compiler omits any template arguments that match the default template arguments for that template. If either of these behaviors make it harder to understand the error message rather than easier, you can use -fno-pretty-templates to disable them. Un-prettify the templates. 💀
  • -frange-for-ext-temps — enable lifetime extension of C++ range based for temporaries. With -std=c++23 and above this is part of the language standard, so the lifetime of the temporaries is extended until the end of the loop by default. This option allows enabling that behavior also in earlier versions of the standard. Time travel, but legal. 🧚
  • -fno-rtti — disable generation of information about every class with virtual functions for use by the C++ run-time type identification features (dynamic_cast and typeid). If you don't use those parts of the language, you can save some space by using this flag. Note that exception handling uses the same information, but G++ generates it as needed. The dynamic_cast operator can still be used for casts that do not require run-time type information, i.e. casts to void * or to unambiguous base classes.
Heads up bestie: mixing code compiled with -frtti with that compiled with -fno-rtti may not work. For example, programs may fail to link if a class compiled with -fno-rtti is used as a base for a class compiled with -frtti. 💔
  • -fsized-deallocation — enable the built-in global declarations
void operator delete (void *, std::size_t) noexcept;
void operator delete[] (void *, std::size_t) noexcept;

as introduced in C++14. This is useful for user-defined replacement deallocation functions that, for example, use the size of the object to make deallocation faster. Enabled by default under -std=c++14 and above. The flag -Wsized-deallocation warns about places that might want to add a definition. 🔥

  • -fstrict-enums — allow the compiler to optimize using the assumption that a value of enumerated type can only be one of the values of the enumeration (as defined in the C++ standard; basically, a value that can be represented in the minimum number of bits needed to represent all the enumerators). This assumption may not be valid if the program uses a cast to convert an arbitrary integer value to the enumerated type. This option has no effect for an enumeration type with a fixed underlying type. Trust the enum. Or don't. 🙈
  • -fstrong-eval-order, -fstrong-eval-order=kind — evaluate member access, array subscripting, and shift expressions in left-to-right order, and evaluate assignment in right-to-left order, as adopted for C++17. -fstrong-eval-order is equivalent to -fstrong-eval-order=all, and is enabled by default with -std=c++17 or later.
    • -fstrong-eval-order=some enables just the ordering of member access and shift expressions, and is the default for C++ dialects prior to C++17.
    • -fstrong-eval-order=none is equivalent to -fno-strong-eval-order.
  • -ftemplate-backtrace-limit=n — set the maximum number of template instantiation notes for a single warning or error to n. The default value is 10. Because 10 notes is already a lot of notes, bestie. 😭
  • -ftemplate-depth=n — set the maximum instantiation depth for template classes to n. A limit on the template instantiation depth is needed to detect endless recursions during template class instantiation. ANSI/ISO C++ conforming programs must not rely on a maximum depth greater than 17 (changed to 1024 in C++11). The default value is 900, as the compiler can run out of stack space before hitting 1024 in some situations. So close to a nice round number but not quite. 💀
  • -fno-threadsafe-statics — do not emit the extra code to use the routines specified in the C++ ABI for thread-safe initialization of local statics. You can use this option to reduce code size slightly in code that doesn't need to be thread-safe. Shrink the binary, accept the chaos. 🎀
  • -fuse-cxa-atexit — register destructors for objects with static storage duration with the __cxa_atexit function rather than the atexit function. This option is required for fully standards-compliant handling of static destructors, but only works if your C library supports __cxa_atexit. Check first, fr fr. ✨
  • -fno-use-cxa-get-exception-ptr — don't use the __cxa_get_exception_ptr runtime routine. This causes std::uncaught_exception to be incorrect, but is necessary if the runtime routine is not available. Trade-off city. 🦋
  • -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. Big claim, very serious, love that for you.

The effect of this is that GCC may, effectively, mark inline methods with __attribute__ ((visibility ("hidden"))) so that they do not appear in the export table of a DSO and do not require a PLT indirection when used within the DSO. Enabling this option can have a dramatic effect on load and link times of a DSO as it massively reduces the size of the dynamic export table when the library makes heavy use of templates. 🔥 Obsessed actually.

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. Respect the boundary. 🌸

  • -fvisibility-ms-compat — okay so this flag tries to use visibility settings to make GCC's C++ linkage model compatible with that of Microsoft Visual Studio. It's giving "let's make MSVC happy" energy.

The flag makes these changes to GCC's linkage model:

  1. It sets the default visibility to "hidden", like -fvisibility=hidden.
  2. Types, but not their members, are not hidden by default.
  3. The One Definition Rule is relaxed for types without explicit visibility specifications that are defined in more than one shared object: those declarations are permitted if they are permitted when this option is not used.

In new code it is better to use -fvisibility=hidden and export those classes that are intended to be externally visible. Unfortunately it is possible for code to rely, perhaps accidentally, on the Visual Studio behavior.

Among the consequences of these changes are that static data members of the same type with the same name but defined in different shared objects are different, so changing one does not change the other; and that pointers to function members defined in different shared objects may not compare equal. When this flag is given, it is a violation of the ODR to define types with the same name differently.

  • -fno-weak — bestie, do not use weak symbol support here. This option says do not use weak symbol support, even if it is provided by the linker. By default, G++ uses weak symbols if they are available. This option exists only for testing, and should not be used by end-users; it results in inferior code and has no benefits. This option may be removed in a future release of G++.
  • -fext-numeric-literals (C++ and Objective-C++ only) — accept the silly little number suffixes as GNU extensions: imaginary, fixed-point, or machine-defined literal number suffixes. 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++ — do not search for header files in the standard directories specific to C++, but do still search the other standard directories. (This option is used when building the C++ library.)
  • -flang-info-include-translate, -flang-info-include-translate-not, -flang-info-include-translate=header — these three are the include-translation gossip squad: they 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 form "user" or "<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 — hello, Compiled Module Interface pathname updates: these inform of Compiled Module Interface pathnames. The first will note all read CMI pathnames. The module form will not 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++) and libc++ 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.
  • -Wcomma-subscript (C++ and Objective-C++ only) — warn about the comma-expression-in-a-subscript situation. Warn about uses of a comma expression within a subscripting expression. This usage was deprecated in C++20 and is going to be removed in C++23. However, 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 that can be disabled 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, enabled with explicit -Wdeprecated.

this warning is upgraded to an error by -pedantic-errors in C++23 mode or later.
  • -Wctad-maybe-unsupported (C++ and Objective-C++ only) — warn when performing class template argument deduction (CTAD) on a type with no explicitly written deduction guides. This warning will point out cases where CTAD succeeded only because the compiler synthesized the implicit deduction guides, which might not be what the programmer intended. Certain style guides allow CTAD only on types that specifically "opt-in"; i.e., on types that are designed to support CTAD. This warning can be suppressed with the following 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) — warn when a class seems unusable because all the constructors or destructors in that class are private, and it has neither friends nor public static member functions. Also warn if there are no non-private methods, and there's at least one private member function that isn't a constructor or destructor.
  • -Wdangling-reference (C++ and Objective-C++ only) — warn 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 dangling

In the example above, two temporaries are created, one for each argument, and a reference to one of the temporaries is returned. However, both temporaries are destroyed at the end of the full expression, so the reference "r" is dangling. This warning also detects 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 are checked as well, but only their object argument:

struct S {
   const S& self () { return *this; }
};
const S& s = S().self(); // s is dangling

Certain functions are safe in this respect, for example "std::use_facet": they take and return a reference, but they don't return one of its arguments, which can fool the warning. Such functions can be excluded from the warning by wrapping them in a "#pragma":

#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wdangling-reference"
const T& foo (const T&) { ... }
#pragma GCC diagnostic pop

The "#pragma" can also surround the class; in that case, the warning will be 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; that is, 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) — warn when "delete" is used to destroy an instance of a class that has virtual functions and non-virtual destructor. It is unsafe to delete an instance of a derived class through a pointer to a base class if the base class does not have a virtual destructor. This warning is enabled by -Wall.
  • -Wdeprecated-copy (C++ and Objective-C++ only) — warn 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. With -Wdeprecated-copy-dtor, also deprecate if the class has a user-provided destructor.
  • -Wno-deprecated-enum-enum-conversion (C++ and Objective-C++ only) — disable the warning about the case 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) — disable the warning about the case 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) — ok so this one's calling out that whole situation where you declare a user-defined literal operator with a space before the suffix, which is lowkey deprecated now. It's on by default in C++23, or if you explicitly pass -Wdeprecated. Look at the glow-up:
string operator "" _i18n(const char*, std::size_t); // deprecated
string operator ""_i18n(const char*, std::size_t); // preferred

See the space? Yeah. Gone. No space, no problem. 💅

  • -Wdeprecated-variadic-comma-omission (C++ and Objective-C++ only) — bestie, the comma before the varargs ... at the end of a function parameter list? Skipping it is deprecated, and this flag will absolutely let you know. On by default in C++26, or with explicit -Wdeprecated. Receipts:
void f1(int...); // deprecated
void f1(int, ...); // preferred
template <typename ...T>
void f2(T...); // ok
template <typename ...T>
void f3(T......); // deprecated

Note that T... in a template is still iconic and fine, but T...... is giving chaos. 😭

  • -Wno-elaborated-enum-base — for C++11 and above, this warns if an (invalid) additional enum-base shows up in an elaborated-type-specifier. Translating from compiler-speak: it fires if an enum with a given underlying type and no enumerator list gets used in a declaration that isn't just a standalone declaration of the enum. Enabled by default, and heads up — this warning gets upgraded to a full-on error with -pedantic-errors.
  • -Wno-init-list-lifetime (C++ and Objective-C++ only) — this one's the "do not warn" version, so passing it tells GCC to chill about uses of std::initializer_list that are likely to result in dangling pointers. Which, honestly, is a lot of them, because the underlying array for an initializer_list is handled like a normal C++ temporary object — so it's so easy to inadvertently keep a pointer to the array past the end of the array's lifetime. The classic ways you get got:
    • If a function returns a temporary initializer_list, or a local initializer_list variable, the array's lifetime ends at the end of the return statement, so the value you 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 the initializer_list sitting in the heap has a dangling pointer.
    • When an initializer_list variable 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 by the next statement, the initializer_list variable has a dangling pointer. Watch:
// 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
  • And when a list constructor stores the begin pointer from the initializer_list argument, that does not extend the lifetime of the array — so if a class variable is constructed from a temporary initializer_list, the pointer is left dangling by the end of the variable declaration statement. Yeah. Undefined behaviour. Not cute. 💀
  • -Winvalid-constexpr — warns when a function never produces a constant expression. Ok so the lore: in C++20 and earlier, for every constexpr function and function template, there had to 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 that restriction, which means it's now possible to have a function or function template marked constexpr for which no invocation satisfies the requirements of a core constant expression. Slay for flexibility, slightly unhinged for diagnostics.

This warning is enabled as a pedantic warning by default in C++20 and earlier. In C++23, -Winvalid-constexpr can be turned on, in which case it will be an ordinary warning. Example time:

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 — verifies that all imported macro definitions are valid at the end of compilation. Not enabled by default, because it requires additional processing to figure out. Genuinely useful when you're preparing sets of header-units and you want to make sure the macros stay consistent. ✨
  • -Wno-literal-suffix (C++ and Objective-C++ only) — the do-not-warn version: silences the warning you'd get when a string or character literal is followed by a ud-suffix that does not begin with an underscore. As a conforming extension, GCC treats such suffixes as separate preprocessing tokens to maintain backwards compatibility with code that uses formatting macros from <inttypes.h>. Exhibit A:
#define __STDC_FORMAT_MACROS
#include <inttypes.h>
#include <stdio.h>

int main() {
  int64_t i64 = 123;
  printf("My int64: %" PRId64"\n", i64);
}

In this case, PRId64 is treated as a separate preprocessing token. Iconic legacy behaviour, honestly.

This option also controls the warnings for when a user-defined literal operator is declared with a literal suffix identifier that doesn't begin with an underscore — those identifiers are reserved for future standardization, so GCC side-eyes them. Both sets of warnings are enabled by default. 👑

  • -Wno-narrowing (C++ and Objective-C++ only) — for C++11 and later standards, narrowing conversions are diagnosed by default, as the standard requires. A narrowing conversion from a constant produces an error, and a narrowing conversion from a non-constant produces a warning — but -Wno-narrowing suppresses the diagnostic entirely. Important caveat though: this does not change the meaning of well-formed code, and narrowing conversions are still considered ill-formed in SFINAE contexts. So don't get it twisted.

With -Wnarrowing in C++98, you get a warning when a narrowing conversion prohibited by C++11 occurs within { }, like:

int i = { 2.2 }; // error: narrowing from double to int

This flag is included in -Wall and -Wc++11-compat.

  • -Wnoexcept (C++ and Objective-C++ only) — warns when a noexcept-expression evaluates to false because 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's chill, it just can't prove it on paper. Rude. 🙈
  • -Wnoexcept-type (C++ and Objective-C++ only) — warns if the C++17 feature making noexcept part of a function type changes the mangled name of a symbol relative to C++14. Enabled by -Wabi and -Wc++17-compat. For 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>. Same vibe, different mangled name, and that's the whole problem. 🌀

  • -Wclass-memaccess (C++ and Objective-C++ only) — warns when the destination of a call to a raw memory function such as 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 such objects may violate invariants maintained by member functions of the class — which is a very polite way of saying "you're about to break everything."

For example, the call to memset below is undefined because it modifies a non-trivial class object — and is, therefore, diagnosed. The safe way to either initialize or clear the storage of objects of such types 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 itself or in an accessible polymorphic base class. In that case it is possible — but unsafe — to delete an instance of a derived class through a pointer to the class itself or base class. This warning is automatically enabled if -Weffc++ is specified. That said, the -Wdelete-non-virtual-dtor option (enabled by -Wall) should be preferred, because it warns about the unsafe cases without false positives. Bestie, take the upgrade. 🎀
  • -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. The use of the register keyword as a storage class specifier has been deprecated in C++11 and removed in C++17. Enabled by default with -std=c++17. RIP to a real one. ⚰️
  • -Wreorder (C++ and Objective-C++ only) — warns when the order of member initializers given in the code doesn't match the order in which they must be executed. For instance:
struct A {
  int i;
  int j;
  A(): j (0), i (1) { }
};

The compiler rearranges the member initializers for i and j to match the declaration order of the members, and emits a warning to that effect. So yes, the order you wrote is not the order that happens. This warning is enabled by -Wall.

  • -Wno-pessimizing-move (C++ and Objective-C++ only) — this warning fires when a call to std::move prevents copy elision. Which is so tragic, because you called std::move to be fast. 😭

A typical scenario when copy elision can occur is when returning in a function with a class return type, when the expression being returned is the name of a non-volatile automatic object, and is not a function parameter, and has the same type as the function return type:

struct T {
...
};
T fn()
{
  T t;
  ...
  return std::move (t);
}

But in this example, the std::move call prevents copy elision. Not the flex you thought it was.

This warning is enabled by -Wall.

  • -Wno-redundant-move (C++ and Objective-C++ only) — warns about redundant calls to std::move; that is, when a move operation would have been performed 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 such as returning a local variable, where copy elision isn't applicable. Consider:

okay so this next snippet is the poster child for a redundant std::move, bestie:

struct T {
...
};
T fn(T t)
{
  ...
  return std::move (t);
}

Here, the std::move call is redundant. Because G++ implements Core Issue 1579, another example is:

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) — this one is like “bestie, why are we copying?” for C++ range-based for loops. It warns when the loop is creating an unnecessary copy, which can happen when the range declaration is not 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 warns 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, to which the reference const double & is bound.

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 eliminated

On the other hand, in this example there is no warning:

struct foo;
void foo ();   // "hides" struct foo
void bar (struct foo&);  // no warning, keyword struct is necessary
  • -Wno-subobject-linkage (C++ and Objective-C++ only) — do not warn if a class type has a base or a field whose type uses the anonymous namespace or depends on a type with no linkage. 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 as well. The compiler doesn't give this warning for types defined in the main .C file, as those are unlikely to have multiple definitions. -Wsubobject-linkage is enabled by default.
  • -Weffc++ (C++ and Objective-C++ only) — warns about violations of the following style guidelines from Scott Meyers' Effective C++ series of books:
    • Define a copy constructor and an assignment operator for classes with dynamically-allocated memory.
    • Prefer initialization to assignment in constructors.
    • Have operator= return a reference to *this.
    • Don't try to return a reference when you must return an object.
    • Distinguish between prefix and postfix forms of increment and decrement operators.
    • Never overload &&, ||, or ,.

This option also enables -Wnon-virtual-dtor, which is also one of the effective C++ recommendations. However, the check is extended to warn about the lack of virtual destructor in accessible non-polymorphic base classes too.

When selecting this option, be aware that the standard library headers do not obey all of these guidelines; use grep -v to filter out those warnings.

  • -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.
  • -Wstrict-null-sentinel (C++ and Objective-C++ only) — warns about the use of an uncasted NULL as sentinel. When compiling only with GCC this is a valid sentinel, as NULL is defined to __null. Although it is a null pointer constant rather than a null pointer, it is guaranteed to be of the same size as a pointer. But this use is not portable across different compilers.
  • -Wno-non-template-friend (C++ and Objective-C++ only) — disables 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 as friend 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.
  • -Wold-style-cast (C++ and Objective-C++ only) — warns if an old-style (C-style) cast to a non-void type is used within a C++ program. The new-style casts (dynamic_cast, static_cast, reinterpret_cast, and const_cast) are less vulnerable to unintended effects and much easier to search for.
  • -Woverloaded-virtual, -Woverloaded-virtual=n (C++ and Objective-C++ only) — warns 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.

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.

  • -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) — warns 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.
  • -Wtemplates (C++ and Objective-C++ only) — warns 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.
  • -Wmismatched-new-delete (C++ and Objective-C++ only) — warns 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.

  • -Wmismatched-tags (C++ and Objective-C++ only) — warns 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.

  • -Wmultiple-inheritance (C++ and Objective-C++ only) — pops off a warning whenever a class is defined with multiple direct base classes 💅. Some coding rules straight-up ban multiple inheritance and this is how you enforce that rule, periodt. Lowkey important: the warning is inactive inside a system header file, such as the STL, so you can still use the STL. And you may also define classes that indirectly use multiple inheritance — that's fine, no callout.
  • -Wvirtual-inheritance — warns when a class is defined with a virtual direct base class 🦋. Same energy as above: some coding rules disallow multiple inheritance and this enforces it. The warning is inactive inside a system header file, such as the STL, so the STL stays usable. One may also define classes that indirectly use virtual inheritance. Slay.
  • -Wno-virtual-move-assign — shuts off warnings about inheriting from a virtual base with a non-trivial C++11 move assignment operator 🙈. Ngl this one is dangerous, and the manpage says so: 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. So like… only turn it off if you actually know what you're doing, bestie.
  • -Wnamespaces — warns when a namespace definition is opened ✨. Some coding rules disallow namespaces and this is the enforcement mechanism. 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. iykyk.
  • -Wno-template-body (C++ and Objective-C++ only) — disables diagnosing errors when parsing a template, and instead issues 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.
  • -Wno-template-id-cdtor (C++ and Objective-C++ only) — disables 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.

  • -Wtemplate-names-tu-local — warns when a template body hides an exposure of a translation-unit-local entity 🧚. In most cases, referring to a translation-unit-local entity (such as an internal linkage declaration) within an entity that is emitted into a module's CMI is an error. However, within the initializer of a variable, or in the body of a non-inline function, this is not an exposure and no error is emitted.

This can cause variable or function templates to 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 attempts to detect cases where this might occur. The presence of an explicit instantiation silences the warning.

This flag is enabled by -Wextra.

  • -Wno-terminate (C++ and Objective-C++ only) — disables the warning about a throw-expression that will immediately result in a call to terminate 💀. Like, you threw it and it's just gonna die? Okay.
  • -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 🫶.

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 🙈. Dead code energy, honestly.
  • -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.

Enabled by default with -std=c++20. Before -std=c++20, enabled with explicit -Wdeprecated.

  • -Waligned-new — warns about a new-expression of a type that requires greater alignment than the alignof(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-value for short) warn about polymorphic class types that are caught by value. With -Wcatch-value=2 warn about all class types that are caught by value. With -Wcatch-value=3 warn about all types that are not caught by reference. -Wcatch-value is 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.
  • -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 -Wextra option.
        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. No cap, that's literally who it's for.

The restrictions on offsetof may be relaxed in a future version of the C++ standard.

  • -Wsized-deallocation (C++ and Objective-C++ only) — so this one's the sized-vs-unsized operator delete watchdog 🐕. It warns when you define an unsized deallocation function
void operator delete (void *) noexcept;
void operator delete[] (void *) noexcept;

without also defining the matching sized deallocation function

void operator delete (void *, std::size_t) noexcept;
void operator delete[] (void *, std::size_t) noexcept;

or vice versa, bestie — either direction trips it. It's enabled by -Wextra along with -fsized-deallocation. 💅

  • -Wsuggest-final-types — warns about types that have virtual methods where your code quality would literally improve if the type were declared with the C++11 final specifier, or — if possible — declared in an anonymous namespace. Why do we care? Because that lets GCC devirtualize the polymorphic calls way more aggressively. 🔥 This warning hits harder with link-time optimization, where the info about the class hierarchy graph is more complete.
  • -Wsuggest-final-methods — same vibe but for virtual methods: it warns when code quality would improve if the method were declared with the C++11 final specifier, or, if possible, if its type were declared in an anonymous namespace or with the final specifier. Also more effective with link-time optimization, where the class hierarchy graph info is more complete. Pro tip from the page itself: consider the suggestions from -Wsuggest-final-types first, then rebuild with your new annotations. 🫶
  • -Wsuggest-override — warns about overriding virtual functions that aren't marked with the override keyword. like, just say override, it's free ✨
  • -Wno-conversion-null (C++ and Objective-C++ only) — do not warn for conversions between NULL and non-pointer types. Heads up: -Wconversion-null is enabled by default, so this is you turning that off. 🙈

🦋 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. That said, you can still use most of the language-independent GNU compiler options too. Case in point — you might compile a file some_class.m like this:

gcc -g -fgnu-runtime -O -c some_class.m

In that example, -fgnu-runtime is an option meant only for Objective-C and Objective-C++ programs; the other options there you can use with any language GCC supports. 💖

Also keep in mind: since Objective-C is an extension of the C language, Objective-C compilations can also use options specific to the C front-end (e.g., -Wtraditional). Similarly, Objective-C++ compilations can 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 — use class-name as the name of the class to instantiate for each literal string you specify with the @"..." syntax. The default class name is NXConstantString if the GNU runtime is being used, and NSConstantString if the NeXT runtime is being used (see below 👀). On Darwin / macOS platforms, the -fconstant-cfstrings option — if it's also present — overrides the -fconstant-string-class setting and causes @"..." literals to be laid out as constant CoreFoundation strings. Note that -fconstant-cfstrings is an alias for the target-specific -mconstant-cfstrings equivalent.
  • -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 not nil. That lets the runtime use more efficient entry points. 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. Currently only supported 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 you don't specify anything, 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 whether any of its instance variables is a C++ object with a non-trivial default constructor. If so, synthesize a special - (id) .cxx_construct instance method that runs the non-trivial default constructors on any such instance variables, in order, and then returns self. Same deal on the other side: check whether any instance variable is a C++ object with a non-trivial destructor, and if so, synthesize a special - (void) .cxx_destruct method that runs all such default destructors, in reverse order.

The - (id) .cxx_construct and - (void) .cxx_destruct methods generated this way only operate on instance variables declared in the current Objective-C class — not the ones inherited from superclasses. It's the Objective-C runtime's job to invoke all such methods across an object's inheritance hierarchy. The - (id) .cxx_construct methods get invoked by the runtime immediately after a new object instance is allocated; the - (void) .cxx_destruct methods get 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 supports 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 C++ offers. This option is required to use the Objective-C keywords @try, @throw, @catch, @finally and @synchronized. 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. Only available with the NeXT runtime; the GNU runtime has a different garbage collection implementation that doesn't 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 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 GCC 4.0 doesn't recognize gets rejected. Useful if you need to make sure your Objective-C code can be compiled with older versions of GCC. 👑
  • -freplace-objc-classes — emit a special marker instructing ld(1) not to statically link in the resulting object file, and let dyld(1) load it in at run time instead. This pairs with the Fix-and-Continue debugging mode, where the object file in question may be recompiled and dynamically reloaded while the program is executing, no restart needed. Currently, Fix-and-Continue functionality is only available in conjunction with the NeXT runtime on Mac OS X 10.3 and later.
  • -fzero-link — when compiling for the NeXT runtime, the compiler ordinarily replaces calls to objc_getClass("...") (when the class name is known at compile time) with static class references that get initialized at load time, which improves run-time performance. Specifying -fzero-link suppresses that behavior and causes calls to objc_getClass("...") to be retained. Handy in Zero-Link debugging mode, since it lets individual class implementations be modified during program execution. The GNU runtime currently always retains calls to objc_get_class("...") regardless of command-line options. 💀
  • -fno-local-ivars — by default, instance variables in Objective-C can be accessed as if they were local variables from within the methods of the class they're declared in. That can lead to shadowing between instance variables and other variables declared either locally inside a class method or globally with the same name 😬. Specifying the -fno-local-ivars flag disables this behavior, thus avoiding those 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 that visibility. 🌸
  • -gen-decls — dump interface declarations for all classes seen in the source file to a file named sourcename.decl. 📝
  • -Wassign-intercept (Objective-C and Objective-C++ only) — warn whenever an Objective-C assignment is being intercepted by the garbage collector.
  • -Wno-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 <span>(Objective-C and Objective-C++ only)</span> — okay so if a class says it implements a protocol, GCC is gonna warn you about every method in that protocol the class doesn't implement. The default vibe is to warn on every method that's not explicitly implemented in the class, even if it's inherited from the superclass. But if you turn on -Wno-protocol, then inherited-from-superclass methods count as implemented and you get no warning for them. Chill.
  • -Wobjc-root-class <span>(Objective-C and Objective-C++ only)</span> — warns you if a class interface has no superclass 💀. Like most classes are gonna inherit from something, NSObject or Object for example. So if you're intentionally declaring a root class, you can hush the warning by marking the interface with __attribute__((objc_root_class)). Slay.
  • -Wselector <span>(Objective-C and Objective-C++ only)</span> — warns if it finds multiple methods of different types for the same selector while compiling. The check runs on the method list at the final stage of compilation. Also, it checks each selector that shows up in a @selector(...) expression, and a corresponding method for that selector has been found during compilation.
🫶 Important detail bestie: because these checks scan the method table only at the end of compilation, the warnings won't be produced if the final stage never happens — like if an error pops up during compilation, or if you're using -fsyntax-only. No final stage, no warnings, periodt.
  • -Wstrict-selector-match <span>(Objective-C and Objective-C++ only)</span> — warns if multiple methods with differing argument and/or return types exist for a given selector, when you're trying to send a message using that selector to a receiver of type id or Class. When this flag is off (which is the default behavior, jsyk), the compiler skips those warnings if any differences it found are confined to types that share the same size and alignment. Iconic.
  • -Wundeclared-selector <span>(Objective-C and Objective-C++ only)</span> — warns if it finds a @selector(...) expression pointing at an undeclared selector. A selector counts as undeclared if no method with that name has been declared before the @selector(...) expression — whether explicitly in an @interface or @protocol declaration, or implicitly in an @implementation section. This option always runs its checks the moment it finds a @selector(...) expression, whereas -Wselector only checks in the final stage of compilation. It also enforces the coding-style convention that methods and selectors must be declared before being used 💅.
  • -print-objc-runtime-info — generates a C header describing the largest structure that's passed by value, if there is one. That's it, that's the whole thing ✨.

🌈 Options Controlling OpenMP and OpenACC 🌈

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. Look at us, being all multilingual and fancy 🦋.

  • -foffload=disable
  • -foffload=default
  • -foffload=target-list — specifies which OpenMP and OpenACC offload targets code should be generated for. 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 acc in C/C++ and !$acc in free-form Fortran and !$acc, c$acc and *$acc in fixed-form Fortran. This option implies -pthread, and thus is only supported on targets that have support for -pthread. Lowkey a package deal 🎀.
  • -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. Cute and flexible, we love that 💖.
  • -fopenmp — enables handling of OpenMP directives #pragma omp, [[omp::directive(...)]], [[omp::decl(...)]], and [[omp::sequence(...)]] in C/C++. In Fortran, it enables !$omp and the conditional compilation sentinel !$. In fixed source form Fortran, the sentinels can also start with c or *.

This option implies -pthread, and thus is only supported on targets that have support for -pthread. And -fopenmp implies -fopenmp-simd. Two-for-one situation, we stan 💅.

  • -fopenmp-simd — enables handling of OpenMP's simd, declare simd, declare reduction, assume, ordered, scan and loop directive, and of combined or composite directives with simd as constituent with #pragma omp, [[omp::directive(...)]], [[omp::sequence(...)]] and [[omp::decl(...)]] in C/C++ and !$omp in Fortran. It additionally enables the conditional compilation sentinel !$ in Fortran. In fixed source form Fortran, the sentinels can also start with c or *. Other OpenMP directives are ignored.
😭 Unless -fopenmp is additionally specified, the loop region binds to the current task region, independent of the specified bind clause. Don't say I didn't warn you.
  • -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. And for the record: -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's disabled by default. The way that detail matters, 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. But ngl, some language front ends may not honor these options 🙈.

  • -fmessage-length=n — tries to format error messages so 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. Sorry bestie, different rules.
  • -fdiagnostics-plain-output — 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-output is 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

In 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. Maintaining the vibe check forever 💜.

  • -fdiagnostics-show-location=once — only meaningful in line-wrapping mode. Instructs the diagnostic messages reporter to emit source location information once; that is, if 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. Iconic, honestly 🎀.
  • -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 that's too long to fit on a single line. Every single line, bestie. Main character energy for the prefix 🌈.
  • -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, and 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 ✨.

OK so 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. (Peep the section in the documentation of your text terminal for permitted values and their meanings as character attributes.) These substring values are integers in decimal representation and can be concatenated with semicolons. Common values to concatenate include 1 for bold, 4 for underline, 5 for blink, 7 for inverse, 39 for default foreground color, 30 to 37 for foreground colors, 90 to 97 for 16-color mode foreground colors, 38;5;0 to 38;5;255 for 88-color and 256-color modes foreground colors, 49 for default background color, 40 to 47 for background colors, 100 to 107 for 16-color mode background colors, and 48;5;0 to 48;5;255 for 88-color and 256-color modes background colors.

The default GCC_COLORS is

error=01;31:warning=01;35:note=01;36: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;34

where 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. Setting GCC_COLORS to the empty string disables colors. Supported capabilities are as follows.

  • error= — SGR substring for error: markers.
  • warning= — SGR substring for warning: markers.
  • note= — SGR substring for note: markers.
  • path= — SGR substring for colorizing paths of control-flow events as printed via -fdiagnostics-path-format=, such as the identifiers of individual events and lines indicating interprocedural calls and returns.
  • range1= — SGR substring for first additional range.
  • range2= — SGR substring for second additional range.
  • locus= — SGR substring for location information, file:line or file:line:column etc.
  • quote= — SGR substring for information printed within quotes.
  • fnname= — SGR substring for names of C++ functions.
  • targs= — SGR substring for C++ function template parameter bindings.
  • fixit-insert= — SGR substring for fix-it hints suggesting text to be inserted or replaced.
  • fixit-delete= — SGR substring for fix-it hints suggesting text to be deleted.
  • diff-filename= — SGR substring for filename headers within generated patches.
  • diff-hunk= — SGR substring for the starts of hunks within generated patches.
  • diff-delete= — SGR substring for deleted lines within generated patches.
  • diff-insert= — SGR substring for inserted lines within generated patches.
  • type-diff= — SGR substring for highlighting mismatching types within template arguments in the C++ frontend.
  • valid= — SGR substring for highlighting valid elements within text art diagrams.
  • invalid= — SGR substring for highlighting invalid elements within text art diagrams.
  • highlight-a=, highlight-b= — SGR substrings for contrasting two different things within diagnostics, such as a pair of mismatching types. See -fdiagnostics-show-highlight-colors.
  • -fdiagnostics-urls[=WHEN] — Use escape sequences to embed URLs in diagnostics. For example, when -fdiagnostics-show-option emits text showing the command-line option controlling a diagnostic, embed a URL for documentation of that option.

WHEN is never, always, or auto. auto makes GCC use URL escape sequences only when the standard error is a terminal, and when not executing in an emacs shell or any graphical terminal which is known to be incompatible with this feature, see below.

The default depends on how the compiler has been configured. It can be any of the above WHEN options.

GCC can also be configured (via the --with-diagnostics-urls=auto-if-env configure-time option) so that the default is affected by environment variables. Under such a configuration, GCC defaults to using auto if either GCC_URLS or TERM_URLS environment variables are present and non-empty in the environment of the compiler, or never if neither are.

However, even with -fdiagnostics-urls=always the behavior is dependent on those environment variables: If GCC_URLS is set to empty or no, do not embed URLs in diagnostics. If set to st, URLs use ST escape sequences. If set to bel, the default, URLs use BEL escape sequences. Any other non-empty value enables the feature. If GCC_URLS is not set, use TERM_URLS as a fallback. Note: ST is an ANSI escape sequence, string terminator ESC \, BEL is an ASCII character, CTRL-G that usually sounds like a beep.

At this time GCC tries to detect also 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 on the screen. That list is currently xfce4-terminal, certain known to be buggy gnome-terminal versions, the linux console, and mingw. This check can be skipped with -fdiagnostics-urls=always.

  • -fno-diagnostics-show-option — By default, each diagnostic emitted includes text indicating the command-line option that directly controls the diagnostic (if such an option is known to the diagnostic machinery). Specifying -fno-diagnostics-show-option suppresses that behavior.
  • -fno-diagnostics-show-caret — By default, each diagnostic emitted includes the original source line and a caret ^ indicating the column. This option suppresses this information. The source line is truncated to n characters, if the -fmessage-length=n option is given. When the output is done to the terminal, the width is limited to the width given by the COLUMNS environment variable or, if not set, to the terminal width.
  • -fno-diagnostics-show-labels — By default, when printing source code (via -fdiagnostics-show-caret), diagnostics can label ranges of source code with pertinent information, such as the types of expressions:
            printf ("foo %s bar", long_i + long_j);
                         ~^       ~~~~~~~~~~~~~~~
                          |              |
                          char *         long int

This option suppresses the printing of these labels (in the example above, the vertical bars and the "char *" and "long int" text).

  • -fno-diagnostics-show-event-links — By default, when printing execution paths (via -fdiagnostics-path-format=inline-events), GCC will print lines connecting related events, such as 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 such connector lines.

  • -fno-diagnostics-show-cwe — Diagnostic messages can optionally have an associated CWE identifier (https://cwe.mitre.org/index.html). GCC itself only provides such metadata for some of the -fanalyzer diagnostics. GCC plugins may also provide diagnostics with such metadata. By default, if this information is present, it will be printed with the diagnostic. This option suppresses the printing of this metadata.
  • -fno-diagnostics-show-rules — Diagnostic messages can optionally have rules associated with them, such as from a coding standard, or a specification. GCC itself does not do this for any of its diagnostics, but plugins may do so. By default, if this information is present, it will be printed with the diagnostic. This option suppresses the printing of this metadata.
  • -fno-diagnostics-show-highlight-colors — GCC can use color for emphasis and contrast when printing diagnostic messages and quoting the user's 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 %i and "int" in the message and the "int" in the quoted source are colored using highlight-a (bold green by default), and
  • the "const char *" in the message and in the quoted source are both colored using highlight-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 additional 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 this left margin.
  • -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-parseable-fixits — bestie, this one makes GCC spill its fix-it hints in a machine-parseable format so your IDE can actually read them 💅. For every fix-it, a line gets printed right after the relevant diagnostic, and it starts with the string "fix-it:". Obsessed. Example:
fix-it:"test.c":{45:3-45:21}:"gtk_widget_show_all"

The location is a half-open range, and it's expressed as a count of bytes, starting at byte 1 for the initial column. So in that example above, bytes 3 through 20 of line 45 of "test.c" are getting replaced with the given string:

00000000011111111112222222222
12345678901234567890123456789
  gtk_widget_showall (dlg);
  ^^^^^^^^^^^^^^^^^^
  gtk_widget_show_all

The 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"). Ngl the escaping lore is real.

An empty replacement string means the given range is to be removed 🫶. And an empty range (e.g. "45:3-45:3") means the string is to be inserted at the given position. Cute.

  • -fdiagnostics-generate-patch — this one prints fix-it hints to stderr in unified diff format, after any diagnostics are printed. Like, a whole glow-up for your terminal ✨:
--- test.c
+++ test.c
@ -42,5 +42,5 @

 void show_cb(GtkDialog *dlg)
 {
-  gtk_widget_showall(dlg);
+  gtk_widget_show_all(dlg);
 }

The diff may or may not be colorized, following the same rules as for diagnostics (see -fdiagnostics-color). So don't come for GCC if the vibes are monochrome.

  • -fdiagnostics-show-template-tree — okay so in the C++ frontend, when printing diagnostics that show mismatching template types, you get stuff like:
  could not convert 'std::map<int, std::vector<double> >()'
    from 'map<[...],vector<double>>' to 'map<[...],vector<float>>

and the -fdiagnostics-show-template-tree flag enables printing a tree-like structure showing the common and the differing parts of the types, like:

  map<
    [...],
    vector<
      [double != float]>>

The parts that differ are highlighted with color ("double" and "float" in this case). Iconic behavior frankly 🌈.

  • -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 simplify the error message. 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 — no more hiding, we see everything 🔥. This flag also affects the output of the -fdiagnostics-show-template-tree flag, fyi.

  • -fdiagnostics-path-format=KIND — specify how to print paths of control-flow events for diagnostics that have such a path associated with them. Pick your fighter 🎀:

KIND is none, separate-events, or inline-events, the default.

none means to not print diagnostic paths. Respectful. Minimalist.

separate-events means to print a separate "note" diagnostic for each event within the diagnostic. Like this:

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 1

inline-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. Bestie it's giving annotated director's cut 😭.

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 1

Interprocedural control flow is shown by grouping the events by stack frame, and using indentation to show how stack frames are nested, pushed, and popped. The way this is so structured?? 🧚

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 additional information when printing control-flow paths associated with a diagnostic. Extra lore, basically 🦋.

If this is option is provided then the stack depth will be printed for each run of events within -fdiagnostics-path-format=inline-events. If provided with -fdiagnostics-path-format=separate-events, then the stack depth and function declaration will be 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, devs only, no cap — it's giving internal tooling 💀.

  • -fno-show-column — do not print column numbers in diagnostics. This may be necessary if diagnostics are being scanned by a program that does not understand the column numbers, such as dejagnu. Respectfully, some tools just can't handle the truth 🙈.
  • -fdiagnostics-column-unit=UNIT — select the units for the column number. This affects traditional diagnostics (in the absence of -fno-show-column), as well as JSON format diagnostics if requested.

The default UNIT, display, considers the number of display columns occupied by each character. This may be larger than the number of bytes required to encode the character, in the case of tab characters, or it may 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. The math is mathing 📐.

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. Retro era, we love a throwback ⭐.

  • -fdiagnostics-column-origin=ORIGIN — select the origin for column numbers, i.e. the column number assigned to the first column. The default value of 1 corresponds to traditional GCC behavior and to the GNU style guide. Some utilities may perform better with an origin of 0; any non-negative value may be specified. Zero-indexed girlies rise 🌸.
  • -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. These diagnostics are flagged so that GCC will escape bytes that are not printable ASCII when printing their pertinent source lines. Encoding drama, we've all been there 😭.

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>after

Setting 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. Art is art, bestie 🎨.

This option selects which characters should be used for printing such diagrams, if any. CHARSET is none, ascii, unicode, or emoji. Yes, emoji is an option. We are so back 💖.

  • The none value just… suppresses the diagrams entirely. no art for you 💀
  • The ascii value keeps them pure ASCII ("ASCII art"), very retro, very cute.
  • The unicode value lets it use unicode drawing characters conservatively (think box-drawing characters) 🎀
  • The emoji value goes even further and adds emoji to the output (like emitting U+26A0 WARNING SIGN followed by U+FE0F VARIATION SELECTOR-16 to pick the emoji variant of the character) ✨

The default is emoji, except when the environment variable LANG is set to C — then the default is ascii.

💅 -fdiagnostics-format=FORMAT

Pick a whole different format for printing diagnostics, bestie. FORMAT is one of text, sarif-stderr, sarif-file, json, json-stderr, or json-file.

Heads up: using this option replaces any extra "output sinks" you added with -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.

The various json, json-stderr, and json-file values are deprecated and refer to a legacy JSON-based output format 😭 The json format is a synonym for json-stderr. The json-stderr and json-file formats are identical, apart from where the JSON is emitted to. With json-stderr, the JSON is emitted to stderr, whereas with json-file it is written to source.gcc.json.

🦋 -fdiagnostics-add-output=DIAGNOSTICS-OUTPUT-SPEC

This one adds an additional "output sink" for emitting diagnostics. more sinks = more vibes 🫶

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. so like… don't get attached 🙈

SCHEME can be:

text

Emit diagnostics to stderr using GCC's classic text output format.

Supported keys are:

  • color=[yes|no] — override colorization settings from -fdiagnostics-color for this text output.
  • experimental-nesting=[yes|no] — enable an experimental mode that emphasizes hierarchical relationships within diagnostics messages, displaying location information on separate lines.
  • experimental-nesting-show-locations=[yes|no] — if experimental-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.
  • experimental-nesting-show-levels=[yes|no] — this is a debugging option for use with experimental-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.

Supported keys 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 to source.sarif.
  • version=[2.1|2.2-prerelease] — specify the version of SARIF to use for the output. If not specified, defaults to 2.1. 2.2-prerelease uses an unofficial draft of the future SARIF 2.2 specification and should only be used for experimentation in this release.

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.sarif

would add a pair of outputs, each writing to a different file, using versions 2.1 and 2.2 of the SARIF standard respectively. obsessed 🔥

In EBNF:

<diagnostics-output-specifier> = <diagnostics-output-name>
                                   | <diagnostics-output-name>, ":", <key-value-pairs>;

<diagnostics-output-name> = "text" | "sarif";

<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 basically the same as -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=. it's giving clean slate ✨

💜 -fno-diagnostics-json-formatting

So by default, when JSON is emitted for diagnostics (via -fdiagnostics-format=sarif-stderr, -fdiagnostics-format=sarif-file, -fdiagnostics-format=json, -fdiagnostics-format=json-stderr, -fdiagnostics-format=json-file), GCC adds newlines and indentation to visually emphasize the hierarchical structure of the JSON. pretty JSON, we love her 🎀

Use -fno-diagnostics-json-formatting to suppress that whitespace. ngl it must be passed before the option it is to affect.

This is intended for compatibility with tools that do not expect the output to contain newlines, like the output 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 not errors, they're just… red flags 🚩

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 on the syntax, 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-errors is also specified, then -Wfatal-errors takes precedence over this option.
  • -w — inhibit all warning messages. silencing the haters fr fr
  • -Werror — turn all warnings into errors. glow-up of the century, every warning is now an error 😤
  • -Werror= — turn the specified warning into an error. The specifier for a warning is appended; for example -Werror=switch turns the warnings controlled by -Wswitch into errors. This switch takes a negative form, to be used to negate -Werror for specific warnings; for example -Wno-error=switch makes -Wswitch warnings not be errors, even when -Werror is in effect.

The warning message for each controllable warning includes the option that controls the warning. That option can then be used with -Werror= and -Wno-error= as described above. (Printing of the option in the warning message can be disabled using the -fno-diagnostics-show-option flag.)

Note that specifying -Werror=foo automatically implies -Wfoo. However, -Wno-error=foo does not imply anything. it's strictly a "no" 🫶

  • -Wfatal-errors — this option causes the compiler to abort compilation on the first error occurred rather than trying to keep going and printing further error messages. one strike and you're out 💀

You can request many specific warnings with options beginning with -W, for example -Wunused-variable to request warnings on declarations of variables that are never used. Each of these specific warning options also has a negative form beginning with -Wno- to turn off warnings; for example, -Wno-unused-variable. This manual lists only one of the two forms, whichever is not the default. For further language-specific options also refer to C++ Dialect Options and Objective-C and Objective-C++ Dialect Options. Additional warnings can be produced by enabling the static analyzer;

Some options, such as -Wall and -Wextra, turn on other options, such as -Wunused, which may turn on further options, such as -Wunused-variable. The combined effect of positive and negative forms is that more specific options have priority over less specific ones, independently of their position in the command line. For options of the same specificity, the last one takes effect. Options enabled or disabled via pragmas take effect as if they appeared at the end of the command line. it's giving hierarchy of needs 🧚

When an unrecognized warning option is requested (e.g., -Wunknown-warning), GCC gives an error stating that the option is not recognized. However, if the -Wno- form is used, the behavior is slightly different: no diagnostic is produced for -Wno-unknown-warning unless other diagnostics are being produced. This allows the use of new -Wno- options with old compilers, but if something goes wrong, the compiler warns that an unrecognized option is present.

The effectiveness of some warnings depends on optimizations also being enabled. For example, -Wsuggest-final-types is more effective with link-time optimization. Some other warnings may not be issued at all unless optimization is enabled. While optimization in general improves the efficacy of warnings about control and data-flow problems, in some cases it may also cause false positives. 😭 the optimizer giveth and the optimizer taketh away

⭐ -Wpedantic, -pedantic

Issue all the warnings demanded by strict ISO C and ISO C++; diagnose all programs that use forbidden extensions, and some other programs that do not follow ISO C and ISO C++. This follows the version of the ISO C or C++ standard specified by any -std option used.

Valid ISO C and ISO C++ programs should compile properly with or without this option (though a rare few require -ansi or a -std option specifying the version of the standard). However, without this option, certain GNU extensions and traditional C and C++ features are supported as well. With this option, they are diagnosed (or rejected with -pedantic-errors). no cap, it gets strict 🔥

-Wpedantic does not cause warning messages for use of the alternate keywords whose names begin and end with __. This 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__. However, only system header files should use these escape routes; application programs should avoid them. 🙈

Some warnings about non-conforming programs are controlled by options other than -Wpedantic; in many cases they are implied by -Wpedantic but can be disabled separately by their specific option, e.g. -Wpedantic -Wno-pointer-sign.

Okay so real talk: when the standard you picked with -std is a GNU extended dialect of C — think gnu90 or gnu99 — there's a base standard lurking under it, aka the ISO C version that dialect is built on. -Wpedantic will yell at you exactly where that base standard says it must. (And like, it would be nonsense to only warn about features that aren't in the GNU C dialect you chose, because by definition the GNU dialects of C include everything the compiler supports with that option — there'd be literally nothing to warn about. 💀)

-pedantic-errors 💅

This one is -Wpedantic's unhinged sibling: it throws an error any time the base standard (peep -Wpedantic above) requires a diagnostic — including some cases where there's undefined behavior at compile-time, and some other cases that don't even stop you compiling programs that are totes valid per the standard.

Careful bestie: this is not the same as -Werror=pedantic. That other option is honestly not likely to be useful, because it only makes errors out of the diagnostics controlled by -Wpedantic, whereas this one also hits the required diagnostics that are always on or controlled by options other than -Wpedantic.

So if you want the required diagnostics that are warnings by default to become errors, but you do not want to switch on the -Wpedantic diagnostics? You can do -pedantic-errors -Wno-pedantic (or -pedantic-errors -Wno-error=pedantic if you want them enabled but only as warnings). Slay, options.

Some required diagnostics are errors by default, but you can knock them down to warnings with -fpermissive or their own specific warning option, e.g. -Wno-error=narrowing.

And some diagnostics for non-ISO practices are controlled by specific warning options other than -Wpedantic, but -pedantic-errors still escalates them to 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 🫶

The chill pill option ✨ — it downgrades some required diagnostics about nonconformant code from errors down to warnings. So yeah, -fpermissive lets some nonconforming code actually compile. Some C++ diagnostics are controlled only by this flag, but it also downgrades some C and C++ diagnostics that already have their own flag:

  • -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)

Fun fact: -fpermissive is the default for historic C language modes (-std=c89, -std=gnu89, -std=c90, -std=gnu90). Vintage vibes, honestly.

-Wall 🔥

The classic. The one everybody types. This turns on all the warnings about constructions that some users consider questionable and that are easy to avoid (or tweak so the warning goes away) — even when macros are in the mix. It also enables some language-specific warnings described in C++ Dialect Options and Objective-C and Objective-C++ Dialect Options. Iconic behavior.

-Wall turns on this whole girliepop list of 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
Heads up babes: not every warning flag is implied by -Wall. Some of them warn about constructions users generally don't think are questionable but which occasionally you might wanna check for; others warn about stuff that's necessary or hard to avoid in some cases, with no simple way to change the code and silence it. Some get enabled by -Wextra, but many of them you gotta turn on individually, one by one. 🙈

-Wextra ✨

This one unlocks some extra warning flags that -Wall doesn't. (Fun history moment: this option used to be called -W. The older name is still supported, but the newer name is more descriptive. Glow-up.)

  • -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)
  • -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
  • -Wunused-parameter (only with -Wunused or -Wall)
  • -Wunused-but-set-parameter (only with -Wunused or -Wall)

The option -Wextra also prints warning messages for these situations:

  • A pointer is compared against integer zero with <, <=, >, or >=.
  • (C++ only) An enumerator and a non-enumerator both appear in a conditional expression.
  • (C++ only) Ambiguous virtual bases.
  • (C++ only) Subscripting an array that has been declared register.
  • (C++ only) Taking the address of a variable that has been declared register.
  • (C++ only) A base class is not initialized in the copy constructor of a derived class.

-Wabi (C, Objective-C, C++ and Objective-C++ only) 🦋

Warns you about code affected by ABI changes — including code that may not be compatible with the vendor-neutral C++ ABI and the psABI for the particular target. Those latter warnings are also controlled separately by -Wpsabi, which -Wabi implies. (Two-for-one deal, we love to see it.)

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 you do provide an explicit version number and -fabi-compat-version is not specified, the version number from this option is used for compatibility aliases. If you don't give an explicit version number with this option, but -fabi-compat-version is specified, that version number gets used for C++ ABI warnings. Mix and match, iykyk.

Real talk, no cap: even though an effort has been made to warn about all such cases, there are probably some cases that aren't warned about even though G++ is generating incompatible code. There may also be cases where warnings get emitted even though the code that's generated is compatible. It's giving "we tried".

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. Bestie, that's the whole point.

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 doesn't allow for overloading of functions taking vectors of different sizes.

The mangling was changed in -fabi-version=4.

  • __attribute ((const)) and noreturn were mangled as type qualifiers, and decltype of a plain declaration was folded away.

These mangling issues were fixed in -fabi-version=5.

  • Scoped enumerators passed as args to a variadic function get promoted just like unscoped ones, which makes va_arg big mad and complain. On most targets this lowkey doesn't actually mess with the parameter-passing ABI, since there's literally no way to pass an argument smaller than int anyway.

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.

All that got fixed in -fabi-version=6. ✨

  • Lambdas in default argument scope were mangled wrong, and the ABI changed how nullptr_t gets mangled.

Fixed 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. Rude!

Fixed in -fabi-version=8, which is the default for GCC 5.1. 🎀

  • decltype(nullptr) had an alignment of 1 (bestie, why 😭), which led to unaligned accesses. Note that this did not affect the ABI of a function with a nullptr_t parameter, because parameters get a minimum alignment. So, y'know, it's giving "contained chaos."

Fixed in -fabi-version=9, the default for GCC 5.2. 🌸

  • Target-specific attributes that affect the identity of a type — think ia32 calling conventions on a function type (stdcall, regparm, etc.) — did not affect the mangled name. That caused name collisions when function pointers were used as template arguments. Not it. 💀

Fixed in -fabi-version=10, the default for GCC 6.1. 👑

🌈 -Wpsabi (C, Objective-C, C++ and Objective-C++ only)

-Wpsabi turns on warnings about processor-specific ABI changes — stuff like changes in alignment requirements or how function arguments get 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 it's also implied by -Wabi.

🌈 -Wno-changes-meaning (C++ and Objective-C++ only)

C++ requires that unqualified uses of a name inside a class have the same meaning across the complete scope of the class — so declaring the name after using it is ill-formed. Rude but true:

struct A;
struct B1 { A a; typedef A A; }; // warning, 'A' changes meaning
struct B2 { A a; struct A { }; }; // error, 'A' changes meaning

By 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 you if an array subscript has type char. This is a super common cause of error, because programmers constantly forget that this type is signed on some machines. 🙈 This warning is enabled by -Wall.

🌈 -Wno-coverage-mismatch

Warns you if feedback profiles don't match when you're using the -fprofile-use option. If a source file changes between compiling with -fprofile-generate and compiling with -fprofile-use, the files with the profile feedback can fail to match the source file, and GCC just cannot use the profile feedback info. By default this warning is enabled and treated as an error. -Wno-coverage-mismatch can be used to disable the warning, or -Wno-error=coverage-mismatch can be used to disable the error.

Okay so heads up: disabling the error for this warning can result in poorly optimized code, and it's only useful in the case of very minor changes — like bug fixes to an existing code-base. Completely disabling the warning is not recommended. fr fr, don't do it. 😭

🌈 -Wno-coverage-too-many-conditions

Warns you if -fcondition-coverage is used and an expression has too many terms, so GCC gives up on coverage. Coverage gets given up when there are more terms in the conditional than there are bits in a gcov_type_unsigned. This warning is enabled by default. 💜

🌈 -Wno-coverage-too-many-paths

Warns you if -fpath-coverage is used and a function has too many paths, so GCC gives up on coverage. The giving-up is controlled by -fpath-coverage-limit. This warning is enabled by default. 💜

🌈 -Wno-coverage-invalid-line-number

Warns you in case a function ends earlier than it begins, thanks to invalid linenum macros. The warning is emitted only with --coverage enabled. 👀

By default this warning is enabled and 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 #warning directives. That's it, that's the whole vibe check. 🤫

🌈 -Wdouble-promotion (C, C++, Objective-C and Objective-C++ only)

Gives you a warning when a value of type float gets implicitly promoted to double. Here's the tea: CPUs with a 32-bit "single-precision" floating-point unit implement float in hardware, but emulate double in software. On such a machine, doing computations with double values is much more expensive because of the overhead required for software emulation. 💸

It is genuinely 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. Not me writing this exact bug in college 💀

🌈 -Wduplicate-decl-specifier (C and Objective-C only)

Warns you if a declaration has a duplicate const, volatile, restrict or _Atomic specifier. This warning is enabled by -Wall. ✨

🌈 -Wformat / -Wformat=n

Checks calls to printf and scanf, etc., to make sure the arguments you supplied have types appropriate to the format string you specified, and that the conversions specified in the format string make sense. This covers standard functions, plus others specified by format attributes, in the printf, scanf, strftime and strfmon families (that last one's an X/Open extension, not in the C standard) — or other target-specific families. Which functions get checked without format attributes having been specified depends on the standard version you selected, and those 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 together with -Wformat, warnings are given about format features not in the selected standard version (but not for strfmon formats, since those are not in any version of the C standard).

⭐ -Wformat=1 / -Wformat

Option -Wformat is equivalent to -Wformat=1, and -Wno-format is equivalent to -Wformat=0. Since -Wformat also checks for null format arguments for several functions, -Wformat also implies -Wnonnull. Some aspects of this level of format checking can be disabled by the options: -Wno-format-contains-nul, -Wno-format-extra-args, and -Wno-format-zero-length. -Wformat is enabled by -Wall. 💅

⭐ -Wformat=2

Enables -Wformat plus additional format checks. Currently equivalent to -Wformat -Wformat-nonliteral -Wformat-security -Wformat-y2k. Iconic combo. 👑

🌈 -Wno-format-contains-nul

If -Wformat is specified, do not warn about format strings that contain NUL bytes. 🤐

🌈 -Wno-format-extra-args

If -Wformat is specified, do not warn about excess arguments to a printf or scanf format function. The C standard specifies that such arguments are ignored, so it's giving "extra, but make it legal."

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 in order 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 as sprintf and vsprintf that might overflow the destination buffer. When the exact number of bytes written by a format directive can't be determined at compile-time, it gets estimated based on heuristics that depend on the level argument and on optimization. While enabling optimization will in most cases improve the accuracy of the warning, 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 are assumed 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 to sprintf below is diagnosed because even with both a and b equal 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. 😭

-Wformat-overflow=2

Level 2 is where things get spicy 🌶️ — it 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 -Wformat is specified, do not warn about zero-length formats. The C standard specifies that zero-length formats are allowed. so like… they're legal, bestie, chill 💅

-Wformat-nonliteral

If -Wformat is specified, also warn if the format string is not a string literal and so cannot be checked, unless the format function takes its format arguments as a va_list.

-Wformat-security

If -Wformat is specified, also warn about uses of format functions that represent possible security problems. At present, this warns about calls to printf and scanf functions where the format string is not a string literal and there are no format arguments, as in printf (foo);. This may be a security hole if the format string came from untrusted input and contains %n. 😭 (This is currently a subset of what -Wformat-nonliteral warns about, but in future warnings may be added to -Wformat-security that are not included in -Wformat-nonliteral.)

-Wformat-signedness

If -Wformat is specified, also warn if the format string requires an unsigned argument and the argument is signed and vice versa. mismatch behavior fr fr 🙈

-Wformat-truncation

-Wformat-truncation=level

Warn about calls to formatted input/output functions such as snprintf and vsnprintf that 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 the level argument 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 -Wformat is specified, also warn about strftime formats that may yield only a two-digit year. y2k said "i'm back" 💀

-Wnonnull

Warn about passing a null pointer for arguments marked as requiring a non-null value by the nonnull function attribute.

-Wnonnull is included in -Wall and -Wformat. It can be disabled with the -Wno-nonnull option.

-Wnonnull-compare

Warn when comparing an argument marked with the nonnull function attribute against null inside the function.

-Wnonnull-compare is included in -Wall. It can be disabled with the -Wno-nonnull-compare option.

-Wnull-dereference

Warn 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-checks is active, which is enabled by optimizations in most targets. The precision of the warnings depends on the optimization options used. ngl this one is doing detective work 🔍

-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 a musttail call. 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 a musttail call, unless it goes out of scope already before the musttail call.

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). it's giving "same energy, different pipeline" ✨

-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. shortcuts, love that for us 👑

-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;
  }

Okay so bestie, lowkey the nicest part: this warning is not gonna yell at you when the last statement of a case literally cannot fall through — like when there's a return in there, or a call to a function declared with the noreturn attribute. It knows. It's not a hater. 🫶

And -Wimplicit-fallthrough= also takes into account control flow statements, such as ifs, and it only warns when appropriate. Like, check this out:

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 genuinely desirable (yes, it happens, we don't judge 💅), GCC gives you an attribute — __attribute__ ((fallthrough)) — that you use alongside a null statement to suppress the warning that would normally pop off:

switch (cond)
  {
  case 1:
    bar (0);
    __attribute__ ((fallthrough));
  default:
    ...
  }

C++17 provides a standard way to suppress the -Wimplicit-fallthrough warning using [[fallthrough]]; instead of the GNU attribute. And in C++11 or C++14 you can use [[gnu::fallthrough]];, which is a GNU extension. ✨

Instead of these attributes, it's 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=0 — disables the warning altogether. (gone. deleted. not me forgetting it exists 🙈)
  • -Wimplicit-fallthrough=1 — matches the .* regular expression, so literally any comment is used as a fallthrough comment. chaotic but valid.
  • -Wimplicit-fallthrough=2 — case insensitively matches the .*falls?[ \t-]*thr(ough|u).* regular expression.
  • -Wimplicit-fallthrough=3 — case 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=4 — case sensitively matches one of the following regular expressions:
"-fallthrough"
"@fallthrough@"
"lint -fallthrough[ \t]*"
"[ \t]*FALLTHR(OUGH|U)[ \t]*"
  • -Wimplicit-fallthrough=5 — doesn't recognize any comments as fallthrough comments, only attributes disable the warning. strict queen 👑

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. Like so:

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) — controls whether warnings triggered by the warn_if_not_aligned attribute should be issued. These warnings are enabled by default.
  • -Wignored-qualifiers (C and C++ only) — warns 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. And ISO C prohibits qualified void return types on function definitions, so those return types always receive a warning even without this option. the way ISO C said absolutely not 😭

This warning is also enabled by -Wextra.

  • -Wno-ignored-attributes (C and C++ only) — this option controls warnings when an attribute is ignored. It's 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 — warns 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. so like, now it's an error, no cap.

  • -Wmisleading-indentation (C and C++ only) — warns 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. it's giving "the vibes are lying to you" 🎀

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);

The warning is also not issued after a #line directive, since this typically indicates autogenerated code, and no assumptions can be made about the layout of the file that the directive references.

This warning is enabled by -Wall in C and C++. ✨

  • -Wmissing-attributes — warns when a declaration of a function is missing one or more attributes that a related function is declared with and whose absence may adversely affect the correctness or efficiency of generated code. For example, the warning is issued for declarations of aliases that use attributes to specify less restrictive requirements than those of their targets. This typically represents a potential optimization opportunity. By contrast, the -Wattribute-alias=2 option controls warnings issued when the alias is more restrictive than the target, which could lead to incorrect code generation. Attributes considered include alloc_align, alloc_size, cold, const, hot, leaf, malloc, nonnull, noreturn, nothrow, pure, returns_nonnull, and returns_twice.

In C++, the warning is issued when an explicit specialization of a primary template declared with attribute alloc_align, alloc_size, assume_aligned, format, format_arg, malloc, or nonnull is declared without it. Attributes deprecated, error, and warning suppress the warning. (they said shhh 🤫)

You can use the copy attribute to apply the same set of attributes to a declaration as that on another declaration without explicitly enumerating the attributes. This attribute can be applied to declarations of functions, variables, or types.

-Wmissing-attributes is enabled by -Wall.

For example, since the declaration of the primary function template below makes use of both attribute malloc and alloc_size, the declaration of the explicit specialization of the template is diagnosed because it's missing one of the attributes:

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 is not fully bracketed. In the following example, the initializer for a is not fully bracketed, but the one for b is fully bracketed:
int a[2][2] = { 0, 1, 2, 3 };
int b[2][2] = { { 0, 1 }, { 2, 3 } };

This warning is enabled by -Wall. put your braces on, bestie, it's cold outside 🧣

  • -Wmissing-include-dirs (C, C++, Objective-C, Objective-C++ and Fortran only) — warns if a user-supplied include directory does not exist. This option is disabled by default for C, C++, Objective-C and Objective-C++. For Fortran, it's partially enabled by default by warning for -I and -J, only.
  • -Wno-missing-profile — this option controls warnings if feedback profiles are missing when using the -fprofile-use option. It diagnoses those cases where a new function or a new file is added between compiling with -fprofile-generate and with -fprofile-use, without regenerating the profiles. In these cases, the profile feedback data files do not contain any profile feedback information for the newly added function or file respectively. Also, in the case when profile count data (.gcda) files are removed, GCC cannot use any profile feedback information. In all these cases, warnings are issued to inform you that a profile generation step is due. Ignoring the warning can result in poorly optimized code. -Wno-missing-profile can be used to disable the warning, but this is not recommended and should be done only when non-existent profile data is justified. seriously, don't 🙈
  • -Wmismatched-dealloc — warns for calls to deallocation functions with pointer arguments returned from allocation functions for which the former isn't a suitable deallocator. A pair of functions can be associated as matching allocators and deallocators by use of attribute malloc. Unless disabled by the -fno-builtin option, the standard functions calloc, malloc, realloc, and free, as well as the corresponding forms of C++ operator new and operator delete, are implicitly associated as matching allocators and deallocators. In the following example, mydealloc is the deallocator for pointers returned from myalloc.
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
}

And if you're in C++, there's a sibling option — -Wmismatched-new-delete — that catches the same kind of mismatch when operator new or operator delete are the ones involved 🎀.

Oh and heads up: -Wmismatched-dealloc is part of -Wall, so it's already on your team bestie ✨

  • -Wmultistatement-macros — this one's a lil lifesaver 🫶. It yells at you about unsafe multi-statement macros that look like they're guarded by an if, else, for, switch, or while, but where — plot twist — only the first statement actually ends up guarded after the macro gets expanded 💀.

Like, look at this mess:

#define DOIT x++; y++
if (c)
  DOIT;

That increments y unconditionally, not just when c holds 😭. The way that silently ruins your day. You can usually fix it by wrapping 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 — the "bestie, please add parentheses" warning 🌸. It fires if parentheses are omitted in certain contexts — like when there's an assignment chilling in a spot where a truth value is expected, or when operators get nested in precedence combos that people constantly get confused about 🙈.

It also warns when a comparison like x<=y<=z shows up, because that's actually equivalent to (x<=y ? 1 : 0) <= z — which is a whole different interpretation from what ordinary mathematical notation means, no cap 🧮.

Also flags dangerous uses of the GNU extension to ?: with an omitted middle operand. When the condition in the ?: operator is a boolean expression, the omitted value is always 1 — but often programmers expect it to be a value computed inside the conditional expression instead 😬.

For C++ it also warns about some cases of unnecessary parentheses in declarations, which can be a tell that someone meant a function call but accidentally wrote 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) — warns when a value gets moved to itself with std::move 😭. Such a std::move typically has no effect, so it's giving pointless, honestly.
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 breaks the sequence point rules in the C and C++ standards 🚨.

So here's the lore, bestie: 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 — the parts executed before the sequence point, and the parts executed after it. These occur after the evaluation of a full expression (one that isn't 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's 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, that's your call to make, bestie.)
  • -Wreturn-mismatch — warns about return statements without an expression in functions that don't return void 😭. Also warns about a return statement with an expression in a function whose return type is void, unless the expression type is also void. As a GNU extension, that latter case is accepted without a warning unless -Wpedantic is used.

Attempting to use the return value of a non-void function other than main that flows off the end by reaching the closing curly brace that terminates the function is undefined 💀.

This warning is specific to C and enabled by default. In C99 and later language dialects, it's treated as an error 🚨. It can be downgraded to a warning using -fpermissive (along with other warnings), or for just this warning, with -Wno-error=return-mismatch 🎀

  • -Wreturn-type — warns whenever a function is defined with a return type that defaults to int (unless -Wimplicit-int is active, which takes precedence 👑). Also warns if execution may reach the end of the function body, or if the function doesn't contain any return statement at all 🌸.

Attempting to use the return value of a non-void function other than main that flows off the end by reaching the closing curly brace that terminates the function is undefined 💀.

Unlike in C, in C++, flowing off the end of a non-void function other than main results in undefined behavior even when the value of the function is not used 😵‍💫.

This warning is enabled by default in C++ and by -Wall otherwise ✨

  • -Wno-shift-count-negative — controls warnings if a shift count is negative. This warning is enabled by default 🎀
  • -Wno-shift-count-overflow — controls warnings if a shift count is greater than or equal to the bit width of the type. This warning is enabled by default 🌈
  • -Wshift-negative-value — warns if you left shift a negative value 🙈. This warning is enabled by -Wextra in C99 (and newer) and C++11 to C++17 modes.
  • -Wno-shift-overflow, -Wshift-overflow=n — these options control warnings about left shift overflows 💫
    • -Wshift-overflow=1 — this is the warning level of -Wshift-overflow and is enabled by default in C99 and C++11 modes (and newer). This warning level does not warn about left-shifting 1 into the sign bit. (However, in C, such an overflow is still rejected in contexts where an integer constant expression is required.) No warning is emitted in C++20 mode (and newer), as signed left shifts always wrap 🌀
    • -Wshift-overflow=2 — this warning level also warns about left-shifting 1 into the sign bit, unless C++14 mode (or newer) is active ⚠️
  • -Wswitch — warns whenever a switch statement has an index of enumerated type and lacks a case for one or more of the named codes of that enumeration 🕵️‍♀️. (The presence of a default label prevents this warning.) case labels that don't correspond to enumerators also provoke warnings when this option is used, unless the enumeration is marked with the flag_enum attribute. This warning is enabled by -Wall 👑
  • -Wswitch-default — warns whenever a switch statement doesn't have a default case 😤 (like, give it a fallback bestie, c'mon)
  • -Wswitch-enum — warns whenever a switch statement has an index of enumerated type and lacks a case for one or more of the named codes of that enumeration 🌸. case labels that don't correspond to enumerators also provoke warnings when this option is used, unless the enumeration is marked with the flag_enum attribute. The only difference between -Wswitch and this option is that this option gives a warning about an omitted enumeration code even if there's a default label 💅
  • -Wno-switch-bool — do not warn when a switch statement has an index of boolean type and the case values are outside the range of a boolean type 🙈. Fun fact: it's possible to suppress this warning by casting the controlling expression to a type other than bool. Like this:
switch ((int) (a == 4))
  {
  ...
  }

This warning is enabled by default for C and C++ programs ⭐

  • -Wno-switch-outside-range — this option controls warnings when a switch case has a value that's outside of its respective type range 🚧. This warning is enabled by default for C and C++ programs.
  • -Wno-switch-unreachable — do not warn when a switch statement contains statements between the controlling expression and the first case label, which will never be executed 😭 (rip to that code, gone but not forgotten). For example:
switch (cond)
  {
   i = 15;
  ...
   case 5:
  ...
  }
  • -Wswitch-unreachable — okay so this one does not yell at you if the statement between the controlling expression and the first case label is just a declaration, like this:
switch (cond)
  {
   int i;
  ...
   case 5:
   i = 5;
  ...
  }

This warning is on by default for C and C++ programs. 💅

  • -Wsync-nand (C and C++ only) — warns you when the __sync_fetch_and_nand and __sync_nand_and_fetch built-in functions get used. Bestie, these functions changed semantics in GCC 4.4, so like... be aware. 👀
  • -Wtrivial-auto-var-init — warns when -ftrivial-auto-var-init can't initialize the automatic variable. The classic situation: an automatic variable declared between the controlling expression and the first case label of a switch statement. It's giving "caught in the crossfire" energy 😭
  • -Wunused-but-set-parameter — warns whenever a function parameter gets assigned to but is otherwise unused (aside from its declaration). To suppress it, use the unused attribute. This one's also enabled by -Wunused together with -Wextra. 🫶
  • -Wunused-but-set-variable — warns whenever a local variable is assigned to but otherwise unused (aside from its declaration). Enabled by -Wall. To suppress, use the unused attribute. Also enabled by -Wunused, which is itself enabled by -Wall. Nesting dolls but make it compiler flags 🎀
  • -Wunused-function — warns whenever a static function is declared but never defined, or a non-inline static function is unused. Enabled by -Wall. The "you made this and never called it" warning, fr fr 💀
  • -Wunused-label — warns whenever a label is declared but never used. Enabled by -Wall. To suppress it, use the unused attribute. 🏷️
  • -Wunused-local-typedefs (C, Objective-C, C++ and Objective-C++ only) — warns when a typedef that's locally defined in a function goes unused. Enabled by -Wall. Local typedef found unemployed, more at 11 ✨
  • -Wunused-parameter — warns whenever a function parameter is unused aside from its declaration. Heads up: this option is not enabled by -Wunused unless -Wextra is also specified. To suppress it, use the unused attribute. 💅
  • -Wno-unused-result — don't warn if a caller of a function marked with the warn_unused_result attribute doesn't use its return value. The default is -Wunused-result. So yes, the "you ignored the return value bestie" warning is on unless you turn it off 🫠
  • -Wunused-variable — warns whenever a local or static variable is unused aside from its declaration. This option implies -Wunused-const-variable=1 for C, but not for C++. Enabled by -Wall. To suppress it, 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 it, use the unused attribute. And now the levels, because of course there are levels 👑
    • -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. Level 1 is enabled by either -Wunused-variable or -Wunused for C, but not for C++. In C this declares variable storage, but in C++ it isn't an error since const variables take the place of #defines. 🦋
    • -Wunused-const-variable=2 — this level also warns for unused constant static variables in headers (excluding system headers). It's equivalent to the short form -Wunused-const-variable. This level must be explicitly requested in both C and C++, because it might be hard to clean up all the headers you include. Lowkey a commitment 😅
  • -Wunused-value — warns whenever a statement computes a result that is explicitly not used. To suppress it, cast the unused expression to void. This includes an expression-statement, or the left-hand side of a comma expression, that contains no side effects. For example, an expression such as x[i,j] causes a warning, while x[(void)i,j] does not. Periodt. ✨

This warning is enabled by -Wall.

  • -Wunused — all of the above -Wunused options combined, except those documented as needing to be specified explicitly. 🎀

So if you actually want a warning about an unused function parameter, you gotta either specify -Wextra -Wunused (note that -Wall implies -Wunused), or separately specify -Wunused-parameter and/or -Wunused-but-set-parameter. iykyk.

-Wunused enables only -Wunused-const-variable=1 rather than -Wunused-const-variable, and only for C, not C++.

  • -Wuse-after-free (C, Objective-C, C++ and Objective-C++ only)
  • -Wuse-after-free=n — warns about uses of pointers to dynamically allocated objects that have been rendered indeterminate by a call to a deallocation function. Enabled at all optimization levels, but ngl it may yield different results with optimization than without. 🧚
    • -Wuse-after-free=1 — at level 1, the warning tries to diagnose only unconditional uses of pointers made indeterminate by a deallocation call or a successful call to realloc, regardless of whether or not the call actually resulted in a real reallocation of memory. This includes double-free calls 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, it 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 to free in this function is 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 show up after calls to realloc as an attempt to figure out whether the call relocated the object to a different address. They get diagnosed at a separate level to help you gradually transition 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 obviates needing to adjust the stored pointers after reallocation. Iconic workaround, honestly 👑

-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. But! This warning does not fire 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-static const member appears in a class without constructors. 🫶

In addition, 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 also diagnosed by this warning. (-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 warnings for depends on the precise optimization options and version of GCC you're using. So like... it's giving "results may vary" 💀

Note that there may be no warning about a variable used only to compute a value that itself is never used, because data flow analysis may delete such computations before the warnings even get 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 __atomic Builtins, __sync Builtins, 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 the memory_order enumeration. For example, since the __atomic_store and __atomic_store_n built-ins are only defined for the relaxed, release, and sequentially consistent memory orders, this code gets 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 some other paths exist where the object is not initialized, the compiler emits a warning if it cannot prove those uninitialized paths are not executed at run time. It's the "maybe? perhaps? who's to say" of warnings 😭

Also, if you pass a pointer (or, in C++, a reference) to an uninitialized object into a const-qualified function argument, this warning catches that too. (-Wuninitialized fires for built-in functions that are known to read the object.) And if you annotate the function with the attribute access (none), that's you telling GCC "bestie, this argument isn't used to touch the object" — which dodges the warning entirely. 💅

These warnings only even have a chance in optimizing compilation, because otherwise GCC isn't keeping track of variable state at all. Like, it literally cannot see the vibes. 🙈

They're also optional warnings, because GCC sometimes can't tell when your code is actually correct even though it looks sus. Classic example of how that happens:

{
  int x;
  switch (y)
    {
    case 1: x = 1;
      break;
    case 2: x = 4;
      break;
    case 3: x = 5;
    }
  foo (x);
}

If y is always 1, 2 or 3, then x is always initialized — but GCC doesn't know that. So to shut the warning up, you gotta give it a default case with assert(0) or something similar. 🫶

This option also warns when a non-volatile automatic variable might get changed by a call to longjmp. The compiler only sees the calls to setjmp — it has zero clue where longjmp will be called. And honestly? A signal handler could call it at literally any point in the code. So yeah, you may get a warning even when there's genuinely no problem, because longjmp can't actually be called at the spot that'd cause trouble.

Some of these spurious warnings can be avoided if you declare all the functions you use that never return as noreturn. Cute little trick, no cap. ✨

This warning is enabled by -Wall or -Wextra.

  • -Wunknown-pragmas — Warns you when a #pragma directive shows up 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 -Wall command-line option.
  • -Wno-pragmas — Do not warn about misuses of pragmas, like incorrect parameters, invalid syntax, or pragmas conflicting with each other. See also -Wunknown-pragmas.
  • -Wno-pragma-once-outside-header — Do not warn when #pragma once is used in a file that isn't a header file — like, say, a main file. 🎀
  • -Wno-prio-ctor-dtor — Do not warn if a priority from 0 to 100 is used for a constructor or destructor. The constructor and destructor attributes let you assign a priority to the constructor/destructor so you can control the order of execution before main is 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. Periodt.
  • -Wstrict-aliasing — This one's only active when -fstrict-aliasing is active. It warns about code that might break the strict aliasing rules the compiler is using for optimization. Heads up: the warning does not catch all cases, but it does try to catch the more common pitfalls. It's included in -Wall. It's equivalent to -Wstrict-aliasing=3.
  • -Wstrict-aliasing=n — Same deal: only active when -fstrict-aliasing is active. Warns about code that might break the strict aliasing rules the compiler uses for optimization. Higher levels = higher accuracy (fewer false positives). Higher levels also = more effort, similar to how -O works. -Wstrict-aliasing is equivalent to -Wstrict-aliasing=3.
    • Level 1: Most aggressive, quick, least accurate. Possibly useful when higher levels don't warn but -fstrict-aliasing still breaks the code, since it has very few false negatives. However, 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 (though not as many as level 1), 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. Handles the common pun+dereference pattern in the front end: *(int*)&some_float. If optimization is enabled, it also runs in the back end, where it deals with multiple statement cases using flow-sensitive points-to information. Only warns when the converted pointer is dereferenced. Does not warn about incomplete types.
  • -Wstrict-overflow
  • -Wstrict-overflow=n

This option is only active when signed overflow is undefined. It warns about cases where the compiler optimizes based on the assumption that signed overflow doesn't occur. Note that it does not warn about all cases where the code might overflow — it only warns about cases where the compiler actually implements some optimization. So like, 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, in fact, occurs. So this warning can easily give a false positive: a warning about code that isn't actually a problem. To help focus on what matters, 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 — Warns about cases that are both questionable and easy to avoid. For example, the compiler simplifies x + 1 > x to 1. This level of -Wstrict-overflow is enabled by -Wall; higher levels are not, and must be explicitly requested.
  • -Wstrict-overflow=2 — Also warns 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 warns about other cases where a comparison is simplified. For example: x + 1 > 1 is simplified to x > 0.
  • -Wstrict-overflow=4 — Also warns about other simplifications not covered by the cases above. For example: (x * 10) / 5 is simplified to x * 2.
  • -Wstrict-overflow=5 — Also warns 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 only reported at the highest warning level, because this simplification applies to so many comparisons — so this level gives a very large number of false positives. 💀
  • -Wstring-compare — Warns 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). Calls like that could be mistakes. For example, the call to strcmp below is diagnosed because its result is necessarily non-zero no matter what's actually in the array a:
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

Warns 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 only meaningful 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.

It produces the best results with optimization enabled, but it can detect a small subset of simple buffer overflows even without optimization, in calls to GCC built-in functions like __builtin_memcpy that correspond to the standard functions. In any case, the option warns about just a subset of the buffer overflows detected by the corresponding overflow-checking built-ins, such as __builtin___memcpy_chk, which can perform run-time checking if the access can't 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 a 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 — bestie, this level does its size math with type-zero Object Size Checking to figure out how big your destination objects are. At this setting it will not come for you about writes past the end of subobjects of bigger objects that are reached through pointers — unless it knows the size of the largest surrounding object. And when the destination could be any one of several objects? It just assumes it's the biggest one of them. 💅 On Linux systems, when optimization is on at this setting, it warns for the exact same code as when the _FORTIFY_SOURCE macro is defined to a non-zero value.
  • -Wstringop-overflow=2 — this one rolls with type-one Object Size Checking to work out the sizes of destination objects. At this setting it warns about overflows when you're writing to members of the largest complete objects whose exact size is known. 🎀 But it stays quiet about excessive writes to those same members of unknown objects referenced by pointers, because those pointers might be pointing at arrays with an unknown number of elements, iykyk. This is the default setting of the option, just so you know.
  • -Wstringop-overflow=3 — now we're on type-two Object Size Checking for the destination object sizes. This setting warns about overflowing the smallest object or data member. Heads up: this is the most restrictive setting of the option, and it may legitimately result in warnings even for safe code. 😭
  • -Wstringop-overflow=4 — type-three Object Size Checking, the whole nine yards. This setting 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 drop a warning. Similarly to -Wstringop-overflow=3, this setting of the option may result in warnings for benign code. 💀
  • -Wno-stringop-overread — warns you about calls to string manipulation functions like memchr or strcpy that are determined to read past the end of the source sequence. Ngl, reading past the end is giving major red flag energy. 🚩 Note that option -Wstringop-overread is enabled by default.
  • -Wno-stringop-truncation — okay so this one says 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. 🙈 Let's walk through the vibes:

In the following example, the call to strncat specifies a bound that's less than the length of the source string. So the copy of the source is gonna get truncated, and the call gets 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 super common mistake, so the call is diagnosed. To avoid the warning when the result isn't expected to be NUL-terminated, call memcpy instead. 🫶

void copy (char *d, const char *s)
{
  strncpy (d, s, strlen (s));
}

And 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. So that call gets diagnosed too. 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);
  ...
}

So like, in situations where a character array is intended to store a sequence of bytes with no terminating NUL, that array may be annotated with attribute nonstring to avoid this warning. ✨ But lowkey those arrays are not suitable arguments to functions that expect NUL-terminated strings. To help catch accidental misuses of such arrays, GCC issues warnings unless it can prove that the use is safe.

  • -Wstrict-flex-arrays (C and C++ only) — warns about improper usages of flexible array members according to the level of the strict_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. Note that -Wstrict-flex-arrays is effective only when level is greater than 0.
    • When level=1, warnings are issued for a trailing array reference of a structure that has 2 or more elements, if the trailing array is referenced as a flexible array member.
    • When level=2, in addition to level=1, extra 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, extra 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 way more effective when -ftree-vrp is active (that's the default for -O2 and above), but some warnings may still be diagnosed even without optimization. 🦋

  • -Wsuggest-attribute=attribute-name — warns for cases where adding an attribute might be a glow-up. 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

These warn about functions that might be candidates for the 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 can't prove the function returns normally. And a function returns normally if it doesn't contain an infinite loop and doesn't 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

These 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 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, only possible candidates. GCC guesses that format attributes might be appropriate for any function that calls a function like vprintf or vscanf — but that might not always be the case, and some functions for which format attributes are appropriate may not be detected. 😭

  • -Wsuggest-attribute=cold — warns about functions that might be candidates for the cold attribute. This is based on static detection and generally only warns about functions that always lead to a call to another cold function, like wrappers of C++ throw or fatal error reporting functions leading to abort. 🥶
  • -Walloc-size — warns about calls to allocation functions decorated with attribute alloc_size that specify insufficient size for the target type of the pointer the result is assigned to — including the built-in forms of aligned_alloc, alloca, calloc, malloc, and realloc. Not enough room is not the vibe. 🚫
  • -Walloc-zero — warns about calls to allocation functions decorated with attribute alloc_size that specify zero bytes, including the built-in forms of aligned_alloc, alloca, calloc, malloc, and realloc. Fr fr, because the behavior of these functions when called with a zero size differs among implementations (and in the case of realloc has been deprecated), relying on it may result in subtle portability bugs and should be avoided. 💀
  • -Wcalloc-transposed-args — warns about calls to allocation functions decorated with attribute alloc_size with two arguments, which use the sizeof operator as the earlier size argument and don't use it as the later size argument. This one's a coding style warning, bestie. The first argument to calloc is documented to be the number of elements in the array, while the second argument is the size of each element — so calloc (n, sizeof (int)) is preferred over calloc (sizeof (int), n). If having sizeof in the earlier argument and not the latter is intentional, you can suppress the warning by using calloc (sizeof (struct S) + 0, n) or calloc (1 * sizeof (struct S), 4), or just use sizeof in the later argument as well. ✨
  • -Walloc-size-larger-than=byte-size — warns about calls to functions decorated with attribute alloc_size that 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 of PTRDIFF_MAX on the target. Note that -Walloc-size-larger-than=PTRDIFF_MAX is enabled by default. Warnings controlled by the option can be disabled either by specifying byte-size of SIZE_MAX or more, or by -Wno-alloc-size-larger-than.
  • -Wno-alloc-size-larger-than — disables -Walloc-size-larger-than= warnings. This option is equivalent to -Walloc-size-larger-than=SIZE_MAX or larger. 🙅‍♀️
  • -Walloca — this option warns on all uses of alloca in the source. All of them. Every single one. 👀
  • -Walloca-larger-than=byte-size — ok so bestie, this one warns you every time someone calls alloca with an integer argument that's either zero or just... not bounded by anything controlling it, i.e. nothing limits it to at most byte-size. It ALSO yells at you for alloca calls where the bound is literally unknown. And arguments that aren't integers? Considered unbounded, periodt — even if they look all constrained and well-behaved. 💅

For example, a bounded (good) case of alloca could look like:

void func (size_t n)
{
  void *p;
  if (n <= 1000)
    p = alloca (n);
  else
    p = malloc (n);
  f (p);
}

In the example above, passing -Walloca-larger-than=1000 would not issue a warning, because the call to alloca is known to be at most 1000 bytes. Slay. But 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. Like this:

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, not because of the size itself.

Note: 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. no cap.

-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 — turns off the -Walloca-larger-than= warnings entirely. It's equivalent to -Walloca-larger-than=SIZE_MAX or larger. just... nothing to see here 💅
  • -Warith-conversion — this one makes the compiler warn about implicit conversions from arithmetic operations even when converting the operands to the same type cannot change their values. It affects warnings from -Wconversion, -Wfloat-conversion, and -Wsign-conversion. lowkey pedantic but iconic
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 — warns about out-of-bounds subscripts or offsets into arrays. Enabled by -Wall. It hits way harder when -ftree-vrp is active (the default for -O2 and above), but a subset of instances get issued even without optimization. ✨

By default, the trailing array of a structure will be 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 an one-element array ([1]). As a result, out-of-bounds subscripts or offsets into zero-length arrays or one-element arrays are not warned by default. ngl that's the part everyone forgets 💀

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 to level=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-bounds and is enabled by -Wall; higher levels are not, and must be explicitly requested. 👑

-Warray-bounds=2

This warning level also warns about the intermediate results of pointer arithmetic that may yield out-of-bounds values. Fair warning bestie: this level may give a larger number of false positives and is 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, as in C++ such initializations are an error. 🎀

  • -Warray-compare — warns about equality and relational comparisons between two operands of array type. This comparison was deprecated in C++20. Like:
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-alias option 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-alias also 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-attributes option, 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 flip left-to-right writing direction into right-to-left (and vice versa), which can cause confusion between the logical order and visual order. This may be genuinely dangerous — for instance, it may seem that a piece of code is not commented out, whereas it in fact is. 💀 scary stuff, bestie

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 a 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

Okay so this one? It's calling out suspicious operations on boolean-type expressions. Like bestie, bitwise negation of a boolean is very likely a bug in the program — that's not a vibe, that's a cry for help. 💅 And for C, this warning also catches 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 — so like, read the room.)

This warning is enabled by -Wall. 🫶

-Wduplicated-branches

Warns when an if-else has identical branches. Like, the way that both paths do the exact same thing? Pick a lane, babe. It detects cases like:

if (p != NULL)
  return 0;
else
  return 0;

It doesn't warn when both branches contain just a null statement (no drama there). 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) { ... }

Not the same check twice, queen 😭

-Wframe-address

Warns when __builtin_frame_address or __builtin_return_address is called with an argument greater than 0. 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.

Bestie, heads up: 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-cast and -Wno-pointer-to-int-cast may be used. 🦋

Again: 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=int-conversion.

This warning is upgraded to an error by -pedantic-errors.

-Wzero-as-null-pointer-constant

Warns when a literal 0 is used as null pointer constant. Like girl, we have NULL at home. 🌸

-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. 🙈

For example, the first two stores in function bad are diagnosed because the array elements overlap the subsequent members b and c. The third store is diagnosed by -Warray-bounds because it is 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
}

Option -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 is not warned about, as it can be a legitimate way of obtaining infinities and NaNs. 🧚 Math said what it said.

-Wsystem-headers

Prints warning messages for constructs found in system header files. Warnings from system headers are normally suppressed, on the assumption that they usually do not indicate real problems and would only make the compiler output harder to read. Using this command-line option tells GCC to emit warnings from system headers as if they occurred in user code. However — note this — using -Wall in conjunction with this option does not warn about unknown pragmas in system headers; for that, -Wunknown-pragmas must also be used. 💅

-Wtautological-compare

Warns if a self-comparison always evaluates to true or false. This warning detects various mistakes such as:

int i = 1;
...
if (i > i) { ... }

This warning also warns about bitwise comparisons that always evaluate to true or false, for instance:

if ((a & 16) == 10) { ... }

will always be false. 😭 (Not her comparing herself to herself.)

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-whitespace is equivalent to -Wtrailing-whitespace=blanks and warns just about trailing space and horizontal tab characters. -Wtrailing-whitespace=any warns about those or trailing form feed or vertical tab characters. -Wno-trailing-whitespace or -Wtrailing-whitespace=none disables 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), no warnings are emitted inside of raw string literals. Warnings are also emitted for leading whitespace inside of multi-line comments. -Wleading-whitespace=spaces warns about leading whitespace other than spaces for projects which want to indent just by spaces. -Wleading-whitespace=tabs warns about leading whitespace other than horizontal tabs for projects which want to indent just by horizontal tabs. -Wleading-whitespace=blanks warns 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=none disables the warning, which is the default. This is a coding style warning. 🎀

-Wtrampolines

Warns about trampolines generated for pointers to nested functions. A trampoline is a small piece of data or code that is created at run time on the stack when the address of a nested function is taken, and is used to call the nested function indirectly. For some targets, it is made up of data only and thus requires no special treatment. But, for most targets, it is made up of code and thus requires the stack to be made executable in order for the program to work properly. 😬 Executable stack = not the serve.

-Wfloat-equal

Warns if floating-point values are used in equality comparisons.

The idea behind this is that sometimes it is convenient (for the programmer) to consider floating-point values as approximations to infinitely precise real numbers. If you are doing this, then you need to compute (by analyzing the code, or in some other way) the maximum or likely maximum error that the computation introduces, and allow for it when performing comparisons (and when producing output, but that's a different problem). In particular, instead of testing for equality, you should check to see whether the two values have ranges that overlap; and this is done with the relational operators, so equality comparisons are probably mistaken. 💖

Like, == on floats is giving "trust me bro". Don't.

-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 -Wtraditional warns about directives that traditional C understands but ignores because the # does not appear as the first character on the line. It also suggests you hide directives like #pragma not understood by traditional C by indenting them. Some traditional implementations do not recognize #elif, so this option suggests avoiding it altogether.
  • A function-like macro that appears without arguments.
  • The unary plus operator.
  • The U integer constant suffix, or the F or L floating-point constant suffixes. (Traditional C does support the L suffix on integer constants.) Note, these suffixes appear in macros defined in the system headers of most modern systems, e.g. the _MIN/_MAX macros in <limits.h>. Use of these macros in user code might normally lead to spurious warnings, however GCC's integrated preprocessor has enough context to avoid warning in these cases.
  • A function declared external in one block and then used after the end of the block.
  • A switch statement has an operand of type long.
  • A non-static function declaration follows a static one. This construct is not accepted by some traditional C compilers.

Periodt. 🫶

  • The ISO type of an integer constant has a different width or signedness from its traditional type. This warning only fires if the base of the constant is ten, iykyk. Hex 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, so yeah, that's a whole thing. 💅
  • 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, no cap. This is a subset of the possible conversion warnings; for the full set use -Wtraditional-conversion.
  • Use of ISO C style function definitions. This warning intentionally is not issued for prototype declarations or variadic functions because these ISO C features appear in your code when using libiberty's traditional C compatibility macros, "PARAMS" and "VPARAMS". This warning is also bypassed for nested functions because that feature is already a GCC extension and thus not relevant to traditional C compatibility.
  • -Wtraditional-conversion (C and Objective-C only) — Warn if a prototype causes a type conversion that is different from what would happen to the same argument in the absence of a prototype. This includes conversions of fixed point to floating and vice versa, and conversions changing the width or signedness of a fixed-point argument except when the same as the default promotion. 🧚
  • -Wdeclaration-after-statement (C and Objective-C only) — Warn when a declaration is found after a statement in a block. This construct, known from C++, was introduced with ISO C99 and is by default allowed in GCC. It is not supported by ISO C90.

This warning is upgraded to an error by -pedantic-errors. 💀

  • -Wshadow — Warn whenever a local variable or type declaration shadows another variable, parameter, type, class member (in C++), or instance variable (in Objective-C), or whenever a built-in function is shadowed. Note that in C++, the compiler warns if a local variable shadows an explicit typedef, but not if it shadows a struct/class/enum. If this warning is enabled, it includes also all instances of local shadowing. This means that -Wno-shadow=local and -Wno-shadow=compatible-local are ignored when -Wshadow is used. Same as -Wshadow=global. 👑
  • -Wno-shadow-ivar (Objective-C only) — Do not warn whenever a local variable shadows an instance variable in an Objective-C method. 🙈
  • -Wshadow=global — Warn for any shadowing. Same as -Wshadow. 🌈
  • -Wshadow=local — Warn when a local variable shadows another local variable or parameter. 🌸
  • -Wshadow=compatible-local — Warn when a local variable shadows another local variable or parameter whose type is compatible with that of the shadowing variable. In C++, type compatibility here means the type of the shadowing variable can be converted to that of the shadowed variable. The creation of this flag (in addition to -Wshadow=local) is based on the idea that when a local variable shadows another one of incompatible type, it is most likely intentional, not a bug or typo, as shown in the following example:
for (SomeIterator i = SomeObj.begin(); i != SomeObj.end(); ++i)
{
  for (int i = 0; i < N; ++i)
  {
    ...
  }
  ...
}

Since the two variable "i" in the example above have incompatible types, enabling only -Wshadow=compatible-local does not emit a warning. Because their types are incompatible, if a programmer accidentally uses one in place of the other, type checking is expected to catch that and emit an error or warning. Use of this flag instead of -Wshadow=local can possibly reduce the number of warnings triggered by intentional shadowing. Note that this also means that shadowing "const char *i" by "char *i" does not emit a warning.

This warning is also enabled by -Wshadow=local. 💜

  • -Wlarger-than=byte-size — Warn whenever an object is defined whose size exceeds byte-size. -Wlarger-than=PTRDIFF_MAX is enabled by default. Warnings controlled by the option can be disabled either by specifying byte-size of SIZE_MAX or more or by -Wno-larger-than.

Also warn for calls to bounded functions such as "memchr" or "strnlen" that specify a bound greater than the largest possible object, which is PTRDIFF_MAX bytes by default. These warnings can only be disabled by -Wno-larger-than. 🔥

  • -Wno-larger-than — Disable -Wlarger-than= warnings. The option is equivalent to -Wlarger-than=SIZE_MAX or larger. ✨
  • -Wframe-larger-than=byte-size — Warn if the size of a function frame exceeds byte-size. The computation done to determine the stack frame size is approximate and not conservative. The actual requirements may be somewhat greater than byte-size even if you do not get a warning. In addition, any space allocated via "alloca", variable-length arrays, or related constructs is not included by the compiler when determining whether or not to issue a warning. -Wframe-larger-than=PTRDIFF_MAX is enabled by default. Warnings controlled by the option can be disabled either by specifying byte-size of SIZE_MAX or more or by -Wno-frame-larger-than. 🫶
  • -Wno-frame-larger-than — Disable -Wframe-larger-than= warnings. The option is equivalent to -Wframe-larger-than=SIZE_MAX or larger. 🎀
  • -Wfree-nonheap-object — Warn when attempting 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 to "stpcpy" returns a pointer to the terminating nul character and not to the beginning of the object, the call to "free" below is 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. Any space allocated via "alloca", variable-length arrays, or related constructs is included by the compiler when determining whether or not to issue a warning.

The message is in keeping with the output of -fstack-usage.

  • If the stack usage is fully static but exceeds the specified amount, it's: warning: stack usage is 1120 bytes
  • If the stack usage is (partly) dynamic but bounded, it's: warning: stack usage might be 1648 bytes
  • If the stack usage is (partly) dynamic and not bounded, it's: warning: stack usage might be unbounded

-Wstack-usage=PTRDIFF_MAX is enabled by default. Warnings controlled by the option can be disabled either by specifying byte-size of SIZE_MAX or more or by -Wno-stack-usage. 🦋

  • -Wno-stack-usage — Disable -Wstack-usage= warnings. The option is equivalent to -Wstack-usage=SIZE_MAX or larger. 🌸
  • -Wunsafe-loop-optimizations — Warn if the loop cannot be optimized because the compiler cannot assume anything on the bounds of the loop indices. With -funsafe-loop-optimizations warn if the compiler makes such assumptions. ⭐
  • -Wno-pedantic-ms-format (MinGW targets only) — When used in combination with -Wformat and -pedantic without GNU extensions, this option disables the warnings about non-ISO "printf" / "scanf" format width specifiers "I32", "I64", and "I" used on Windows targets, which depend on the MS runtime. 🎀
  • -Wpointer-arith — Warn about anything that depends on the "size of" a function type or of "void". GNU C assigns these types a size of 1, for convenience in calculations with "void *" pointers and pointers to functions. In C++, warn also when an arithmetic operation involves "NULL". This warning is also enabled by -Wpedantic.

This warning is upgraded to an error by -pedantic-errors. 💀

  • -Wno-pointer-compare — Do not warn if a pointer is compared with a zero character constant. This usually means that the pointer was meant to be dereferenced. For example: 🙈
const char *p = foo ();
if (p == '\0')
  return 42;

Note that the code above is invalid in C++11.

This warning is enabled by default. ✨

  • -Wno-tsan — Disable warnings about unsupported features in ThreadSanitizer.

ThreadSanitizer does not support "std::atomic_thread_fence" and 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, calling abs(3.14) triggers the warning because the appropriate function to call to compute the absolute value of a double argument is "fabs". 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 is 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. 🔥
  • -Wtrigraphs — this one's implied by -Wall 🫶. And if you didn't pass -Wall, it's still on unless trigraphs are enabled. Want trigraph conversion with no warning but still get all the other -Wall goodies? Hit it with -trigraphs -Wall -Wno-trigraphs. Slay.
  • -Wundef — warns when an undefined identifier gets evaluated inside an "#if" directive. Heads up: those identifiers get replaced with zero, so no cap, this can bite you 💀.
  • -Wexpansion-to-defined — warns whenever defined shows up in the expansion of a macro (yes, including the case where the macro is expanded by an #if directive). That usage is not portable, bestie. Also switched on by -Wpedantic and -Wextra.
  • -Wunused-macros — warns about macros defined in the main file that never get used. A macro counts as used if it's 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? Not warned about, periodt.

Note: if a macro is actually used but only inside skipped conditional blocks, the preprocessor still reports it as unused 😭. To dodge the warning, you could improve the scope of the macro's definition — for example, move it into the first skipped block. Or you could just provide a dummy use like this:
#if defined the_macro_causing_the_warning
#endif
  • -Wno-endif-labels — don't warn whenever an "#else" or an "#endif" are followed by text. This sometimes happens in older programs with code of the form:
#if FOO
...
#else FOO
...
#endif FOO

The second and third "FOO" should be in comments, ngl. This warning is on by default.

  • -Wbad-function-cast (C and Objective-C only) — warns when a function call is cast to a non-matching type. Like, warn if a call to a function returning an integer type gets cast to a pointer type. That's giving chaos 🦋.
  • -Wc90-c99-compat (C and Objective-C only) — warns about features not present in ISO C90 but present in ISO C99. For instance: variable length arrays, "long long" type, "bool" type, compound literals, designated initializers, and so on. This option is independent of the standards mode. Warnings are disabled in the expression that follows "__extension__".
  • -Wc99-c11-compat (C and Objective-C only) — warns about features not present in ISO C99 but present in ISO C11. For instance: anonymous structures and unions, "_Atomic" type qualifier, "_Thread_local" storage-class specifier, "_Alignas" specifier, "Alignof" operator, "_Generic" keyword, and so on. Also independent of the standards mode, and warnings are disabled in the expression that follows "__extension__".
  • -Wc11-c23-compat, -Wc11-c2x-compat (C and Objective-C only) — warns about features not present in ISO C11 but present in ISO C23. For instance: omitting the string in "_Static_assert", use of [[]] syntax for attributes, use of decimal floating-point types, and so on. Independent of the standards mode, warnings disabled in the expression that follows "__extension__". Heads up: the name -Wc11-c2x-compat is deprecated 🙈.

When not compiling in C23 mode, these warnings get upgraded to errors by -pedantic-errors. Not a vibe, but it's the rules.

  • -Wc23-c2y-compat (C and Objective-C only) — warns about features not present in ISO C23 but present in ISO C2Y. For instance: "_Generic" selecting with a type name instead of an expression. Independent of the standards mode, warnings disabled in the expression that follows "__extension__".
  • -Wc23-c2y-compat (C and Objective-C only) — yeah, it's listed twice in the original. Not my typo, bestie, I'm just the messenger 💅.

When not compiling in C2Y mode, these warnings are upgraded to errors by -pedantic-errors.

  • -Wc++-compat (C and Objective-C only) — warns about ISO C constructs that live outside the common subset of ISO C and ISO C++, e.g. a request for implicit conversion from "void *" to a pointer to a non-"void" type.
  • -Wc++11-compat (C++ and Objective-C++ only) — warns about C++ constructs whose meaning differs between ISO C++ 1998 and ISO C++ 2011, e.g. identifiers that are fine in ISO C++ 1998 but are keywords in ISO C++ 2011 👑. This warning turns on -Wnarrowing and is enabled by -Wall.
  • -Wc++14-compat (C++ and Objective-C++ only) — warns about C++ constructs whose meaning differs between ISO C++ 2011 and ISO C++ 2014. Enabled by -Wall.
  • -Wc++17-compat (C++ and Objective-C++ only) — warns about C++ constructs whose meaning differs between ISO C++ 2014 and ISO C++ 2017. Enabled by -Wall.
  • -Wc++20-compat (C++ and Objective-C++ only) — warns about C++ constructs whose meaning differs between ISO C++ 2017 and ISO C++ 2020. Enabled by -Wall.
  • -Wno-c++11-extensions (C++ and Objective-C++ only) — don't warn about C++11 constructs in code being compiled with an older C++ standard. Even without this option, some C++11 constructs only get diagnosed if -Wpedantic is used.
  • -Wno-c++14-extensions (C++ and Objective-C++ only) — don't warn about C++14 constructs in code being compiled with an older C++ standard. Even without this option, some C++14 constructs only get diagnosed if -Wpedantic is used.
  • -Wno-c++17-extensions (C++ and Objective-C++ only) — don't warn about C++17 constructs in code being compiled with an older C++ standard. Even without this option, some C++17 constructs only get diagnosed if -Wpedantic is used.
  • -Wno-c++20-extensions (C++ and Objective-C++ only) — don't warn about C++20 constructs in code being compiled with an older C++ standard. Even without this option, some C++20 constructs only get diagnosed if -Wpedantic is used.
  • -Wno-c++23-extensions (C++ and Objective-C++ only) — don't warn about C++23 constructs in code being compiled with an older C++ standard. Even without this option, some C++23 constructs only get diagnosed if -Wpedantic is used.
  • -Wno-c++26-extensions (C++ and Objective-C++ only) — don't warn about C++26 constructs in code being compiled with an older C++ standard. Even without this option, some C++26 constructs only get diagnosed if -Wpedantic is used.
  • -Wcast-qual — warns whenever a pointer gets cast in a way that removes a type qualifier from the target type. Like, warn if a "const char *" is cast to an ordinary "char *" 😭.

It also warns when you make a cast that introduces a type qualifier in an unsafe way. For example, casting "char **" to "const char **" is unsafe, like in this example:

/* p is char ** value.  */
const char **q = (const char **) p;
/* Assignment of readonly string to const char * is OK.  */
*q = "string";
/* Now char** pointer points to read-only memory.  */
**p = 'b';
  • -Wcast-align — warns whenever a pointer is cast such that the required alignment of the target is increased. For example, warn if a "char *" is cast to an "int *" on machines where integers can only be accessed at two- or four-byte boundaries 🔥.
  • -Wcast-align=strict — same deal: warns whenever a pointer is cast such that the required alignment of the target is increased — e.g. warn if a "char *" is cast to an "int *" regardless of the target machine. No exceptions, periodt.
  • -Wcast-function-type — warns when a function pointer is cast to an incompatible function pointer. In a cast involving function types with a variable argument list, only the types of the initial arguments that are actually provided are considered. Any parameter of pointer-type matches any other pointer-type. Any benign differences in integral types are ignored, like "int" vs. "long" on ILP32 targets. Likewise, type qualifiers are ignored. The function type "void (*) (void)" is special and matches everything, which you can use to suppress this warning ✨. In a cast involving pointer to member types, this warning fires whenever the type cast is changing the pointer to member type. This warning is enabled by -Wextra.
  • -Wcast-user-defined — warns when a cast to reference type does not involve a user-defined conversion that the programmer might expect to be called 👀:
struct A { operator const int&(); } a;
auto r = (int&)a; // warning

This warning is enabled by default.

  • -Wwrite-strings — when compiling C, gives string constants the type "const char[length]" so that copying the address of one into a non-"const" "char *" pointer produces a warning. These warnings help you find code at compile time that could try to write into a string constant — but only if you've been very careful about using "const" in declarations and prototypes. Otherwise it's just a nuisance, fr fr. That's why -Wall doesn't request these warnings.

When compiling C++, it warns about the deprecated conversion from string literals to "char *". This warning is enabled by default for C++ programs.

This warning gets upgraded to an error by -pedantic-errors in C++11 mode or later 😳.

  • -Wclobbered — warns for variables that might be changed by "longjmp" or "vfork". Also enabled by -Wextra.
  • -Wno-complain-wrong-lang — by default, language front ends complain when a command-line option is valid but not applicable to that front end. You can disable that with -Wno-complain-wrong-lang, which is mostly useful when invoking a single compiler driver for multiple source files written in different languages, for example:
$ g++ -fno-rtti a.cc b.f90

The driver g++ invokes the C++ front end to compile a.cc and the Fortran front end to compile b.f90. That latter front end diagnoses:

f951: Warning: command-line option '-fno-rtti' is valid for C++/D/ObjC++ but not for Fortran

which may be disabled with -Wno-complain-wrong-lang 🫶.

  • -Wcompare-distinct-pointer-types (C and Objective-C only) — warns if pointers of distinct types are compared without a cast. This warning is enabled by default 🌸.
  • -Wconversion — bestie, this one's the "hey wait, your value just shapeshifted"</p> warning ✨ it yells when there's an implicit conversion that could alter a value. That includes real ↔ integer conversions (think abs (x) when x is double 💀), signed ↔ unsigned (like unsigned ui = -1, which is giving chaos), and conversions to smaller types (like sqrtf (M_PI)). It does not warn for explicit casts like abs ((int) x) and ui = (unsigned) -1, or if the value isn't actually changed by the conversion, like in abs (2.0). Lowkey nice of it. Warnings about signed/unsigned conversions can be turned off with -Wno-sign-conversion, iykyk.

For C++, it also warns about confusing overload resolution for user-defined conversions, and conversions that never use a type conversion operator: conversions to void, the same type, a base class, or a reference to them. Warnings about signed/unsigned conversions are disabled by default in C++ unless -Wsign-conversion is explicitly enabled, bestie.

Warnings about conversion from arithmetic on a small type back to that type are only given with -Warith-conversion. No extra credit without the flag 💅.

  • -Wdangling-else — ok so picture this: there might be confusion about which if statement an else branch belongs to 😭 here's an example of that exact situation:
                   {
                     if (a)
                       if (b)
                         foo ();
                     else
                       bar ();
                   }

In C/C++, every else branch belongs to the innermost possible if statement, which in this example is if (b). This is often not what the programmer expected, as illustrated by the indentation the programmer chose (we've all been there 🫠). When there's potential for this confusion, GCC issues a warning when this flag is specified. To eliminate the warning, add explicit braces around the innermost if statement so there's no way the else can belong to the enclosing if. The resulting code looks like this:

                   {
                     if (a)
                       {
                         if (b)
                           foo ();
                         else
                           bar ();
                       }
                   }

This warning is enabled by -Wparentheses. Slay.

  • -Wdangling-pointer, -Wdangling-pointer=n — this one warns about uses of pointers (or C++ references) to objects with automatic storage duration after their lifetime has ended 💀 it 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. The warning is enabled at all optimization levels but may yield different results with optimization than without. Vibe check: unpredictable but always watching 👀
    • -Wdangling-pointer=1 — at level 1, the warning diagnoses only unconditional uses of dangling pointers.
    • -Wdangling-pointer=2 — at level 2, in addition to unconditional uses the warning also diagnoses conditional uses of dangling pointers.

The short form -Wdangling-pointer is equivalent to -Wdangling-pointer=2, while -Wno-dangling-pointer and -Wdangling-pointer=0 have the same effect of disabling the warnings. -Wdangling-pointer=2 is included in -Wall. Iconic.

This example triggers the warning at level 1; the address of the unnamed temporary is unconditionally referenced outside of its scope:

                   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;
                   }

In the following function, the store of the address of the local variable x in the escaped pointer *p triggers 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 example, the array a is out of scope when the pointer s is used. 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 macros __TIME__, __DATE__, or __TIMESTAMP__ are encountered, since they might prevent bit-wise-identical reproducible compilations. Reproducibility girlies, stay safe out there 💖
  • -Wempty-body — warns if an empty body occurs in an if, else, or do while statement. Like bestie, did you mean to do nothing?? 🙈 This warning is also enabled by -Wextra.
  • -Wno-endif-labels — do not warn about stray tokens after #else and #endif. Living that no-drama life ✨
  • -Wenum-compare — warns about a comparison between values of different enumerated types. In C++, enumerated type mismatches in conditional expressions are also diagnosed and the warning is enabled by default. In C, this warning is enabled by -Wall. Apples to oranges, queen 🍓
  • -Wenum-conversion — warns when a value of enumerated type is implicitly converted to a different enumerated type. This warning is enabled by -Wextra in C.
  • -Wenum-int-mismatch (C and Objective-C only) — warns about mismatches between an enumerated type and an integer type in declarations. 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. However, since the choice of the underlying type of an enumerated type is implementation-defined, such mismatches may cause portability issues 😬 in C++, such mismatches are an error. In C, this warning is enabled by -Wall and -Wc++-compat.

  • -Wjump-misses-init (C, Objective-C only) — warns if a goto statement or a switch statement jumps forward across the initialization of a variable, or jumps backward to a label after the variable has been initialized. This only warns about variables that are initialized when they're declared. Also, this warning is only supported for C and Objective-C; in C++ this sort of branch is an error in any case. Not me jumping over your initialization 😭

-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 when the signed value is converted to unsigned. In C++, this warning is also enabled by -Wall. In C, it is also enabled by -Wextra. The classic "why is this always true" moment 💀
  • -Wsign-conversion — warns for implicit conversions that may change the sign of an integer value, like assigning a signed integer expression to an unsigned integer variable. An explicit cast silences the warning. In C, this option is enabled also by -Wconversion. Periodt 💅
  • -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. It warns, e.g., about:
                   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. Floats losing pixels, honestly 🫶
  • -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 such a case, the computation will not yield the number of elements in the array, which is probably what you intended bestie 😭 it warns, e.g., about:
                   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 the array size but won't work out correctly with pointers. It warns, e.g., about sizeof (ptr) / sizeof (ptr[0]) if ptr is not an array but a pointer 💀 this warning is enabled by -Wall.
  • -Wsizeof-pointer-memaccess — warns for suspicious length parameters to certain string and memory built-in functions if the argument uses sizeof. This warning triggers, for example, for memset (ptr, 0, sizeof (ptr)); if ptr is not an array but a pointer, and suggests a possible fix; or about memcpy (&foo, ptr, sizeof (&foo));. It 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. For example, in the following function the call to strncat specifies the size of the source string as the bound. That is almost certainly a mistake and so the call is 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. Roasted by the compiler, respectfully 🔥

  • -Wno-sizeof-array-argument — do not warn when the sizeof operator is applied to a parameter that is declared as an array in a function definition. This warning is enabled by default for C and C++ programs.
  • -Wmemset-elt-size — warns for suspicious calls to the memset built-in function if 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 indicates the user has omitted a multiplication by the element size. Bestie, you forgot to multiply 😭 this warning is enabled by -Wall.
  • -Wmemset-transposed-args — bestie, this one's the snitch for suspicious memset calls 💅 it fires when the second argument isn't zero while the third argument is zero. Like, the call memset (buf, sizeof buf, 0) gets diagnosed because you obviously meant memset (buf, 0, sizeof buf) — the args got flipped, no cap 😭 and here's the ✨fine print✨: the diagnostic is only emitted if that third argument is a literal zero. If it's an expression that gets folded to zero, or a cast of zero to some type, it's way less likely the args were mistakenly transposed, so no warning, periodt. Enabled by -Wall.
  • -Waddress — warns you about suspicious uses of address expressions 🫶 these include comparing the address of a function or a declared object to the null pointer constant, like so:
        void f (void);
        void g (void)
        {
          if (!f)   // warning: expression evaluates to false
            abort ();
        }

Also comparisons of a pointer to a string literal, like this one:

        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:

        void f (const int *p, int i)
        {
          return p + i == NULL;
        }

Such uses 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 you forgot parentheses in a function call or forgot a dereference in an array expression, iykyk. 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 lowkey suggest you meant to call strcmp. 🚨 The warning is suppressed if the suspicious expression is the result of macro expansion. -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 🧚 this includes using logical operators in contexts where a bit-wise operator is likely expected. 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 🎀 it does not warn if the right operand is considered to be a boolean expression. Its whole purpose is detecting suspicious code like this:
        int a;
        ...
        if (!a > 1) { ... }

You can suppress the warning by wrapping the LHS into parentheses:

        if ((!a) > 1) { ... }

Enabled by -Wall ✨

  • -Waggregate-return — warns if any functions that return structures or unions are defined or called 🌸 (and in languages where you can return an array, this also elicits a warning).
  • -Wno-aggressive-loop-optimizations — do not warn if the compiler detects undefined behavior in a loop with a constant number of iterations 💀 -Waggressive-loop-optimizations is enabled by default.
  • -Wno-attributes — do not warn if an unexpected __attribute__ is used, like unrecognized attributes, function attributes applied to variables, etc. 🙈 this does not stop errors for incorrect use of supported attributes, just so we're clear.

Warnings about ill-formed uses of standard attributes are 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();

And you can disable the warning about all attributes in a namespace using -Wno-attributes=vendor::, which prevents warnings about both of these declarations:

        [[vendor::safe]] void f();
        [[vendor::unsafe]] void f2();
Heads up bestie: -Wno-attributes= does not imply -Wno-attributes. 💅
  • -Wno-builtin-declaration-mismatch — warns if a built-in function is declared with an incompatible signature or as a non-function, or when a built-in function declared with a type that doesn't include a prototype is called with arguments whose promoted types don't match what the function expects 😭 with -Wextra, it also warns when a built-in function that takes arguments is declared without a prototype. The -Wbuiltin-declaration-mismatch warning is enabled by default. To dodge the warning, include the appropriate header so the prototypes of built-in functions come into scope 🫶

Like, the call to memset below gets diagnosed because the function expects a value of type size_t as its argument but the type of 32 is int 💀 with -Wextra, the declaration of the function is diagnosed as well:

        extern void* memset ();
        void f (void *d)
        {
          memset (d, '\0', 32);
        }
  • -Wno-builtin-macro-redefined — do not warn if certain built-in macros are redefined 🎀 this suppresses warnings for redefinition of __TIMESTAMP__, __TIME__, __DATE__, __FILE__, and __BASE_FILE__.
  • -Wfree-labels (C and Objective-C only) — warns if a label is applied to a non-statement, or occurs 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. And it's turned into an error if building for a C version before C23 by -pedantic-errors.

  • -Wheader-guard — warns if a valid preprocessor header multiple inclusion guard has a #define directive right after #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 empty definition. In such a case it's often just a misspelled guard name — either in the #ifndef or #if !defined directive or in the subsequent #define directive 🙈 enabled by -Wall.
  • -Wstrict-prototypes (C and Objective-C only) — warns if a function is declared or defined without specifying the argument types 💅 (an old-style function definition is permitted without a warning if preceded by a declaration that specifies the argument types).
  • -Wold-style-declaration (C and Objective-C only) — warns for obsolescent usages, according to the C Standard, in a declaration 🌀 for example, warns if storage-class specifiers like static aren't the first things in a declaration. Also enabled by -Wextra.
  • -Wold-style-definition (C and Objective-C only) — warns if an old-style function definition is used ✨ the warning happens 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.
  • -Wdeprecated-non-prototype (C and Objective-C only) — warns 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.

Also enabled by -Wc11-c23-compat.

  • -Wmissing-parameter-name (C and Objective-C only) — warns 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.

Also enabled by -Wc11-c23-compat. It gets turned into an error if 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) { }

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 💀 and 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.

  • -Wmissing-prototypes (C and Objective-C only) — warns if a global function is defined without a previous prototype declaration ✨ the warning shows up even if the definition itself provides a prototype. Use this one to detect global functions that don't have a matching prototype declaration in a header file 🫶 and ngl, 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-declarations to detect missing declarations in C++.
  • -Wmissing-variable-declarations (C and Objective-C only) — warns if a global variable is defined without a previous declaration 🌸 use this one to detect global variables that don't have a matching extern declaration in a header file.
  • -Wmissing-declarations — bestie, this one yells at you when a global function gets defined but nobody ever declared it first. And yeah, it still clocks it even when the definition itself hands you a prototype. The whole point is catching global functions that never made it into a header file. Quick nuance tho: in C, no warnings get thrown for functions with previous non-prototype declarations — for those you want -Wmissing-prototypes. In C++, zero warnings for function templates, or inline functions, or functions hanging out in anonymous namespaces. 💅
  • -Wmissing-field-initializers — this one side-eyes a struct initializer that left some fields out. Like, in this code x.h is just implicitly zero, so you get the warning:
struct s { int f, g, h; };
struct s x = { 3, 4 };

In C, this option does not warn about designated initializers, so this little glow-up below stays completely unbothered:

struct s { int f, g, h; };
struct s x = { .f = 3, .g = 4 };

In C, it also doesn't warn about the universal zero initializer { 0 }:

struct s { int f, g, h; };
struct s x = { 0 };

Same energy in C++ — the empty { } initializer doesn't trigger it either, like:

struct s { int f, g, h; };
s x = { };

Heads up: this warning is included in -Wextra. So if you want the other -Wextra warnings without this one, use -Wextra -Wno-missing-field-initializers. 🎀

  • -Wno-missing-requires — okay so by default the compiler warns when a concept-id shows up as a C++20 simple-requirement:
bool satisfied = requires { C<T> };

Here satisfied will be true if C<T> is a valid expression — which, plot twist, it is for all T. So like, presumably the user meant to write:

bool satisfied = requires { requires C<T> };

...so that satisfied is only true if concept C is satisfied for type T. Yeah. You can turn this warning off with -Wno-missing-requires. ✨

  • -Wno-missing-template-keyword — the member access tokens ., -> and :: must be followed by the template keyword when the parent object is dependent and the member being named is a template. No skips:
template <class X>
void DoStuff (X x)
{
  x.template DoSomeOtherStuff<X>(); // Good.
  x.DoMoreStuff<X>(); // Warning, x is dependent.
}

In rare cases you can get false positives. To silence those, wrap the expression in parentheses. Like, 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.
}

This warning can be disabled with -Wno-missing-template-keyword. 🦋

  • -Wno-multichar — do not warn if a multicharacter constant ('FOOF') is used. Usually those are just a typo in the user's code, since they have implementation-defined values, and honestly they should not be used in portable code. Periodt. 💀
  • -Wnormalized=[none|id|nfc|nfkc] — okay, storytime. In ISO C and ISO C++, two identifiers are different if they're different sequences of characters. But sometimes, when you use characters outside the basic ASCII character set, you can end up with two different character sequences that look the same. To avoid the confusion, the ISO 10646 standard sets out some normalization rules which, when applied, ensure that two sequences that look the same get turned into the same sequence. GCC can warn you if you're using identifiers that haven't been normalized, and this option controls that warning. 👑

There are four levels of warning supported by GCC. The default is -Wnormalized=nfc, which warns about any identifier that's not in the ISO 10646 "C" normalized form, NFC. NFC is the recommended form for most uses. It's equivalent to -Wnormalized.

Unfortunately, there are some characters allowed in identifiers by ISO C and ISO C++ that, when turned into NFC, are not allowed in identifiers. Which is to say, there's no way to use these symbols in portable ISO C or C++ and have all your identifiers in NFC. -Wnormalized=id suppresses the warning for these characters. It is hoped that future versions of the standards involved will correct this — which is exactly why this option is not the default.

You can switch the warning off for all characters by writing -Wnormalized=none or -Wno-normalized. 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 these into a standard form as well, 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 it's 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 — do not warn about usage of functions declared with the warning attribute. By default this warning is enabled. -Wno-attribute-warning can be used to disable the warning, or -Wno-error=attribute-warning can be used to disable the error when compiled with the -Werror flag. 🫶
  • -Wno-deprecated — do not warn about usage of deprecated features. Fr fr, no commentary on the old stuff. 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. 😭
  • -Wno-deprecated-declarations — do not warn about uses of functions, variables, and types marked as deprecated by using the deprecated attribute. Respectfully, they told us, and we're moving on.
  • -Wno-overflow — do not warn about compile-time overflow in constant expressions. The math is not mathing and we're staying quiet about it. 💜
  • -Wno-odr — warn about One Definition Rule violations during link-time optimization. Enabled by default. 🔥
  • -Wopenacc-parallelism — warn about potentially suboptimal choices related to OpenACC parallelism. Not a vibe check fail, just a heads-up that you could slay harder. ⭐
  • -Wno-openmp — warn about suspicious OpenMP code. 👀
  • -Wopenmp-simd — warns you if the vectorizer cost model overrides the OpenMP simd directive set by the user. The -fsimd-cost-model=unlimited option can be used to relax the cost model. 🧚
  • -Woverride-init (C and Objective-C only) — warn if an initialized field without side effects is overridden when using designated initializers. This warning is included in -Wextra, so to get the other -Wextra warnings without this one, use -Wextra -Wno-override-init. 🌈
  • -Wno-override-init-side-effects (C and Objective-C only) — do not 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 basically no benefit — it's giving effort with zero payoff. For instance, in this code, the variable f.x in struct bar is misaligned even though struct bar doesn't 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 — history lesson: the 4.1, 4.2 and 4.3 series of GCC ignore the packed attribute on bit-fields of type char. 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's no longer a 4-bit padding between field a and b in this structure:
struct foo
{
  char a:4;
  char b:8;
} __attribute__ ((packed));

This warning is enabled by default. Use -Wno-packed-bitfield-compat to disable this warning. 💅

  • -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 on struct 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. Decluttering era. ✨
  • -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. Say it once, bestie, we heard you the first time. 😭
  • -Wrestrict — warn when an object referenced by a restrict-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 the strcpy function below attempts to truncate the string by replacing its initial characters with the last four. However, because the call writes the terminating NUL into a[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's included in -Wall. 🦋

  • -Wnested-externs (C and Objective-C only) — warn if an extern declaration is encountered within a function. Like, why is that in there? 🙈
  • -Winline — warns you when a function you declared inline just… doesn't get inlined bestie 😭. Fair warning tho: even with this on, the compiler stays quiet about failures to inline functions declared in system headers. Rude but true.

The compiler is out here using a whole buffet of heuristics to decide whether to inline something — like how big the function is, and how much inlining it's already done in the current function. So basically, teeny tiny "insignificant" changes in your source can make -Winline warnings appear or vanish. It's giving chaotic, ngl 💅.

  • -Winterference-size — warns you about using C++17 std::hardware_destructive_interference_size without telling it a value via --param destructive-interference-size. Also side-eyes questionable values for that option 👀.

So this variable is meant for controlling class layout to dodge false sharing in concurrent code, like:

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 meant to be far enough apart that storing to one won't force accesses to two to reload the cache line. Obsessed with that for them 💖.

By default, --param destructive-interference-size and --param constructive-interference-size get set based on whatever -mtune option you're rocking — typically the L1 cache line size for the target CPU, sometimes a range if you're tuning for a generic target. So every translation unit that depends on ABI compatibility for these variables must be compiled with the same -mtune (or -mcpu). No mixing and matching, bestie 🚫.

If ABI stability matters to you — like, say, the use is in a library header — you should probs just not use the hardware interference size variables at all. Or alternatively, force a particular value with --param.

And if you're confident your use doesn't affect ABI outside a single build of your project, you can turn the warning off with -Wno-interference-size. Slay, your call 🫶.

  • -Wint-in-bool-context — clocks suspicious use of integer values where booleans are expected 😳. Like conditional expressions (?:) using non-boolean integer constants in a boolean context, e.g. if (a <= b ? 2 : 3). Or left shifting signed integers in a boolean context, like for (a = 0; 1 << a; a++);. Same energy for all kinds of multiplications regardless of data type. This one is enabled by -Wall ✨.
  • -Wno-int-to-pointer-cast — suppresses warnings from casts to pointer type of an integer of a different size. In C++, casting to a pointer type of smaller size is straight-up an error, no cap. Wint-to-pointer-cast is enabled by default.
  • -Wno-pointer-to-int-cast (C and Objective-C only) — suppresses warnings from casts from a pointer to an integer type of a different size 🙈.
  • -Winvalid-pch — warns if a precompiled header is sitting in the search path but cannot be used. It's giving "found the thing, can't use the thing" 💀.
  • -Winvalid-utf8 — warns if an invalid UTF-8 character is found. This warning is on by default for C++23 if -finput-charset=UTF-8 is used, and gets turned into an error with -pedantic-errors 🔥.
  • -Wno-unicode — don't diagnose invalid forms of delimited or named escape sequences which are treated as separate tokens. Wunicode is enabled by default ✨.
  • -Wlong-long — warns if the long long type is used. Enabled by either -Wpedantic or -Wtraditional in ISO C90 and C++98 modes. To inhibit the warning messages, use -Wno-long-long.

This warning is 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 -Wpedantic or -Wtraditional. To inhibit the warning messages, use -Wno-variadic-macros.
  • -Wno-varargs — don't warn upon questionable usage of the macros used to handle variable arguments like va_start. These warnings are enabled by default 👑
  • -Wvector-operation-performance — warns if a vector operation is not implemented via the SIMD capabilities of the architecture. Mainly useful for performance tuning 🏎️. A vector operation can be implemented:
    • "piecewise" — the scalar operation is performed on every vector element
    • "in parallel" — the vector operation is implemented using scalars of wider type, which normally is more performance efficient 💅
    • "as a single scalar" — the vector fits into a scalar type
  • -Wvla — warns if a variable-length array is used in the code. -Wno-vla prevents the -Wpedantic warning of the variable-length array.

This warning is upgraded to an error by -pedantic-errors 💀.

  • -Wvla-larger-than=byte-size — if this option is used, the compiler warns for declarations of variable-length arrays whose size is either unbounded, or bounded by an argument that allows the array size to exceed byte-size bytes. Similar to how -Walloca-larger-than=byte-size works, but with variable-length arrays 🦋.

Note that GCC may optimize small variable-length arrays of a known value into plain arrays, so this warning may not get triggered for such arrays. Sneaky 🧚.

-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. The option is equivalent to -Wvla-larger-than=SIZE_MAX or 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 such parameters by warnings such as -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 the following redeclarations because the first one allows an array of any size to be passed to f, while the second one specifies that 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 as well:

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. 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 appears the code meant exponentiation. Specifically, the warning occurs 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. Not the same thing bestie 😭.

This warning can be silenced 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 doesn't generally mean anything'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. It's giving "I would but I'm tired" 💀.
  • -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 -Wall and by -Wpedantic, which can be disabled with -Wno-pointer-sign.

This warning is upgraded to an error by -pedantic-errors 🔥.

  • -Wstack-protector — this one is only active when -fstack-protector is active. It warns about functions that are not protected against stack smashing 🛡️.
  • -Woverlength-strings — warns about string constants that are 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 doesn't specify a normative minimum maximum, so we don't 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 does not have a suffix. When used together with -Wsystem-headers it warns about such constants in system header files. This can be useful when preparing code to use with the FLOAT_CONST_DECIMAL64 pragma from the decimal floating-point extension to C99 ✨.
  • -Wno-lto-type-mismatch — during link-time optimization, don't warn about type mismatches in global declarations from different compilation units. Requires -flto to 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 has been marked with the designated_init attribute 🙈.

✨ Options That Control Static Analysis ✨

  • -fanalyzer — bestie, this one turns on a whole static analysis of your program flow 🔍 it goes looking for "interesting" interprocedural paths through the code and pops warnings for problems it finds on them.

Heads up though: this analysis is much more expensive than other GCC warnings. like, way more 💸

In technical terms (ooh fancy), it performs coverage-guided symbolic execution of the code being compiled. And it's neither sound nor complete: it can have false positives and false negatives. It's a bug-finding tool, not a tool for proving your program is correct. periodt.

The analyzer is only suitable for use on C code in this release. C only, no cap.

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-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-literal
🫶 This option is only available if GCC was configured with analyzer support enabled. no support, no analyzer, that's the rule.
  • -Wanalyzer-symbol-too-complex — so if -fanalyzer is enabled, the analyzer uses various heuristics to try and track the state of memory, but sufficiently complicated code can absolutely defeat them 💀

By default, the analysis silently stops tracking values of expressions once they exceed the threshold defined by --param analyzer-max-svalue-depth=value, and falls back to an imprecise representation for those expressions. The -Wanalyzer-symbol-too-complex option is the one that warns you when this happens. so you're not left wondering, yk?

  • -Wanalyzer-too-complex — same deal, but for control flow and data flow: if -fanalyzer is enabled, the analyzer uses various heuristics to attempt to explore the control flow and data flow in the program, and again, sufficiently complicated code can defeat them 😭

By default, the analysis silently stops if the code is too complicated for the analyzer to fully explore and it reaches an internal limit. The -Wanalyzer-too-complex option warns if this occurs. iconic of it to tell you, honestly.

  • -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). math girlies stay winning.

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-check to disable it.

This diagnostic warns for paths through the code in which a pointer is checked for "NULL" after it has already been dereferenced 🙈 which kinda suggests the pointer could have been NULL the whole time. 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.

💀 ngl this diagnostic is unlikely to be reported when any level of optimization is enabled, because GCC's optimization logic will typically consider such checks for NULL as being redundant and optimize them away before the analyzer even "sees" them. So if you're trying to trigger this one, disable optimization. iykyk.
  • -Wno-analyzer-double-fclose — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-double-fclose to disable it.

This diagnostic warns for paths through the code in which a "FILE *" can have fclose called on it more than once. once was enough bestie, we promise.

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-free to 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-file to disable it.

This diagnostic warns for paths through the code in which a security-sensitive value is written to an output file. like... writing a password to a log file?? ma'am 😭

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 -fanalyzer and the use of a plugin to specify a function that copies across a "trust boundary". Use -Wno-analyzer-exposure-through-uninit-copy to disable it.

This diagnostic warns for "infoleaks" — paths through the code in which uninitialized values are copied across a security boundary. think: code within an OS kernel that copies a partially-initialized struct on the stack to user space. not the vibe 🚨

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-mismatch to disable it.

This diagnostic warns for paths through code in which a read on a write-only file descriptor is attempted, or vice versa. pick a lane babe.

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-close to 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-leak to disable it.

This diagnostic warns for paths through code in which an open file descriptor is leaked. close ur fds, babes 🚪

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-mismatch to 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. it's giving trying to skip a step ✨

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-mismatch to 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. wrong fd, wrong energy 🫠

  • -Wno-analyzer-fd-use-after-close — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-fd-use-after-close to 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-check to disable it.

This diagnostic warns for paths through code in which a file descriptor is used without being checked for validity. like bestie, did u even verify 😭

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-leak to 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 — okay so this warning is a -fanalyzer girlie: -fanalyzer enables it, and -Wno-analyzer-free-of-non-heap disables it.

It warns on paths through your code where free gets called on a non-heap pointer — think an on-stack buffer, or a global. 💀 That's a no from the memory allocator, bestie.

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 needs -fanalyzer to be enabled, and -Wno-analyzer-imprecise-fp-arithmetic turns it off.

It warns on paths through the code where floating-point arithmetic is used in places that need precise computation. At the moment, it only warns on use of floating-point operands inside the calculation of an allocation size. 🎀

  • -Wno-analyzer-infinite-loop — this warning requires -fanalyzer, which enables it; -Wno-analyzer-infinite-loop disables it.

This diagnostic warns on paths through the code that 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 this warning gets emitted is when there's an execution path through a loop where taking that path on one iteration implies the same path will be taken on all subsequent iterations. iykyk, that's the loop equivalent of never leaving the group chat.

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 about the above case because if opcode ever matches none of the cases, the switch will follow the implicit default case. That makes the body of the loop a no-op with cpu_state.pc unchanged, so the same value of opcode gets used on all subsequent iterations — leading to an infinite loop. 😭

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-recursion to disable it.

This diagnostic warns on paths through the code that appear to lead to infinite recursion. 🪞

Specifically, when the analyzer sees a recursive call, it compares the state of memory at the entry to the new frame with the state at the entry to the previous frame of that function on the stack. The warning is issued if nothing in memory appears to be changing. Any changes observed to parameters or globals are assumed to lead to termination of the recursion, and that suppresses the warning.

This diagnostic is likely to miss cases of infinite recursion that are converted to iteration by the optimizer before the analyzer sees them. So optimization should be disabled when you're trying to trigger this diagnostic. fr fr, turn optimizations off for this vibe check.

Compare with -Winfinite-recursion, which provides 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-null to disable it.

This diagnostic warns on paths through the code where a NULL function pointer is called. Absolutely not the main character energy we're going for.

  • -Wno-analyzer-malloc-leak — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-malloc-leak to disable it.

This diagnostic warns on paths through the code where 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-deallocation to disable it.

This diagnostic warns on paths through the code where the wrong deallocation function is called on a pointer value, based on which function was used to allocate that pointer value. It warns about mismatches between free, scalar delete and vector delete[], and those marked as allocator/deallocator pairs using attribute malloc. Matching your allocator to your deallocator is literally the bare minimum, bestie.

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-bounds to disable it.

This diagnostic warns on paths through the code where a buffer is definitely read or written out-of-bounds. It applies to cases where the analyzer can determine a constant offset and to accesses past the end of a buffer, also with 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 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. It's giving forensic evidence, but make it ASCII. 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-buffers to disable it.

This diagnostic warns on paths through the code where 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:

  • memcpy
  • strcat
  • strcpy

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-argument to disable it.

This diagnostic warns on paths through the code where a possibly-NULL value is passed to a function argument marked with __attribute__((nonnull)) as requiring a non-NULL value. Like, the function said "no NULLs allowed" and you showed up with a maybe-NULL. Rude.

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-dereference to disable it.

This diagnostic warns on paths through the code where a possibly-NULL value is dereferenced. Dereferencing a maybe-NULL is not a slay, it's a scream.

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-argument to disable it.

This diagnostic warns on paths through the code where a value known to be NULL is passed to a function argument marked with __attribute__((nonnull)) as requiring a non-NULL value. Known NULL? Bestie, that's not a warning, that's a prophecy.

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-dereference to disable it.

This diagnostic warns on paths through the code where 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-var to disable it.

This diagnostic warns on paths through the code where a call to putenv is passed a pointer to an automatic variable or an on-stack buffer. Stack memory said "I'm temporary," and putenv said "I'll keep it forever." Iconic? No. Dangerous.

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-negative to disable it.

This diagnostic warns on paths through the code where a shift is attempted with a negative count. It's analogous to the -Wshift-count-negative diagnostic implemented in the C/C++ front ends, but it's 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-overflow to disable it.

This diagnostic warns on paths through the code where a shift is attempted with a count greater than or equal to the precision of the operand's type. It's analogous to the -Wshift-count-overflow diagnostic implemented in the C/C++ front ends, but it's 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-buffer to disable it.

This diagnostic warns on paths through the code where longjmp is called to rewind to a jmp_buf relating to a setjmp call in a function that has returned. That buffer is stale, bestie. It's giving expired leftovers in the back of the fridge. 🫶

When setjmp gets called on a jmp_buf to record a rewind spot, it grabs the stack frame along with it. But 💀 that stack frame becomes invalid the literal second the function containing the setjmp call returns. So if you try to rewind back to it via longjmp? You're now referencing a stack frame that doesn't exist anymore, and bestie that's a one-way ticket to a crash (or worse 😭). Don't do it.

  • -Wno-analyzer-tainted-allocation-size — this one needs -fanalyzer, which turns it on in the first place; use -Wno-analyzer-tainted-allocation-size to turn it back off.

It warns you about paths through your code where a value that could be under an attacker's control gets used as the size of an allocation without being sanitized first — which means an attacker could inject a comically huge 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 -fanalyzer which enables it; use -Wno-analyzer-tainted-assertion to disable it.

It warns about paths through the code where a value that could be under an attacker's control is used as part of a condition without being sanitized first, and that condition guards a call to a function marked with attribute noreturn (like the function __builtin_unreachable). Those functions usually mean the program is terminating abnormally — think assertion failure handlers. For example:

        assert (some_tainted_value < SOME_LIMIT);

In these situations:

  • when assertion-checking is enabled: an attacker could trigger a denial of service by injecting an assertion failure 💀
  • when assertion-checking is disabled, like by defining NDEBUG: an attacker could inject data that subverts the process, since it presumably violates a precondition the code is assuming. 🙈
Heads up: when assertion-checking is disabled, the assertions usually get removed by the preprocessor before the analyzer even has a chance to "see" them, so this diagnostic can only generate warnings on builds where assertion-checking is enabled.

For the purpose of this warning, any function marked with attribute noreturn counts as a possible assertion failure handler, __builtin_unreachable included. And ngl, those functions are sometimes removed by the optimizer before the analyzer "sees" them — so optimization should be disabled when you're trying to trigger this diagnostic.

See CWE-617: Reachable Assertion (https://cwe.mitre.org/data/definitions/617.html).

The warning can also report problematic constructions like this one:

        switch (some_tainted_value) {
        case 0:
          /* [...etc; various valid cases omitted...] */
          break;

        default:
          __builtin_unreachable (); /* BUG: attacker can trigger this  */
        }

...even though, strictly speaking, that's not an assertion failure. 🤷

  • -Wno-analyzer-tainted-array-index — this warning requires -fanalyzer which enables it; use -Wno-analyzer-tainted-array-index to disable it.

It warns about paths through the code where a value that could be under an attacker's control is used as the index of an array access 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-tainted-divisor — this warning requires -fanalyzer which enables it; use -Wno-analyzer-tainted-divisor to disable it.

It warns about paths through the code where a value that could be under an attacker's control 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 — this warning requires -fanalyzer which enables it; use -Wno-analyzer-tainted-offset to disable it.

It warns about paths through the code 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 — this warning requires -fanalyzer which enables it; use -Wno-analyzer-tainted-size to disable it.

It warns about paths through the code where 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-undefined-behavior-ptrdiff — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-undefined-behavior-ptrdiff to disable it.

It warns about paths through the code where a pointer subtraction happens but the pointers refer to different chunks of memory. That code is relying on undefined behavior, because pointer subtraction is only defined when both pointers point inside (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 — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-undefined-behavior-strtok to disable it.

It warns about paths through the code where a call to strtok is made 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 — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-unsafe-call-within-signal-handler to disable it.

It warns about paths through the code where a function known to be async-signal-unsafe (like fprintf) is 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 — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-use-after-free to disable it.

It warns about paths through the code where a pointer gets used after a deallocator was 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 — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-use-of-pointer-in-stale-stack-frame to disable it.

It warns about paths through the code where a pointer is dereferenced but it points to a variable sitting in a stale stack frame. 🧟

  • -Wno-analyzer-va-arg-type-mismatch — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-va-arg-type-mismatch to disable it.

It warns about interprocedural paths through the code where the analyzer catches an attempt to use va_arg to extract a value passed to a variadic call, but using a type that does not match the type of the expression 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 — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-va-list-exhausted to disable it.

It warns about interprocedural paths through the code where the analyzer catches an attempt to use va_arg to access the next value passed to a variadic call — except all 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 — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-va-list-leak to disable it.

It warns about interprocedural paths through the code where the analyzer detects that va_start or va_copy was called on a va_list with no matching call to va_end. 💧

  • -Wno-analyzer-va-list-use-after-va-end — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-va-list-use-after-va-end to disable it.

It warns about interprocedural paths through the code where the analyzer detects an attempt to use a va_list after va_end has already been called on it. va_list. 💀

  • -Wno-analyzer-write-to-const — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-write-to-const to disable it.

It warns about paths through the code where the analyzer detects an attempt to write through a pointer to a const object. That said, the analyzer doesn't prioritize detecting these paths, so false negatives are more likely here than with the other warnings. 🙈

  • -Wno-analyzer-write-to-string-literal — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-write-to-string-literal to disable it.

It warns about paths through the code where the analyzer detects an attempt to write through a pointer to a string literal. Same disclaimer tho: the analyzer doesn't prioritize detecting these paths, so false negatives are more likely relative to other warnings. 🎀

  • -Wno-analyzer-use-of-uninitialized-value — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-use-of-uninitialized-value to disable it.

It warns about paths through the code where an uninitialized value gets used. 👻

See CWE-457: Use of Uninitialized Variable (https://cwe.mitre.org/data/definitions/457.html).

The analyzer has hardcoded knowledge about the behavior of the following memory-management functions:

…and we're back, bestie, picking up the list right where we left off 💅

  • 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
  • calloc
  • free
  • malloc
  • memset
  • operator delete
  • operator delete []
  • operator new
  • operator new []
  • realloc
  • strdup
  • strndup

then of these functions for working with file descriptors:

  • open
  • close
  • creat
  • dup, dup2 and dup3
  • isatty
  • pipe, and pipe2
  • read
  • write
  • socket, bind, listen, accept, and connect

then of these 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
  • fopen
  • fclose
  • ferror
  • fgets
  • fgets_unlocked
  • fileno
  • fread
  • getc
  • getchar
  • fprintf
  • printf
  • fwrite

and of these 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 error and error_at_line
  • getpass
  • longjmp
  • putenv
  • setjmp
  • siglongjmp
  • signal
  • sigsetjmp
  • strcat
  • strchr
  • strlen

plus — and this is kinda the fun bit — allll sorts of functions with an __analyzer_ prefix have special meaning to the analyzer, and those are described in the GCC Internals manual ✨

🎀 the parameters that steer the exploration

  • --param analyzer-bb-explosion-factor=value
  • --param analyzer-max-enodes-per-program-point=value
  • --param analyzer-max-recursion-depth=value
  • --param analyzer-min-snodes-for-call-summary=value

💜 and here's the gaggle of options that control the analyzer

  • -fanalyzer-call-summaries — simplifies interprocedural analysis by computing the effect of certain calls instead of exploring every single path through the function from that callsite to each possible return. fr fr, it's a shortcut. if it's enabled, call summaries only get used for functions with more than one call site, and that are sufficiently complicated (as per --param analyzer-min-snodes-for-call-summary=value).
  • -fanalyzer-checker=name** — restricts the analyzer to run just the named checker, and enables it. one chec*ker* only, no scrolling 💅
  • -fanalyzer-debug-text-art-headings — this one's for analyzer developers 🫶. if enabled, the analyzer adds extra annotations to any diagrams it generates.
  • -fno-analyzer-feasibility — for analyzer developers, again.

by default the analyzer verifies that there's a feasible control flow path for each diagnostic it emits: like, that the conditions that hold are not mutually exclusive, okaaay? diagnostics where no feasible path can be found get rejected. you can suppress that filtering with -fno-analyzer-feasibility, for debugging issues in this code.

  • -fanalyzer-fine-grained — analyzer developers, gather round.

internally the analyzer builds an "exploded graph" that combines control flow graphs with data flow information.

by default, an edge in this graph can contain the effects of a run of multiple statements inside a basic block. with -fanalyzer-fine-grained, each statement gets its own edge. like, individual attention for every statement, we love that for them 🦋

  • -fanalyzer-show-duplicate-count — analyzer developers only: 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 were suppressed by the deduplication logic. so it's like "bestie there were 12 more of these, i'm sparing you" 😭
  • -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 stays hidden 💀
  • -fno-analyzer-state-merge — analyzer developers, yes you again.

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 — analyzer developers, bestie, same hat.

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 — analyzer developers, ok so this one's spicy 🌸

by default the analyzer will stop exploring an execution path after encountering certain diagnostics, in order to avoid potentially issuing a 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 will continue to explore such paths even after such diagnostics, which may be helpful for debugging issues in the analyzer, or for microbenchmarks for detecting undefined behavior. so like, it keeps walking into the wall on purpose, but for science 🔥

  • -fanalyzer-transitivity — this one enables transitivity of constraints within the analyzer. iykyk ✨
  • -fno-analyzer-undo-inlining — analyzer developers, one more for the pile.

-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. 😭 rude.

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 — analyzer developers, babes. it enables more verbose, lower-level detail in the descriptions of control flow within diagnostic paths. more words, more detail, iconic 🎀
  • -fanalyzer-verbose-state-changes — analyzer developers again! it enables more verbose, lower-level detail in the descriptions of events relating to state machines within diagnostic paths.
  • -fanalyzer-verbosity=level** — okay this one actually controls the complexity of the control flow paths that are emitted for analyzer diagnostics. the level can be one of these:
    • 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-free diagnostic, both calls to free will be shown. both of them, no cap 💀
    • 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. all of them. not me being extra 🙈
    • 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 to file.analyzer.txt. heads up though: -fdump-analyzer-stderr overrides this option.
  • -fdump-analyzer-stderr — dumps internal details about what the analyzer is doing to stderr. this option overrides -fdump-analyzer. it wins the lil override battle 💅
  • -fdump-analyzer-callgraph — dumps a representation of the call graph suitable for viewing with GraphViz to file.callgraph.dot. pretty graph goes brrr 🦋
  • -fdump-analyzer-exploded-graph — dumps a representation of the "exploded graph" suitable for viewing with GraphViz to file.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" to file.eg.txt.
  • -fdump-analyzer-exploded-nodes-3 — dumps a textual representation of the "exploded graph" to one dump file per node, to file.eg-id.txt. this is typically a large number of dump files. like, genuinely so many 😭
  • -fdump-analyzer-exploded-paths — dumps a textual representation of the "exploded path" for each diagnostic to file.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 form file.*.fg.dot, file.*.tg.dot, and file.*.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 form file.*.infinite-loop.dot. going in circles, but make it art 🌈
  • -fdump-analyzer-json — dumps a compressed JSON representation of analyzer internals to file.analyzer.json.gz. the precise format is subject to change — so like, don't get too attached to it 🫶
  • -fdump-analyzer-state-purge — okay so this is basically -fdump-analyzer-supergraph's sibling, bestie 🫶 it dumps a representation of the "supergraph" that's suitable for viewing with GraphViz, but it also annotates the graph with info on what state will be purged at each node. The graph gets written to file.state-purge.dot. cute, annotated, purged 💅
  • -fdump-analyzer-supergraph — dumps representations of the "supergraph" suitable for viewing with GraphViz, into file.supergraph.dot and into file.supergraph-eg.dot ✨ these show, like, all of the control flow graphs in the program, with interprocedural edges for calls and returns. The second dump contains annotations showing nodes in the "exploded graph" plus the diagnostics associated with them. very demure, very exploded 🦋
  • -fdump-analyzer-untracked — emits custom warnings with internal details, and these are intended for analyzer developers 👑 iykyk, it's giving "for the devs only" energy

🐛 Options for Debugging Your Program 🐛

So to tell GCC to emit extra information for use by a debugger, okay, in almost all cases you literally only need to add -g to your other options 🎀 and like, 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. periods, bestie.

GCC allows you to use -g with -O. But the shortcuts taken by optimized code may occasionally be surprising 😭 like: 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. not the optimizer moving your code around 🙈 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 ✨ like, it's giving "actually helpful for once".

  • -g — produces debugging information in the operating system's native format (one of stabs, COFF, XCOFF, or DWARF). GDB can work with this debugging information 💖

On most systems that use stabs format, -g enables use of extra debugging information that only GDB can use; this extra information makes debugging work better in GDB but probably makes other debuggers crash or refuse to read the program 💀 if you want to control for certain whether to generate the extra information, use -gvms (see below). no cap, that's the tradeoff.

  • -ggdb — produces debugging information for use by GDB. This means to use the most expressive format available (DWARF, stabs, or the native format if neither of those are supported), including GDB extensions if at all possible 👑 maximum expressiveness, we love to see it.
  • -gdwarf, -gdwarf-version — produce debugging information in DWARF format (if that is supported). The value of version may be either 2, 3, 4 or 5; the default version for most targets is 5 (with the exception of VxWorks, TPF and Darwin / macOS, which default to version 2, and AIX, which defaults to version 4) 🌸

Note that with DWARF Version 2, some ports require and always use some non-conflicting DWARF 3 extensions in the unwind tables.

Version 4 may require GDB 7.0 and -fvar-tracking-assignments for maximum benefit. Version 5 requires GDB 8.0 or higher. ngl, check your GDB version before you come for DWARF 5 🙈

GCC no longer supports DWARF Version 1, which is substantially different than Version 2 and later 🚫 for historical reasons, some other DWARF-related options such as -fno-dwarf2-cfi-asm retain a reference to DWARF Version 2 in their names, but apply to all currently-supported versions of DWARF. it's giving "the name is a lie but the behaviour is fine" 😭

  • -gbtf — requests BTF debug information. BTF is the default debugging format for the eBPF target 🦋 on other targets, like x86, BTF debug information can be generated along with DWARF debug information when both of the debug formats are enabled explicitly via their respective command line options. team eBPF stays winning fr fr.
  • -gprune-btf, -gno-prune-btf — prune BTF information before emission. When pruning, only type information for types used by global variables and file-scope functions will be emitted. If compiling for the BPF target with BPF CO-RE enabled, type information will also be emitted for types used in BPF CO-RE relocations 🌈 in addition, struct and union types which are only referred to via pointers from members of other struct or union types shall be pruned and replaced with BTF_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 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 — request CTF debug information and use level to specify how much CTF debug information should be produced. If -gctf is specified without a value for level, the default level of CTF debug information is 2 ✨

CTF debug information can be generated along with DWARF debug information when both of the debug formats are enabled explicitly via their respective command line options.

Level 0 produces no CTF debug information at all. Thus, -gctf0 negates -gctf 🙅

Level 1 produces CTF information for tracebacks only. This includes callsite information, but does not include type information.

Level 2 produces type information for entities (functions, data objects etc.) at file-scope or global-scope only. iconic scope-restricted behaviour tbh 💅

  • -gvms — produces debugging information in Alpha/VMS debug format (if that is supported). This is the format used by DEBUG on Alpha/VMS systems 🎀 retro.
  • -gcodeview — produces debugging information in CodeView debug format (if that is supported). This is the format used by Microsoft Visual C++ on Windows windows girlies rise 🪟
  • -glevel, -ggdblevel, -gvmslevel — request debugging information and also use level to specify how much information. The default level is 2.

Level 0 produces no debug information at all. Thus, -g0 negates -g 🚫

Level 1 produces minimal information, enough for making backtraces in parts of the program that you don't plan to debug. This includes descriptions of functions and external variables, and line number tables, but no information about local variables. the "just enough" era.

Level 3 includes extra information, such as all the macro definitions present in the program. Some debuggers support macro expansion when you use -g3 🔥 macro girlies, this one's for you.

If you use multiple -g options, with or without level numbers, the last such option is the one that is effective 👑 last one wins, always.

-gdwarf does not accept a concatenated debug level, to avoid confusion with -gdwarf-level. Instead use an additional -glevel option to change the debug level for DWARF 🙈 it's giving "we learned from the ambiguity, bestie".

  • -fno-eliminate-unused-debug-symbols — by default, no debug information is produced for symbols that are not actually used. Use this option if you want debug information for all symbols 💖 all of them. every single one.
  • -femit-class-debug-always — instead of emitting debugging information for a C++ class in only one object file, emit it in all object files using the class. This option should be used only with debuggers that are unable to handle the way GCC normally emits debugging information for classes because using this option increases the size of debugging information by as much as a factor of two 😭 two!! double!! so use it with caution, bestie.
  • -fno-merge-debug-strings — directs the linker to not merge together strings in the debugging information that are identical in different object files. Merging is not supported by all assemblers or linkers. Merging decreases the size of the debug information in the output file at the cost of increasing link processing time. Merging is enabled by default ✨ tradeoffs, we love them.
  • -fdebug-prefix-map=old=new — when compiling files residing in directory old, record debugging information describing them as if the files resided in directory new instead 🦋 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 . for new. 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-map and -fcanon-prefix-map 💅
  • -fvar-tracking — runs the variable tracking pass. It computes where variables are stored at each position in code. Better debugging information is then generated (if the debugging information format supports this information) 👑

It is enabled by default when compiling with optimization (-Os, -O, -O2, ...), debugging information (-g) and the debug info format supports it. auto-enabled when the vibes align ✨

  • -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 information while optimizing. Use of -gdwarf-4 is recommended along with it 🎀

It can be enabled even if var-tracking is disabled, in which case annotations are created and maintained, but discarded at the end 😭 so much effort for nothing, honestly. By default, this flag is enabled together with -fvar-tracking, except when selective scheduling is enabled.

  • -gsplit-dwarf — if DWARF debugging information is enabled, separate as much debugging information as possible into a separate output file with the extension .dwo 🌸 this option allows the build system to avoid linking files with debug information. To be useful, this option requires a debugger capable of reading .dwo files 👀 no .dwo-reading debugger, no benefit, periodt.
  • -gdwarf32, -gdwarf64 — if DWARF debugging information is enabled, the -gdwarf32 selects the 32-bit DWARF format and the -gdwarf64 selects the 64-bit DWARF format. The default is target specific — on most targets it is -gdwarf32 though. The 32-bit DWARF format is smaller, but can't support more than 2GiB of debug information in any of the DWARF debug information sections 💀 the 64-bit DWARF format allows larger debug information and might not be well supported by all consumers yet 🫶 use with awareness.
  • -gdescribe-dies — add description attributes to some DWARF DIEs that have no name attribute, such as artificial variables, external references and call site parameter DIEs 🌈 little descriptions for the nameless ones, we love a thoughtful annotation.
  • -gpubnames — Generate DWARF ".debug_pubnames" and ".debug_pubtypes" sections. 💖
  • -ggnu-pubnames — Generate ".debug_pubnames" and ".debug_pubtypes" sections in a format suitable for conversion into a GDB index. This option is only useful with a linker that can produce GDB index version 7. 🦋
  • -fdebug-types-section — When using DWARF Version 4 or higher, type DIEs can be put into their own ".debug_types" section instead of making them part of the ".debug_info" section. It is more efficient to put them in a separate comdat section since the linker can then remove duplicates. But not all DWARF consumers support ".debug_types" sections yet and on some objects ".debug_types" produces larger instead of smaller debugging information. 😭
  • -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 the DW_AT_producer attribute in DWARF debugging information. The options are concatenated with spaces separating them from each other and from the compiler version. It is enabled by default. See also -frecord-gcc-switches for another way of storing compiler options into the object file. ✨
  • -gstrict-dwarf — Disallow using extensions of later DWARF standard version than selected with -gdwarf-version. On most targets using non-conflicting DWARF extensions from later standard versions is allowed. 🔥
  • -gno-strict-dwarf — Allow using extensions of later DWARF standard version than selected with -gdwarf-version. 🫶
  • -gas-loc-support — Inform the compiler that the assembler supports ".loc" directives. It may then use them for the assembler to 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 will be enabled by default if, at GCC configure time, the assembler was found to support such directives. 💅
  • -gno-as-loc-support — Force GCC to generate DWARF2+ line number tables internally, if DWARF2+ line number tables are to be generated. 🌸
  • -gas-locview-support — Inform the compiler that the assembler supports "view" assignment and reset assertion checking in ".loc" directives. This option will be enabled by default if, at GCC configure time, the assembler was found to support them. 🎀
  • -gno-as-locview-support — Force GCC to assign view numbers internally, if -gvariable-location-views are 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 option causes GCC to create markers in the internal representation at the beginning of statements, and to keep them roughly in place throughout compilation, using them to guide the output of "is_stmt" markers in the line number table. This is 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 enables debug information consumers to 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 will cause 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 will force GCC to output internal line number tables, and if -ginternal-reset-location-views is not enabled, that will most certainly lead 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 insn 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 will flag 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 of none (don't compress debug sections), or zlib (use zlib 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, -gz is 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_frame" section 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. ✨

okay so when you don't pass any optimization option, the compiler is in full "don't touch my debugging" mode 💅 its whole goal is to keep compilation cheap and make debugging actually behave the way you expect. statements are independent — like, if you stop the program with a breakpoint between statements, you can assign a new value to any variable or move the program counter to any other statement in the function and get exactly the results you'd expect from the source code. no cap.

flip on optimization flags and the compiler starts trying to improve performance and/or code size, but it's gonna cost you compilation time and possibly your ability to debug the program 😭.

the compiler optimizes based on what it knows about the program. if you compile multiple files at once into a single output file, it can use info from all of them when compiling each one — cross-file glow-up ✨.

not every optimization has its own flag, bestie. only the ones with a flag are listed here.

most optimizations are fully off at -O0 or if no -O level is set on the command line — even if you specify individual optimization flags. same vibe with -Og: it suppresses a bunch of optimization passes.

depending on the target and how GCC was configured, each -O level might enable a slightly different set than what's listed here. run GCC with -Q --help=optimizers to see the exact set enabled at each level 🔍.

  • -O, -O1 — Optimize. optimizing compilation takes somewhat more time and a lot more memory for a large function.

with -O, the compiler tries to reduce code size and execution time without doing any optimizations that take a great deal of compilation time.

-O is the recommended optimization level for large machine-generated code — it's the sensible balance between compile time and memory use. higher optimization levels do optimizations with greater algorithmic complexity than -O.

-O turns on the following optimization flags:

-fauto-inc-dec -fbranch-count-reg -fcombine-stack-adjustments
-fcompare-elim -fcprop-registers -fdce -fdefer-pop -fdelayed-branch
-fdse -fforward-propagate -fguess-branch-probability -fif-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. GCC performs nearly all supported optimizations that don't involve a space-speed tradeoff. compared to -O, this option increases both compilation time and the performance of the generated code.

-O2 turns on all optimization flags specified by -O1. it also turns on these:

-falign-functions  -falign-jumps -falign-labels  -falign-loops
-fcaller-saves -fcode-hoisting -fcrossjumping -fcse-follow-jumps
-fcse-skip-blocks -fdelete-null-pointer-checks -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 -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-cheap
please note the warning under -fgcse about invoking -O2 on programs that use computed gotos ⚠️
  • -O3 — Optimize yet more. -O3 turns on all optimizations specified by -O2 and also turns on 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 — Reduce compilation time and make debugging produce the expected results. this is the default.

at -O0, GCC completely disables most optimization passes — they are not run even if you explicitly enable them on the command line, or are listed by -Q --help=optimizers as being enabled by default. many optimizations performed by GCC depend on code analysis or canonicalization passes that are enabled by -O, and it wouldn't be useful to run individual optimization passes in isolation.

  • -Os — Optimize for size. -Os enables all -O2 optimizations except those that often increase code size:
-falign-functions  -falign-jumps -falign-labels  -falign-loops
-fprefetch-loop-arrays  -freorder-blocks-algorithm=stc

it also enables -finline-functions, causes the compiler to tune for code size rather than execution speed, and performs further optimizations designed to reduce code size.

  • -Ofast — Disregard strict standards compliance. -Ofast enables all -O3 optimizations. it also enables optimizations that are not valid for all standard-compliant programs. it turns on -ffast-math, -fallow-store-data-races and the Fortran-specific -fstack-arrays (unless -fmax-stack-var-size is specified), and -fno-protect-parens. it turns off -fsemantic-interposition.
  • -Og — Optimize debugging experience. -Og should be the optimization level of choice for the standard edit-compile-debug cycle — it offers a reasonable level of optimization while maintaining fast compilation and a good debugging experience. it's a better choice than -O0 for producing debuggable code because some compiler passes that collect debug information are disabled at -O0.

like -O0, -Og completely disables a number of optimization passes so that individual options controlling them have no effect. otherwise -Og enables all -O1 optimization flags except for those that may 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 may increase the number of instructions executed if those instructions require fewer bytes to encode. -Oz behaves similarly to -Os, including enabling most -O2 optimizations.

if you use multiple -O options, with or without level numbers, the last such option is the one that's effective. periodt.

options of the form -fflag specify machine-independent flags. most flags have both positive and negative forms; the negative form of -ffoo is -fno-foo. in the table below, only one of the forms is listed — the one you typically use. you can figure out the other form by either removing no- or adding it. iykyk 💅

the following options control specific optimizations. they're either activated by -O options or are related to ones that are. you can use these flags in the rare cases when you want to "fine-tune" the optimizations being performed.

  • -fno-defer-pop — for machines that must pop arguments after a function call, always pop the arguments as soon as each function returns. at levels -O1 and higher, -fdefer-pop is the default; this lets the compiler let arguments accumulate on the stack for several function calls and pop them all at once.
  • -fforward-propagate — perform a forward propagation pass on RTL. the pass tries to combine two instructions and checks if the result can be simplified. if loop unrolling is active, two passes are performed and the second is scheduled after loop unrolling.

this option is enabled by default at optimization levels -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 such cases and avoid the penalty by changing the order of the load and store and then fixing up the loaded value.

disabled by default.

  • -ffp-contract=style — -ffp-contract=off disables floating-point expression contraction. -ffp-contract=fast enables floating-point expression contraction such as forming fused multiply-add operations if the target has native support for them. -ffp-contract=on enables floating-point expression contraction if allowed by the language standard. this is 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), -ffp-contract=fast otherwise.

  • -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 cannot 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 replace them with a faster implementation. 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, generate that CRC instruction. otherwise, if the target supports a carry-less-multiplication instruction, generate CRC using it; otherwise generate table-based CRC.

enabled by default at -O2 and higher.

  • -foptimize-sibling-calls — ok so this one optimizes sibling and tail recursive calls 💫 it's like the compiler looking at your recursion and going "bestie I can make this cuter". Enabled at levels -O2, -O3, -Os.
  • -foptimize-strlen — turns various standard C string functions — like "strlen", "strchr" or "strcpy" — and their "_FORTIFY_SOURCE" counterparts into faster alternatives 🔥 so your string wrangling gets a little glow-up. Enabled at levels -O2, -O3.
  • -finline-stringops[=fn] — expands memory and string operations inline (for now, only "memset"), even when the length is variable or big enough as to require looping 😳 most useful when you're chilling with -ffreestanding and -fno-builtin.

In some circumstances, it enables the compiler to generate code that takes advantage of known alignment and length multipliers — but even then it may be less efficient than optimized runtime implementations, and grow code size so much that even a less performant but shared implementation runs faster due to better use of code caches 💀 ngl that's the whole tea. This option is disabled by default.

  • -fno-inline — do not expand any functions inline apart from those marked with the "always_inline" attribute 🙅‍♀️ this is the default when not optimizing.

Single functions can be exempted from inlining by marking them with the "noinline" attribute.

  • -finline-small-functions — integrates functions into their callers when their body is smaller than expected function call code (so the overall size of the program gets smaller ✨). The compiler heuristically decides which functions are simple enough to be worth integrating this way. This inlining applies to all functions, even those not declared inline 👀

Enabled at levels -O2, -O3, -Os.

  • -findirect-inlining — inlines also indirect calls that are discovered to be known at compile time thanks to previous inlining 🧠 this option has any effect only when inlining itself is turned on by the -finline-functions or -finline-small-functions options.

Enabled at levels -O2, -O3, -Os.

  • -finline-functions — considers all functions for inlining, even if they are 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 all "static" functions called once for inlining into their caller even if they are not marked "inline" 🎀 if a call to a given function is integrated, then the function is not output as assembler code in its own right.

Enabled at levels -O1, -O2, -O3 and -Os, but not -Og.

  • -fearly-inlining — inlines functions marked by "always_inline", plus functions whose body seems smaller than the function call overhead, early — before doing -fprofile-generate instrumentation and the real inlining pass 🏃‍♀️💨 doing so makes profiling significantly cheaper and usually inlining faster on programs having large chains of nested wrapper functions. icon behavior fr.

Enabled by default.

  • -fipa-sra — performs interprocedural scalar replacement of aggregates, removal of unused parameters and replacement of parameters passed by reference by parameters passed by value 🌸 very "clean up your room" of it.

Enabled at levels -O2, -O3 and -Os.

  • -finline-limit=n — by default, GCC limits the size of functions that can be inlined. This flag allows coarse control of this limit 🎛️ n is the size of functions that can be inlined in number of pseudo instructions.

Inlining is actually controlled by a number of parameters, which may be specified individually by using --param name=value. The -finline-limit=n option sets some of these parameters as follows:

  • max-inline-insns-single — is set to n/2.
  • max-inline-insns-auto — is set to n/2.

See below for a documentation of the individual parameters controlling inlining and for the defaults of these parameters 📖

Note: there may be no value to -finline-limit that results in default behavior.
Note: pseudo instruction represents, in this particular context, an abstract measurement of function's size. In no way does it represent a count of assembly instructions and as such its exact meaning might change from one release to an another 😭 (yes, "an another", the manpage said it, not me).
  • -fno-keep-inline-dllexport — this is a more fine-grained version of -fkeep-inline-functions, which applies only to functions that are declared using the "dllexport" attribute or declspec 🎯
  • -fkeep-inline-functions — in C, emit "static" functions that are declared "inline" into the object file, even if the function has been inlined into all of its callers 💾 this switch does not affect functions using the "extern inline" extension in GNU C90. In C++, emit any and all inline functions into the object file.
  • -fkeep-static-functions — emits "static" functions into the object file, even if the function is never used 🫶 she stays on the roster regardless.
  • -fkeep-static-consts — emits variables declared "static const" when optimization isn't turned on, even if the variables aren't referenced 👑

GCC enables this option by default. If you want to force the compiler to check if a variable is referenced, regardless of whether or not optimization is turned on, use the -fno-keep-static-consts option.

  • -fmerge-constants — 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. In addition to -fmerge-constants this considers e.g. even constant initialized arrays or initialized constant variables with integral or floating-point types.
Languages like C or C++ require each variable, including multiple instances of the same variable in recursive calls, to have distinct locations, so using this option results in non-conforming behavior 💀 proceed with caution bestie.
  • -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. it's giving choreography.
  • -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-sched enabled.
  • -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 upon 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-reg option 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

  • -fno-zero-initialized-in-bss — if the target supports a BSS section, GCC by default puts variables that are initialized to zero into BSS 🌈 this can save space in the resulting code.

This option turns off this behavior because some programs explicitly rely on variables going to the data section — e.g., so that the resulting executable can find the beginning of that section and/or make assumptions based on that 🕵️‍♀️

The default is -fzero-initialized-in-bss 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 using a type that occupies multiple registers, such as "long long" on a 32-bit system, split the registers apart and allocate them independently ✂️ this normally generates better code for those types, but may make debugging more difficult 😭 (trade-offs, always trade-offs).

Enabled at levels -O1, -O2, -O3, -Os.

  • -fsplit-wide-types-early — fully split wide types early, instead of very late ⏰ this option has no effect unless -fsplit-wide-types is turned on.

This is the default on some targets.

  • -fcse-follow-jumps — in common subexpression elimination (CSE), scan through jump instructions when the target of the jump is not reached by any other path 🛤️ for example, when CSE encounters an "if" statement with an "else" clause, CSE follows the jump when the condition tested is false.

Enabled at levels -O2, -O3, -Os.

  • -fcse-skip-blocks — this is similar to -fcse-follow-jumps, but causes CSE to follow jumps that conditionally skip over blocks 🦘 when CSE encounters a simple "if" statement with no else clause, -fcse-skip-blocks causes CSE to follow the jump around the body of the "if".

Enabled at levels -O2, -O3, -Os.

  • -frerun-cse-after-loop — re-runs common subexpression elimination after loop optimizations are performed 🔁 like a second pass of the vibe check after the loops have been glamified.

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 💡 iykyk.

Enabled at levels -O2, -O3, -Os.

  • -fgcse-lm — okay so when this one's on, global common subexpression elimination straight up tries to move loads that are only killed by stores into themselves 💅. Translation: a loop with a load/store sequence can become a load outside the loop plus a copy/store inside it. Slay.

Lowkey enabled by default whenever -fgcse is 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-lm and a loop with a load/store sequence can become a load before the loop and a store after the loop. Iconic teamwork fr fr.

Not enabled at any optimization level, bestie — you gotta ask for it 🙈.

  • -fgcse-las — with this on, the global common subexpression elimination pass eliminates redundant loads that come after stores to the same memory location (both partial and full redundancies, no cap) 🔥.

Not enabled at any optimization level either 💀.

  • -fgcse-after-reload — enables a redundant load elimination pass that runs after reload, and its whole purpose is cleaning up redundant spilling 🧚. Very "post-party cleanup" energy.

Enabled by -O3, -fprofile-use and -fauto-profile ✨.

  • -faggressive-loop-optimizations — this tells the loop optimizer to use language constraints to derive bounds for the number of iterations of a loop 👑. It assumes your loop code doesn't invoke undefined behavior — like, no signed integer overflows, no out-of-bound array accesses, none of that 😭. Those iteration bounds then guide loop unrolling, peeling, and loop exit test optimizations.
Heads up: the whole thing rests on that assumption, so if your code does the undefined-behavior thing, that's on you bestie 💀.

Enabled by default 🫶.

  • -funconstrained-commons — tells the compiler that variables declared in common blocks (e.g. Fortran) may later be overridden with longer trailing arrays 🌸. This prevents certain optimizations that depend on knowing the array bounds. Basically "don't assume you know how big this is, because you don't" 💅.
  • -fcrossjumping — performs the cross-jumping transformation, which unifies equivalent code and saves code size ✨. The resulting code may or may not perform better than without cross-jumping — no promises, it's giving "aesthetic win, maybe a speed win" 🎀.

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 (respectfully, not every chip can). Enabled by default at -O1 and higher on architectures that do support this 💖.
  • -fdce — performs dead code elimination (DCE) on RTL 💀. Enabled by default at -O1 and higher.
  • -fdse — performs dead store elimination (DSE) on RTL 🌈. Enabled by default at -O1 and higher.
  • -fif-conversion — attempts to transform conditional jumps into branch-less equivalents, which includes using conditional moves, min, max, set flags and abs instructions, plus some tricks doable by 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 — the C++ ABI requires multiple entry points for constructors and destructors: one for a base subobject, one for a complete object, and one for a virtual destructor that calls operator delete afterwards 😭. For a hierarchy with virtual bases, the base and complete variants are clones — meaning two copies of the function, rip. With this option, the base and complete variants are changed to be thunks that call a common implementation 👑

Enabled by -Os ✨.

  • -fdelete-null-pointer-checks — assumes programs can't safely dereference null pointers, and that no code or data element lives at address zero 🚫. This option 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 null pointer checks — those passes assume a memory access to address zero always traps, so if a pointer is checked after it's already been dereferenced, it can't be null 🤯.
But bestie, in some environments that assumption is just 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 👑. It does this 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 the analysis of the type inheritance graph, it figures out the set of likely targets for a given call; if that set is small — preferably of size 1 — it changes the call 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. Self-aware king behavior 👑.
  • -fdevirtualize-at-ltrans — streams extra information needed for aggressive devirtualization when running the link-time optimizer in local transformation mode ✨. This enables more devirtualization but significantly increases the size of streamed data, so for that reason it's disabled by default 🙈.
  • -fexpensive-optimizations — performs a number of minor optimizations that are relatively expensive 💸. Worth it though.

Enabled at levels -O2, -O3, -Os.

  • -free — attempts to remove redundant extension instructions 🎀. Especially helpful for x86-64, which implicitly zero-extends in 64-bit registers after writing to their lower 32-bit half — the way that architecture just does that, iykyk.

Enabled for Alpha, AArch64, LoongArch, PowerPC, RISC-V, SPARC, h83000 and x86 at levels -O2, -O3, -Os 💖.

  • -fno-lifetime-dse — okay, C++ lore time: 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; if your code relies on the value of the object storage persisting beyond the object's lifetime, you can use this flag to disable the optimization 🫶.

To preserve stores before the constructor starts (e.g. because your operator new clears the object storage) but still treat the object as dead after the destructor, you can use -flifetime-dse=1. The default behavior can be explicitly selected with -flifetime-dse=2, and -flifetime-dse=0 is equivalent to -fno-lifetime-dse ✨.

  • -flive-range-shrinkage — attempts to decrease register pressure through register live range shrinkage 💅. Helpful for fast processors with small or moderate size register sets — small register file gang rise up 🫶.
  • -fira-algorithm=*algorithm — uses the specified coloring algorithm for the integrated register allocator 🎨. The algorithm argument can be priority, which specifies Chow's priority coloring, or CB, which specifies Chaitin-Briggs coloring. Chaitin-Briggs coloring isn't 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 the specified regions for the integrated register allocator 🗺️. The region argument should be one of the following:
    • all — use all loops as register allocation regions. This can give the best results for machines with a small and/or irregular register set 👑.
    • mixed — use all loops except loops with small register pressure as the regions. This value usually gives the best results in most cases and for most architectures, and is enabled by default when compiling with optimization for speed (-O, -O2, ...) 💅.
    • one — use all functions as a single region. This typically results in the smallest code size, and is enabled by default for -Os or -O0 🎀.
  • -fira-hoist-pressure — uses IRA to evaluate register pressure in the code hoisting pass for decisions to hoist expressions 🦋. This usually results in smaller code, but it can slow the compiler down 😭.

This option is enabled at level -Os for all targets ✨.

  • -fira-loop-pressure — uses IRA to evaluate register pressure in loops for decisions to move loop invariants 🔄. This usually results in generation of faster and smaller code on machines with large register files (>= 32 registers), but it can slow the compiler down 😭.

This option is enabled at level -O3 for some targets 💖.

  • -fno-ira-share-save-slots — 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. Enjoy the extra real estate, I guess 🏠.
  • -fno-ira-share-spill-slots — disables sharing of stack slots allocated for pseudo-registers 🚫. Each pseudo-register that doesn't get a hard register gets a separate stack slot, and as a result function stack frames are larger. Same bigger-frame situation, different reason 💀.
  • -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 — if supported for the target machine, attempts to reorder instructions to eliminate execution stalls due to required data being unavailable ⏳. This helps machines that have slow floating point or memory load instructions by allowing other instructions to be issued until the result of the load or floating-point instruction is required 💅. Multitasking, but for CPUs.

okay so picking up right where we left off — this one's conventionally enabled at optimization levels -O2 and -O3, but bestie, a lot of targets override that. Like on x86 it's disabled at all levels, while on AArch64 it only turns on at -O3. it's giving "it depends" 😭

  • -fschedule-insns2 — same energy as -fschedule-insns, but it asks for an extra pass of instruction scheduling after register allocation has already happened. this one absolutely slaps on machines with a pretty small number of registers, and where memory load instructions take more than one cycle 🫶

Enabled at levels -O2, -O3, -Os.

  • -fno-sched-interblock — turns off instruction scheduling across basic blocks, which is normally on when scheduling happens before register allocation, i.e. with -fschedule-insns or at -O2 or higher.
  • -fno-sched-spec — turns off speculative motion of non-load instructions, which is normally on when scheduling happens before register allocation, i.e. with -fschedule-insns or at -O2 or higher.
  • -fsched-pressure — enables register-pressure-sensitive insn scheduling before register allocation. this only makes sense when scheduling before register allocation is actually turned on, i.e. with -fschedule-insns or at -O2 or higher. using this can legitimately improve the generated code and shrink it, by stopping register pressure from climbing above the number of available hard registers and the spills that follow in register allocation 🔥
  • -fsched-spec-load — allows speculative motion of some load instructions. again, only makes sense when you're scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.
  • -fsched-spec-load-dangerous — allows speculative motion of more load instructions. yes, the flag is literally called dangerous, and yeah, same rule: only makes sense when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher 💀
  • -fsched-stalled-insns, -fsched-stalled-insns=n — defines how many insns (if any) can be moved prematurely out of the queue of stalled insns and into the ready list during the second scheduling pass. -fno-sched-stalled-insns means no insns get moved prematurely, and -fsched-stalled-insns=0 means there's no limit at all on how many queued insns can be moved early. -fsched-stalled-insns with no value is equivalent to -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-insns is being used. -fno-sched-stalled-insns-dep is equivalent to -fsched-stalled-insns-dep=0, and -fsched-stalled-insns-dep with no value is equivalent to -fsched-stalled-insns-dep=1 💅
  • -fsched2-use-superblocks — when scheduling after register allocation, use superblock scheduling. this lets motion happen across basic block boundaries, which gives you faster schedules. heads up though: this option is experimental, because not every machine description GCC uses models the CPU closely enough to avoid unreliable results from the algorithm 🙈

This only makes sense when scheduling after register allocation, i.e. with -fschedule-insns2 or at -O2 or higher.

  • -fsched-group-heuristic — enables the group heuristic in the scheduler. it favours the instruction that belongs to a schedule group. enabled by default whenever scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher 🌸
  • -fsched-critical-path-heuristic — enables the critical-path heuristic in the scheduler. it favours instructions sitting on the critical path. enabled by default whenever scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher ⭐
  • -fsched-spec-insn-heuristic — enables the speculative instruction heuristic in the scheduler. it favours speculative instructions that have greater dependency weakness. enabled by default whenever scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher 🦋
  • -fsched-rank-heuristic — enables the rank heuristic in the scheduler. it favours the instruction belonging to a basic block with greater size or frequency. enabled by default whenever scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher 👑
  • -fsched-last-insn-heuristic — enables the last-instruction heuristic in the scheduler. it favours the instruction that's less dependent on the last instruction scheduled. enabled by default whenever scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher 🌈
  • -fsched-dep-count-heuristic — enables the dependent-count heuristic in the scheduler. it favours the instruction that has more instructions depending on it. enabled by default whenever scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher 🎀
  • -freschedule-modulo-scheduled-loops — modulo scheduling happens before traditional scheduling. so if a loop is modulo scheduled, later scheduling passes might go and 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. here selective scheduling runs instead of the second scheduler pass.
  • -fsel-sched-pipelining — enables software pipelining of innermost loops during selective scheduling. does absolutely nothing unless one of -fselective-scheduling or -fselective-scheduling2 is turned on 😭
  • -fsel-sched-pipelining-outer-loops — when pipelining loops during selective scheduling, also pipeline outer loops. does nothing unless -fsel-sched-pipelining is turned on.
  • -fsemantic-interposition — some object formats, like ELF, let the dynamic linker interpose symbols. meaning: 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. it's a genuinely useful feature — like, rewriting memory allocation functions with a debugging implementation? iconic — but it's 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). similarly, if interposition happens for variables, the constructor of the variable will be the same. the flag has no effect for functions explicitly declared inline (where interposition is never allowed to change semantics) and for symbols explicitly declared weak.

  • -fshrink-wrap — emits 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 -O and higher.
  • -fshrink-wrap-separate — shrink-wraps separate parts of the prologue and epilogue separately, so those parts only get executed when needed. on by default, but it does nothing unless -fshrink-wrap is also turned on and the target supports it.
  • -fcaller-saves — enables allocation of values to registers that get clobbered by function calls, by emitting extra instructions to save and restore those registers around such calls. this allocation only happens when it looks like it'll result in 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 -O1 and higher.

  • -fipa-ra — uses 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 it's disabled if the generated code will 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 — attempts to minimize stack usage. the compiler tries to use less stack space, even if that makes the program slower 🙃 this option implies setting the large-stack-frame parameter to 100 and the large-stack-frame-growth parameter to 400.
  • -ftree-reassoc — performs reassociation on trees. enabled by default at -O1 and higher.
  • -fcode-hoisting — performs code hoisting. code hoisting tries to move the evaluation of expressions 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 code speed too. enabled by default at -O2 and higher 🔥
  • -ftree-pre — performs partial redundancy elimination (PRE) on trees. enabled by default at -O2 and -O3.
  • -ftree-partial-pre — makes partial redundancy elimination (PRE) more aggressive. enabled by default at -O3 💅
  • -ftree-forwprop — performs forward propagation on trees. enabled by default at -O1 and higher.
  • -ftree-fre — performs full redundancy elimination (FRE) on trees. the difference between FRE and PRE is that FRE only considers expressions computed on all paths leading to the redundant computation. this analysis is faster than PRE, though it exposes fewer redundancies. enabled by default at -O1 and higher.
  • -ftree-phiprop — performs hoisting of loads from conditional pointers on trees. enabled by default at -O1 and higher 🧚

okay so we’re still deep in flag city and these next ones are lowkey giving compiler-girlie magic ✨ stay with me bestie, they’re all important 💅

  • -fhoist-adjacent-loads — speculatively hoists loads from both branches of an if-then-else when those loads are from adjacent locations in the same structure and the target architecture has a conditional move instruction. This flag is enabled by default at -O2 and higher 🫶
  • -ftree-copy-prop — performs copy propagation on trees, and this pass eliminates unnecessary copy operations. This flag is enabled by default at -O1 and higher ✨
  • -fipa-pure-const — discovers which functions are pure or constant. Enabled by default at -O1 and higher 💖
  • -fipa-reference — discovers which static variables do not escape the compilation unit. Enabled by default at -O1 and higher 🌸
  • -fipa-reference-addressable — discovers read-only, write-only and non-addressable static variables. Enabled by default at -O1 and higher 🦋
  • -fipa-reorder-for-locality — groups call chains close together in the binary layout to improve code locality and minimize jump distances between frequently called functions. Unlike -freorder-functions, this pass considers the call chains between functions and groups them together, rather than grouping all hot/normal/cold/never-executed functions into separate sections. Unlike -fprofile-reorder-functions, it aims to improve code locality throughout the runtime of the program rather than focusing on program startup. This option is incompatible with an explicit -flto-partition= option since it enforces a custom partitioning scheme. If using this option it is 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 — performs interprocedural pointer analysis and interprocedural modification and reference analysis. Heads up bestie: this option can cause excessive memory and compile-time usage on large compilation units 😭 It is not enabled by default at any optimization level.
  • -fipa-profile — performs interprocedural profile propagation. The functions called only from cold functions are marked as cold. Also functions executed once (such as "cold", "noreturn", static constructors or destructors) are identified. Cold functions and loop less parts of functions executed once are then optimized for size. Enabled by default at -O1 and higher 💜
  • -fipa-modref — performs 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. This flag is enabled by default at -O1 and higher 🌈
  • -fipa-cp — performs interprocedural constant propagation. This optimization analyzes the program to determine when values passed to functions are constants and then optimizes accordingly. This optimization can substantially increase performance if the application has constants passed to functions. This flag is enabled by default at -O2, -Os and -O3. It is also enabled by -fprofile-use and -fauto-profile 🔥
  • -fipa-cp-clone — performs function cloning to make interprocedural constant propagation stronger. When enabled, interprocedural constant propagation performs function cloning when externally visible function can be called with constant arguments. Because this optimization can create multiple copies of functions, it may significantly increase code size (see --param ipa-cp-unit-growth=value). This flag is enabled by default at -O3. It is also enabled by -fprofile-use and -fauto-profile 🎀
  • -fipa-bit-cp — when enabled, performs interprocedural bitwise constant propagation. This flag is enabled by default at -O2 and by -fprofile-use and -fauto-profile. It requires that -fipa-cp is enabled 🧚
  • -fipa-vrp — when enabled, performs interprocedural propagation of value ranges. This flag is enabled by default at -O2. It requires that -fipa-cp is enabled 🍓
  • -fipa-icf — performs Identical Code Folding for functions and read-only variables. The optimization reduces code size and may disturb unwind stacks by replacing a function by equivalent one with a different name. The optimization works more effectively with link-time optimization enabled.

Although the behavior is similar to the Gold Linker's ICF optimization, GCC ICF works on different levels and thus the optimizations are not same — there are equivalences that are found only by GCC and equivalences found only by Gold.

This flag is enabled by default at -O2 and -Os 💖

  • -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 to produce output suitable for live-patching.

If the compiler's optimization uses a function's body or information extracted from its body to optimize/change another function, the latter is called an impacted function of the former. If a function is 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 function, 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 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 the following 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-patching is 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. It isolates those paths from the main control flow and turns the statement with erroneous or undefined behavior into a trap. This flag is enabled by default at -O2 and higher and depends on -fdelete-null-pointer-checks also being enabled 🙈
  • -fisolate-erroneous-paths-attribute — detects paths that trigger erroneous or undefined behavior due to a null value being used in a way forbidden by a "returns_nonnull" or "nonnull" attribute. 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 -O2 in the future 👀
  • -ftree-sink — performs forward store motion on trees. This flag is enabled by default at -O1 and 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 -O1 and higher, except for -Og. It requires that -ftree-ccp is 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 -O1 and 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. The flag is enabled by default at -O1 and higher 💫
  • -fssa-phiopt — performs pattern matching on SSA PHI nodes to optimize conditional code. This pass is enabled by default at -O1 and higher, except for -Og 🦋
  • -ftree-switch-conversion — performs conversion of simple initializations in a switch to initializations from a scalar array. This flag is enabled by default at -O2 and higher 🎀
  • -ftree-tail-merge — looks for identical code sequences. When found, it replaces one with a jump to the other. This optimization is known as tail merging or cross jumping. This flag is enabled by default at -O2 and higher. The compilation time in this pass can be limited using max-tail-merge-comparisons parameter and max-tail-merge-iterations parameter ⏳
  • -ftree-cselim — performs conditional store elimination on trees. This flag is enabled by default at -O1 and higher on targets that have conditional move instructions 💛
  • -ftree-dce — performs dead code elimination (DCE) on trees. This flag is enabled by default at -O1 and higher 🧹
  • -ftree-builtin-call-dce — performs conditional dead code elimination (DCE) for calls to built-in functions that may set "errno" but are otherwise free of side effects. This flag is enabled by default at -O2 and higher if -Os is not also specified 🌈
  • -ffinite-loops — bestie, this one says: assume a loop with an exit will eventually take that exit instead of vibing forever ♾️. That lets the compiler delete loops that otherwise have zero side-effects, because it's not out here considering "what if it just loops eternally" as a thing. 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, all based on a dominator tree traversal 🌳. It also does jump threading (to reduce jumps to jumps, no cap). Enabled by default at -O1 and higher. 💅
  • -ftree-dse — dead store elimination (DSE) on trees 🪦. A dead store is when you store into a memory location and then some other store overwrites it later with no intervening loads — so like, the first store literally did nothing? embarrassed for it. In that case the earlier store gets deleted. Enabled by default at -O1 and higher.
  • -ftree-ch — loop header copying on trees 📋. This one's a slay because it increases the effectiveness of code motion optimizations, plus it saves one jump. Enabled by default at -O1 and higher. It is not enabled for -Os, since it usually increases code size (we stay skinny at -Os, periodt).
  • -ftree-loop-optimize — performs loop optimizations on trees 🔁. Enabled by default at -O1 and higher.
  • -ftree-loop-linear, -floop-strip-mine, -floop-block — performs loop nest optimizations 🧚. Same as -floop-nest-optimize. To use this code transformation, GCC has to be configured with --with-isl to enable the Graphite loop transformation infrastructure. iykyk 🔥
  • -fgraphite-identity — enables the identity transformation for graphite 🎀. For every SCoP we generate the polyhedral representation and transform it back to gimple. Using -fgraphite-identity we can check the costs or benefits of the GIMPLE -> GRAPHITE -> GIMPLE transformation. Some minimal optimizations are also performed by the code generator isl, like index splitting and dead code elimination in loops.
  • -floop-nest-optimize — enables the isl based loop nest optimizer 🌀. It's a generic loop nest optimizer based on the Pluto optimization algorithms, and it calculates a loop structure optimized for data-locality and parallelism. Heads up though: this option is experimental 🧪.
  • -floop-parallelize-all — uses the Graphite data dependence analysis to find loops that can be parallelized 🌈. It parallelizes all the loops that can be analyzed to not contain loop carried dependences — without checking that it's actually profitable to parallelize them. Bold. Iconic. Slightly feral 💀.
  • -ftree-coalesce-vars — while transforming the program out of the SSA representation, it attempts to reduce copying by coalescing versions of different user-defined variables, instead of just compiler temporaries 🫶. Warning bestie: this may severely limit the ability to debug an optimized program compiled with -fno-var-tracking-assignments 😭. In the negated form, this flag prevents SSA coalescing of user variables. Enabled by default if optimization is enabled, and it does very little otherwise.
  • -ftree-loop-if-convert — attempts to transform conditional jumps in the innermost loops into branch-less equivalents 🦋. The intent is to remove control-flow from the innermost loops so the vectorization pass can handle these loops better. Enabled by default if vectorization is enabled.
  • -ftree-loop-distribution — performs loop distribution 🌸. This can improve cache performance on big loop bodies and allow further loop optimizations, like parallelization or vectorization, to take place. For example, the loop
        DO I = 1, N
          A(I) = B(I) + C
          D(I) = E(I) * F
        ENDDO

is transformed to

        DO I = 1, N
           A(I) = B(I) + C
        ENDDO
        DO I = 1, N
           D(I) = E(I) * F
        ENDDO

Enabled by default at -O3. It's also enabled by -fprofile-use and -fauto-profile.

  • -ftree-loop-distribute-patterns — performs loop distribution of patterns that can be code generated with calls to a library 📚. Enabled by default at -O2 and higher, and by -fprofile-use and -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
        ENDDO

is transformed to

        DO I = 1, N
           A(I) = 0
        ENDDO
        DO I = 1, N
           B(I) = A(I) + I
        ENDDO

and the initialization loop is transformed into a call to memset zero.

  • -floop-interchange — performs loop interchange outside of graphite 🔄. This flag can improve cache performance on loop nest and allow further loop optimizations, like vectorization, to take place. 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. It's also enabled 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. It's also enabled by -fprofile-use and -fauto-profile.
  • -ftree-loop-im — performs loop invariant motion on trees 🚚. This pass moves only invariants that are hard to handle at RTL level (function calls, operations that expand to nontrivial sequences of insns). With -funswitch-loops it also moves operands of conditions that are invariant out of the loop, so that we can use just trivial invariantness analysis in loop unswitching. The pass also includes store motion.
  • -ftree-loop-ivcanon — creates a canonical counter for number of iterations in loops for which determining number of iterations requires complicated analysis 🔢. Later optimizations then may determine the number easily. Useful especially in connection with unrolling.
  • -ftree-scev-cprop — performs final value replacement ✨. If a variable is modified in a loop in such a way that its value when exiting the loop can be determined using only its initial value and the number of loop iterations, replace uses of the final value by such a computation — provided it's sufficiently cheap. This reduces data dependencies and may allow further simplifications. Enabled by default at -O1 and higher.
  • -fivopts — performs induction variable optimizations (strength reduction, induction variable merging and induction variable elimination) on trees 💪. Enabled by default at -O1 and higher.
  • -ftree-parallelize-loops=_n_ — parallelizes loops, i.e., splits their iteration space to run in n threads 🧵. This is only possible for loops whose iterations are independent and can be arbitrarily reordered. The optimization is only profitable on multiprocessor machines, for loops that are CPU-intensive, rather than constrained e.g. by memory bandwidth. This option implies -pthread, and thus is only supported on targets that have support for -pthread.
  • -ftree-pta — performs function-local points-to analysis on trees 🎯. Enabled by default at -O1 and higher, except for -Og.
  • -ftree-sra — performs scalar replacement of aggregates 🧩. This pass replaces structure references with scalars to prevent committing structures to memory too early. Enabled by default at -O1 and higher, except for -Og.
  • -fstore-merging — performs merging of 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 -O2 and higher as well as -Os.
  • -ftree-ter — performs temporary expression replacement during the SSA->normal phase 🔁. Single use/single def temporaries are replaced at their use location with their defining expression. This results in non-GIMPLE code, but gives the expanders much more complex trees to work on, resulting in better RTL generation. Enabled by default at -O1 and higher.
  • -ftree-slsr — performs straight-line strength reduction on trees ⚡. This recognizes related expressions involving multiplications and replaces them by less expensive calculations when possible. Enabled by default at -O1 and higher.
  • -ftree-vectorize — performs vectorization on trees 🌟. This flag enables -ftree-loop-vectorize and -ftree-slp-vectorize if not explicitly specified.
  • -ftree-loop-vectorize — performs loop vectorization on trees 🔂. Enabled by default at -O2 and by -ftree-vectorize, -fprofile-use, and -fauto-profile.
  • -ftree-slp-vectorize — performs basic block vectorization on trees 🧱. Enabled by default at -O2 and by -ftree-vectorize, -fprofile-use, and -fauto-profile.
  • -ftrivial-auto-var-init=_choice_ — initializes automatic variables with either a pattern or with zeroes to increase the security and predictability of a program by preventing uninitialized memory disclosure and use 🔐. But fr fr, note this: GCC still considers an automatic variable that doesn't have an explicit initializer as uninitialized — -Wuninitialized and -Wanalyzer-use-of-uninitialized-value will still report warning messages on such automatic variables, and the compiler will perform optimization as if the variable were uninitialized. With this option, GCC will also initialize any padding of automatic variables that have structure or union types to zeroes. However, the current implementation cannot initialize automatic variables that are declared between the controlling expression and the first case of a switch statement 🙈. Use -Wtrivial-auto-var-init to report all such cases.

The three values of choice are:

  • uninitialized — doesn’t initialize any automatic variables. This is C and C++’s default. 💅
  • pattern — initializes automatic variables with values that 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 — initializes automatic variables with zeroes.

The default is uninitialized.

Quick note, bestie: 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. ✨

  • -fvect-cost-model=model — alters the cost model used for vectorization. The model argument should be one of unlimited, dynamic, cheap, or very-cheap. With the unlimited model, the vectorized code-path is assumed to be profitable. With the dynamic model, a runtime check guards the vectorized code-path to enable it only for iteration counts that will likely execute faster than when executing the original scalar loop. The cheap model disables vectorization of loops where doing so would be cost prohibitive, for example due to required runtime checks for data dependence or alignment, but otherwise is equal to the dynamic model. The very-cheap model disables vectorization of loops when any runtime check for data dependence or alignment is required; it also disables vectorization of epilogue loops but otherwise is equal to the cheap model.

The default cost model depends on other optimization flags and is either dynamic or cheap. 💅

  • -fsimd-cost-model=model — alters the cost model used for vectorization of loops marked with the OpenMP simd directive. The model argument should be one of unlimited, dynamic, cheap. All values of model have the same meaning as described in -fvect-cost-model, and by default a cost model defined with -fvect-cost-model is used.
  • -ftree-vrp — 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. This is enabled by default at -O2 and higher. Null pointer check elimination is only done if -fdelete-null-pointer-checks is enabled. 👑
  • -fsplit-paths — splits paths leading to loop backedges. This can improve dead code elimination and common subexpression elimination. This is enabled by default at -O3 and 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. 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 — inlines parts of functions. This option has any effect only when inlining itself is turned on by the -finline-functions or -finline-small-functions options. Enabled at 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 is 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 as sprintf, snprintf, vsprintf, and vsnprintf (but not printf of fprintf). 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-value is in effect, both the branch and the body of the if statement (but not the call to snprint) can be optimized away when i is 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-peephole and -fno-peephole2 is in how they are implemented in the compiler; some targets use one, some use the other, a few use both. -fpeephole is enabled by default. -fpeephole2 enabled at levels -O2, -O3, -Os. 💀
  • -fno-guess-branch-probability — do not guess branch probabilities using heuristics. GCC uses heuristics to guess branch probabilities if they are not provided by profiling feedback (-fprofile-arcs). These heuristics are based on the control flow graph. If some branch probabilities are specified by __builtin_expect, then the heuristics are used to guess branch probabilities for the rest of the control flow graph, taking the __builtin_expect info into account. The interactions between the heuristics and __builtin_expect can be complex, and in some cases, it may be useful to disable the heuristics so that the effects of __builtin_expect are easier to understand. It is 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** — uses the specified algorithm for basic block reordering. The algorithm argument can be simple, which does not increase code size (except sometimes due to secondary effects like alignment), or stc, 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 .o files, to improve paging and cache locality performance. This optimization is automatically turned off in the presence of exception handling or unwind tables (on targets using setjump/longjump or target specific scheme), for linkonce sections, for functions with a user-defined section attribute and on any architecture that does not support named sections. When -fsplit-stack is used this option is not enabled by default (to avoid linker errors), but may be enabled explicitly (if using a working linker). Enabled for x86 at levels -O2, -O3, -Os. 🍓
  • -freorder-functions — reorders functions in the object file in order to improve code locality. Unlike -fipa-reorder-for-locality this option prioritises grouping all functions within a category (hot/normal/cold/never-executed) together. This is implemented by using special subsections .text.hot for most frequently executed functions and .text.unlikely for unlikely executed functions. Reordering is done by the linker so object file format must support named sections and linker must place them in a reasonable way. This option isn’t effective unless you either provide profile feedback (see -fprofile-arcs for details) or manually annotate functions with hot or cold attributes. Enabled at levels -O2, -O3, -Os. periodt. 👑
  • -fstrict-aliasing — bestie, this one lets the compiler assume the strictest aliasing rules that apply to whatever language you're compiling. For C (and C++, obvi), it turns on optimizations based on the type of expressions — specifically, an object of one type is assumed to never live at the same address as an object of a different type, unless the types are, like, almost the same. For example, an unsigned int can alias an int, but not a void* or a double. A character type may alias any other type 💅

Pay special attention to code like this, fr fr:

union a_union {
  int i;
  double d;
};

int f() {
  union a_union t;
  t.d = 3.0;
  return t.i;
}

The practice of reading from a different union member than the one most recently written to (called "type-punning") is common. Even with -fstrict-aliasing, type-punning is allowed, provided the memory is accessed through the union type. So the code above works as expected 🫶 However, this code might not:

int f() {
  union a_union t;
  int* ip;
  t.d = 3.0;
  ip = &t.i;
  return *ip;
}

Similarly, access by taking the address, casting the resulting pointer and dereferencing the result has undefined behavior, even if the cast uses a union type, e.g.:

int f() {
  double d = 3.0;
  return ((union a_union *) &d)->i;
}

The -fstrict-aliasing option is enabled at levels -O2, -O3, -Os.

  • -fipa-strict-aliasing — this one controls whether the rules of -fstrict-aliasing get applied across function boundaries 🦋 Note that if multiple functions get inlined into a single function, the memory accesses are no longer considered to be crossing a function boundary, iykyk.

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 — align the start of functions to the next power-of-two greater than or equal to n, skipping up to m-1 bytes ✨ This ensures that 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 alignment is required for all functions, use -fmin-function-alignment instead.

If m is not specified, it defaults to n.

Examples, because yes we love a concrete demo: -falign-functions=32 aligns functions to the next 32-byte boundary, -falign-functions=24 aligns to the next 32-byte boundary only if this can be done by skipping 23 bytes or less, -falign-functions=32:7 aligns to the next 32-byte boundary only if this 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 this can be done by skipping 6 bytes or less, otherwise aligns to the next 32-byte boundary if this can be done by skipping 2 bytes or less. If m2 is not specified, it defaults to n2.

Some assemblers only support this flag when n is a power of two; in that case, it is rounded up 🔼

-fno-align-functions and -falign-functions=1 are equivalent and mean that functions are not aligned.

If n is not specified or is zero, use a machine-dependent default. The maximum allowed n option value is 65536.

Enabled at levels -O2, -O3.

  • -flimit-function-alignment — with this enabled, 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 — align all branch targets to a power-of-two boundary 💫

Parameters of this option are analogous to the -falign-functions option. -fno-align-labels and -falign-labels=1 are equivalent and mean that labels are not aligned.

If -falign-loops or -falign-jumps are applicable and are greater than this value, then their values are used instead.

If n is not specified or is zero, use a machine-dependent default which is very likely to be 1, meaning no alignment. 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 — align 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.

Parameters of this option are analogous to the -falign-functions option. -fno-align-loops and -falign-loops=1 are equivalent and mean that loops are not aligned. The maximum allowed n option value is 65536.

If n is not specified or is zero, use a machine-dependent default.

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 — align 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 — slay, no wasted cycles. This is 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.

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 is not specified or is zero, use a machine-dependent default. The maximum allowed n option value is 65536.

Enabled at levels -O2, -O3.

  • -fmin-function-alignment — specify minimal alignment of functions to the next power-of-two greater than or equal to n. Unlike -falign-functions this alignment is applied also to all functions (even those considered cold) 👑 The alignment is also not affected by -flimit-function-alignment.
  • -fno-allocation-dce — do not remove unused C++ allocations (using operator new and operator delete) in dead code elimination 🙈

See also -fmalloc-dce.

  • -fallow-store-data-races — allow the compiler to perform optimizations that may introduce new data races on stores, without proving that the variable cannot be concurrently accessed by other threads 😭 Does not affect optimization of local data. It is safe to use this option if it is known that global data will not 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. Such 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, bestie 😌 -funit-at-a-time has no effect, while -fno-unit-at-a-time implies -fno-toplevel-reorder and -fno-section-anchors.

Enabled by default.

  • -fno-toplevel-reorder — do not reorder top-level functions, variables, and asm statements 🙅‍♀️ Output them in the same order that they appear in the input file. When this option is used, unreferenced static variables are not removed. This option is intended to support existing code that relies on a particular ordering. For new code, it is better to use attributes when possible ✨

-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_unreachable into 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 — constructs webs as commonly used for register allocation purposes and assigns each web an individual pseudo register 🕸️ This allows the register allocation pass to operate on pseudos directly, but also strengthens several other optimization passes, such as CSE, loop optimizer and trivial dead code remover. It can, however, make debugging impossible, since variables no longer stay in a "home register" 😭 (iconic tradeoff, ngl)

Enabled by default with -funroll-loops.

  • -fwhole-program — assume that the current compilation unit represents the whole program being compiled 🌍 All public functions and variables with the exception of main and those merged by attribute externally_visible become static functions and in effect are optimized more aggressively by interprocedural optimizers.

With -flto this option has a limited use. In most cases the precise list of symbols used or exported from the binary is known the resolution info passed to the link-time optimizer by the linker plugin. It is still useful if no linker plugin is used or during incremental link step when final code is produced (with -flto -flinker-output=nolto-rel).

  • -flto[=n] — this option runs the standard link-time optimizer 🔗 When invoked with source code, it generates GIMPLE (one of GCC's internal representations) and writes it to special ELF sections in the object file. When the object files are linked together, all the function bodies are read from these ELF sections and instantiated as if they had been part of the same translation unit. Literally the whole "one big happy file" fantasy, but optimized 💖

okay so bestie, -flto is the link-time optimizer and it is not a one-and-done vibe — you gotta feed it optimization options at compile time and at final link. fr fr, the recommended move is to compile every file that’s in the same link with the same options, and then also specify those options at link time. Like this:

gcc -c -O2 -flto foo.c
gcc -c -O2 -flto bar.c
gcc -o myprog -flto -O2 foo.o bar.o

The first two GCC invocations save a bytecode representation of GIMPLE into special ELF sections inside foo.o and bar.o ✨. The final invocation reads that GIMPLE bytecode from foo.o and bar.o, merges the two files into one single internal image, and compiles the result as usual. Because both foo.o and bar.o get merged into a single image, all the interprocedural analyses and optimizations in GCC work across the two files as if they were one file. That means, for example, the inliner can inline functions in bar.o into functions in foo.o, and vice-versa 🫶.

Another (simpler!) way to enable link-time optimization is:

gcc -o myprog -flto -O2 foo.c bar.c

The above generates bytecode for foo.c and bar.c, merges them together into a single GIMPLE representation, and optimizes them as usual to produce myprog 💅.

The important thing to keep in mind is that to enable link-time optimizations, you need to use the GCC driver to perform the link step. GCC automatically performs link-time optimization if any of the objects involved were compiled with the -flto command-line option. You can always override the automatic decision to do link-time optimization by passing -fno-lto to the link command 👑.

To make whole program optimization effective, it is necessary to make certain whole program assumptions. The compiler needs to know what functions and variables can be accessed by libraries and runtime outside of the link-time optimized unit. When supported by the linker, the linker plugin (see -fuse-linker-plugin) passes information to the compiler about used and externally visible symbols. When the linker plugin is not available, -fwhole-program should be used to allow the compiler to make these assumptions, which leads to more aggressive optimization decisions 🔥.

When a file is compiled with -flto without -fuse-linker-plugin, the generated object file is larger than a regular object file because it contains GIMPLE bytecodes and the usual final code (see -ffat-lto-objects). This means object files with LTO information can be linked as normal object files; if -fno-lto is passed to the linker, no interprocedural optimizations are applied. Note that when -fno-fat-lto-objects is enabled the compile stage is faster, but you cannot perform a regular, non-LTO link on them 🙈.

When producing the final binary, GCC only applies link-time optimizations to those files that contain bytecode. Therefore, you can mix and match object files and libraries with GIMPLE bytecodes and final object code. GCC automatically selects which files to optimize in LTO mode and which files to link without further processing 🌈.

Generally, options specified at link time override those specified at compile time, although in some cases GCC attempts to infer link-time options from the settings used to compile the input files. iykyk.

If you do not specify an optimization level option -O at link time, then GCC uses the highest optimization level used when compiling the object files. Note that it is 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 be performed only at compile time and not at link time 😭.

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
  • all the -m target flags

The following options -fPIC, -fpic, -fpie and -fPIE are combined based on the following scheme:

B<-fPIC> + B<-fpic> = B<-fpic>
B<-fPIC> + B<-fno-pic> = B<-fno-pic>
B<-fpic/-fPIC> + (no option) = (no option)
B<-fPIC> + B<-fPIE> = B<-fPIE>
B<-fpic> + B<-fPIE> = B<-fpie>
B<-fPIC/-fpic> + B<-fpie> = B<-fpie>

Certain ABI-changing flags are required to match in all compilation units, and trying to override this at link time with a conflicting value is ignored. This includes options such as -freg-struct-return and -fpcc-struct-return 💀.

Other options such as -ffp-contract, -fno-strict-overflow, -fwrapv, -fno-trapv or -fno-strict-aliasing are passed through to the link stage and merged conservatively for conflicting translation units. Specifically, -fno-strict-overflow, -fwrapv and -fno-trapv take precedence; and for example -ffp-contract=off takes precedence over -ffp-contract=fast. You can override them at 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. Note that code transforms such as inlining can lead to warnings being enabled or disabled for regions of code not 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. Similar diagnostics may be raised for other languages.

heads up bestie: that’s still undefined behavior at run time — the diagnostic being non-fatal does not make it safe. 😭

Another feature of LTO is that it is possible to apply interprocedural optimizations on files written in different languages:

gcc -c -flto foo.c
g++ -c -flto bar.cc
gfortran -c -flto baz.f90
g++ -o myprog -flto -O3 foo.o bar.o baz.o -lgfortran

Notice that the final link is done with g++ to get the C++ runtime libraries, and -lgfortran is added to get the Fortran runtime libraries. In general, when mixing languages in LTO mode, you should use the same link command options as when mixing languages in a regular (non-LTO) compilation 💖.

If object files containing GIMPLE bytecode are stored in a library archive, say libfoo.a, it is possible to extract and use them in an LTO link if you are using a linker with plugin support. To create static libraries suitable for LTO, use gcc-ar and gcc-ranlib instead of ar and ranlib; to show the symbols of object files with GIMPLE bytecode, use gcc-nm. Those commands require that ar, ranlib and nm have been compiled with plugin support. At link time, use the flag -fuse-linker-plugin to ensure that the library participates in the LTO optimization process:

gcc -o myprog -O2 -flto -fuse-linker-plugin a.o b.o -lfoo

With the linker plugin enabled, the linker extracts the needed GIMPLE files from libfoo.a and passes them on to the running GCC to make them part of the aggregated GIMPLE image to be optimized ✨.

If you are not using a linker with plugin support and/or do not enable the linker plugin, then the objects inside libfoo.a are extracted and linked as usual, but they do not participate in the LTO optimization process. In order to make a static library suitable for both LTO optimization and usual linkage, compile its object files with -flto -ffat-lto-objects 🫶.

Link-time optimizations do not require the presence of the whole program to operate. If the program does not require any symbols to be exported, it is possible to combine -flto and -fwhole-program to allow the interprocedural optimizers to use more aggressive assumptions which may lead to improved optimization opportunities. Use of -fwhole-program is not needed when the linker plugin is active (see -fuse-linker-plugin) 💅.

The current implementation of LTO makes no attempt to generate bytecode that is portable between different types of hosts. The bytecode files are versioned and there is a strict version check, so bytecode files generated in one version of GCC do not work with an older or newer version of GCC. no cap, version match or it’s a no-go. 😭

Link-time optimization does not work well with generation of debugging information on systems other than those using a combination of ELF and DWARF 💀.

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 ⭐.

  • -flto-partition=alg — bestie, this one picks the partitioning algorithm the link-time optimizer uses. The value is one of these:
    • 1to1 — partitions mirror the original source files, cute and literal 💅
    • balanced — splits into equally sized chunks whenever possible (this is the default, the main character)
    • max — makes a new partition for every symbol where possible
    • cache — balances chunk sizes while keeping related symbols together for better caching in incremental LTO, obsessed 🫶
    • none — disables partitioning and streaming completely, like, fully off

The default value is balanced. So 1to1 can be used as a workaround for various code ordering issues, but the max partitioning is intended for internal testing only — do not come for it 🙈. 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. Two different things, iykyk ✨

  • -flto-incremental=path — enables incremental LTO, with its cache in the given existing directory. Can significantly shorten edit-compile cycles with LTO. Like, actually life-changing for your build times fr fr 🔥

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. It's giving smart ✨

Multiple GCC instances can use the same cache in parallel. Sharing is caring, no cap 💖

  • -flto-incremental-cache-size=n — specifies number of cache entries in incremental LTO after which to prune old entries. Heads up though: this is a soft limit, so temporarily there may be more entries. It's not that deep, it's just a vibe check on the cache 🎀
  • -flto-compression-level=n — this option specifies the level of compression used for intermediate language written to LTO object files, and it's only meaningful in conjunction with LTO mode (-flto). GCC currently supports two LTO compression algorithms:
    • zstd — valid values are 0 (no compression) to 19 (maximum compression)
    • 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. Squish responsibly 🍓

  • -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. Ngl, that's a real requirement, can't skip it 💀

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. Go read the lore 📖

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). Free win, we love to see it 👑

  • -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. Chunky, but versatile 🧚

-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. Yeaaah, that's the catch 😭

Note that modern binutils provide plugin auto-load mechanism. Installing the linker plugin into $libdir/bfd-plugins has the same effect as usage of the command wrappers (gcc-ar, gcc-nm and gcc-ranlib). Convenient, we stan ✨

The default is -fno-fat-lto-objects on targets with linker plugin support.

  • -fcompare-elim — after register allocation and post-register allocation instruction splitting, identify arithmetic instructions that compute processor flags similar to a comparison operation based on that arithmetic. If possible, eliminate the explicit comparison operation. Sneaky optimization, love her 💅

This pass only applies to certain targets that cannot explicitly represent the comparison operation before register allocation is complete. Target-specific queen 🎀

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. Bye bye extra adds 👋

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. Occasionally eliminate the copy — she's got range 🦋

Enabled at levels -O1, -O2, -O3, -Os.

  • -fprofile-correction — okay so profiles collected using an instrumented binary for multi-threaded programs may be inconsistent due to missed counter updates. Literally the data's a little messy 😭. 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. Not her being dramatic about it 💀

This option is enabled by -fauto-profile.

  • -fprofile-partial-training — with -fprofile-use, all portions of programs not executed during train run 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, program may contain functions specific for a given hardware and training may not cover all hardware configurations program is run on.) With -fprofile-partial-training profile feedback is ignored for all functions not executed during the train run, leading them to be optimized as if they were compiled without profile feedback. This leads to better performance when train run is not representative but also leads to significantly bigger code. So like, pick your fighter: faster or smaller 🫶
  • -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-functions

Before you can use this option, you must first generate profiling information. Like, non-negotiable, bestie — no profile, no party 🎉

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-correction

path is the name of a file containing AutoFDO profile information. If omitted, it defaults to fbdata.afdo in the current directory. Defaults, we love a fallback 💖

Producing an AutoFDO profile data file requires running your program with the perf utility on a supported GNU/Linux target system. For more information, see <https://perf.wiki.kernel.org/>.

E.g.

perf record -e br_inst_retired:near_taken -b -o perf.data \
    -- your_program

Then use the create_gcov tool to convert the raw profile data to a format that can be used by GCC. You must also supply the unstripped binary for your program to this tool. See <https://github.com/google/autofdo>. Don't forget the unstripped one, that's the whole point 🧚

E.g.

create_gcov --binary=your_program.unstripped --profile=perf.data \
    --gcov=profile.afdo

The following options control compiler behavior regarding floating-point arithmetic. These options trade off between speed and correctness. All must be specifically enabled. Like, none of these are on by default — you gotta ask for them, bestie 🌸

  • -fexcess-precision=style — this option allows control over excess precision on machines where floating-point operations occur in a format with more precision or range than the IEEE standard and interchange floating-point types. An 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, but some programs may rely on the requirements of the C or C++ language standards for handling IEEE values. Highkey the details matter here ✨

By default, -fexcess-precision=fast is in effect; this means that 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. Wild, right? 😭 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. Sixteen, no excess, periodt 💅

  • -fexcess-precision=standard — bestie, pls note: this one is not implemented for languages other than C or C++. On x86 it's a whole no-op if -mfpmath=sse or -mfpmath=sse+387 is in play 🫠 in the former case, IEEE semantics apply without excess precision, and in the latter, rounding is just... 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. Ngl this option has generally been subsumed by -fexcess-precision=standard, which is more general (slay, more general icon). If you do use -ffloat-store, you may need to modify your program to explicitly store intermediate computations in temporary variables, since -ffloat-store handles rounding to IEEE format only on assignments and not casts — unlike -fexcess-precision=standard, which does both. ✨
  • -ffast-math — the group-chat bundle 🔥 it sets -fno-math-errno, -funsafe-math-optimizations, -ffinite-math-only, -fno-rounding-math, -fno-signaling-nans, -fcx-limited-range and -fexcess-precision=fast.

This option causes the preprocessor macro __FAST_MATH__ to be defined.

Not turned on by any -O option besides -Ofast, since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions 😭 it may, however, yield faster code for programs that do not require the guarantees of these specifications.

  • -fno-math-errno — do not set errno after calling math functions that are executed with a single instruction, e.g. sqrt. A program that relies on IEEE exceptions for math error handling may want this flag for speed while maintaining IEEE arithmetic compatibility.

Not turned on by any -O option besides -Ofast, since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that do not require the guarantees of these specifications.

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. Periodt.

  • -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. 😳 no cap, "unsafe" is literally in the name.

Not turned on by any -O option besides -Ofast, since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that do not require the guarantees of these specifications. Enables -fno-signed-zeros, -fno-trapping-math, -fassociative-math and -freciprocal-math.

The default is -fno-unsafe-math-optimizations.

  • -fassociative-math — allow re-association of operands in series of floating-point operations. Okay so this one violates the ISO C and C++ language standard by possibly changing computation result 🙈 NOTE: re-ordering may change the sign of zero as well as ignore NaNs and inhibit or create underflow or overflow — and thus cannot be used on code that relies on rounding behavior like (x + 2**52) - 2**52. May also reorder floating-point comparisons, and thus may not be used when ordered comparisons are required. This option requires that both -fno-signed-zeros and -fno-trapping-math be in effect. Moreover, it doesn't make much sense with -frounding-math. For Fortran the option is automatically enabled when both -fno-signed-zeros and -fno-trapping-math are in effect.

The default is -fno-associative-math.

  • -freciprocal-math — allow the reciprocal of a value to be used instead of dividing by the value if this enables optimizations. For example x / y can be replaced with x * (1/y), which is useful if (1/y) is subject to common subexpression elimination. Note that this loses precision and increases the number of flops operating on the value. 💅 trade-offs, babes.

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.

Not turned on by any -O option besides -Ofast, since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that do not require the guarantees of these specifications.

The default is -fno-finite-math-only.

  • -fno-signed-zeros — allow optimizations for floating-point arithmetic that ignore the signedness of zero. IEEE arithmetic specifies the behavior of distinct +0.0 and -0.0 values, which then prohibits simplification of expressions such as x+0.0 or 0.0*x (even with -ffinite-math-only). This option implies that the sign of a zero result isn't significant.

The default is -fsigned-zeros.

  • -fno-trapping-math — compile code assuming that floating-point operations cannot generate user-visible traps. These traps include division by zero, overflow, underflow, inexact result and invalid operation. This option requires that -fno-signaling-nans be in effect. Setting this option may allow faster code if one relies on "non-stop" IEEE arithmetic, for example.

Not turned on by any -O option besides -Ofast, since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions.

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. It disables constant folding of floating-point expressions at compile time (which may be affected by rounding mode) and arithmetic transformations that are unsafe in the presence of sign-dependent rounding modes.

The default is -fno-rounding-math.

This option is experimental and does not currently guarantee to disable all GCC optimizations that are affected by rounding mode. 👀

Future versions of GCC may provide finer control of this setting using C99's FENV_ACCESS pragma. This command-line option will be used along with -ftrapping-math to specify the default state for FENV_ACCESS.

  • -fsignaling-nans — compile code assuming that IEEE signaling NaNs may generate user-visible traps during floating-point operations. Setting this option disables optimizations that may change the number of exceptions visible with signaling NaNs. This option implies -ftrapping-math.

This option causes the preprocessor macro __SUPPORT_SNAN__ to be defined. 🌸

The default is -fno-signaling-nans.

This option is experimental and does not currently guarantee to disable all GCC optimizations that affect signaling NaN behavior. 😭
  • -fno-fp-int-builtin-inexact — do not allow the built-in functions ceil, floor, round and trunc, and their float and long double variants, to generate code that raises the inexact floating-point exception for noninteger arguments. ISO C99 and C11 allow these functions to raise the inexact exception, 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 -ffp-int-builtin-inexact, allowing the exception to be raised, unless C23 or a later 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 be raised if the library implementation does not follow TS 18661. 💀

  • -fsingle-precision-constant — treat floating-point constants as single precision instead of implicitly converting them to double-precision constants. slay, less precision by choice ✨
  • -fcx-limited-range — when enabled, this option states that a range reduction step is not needed when performing complex division. Also, there is no checking whether the result of a complex multiplication or division is NaN + I*NaN, with an attempt to rescue the situation in that case. The default is -fno-cx-limited-range, but is enabled by -ffast-math.

This option controls the default setting of the ISO C99 CX_LIMITED_RANGE pragma. Nevertheless, the option applies to all languages. 🌈

  • -fcx-fortran-rules — complex multiplication and division follow Fortran rules. Range reduction is done as part of complex division, but there is no checking whether the result of a complex multiplication or division is NaN + I*NaN, with an attempt to rescue the situation in that case.

The default is -fno-cx-fortran-rules.

The following options control optimizations that may improve performance, but are not enabled by any -O options 🫶 this section includes experimental options that may produce broken code — bestie, read that twice. 💀

  • -fbranch-probabilities — after running a program compiled with -fprofile-arcs, you can compile it a second time using -fbranch-probabilities, to improve optimizations based on the number of times each branch was taken. When a program compiled with -fprofile-arcs exits, it saves arc execution counts to a file called sourcename.gcda for each source file. The information in this data file is very dependent on the structure of the generated code, so you must use the same source code and the same optimization options for both compilations 🔥 see details about the file naming in -fprofile-arcs.

With -fbranch-probabilities, GCC drops a REG_BR_PROB note on each JUMP_INSN and CALL_INSN — little receipts for the optimizer 💅. These can be used to improve optimization. Right now they're 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. No cap, actual data 🎀.

Enabled by -fprofile-use and -fauto-profile.

  • -fprofile-values — If you pair it with -fprofile-arcs, it adds code so that some data about values of expressions in the program is gathered. 📊

With -fbranch-probabilities, it reads back the data gathered from profiling values of expressions for usage in optimizations.

Enabled by -fprofile-generate, -fprofile-use, and -fauto-profile.

  • -fprofile-reorder-functions — Function reordering based on profile instrumentation: it 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. Main character energy for your startup time ✨.

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. Like, it knows now 🔥.

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". 😭 So like, great for speed, mildly cursed for your debugger.

Enabled by default with -funroll-loops.

  • -fschedule-fusion — Performs a target dependent pass over the instruction stream to schedule instructions of same type together, because the target machine can execute them more efficiently if they are adjacent to each other in the instruction flow. Bestie grouping 🫶.

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. It's giving "make everyone else's life easier" 💖.

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-loops implies -frerun-cse-after-loop, -fweb and -frename-registers. It also turns on complete loop peeling (i.e. complete removal of loops with a small constant number of iterations). This option makes code larger, and may or may not make it run faster. 🙈 The eternal tradeoff, ngl.

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-loops implies the same options as -funroll-loops.
  • -fpeel-loops — Peels loops for which there is enough information that they do not roll much (from profile feedback 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 — Control 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 "free" call or compared with "NULL". If -fmalloc-dce=1 is used, only calls to "free" are allowed, while with -fmalloc-dce=2 also comparisons with "NULL" pointer are 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 -O1 and 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-im has to be enabled as well. Enabled at level -O1 and 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 becomes two, slay.

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. So like:
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. ⚠️
  • -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. 🧚

  • -fzero-call-used-regs=choice — Zero call-used registers at function return to increase program security by either mitigating Return-Oriented Programming (ROP) attacks or preventing information leakage through registers. 🔒

The possible values of choice are the same as for the "zero_call_used_regs" attribute. The default is skip.

You can control this behavior for a specific function by using the function attribute "zero_call_used_regs".

  • --param name=value — In some places, GCC uses various constants to control the amount of optimization that is done. For example, GCC does not inline functions that contain more than a certain number of instructions. You can control some of these constants on the command line using the --param option. 🎛️

The names of specific parameters, and the meaning of the values, are tied to the internals of the compiler, and are subject to change without notice in future releases. 👀 iykyk, don't get too attached.

In order to get the minimal, maximal and default values of a parameter, use the --help=param -Q options.

In each case, the value is an integer. The following choices of name are recognized for all targets:

phiopt-factor-max-stmts-live

When factoring statements out of if/then/else, this is the max # of statements after the defining statement to be allow to extend the lifetime of a name.

predictable-branch-outcome

When branch is predicted to be taken with probability lower than this threshold (in percent), then it is considered well predictable.

max-rtl-if-conversion-insns

RTL if-conversion tries to remove conditional branches around a block and replace them with conditionally executed instructions. This parameter gives the maximum number of instructions in a block which should be considered for if-conversion. The compiler will also use other heuristics to decide whether if-conversion is likely to be profitable.

file-cache-files

Max number of files in the file cache. The file cache is used to print source lines in diagnostics and do some source checks like -Wmisleading-indentation.

file-cache-lines

Max number of lines to index into file cache. When 0 this is automatically sized. The file cache is used to print source lines in diagnostics and do some source checks like -Wmisleading-indentation. 🌸

max-rtl-if-conversion-predictable-cost

So RTL if-conversion is out here trying to delete conditional branches wrapping a whole block and swap them for conditionally executed instructions instead — cute, right? 💅 These parameters hand it the maximum permissible cost for the sequence it'd generate, and the answer depends on whether the branch is statically determined to be predictable or not. Units? Same as the ones for the GCC internal seq_cost metric, no cap. The compiler will try to give you a reasonable default for this parameter using the BRANCH_COST target macro ✨

max-crossjump-edges

The max number of incoming edges to even consider for cross-jumping 🌀. Heads up: the algorithm used by -fcrossjumping is O(N^2) in the number of edges incoming to each block, so like… math. Bumping this value up means more aggressive optimization, which makes compilation time go up with probably small improvement in executable size. Trade-off city 💀

min-crossjump-insns

The minimum number of instructions that have to match at the end of two blocks before cross-jumping gets performed on them 🎀. Tiny little exception tho: this value is ignored in the case where all instructions in the block being cross-jumped from are matched. Slay.

max-grow-copy-bb-insns

The maximum code size expansion factor when copying basic blocks instead of jumping 🦋. The expansion is relative to a jump instruction, iykyk.

max-goto-duplication-insns

The max number of instructions to duplicate to a block that jumps to a computed goto 👑. Here's the lore: to avoid O(N^2) behavior in a number of passes, GCC factors computed gotos early in the compilation process, and unfactors them as late as possible. And only computed jumps at the end of a basic blocks with no more than max-goto-duplication-insns are unfactored. Periodt.

The maximum number of instructions to consider when hunting for an instruction to fill a delay slot 🔍. If more than this arbitrary number of instructions get searched, the time savings from filling the delay slot are minimal, so it just stops searching. Increasing values mean more aggressive optimization, making compilation time increase with probably small improvement in execution time. Ngl, it's giving diminishing returns.

When trying to fill delay slots, this is the maximum number of instructions to consider when searching for a block with valid live register information 🫶. Increasing this arbitrarily chosen value means more aggressive optimization, increasing the compilation time. Also, big mood: this parameter should be removed when the delay slot code is rewritten to maintain the control-flow graph. So it's on borrowed time fr fr 💀

max-gcse-memory

The approximate maximum amount of memory in kB that can be allocated in order to perform the global common subexpression elimination optimization 🧠. If more memory than specified is required, the optimization is just… not done. Bye 🙈

max-gcse-insertion-ratio

If the ratio of expression insertions to deletions is larger than this value for any expression, then RTL PRE inserts or removes the expression and thus leaves partially redundant computations in the instruction stream 💫. That's the whole rule, bestie.

max-pending-list-length

The maximum number of pending dependencies scheduling allows before flushing the current state and starting over 🔄. Real talk: large functions with few branches or calls can create excessively large lists which needlessly consume memory and resources. Not the vibe.

max-modulo-backtrack-attempts

The maximum number of backtrack attempts the scheduler should make when modulo scheduling a loop 🎡. Larger values can exponentially increase compilation time. We love a warning.

max-inline-functions-called-once-loop-depth

Maximal loop depth of a call considered by inline heuristics that tries to inline all functions called once 🌸. Short and sweet.

max-inline-functions-called-once-insns

Maximal estimated size of functions produced while inlining functions called once ✨. Also short and sweet.

max-inline-insns-single

Several parameters control the tree inliner used in GCC 🌳. This number sets the maximum number of instructions (counted in GCC's internal representation) in a single function that the tree inliner considers for inlining. And ngl it's picky — this only affects functions declared inline and methods implemented in a class declaration (C++).

max-inline-insns-auto

When you use -finline-functions (included in -O3), a lot of functions that would otherwise not be considered for inlining by the compiler get investigated 👀. To those functions, a different (more restrictive) limit compared to functions declared inline can be applied (--param max-inline-insns-auto). More restrictive, because of course 💅

max-inline-insns-small

This is the bound applied to calls that are considered relevant with -finline-small-functions 🎀.

max-inline-insns-size

This is the bound applied to calls that are optimized for size 📏. And like, small growth may be desirable to anticipate optimization opportunities exposed by inlining. Sometimes you gotta spend instructions to make instructions, yk?

uninlined-function-insns

Number of instructions accounted by inliner for function overhead such as function prologue and epilogue 🧾. Gotta budget for the boring parts.

uninlined-function-time

Extra time accounted by inliner for function overhead such as time needed to execute function prologue and epilogue ⏱️. Same energy, but in time.

inline-heuristics-hint-percent

The scale (in percents) applied to inline-insns-single, inline-insns-single-O2, inline-insns-auto when inline heuristics hints that inlining is very profitable (will enable later optimizations) 🔥. Basically the hype multiplier.

uninlined-thunk-insns

uninlined-thunk-time

Same as --param uninlined-function-insns and --param uninlined-function-time but applied to function thunks 🫶. Thunks get the same treatment, no discrimination here.

inline-min-speedup

When estimated performance improvement of caller + callee runtime exceeds this threshold (in percent), the function can be inlined regardless of the limit on --param max-inline-insns-single and --param max-inline-insns-auto 🚀. Like, if it's that good, the limits simply don't apply. Ate.

large-function-insns

The limit specifying really large functions 🐘. For functions larger than this limit after inlining, inlining is constrained by --param large-function-growth. This parameter is useful primarily to avoid extreme compilation time caused by non-linear algorithms used by the back end. Non-linear algorithms my beloathed 😭

large-function-growth

Specifies maximal growth of large functions caused by inlining in percents 📈. Example, because numbers are real: parameter value 100 limits large function growth to 2.0 times the original size.

large-unit-insns

The limit specifying large translation unit 🗃️. Growth caused by inlining of units larger than this limit is limited by --param inline-unit-growth. For small units this might be too tight. For example, consider a unit consisting of function A that is inline and B that just calls A three times. If B is small relative to A, the growth of unit is 300% and yet such inlining is very sane 🧚. For very large units consisting of small inlineable functions, however, the overall unit growth limit is needed to avoid exponential explosion of code size. Thus for smaller units, the size is increased to --param large-unit-insns before applying --param inline-unit-growth. Clever little loophole, honestly.

lazy-modules

Maximum number of concurrently open C++ module files when lazy loading 📂.

inline-unit-growth

Specifies maximal overall growth of the compilation unit caused by inlining 🌱. Example time: parameter value 20 limits unit growth to 1.2 times the original size. And heads up — cold functions (either marked cold via an attribute or by profile feedback) are not accounted into the unit size. They get a pass 💜

ipa-cp-unit-growth

Specifies maximal overall growth of the compilation unit caused by interprocedural constant propagation 🔮. Example, once more: parameter value 10 limits unit growth to 1.1 times the original size.

ipa-cp-large-unit-insns

The size of translation unit that IPA-CP pass considers large 📦. That's it, that's the whole param.

large-stack-frame

The limit specifying large stack frames 🥞. While inlining, the algorithm is trying to not grow past this limit too much. "Trying" being the operative word, bestie.

large-stack-frame-growth

Specifies maximal growth of large stack frames caused by inlining in percents 📊. Example: parameter value 1000 limits large stack frame growth to 11 times the original size. Eleven. Times. 💀

max-inline-insns-recursive

max-inline-insns-recursive-auto

Specifies the maximum number of instructions an out-of-line copy of a self-recursive inline function can grow into by performing recursive inlining 🌀🌀. And here's the split, so listen up: --param max-inline-insns-recursive applies to functions declared inline. For functions not declared inline, recursive inlining happens only when -finline-functions (included in -O3) is enabled; --param max-inline-insns-recursive-auto applies instead 🫶

max-inline-recursive-depth

max-inline-recursive-depth-auto

Specifies the maximum recursion depth used for recursive inlining 🕳️. Same deal as above: --param max-inline-recursive-depth applies to functions declared inline. For functions not declared inline, recursive inlining happens only when -finline-functions (included in -O3) is enabled; --param max-inline-recursive-depth-auto applies instead ✨

min-inline-recursive-probability

Okay so recursive inlining is profitable only for function having deep recursion in average and can hurt for function having little recursion depth by increasing the prologue size or complexity of function body to other optimizers 😭. When profile feedback is available (see -fprofile-generate) the actual recursion depth can be guessed from the probability that function recurses via a given call expression. This parameter limits inlining only to call expressions whose probability exceeds the given threshold (in percents) 🎯

  • early-inlining-insns — picks how much growth the early inliner is allowed to make 🌱 and like, the effect is that code with a big ol' abstraction penalty gets way more inlining. slay.
  • max-early-inliner-iterations — caps how many iterations the early inliner does. basically it bounds how many nested indirect calls the early inliner can resolve — any deeper chains still get handled by late inlining, so no cap, no drama 🙈
  • comdat-sharing-probability — the probability, in percent, that a C++ inline function with comdat visibility gets shared across multiple compilation units 💫
  • modref-max-bases, modref-max-refs, modref-max-accesses — these three besties specify the maximal number of base pointers, references and accesses stored for a single function by mod/ref analysis 🎀
  • modref-max-tests — the maximal number of tests the alias oracle can perform to disambiguate memory locations using the mod/ref info. and like, this param ought to be bigger than --param modref-max-bases and --param modref-max-refs, ok? iykyk 🔍
  • modref-max-depth — the maximum depth of the DFS walk used by modref escape analysis. setting it to 0 disables the analysis completely, like fully off, no vibe 💀
  • modref-max-escape-points — the maximum number of escape points tracked by modref per SSA-name ✨
  • modref-max-adjustments — the maximum number of times the access range is enlarged during modref dataflow analysis 📈
  • profile-func-internal-id — a parameter controlling whether to use the function internal id in profile database lookup. if the value is 0, the compiler uses an id based on the function assembler name and filename, which makes old profile data more tolerant to source changes such as function reordering etc. 🫶
  • min-vect-loop-bound — the minimum number of iterations under which loops are not vectorized when -ftree-vectorize is used. after vectorization the iteration count needs to be greater than the value you specify here to actually allow the vectorization, no cap 🌈
  • gcse-cost-distance-ratio — scaling factor in the calculation of the maximum distance an expression can be moved by GCSE optimizations. currently supported only in the code hoisting pass. the bigger the ratio, the more aggressive code hoisting gets with simple expressions — i.e. the expressions that cost less than gcse-unrestricted-cost. specifying 0 disables hoisting of simple expressions entirely 🙅‍♀️
  • gcse-unrestricted-cost — a cost, roughly measured as the cost of a single typical machine instruction, at which GCSE optimizations stop constraining how far an expression can travel. again, currently supported only in the code hoisting pass. the lesser the cost, the more aggressive the hoisting gets. specifying 0 lets all expressions travel unrestricted distances 🔥
  • max-hoist-depth — the depth of search in the dominator tree for expressions to hoist. this exists to avoid quadratic behavior in the hoisting algorithm. a value of 0 doesn't limit the search, but heads up, it may slow down compilation of huge functions ⚠️
  • max-tail-merge-comparisons — the maximum amount of similar bbs to compare a bb with. used to dodge quadratic behavior in tree tail merging 💅
  • max-tail-merge-iterations — the maximum amount of iterations of the pass over the function. used to limit compilation time in tree tail merging ⏳
  • store-merging-allow-unaligned — lets the store merging pass introduce unaligned stores if it's legal to do so 👑
  • max-stores-to-merge — the maximum number of stores to attempt to merge into wider stores in the store merging pass 🧚
  • max-store-chains-to-track — the maximum number of store chains to track at the same time while trying to merge them into wider stores in the store merging pass 🎀
  • max-stores-to-track — the maximum number of stores to track at the same time in the attempt to merge them into wider stores in the store merging pass 🍓
  • max-unrolled-insns — the maximum number of instructions a loop may have to be unrolled. if a loop does get unrolled, this parameter also determines how many times the loop code is unrolled 💫
  • max-average-unrolled-insns — the maximum number of instructions biased by probabilities of their execution that a loop may have to be unrolled. same deal: if a loop is unrolled, this also determines how many times the loop code is unrolled 🌸
  • max-unroll-times — the maximum number of unrollings of a single loop 🔁
  • max-peeled-insns — the maximum number of instructions a loop may have to be peeled. if a loop is peeled, this parameter also determines how many times the loop code is peeled 🍌
  • max-peel-times — the maximum number of peelings of a single loop ✂️
  • max-peel-branches — the maximum number of branches on the hot path through the peeled sequence 🔥
  • max-completely-peeled-insns — the maximum number of insns of a completely peeled loop 💅
  • max-completely-peel-times — the maximum number of iterations of a loop to be suitable for complete peeling 🌀
  • max-completely-peel-loop-nest-depth — the maximum depth of a loop nest suitable for complete peeling 🪆
  • max-unswitch-insns — the maximum number of insns of an unswitched loop 🎚️
  • max-unswitch-depth — the maximum depth of a loop nest to be unswitched 🕳️
  • lim-expensive — the minimum cost of an expensive expression in loop invariant motion 💰
  • min-loop-cond-split-prob — when FDO profile information is available, min-loop-cond-split-prob specifies the minimum threshold for the probability of a semi-invariant condition statement to trigger a loop split 📊
  • iv-consider-all-candidates-bound — a bound on the number of candidates for induction variables, below which all candidates are considered for each use in induction variable optimizations. if there are more candidates than this, only the most relevant ones get considered, to avoid quadratic time complexity 😮‍💨
  • iv-max-considered-uses — the induction variable optimizations give up on loops that contain more induction variable uses 🏳️
  • iv-always-prune-cand-set-bound — if the number of candidates in the set is smaller than this value, always try to remove unnecessary ivs from the set when adding a new one 🧹
  • avg-loop-niter — average number of iterations of a loop 🔄
  • dse-max-object-size — maximum size (in bytes) of objects tracked bytewise by dead store elimination. larger values may result in larger compilation times, so like, tradeoff 💀
  • dse-max-alias-queries-per-store — maximum number of queries into the alias oracle per store. larger values result in larger compilation times and may result in more removed dead stores ⚖️
  • scev-max-expr-size — bound on the size of expressions used in the scalar evolutions analyzer. large expressions slow the analyzer down 🐌
  • scev-max-expr-complexity — bound on the complexity of the expressions in the scalar evolutions analyzer. complex expressions also slow the analyzer down, yeah it's the same vibe 😭
  • max-tree-if-conversion-phi-args — maximum number of arguments in a PHI supported by TREE if conversion, unless the loop is marked with the simd pragma 👀
  • vect-max-layout-candidates — the maximum number of possible vector layouts (such as permutations) to consider when optimizing to-be-vectorized code 🧩
  • vect-max-version-for-alignment-checks — the maximum number of run-time checks that can be performed when doing loop versioning for alignment in the vectorizer 📐
  • vect-max-version-for-alias-checks — the maximum number of run-time checks that can be performed when doing loop versioning for alias in the vectorizer 🪞
  • vect-max-peeling-for-alignment — the maximum number of loop peels to enhance access alignment for the vectorizer. value -1 means no limit 🌈
  • max-iterations-to-track — the maximum number of iterations of a loop that the brute-force algorithm for analysis of the number of iterations of the loop tries to evaluate 💪
  • hot-bb-count-fraction — the denominator n of the fraction 1/n of the maximal execution count of a basic block in the entire program that a basic block needs to at least have to be considered hot. the default is 10000, which means a basic block is considered hot if its execution count is greater than 1/10000 of the maximal execution count. 0 means it's never considered hot. used in non-LTO mode 🔥
  • hot-bb-count-ws-permille — the number of most executed permilles, ranging from 0 to 1000, of the profiled execution of the entire program that a basic block's execution count must be part of to be considered hot. the default is 990, which means a basic block is considered hot if its execution count contributes to the upper 990 permilles, or 99.0%, of the profiled execution of the entire program. 0 means it's never considered hot. used in LTO mode 🌡️
  • hot-bb-frequency-fraction — the denominator n of the fraction 1/n of the execution frequency of the entry block of a function that a basic block of that function needs to at least have to be considered hot. the default is 1000, which means a basic block is considered hot in a function if it's executed more frequently than 1/1000 of the frequency of the function's entry block. 0 means it's never considered hot 🥵
  • unlikely-bb-count-fraction — okay so this one sets the denominator n of the fraction 1/n of the number of profiled runs of the entire program. Below that fraction, a basic block's execution count has to be for it to be counted as unlikely executed. Default is 20, meaning a basic block lowkey counts as unlikely executed if it runs in fewer than 1/20, or 5%, of the program's runs. Set it to 0 and it's always considered unlikely executed. iykyk.
  • max-predicted-iterations — the max number of loop iterations we predict statically, bestie. Super useful when a function has one loop with a known bound and another loop with an unknown bound — the known iteration count gets predicted correctly, while the unknown one averages out to roughly 10. So the loop without bounds ends up looking artificially cold next to the other one. 💅
  • builtin-expect-probability — controls the probability of the expression having the specified value. Takes a percentage (i.e. 0 ... 100) as input. Slay.
  • builtin-string-cmp-inline-length — the maximum length of a constant string for a builtin string cmp call that's eligible for inlining. ✨
  • align-threshold — selects the fraction of the maximal frequency of executions of a basic block in a function, which is then used to align that basic block.
  • align-loop-iterations — a loop that's expected to iterate at least the selected number of iterations gets aligned. 🎀
  • tracer-dynamic-coverage, tracer-dynamic-coverage-feedback — this value limits superblock formation once the given percentage of executed instructions is covered, which keeps unnecessary code size expansion in check. The tracer-dynamic-coverage-feedback parameter is used only when profile feedback is available — the real profiles (as opposed to statically estimated ones) are way less balanced, so the threshold can be a larger value.
  • tracer-max-code-growth — stops tail duplication once code growth hits the given percentage. It's kind of an artificial limit ngl, since most of the duplicates get eliminated later in cross jumping, so you can set it to much higher values than the code growth you actually want. 🦋
  • tracer-min-branch-ratio — stops reverse growth when the reverse probability of the best edge is less than this threshold (in percent).
  • tracer-min-branch-probability, tracer-min-branch-probability-feedback — stops forward growth if the best edge has a probability lower than this threshold. Just like tracer-dynamic-coverage, there are two parameters here: tracer-min-branch-probability-feedback is used for compilation with profile feedback, and tracer-min-branch-probability for compilation without. The profile-feedback value needs to be more conservative (higher) to make the tracer effective. 💖
  • stack-clash-protection-guard-size — specifies the size of the operating system provided stack guard as 2 raised to num bytes. Higher values may reduce the number of explicit probes, but heads up: a value larger than the operating system provided guard will leave your code vulnerable to stack clash style attacks. 😭
Not me warning you twice about stack clash attacks in one page. Keep your guard size sane, bestie.
  • stack-clash-protection-probe-interval — stack clash protection involves probing stack space as it's allocated, and this param controls the maximum distance between probes into the stack as 2 raised to num bytes. Higher values may reduce the number of explicit probes, but again — a value larger than the operating system provided guard leaves code vulnerable to stack clash style attacks. 🫶
  • max-cse-path-length — the maximum number of basic blocks on a path that CSE considers.
  • max-cse-insns — the maximum number of instructions CSE processes before flushing.
  • ggc-min-expand — GCC uses a garbage collector to manage its own memory allocation, and this parameter specifies the minimum percentage by which the GC's heap should be allowed to expand between collections. Tuning this might improve compilation speed; it has no effect on code generation. The default is 30% + 70% * (RAM/1GB) with an upper bound of 100% when RAM >= 1GB. If getrlimit is available, "RAM" means the smallest of actual RAM and RLIMIT_DATA or RLIMIT_AS. If GCC can't calculate RAM on a particular platform, the lower bound of 30% is used. Setting this parameter and ggc-min-heapsize to zero causes a full collection to occur at every opportunity — extremely slow, but useful for debugging. 💀
  • ggc-min-heapsize — the minimum size of the garbage collector's heap before it even bothers collecting garbage. The first collection happens after the heap expands by ggc-min-expand% beyond ggc-min-heapsize. Again, tuning this may improve compilation speed and has no effect on code generation. The default is the smaller of RAM/8, RLIMIT_RSS, or a limit that tries to make sure RLIMIT_DATA or RLIMIT_AS aren't exceeded — with a lower bound of 4096 (four megabytes) and an upper bound of 131072 (128 megabytes). If GCC can't calculate RAM on a particular platform, the lower bound is used. Setting this parameter very large effectively disables garbage collection. Setting this parameter and ggc-min-expand to zero causes a full collection to occur at every opportunity. 🌸
  • max-reload-search-insns — the maximum number of instruction reload should look backward for an equivalent register. Increasing values mean more aggressive optimization, so compilation time goes up — with probably slightly better performance. Tradeoffs, we love to see them.
  • max-cselib-memory-locations — the maximum number of memory locations cselib should take into account. Increasing values mean more aggressive optimization, making compilation time increase with probably slightly better performance.
  • max-sched-ready-insns — the maximum number of instructions ready to be issued that the scheduler should consider at any given time during the first scheduling pass. Increasing values mean more thorough searches, making compilation time increase with probably little benefit. Lowkey not worth it, but go off. 💅
  • max-sched-region-blocks — the maximum number of blocks in a region to be considered for interblock scheduling.
  • max-pipeline-region-blocks — the maximum number of blocks in a region to be considered for pipelining in the selective scheduler.
  • max-sched-region-insns — the maximum number of insns in a region to be considered for interblock scheduling.
  • max-pipeline-region-insns — the maximum number of insns in a region to be considered for pipelining in the selective scheduler.
  • min-spec-prob — the minimum probability (in percents) of reaching a source block for interblock speculative scheduling.
  • max-sched-extend-regions-iters — the maximum number of iterations through CFG to extend regions. A value of 0 disables region extensions. 🧚
  • max-sched-insn-conflict-delay — the maximum conflict delay for an insn to be considered for speculative motion.
  • sched-spec-prob-cutoff — the minimal probability of speculation success (in percents), so that speculative insns get scheduled.
  • sched-state-edge-prob-cutoff — the minimum probability an edge must have for the scheduler to save its state across it.
  • sched-mem-true-dep-cost — minimal distance (in CPU cycles) between a store and a load targeting the same memory locations.
  • selsched-max-lookahead — the maximum size of the lookahead window of selective scheduling. It's basically the depth of search for available instructions.
  • selsched-max-sched-times — the maximum number of times an instruction is scheduled during selective scheduling. This is the limit on the number of iterations through which the instruction may be pipelined.
  • selsched-insns-to-rename — the maximum number of best instructions in the ready list that are considered for renaming in the selective scheduler. Iconic behavior honestly.
  • sms-min-sc — the minimum value of stage count that the swing modulo scheduler generates.
  • max-last-value-rtl — the maximum size, measured as number of RTLs, that can be recorded in an expression in the combiner for a pseudo register as the last known value of that register.
  • max-combine-insns — the maximum number of instructions the RTL combiner tries to combine.
  • max-combine-search-insns — the maximum number of instructions that the RTL combiner searches in order to find the next use of a given register definition. If this limit is reached without finding such a use, the combiner will stop trying to optimize the definition. Currently this limit only applies after certain successful combination attempts, but it could be extended to other cases in the future. 👑
  • integer-share-limit — small integer constants can use a shared data structure, which reduces the compiler's memory usage and increases its speed. This sets the maximum value of a shared integer constant. Fr fr, free wins.
  • ssp-buffer-size — the minimum size of buffers (i.e. arrays) that receive stack smashing protection when -fstack-protector is used.
  • min-size-for-stack-sharing — the minimum size of variables taking part in stack slot sharing when not optimizing.
  • max-jump-thread-duplication-stmts — maximum number of statements allowed in a block that needs to be duplicated when threading jumps.
  • max-jump-thread-paths — the maximum number of paths to consider when searching for jump threading opportunities. When arriving at a block, incoming edges are only considered if the number of paths to be searched so far multiplied by the number of incoming edges doesn't exhaust the specified maximum number of paths to consider. Math-checked, bestie. ✨🌈
  • max-fields-for-field-sensitive — the max number of fields in a struct that'll still get treated in a field-sensitive way during pointer analysis. Bigger than this? It stops being ~special~ about individual fields.
  • prefetch-latency — an estimate of how many instructions (on average) get executed before a prefetch actually finishes. The prefetch distance runs proportional to this constant, so it's kind of the whole personality of your prefetching. Warning tho: cranking this up can also mean less streams get prefetched (peep simultaneous-prefetches 👀).
  • simultaneous-prefetches — the max number of prefetches allowed to vibe at the same time.
  • l1-cache-line-size — size of a cache line in the L1 data cache, in bytes.
  • l1-cache-size — size of the L1 data cache, in kilobytes.
  • l2-cache-size — size of the L2 data cache, in kilobytes.
  • prefetch-dynamic-strides — should the loop array prefetch pass drop software prefetch hints for strides that are non-constant? Sometimes it genuinely helps, but ngl, a non-constant stride makes it hard to know when there's a clear benefit to bothering.
    • Set it to 1 if hints should be issued for non-constant strides.
    • Set it to 0 if hints should only be issued for strides known to be constant and below prefetch-minimum-stride.
  • prefetch-minimum-stride — the minimum constant stride, in bytes, before we even start using prefetch hints. Stride shorter than this threshold? No hints, bye. ✋
    • This one's a lifesaver for processors that have hardware prefetchers, 'cause hardware and software prefetchers can absolutely beef with each other. If your hardware prefetchers have a maximum stride they can handle, plug it in here — makes the software prefetchers way more useful.
    • A value of -1 means there's no threshold at all, so prefetch hints can go out for any constant stride.
    • Only useful for strides that are known and constant, fr.
  • destructive-interference-size
  • constructive-interference-size — the values for the C++17 variables std::hardware_destructive_interference_size and std::hardware_constructive_interference_size.
    • The destructive interference size is the minimum recommended offset between two independent, concurrently-accessed objects. The constructive interference size is the maximum recommended size of contiguous memory accessed together. Typically both equal the size of an L1 cache line for the target, in bytes. On a generic target covering a range of L1 cache line sizes, typically constructive = the small end of the range and destructive = the large end.
    • Destructive interference size is meant for layout, so it has ABI impact 😬. The default value is not expected to be stable, and on some targets it varies with -mtune — so using this variable anywhere ABI stability matters (like the public interface of a library) is strongly discouraged. If you do use it there, users can stabilize the value with this option.
    • Constructive interference size is less sensitive, since it's typically only used in a static_assert to make sure a type fits within a cache line.
    • See also -Winterference-size.
  • loop-interchange-max-num-stmts — the max number of stmts in a loop for it to get interchanged.
  • loop-interchange-stride-ratio — the minimum ratio between the strides of two loops before interchange is actually profitable.
  • min-insn-to-prefetch-ratio — the minimum ratio between the number of instructions and the number of prefetches needed to enable prefetching in a loop.
  • prefetch-min-insn-to-mem-ratio — the minimum ratio between the number of instructions and the number of memory references needed to enable prefetching in a loop.
  • use-canonical-types — should the compiler use the "canonical" type system? Should always be 1, which uses a more efficient internal mechanism for comparing types in C++ and Objective-C++. But if bugs in the canonical type system are causing compilation failures, set this to 0 to disable canonical types. (Escape hatch energy. 🚪)
  • switch-conversion-max-branch-ratio — switch initialization conversion refuses to create arrays bigger than switch-conversion-max-branch-ratio times the number of branches in the switch. It's giving "we're not allocating that".
  • switch-lower-slow-alg-max-cases — the max number of cases before the slow switch lowering algorithms get used.
  • max-partial-antic-length — the max length of the partial antic set computed during the tree partial redundancy elimination optimization (-ftree-pre) when optimizing at -O3 and above. For some kinds of source code the enhanced partial redundancy elimination optimization can straight-up run away and eat all the memory available on the host machine 😭. This parameter caps the length of the sets computed, which stops the runaway behavior. Setting it to 0 allows an unlimited set length (choose chaos bestie).
  • rpo-vn-max-loop-depth — the max loop depth that gets value-numbered optimistically. When the limit hits, the innermost rpo-vn-max-loop-depth loops and the outermost loop in the loop nest get value-numbered optimistically, and the remaining ones don't.
  • sccvn-max-alias-queries-per-access — the max number of alias-oracle queries we perform when hunting for redundancies in loads and stores. Hit this limit and the search is aborted, and that load or store is not considered redundant. The number of queries is algorithmically limited to the number of stores on all paths from the load to the function entry.
  • ira-max-loops-num — IRA uses regional register allocation by default. If a function has more loops than the number this parameter gives, only at most that many of the most frequently-executed loops form regions for regional register allocation.
  • ira-max-conflict-table-size — IRA already uses a sophisticated algorithm to compress the conflict table, but the table can still need excessive amounts of memory for huge functions. If a function's conflict table could be more than the size in MB given here, the register allocator instead uses a faster, simpler, lower-quality algorithm that doesn't need a pseudo-register conflict table built at all. Trade-off city. 💀
  • ira-loop-reserved-regs — IRA can be used to evaluate more accurate register pressure in loops when deciding whether to move loop invariants (see -O3). This parameter gives the number of available registers reserved for some other purposes. Default is the best found from numerous experiments, i.e. the scientists did the homework for you. 🧪
  • ira-consider-dup-in-all-alts — makes IRA heavily consider the matching constraint (duplicated operand number) in all available alternatives for the preferred register class. If it's 0, IRA only respects the matching constraint when it's in the only available alternative with an appropriate register class. Otherwise, IRA checks all available alternatives for the preferred register class — even after finding some choice with an appropriate register class — and respects the qualified matching constraint it found.
  • ira-simple-lra-insn-threshold — approximate function insn number in 1K units that triggers simple local RA.
  • lra-inheritance-ebb-probability-cutoff — LRA tries to reuse values reloaded in registers in subsequent insns, and this optimization is called inheritance. EBB is used as the region to do it in. This parameter defines the minimal fall-through edge probability, in percentage, used to add a BB to the inheritance EBB in LRA. The default value was chosen from numerous runs of SPEC2000 on x86-64. 👑
  • loop-invariant-max-bbs-in-loop — loop invariant motion can get expensive, both in compile time and in compile-time memory, with very large loops. Loops with more basic blocks than this parameter won't get loop invariant motion optimization performed on them. Sorry babe, too big.
  • loop-max-datarefs-for-datadeps — building data dependencies is expensive for very large loops, so this parameter limits the number of data references in loops considered for data dependence analysis. These large loops are not handled by the optimizations that use loop data dependencies.
  • max-vartrack-size — sets a max number of hash table slots to use during variable tracking dataflow analysis of any function. Exceed the limit with variable tracking at assignments enabled? The analysis for that function gets retried without it, after removing all debug insns from the function. Exceed the limit even without debug insns? Var tracking analysis is completely disabled for that function. Setting it to 0 makes it unlimited.
  • max-vartrack-expr-depth — sets a max number of recursion levels when trying to map variable names or debug temporaries to value expressions. It's literally trading compile time for more complete debug info. Set it too low and value expressions that are available and could be represented in debug info might end up unused; set it higher and the compiler may find more complex debug expressions, but compile time and memory use may grow. Balance, bestie. ⚖️
  • max-debug-marker-count — sets a threshold on the number of debug markers (e.g. begin stmt markers) to avoid a complexity explosion at inlining or when expanding to RTL. If a function has more such gimple stmts than the set limit, those stmts get dropped from the inlined copy of the function and from its RTL expansion.

okay bestie, more --param lore incoming, keep up 🫶

  • min-nondebug-insn-uid — the uid number where nondebug insns start 🎀. Everything below this value is exclusively reserved for the debug insns that -fvar-tracking-assignments creates, but ngl, if that reserved range runs out, debug insns are allowed to grab (non-overlapping) uids above this number too.
  • ipa-sra-deref-prob-threshold — IPA-SRA only swaps out a pointer that's known not to be NULL for one or more fresh params when the probability (in percent, measured relative to function entry) that it actually gets dereferenced is higher than this parameter 💅.
  • ipa-sra-ptr-growth-factor — same energy, but for pointers to aggregates: IPA-SRA only replaces it with new params when their cumulative size is less than or equal to this factor times the size of the original pointer parameter 🌸.
  • ipa-sra-ptrwrap-growth-factor — the extra max allowed growth of the total size of the new params IPA-SRA replaces a pointer-to-aggregate with, but only when that pointer points to a local variable that the caller only writes to and then passes as an argument to other functions 👑.
  • ipa-sra-max-replacements — max pieces of an aggregate that IPA-SRA tracks. And because of that, it's lowkey also the max number of replacements any single formal parameter can get 🎀.
  • sra-max-scalarization-size-Ospeed, sra-max-scalarization-size-Osize — both of the Scalar Reduction of Aggregates passes (SRA and IPA-SRA) are out here trying to replace scalar parts of aggregates with independent scalar variables, and these two control the max size (in storage units) of an aggregate that even gets considered for replacement when compiling for speed (sra-max-scalarization-size-Ospeed) or for size (sra-max-scalarization-size-Osize) respectively 💖.
  • sra-max-propagations — max number of artificial accesses that SRA will track per one local variable, all so that copy propagation can happen 🦋.
  • tm-max-aggregate-size — when copying thread-local variables inside a transaction, this is the size in bytes past which the variables get saved with the logging functions instead of save/restore code sequence pairs. Only applies when you're using -fgnu-tm ⭐.
  • graphite-max-nb-scop-params — to stop Graphite loop transforms from going exponential, the number of parameters in a Static Control Part (SCoP) gets capped. A value of zero lifts that bound. FYI, a variable counts as a SCoP parameter when its value is unknown at compile time and it's defined outside the SCoP 🌈.
  • hardcfr-max-blocks — disables -fharden-control-flow-redundancy for functions that have more blocks than the value you give. Zero removes any limit 💀.
  • hardcfr-max-inline-blocks — forces -fharden-control-flow-redundancy to use out-of-line checking for functions with more basic blocks than the value you give 🔥.
  • loop-block-tile-size — the loop blocking / strip mining transforms (turned on with -floop-block or -floop-strip-mine) strip mine every loop in the loop nest by a given number of iterations, and this parameter is how you change the strip length 🧚.
  • ipa-jump-function-lookups — how many statements get visited during jump function offset discovery 👑.
  • ipa-cp-value-list-size — IPA-CP tries to track every possible value and type passed to a function's parameter so it can propagate them and do its devirtualization thing, and this is the max number of values and types it stores per one formal parameter of a function 🍓.
  • ipa-cp-eval-threshold — IPA-CP gives itself a score for its cloning profitability heuristics and only actually does the cloning opportunities whose scores go above this parameter 💅.
  • ipa-cp-max-recursive-depth — max depth of recursive cloning for a self-recursive function 🦋.
  • ipa-cp-min-recursive-probability — recursive cloning happens only when the probability of the call actually being executed goes above this parameter 🌸.
  • ipa-cp-profile-count-base — when you're using the -fprofile-use option, IPA-CP takes the measured execution count of a call graph edge sitting at this percentage position in its histogram and treats it as the basis for its heuristics calculation 💖.
  • ipa-cp-recursive-freq-factor — how many times interprocedural copy propagation expects recursive functions to call themselves 🙈.
  • ipa-cp-recursion-penalty — the percentage penalty recursive functions get handed when they're evaluated for cloning 🎀.
  • ipa-cp-single-call-penalty — the percentage penalty that functions containing a single call to another function get handed when they're evaluated for cloning ✨.
  • ipa-max-agg-items — IPA-CP can also propagate a number of scalar values that were passed in an aggregate, and this controls the max number of such values per one parameter 🫶.
  • ipa-cp-loop-hint-bonus — when IPA-CP figures out that a cloning candidate would make a loop's iteration count known, it adds a bonus of this amount to that candidate's profitability score 🔥.
  • ipa-max-loop-predicates — max number of different predicates IPA will use to describe when loops in a function have known properties 🌈.
  • ipa-max-aa-steps — while analyzing function bodies, IPA-CP busts out alias analysis to track values pointed to by function parameters. But so it doesn't spend forever on gigantic functions, it gives up and considers all memory clobbered once it's examined this many statements that modify memory 💀.
  • ipa-max-switch-predicate-bounds — max number of boundary endpoints of the case ranges in a switch statement. For a switch that goes over this limit, IPA-CP won't build the cloning cost predicate (the thing used to estimate cloning benefit) for that switch's default case 🎀.
  • ipa-max-param-expr-ops — IPA-CP analyzes conditional statements that reference some function parameter to estimate the benefit of cloning on a certain constant value. But if the number of operations in a parameter expression goes over this, the expression gets treated as a complicated one and IPA analysis just won't handle it 🙈.
  • lto-partitions — the number of partitions you want produced during WHOPR compilation. Bestie, that number should exceed the number of CPUs used for compilation 💅.
  • lto-min-partition — minimal partition size for WHOPR (in estimated instructions). This is what stops tiny programs from getting split into way too many partitions 🌸.
  • lto-max-partition — max partition size for WHOPR (in estimated instructions), giving an upper bound on each individual partition's size. Meant to be used only with balanced partitioning 🎀.
  • lto-partition-locality-frequency-cutoff — the denominator n of the fraction 1/n of the execution frequency of a callee that's needed for it to get cloned for a particular caller. Special value of 0 dictates always cloning, no cut-off at all 💖.
  • lto-partition-locality-size-cutoff — the size cut-off for a callee (including its inlined calls) to get cloned for a particular caller ✨.
  • lto-max-locality-partition — max size of a locality partition for LTO (in estimated instructions). A value of 0 means the default value gets used 🦋.
  • lto-max-streaming-parallelism — max number of parallel processes used for LTO streaming 🔥.
  • cxx-max-namespaces-for-diagnostic-help — max number of namespaces to consult for suggestions when C++ name lookup fails on an identifier 👑.
  • sink-frequency-threshold — max relative execution frequency (in percents) of the target block compared to a statement's original block for that statement to be allowed to sink. Bigger numbers = more aggressive statement sinking. Statements with memory operands get a small positive adjustment applied, because those are even more profitable to sink 💅.
  • max-stores-to-sink — max number of conditional store pairs that can be sunk. Set it to 0 if either vectorization (-ftree-vectorize) or if-conversion (-ftree-loop-if-convert) is disabled ⭐.
  • case-values-threshold — the smallest number of different values for which it's best to use a jump-table instead of a tree of conditional branches. If the value is 0, the default for the machine gets used 🌸.
  • jump-table-max-growth-ratio-for-size — max code size growth ratio when expanding into a jump table (in percent). This one's used when optimizing for size 🎀.
  • jump-table-max-growth-ratio-for-speed — max code size growth ratio when expanding into a jump table (in percent). This one's used when optimizing for speed ✨.
  • tree-reassoc-width — max number of instructions executed in parallel in a reassociated tree. If it has a non-zero value, it overrides the target-dependent heuristics used by default 🔥.
  • sched-pressure-algorithm — picks between the two available implementations of -fsched-pressure. Algorithm 1 is the original implementation and is the one more likely to prevent instructions from being reordered. Algorithm 2 was designed as a compromise between algorithm 1's relatively conservative approach and the default scheduler's rather aggressive approach — it leans way harder on having a regular register file and accurate register pressure classes. See haifa-sched.cc in the GCC sources for more details. The default choice depends on the target 💜.
  • max-slsr-cand-scan — max number of existing candidates that get considered when looking for a basis for a new straight-line strength reduction candidate 🦋.
  • asan-globals — turns on buffer overflow detection for global objects 💅 this protection is enabled by default if you’re using -fsanitize=address; to disable global objects protection use --param asan-globals=0.
  • asan-stack — buffer overflow detection for stack objects, bestie 🫶 this is enabled by default when using -fsanitize=address; to disable stack protection use --param asan-stack=0.
  • asan-instrument-reads — buffer overflow detection for memory reads, no cap ✨ enabled by default when using -fsanitize=address; to disable memory reads protection use --param asan-instrument-reads=0.
  • asan-instrument-writes — buffer overflow detection for memory writes 💅 enabled by default when using -fsanitize=address; to disable memory writes protection use --param asan-instrument-writes=0.
  • asan-memintrin — turns on detection for built-in functions 🧚 enabled by default when using -fsanitize=address; to disable built-in functions protection use --param asan-memintrin=0.
  • asan-use-after-return — turns on detection of use-after-return 🎀 enabled by default when using the -fsanitize=address option; to disable it use --param asan-use-after-return=0. Note: by default the check is disabled at run time — to enable it, add detect_stack_use_after_return=1 to the environment variable ASAN_OPTIONS 😭.
  • asan-instrumentation-with-call-threshold — if the number of memory accesses in a function being instrumented is greater or equal to this number, use callbacks instead of inline checks 👑 e.g. to disable inline code use --param asan-instrumentation-with-call-threshold=0.
  • asan-kernel-mem-intrinsic-prefix — if nonzero, prefix calls to memcpy, memset and memmove with __asan_ or __hwasan_ for -fsanitize=kernel-address or -fsanitize=kernel-hwaddress, respectively 💀.
  • hwasan-instrument-stack — hwasan instrumentation of statically sized stack-allocated variables 🦋 enabled by default when using -fsanitize=hwaddress and disabled by default when using -fsanitize=kernel-hwaddress; to disable stack instrumentation use --param hwasan-instrument-stack=0, and to enable it use --param hwasan-instrument-stack=1.
  • hwasan-random-frame-tag — when using stack instrumentation, decide tags for stack variables using a deterministic sequence beginning at a random tag for each frame 🌈 with this parameter unset, tags are chosen using the same sequence but beginning from 1; this is enabled by default for -fsanitize=hwaddress and unavailable for -fsanitize=kernel-hwaddress; to disable it use --param hwasan-random-frame-tag=0.
  • hwasan-instrument-allocas — hwasan instrumentation of dynamically sized stack-allocated variables 💜 enabled by default when using -fsanitize=hwaddress and disabled by default when using -fsanitize=kernel-hwaddress; to disable instrumentation of such variables use --param hwasan-instrument-allocas=0, and to enable it use --param hwasan-instrument-allocas=1.
  • hwasan-instrument-reads — hwasan checks on memory reads 🌸 instrumentation of reads is enabled by default for both -fsanitize=hwaddress and -fsanitize=kernel-hwaddress; to disable checking memory reads use --param hwasan-instrument-reads=0.
  • hwasan-instrument-writes — hwasan checks on memory writes ✨ instrumentation of writes is enabled by default for both -fsanitize=hwaddress and -fsanitize=kernel-hwaddress; to disable checking memory writes use --param hwasan-instrument-writes=0.
  • hwasan-instrument-mem-intrinsics — hwasan instrumentation of builtin functions 🎀 instrumentation of these builtin functions is enabled by default for both -fsanitize=hwaddress and -fsanitize=kernel-hwaddress; to disable instrumentation of builtin functions use --param hwasan-instrument-mem-intrinsics=0.
  • use-after-scope-direct-emission-threshold — if the size of a local variable in bytes is smaller or equal to this number, directly poison (or unpoison) shadow memory instead of using run-time callbacks 😭.
  • tsan-distinguish-volatile — emit special instrumentation for accesses to volatiles 💅.
  • tsan-instrument-func-entry-exit — emit instrumentation calls to __tsan_func_entry() and __tsan_func_exit() 🔥.
  • max-fsm-thread-path-insns — maximum number of instructions to copy when duplicating blocks on a finite state automaton jump thread path 🧚.
  • threader-debug — threader-debug=[none|all] enables verbose dumping of the threader solver 🙈.
  • parloops-chunk-size — chunk size of omp schedule for loops parallelized by parloops 💜.
  • parloops-schedule — schedule type of omp schedule for loops parallelized by parloops (static, dynamic, guided, auto, runtime) 🌈.
  • parloops-min-per-thread — the minimum number of iterations per thread of an innermost parallelized loop for which the parallelized variant is preferred over the single threaded one 🫶 note that for a parallelized loop nest the minimum number of iterations of the outermost loop per thread is two.
  • max-ssa-name-query-depth — maximum depth of recursion when querying properties of SSA names in things like fold routines ⭐ one level of recursion corresponds to following a use-def chain.
  • max-speculative-devirt-maydefs — the maximum number of may-defs we analyze when looking for a must-def specifying the dynamic type of an object that invokes a virtual call we may be able to devirtualize speculatively 💀.
  • ranger-debug — specifies the type of debug output to be issued for ranges 🙈.
  • unroll-jam-min-percent — the minimum percentage of memory references that must be optimized away for the unroll-and-jam transformation to be considered profitable 👑.
  • unroll-jam-max-unroll — the maximum number of times the outer loop should be unrolled by the unroll-and-jam transformation 🎀.
  • max-rtl-if-conversion-unpredictable-cost — maximum permissible cost for the sequence that would be generated by the RTL if-conversion pass for a branch that is considered unpredictable 🦋.
  • max-variable-expansions-in-unroller — if -fvariable-expansion-in-unroller is used, the maximum number of times that an individual variable will be expanded during loop unrolling 💜.
  • partial-inlining-entry-probability — maximum probability of the entry BB of split region (in percent relative to entry BB of the function) to make partial inlining happen ✨.
  • max-tracked-strlens — maximum number of strings for which strlen optimization pass will track string lengths 🌸.
  • gcse-after-reload-partial-fraction — the threshold ratio for performing partial redundancy elimination after reload 💅.
  • gcse-after-reload-critical-fraction — the threshold ratio of critical edges execution count that permit performing redundancy elimination after reload 🔥.
  • max-loop-header-insns — the maximum number of insns in loop header duplicated by the copy loop headers pass 👑.
  • vect-epilogues-nomask — enable loop epilogue vectorization using smaller vector size 🌈.
  • vect-partial-vector-usage — controls when the loop vectorizer considers using partial vector loads and stores as an alternative to falling back to scalar code 🎀 0 stops the vectorizer from ever using partial vector loads and stores; 1 allows partial vector loads and stores if vectorization removes the need for the code to iterate; 2 allows partial vector loads and stores in all loops. The parameter only has an effect on targets that support partial vector loads and stores.
  • vect-inner-loop-cost-factor — the maximum factor which the loop vectorizer applies to the cost of statements in an inner loop relative to the loop being vectorized 🦋 the factor applied is the maximum of the estimated number of iterations of the inner loop and this parameter; the default value of this parameter is 50.
  • vect-induction-float — enable loop vectorization of floating point inductions ✨.
  • vect-force-slp — force the use of SLP when vectorizing, fail if not possible 💀.
  • vrp-block-limit — maximum number of basic blocks before VRP switches to a lower memory algorithm 🧚.
  • vrp-sparse-threshold — maximum number of basic blocks before VRP uses a sparse bitmap cache 🌸.
  • vrp-switch-limit — maximum number of outgoing edges in a switch before VRP will not process it 🫶.
  • vrp-vector-threshold — maximum number of basic blocks for VRP to use a basic cache vector 💜.
  • avoid-fma-max-bits — maximum number of bits for which we avoid creating FMAs 💅.
  • fully-pipelined-fma — whether the target fully pipelines FMA instructions; if non-zero, reassociation considers the benefit of parallelizing FMA's multiplication part and addition part, assuming FMUL and FMA use the same units that can also do FADD 🔥.
  • sms-loop-average-count-threshold — a threshold on the average loop count considered by the swing modulo scheduler 🌈.
  • sms-dfa-history — the number of cycles the swing modulo scheduler considers when checking conflicts using DFA ⭐.
  • graphite-allow-codegen-errors — whether codegen errors should be ICEs when -fchecking 😭.
  • sms-max-ii-factor — a factor for tuning the upper bound that swing modulo scheduler uses for scheduling a loop 🎀.
  • lra-max-considered-reload-pseudos — the max number of reload pseudos which are considered during spilling a non-reload pseudo 👑.
  • max-pow-sqrt-depth — the max depth of sqrt chains GCC will go to when it's synthesizing an exponentiation by a real constant ✨
  • max-dse-active-local-stores — biggest number of active local stores allowed in RTL dead store elimination 💅
  • asan-instrument-allocas — turns on the asan allocas/VLAs protection 🛡️
  • max-iterations-computation-cost — a bound on the cost of an expression when GCC computes the number of iterations, no cap on the vibes 🌀
  • max-isl-operations — max number of isl operations. 0 means unlimited, go off 👑
  • graphite-max-arrays-per-scop — the max number of arrays per scop, periodt.
  • max-vartrack-reverse-op-size — max size of the loc list for which reverse ops should be added 🔄
  • fsm-scale-path-stmts — a scale factor applied to the number of statements in a threading path that crosses a loop backedge, when it's being compared against --param=max-jump-thread-duplication-stmts 🧵
  • uninit-control-dep-attempts — max number of nested calls made while searching for control dependencies during uninitialized variable analysis 🔍
  • uninit-max-chain-len — max number of predicates anded for each predicate ored in the normalized predicate chain.
  • uninit-max-num-chains — max number of predicates ored in the normalized predicate chain.
  • uninit-max-prune-work — max amount of work done to prune paths where the variable is always initialized ✂️
  • sched-autopref-queue-depth — hardware autoprefetcher scheduler model control flag. It's the number of lookahead cycles the model looks into; at ' ' only enable the instruction sorting heuristic 👀
  • loop-versioning-max-inner-insns — the max number of instructions an inner loop can have before the loop versioning pass decides it's too big to copy 🚫
  • loop-versioning-max-outer-insns — the max number of instructions an outer loop can have before the loop versioning pass decides it's too big to copy, discounting any instructions in inner loops that directly benefit from versioning 💫
  • ssa-name-def-chain-limit — the max number of SSA_NAME assignments GCC will follow when determining a property of a variable, like its value. This caps the number of iterations or recursive calls GCC performs when optimizing certain statements, or when it's checking their validity before issuing diagnostics 🧠
  • store-merging-max-size — max size of a single store merging region, in bytes 📏
  • store-forwarding-max-distance — max instruction distance that a small store forwarded to a larger load may stall. Value '0' disables the cost checks for the avoid-store-forwarding pass 🚦
  • hash-table-verification-limit — the number of elements for which hash table verification is done, for each searched element.
  • max-find-base-term-values — max number of VALUEs handled during a single find_base_term call.
  • analyzer-max-enodes-per-program-point — the max number of exploded nodes per program point within the analyzer, before it terminates analysis of that point 💥
  • analyzer-max-constraints — the max number of constraints per state.
  • analyzer-min-snodes-for-call-summary — the min number of supernodes within a function for the analyzer to even consider summarizing its effects at call sites 📋
  • analyzer-max-enodes-for-full-dump — the max depth of exploded nodes that should appear in a dot dump before switching to a less verbose format 🗜️
  • analyzer-max-recursion-depth — the max number of times a callsite can appear in a call stack within the analyzer, before it terminates analysis of a call that would recurse deeper 🪜
  • analyzer-max-svalue-depth — the max depth of a symbolic value, before GCC just approximates the value as unknown 🤷‍♀️
  • analyzer-max-infeasible-edges — the max number of infeasible edges to reject before declaring a diagnostic as infeasible ⛔
  • gimple-fe-computed-hot-bb-threshold — the number of executions of a basic block which counts as hot 🔥 This parameter is used only in GIMPLE FE.
  • analyzer-bb-explosion-factor — the max number of 'after supernode' exploded nodes within the analyzer per supernode, before terminating analysis 💣
  • analyzer-text-art-string-ellipsis-threshold — the number of bytes at which string literals get ellipsized in analyzer text art diagrams ✂️
  • analyzer-text-art-ideal-canvas-width — the ideal width, in characters, of text art diagrams generated by the analyzer 🎨
  • analyzer-text-art-string-ellipsis-head-len — the number of literal bytes to show at the head of a string literal in text art when it's being ellipsized.
  • analyzer-text-art-string-ellipsis-tail-len — the number of literal bytes to show at the tail of a string literal in text art when it's being ellipsized 🍬
  • ranger-logical-depth — max depth of logical expression evaluation the ranger will look through when evaluating outgoing edge ranges 🧚
  • ranger-recompute-depth — max depth of instruction chains considered for recomputation in the outgoing range calculator.
  • relation-block-limit — max number of relations the oracle will register in a basic block.
  • transitive-relations-work-bound — the work bound when discovering transitive relations from relations that already exist 🔗
  • min-pagesize — minimum page size, for warning and early break vectorization purposes.
  • openacc-kernels — specifies the mode of OpenACC kernels constructs handling. With --param=openacc-kernels=decompose, OpenACC kernels constructs are decomposed into parts — a sequence of compute constructs, each handled individually. This is work in progress, heads up 🚧 With --param=openacc-kernels=parloops, OpenACC kernels constructs are handled by the parloops pass, en bloc. This is the current default 👑
  • openacc-privatization — controls whether the -fopt-info-omp-note and applicable -fdump-tree-*-details options emit OpenACC privatization diagnostics. With --param=openacc-privatization=quiet, don't diagnose — that's the current default. With --param=openacc-privatization=noisy, do diagnose 📢
  • cycle-accurate-model — specifies whether GCC should assume the scheduling description is mostly a cycle-accurate model of the target processor the code is meant to run on, in the absence of cache misses. Nonzero means the selected scheduling model is accurate and likely describes an in-order processor, and that scheduling should aggressively spill to try to fill any pipeline bubbles — this is the current default 🌈 Zero means the scheduling description might not be available/accurate, or maybe not applicable at all, like for modern out-of-order processors.

The following choices of name are available on AArch64 targets:

  • aarch64-vect-compare-costs — when vectorizing, consider using multiple different approaches and let the cost model pick the cheapest one ✨ This includes:
    • Trying both SVE and Advanced SIMD, when SVE is available.
    • Trying to use 64-bit Advanced SIMD vectors for the smallest data elements, rather than using 128-bit vectors for everything.
    • Trying to use "unpacked" SVE vectors for smaller elements. This includes storing smaller elements in larger containers and accessing elements with extending loads and truncating stores.
  • aarch64-float-recp-precision — the number of Newton iterations for calculating the reciprocal for float type. The precision of division is proportional to this param when division approximation is enabled. The default value is 1 🍓
  • aarch64-double-recp-precision — the number of Newton iterations for calculating the reciprocal for double type. The precision of division is proportional to this param when division approximation is enabled. The default value is 2 🍓🍓
  • aarch64-autovec-preference — forces an ISA selection strategy for auto-vectorization 💪
    • 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 ⚖️
  • aarch64-ldp-policy — fine-grained policy for load pairs 🎀 With --param=aarch64-ldp-policy=default, use the policy of the tuning structure — this is the current default. With --param=aarch64-ldp-policy=always, emit ldp regardless of alignment. With --param=aarch64-ldp-policy=never, do not emit ldp. With --param=aarch64-ldp-policy=aligned, emit ldp only if the source pointer is aligned to at least double the alignment of the type.
  • aarch64-stp-policy — fine-grained policy for store pairs 🎀 With --param=aarch64-stp-policy=default, use the policy of the tuning structure — this is the current default. With --param=aarch64-stp-policy=always, emit stp regardless of alignment. With --param=aarch64-stp-policy=never, do not emit stp. With --param=aarch64-stp-policy=aligned, emit stp only if the source pointer is aligned to at least double the alignment of the type.
  • aarch64-ldp-alias-check-limit — a limit on the number of alias checks performed by the AArch64 load/store pair fusion pass when it's attempting to form an ldp/stp 👯 Higher values make the pass more aggressive at re-ordering loads over stores — at the expense of increased compile time, so, tradeoffs bestie 😭
  • aarch64-ldp-writeback — okay so this param decides which writeback opportunities the AArch64 load/store pair fusion pass even tries to handle 💅 a value of zero = writeback handling is off, done, not happening. One means we try to form pairs involving one or more existing individual writeback accesses where possible. A value of two means we also try to opportunistically form writeback opportunities by folding in trailing destructive updates of the base register used by a pair. yes bestie, two is the ambitious one ✨
  • aarch64-loop-vect-issue-rate-niters — so the tuning for some AArch64 CPUs tries to take both latencies and issue rates into account when deciding whether a loop should be vectorized using SVE, vectorized using Advanced SIMD, or not vectorized at all. If this parameter is set to n, GCC will not use this heuristic for loops that are known to execute in fewer than n Advanced SIMD iterations. tiny loops don't get the whole vibe check 🦋
  • aarch64-vect-unroll-limit — the vectorizer will use available tuning information to determine whether it would be beneficial to unroll the main vectorized loop and by how much. This parameter sets the upper bound of how much the vectorizer will unroll the main loop. The default value is four. main character loop gets four glow-ups max, periodt 🎀

The following choices of name are available on GCN targets:

  • gcn-preferred-vectorization-factor — preferred vectorization factor: default, 32, 64.

The following choices of name are available on i386 and x86_64 targets:

  • x86-stlf-window-ninsns — instructions number above which STFL stall penalty can be compensated.
  • x86-stv-max-visits — the maximum number of use and def visits when discovering a STV chain before the discovery is aborted.

✨ Program Instrumentation Options ✨

GCC supports a number of command-line options that control adding run-time instrumentation to the code it normally generates. Like, for example, one purpose of instrumentation is collect profiling statistics for use in finding program hot spots, code coverage analysis, or profile-guided optimizations 📊 another class of program instrumentation is adding run-time checking to detect programming errors like invalid pointer dereferences or out-of-bounds array accesses, as well as deliberately hostile attacks such as stack smashing or C++ vtable hijacking. rude. and there's also a general hook which can be used to implement other forms of tracing or function-level instrumentation for debug or program analysis purposes. it's giving ✨observability✨

  • -p
  • -pg — generate extra code to write profile information suitable for the analysis program prof (for -p) or gprof (for -pg). You must use this option when compiling the source files you want data about, and you must also use it when linking 🔥

You can use the function attribute no_instrument_function to suppress profiling of individual functions when compiling with these options. (yes, you can opt specific functions out of the spotlight 💅)

  • -fprofile-arcs — add code so that program flow arcs are instrumented. During execution the program records how many times each branch and call is executed and how many times it is taken or returns. 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. The data may be used for profile-directed optimizations (-fbranch-probabilities), or for test coverage analysis (-ftest-coverage). Each object file's auxname is generated from the name of the output file, if explicitly specified and it is not the final executable, otherwise it is the basename of the source file. In both cases any suffix is removed (e.g. foo.gcda for input file dir/foo.c, or dir/foo.gcda for output file specified as -o dir/foo.o). naming lore 🫶

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 with "gcov --conditions". every term pulling its weight, no passengers 🦋
  • -fpath-coverage — add code so that the paths taken are tracked. During execution the program records the prime paths taken. 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 with "gcov --prime-paths --prime-paths-lines --prime-paths-source",
  • -fpath-coverage-limit=limit — the threshold at which point -fpath-coverage gives up on instrumenting a function. This limit is approximate and conservative, as 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 is 250000. (path explosion is so real, we're being so brave about it 😭)
  • --coverage — this option is used to compile and link code instrumented for coverage analysis. The option is a synonym for -fprofile-arcs -ftest-coverage (when compiling) and -lgcov (when linking). See the documentation for those options for more details.
    • Compile the source files with -fprofile-arcs plus optimization and code generation options. For test coverage analysis, use the additional -ftest-coverage option. You do not need to profile every source file in a program.
    • Compile the source files additionally with -fprofile-abs-path to create absolute path names in the .gcno files. This allows gcov to find the correct sources in projects where compilations occur with different working directories.
    • Link your object files with -lgcov or -fprofile-arcs (the latter implies the former).
    • Run the program on a representative workload to generate the arc profile information. This may be repeated any number of times. You can run concurrent instances of your program, and provided that the file system supports locking, the data files will be correctly updated. Unless a strict ISO C dialect option is in effect, "fork" calls 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. (loyalty 💖)

  • For profile-directed optimizations, compile the source files again with the same optimization and code generation options plus -fbranch-probabilities.
  • For test coverage analysis, use gcov to produce human readable information from the .gcno and .gcda files. Refer to the gcov documentation for further information.

With -fprofile-arcs, for each function of your program GCC creates a program flow graph, then finds a spanning tree for the graph. Only arcs that are not on the spanning tree have to be instrumented: the compiler adds code to count the number of times that these arcs are executed. When an arc is the only exit or only entrance to a block, the instrumentation code can be added to the block; otherwise, a new basic block must be created to hold the instrumentation code. spanning tree = the main characters, the rest get counted 👑

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. okay that definition ate, honestly 🌈

  • -ftest-coverage — produce a notes file that the gcov code-coverage utility can use to show program coverage. Each source file's note file is called auxname.gcno. Refer to the -fprofile-arcs option above for a description of auxname and instructions on how to generate test coverage data. Coverage data matches the source files more closely if you do not optimize. ngl, no-opt is the move here 💀
  • -fprofile-abs-path — automatically convert relative source file names to absolute path names in the .gcno files. This allows gcov to find the correct sources in projects where compilations occur with different working directories. absolute paths, absolute icons 🎀
  • -fprofile-dir=path — set the directory to search for the profile data files in to path. This option affects only the profile data generated by -fprofile-generate, -ftest-coverage, -fprofile-arcs and used by -fprofile-use and -fbranch-probabilities and its related options. Both absolute and relative paths can be used. By default, GCC uses the current directory as path, thus the profile data file appears in the same directory as the object file. In order to prevent the file name clashing, if the object file name is not an absolute path, we mangle the absolute path of the sourcename.gcda file and use it as the file name of a .gcda file. 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-generate both when compiling and when linking your program. both times bestie, no skipping 💜

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

Okay so instead of using a constructor/destructor, this registers the profile info in the specified section 💅 The section name is name if you pass it, otherwise it just defaults to ".gcov_info" (the default ate, honestly). A pointer to the profile info generated by -fprofile-arcs gets placed in that section for each translation unit. Heads up tho: this option disables the profile information registration through a constructor, and it disables the profile information processing through a destructor. This one is not meant for hosted environments like GNU/Linux — it targets freestanding environments (think embedded systems 👀) with limited resources which don't support constructors/destructors or the C library file I/O.

The linker could collect the input sections in a continuous memory block and define start and end symbols. A GNU linker script example which defines a linker output section follows:

                   .gcov_info      :
                   {
                     PROVIDE (__gcov_info_start = .);
                     KEEP (*(.gcov_info))
                     PROVIDE (__gcov_info_end = .);
                   }

The program could 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 ✨

  • -fprofile-note=path

If path is specified, GCC saves .gcno file into path location. ⚠️ But if you combine the option with multiple source files, the .gcno file will be overwritten — yeah, it just gets clobbered, no cap.

  • -fprofile-prefix-path=path

This option can be used in combination with profile-generate=profile_dir and profile-use=profile_dir to inform GCC where is the base directory of the built source tree. By default profile_dir will contain files with mangled absolute paths of all object files in the built project. That's lowkey a problem when the directory used to build the instrumented binary differs from the directory used to build the binary optimized with profile feedback, because the profile data will not be found during the optimized build 😭. In such setups, -fprofile-prefix-path=path with path pointing 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. slay 🫶

  • -fprofile-prefix-map=old=new

When compiling files residing in directory old, record profiling information (with --coverage) describing them as if the files resided in directory new instead. See also -ffile-prefix-map and -fcanon-prefix-map 💫

  • -fprofile-update=method

Alter the update method for an application instrumented for profile feedback based optimization. The method argument should be one of single, atomic or prefer-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 doesn't properly join all threads (or creates a detached thread), a profile file can still be 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 doesn't 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. And ngl, 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.*\.c will instrument only main.c and all C files starting with 'module'. Obsessed with the precision honestly ✨

  • -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. Bye bye /usr/ 👋

  • -fprofile-reproducible=[multithreaded|parallel-runs|serial]

Control level of reproducibility of profile gathered by "-fprofile-generate". This makes it possible to rebuild program with 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's 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 depend — 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"). Heads up though: this reduces quality of gathered data, in particular of indirect call profiling 💅

  • -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. See <https://github.com/google/sanitizers/wiki/AddressSanitizer> for more details. The run-time behavior can be influenced using the ASAN_OPTIONS environment variable. When set to "help=1", the available options are shown at startup of the instrumented program. See <https://github.com/google/sanitizers/wiki/AddressSanitizerFlags#run-time-flags> for a list of supported options.

The option cannot be combined with -fsanitize=thread or -fsanitize=hwaddress 🚫

Note that the only targets -fsanitize=hwaddress is currently supported on are x86-64 (only with "-mlam=u48" or "-mlam=u57" options) and AArch64, in both cases only in 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's 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 🐧 See <https://github.com/google/kernel-sanitizers> for more details.

  • -fsanitize=hwaddress — okay so this one turns on Hardware-assisted AddressSanitizer, which is so smart bestie, it uses a hardware ability to ignore the top byte of a pointer so it can detect memory errors with a low memory overhead 💅. Memory access instructions get instrumented to detect out-of-bounds and use-after-free bugs. Heads up, this option also enables -fsanitize-address-use-after-scope. More deets at <https://clang.llvm.org/docs/HardwareAssistedAddressSanitizerDesign.html> 🔥. The run-time behavior can be influenced using the HWASAN_OPTIONS environment variable — when it's set to "help=1", the available options are shown at startup of the instrumented program ✨. The option cannot be combined with -fsanitize=thread or -fsanitize=address, and is currently only available on AArch64 🫶.
  • -fsanitize=kernel-hwaddress — this one enables Hardware-assisted AddressSanitizer for compilation of the Linux kernel 🐧. It's similar to -fsanitize=kernel-address but uses an alternate instrumentation method, and similar to -fsanitize=hwaddress but with instrumentation differences necessary for compiling the Linux kernel. Those differences are to avoid hwasan library initialization calls and to account for the stack pointer having a different value in its top byte 🎀.
Note: this option has different defaults to the -fsanitize=hwaddress. Instrumenting the stack and alloca calls are not on by default but are still possible by specifying the command-line options --param hwasan-instrument-stack=1 and --param hwasan-instrument-allocas=1 respectively. Using a random frame tag is not implemented for kernel instrumentation 💀.
  • -fsanitize=pointer-compare — instruments comparison operation (<, <=, >, >=) with pointer operands 🔍. The option must be combined with either -fsanitize=kernel-address or -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 variable ASAN_OPTIONS. Using "detect_invalid_pointer_pairs=1" detects invalid operation only when both pointers are non-null 👀.
  • -fsanitize=pointer-subtract — instruments subtraction with pointer operands 🧮. The option must be combined with either -fsanitize=kernel-address or -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 variable ASAN_OPTIONS. Using "detect_invalid_pointer_pairs=1" detects 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 (e.g. stack buffer overflows) 🛡️✨. It works by saving a function's return address to a separately allocated shadow call stack in the function prologue and restoring the return address from the shadow call stack in the function epilogue. Instrumentation only occurs in functions that need to save the return address to the stack. Iconic behavior honestly 🎀.

Currently it only supports the aarch64 platform. It is specifically designed for linux kernels that enable the CONFIG_SHADOW_CALL_STACK option 🐧. For the 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. 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 ⚠️. Lowkey terrifying but okay.

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 💖.

When compiling with -fsanitize=thread, you should also use -g to produce 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 overrides "malloc" and other allocator functions. See <https://github.com/google/sanitizers/wiki/AddressSanitizerLeakSanitizer> for more details. The run-time behavior can be influenced using the LSAN_OPTIONS environment variable. 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 the UBSAN_OPTIONS environment variable ✨. Current suboptions are:
    • -fsanitize=shift — this option enables checking that the result of a shift operation is not undefined. Note that what exactly is considered undefined differs slightly between C and C++, as well as between ISO C90 and C99, etc. This option has two suboptions, -fsanitize=shift-base and -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_unreachable" call into a diagnostics message call instead. When reaching the "__builtin_unreachable" call, the behavior is undefined 💀.
    • -fsanitize=vla-bound — this option instructs the compiler to check that the size of a variable length array is positive ✅.
    • -fsanitize=null — this option enables pointer checking 👀. Particularly, the application built with this option turned on will issue an error message when it tries to dereference a NULL pointer, or if a reference (possibly an rvalue reference) is bound to a NULL pointer, or if a method is invoked on an object pointed by a NULL pointer 🙈.
    • -fsanitize=return — this option enables return statement checking 🔙. Programs built with this option turned on will issue an error message when the end of a non-void function is reached without actually returning a value. This option works in C++ only 🎀.
    • -fsanitize=signed-integer-overflow — this option enables signed integer overflow checking 🔥. We check that the result of "+", "*", and both unary and binary "-" does not overflow in the signed arithmetics. This also detects "INT_MIN / -1" signed 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-arrays" or "-fstrict-flex-arrays=" options or "strict_flex_array" attributes 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 insufficiently aligned target, or when a method or constructor is invoked on insufficiently aligned object 📐✨.
  • -fsanitize=object-size — this option enables instrumentation of memory references using the "__builtin_dynamic_object_size" function 🔍. Various out of bounds pointer accesses are detected.
  • -fsanitize=float-divide-by-zero — detect floating-point division by zero 🌊➗. Unlike other similar options, -fsanitize=float-divide-by-zero is not enabled by -fsanitize=undefined, since floating-point division by zero can be a legitimate way of obtaining infinities and NaNs 👑.
  • -fsanitize=float-cast-overflow — this option enables floating-point type to integer conversion checking 🔄. We check that the result of the conversion does not overflow. Unlike other similar options, -fsanitize=float-cast-overflow is not enabled by -fsanitize=undefined. This option does not work well with "FE_INVALID" exceptions enabled ⚠️.
  • -fsanitize=nonnull-attribute — this option enables instrumentation of calls, checking whether null values are not passed to arguments marked as requiring a non-null value by the "nonnull" function attribute 🚫🙅‍♀️.
  • -fsanitize=returns-nonnull-attribute — okay so this one instruments return statements inside functions that are marked with the "returns_nonnull" function attribute, specifically to catch the vibe-ruining moment where someone returns a null value from a function that pinky-promised it wouldn't. 🙈
  • -fsanitize=bool — instruments loads from bool. If something that is not 0/1 gets loaded, a run-time error is issued. no cap, bool should be bool. 💅
  • -fsanitize=enum — instruments loads from an enum type. Load a value outside the range of values for that enum type and boom, run-time error issued. ✨
  • -fsanitize=vptr — this one instruments C++ member function calls, member accesses, and some conversions between pointers to base and derived classes, all to verify the referenced object actually has the correct dynamic type. like a vibe check but for your object's true identity 🦋
  • -fsanitize=pointer-overflow — instruments pointer arithmetics. If the pointer arithmetics overflows, a run-time error is issued. pointer math said "I'm fine" and it was not fine 😭
  • -fsanitize=builtin — instruments arguments to selected builtin functions. Pass an invalid value to such arguments and a run-time error is issued. E.g. passing 0 as the argument to __builtin_ctz or __builtin_clz invokes undefined behavior and is diagnosed by this option. don't do it bestie 💀
heads up! sanitizers tend to increase the rate of false positive warnings, most notably those around -Wmaybe-uninitialized. We recommend against combining -Werror and [the use of] sanitizers. like, they're both icons but they do NOT belong in the same group chat 🫶

While -ftrapv causes traps for signed overflows to be emitted, -fsanitize=undefined gives a diagnostic message. This currently works only for the C family of languages. 🎀

  • -fno-sanitize=all — disables all previously enabled sanitizers. and nope, -fsanitize=all is not allowed, because some sanitizers cannot be used together. they're giving exes at the same party 💀
  • -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. scientist era 🔬
  • -fsanitize-sections=s1,s2,... — sanitize global variables in selected user-defined sections. si may contain wildcards. ⭐
  • -fsanitize-recover[=opts] — -fsanitize-recover= controls error recovery mode for the sanitizers mentioned in the comma-separated list of opts. Enable this for a sanitizer component and it attempts to keep running the program as if no error happened 😳 which means multiple runtime errors can be reported in a single program run, and the exit code of the program may indicate success even when errors have been reported. kind of a "we're fine, everything's fine" moment ngl. The -fno-sanitize-recover= option can be used to alter this behavior: only the first detected error is reported and the program then exits with a non-zero exit code. 💅

Currently this feature only works for -fsanitize=undefined (and its suboptions except for -fsanitize=unreachable and -fsanitize=return), -fsanitize=float-cast-overflow, -fsanitize=float-divide-by-zero, -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 a recovery mode is turned on the compiler side, it needs to be also 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". This can be overridden through setting the "halt_on_error" flag in the corresponding environment variable. 🔥

deprecated alert 🚨 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 — enable sanitization of local variables to detect use-after-scope bugs. The option sets -fstack-reuse to none. caught in 4k 💅
  • -fsanitize-trap[=opts] — the -fsanitize-trap= option instructs the compiler to report, for the sanitizers mentioned in the comma-separated list of opts, undefined behavior using "__builtin_trap" rather than a "libubsan" library routine. If this option is enabled for a certain sanitizer, it takes precedence over the -fsanitizer-recover= for that sanitizer — "__builtin_trap" will be emitted and be fatal regardless of whether recovery is enabled or disabled using -fsanitize-recover=. 💀

The advantage of this is that the "libubsan" library is not needed and is not linked in, so this is usable even in freestanding environments. slay for the no-dependencies girlies ✨

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 — -fsanitize-trap=vptr is not allowed. 🙈

  • -fsanitize-undefined-trap-on-error — this is the deprecated equivalent of -fsanitize-trap=all. old news, still here 🎀
  • -fsanitize-coverage=trace-pc — enable coverage-guided fuzzing code instrumentation. Inserts a call to "__sanitizer_cov_trace_pc" into every basic block. every. single. one. 🦋
  • -fsanitize-coverage=trace-cmp — enable dataflow guided fuzzing code instrumentation. Inserts a call to "__sanitizer_cov_trace_cmp1", "__sanitizer_cov_trace_cmp2", "__sanitizer_cov_trace_cmp4" or "__sanitizer_cov_trace_cmp8" for integral comparison with both operands variable, or "__sanitizer_cov_trace_const_cmp1", "__sanitizer_cov_trace_const_cmp2", "__sanitizer_cov_trace_const_cmp4" or "__sanitizer_cov_trace_const_cmp8" for integral comparison with one operand constant, and "__sanitizer_cov_trace_cmpf" or "__sanitizer_cov_trace_cmpd" for float or double comparisons, plus "__sanitizer_cov_trace_switch" for switch statements. that's a whole lot of names, I know, but they're all icons 💅
  • -fcf-protection=[full|branch|return|none|check]
  • -fcf-protection — enable 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). security guard energy 👑

The -fcf-protection= keywords are interpreted as follows. 📖

The value "branch" tells 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 "return" implements checking of validity at the point of returning from a function.

The value "full" is an alias for specifying both "branch" and "return". the full package, we love her 💖

The value "check" is used for the final link with link-time optimization (LTO). An error is issued if LTO object files are compiled with different -fcf-protection values. The value "check" is ignored at the compile time.

The value "none" turns 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. order matters bestie ✨

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". bits are doing bits things 🔢

You can also use the "nocf_check" attribute to identify which functions and calls should be skipped from instrumentation. exceptions exist, and they're valid 🫶

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_trap" if the results do not match. Use with -fharden-conditional-branches to cover all conditionals. double-checking era 💅
  • -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_trap" if the result is unexpected. Use with -fharden-compares to cover all conditionals. bestie duo 🤝
  • -fharden-control-flow-redundancy — emit 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). Tuning options --param hardcfr-max-blocks and --param hardcfr-max-inline-blocks are available. so many knobs and I love that for them 🎀

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. theory only, no cap 📚

  • -fhardcfr-skip-leaf — disable -fharden-control-flow-redundancy in leaf functions. leaf functions get a pass 🌸
  • -fhardcfr-check-exceptions — when -fharden-control-flow-redundancy is 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-exceptions to disable it. ✨ default-on behavior, bestie ✨
  • -fhardcfr-check-returning-calls — when -fharden-control-flow-redundancy is 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. tail-call girlies stay winning 💅

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] — when -fharden-control-flow-redundancy is active, check the recorded execution path against the control flow graph before noreturn calls, either all of them (always), those that aren't expected to return control to the caller through an exception (no-xthrow, the default), those that may not return control to the caller through an exception either (nothrow), or none of them (never). the whole pick-your-fighter menu 🌈

Checking before a noreturn function that may return control to the caller through an exception may cause checking to be performed more than once, if the exception is caught in the caller, whether by a handler or a cleanup. When -fhardcfr-check-exceptions is also enabled, the compiler will avoid associating a noreturn call with the implicitly-added cleanup handler, since it would be redundant with the check performed before the call, but other handlers or cleanups in the function, if activated, will modify the recorded execution path and check it again when another checkpoint is hit. The checkpoint may even be another noreturn call, so checking may end up performed multiple times. yes, it can get a little extra like that 😭

Various optimizers may cause calls to be marked as noreturn and/or nothrow, even in the absence of the corresponding attributes, which may affect the placement of checks before calls, as well as 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, have made no-xthrow the default setting for this option: it excludes from the noreturn treatment only internal functions used to (re)raise exceptions, that are not affected by these optimizations. iykyk default lore

  • -fhardened — enable a set of flags for C and C++ that improve the security of the generated code without affecting its ABI. The precise flags enabled may change between major releases of GCC, but are currently:
-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)

The list of options enabled by -fhardened can be generated using the --help=hardened option. handy little cheat code ✨

When the system glibc is older than 2.35, -D_FORTIFY_SOURCE=2 is used instead.

This option is intended to be used in production builds, not merely in debug builds. production queen behavior, no cap 💅

Currently, -fhardened is only supported on GNU/Linux targets.

-fhardened only enables a particular option if it wasn't already specified anywhere on the command line. For instance, -fhardened -fstack-protector will only enable -fstack-protector, but not -fstack-protector-strong. it respects your choices, periodt

  • -fstack-protector — emit extra code to check for buffer overflows, such as stack smashing attacks. This is done by adding a guard variable to functions with vulnerable objects. This includes functions that call alloca, and functions with buffers larger than or equal to 8 bytes. The guards are initialized when a function is entered and then checked when the function exits. If a guard check fails, an error message is printed and the program exits. Only variables that are actually allocated on the stack are considered, optimized away variables or variables allocated in registers don't count. guard-girlie behavior 🛡️
  • -fstack-protector-all — like -fstack-protector except that all functions are protected. everybody gets a guard, no exceptions 💅
  • -fstack-protector-strong — like -fstack-protector but includes additional functions to be protected — those that have local array definitions, or have references to local frame addresses. Only variables that are actually allocated on the stack are considered, optimized away variables or variables allocated in registers don't count. strong-mode slay 🌿
  • -fstack-protector-explicit — like -fstack-protector but only protects those functions which have the stack_protect attribute. only the ones with the little hand-written invite 🎀
  • -fstack-check — generate code to verify that you do not go beyond the boundary of the stack. You should specify this flag if you are running in an environment with multiple threads, but you only rarely need to specify it in a single-threaded environment since stack overflow is automatically detected on nearly all systems if there is only one stack.

Note that this switch does not actually cause checking to be done; the operating system or the language runtime must do that. The switch causes generation of code to ensure that they see the stack being extended. it’s giving “I set up the vibes, someone else does the actual checking” 💀

You can additionally specify a string parameter: no means no checking, generic means force the use of old-style checking, specific means use the best checking method and is equivalent to bare -fstack-check.

Old-style checking is a generic mechanism that requires no specific target support in the compiler but comes with the following drawbacks:

  1. Modified allocation strategy for large objects: they are always allocated dynamically if their size exceeds a fixed threshold. Note this may change the semantics of some code. lowkey a big deal 😬
  2. Fixed limit on the size of the static frame of functions: when it is topped by a particular function, stack checking is not reliable and a warning is issued by the compiler.
  3. Inefficiency: because of both the modified allocation strategy and the generic implementation, code performance is hampered.

Note that old-style stack checking is also the fallback method for specific if no target support has been added in the compiler.

-fstack-check= is designed for Ada's needs to detect infinite recursion and stack overflows. specific is an excellent choice when compiling Ada code. It is not generally sufficient to protect against stack-clash attacks. To protect against those you want -fstack-clash-protection. different stack problem, different stack remedy, bestie

  • -fstack-clash-protection — generate code to prevent stack clash style attacks. When this option is enabled, the compiler will only allocate one page of stack space at a time and each page is accessed immediately after allocation. Thus, it prevents allocations from jumping over any stack guard page provided by the operating system. one page at a time, no sneaky leaps 🦋

Most targets do not fully support stack clash protection. However, on those targets -fstack-clash-protection will protect dynamic stack allocations. -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 — generate code to ensure that the stack does not grow beyond a certain value, either the value of a register or the address of a symbol. If a larger stack is required, a signal is raised at run time. For most targets, the signal is raised before the stack overruns the boundary, so it is possible to catch the signal without taking special precautions. stack boundary drama, but make it catchable ✨

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. local override, because sometimes rules are not the vibe

  • -fsplit-stack — generate code to automatically split the stack before it overflows. The resulting program has a discontiguous stack which can only overflow if the program is unable to allocate any more memory. This is most useful when running threaded programs, as it is no longer necessary to calculate a good stack size to use for each thread. This is currently only implemented for the x86 targets running GNU/Linux. threaded-program girlies, this one’s for you 🧵

When code compiled with -fsplit-stack calls code compiled without -fsplit-stack, there may not be much stack space available for the latter code to run. If compiling all code, including library code, with -fsplit-stack is not an option, then the linker can fix up these calls so that the code compiled without -fsplit-stack always has a large stack. Support for this is implemented in the gold linker in GNU binutils release 2.21 and later. linker fix-up era, iconic

  • -fstrub=disable — disable stack scrubbing entirely, ignoring any strub attributes. See (literally just See hanging there, no cap 💀)
  • -fstrub=strict — functions default to strub mode disabled, and apply strictly the restriction that only functions associated with strub-callable modes (at-calls, callable and always_inline internal) are callable by functions with strub-enabled modes (at-calls and internal). strict mode is strict, bestie 🎀
  • -fstrub=relaxed — restore the default stack scrub (strub) setting, namely, strub is only enabled as required by strub attributes associated with function and data types. Relaxed means that strub contexts are only prevented from calling functions explicitly associated with strub mode disabled. This option is only useful to override other -fstrub=* options that precede it in the command line. chill mode, but only for overriding the earlier -fstrub=* choices ✨

-fstrub=at-calls

Turns on the at-calls strub mode where it's viable, bestie. Ngl, the main character use case here is testing — it puts the whole strub machinery through its paces in scenarios that are strictly local to one translation unit. 🧪

Here's the catch though: this strub mode modifies function interfaces, so any function that's visible to other translation units, or that has its address taken, will not be affected by this option. Optimization options can affect viability too. Peep the strub attribute documentation for the deets on viability and eligibility requirements. iykyk 💅

-fstrub=internal

Enables the internal strub mode where viable. Again, primary use is testing — this one's designed to thoroughly exercise the parts of the strub machinery that implement the less efficient but interface-preserving strub mode. Functions that wouldn't be affected by this option are, like, genuinely uncommon. 🦋

-fstrub=all

Enables some strub mode where viable. When both strub modes are viable, at-calls wins — it's the favorite. -fdump-ipa-strubm adds function attributes that tell you which mode was selected for each function. Primary use: testing, to thoroughly exercise the strub machinery. Obsessed with the thoroughness ngl. ✨

-fvtable-verify=[std|preinit|none]

Only available when you're compiling C++ code, so C girlies this one's not for you. 🙈 It turns on (or off, if you're using -fvtable-verify=none) the security feature that verifies at run time — for every virtual call — that the vtable pointer through which the call is made is valid for the type of the object, and has not been corrupted or overwritten. If an invalid vtable pointer gets detected at run time, an error is reported and execution of the program is immediately halted. 💀 Like, no mercy.

This option causes run-time data structures to be built at program startup, which are then used for verifying the vtable pointers. The options std and preinit control the timing of when these data structures get built. In both cases the data structures are built before execution reaches main. 🌸

  • -fvtable-verify=std — data structures are built after shared libraries have been loaded and initialized.
  • -fvtable-verify=preinit — they're built before shared libraries have been loaded and initialized.

If this option appears multiple times on the command line with different values specified, none takes highest priority over both std and preinit; preinit takes priority over std. ✨

-fvtv-debug

When you use this alongside -fvtable-verify=std or -fvtable-verify=preinit, it causes debug versions of the runtime functions for the vtable verification feature to be called. This flag also causes the compiler to log info about which vtable pointers it finds for each class. 📝

That info is written to a file named vtv_set_ptr_data.log in the directory named by the environment variable VTV_LOGS_DIR if that's defined — or the current working directory otherwise.

⚠️ Heads up bestie: this feature appends data to the log file. If you want a fresh log file, be sure to delete any existing one first. 🗑️

-fvtv-counts

This one's a debugging flag. 🐛 When used alongside -fvtable-verify=std or -fvtable-verify=preinit, it causes the compiler to keep track of the total number of virtual calls it encounters and the number of verifications it inserts. It also counts the number of calls to certain run-time library functions that it inserts and logs this info for each compilation unit.

The compiler writes all that to a file named vtv_count_data.log in the directory named by the environment variable VTV_LOGS_DIR if that's defined — or the current working directory otherwise. It also counts the size of the vtable pointer sets for each class and writes this to vtv_class_set_sizes.log in the same directory. 📊

⚠️ Same warning, same energy: this feature appends data to the log files. To get fresh log files, be sure to delete any existing ones. 🗑️

-finstrument-functions

Generate instrumentation calls for entry and exit to functions. 🎀 Just after function entry and just before function exit, the following profiling functions are called with the address of the current function and its call site:

void __cyg_profile_func_enter (void *this_fn,
                               void *call_site);
void __cyg_profile_func_exit  (void *this_fn,
                               void *call_site);

(On some platforms, __builtin_return_address does not work beyond the current function, so the call site info may not be available to the profiling functions otherwise.) 😭

The first argument is the address of the start of the current function, which may be looked up exactly in the symbol table. 🔍

This instrumentation is also done for functions expanded inline in other functions — the profiling calls indicate where, conceptually, the inline function is entered and exited. This means that addressable versions of such functions must be available. If all your uses of a function are expanded inline, this may mean an additional expansion of code size. If you use extern inline in your C code, an addressable version of such functions must be provided. (This is normally the case anyway, but if you get lucky and the optimizer always expands the functions inline, you might have gotten away without providing static copies.) 🍀

A function may be given the attribute no_instrument_function, in which case this instrumentation is not done. This can be used, for example, for the profiling functions listed above, high-priority interrupt routines, and any functions from which the profiling functions cannot safely be called — perhaps signal handlers, if the profiling routines generate output or allocate memory. 💜

-finstrument-functions-once

Similar to -finstrument-functions, but the profiling functions are called only 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 this first entry. 🎯

The definition of "once" for the purpose of this option is a little vague, because the implementation is not protected against data races. As a result, 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. That is to say, 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 that are excluded from instrumentation (see the description of -finstrument-functions). If the file that contains a function definition matches with one of file, then that function is not instrumented. The match is done on substrings: if the file parameter is a substring of the file name, it's considered to be a match. ✂️

For example:

-finstrument-functions-exclude-file-list=/bits/stl,include/sys

excludes any inline function defined in files whose pathnames contain /bits/stl or include/sys.

If, for some reason, you want to include letter , in one of sym, write ,. For example, -finstrument-functions-exclude-file-list=',,tmp' (note the single quote surrounding the option). 🙈

-finstrument-functions-exclude-function-list=sym,sym,...

Similar to -finstrument-functions-exclude-file-list, but this option sets the list of function names to be excluded from instrumentation. The function name to be matched is its user-visible name, such as vector<int> blah(const vector<int> &), not the internal mangled name (e.g., _Z4blahRSt6vectorIiSaIiEE). 💅

The match is done on substrings: if the sym parameter is a substring of the function name, it's considered to be a match. For C99 and C++ extended identifiers, the function name must be given in UTF-8, not using universal character names. 🌈

-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 to 0, so the function entry points to the address just at the first NOP. 🩹

The NOP instructions reserve extra space which can be used to patch in any desired instrumentation at run time — provided that the code segment is writable. The amount of space is controllable indirectly via the number of NOPs; the NOP instruction used corresponds to the instruction emitted by the internal GCC back-end interface gen_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. 📍

💡 Note that the value of __attribute__ ((patchable_function_entry (N,M))) takes precedence over command-line option -fpatchable-function-entry=N,M. This can be used to increase the area size or to remove it completely on a single function. If N=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 this. 🔧

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 not using -msplit-patch-nops. 👑

🌸 Options Controlling the Preprocessor 🌸

These options control the C preprocessor, which is run on each C source file before actual compilation. 🧚

If you use the -E option, nothing is done except preprocessing. Some of these options make sense only together with -E, because they cause the preprocessor output to be unsuitable for actual compilation. 😌

In addition to the options listed here, there are a number of options to control search paths for include files documented in Directory Options. Options to control preprocessor diagnostics are listed in Warning Options. 🎀

  • -D name — Predefine name as a macro, with definition 1. Slay, that's it. 💖
  • -D name=definition — bestie, this is the define flag 💅. The contents of definition get tokenized and processed exactly as if they'd shown up during translation phase three in a #define directive. Huge caveat though: the definition gets truncated by embedded newline characters — so no sneaky multi-line macros, okay? 😭

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 such as spaces that have a meaning in the shell syntax.

Wanna define a function-like macro right there on the command line? Write its argument list with surrounding parentheses before the equals sign (if any). Parentheses are meaningful to most shells, so quote the option, mwah 💋. With sh and csh, -D'name(args...)=definition' works.

And the ordering lore: -D and -U options are processed in the order they are given on the command line. All -imacros file and -include file options are processed after all -D and -U options. Clock it ⏰.

  • -U name — cancel any previous definition of name, whether it was built in or provided with a -D option. It's giving "undo, but make it preprocessor" ✨.
  • -include file — process file as if #include "file" appeared as the first line of the primary source file 🫶. But plot twist: the first directory searched for file is the preprocessor's working directory instead of the directory containing the main source file. If it's not found there, it's searched for in the remainder of the #include "..." search chain as normal.

Multiple -include options? The files get included in the order they appear on the command line. Order is everything, no cap 🎀.

  • -imacros file — literally exactly like -include, except any output produced by scanning file is thrown away 🗑️. Macros it defines remain defined. This is the whole point: it lets you acquire all the macros from a header without also processing its declarations. Chef's kiss 👑

Also, all files specified by -imacros are processed before all files specified by -include. Rules are rules 💅.

  • -undef — do not predefine any system-specific or GCC-specific macros. The standard predefined macros remain defined though, so we're not fully feral 🌸.
  • -pthread — define additional macros required for using the POSIX threads library 🧵. Use this option consistently for both compilation and linking — like, both, bestie, don't do one and not the other 🙈. Supported on GNU/Linux targets, most other Unix derivatives, and also on x86 Cygwin and MinGW targets. Iconic cross-platform behavior tbh.
  • -M — instead of outputting the result of preprocessing, output a rule suitable for make describing the dependencies of the main source file 🫡. The preprocessor outputs one make rule containing the object file name for that source file, a colon, and the names of all the included files, including those coming from -include or -imacros command-line options.

Unless specified explicitly (with -MT or -MQ), the object file name consists of the name of the source file with any suffix replaced with object file suffix and with any leading directory parts removed. If there are many included files then the rule is split into several lines using \-newline. The rule has no commands.

Heads up ⚠️: 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. Sneaky but slay.

  • -MM — like -M but do not mention header files that are found in system header directories, nor header files that are included, directly or indirectly, from such a header 🚫📁.

This implies that the choice of angle brackets or double quotes in an #include directive does not in itself determine whether that header appears in -MM dependency output. So don't come for me about < vs " — that's not the vibe check happening here 💀.

  • -MF file — when used with -M or -MM, specifies a file to write the dependencies to 📝. If no -MF switch is given the preprocessor sends the rules to the same place it would send preprocessed output.

When used with the driver options -MD or -MMD, -MF overrides the default dependency output file.

And if file is -, then the dependencies are written to stdout 🖨️. Dash supremacy.

  • -MG — in conjunction with an option such as -M requesting dependency generation, -MG assumes missing header files are generated files and adds them to the dependency list without raising an error 🧚. The dependency filename is taken directly from the #include directive without prepending any path. -MG also suppresses preprocessed output, as a missing header file renders this useless.

This feature is used in automatic updating of makefiles. Very "I made this for the build system girlies" of it 🎀.

  • -Mno-modules — disable dependency generation for compiled module interfaces 🔒. That's it, that's the whole thing.
  • -MP — this option instructs CPP to add a phony target for each dependency other than the main file, causing each to depend on nothing 👻. These dummy rules work around errors make gives if you remove header files without updating the Makefile to match. Rude of make honestly.

This is typical output:

test.o: test.c test.h

test.h:
  • -MT target — change the target of the rule emitted by dependency generation 🎯. By default CPP takes the name of the main input file, deletes any directory components and any file suffix such as .c, and appends the platform's usual object suffix. The result is the target.

An -MT option 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. Your choice, main character 💅.

For example, -MT '$(objpfx)foo.o' might give

$(objpfx)foo.o: foo.c
  • -MQ target — same as -MT, but it quotes any characters which are special to Make 🧼. So -MQ '$(objpfx)foo.o' gives
$$(objpfx)foo.o: foo.c

The default target is automatically quoted, as if it were given with -MQ. Safety first, periodt.

  • -MD — -MD is equivalent to -M -MF file, except that -E is not implied 🔥. The driver determines file based on whether an -o option is given. If it is, the driver uses its argument but with a suffix of .d, otherwise it takes the name of the input file, removes any directory components and suffix, and applies a .d suffix.

If -MD is used in conjunction with -E, any -o switch is understood to specify the dependency output file, but if used without -E, each -o is understood to specify a target object file. Context matters bestie 🙈.

Since -E is not implied, -MD can be used to generate a dependency output file as a side effect of the compilation process. Sneaky little bonus, we love her 👑

  • -MMD — like -MD except mention only user header files, not system header files 🦋. Keep your circle small.
  • -fpreprocessed — indicate to the preprocessor that the input file has already been preprocessed ✅. This suppresses things like macro expansion, trigraph conversion, escaped newline splicing, and processing of most directives. The preprocessor still recognizes and removes comments, so that you can pass a file preprocessed with -C to the compiler without problems. In this mode the integrated preprocessor is little more than a tokenizer for the front ends. Glorified tokenizer era 💀.

-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. Write that down 📝.

  • -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. Layered, gorgeous, no notes 💅✨.

  • -fdollars-in-identifiers — accept $ in identifiers 💸. Chaotic. Love it.
  • -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++. Default-on royalty 👑.
  • -fno-canonical-system-headers — when preprocessing, do not shorten system header paths with canonicalization 🛣️. Sometimes you want the long way around, bestie.
  • -fmax-include-depth=depth — set the maximum depth of the nested #include. The default is 200 🌀. Don't nest infinitely, it's giving recursion nightmare.
  • -fsearch-include-path[=kind] — look for input files on the #include path, 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.cc

kind defaults to user, which looks on the #include "..." search path; you can also explicitly specify system for the #include <...> search path 🎯. Choose your fighter.

  • -ftabstop=width — set the distance between tab stops 📏. This helps the preprocessor report correct column numbers in warnings or errors, even if tabs appear on the line. If the value is less than 1 or greater than 100, the option is ignored 🚫. The default is 8. Tabs are still the enemy but at least now they're measured 🫶.
  • -ftrack-macro-expansion[=level] — track locations of tokens across macro expansions 🧭. This allows the compiler to emit a diagnostic about the current macro expansion stack when a compilation error occurs in a macro expansion. Fair warning: using this option makes the preprocessor and the compiler consume more memory 😭.

The level parameter can be used to choose the level of precision of token location tracking, thus decreasing the memory consumption if necessary:

  • Value 0 of level de-activates this option.
  • Value 1 tracks token locations in a degraded mode for the sake of minimal memory overhead. In this mode all tokens resulting from the expansion of an argument of a function-like macro have the same location.
  • Value 2 tracks token locations completely. This value is the most memory hungry 🍔.

When this option is given no argument, the default parameter value is 2.

Heads up bestie: -ftrack-macro-expansion=2 is activated by default. So you're already living that memory-hungry lifestyle, no cap 💀👑
  • -fmacro-prefix-map=old=new — okay so when the preprocessor is chewing through files that live in directory old, it'll expand the __FILE__ and __BASE_FILE__ macros like those files were chilling in directory new instead 🗂️. The iconic use case? Turning an absolute path into a relative one by setting new to ., which lowkey makes your builds reproducible and location-independent, and honestly we love that for us. This one also affects __builtin_FILE() during compilation, bestie. Also see -ffile-prefix-map and -fcanon-prefix-map 🫶.
  • -fexec-charset=charset — sets the execution character set, i.e. what's used for string and character constants ✨. Default is UTF-8, periodt. charset can be literally any encoding your system's iconv library routine supports, so go off 💅.
  • -fwide-exec-charset=charset — sets the wide execution character set, used for wide string and character constants 🌈. The default is one of UTF-32BE, UTF-32LE, UTF-16BE, or UTF-16LE — whichever one matches the width of wchar_t and the big-endian or little-endian byte order being used for code generation. Same deal as -fexec-charset: charset can be any encoding your system's iconv library routine supports. BUT ⚠️ you will have problems with encodings that don't fit exactly in wchar_t, so vibe-check that first, ngl.
  • -finput-charset=charset — sets the input character set, used to translate from the character set of the input file to the source character set GCC uses 🦋. If the locale doesn't specify, or GCC can't get that info from the locale, the default is UTF-8. This can be overridden by either the locale or this command-line option — and currently the command-line option takes precedence if there's a conflict. charset can be any encoding supported by the system's iconv library routine 💜.
  • -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 used to create it, because those files are not consulted when a precompiled header is used. Makes sense, slay.
  • -fpch-preprocess — this option lets you use a precompiled header together with -E 🎀. It inserts a special #pragma, specifically #pragma GCC pch_preprocess "filename", in the output to mark where the precompiled header was found, plus its filename. When -fpreprocessed is in use, GCC recognizes this #pragma and loads the PCH.

This option is off by default, because the resulting preprocessed output is only really suitable as input to GCC — it's not giving shareable-content energy, bestie. 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 is available in a different location. 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 it's enabled, the preprocessor emits, after the initial linemarker, a second linemarker with the current working directory followed by two slashes. GCC uses this directory — when it's present in the preprocessed input — as the directory emitted as the current working directory in some debugging information formats. This option is implicitly enabled if debugging information is enabled, but you can inhibit it with the negated form -fno-working-directory. And if the -P flag is present on the command line, this option has no effect, since no #line directives are emitted whatsoever — zero, zilch, nada 💀.
  • -A predicate=answer — make an assertion with the predicate predicate and answer answer ✅. This form is preferred over the older -A predicate(answer) form (which is still supported, btw) because it doesn't use shell special characters. iykyk.
  • -A -predicate=answer — cancel an assertion with the predicate predicate and answer answer ❌. Undoing your own moves, we respect it.
  • -C — do not discard comments 🗯️. All comments are passed through to the output file, except for comments in processed directives, which get deleted along with the directive.
Be prepared for side effects when using -C — it causes the preprocessor to treat comments as tokens in their own right 😭. For example, comments appearing at the start of what would be a directive line have the effect of turning that line into an ordinary source line, since the first token on the line is no longer a #. Main character energy, honestly.
  • -CC — do not discard comments, including during macro expansion 💅. This is like -C, except that comments contained within macros are also passed through to the output file where the macro is expanded.

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 remainder of the source line 🙈. Very "we fixed it before it fixed us" of it.

The -CC option is generally used to support lint comments, fr fr.

  • -P — inhibit generation of linemarkers in the output from the preprocessor 🚫. This might be useful when you're running the preprocessor on something that is not C code and will be sent to a program which might be confused by the linemarkers. Protecting the other programs' peace, iconic.
  • -traditional
  • -traditional-cpp — try to imitate the behavior of pre-standard C preprocessors, as opposed to ISO C preprocessors 🧓. See the GNU CPP manual for the details, bestie.
Note that 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. So don't just sprinkle it around and expect magic ✨.
  • -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 which only permit very short file names, such as MS-DOS 🦕. A little fossil-support moment, we love to see it.
  • -H — print the name of each header file used, in addition to other normal activities 📚. Each name is indented to show how deep in the #include stack it is. Precompiled header files are also printed, even if they're found to be invalid — an invalid precompiled header file is printed with ...x and a valid one with ...! 💀✅. Chef's kiss diagnostic UX.
  • -dletters — says to make debugging dumps during compilation as specified by letters 🐛. The flags documented here are the ones relevant to the preprocessor. Other letters are interpreted by the compiler proper, or reserved for future versions of GCC, and so are silently ignored 🤐. And if you specify letters whose behavior conflicts, the result is undefined — so like, don't 😭.
    • -dM — instead of the normal output, generate a list of #define directives for all the macros defined during the execution of the preprocessor, including predefined macros 👑. This gives you a way of finding out what is predefined in your version of the preprocessor. Assuming you have no file foo.h, the command
        touch foo.h; cpp -dM foo.h

shows all the predefined macros. Sneaky but effective.

If you use -dM without the -E option, -dM is interpreted as a synonym for -fdump-rtl-mach. Plot twist 🌀.
  • -dD — like -dM except that it outputs both the #define directives and the result of preprocessing 📝. Both kinds of output go to the standard output file.
  • -dN — like -dD, but emit only the macro names, not their expansions 🤫. Minimalist queen.
  • -dI — output #include directives in addition to the result of preprocessing 📎.
  • -dU — like -dD, except that only macros that are expanded, or whose definedness is tested in preprocessor directives, are output 🎯. The output is delayed until the use or test of the macro, and #undef directives are also output for macros tested but undefined at the time. Very "only speak when spoken to" of it.
  • -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. Basically a token-by-token receipts moment.

When used from GCC without -E, this option has no effect. Nothing. Nada. 💀

  • -Wp,option — you can use -Wp,option to bypass the compiler driver and pass option directly through to the preprocessor 🏃‍♀️💨. If option contains commas, it's split into multiple options at the commas. However — and this is the important bit — many options are modified, translated or interpreted by the compiler driver before being passed to the preprocessor, and -Wp forcibly bypasses this phase 😬. The preprocessor's direct interface is undocumented and subject to change, so whenever possible you should avoid using -Wp and let the driver handle the options instead. Trust the driver, bestie 🫶.
  • -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. Don't ask, just do it.
  • -no-integrated-cpp — perform preprocessing as a separate pass before compilation 🚂. By default, GCC performs preprocessing as an integrated part of input tokenization and parsing. If this option is provided, the appropriate language front end (cc1, cc1plus, or cc1obj for C, C++, and Objective-C, respectively) is instead invoked twice — once for preprocessing only and once for actual compilation of the preprocessed input. This option may be useful in conjunction with the -B or -wrapper options to specify an alternate preprocessor or perform additional processing of the program source between normal preprocessing and compilation 🎀. Very "do it in two steps because we can" energy.

🌸 Passing Options to the Assembler 🌸

You can pass options to the assembler, bestie — here's how 💜

  • -Wa,option — pass option as an option to the assembler 🔧. If option contains commas, it's split into multiple options at the commas.
  • -Xassembler option — pass option as an option to the assembler 📦. You can use this to supply system-specific assembler options that GCC does not recognize.
If you want to pass an option that takes an argument, you must use -Xassembler twice, once for the option and once for the argument 🔁. Same rule as -Xpreprocessor, no cap.

🔗 Options for Linking 🔗

okay bestie so these options literally only matter when the compiler is actually linking object files into an executable output file, mmmkay? If it's not doing a link step they are meaningless. 💅

  • object-file-name — a file name that doesn't end in a special recognized suffix gets treated as naming an object file or a library. (the linker tells object files apart from libraries by peeking at the file contents, sneaky ✨) If linking is done, these object files get used as input to the linker.
  • -c, -S, -E — if any of these is used, the linker is not run, and object file names should not be used as arguments. like, don't even try it 🙈
  • -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. But for debugging the compiler, and if incremental linking with a non-LTO object file is what you want, it may be useful to control the type manually. 👑
    • if type is exec, code generation produces a static binary. in this case -fpic and -fpie are both disabled.
    • if type is dyn, code generation produces a shared library. in this case -fpic or -fPIC is preserved, but not enabled automatically. this allows you to build shared libraries without position-independent code on architectures where that's possible, i.e. on x86. 🎀
    • if type is pie, code generation produces an -fpie executable. this results in similar optimizations as exec except that -fpie is not disabled if specified at compilation time.
    • if type is rel, the compiler assumes that incremental linking is done. the sections containing intermediate code for link-time optimization are merged, pre-optimized, and output to the resulting object file. in addition, if -ffat-lto-objects is 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. at link time the result of incremental linking also loads faster than a static library assuming that the majority of objects in the library are used. 🚀
    • finally nolto-rel configures 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 benefits of whole program optimizations are lost. 😭

and ngl, during the incremental link (by -r) the linker plugin defaults to rel. With current interfaces to GNU Binutils it is however not possible to incrementally link LTO objects and non-LTO objects into a single mixed object file. if any of object files in incremental link cannot be used for link-time optimization, the linker plugin issues a warning and uses nolto-rel. to maintain whole program optimization, it is recommended to link such objects into static library instead. alternatively it is possible to use H.J. Lu's binutils with support for mixed objects. iykyk 🫶

  • -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.
  • -llibrary
  • -l library — search the library named library when linking. (the second alternative, with the library as a separate argument, is only for POSIX compliance and is not recommended — bestie pls don't 🙈)

the -l option is passed directly to the linker by GCC. refer to your linker documentation for exact details. the general description below applies to the GNU linker.

the linker searches a standard list of directories for the library. the directories searched 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 the -static option is used.

✨ and it makes a difference where in the command you write this option — the linker searches and processes libraries and object files in the order they are specified. thus, foo.o -lz bar.o searches library z after file foo.o but before bar.o. if bar.o refers to functions in z, those functions may not be loaded. 😬 order matters, periodt.

  • -lobjc — you need this special case of the -l option in order to link an Objective-C or Objective-C++ program. 🍓
  • -nostartfiles — do not use the standard system startup files when linking. the standard system libraries are used normally, unless -nostdlib, -nolibc, or -nodefaultlibs is used.
  • -nodefaultlibs — do not use the standard system libraries when linking. only the libraries you specify are passed to the linker, and options specifying linkage of the system libraries, such as -static-libgcc or -shared-libgcc, are ignored. the standard startup files are used normally, unless -nostartfiles is used.
the compiler may generate calls to memcmp, memset, memcpy and memmove. these entries are usually resolved by entries in libc. these entry points should be supplied through some other mechanism when this option is specified. 👀
  • -nolibc — do not use the C library or system libraries tightly coupled with it when linking. 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 -lc from the link command line, as well as system libraries that normally go with it and become meaningless when absence of a C library is assumed, for example -lpthread or -lm in some configurations. this is intended for bare-board targets when there is indeed no C library available. 🏝️
  • -nostdlib — do not use the standard system startup files or libraries when linking. no startup files and only the libraries you specify are passed to the linker, and options specifying linkage of the system libraries, such as -static-libgcc or -shared-libgcc, are ignored.
the compiler may generate calls to memcmp, memset, memcpy and memmove. these entries are usually resolved by entries in libc. these entry points should be supplied through some other mechanism when this option is specified.

one of the standard libraries bypassed by -nostdlib and -nodefaultlibs is libgcc.a, a library of internal subroutines which GCC uses to overcome shortcomings of particular machines, or special needs for some languages.

in most cases, you need libgcc.a even when you want to avoid other standard libraries. in other words, when you specify -nostdlib or -nodefaultlibs you should usually specify -lgcc as well. this ensures that you have no unresolved references to internal GCC library subroutines. (an example of such an internal subroutine is __main, used to ensure C++ constructors are called.) 💖

  • -nostdlib++ — do not implicitly link with standard C++ libraries.
  • -e 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 — 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. like, don't skip that part bestie 🌸
  • -no-pie — don't produce a dynamically linked position independent executable.
  • -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 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-dynamic to the ELF linker, on targets that support it. this instructs the linker to add all symbols, not only used ones, to the dynamic symbol table. this option is needed for some uses of dlopen or to allow obtaining backtraces from within a program. 🔍
  • -s — remove all symbol table and relocation information from the executable.
  • -static — on systems that support dynamic linking, this overrides -pie and prevents linking with the shared libraries. on other systems, this option has no effect.
  • -shared — produce a shared object which can then be linked with other objects to form an executable. not all systems support this option. for predictable results, you must also specify the same set of options used for compilation (-fpic, -fPIC, or model suboptions) when you specify this linker option. [1] 💅
  • -shared-libgcc
  • -static-libgcc — on systems that provide libgcc as a shared library, these options force the use of either the shared or static version, respectively. if no shared version of libgcc was built when the compiler was configured, these options have no effect.

there are several situations in which an application should use the shared libgcc instead of the static version. the most common of these is when the application wishes to throw and catch exceptions across different shared libraries. in that case, each of the libraries as well as the application itself should use the shared libgcc. 😤

therefore, the G++ driver automatically adds -shared-libgcc whenever you build a shared library or a main executable, because C++ programs typically use exceptions, so this is the right thing to do. slay 🌈

Okay so if, instead, you use the GCC driver to create shared libraries, you may find that they are not always linked with the shared libgcc. If GCC finds, at its configuration time, that you have a non-GNU linker or a GNU linker that does not support option --eh-frame-hdr, it links the shared version of libgcc into shared libraries by default. Otherwise, it takes advantage of the linker and optimizes away the linking with the shared version of libgcc, linking with the static version of libgcc by default. This allows exceptions to propagate through such shared libraries, without incurring relocation costs at library load time.

However, if a library or main executable is supposed to throw or catch exceptions, you must link it using the G++ driver, or using the option -shared-libgcc, such that it is linked with the shared libgcc.

  • -static-libasan — when the -fsanitize=address option is used to link a program, the GCC driver automatically links against libasan. If libasan is available as a shared library, and the -static option is not used, then this links against the shared version of libasan. The -static-libasan option directs the GCC driver to link libasan statically, without necessarily linking other libraries statically.
  • -static-libtsan — when the -fsanitize=thread option is used to link a program, the GCC driver automatically links against libtsan. If libtsan is available as a shared library, and the -static option is not used, then this links against the shared version of libtsan. The -static-libtsan option directs the GCC driver to link libtsan statically, without necessarily linking other libraries statically.
  • -static-liblsan — when the -fsanitize=leak option is used to link a program, the GCC driver automatically links against liblsan. If liblsan is available as a shared library, and the -static option is not used, then this links against the shared version of liblsan. The -static-liblsan option directs the GCC driver to link liblsan statically, without necessarily linking other libraries statically.
  • -static-libubsan — when the -fsanitize=undefined option is used to link a program, the GCC driver automatically links against libubsan. If libubsan is available as a shared library, and the -static option is not used, then this links against the shared version of libubsan. The -static-libubsan option directs the GCC driver to link libubsan statically, without necessarily linking other libraries statically.
  • -static-libstdc++ — when the g++ program is used to link a C++ program, it normally automatically links against libstdc++. If libstdc++ is available as a shared library, and the -static option is not used, then this links against the shared version of libstdc++. That is normally fine. However, it is sometimes useful to freeze the version of libstdc++ used by the program without going all the way to a fully static link. The -static-libstdc++ option directs the g++ driver to link libstdc++ statically, without necessarily linking other libraries statically.
  • -symbolic — bind references to global symbols when building a shared object. Warn about any unresolved references (unless overridden by the link editor option -Xlinker -z -Xlinker defs). Only a few systems support this option.
  • -T script — use script as the linker script. This option is supported by most systems using the GNU linker. On some targets, such as bare-board targets without an operating system, the -T option may be required when linking to avoid references to undefined symbols.
  • -Xlinker option — pass option as an option to the linker. You can use this to supply system-specific linker options that GCC does not recognize.

If you want to pass an option that takes a separate argument, you must use -Xlinker twice, once for the option and once for the argument. For example, to pass -assert definitions, you must write -Xlinker -assert -Xlinker definitions. It does not work to write -Xlinker "-assert definitions", because this passes the entire string as a single argument, which is not what the linker expects.

When using the GNU linker, it is usually more convenient to pass arguments to linker options using the option=value syntax than as separate arguments. For example, you can specify -Xlinker -Map=output.map rather than -Xlinker -Map -Xlinker output.map. Other linkers may not support this syntax for command-line options.

  • -Wl,option — pass option as an option to the linker. If option contains commas, it is split into multiple options at the commas. You can use this syntax to pass an argument to the option. For example, -Wl,-Map,output.map passes -Map output.map to the linker. When using the GNU linker, you can also get the same effect with -Wl,-Map=output.map.
  • -u symbol — pretend the symbol symbol is undefined, to force linking of library modules to define it. You can use -u multiple times with different symbols to force loading of additional library modules.
  • -z keyword — -z is passed directly on to the linker along with the keyword keyword. See the section in the documentation of your linker for permitted values and their meanings.

These options specify directories to search for header files, for libraries and for parts of the compiler:

  • -I dir — add the directory dir to the list of directories to be searched for header files during preprocessing.
  • -iquote dir — add the directory dir to the header search list, but only for 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.
  • -isystem dir — add the directory dir to the header search list, and also mark it as a system directory.
  • -idirafter dir — add the directory dir to the header search list after the standard system directories, and also mark it as a system directory.

For all of these, if dir begins with = or $SYSROOT, then the = or $SYSROOT is replaced by the sysroot prefix; see --sysroot and -isysroot.

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:

  1. For the quote form of the include directive, the directory of the current file is searched first.
  2. For the quote form of the include directive, the directories specified by -iquote options are searched in left-to-right order, as they appear on the command line.
  3. Directories specified with -I options are scanned in left-to-right order.
  4. Directories specified with -isystem options are scanned in left-to-right order.
  5. Standard system directories are scanned.
  6. Directories specified with -idirafter options 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.

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- — split the include path. This option has been deprecated. Please use -iquote instead for -I directories 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-.

  • -iprefix prefix — specify prefix as the prefix for subsequent -iwithprefix options. If the prefix represents a directory, you should include the final /.
  • -iwithprefix dir — append dir to the prefix specified previously with -iprefix, and add the resulting directory to the include search path. It puts it where -idirafter would.
  • -iwithprefixbefore dir — append dir to the prefix specified previously with -iprefix, and add the resulting directory to the include search path. It puts it in the same place -I would.
  • -isysroot dir — this option is like the --sysroot option, but applies only to header files (except for Darwin targets, where it applies to both header files and libraries). See the --sysroot option for more information.
  • -imultilib dir — use dir as a subdirectory of the directory containing target-specific C++ headers.
  • -nostdinc — do not search the standard system directories for header files. Only the directories explicitly specified with -I, -iquote, -isystem, and/or -idirafter options (and the directory of the current file, if appropriate) are searched.
  • -nostdinc++ — do not search for header files in the C++-specific standard directories, but do still search the other standard directories. (This option is used when building the C++ library.)
  • --embed-dir=dir — append the dir directory to the list of searched directories for the #embed preprocessing directive or __has_embed macro. There are no default directories for #embed.

If dir begins with = or $SYSROOT, then the = or $SYSROOT is replaced by the sysroot prefix; see --sysroot and -isysroot.

  • -iplugindir=dir — set the directory to search for plugins that are passed by -fplugin=name instead of -fplugin=path/name.so. This option is not meant to be used by the user, but only passed by the driver.
  • -Ldir — add directory dir to the list of directories to be searched for -l.
  • -Bprefix — this option specifies where to find the executables, libraries, include files, and data files of the compiler itself.

okay so 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. bestie, it is literally doing a little path scavenger hunt 🔎✨

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 way that it keeps trying until it finds the vibe 💅

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. no cap, that is the whole search arc 🫶

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. iconic little bootstrap behavior, fr fr 🧚

  • -no-canonical-prefixes — do not expand any symbolic links, resolve references to /../ or /./, or make the path absolute when generating a relative prefix. 📁
  • --sysroot=dir — use dir as the logical root directory for headers and libraries. For example, if the compiler normally searches for headers in /usr/include and libraries in /usr/lib, it instead searches dir/usr/include and dir/usr/lib. slay, now the whole filesystem is wearing your chosen root as a little outfit 👑

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. heads up, the library side can ghost you there 💀

  • --no-sysroot-suffix — for some targets, a suffix is added to the root directory specified with --sysroot, depending on the other options used, so that headers may for example be found in dir/suffix/usr/include instead of dir/usr/include. This option disables the addition of such a suffix. 🦋

💅 Options for Code Generation Conventions 💅

These machine-independent options control the interface conventions used in code generation. yes bestie, this is the conventions section and it is lowkey iconic ✨

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-level can be all, named_vars, or none. all enables stack reuse for all local variables and temporaries, named_vars enables the reuse only for user defined local variables with names, and none disables stack reuse completely. The default value is all. The option is needed when the program extends the lifetime of a scoped local variable or a compiler generated temporary beyond the end point defined by the language. When a lifetime of a variable ends, and if the variable lives in memory, the optimizing compiler has the freedom to reuse its stack space with other temporaries or scoped local variables whose live range does not overlap with it. Legacy code extending local lifetime is likely to break with the stack reuse optimization. bestie, this is the “your old code might get cooked” warning 😭

For example,

int *p;
{
  int local1;

  p = &local1;
  local1 = 10;
  ....
}
{
   int local2;
   local2 = 20;
   ...
}

if (*p == 10)  // out of scope use of local1
  {

  }

Another example:

struct A
{
    A(int k) : i(k), j(k) { }
    int i;
    int j;
};

A *ap;

void foo(const A& ar)
{
   ap = &ar;
}

void bar()
{
   foo(A(10)); // temp object's lifetime ends when foo returns

   {
     A a(20);
     ....
   }
   ap->i+= 10;  // ap references out of scope temp whose space
                // is reused with a. What is the value of ap->i?
}

The lifetime of a compiler generated temporary is well defined by the C++ standard. When a lifetime of a temporary ends, and if the temporary lives in memory, the optimizing compiler has the freedom to reuse its stack space with other temporaries or scoped local variables whose live range does not overlap with it. However some of the legacy code relies on the behavior of older compilers in which temporaries' stack space is not reused, the aggressive stack reuse can lead to runtime errors. This option is used to control the temporary stack reuse optimization. 🍓

  • -ftrapv — this option generates traps for signed overflow on addition, subtraction, multiplication operations. The options -ftrapv and -fwrapv override each other, so using -ftrapv -fwrapv on the command-line results in -fwrapv being effective. Note that only active options override, so using -ftrapv -fwrapv -fno-wrapv on the command-line results in -ftrapv being effective. the options are literally fighting for main character energy ⚔️
  • -fwrapv — this option instructs the compiler to assume that signed arithmetic overflow of addition, subtraction and multiplication wraps around using twos-complement representation. This flag enables some optimizations and disables others. The options -ftrapv and -fwrapv override each other, so using -ftrapv -fwrapv on the command-line results in -fwrapv being effective. Note that only active options override, so using -ftrapv -fwrapv -fno-wrapv on the command-line results in -ftrapv being effective. 🔥
  • -fwrapv-pointer — this option 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. pointer overflow said “I wrap,” and the compiler said “noted” 🌀
  • -fstrict-overflow — this option implies -fno-wrapv -fno-wrapv-pointer and 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. exceptions are the plot twists, and frame unwind info is the receipts 🎀
  • -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. not every platform is gonna have the support, bestie — vibe check first 🙈
  • -fdelete-dead-exceptions — consider that instructions that may throw exceptions but don't otherwise contribute to the execution of the program can be optimized away. This does not affect calls to functions except those with the "pure" or "const" attributes. This option is enabled by default for the Ada and C++ compilers, as permitted by the language specifications. Optimization passes that cause dead exceptions to be removed are enabled independently at different optimization levels. dead exceptions getting deleted is kind of a slay, ngl 💅
  • -funwind-tables — similar to -fexceptions, except that it just generates any needed static data, but does not affect the generated code in any other way. You normally do not need to enable this option; instead, a language processor that needs this handling enables it on your behalf. let the language processor handle its own business 🦋
  • -fasynchronous-unwind-tables — generate unwind table in DWARF format, if supported by target machine. The table is exact at each instruction boundary, so it can be used for stack unwinding from asynchronous events (such as debugger or garbage collector). exact at every instruction boundary is giving precision queen 👑
  • -fno-gnu-unique — on systems with recent GNU assembler and C library, the C++ compiler uses the "STB_GNU_UNIQUE" binding to make sure that definitions of template static data members and static local variables in inline functions are unique even in the presence of "RTLD_LOCAL"; this is necessary to avoid problems with a library used by two different "RTLD_LOCAL" plugins depending on a definition in one of them and therefore disagreeing with the other one about the binding of the symbol. But this causes "dlclose" to be ignored for affected DSOs; if your program relies on reinitialization of a DSO via "dlclose" and "dlopen", you can use -fno-gnu-unique. so real talk: this can make "dlclose" get ignored for affected DSOs, and that is the whole reason this escape hatch exists 🫶
  • -fpcc-struct-return — okay so this one says return “short” struct and union values in memory like longer ones, instead of in registers. It’s less efficient, but the whole slay is intercallability between GCC-compiled files and files compiled with other compilers, especially the Portable C Compiler (pcc). The exact convention for returning structures in memory depends on the target configuration macros. Short structures and unions are those whose size and alignment match that of some integer type.
⚠️ Warning: code compiled with the -fpcc-struct-return switch is not binary compatible with code compiled with the -freg-struct-return switch. Use it to conform to a non-default application binary interface.
  • -freg-struct-return — returns struct and union values in registers when possible. This is more efficient for small structures than -fpcc-struct-return. If you specify neither -fpcc-struct-return nor -freg-struct-return, GCC defaults to whichever convention is standard for the target. If there is no standard convention, GCC defaults to -fpcc-struct-return, except on targets where GCC is the principal compiler. In those cases, we can choose the standard, and we chose the more efficient register return alternative.
⚠️ Warning: code compiled with the -freg-struct-return switch is not binary compatible with code compiled with the -fpcc-struct-return switch. Use it to conform to a non-default application binary interface.
  • -fshort-enums — allocate to an enum type only as many bytes as it needs for the declared range of possible values. Specifically, the enum type is equivalent to the smallest integer type that has enough room. This option has no effect for an enumeration type with a fixed underlying type.
⚠️ Warning: the -fshort-enums switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface.
  • -fshort-wchar — override the underlying type for wchar_t to be short unsigned int instead of the default for the target. This option is useful for building programs to run under WINE.
⚠️ Warning: the -fshort-wchar switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface.
  • -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 the extern keyword, 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. -fcommon places 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 — ignore the #ident directive. simple as that, bestie 💅
  • -finhibit-size-directive — don’t output a .size assembler directive, or anything else that would cause trouble if the function is split in the middle, and the two halves are placed at locations far apart in memory. This option is used when compiling crtstuff.c; you should not need to use it for anything else.
  • -fverbose-asm — put extra commentary information in the generated assembly code to make it more readable. This option is generally only of use to those who actually need to read the generated assembly code (perhaps while debugging the compiler itself). -fno-verbose-asm, the default, causes the extra information to be omitted and is useful when comparing two assembler files.

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,"",@progbits

The 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. This switch is only implemented on some targets and the exact format of the recording is target and binary file format dependent, but it usually takes the form of a section containing ASCII text. This switch is related to the -fverbose-asm switch, but that switch only records information in the assembler output file as comments, so it never reaches the object file. See also -grecord-gcc-switches for another way of storing compiler options into the object file. ✨
  • -fpic — generate position-independent code (PIC) suitable for use in a shared library, if supported for the target machine. Such code accesses all constant addresses through a global offset table (GOT). The dynamic loader resolves the GOT entries when the program starts (the dynamic loader is not part of GCC; it is part of the operating system). If the GOT size for the linked executable exceeds a machine-specific maximum size, you get an error message from the linker indicating that -fpic does not work; in that case, recompile with -fPIC instead. (These maximums are 8k on the SPARC, 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. When this flag is set, the macros __pic__ and __PIC__ are defined to 1. 🦋
  • -fPIC — if supported for the target machine, emit position-independent code, suitable for dynamic linking and avoiding any limit on the size of the global offset table. This option makes a difference on AArch64, m68k, PowerPC and SPARC. Position-independent code requires special support, and therefore works only on certain machines. When this flag is set, the macros __pic__ and __PIC__ are defined to 2. 💜
  • -fpie, -fPIE — these options are similar to -fpic and -fPIC, but the generated position-independent code can be only linked into executables. Usually these options are used to compile code that will be linked using the -pie GCC option. -fpie and -fPIE both define the macros __pie__ and __PIE__. The macros have the value 1 for -fpie and 2 for -fPIE. 👑
  • -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 leads to 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 — do not use jump tables for switch statements even where it would be more efficient than other code generation strategies. This option is of use in conjunction with -fpic or -fPIC for building code that forms part of a dynamic linker and cannot reference the address of a jump table. On some targets, jump tables do not require a GOT and this option is not needed. 💀
  • -fno-bit-tests — so sometimes GCC looks at your switch statement and goes "omg I could totally use bit tests for this, it's literally faster" and this flag is you going "no bestie, not today 💅". It says: do not use bit tests for switch statements even where it would be more efficient than other code generation strategies. Efficiency who? Never heard of her. We said no.
  • -ffixed-reg — treat the register named reg as a fixed register, meaning generated code should never refer to it (except maybe as a stack pointer, frame pointer, or in some other fixed role). Like, that register is not in the group chat anymore, she's reserved 🚫

reg must be the name of a register, and the accepted names are machine-specific — they're defined in the REGISTER_NAMES macro in the machine description macro file. iykyk.

Heads up: this flag does not have a negative form, because it specifies a three-way choice. There's no -fno-fixed-reg to un-fix your vibe.
  • -fcall-used-reg — treat the register named reg as an allocable register that gets clobbered by function calls. It may be allocated for temporaries or variables that don't live across a call. Functions compiled this way do not save and restore the register reg — she's on her own out there 😭

It is an error to use this flag with the frame pointer or stack pointer. And using this flag for other registers that have fixed pervasive roles in the machine's execution model produces disastrous results. Not "oops" disastrous, like, disastrous disastrous 💀

Same deal: no 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. So thoughtful of them 🫶

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.

And a different sort of disaster results from using this flag for a register in which function values may be returned. Yeah. Two flavours of disaster, both iconic in the worst way.

Again: no negative form, three-way choice, you know the drill.
  • -fpack-struct[=n] — with no value given, pack all structure members together without holes. When you do give a value (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. Cozy. Very no-gaps-allowed energy.
⚠️ Warning: the -fpack-struct switch causes GCC to generate code that is not binary compatible with code generated without that switch. On top of that, it makes the code suboptimal. Use it to conform to a non-default application binary interface. That's it. That's the only reason. Don't do it for fun 🫠
  • -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. Old-school assembly out here needing a glow-up ✨
⚠️ 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. And heads up, not all targets provide complete support for this switch — check before you commit.
  • -ftls-model=model — alters the thread-local storage model to be used. The model argument should be one of global-dynamic, local-dynamic, initial-exec or local-exec. Note that the choice is subject to optimization: the compiler may use a more efficient model for symbols not visible outside of the translation unit, or if -fpic is not given on the command line. She'll upgrade you if she can, bestie 💅

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. Literally nothing. She's not even gonna try 🧚

So what even is a trampoline? A trampoline is a small piece of code 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. Which means it requires the stack to be made executable in order for the program to work properly. Yeah. Executable stack. Breathe.

-fno-trampolines is enabled by default on a language-by-language basis, so the compiler can 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 -ftrampolines and code compiled with -fno-trampolines are 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. Like, the whole program. Every file. No halfsies.

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 the stack. But certain platforms (such as the Apple M1) do not permit an executable stack 🙅‍♀️ Compiling with -ftrampoline-impl=heap generates calls to __gcc_nested_func_ptr_created and __gcc_nested_func_ptr_deleted in 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 you setjmp, instantiate nested functions, and then longjmp back to a state prior to having allocated those nested functions. The vibes are, uh, undefined-ish. Be careful out there 😭
  • -fvisibility=[default|internal|hidden|protected] — sets the default ELF image symbol visibility to the specified option, and all symbols get 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. Like, strongly. She's begging 🥺

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. Everyone's invited to the party, I guess 🎉

A good explanation of the benefits of 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/>). But here's the superior solution this option makes possible: instead of marking things hidden when the default is public, make the default hidden and mark things public. That's 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. Slay 🌈

For those adding visibility support to existing code, you may find #pragma GCC visibility of use. It 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's not the default. I.e., declarations only for use within the local DSO should always be marked explicitly as hidden, so as to avoid PLT indirection overheads — and 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. No exceptions, bestie, that's the spec.

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. Give them their safe space 🫶

extern declarations are not affected by -fvisibility, so a lot of code can be recompiled with -fvisibility=hidden with no modifications. However, this means that calls to extern functions with no explicit visibility use the PLT, so it is more effective to use __attribute ((visibility)) and/or #pragma GCC visibility to tell the compiler which extern declarations should be treated as hidden.

Note that -fvisibility does affect C++ vague linkage entities. This means that, for instance, an exception class that is thrown between DSOs must be explicitly marked with default visibility so that the type_info nodes are unified between the DSOs. Unify those nodes, queen 👑

An overview of these techniques, their benefits and how to use them is at <https://gcc.gnu.org/wiki/Visibility>.

  • -fstrict-volatile-bitfields — use this option if accesses to volatile bit-fields (or other structure fields, although the compiler usually honors those types anyway) should use a single access of the width of the field's type, aligned to a natural alignment if possible. For example, targets with memory-mapped peripheral registers might require all such accesses to be 16 bits wide; with this flag you can declare all peripheral bit-fields as unsigned short (assuming short is 16 bits on these targets) to force GCC to use 16-bit accesses instead of, perhaps, a more efficient 32-bit access. Fancy? No. Correct? Yes 💅

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. Rude. Touching things that aren't hers 😤

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. She's doing her best, okay 🥺

✨ Quick heads-up, bestie: 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. The default value of this option is determined by the application binary interface for the target processor. 💅
  • -fsync-libcalls — 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 is enabled, so the only useful form is -fno-sync-libcalls. This option is used in the implementation of the libatomic runtime library.
  • -fzero-init-padding-bits=value — guarantees 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 a 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 it guarantees padding bits outside of the initialized member are zero initialized). This was the GCC behavior before GCC 15, and in the above example it guarantees zero initialization of the last sizeof (unsigned long long) - 1 bytes in the union.

-fzero-init-padding-bits=all additionally guarantees 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 all about command-line options that are primarily of interest to GCC developers — including 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; options that print statistics such as memory use and execution time; and options that print information about GCC’s configuration, such as where it searches for libraries. You should rarely need any of these for ordinary compilation and linking tasks, fr fr.

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 the 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
  • -fdump-rtl-pass
  • -fdump-rtl-pass=filename — says to make debugging dumps during compilation at times specified by letters. This is used for debugging the RTL-based passes of the compiler.

Some -dletters switches have different meaning when -E is used for preprocessing.

Debug dumps can be enabled with a -fdump-rtl switch or some -d option letters. Here are the possible letters for use in pass and letters, and their meanings:

  • -fdump-rtl-alignments — dump after branch alignments have been computed.
  • -fdump-rtl-asmcons — dump after fixing rtl statements that have unsatisfied in/out constraints.
  • -fdump-rtl-auto_inc_dec — dump after auto-inc-dec discovery. This pass is only run on architectures that have auto inc or auto dec instructions.
  • -fdump-rtl-barriers — dump after cleaning up the barrier instructions.
  • -fdump-rtl-bbpart — dump after partitioning hot and cold basic blocks.
  • -fdump-rtl-bbro — dump after block reordering.
  • -fdump-rtl-btl1
  • -fdump-rtl-btl2 — -fdump-rtl-btl1 and -fdump-rtl-btl2 enable dumping after the two branch target load optimization passes.
  • -fdump-rtl-bypass — dump after jump bypassing and control flow optimizations.
  • -fdump-rtl-combine — dump after the RTL instruction combination pass.
  • -fdump-rtl-compgotos — dump after duplicating the computed gotos.
  • -fdump-rtl-ce1
  • -fdump-rtl-ce2
  • -fdump-rtl-ce3 — -fdump-rtl-ce1, -fdump-rtl-ce2, and -fdump-rtl-ce3 enable dumping after the three if conversion passes.
  • -fdump-rtl-cprop_hardreg — dump after hard register copy propagation.
  • -fdump-rtl-csa — dump after combining stack adjustments.
  • -fdump-rtl-cse1
  • -fdump-rtl-cse2 — -fdump-rtl-cse1 and -fdump-rtl-cse2 enable dumping after the two common subexpression elimination passes.
  • -fdump-rtl-dce — dump after the standalone dead code elimination passes.
  • -fdump-rtl-dbr — dump after delayed branch scheduling.
  • -fdump-rtl-dce1
  • -fdump-rtl-dce2 — -fdump-rtl-dce1 and -fdump-rtl-dce2 enable dumping after the two dead store elimination passes.
  • -fdump-rtl-eh — dump after finalization of EH handling code.
  • -fdump-rtl-eh_ranges — dump after conversion of EH handling range regions.
  • -fdump-rtl-expand — dump after RTL generation.
  • -fdump-rtl-fwprop1
  • -fdump-rtl-fwprop2 — -fdump-rtl-fwprop1 and -fdump-rtl-fwprop2 enable dumping after the two forward propagation passes.
  • -fdump-rtl-gcse1
  • -fdump-rtl-gcse2 — -fdump-rtl-gcse1 and -fdump-rtl-gcse2 enable dumping after global common subexpression elimination.
  • -fdump-rtl-init-regs — dump after the initialization of the registers.
  • -fdump-rtl-initvals — dump after the computation of the initial value sets.
  • -fdump-rtl-into_cfglayout — dump after converting to cfglayout mode.
  • -fdump-rtl-ira — dump after iterated register allocation.
  • -fdump-rtl-jump — dump after the second jump optimization.
  • -fdump-rtl-loop2 — -fdump-rtl-loop2 enables dumping after the rtl loop optimization passes.
  • -fdump-rtl-mach — dump after performing the machine dependent reorganization pass, if that pass exists.
  • -fdump-rtl-mode_sw — dump after removing redundant mode switches.
  • -fdump-rtl-rnreg — dump after register renumbering.
  • -fdump-rtl-outof_cfglayout — dump after converting from cfglayout mode.
  • -fdump-rtl-peephole2 — dump after the peephole pass.
  • -fdump-rtl-postreload — dump after post-reload optimizations.
  • -fdump-rtl-pro_and_epilogue — dump after generating the function prologues and epilogues.
  • -fdump-rtl-sched1
  • -fdump-rtl-sched2 — -fdump-rtl-sched1 and -fdump-rtl-sched2 enable dumping after the basic block scheduling passes.
  • -fdump-rtl-ree — dump after sign/zero extension elimination.
  • -fdump-rtl-seqabstr — dump after common sequence discovery.
  • -fdump-rtl-shorten — dump after shortening branches.
  • -fdump-rtl-split1
  • -fdump-rtl-split2
  • -fdump-rtl-split3
  • -fdump-rtl-split4
  • -fdump-rtl-split5 — these options enable dumping after five rounds of instruction splitting.
  • -fdump-rtl-sms — dump after modulo scheduling. This pass is only run on some architectures.
  • -fdump-rtl-stack — dump after conversion from GCC’s “flat register file” registers to the x87’s stack-like registers. This pass is only run on x86 variants.
  • -fdump-rtl-subreg1
  • -fdump-rtl-subreg2 — -fdump-rtl-subreg1 and -fdump-rtl-subreg2 enable dumping after the two subreg expansion passes.
  • -fdump-rtl-vartrack — dump after variable tracking.
  • -fdump-rtl-vregs — dump after converting virtual registers to hard registers.
  • -fdump-rtl-web — dump after live range splitting.
  • -fdump-rtl-regclass
  • -fdump-rtl-subregs_of_mode_init
  • -fdump-rtl-subregs_of_mode_finish
  • -fdump-rtl-dfinit
  • -fdump-rtl-dfinish — these dumps are defined but always produce empty files.
  • -da
  • -fdump-rtl-all — produce all the dumps listed above.
  • -dA — slaps annotations on the assembler output with all kinds of miscellaneous debugging tea ☕. unhinged little notes everywhere, but iconic.
  • -dD — dumps every macro definition at the end of preprocessing, on top of the normal output. like, all of them. no holding back 💅
  • -dH — produces a core dump whenever an error occurs. yes bestie, an actual core dump. dramatic? maybe. useful for debugging? absolutely 🫶
  • -dp — annotates the assembler output with a comment showing which pattern and alternative is used. the length and cost of each instruction also get printed. little receipts for every instruction, we love to see it ✨
  • -dP — dumps the RTL into the assembler output as a comment before each instruction. also flips on the -dp annotation, so you get both. package deal 🎀
  • -dx — just generates RTL for a function instead of actually compiling it. usually hangs out with -fdump-rtl-expand, they're besties.

💖 -fdump-debug

Dumps the debugging information that gets generated during the debug generation phase. peak lore, honestly.

🌸 -fdump-earlydebug

Same vibe, but for the early debug generation phase. it's giving "debugging but make it early" ✨

🦋 -fdump-noaddr

When it's doing debugging dumps, this suppresses address output. why do we care? because it makes it way more feasible to run diff on debugging dumps from compiler invocations using different compiler binaries and/or different text / bss / data / heap / stack / dso start locations. imagine diffing dumps that shift around for no reason — not on our watch 💅

🎀 -freport-bug

If an internal compiler error (ICE) happens, this collects and dumps debug information into a temporary file. so when the compiler eats it, you get receipts 🔥

⭐ -fdump-unnumbered

When doing debugging dumps, suppresses instruction numbers and address output. makes diffing dumps way more feasible across compiler invocations with different options — especially with and without -g. honestly a lifesaver, no cap 😭

When doing debugging dumps (peep the -d option above), this suppresses instruction numbers for the links to the previous and next instructions in a sequence. clean little links, we stan 🫶

👑 -fdump-ipa-switch

👑 -fdump-ipa-switch-options

These control the dumping at various stages of inter-procedural analysis language tree into a file. the file name? generated by appending a switch-specific suffix to the source file name — and the file gets created in the same directory as the output file. so organized of it 💅

The following dumps are possible:

  • all — enables all the inter-procedural analysis dumps. go big or go home ✨
  • cgraph — dumps info about call-graph optimization, unused function removal, and inlining decisions. the whole social network of your functions.
  • inline — dump after function inlining. it's giving "who got absorbed into whom" 💀
  • strubm — dump after selecting "strub" modes, and recording those selections as function attributes.
  • strub — dumps "strub" transformations: interface changes, function wrapping, and insertion of builtin calls for stack scrubbing and watermarking. so much going on, obsessed.

Additionally, the options -optimized, -missed, -note, and -all can be provided, with the same meaning as for -fopt-info — and they default to -optimized.

For example, -fdump-ipa-inline-optimized-missed will emit information on callsites that were inlined, along with callsites that were not. equal opportunity reporting 🫶

By default, the dump contains messages about successful optimizations (equivalent to -optimized) together with low-level details about the analysis.

🍓 -fdump-ipa-clones

Creates a dump file containing info about the creation of call graph node clones and the removals of call graph nodes during inter-procedural optimizations and transformations. its main intended use? tools that create live-patches can figure out 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 the suffix ipa-clones to the source file name, and the file is created in the same directory as the output file. each entry in the file sits on its own separate line, with semicolon-separated fields. so结构化 of it 💅

In the case of call graph clone creation, the individual fields are:

  1. String "Callgraph clone".
  2. Name of the function being cloned as it is presented to the assembler.
  3. 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 it's likely to be different in the two phases. rude, but okay.
  4. The file name of the source file where the function is defined.
  5. The line on which the function definition is located.
  6. The column where the function definition is located.
  7. Name of the new function clone as it is presented to the assembler.
  8. A number that uniquely represents the new function clone in the call graph. again: unique only within a compilation unit or within whole-program analysis, and likely different in the two phases 😭
  9. The file name of the source file where the source code location of the new clone points to.
  10. The line to which the source code location of the new clone points to.
  11. The column to which the source code location of the new clone points to.
  12. A string that determines the reason for cloning.

In the case of call graph clone removal, the individual fields are:

  1. String "Callgraph removal".
  2. Name of the function being removed as it would be presented to the assembler.
  3. A number that uniquely represents the function being cloned in the call graph. (same caveat bestie — unique only within a compilation unit or within whole-program analysis, likely different in the two phases.)
  4. The file name of the source file where the function is defined.
  5. The line on which the function definition is located.
  6. The column where the function definition is located.

🧚 -fdump-lang

Dumps language-specific information. the file name is made by appending .lang to the source file name. cute suffix, honestly 🎀

🧚‍♀️ -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 exactly as described in the -fdump-tree option. the following switch values are accepted:

  • all — enable all language-specific dumps. all of them. the whole menu 🍓
  • class — dump class hierarchy information. virtual table information gets emitted unless 'slim' is specified. this option is applicable to C++ only.
  • module — dump module information. the options lineno (locations), graph (reachability), blocks (clusters), uid (serialization), alias (mergeable), asmname (Elrond), eh (mapper) & vops (macros) may provide additional information. this option is applicable to C++ only. (yes, Elrond. we don't make the rules 💀)
  • raw — dump the raw internal tree data. this option is applicable to C++ only.

💜 -fdump-passes

Prints the list of optimization passes that are turned on and off by the current command-line options — straight to stderr. a little behind-the-scenes tour of what your flags actually did 👑

🌟 -fdump-statistics-option

Enables and controls dumping of pass statistics into a separate file. the file name is generated by appending a suffix ending in .statistics to the source file name, and the file is created in the same directory as the output file. if the -option form is used, -stats causes counters to be summed over the whole compilation unit, while -details dumps every event as the passes generate them. the default with no option is to sum counters for each function compiled. numbers, but make it organized ✨

🌈 -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 -options form is used, options is a list of - separated options which control the details of the dump. not all options are applicable to all dumps — the ones that aren't meaningful just get ignored, no drama 💅

The following options are available:

  • address — print the address of each node. usually this isn't meaningful since it changes according to the environment and source file. its primary use is tying up a dump file with a debug environment. basically a little anchor back to reality ⚓
  • 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 working backward from mangled names in the assembly file. detective work, slay 🔍
  • slim — when dumping front-end intermediate representations, this inhibits dumping of members of a scope or body of a function merely because that scope has been reached. only dump such items when they're directly reachable by some other path. also: when dumping pretty-printed trees, it inhibits dumping the bodies of control structures. and when dumping RTL, it prints the RTL in slim (condensed) form instead of the default LISP-like representation. three-for-one, iconic ✨
  • raw — print a raw representation of the tree. by default, trees are pretty-printed into a C-like representation.
  • details — enable more detailed dumps (not honored by every dump option). also includes information from the optimization passes.
  • stats — enable dumping various statistics about the pass (not honored by every dump option).
  • blocks — enable showing basic block boundaries (disabled in raw dumps).
  • graph — for each of the other indicated dump files (-fdump-rtl-pass), dump a representation of the control flow graph suitable for viewing with GraphViz to file.passid.pass.dot. each function in the file is pretty-printed as a subgraph, so GraphViz can render them all in a single plot 🎨. this option currently only works for RTL dumps, and the RTL is always dumped in slim form.
  • vops — enable showing virtual operands for every statement.
  • lineno — enable showing line numbers for statements.
  • uid — enable showing the unique ID ("DECL_UID") for each variable.
  • verbose — enable showing the tree dump for each statement.
  • eh — enable showing the EH region number holding each statement.
  • scev — enable showing scalar evolution analysis details.
  • optimized — enable showing optimization information (only available in certain passes).
  • missed — enable showing missed optimization information (only available in certain passes). not me getting rejected by the optimizer 😭
  • note — enable other detailed optimization information (only available in certain passes).
  • all — turns on, like, every option 💅 except raw, slim, verbose and lineno. those four are the ones that stay in their lane.
  • optall — turns on all the optimization options, i.e. optimized, missed, and note. the optimization girlies only ✨

so to figure out which tree dumps even exist, or to hunt down the dump for a pass you actually care about, do this lil three-step ritual 🎀

  1. Invoke GCC with -fdump-passes and then go digging through the stderr output for a code that matches the pass you're curious about. for example, the codes tree-evrp, tree-vrp1, and tree-vrp2 correspond to the three Value Range Propagation passes. that number hanging off the end? it's there to distinguish distinct invocations of the same pass. iykyk 🦋
  2. To actually make the dump file appear, append the pass code onto the -fdump- option prefix and invoke GCC with it. like, to enable the dump from the Early Value Range Propagation pass, invoke GCC with -fdump-tree-evrp. optionally you can specify the name of the dump file too — if you don't, GCC creates it as described below 👇
  3. Find the pass dump in a file whose name is made 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 t for tree passes (and the letter r for RTL passes), and finally the pass code. for example, the Early VRP pass dump might be chilling in a file named myfile.c.038t.evrp in the current working directory. ngl, heads up: those numeric codes are not stable and may change from one version of GCC to another 🙈 so don't go hardcoding them.

🫶 -fopt-info / -fopt-info-options / -fopt-info-options=filename

Controls optimization dumps from all the various optimization passes. if you use the -options form, then options is a list of - separated option keywords that select the dump details and which optimizations get included 💖

These options split into three groups, and honestly it's giving very organized:

  1. options describing what kinds of messages should be emitted,
  2. options describing the verbosity of the dump, and
  3. options describing which optimizations should be included.

You can freely mix options from each group because they're non-overlapping — no drama. but in case of any conflicts, the later options override the earlier options on the command line. last one wins, periodt 👑

🌸 the "what messages get emitted" girlies

  • optimized — print info when an optimization is successfully applied. it's up to each pass to decide which information is relevant, so like, don't blame me. 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, like certain transformations, plus more detailed messages about decisions etc.
  • all — print detailed optimization information. this includes optimized, missed, and note. the whole set, bestie 💅

🦋 the "how verbose is this dump" one

  • 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. compiler-dev lore, not for us mere mortals ✨

🎀 the "which optimizations get included" keywords

one or more of these 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: dump messages about successful optimizations from all the passes, while omitting messages that are treated as "internals". the chill default ✨

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 🙈

⚠️ okay so real talk: the output filename is overwritten in case of multiple translation units. if you want a combined output from multiple translation units, use stderr instead. don't say i didn't warn you 💀

In the following example, the optimization info is output to stderr:

gcc -O3 -fopt-info

This example:

gcc -O3 -fopt-info-missed=missed.all

outputs missed optimization report from all the passes into missed.all, and this one:

gcc -O2 -ftree-vectorize -fopt-info-vec-missed

prints information about missed optimization opportunities from vectorization passes on stderr. note that -fopt-info-vec-missed is equivalent to -fopt-info-missed-vec. the order of the optimization group names and message types listed after -fopt-info does not matter — it's giving very go-with-the-flow 🌈

As another example,

gcc -O3 -fopt-info-inline-optimized-missed=inline.txt

outputs information about missed optimizations as well as optimized locations from all the inlining passes into inline.txt 🎀

Finally, consider:

gcc -fopt-info-vec-missed=vec.miss -fopt-info-loop-optimized=loop.opt

Here the two output filenames vec.miss and loop.opt are in conflict since only one output file is allowed. in this case, only the first option takes effect and the subsequent options are ignored. thus only vec.miss is produced, which contains dumps from the vectorizer about missed opportunities. sad for loop.opt, but rules are rules 💅

⭐ -fsave-optimization-record

Write a SRCFILE.opt-record.json.gz file detailing what optimizations were performed, for those optimizations that support -fopt-info. yes, it's gzipped JSON, she's fancy 🧚

⚠️ This option is experimental and the format of the data within the compressed JSON file is subject to change. no cap, don't build your whole toolchain around it.

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. very meta, very inception 🌀

💜 -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 output basic block probabilities, detailed ready list information and unit/insn info. For n greater than two, it includes RTL at abort point, control-flow and regions info. And for n over four, -fsched-verbose also includes dependence info. so like, the bigger the n, the more it will not shut up 💀

👑 -fenable-kind-pass / -fdisable-kind-pass=range-list

This is a whole set of options 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 — this is internals-only territory, bestie 🚧

-fdisable-ipa-pass

Disable IPA pass pass. pass is the pass name. if the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from 1. 🔢

-fdisable-rtl-pass / -fdisable-rtl-pass=range-list

Disable RTL pass pass. pass is the pass name. if the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from 1. range-list is a comma-separated list of function ranges or assembler names. each range is a number pair separated by a colon. the range is inclusive in both ends. if the range is trivial, the number pair can be simplified as a single number. if the function's call graph node's uid falls within one of the specified ranges, the pass is disabled for that function. the uid is shown in the function header of a dump file, and the pass names can be dumped by using option -fdump-passes. iykyk 🦋

-fdisable-tree-pass / -fdisable-tree-pass=range-list

Disable tree pass pass. See -fdisable-rtl for the description of option arguments 👀

-fenable-ipa-pass

Enable IPA pass pass. pass is the pass name. if the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from 1. ✨

-fenable-rtl-pass / -fenable-rtl-pass=range-list

Enable RTL pass pass. See -fdisable-rtl for option argument description and examples.

-fenable-tree-pass / -fenable-tree-pass=range-list

Enable tree pass pass. See -fdisable-rtl for the description of option arguments.

Here are some examples showing uses of these options.

More of the same energy — the rest of the examples, still slaying exactly as written:

# 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 — turns on internal consistency checking ✨. The default? Honestly depends on how your compiler was configured, ngl. Peep this though: -fchecking=2 turns on further internal consistency checking, and that one might actually affect code generation 👀.
  • -frandom-seed=string — gives GCC a seed to use instead of random numbers when it generates certain symbol names that have to be different in every compiled file. It's also what places those unique stamps in coverage data files and in the object files that produce them. Basically: use -frandom-seed and you can produce reproducibly identical object files, periodt 💅.

The string can be a number (decimal, octal or hex) or just an arbitrary string — in which case it gets converted to a number by computing CRC32. Wild but ok.

The string should be different for every file you compile. Fr fr, don't reuse it.
  • -save-temps — stores the usual "temporary" intermediate files permanently 🎀, naming them as auxiliary output files, as described under -dumpbase and -dumpdir.

Lowkey iconic detail: when you use it together with the -x command-line option, -save-temps is sensible enough to avoid overwriting an input source file that has the same extension as an intermediate file. If you still want that corresponding intermediate file, just rename the source file before using -save-temps. Easy.

  • -save-temps=cwd — same energy as -save-temps -dumpdir ./.
  • -save-temps=obj — equivalent to -save-temps -dumpdir outdir/, where outdir/ is the directory of the output file you specified after the -o option, including any directory separators. And if you didn't use -o? Then -save-temps=obj just behaves like -save-temps=cwd. She adapts 💁
  • -time[=file] — reports the CPU time taken by each subprocess in the compilation sequence. For C source files, that's the compiler proper and the assembler (plus the linker if linking is done).

Without an output file, it looks like this:

        # cc1 0.12 0.01
        # as 0.00 0.01

First number on each line = the "user time", i.e. time spent executing the program itself. Second number = "system time", i.e. time spent executing operating system routines on behalf of the program. Both in seconds ⏱️.

With an output file specified, the output is appended to the named file, and it looks like this:

        0.12 0.01 cc1 <options>
        0.00 0.01 as <options>

The "user time" and "system time" get moved before the program name, and the options passed to the program are displayed — so later you can actually tell what file was being compiled and with which options. Main character detective work 🔍

  • -fdump-final-insns[=file] — dumps the final internal representation (RTL) to file. If you leave the optional argument out (or if file is .), the dump file's name is determined by appending .gkd to the dump base name — see -dumpbase.
  • -fcompare-debug[=opts] — if no error occurs during compilation, GCC runs the compiler a second time, adding opts and -fcompare-debug-second to the arguments passed to that second compilation. It dumps the final internal representation in both compilations and prints an error if they differ. Very "proofread your own essay" of it 🫶

If you omit the equal sign, 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 that string is used for opts; otherwise the default -gtoggle is used.

-fcompare-debug= — with the equal sign but without opts — is equivalent to -fno-compare-debug, which disables the dumping of the final representation and the second compilation, preventing even GCC_COMPARE_DEBUG from taking effect. So it fully cancels out 👑

Want to verify full coverage while -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). And if you'd like just a warning instead of that whole situation, setting GCC_COMPARE_DEBUG to -w%n-fcompare-debug not overridden will do the trick.

  • -fcompare-debug-second — implicitly passed to the compiler for that 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 get renamed so they contain the .gk additional extension during the second compilation, so we don't overwrite the ones from the first 🌸

And 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. That's it. That's the whole gig 💀

  • -gtoggle — turns off generation of debug info if leaving out this option would generate it, or turns 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 only does so once, no matter how many times you give it. Mainly intended to be used with -fcompare-debug.
  • -fvar-tracking-assignments-toggle — toggles -fvar-tracking-assignments, in the exact same way that -gtoggle toggles -g. Symmetry, we love to see it ✨
  • -Q — when used on the command line prior to --help=, -Q acts as a modifier to the help output.

Otherwise? It makes the compiler print out each function name as it is compiled, and print some statistics about each pass when it finishes. Chatty king 🗣️

  • -ftime-report — makes the compiler print some statistics to stderr about the time consumed by each pass when it finishes.

Quick heads up though: 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 the SARIF output. And the precise format of that JSON data is subject to change — the values may not exactly match the ones emitted to stderr, because they get written out at a slightly different place within the compiler. No cap, that's just how it is 🙈

  • -ftime-report-details — records the time consumed by infrastructure parts separately for each pass. Detail-oriented queen 💅
  • -fira-verbose=n — controls the verbosity of the dump file for the integrated register allocator. Default value is 5. And if the value n is greater than or equal to 10, the dump output is sent to stderr using the same format as n minus 10. Math but make it compiler.
  • -flto-report — prints a report with internal details on the workings of the link-time optimizer. Heads up: 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 prints for the WPA phase of link-time optimization. Scoped. Focused. 🎯
  • -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 one 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.

Just like "TFLAGS", this lets the target libraries be built for portable baseline environments while the compiler defaults to more demanding ones. Why is that useful? Because users can easily override the defaults the compiler is configured to use when building their own programs — if those defaults aren't ideal for their target environment — whereas rebuilding the runtime libraries is usually not as easy or desirable 🫠

Unlike "TFLAGS", the use of specs enables different flags to be selected for different multilibs. The way to accomplish that is to build with make TFLAGS=-fmultiflags, after configuring --with-specs=%{fmultiflags:...}.

Also: this option is discarded by the driver once it's done processing the driver self spec. Bye bestie 👋

Bonus use: it's also useful to check that "TFLAGS" are being used to build all target libraries — configure a non-bootstrap compiler --with-specs='%{!fmultiflags:%emissing TFLAGS}' and build the compiler and target libraries.

  • -fprofile-report — makes the compiler print some statistics about consistency of the (estimated) profile and the effect of individual passes. Vibe check for your profile 📊
  • -fstack-usage — makes the compiler output stack usage information for the program, on a per-function basis. The filename for the dump is made by appending .su to the auxname. auxname is generated from the name of the output file, if it was explicitly specified and it is 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 stack adjustments are otherwise made in the function. And the second field? That's exactly this fixed number of bytes 🍓

okay so the qualifier dynamic means the function is out here doing stack stuff on the fly: besides the static allocation described above, stack adjustments happen in the body of the function — like pushing/popping arguments around function calls. If the qualifier bounded is also present, the amount of those adjustments is bounded at compile time, and the second field is an upper bound on the total amount of stack used by the function. If bounded isn’t present, the amount of those adjustments is not bounded at compile time, and the second field only represents the bounded part.

  • -fstats — emit statistics about front-end processing at the end of the compilation. This option is supported only by the C++ front end, and the info is generally only useful to the G++ development team. 💅
  • -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 of name:lower_bound1-upper_bound1 [:lower_bound2-upper_bound2...] tuples, which sets the name of the counter and 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 second, third, fourth, tenth and eleventh invocation. For dbg_cnt(tail_call) true is returned for first 10 invocations. 🧚
  • -print-file-name=library — print the full absolute name of the library file library that would be used when linking — and don’t do anything else. With this option, GCC does not compile or link anything; it just prints the file name. ✨
  • -print-multi-directory — print the directory name corresponding to the multilib selected by any other switches present in the command line. This directory is supposed to exist in GCC_EXEC_PREFIX. 💖
  • -print-multi-lib — print the mapping from multilib directory names to compiler switches that enable them. The directory name is separated from the switches by ;, and each switch starts with an @ instead of the -, without spaces between multiple switches. This is supposed to ease shell processing. 🐚
  • -print-multi-os-directory — print the path to OS libraries for the selected multilib, relative to some lib subdirectory. If OS libraries are present in the lib subdirectory and no multilibs are used, this is usually just ., if OS libraries are present in libsuffix sibling directories this prints e.g. ../lib64, ../lib or ../lib32, or if OS libraries are present in lib/subdir subdirectories it prints e.g. amd64, sparcv9 or ev6. 🦋
  • -print-multiarch — print the path to OS libraries for the selected multiarch, relative to some lib subdirectory.
  • -print-prog-name=program — like -print-file-name, but searches for a program such as cpp. 🔎
  • -print-libgcc-file-name — same as -print-file-name=libgcc.a.

This is useful when you use -nostdlib or -nodefaultlibs but you do want to link with libgcc.a. You can do:

gcc -nostdlib <files>... `gcc -print-libgcc-file-name`
  • -print-search-dirs — print the name of the configured installation directory and a list of program and library directories gcc searches — and don’t do anything else. 🔍

This is useful when gcc prints the error message installation problem, cannot exec cpp0: No such file or directory. To resolve this you either need to put cpp0 and the other compiler components where gcc expects to find them, or you can set the environment variable GCC_EXEC_PREFIX to the directory where you installed them. Don’t forget the trailing /. 🫶

  • -print-sysroot — print the target sysroot directory that is used during compilation. This is the target sysroot specified either at configure time or using the --sysroot option, possibly with an extra suffix that depends on compilation options. If no target sysroot is specified, the option prints nothing. 🌸
  • -print-sysroot-headers-suffix — print the suffix added to the target sysroot when searching for headers, or give an error if the compiler is not configured with such a suffix — and don’t do anything else.
  • -dumpmachine — print the compiler’s target machine (for example, i686-pc-linux-gnu) — and don’t do anything else. 👑
  • -dumpversion — print the compiler version (for example, 3.0, 6.3.0 or 7) — and don’t do anything else. This is the compiler version used in filesystem paths and specs. Depending on how the compiler has been 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 is used when GCC itself is being built.) 🛠️

🌍 Machine-Dependent Options 🌍

Each target machine supported by GCC can have its own options — for example, to allow you to compile for a particular processor variant or ABI, or to control optimizations specific to that machine. 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 are ilp32 for SysV-like data model where int, long int and pointers are 32 bits, and lp64 for SysV-like data model where int is 32 bits, but long int and 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. 😭

🚨 The ilp32 model is deprecated. 🚨

  • -mbig-endian — generate big-endian code. This is the default when GCC is configured for an aarch64_be-*-* target. 🐘
  • -mgeneral-regs-only — generate code which uses only the general-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 little-endian code. This is the default when GCC is configured for an aarch64-*-* but not an aarch64_be-*-* target. 🐭
  • -mcmodel=tiny — generate code for the tiny code model. The program and its statically defined symbols must be within 1MB of each other. Programs can be statically or dynamically linked.
  • -mcmodel=small — generate code for the small code model. The program and its statically defined symbols must be within 4GB of each other. Programs can be statically or dynamically linked. This is the default code model. ✨
  • -mcmodel=large — generate code for the large code model. This makes no assumptions about addresses and sizes of sections. Programs can be statically linked only. The -mcmodel=large option is incompatible with -mabi=ilp32, -fpic and -fPIC. 💀
  • -mtp=name — specify the system register to use as a thread pointer. The valid values are tpidr_el0, tpidrro_el0, tpidr_el1, tpidr_el2, tpidr_el3. For backwards compatibility the aliases el0, el1, el2, el3 are also accepted. The default setting is tpidr_el0. It is recommended to compile all code intended to interoperate with the same value of this option to avoid accessing a different thread pointer from the wrong exception level. 🧵
  • -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.
  • -momit-leaf-frame-pointer, -mno-omit-leaf-frame-pointer — omit or keep the frame pointer in leaf functions. The former behavior is the default. 🍃
  • -mstack-protector-guard=guard, -mstack-protector-guard-reg=reg, -mstack-protector-guard-offset=offset — generate stack protection code using canary at guard. Supported locations are global for a global canary or sysreg for a canary in an appropriate system register.

With the latter 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. There is no default register or offset as this is entirely for use within the Linux kernel. 🐧

  • -mtls-dialect=desc — use TLS descriptors as the thread-local storage mechanism for dynamic accesses of TLS variables. This is the default. 🧸
  • -mtls-dialect=traditional — use traditional TLS as the thread-local storage mechanism for dynamic accesses of TLS variables.
  • -mtls-size=size — specify bit size of immediate TLS offsets. Valid values are 12, 24, 32, 48. This option requires binutils 2.26 or newer.
  • -mfix-cortex-a53-835769, -mno-fix-cortex-a53-835769 — enable or disable the workaround for the ARM Cortex-A53 erratum number 835769. This involves inserting a NOP instruction between memory instructions and 64-bit integer multiply-accumulate instructions. This flag will be ignored if an architecture or cpu is specified on the command line which does not need the workaround. 🛠️
  • -mfix-cortex-a53-843419, -mno-fix-cortex-a53-843419 — enable or disable the workaround for the ARM Cortex-A53 erratum number 843419. This erratum workaround is made at link time and this will only pass the corresponding flag to the linker. 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-math or -funsafe-math-optimizations is 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. 🌈
  • -mlow-precision-sqrt, -mno-low-precision-sqrt — turn the square root approximation on or off 💅. Heads up though: this only does anything if you're also using -ffast-math or -funsafe-math-optimizations. Turning it on drops the precision of square root results to about 16 bits for single precision and about 32 bits for double precision. And if you enable it, it sneakily implies -mlow-precision-recip-sqrt too 🔥.
  • -mlow-precision-div, -mno-low-precision-div — same vibe but for division ✨. Again, only has an effect if -ffast-math or -funsafe-math-optimizations is used as well. Enabling it reduces division result precision to about 16 bits for single precision and 32 bits for double precision.
  • -mtrack-speculation, -mno-track-speculation — toggles generating extra code that tracks speculative execution through conditional branches 👀. The compiler can then use that tracking state when expanding calls to __builtin_speculation_safe_copy so it can generate a more efficient code sequence. Bestie, this is the security-girlie option.
  • -moutline-atomics, -mno-outline-atomics — toggles calls to out-of-line helpers that implement atomic operations 🎀. At runtime those helpers check whether the LSE instructions from ARMv8.1-A can be used; if not, they fall back to the load/store-exclusive instructions that exist in the base ARMv8.0 ISA.

This one only applies when you're compiling for the base ARMv8.0 instruction set. If you're on a later revision — like -march=armv8.1-a or -march=armv8-a+lse — the ARMv8.1-Atomics instructions get used directly. Same deal when you use -mcpu= and the selected cpu supports the lse feature. And yes, this option is on by default ⭐.

  • *-march=name* — tell GCC the name of the target architecture, plus optionally one or more feature modifiers 💜. The form is:
-march=arch{+[no]feature}*

Here's the rundown of what arch values are allowed and which features they switch on by default (yes I'm formatting it as a list because tables are not the vibe):

  • armv8-a : Armv8-A : includes +fp, +simd
  • armv8.1-a : Armv8.1-A : armv8-a, +crc, +lse, +rdma
  • armv8.2-a : Armv8.2-A : armv8.1-a
  • armv8.3-a : Armv8.3-A : armv8.2-a, +pauth, +fcma, +jscvt
  • armv8.4-a : Armv8.4-A : armv8.3-a, +flagm, +fp16fml, +dotprod, +rcpc2
  • armv8.5-a : Armv8.5-A : armv8.4-a, +sb, +ssbs, +predres, +frintts, +flagm2
  • armv8.6-a : Armv8.6-A : armv8.5-a, +bf16, +i8mm
  • armv8.7-a : Armv8.7-A : armv8.6-a, +wfxt, +xs
  • armv8.8-a : Armv8.8-a : armv8.7-a, +mops
  • armv8.9-a : Armv8.9-a : armv8.8-a
  • armv9-a : Armv9-A : armv8.5-a, +sve, +sve2
  • armv9.1-a : Armv9.1-A : armv9-a, +bf16, +i8mm
  • armv9.2-a : Armv9.2-A : armv9.1-a, +wfxt, +xs
  • armv9.3-a : Armv9.3-A : armv9.2-a, +mops
  • armv9.4-a : Armv9.4-A : armv9.3-a, +sve2p1
  • armv9.5-a : Armv9.4-A : armv9.4-a, cpa, +faminmax, +lut
  • armv8-r : Armv8-R : armv8-r

The value native is available on native AArch64 GNU/Linux and makes the compiler pick the architecture of the host system 🖥️. This option has no effect if the compiler can't recognize the host's architecture, sorry babe. And when -march=native is given and no other -mcpu or -mtune is given, GCC picks the host CPU as the CPU to tune for as well as selecting architecture features from it. Which basically means -march=native gets treated as -mcpu=native ✨.

The allowed values for feature are listed in the sub-section on aarch64-feature-modifiers,,-march and -mcpu Feature Modifiers. If you specify conflicting feature modifiers, the right-most feature wins 🫶.

GCC uses name to decide what kind of instructions it can emit when generating assembly code. And if -march is specified without -mtune or -mcpu also being specified, the code gets tuned to perform well across a range of target processors implementing the target architecture 🌈.

  • *-mtune=name* — name the target processor GCC should tune code performance for 👑. The permissible values 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, neoverse-n3, olympus, 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, 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, cobalt-100, apple-m1, apple-m2, apple-m3 and native 💖.

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 mean GCC should tune for a big.LITTLE system 🧚.

The value neoverse-512tvb means GCC should tune for Neoverse cores that (a) implement SVE and (b) have a total vector bandwidth of 512 bits per cycle. In other words, it's telling GCC to 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). That's more general than tuning for one specific core like Neoverse V1, but more specific than the default tuning described below 💅.

Also, on native AArch64 GNU/Linux systems, the value native tunes performance to the host system. This option has no effect if the compiler can't recognize the host's processor 🙈.

Where none of -mtune=, -mcpu= or -march= are specified, the code gets tuned to perform well across a range of target processors 🌸.

Ngl, this option cannot be suffixed by feature modifiers. Periodt.
  • *-mcpu=name* — name the target processor, optionally suffixed by one or more feature modifiers 🔥. Form is:
-mcpu=cpu{+[no]feature}*

where the permissible values for cpu are the same as the ones available for -mtune. The permissible values for feature are documented in the sub-section on aarch64-feature-modifiers,,-march and -mcpu Feature Modifiers. Conflicting feature modifiers? Right-most wins, always 💜.

GCC uses name to decide what kind of instructions it can emit when generating assembly code (as if by -march) and to decide the target processor to tune for performance (as if by -mtune). If you use this option together with -march or -mtune, those options take precedence over the appropriate part of this one 👀.

-mcpu=neoverse-512tvb is special because it doesn't refer to a specific core — it refers to all Neoverse cores that (a) implement SVE and (b) have a total vector bandwidth of 512 bits a cycle. Unless overridden by -march, -mcpu=neoverse-512tvb generates code that can run on a Neoverse V1 core, since Neoverse V1 is the first Neoverse core with these properties. And unless overridden by -mtune, -mcpu=neoverse-512tvb tunes code the same way as -mtune=neoverse-512tvb ⭐.

  • -moverride=string — override tuning decisions the back-end made in response to a -mtune= switch 🎀. The syntax, semantics and accepted values for string are **not guaranteed to be consistent across releases.
Fr fr, this option is only intended to be useful when developing GCC. Like, not for you, bestie.
  • **-mverbose-cost-dump — enable verbose cost model dumping in the debug dump files 💀. This option exists for use in debugging the compiler.
  • -mpc-relative-literal-loads, -mno-pc-relative-literal-loads — turn PC-relative literal loads on or off ✨. With this, literal pools are accessed using a single instruction and are emitted after each function. That caps the maximum size of functions at 1MB 🫶. It's enabled by default for -mcmodel=tiny.
  • *-msign-return-address=scope* — pick the function scope that return address signing applies to 🔐. Permissible values are none, which disables return address signing, non-leaf, which enables pointer signing for functions that aren't leaf functions, and all, which enables pointer signing for all functions. Default is none.
This option has been deprecated by -mbranch-protection 😭.
  • -mbranch-protection=none|standard|pac-ret[+leaf+b-key]|bti|gcs — choose which branch protection features to use 🛡️. none is the default and turns off all types of branch protection. standard turns on all types of branch protection features — and if a feature has additional tuning options, standard sets it to its standard level. pac-ret[+leaf] turns on return address signing to its standard level: signing functions that save the return address to memory (non-leaf functions will practically always do this) using the a-key. The optional argument leaf extends the signing to include leaf functions. The optional argument b-key signs functions with the B-key instead of the A-key. 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) 💪. opts is a comma-separated list of these options:
    • retbr
    • blr

Also, -mharden-sls=all enables all SLS hardening while -mharden-sls=none disables all SLS hardening ✨.

  • *-mearly-ra=scope* — decide when to enable an early register allocation pass 🧚. This pass runs before 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 strided LD1 and ST1 🌸.

The possible values of scope are: all, which runs the pass on all functions; strided, which runs the pass on functions that have access to strided multi-register instructions; and none, which disables the pass 💅

-mearly-ra=all is the default for -O2 and above, and for -Os. -mearly-ra=none is the default otherwise. iykyk ✨

  • -mearly-ldp-fusion — turns on the copy of the AArch64 load/store pair fusion pass that runs before register allocation. It's enabled by default at -O and above 🎀
  • -mlate-ldp-fusion — turns on the copy of that same AArch64 load/store pair fusion pass, but the one that runs after register allocation. Also enabled by default at -O and above 🦋
  • *-msve-vector-bits=bits* — tells GCC how many bits live in an SVE vector register. Heads up bestie: this only does anything when SVE is actually enabled 🙈

GCC supports two whole flavors of SVE code generation: "vector-length agnostic" output that works with any size of vector register, and "vector-length specific" output that 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. Right now -msve-vector-bits=128 also generates vector-length agnostic output for big-endian targets. Every other value generates vector-length specific code. Fair warning though: the behavior of these values may change in future releases, and no value except scalable should be relied on for producing code that's portable across different hardware SVE vector lengths ⚠️

The default is -msve-vector-bits=scalable, which produces vector-length agnostic code 💖

  • -Wexperimental-fmv-target — warns you about use of experimental Function Multi Versioning. The Arm C Language Extension spec for Function Multi Versioning is beta and subject to change, so literally any usage of FMV comes with the caveat that future behavior changes and incompatibility are likely 😭

🌸 -march and -mcpu Feature Modifiers 🌸

Feature modifiers used with -march and -mcpu can be any of the following — and also their inverses, nofeature 👑

  • crc — enables the CRC extension. On by default for -march=armv8.1-a.
  • crypto — enables the Crypto extension. This also enables Advanced SIMD and floating-point instructions.
  • fp — enables floating-point instructions. On by default for all possible values for options -march and -mcpu.
  • simd — enables Advanced SIMD instructions. This also enables floating-point instructions. On by default for all possible values for options -march and -mcpu.
  • sve — enables Scalable Vector Extension instructions. This also enables Advanced SIMD and floating-point instructions.
  • lse — enables Large System Extension instructions. On by default for -march=armv8.1-a.
  • rdma — enables Round Double Multiply Accumulate instructions. On by default for -march=armv8.1-a.
  • fp16 — enables the FP16 extension. This also enables floating-point instructions.
  • fp16fml — enables the FP16 fmla extension. This also enables FP16 extensions and floating-point instructions. Enabled by default for -march=armv8.4-a. Using this option with architectures prior to Armv8.2-A is not supported 💀
  • rcpc — enables the RCpc extension. This enables the use of the LDAPR instructions for load-acquire atomic semantics, and passes it on to the assembler, which lets inline asm statements use instructions from the RCpc extension.
  • dotprod — enables the Dot Product extension. This also enables Advanced SIMD instructions.
  • aes — enables the Armv8-a aes and pmull crypto extension. This also enables Advanced SIMD instructions.
  • sha2 — enables the Armv8-a sha2 crypto extension. This also enables Advanced SIMD instructions.
  • sha3 — enables the sha512 and sha3 crypto extension. This also enables Advanced SIMD instructions. Using this with architectures prior to Armv8.2-A is not supported 💀
  • sm4 — enables the sm3 and sm4 crypto extension. This also enables Advanced SIMD instructions. Using this with architectures prior to Armv8.2-A is not supported 💀
  • profile — enables the Statistical Profiling extension. This one is only about enabling the extension at the assembler level and does not affect code generation, no cap.
  • rng — enables the Armv8.5-a Random Number instructions. Assembler-level only, doesn't affect code generation.
  • memtag — enables the Armv8.5-a Memory Tagging Extensions. Using this with architectures prior to Armv8.5-A is not supported 💀
  • sb — enables the Armv8-a Speculation Barrier instruction. Assembler-level only, doesn't affect code generation. Enabled by default for -march=armv8.5-a.
  • ssbs — enables the Armv8-a Speculative Store Bypass Safe instruction. Assembler-level only, doesn't affect code generation. Enabled by default for -march=armv8.5-a.
  • predres — enables the Armv8-a Execution and Data Prediction Restriction instructions. Assembler-level only, doesn't affect code generation. Enabled 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 — enables the Transactional Memory Extension.
  • i8mm — enables 8-bit Integer Matrix Multiply instructions. This also enables Advanced SIMD and floating-point instructions. Enabled by default for -march=armv8.6-a. Using it with architectures prior to Armv8.2-A is not supported 💀
  • f32mm — enables 32-bit Floating point Matrix Multiply instructions. This also enables SVE instructions. Using it with architectures prior to Armv8.2-A is not supported 💀
  • f64mm — enables 64-bit Floating point Matrix Multiply instructions. This also enables SVE instructions. Using it with architectures prior to Armv8.2-A is not supported 💀
  • bf16 — enables brain half-precision floating-point instructions. This also enables Advanced SIMD and floating-point instructions. Enabled by default for -march=armv8.6-a. Using it with architectures prior to Armv8.2-A is not supported 💀
  • ls64 — enables the 64-byte atomic load and store instructions for accelerators.
  • mops — enables the instructions that accelerate memory operations like memcpy, memmove, memset. Enabled by default for -march=armv8.8-a ✨
  • flagm — enables the Flag Manipulation instructions Extension.
  • flagm2 — enables the FlagM2 flag conversion instructions.
  • pauth — enables the Pointer Authentication Extension.
  • cssc — enables the Common Short Sequence Compression instructions.
  • sme — enables the Scalable Matrix Extension. Only supported when SVE2 is also enabled.
  • sme-i16i64 — enables the FEAT_SME_I16I64 extension to SME. This also enables SME instructions.
  • sme-f64f64 — enables the FEAT_SME_F64F64 extension to SME. This also enables SME instructions.
  • sme2 — enables the Scalable Matrix Extension 2. This also enables SME instructions.
  • sme-b16b16 — enables the FEAT_SME_B16B16 extension to SME. This also enables SME2 and SVE_B16B16 instructions.
  • sme-f16f16 — enables the FEAT_SME_F16F16 extension to SME. This also enables SME2 instructions.
  • sme2p1 — enables the Scalable Matrix Extension version 2.1. This also enables SME2 instructions.
  • fcma — enables the complex number SIMD extensions.
  • jscvt — enables the fjcvtzs JavaScript conversion instruction.
  • frintts — enables floating-point round to integral value instructions.
  • wfxt — enables wfet and wfit instructions.
  • xs — enables the XS memory attribute extension.
  • lse128 — enables the LSE128 128-bit atomic instructions extension. This also enables LSE instructions.
  • d128 — enables support for 128-bit system register read/write instructions. This also enables the LSE128 extension.
  • gcs — enables support for the Armv9.4-a Guarded Control Stack extension.
  • the — enables support for the Armv8.9-a/9.4-a translation hardening extension.
  • rcpc2 — enables the RCpc2 extension.
  • rcpc3 — enables the RCpc3 (Release Consistency) extension.
  • fp8 — enables the fp8 (8-bit floating point) extension.
  • fp8fma — enables the fp8 (8-bit floating point) multiply accumulate extension.
  • ssve-fp8fma — enables the fp8 (8-bit floating point) multiply accumulate extension in streaming mode.
  • fp8dot4 — enables the fp8 (8-bit floating point) to single-precision 4-way dot product extension.
  • ssve-fp8dot4 — enables the fp8 (8-bit floating point) to single-precision 4-way dot product extension in streaming mode.
  • fp8dot2 — enables the fp8 (8-bit floating point) to half-precision 2-way dot product extension.
  • ssve-fp8dot2 — enables the fp8 (8-bit floating point) to half-precision 2-way dot product extension in streaming mode.
  • faminmax — enables the Floating Point Absolute Maximum/Minimum extension.
  • lut — enables the Lookup Table extension.
  • cpa — enables the Checked Pointer Arithmetic instructions.
  • sve-b16b16 — enables the SVE non-widening brain floating-point (bf16) extension. This only has an effect when sve2 or sme2 are also enabled 👑
Feature crypto implies aes, sha2, and simd, which implies fp. Conversely, nofp implies nosimd, which implies nocrypto, noaes and nosha2 🌀

🧚 Adapteva Epiphany Options 🧚

These -m options are defined for Adapteva Epiphany:

  • -mhalf-reg-file — don't allocate any register in the range r32–r63. That lets your code run on hardware variants that lack these registers 💜
  • -mprefer-short-insn-regs — tells the compiler to prefer registers that unlock short instructions. ✨ Cute in theory, but it can bump up your instruction count, so overall code size might go down or up. It's giving gamble, honestly.
  • -mbranch-cost=num — sets the cost of a branch to roughly num "simple" instructions. Heads up bestie: this is a heuristic only, and there's no promise it gives consistent results across releases. 🙈
  • -mcmove — turns on generation of conditional moves. periodt.
  • -mnops=num — emits num NOPs before every other generated instruction. Yes, really. That's the whole vibe.
  • -mno-soft-cmpsf — for single-precision floating-point comparisons, this emits an fsub instruction and just tests the flags. Faster than a software comparison, sure — but it can get incorrect results when NaNs are in the mix, or when two different small numbers get compared such that their difference comes out as zero. 😭 The default is -msoft-cmpsf, which does the slower-but-IEEE-compliant software comparison thing. Safety first, diva.
  • -mstack-offset=num — sets the offset between the top of the stack and the stack pointer. Like, a value of 8 means the eight bytes in the range sp+0...sp+7 are fair game for leaf functions with no stack allocation needed. Values other than 8 or 16 are untested and unlikely to work, ngl.
Okay so this one changes the ABI. Compiling a program with a different stack offset than the libraries were compiled with generally does not work. It's handy if you wanna evaluate whether a different stack offset would give you better code — but to actually build working programs with one, the recommendation is to configure the toolchain with the appropriate --with-stack-offset=num option. 🫶
  • -mno-round-nearest — makes the scheduler assume the rounding mode has been set to truncating. The default is -mround-nearest. 🌸
  • -mlong-calls — unless an attribute says otherwise, assume every call might land outside the offset range of the b / bl instructions, so the function address gets loaded into a register before doing an (otherwise direct) call. This is the default. 👑
  • -mshort-calls — unless an attribute says otherwise, assume all direct calls are within range of the b / bl instructions, so those get used for direct calls. The default is -mlong-calls. (yes, that's the default, don't come for me)
  • -msmall16 — assume addresses can be loaded as 16-bit unsigned values. This does not apply to function addresses, which are still living under -mlong-calls semantics. ✨
  • -mfp-mode=mode — sets the prevailing mode of the floating-point unit. This decides the FP mode that's provided and expected at function call and return time. Matching it to the mode you mostly need at function start can make your programs smaller and faster by dodging unnecessary mode switches. 🔥

mode can be one of these:

  • caller — any mode at function entry is valid, and it's retained or restored when the function returns and when it calls other functions. Super useful for compiling libraries or other compilation units you might wanna drop into different programs with different prevailing FPU modes — the convenience of one single object file outweighs the size and speed overhead of any extra mode switching, compared to picking a more specific prevailing FPU mode. 🎀
  • truncate — the mode used for floating-point calculations with truncating (i.e. round towards zero) rounding mode. That includes conversion from floating point to integer. 💜
  • round-nearest — the mode used for floating-point calculations with round-to-nearest-or-even rounding mode. 🌈
  • int — the mode used to do integer calculations in the FPU, e.g. integer multiply, or integer multiply-and-accumulate. ⭐

The default is -mfp-mode=caller.

  • -mno-split-lohi
  • -mno-postinc
  • -mno-postmodify — code generation tweaks that disable, respectively: splitting of 32-bit loads, generation of post-increment addresses, and generation of post-modify addresses. The defaults are msplit-lohi, -mpost-inc, and -mpost-modify. 🧚
  • -mnovect-double — changes the preferred SIMD mode to SImode. The default is -mvect-double, which uses DImode as the preferred SIMD mode. 💅
  • -max-vect-align=num — the maximum alignment for SIMD vector mode types. num may be 4 or 8. The default is 8.
Note that this is an ABI change — even though lots of library function interfaces are unaffected if they don't use SIMD vector modes in places that affect size and/or alignment of the relevant types. 💀
  • -msplit-vecmove-early — splits 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. It's giving "sounded good on paper". 😭
  • -m1reg-reg — specifies a register to hold the constant -1, which makes loading small negative constants and certain bitmasks faster. Allowable values for reg are r43 and r63, which specify use of that register as a fixed register, and none, which means no register gets used for this. The default is -m1reg-none. ✨

🌸 AMD GCN Options 🌸

These options are defined specifically for the AMD GCN port. GPU girlies rise 🫶

  • -march=gpu
  • -mtune=gpu — set architecture type or tuning for gpu. Supported values for gpu are:
    • gfx900 — compile for GCN5 Vega 10 devices (gfx900). 👑
    • gfx902 — compile for GCN5 Vega gfx902 devices. (Experimental)
    • gfx904 — compile for GCN5 Vega gfx904 devices. (Experimental)
    • gfx906 — compile for GCN5 Vega 20 devices (gfx906).
    • gfx908 — compile for CDNA1 Instinct MI100 series devices (gfx908).
    • gfx909 — compile for GCN5 Vega gfx909 devices. (Experimental)
    • gfx90a — compile for CDNA2 Instinct MI200 series devices (gfx90a).
    • gfx90c — compile for GCN5 Vega 7 devices (gfx90c).
    • gfx9-generic — compile generic code for Vega devices, executable on this subset of GFX9 devices: gfx900, gfx902, gfx904, gfx906, gfx909 and gfx90c. (Experimental) 🌈
    • gfx1030 — compile for RDNA2 gfx1030 devices (GFX10 series).
    • gfx1031 — compile for RDNA2 gfx1031 devices (GFX10 series). (Experimental)
    • gfx1032 — compile for RDNA2 gfx1032 devices (GFX10 series). (Experimental)
    • gfx1033 — compile for RDNA2 gfx1033 devices (GFX10 series). (Experimental)
    • gfx1034 — compile for RDNA2 gfx1034 devices (GFX10 series). (Experimental)
    • gfx1035 — compile for RDNA2 gfx1035 devices (GFX10 series). (Experimental)
    • gfx1036 — compile for RDNA2 gfx1036 devices (GFX10 series). ⭐
    • gfx10-3-generic — compile generic code for GFX10-3 devices, executable on gfx1030, gfx1031, gfx1032, gfx1033, gfx1034, gfx1035, and gfx1036. (Experimental)
    • gfx1100 — compile for RDNA3 gfx1100 devices (GFX11 series).
    • gfx1101 — compile for RDNA3 gfx1101 devices (GFX11 series). (Experimental)
    • gfx1102 — compile for RDNA3 gfx1102 devices (GFX11 series). (Experimental)
    • gfx1103 — compile for RDNA3 gfx1103 devices (GFX11 series). 🔥
    • gfx1150 — compile for RDNA3 gfx1150 devices (GFX11 series). (Experimental)
    • gfx1151 — compile for RDNA3 gfx1151 devices (GFX11 series). (Experimental)
    • gfx1152 — compile for RDNA3 gfx1152 devices (GFX11 series). (Experimental)
    • gfx1153 — compile for RDNA3 gfx1153 devices (GFX11 series). (Experimental)
    • gfx11-generic — compile generic code for GFX11 devices, executable on gfx1100, gfx1101, gfx1102, gfx1103, gfx1150, gfx1151, gfx1152, and gfx1153. (Experimental) 🦋
  • -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 is any, for devices that support it. 🎀
  • -mstack-size=bytes — how many bytes of stack space get requested for each GPU thread (wave-front). Beware: there may be many threads and limited memory available 😭 — and the size of the stack allocation may also impact run-time performance. The default is 32KB when using OpenACC or OpenMP, and 1MB otherwise.
  • -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 let the user configure it. The compiled code must match the device mode. The default is -mxnack=any on devices that support Unified Shared Memory, and -mxnack=no otherwise. 💅

🦋 ARC Options 🦋

The following options control the architecture variant for which code is being compiled:

  • -mbarrel-shifter — generate instructions supported by barrel shifter. This is the default unless -mcpu=ARC601 or -mcpu=ARCEM is in effect. ✨
  • -mjli-always — force calling a function using the jli_s instruction. Valid only for ARCv2 architecture, bestie.
  • -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 the ARC HS38 CPU ✨
  • hs38_linux — ARC HS38 CPU, but with all the hardware extensions turned on 💅
  • hs4x — compile for the ARC HS4x CPU 🦋
  • hs4xd — compile for the ARC HS4xD CPU 🎀
  • hs4x_rel31 — ARC HS4x CPU, release 3.10a specifically, no cap
  • arc600_norm — ARC 600 CPU with "norm" instructions enabled 🌸
  • arc600_mul32x16 — ARC 600 CPU with "norm" and 32x16-bit multiply instructions enabled 💖
  • arc600_mul64 — ARC 600 CPU with "norm" and "mul64"-family instructions enabled ✨
  • arc601_norm — ARC 601 CPU with "norm" instructions enabled
  • arc601_mul32x16 — ARC 601 CPU with "norm" and 32x16-bit multiply instructions enabled 🫶
  • arc601_mul64 — ARC 601 CPU with "norm" and "mul64"-family instructions enabled
  • nps400 — compile for the ARC 700 on the NPS400 chip 💀
  • em_mini — ARC EM minimalist configuration, featuring the reduced register set. tiny icon energy 🧚
  • -mdpfp, -mdpfp-compact — generate double-precision FPX instructions, tuned for the compact implementation 🎀
  • -mdpfp-fast — double-precision FPX instructions, but tuned for the fast implementation 🔥
  • -mno-dpfp-lrsr — disables "lr" and "sr" instructions from touching FPX extension aux registers, periodt
  • -mea — generate extended arithmetic instructions. Right now only "divaw", "adds", "subs", and "sat16" are supported. Only valid for -mcpu=ARC700 ngl
  • -mno-mpy — don't generate "mpy"-family instructions for ARC700. Heads up bestie, this option is deprecated 💀
  • -mmul32x16 — generate 32x16-bit multiply and multiply-accumulate instructions ✨
  • -mmul64 — generate "mul64" and "mulu64" instructions. Only valid for -mcpu=ARC600
  • -mnorm — generate "norm" instructions. This is the default if -mcpu=ARC700 is in effect 👑
  • -mspfp, -mspfp-compact — generate single-precision FPX instructions, tuned for the compact implementation 🌸
  • -mspfp-fast — single-precision FPX instructions, tuned for the fast implementation ⭐
  • -msimd — enable generation of ARC SIMD instructions via target-specific builtins. Only valid for -mcpu=ARC700 🔥
  • -msoft-float — okay so this option is literally ignored; it's only here for compatibility purposes 🙈 Software floating-point code is emitted by default, and that default can be overridden by the FPX options: -mspfp, -mspfp-compact, or -mspfp-fast for single precision, and -mdpfp, -mdpfp-compact, or -mdpfp-fast for double precision
  • -mswap — generate "swap" instructions 💫
  • -matomic — lets you use the locked load/store conditional extension to implement atomic memory built-in functions. Not available for ARC 6xx or ARC EM cores 🚫
  • -mdiv-rem — enable "div" and "rem" instructions for ARCv2 cores ✨
  • -mcode-density — enable code density instructions for ARC EM. This one's on by default for ARC HS 💅
  • -mll64 — enable double load/store operations for ARC HS cores 🦋
  • *-mtp-regno=regno* — specify the thread pointer register number
  • *-mmpy-option=multo* — compile ARCv2 code with a multiplier design option. You can pass multo as either a string or a numeric value. wlh1 is the default value. The recognized values are:
    • 0, none — no multiplier available 🙅
    • 1, w — 16x16 multiplier, fully pipelined. Enables these instructions: "mpyw" and "mpyuw"
    • 2, wlh1 — 32x32 multiplier, fully pipelined (1 stage). Additionally enables: "mpy", "mpyu", "mpym", "mpymu", and "mpy_s" ✨
    • 3, wlh2 — 32x32 multiplier, fully pipelined (2 stages). Additionally enables: "mpy", "mpyu", "mpym", "mpymu", and "mpy_s"
    • 4, wlh3 — two 16x16 multipliers, blocking, sequential. Additionally enables: "mpy", "mpyu", "mpym", "mpymu", and "mpy_s"
    • 5, wlh4 — one 16x16 multiplier, blocking, sequential. Additionally enables: "mpy", "mpyu", "mpym", "mpymu", and "mpy_s"
    • 6, wlh5 — one 32x4 multiplier, blocking, sequential. Additionally enables: "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, iykyk

  • *-mfpu=fpu* — enables support for specific floating-point hardware extensions for ARCv2 cores. Supported values for fpu are:
    • 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. Single-precision floating-point is also enabled. Only available for ARC EM ✨
    • fpuda_div — same double-precision-assist situation, plus the single-precision floating-point, square-root, and divide extensions are also enabled. Only available for ARC EM
    • fpuda_fma — double-precision assist again, plus single-precision floating-point and fused multiply and add hardware extensions are also enabled. Only available for ARC EM
    • fpuda_all — double-precision assist, and all single-precision floating-point hardware extensions are also enabled. 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 option fpus_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 option fpus_fma. Not available for ARC EM
    • fpus_all — enables support for all single-precision floating-point hardware extensions 👑
    • 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 the general-purpose registers that the processor automatically saves/restores on interrupt entry and exit. register-range is two registers separated by a dash. The range always starts with "r0", and the upper limit is the "fp" register. blink and lp_count are optional. This option is only valid for ARC EM and ARC HS cores ⭐
  • *-mrgf-banked-regs=number* — specifies how many registers get replicated in the second register bank on entry to a fast interrupt. Fast interrupts are the ones with the highest priority level P0. These interrupts save only PC and STATUS32 registers, to avoid memory transactions during interrupt entry and exit sequences. Use this when you're using fast interrupts in an ARC V2 family processor. Permitted values are 4, 8, 16, and 32 🌈
  • *-mlpc-width=width* — specify the width of the "lp_count" register. Valid values for width are 8, 16, 20, 24, 28 and 32 bits. The default width is fixed to 32 bits. If the width is less than 32, the compiler won't try to transform loops in your program into the zero-delay loop mechanism unless it knows the "lp_count" register can hold the required loop-counter value. Depending on the width you specify, the compiler and run-time library might still use the loop mechanism for various needs. This option defines the macro "__ARC_LPC_WIDTH__" with the value of width 🎀
  • -mrf16 — tells 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 they also define preprocessor macro symbols 🫶

  • -mdsp-packa — passed down to the assembler to enable the DSP Pack A extensions. Also sets the preprocessor symbol "__Xdsp_packa". This option is deprecated 💀
  • -mdvbf — passed down to the assembler to enable the dual Viterbi butterfly extension. Also sets the preprocessor symbol "__Xdvbf". This option is deprecated
  • -mlock — passed down to the assembler to enable the locked load/store conditional extension. Also sets the preprocessor symbol "__Xlock" 🔒
  • -mmac-d16 — passed down to the assembler. Also sets the preprocessor symbol "__Xxmac_d16". This option is deprecated
  • -mmac-24 — passed down to the assembler. Also sets the preprocessor symbol "__Xxmac_24". This option is deprecated
  • -mrtsc — passed down to the assembler to enable the 64-bit time-stamp counter extension instruction. Also sets the preprocessor symbol "__Xrtsc". This option is deprecated 😭
  • -mswape — passed down to the assembler to enable the swap byte ordering extension instruction. Also sets the preprocessor symbol "__Xswape" 🦋
  • -mtelephony — passed down to the assembler to enable dual- and single-operand instructions for telephony. Also sets the preprocessor symbol "__Xtelephony". This option is deprecated
  • -mxy — Handed straight down to the assembler so it can enable the XY memory extension. It also sets the preprocessor symbol __Xxy. Slay.

✨ The following options control how the assembly code is annotated ✨

  • -misize — Annotates assembler instructions with estimated addresses.
  • -mannotate-align — Literally does nothing. It’s preserved for backward compatibility, bestie.

🦋 The following options are passed through to the linker 🦋

  • -marclinux — Passed through to the linker to specify use of the arclinux emulation. This option is enabled by default in tool chains built for arc-linux-uclibc and arceb-linux-uclibc targets when profiling is not requested.
  • -marclinux_prof — Passed through to the linker to specify use of the arclinux_prof emulation. This option is enabled by default in tool chains built for arc-linux-uclibc and arceb-linux-uclibc targets when profiling is requested.

🌸 The following options control the semantics of generated code 🌸

  • -mlong-calls — Generates calls as register indirect calls, which gives access to the full 32-bit address range. Iconic.
  • -mmedium-calls — Don’t use less than the 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 so the 25-bit range can be used instead of 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 — Puts definitions of externally-visible data in a small data section if that data is no bigger than num bytes. The default value of num is 4 for any ARC configuration, or 8 when 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 — Use ordinarily cached memory accesses for volatile references. This is the default.
  • -mno-volatile-cache — Enable cache bypass for volatile references.

🎀 The following options fine tune code generation 🎀

  • -malign-call — Does nothing. Preserved for backward compatibility.
  • -mauto-modify-reg — Enables the use of pre/post modify with register displacement.
  • -mbbit-peephole — Does nothing. Preserved for backward compatibility.
  • -mno-brcc — This option disables a target-specific pass in arc_reorg to generate compare-and-branch (brcc) instructions. It has no effect on generation of these instructions driven by the combiner pass.
  • -mcase-vector-pcrel — Use PC-relative switch case tables to enable case table shortening. This is the default for -Os.
  • -mcompact-casesi — Enable compact casesi pattern. This is the default for -Os, and only available for ARCv1 cores. This option is deprecated.
  • -mno-cond-exec — Disable the ARCompact-specific pass to generate conditional execution instructions.

Due to delay slot scheduling and interactions between operand numbers, literal sizes, instruction lengths, and the support for conditional execution, the target-independent pass to generate conditional execution is often lacking, so the ARC port has kept a special pass around that tries to find more conditional execution 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.
  • -mexpand-adddi — Expand adddi3 and subdi3 at RTL generation time into add.f, adc etc. This option is deprecated.
  • -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 — Does nothing. Preserved for backward compatibility.
  • -mlra-priority-none — Don’t indicate any priority for target registers.
  • -mlra-priority-compact — Indicate target register priority for r0..r3 / r12..r15.
  • -mlra-priority-noncompact — Reduce target register priority for r0..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 a call to a special function in libgcc; this is referred to as a millicode call. As these calls can pose performance issues, and/or cause linking issues when linking in a nonstandard way, this option is provided to turn on or off millicode call generation.
  • -mcode-density-frame — This option enables the compiler to emit enter and leave instructions. These instructions are only valid for CPUs with the code-density feature.
  • -mmixed-code — Does nothing. Preserved for backward compatibility.
  • -mq-class — This option is deprecated. Enable q instruction alternatives. This is the default for -Os.
  • -mRcq — Does nothing. Preserved for backward compatibility.
  • -mRcw — Does nothing. Preserved for backward compatibility.
  • -msize-level=level — Fine-tune size optimization with regards to instruction lengths and alignment. The recognized values for level are:
    • 0 — No size optimization. This level is deprecated and treated like 1.
    • 1 — Short instructions are used opportunistically.
    • 2 — In addition, alignment of loops and of code after barriers are dropped.
    • 3 — In addition, optional data alignment is dropped, and the option Os is enabled.

This defaults to 3 when -Os is in effect. Otherwise, the behavior when this is not set is equivalent to level 1.

  • -mtune=cpu — Set instruction scheduling parameters for cpu, overriding any implied by -mcpu=. Supported values for cpu are:
    • ARC600 — Tune for ARC600 CPU.
    • ARC601 — Tune for ARC601 CPU.
    • ARC700 — Tune for ARC700 CPU with standard multiplier block.
    • ARC700-xmac — Tune for ARC700 CPU with XMAC block.
    • ARC725D — Tune for ARC725D CPU.
    • ARC750D — Tune for ARC750D CPU.
    • core3 — Tune for ARCv2 core3 type CPU. This option enables usage of dbnz instruction.
    • release31a — Tune for ARC4x release 3.10a.
  • -mmultcost=num — Cost to assume for a multiply instruction, with 4 being equal to a normal instruction.
  • -munalign-prob-threshold=probability — Does nothing. Preserved for backward compatibility.

💀 The following options are maintained for backward compatibility, but are now deprecated and will be removed in a future release 💀

  • -margonaut — Obsolete FPX.
  • -mbig-endian, -EB — Compile code for big-endian targets. Use of these options is now deprecated. Big-endian code is supported by configuring GCC to build arceb-elf32 and arceb-linux-uclibc targets, for which big endian is the default.
  • -mlittle-endian, -EL — Compile code for little-endian targets. Use of these options is now deprecated. Little-endian code is supported by configuring GCC to build arc-elf32 and arc-linux-uclibc targets, for which little endian is the default.
  • -mbarrel_shifter — Replaced by -mbarrel-shifter.
  • -mdpfp_compact — Replaced by -mdpfp-compact.
  • -mdpfp_fast — Replaced by -mdpfp-fast.
  • -mdsp_packa — Replaced by -mdsp-packa.
  • -mEA — Replaced by -mea.
  • -mmac_24 — Replaced by -mmac-24.
  • -mmac_d16 — Replaced by -mmac-d16.
  • -mspfp_compact — Replaced by -mspfp-compact.
  • -mspfp_fast — Replaced by -mspfp-fast.
  • -mtune=cpu — Values arc600, arc601, arc700 and arc700-xmac for cpu are replaced by ARC600, ARC601, ARC700 and ARC700-xmac respectively.
  • -multcost=num — Replaced by -mmultcost.

👑 ARM Options 👑

These -m options are defined for the ARM port:

  • -mabi=name — Generate code for the specified ABI. Permissible values are: apcs-gnu, atpcs, aapcs, aapcs-linux and iwmmxt.
  • -mapcs-frame — Generate a stack frame that is compliant with the ARM Procedure Call Standard for all functions, even if this is not strictly necessary for correct execution of the code. Specifying -fomit-frame-pointer with this option causes the stack frames not to be generated for leaf functions. The default is -mno-apcs-frame. This option is deprecated.
  • -mapcs — This is a synonym for -mapcs-frame and is deprecated.
  • -mthumb-interwork — Generate code that supports calling between the ARM and Thumb instruction sets. Without this option, on pre-v5 architectures, the two instruction sets cannot be reliably used inside one program. The default is -mno-thumb-interwork, since slightly larger code is generated when -mthumb-interwork is specified. In AAPCS configurations this option is meaningless.
  • -mno-sched-prolog — Prevent the reordering of instructions in the function prologue, or the merging of those instructions with the instructions in the function’s body. This means that all functions start with a recognizable set of instructions (or in fact one of a choice from a small set of different function prologues), and this information can be used to locate the start of functions inside an executable piece of code. The default is -msched-prolog.
  • -mfloat-abi=name — Specifies which floating-point ABI to use. Permissible values are: soft, softfp and hard.

Specifying soft causes GCC to generate output containing library calls for floating-point operations. softfp allows the generation of code using hardware floating-point instructions, but still uses the soft-float calling conventions. hard allows generation of floating-point instructions and uses FPU-specific calling conventions.

The default depends on the specific target configuration. Note that the hard-float and soft-float ABIs are not link-compatible; you must compile your entire program with the same ABI, and link with a compatible set of libraries.

  • -mgeneral-regs-only — codegen that touches only the general-purpose registers ✨ like it refuses to even look at the floating-point and Advanced SIMD registers. Lowkey a glow-up for interrupt handlers. Important though: this puts zero restrictions on the assembler — it only ties the compiler's hands, not yours 💅
  • -mlittle-endian — generate code for a processor running in little-endian mode. This is the default for all standard configurations, so yeah, it's the main character 👑
  • -mbig-endian — generate code for a processor running in big-endian mode. The default is to compile code for a little-endian processor, so you have to ask for this one bestie 🦋
  • -mbe8, -mbe32 — when you're linking a big-endian image, these pick between BE8 and BE32 formats. For little-endian images the option does absolutely nothing and is just ignored 🙈 The default depends on the selected target architecture: ARMv6 and later architectures default to BE8, older architectures default to BE32. Oh and — BE32 format has been deprecated by ARM, so keep that in mind fr fr 💀
  • -march=name[+extension...] — this names the target ARM architecture, and GCC uses that name to figure out what kind of instructions it's allowed to emit when generating assembly code. It can be used together with, or instead of, the -mcpu= option — they're not mutually exclusive, they're a duo 🫶

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, iwmmxt and iwmmxt2.

Also recognized but with deprecated support are the following architectures, which lack support for the Thumb execution state: armv4 ⚠️ (iconic but old, bestie)

💫 the extension situation

Lots of these architectures support extensions, and you add them by appending +extension to the architecture name. Extension options are processed in order and capabilities accumulate — it's giving stacking buffs 🌸 An extension will also enable any necessary base extensions it depends on. Example: +crypto will always enable +simd.

The one exception to this additive construction is extensions prefixed with +no... — those disable the specified option and any other extensions that may depend on the presence of that extension. So it's not just "add a thing", it's "yeet a thing and everything that needed it" 😭

For example:

-march=armv7-a+simd+nofp+vfpv4

is equivalent to writing

-march=armv7-a+vfpv4

since the +simd option is entirely disabled by the +nofp option that follows it. Order matters, periodt ✨

Most extension names are generically named, but their effect depends on the architecture you applied them to — same name, different personality 💅 For example, +simd can be applied to both armv7-a and armv8-a architectures, but it enables the original ARMv7-A Advanced SIMD (Neon) extensions for armv7-a and the ARMv8-A variant for armv8-a.

The list below has the supported extensions for each architecture. Architectures not mentioned support no extensions — if you don't see your fave, it's not on the list 🎀

🧚 armv5te, armv6, armv6j, armv6k, armv6kz, armv6t2, armv6z, armv6zk

  • +fp — the VFPv2 floating-point instructions. The extension +vfpv2 can 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-d16 can be used as an alias for this extension. Heads up: floating-point is not supported by the base ARMv7-M architecture, but it 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-d16 can be used as an alias for this extension.
  • +simd — the Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions. The extensions +neon and +neon-vfpv3 can 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, with 32 double-precision registers.
  • +neon-fp16 — the Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions, with the half-precision floating-point conversion operations.
  • +neon-vfpv4 — the Advanced SIMD (Neon) v2 and the VFPv4 floating-point instructions.
  • +nosimd — disable the Advanced SIMD instructions (does not disable floating point 👀).
  • +nofp — disable the floating-point and Advanced SIMD instructions.

🧚 armv7ve

The extended version of the ARMv7-A architecture with support for virtualization ✨

  • +fp — the VFPv4 floating-point instructions, with 16 double-precision registers. The extension +vfpv4-d16 can be used as an alias for this extension.
  • +simd — the Advanced SIMD (Neon) v2 and the VFPv4 floating-point instructions. The extension +neon-vfpv4 can 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. The extension +neon-vfpv3 can be used as an alias for this extension.
  • +neon-fp16 — the Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions, with the half-precision floating-point conversion operations.
  • +nosimd — disable the Advanced SIMD instructions (does not disable floating point).
  • +nofp — disable the floating-point and Advanced SIMD instructions.

🧚 armv8-a

  • +crc — the Cyclic Redundancy Check (CRC) instructions.
  • +simd — the ARMv8-A Advanced SIMD and floating-point instructions.
  • +crypto — the cryptographic instructions.
  • +nocrypto — disable the cryptographic instructions.
  • +nofp — disable the floating-point, Advanced SIMD and cryptographic instructions.
  • +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.
  • +nocrypto — disable the cryptographic instructions.
  • +nofp — disable 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

  • +fp16 — the half-precision floating-point data processing instructions. This also enables the Advanced SIMD and floating-point instructions.
  • +fp16fml — the half-precision floating-point fmla extension. 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 — enable the Dot Product extension. This also enables Advanced SIMD instructions.
  • +nocrypto — disable the cryptographic extension.
  • +nofp — disable 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. (yes, "Brain", that's what it says bestie, I don't make the rules 😭)

🧚 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-point fmla extension.
  • +simd — the ARMv8.3-A Advanced SIMD and floating-point instructions, plus the Dot Product extension. slay 💅
  • +crypto — the cryptographic instructions. this also enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension. ✨
  • +nocrypto — disable the cryptographic extension. bye crypto 👋
  • +nofp — disable the floating-point, Advanced SIMD and cryptographic instructions. no floaties for u 🫶
  • +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. math girlie behavior 🧮
  • +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-point fmla extension. 💖
  • +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 — disable the cryptographic extension.
  • +nofp — disable 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-point fmla extension.
  • +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 — disable the cryptographic extension.
  • +nofp — disable 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. the extension +vfpv3xd can be used as an alias for this extension. aliases are iconic, periodt.
  • +fp — the VFPv3 floating-point instructions with 16 double-precision registers. the extension +vfpv3-d16 can 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. say that five times fast 😭
  • +vfpv3-d16-fp16 — the VFPv3 floating-point instructions with 16 double-precision registers and the half-precision floating-point conversion operations.
  • +nofp — disable the floating-point extension.
  • +idiv — the ARM-state integer division instructions.
  • +noidiv — disable 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.
  • +nofp — disable 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 — disable the floating-point extension.
  • +cdecp0, +cdecp1, ... , +cdecp7 — enable the Custom Datapath Extension (CDE) on selected coprocessors according to the numbers given in the options in the range 0 to 7. pick ur fave coprocessor 💅
  • +pacbti — enable the Pointer Authentication and Branch Target Identification Extension. security king 👑

armv8-m.main

  • +dsp — the DSP instructions.
  • +nodsp — disable the DSP extension.
  • +fp — the single-precision floating-point instructions.
  • +fp.dp — the single- and double-precision floating-point instructions.
  • +nofp — disable the floating-point extension.
  • +cdecp0, +cdecp1, ... , +cdecp7 — enable the Custom Datapath Extension (CDE) on selected coprocessors according to the numbers given in the options in the range 0 to 7.

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 — disable the cryptographic instructions.
  • +nofp — disable the floating-point, Advanced SIMD and cryptographic instructions.
  • -march=native — tells the compiler to auto-detect the architecture of the build computer. rn 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. no drama, it just vibes.
  • -march=unset — tells the compiler to ignore any -march=... 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. not me clearing the group chat history 🫣

-mtune=name

this option specifies the name of the target ARM processor for which GCC should tune the performance of the code. for some ARM implementations better performance can be obtained by using this option. the 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. that's a lot of names, but we love a thorough queen 👑

additionally, this option can specify that GCC should tune the performance of the code for a big.LITTLE system. the permissible names 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. big.LITTLE energy fr fr 🦋

-mtune=generic-arch specifies that GCC should tune the performance for a blend of processors within architecture arch. the aim is to generate code that run well on the current most popular processors, balancing between optimizations that benefit some CPUs in the range, and avoiding performance pitfalls of other CPUs. the effects of this option may change in future GCC versions as CPU models come and go. it's giving adaptable 💅

-mtune permits the same extension options as -mcpu, but the extension options do not affect the tuning of the generated code. noted.

-mtune=native causes the compiler to auto-detect the CPU of the build computer. at present, this feature is only supported on GNU/Linux, and not all architectures are recognized. if the auto-detect is unsuccessful the option has no effect. same vibe as -march=native, just CPU-flavored ✨

-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 -mcpu is the main character, but -march and -mtune can steal the scene when they show up 😭👑

So yeah — lots of the CPUs we support have optional architectural extensions, and where that's the case those extensions are normally on by default. But if there are implementations out there missing the extension, you can use the extension syntax to turn off the ones that got left out. 🫶

For floating-point and Advanced SIMD (Neon) instructions you also gotta factor in -mfloat-abi and -mfpu: floating-point and Advanced SIMD instructions only get used if -mfloat-abi isn't set to soft, and any -mfpu setting other than auto overrides the available floating-point and SIMD extension instructions. iykyk 💅

Example time: cortex-a9 ships in three major configurations — integer only, with just a floating-point unit, or with floating-point and Advanced SIMD. Default is all the instructions enabled, but +nosimd and +nofp let you disable just the SIMD, or both SIMD and floating-point, respectively. Slay. ✨

Permissible names for this option are the same as for -mtune.

The following extension options are common to the listed CPUs:

  • +nodsp — Disable the DSP instructions on cortex-m33, cortex-m35p, cortex-m52, cortex-m55 and cortex-m85. Also disables the M-Profile Vector Extension (MVE) integer and single precision floating-point instructions on cortex-m52, cortex-m55 and cortex-m85.
  • +nopacbti — Disable the Pointer Authentication and Branch Target Identification Extension on cortex-m52 and cortex-m85.
  • +nomve — Disable the M-Profile Vector Extension (MVE) integer and single precision floating-point instructions on cortex-m52, cortex-m55 and cortex-m85.
  • +nomve.fp — Disable the M-Profile Vector Extension (MVE) single precision floating-point instructions on cortex-m52, cortex-m55 and cortex-m85.
  • +cdecp0, +cdecp1, ... , +cdecp7 — Enable the Custom Datapath Extension (CDE) on selected coprocessors according to the numbers given in the options, in the range 0 to 7, on cortex-m52, cortex-m55 and star-mc1.
  • +nofp — Disables the floating-point instructions on arm9e, 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-m55 and cortex-m85. Disables the floating-point and SIMD instructions on generic-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-a53 and cortex-a55.
  • +nofp.dp — Disables the double-precision component of the floating-point instructions on cortex-r5, cortex-r7, cortex-r8, cortex-r52, cortex-r52plus and cortex-m7.
  • +nosimd — Disables the SIMD (but not floating-point) instructions on generic-armv7-a, cortex-a5, cortex-a7 and cortex-a9.
  • +crypto — Enables the cryptographic instructions on cortex-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-a53 and cortex-a75.cortex-a55.

Also (and this is lowkey important 💜) the generic-armv7-a pseudo target defaults to VFPv3 with 16 double-precision registers. It supports these extension options: mp, sec, vfpv3-d16, vfpv3, vfpv3-d16-fp16, vfpv3-fp16, vfpv4-d16, vfpv4, neon, neon-vfpv3, neon-fp16, neon-vfpv4. 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 info. 👑

-mcpu=native makes the compiler auto-detect the CPU of the build computer. Right now this feature is only supported on GNU/Linux, and not all architectures are recognized. If the auto-detect flops, 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. Super useful for avoiding warnings about conflicting CPU and architecture options when the two produce different architecture specifications. no cap 🙈

  • -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. Note that neon is an alias for neon-vfpv3 and vfp is an alias for vfpv2.

The setting auto is the default and it's special ✨ — it makes the compiler pick the floating-point and Advanced SIMD instructions based on the settings of -mcpu and -march.

If the selected floating-point hardware includes the NEON extension (e.g. -mfpu=neon), heads up: floating-point operations are not generated by GCC's auto-vectorization pass unless -funsafe-math-optimizations is also specified. Why? Because NEON hardware doesn't fully implement the IEEE 754 standard for floating-point arithmetic (in particular, denormal values are treated as zero), so using NEON instructions may lead to a loss of precision. 💀

You can also set the fpu name at function level by using the target("fpu=") function attributes or pragmas.

  • -mfp16-format=name — Specify the format of the __fp16 half-precision floating-point type. Permissible names are none, ieee, and alternative; the default is none, in which case the __fp16 type is not defined.
  • -mstructure-size-boundary=n — The sizes of all structures and unions are rounded up to a multiple of the number of bits set by this option. Permissible values are 8, 32 and 64. The default value varies for different toolchains. For the COFF targeted toolchain the default value is 8. A value of 64 is only allowed if the underlying ABI supports it.

Specifying a larger number can produce faster, more efficient code, but can also increase the size of the program. Different values are potentially incompatible — code compiled with one value 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.
  • -mabort-on-noreturn — Generate a call to the function abort at the end of a noreturn function. It gets executed if the function tries to return.
  • -mlong-calls, -mno-long-calls — Tells the compiler to perform function calls by first loading the address of the function into a register and then performing a subroutine call on this register. This switch is needed if the target function lies outside of the 64-megabyte addressing range of the offset-based version of subroutine call instruction. 🔥

Even if this switch is enabled, not all function calls are turned into long calls. The heuristic: static functions, functions that have 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. Exceptions to the 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, as does placing the function calls within the scope of a #pragma long_calls_off directive. Note these switches have no effect on how the compiler generates code to handle function calls via function pointers. 🦋

  • -msingle-pic-base — Treat the register used for PIC addressing as read-only, rather than loading it in the prologue for each function. The runtime system is responsible for initializing this register with an appropriate value before execution begins.
  • -mpic-register=reg — Specify the register to be used for PIC addressing. For the standard PIC base case, the default is any suitable register determined by the compiler. For the single PIC base case, the default is R9 if the target is EABI based or stack-checking is enabled, otherwise the default is R10.
  • -mpic-data-is-text-relative — Assume that the displacement between the text and data segments is fixed at static link time. This permits using 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 disabled on such targets, it will enable -msingle-pic-base by default. ✨
  • -mpoke-function-name — Write the name of each function into the text section, directly preceding the function prologue. The generated code is similar to this:
             t0
                 .ascii "arm_poke_function_name", 0
                 .align
             t1
                 .word 0xff000000 + (t1 - t0)
             arm_poke_function_name
                 mov     ip, sp
                 stmfd   sp!, {fp, ip, lr, pc}
                 sub     fp, ip, #4

When performing a stack backtrace, code can inspect the value of pc stored at fp + 0. If the trace function then looks at location pc - 12 and the top 8 bits are set, then we know that there's a function name embedded immediately preceding this location and has length ((pc[-3]) & 0xff000000). 🕵️‍♀️

  • -mthumb, -marm — Select between generating code that executes in ARM and Thumb states. The default for most configurations is to generate code that executes in ARM state, but the default can be changed by configuring GCC with the --with-mode=state configure option.

You can also override the ARM and Thumb mode for each function by using the target("thumb") and target("arm") function attributes or pragmas. 💅

  • -mflip-thumb — ok this one literally just flips ARM/Thumb modes on alternating functions 🎀 it exists for regression testing of mixed Thumb/ARM code generation and is not meant for ordinary use in compiling code, so like… bestie, don't.
  • -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 doesn't call any other functions, cute). Default is -mno-tpcs-frame.
  • -mtpcs-leaf-frame — same energy but for all leaf functions. Default is -mno-apcs-leaf-frame — yes, that's what it says, no I'm not fixing it 💅.
  • -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 lets those functions be called from non-interworking code. Not valid in AAPCS configurations because interworking is enabled by default there.
  • -mcaller-super-interworking — lets calls via function pointers (including virtual functions) execute correctly regardless of whether the target code was compiled for interworking or not 🫶 there's a small overhead in the cost of executing a function pointer if this is on. Also not valid in AAPCS configurations, same reason as above.
  • -mtp=name — picks the access model for the thread local storage pointer 💖 the model soft generates calls to "__aeabi_read_tp". Other accepted models are tpidrurw, tpidruro and tpidrprw, which fetch the thread pointer from the corresponding system register directly (supported from the arm6k architecture and later). Those system registers get accessed through the CP15 co-processor interface, and the argument cp15 is also accepted as a convenience alias of tpidruro. The argument auto uses the best available method for the selected processor. Default setting is auto.
  • -mtls-dialect=dialect — specifies the dialect for accessing thread local storage, and there are exactly two: gnu and gnu2 🌸 gnu selects the original GNU scheme for supporting local and global dynamic TLS models. gnu2 selects the GNU descriptor scheme, which gives better performance for shared libraries. The GNU descriptor scheme is compatible with the original scheme but does require new assembler, linker and library support. Initial and local exec TLS models are unaffected by this option and always use the original scheme.
  • -mword-relocations — only generate absolute relocations on word-sized values (i.e. R_ARM_ABS32). Enabled by default on targets (uClinux, SymbianOS) where the runtime loader imposes this restriction, and when -fpic or -fPIC is specified. This option conflicts with -mslow-flash-data ⚠️
  • -mfix-cortex-m3-ldrd — some Cortex-M3 cores can cause data corruption when "ldrd" instructions with overlapping destination and base registers are used 😭 this option avoids generating those instructions. Enabled by default when -mcpu=cortex-m3 is specified.
  • -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 🔥 enabled by default when either -mcpu=cortex-a57 or -mcpu=cortex-a72 is specified.
  • -munaligned-access, -mno-unaligned-access — enables (or disables) reading and writing of 16- and 32-bit values from addresses that aren't 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 isn't 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 ✨

  • -mneon-for-64bits — this option is deprecated and has no effect 💀 it's giving ghost of options past.
  • -mslow-flash-data — assumes loading data from flash is slower than fetching instructions, so literal load is minimized for better performance. Only supported when compiling for ARMv7 M-profile, and 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, and 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. On by default for ARMv8-A Thumb mode.
  • -mprint-tune-info — prints CPU tuning information as a comment in the assembler file 🎀 this one is only for regression testing of the compiler and is not intended for ordinary use in compiling code. Disabled by default.
  • -mverbose-cost-dump — enables verbose cost model dumping in the debug dump files. Provided for use in debugging the compiler, iykyk.
  • -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 attribute "SHF_ARM_PURECODE" 💜 only available when generating non-pic code for M-profile targets.
  • -mcmse — generates secure code as 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 — mitigates against a potential security issue with the "VLLDM" instruction in some M-profile devices when using CMSE (CVE-2021-365465) 😭 enabled by default when the option -mcpu= is used with "cortex-m33", "cortex-m35p", "cortex-m52", "cortex-m55", "cortex-m85" or "star-mc1". The option -mno-fix-cmse-cve-2021-35465 can be used to disable the mitigation.
  • -mstack-protector-guard=guard, -mstack-protector-guard-offset=offset — generate stack protection code using a canary at guard. Supported locations are global for a global canary, or tls for a canary accessible via the TLS register 🌸 the option -mstack-protector-guard-offset= is for use with -fstack-protector-guard=tls and not for use in user-land code.
  • -mfdpic, -mno-fdpic — select the FDPIC ABI, which uses 64-bit function descriptors to represent pointers to functions 🦋 when the compiler is configured for "arm-*-uclinuxfdpiceabi" targets, this option is on by default and implies -fPIE if none of the PIC/PIE-related options is provided. On other targets, it only enables the FDPIC-specific code generation features, and you (yes, you) should explicitly provide the PIC/PIE-related options as needed.
Note that static linking is not supported, because it would still involve the dynamic linker when the program self-relocates. If such behavior is acceptable, use -static and -Wl,-dynamic-linker options.

The opposite -mno-fdpic option is useful (and required) to build the Linux kernel using the same ("arm-*-uclinuxfdpiceabi") toolchain as the one used to build the userland programs 💅

  • -mbranch-protection=none|standard|pac-ret[+leaf][+bti]|bti[+pac-ret[+leaf]] — enable branch protection features (armv8.1-m.main only) 🔥
    • none — generate code without branch protection or return address signing.
    • standard[+leaf] — generate code with all branch protection features enabled at their standard level.
    • pac-ret[+leaf] — generate code with return address signing set to its standard level, which is to sign all functions that save the return address to memory.
    • leaf — when return address signing is enabled, also sign leaf functions even if they don't write the return address to memory.
    • +bti — add landing-pad instructions at the permitted targets of indirect branch instructions.

If the +pacbti architecture extension isn't enabled, 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 will remain backwards-compatible with earlier versions of the architecture, but the additional 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.

🌟 AVR Options 🌟

These options are defined for AVR implementations:

  • -mmcu=mcu — specify the AVR instruction set architecture (ISA) or device type. The default for this option is "avr2" 🎀

The following AVR devices and ISAs are supported. Note: 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 (https://gcc.gnu.org/wiki/avr-gcc#spec-files) file "specs-mcu" 💖 only the latter is provided by the compiler, according to the supported "mcu"s below. The rest is supported by AVR-LibC (https://github.com/avrdudes/avr-libc/), or by means of "atpack" (https://gcc.gnu.org/wiki/avr-gcc#atpack) files from the hardware manufacturer.

"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

So these are the "Classic" girlies with up to 8 KiB of program memory, and they do have the MOVW instruction 💅 — 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, 16 KiB up to 64 KiB of program memory. mcu = at76c711, at43usb355, periodt.

avr31

"Classic" devices with 128 KiB of program memory — big girls. mcu = atmega103, at43usb320.

avr35

"Classic" devices with 16 KiB up to 64 KiB of program memory, and they've got the MOVW instruction — ate. 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, 16 KiB up to 64 KiB of program memory — this list is long, bestie, grab a snack 🍓 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 a 3-byte PC — that's the whole point, it means 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, plus the program memory is visible in the RAM address space (iconic behaviour, tbh) 🌈 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, avr32da28, avr32da28s, avr32da32, avr32da32s, avr32da48, avr32da48s, avr32db28, avr32db32, avr32db48, avr32dd14, avr32dd20, avr32dd28, avr32dd32, avr32du14, avr32du20, avr32du28, avr32du32, avr32ea28, avr32ea32, avr32ea48, 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 — she's got range 💖 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 — smol but valid 🎀 mcu = attiny4, attiny5, attiny9, attiny10, attiny102, attiny104, attiny20, attiny40.

avr1

Okay so this ISA is implemented by the minimal AVR core and it's supported for assembler only — no cap, that's just the situation. mcu = attiny11, attiny12, attiny15, attiny28, at90s1200.

  • -mabsdata — tells the compiler to assume that all the data in static storage can be reached with LDS / STS instructions. Heads up though: this option only does anything on reduced Tiny devices like ATtiny40. Also peep the absdata AVR Variable Attributes variable attribute 👀
  • -mcvt — use a compact vector table 🔥 Some devices support a CVT with only four entries: 0=Reset, 1=NMI, 2=Prio1 IRQ, 3=Prio0 IRQs. With this option the startup code gets linked from crtmcu-cvt.o instead of the usual crtmcu.o. And it's not just a compact vector table — the startup code will also set the bit CPUINT_CTRLA.CPUINT_CVT, which enables the CVT on the device. ✨

If you don't want the startup code setting CPUINT_CTRLA.CPUINT_CVT, then you can satisfy the symbol __init_cvt so that the respective code is no longer pulled in from libmcu.a. For example, you can link with -Wl,--defsym,__init_cvt=0 💅

The CVT startup code is available since AVR-LibC v2.3 (https://github.com/avrdudes/avr-libc/issues/1010).

  • -mdouble=bits
  • -mlong-double=bits — these set the size (in bits) of the double or long double type, respectively. Possible values for bits are 32 and 64. Now whether or not a specific 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 the same applies for the default values of the options. So like, check your config, bestie 🦋
  • -mgas-isr-prologues — interrupt service routines (ISRs) may use the __gcc_isr pseudo instruction supported by GNU Binutils. If this option is on, you can still disable the feature for individual ISRs by means of the AVR Function Attributes no_gccisr function attribute. This feature is activated per default if optimization is on (but not with -Og, @pxref{Optimize Options}), and if GNU Binutils support PR21683 (https://sourceware.org/PR21683).
  • -mint8 — assumes int is an 8-bit integer. Yeah, this affects the sizes of all the types: a char is 1 byte, an int is 1 byte, a long is 2 bytes, and long long is 4 bytes. Please note that this option does not conform to the C standards 😭 but it results in smaller code size, so, trade-offs.
  • -mmain-is-OS_task — do not save registers in main. The effect is the same like attaching the attribute AVR Function Attributes OS_task to main. It's activated per default if optimization is on.
  • -mno-call-main — don't run main by means of:
XCALL  main
XJMP   exit

Instead, put main in section .init9 (https://avrdudes.github.io/avr-libc/avr-libc-user-manual/mem_sections.html#sec_dot_init) so that no call is required. By setting this option the user asserts that main will not return ⚠️

This option can be used for devices with very limited resources in order to save a few bytes of code and stack space. It will work as expected since AVR-LibC v2.3 (https://github.com/avrdudes/avr-libc/issues/1012). With older versions, there will be no performance gain. Ngl, kind of a nothing-burger on old versions.

  • -mno-interrupts — the code we spit out is not compatible with hardware interrupts, bestie. The trade-off? Smaller code size. That's it. 💅
  • -mrelax — try to swap "CALL" resp. "JMP" instructions for the shorter "RCALL" resp. "RJMP" ones when it applies. Setting -mrelax just adds the --mlink-relax option to the assembler's command line and the --relax option to the linker's command line. 🎀

Jump relaxing is literally done by the linker, because jump offsets aren't known before the code is actually located. So the assembler code the compiler hands over is the same — but the instructions in the executable may differ from the ones in the assembler code. ✨

Relaxing must be turned on if linker stubs are needed — see the section on "EIND" and linker stubs below. ⚠️
  • -mrodata-in-ram, -mno-rodata-in-ram — put the ".rodata" sections for read-only data in RAM resp. in program memory. For most devices there's genuinely no choice and this option kinda acts like an assertion, ngl. 🫶

Since v14 and for the AVR64* and AVR128* devices, ".rodata" lives in flash memory per default — provided the required GNU Binutils support (PR31124 ("https://sourceware.org/PR31124")) is available. In that case, -mrodata-in-ram can be used 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 the whole vibe. 🧚
  • -mfract-convert-truncate — allow truncation instead of rounding towards zero for fractional fixed-point types. Iconic little permission slip, honestly. 💜
  • -nodevicelib — don't link against AVR-LibC's device specific library "libmcu.a". 🙈
Notice that since AVR-LibC v2.3, that library contains code that is essential for the correct functioning of a program. In particular, it contains parts of the startup code like: "__init_sp" ("https://github.com/avrdudes/avr-libc/issues/1011") to initialize the stack pointer with 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 call "main" and "exit", and "__do_flmap_init" ("https://github.com/avrdudes/avr-libc/issues/931") to set up FLMAP according to symbol "__flmap". 😭
  • -nodevicespecs — don't add -specs=device-specs/specs-mcu to the compiler driver's command line. The user takes responsibility for supplying the sub-processes like compiler proper, assembler and linker with appropriate command line options. That means you have to supply her private device specs file by means of -specs=path-to-specs-file. There's no more need for option -mmcu=mcu. 👑

This option can also serve as a replacement for the older way of specifying custom device-specs files that needed -B some-path to point to a directory which contains 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 __vector prefix. Enabled by default, so it's already on the case for you. 💅

✨ AVR Optimization Options ✨

Okay so the following options are pure optimization options, no cap. 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. Slight main-character exception energy, honestly. 💀

  • -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. See also the -fdefer-pop Optimize Options,,optimization option. 🎀

Popping the arguments after the function call can be expensive on AVR, so accumulating the stack space might lead to smaller executables, because arguments need not be removed from the stack after such a function call. 🫶

This option can lead to reduced code size for functions that perform several calls to functions that get their arguments on the stack — like calls to printf-like functions. Slay. ✨

  • *-mbranch-cost=cost* — set the branch costs for conditional branch instructions to cost. Reasonable values for cost are small, non-negative integers. The default branch cost is 0. 🦋
  • -mcall-prologues — function prologues/epilogues are expanded as calls to appropriate subroutines. Code size is smaller. That's the trade, bestie. 💜
  • -mfuse-add, -mno-fuse-add, *-mfuse-add=level* — optimize indirect memory accesses on reduced Tiny devices. The default uses "level=1" for optimizations -Og and -O1, and "level=2" for higher optimizations. Valid values for level are 0, 1 and 2. 🌸
  • -mfuse-move, -mno-fuse-move, *-mfuse-move=level* — run a post reload optimization pass that tries to fuse move instructions and to split multi-byte instructions into 8-bit operations. The default uses "level=3" for optimization -O1, and "level=23" for higher optimizations. Valid values for level are in the range 0 ... 23, which is a 3:2:2:2 mixed radix value. Each digit controls some aspect of the optimization. Numbers with lore, fr fr. 🔢
  • -mfuse-move2 — run a post combine optimization pass that tries to fuse move instructions. 🧚
  • -mstrict-X — use address register "X" in a way proposed by the hardware. This means that "X" is only used in indirect, post-increment or pre-decrement addressing. 👑

Without this option, the "X" register may be used in the same way as "Y" or "Z" — which then gets emulated by additional instructions. For example, loading a value with "X+const" addressing with a small non-negative "const < 64" to a register Rn is performed as:

        adiw r26, const   ; X += const
        ld   <Rn>, X        ; <Rn> = *X
        sbiw r26, const   ; X -= const
  • -msplit-bit-shift — split multi-byte shifts with a constant offset into a shift with a byte offset and a residual shift with a non-byte offset. This optimization is turned on per default for -O2 and higher, including -Os but excluding -Oz. Splitting of shifts with a constant offset that is a multiple of 8 is controlled by -mfuse-move. ✨
  • -msplit-ldst — split multi-byte loads and stores into several byte loads and stores. This optimization is turned on per default for -O2 and higher. 💅
  • -muse-nonzero-bits — enable optimizations that are only possible when some bits in a register are always zero. This optimization is turned on per default for -O2 and 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's 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 are subject to some limitations:

  • The compiler never sets "EIND". 🙈
  • The compiler uses "EIND" implicitly in "EICALL"/"EIJMP" instructions, or might read "EIND" directly in order to emulate an indirect call/jump by means of a "RET" instruction. 🌀
  • The compiler assumes that "EIND" never changes during the startup code or during the application. In particular, "EIND" is not saved/restored in function or interrupt service routine prologue/epilogue. 💀
  • For indirect calls to functions and computed goto, the linker generates stubs. Stubs are jump pads, sometimes also called trampolines. So the indirect call/jump jumps to such a stub. The stub contains a direct jump to the desired address. 🎯
  • Linker relaxation must be turned on so that the linker generates the stubs correctly in all situations. See the compiler option -mrelax and the linker option --relax. There are corner cases where the linker is supposed to generate stubs but aborts without relaxation and without a helpful error message. 😭
  • The default linker script is arranged for code with "EIND = 0". If code is supposed to work for a setup with "EIND != 0", a custom linker script has to be used in order to place the sections whose name start with .trampolines into the segment where "EIND" points to. 🧭
  • The startup code from libgcc never sets "EIND". Notice that startup code is a blend of code from libgcc and AVR-LibC. For the impact of AVR-LibC on "EIND", see the AVR-LibC user manual ("https://avrdudes.github.io/avr-libc/avr-libc-user-manual/"). 💜
  • It is legitimate for user-specific startup code to set up "EIND" early — for example by means of initialization code located in section .init3. Such code runs prior to general startup code that initializes RAM and calls constructors, but after the bit of startup code from AVR-LibC that sets "EIND" to the segment where the vector table is located. 🌸
        #include <avr/io.h>

        static void
        __attribute__((section(".init3"),naked,used,no_instrument_function))
        init3_set_eind (void)
        {
          __asm volatile ("ldi r24,pm_hh8(__trampolines_start)\n\t"
                          "out %i0,r24" :: "n" (&EIND) : "r24","memory");
        }

The "__trampolines_start" symbol is defined in the linker script. 📜

  • Stubs are generated automatically by the linker if the following two conditions are met:
    • <The address of a label is taken by means of the "gs" modifier> (short for generate stubs) like so:
                LDI r24, lo8(gs(<func>))
                LDI r25, hi8(gs(<func>))
  • <The final location of that label is in a code segment> outside the segment where the stubs are located.
  • The compiler emits such "gs" modifiers for code labels in the following situations: 🎀
    • <Taking address of a function or code label.>
    • <Computed goto.>
    • <If prologue-save function is used, see -mcall-prologues> command-line option.
    • <Switch/case dispatch tables. If you do not want such dispatch> tables you can specify the -fno-jump-tables command-line option.
  • C and C++ constructors/destructors that get called during startup/shutdown.
  • If the tools hit a gs() modifier explained above.
  • Jumping straight to non-symbolic addresses like this? Yeah, no, literally not supported:
int main (void)
{
    /* Call function at word address 0x2 */
    return ((int(*)(void)) 0x2)();
}

Instead you gotta set up a stub, i.e. call the function 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 then link the application with -Wl,--defsym,func_4=0x4. Or, like, alternatively func_4 can be defined in the linker script. Either works 💅

🌸 Handling of the RAMPD, RAMPX, RAMPY and RAMPZ Special Function Registers 🌸

So some AVR devices have memories bigger than the 64 KiB range that 16-bit pointers can reach. To get to memory outside that 64 KiB range, the content of a RAMP register gets used as the high part of the address: the X, Y, Z address register gets concatenated with the RAMPX, RAMPY, RAMPZ special function register, respectively, to build a wide address. Same energy for RAMPD, which pairs up with direct addressing.

  • The startup code initializes the RAMP special function registers with zero.
  • If a named address space other than generic or __flash is used (iykyk, that's the AVR Named Address Spaces thing), then RAMPZ is set as needed before the operation.
  • If the device supports RAM larger than 64 KiB and the compiler needs to change RAMPZ to accomplish an operation, RAMPZ is reset to zero after the operation.
  • If the device comes with a specific RAMP register, the ISR prologue/epilogue saves/restores that SFR and initializes it with zero in case the ISR code might (implicitly) use it.
  • RAM larger than 64 KiB is not supported by GCC for AVR targets. So if you're using inline assembler to read from locations outside the 16-bit address range and you change one of the RAMP registers, you must reset it to zero after the access. No cap, just do it 🙈

🦋 AVR Built-in Macros 🦋

GCC defines several built-in macros so your code can vibe-check for the presence or absence of features. Almost all of the built-in macros below get deduced from device capabilities, which means they're triggered by the -mmcu= command-line option.

For even more AVR-specific built-in macros, go see AVR Named Address Spaces and AVR Built-in Functions ✨

  • __AVR_ARCH__ — build-in macro that resolves to a decimal number that identifies the architecture and depends on the -mmcu=mcu option. Possible values are:
2, 25, 3, 31, 35, 4, 5, 51, 6

for mcu=avr2, avr25, avr3, avr31, avr35, avr4, avr5, avr51, avr6,

respectively and

100, 102, 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=device defines this built-in macro, which reflects the device's name. For example, -mmcu=atmega8 defines the built-in macro __AVR_ATmega8__, -mmcu=attiny261a defines __AVR_ATtiny261A__, etc.

The built-in macros' names follow the scheme __AVR_Device__ where Device is the device name as from the AVR user manual. The difference between Device in the built-in macro and device in -mmcu=device is that the latter is always lowercase.

If device is not a device but only a core architecture like avr51, this macro is not defined.

  • __AVR_DEVICE_NAME__ — setting -mmcu=device defines this built-in macro to the device's name. For example, with -mmcu=atmega8 the macro is defined to atmega8.

If device is not a device but only a core architecture like avr51, this macro is not defined.

  • __AVR_CVT__ — the code is being compiled with option -mcvt to use a compact vector table.
  • __AVR_XMEGA__ — the device / architecture belongs to the XMEGA family of devices.
  • __AVR_HAVE_ADIW__ — the device has the ADIW and SBIW instructions.
  • __AVR_HAVE_ELPM__ — the device has the ELPM instruction.
  • __AVR_HAVE_ELPMX__ — the device has the ELPM Rn,Z and ELPM Rn,Z+ instructions.
  • __AVR_HAVE_LPMX__ — the device has the LPM Rn,Z and LPM Rn,Z+ instructions.
  • __AVR_HAVE_MOVW__ — the device has the MOVW instruction to perform 16-bit register-register moves.
  • __AVR_HAVE_MUL__ — the device has a hardware multiplier.
  • __AVR_HAVE_JMP_CALL__ — the device has the JMP and CALL instructions. This is the case for devices with more than 8 KiB of program memory.
  • __AVR_HAVE_EIJMP_EICALL__
  • __AVR_3_BYTE_PC__

The device has the EIJMP and EICALL instructions. This is the case for devices with more than 128 KiB of program memory. This also means that 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__
  • __AVR_HAVE_16BIT_SP__

The stack pointer (SP) register is treated as 8-bit respectively 16-bit register by the compiler. The definition of these macros is affected by -mtiny-stack.

  • __AVR_HAVE_SPH__
  • __AVR_SP8__

The device has the SPH (high part of stack pointer) special function register or has an 8-bit stack pointer, respectively. The definition of these macros is affected by -mmcu= and in the cases of -mmcu=avr2 and -mmcu=avr25 also by -msp8.

  • __AVR_HAVE_RAMPD__
  • __AVR_HAVE_RAMPX__
  • __AVR_HAVE_RAMPY__
  • __AVR_HAVE_RAMPZ__

The device has the RAMPD, RAMPX, RAMPY, RAMPZ special function register, respectively.

  • __NO_INTERRUPTS__ — this macro reflects the -mno-interrupts command-line option.
  • __AVR_ERRATA_SKIP__
  • __AVR_ERRATA_SKIP_JMP_CALL__

Some AVR devices (AT90S8515, ATmega103) must not skip 32-bit instructions because of a hardware erratum. Skip instructions are SBRS, SBRC, SBIS, SBIC and CPSE. The second macro is only defined if __AVR_HAVE_JMP_CALL__ is also set.

  • __AVR_ISA_RMW__ — the device has Read-Modify-Write instructions (XCH, LAC, LAS and LAT).
  • __AVR_SFR_OFFSET__=offset — instructions that can address I/O special function registers directly like IN, OUT, SBI, etc. may use a different address as if addressed by an instruction to access RAM like LD or STS. This offset depends on the device architecture and has to be subtracted from the RAM address in order to get the respective I/O address.
  • __AVR_SHORT_CALLS__ — the -mshort-calls command line option is set.
  • __AVR_PM_BASE_ADDRESS__=addr — some devices support reading from flash memory by means of LD* instructions. The flash memory is seen 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 defined, the address space is linear and there's no need to put .rodata into RAM. This is handled by the default linker description file, and is currently available for avrtiny and avrxmega3. Even more convenient: there's no need to use address spaces like __flash or features like attribute progmem and pgm_read_*.
  • __AVR_HAVE_FLMAP__ — this macro is defined provided the following conditions are met:
    • The device has the NVMCTRL_CTRLB.FLMAP bitfield. This applies to the AVR64* and AVR128* devices.
    • It's not known at assembler-time which emulation will be used.

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 is located in RAM; and defined to 0, otherwise.

  • __WITH_AVRLIBC__ — the compiler is configured to be used together with AVR-LibC. See the --with-avrlibc configure option.
  • __HAVE_SIGNAL_N__ — the compiler supports the signal(num) and interrupt(num) function attributes with an argument num that specifies the number of the interrupt service routine.
  • __HAVE_DOUBLE_MULTILIB__ — defined if -mdouble= acts as a multilib option.
  • __HAVE_DOUBLE32__
  • __HAVE_DOUBLE64__

Defined if the compiler supports 32-bit double resp. 64-bit double. The actual layout is specified by option -mdouble=.

  • __DEFAULT_DOUBLE__ — the size in bits of double if -mdouble= is not set. To test the layout of double in a program, use the built-in macro __SIZEOF_DOUBLE__.
  • __HAVE_LONG_DOUBLE32__
  • __HAVE_LONG_DOUBLE64__
  • __HAVE_LONG_DOUBLE_MULTILIB__
  • __DEFAULT_LONG_DOUBLE__

Same as above, but for long double instead of double.

  • __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__
  • __WITH_LIBF7_MATH__
  • __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 🧚

The following options are used internally by the compiler and to communicate between device specs files and the compiler proper. You do not need to set these by hand — in particular, they are not optimization options. Using these in 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 files (https://gcc.gnu.org/wiki/avr-gcc#spec-files).

  • -mn-flash=num — assume that the flash memory has a size of num times 64 KiB. This determines which __flashN address spaces are available.
  • -mflmap — the device has the FLMAP bit field located in special function register NVMCTRL_CTRLB.
  • -mrmw — assume that the device supports the Read-Modify-Write instructions XCH, LAC, LAS and LAT.
  • -mshort-calls — okay so this one tells GCC to assume that RJMP and RCALL can reach the whole program memory 🎯. It's the option used for multilib generation and selection for devices from architecture avrxmega3, iykyk.
  • -mskip-bug — generates code without skips (CPSE, SBRS, SBRC, SBIS, SBIC) over 32-bit instructions 🫶. No skips, no drama.
  • -msp8 — treats the stack pointer register like it's an 8-bit register, like, assumes the high byte of the stack pointer is just zero 💅. The compiler uses this one to select and build multilibs for architectures avr2 and avr25 — those architectures mix devices with and without SPH, so yeah, chaos, and this is the fix.

🖤 Blackfin Options 🖤

  • -mcpu=cpu[-sirevision] — names the target Blackfin processor, bestie. Right now cpu can be one of: bf512, bf514, bf516, bf518, bf522, bf523, bf524, bf525, bf526, bf527, bf531, bf532, bf533, bf534, bf536, bf537, bf538, bf539, bf542, bf544, bf547, bf548, bf549, bf542m, bf544m, bf547m, bf548m, bf549m, bf561, bf592.

The optional sirevision specifies the silicon revision of the target Blackfin processor, and any workarounds available for the targeted silicon revision get 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 — and okay, careful here 🙈: if sirevision is none, __SILICON_REVISION__ is not defined; if sirevision is any, __SILICON_REVISION__ is defined to be 0xffff. If this optional sirevision isn't used at all, GCC just assumes the latest known silicon revision of the targeted Blackfin processor. Easy.

GCC also 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 — but only if -msim is not given.

Without this option, bf532 is used as the processor by default 🌸.

Real talk: support for bf561 is incomplete 😭. For bf561, only the preprocessor macro is defined. That's it. That's the whole thing.
  • -msim — says the program is gonna run on the simulator, so the simulator BSP provided by libgloss gets linked in 🎀. Only has an effect for the bfin-elf toolchain. Also, certain other options — like -mid-shared-library and -mfdpic — imply -msim, sneaky little things.
  • -momit-leaf-frame-pointer — don't keep the frame pointer in a register for leaf functions 🍃. This avoids all those instructions to save, set up and restore frame pointers, and frees up an extra register in leaf functions. Slay, honestly.
  • -mspecld-anomaly — when enabled, the compiler makes sure the generated code doesn't contain speculative loads after jump instructions 🔥. If you use this option, __WORKAROUND_SPECULATIVE_LOADS is defined.
  • -mno-specld-anomaly — don't generate the extra code to prevent speculative loads from happening. That's the whole vibe.
  • -mcsync-anomaly — when enabled, the compiler ensures the generated code doesn't contain CSYNC or SSYNC instructions too soon after conditional branches ⭐. If this option is used, __WORKAROUND_SPECULATIVE_SYNCS is defined.
  • -mno-csync-anomaly — don't generate extra code to stop CSYNC or SSYNC instructions from occurring too soon after a conditional branch.
  • -mlow64k — when enabled, the compiler is free to take advantage of the knowledge that the entire program fits into the low 64k of memory 💜. Cute and small.
  • -mno-low64k — assume the program is arbitrarily large. And this one? This is the default 👑.
  • -mstack-check-l1 — does stack checking using information placed into L1 scratchpad memory by the uClinux kernel 🧚.
  • -mid-shared-library — generates 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. And with a bfin-elf target, this option implies -msim too.
  • -mno-id-shared-library — generates code that does not assume ID-based shared libraries are being used. This is the default 💅.
  • -mleaf-id-shared-library — generates 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 lets the compiler use faster code for jumps and calls. Speed, bestie.
  • -mno-leaf-id-shared-library — do not assume the code being compiled won't link against any ID shared libraries. Result: slower code for jump and call insns 😭.
  • -mshared-library-id=n — specifies the identification number of the ID-based shared library being compiled 🎯. Specifying a value of 0 generates more compact code; specifying other values forces the allocation of that number to the current library, but is no more space- or time-efficient than just... omitting this option. So like, why.
  • -msep-data — generates code that allows the data segment to be located in a different area of memory from the text segment 🦋. This allows for execute in place in an environment without virtual memory management, by eliminating relocations against the text section.
  • -mno-sep-data — generates code that assumes the data segment follows the text segment. This is the default 🌸.
  • -mlong-calls, -mno-long-calls — tells the compiler to perform function calls by first loading the address of the function into a register, and then performing a subroutine call on this register 💖. You need this switch if the target function lies outside of the 24-bit addressing range of the offset-based version of subroutine call instruction.

This feature is not enabled by default. Specifying -mno-long-calls restores the default behavior. And heads up 🙈: these switches have no effect on how the compiler generates code to handle function calls via function pointers.

  • -mfast-fp — links with the fast floating-point library ⚡. This library relaxes some of the IEEE floating-point standard's rules for checking inputs against Not-a-Number (NAN), in the interest of performance. Trade-offs, bestie.
  • -minline-plt — enables inlining of PLT entries in function calls to functions that are not known to bind locally 🎀. It has no effect without -mfdpic. None. Zero. Nada.
  • -mmulticore — builds a standalone application for multicore Blackfin processors 👑. This option causes proper start files and link scripts supporting multicore to be used, and defines the macro __BFIN_MULTICORE. It can only be used with -mcpu=bf561[-sirevision].

This option can be used with -mcorea or -mcoreb, which selects the one-application-per-core programming model. Without -mcorea or -mcoreb, the single-application/dual-core programming model is used — and 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 used.

  • -mcorea — builds a standalone application for Core A of BF561 when using the one-application-per-core programming model 🅰️. Proper start files and link scripts are used to support Core A, and the macro __BFIN_COREA is defined. This option can only be used in conjunction with -mmulticore. Periodt.
  • -mcoreb — builds a standalone application for Core B of BF561 when using the one-application-per-core programming model 🅱️. Proper start files and link scripts are used to support Core B, and the macro __BFIN_COREB is defined. When this option is used, coreb_main should be used instead of main. Also can only be used in conjunction with -mmulticore.
  • -msdram — builds a standalone application for SDRAM 💾. Proper start files and link scripts are used to put the application into SDRAM, and the macro __BFIN_SDRAM is defined. The loader should initialize SDRAM before loading the application — don't skip that step, bestie 🫶.
  • -micplb — assumes 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 — 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. Permissible names are: c62x, c64x, c64x+, c67x, c67x+, c674x.
  • -mbig-endian — generate code for a big-endian target 🐘.
  • -mlittle-endian — generate code for a little-endian target 🐭. This is the default.
  • -msim — choose startup files and linker script suitable for the simulator 🎮.
  • -msdata=default — put small global and static data in the .neardata section, which is pointed to by register B14 📍. Put small uninitialized global and static data in the .bss section, which is adjacent to the .neardata section. Put small read-only data into the .rodata section. The corresponding sections used for large pieces of data are .fardata, .far and .const — big data gets its own neighborhoods 💅.
  • -msdata=all — put all data, not just small objects, into the sections reserved for small data, and use addressing relative to the B14 register to access them 🔥.
  • -msdata=none — make no use of the sections reserved for small data, and use absolute addresses to access all data 🙅. Put all initialized global and static data in the .fardata section, all uninitialized data in the .far section, and all constant data into the .const section.

🦋 CRIS Options 🦋

These options are defined specifically for the CRIS ports, iykyk 🫶.

  • -march=architecture-type, -mcpu=architecture-type — generate code for the specified architecture ✨. The choices for architecture-type are v3, v8 and v10, for respectively ETRAX 4, ETRAX 100, and ETRAX 100 LX. Default is v0.
  • -mtune=architecture-type — tune to architecture-type everything applicable about the generated code, except for the ABI and the set of available instructions 🎀. The choices for architecture-type are the same as for -march=architecture-type.
  • -mmax-stack-frame=n — warn when the stack frame of a function exceeds n bytes ⚠️. Keep an eye on it.
  • -metrax4, -metrax100 — these two are literally just synonyms for -march=v3 and -march=v8 respectively 💖.
  • -mmul-bug-workaround, -mno-mul-bug-workaround — work around a bug in the muls and mulu instructions for CPU models where it applies 🐛. This option is disabled by default.
  • -mpdebug — turns on CRIS-specific verbose debug info in the assembly code ✨ and also (bonus chaos) kills the #NO_APP formatted-code indicator that normally goes to the assembler at the top of the assembly file.
  • -mcc-init — bestie, stop reusing condition-code results from previous instructions. Always emit compare and test instructions before condition codes get used. No leftovers 😤
  • -mno-side-effects — don't emit instructions with side effects in any addressing mode other than post-increment. That's it, that's the whole vibe check.
  • -mstack-align, -mno-stack-align, -mdata-align, -mno-data-align, -mconst-align, -mno-const-align — these (and their no- twins) arrange — or eliminate the arrangement of — the stack frame, individual data and constants so they're aligned for the maximum single data access size of the CPU model you picked 👑. Default is to arrange for 32-bit alignment. Important: ABI details like structure layout are not affected by these options, so no surprises there, iykyk.
  • -m32-bit, -m16-bit, -m8-bit — same energy as the stack/data/const-align options above, but these arrange for stack frame, writable data and constants to all be 32-bit, 16-bit or 8-bit aligned respectively. Default is 32-bit alignment 💅
  • -mno-prologue-epilogue, -mprologue-epilogue — with -mno-prologue-epilogue, the normal function prologue and epilogue that set up the stack frame get yeeted, and no return instructions or return sequences are generated 🫠
Real talk: only use this one while visually inspecting the compiled code. You get no warnings or errors even when call-saved registers must be saved, or when storage for local variables needs to be allocated. Silent danger, fr fr 💀
  • -melf — legacy no-op option. It's giving "just here for the vibes" ✨
  • -sim — arranges to link with input-output functions from a simulator library. Code, initialized data and zero-initialized data get allocated consecutively 🌸
  • -sim2 — like -sim, but it also passes linker options to locate initialized data at 0x40000000 and zero-initialized data at 0x80000000 🎯

🦋 C-SKY Options 🦋

GCC has these options when compiling for C-SKY V2 processors 💖

  • -march=arch — pick the C-SKY target architecture. Valid values for arch are: ck801, ck802, ck803, ck807, and ck810. Default is ck810 ⭐
  • -mcpu=cpu — pick 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, and ck810ftv. Yes, that's a lot of CPUs. Yes, we listed every single one 😭 no skimping here.
  • -mbig-endian, -EB, -mlittle-endian, -EL — select big- or little-endian code. Default is little-endian 🐣
  • -mfloat-abi=name — specifies which floating-point ABI to use. Permissible values: soft, softfp and hard.
    • soft makes GCC generate output containing library calls for floating-point operations.
    • softfp lets it generate code using hardware floating-point instructions, but still uses the soft-float calling conventions.
    • hard allows generation of floating-point instructions and uses FPU-specific calling conventions.

The default depends on the specific target configuration. Note that the hard-float and soft-float ABIs are not link-compatible — you must compile your entire program with the same ABI and link with a compatible set of libraries. No mixing, bestie 🚫

  • -mhard-float, -msoft-float — select hardware or software floating-point implementations. Default is soft float 💜
  • -mdouble-float, -mno-double-float — when -mhard-float is in effect, enable generation of double-precision float instructions. This is the default except when compiling for CK803 🙈
  • -mfdivdu, -mno-fdivdu — when -mhard-float is in effect, enable generation of frecipd, fsqrtd, and fdivd instructions. This is the default except when compiling for CK803 ✨
  • -mfpu=fpu — select the floating-point processor. Can only be used with -mhard-float. Values for fpu are fpv2_sf (equivalent to -mno-double-float -mno-fdivdu), fpv2 (-mdouble-float -mno-divdu), and fpv2_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 👑
  • -mmp — enable multiprocessor instructions; default is off.
  • -mcp — enable coprocessor instructions; default is off.
  • -mcache — enable coprocessor instructions; default is off. (Yep, same wording as above — we're just relaying the lore 💅)
  • -msecurity — enable C-SKY security instructions; default is off.
  • -mtrust — enable C-SKY trust instructions; default is off.
  • -mdsp, -medsp, -mvdsp — 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 16-bit instructions can be used. 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 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. This option defaults to on.
  • -mmultiple-stld, -mstm, -mno-multiple-stld, -mno-stm — 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 option is the default and is required for correct code generation on CK801 and CK802; it's optional on other processors.
  • -mstack-size, -mno-stack-size — emit .stack_size directives for each function in the assembly output. This option defaults to off.
  • -mccrt, -mno-ccrt — generate code for the C-SKY compiler runtime instead of libgcc. This option defaults to off.
  • -mbranch-cost=n — set the branch costs to roughly n instructions. Default is 1 ⚖️
  • -msched-prolog, -mno-sched-prolog — permit scheduling of function prologue and epilogue sequences.
Careful, bestie: using this can result in code that is not compliant with the C-SKY V2 ABI prologue requirements and that cannot be debugged or backtraced 😭 It's disabled by default.
  • -msim — links the library libsemi.a, which is compatible with the simulator. Applicable to ELF compiler only 🦋

🎀 Cygwin and MinGW Options 🎀

These additional options are available for Microsoft Windows targets 💻✨

  • -mconsole — specifies that a console application is to be generated, by telling the linker to set the PE header subsystem type required for console applications. 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 that CRT library and links with that 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.

Generally speaking, changing the CRT DLL requires recompiling the entire MinGW CRT.

This option is for experimental and testing purposes only 🙈 Available for MinGW targets.
  • -mdll — available for Cygwin and MinGW targets. Specifies that a DLL — a dynamic link library — is to be generated, enabling the selection of the required runtime startup object and entry point 🧩
  • -mnop-fun-dllimport — available for Cygwin and MinGW targets. Specifies that the dllimport attribute should be ignored 💅
  • -mthreads — available for MinGW targets. Specifies that MinGW-specific thread support is to be used 🧵
  • -municode — available for MinGW-w64 targets. Causes the UNICODE preprocessor macro to be predefined, and chooses Unicode-capable runtime startup code 🌈
  • -mwin32 — available for Cygwin and MinGW targets. Specifies that the typical Microsoft Windows predefined macros are to be set in the pre-processor, but does not influence the choice of runtime library/startup code.
  • -mwindows — available for Cygwin and MinGW targets. Specifies that a GUI application is to be generated by telling the linker to set the PE header subsystem type appropriately 🪟✨
  • -fno-set-stack-executable — available for MinGW targets. 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, since there the User32 API — which is used to set executable privileges — isn't available 💀 (kernel mode said no thanks to User32).
  • -fwritable-relocated-rdata — okay so this one is available for MinGW and Cygwin targets only. It specifies that relocated-data in a read-only section gets put into the .data section. That's necessary for older runtimes that don't support modification of .rdata sections for pseudo-relocation. 💅
  • -mpe-aligned-commons — Cygwin and MinGW again, bestie. It says 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. ✨

See also under x86 Options for standard options. 👀

🌈 Darwin Options 💖

These options are defined for all architectures running the Darwin operating system. 🍓

FSF GCC on Darwin does not create "fat" object files; it creates an object file for the single architecture that GCC was built to target. Apple's GCC on Darwin does create "fat" files if multiple -arch options are used; it does so by running the compiler or linker multiple times and joining the results together with lipo. 😭

The subtype of the file created (like ppc7400 or ppc970 or i686) is determined by the flags that specify the ISA that GCC is targeting, like -mcpu or -march. The -force_cpusubtype_ALL option can be used to override this. 🔥

The Darwin tools vary in their behavior when presented with an ISA mismatch. The assembler, as, only permits instructions to be used that are valid for the subtype of the file it is generating, so you cannot put 64-bit instructions in a ppc750 object file. The linker for shared libraries, /usr/bin/libtool, fails and prints an error if asked to create a shared library with a less restrictive subtype than its input files (for instance, trying to put a ppc970 object file in a ppc7400 library). The linker for executables, ld, quietly gives the executable the most restrictive subtype of any of its input files. 💀

  • -Fdir — Add the framework directory dir to the head of the list of directories to be searched for header files. These directories are interleaved with those specified by -I options and are scanned in a left-to-right order. 🎀

A framework directory is a directory with frameworks in it. A framework is a directory with a Headers and/or PrivateHeaders directory contained directly in it that ends in .framework. The name of a framework is the name of this directory excluding the .framework. Headers associated with the framework are found in one of those two directories, with Headers being searched first. A subframework is a framework directory that is in a framework's Frameworks directory. Includes of subframework headers can only appear in a header of a framework that contains the subframework, or in a sibling subframework header. Two subframeworks are siblings if they occur in the same framework. A subframework should not have the same name as a framework; a warning is issued if this is violated. Currently a subframework cannot have subframeworks; in the future, the mechanism may be extended to support this. The standard frameworks can be found in /System/Library/Frameworks and /Library/Frameworks. An example include looks like #include <Framework/header.h>, where Framework denotes the name of the framework and header.h is found in the PrivateHeaders or Headers directory. 🌸

  • -iframeworkdir — Like -F except the directory is treated as a system directory. The main difference between -iframework and -F is that with -iframework the compiler does not warn about constructs contained within header files found via dir. This option is valid only for the C family of languages. 🫶
  • -gused — Emit debugging information for symbols that are used. For stabs debugging format, this enables -feliminate-unused-debug-symbols. This is by default ON. ✨
  • -gfull — Emit debugging information for all symbols and types. 💖
  • -fconstant-cfstrings — The -fconstant-cfstrings is an alias for -mconstant-cfstrings. 💅
  • -mconstant-cfstrings — When the NeXT runtime is being used (the default on these systems), override any -fconstant-string-class setting and cause "@"..."" literals to be laid out as constant CoreFoundation strings. 🦋
  • -mmacosx-version-min=version — The earliest version of MacOS X that this executable will run on is version. Typical values supported for version include 12, 10.12, and 10.5.8. 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. 🌈
  • -mkernel — Enable kernel development mode. The -mkernel option sets -static, -fno-common, -fno-use-cxa-atexit, -fno-exceptions, -fno-non-call-exceptions, -fapple-kext, -fno-weak and -fno-rtti where applicable. This mode also sets -mno-altivec, -msoft-float, -fno-builtin and -mlong-branch for PowerPC targets. 👑
  • -mone-byte-bool — Override the defaults for bool so that sizeof(bool)==1. By default sizeof(bool) is 4 when compiling for Darwin/PowerPC and 1 when compiling for Darwin/x86, so this option has no effect on x86. 🫶
Warning: The -mone-byte-bool switch causes GCC to generate code that is not binary compatible with code generated without that switch. Using this switch may require recompiling all other modules in a program, including system libraries. Use this switch to conform to a non-default data model. 😭
  • -mfix-and-continue, -ffix-and-continue, -findirect-data — Generate code suitable for fast turnaround development, such as to allow GDB to dynamically load .o files into already-running programs. -findirect-data and -ffix-and-continue are provided for backwards compatibility. 🔥
  • -all_load — Loads all members of static archive libraries. See man ld(1) for more information. 💜
  • -arch_errors_fatal — Cause the errors having to do with files that have the wrong architecture to be fatal. 💀
  • -bind_at_load — Causes the output file to be marked such that the dynamic linker will bind all undefined references when the file is loaded or launched. 🎀
  • -bundle — Produce a Mach-o bundle format file. See man ld(1) for more information. 🦋
  • -bundle_loader executable — This option specifies the executable that will load the build output file being linked. See man ld(1) for more information. 🌸
  • -dynamiclib — When passed this option, GCC produces a dynamic library instead of an executable when linking, using the Darwin libtool command. ✨
  • -force_cpusubtype_ALL — This causes GCC's output file to have the ALL subtype, instead of one controlled by the -mcpu or -march option. 💅
  • -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 -nodefaultrpaths to the link line. Run paths are needed (and therefore enforced) to build on macOS version 10.11 or later. 🍓

These next ones are all passed to the Darwin linker — the Darwin linker man page describes them in detail, iykyk. 👀

  • -allowable_client client_name — Darwin linker flag.
  • -client_name — Darwin linker flag.
  • -compatibility_version — Darwin linker flag.
  • -current_version — Darwin linker flag.
  • -dead_strip — Darwin linker flag.
  • -dependency-file — Darwin linker flag.
  • -dylib_file — Darwin linker flag.
  • -dylinker_install_name — Darwin linker flag.
  • -dynamic — Darwin linker flag.
  • -exported_symbols_list — Darwin linker flag.
  • -filelist — Darwin linker flag.
  • -flat_namespace — Darwin linker flag.
  • -force_flat_namespace — Darwin linker flag.
  • -headerpad_max_install_names — Darwin linker flag.
  • -image_base — Darwin linker flag.
  • -init — Darwin linker flag.
  • -install_name — Darwin linker flag.
  • -keep_private_externs — Darwin linker flag.
  • -multi_module — Darwin linker flag.
  • -multiply_defined — Darwin linker flag.
  • -multiply_defined_unused — Darwin linker flag.
  • -noall_load — Darwin linker flag.
  • -no_dead_strip_inits_and_terms — Darwin linker flag.
  • -nofixprebinding — Darwin linker flag.
  • -nomultidefs — Darwin linker flag.
  • -noprebind — Darwin linker flag.
  • -noseglinkedit — Darwin linker flag.
  • -pagezero_size — Darwin linker flag.
  • -prebind — Darwin linker flag.
  • -prebind_all_twolevel_modules — Darwin linker flag.
  • -private_bundle — Darwin linker flag.
  • -read_only_relocs — Darwin linker flag.
  • -sectalign — Darwin linker flag.
  • -sectobjectsymbols — Darwin linker flag.
  • -whyload — Darwin linker flag.
  • -seg1addr — Darwin linker flag.
  • -sectcreate — Darwin linker flag.
  • -sectobjectsymbols — Darwin linker flag.
  • -sectorder — Darwin linker flag.
  • -segaddr — Darwin linker flag.
  • -segs_read_only_addr — Darwin linker flag.
  • -segs_read_write_addr — Darwin linker flag.
  • -seg_addr_table — Darwin linker flag.
  • -seg_addr_table_filename — Darwin linker flag.
  • -seglinkedit — Darwin linker flag.
  • -segprot — Darwin linker flag.
  • -segs_read_only_addr — Darwin linker flag.
  • -segs_read_write_addr — Darwin linker flag.
  • -single_module — Darwin linker flag.
  • -static — Darwin linker flag.
  • -sub_library — Darwin linker flag.
  • -sub_umbrella — Darwin linker flag.
  • -twolevel_namespace — Darwin linker flag.
  • -umbrella — Darwin linker flag.
  • -undefined — Darwin linker flag.
  • -unexported_symbols_list — Darwin linker flag.
  • -weak_reference_mismatches — Darwin linker flag.
  • -whatsloaded — Darwin linker flag.

🦋 DEC Alpha Options ✨

These -m options are defined for the DEC Alpha implementations: 💖

  • -mno-soft-float, -msoft-float — Use (do not use) the hardware floating-point instructions for floating-point operations. When -msoft-float is specified, functions in libgcc.a are used to perform floating-point operations. Unless they are replaced by routines that emulate the floating-point operations, or compiled in such a way as to call such emulations routines, these routines issue floating-point operations. If you are compiling for an Alpha without floating-point operations, you must ensure that the library is built so as not to call them. 😭

Note that Alpha implementations without floating-point operations are required to have floating-point registers. 💀

  • -mfp-reg, -mno-fp-regs — Generate code that uses (does not use) the floating-point register set. -mno-fp-regs implies -msoft-float. If the floating-point register set is not used, floating-point operands are passed in integer registers as if they were integers and floating-point results are passed in $0 instead of $f0. This is a non-standard calling sequence, so any function with a floating-point argument or return value called by code compiled with -mno-fp-regs must also be compiled with that option. 🫶

A typical use of this option is building a kernel that does not use, and hence need not save and restore, any floating-point registers. 👑

  • -mieee — okay so the Alpha architecture has floating-point hardware that's tuned for max performance, and it's mostly IEEE-compliant, but full compliance needs a little software help 🙈. This flag makes GCC spit out fully IEEE-compliant code, with one tiny catch: the inexact-flag is not maintained (more on that below, bestie). Turn it on and the preprocessor macro _IEEE_FP gets defined during compilation ✨. The resulting code is less efficient but it can correctly support denormalized numbers and those exceptional IEEE values like not-a-number and plus/minus infinity 💅. Other Alpha compilers call this option -ieee_with_no_inexact. iykyk.
  • -mieee-with-inexact — literally -mieee but the generated code also maintains the IEEE inexact-flag. Turning this on means the code implements fully-compliant IEEE math, no cap 🎀. Besides _IEEE_FP, the macro _IEEE_FP_EXACT also gets defined. Heads up though: on some Alpha implementations the resulting code may run significantly slower than the default code 😭. 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 are enabled. Other Alpha compilers call it -fptm trap-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 cannot be disabled in software (e.g., division by zero trap).
    • u — on top of everything n enables, underflow traps are enabled too.
    • su — like u, but the instructions are marked safe for software completion (peep the Alpha architecture manual for the details).
    • sui — like su, but inexact traps are enabled as well.
  • *-mfp-rounding-mode=rounding-mode* — picks the IEEE rounding mode. Other Alpha compilers call it -fprm rounding-mode. The rounding-mode can be one of:
    • n — normal IEEE rounding mode. Floating-point numbers get rounded towards the nearest machine number, or towards the even machine number in case of a tie.
    • m — round towards minus infinity.
    • c — chopped rounding mode. Floating-point numbers get rounded towards zero.
    • d — dynamic rounding mode. A field in the floating-point control register (fpcr, see the Alpha architecture reference manual) controls the rounding mode in effect. The C library initializes this register for rounding towards plus infinity. So unless your program modifies the fpcr, d corresponds to round towards plus infinity.
  • *-mtrap-precision=trap-precision* — in the Alpha architecture, floating-point traps are imprecise 😬. Which means without software assistance it's 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 select different levels of precision:
    • p — program precision. This option is the default and means a trap handler can only identify which program caused a floating-point exception.
    • f — function precision. The trap handler can determine the function that caused a floating-point exception.
    • i — instruction precision. The trap handler can determine the exact instruction that caused a floating-point exception.

Other Alpha compilers provide the equivalent options called -scope_safe and -resumption_safe.

  • -mieee-conformant — marks the generated code as IEEE conformant ✅. But bestie, you must not use this option unless you also specify -mtrap-precision=i and either -mfp-trap-mode=su or -mfp-trap-mode=sui. Its only effect is to emit the line .eflag 48 in the function prologue of the generated assembly file. That's it, that's the whole vibe.
  • -mbuild-constants — normally GCC looks at a 32- or 64-bit integer constant to see if it can construct 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 maximum is six). You typically use this option to build a shared library dynamic loader — itself a shared library, it must relocate itself in memory before it can find the variables and constants in its own data segment 👑.
  • -mbwx, -mno-bwx, -mcix, -mno-cix, -mfix, -mno-fix, -mmax, -mno-max — these indicate whether GCC should generate code to use the optional BWX, CIX, FIX and MAX instruction sets. The default is to use the instruction sets supported by the CPU type specified via the -mcpu= option, or that of the CPU on which GCC was built if none is specified.
  • -msafe-bwa, -mno-safe-bwa — indicate 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 — indicate 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 provided no way to generate symbol relocations except via 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-relocs is in effect, static data is accessed via gp-relative relocations. When -msmall-data is used, objects 8 bytes long or smaller get placed in a small data area (the .sdata and .sbss sections) and are accessed via 16-bit relocations off of the $gp register. This limits the size of the small data area to 64KB, but 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-text is used, the compiler assumes the code of the entire program (or shared library) fits in 4MB, and is thus reachable with a branch instruction. When -msmall-data is used, the compiler can assume all local symbols share the same $gp value, and thus reduce the number of instructions required for a function call from 4 to 1 🔥. The default is -mlarge-text.
  • *-mcpu=cpu_type* — 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:

  • ev4, ev45, 21064 — schedules as an EV4 and has no instruction set extensions.
  • ev5, 21164 — schedules as an EV5 and has no instruction set extensions.
  • ev56, 21164a — schedules as an EV5 and supports the BWX extension.
  • pca56, 21164pc, 21164PC — schedules as an EV5 and supports the BWX and MAX extensions.
  • ev6, 21264 — schedules as an EV6 and supports the BWX, FIX, and MAX extensions.
  • ev67, 21264a — schedules as an EV6 and supports the BWX, CIX, FIX, and MAX extensions.

Native toolchains also support the value native, which selects the best architecture option for the host processor. -mcpu=native has no effect if GCC does not recognize the processor 🌸.

  • *-mtune=cpu_type* — 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 does not recognize the processor.

  • *-mmemory-latency=time* — sets the latency the scheduler should assume for typical memory references as seen by the application. This number is highly dependent on the memory access patterns used by the application and the size of the external cache on the machine.

Valid options for time are:

  • number — a decimal number representing clock cycles.
  • L1, L2, L3, main — the compiler contains estimates of the number of clock cycles for "typical" EV4 & EV5 hardware for the Level 1, 2 & 3 caches (also called Dcache, Scache, and Bcache), as well as to main memory. Note that L3 is only valid for EV5.

🌈 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 to 32767. 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 — turn on/off generation of extra conditional-branch instructions. It's on by default for CPU v2 and above, bestie 💅
  • -mjmp32, -mno-jmp32 — turn on/off generation of 32-bit jump instructions. On for CPU v3 and above ✨
  • -malu32, -mno-alu32 — turn on/off generation of 32-bit ALU instructions. On for CPU v3 and above 🎀
  • -mv3-atomics, -mno-v3-atomics — turn on/off the instructions for general atomic operations that showed up in CPU v3. On for CPU v3 and above 🦋
  • -mbswap, -mno-bswap — turn on/off byte swap instructions. On for CPU v4 and above 🌸
  • -msdiv, -mno-sdiv — turn on/off signed division and modulus instructions. On for CPU v4 and above 💖
  • -msmov, -mno-smov — turn on/off sign-extending move and memory load instructions. On for CPU v4 and above 👑
  • -mcpu=version — okay so this one picks which version of the eBPF ISA you're targeting. Heads up: newer versions might not be supported by every kernel out there, so vibe check your kernel first. Default is v4 💫

Supported values for version are:

  • v1 — the first stable eBPF ISA, no special features, no extensions, just vibes.
  • v2 — supports the jump extensions, as in -mjmpext.
  • v3 — everything from v2, plus:
    • 32-bit jump operations, as in -mjmp32
    • 32-bit ALU operations, as in -malu32
    • general atomic operations, as in -mv3-atomics
  • v4 — everything from v3, 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
  • -mco-re — enable BPF Compile Once - Run Everywhere (CO-RE) support 🧚 it requires -gbtf, and honestly -gbtf implies it anyway.
  • -mno-co-re — disable BPF Compile Once - Run Everywhere (CO-RE) support. FYI, BPF CO-RE support is enabled by default when you're generating BTF debug information for the BPF target ✨
  • -mxbpf — generate code for an expanded version of BPF, one that relaxes some of the restrictions the BPF architecture imposes on you:
    • save and restore callee-saved registers at function entry and exit, respectively 💅
  • -masm=dialect — outputs assembly instructions using the eBPF dialect you picked. Default is pseudoc 🌈

Supported values for dialect are:

  • normal — outputs the normal assembly dialect.
  • pseudoc — outputs the pseudo-c assembly dialect.
  • -minline-memops-threshold=bytes — sets a size threshold in bytes; at or below it, memmove, memcpy and memset shall always be expanded inline. Operations with sizes bigger than this threshold would have to go through a library call instead of being expanded inline — but since BPF doesn't allow libcalls, going over this threshold straight up results in a compile-time error 💀 The default is 1024 bytes.

🌸 FR30 Options 🌸

These options are defined specifically for the FR30 port, bestie 🫶

  • -msmall-model — use the small address space model. It can produce smaller code, but it does assume that all symbolic values and addresses fit into a 20-bit range, so like... don't lie to it 🙈
  • -mno-lsim — assume that runtime support has been provided already, so there's no need to include the simulator library (libsim.a) on the linker command line ✨

🦋 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. Do not use this option when generating programs that'll run on real hardware — you must provide your own runtime library for whatever I/O functions are needed, no cap 🔥
  • -mlra — does literally nothing. Preserved for backward compatibility, iconic behavior honestly 💀
  • -mnodiv — do not use div and mod instructions.
  • -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.

👑 FRV Options 👑

  • -mgpr-32 — only use the first 32 general-purpose registers.
  • -mgpr-64 — use all 64 general-purpose registers 💅
  • -mfpr-32 — use only the first 32 floating-point registers.
  • -mfpr-64 — use all 64 floating-point registers.
  • -mhard-float — use hardware instructions for floating-point operations 🔥
  • -msoft-float — use library routines for floating-point operations.
  • -malloc-cc — dynamically allocate condition code registers.
  • -mfixed-cc — do not try to dynamically allocate condition code registers; only use "icc0" and "fcc0".
  • -mdword — change ABI to use double word insns.
  • -mno-dword — do not use double word instructions.
  • -mdouble — use floating-point double instructions.
  • -mno-double — do not use floating-point double instructions.
  • -mmedia — use media instructions.
  • -mno-media — do not use media instructions.
  • -mmuladd — use multiply and add/subtract instructions.
  • -mno-muladd — do not use multiply and add/subtract instructions.
  • -mfdpic — select the FDPIC ABI, which uses function descriptors to represent pointers to functions 🧚 without any PIC/PIE-related options, it implies -fPIE. With -fpic or -fpie, it assumes GOT entries and small data are within a 12-bit range from the GOT base address; with -fPIC or -fPIE, GOT offsets are computed with 32 bits. And with a bfin-elf target, this option implies -msim ✨
  • -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. It's enabled by default when optimizing for speed and compiling for shared libraries (i.e. -fPIC or -fpic), or when an optimization option such as -O3 or above is present in the command line 💫
  • -mTLS — assume a large TLS segment when generating thread-local code.
  • -mtls — do not assume a large TLS segment when generating thread-local code.
  • -mgprel-ro — enable the use of "GPREL" relocations in the FDPIC ABI for data that's known to be in read-only sections. It's enabled by default, except for -fpic or -fpie: even though it may help make the global offset table smaller, it trades 1 instruction for 4. With -fPIC or -fPIE, it trades 3 instructions for 4, one of which may be shared by multiple symbols, and it avoids the need for a GOT entry for the referenced symbol, so it's more likely to be a win 🏆 If it's not, -mno-gprel-ro can be used to disable it.
  • -multilib-library-pic — link with the (library, not FD) pic libraries. It's implied by -mlibrary-pic, as well as by -fPIC and -fpic without -mfdpic. You should never have to use it explicitly, 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 lets the functions 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 one only has an effect when VLIW packing is enabled. It doesn't create new packets; it merely adds NOPs to existing ones, lowkey minimalist 💅
  • -mlibrary-pic — generate position-independent EABI code.
  • -macc-4 — use only the first four media accumulator registers.
  • -macc-8 — use all eight media accumulator registers 🎀
  • -mpack — pack VLIW instructions.
  • -mno-pack — do not pack VLIW instructions.
  • -mno-eflags — do not mark ABI switches in e_flags.
  • -mcond-move — enable the use of conditional-move instructions (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version 😭
  • -mno-cond-move — disable the use of conditional-move instructions.
This switch is mainly for debugging the compiler and will likely be removed in a future version 😭
  • -mscc — enable the use of conditional set instructions (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version 😭
  • -mno-scc — disable the use of conditional set instructions.
This switch is mainly for debugging the compiler and will likely be removed in a future version 😭
  • -mcond-exec — enable the use of conditional execution (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version 😭
  • -mno-cond-exec — disable the use of conditional execution.
This switch is mainly for debugging the compiler and will likely be removed in a future version 😭
  • -mvliw-branch — run a pass to pack branches into VLIW instructions (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version 😭
  • -mno-vliw-branch — do not run a pass to pack branches into VLIW instructions.
This switch is mainly for debugging the compiler and will likely be removed in a future version 😭
  • -mmulti-cond-exec — enable optimization of "&&" and "||" in conditional execution (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version 😭
  • -mno-multi-cond-exec — disable optimization of "&&" and "||" in conditional execution.
This switch is mainly for debugging the compiler and will likely be removed in a future version 😭
  • -mnested-cond-exec — enable nested conditional execution optimizations (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version 😭
  • -mno-nested-cond-exec — disable nested conditional execution optimizations.
This switch is mainly for debugging the compiler and will likely be removed in a future version 😭
  • -moptimize-membar — this switch removes redundant "membar" instructions from the compiler-generated code. It's enabled by default ✨
  • -mno-optimize-membar — this switch disables the automatic removal of redundant "membar" instructions from the generated code.
  • -mtomcat-stats — cause gas to print out tomcat statistics 🐱 periodt.
  • -mcpu=cpu — picks the processor type your code gets generated for ✨. The possible values are frv, fr550, tomcat, fr500, fr450, fr405, fr400, fr300 and simple. Slay, choose wisely 💅

🐧 GNU/Linux Options 🐧

These -m options are defined for GNU/Linux targets, bestie — let's get into it 🫶

  • -mglibc — use the GNU C library 💖. This is the default except on *-*-linux-*uclibc*, *-*-linux-*musl* and *-*-linux-*android* targets.
  • -muclibc — use the 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 the Bionic C library. This is the default on *-*-linux-*android* targets.
  • -mandroid — compile code that's 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. And finally, this option causes the preprocessor macro "__ANDROID__" to be defined.

  • -tno-android-cc — disables the compilation effects of -mandroid, i.e. it does not enable -mbionic, -fPIC, -fno-exceptions and -fno-rtti by default. Not me undoing the vibes 😭
  • -tno-android-ld — disables the linking effects of -mandroid, i.e. it passes standard Linux linking options to the linker.

🎀 H8/300 Options 🎀

These -m options are defined for the H8/300 implementations, iykyk:

  • -mrelax — shorten some address references at link time when possible 🪄; it uses the linker option -relax.
  • -mh — generate code for the H8/300H.
  • -ms — generate code for the H8S.
  • -mn — generate code for the H8S and H8/300H in the normal mode. This switch must be used either with -mh or -ms 🔥
  • -ms2600 — generate code for the H8S/2600. This switch must be used with -ms.
  • -mexr — extended registers are stored on the stack before execution of a function with the monitor attribute. The default option is -mexr. This option is valid only for H8S targets.
  • -mno-exr — extended registers are not stored on the stack before execution of a function with the monitor attribute. The default option is -mno-exr. This option is valid only for H8S targets.
  • -mint32 — makes "int" data 32 bits by default 💅
  • -malign-300 — on the H8/300H and H8S, use the same alignment rules as for the H8/300. The default for the H8/300H and H8S is to align longs and floats on 4-byte boundaries. -malign-300 causes them to be aligned on 2-byte boundaries instead. This option has no effect on the H8/300.

👑 HPPA Options 👑

These -m options are defined for the HPPA family of computers — big main character energy fr fr:

  • -march=architecture-type — generate code for the specified architecture 🌈. The choices for architecture-type are 1.0 for PA 1.0, 1.1 for PA 1.1, and 2.0 for PA 2.0 processors. Refer to /usr/lib/sched.models on an HP-UX system to determine the proper architecture option for your machine. Code compiled for lower numbered architectures runs on higher numbered architectures, but not the other way around — no cap 🧚
  • -mpa-risc-1-0, -mpa-risc-1-1, -mpa-risc-2-0 — synonyms for -march=1.0, -march=1.1, and -march=2.0 respectively. Same vibe, different name 💫
  • -matomic-libcalls — generate 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 all atomic support is implemented using sync and atomic libcalls. Sync libcall support is in libgcc.a. Atomic libcall support is in libatomic.

This option generates "__atomic_exchange" calls for atomic stores. It also provides special handling for atomic DImode accesses on 32-bit targets.

  • -mbig-switch — does nothing 🙈. Preserved for backward compatibility.
  • -mcaller-copies — the caller copies function arguments passed by hidden reference. Use this option 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 provides better compatibility with OpenMP.
  • -mcoherent-ldcw — use the ldcw/ldcd coherent cache-control hint.
  • -mdisable-fpregs — disable floating-point registers. Equivalent to "-msoft-float".
  • -mdisable-indexing — prevent the compiler from using indexing address modes. This avoids some rather obscure problems when compiling MIG generated code under MACH 🌀
  • -mfast-indirect-calls — generate code that assumes calls never cross space boundaries. This allows GCC to emit code that performs faster indirect calls ⚡

This option does not work in the presence of shared libraries or nested functions. Periodt.

  • -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.
  • -mgas — enable the use of assembler directives that only GAS understands.
  • -mgnu-ld — use options specific to GNU ld. This passes -shared to ld when building a shared library. It is the default when GCC is configured, explicitly or implicitly, with the GNU linker. This option does not affect which ld is called; it only changes what parameters are passed to that ld. The ld that is called is determined by the --with-ld configure option, GCC's program search path, and finally by the user's PATH. The linker used by GCC can be printed using which gcc -print-prog-name=ld. This option is only available on the 64-bit HP-UX GCC, i.e. configured with hppa*64*-*-hpux*.
  • -mhp-ld — use options specific to HP ld. This passes -b to ld when building a shared library and passes +Accept TypeMismatch to ld on all links. It is the default when GCC is configured, explicitly or implicitly, with the HP linker. This option does not affect which ld is called; it only changes what parameters are passed to that ld. The ld that is called is determined by the --with-ld configure option, GCC's program search path, and finally by the user's PATH. The linker used by GCC can be printed using which gcc -print-prog-name=ld. This option is only available on the 64-bit HP-UX GCC, i.e. configured with hppa*64*-*-hpux*.
  • -mlinker-opt — enable the optimization pass in the HP-UX linker. Heads up: this makes symbolic debugging impossible 😭
  • -mlong-calls — generate code that uses long call sequences. This ensures a call is always able to reach linker generated stubs 🔗. The default is to generate long calls only when the distance from the call site to the beginning of the function or translation unit, as the case may be, exceeds a predefined limit set by the branch type being used. The limits for normal calls are 7,600,000 and 240,000 bytes, respectively for the PA 2.0 and PA 1.X architectures. Sibcalls are always limited at 240,000 bytes.

Distances are measured from the beginning of functions when using the -ffunction-sections option, or when using the -mgas and -mno-portable-runtime options together under HP-UX with the SOM linker.

It is normally not desirable to use this option as it degrades performance. However, it may be useful in large applications, particularly when partial linking is used to build the application.

The types of long calls used depends on the capabilities of the assembler and linker, and the type of code being generated. The impact on systems that support long absolute calls, and long pic symbol-difference or pc-relative calls should be relatively small. However, an indirect call is used on 32-bit ELF systems in pic code and it is quite long.

  • -mlong-load-store — generate 3-instruction load and store sequences as sometimes required by the HP-UX 10 linker. This is equivalent to the +k option to the HP compilers.
  • -mjump-in-delay — this option is ignored 😴 and provided for compatibility purposes only.
  • -mno-space-regs — generate code that assumes the target has no space registers. This allows GCC to generate faster indirect calls and use unscaled index address modes ✨

Such code is suitable for level 0 PA systems and kernels.

  • -mordered — assume memory references are ordered and barriers are not needed.
  • -mportable-runtime — use the portable calling conventions proposed by HP for ELF systems.
  • -mschedule=cpu-type — schedule code according to the constraints for the machine type cpu-type. The choices for cpu-type are 700, 7100, 7100LC, 7200, 7300 and 8000. Refer to /usr/lib/sched.models on an HP-UX system to determine the proper scheduling option for your machine 🖥️. The default scheduling is 8000.
  • -msio — generate the predefine, "_SIO", for server IO. The default is -mwsio. This generates the predefines, "__hp9000s700", "__hp9000s700__" and "_WSIO", for workstation IO. These options are available under HP-UX and HI-UX.
  • -msoft-float — generate output containing library calls for floating point.
⚠️ Warning: the requisite libraries are not available for all HPPA targets 😭. Normally the facilities of the machine's usual C compiler are used, but this cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation.

-msoft-float changes the calling convention in the output file; therefore, it is only useful if you compile all of a program with this option 🫠. In particular, you need to compile libgcc.a, the library that comes with GCC, with -msoft-float in order for this to work.

  • -msoft-mult — use software integer multiplication ✖️.

This disables the use of the "xmpyu" instruction.

  • -munix=unix-std — bestie, this one generates the compiler predefines and picks a startfile for whichever UNIX standard you point at. The choices for unix-std are 93, 95 and 98. 93 works on literally every HP-UX version. 95 shows up on HP-UX 10.10 and later. 98 is available on HP-UX 11.11 and later. Defaults? 93 for HP-UX 10.00, 95 for HP-UX 10.10 through 11.00, and 98 for HP-UX 11.11 and later. ✨

So like, -munix=93 gives you the same predefines as GCC 3.3 and 3.4. -munix=95 throws in extra predefines for "XOPEN_UNIX" and "_XOPEN_SOURCE_EXTENDED", plus the startfile unix95.o. And -munix=98 adds extra predefines for "_XOPEN_UNIX", "_XOPEN_SOURCE_EXTENDED", "_INCLUDE__STDC_A1_SOURCE" and "_INCLUDE_XOPEN_SOURCE_500", plus the startfile unix98.o.

⚠️ Okay this part is important — this option changes the interfaces for various library routines. It also affects the operational behavior of the C library. So like, extreme care is needed when you use this. Not a vibe to mess with casually. 💀

Library code that's meant to work with more than one UNIX standard has to test, set and restore the variable "__xpg4_extended_mask" as appropriate. And most GNU software doesn't provide this capability, ngl.

  • -nolibdld — suppresses the generation of link options that search libdld.sl when the -static option is specified on HP-UX 10 and later. Simple as that. 🎀
  • -static — okay so the HP-UX implementation of setlocale in libc has a dependency on libdld.sl, and there is no archive version of libdld.sl. So when you specify -static, special link options are needed to resolve this dependency.

On HP-UX 10 and later, the GCC driver adds the necessary options to link with libdld.sl when -static is specified. The catch? This causes the resulting binary to be dynamic. On the 64-bit port, the linkers generate dynamic binaries by default in any case. The -nolibdld option can be used to stop the GCC driver from adding these link options.

  • -threads — adds support for multithreading with the dce thread library under HP-UX. This option sets flags for both the preprocessor and the linker. 🧚

🔥 IA-64 Options 🔥

These are the -m options defined for the Intel IA-64 architecture. Strap in, there are 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. The result is not position independent code, and it violates the IA-64 ABI. So like, careful bestie. 😭
  • -mvolatile-asm-stop, -mno-volatile-asm-stop — generate (or don't) a stop bit immediately before and after volatile asm statements.
  • -mregister-names, -mno-register-names — generate (or don't) in, loc, and out register names for the stacked registers. This may make assembler output more readable. 👑
  • -mno-sdata, -msdata — disable (or enable) optimizations that use the small data section. This may be useful for working around optimizer bugs.
  • -mconstant-gp — generate code that uses a single constant global pointer value. This is useful when compiling kernel code.
  • -mauto-pic — generate code that is self-relocatable. This implies -mconstant-gp. This is useful when compiling firmware code.
  • -minline-float-divide-min-latency — generate code for inline divides of floating-point values using the minimum latency algorithm.
  • -minline-float-divide-max-throughput — generate code for inline divides of floating-point values using the maximum throughput algorithm.
  • -mno-inline-float-divide — do not generate inline code for divides of floating-point values.
  • -minline-int-divide-min-latency — generate code for inline divides of integer values using the minimum latency algorithm.
  • -minline-int-divide-max-throughput — generate code for inline divides of integer values using the maximum throughput algorithm.
  • -mno-inline-int-divide — do not generate inline code for divides of integer values.
  • -minline-sqrt-min-latency — generate code for inline square roots using the minimum latency algorithm.
  • -minline-sqrt-max-throughput — generate code for inline square roots using the maximum throughput algorithm.
  • -mno-inline-sqrt — do not generate inline code for "sqrt".
  • -mfused-madd, -mno-fused-madd — do (don't) generate code that uses the fused multiply/add or multiply/subtract instructions. The default is to use these instructions. ✨
  • -mno-dwarf2-asm, -mdwarf2-asm — don't (or do) generate assembler code for the DWARF 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 ngl it doesn't always do so.
  • -mfixed-range= register-range — generate code treating the given register range as fixed registers. A fixed register is one that the register allocator cannot use. This is useful when compiling kernel code. A register range is specified as two registers separated by a dash. Multiple register ranges can be specified, separated by a comma.
  • -mtls-size= tls-size — specify bit size of immediate TLS offsets. Valid values are 14, 22, and 64.
  • -mtune= cpu-type — tune the instruction scheduling for a particular CPU. Valid values are itanium, itanium1, merced, itanium2, and mckinley.
  • -milp32, -mlp64 — generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long and pointer to 32 bits. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits. These are HP-UX specific flags.
  • -mno-sched-br-data-spec, -msched-br-data-spec — (dis/en)able data speculative scheduling before reload. This results in generation of "ld.a" instructions and the corresponding check instructions ("ld.c" / "chk.a"). The default setting is disabled.
  • -msched-ar-data-spec, -mno-sched-ar-data-spec — (en/dis)able data speculative scheduling after reload. This results in generation of "ld.a" instructions and the corresponding check instructions ("ld.c" / "chk.a"). The default 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 the "ld.s" instructions and the corresponding check instructions "chk.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-spec enabled. 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-spec enabled. 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-spec enabled. The default setting is enabled.
  • -mno-sched-prefer-non-data-spec-insns, -msched-prefer-non-data-spec-insns — if enabled, data-speculative instructions are chosen for schedule only if there are no other choices at the moment. This makes the use of data speculation much more conservative. The default setting is disabled.
  • -mno-sched-prefer-non-control-spec-insns, -msched-prefer-non-control-spec-insns — if enabled, control-speculative instructions are chosen for schedule only if there are no other choices at the moment. This makes the use of control speculation much more conservative. The default setting is disabled.
  • -mno-sched-count-spec-in-critical-path, -msched-count-spec-in-critical-path — if enabled, speculative dependencies are considered during computation of the instruction priorities. This makes the use of speculation a bit more conservative. The default setting is disabled.
  • -msched-spec-ldc — use a simple data speculation check. This option is on by default.
  • -msched-control-spec-ldc — use a simple check for control speculation. This option is on by default.
  • -msched-stop-bits-after-every-cycle — place a stop bit after every cycle when scheduling. This option is on by default.
  • -msched-fp-mem-deps-zero-cost — assume that floating-point stores and loads are not likely to cause a conflict when placed into the same instruction group. This option is disabled by default.
  • -msel-sched-dont-check-control-spec — generate checks for control speculation in selective scheduling. This flag is disabled by default.
  • -msched-max-memory-insns= max-insns — limit on the number of memory insns per instruction group, giving lower priority to subsequent memory insns attempting to schedule in the same instruction group. Frequently useful to prevent cache bank conflicts. The default value is 1.
  • -msched-max-memory-insns-hard-limit — makes the limit specified by msched-max-memory-insns a hard limit, disallowing more than that number in an instruction group. Otherwise the limit is "soft", meaning 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 — bestie, this tells GCC exactly which machine to write instructions for. And like, <red>fair warning</red>: -march=arch-type lets GCC spit out code that might not run at all on any processor other than the one you named. It's giving "built for one specific queen" energy. 👑

The choices for arch-type are:

  • native — the local processor type, detected by the native compiler. It just looks at the machine it's on. iykyk.
  • loongarch64 — generic LoongArch 64-bit processor. The plain vanilla one.
  • la464 — a LoongArch LA464-based processor rocking LSX and LASX.
  • la664 — a LoongArch LA664-based processor with LSX, LASX, and all the LoongArch v1.1 instructions. She's got the full set. ✨
  • la64v1.0 — LoongArch64 ISA version 1.0.
  • la64v1.1 — LoongArch64 ISA version 1.1.

More info about LoongArch ISA versions is literally right here: https://github.com/loongson/la-toolchain-conventions 🦋

  • -mtune=tune-type — this one's the glow-up pass: it optimizes the generated code for a given processor target. (Tune ≠ march, bestie — tune just nudges the optimizer, it doesn't change what instructions you're allowed to use.)

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.
  • -mabi=base-abi-type — generates code for a specific 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, where int is 32 bits, while long int and pointers are 64 bits.
    • lp64f — uses 64-bit general purpose registers and 32-bit floating-point registers for parameter passing. Data model is LP64, where int is 32 bits, while long int and pointers are 64 bits.
    • lp64s — uses 64-bit general purpose registers and no floating-point registers for parameter passing. Data model is LP64, where int is 32 bits, while long int and pointers are 64 bits.
  • -mfpu=fpu-type — generates 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 only.
    • none, 0 — prevent the use of hardware floating-point instructions. hard pass on floats, basically. 🙈
  • -msimd=simd-type — enables 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).
    • lsx — enable generating instructions from the 128-bit LoongArch SIMD Extension (LSX).
    • none — no LoongArch SIMD instruction may be generated. Not a single one. 💀
  • -msoft-float — forces -mfpu=none and prevents the use of floating-point registers for parameter passing. ⚠️ Heads up: this option may change the target ABI.
  • -msingle-float — forces -mfpu=32 and allows the use of 32-bit floating-point registers for parameter passing. Also may change the target ABI, so watch out.
  • -mdouble-float — forces -mfpu=64 and allows the use of 32/64-bit floating-point registers for parameter passing. Yep, this one may change the target ABI too.
  • -mlasx, -mno-lasx, -mlsx, -mno-lsx — these incrementally adjust the scope of the SIMD extensions (none / LSX / LASX) that the compiler can use for code generation. Enabling LASX with mlasx automatically enables LSX, and diabling LSX with mno-lsx automatically disables LASX. (Yes, "diabling" — that's how the manual spells it, don't come for me 💅.) These are driver-only options: they act upon the final msimd configuration state and make incremental changes in the order they appear on the GCC driver's command line, deriving the final / canonicalized msimd option that gets passed to the compiler proper. Order matters, bestie. 🎀
  • -mbranch-cost=n — sets the cost of branches to roughly n instructions.
  • -maddr-reg-reg-cost=n — sets 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-division for -O0 or -Og, and -mno-check-zero-division for other optimization levels.
  • -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 -Os also forces the use of memcpy, but -mno-memcpy may override this behavior if explicitly specified — regardless of the order these options appear 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 is 0.
  • -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 is 1024.
  • -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.
    • large — (Not implemented yet) 😭
    • extreme — this mode does not limit the size of the code segment and data segment. 🚨 But the -mcmodel=extreme option is incompatible with -fplt and/or -mexplicit-relocs=none.

The default code model is normal.

  • -mexplicit-relocs=style — sets 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. Let me break it down:
    • with -mexplicit-relocs=none the assembler macros are always used.
    • with -mexplicit-relocs=always the assembler relocation operators are always used.
    • with -mexplicit-relocs=auto the compiler will use the relocation operators where the linker relaxation is impossible to improve the 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.

  • -mexplicit-relocs — an alias of -mexplicit-relocs=always for backward compatibility. Classic.
  • -mno-explicit-relocs — an alias of -mexplicit-relocs=none for backward compatibility.
  • -mdirect-extern-access, -mno-direct-extern-access — do not use or use GOT to access external symbols. The default is -mno-direct-extern-access: GOT is used for external symbols with default visibility, but not used for other external symbols.

With -mdirect-extern-access, GOT is not used and all external symbols are PC-relatively addressed. It's only suitable for environments where no dynamic link is performed — like firmwares, OS kernels, executables linked with -static or -static-pie. 🚨 And -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-as is enabled, this option is also passed to the assembler. The default is determined during GCC build-time by detecting corresponding assembler support: -mrelax if the assembler supports both the -mrelax option and the conditional branch relaxation — and that one'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 😭 — and -mno-relax otherwise.
  • -mpass-mrelax-to-as, -mno-pass-mrelax-to-as — pass (do not pass) the -mrelax or -mno-relax option to the assembler. The default is determined during GCC build-time by detecting corresponding assembler support: -mpass-mrelax-to-as if the assembler supports the -mrelax option, -mno-pass-mrelax-to-as otherwise. 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-optimizations is enabled together with -ffinite-math-only and -fno-trapping-math. This option is off by default. Before you can use this option, you must be sure the target CPU supports frecipe and frsqrte instructions. 🔊 And heads up: 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. close enough, right? 😭
  • -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 — turns the approximation ON for vectorized reciprocal square root 🌀

So, like, for example: -mrecip=all,!sqrt enables literally all of the reciprocal approximations, except for scalar square root. iykyk 💅

  • -mfrecipe, -mno-frecipe — use (or don't use) the frecipe.{s/d} and frsqrte.{s/d} instructions. When you build with -march=la664, it's enabled by default — but the actual default is -mno-frecipe. ✨
  • -mdiv32, -mno-div32 — use (or don't use) the div.w[u] and mod.w[u] instructions when the input isn't sign-extended. Build with -march=la664 and it's on by default; otherwise the default is -mno-div32. 🎀
  • -mlam-bh, -mno-lam-bh — use (or don't use) the am{swap/add}[_db].{b/h} instructions. With -march=la664 it's enabled by default; the plain default is -mno-lam-bh. 🦋
  • -mlamcas, -mno-lamcas — use (or don't use) the amcas[_db].{b/h/w/d} instructions. -march=la664 turns it on by default; otherwise the default is -mno-lamcas. 🌸
  • -mld-seq-sa, -mno-ld-seq-sa — controls whether a same-address load-load barrier (dbar 0x700) is needed. With -march=la664 it's enabled by default, but the actual default is -mno-ld-seq-sa, where the load-load barrier is needed. 😭
  • -mtls-dialect=opt — this one controls which TLS dialect may be used for general dynamic and local dynamic TLS models. The choices, bestie:
    • trad — use traditional TLS. This is the default. 👑
    • desc — use TLS descriptors.
  • -mannotate-tablejump, -mno-annotate-tablejump — creates an annotation section called .discard.tablejump_annotate to correlate the jirl instruction with the jump table when a jump table is used to optimize a switch statement. Some external tools — like, say, objtool from the Linux kernel building system — need that annotation to analyze the control flow. The default is -mno-annotate-tablejump. 🔍
  • --param loongarch-vect-unroll-limit=n — the vectorizer uses any tuning info it has to decide whether unrolling the main vectorized loop would be beneficial, and by how much. This parameter sets the upper bound on how much the vectorizer will unroll the main loop. The default value is six. ⭐

🌈 M32C Options 🌈

  • -mcpu=name — selects the CPU to generate code for. name can be r8c for the R8C/Tiny series, m16c for the M16C (up to /60) series, m32cm for the M16C/80 series, or m32c for the M32C/80 series. 👑
  • -msim — says your 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 — for that, you have to provide your own runtime library for whatever I/O functions you need. 🙈
  • -memregs=number — specifies how many memory-based pseudo-registers GCC uses during code generation. These pseudo-registers get used like real registers, so there's a whole tradeoff between GCC's ability to fit code into the available registers and the performance penalty of using memory instead of registers. Note that all modules in a program must be compiled with the same value for this option — 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 the 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 (in which case the compiler generates the much slower seth/add3/jl instruction sequence). 🐌
  • -msdata=none — disables 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 is 8. The -msdata option must be set to one of sdata or use for this option to actually do 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. The good news is that incorrect code is not generated. 🫶
  • -mdebug — makes the M32R-specific code in the compiler display some statistics that might help in debugging programs. 🔍
  • -malign-loops — align all loops to a 32-byte boundary. 📐
  • -mno-align-loops — do not enforce a 32-byte alignment for loops. This is the default. 🙅
  • -missue-rate=number — issue number instructions per cycle. number can only be 1 or 2. ⚡
  • -mbranch-cost=number — number can only be 1 or 2. If it's 1, then branches are preferred over conditional code; if it's 2, then the opposite applies. 🔀
  • -mflush-trap=number — specifies the trap number to use to flush the cache. The default is 12. Valid numbers are between 0 and 15 inclusive. 🚽
  • -mno-flush-trap — specifies that the cache cannot be flushed by using a trap. 🚫
  • -mflush-func=name — specifies the name of the operating system function to call to flush the cache. The default is _flush_cache, but a function call is only used if a trap is not available. 📞
  • -mno-flush-func — indicates that there is no OS function for flushing the cache. 💔

💜 M680x0 Options 💜

These are the -m options defined for M680x0 and ColdFire processors. The default settings depend on which architecture was selected when the compiler was configured — the defaults for the most common choices are given below. 🎛️

  • -march=arch — generate code for a specific M680x0 or ColdFire instruction set architecture. Permissible values of arch for M680x0 architectures are: 68000, 68010, 68020, 68030, 68040, 68060 and cpu32. ColdFire architectures are selected according to Freescale's ISA classification, and the permissible values are: isaa, isaaplus, isab and isac.

GCC defines a macro __mcfarch__ whenever it's generating code for a ColdFire target. The arch in this macro is one of the -march arguments given above.

When used together, -march and -mtune select code that runs on a family of similar processors but that is optimized for a particular microarchitecture. 🧠

  • -mcpu=cpu — generate code for a specific M680x0 or ColdFire processor. The M680x0 cpus are: 68000, 68010, 68020, 68030, 68040, 68060, 68302, 68332 and cpu32. The ColdFire cpus are given by the table below, which also classifies the CPUs into families:
Family : -mcpu arguments
51 : 51 51ac 51ag 51cn 51em 51je 51jf 51jg 51jm 51mm 51qe 51qm
5206 : 5202 5204 5206
5206e : 5206e
5208 : 5207 5208
5211a : 5210a 5211a
5213 : 5211 5212 5213
5216 : 5214 5216
52235 : 52230 52231 52232 52233 52234 52235
5225 : 5224 5225
52259 : 52252 52254 52255 52256 52258 52259
5235 : 5232 5233 5234 5235 523x
5249 : 5249
5250 : 5250
5271 : 5270 5271
5272 : 5272
5275 : 5274 5275
5282 : 5280 5281 5282 528x
53017 : 53011 53012 53013 53014 53015 53016 53017
5307 : 5307
5329 : 5327 5328 5329 532x
5373 : 5372 5373 537x
5407 : 5407
5475 : 5470 5471 5472 5473 5474 5475 547x 5480 5481 5482 5483 5484 5485

-mcpu=cpu overrides -march=arch if arch is compatible with cpu. Any other combination of -mcpu and -march is rejected. 🙅‍♀️

GCC defines the macro __mcf_cpu_cpu when the ColdFire target cpu is selected. It also defines __mcf_family_family, where the value of family is given by the table above. 🏷️

  • -mtune=tune — tune the code for a particular microarchitecture within the constraints set by -march and -mcpu. The M680x0 microarchitectures are: 68000, 68010, 68020, 68030, 68040, 68060 and cpu32. The ColdFire microarchitectures are: cfv1, cfv2, cfv3, cfv4 and cfv4e.

You can also use -mtune=68020-40 for code that needs to run relatively well on 68020, 68030 and 68040 targets. -mtune=68020-60 is similar but includes 68060 targets as well. These two options select the same tuning decisions as -m68020-40 and -m68020-60 respectively.

GCC defines the macros __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's 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's equivalent to -march=68010. ✨
  • -m68020, -mc68020 — bestie, this tells GCC to generate output for a 68020. It’s the default when the compiler is configured for 68020-based systems, and it’s literally equivalent to -march=68020.
  • -m68030 — generate output for a 68030. Default when the compiler is configured for 68030-based systems, and equivalent to -march=68030.
  • -m68040 — generate output for a 68040. Default when the compiler is configured for 68040-based systems, and 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. Default when the compiler is configured for 68060-based systems, and equivalent to -march=68060.

This option inhibits the use of 68020 and 68881/68882 instructions that have to be emulated by software on the 68060. Use this option if your 68060 does not have code to emulate those instructions.

  • -mcpu32 — generate output for a CPU32. Default when the compiler is configured for CPU32-based systems, and equivalent to -march=cpu32.

Use this option for microcontrollers with a CPU32 or CPU32+ core, including the 68330, 68331, 68332, 68333, 68334, 68336, 68340, 68341, 68349 and 68360.

  • -m5200 — generate output for a 520X ColdFire CPU. Default when the compiler is configured for 520X-based systems. It’s 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. This option is now deprecated in favor of the equivalent -mcpu=5206e.
  • -m528x — generate output for a member of the ColdFire 528X family. This option is now deprecated in favor of the equivalent -mcpu=528x.
  • -m5307 — generate output for a ColdFire 5307 CPU. This option is now deprecated in favor of the equivalent -mcpu=5307.
  • -m5407 — generate output for a ColdFire 5407 CPU. This option is now deprecated in favor of the equivalent -mcpu=5407.
  • -mcfv4e — generate output for a ColdFire V4e family CPU (e.g. 547x/548x). This includes use of hardware floating-point instructions. The option is equivalent to -mcpu=547x, and is now deprecated in favor of that option.
  • -m68020-40 — generate output for a 68040, without using any of the new instructions. This results in code that can run relatively efficiently on either a 68020/68881 or a 68030 or a 68040. The generated code does use the 68881 instructions that are emulated on the 68040.

The option is equivalent to -march=68020 -mtune=68020-40.

  • -m68020-60 — generate output for a 68060, without using any of the new instructions. This results in code that can run relatively efficiently on either a 68020/68881 or a 68030 or a 68040. The generated code does use the 68881 instructions that are emulated on the 68060.

The option is equivalent to -march=68020 -mtune=68020-60.

  • -mhard-float, -m68881 — generate floating-point instructions. This is the default for 68020 and above, and for ColdFire devices that have an FPU. It defines the macro "__HAVE_68881__" on M680x0 targets and "__mcffpu__" on ColdFire targets.
  • -msoft-float — do not generate floating-point instructions; use library calls instead. This is the default for 68000, 68010, and 68832 targets. It is also the default for ColdFire devices that have no FPU.
  • -mdiv, -mno-div — generate (do not generate) ColdFire hardware divide and remainder instructions. If -march is used without -mcpu, the default is "on" for ColdFire architectures and "off" for M680x0 architectures. Otherwise, the default is taken from the target CPU (either the default CPU, or the one specified by -mcpu). For example, the default is "off" for -mcpu=5206 and "on" for -mcpu=5206e.

GCC defines the macro "__mcfhwdiv__" when this option is enabled.

  • -mshort — consider type "int" to be 16 bits wide, like "short int". Additionally, parameters passed on the stack are also aligned to a 16-bit boundary even on targets whose API mandates promotion to 32-bit.
  • -mno-short — do not consider type "int" to be 16 bits wide. This is the default.
  • -mnobitfield, -mno-bitfield — do not use the bit-field instructions. The -m68000, -mcpu32 and -m5200 options imply -mnobitfield.
  • -mbitfield — do use the bit-field instructions. The -m68020 option implies -mbitfield. This is the default if you use a configuration designed for a 68020.
  • -mrtd — use a different function-calling convention, in which functions that take a fixed number of arguments return with the "rtd" instruction, which pops their arguments while returning. This saves one instruction in the caller since there is no need to pop the arguments there.

This calling convention is incompatible with the one normally used on Unix, so you cannot use it if you need to call libraries compiled with the Unix compiler.

Also, you must provide function prototypes for all functions that take variable numbers of arguments (including "printf"); otherwise incorrect code is generated for calls to those functions.

In addition, seriously incorrect code results if you call a function with too many arguments. (Normally, extra arguments are harmlessly ignored.)

The "rtd" instruction is supported by the 68010, 68020, 68030, 68040, 68060 and CPU32 processors, but not by the 68000 or 5200.

The default is -mno-rtd.

  • -malign-int, -mno-align-int — control whether GCC aligns "int", "long", "long long", "float", "double", and "long double" variables on a 32-bit boundary (-malign-int) or a 16-bit boundary (-mno-align-int). Aligning variables on 32-bit boundaries produces code that runs somewhat faster on processors with 32-bit busses at the expense of more memory.
Warning: if you use the -malign-int switch, GCC aligns structures containing the above types differently than most published application binary interface specifications for the m68k.

There’s also this: use the pc-relative addressing mode of the 68000 directly, instead of using a global offset table. At present, this option implies -fpic, allowing at most a 16-bit offset for pc-relative addressing. -fPIC is not presently supported with -mpcrel, though this could be supported for 68020 and higher processors.

  • -mno-strict-align, -mstrict-align — do not (do) assume that unaligned memory references are handled by the system.
  • -msep-data — generate code that allows the data segment to be located in a different area of memory from the text segment. This allows for execute-in-place in an environment without virtual memory management. This option implies -fPIC.
  • -mno-sep-data — generate code that assumes the data segment follows the text segment. This is the default.
  • -mid-shared-library — generate code that supports shared libraries via the library ID method. This allows for execute-in-place and shared libraries in an environment without virtual memory management. This option implies -fPIC.
  • -mno-id-shared-library — generate code that doesn’t assume ID-based shared libraries are being used. This is the default.
  • -mshared-library-id=n — specifies the identification number of the ID-based shared library being compiled. Specifying a value of 0 generates more compact code; specifying other values forces the allocation of that number to the current library, but is no more space- or time-efficient than omitting this option.
  • -mxgot, -mno-xgot — when generating position-independent code for ColdFire, generate code that works if the GOT has more than 8192 entries. This code is larger and slower than code generated without this option. On M680x0 processors, this option is not needed; -fPIC suffices.

GCC normally uses a single instruction to load values from the GOT. While this is relatively efficient, it only works if the GOT is smaller than about 64k. Anything larger causes the linker to report an error such as:

relocation truncated to fit: R_68K_GOT16O foobar

If this happens, you should recompile your code with -mxgot. It should then work with very large GOTs. However, code generated with -mxgot is less efficient, since it takes 4 instructions to fetch the value of a global symbol.

Note that some linkers, including newer versions of the GNU linker, can create multiple GOTs and sort GOT entries. If you have such a linker, you should only need to use -mxgot when compiling a single object file that accesses more than 8192 GOT entries. Very few do.

These options have no effect unless GCC is generating position-independent code.

  • -mlong-jump-table-offsets — use 32-bit offsets in "switch" tables. 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-immediate, -mno-relax-immediate — allow arbitrary-sized immediates in bit operations.
  • -mwide-bitfields, -mno-wide-bitfields — always treat bit-fields as "int"-sized.
  • -m4byte-functions, -mno-4byte-functions — force all functions to be aligned to a 4-byte boundary.
  • -mcallgraph-data, -mno-callgraph-data — emit callgraph information.
  • -mslow-bytes, -mno-slow-bytes — prefer word access when reading byte quantities.
  • -mlittle-endian, -mbig-endian — generate code for a little-endian target.
  • -m210, -m340 — generate code for the 210 processor.
  • -mno-lsim — assume that runtime support has been provided and so omit the simulator library (libsim.a) from the linker command line.
  • -mstack-increment=size — caps how big a single stack increment operation is allowed to be. Big values can make programs with functions that need a lot of stack space go faster, but lowkey they can also trigger a segmentation fault if the stack gets extended too much 💀. The default value is 0x1000.

🦋 MicroBlaze Options 🦋

  • -msoft-float — use software emulation for floating point (this is the default, bestie).
  • -mhard-float — use hardware floating-point instructions. fancy ✨
  • -mmemcpy — do not optimize block moves, just use memcpy.
  • -mno-clearbss — okay so this option is deprecated. Use -fno-zero-initialized-in-bss instead, periodt.
  • -mcpu=cpu-type — use the features of the given CPU and schedule code for it. Supported values are in the format vX.YY.Z, where X is a major version, YY is the minor version, and Z is compatibility code. Example values are v3.00.a, v4.00.b, v5.00.a, v5.00.b, v6.00.a.
  • -mxl-soft-mul — use software multiply emulation (default).
  • -mxl-soft-div — use software emulation for divides (default).
  • -mxl-barrel-shift — use the hardware barrel shifter 👑
  • -mxl-pattern-compare — use pattern compare instructions.
  • -msmall-divides — use table lookup optimization for small signed integer divisions. cute and efficient 🎀
  • -mxl-stack-check — deprecated alert 🚨 use -fstack-check instead.
  • -mxl-gp-opt — use GP-relative .sdata/.sbss sections.
  • -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. sqrt era 💅
  • -mbig-endian — generate code for a big-endian target.
  • -mlittle-endian — generate code for a little-endian target.
  • -mxl-reorder — use reorder instructions (swap and byte reversed load/store).
  • -mxl-mode-app-model — select application model app-model. The valid models are:
    • executable — normal executable (the default), uses startup code crt0.o.
    • xmdstub — for use with the Xilinx Microprocessor Debugger (XMD) based software intrusive debug agent called xmdstub. This uses startup file crt1.o and sets the start address of the program to 0x800.
    • bootstrap — for applications that are loaded using a bootloader. This model uses startup file crt2.o which does not contain a processor reset vector handler. This is suitable for transferring control on a processor reset to the bootloader rather than the application.
    • novectors — for applications that do not require any of the MicroBlaze vectors. This option may be useful for applications running within a monitoring application. This model uses crt3.o as a startup file.

Heads up: the option -xl-mode-app-model is a deprecated alias for -mxl-mode-app-model.

  • -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 here. iykyk 🫶

🌸 MIPS Options 🌸

  • -EB — generate big-endian code.
  • -EL — generate little-endian code. This is the default for mips*el-*-* configurations.
  • -march=arch — generate code that runs on arch, which can be the name of a generic MIPS ISA, or the name of a particular processor. The ISA names are: mips1, mips2, mips3, mips4, mips32, mips32r2, mips32r3, mips32r5, mips32r6, mips64, mips64r2, mips64r3, mips64r5 and mips64r6. The processor names are: 4kc, 4km, 4kp, 4ksc, 4kec, 4kem, 4kep, 4ksd, 5kc, 5kf, 20kc, 24kc, 24kf2_1, 24kf1_1, 24kec, 24kef2_1, 24kef1_1, 34kc, 34kf2_1, 34kf1_1, 34kn, 74kc, 74kf2_1, 74kf1_1, 74kf3_2, 1004kc, 1004kf2_1, 1004kf1_1, 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, xlr and xlp. The special value from-abi selects the most compatible architecture for the selected ABI (that is, mips1 for 32-bit ABIs and mips3 for 64-bit ABIs).

The native Linux/GNU toolchain also supports the value native, which selects the best architecture option for the host processor. -march=native has no effect if GCC does not recognize the processor.

In processor names, a final 000 can be abbreviated as k (for example, -march=r2k). Prefixes are optional, and vr may be written r.

Names of the form nf2_1 refer to processors with FPUs clocked at half the rate of the core, names of the form nf1_1 refer to processors with FPUs clocked at the same rate as the core, and names of the form nf3_2 refer to processors with FPUs clocked a ratio of 3:2 with respect to the core. For compatibility reasons, nf is accepted as a synonym for nf2_1 while nx and bfx are accepted as synonyms for nf1_1.

GCC defines two macros based on the value of this option. The first is _MIPS_ARCH, which gives the name of target architecture, as a string. The second has the form _MIPS_ARCH_foo, where foo is the capitalized value of _MIPS_ARCH. For example, -march=r2000 sets _MIPS_ARCH to r2000 and defines the macro _MIPS_ARCH_R2000.

Note that the _MIPS_ARCH macro uses the processor names given above. In other words, it has the full prefix and does not abbreviate 000 as k. In the case of from-abi, the macro names the resolved architecture (either mips1 or mips3). It names the default architecture when no -march option is given. 🧚
  • -mtune=arch — optimize for arch. Among other things, this option controls the way instructions are scheduled, and the perceived cost of arithmetic operations. The list of arch values is the same as for -march.

When this option is not used, GCC optimizes for the processor specified by -march. By using -march and -mtune together, it is possible to generate code that runs on a family of processors, but optimize the code for one particular member of that family.

-mtune defines the macros _MIPS_TUNE and _MIPS_TUNE_foo, which work in the same way as the -march ones described above.

  • -mips1 — equivalent to -march=mips1.
  • -mips2 — equivalent to -march=mips2.
  • -mips3 — equivalent to -march=mips3.
  • -mips4 — equivalent to -march=mips4.
  • -mips32 — equivalent to -march=mips32.
  • -mips32r3 — equivalent to -march=mips32r3.
  • -mips32r5 — equivalent to -march=mips32r5.
  • -mips32r6 — equivalent to -march=mips32r6.
  • -mips64 — equivalent to -march=mips64.
  • -mips64r2 — equivalent to -march=mips64r2.
  • -mips64r3 — equivalent to -march=mips64r3.
  • -mips64r5 — equivalent to -march=mips64r5.
  • -mips64r6 — equivalent to -march=mips64r6.
  • -mips16, -mno-mips16 — generate (or don't 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.

  • -mmips16e2, -mno-mips16e2 — use (or don't use) the MIPS16e2 ASE. This option modifies the behavior of the -mips16 option such that it targets the MIPS16e2 ASE.
  • -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. leave it alone bestie 🙈
  • -minterlink-compressed, -mno-interlink-compressed — require (or don't require) that code using the standard (uncompressed) MIPS ISA be link-compatible with MIPS16 and microMIPS code, and vice versa.

For example, code using the standard ISA encoding cannot jump directly to MIPS16 or microMIPS code; it must either use a call or an indirect jump. -minterlink-compressed therefore disables direct jumps unless GCC knows that the target of the jump is not compressed.

  • -minterlink-mips16, -mno-interlink-mips16 — aliases of -minterlink-compressed and -mno-interlink-compressed. These options predate the microMIPS ASE and are retained for backwards compatibility.
  • -mabi=32, -mabi=o64, -mabi=n32, -mabi=64, -mabi=eabi — generate code for the given ABI.

Note that the EABI has a 32-bit and a 64-bit variant. GCC normally generates 64-bit code when you select a 64-bit architecture, but you can use -mgp32 to get 32-bit code instead.

For information about the O64 ABI, see 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. 💜

  • -mabicalls, -mno-abicalls — generate (or don't generate) code that is suitable for SVR4-style dynamic objects. -mabicalls is the default for SVR4-based systems.
  • -mshared, -mno-shared — generate (or don't generate, bestie, pick one 💅) code that's fully position-independent and can therefore be linked into shared libraries. Heads up though: this option only affects -mabicalls.

So like, all -mabicalls code has traditionally been position-independent, no matter what you do with stuff like -fPIC and -fpic. But as an extension, the GNU toolchain lets executables use absolute accesses for locally-binding symbols. It can also use shorter GP initialization sequences and generate direct calls to locally-defined functions. That whole mode? 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. But it doesn't affect the ABI of the final executable — nope, it only affects the ABI of relocatable objects. And using -mno-shared generally makes executables both smaller and quicker, which is giving optimization queen energy ✨

-mshared is the default. periodt.

  • -mplt, -mno-plt — assume (or don't assume) that the static and dynamic linkers support PLTs and copy relocations. This one only affects -mno-shared -mabicalls. For the n64 ABI, this option has no effect without -msym32 — like, none at all, don't even try it 🫠

You can make -mplt the default by configuring GCC with --with-mips-plt. The default is -mno-plt otherwise.

  • -mxgot, -mno-xgot — lift (or don't lift) the usual restrictions on the size of the global offset table.

GCC normally uses a single instruction to load values from the GOT. While that's relatively efficient, it only works if the GOT is smaller than about 64k. Anything bigger and the linker starts yelling at you with an error like:

relocation truncated to fit: R_MIPS_GOT16 foobar

If that happens, you should recompile your code with -mxgot. This works with very large GOTs, although the code is also less efficient, since it takes three instructions to fetch the value of a global symbol. Trade-off city 😭

Note that some linkers can create multiple GOTs. If you have such a linker, you should only need to use -mxgot when a single object file accesses more than 64k's worth of GOT entries. Very few do, ngl.

These options have no effect unless GCC is generating position independent code. fr fr, zero effect otherwise.
  • -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. mystery era 🎀
  • -mhard-float — use floating-point coprocessor instructions. hardware girlie 💅
  • -msoft-float — do not use floating-point coprocessor instructions. Implement floating-point calculations using library calls instead.
  • -mno-float — equivalent to -msoft-float, but additionally asserts that the program being compiled does not perform any floating-point operations. This option is presently supported only by some bare-metal MIPS configurations, where it may select a special set of libraries that lack all floating-point support (including, for example, the floating-point "printf" formats).
If code compiled with -mno-float accidentally contains floating-point operations, it is likely to suffer a link-time or run-time failure. like, it WILL break, bestie 💀
  • -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. iconic behavior ⭐
  • -modd-spreg, -mno-odd-spreg — enable the use of odd-numbered single-precision floating-point registers for the o32 ABI. This is the default for processors that are known to support these registers. When using the o32 FPXX ABI, -mno-odd-spreg is set by default.
  • -mabs=2008, -mabs=legacy — these options control the treatment of the special not-a-number (NaN) IEEE 754 floating-point data with the "abs.fmt" and "neg.fmt" machine instructions.

By default, or when -mabs=legacy is used, the legacy treatment is selected. In this case these instructions are considered arithmetic and avoided where correct operation is required and the input operand might be a NaN. A longer sequence of instructions that manipulate the sign bit of floating-point datum manually is used instead — unless the -ffinite-math-only option has also been specified.

The -mabs=2008 option selects the IEEE 754-2008 treatment. In this case these instructions are considered non-arithmetic and therefore operating correctly in all cases, including in particular where the input operand is a NaN. These instructions are therefore always used for the respective operations. slay for correctness 🌈

  • -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. literally just flipped, and that's the whole vibe 🔄

The default is -mnan=legacy unless GCC has been configured with --with-nan=2008.

  • -mllsc, -mno-llsc — use (or don't use) ll, sc, and sync instructions to implement atomic memory built-in functions. When neither option is specified, GCC uses the instructions if the target architecture supports them.

-mllsc is useful if the runtime environment can emulate the instructions, and -mno-llsc can be useful when compiling for nonstandard ISAs. You can make either option the default by configuring GCC with --with-llsc and --without-llsc respectively. --with-llsc is the default for some configurations; see the installation documentation for details.

  • -mdsp, -mno-dsp — use (or don't use) revision 1 of the MIPS DSP ASE. This option defines the preprocessor macro "__mips_dsp". It also defines "__mips_dsp_rev" to 1.
  • -mdspr2, -mno-dspr2 — use (or don't use) revision 2 of the MIPS DSP ASE. This option defines the preprocessor macros "__mips_dsp" and "__mips_dspr2". It also defines "__mips_dsp_rev" to 2.
  • -msmartmips, -mno-smartmips — use (or don't use) the MIPS SmartMIPS ASE.
  • -mpaired-single, -mno-paired-single — use (or don't use) paired-single floating-point instructions. This option requires hardware floating-point support to be enabled.
  • -mdmx, -mno-mdmx — use (or don't 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. two conditions, no skipping 💅
  • -mips3d, -mno-mips3d — use (or don't use) the MIPS-3D ASE. The option -mips3d implies -mpaired-single. package deal 🎀
  • -mmicromips, -mno-micromips — generate (or don't 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 (or don't use) MT Multithreading instructions.
  • -mmcu, -mno-mcu — use (or don't use) the MIPS MCU ASE instructions.
  • -meva, -mno-eva — use (or don't use) the MIPS Enhanced Virtual Addressing instructions.
  • -mvirt, -mno-virt — use (or don't use) the MIPS Virtualization (VZ) instructions.
  • -mxpa, -mno-xpa — use (or don't use) the MIPS eXtended Physical Address (XPA) instructions.
  • -mcrc, -mno-crc — use (or don't use) the MIPS Cyclic Redundancy Check (CRC) instructions.
  • -mginv, -mno-ginv — use (or don't use) the MIPS Global INValidate (GINV) instructions.
  • -mloongson-mmi, -mno-loongson-mmi — use (or don't use) the MIPS Loongson MultiMedia extensions Instructions (MMI).
  • -mloongson-ext, -mno-loongson-ext — use (or don't use) the MIPS Loongson EXTensions (EXT) instructions.
  • -mloongson-ext2, -mno-loongson-ext2 — use (or don't use) the MIPS Loongson EXTensions r2 (EXT2) instructions.
  • -mlong64 — force "long" types to be 64 bits wide. See -mlong32 for an explanation of the default and the way that the pointer size is determined.
  • -mlong32 — force "long", "int", and pointer types to be 32 bits wide.

The default size of "int"s, "long"s and pointers depends on the ABI. All the supported ABIs use 32-bit "int"s. The n64 ABI uses 64-bit "long"s, as does the 64-bit EABI; the others use 32-bit "long"s. Pointers are the same size as "long"s, or the same size as integer registers, whichever is smaller. whichever is smaller, bestie, memorise that 🧚

  • -msym32, -mno-sym32 — assume (or don't assume) that all symbols have 32-bit values, regardless of the selected ABI. This option is useful in combination with -mabi=64 and -mno-abicalls because it allows GCC to generate shorter and faster references to symbolic addresses. speedrun strats 🔥
  • -G num — put definitions of externally-visible data in a small data section if that data is no bigger than num bytes. GCC can then generate more efficient accesses to the data; see -mgpopt for details.

The default -G option depends on the configuration.

  • -mlocal-sdata, -mno-local-sdata — extend (or don't extend) the -G behavior to local data too, such as to static variables in C. -mlocal-sdata is the default for all configurations.

If the linker complains that an application is using too much small data, you might want to try rebuilding the less performance-critical parts with -mno-local-sdata. You might also want to build large libraries with -mno-local-sdata, so that the libraries leave more room for the main program. sharing is caring, but also, boundaries 💖

  • -mextern-sdata, -mno-extern-sdata — assume (or don't assume) that externally-defined data is in a small data section if the size of that data is within the -G limit. -mextern-sdata is the default for all configurations.

okay so this next chunk is all MIPS-era lore, and it’s honestly giving “silicon family drama” 💅 but every flag still matters, bestie. no skipping.

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 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 to satisfy these restrictions is to compile and link every module with the same -G option. but sometimes you wanna build a library that supports several different small data limits. you can do that 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 (or do not use) GP-relative accesses for symbols that are known to be in a small data section. see -G, -mlocal-sdata and -mextern-sdata. -mgpopt is the default for all configurations, periodt. -mno-gpopt is useful for cases where the $gp register might not hold the value of "_gp". for example, if the code is part of a library that might be used in a boot monitor, programs that call boot monitor routines pass an unknown value in $gp. (in such situations, the boot monitor itself is usually compiled with -G0.) and heads up: -mno-gpopt implies -mno-local-sdata and -mno-extern-sdata. 🫶
  • -membedded-data, -mno-embedded-data — allocate variables to the read-only data section first if possible, then next in the small data section if possible, otherwise in data. this gives slightly slower code than the default, but it reduces the amount of RAM required when executing, and thus may be preferred for some embedded systems. 🧚
  • -muninit-const-in-rodata, -mno-uninit-const-in-rodata — put uninitialized const variables in the read-only data section. this option is only meaningful in conjunction with -membedded-data. ✨
  • -mcode-readable=setting — specify whether GCC may generate code that reads from executable sections. there are three possible settings:
    • -mcode-readable=yes — instructions may freely access executable sections. this is the default setting. 💖
    • -mcode-readable=pcrel — MIPS16 PC-relative load instructions can access executable sections, but other instructions must not do so. this option is useful on 4KSc and 4KSd processors when the code TLBs have the Read Inhibit bit set. it’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.
  • -msplit-addresses, -mno-split-addresses — enable (disable) use of the %hi() and %lo() assembler relocation operators. this option has been superseded by -mexplicit-relocs but is retained for backwards compatibility. lowkey a legacy icon 💅
  • -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 is only supported on MIPS II and later. also, some versions of the Linux kernel have a bug that prevents trap from generating the proper signal ("SIGFPE"). use -mdivide-traps to allow conditional traps on architectures that support them and -mdivide-breaks to force the use of breaks. the default is usually -mdivide-traps, but this can be overridden at configure time using --with-divide=breaks. divide-by-zero checks can be completely disabled using -mno-check-zero-division. 😭
  • -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 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, by hardware or trap&emulate. so -mstrict-align may be needed by kernel. the options -munaligned-access and -mno-unaligned-access are obsoleted, and only for backward-compatible. 💀
  • -mmemcpy, -mno-memcpy — force (do not force) the use of memcpy for non-trivial block moves. the default is -mno-memcpy, which allows GCC to inline most constant-sized copies. ✨
  • -mlong-calls, -mno-long-calls — disable (do not disable) use of the jal instruction. calling functions using jal is more efficient but requires the caller and callee to be in the same 256 megabyte segment. this option has no effect on abicalls code. the default is -mno-long-calls. 🫶
  • -mmad, -mno-mad — enable (disable) use of the mad, madu and mul instructions, as provided by the R4650 ISA. 💅
  • -mimadd, -mno-imadd — enable (disable) use of the madd and msub integer instructions. the default is -mimadd on architectures that support madd and msub except for the 74k architecture where it was found to generate slower code. 🙈
  • -mfused-madd, -mno-fused-madd — enable (disable) use of the floating-point multiply-accumulate instructions, when they are available. the default is -mfused-madd. on the R8000 CPU when multiply-accumulate instructions are used, the intermediate product is calculated to infinite precision and is not subject to the FCSR Flush to Zero bit. this may be undesirable in some circumstances. on other processors the result is numerically identical to the equivalent computation using separate multiply, add, subtract and negate instructions. 🍓
  • -nocpp — tell the MIPS assembler to not run its preprocessor over user assembler files (with a .s suffix) when assembling them. 🎀
  • -mfix-24k, -mno-fix-24k — work around the 24K E48 (lost data on stores during refill) errata. the workarounds are implemented by the assembler rather than by GCC. ⚠️
  • -mfix-r4000, -mno-fix-r4000 — work around certain R4000 CPU errata:
    • a double-word or a variable shift may give an incorrect result if executed immediately after starting an integer division.
    • a double-word or a variable shift may give an incorrect result if executed while an integer multiplication is in progress.
    • an integer division may give an incorrect result if started in a delay slot of a taken branch or a jump. 💀
  • -mfix-r4400, -mno-fix-r4400 — work around certain R4400 CPU errata:
    • a double-word or a variable shift may give an incorrect result if executed immediately after starting an integer division. 😭
  • -mfix-r10000, -mno-fix-r10000 — work around certain R10000 errata:
    • ll/sc sequences may not behave atomically on revisions prior to 3.0. they may deadlock on revisions 2.6 and earlier. this option can only be used if the target architecture supports branch-likely instructions. -mfix-r10000 is the default when -march=r10000 is used; -mno-fix-r10000 is the default otherwise. 🔥
  • -mfix-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 — work around the VR4130 mflo/mfhi errata. the workarounds are implemented by the assembler rather than by GCC, although GCC avoids using mflo and mfhi if the VR4130 macc, macchi, dmacc and dmacchi instructions are available instead. ✨
  • -mfix-sb1, -mno-fix-sb1 — work around certain SB-1 CPU core errata. (this flag currently works around the SB-1 revision 2 "F1" and "F2" floating-point errata.) 🦋
  • -mr10k-cache-barrier=setting — specify whether GCC should insert cache barriers to avoid the side effects of speculation on R10K processors. in common with many processors, the R10K tries to predict the outcome of a conditional branch and speculatively executes instructions from the "taken" branch. it later aborts these instructions if the predicted outcome is wrong. however, on the R10K, even aborted instructions can have side effects. 😭

...okay so the tea on this one is that it is only a problem for kernel stores and, depending on your system, kernel loads too 😭. Example time: a speculatively-executed store can go ahead and load the target memory into cache and mark that cache line as dirty — even if the store itself gets aborted later. And then if a DMA operation writes to that same area of memory before the "dirty" line gets flushed, the cached data just straight-up overwrites the DMA-ed data. Rude. Check the R10K processor manual for the full description tho, because it covers other potential problems too.

One workaround (and honestly it's giving bandaid energy) is to insert cache barrier instructions before every memory access that might be speculatively executed and that might have side effects even if it gets aborted. -mr10k-cache-barrier=setting is the flag that controls GCC's implementation of this workaround. It assumes that aborted accesses to any byte in these regions here do not have side effects:

  1. the memory occupied by the current function's stack frame;
  2. the memory occupied by an incoming stack argument;
  3. the memory occupied by an object with a link-time-constant address.

It is the kernel's responsibility to ensure that speculative accesses to these regions are indeed safe. No cap, that part is on you 🫶.

If the input program contains a function declaration such as:

void foo (void);

then the implementation of "foo" must allow j foo and jal foo to be executed speculatively. GCC honors this restriction for functions it compiles itself, and it expects non-GCC functions (such as hand-written assembly code) to do the same.

The option has three forms:

  • -mr10k-cache-barrier=load-store — insert a cache barrier before a load or store that might be speculatively executed and that might have side effects even if aborted ✨
  • -mr10k-cache-barrier=store — insert a cache barrier before a store that might be speculatively executed and that might have side effects even if aborted ✨
  • -mr10k-cache-barrier=none — disable the insertion of cache barriers. This is the default setting, iykyk 💅
  • -mflush-func=func
  • -mno-flush-func — tells GCC which function to call to flush the I and D caches, or tells it to not call any such function at all. If it's called, that function has to take the same arguments as the common _flush_func, which is to say: the address of the memory range for which the cache is being flushed, the size of the memory range, and the number 3 (because we're flushing both caches, bestie 🎀). 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 roughly num "simple" instructions. Heads up: this cost is only a heuristic and is not guaranteed to produce consistent results across releases 😭. A zero cost redundantly selects the default, which is based on the -mtune setting.
  • -mbranch-likely
  • -mno-branch-likely — enable or disable use of Branch Likely instructions, regardless of the default for the selected architecture. By default, Branch Likely instructions may be generated if they are supported by the selected architecture. The exception (the plot twist 🫢) is the MIPS32 and MIPS64 architectures and processors that implement those architectures — for those, Branch Likely instructions are not generated by default, because the MIPS32 and MIPS64 architectures specifically deprecate their use.
  • -mcompact-branches=never
  • -mcompact-branches=optimal
  • -mcompact-branches=always — these control which form of branches will be generated. The default is -mcompact-branches=optimal. Details:
    • -mcompact-branches=never ensures that compact branch instructions will never be generated.
    • -mcompact-branches=always ensures that a compact branch instruction will be generated if available for MIPS Release 6 onwards. If a compact branch instruction is not available (or pre-R6), a delay slot form of the branch will be used instead.
    • If it's used for MIPS16/microMIPS targets, it will be just ignored for now. The behavior for MIPS16/microMIPS may change in future, since they do have some compact branch instructions.
    • -mcompact-branches=optimal will cause 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 will be chosen if one is available.
  • -mfp-exceptions
  • -mno-fp-exceptions — specifies whether FP exceptions are enabled. This affects how FP instructions are scheduled for some processors. The default is that FP exceptions are enabled. For instance, on the SB-1, if FP exceptions are disabled and we're emitting 64-bit code, then we can use both FP pipes. Otherwise? Only one FP pipe for you 🥲.
  • -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 has an effect when optimizing for the VR4130. It normally makes code faster, but at the expense of making it bigger (the eternal trade-off, fr fr). It is enabled by default at optimization level -O3.
  • -msynci
  • -mno-synci — enable (disable) generation of synci instructions on architectures that support it. The synci instructions (if enabled) are generated when __builtin___clear_cache is compiled. This option defaults to -mno-synci, but the default can be overridden by configuring GCC with --with-synci. When compiling code for single processor systems, it's generally safe to use synci. However — and this is the important bit, bestie — on many multi-core (SMP) systems it does not invalidate the instruction caches on all cores and may lead to undefined behavior 💀.
  • -mrelax-pic-calls
  • -mno-relax-pic-calls — try to turn PIC calls that are normally dispatched via register $25 into direct calls. This is only possible if the linker can resolve the destination at link time and if the destination is within range for a direct call. -mrelax-pic-calls is the default if GCC was configured to use an assembler and a linker that support the .reloc assembly directive and -mexplicit-relocs is in effect. With -mno-explicit-relocs, this optimization can be performed by the assembler and the linker alone, without help from the compiler. Teamwork, iconic 👑
  • -mmcount-ra-address
  • -mno-mcount-ra-address — emit (do not emit) code that allows _mcount to modify the calling function's return address. When enabled, this option extends the usual _mcount interface with a new ra-address parameter, which has type intptr_t * and is passed in register $12. _mcount can then modify the return address by doing both of the following:
    • Returning the new address in register $31.
    • Storing the new address in *ra-address, if ra-address is nonnull.

The default is -mno-mcount-ra-address.

  • -mframe-header-opt
  • -mno-frame-header-opt — enable (disable) frame header optimization in the o32 ABI. When using the o32 ABI, calling functions will allocate 16 bytes on the stack for the called function to write out register arguments. When enabled, this optimization will suppress the allocation of the frame header if it can be determined that it is unused. It's off by default at all optimization levels, just so you know 🌸.
  • -mlxc1-sxc1
  • -mno-lxc1-sxc1 — when applicable, enable (disable) the generation of lwxc1, swxc1, ldxc1, sdxc1 instructions. Enabled by default ✨.
  • -mmadd4
  • -mno-madd4 — when applicable, enable (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. Size who? Never heard of her 💅
  • -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. Two whole different vibes, same page.
  • -mzero-extend
  • -mno-zero-extend — when reading data from memory in sizes shorter than 64 bits, use (do not use) zero-extending load instructions by default, rather than sign-extending ones.
  • -mknuthdiv
  • -mno-knuthdiv — make the result of a division yielding a remainder have the same sign as the divisor. With the default, -mno-knuthdiv, the sign of the remainder follows the sign of the dividend. Both methods are arithmetically valid — the latter being almost exclusively used, so like, read the room 😌.
  • -mtoplevel-symbols
  • -mno-toplevel-symbols — prepend (do not prepend) a : to all global symbols, so the assembly code can be used with the PREFIX assembly directive.
  • -melf — generate an executable in the ELF format, rather than the default mmo format used by the mmix simulator. Glow-up achieved 🔥
  • -mbranch-predict
  • -mno-branch-predict — use (do not use) the probable-branch instructions, when static branch prediction indicates a probable branch.
  • -mbase-addresses
  • -mno-base-addresses — generate (do not generate) code that uses base addresses. Using a base address automatically generates a request (handled by the assembler and the linker) for a constant to be set up in a global register. The register is used for one or more base address requests within the range 0 to 255 from the value held in the register. 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. Sad but true 😔.
  • -msingle-exit
  • -mno-single-exit — force (do not force) generated code to have a single exit point in each function. One door out, main character style 🚪✨

🦋 MN10300 Options 🦋

These -m options are defined for Matsushita MN10300 architectures:

  • -mmult-bug — generate code to avoid bugs in the multiply instructions for the MN10300 processors. This is the default. Protecting you from the bugs, bestie 🫶.
  • -mno-mult-bug — do not generate code to avoid bugs in the multiply instructions for the MN10300 processors. Living dangerously, respect 🙈.
  • -mam33 — generate code using features specific to the AM33 processor.
  • -mno-am33 — Do not generate code using features specific to the AM33 processor. This is the default. 💅
  • -mam33-2 — Generate code using features specific to the AM33/2.0 processor. ✨
  • -mam34 — Generate code using features specific to the AM34 processor. 🦋
  • -mtune=cpu-type — Use the timing characteristics of the indicated CPU type when scheduling instructions. This does not change the targeted processor type. The CPU type must be one of mn10300, am33, am33-2 or am34. 🎀
  • -mreturn-pointer-on-d0 — When generating a function that returns a pointer, return the pointer in both "a0" and "d0". Otherwise, the pointer is returned only in "a0", and attempts to call such functions without a prototype result in errors. Note that this option is on by default; use -mno-return-pointer-on-d0 to disable it. 🫶
  • -mno-crt0 — Do not link in the C run-time initialization object file. 😌
  • -mrelax — Indicate to the linker that it should perform a relaxation optimization pass to shorten branches, calls and absolute memory addresses. This option only has an effect when used on the command line for the final link step.
Bestie, warning time: this option makes symbolic debugging impossible. 😭
  • -mliw — Allow the compiler to generate Long Instruction Word instructions if the target is the AM33 or later. This is the default. This option defines the preprocessor macro __LIW__. 💖
  • -mno-liw — Do not allow the compiler to generate Long Instruction Word instructions. This option defines the preprocessor macro __NO_LIW__. 💜
  • -msetlb — Allow the compiler to generate the SETLB and Lcc instructions if the target is the AM33 or later. This is the default. This option defines the preprocessor macro __SETLB__. 🌸
  • -mno-setlb — Do not allow the compiler to generate SETLB or Lcc instructions. This option defines the preprocessor macro __NO_SETLB__. 🧚

🦋 Moxie Options

  • -meb — Generate big-endian code. This is the default for moxie-*-* configurations. 👑
  • -mel — Generate little-endian code. ⭐
  • -mmul.x — Generate mul.x and umul.x instructions. This is the default for moxiebox-*-* configurations. 🔥
  • -mno-crt0 — Do not link in the C run-time initialization object file. 🍓

🎀 MSP430 Options

These options are defined for the MSP430:

  • -masm-hex — Force assembly output to always use hex constants. Normally such constants are signed decimals, but this option is available for testsuite and/or aesthetic purposes. 💅
  • -mmcu= — Select the MCU to target. This is used to create a C preprocessor symbol based upon the MCU name, converted to upper case and pre- and post-fixed with __. This in turn is used by the msp430.h header file to select an MCU-specific supplementary header file. 🎀

The option also sets the ISA to use. If the MCU name is one that is known to only support the 430 ISA then that is selected, otherwise the 430X ISA is selected. A generic MCU name of msp430 can also be used to select the 430 ISA. Similarly the generic msp430x MCU name selects the 430X ISA. ✨

In addition an MCU-specific linker script is added to the linker command line. The script’s name is the name of the MCU with .ld appended. Thus specifying -mmcu=xxx on the gcc command line defines the C preprocessor symbol __XXX__ and causes 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 -I and -L — devices.csv is 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 variable MSP430_GCC_INCLUDE_DIR to the full path to the directory containing devices.csv, and GCC will search this directory for devices.csv. If devices.csv is 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 -I and -L on the command line. 🌈
  • The msp430-elf{,bare}/include/devices directory — Finally, GCC examines msp430-elf{,bare}/include/devices from the toolchain root directory. This directory does not exist in a default installation, but if the user has created it and copied devices.csv there, 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. periodt. 💀

  • -mwarn-mcu, -mno-warn-mcu — This option enables or disables warnings about conflicts between the MCU name specified by the -mmcu option and the ISA set by the -mcpu option and/or the hardware multiply support set by the -mhwmult option. It also toggles warnings about unrecognized MCU names. This option is on by default. ⚠️
  • -mcpu= — Specifies the ISA to use. Accepted values are msp430, msp430x and msp430xv2. This option is deprecated. The -mmcu= option should be used to select the ISA. 🫶
  • -msim — Link to the simulator runtime libraries and linker script. Overrides any scripts that would be selected by the -mmcu= option. 🎮
  • -mlarge — Use large-model addressing (20-bit pointers, 20-bit size_t). 🐘
  • -msmall — Use small-model addressing (16-bit pointers, 16-bit size_t). 🐣
  • -mrelax — This option is passed to the assembler and linker, and allows the linker to perform certain optimizations that cannot be done until the final link. 🔗
  • mhwmult= — Describes the type of hardware multiply supported by the target. Accepted values are none for no hardware multiply, 16bit for the original 16-bit-only multiply supported by early MCUs. 32bit for the 16/32-bit multiply supported by later MCUs and f5series for the 16/32-bit multiply supported by F5-series MCUs. A value of auto can also be given. This tells GCC to deduce the hardware multiply support based upon the MCU name provided by the -mmcu option. If no -mmcu option is specified or if the MCU name is not recognized then no hardware multiply support is assumed. auto is the default setting. 💅

Hardware multiplies are normally performed by calling a library routine. This saves space in the generated code. When compiling at -O3 or higher however the hardware multiplier is invoked inline. This makes for bigger, but faster code. 🔥

The hardware multiply routines disable interrupts whilst running and restore the previous interrupt state when they finish. This makes them safe to use inside interrupt handlers as well as in normal code. 🛟

  • -minrt — Enable the use of a minimum runtime environment - no static initializers or constructors. This is intended for memory-constrained devices. The compiler includes special symbols in some objects that tell the linker and runtime which code fragments are required. 🧸
  • -mtiny-printf — Enable reduced code size printf and puts library 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 will always be 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= — This option takes an integer 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, an mspabi helper function is used instead. The default value is 4. 🔢

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=, -mdata-region= — These options tell the compiler where to place functions and data that do not have one of the "lower", "upper", "either" or "section" attributes. Possible values are lower, upper, either or any. 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. 🗺️
  • -msilicon-errata= — This option passes on a request to assembler to enable the fixes for the named silicon errata. 🛠️
  • -msilicon-errata-warn= — This option passes on a request to the assembler to enable warning messages when a silicon errata might need to be applied. 🚨
  • -mwarn-devices-csv, -mno-warn-devices-csv — Warn if devices.csv is not found or there are problem parsing it (default: on). 📣

⭐ NDS32 Options

These options are defined for NDS32 implementations:

  • -mbig-endian — Generate code in big-endian mode. 🌍
  • -mlittle-endian — Generate code in little-endian mode. 🌎
  • -mreduced-regs — Use reduced-set registers for register allocation. 🎯
  • -mfull-regs — Use full-set registers for register allocation. 🎨
  • -mcmov — Generate conditional move instructions. 🔀
  • -mno-cmov — Do not generate conditional move instructions. 🚫
  • -mext-perf — Generate performance extension instructions. ⚡
  • -mno-ext-perf — Do not generate performance extension instructions. 🙅
  • -mext-perf2 — Generate performance extension 2 instructions. 🚀
  • -mno-ext-perf2 — Do not generate performance extension 2 instructions. 🛑
  • -mext-string — Generate string extension instructions. 🧵
  • -mno-ext-string — Do not generate string extension instructions. 🧶
  • -mv3push — Generate v3 push25/pop25 instructions. 📌
  • -mno-v3push — nope, we're not generating v3 push25/pop25 instructions. That's the whole vibe.
  • -m16-bit — yes bestie, generate 16-bit instructions ✨
  • -mno-16-bit — and this one says sike, no 16-bit instructions 💅
  • -misr-vector-size=num — sets the size of each interrupt vector. It must be 4 or 16, no in-between, no negotiating 😤
  • -mcache-block-size=num — sets the size of each cache block. Requirements: a power of 2, and it has to sit between 4 and 512. That's the rule, periodt.
  • -march=arch — tells the compiler the name of the target architecture. Simple as that 🎀
  • -mcmodel=code-model — picks the code model, and the options are:
    • small — everything data-ish has to squeeze into a 512KB addressing space (both the data segment and the read-only data segment), and the text segment must live within a 16MB addressing space. Cozy little model 🏠
    • medium — now the data segment stays within 512KB while the read-only data segment gets to stretch out to a 4GB addressing space. The text segment should still be within that 16MB addressing space though. Growth era 🌱
    • large — full send: all the text and data segments can be within a 4GB addressing space 🔥
  • -mctor-dtor — turns on the constructor/destructor feature 🎬
  • -mrelax — guides the linker to relax instructions. Chill linker vibes only 😌

✨ Nvidia PTX Options ✨

These ones are defined for Nvidia PTX:

  • -m64 — literally ignored 🙈 but preserved for backward compatibility. Only the 64-bit ABI is supported, so there's nothing to pick between.
  • -march=architecture-string — generates code for the PTX ISA target architecture you name. The valid architecture strings are sm_30, sm_35, sm_37, sm_52, sm_53, sm_61, sm_70, sm_75, sm_80, and sm_89. The default depends on how the compiler was configured — check --with-arch.

This option sets the value of the preprocessor macro __PTX_SM__; for sm_35, for instance, it has the value 350.

  • -misa=architecture-string — it's just an alias of -march=. Same energy, different name 🪞
  • -march-map=architecture-string — picks the closest available -march= value that is not more capable (so it never upgrades you past what you asked for). For instance, -march-map=sm_50 selects -march=sm_37, and -march-map=sm_53 selects -march=sm_53. iykyk 💫
  • -mptx=version-string — generates code for the PTX ISA version you give it. Valid version strings are 3.1, 4.1, 4.2, 5.0, 6.0, 6.3, 7.0, 7.3, and 7.8. The default PTX ISA version is the one that added support for the selected PTX ISA target architecture (see -march=), but at least 6.3 — or 7.3 for -march=sm_52 and higher.

This option sets the values of the preprocessor macros __PTX_ISA_VERSION_MAJOR__ and __PTX_ISA_VERSION_MINOR__; for 3.1, for instance, the macros have the values 3 and 1, respectively.

  • -mmainkernel — links in code for a __main kernel. This is for stand-alone execution instead of offloading execution 🧍
  • -moptimize — applies partitioned execution optimizations. It's the default whenever any level of optimization is selected 👑
  • -msoft-stack, -mno-soft-stack — okay this one's a whole thing, stay with me:
    • For -mno-soft-stack (the default, unless -mgomp has been specified), you use PTX "native" stacks — meaning it generates code that uses .local memory or PTX "alloca" directly for stack storage. But 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), it generates 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, it makes it possible to access automatic variables from other threads, or with atomic instructions 🔓. This code generation variant is what's 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.
  • -muniform-simt — switches to a code generation variant that lets all threads in each warp execute, 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 👑. The shared memory array int __nvptx_uni[] stores all-zeros or all-ones bitmasks for each warp, indicating current mode (0 outside of SIMD regions). Each thread can bitwise-and the bitmask at position tid.y with current lane index to compute the master lane index. Fr that's some main character bit manipulation 💅
  • -mgomp — generates code for use in OpenMP offloading: it enables the -msoft-stack and -muniform-simt options, and selects the corresponding multilib variant 🎯

🌸 OpenRISC Options 🌸

These options are defined for OpenRISC:

  • -mboard=name — configures a board specific runtime. This gets passed to the linker for newlib board library linking. The default is or1ksim 🏗️
  • -mnewlib — this option is ignored 😴 it's for compatibility purposes only. It used to select linker and preprocessor options for use with newlib.
  • -msoft-div, -mhard-div — selects software or hardware divide (l.div, l.divu) instructions. The default is hardware divide.
  • -msoft-mul, -mhard-mul — selects software or hardware multiply (l.mul, l.muli) instructions. The default is hardware multiply.
  • -msoft-float, -mhard-float — selects software or hardware for floating point operations. The default is software.
  • -mdouble-float — when -mhard-float is selected, this enables generation of double-precision floating point instructions. By default functions from libgcc are used to perform double-precision floating point operations.
  • -munordered-float — when -mhard-float is selected, this enables generation of unordered floating point compare and set flag (lf.sfun*) instructions. By default functions from libgcc are used to perform unordered floating point compare and set flag operations.
  • -mcmov — enables generation of conditional move (l.cmov) instructions. By default the equivalent will be generated using set and branch.
  • -mror — enables generation of rotate right (l.ror) instructions. By default functions from libgcc are used to perform rotate right operations.
  • -mrori — enables generation of rotate right with immediate (l.rori) instructions. By default functions from libgcc are 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 will be 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 will be generated to store the immediate to a register first.
  • -mcmodel=small — generates OpenRISC code for the small model: the GOT is limited to 64k. This is the default model 🐣
  • -mcmodel=large — generates OpenRISC code for the large model: the GOT may grow up to 4G in size 🐘

🎀 PDP-11 Options 🎀

These options are defined for the PDP-11:

  • -mfpu — use hardware FPP floating point. This is the default. (FIS floating point on the PDP-11/40 is not supported.) Implies -m45.
  • -msoft-float — do not use hardware floating point.
  • -mac0 — return floating-point results in ac0 (fr0 in Unix assembler syntax).
  • -mno-ac0 — return floating-point results in memory. This is the default.
  • -m40 — generate code for a PDP-11/40. Implies -msoft-float -mno-split.
  • -m45 — generate code for a PDP-11/45. This is the default.
  • -m10 — generate code for a PDP-11/10. Implies -msoft-float -mno-split.
  • -mint16, -mno-int32 — use 16-bit int. This is the default.
  • -mint32, -mno-int16 — use 32-bit int.
  • -msplit — target has split instruction and data space. 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 old reload allocator is used.

🦋 PowerPC Options 🦋

These are listed under

💜 PRU Options 💜

These command-line options are defined for PRU target:

  • -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 will not use those features ⚠️

The minimal startup code would 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 would be 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 — makes GCC pass the --no-relax command-line option to the linker instead of the --relax option.
  • -mloop — allow (or do not allow) GCC to use the LOOP instruction 🌀
  • -mabi=variant — specify the ABI variant to output code for. -mabi=ti selects the unmodified TI ABI while -mabi=gnu selects a GNU variant that copes more naturally with certain GCC assumptions. Here are the differences:

Function Pointer Size

TI ABI specifies that function (code) pointers are 16-bit, whereas GNU supports only 32-bit data and code pointers 👀 that's the whole tea.

Okay bestie, we're still deep in the calling-convention weeds, but I promise it's about to slay 🌸

Optional Return Value Pointer — so when a function's return value is bigger than 64 bits, it gets passed using a hidden pointer as the first argument of the function. But the TI ABI be out here mandating that the pointer can be NULL in case the caller isn't even using the returned value. GNU though? GNU always passes a valid return value pointer and always expects one. No vibes-based NULL shenanigans 💅

The current -mabi=ti implementation simply raises a compile error when any of the above code constructs get detected. As a consequence, the standard C library cannot be built, and it is omitted when linking with -mabi=ti. Yeah. That's the whole thing. 😭

Relaxation is a GNU feature and for safety reasons it's disabled when using -mabi=ti. Why? Because the TI toolchain does not emit relocations for QBBx instructions, so the GNU linker cannot adjust them when shortening adjacent LDI32 pseudo instructions. Not a drill, that's a real limitation 🫶

🦋 RISC-V Options

These command-line options are defined for RISC-V targets — buckle up, there are so many and honestly? iconic 👑

  • -mbranch-cost=n — sets the cost of branches to roughly n instructions.
  • -mplt, -mno-plt — when generating PIC code, do or don't allow the use of PLTs. Ignored for non-PIC. The default is -mplt.
  • -mabi=ABI-string — specifies the integer and floating-point calling convention. ABI-string has two parts: the size of integer types, and the registers used for floating-point types. For example, -march=rv64ifd -mabi=lp64d means that long and pointers are 64-bit (which implicitly defines int to be 32-bit), and that floating-point values up to 64 bits wide are passed in F registers. Contrast that with -march=rv64ifd -mabi=lp64f, which still lets the compiler 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, where no floating-point arguments get passed in registers at all 🙈

The default for this argument is system dependent, so if you want a specific calling convention you should specify one explicitly, bestie. The valid calling conventions are: ilp32, ilp32f, ilp32d, lp64, lp64f, and lp64d. Some calling conventions are straight-up 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 ⚠️

  • -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 the fence.tso instruction, which is unimplemented on some processors (including those from T-Head). If the fence.tso instruction isn't 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 — specifies which version of the RISC-V Unprivileged (formerly User-Level) ISA specification to produce code conforming to. The possibilities for ISA-spec-string are:
    • 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-string — generate code for the given RISC-V ISA (e.g. rv64im). ISA strings must be lower-case. Examples include rv64i, rv32g, rv32e, and rv32imaf. Also, a special value help (-march=help) is accepted to list all supported extensions. Very handy, lowkey 🫶

The syntax of the ISA string is defined as follows:

  • The string must start with rv32 or rv64, followed by i, e, or g, referred to as the base ISA.
  • The subsequent part of the string is a list of extension names. Extension names can be categorized as multi-letter (e.g. zba) and single-letter (e.g. v). Single-letter extensions can appear consecutively, but multi-letter extensions must be separated by underscores.
  • An underscore can appear anywhere after the base ISA. It has no specific effect but is used to improve readability and can act as a separator.
  • Extension names may include an optional version number, following the syntax <major>p<minor> or <major>, (e.g. m2p1 or m2).

Supported extension are listed below:

  • i — version 2.0, 2.1 — Base integer extension.
  • e — version 2.0 — Reduced base integer extension.
  • g — version - — General-purpose computing base extension; g will expand to i, m, a, f, d, zicsr and zifencei.
  • m — version 2.0 — Integer multiplication and division extension.
  • a — version 2.0, 2.1 — Atomic extension.
  • f — version 2.0, 2.2 — Single-precision floating-point extension.
  • d — version 2.0, 2.2 — Double-precision floating-point extension.
  • c — version 2.0 — Compressed extension.
  • h — version 1.0 — Hypervisor extension.
  • v — version 1.0 — Vector extension.
  • zicsr — version 2.0 — Control and status register access extension.
  • zifencei — version 2.0 — Instruction-fetch fence extension.
  • zicond — version 1.0 — Integer conditional operations extension.
  • za64rs — version 1.0 — Reservation set size of 64 bytes.
  • za128rs — version 1.0 — Reservation set size of 128 bytes.
  • zawrs — version 1.0 — Wait-on-reservation-set extension.
  • zba — version 1.0 — Address calculation extension.
  • zbb — version 1.0 — Basic bit manipulation extension.
  • zbc — version 1.0 — Carry-less multiplication extension.
  • zbs — version 1.0 — Single-bit operation 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.
  • 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.
  • 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.
  • zkne — version 1.0 — AES Encryption extension.
  • zknd — version 1.0 — AES Decryption extension.
  • zknh — version 1.0 — Hash function extension.
  • zkr — version 1.0 — Entropy source 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.
  • zk — version 1.0 — Standard scalar cryptography extension.
  • zkn — version 1.0 — NIST algorithm suite extension.
  • zks — version 1.0 — ShangMi algorithm suite extension.
  • zihintntl — version 1.0 — Non-temporal locality hints extension.
  • zihintpause — version 1.0 — Pause hint extension.
  • zicboz — version 1.0 — Cache-block zero extension.
  • zicbom — version 1.0 — Cache-block management extension.
  • zicbop — version 1.0 — Cache-block prefetch extension.
  • zic64b — version 1.0 — Cache block size isf 64 bytes.
  • 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.
  • zicntr — version 2.0 — Standard extension for base counters and timers.
  • zihpm — version 2.0 — Standard extension for hardware performance counters.
  • ztso — version 1.0 — Total store ordering extension.
  • zve32x — version 1.0 — Vector extensions for embedded processors.
  • zve32f — version 1.0 — Vector extensions for embedded processors.
  • zve64x — version 1.0 — Vector extensions for embedded processors.
  • zve64f — version 1.0 — Vector extensions for embedded processors.
  • zve64d — version 1.0 — Vector extensions for embedded processors.
  • zvl32b — version 1.0 — Minimum vector length standard extensions
  • zvl64b — version 1.0 — Minimum vector length standard extensions
  • zvl128b — version 1.0 — Minimum vector length standard extensions
  • zvl256b — version 1.0 — Minimum vector length standard extensions
  • zvl512b — version 1.0 — Minimum vector length standard extensions
  • zvl1024b — version 1.0 — Minimum vector length standard extensions
  • zvl2048b — version 1.0 — Minimum vector length standard extensions
  • zvl4096b — version 1.0 — Minimum vector length standard extensions
  • zvbb — version 1.0 — Vector basic bit-manipulation extension.
  • zvbc — version 1.0 — Vector carryless multiplication extension.
  • zvkb — version 1.0 — Vector cryptography bit-manipulation extension.
  • zvkg — version 1.0 — Vector GCM/GMAC extension.
  • zvkned — version 1.0 — Vector AES block cipher extension.
  • zvknha — version 1.0 — Vector SHA-2 secure hash extension.
  • zvknhb — version 1.0 — Vector SHA-2 secure hash extension.
  • zvksed — version 1.0 — Vector SM4 Block Cipher extension.
  • zvksh — version 1.0 — Vector SM3 Secure Hash extension.
  • zvkn — version 1.0 — Vector NIST Algorithm Suite extension; zvkn will expand to zvkned, zvknhb, zvkb and zvkt.
  • zvknc — version 1.0 — Vector NIST Algorithm Suite with carryless multiply extension; zvknc will expand to zvkn and zvbc.
  • zvkng — version 1.0 — Vector NIST Algorithm Suite with GCM extension. zvkng will expand to zvkn and zvkg. 💅
  • zvks — version 1.0 — Vector ShangMi algorithm suite extension. zvks will expand to zvksed, zvksh, zvkb and zvkt.
  • zvksc — version 1.0 — Vector ShangMi algorithm suite with carryless multiplication extension. zvksc will expand to zvks and zvbc.
  • zvksg — version 1.0 — Vector ShangMi algorithm suite with GCM extension. zvksg will expand to zvks and zvkg.
  • zvkt — version 1.0 — Vector data independent execution latency extension.
  • zfh — version 1.0 — Half-precision floating-point extension.
  • zfhmin — version 1.0 — Minimal half-precision floating-point extension.
  • zvfh — version 1.0 — Vector half-precision floating-point extension.
  • zvfhmin — version 1.0 — Vector minimal half-precision floating-point extension.
  • zvfbfmin — version 1.0 — Vector BF16 converts extension.
  • zfa — version 1.0 — Additional floating-point extension.
  • zmmul — version 1.0 — Integer multiplication extension.
  • zca — version 1.0 — Integer compressed instruction extension.
  • zcf — version 1.0 — Compressed single-precision floating point loads and stores extension.
  • zcd — version 1.0 — Compressed double-precision floating point loads and stores extension.
  • zcb — version 1.0 — Simple compressed instruction extension.
  • zce — version 1.0 — Compressed instruction extensions for embedded processors.
  • zcmp — version 1.0 — Compressed push pop extension.
  • zcmt — version 1.0 — Table jump instruction extension.
  • smaia — version 1.0 — Advanced interrupt architecture extension.
  • smepmp — version 1.0 — PMP Enhancements for memory access and execution prevention on Machine mode.
  • smstateen — version 1.0 — State enable extension.
  • ssaia — version 1.0 — Advanced interrupt architecture extension for supervisor-mode.
  • sscofpmf — version 1.0 — Count overflow & filtering extension.
  • ssstateen — version 1.0 — State-enable extension for supervisor-mode.
  • sstc — version 1.0 — Supervisor-mode timer interrupts extension.
  • svinval — version 1.0 — Fine-grained address-translation cache invalidation extension.
  • svnapot — version 1.0 — NAPOT translation contiguity extension.
  • svpbmt — version 1.0 — Page-based memory types extension.
  • xcvmac — version 1.0 — Core-V multiply-accumulate extension.
  • xcvalu — version 1.0 — Core-V miscellaneous ALU extension.
  • xcvelw — version 1.0 — Core-V event load word extension.
  • xtheadba — version 1.0 — T-head address calculation extension.
  • xtheadbb — version 1.0 — T-head basic bit-manipulation extension.
  • xtheadbs — version 1.0 — T-head single-bit instructions extension.
  • xtheadcmo — version 1.0 — T-head cache management operations extension.
  • xtheadcondmov — version 1.0 — T-head conditional move extension.
  • xtheadfmemidx — version 1.0 — T-head indexed memory operations for floating-point registers extension.
  • xtheadfmv — version 1.0 — T-head double floating-point high-bit data transmission extension.
  • xtheadint — version 1.0 — T-head acceleration interruption extension.
  • xtheadmac — version 1.0 — T-head multiply-accumulate extension.
  • xtheadmemidx — version 1.0 — T-head indexed memory operation extension.
  • xtheadmempair — version 1.0 — T-head two-GPR memory operation extension.
  • xtheadsync — version 1.0 — T-head multi-core synchronization extension.
  • xventanacondops — version 1.0 — Ventana integer conditional operations extension.

Okay so quick lil recap on how this all hangs together, bestie — when -march= is not specified, use the setting from -mcpu.

And if both -march and -mcpu= are not specified? The default for this argument is system dependent, and users who want a specific architecture extension should specify one explicitly. Like, don't leave it up to fate, okay? 😭

Also, when the RISC-V specifications define an extension as depending on other extensions, GCC will implicitly add the dependent extensions to the enabled extension set if they weren't added explicitly. So it's got your back 🫶

  • -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-xp450, sifive-x670, thead-c906, tt-ascalon-d8, xiangshan-nanhu. Note that -mcpu does not override -march or -mtune. Periodt. 💅
  • -mtune=processor-string — optimize the output for the given processor, specified by microarchitecture or particular CPU name. Permissible values for this option are: generic-ooo, rocket, sifive-3-series, sifive-5-series, sifive-7-series, size, sifive-p400-series, sifive-p600-series, and all valid options for -mcpu=. When -mtune= is not specified, use the setting from -mcpu, the default is rocket if both are not 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. Lowkey genius ngl ✨
  • -mpreferred-stack-boundary=num — attempt to keep the stack boundary aligned to a 2 raised to num byte boundary. If -mpreferred-stack-boundary is not specified, the default is 4 (16 bytes or 128-bits).
⚠️ Warning: 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. Not a vibe check you wanna fail 😭
  • -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. Slay 💅
  • -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 will only be done if the string is properly aligned and instructions for accelerated processing are available. The default is to not inline strlen calls.
  • -minline-strcmp, -mno-inline-strcmp — do or do not attempt to inline strcmp calls if possible. Inlining will only be done if the strings are properly aligned and instructions for accelerated processing are available. The default is to not inline strcmp calls. The --param riscv-strcmp-inline-limit=n parameter controls the maximum number of bytes compared by the inlined code. The default value is 64.
  • -minline-strncmp, -mno-inline-strncmp — do or do not attempt to inline strncmp calls if possible. Inlining will only be done if the strings are properly aligned and instructions for accelerated processing are available. The default is to not inline strncmp calls. The --param riscv-strcmp-inline-limit=n parameter controls the maximum number of bytes compared by the inlined code. The default value is 64.

(yes, same param for all the strcmp-flavoured ones, that's not a typo on my end 🫶)

  • -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.
  • -mstrict-align, -mno-strict-align — do not or do generate unaligned memory accesses. The default is set depending on whether the processor we are optimizing for supports fast unaligned access or not.
  • -mscalar-strict-align, -mno-scalar-strict-align — do not or do generate unaligned memory accesses. The default is set depending on whether the processor we are optimizing for supports fast unaligned access or not. This is an alias for -mstrict-align.
  • -mvector-strict-align, -mno-vector-strict-align — do not or do generate unaligned vector memory accesses. The default is set to off unless the processor we are optimizing for explicitly supports element-misaligned vector memory access.
  • -mcmodel=medlow — generate code for the medium-low code model. The program and its statically defined symbols must 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. This is the default code model unless GCC has been configured with --with-cmodel= specifying a different default code model.
  • -mcmodel=medany — generate code for the medium-any code model. 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. Read that twice bestie 🔥
  • -mcmodel=large — generate code for a large code model, which has no restrictions on size or placement of symbols.
  • -mexplicit-relocs, -mno-exlicit-relocs — use or do not use assembler relocation operators when dealing with symbolic addresses. The alternative is to use assembler macros instead, which may limit optimization. (and yeah, -mno-exlicit-relocs is spelled exactly like that up there — I'm not touching it, that's above my pay grade 💀)
  • -mrelax, -mno-relax — take advantage of linker relaxations to reduce the number of instructions required to materialize symbol addresses. The default is to take advantage of linker relaxations.
  • -mriscv-attribute, -mno-riscv-attribute — tells GCC to (or not to, bestie) drop a RISC-V attribute into the ELF objects so all that extra info gets recorded 💾. Heads up tho: this one needs at least binutils 2.32, no earlier, periodt.
  • -mcsr-check, -mno-csr-check — flips CSR checking on or off. That's it, that's the whole vibe 🎀
  • -malign-data=type — controls how GCC lines up (aligns) variables and constants that are array, structure, or union types. The supported values for type are:
    • xlen — uses the x register width as the alignment value. Also, lowkey, this is the default ✨
    • natural — uses natural alignment instead.
  • -mbig-endian — generates big-endian code 🌈. This is the default when GCC was configured for a riscv64be-*-* or riscv32be-*-* target.
  • -mlittle-endian — generates little-endian code 🦋. This is the default when GCC was configured for a riscv64-*-* or riscv32-*-* target — but not a riscv64be-*-* or riscv32be-*-* one, k?
  • -mstack-protector-guard=guard, -mstack-protector-guard-reg=reg, -mstack-protector-guard-offset=offset — generates stack protection code with the canary living at guard 🐤. The supported locations are:
    • global — for a global canary
    • tls — for a per-thread canary in the TLS block

With that second choice (tls), the options -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset go even further and say which register to use as the base register for reading the canary, and what offset from that base register to read it from. There's literally no default register or offset, because this whole thing is entirely for use inside the Linux kernel 🐧 — iykyk.

  • -mtls-dialect=desc — uses TLS descriptors as the thread-local storage mechanism for dynamic accesses of TLS variables ✨
  • -mtls-dialect=trad — uses traditional TLS as the thread-local storage mechanism for dynamic accesses of TLS variables. And this one? This is the default 💅

🎀 RL78 Options 🎀

  • -msim — links in extra target libraries so things can actually run inside a simulator 🖥️✨
  • -mmul=none, -mmul=g10, -mmul=g13, -mmul=g14, -mmul=rl78 — picks what kind of hardware multiplication and division support you want. Let's break it down, bestie:
    • none is the simplest — it uses software for both multiplication and division. And yeah, this is the default.
    • g13 is for the hardware multiply/divide peripheral found on the RL78/G13 (S2 core) targets.
    • g14 selects the multiplication and division instructions supported by the RL78/G14 (S3 core) parts.
    • rl78 is an alias for g14, and — no cap — the value mg10 is an alias for none.

On top of all that, a C preprocessor macro gets defined based on how you set this option 👑. The possible values are: __RL78_MUL_NONE__, __RL78_MUL_G13__ or __RL78_MUL_G14__.

  • -mcpu=g10, -mcpu=g13, -mcpu=g14, -mcpu=rl78 — says which RL78 core to target. The default is the G14 core, also known as an S3 core or just RL78. Here's the tea:
    • The G13 or S2 core does not have multiply or divide instructions — it uses a hardware peripheral for those operations instead.
    • The G10 or S1 core doesn't have register banks, so it uses a different calling convention entirely.

If you set this option, it also selects the type of hardware multiply support to use — unless you override it with an explicit -mmul=none on the command line 🔥. So specifying -mcpu=g13 enables the G13 hardware multiply peripheral, and specifying -mcpu=g10 disables hardware multiplications altogether.

Note time 📝: even though the RL78/G14 core is the default target, specifying -mcpu=g14 or -mcpu=rl78 on the command line does change the behaviour of the toolchain, because it also enables G14 hardware multiply support. If you don't specify these options on the command line, then software multiplication routines get used even though the code targets the RL78 core. Why? Backwards compatibility with older toolchains that didn't have hardware multiply and divide support. Iconic legacy behaviour, honestly 💀

Also: a C preprocessor macro is defined based on this option's setting. Possible values: __RL78_G10__, __RL78_G13__ or __RL78_G14__.

  • -mg10, -mg13, -mg14, -mrl78 — these are aliases for the corresponding -mcpu= option, provided for backwards compatibility 🫶
  • -mallregs — lets the compiler use all the available registers 🌈. By default the registers r24..r31 are reserved for use in interrupt handlers, but with this option enabled those registers can be used in ordinary functions too. More registers, more problems? No, more registers, more slay.
  • -m64bit-doubles, -m32bit-doubles — makes the double data type be 64 bits (-m64bit-doubles) or 32 bits (-m32bit-doubles) in size. The default is -m32bit-doubles ⭐
  • -msave-mduc-in-interrupts, -mno-save-mduc-in-interrupts — says interrupt handler functions should preserve the MDUC registers 🛟. You only need this 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, because that makes the interrupt handlers faster ⚡. And the target option -mg13 needs to be passed for this to work, since the feature is only available on the G13 target (S2 core). Also worth knowing: the MDUC registers will only be saved if the interrupt handler performs a multiplication or division operation, or if it calls another function. Conditional saving, we love to see it 💅

👑 IBM RS/6000 and PowerPC Options 👑

These -m options are defined for the IBM RS/6000 and PowerPC 💖:

  • -mpowerpc-gpopt, -mno-powerpc-gpopt
  • -mpowerpc-gfxopt, -mno-powerpc-gfxopt
  • -mpowerpc64, -mno-powerpc64
  • -mmfcrf, -mno-mfcrf
  • -mpopcntb, -mno-popcntb
  • -mpopcntd, -mno-popcntd
  • -mfprnd, -mno-fprnd
  • -mcmpb, -mno-cmpb
  • -mhard-dfp, -mno-hard-dfp

You use these options to specify which instructions are available on the processor you're using 🎯. The default value of these options is determined when configuring GCC, and specifying -mcpu=cpu_type overrides the specification of these options. Real talk: we recommend you use the -mcpu=cpu_type option rather than the options listed above 💅

Here's what each one actually does, because you deserve the details, bestie:

Specifying -mpowerpc-gpopt allows GCC to use the optional PowerPC architecture instructions in the General Purpose group, including floating-point square root. Specifying -mpowerpc-gfxopt allows GCC to use the optional PowerPC architecture instructions in the Graphics group, including floating-point select ✨

The -mmfcrf option allows GCC to generate the move from condition register field instruction implemented on the POWER4 processor and other processors that support the PowerPC V2.01 architecture. The -mpopcntb option allows GCC to generate the popcount and double-precision FP reciprocal estimate instruction implemented on the POWER5 processor and other processors that support the PowerPC V2.02 architecture. The -mpopcntd option allows GCC to generate the popcount instruction implemented on the POWER7 processor and other processors that support the PowerPC V2.06 architecture. The -mfprnd option allows GCC to generate the FP round to integer instructions implemented on the POWER5+ processor and other processors that support the PowerPC V2.03 architecture. The -mcmpb option allows GCC to generate the compare bytes instruction implemented on the POWER6 processor and other processors that support the PowerPC V2.05 architecture. The -mhard-dfp option allows GCC to generate the decimal floating-point instructions implemented on some POWER processors 🔥

And the -mpowerpc64 option allows GCC to generate the additional 64-bit instructions found in the full PowerPC64 architecture, and to treat GPRs as 64-bit, doubleword quantities. GCC defaults to -mno-powerpc64.

  • -mcpu=cpu_type — sets the architecture type, register usage, and instruction scheduling parameters for machine type cpu_type 🧠. The supported values for cpu_type are:
401, 403, 405, 405fp, 440, 440fp, 464, 464fp, 476, 476fp, 505,
601, 602, 603, 603e, 604, 604e, 620, 630, 740, 7400, 7450, 750, 801,
821, 823, 860, 970, 8540, a2, e300c2, e300c3, e500mc, e500mc64,
e5500, e6500, ec603e, G3, G4, G5, titan, power3, power4, power5,
power5+, power6, power6x, power7, power8, power9, power10, power11,
powerpc, powerpc64, powerpc64le, rs64, and native.

Okay so about those special ones: -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 the cpu type detects and selects the architecture option that corresponds to the host processor of the system performing the compilation. But — important — -mcpu=native has no effect if GCC doesn't recognize the processor 🙈

The other options specify a specific processor. Code generated under those options runs best on that processor, and may not run at all on others 💀 (yes, that's a real warning, don't come for me).

The -mcpu options automatically enable or disable the following options:

-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 -mrop-protect

The particular options set for any particular CPU varies between compiler versions, depending on what setting seems to produce optimal code for that CPU — it doesn't necessarily reflect the actual hardware's capabilities 👀. If you want to set an individual option to a particular value, you can specify it after the -mcpu option, like -mcpu=970 -mno-altivec.

On AIX, the -maltivec and -mpowerpc64 options are not enabled or disabled by the -mcpu option at present, because AIX doesn't have full support for these options. You can still enable or disable them individually if you're sure it'll work in your environment 🫶

  • -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_type does 🌸. Same values for cpu_type as -mcpu. And if you specify both? The code generated uses the architecture and registers set by -mcpu, but the scheduling parameters set by -mtune. Best of both, honestly.
  • -mcmodel=small — generates PowerPC64 code for the small model, where the TOC is limited to 64k 📏✨
  • -mcmodel=medium — bestie this one makes PowerPC64 code for the medium model: the TOC plus other static data can be up to a total of 4G in size 💖 and guess what, this is literally the default for 64-bit Linux. Periodt.
  • -mcmodel=large — the large model, okurr: the TOC can be up to 4G in size, and other data and code is only limited by the 64-bit address space. Big main character energy, no cap.
  • -maltivec, -mno-altivec — generate code that uses (or does not use) AltiVec instructions, and also enable the use of built-in functions that let you poke the AltiVec instruction set more directly ✨. You may also need to set -mabi=altivec to 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.

  • -mvrsave, -mno-vrsave — generate VRSAVE instructions when generating AltiVec code 🎀.
  • -msecure-plt — generates code that lets ld and ld.so build executables and shared libraries with non-executable .plt and .got sections 🔥. This is a PowerPC 32-bit SYSV ABI option.
  • -mbss-plt — generates code that uses a BSS .plt section that ld.so fills in, and requires .plt and .got sections that are both writable and executable 😭. This is a PowerPC 32-bit SYSV ABI option.
  • -msplit-patch-nops — when adding NOPs for a patchable area via the -fpatchable-function-entry option, this emits 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 the global and local entry point further apart. If only a single entry point is generated this option has no effect — so like, nothing happens, iykyk.
  • -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 use of built-in functions that allow more direct access to the VSX instruction set ⭐.
  • -mcrypto, -mno-crypto — enable the use (disable) of the built-in functions that allow direct access to the cryptographic instructions that were added in version 2.07 of the PowerPC ISA 🔐 (well, that's my emoji, the facts are hers).
  • -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 keep) some integer operations adjacent so that the instructions can be fused together on power8 and later processors 👑.
  • -mquad-memory, -mno-quad-memory — generate code that uses (does not use) the non-atomic quad word memory instructions. Heads up bestie: the -mquad-memory option requires use of 64-bit mode ⚠️.
  • -mquad-memory-atomic, -mno-quad-memory-atomic — generate code that uses (does not use) the atomic quad word memory instructions. And same deal: the -mquad-memory-atomic option requires use of 64-bit mode ⚠️.
  • -mfloat128, -mno-float128 — enable/disable the __float128 keyword 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 will also enable the generation of ISA 3.0 IEEE 128-bit floating point instructions ✨. Otherwise, if you do not specify to generate ISA 3.0 instructions or you are targeting a 32-bit big endian system, IEEE 128-bit floating point will be done with software emulation.

  • -mfloat128-hardware, -mno-float128-hardware — enable/disable using ISA 3.0 hardware instructions to support the __float128 data 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 sets int, long and pointer to 32 bits and generates code that runs on any PowerPC variant. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits, and generates code for PowerPC64, as for -mpowerpc64.
  • -mfull-toc, -mno-fp-in-toc, -mno-sum-in-toc, -mminimal-toc — these modify generation of the TOC (Table Of Contents), which is created for every executable file 👀. The -mfull-toc option is selected by default. In that case, GCC allocates at least one TOC entry for each unique non-automatic variable reference in your program. GCC also places floating-point constants in the TOC. However — and this is the plot twist — only 16,384 entries are available in the TOC 😭.

If you receive a linker error message saying you have overflowed the available TOC space, you can reduce the amount of TOC space used with the -mno-fp-in-toc and -mno-sum-in-toc options 🫶. -mno-fp-in-toc prevents GCC from putting floating-point constants in the TOC, and -mno-sum-in-toc forces GCC to generate code to calculate the sum of an address and a constant at run time instead of putting that sum into the TOC. You may specify one or both of these options. Each causes GCC to produce very slightly slower and larger code at the expense of conserving TOC space.

If you still run out of space in the TOC even when you specify both of these options, specify -mminimal-toc instead 💅. This option causes GCC to make only one TOC entry for every file. When you specify this option, GCC produces code that is slower and larger but which uses extremely little TOC space. You may wish to use this option only on files that contain less frequently-executed code.

  • -maix64, -maix32 — enable 64-bit AIX ABI and calling convention: 64-bit pointers, 64-bit long type, and the whole infrastructure needed to support them 🔥. Specifying -maix64 implies -mpowerpc64, while -maix32 disables the 64-bit ABI and implies -mno-powerpc64. GCC defaults to -maix32.
  • -mxl-compat, -mno-xl-compat — produce code that conforms more closely to IBM XL compiler semantics when using AIX-compatible ABI 👑. Pass floating-point arguments to prototyped functions beyond the register save area (RSA) on the stack in addition to argument FPRs. Do not assume that most significant double in 128-bit long double value is properly rounded when comparing values and converting to double. Use XL symbol names for long double support routines.

The AIX calling convention was extended but not initially documented to handle an obscure K&R C case of calling a function that takes the address of its arguments with fewer arguments than declared 👀. IBM XL compilers access floating-point arguments that do not fit in the RSA from the stack when a subroutine is compiled without optimization. Because always storing floating-point arguments on the stack is inefficient and rarely needed, this option is not enabled by default and only is necessary when calling subroutines compiled by IBM XL compilers without optimization.

  • -mpe — support IBM RS/6000 SP Parallel Environment (PE) 🦋. Link an application written to use message passing with special startup code to enable the application to run. The system must have PE installed in the standard location (/usr/lpp/ppe.poe/), or the specs file must be overridden with the -specs= option to specify the appropriate directory location. The Parallel Environment does not support threads, so the -mpe option and the -pthread option are incompatible ⚠️ — do not try it, bestie, it's not giving.
  • -malign-natural, -malign-power — on AIX, 32-bit Darwin, and 64-bit PowerPC GNU/Linux, the option -malign-natural overrides the ABI-defined alignment of larger types, such as floating-point doubles, on their natural size-based boundary 🌈. The option -malign-power instructs GCC to follow the ABI-specified alignment rules. GCC defaults to the standard alignment defined in the ABI.

On 64-bit Darwin, natural alignment is the default, and -malign-power is not supported 💀.

  • -msoft-float, -mhard-float — generate code that does not use (uses) the floating-point register set 🎀. Software floating-point emulation is provided if you use the -msoft-float option, and pass the option to GCC when linking. Don't forget the linking part, that's the whole vibe.
  • -mmultiple, -mno-multiple — generate code that uses (does not use) the load multiple word instructions and the store multiple word instructions ⭐. These instructions are generated by default on POWER systems, and not generated on PowerPC systems. Do not use -mmultiple on little-endian PowerPC systems, since those instructions do not work when the processor is in little-endian mode 😭. The exceptions are PPC740 and PPC750 which permit these instructions in little-endian mode.
  • -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 ⚠️ — yeah, that's a real one, 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 💅.
  • -mfused-madd, -mno-fused-madd — generate code that uses (or doesn't use) the floating-point multiply-and-accumulate instructions 🌀✨. These get generated by default if hardware floating point is in play. Heads up though: the machine-dependent -mfused-madd option is now mapped to the machine-independent -ffp-contract=fast option, and -mno-fused-madd is mapped to -ffp-contract=off. iykyk, the old name still works, it's just rebranded 💅.
  • -mmulhw, -mno-mulhw — generate code that uses (doesn't use) the half-word multiply and multiply-accumulate instructions on the IBM 405, 440, 464 and 476 processors 💖. These are generated by default when you're targeting those processors.
  • -mdlmzb, -mno-dlmzb — generate code that uses (doesn't use) the string-search dlmzb instruction on the IBM 405, 440, 464 and 476 processors 🔍. Also generated by default when targeting those processors.
  • -mno-bit-align, -mbit-align — on System V.4 and embedded PowerPC systems, don't (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. Little structure, big slay 📦✨.

  • -mno-strict-align, -mstrict-align — on System V.4 and embedded PowerPC systems, don't (do) assume that unaligned memory references are handled by the system.
  • -mrelocatable, -mno-relocatable — generate code that allows (doesn't allow) a static executable to be relocated to a different address at run time 🏃‍♀️. A simple embedded PowerPC system loader should relocate the entire contents of ".got2" and 4-byte locations listed in the ".fixup" section — that's a table of 32-bit addresses generated by this option. For this to actually work, all objects linked together must be compiled with -mrelocatable or -mrelocatable-lib. Also -mrelocatable code aligns the stack to an 8-byte boundary.
  • -mrelocatable-lib, -mno-relocatable-lib — like -mrelocatable, -mrelocatable-lib generates a ".fixup" section so static executables can be relocated at run time, but -mrelocatable-lib does not use the smaller stack alignment of -mrelocatable 🫶. Objects compiled with -mrelocatable-lib may be linked with objects compiled with any combination of the -mrelocatable options — it's giving "plays well with others" energy.
  • -mno-toc, -mtoc — on System V.4 and embedded PowerPC systems, don't (do) assume that register 2 contains a pointer to a global area pointing to the addresses used in the program.
  • -mlittle, -mlittle-endian — on System V.4 and embedded PowerPC systems, compile code for the processor in little-endian mode 🐣. The -mlittle-endian option is the same as -mlittle.
  • -mbig, -mbig-endian — on System V.4 and embedded PowerPC systems, compile code for the processor in big-endian mode 🐘. The -mbig-endian option is the same as -mbig.
  • -mdynamic-no-pic — on Darwin / macOS systems, compile code so that it's not relocatable, but its external references are relocatable 🍎. The resulting code is suitable for applications, but not shared libraries.
  • -msingle-pic-base — treat the register used for PIC addressing as read-only, rather than loading it in the prologue for each function 🔒. The runtime system is responsible for initializing this register with an appropriate value before execution begins. Not your job, bestie.
  • *-mprioritize-restricted-insns=priority* — this option controls the priority that gets assigned to dispatch-slot restricted instructions during the second scheduling pass. The argument priority takes the value 0, 1, or 2 to assign no, highest, or second-highest (respectively) priority to dispatch-slot restricted instructions. Ranked choice but for instructions 🗳️.
  • *-msched-costly-dep=dependence_type* — controls which dependences are considered costly by the target during instruction scheduling 💸. The argument dependence_type takes one of these 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* — controls which NOP insertion scheme is used during the second scheduling pass 🧩. The argument scheme takes one of these values:
    • no — don't insert NOPs.
    • pad — pad with NOPs any dispatch group that has vacant issue slots, according to the scheduler's grouping.
    • regroup_exact — insert NOPs to force costly dependent insns into separate groups. Insert exactly as many NOPs as needed to force an insn to a new group, according to the estimated processor grouping.
    • number — insert NOPs to force costly dependent insns into separate groups. Insert number NOPs to force an insn to a new group.
  • -mcall-sysv — on System V.4 and embedded PowerPC systems, compile code using calling conventions that adhere to the March 1995 draft of the System V Application Binary Interface, PowerPC processor supplement 📜. This is the default unless you configured GCC using powerpc-*-eabiaix.
  • -mcall-sysv-eabi, -mcall-eabi — specify both -mcall-sysv and -meabi options. Two-for-one, we love a bundle 🎀.
  • -mcall-sysv-noeabi — specify both -mcall-sysv and -mno-eabi options.
  • -mcall-aixdesc — on System V.4 and embedded PowerPC systems, compile code for the AIX operating system.
  • -mcall-linux — on System V.4 and embedded PowerPC systems, compile code for the Linux-based GNU system 🐧.
  • -mcall-freebsd — on System V.4 and embedded PowerPC systems, compile code for the FreeBSD operating system 😈.
  • -mcall-netbsd — on System V.4 and embedded PowerPC systems, compile code for the NetBSD operating system.
  • -mcall-openbsd — on System V.4 and embedded PowerPC systems, compile code for the OpenBSD operating system 🐡.
  • *-mtraceback=traceback_type* — select the type of traceback table. Valid values for traceback_type are full, part, and no.
  • -maix-struct-return — return all structures in memory (as specified by the AIX ABI) 📥.
  • -msvr4-struct-return — return structures smaller than 8 bytes in registers (as specified by the SVR4 ABI) 📤.
  • *-mabi=abi-type* — extend the current ABI with a particular extension, or remove such extension. Valid values are: altivec, no-altivec, ibmlongdouble, ieeelongdouble, elfv1, elfv2, and for AIX: vec-extabi, vec-default 🧬.
  • -mabi=ibmlongdouble — change the current ABI to use IBM extended-precision long double. This is not likely to work if your system defaults to using IEEE extended-precision long double 😭. If you change the long double type from IEEE extended-precision, the compiler issues a warning unless you use the -Wno-psabi option. Requires -mlong-double-128 to be enabled.
  • -mabi=ieeelongdouble — change the current ABI to use IEEE extended-precision long double ⚡. This is not likely to work if your system defaults to using IBM extended-precision long double. If you change the long double type from IBM extended-precision, the compiler issues a warning unless you use the -Wno-psabi option. Requires -mlong-double-128 to be enabled.
  • -mabi=elfv1 — change the current ABI to use the ELFv1 ABI. This is the default ABI for big-endian PowerPC 64-bit Linux 🐘. Overriding the default ABI requires special system support and is likely to fail in spectacular ways 💀. You've been warned, respectfully.
  • -mabi=elfv2 — change the current ABI to use the ELFv2 ABI. This is the default ABI for little-endian PowerPC 64-bit Linux 🐣. Overriding the default ABI requires special system support and is likely to fail in spectacular ways 💀. Same energy, different endianness.
  • -mgnu-attribute, -mno-gnu-attribute — emit .gnu_attribute assembly directives to set tag/value pairs in a .gnu.attributes section that specify ABI variations in function parameters or return values 🏷️.
  • -mprototype, -mno-prototype — on System V.4 and embedded PowerPC systems, assume that all calls to variable argument functions are properly prototyped. Otherwise, the compiler must insert an instruction before every non-prototyped call to set or clear bit 6 of the condition code register ("CR") to indicate whether floating-point values are passed in the floating-point registers in case the function takes variable arguments 🤔. With -mprototype, only calls to prototyped variable argument functions set or clear the bit.
  • -msim — on embedded PowerPC systems, assume that the startup module is called sim-crt0.o and that the standard C libraries are libsim.a and libc.a. This is the default for powerpc-*-eabisim configurations 🎮.
  • -mmvme — on embedded PowerPC systems, assume that the startup module is called crt0.o and the standard C libraries are libmvme.a and libc.a.
  • -mads — on embedded PowerPC systems, assume that the startup module is called crt0.o and the standard C libraries are libads.a and libc.a.
  • -myellowknife — on embedded PowerPC systems, assume that the startup module is called crt0.o and the standard C libraries are libyk.a and libc.a 💛🔪. (Yes, that's the real name. Iconic.)
  • -mvxworks — on System V.4 and embedded PowerPC systems, specify that you're compiling for a VxWorks system.
  • -memb — on embedded PowerPC systems, set the "PPC_EMB" bit in the ELF flags header to indicate that eabi extended relocations are used 🏁.
  • -meabi, -mno-eabi — on System V.4 and embedded PowerPC systems, do (do not) adhere to the Embedded Applications Binary Interface (EABI), which is a set of modifications to the System V.4 specifications 📚. Selecting -meabi means the stack is aligned to an 8-byte boundary, a function "__eabi" is called from "main" to set up the EABI environment, and the -msdata option can use both "r2" and "r13" to point to two separate small data areas. Selecting -mno-eabi means the stack is aligned to a 16-byte boundary, no EABI initialization function is called from "main", and the -msdata option only uses "r13" to point to a single small data area. The -meabi option is on by default if you configured GCC using one of the powerpc*-*-eabi* options ✨.
  • -msdata=eabi — on System V.4 and embedded PowerPC systems, this one's giving "small data era" ✨ it puts small initialized const global and static data into the .sdata2 section (which register r2 points at), and small initialized non-const global and static data into the .sdata section (register r13's whole vibe). Small uninitialized global and static data goes in .sbss, right next to .sdata. Heads up though: -msdata=eabi is incompatible with -mrelocatable 😭 and it also sets -memb while it's at it.
  • -msdata=sysv — on System V.4 and embedded PowerPC systems, small global and static data goes into .sdata (pointed to by register r13), and small uninitialized global and static data goes into .sbss, which sits adjacent to .sdata. Also incompatible with -mrelocatable, no cap 🫶
  • -msdata=default, -msdata — on System V.4 and embedded PowerPC systems: if -meabi is used, it compiles code exactly like -msdata=eabi; otherwise it compiles like -msdata=sysv. Basically "depends on the situation" energy 💅
  • -msdata=data — on System V.4 and embedded PowerPC systems, small global data goes into .sdata, and small uninitialized global data goes into .sbss. But plot twist: it does not use register r13 to address that small data. This is the default behavior unless some other -msdata option is used 👑
  • -msdata=none, -mno-sdata — on embedded PowerPC systems, all initialized global and static data goes into .data, and all uninitialized data goes into .bss. Simple. Clean. Periodt.
  • -mreadonly-in-sdata — puts read-only objects in .sdata too. And yes bestie, this is the default already 🌸
  • -mblock-move-inline-limit=num — inlines all block moves (think calls to memcpy or structure copies) that are less than or equal to num bytes. The minimum for num is 32 bytes on 32-bit targets and 64 bytes on 64-bit targets, and the default value is target-specific 🦋
  • -mblock-compare-inline-limit=num — generates non-looping inline code for all block compares (like memcmp calls or structure compares) less than or equal to num bytes. If num is 0, all inline expansion (non-loop and loop) of block compare gets disabled. Default value is target-specific ⭐
  • -mblock-compare-inline-loop-limit=num — generates an inline expansion using loop code for all block compares 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 get compared before memcmp is called for the rest of the block. Default value is target-specific 💜
  • -mstring-compare-inline-limit=num — compares at most num string bytes with inline code. If the difference or end of string isn't found at the end of that inline compare, a call to strcmp or strncmp handles the rest. 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. Default num is 8. The -G num switch also gets passed to the linker, and all modules should be compiled with the same -G num value — no mixing, bestie 🙈
  • -mregnames, -mno-regnames — on System V.4 and embedded PowerPC systems, do (or don't) emit register names in the assembly language output using symbolic forms ✨
  • -mlongcall, -mno-longcall — by default it assumes all calls are far away, so a longer and more expensive calling sequence is required. This is required for calls farther than 32 megabytes (33,554,432 bytes) from the current location. A short call gets generated if the compiler knows the call can't be that far away. You can override this setting with the shortcall function attribute, or with #pragma longcall(0).

Some linkers can detect out-of-range calls and generate glue code on the fly. On those systems, long calls are unnecessary and just generate slower code. As of this writing, the AIX linker can do this, and so can the GNU linker for PowerPC/64. It's planned to add this feature to the GNU linker for 32-bit PowerPC systems too 🔥

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 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 gets 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 the 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 only, iykyk 👀)

  • -mpltseq, -mno-pltseq — implement (or don't implement) -fno-plt and 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 that support is detected when configuring GCC — and, for 32-bit PowerPC, if GCC is configured with --enable-secureplt. Warning bestie: -mpltseq code and -mbss-plt 32-bit PowerPC relocatable objects may not be linked together 😭
  • -mtls-markers, -mno-tls-markers — mark (or don't mark) calls to __tls_get_addr with a relocation specifying the function argument. That relocation lets the linker reliably associate the function call with the argument setup instructions for TLS optimization, which in turn lets GCC schedule the sequence better 💅
  • -mrecip, -mno-recip — enables use of the reciprocal estimate and reciprocal square root estimate instructions with extra Newton-Raphson steps to bump up precision, instead of doing a divide or square root and divide for floating-point arguments. You should use -ffast-math when using -mrecip (or at least -funsafe-math-optimizations, -ffinite-math-only, -freciprocal-math and -fno-trapping-math). And ngl, note this: while the throughput of the sequence is generally higher than the throughput of the non-reciprocal instruction, the precision of the sequence can drop by up to 2 ulp (i.e. the inverse of 1.0 equals 0.99999994) for reciprocal square roots 😬
  • -mrecip=opt — controls which reciprocal estimate instructions may be used. opt is a comma-separated list of options, which may be preceded by a ! to invert the option:
    • all — enable all estimate instructions.
    • default — enable the default instructions, equivalent to -mrecip.
    • none — disable all estimate instructions, equivalent to -mno-recip.
    • div — enable the reciprocal approximation instructions for both single and double precision.
    • divf — enable the single-precision reciprocal approximation instructions.
    • divd — enable the double-precision reciprocal approximation instructions.
    • rsqrt — enable the reciprocal square root approximation instructions for both single and double precision.
    • rsqrtf — enable the single-precision reciprocal square root approximation instructions.
    • rsqrtd — enable the double-precision reciprocal square root approximation instructions.

So, for example, -mrecip=all,!rsqrtd enables all of the reciprocal estimate instructions, except for the FRSQRTE, XSRSQRTEDP, and XVRSQRTEDP instructions which handle the double-precision reciprocal square root calculations 🎀

  • -mrecip-precision, -mno-recip-precision — assume (or don't assume) that the reciprocal estimate instructions provide higher-precision estimates than the PowerPC ABI mandates. Selecting -mcpu=power6, -mcpu=power7 or -mcpu=power8 automatically selects -mrecip-precision. The double-precision square root estimate instructions are not generated by default on low-precision machines, since they don't provide an estimate that converges after three steps 🦋
  • -mveclibabi=type — specifies the ABI type to use for vectorizing intrinsics using an external library. The only type supported at present is mass, which says to use IBM's Mathematical Acceleration Subsystem (MASS) libraries for vectorizing intrinsics using external libraries. GCC currently emits calls to acosd2, acosf4, acoshd2, acoshf4, asind2, asinf4, asinhd2, asinhf4, atan2d2, atan2f4, atand2, atanf4, atanhd2, atanhf4, cbrtd2, cbrtf4, cosd2, cosf4, coshd2, coshf4, erfcd2, erfcf4, erfd2, erff4, exp2d2, exp2f4, expd2, expf4, expm1d2, expm1f4, hypotd2, hypotf4, lgammad2, lgammaf4, log10d2, log10f4, log1pd2, log1pf4, log2d2, log2f4, logd2, logf4, powd2, powf4, sind2, sinf4, sinhd2, sinhf4, sqrtd2, sqrtf4, tand2, tanf4, tanhd2, and tanhf4 when generating code for power7. Both -ftree-vectorize and -funsafe-math-optimizations must also be enabled, and the MASS libraries must be specified at link time 🔥
  • -mfriz, -mno-friz — generate (or don't generate) the friz instruction when the -funsafe-math-optimizations option is used to optimize rounding of floating-point values to 64-bit integers and back to floating point. Caveat tho: the friz instruction does not return the same value if the floating-point number is too large to fit in an integer 💀
  • -mpointers-to-nested-functions, -mno-pointers-to-nested-functions — bestie, this one generates (or doesn't generate) the code that loads up the static chain register "r11" when you're calling through a pointer on AIX and 64-bit Linux systems, where a function pointer points to a 3-word descriptor that gives you the function address, the TOC value to load into register "r2", and the static chain value to load into register "r11". The -mpointers-to-nested-functions is on by default, periodt. But heads up — you literally 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. So like… don't do that to yourself 💀
  • -msave-toc-indirect, -mno-save-toc-indirect — generates (or doesn'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 just before the call through the pointer. The -mno-save-toc-indirect option is the default 🎀
  • -mcompat-align-parm, -mno-compat-align-parm — generates (or doesn't generate) code to pass structure parameters with a maximum alignment of 64 bits, for compatibility with older versions of GCC.

So the lore: 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 been corrected in more recent versions of GCC ✨ This option can be used to generate code that's compatible with functions compiled with older versions of GCC — a little blast-from-the-past moment, iykyk.

The -mno-compat-align-parm option is the default.

  • -mstack-protector-guard=guard**, **-mstack-protector-guard-reg=*reg*, *-mstack-protector-guard-offset=offset*** — generates stack protection code using a canary at *guard*. Supported locations are globalfor global canary, ortls` for per-thread canary in the TLS block (that's the default with GNU libc version 2.4 or later) 🦋

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. The default for those is as specified in the relevant ABI. No cap 💅

  • -mpcrel, -mno-pcrel — generates (or doesn't generate) pc-relative addressing. The -mpcrel option requires that the medium code model (-mcmodel=medium) and prefixed addressing (-mprefixed) options are enabled. It's giving very "you can't sit with us unless you bring your friends" energy 😭
  • -mprefixed, -mno-prefixed — generates (or doesn't generate) addressing modes using prefixed load and store instructions. The -mprefixed option requires that the option -mcpu=power10 (or later) is enabled ⭐
  • -mmma, -mno-mma — generates (or doesn't generate) the MMA instructions. The -mma option requires that the option -mcpu=power10 (or later) is enabled. Slay, but read the prerequisites first 🫶
  • -mrop-protect, -mno-rop-protect — generates (or doesn't generate) ROP protection instructions when the target processor supports them. Currently this option disables the shrink-wrap optimization (-fshrink-wrap). Lowkey important tradeoff, ngl 🔥
  • -mprivileged, -mno-privileged — generates (or doesn't generate) code that will run in privileged state 👑
  • -mblock-ops-unaligned-vsx, -mno-block-ops-unaligned-vsx — generates (or doesn't generate) unaligned vsx loads and stores for inline expansion of "memcpy" and "memmove" 🧚
  • --param rs6000-vect-unroll-limit= — the vectorizer will check with target information to determine whether it would be beneficial to unroll the main vectorized loop and by how much. This parameter sets the upper bound of how much the vectorizer will unroll the main loop. The default value is four. Four! That's the number, don't come for me 🍓

🌈 RX Options

These command-line options are defined for RX targets, bestie. Buckle up, there's a whole little ecosystem here 💖

  • -m64bit-doubles, -m32bit-doubles — makes the "double" data type be 64 bits (-m64bit-doubles) or 32 bits (-m32bit-doubles) in size. The default is -m32bit-doubles. Note that RX floating-point hardware only works on 32-bit values, which is exactly why the default is -m32bit-doubles. It's giving "the hardware said no" 😭
  • -fpu, -nofpu — enables (-fpu) or disables (-nofpu) the use of RX floating-point hardware. The default is enabled for the RX600 series and disabled for the RX200 series ✨

Floating-point instructions are only generated for 32-bit floating-point values, however, so the FPU hardware is not used for doubles if the -m64bit-doubles option is used.

Heads up: if the -fpu option is enabled then -funsafe-math-optimizations is also enabled automatically. This is because the RX FPU instructions are themselves unsafe. Like, unsafe unsafe. No cap 💀
  • -mcpu=name — selects the type of RX CPU to be targeted. Currently three types are supported: the generic RX600 and RX200 series hardware, and the specific RX610 CPU. The default is RX600 👑

The only difference between RX600 and RX610 is that the RX610 does not support the "MVTIPL" instruction. That's it, that's the whole drama.

The RX200 series does not have a hardware floating-point unit, and so -nofpu is enabled by default when this type is selected.

  • -mbig-endian-data, -mlittle-endian-data — store data (but not code, bestie, read carefully) in the big-endian format. The default is -mlittle-endian-data, i.e. to store data in the little-endian format 🦋
  • -msmall-data-limit=N — specifies the maximum size in bytes of global and static variables which can be placed into the small data area. Using the small data area can lead to smaller and faster code, but the size of the area is limited and it is up to the programmer to ensure that the area does not overflow 🙈 Also, when the small data area is used, one of the RX's registers (usually "r13") is reserved for use pointing to this area, so it is no longer available for use by the compiler. This could result in slower and/or larger code if variables are pushed onto the stack instead of being held in this register. Tradeoffs, tradeoffs.

Note that common variables (variables that have not been initialized) and constants are not placed into the small data area, as they are assigned to other sections in the output executable.

The default value is zero, which disables this feature. Note that this feature is not enabled by default with higher optimization levels (-O2 etc) because of the potentially detrimental effects of reserving a register. It is up to the programmer to experiment and discover whether this feature is of benefit to their program. See the description of the -mpid option for a description of how the actual register to hold the small data area pointer is chosen. Teamwork, bestie 🤝

  • -msim, -mno-sim — use the simulator runtime. The default is to use the libgloss board-specific runtime 🎀
  • -mas100-syntax, -mno-as100-syntax — when generating assembler output, use a syntax that's compatible with Renesas's AS100 assembler. This syntax can also be handled by the GAS assembler, but it has some restrictions, so it's not generated by default. Restrictive queen 💅
  • -mmax-constant-size=N — specifies the maximum size, in bytes, of a constant that can be used as an operand in a RX instruction. Although the RX instruction set does allow constants of up to 4 bytes in length to be used in instructions, a longer value equates to a longer instruction. Thus in some circumstances it can be beneficial to restrict the size of constants that are used in instructions. Constants that are too big are instead placed into a constant pool and referenced via register indirection.

The value N can be between 0 and 4. A value of 0 (the default) or 4 means that constants of any size are allowed. So like, 0 and 4 mean "go wild" 🌸

  • -mrelax — enable linker relaxation. Linker relaxation is a process whereby the linker attempts to reduce the size of a program by finding shorter versions of various instructions. Disabled by default. It's giving "the linker is doing its little glow-up" ✨
  • -mint-register=N — specify the number of registers to reserve for fast interrupt handler functions. The value N can be between 0 and 4. A value of 1 means that register "r13" is reserved for the exclusive use of fast interrupt handlers. A value of 2 reserves "r13" and "r12". A value of 3 reserves "r13", "r12" and "r11", and a value of 4 reserves "r13" through "r10". A value of 0, the default, does not reserve any registers. Count them, bestie 🔢
  • -msave-acc-in-interrupts — specifies that interrupt handler functions should preserve the accumulator register. This is only necessary if normal code might use the accumulator register, for example because it performs 64-bit multiplications. The default is to ignore the accumulator, as this makes the interrupt handlers faster 🔥
  • -mpid, -mno-pid — enables the generation of position independent data. When enabled, any access to constant data is done via an offset from a base address held in a register. This allows the location of constant data to be determined at run time without requiring the executable to be relocated, which is a benefit to embedded applications with tight memory constraints. Data that can be modified is not affected by this option. Immutable data only, iykyk 🦋

Note that using this feature reserves a register, usually "r13", for the constant data base address. This can result in slower and/or larger code, especially in complicated functions. Sad but true.

The actual register chosen to hold the constant data base address depends upon whether the -msmall-data-limit and/or the -mint-register command-line options are enabled. Starting with register "r13" and proceeding downwards, registers are allocated first to satisfy the requirements of -mint-register, then -mpid and finally -msmall-data-limit. Thus it is possible for the small data area register to be "r8" if both -mint-register=4 and -mpid are specified on the command line. Register allocation drama, iconic honestly 👑

By default this feature is not enabled. The default can be restored via the -mno-pid command-line option.

  • -mno-warn-multiple-fast-interrupts, -mwarn-multiple-fast-interrupts — prevents GCC from issuing a warning message if it finds more than one fast interrupt handler when it's compiling a file. The default is to issue a warning for each extra fast interrupt handler found, as the RX only supports one such interrupt. One (1) interrupt, bestie. That's the limit 😭
  • -mallow-string-insns, -mno-allow-string-insns — turns the string-manipulation instructions SMOVF, SCMPU, SMOVB, SMOVU, SUNTIL, SWHILE on or off, plus the RMPA instruction too. ⚠️ These bad boys may prefetch data, and that is not safe if you're touching an I/O register. (Section 12.2.7 of the RX62N Group User's Manual has the receipts, iykyk.)

The default is to allow them, but bestie, GCC genuinely cannot reliably detect every situation where a string instruction might end up hitting an I/O register, so it can't just disable them for you automatically. Instead it's on you, the programmer, to pass -mno-allow-string-insns if your 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 one's your bestie when code size exceeds the range of BSR instructions. Note that -mno-jsr does not mean "never use JSR" — it means any type of branch may be used. 🌀
Note: the generic GCC command-line option -ffixed-reg has special significance to the RX port when used with the interrupt function attribute. That 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-reg or -mint-register command-line options. 👑

🌈 S/390 and zSeries Options 🌈

These are the -m options defined for the S/390 and zSeries architecture. Big iron vibes only. 🏗️

  • -mhard-float, -msoft-float — use (or don't use) the hardware floating-point instructions and registers for floating-point operations. With -msoft-float, functions in libgcc.a do the floating-point heavy lifting. With -mhard-float, 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. With -mno-hard-dfp, functions in libgcc.a perform decimal-floating-point operations. With -mhard-dfp, the compiler generates decimal-floating-point hardware instructions. This is the default for -march=z9-ec or higher. 🔢
  • -mlong-double-64, -mlong-double-128 — these switches control the size of the long double type. A size of 64 bits makes long double equivalent to double. 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 may be needed so you can debug with tools that don't understand DWARF call frame information. When -mno-packed-stack is in effect, the backchain pointer goes at the bottom of the stack frame; when -mpacked-stack is in effect, the backchain is placed into the topmost word of the 96/160 byte register save area. 🧵

In general, code compiled with -mbackchain is call-compatible with code compiled with -mno-backchain; however, using the backchain for debugging usually requires the whole binary be built with -mbackchain. Note that the combination of -mbackchain, -mpacked-stack and -mhard-float is not supported. In order to build a linux kernel, use -msoft-float. 🐧

The default is to not maintain the backchain.

  • -mpacked-stack, -mno-packed-stack — use (or don't use) the packed stack layout. With -mno-packed-stack, 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. With -mpacked-stack, register save slots are densely packed at the top of the register save area; unused space gets reused for other things, which is way more efficient with the available stack space. However, when -mbackchain is also in effect, the topmost word of the save area is always used to store the backchain, and the return address register is always saved two words below the backchain. 📦

As long as the stack frame backchain isn't used, code generated with -mpacked-stack is call-compatible with code generated with -mno-packed-stack. Note that some non-FSF releases of GCC 2.95 for S/390 or zSeries generated code that uses the stack frame backchain at run time, not just for debugging — such code is not call-compatible with code compiled with -mpacked-stack. Also note that the combination of -mbackchain, -mpacked-stack and -mhard-float is not supported. In order to build a linux kernel, use -msoft-float. 💀

The default is to not use the packed stack layout.

  • -msmall-exec, -mno-small-exec — generate (or don't generate) code using the bras instruction to do subroutine calls. This only works reliably if the total executable size does not exceed 64k. The default is to use the basr instruction instead, which doesn't have that limitation. 🐜
  • -m64, -m31 — with -m31, generate code compliant to the GNU/Linux for S/390 ABI. With -m64, generate code compliant to the GNU/Linux for zSeries ABI. This lets GCC in particular generate 64-bit instructions. For the s390 targets the default is -m31, while the s390x targets default to -m64. 🔢
  • -mzarch, -mesa — with -mzarch, generate code using the instructions available on z/Architecture. With -mesa, generate code using the instructions available on ESA/390. Note that -mesa is not possible with -m64. When generating code compliant to the GNU/Linux for S/390 ABI, the default is -mesa. When generating code compliant to the GNU/Linux for zSeries ABI, the default is -mzarch. 🏛️
  • -mhtm, -mno-htm — -mhtm enables a set of builtins making use of instructions available with the transactional execution facility introduced with the IBM zEnterprise EC12 machine generation S/390 System z Built-in Functions. -mhtm is enabled by default when using -march=zEC12. ⚡
  • -mvx, -mno-vx — with -mvx, generate code using the instructions available with the vector extension facility introduced with the IBM z13 machine generation. This option changes the ABI for some vector type values with regard to alignment and calling conventions. In case 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 available with 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 will not be expanded when requesting strict standard compliance, e.g. with -std=c99. In addition to the GCC low-level builtins, -mzvector enables a set of builtins added for compatibility with AltiVec-style implementations like Power and Cell. In order to make use of these builtins, the header file vecintrin.h needs to be included. -mzvector is disabled by default. 🎛️
  • -mmvcle, -mno-mvcle — generate (or don't generate) code using the mvcle instruction to perform block moves. When -mno-mvcle is specified, use an mvc loop instead. This is the default unless optimizing for size. 🚚
  • -mdebug, -mno-debug — print (or don't print) additional debug information when compiling. The default is to not print debug information. 🐛
  • -march=cpu-type — generate code that runs on cpu-type, which is the name of a system representing a certain processor type. Possible values for cpu-type are 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 doesn't 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 (or 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 (or 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. ⏭️
  • -mfused-madd, -mno-fused-madd — generate code that uses (or 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 have to be a real problem when the program runs. It's meant to identify functions that most probably cause a stack overflow, and it's useful in an environment with limited stack size, e.g. the linux kernel. 🚨
  • -mwarn-dynamicstack — emit a warning if the function calls alloca or uses dynamically-sized arrays. This is generally a bad idea with a limited stack size. 🙈
  • -mstack-guard=stack-guard, -mstack-size=stack-size — bestie, if you pass these two, the S/390 back end sprinkles extra instructions into the function prologue that trigger a trap if the stack size is stack-guard bytes above stack-size (and yes, remember the stack on S/390 grows downward ⬇️). If you skip stack-guard, the compiler picks the smallest power of 2 that's larger than the frame size of the compiled function. These are meant to help you debug stack overflow problems — the extra code is only a little bit of overhead, so it can also go in production-like systems without greater performance degradation 👑. The values you give have to be exact powers of 2, and stack-size has to be greater than stack-guard without exceeding 64k. To be efficient, the extra code assumes the stack starts at an address aligned to the value given by stack-size. Also stack-guard can only be used in conjunction with stack-size.
  • -mhotpatch=pre-halfwords,post-halfwords — turn this on and a "hot-patching" function prologue gets 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 get appended, using the largest NOP-like instructions the architecture allows (maximum 1000000). If both arguments are zero, hotpatching is disabled. You can override this per-function with the "hotpatch" attribute 🎀.

✨ SH Options ✨

These -m options are defined for the SH implementations, and there are... a lot of them. Buckle up, bestie 💅

  • -m1 — generate code for the SH1.
  • -m2 — generate code for the SH2.
  • -m2e — generate code for the SH2e.
  • -m2a-nofpu — generate code for the SH2a without FPU, or for a SH2a-FPU in such a way that the floating-point unit is not used.
  • -m2a-single-only — generate code for the SH2a-FPU, in such a way that no double-precision floating-point operations are used.
  • -m2a-single — generate code for the SH2a-FPU assuming the floating-point unit is in single-precision mode by default.
  • -m2a — generate code for the SH2a-FPU assuming the floating-point unit is in double-precision mode by default.
  • -m3 — generate code for the SH3.
  • -m3e — generate code for the SH3e.
  • -m4-nofpu — generate code for the SH4 without a floating-point unit.
  • -m4-single-only — generate code for the SH4 with a floating-point unit that only supports single-precision arithmetic.
  • -m4-single — generate code for the SH4 assuming the floating-point unit is in single-precision mode by default.
  • -m4 — generate code for the SH4.
  • -m4-100 — generate code for SH4-100.
  • -m4-100-nofpu — generate code for SH4-100 in such a way that the floating-point unit is not used.
  • -m4-100-single — generate code for SH4-100 assuming the floating-point unit is in single-precision mode by default.
  • -m4-100-single-only — generate code for SH4-100 in such a way that no double-precision floating-point operations are used.
  • -m4-200 — generate code for SH4-200.
  • -m4-200-nofpu — generate code for SH4-200 in such a way that the floating-point unit is not used.
  • -m4-200-single — generate code for SH4-200 assuming the floating-point unit is in single-precision mode by default.
  • -m4-200-single-only — generate code for SH4-200 in such a way that no double-precision floating-point operations are used.
  • -m4-300 — generate code for SH4-300.
  • -m4-300-nofpu — generate code for SH4-300 in such a way that the floating-point unit is not used.
  • -m4-300-single — generate code for SH4-300 in such a way that no double-precision floating-point operations are used.
  • -m4-300-single-only — generate code for SH4-300 in such a way that no double-precision floating-point operations are used.
  • -m4-340 — generate code for SH4-340 (no MMU, no FPU).
  • -m4-500 — generate code for SH4-500 (no FPU). Passes -isa=sh4-nofpu to the assembler.
  • -m4a-nofpu — generate code for the SH4al-dsp, or for a SH4a in such a way that the floating-point unit is not used.
  • -m4a-single-only — generate code for the SH4a, in such a way that no double-precision floating-point operations are used.
  • -m4a-single — generate code for the SH4a assuming the floating-point unit is in single-precision mode by default.
  • -m4a — generate code for the SH4a.
  • -m4al — same as -m4a-nofpu, except that it implicitly passes -dsp to the assembler. GCC doesn't generate any DSP instructions at the moment 🫠.
  • -mb — compile code for the processor in big-endian mode.
  • -ml — compile code for the processor in little-endian mode.
  • -mdalign — align doubles at 64-bit boundaries. Heads up though: this changes the calling conventions, and thus some functions from the standard C library do not work unless you recompile it first with -mdalign 😭.
  • -mrelax — shorten some address references at link time, when possible; uses the linker option -relax.
  • -mbigtable — use 32-bit offsets in switch tables. The default is to use 16-bit offsets.
  • -mbitops — enable the use of bit manipulation instructions on SH2A.
  • -mfmovd — enable the use of the instruction fmovd. Check -mdalign for alignment constraints 👀.
  • -mrenesas — comply with the calling conventions defined by Renesas.
  • -mno-renesas — comply with the calling conventions defined for GCC before the Renesas conventions were available. This option is the default for all targets of the SH toolchain.
  • -mnomacsave — mark the MAC register as call-clobbered, even if -mrenesas is given.
  • -mieee, -mno-ieee — control the IEEE compliance of floating-point comparisons, which affects the handling of cases where the result of a comparison is unordered. By default -mieee is implicitly enabled. If -ffinite-math-only is enabled, -mno-ieee is implicitly set, which results in faster floating-point greater-equal and less-equal comparisons. The implicit settings can be overridden by specifying either -mieee or -mno-ieee.
  • -minline-ic_invalidate — inline code to invalidate instruction cache entries after setting up nested function trampolines. This option has no effect if -musermode is in effect and the selected code generation option (e.g. -m4) does not allow the use of the icbi instruction. If the selected code generation option does not allow the use of the icbi instruction, and -musermode is not in effect, the inlined code manipulates the instruction cache address array directly with an associative write. 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.
  • -mpadstruct — this option is deprecated. It pads structures to multiples of 4 bytes, which is incompatible with the SH ABI.
  • -matomic-model=model — sets the model of atomic operations and additional parameters as a comma separated list. For details on the atomic built-in functions see __atomic Builtins. The following models and parameters are supported:
    • none — disable compiler generated atomic sequences and emit library calls for atomic operations. This is the default if the target is not sh*-*-linux*.
    • soft-gusa — generate GNU/Linux compatible gUSA software atomic sequences for the atomic built-in functions. The generated atomic sequences require additional support from the interrupt/exception handling code of the system and are only suitable for SH3* and SH4* single-core systems. This option is enabled by default when the target is sh*-*-linux* and SH3* or SH4*. When the target is SH4A, this option also partially utilizes the hardware atomic instructions movli.l and movco.l to create more efficient code, unless strict is specified.
    • soft-tcb — generate software atomic sequences that use a variable in the thread control block. This is a variation of the gUSA sequences which can also be used on SH1* and SH2* targets. The generated atomic sequences require additional support from the interrupt/exception handling code of the system and are only suitable for single-core systems. When using this model, the gbr-offset= parameter has to be specified as well.
    • soft-imask — generate software atomic sequences that temporarily disable interrupts by setting SR.IMASK = 1111. This model works only when the program runs in privileged mode and is only suitable for single-core systems. Additional support from the interrupt/exception handling code of the system is not required. This model is enabled by default when the target is sh*-*-linux* and SH1* or SH2*.
    • hard-llcs — generate hardware atomic sequences using the movli.l and movco.l instructions only. This is only available on SH4A and is suitable for multi-core systems. Since the hardware instructions support only 32 bit atomic variables, access to 8 or 16 bit variables is emulated with 32 bit accesses. Code compiled with this option is also compatible with other software atomic model interrupt/exception handling systems if executed on an SH4A system. Additional support from the interrupt/exception handling code of the system is not required for this model.
    • gbr-offset= — this parameter specifies the offset in bytes of the variable in the thread control block structure that should be used by the generated atomic sequences when the soft-tcb model has been selected. For other models this parameter is ignored. The specified value must be an integer multiple of four and in the range 0-1020.
    • strict — this parameter prevents mixed usage of multiple atomic models, even if they are compatible, and makes the compiler generate atomic sequences of the specified model only 🔒.
  • -mtas — this one makes the compiler emit the "tas.b" opcode for "__atomic_test_and_set". ⚠️ but like, be careful bestie: depending on your exact hardware and software setup this can lowkey tank overall performance because of the operand cache line flushes that the "tas.b" instruction implies. and on multi-core SH4A processors, "tas.b" must be used with caution since it can literally cause data corruption for certain cache configurations 😭 so yeah, not a casual vibe.
  • -mprefergot — when generating position-independent code, this makes function calls go through the Global Offset Table instead of the Procedure Linkage Table. periodt.
  • -musermode, -mno-usermode — don't allow (or, with the -mno- version, do allow) the compiler to generate privileged mode code. specifying -musermode also implies -mno-inline-ic_invalidate if the inlined code wouldn't work in user mode. -musermode is the default when the target is "sh*-*-linux*". but if the target is SH1* or SH2*, -musermode has no effect at all, since there's literally no user mode. ✨
  • -multcost=number** **— set the cost to assume for a multiply insn. that's it, that's the whole thing 🎀
  • -mdiv=strategy** *— set the division strategy for integer division operations. *strategy can be one of these icons:
    • call-div1 — calls a library function that uses the single-step division instruction "div1" to do 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. if you specify this for targets that don't have a double precision FPU, it defaults to "call-div1".
    • call-table — calls a library function that uses a lookup table for small divisors and the "div1" instruction with case distinction for larger divisors. division by zero calculates an unspecified result and doesn't trap. this is the default for SH4. specifying this for targets that don't have dynamic shift instructions defaults to "call-div1".

when a division strategy hasn't been specified, the default strategy is picked based on the current target. for SH2A the default strategy is to use the "divs" and "divu" instructions instead of library function calls. iykyk 💅

  • -maccumulate-outgoing-args — reserve space once for outgoing arguments in the function prologue instead of around each call. generally a glow-up for performance and size. also needed for unwinding to avoid changing the stack frame around conditional code.
  • -mdivsi3_libfunc=name** *— set the name of the library function used for 32-bit signed division to *name. this only affects the name used in the call division strategies, and the compiler still expects the same sets of input/output/clobbered registers as if this option weren't there. no cap.
  • -mfixed-range=register-range** **— generate 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. 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 it can. if not specified, the value is selected depending on the processor type being compiled for.
  • -mzdcbranch, -mno-zdcbranch — assume (or don't assume) that zero displacement conditional branch instructions "bt" and "bf" are fast. if -mzdcbranch is specified, the compiler prefers zero displacement branch code sequences. this is enabled by default when generating code for SH4 and SH4A. it can be explicitly disabled by specifying -mno-zdcbranch.
  • -mcbranch-force-delay-slot — force the usage of delay slots for conditional branches, which stuffs the delay slot with a "nop" if a suitable instruction can't be found. by default this option is disabled. it can be enabled to work around hardware bugs as found in the original SH7055. kinda giving "fixing the vibe manually" energy 💀
  • -mfused-madd, -mno-fused-madd — generate code that uses (or doesn't use) the floating-point multiply and accumulate instructions. these instructions are generated by default if hardware floating point is used. the machine-dependent -mfused-madd option is now mapped to the machine-independent -ffp-contract=fast option, and -mno-fused-madd is mapped to -ffp-contract=off. glow-up migration arc ✨
  • -mfsca, -mno-fsca — allow or disallow the compiler to emit the "fsca" instruction for sine and cosine approximations. -mfsca must be used in combination with -funsafe-math-optimizations. it's 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. -mfsrra must be used in combination with -funsafe-math-optimizations and -ffinite-math-only. it's enabled by default when generating code for SH4A. using -mno-fsrra disables reciprocal square root approximations even if -funsafe-math-optimizations and -ffinite-math-only are in effect.
  • -mpretend-cmove — prefer zero-displacement conditional branches for conditional move instruction patterns. this can result in faster code on the SH4 processor. main character optimization behavior, honestly 👑
  • -mfdpic — generate code using the FDPIC ABI.

🌞 Solaris 2 Options 🌞

these -m options are supported on Solaris 2, bestie:

  • -mclear-hwcap — -mclear-hwcap tells the compiler to remove the hardware capabilities generated by the Solaris assembler. this is only necessary when object files use ISA extensions not supported by the current machine, but check at runtime whether or not to use them.
  • -mimpure-text — -mimpure-text, used in addition to -shared, tells the compiler to not pass -z text to the linker when linking a shared object. using this option, you can link position-dependent code into a shared object.

-mimpure-text suppresses the "relocations remain against allocatable but non-writable sections" linker error message. however 😬 the necessary relocations trigger copy-on-write, and the shared object is not actually shared across processes. instead of using -mimpure-text, you should compile all source code with -fpic or -fPIC. trust, it's the better move.

these switches are supported in addition to the above on Solaris 2:

  • -pthreads — this is a synonym for -pthread. slay, same vibe.

✨ 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 global register 1, each global register 2 through 4 is then treated as an allocable register that's 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 doesn't 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. no drama.

  • -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 bestie: the requisite libraries are not available for all SPARC targets. normally the facilities of the machine's usual C compiler are used, but this can't be done directly in cross-compilation. you must make your own arrangements to provide suitable library functions for cross-compilation. the embedded targets sparc-*-aout and sparclite-*-* do provide software floating-point support.

-msoft-float changes the calling convention in the output file; therefore, 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. don't skip this step, fr fr.

  • -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. it's giving "we have no choice" energy 😭

  • -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're 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's not the default because it results in a performance loss, especially for floating-point code. lowkey a trade-off, ngl.

  • -muser-mode, -mno-user-mode — don't 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 — okay so with -mfaster-structs, the compiler just assumes structures are 8-byte aligned 💅. That unlocks pairs of ldd and std instructions for copies in structure assignment, instead of literally twice as many ld and st pairs. But bestie, listen — this changed alignment directly violates the SPARC ABI. So it's only for targets where the developer fully acknowledges that their resulting code is not directly in line with the rules of the ABI 🙈. Not a vibe you pick by accident.
  • -mstd-struct-return, -mno-std-struct-return — with -mstd-struct-return, the compiler generates checking code in functions returning structures or unions, so it can 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. Those are v7, v8, sparclite, sparclet, v9.

Here's the full glow-up list of each supported architecture and their supported implementations ✨:

  • v7 — cypress, leon3v7
  • v8 — supersparc, hypersparc, leon, leon3, leon5
  • sparclite — f930, f934, sparclite86x
  • sparclet — tsc701
  • v9 — ultrasparc, ultrasparc3, niagara, niagara2, niagara3, niagara4, niagara7, m8

By default (unless configured otherwise), GCC generates code for the V7 variant of the SPARC architecture. With -mcpu=cypress, the compiler additionally optimizes it for the Cypress CY7C602 chip, as used in the SPARCStation/SPARCServer 3xx series 🔥. This is also appropriate for the older SPARCStation 1, 2, IPX etc.

With -mcpu=v8, GCC generates code for the V8 variant of the SPARC architecture. The only difference from V7 code is that the compiler emits the integer multiply and integer divide instructions which exist in SPARC-V8 but not in SPARC-V7. With -mcpu=supersparc, the compiler additionally optimizes it for the SuperSPARC chip, as used in the SPARCStation 10, 1000 and 2000 series.

With -mcpu=sparclite, GCC generates code for the SPARClite variant of the SPARC architecture. This adds the integer multiply, integer divide step and scan ("ffs") instructions which exist in SPARClite but not in SPARC-V7 🌸. With -mcpu=f930, the compiler additionally optimizes it for the Fujitsu MB86930 chip, which is the original SPARClite, with no FPU. With -mcpu=f934, the compiler additionally optimizes it for the Fujitsu MB86934 chip, which is the more recent SPARClite with FPU.

With -mcpu=sparclet, GCC generates code for the SPARClet variant of the SPARC architecture. This adds the integer multiply, multiply/accumulate, integer divide step and scan ("ffs") instructions which exist in SPARClet but not in SPARC-V7. With -mcpu=tsc701, the compiler additionally optimizes it for the TEMIC SPARClet chip.

With -mcpu=v9, GCC generates code for the V9 variant of the SPARC architecture. This adds 64-bit integer and floating-point move instructions, 3 additional floating-point condition code registers and conditional move instructions 👑. With -mcpu=ultrasparc, the compiler additionally optimizes it for the Sun UltraSPARC I/II/IIi chips. With -mcpu=ultrasparc3, the compiler additionally optimizes it for the Sun UltraSPARC III/III+/IIIi/IIIi+/IV/IV+ chips. With -mcpu=niagara, the compiler additionally optimizes it for Sun UltraSPARC T1 chips. With -mcpu=niagara2, the compiler additionally optimizes it for Sun UltraSPARC T2 chips. With -mcpu=niagara3, the compiler additionally optimizes it for Sun UltraSPARC T3 chips. With -mcpu=niagara4, the compiler additionally optimizes it for Sun UltraSPARC T4 chips. With -mcpu=niagara7, the compiler additionally optimizes it for Oracle SPARC M7 chips. With -mcpu=m8, the compiler additionally optimizes it for Oracle M8 chips 💖.

  • -mtune=cpu_type — set the instruction scheduling parameters for machine type cpu_type, but *do not set the instruction set or register set that the option -mcpu=cpu_type* does. Subtle but important, bestie 🫶.

The same values for -mcpu=cpu_type can be used for -mtune=cpu_type, but the only useful values are those that select a particular CPU implementation. Those are cypress, supersparc, hypersparc, leon, leon3, leon3v7, leon5, f930, f934, sparclite86x, tsc701, ultrasparc, ultrasparc3, niagara, niagara2, niagara3, niagara4, niagara7 and m8. With native Solaris and GNU/Linux toolchains, native can also be used.

  • -mv8plus, -mno-v8plus — with -mv8plus, GCC generates code for the SPARC-V8+ ABI. The difference from the V8 ABI is that the global and out registers are considered 64 bits wide ⭐. This is enabled by default on Solaris in 32-bit mode for all SPARC-V9 processors.
  • -mvis, -mno-vis — with -mvis, GCC generates code that takes advantage of the UltraSPARC Visual Instruction Set extensions 🔥. The default is -mno-vis.
  • -mvis2, -mno-vis2 — with -mvis2, GCC generates code that takes advantage of version 2.0 of the UltraSPARC Visual Instruction Set extensions. The default is -mvis2 when targeting a cpu that supports such instructions, such as UltraSPARC-III and later. Setting -mvis2 also sets -mvis 💫.
  • -mvis3, -mno-vis3 — with -mvis3, GCC generates code that takes advantage of version 3.0 of the UltraSPARC Visual Instruction Set extensions. The default is -mvis3 when targeting a cpu that supports such instructions, such as niagara-3 and later. Setting -mvis3 also sets -mvis2 and -mvis.
  • -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 ✨. The default is -mvis3b when targeting a cpu that supports such instructions, such as niagara-7 and later. Setting -mvis3b also sets -mvis3, -mvis2 and -mvis.
  • -mvis4, -mno-vis4 — with -mvis4, GCC generates code that takes advantage of version 4.0 of the UltraSPARC Visual Instruction Set extensions. The default is -mvis4 when targeting a cpu that supports such instructions, such as niagara-7 and later. Setting -mvis4 also sets -mvis3b, -mvis3, -mvis2 and -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. The default is -mvis4b when targeting a cpu that supports such instructions, such as m8 and later. Setting -mvis4b also sets -mvis4, -mvis3b, -mvis3, -mvis2 and -mvis.
  • -mcbcond, -mno-cbcond — with -mcbcond, GCC generates code that takes advantage of the UltraSPARC Compare-and-Branch-on-Condition instructions. The default is -mcbcond when targeting a CPU that supports such instructions, such as Niagara-4 and later 💜.
  • -mfmaf, -mno-fmaf — with -mfmaf, GCC generates code that takes advantage of the UltraSPARC Fused Multiply-Add Floating-point instructions. The default is -mfmaf when targeting a CPU that supports such instructions, such as 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. The default is -mfsmuld when 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. The default is -mpopc when targeting a CPU that supports such an instruction, such as Niagara-2 and later 🍓.
  • -msubxc, -mno-subxc — with -msubxc, GCC generates code that takes advantage of the UltraSPARC Subtract-Extended-with-Carry instruction. The default is -msubxc when targeting a CPU that supports such an instruction, such as Niagara-7 and later.
  • -mfix-at697f — enable the documented workaround for the single erratum of the Atmel AT697F processor (which corresponds to erratum #13 of the AT697E processor) 🔧.
  • -mfix-ut699 — enable the documented workarounds for the floating-point errata and the data cache nullify errata of the UT699 processor.
  • -mfix-ut700 — enable the documented workaround for the back-to-back store errata of the UT699E/UT700 processor.
  • -mfix-gr712rc — enable 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 sets int, long and pointer to 32 bits. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits 💅.
  • *-mcmodel=which* — set the code model to one of:
    • medlow — the Medium/Low code model: 64-bit addresses, programs must be linked in the low 32 bits of memory. Programs can be statically or dynamically linked.
    • medmid — the Medium/Middle code model: 64-bit addresses, programs must be linked in the low 44 bits of memory, the text and data segments must be less than 2GB in size and the data segment must be located within 2GB of the text segment.
    • medany — the Medium/Anywhere code model: 64-bit addresses, programs may be linked anywhere in memory, the text and data segments must be less than 2GB in size and the data segment must be located within 2GB of the text segment.
  • embmedany — the Medium/Anywhere code model for embedded systems 🛰️: 64-bit addresses, and both the text and data segments have to be less than 2GB in size, each starting literally anywhere in memory (decided at link time, no cap). The global register %g4 points at the base of the data segment. Programs get statically linked, and PIC is not supported, periodt.
  • -mmemory-model= mem-model — sets the memory model in force on the processor. Pick one bestie:
    • default — the default memory model for the processor and operating system 👑
    • rmo — Relaxed Memory Order
    • pso — Partial Store Order
    • tso — Total Store Order
    • sc — Sequential Consistency

These memory models are formally defined in Appendix D of the SPARC-V9 architecture manual, and they're set in the processor's PSTATE.MM field ✨

  • -mstack-bias, -mno-stack-bias — with -mstack-bias, GCC assumes the stack pointer (and the frame pointer, if there is one) is offset by -2047, which you have to add back when making stack frame references. That's the default in 64-bit mode. Otherwise, assume no such offset exists 🫶

💫 Options for System V 💫

These extra options are available on System V Release 4 for compatibility with other compilers on those systems, iykyk:

  • -G — create a shared object. It's recommended that -symbolic or -shared be used instead 🙈
  • -Qy — identify the versions of each tool used by the compiler, in a .ident assembler directive in the output.
  • -Qn — refrain from adding .ident directives to the output file (this is the default).
  • -YP, dirs — search the directories dirs, and no others, for libraries specified with -l.
  • -Ym, dir — look in the directory dir to find the M4 preprocessor. The assembler uses this option.

🌸 V850 Options 🌸

These -m options are defined for V850 implementations, bestie:

  • -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, then calls indirect through that pointer 🔥
  • -mno-ep, -mep — do not optimize (do optimize) basic blocks that use the same index pointer 4 or more times, copying the pointer into the ep register and using the shorter sld and sst instructions. The -mep option is on by default if you optimize — free slay.
  • -mno-prolog-function, -mprolog-function — do not use (do use) external functions to save and restore registers at the prologue and epilogue of a function. The external functions are slower, but use less code space if more than one function saves the same number of registers. The -mprolog-function option is on by default if you optimize.
  • -mspace — try to make the code as small as possible. At present this just turns on the -mep and -mprolog-function options. Tiny code energy 🐜
  • -mtda= n — put static or global variables whose size is n bytes or less into the tiny data area that register ep points to. The tiny data area can hold up to 256 bytes in total (128 bytes for byte references).
  • -msda= n — put static or global variables whose size is n bytes or less into the small data area that register gp points to. The small data area can hold up to 64 kilobytes.
  • -mzda= n — put static or global variables whose size is n bytes or less into the first 32 kilobytes of memory.
  • -mv850 — specify that the target processor is the V850.
  • -mv850e3v5 — specify that the target processor is the V850E3V5. The preprocessor constant __v850e3v5__ is defined if this option is used ✨
  • -mv850e2v4 — specify that the target processor is the V850E3V5. This is an alias for the -mv850e3v5 option.
  • -mv850e2v3 — specify that the target processor is the V850E2V3. The preprocessor constant __v850e2v3__ is defined if this option is used.
  • -mv850e2 — specify that the target processor is the V850E2. The preprocessor constant __v850e2__ is defined if this option is used.
  • -mv850e1 — specify that the target processor is the V850E1. The preprocessor constants __v850e1__ and __v850e__ are defined if this option is used.
  • -mv850es — specify that the target processor is the V850ES. This is an alias for the -mv850e1 option.
  • -mv850e — specify that the target processor is the V850E. The preprocessor constant __v850e__ is defined if this option is used.

If neither -mv850 nor -mv850e nor -mv850e1 nor -mv850e2 nor -mv850e2v3 nor -mv850e3v5 are defined, then a default target processor is chosen and the relevant __v850*__ preprocessor constant is defined 🎀

The preprocessor constants __v850 and __v851__ are always defined, regardless of which processor variant is the target. No cap, always on.

  • -mdisable-callt, -mno-disable-callt — this option suppresses generation of the CALLT instruction for the v850e, v850e1, v850e2, v850e2v3 and v850e3v5 flavors of the v850 architecture.

This option is enabled by default when the RH850 ABI is in use (see -mrh850-abi), and disabled by default when the GCC ABI is in use. If CALLT instructions are being generated, then the C preprocessor symbol __V850_CALLT__ is defined.

  • -mrelax, -mno-relax — pass on (or do not pass on) the -mrelax command-line option to the assembler.
  • -mlong-jumps, -mno-long-jumps — disable (or re-enable) the generation of PC-relative jump instructions.
  • -msoft-float, -mhard-float — disable (or re-enable) the generation of hardware floating point instructions. This option is only significant when the target architecture is V850E2V3 or higher. If hardware floating point instructions are being generated, then the C preprocessor symbol __FPU_OK__ is defined; otherwise the symbol __NO_FPU__ is defined.
  • -mloop — enables the use of the e3v5 LOOP instruction. The use of this instruction is not enabled by default when the e3v5 architecture is selected, because its use is still experimental 🙈
  • -mrh850-abi, -mghs — enables support for the RH850 version of the V850 ABI. This is the default. With this version of the ABI, the following rules apply:
    • Integer sized structures and unions are returned via a memory pointer rather than a register.
    • Large structures and unions (more than 8 bytes in size) are passed by value.
    • Functions are aligned to 16-bit boundaries.
    • The -m8byte-align command-line option is supported.
    • The -mdisable-callt command-line option is enabled by default. The -mno-disable-callt command-line option is not supported 🚫

When this version of the ABI is enabled, the C preprocessor symbol __V850_RH850_ABI__ is defined.

  • -mgcc-abi — enables support for the old GCC version of the V850 ABI. With this version of the ABI, the following rules apply:
    • Integer sized structures and unions are returned in register r10.
    • Large structures and unions (more than 8 bytes in size) are passed by reference.
    • Functions are aligned to 32-bit boundaries, unless optimizing for size.
    • The -m8byte-align command-line option is not supported.
    • The -mdisable-callt command-line option is supported but not enabled by default.

When this version of the ABI is enabled, the C preprocessor symbol __V850_GCC_ABI__ is defined.

  • -m8byte-align, -mno-8byte-align — enables support for double and long long types to be aligned on 8-byte boundaries. The default is to restrict the alignment of all objects to at most 4-bytes. When -m8byte-align is in effect, the C preprocessor symbol __V850_8BYTE_ALIGN__ is defined.
  • -mbig-switch — generate code suitable for big switch tables. Use this option only if the assembler/linker complain about out of range branches within a switch table ⚠️
  • -mapp-regs — this option causes r2 and r5 to be used in the code generated by the compiler. This setting is the default.
  • -mno-app-regs — this option causes r2 and r5 to be treated as fixed registers.

🦋 VAX Options 🦋

These -m options are defined for the VAX, vintage icon behavior:

  • -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 😭

🌈 Visium Options 🌈

  • -mdebug — a program which performs file I/O and is destined to run on an MCM target should be linked with this option. It causes the libraries libc.a and libdebug.a to be linked. The program should be run on the target under the control of the GDB remote debugging stub 🐛
  • -msim — a program which performs file I/O and is destined to run on the simulator should be linked with option. This causes libraries libc.a and libsim.a to be linked.
  • -mfpu, -mhard-float — generate code containing floating-point instructions. This is the default.
  • -mno-fpu, -msoft-float — generate code containing library calls for floating-point.
Heads up bestie: -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. Miss that and it's giving chaos 💀
  • -mcpu= cpu_type — set the instruction set, register set, and instruction scheduling parameters for machine type cpu_type. Supported values for cpu_type are mcm, gr5 and gr6.
    • mcm is a synonym of gr5 present for backward compatibility.
    • By default (unless configured otherwise), GCC generates code for the GR5 variant of the Visium architecture.
    • With -mcpu=gr6, GCC generates code for the GR6 variant of the Visium architecture. The only difference from GR5 code is that the compiler will generate block move instructions 👑
  • -mtune=cpu_type — sets the instruction scheduling params for machine type cpu_type, but does not touch the instruction set or register set the way -mcpu=cpu_type would. like, vibes-tuning only, no hardware glow-up 💅
  • -msv-mode — generates code for supervisor mode, where access to general registers is totally unrestricted. this is the default, bestie ✨
  • -muser-mode — generates code for user mode, where 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. so like… know your register limits, iykyk 😭

🧚 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 instead. pick your fighter fr fr 👑
  • -mdebug-main=prefix — flags the first routine whose name starts with prefix as the main routine for the debugger. first one there gets main character energy 🎀
  • -mmalloc64 — default to 64-bit memory allocation routines. big allocations, big slay 💖
  • -mpointer-size=size — sets the default size of pointers. possible options for size are 32 or short for 32 bit pointers, 64 or long for 64 bit pointers, and no for supporting only 32 bit pointers. that last option disables pragma pointer_size. ngl the no one is the chaotic option 🙈

🌈 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 — so go check those too, bestie 🫶

  • -mrtp — GCC can generate code for both VxWorks kernels and real time processes (RTPs). this option switches from the former to the latter. it also defines the preprocessor macro __RTP__. glow-up from kernel to RTP, periodt 🔥
  • -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. so like… it's giving "PowerPC only, version-locked" 💀
  • -non-static — link an RTP executable against shared libraries rather than static libraries. the options -static and -shared can also be used for RTPs; -static is the default. shared girlies rise 🌸
  • -Bstatic
  • -Bdynamic — these options are passed down to the linker. they are defined for compatibility with Diab. middleman behavior, no cap 🎀
  • -Xbind-lazy — enable lazy binding of function calls. this option is equivalent to -Wl,-z,now and is defined for compatibility with Diab. yes, exactly like that, copy-paste energy, don't ask me 😭
  • -Xbind-now — disable lazy binding of function calls. this option is the default and is defined for compatibility with Diab. default queen behavior 👑

💜 x86 Options 💜

these -m options are defined for the x86 family of computers. buckle up bestie, this list is long 🦋

  • -march=cpu-type — generate instructions for the machine type cpu-type. in contrast to -mtune=cpu-type, which merely tunes the generated code for the specified cpu-type, -march=cpu-type allows GCC to generate code that may not run at all on processors other than the one indicated ⚠️. specifying -march=cpu-type implies -mtune=cpu-type, 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=native enables all instruction subsets supported by the local machine (hence the result might not run on different machines 😬). using -mtune=native produces code optimized for the local machine under the constraints of the selected instruction set. very "this is about ME" of it ✨
  • 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. teamwork makes the dream work 🫶

  • i386 — original Intel i386 CPU. the og 👑
  • i486 — Intel i486 CPU. (no scheduling is implemented for this chip.) zero scheduling, all vibes 💀
  • 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 as generic. context-dependent icon 💅
  • 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. laptop core 🔋
  • 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 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, full 64-bit extensions, and honestly kind of everything: 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. she's serving a whole buffet, no crumbs 💅

🚀 rocketlake

Intel Rocket Lake CPU, 64-bit extensions, and the roster goes: 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

okay these four share one whole vibe — Intel Alder Lake / Raptor Lake / Meteor Lake / Gracemont CPUs, 64-bit extensions, and: MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, 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 and AVX-VNNI instruction set support. four names, one icon 💅✨

🏹 arrowlake

Intel Arrow Lake CPU, 64-bit extensions, and the full list is: MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, 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 and CMPCCXADD instruction set support.

🌙 arrowlake-s, lunarlake

Intel Arrow Lake S / Lunar Lake CPUs, 64-bit extensions, and the résumé reads: MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, 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, SM3 and SM4 instruction set support. the extra four at the end are the moment, lowkey 🦋

🐆 pantherlake

Intel Panther Lake CPU, 64-bit extensions, and: MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, 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 and PREFETCHI instruction set support.

💎 sapphirerapids, emeraldrapids

Intel Sapphire Rapids / Emerald Rapids CPUs, 64-bit extensions, and girlypop brought the whole collection: 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 and AVX512BF16 instruction set support.

🪨 graniterapids

Intel Granite Rapids CPU, 64-bit extensions, with: 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-FP16 and PREFETCHI instruction set support.

🪨💅 graniterapids-d

Intel Granite Rapids D CPU, 64-bit extensions, and: 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 and AMX-COMPLEX instruction set support.

💎✨ diamondrapids

Intel Diamond Rapids CPU, 64-bit extensions, and bestie this list is a paragraph, brace yourself: 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, AMX-TF32, AMX-TRANSPOSE, MOVRS, AMX-MOVRS and USER_MSR instruction set support. she ate and left no crumbs, fr fr 🔥

🍒 bonnell, atom

Intel Bonnell CPU, 64-bit extensions, and honestly a cute minimal lil set: MOVBE, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support. small but iconic 🎀

🥈 silvermont, slm

Intel Silvermont CPU, 64-bit extensions, with: MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, PCLMUL, PREFETCHW and RDRND instruction set support.

🥇 goldmont

Intel Goldmont CPU, 64-bit extensions, and: 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 and FSGSBASE instruction set support.

🥇➕ goldmont-plus

Intel Goldmont Plus CPU, 64-bit extensions, and it's the goldmont list with bonus content: 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 and SGX instruction set support.

🌊 tremont

Intel Tremont CPU, 64-bit extensions, serving: 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, CLDEMOTE and WAITPKG instruction set support.

🌲 sierraforest

Intel Sierra Forest CPU, 64-bit extensions, and: MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, 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, ENQCMD and UINTR instruction set support.

🏞️ grandridge

Intel Grand Ridge CPU, 64-bit extensions, with: MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, 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, ENQCMD and UINTR instruction set support. yes it's the same list as the one above, don't come for me, that's literally what the page says 💀

💧🌲 clearwaterforest

Intel Clearwater Forest CPU, 64-bit extensions, and: MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, 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_MSR and PREFETCHI instruction set support.

🕹️ k6

AMD K6 CPU with MMX instruction set support. a humble queen 👑

🕹️✨ k6-2, k6-3

Improved versions of the AMD K6 CPU, with MMX and 3DNow! instruction set support.

🏛️ athlon, athlon-tbird

AMD Athlon CPU with MMX, 3dNOW!, enhanced 3DNow! and SSE prefetch instructions support.

🏛️👑 athlon-4, athlon-xp, athlon-mp

Improved AMD Athlon CPU with MMX, 3DNow!, enhanced 3DNow! and full SSE instruction set support.

🖥️ k8, opteron, athlon64, athlon-fx

Processors based on the AMD K8 core, with x86-64 instruction set support — including the AMD Opteron, Athlon 64, and Athlon 64 FX processors. (Real talk: this one supersets MMX, SSE, SSE2, 3DNow!, enhanced 3DNow! and 64-bit instruction set extensions.) vintage but still slaying 🫶

  • k8-sse3, opteron-sse3, athlon64-sse3 — glow-up versions of the AMD K8 cores, now with SSE3 instruction set support. same energy, more instructions 💅
  • amdfam10, barcelona — CPUs based on AMD Family 10h cores with x86-64 instruction set support. (This supersets MMX, SSE, SSE2, SSE3, SSE4A, 3DNow!, enhanced 3DNow!, ABM and 64-bit instruction set extensions.) she's got range fr fr ✨
  • bdver1 — CPUs based on AMD Family 15h cores with x86-64 instruction set support. (This supersets FMA4, AVX, XOP, LWP, AES, PCLMUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.)
  • bdver2 — AMD Family 15h core based CPUs with x86-64 instruction set support. (This supersets BMI, TBM, F16C, FMA, FMA4, AVX, XOP, LWP, AES, PCLMUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.)
  • bdver3 — AMD Family 15h core based CPUs with x86-64 instruction set support. (This supersets BMI, TBM, F16C, FMA, FMA4, FSGSBASE, AVX, XOP, LWP, AES, PCLMUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.)
  • bdver4 — AMD Family 15h core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, TBM, F16C, FMA, FMA4, FSGSBASE, AVX, AVX2, XOP, LWP, AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.) the BMI2 and AVX2 additions are lowkey the moment 👑
  • znver1 — AMD Family 17h core based CPUs with x86-64 instruction set support. (This supersets BMI, 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 64-bit instruction set extensions.)
  • znver2 — AMD Family 17h core based CPUs with x86-64 instruction set support. (This supersets BMI, 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 64-bit instruction set extensions.)
  • znver3 — AMD Family 19h core based CPUs with x86-64 instruction set support. (This supersets BMI, 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 64-bit instruction set extensions.)
  • znver4 — AMD Family 19h core based CPUs with x86-64 instruction set support. (This supersets BMI, 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 and 64-bit instruction set extensions.) ok the AVX512 family showed all the way up, that's dedication 😭🔥
  • znver5 — AMD Family 1ah core based CPUs with x86-64 instruction set support. (This supersets BMI, 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 and 64-bit instruction set extensions.) the main character of the AMD list, no notes 💜
  • btver1 — CPUs based on AMD Family 14h cores with x86-64 instruction set support. (This supersets MMX, SSE, SSE2, SSE3, SSSE3, SSE4A, CX16, ABM and 64-bit instruction set extensions.)
  • btver2 — CPUs based on AMD Family 16h cores with x86-64 instruction set support. This includes MOVBE, F16C, BMI, AVX, PCLMUL, AES, SSE4.2, SSE4.1, CX16, ABM, SSE4A, SSSE3, SSE3, SSE2, SSE, MMX and 64-bit instruction set extensions.
  • winchip-c6 — IDT WinChip C6 CPU, dealt in same way as i486 with additional MMX instruction set support.
  • winchip2 — IDT WinChip 2 CPU, dealt in same way as i486 with additional MMX and 3DNow! instruction set support. yes the exclamation mark is part of the name bestie, don't @ me 💀
  • c3 — VIA C3 CPU with MMX and 3DNow! instruction set support. (No scheduling is implemented for this chip.) 🔴 heads up: no scheduling for this chip
  • c3-2 — VIA C3-2 (Nehemiah/C5XL) CPU with MMX and SSE instruction set support. (No scheduling is implemented for this chip.)
  • c7 — VIA C7 (Esther) CPU with MMX, SSE, SSE2 and SSE3 instruction set support. (No scheduling is implemented for this chip.)
  • samuel-2 — VIA Eden Samuel 2 CPU with MMX and 3DNow! instruction set support. (No scheduling is implemented for this chip.)
  • nehemiah — VIA Eden Nehemiah CPU with MMX and SSE instruction set support. (No scheduling is implemented for this chip.)
  • esther — VIA Eden Esther CPU with MMX, SSE, SSE2 and SSE3 instruction set support. (No scheduling is implemented for this chip.)
  • eden-x2 — VIA Eden X2 CPU with x86-64, MMX, SSE, SSE2 and SSE3 instruction 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, AVX and AVX2 instruction set support. (No scheduling is implemented for this chip.)
  • nano — Generic VIA Nano CPU with x86-64, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support. (No scheduling is implemented for this chip.)
  • nano-1000 — VIA Nano 1xxx CPU with x86-64, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support. (No scheduling is implemented for this chip.)
  • nano-2000 — VIA Nano 2xxx CPU with x86-64, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support. (No scheduling is implemented for this chip.)
  • nano-3000 — VIA Nano 3xxx CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3 and SSE4.1 instruction set support. (No scheduling is implemented for this chip.)
  • nano-x2 — VIA Nano Dual Core CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3 and SSE4.1 instruction set support. (No scheduling is implemented for this chip.)
  • nano-x4 — VIA Nano Quad Core CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3 and SSE4.1 instruction 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, RDSEED instruction set support. 👀 While the CPUs do support AVX and F16C, these aren't enabled by -march=lujiazui for performance reasons.
  • 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, LZCNT instruction set support. (unlike lujiazui, this one does hand you AVX and F16C, love that for her 🫶)
  • 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, LZCNT instruction set support.
  • geode — AMD Geode embedded processor with MMX and 3DNow! instruction set support. tiny legend 🎀

🎛️ -mtune=cpu-type

okay so this is the vibe dial, not the what-can-I-actually-run dial. It tunes to cpu-type everything applicable about the generated code, except for the ABI and the set of available instructions.

So while picking a specific cpu-type schedules things appropriately for that particular chip, the compiler does not generate any code that cannot run on the default machine type — unless you also use a -march=cpu-type option. no cap, they're a team 🤝

Example, literally: if GCC is configured for i686-pc-linux-gnu then -mtune=pentium4 generates code that is tuned for Pentium 4 but still runs on i686 machines. slay, backwards compatible queen ✨

The choices for cpu-type are the same as for -march. In addition, -mtune supports 2 extra choices for cpu-type:

  • generic — Produce code optimized for the most common IA32/AMD64/EM64T processors.

If you know the CPU on which your code will run, then you should use the corresponding -mtune or -march option instead of -mtune=generic. But, if you do not know exactly what CPU users of your application will have, then you should use this option. bestie this is your "idk so I'm being nice to everyone" option 💖

As new processors are deployed in the marketplace, the behavior of this option will change. Therefore, if you upgrade to a newer version of GCC, code generation controlled by this option will change to reflect the processors that are most common at the time that version of GCC is released. so yes, it moves with the times 🌈

There is no -march=generic option because -march indicates the instruction set the compiler can use, and there is no generic instruction set applicable to all processors. In contrast, -mtune indicates the processor (or, in this case, collection of processors) for which the code is optimized. that's the whole difference, iykyk 🧚

  • intel — Produce code optimized for the most current Intel processors, which are Haswell and Silvermont for this version of GCC.

If you know the CPU on which your code will run, then you should use the corresponding -mtune or -march option instead of -mtune=intel. But, if you want your application performs better on both Haswell and Silvermont, then you should use this option. (yes that's the original wording, I'm not touching it, it's giving iconic 🎀)

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. periodt. 🔥

  • -mcpu=cpu-type — lowkey a deprecated synonym for -mtune. like it still works but it's giving "old news" ✨

🫶 the -mfpmath=unit situation

  • -mfpmath=unit — tells GCC to generate floating-point arithmetic for the unit you pick 💅 the choices for unit are:
    • 387 — uses the standard 387 floating-point coprocessor, which is literally on the majority of chips and emulated everywhere else. code compiled with this runs almost everywhere 🦋 the tea here: temporary results get computed in 80-bit precision instead of whatever precision the type specifies, so your results come out slightly different compared to most other chips 😭 peep -ffloat-store for the full detailed description. this is the default choice for non-Darwin x86-32 targets, periodt.
    • sse — uses the scalar floating-point instructions that live in the SSE instruction set ✨ this set is supported by Pentium III and newer chips, and on the AMD side by Athlon-4, Athlon XP and Athlon MP chips. the earlier version of SSE only supports single-precision arithmetic, so double and extended-precision arithmetic still get done with 387 🙈 a later version — only in Pentium 4 and AMD x86-64 chips — supports double-precision arithmetic too.

for the x86-32 compiler you literally must use the -march=cpu-type, -msse or -msse2 switches to enable SSE extensions and make this option actually do something 👑 for the x86-64 compiler, these extensions are enabled by default, no cap.

the resulting code should be considerably faster in the majority of cases and dodges the numerical instability problems 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 for x86-32 targets with the SSE2 instruction set when -ffast-math is enabled 🌸

  • sse,387, sse+387, both — 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, the execution resources too. use this one with care bestie, it is still experimental, because the GCC register allocator does not model separate functional units well, which results in unstable performance 😬
  • -masm=dialect — outputs assembly instructions using the dialect you picked 💖 also affects which dialect gets used for basic asm and extended asm. supported choices (in dialect order) are att or intel. the default is att. Darwin does not support intel, sorry bestie 🙈
  • -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. iconic behaviour tbh.
  • -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 get used, but you can't do that 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 still be emitted even if -msoft-float is used 👀

  • -mno-fp-ret-in-387 — does not use the FPU registers for return values of functions 🌸 the usual calling convention has functions return values of types float and double in an FPU register, even if there's no FPU — the idea being that the operating system should emulate an FPU. the option -mno-fp-ret-in-387 causes such values to be returned in ordinary CPU registers instead 💅
  • -mno-fancy-math-387 — some 387 emulators do not support the sin, cos and sqrt instructions for the 387 🎀 specify this option to avoid generating those instructions. this option gets overridden when -march indicates that the target CPU always has an FPU and so the instruction does not need emulation ✨ these instructions are not generated unless you also use the -funsafe-math-optimizations switch, iykyk.
  • -malign-double, -mno-align-double — controls whether GCC aligns double, long double, and long long variables on a two-word boundary or a one-word boundary 💖 aligning double variables on a two-word boundary produces code that runs somewhat faster on a Pentium, at the expense of more memory. on x86-64, -malign-double is enabled by default 👑
⚠️ warning: if you use the -malign-double switch, structures containing the above types are aligned differently than the published application binary interface specifications for the x86-32 and are not binary compatible with structures in code compiled without that switch 💀
  • -m96bit-long-double, -m128bit-long-double — these switches control the size of the long double type 🌈 the x86-32 application binary interface specifies the size to be 96 bits, so -m96bit-long-double is the default in 32-bit mode. modern architectures (Pentium and newer) prefer long double to be aligned to an 8- or 16-byte boundary 🦋 in arrays or structures conforming to the ABI, this is not possible. so specifying -m128bit-long-double aligns long double to a 16-byte boundary by padding the long double with an additional 32-bit zero ✨ in the x86-64 compiler, -m128bit-long-double is the default choice since 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 containing long double variables, as well as modifying the function calling convention for functions taking long double. hence they are not binary-compatible with code compiled without that switch 💀
  • -mlong-double-64, -mlong-double-80, -mlong-double-128 — these switches control the size of the long double type 💅 a size of 64 bits makes the long double type equivalent to the double type — this is the default for 32-bit Bionic C library. a size of 128 bits makes the long double type equivalent to the __float128 type — this is the default for 64-bit Bionic C library 🌸
⚠️ warning: if you override the default value for your target ABI, this changes the size of structures and arrays containing long double variables, as well as modifying the function calling convention for functions taking long double. hence they are not binary-compatible with code compiled without that switch 😭
  • -malign-data=type — controls how GCC aligns variables ✨ supported values for type are: compat uses the increased alignment value compatible with GCC 4.8 and earlier, abi uses the alignment value as specified by the psABI, and cacheline uses the increased alignment value to match the cache line size 🎀 compat is the default.
  • -mlarge-data-threshold=threshold — when -mcmodel=medium or -mcmodel=large is specified, data objects larger than threshold get placed in large data sections 🦋 the default is 65535.
  • -mrtd — uses a different function-calling convention, in which functions that take a fixed number of arguments return with the ret num instruction, which pops their arguments while returning 🔥 this saves one instruction in the caller since there'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.
⚠️ warning: this calling convention is incompatible with the one normally used on Unix, so you cannot use it if you need to call libraries compiled with the Unix compiler 💀

also, you must provide function prototypes for all functions that take variable numbers of arguments (including printf) — otherwise incorrect code gets generated for calls to those functions 😭 in addition, seriously incorrect code results if you call a function with too many arguments. (normally, extra arguments are harmlessly ignored 🙈)

  • -mregparm=num — controls how many registers are used to pass integer arguments 🌸 by default, no registers are used to pass arguments, and at most 3 registers can be used. you can control this behavior for a specific function by using the function attribute regparm ✨
⚠️ warning: if you use this switch, and num is nonzero, then you must build all modules with the same value, including any libraries. this includes the system libraries and startup modules 💀
  • -msseregparm — uses SSE register passing conventions for float and double arguments and return values 💖 you can control this behavior for a specific function by using the function attribute sseregparm.
⚠️ warning: if you use this switch then you must build all modules with the same value, including any libraries. this includes the system libraries and startup modules 😬
  • -mvect8-ret-in-mem — 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 — sets 80387 floating-point precision to 32, 64 or 80 bits ✨ when -mpc32 is specified, the significands of results of floating-point operations get rounded to 24 bits (single precision); -mpc64 rounds the significands of results of floating-point operations to 53 bits (double precision); and -mpc80 rounds the significands of results of floating-point operations to 64 bits (extended double precision), which is the default 👑 when this option is used, floating-point operations in higher precisions are not available to the programmer without setting the FPU control word explicitly 😭

Setting the rounding of floating-point operations to less than the default 80 bits can speed some programs by 2% or more 🔥 — like, genuinely. But heads up bestie: some mathematical libraries straight-up assume extended-precision (80-bit) floating-point operations are enabled by default. Routines in those libraries could suffer a significant loss of accuracy, typically through the so-called "catastrophic cancellation", when you use this option to set the precision to less than extended precision. So it's giving "free speed, hidden cost" vibes. 💀

  • -mdaz-ftz — the flush-to-zero (FTZ) and denormals-are-zero (DAZ) flags in the MXCSR register are what control floating-point calculations. SSE and AVX instructions, including scalar and vector instructions, could benefit from enabling the FTZ and DAZ flags when -mdaz-ftz is specified ✨. Don't set FTZ/DAZ flags when -mno-daz-ftz or -shared is specified — and yes, -mdaz-ftz will set FTZ/DAZ flags even with -shared. 🙈
  • -mstackrealign — realign the stack at entry. On the x86, the -mstackrealign option generates an alternate prologue and epilogue that realigns the run-time stack if necessary 🫶. This supports mixing legacy codes that keep 4-byte stack alignment with modern codes that keep 16-byte stack alignment for SSE compatibility. See also the attribute "force_align_arg_pointer", applicable to individual functions. iykyk.
  • -mpreferred-stack-boundary=num — attempt to keep the stack boundary aligned to a 2 raised to num byte boundary. If -mpreferred-stack-boundary is not specified, the default is 4 (16 bytes or 128 bits).
⚠️ Warning, and I mean it: when generating code for the x86-64 architecture with SSE extensions disabled, -mpreferred-stack-boundary=3 can be used to keep the stack boundary aligned to an 8 byte boundary. Since x86-64 ABI requires 16 byte stack alignment, this is ABI incompatible and intended to be used in controlled environments where stack space is an 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. On top of that, variable arguments are handled incorrectly for 16 byte aligned objects (including x87 long double and __int128), leading to wrong results. You must build all modules with -mpreferred-stack-boundary=3, including any libraries. This includes the system libraries and startup modules. Periodt. 💅
  • -mincoming-stack-boundary=num — assume the incoming stack is aligned to a 2 raised to num byte boundary. If -mincoming-stack-boundary is not specified, the one specified by -mpreferred-stack-boundary is used.

On Pentium and Pentium Pro, "double" and "long double" values should be aligned to an 8-byte boundary (see -malign-double) or suffer significant run time performance penalties 🐌. On Pentium III, the Streaming SIMD Extension (SSE) data type "__m128" may not work properly if it is not 16-byte aligned.

To ensure proper alignment of this values on the stack, the stack boundary must be as aligned as that required by any value stored on the stack 🧚. Further, every function must be generated such that it keeps the stack aligned. Thus calling a function compiled with a higher preferred stack boundary from a function compiled with a lower preferred stack boundary most likely misaligns the stack. It is recommended that libraries that use callbacks always use the default setting.

This extra alignment does consume extra stack space, and generally increases code size. Code that is sensitive to stack space usage, such as embedded systems and operating system kernels, may want to reduce the preferred alignment to -mpreferred-stack-boundary=2. Little-and-precious energy. 🎀

Okay so now the extended-instruction-set menu — the whole lineup, here it is, no cuts, no edits:

-mmmx  -msse  -msse2  -msse3  -mssse3  -msse4  -msse4a  -msse4.1  -msse4.2
-mavx  -mavx2  -mavx512f  -mavx512cd  -mavx512vl  -mavx512bw  -mavx512dq
-mavx512ifma  -mavx512vbmi  -msha  -maes  -mpclmul  -mclflushopt  -mclwb
-mfsgsbase  -mptwrite  -mrdrnd  -mf16c  -mfma  -mpconfig  -mwbnoinvd  -mfma4
-mprfchw  -mrdpid  -mrdseed  -msgx  -mxop  -mlwp  -m3dnow  -m3dnowa  -mpopcnt
-mabm  -madx  -mbmi  -mbmi2  -mlzcnt  -mfxsr  -mxsave  -mxsaveopt  -mxsavec
-mxsaves  -mrtm  -mhle  -mtbm  -mmwaitx  -mclzero  -mpku  -mavx512vbmi2
-mavx512bf16  -mavx512fp16  -mgfni  -mvaes  -mwaitpkg  -mvpclmulqdq
-mavx512bitalg  -mmovdiri  -mmovdir64b  -menqcmd  -muintr  -mtsxldtrk
-mavx512vpopcntdq  -mavx512vp2intersect  -mavx512vnni  -mavxvnni  -mcldemote
-mserialize  -mamx-tile  -mamx-int8  -mamx-bf16  -mhreset  -mkl  -mwidekl
-mavxifma  -mavxvnniint8  -mavxneconvert  -mcmpccxadd  -mamx-fp16  -mprefetchi
-mraoint  -mamx-complex  -mavxvnniint16  -msm3  -msha512  -msm4  -mapxf
-musermsr  -mavx10.1  -mavx10.1-256  -mavx10.1-512  -mavx10.2  -mamx-avx512
-mamx-tf32  -mamx-transpose  -mamx-fp8  -mmovrs  -mamx-movrs

These switches enable the use of instructions in the MMX, SSE, AVX512CD, AVX512VL, AVX512BW, AVX512DQ, AVX512IFMA, AVX512VBMI, SHA, AES, PCLMUL, CLFLUSHOPT, CLWB, FSGSBASE, PTWRITE, RDRND, F16C, FMA, PCONFIG, WBNOINVD, FMA4, PREFETCHW, RDPID, RDSEED, SGX, XOP, LWP, 3DNow!, enhanced 3DNow!, POPCNT, ABM, ADX, BMI, BMI2, LZCNT, FXSR, XSAVE, XSAVEOPT, XSAVEC, XSAVES, RTM, HLE, TBM, MWAITX, CLZERO, PKU, AVX512VBMI2, GFNI, VAES, WAITPKG, VPCLMULQDQ, AVX512BITALG, MOVDIRI, MOVDIR64B, AVX512BF16, ENQCMD, AVX512VPOPCNTDQ, AVX512VNNI, SERIALIZE, UINTR, HRESET, AMXTILE, AMXINT8, AMXBF16, KL, WIDEKL, AVXVNNI, AVX512-FP16, AVXIFMA, AVXVNNIINT8, AVXNECONVERT, CMPCCXADD, AMX-FP16, PREFETCHI, RAOINT, AMX-COMPLEX, AVXVNNIINT16, SM3, SHA512, SM4, APX_F, USER_MSR, AVX10.1, AVX10.2, AMX-AVX512, AMX-TF32, AMX-TRANSPOSE, AMX-FP8, MOVRS, AMX-MOVRS or CLDEMOTE extended instruction sets 👑. Each has a corresponding -mno- option to disable use of these instructions — yes, one for each, don't worry.

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 🦋.

Note that -msse4 enables both SSE4.1 and SSE4.2 support, while -mno-sse4 turns off those features; neither form of the option affects SSE4A support, controlled separately by -msse4a. Different lane, bestie.

To generate SSE/SSE2 instructions automatically from floating-point code (as opposed to 387 instructions), see -mfpmath=sse ✨.

GCC depresses SSEx instructions when -mavx is used. Instead, it generates new AVX instructions or AVX equivalence for all SSEx instructions when needed.

These options enable GCC to use these extended instructions in generated code, even without -mfpmath=sse 😌. Applications that perform run-time CPU detection must compile separate files for each supported architecture, using the appropriate flags. In particular, the file containing the CPU detection code should be compiled without these options. Big main-character-energy move: build the detector without the flags it's detecting. 💅

  • -mdump-tune-features — this option instructs GCC to dump the names of the x86 performance tuning features and default settings 🎀. The names can be used in -mtune-ctrl=feature-list.
  • -mtune-ctrl=feature-list — this option is used to do fine grain control of x86 code generation features. feature-list is a comma separated list of feature names. See also -mdump-tune-features. When specified, the feature is turned on if it is not preceded with ^, otherwise, it is turned off.
⚠️ -mtune-ctrl=feature-list is intended to be used by GCC developers. Using it may lead to code paths not covered by testing and can potentially result in compiler ICEs or runtime errors. You've been warned, don't come crying to me. 😭
  • -mno-default — this option instructs GCC to turn off all tunable features. See also -mtune-ctrl=feature-list and -mdump-tune-features.
  • -mcld — this option instructs GCC to emit a "cld" instruction in the prologue of functions that use string instructions. String instructions depend on the DF flag to select between autoincrement or autodecrement mode. While the ABI specifies the DF flag to be cleared on function entry, some operating systems violate this specification by not clearing the DF flag in their exception dispatchers 💀. The exception handler can be invoked with the DF flag set, which leads to wrong direction mode when string instructions are used. This option can be enabled by default on 32-bit x86 targets by configuring GCC with the --enable-cld configure option. Generation of "cld" instructions can be suppressed with the -mno-cld compiler option in this case.
  • -mvzeroupper — this option instructs GCC to emit a "vzeroupper" instruction before a transfer of control flow out of the function to minimize the AVX to SSE transition penalty as well as remove unnecessary "zeroupper" intrinsics. Clean. 🧚
  • -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 — 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-math is 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-math is in effect 🙈. These issues can be mitigated by manually sanitizing the upper part of the partial vector argument register or by using -mdaz-ftz to set denormals-are-zero (DAZ) flag in the MXCSR register.

This option is enabled by default. 🫶

  • -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 — tells GCC the max number of bits that can be stored to memory efficiently is bits, bestie. The valid bits are 128, 256 and 512. That's it, that's the whole setting:
    • none — no extra limitations beyond whatever the selected platform already says, we're not adding anything 💅
    • 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 optimizes functions that have the noreturn attribute or the _Noreturn specifier by not saving the callee-saved registers used inside the function in the prologue (except the BP register, it stays). Sounds iconic but lowkey it can mess with debugging of the caller of that noreturn function, or anything further up the call stack 😭 which is exactly why it is not enabled by default.
  • -mcx16 — lets GCC generate CMPXCHG16B instructions in 64-bit code so compare-and-exchange ops work on 16-byte aligned 128-bit objects. Great for atomic updates of data structures 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.
  • -msahf — enables generation of SAHF instructions in 64-bit code. History moment 📜: early Intel Pentium 4 CPUs with Intel 64 support, before the Pentium 4 G1 step in December 2005, didn't have the LAHF and SAHF instructions that AMD64 supports. Those two are load and store instructions (respectively) for certain status flags. In 64-bit mode, SAHF is used to optimize the fmod, drem and remainder built-in functions — see Other Builtins for the tea ☕
  • -mmovbe — enables use of the movbe instruction to optimize byte swapping of four and eight byte entities. Swap era ✨
  • -mshstk — turns on the shadow stack built-in functions from x86 Control-flow Enforcement Technology (CET). CET said protect the stack 💅
  • -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 — bestie, this one's spicy 🌶️ It enables use of RCPSS and RSQRTSS instructions (plus their vectorized variants RCPPS and RSQRTPS) with an extra Newton-Raphson step to bump the precision — instead of DIVSS and SQRTSS (and their vectorized variants) for single-precision floating-point arguments. But these instructions are generated only when -funsafe-math-optimizations is enabled together with -ffinite-math-only and -fno-trapping-math. And ngl, while the throughput of the sequence is higher than the throughput of the non-reciprocal instruction, the precision can drop by up to 2 ulp (i.e. the inverse of 1.0 equals 0.99999994) 🙈

Also important: GCC already implements 1.0f/sqrtf(x) in terms of RSQRTSS (or RSQRTPS) with -ffast-math (or that option combination above), and doesn't need -mrecip.

And one more: GCC emits that same sequence with the additional Newton-Raphson step for vectorized single-float division and vectorized sqrtf(x) already with -ffast-math (or the combo above), and doesn't need -mrecip either. iykyk.

  • -mrecip=opt — controls which reciprocal estimate instructions are allowed. opt is a comma-separated list of options, and any of them can have a ! in front 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. Cute and precise 🎀

  • -mveclibabi=type — specifies the ABI type to use for vectorizing intrinsics using an external library. Supported values for type are svml for the Intel short vector math library, aocl for the math library (LibM) from AMD Optimizing CPU Libraries (AOCL), and acml for the end-of-life AMD core math library (AOCL-LibM is its successor, for the record). To use this option, both -ftree-vectorize and -funsafe-math-optimizations have to be enabled, and an SVML or ACML ABI-compatible library must be specified at link time ⭐

Right now GCC emits calls to these, for the corresponding function type, when -mveclibabi=svml is used:

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, vmlsAcos4

These for the corresponding function type when -mveclibabi=aocl 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

And these for the corresponding function type when -mveclibabi=acml is used:

__vrd2_sin, __vrd2_cos, __vrd2_exp, __vrd2_log, __vrd2_log2,
__vrd2_log10, __vrs4_sinf, __vrs4_cosf, __vrs4_expf, __vrs4_logf,
__vrs4_log2f, __vrs4_log10f, __vrs4_powf
  • -mabi=name — generate code for the specified calling convention. The permissible values are sysv for the ABI used on GNU/Linux and other systems, and ms for the Microsoft ABI. Default behaviour: Microsoft ABI when targeting Microsoft Windows, SysV ABI on all other systems 💫 You can control this per-function with the function attributes ms_abi and sysv_abi.
  • -mforce-indirect-call — forces all calls to functions to be indirect. Useful when you're using Intel Processor Trace, where it generates more precise timing information for function calls ✨
  • -mmanual-endbr — inserts the ENDBR instruction at function entry only via the cf_check function attribute. Useful together with the option -fcf-protection=branch when you want to control ENDBR insertion at the function entry.
  • -mcet-switch — by default, CET instrumentation is turned off on switch statements that use a jump table, and indirect branch track is disabled. Now, since jump tables live in read-only memory, this does not result in a direct loss of hardening 🛡️ but if the jump table index is attacker-controlled, that indirect jump may not be constrained by CET. This option turns on CET instrumentation so indirect branch track is enabled for switch statements with jump tables, which leads to the jump targets reachable via any indirect jumps.
  • -mcall-ms2sysv-xlogues — because the 64-bit ABIs differ, 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 those registers is emitted inline, which gives you fairly lengthy prologues and epilogues 😮‍💨 Using -mcall-ms2sysv-xlogues emits prologues and epilogues that use stubs in the static portion of libgcc to do those saves and restores, so you get reduced function size at the cost of a few extra instructions.
  • -mtls-dialect=type — generate code to access thread-local storage using the gnu or gnu2 conventions. gnu is the conservative default; gnu2 is more efficient, but it may add compile- and run-time requirements that cannot be satisfied on all systems 🦋
  • -mpush-args, -mno-push-args — use PUSH operations to store outgoing parameters. This method is shorter and usually equally fast as the method using SUB/MOV operations, and it's enabled by default. In some cases disabling it may improve performance, thanks to improved scheduling and reduced dependencies 👑
  • -maccumulate-outgoing-args — if enabled, the maximum amount of space required for outgoing arguments is computed in the function prologue. This is faster on most modern CPUs because of reduced dependencies, improved scheduling and reduced stack usage when the preferred stack boundary is not equal to 2. The drawback? A notable increase in code size 💀 This switch implies -mno-push-args.
  • -mthreads — supports thread-safe exception handling on MinGW. Programs that rely on thread-safe exception handling must compile and link all their code with the -mthreads option. When compiling, -mthreads defines -D_MT; when linking, it links in a special thread helper library -lmingwthrd which cleans up per-thread exception-handling data 🧚
  • -mms-bitfields, -mno-ms-bitfields — turn bit-field layout that’s compatible with the native Microsoft Windows compiler on or off 💅

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. Especially 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. 😭

This option is enabled by default for Microsoft Windows targets. This behavior can also be controlled locally by use of variable or type attributes. For more information, see x86 Variable Attributes and x86 Type Attributes. ✨

The Microsoft structure layout algorithm is fairly simple with the exception of the bit-field packing. The padding and alignment of members of structures and whether a bit-field can straddle a storage-unit boundary are determined by these rules:

  1. Structure members are stored sequentially in the order in which they are declared: the first member has the lowest memory address and the last member the highest.
  2. Every data object has an alignment requirement. The alignment requirement for all data except structures, unions, and arrays is either the size of the object or the current packing size (specified 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 is allocated an offset so that:
offset % alignment_requirement == 0
  1. Adjacent bit-fields are 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.

MSVC interprets zero-length bit-fields in the following ways:

  1. If a zero-length bit-field is inserted between two bit-fields that are normally coalesced, the bit-fields are not coalesced.

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 the zero-length bit-field were removed, t1's size would be 4 bytes.

  1. 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, bar is 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 this into account, it is important to note the following:

  1. 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 whole. For example, t2 has a size of 4 bytes, since the zero-length bit-field follows a normal bit-field, and is of type short.
  2. 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.

  1. 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.

  • -mno-align-stringops — don’t align the destination of inlined string operations. This switch reduces code size and improves performance in case the destination is already aligned, but GCC doesn’t know about it. 🫶
  • -minline-all-stringops — by default GCC inlines string operations only when the destination is known to be aligned to at least a 4-byte boundary. This enables more inlining and increases code size, but may improve performance of code that depends on fast memcpy and memset for short lengths. The option enables inline expansion of strlen for all pointer alignments. ⚡
  • -minline-stringops-dynamically — for string operations of unknown size, use run-time checks with inline code for small blocks and a library call for large blocks. 🧚
  • -mstringop-strategy=alg — override the internal decision heuristic for the particular algorithm to use for inlining string operations. The allowed values for alg are:
    • rep_byte
    • rep_4byte
    • rep_8byte

Expand using i386 rep prefix of the specified size.

  • byte_loop
  • loop
  • unrolled_loop

Expand into an inline loop.

  • libcall

Always use a library call.

  • -mmemcpy-strategy=strategy — override the internal decision heuristic to decide if __builtin_memcpy should be inlined and what inline algorithm to use when the expected size of the copy operation is known. strategy is a comma-separated list of alg:max_size:dest_align triplets. alg is specified in -mstringop-strategy, max_size specifies the max byte size with which inline algorithm alg is allowed. For the last triplet, the max_size must be -1. The max_size of the triplets in the list must be specified in increasing order. The minimal byte size for alg is 0 for the first triplet and max_size + 1 of the preceding range. 💖
  • -mmemset-strategy=strategy — the option is similar to -mmemcpy-strategy= except that it is to control __builtin_memset expansion. 🌸
  • -momit-leaf-frame-pointer — don’t keep the frame pointer in a register for leaf functions. This avoids the instructions to save, set up, and restore frame pointers and makes an extra register available in leaf functions. The option -fomit-leaf-frame-pointer removes the frame pointer for leaf functions, which might make debugging harder. 😭
  • -mtls-direct-seg-refs, -mno-tls-direct-seg-refs — controls whether TLS variables may be accessed with offsets from the TLS segment register (%gs for 32-bit, %fs for 64-bit), or whether the thread base pointer must be added. Whether or not this is valid depends on the operating system, and whether it maps the segment to cover the entire TLS area. 🦋

For systems that use the GNU C Library, the default is on.

  • -msse2avx, -mno-sse2avx — specify that the assembler should encode SSE instructions with VEX prefix. The option -mavx turns this on by default. ✨
  • -mfentry, -mno-fentry — if profiling is active (-pg), put the profiling counter call before the prologue. Note: On x86 architectures the attribute ms_hook_prologue isn’t possible at the moment for -mfentry and -pg. 💀
  • -mrecord-mcount, -mno-record-mcount — if profiling is active (-pg), generate a __mcount_loc section that contains pointers to each profiling call. This is useful for automatically patching and out calls. 👑
  • -mnop-mcount, -mno-nop-mcount — if profiling is active (-pg), generate the calls to the profiling functions as NOPs. This is useful when they should be patched in later dynamically. This is likely only useful together with -mrecord-mcount. 🌈
  • -minstrument-return=type — instrument function exit in -pg -mfentry instrumented functions with call to specified function. This only instruments true returns ending with ret, but not sibling calls ending with jump. Valid types are none to not instrument, call to generate a call to __return__, or nop5 to generate a 5 byte nop. 🎀
  • -mrecord-return, -mno-record-return — generate a __return_loc section pointing to all return instrumentation code. 🔥
  • -mfentry-name=name — set name of __fentry__ symbol called at function entry for -pg -mfentry functions. ⭐
  • -mfentry-section=name — set name of section to record -mrecord-mcount calls (default __mcount_loc). 💜
  • -mskip-rax-setup, -mno-skip-rax-setup — when generating code for the x86-64 architecture with SSE extensions disabled, -mskip-rax-setup can be used to skip setting up RAX register when there are no variable arguments passed in vector registers. 🍓
Warning: Since RAX register is used to avoid unnecessarily saving vector registers on stack when passing variable arguments, the impacts of this option are callees may waste some stack space, misbehave or jump to a random location. GCC 4.4 or newer don’t have those issues, regardless the RAX register value.
  • -m8bit-idiv, -mno-8bit-idiv — on some processors, like Intel Atom, 8-bit unsigned integer divide is much faster than 32-bit/64-bit integer divide. This option generates a run-time check. If both dividend and divisor are within range of 0 to 255, 8-bit unsigned integer divide is used instead of 32-bit/64-bit integer divide. 🙈
  • -mavx256-split-unaligned-load, -mavx256-split-unaligned-store — split 32-byte AVX unaligned load and store. 🌸
  • -mstack-protector-guard=guard, -mstack-protector-guard-reg=reg, -mstack-protector-guard-offset=offset, -mstack-protector-guard-symbol=symbol — generate stack protection code using canary at guard. Supported locations are global for global canary or tls for per-thread canary in the TLS block (the default). This option has effect only when -fstack-protector or -fstack-protector-all is specified. 💅

So with that latter choice, the options -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset are the ones that go even further and specify which segment register (%fs or %gs) gets used as the base register for reading the canary, plus what offset from that base register you're reading at. The default for both of those? Whatever the relevant ABI says. Not us, the ABI said it 💅

  • -mstack-protector-guard-symbol=symbol — overrides that offset with a symbol reference to a canary sitting in the TLS block ✨
  • -mgeneral-regs-only — generates code that uses only the general-purpose registers. The way this works is it stops the compiler from touching floating-point, vector, mask and bound registers at all. None of them. Not invited 🚫
  • -mrelax-cmpxchg-loop — okay so when it's emitting a compare-and-swap loop for __sync Builtins and __atomic Builtins that are lacking a native instruction, this optimizes for the highly contended case by issuing an atomic load before the CMPXCHG instruction, and it uses the PAUSE instruction to save CPU power when it restarts the loop. Little energy-saver moment 🔥
  • -mindirect-branch=choice — converts indirect call and jump with choice. The choices, bestie, are:
    • keep — the default, keeps indirect call and jump unmodified 🎀
    • thunk — converts indirect call and jump to call and return thunk
    • thunk-inline — converts indirect call and jump to inlined call and return thunk
    • thunk-extern — converts 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 attribute indirect_branch 👑

Note that -mcmodel=large is incompatible with -mindirect-branch=thunk and -mindirect-branch=thunk-extern, since the thunk function may not be reachable in the large code model. Like, it literally can't get there 😭
Note that -mindirect-branch=thunk-extern is compatible with -fcf-protection=branch, since the external thunk can be made to enable control-flow check ✨
  • -mfunction-return=choice — converts function return with choice. Default is keep, which keeps function return unmodified. thunk converts function return to call and return thunk. thunk-inline converts function return to inlined call and return thunk. thunk-extern converts function return to external call and return thunk provided in a separate object file. Per-function control? That's the function attribute function_return 🫶
Note that -mindirect-return=thunk-extern is compatible with -fcf-protection=branch, since the external thunk can be made to enable control-flow check. (yes, that's exactly how it's written, don't come for me 💀)
Note that -mcmodel=large is incompatible with -mfunction-return=thunk and -mfunction-return=thunk-extern, since the thunk function may not be reachable in the large code model 🌸
  • -mindirect-branch-register — forces indirect call and jump via register. Simple as that 💅
  • -mharden-sls=choice — generates code to mitigate against straight line speculation (SLS) with choice. Default is none, which disables all SLS hardening. Then:
    • return — enables SLS hardening for function returns
    • indirect-jmp — enables SLS hardening for indirect jumps
    • all — enables all SLS hardening 👑
  • -mindirect-branch-cs-prefix — adds 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, to allow them to be replaced with lfence; call *%r8-r15 or lfence; jmp *%r8-r15 at run-time. Six bytes of pure intent ✨
  • -mapx-inline-asm-use-gpr32 — for inline asm support with APX, the EGPR feature was disabled by default to prevent potential illegal instruction with EGPR occurring. To invoke egpr usage in inline asm, use the new compiler option -mapx-inline-asm-use-gpr32, and the user should ensure the instruction supports EGPR. That's on you, bestie 🙈
  • -mevex512, -mno-evex512 — enables/disables 512-bit vector. It will be default on if AVX512F is enabled 🦋

These -m switches are supported in addition to the above on x86-64 processors in 64-bit environments. Yes, there's more. There's always more 💀

  • -m32, -m64, -mx32, -m16, -miamcu — generate code for a 16-bit, 32-bit or 64-bit environment. Let's break it down:
    • The -m32 option sets int, 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.
    • The -miamcu option generates code which conforms to Intel MCU psABI. It requires the -m32 option to be turned on 🎀
  • -mno-red-zone — do not use a so-called "red zone" for x86-64 code. Iconic concept, let me explain: the red zone is mandated by the x86-64 ABI, and it's a 128-byte area beyond the location of the stack pointer that is not modified by signal or interrupt handlers, and therefore can be used for temporary data without adjusting the stack pointer. The flag -mno-red-zone disables this red zone. Bye red zone 🦋
  • -mcmodel=small — generates 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, main character energy 👑
  • -mcmodel=kernel — generates 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. Has to, no negotiating 🔥
  • -mcmodel=medium — generates code for the medium model: the program is linked in the lower 2 GB of the address space. Small symbols are also placed there. Symbols with sizes larger than -mlarge-data-threshold are put into large data or BSS sections and can be located above 2GB. Programs can be statically or dynamically linked 💜
  • -mcmodel=large — generates code for the large model. This model makes no assumptions about addresses and sizes of sections. Zero assumptions, zero notes 🌈
  • -maddress-mode=long — generates 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 — generates 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 — emit GNU_PROPERTY_X86_ISA_1_NEEDED GNU property for Linux target to indicate the micro-architecture ISA level required to execute the binary. It's giving "price of admission" 💅
  • -mno-direct-extern-access — without -fpic nor -fPIC, always use the GOT pointer to access external symbols. With -fpic or -fPIC, treat access to protected symbols as local symbols. The default is -mdirect-extern-access.
Warning: shared libraries compiled with -mno-direct-extern-access and executable compiled with -mdirect-extern-access may not be binary compatible if protected symbols are used in shared libraries and executable. Not me being dramatic, this is literally a warning 😭
  • -munroll-only-small-loops — controls conservative small loop unrolling. It is default enabled by O2, and unrolls loop with less than 4 insns by 1 time. Explicit -f[no-]unroll-[all-]loops would disable this flag to avoid any unintended unrolling behavior that user does not want. Respecting your boundaries 🎀
  • -mlam=choice — LAM (linear-address masking) allows special bits in the pointer to be used for metadata. The default is none. With u48, pointer bits in positions 62:48 can be used for metadata; with u57, pointer bits in positions 62:57 can be used for metadata. Bits with a wee little job 🌸

🌈 x86 Windows Options 🌈

(heads up bestie — this heading is here but there are literally no options listed under it in this stretch of the page, so we're just waving as we walk past) 👋

🦋 Xstormy16 Options 🦋

These options are defined for Xstormy16:

  • -msim — choose startup files and linker script suitable for the simulator ✨

🧚 Xtensa Options 🧚

These options are supported for Xtensa targets:

  • -mconst16, -mno-const16 — enable or disable use of CONST16 instructions for loading constant values. The CONST16 instruction is currently not a standard option from Tensilica. When enabled, CONST16 instructions are always used in place of the standard L32R instructions. The use of CONST16 is enabled by default only if the L32R instruction is not available 💜
  • -mfused-madd, -mno-fused-madd — enable or disable use of fused multiply/add and multiply/subtract instructions in the floating-point option. This has no effect if the floating-point option is not also enabled. Disabling fused multiply/add and multiply/subtract instructions forces the compiler to use separate instructions for the multiply and add/subtract operations. Why would you want that? Because this may be desirable in some cases where strict IEEE 754-compliant results are required: the fused multiply add/subtract instructions do not round the intermediate result, thereby producing results with more bits of precision than specified by the IEEE standard. And disabling fused multiply add/subtract instructions also ensures that the program output is not sensitive to the compiler's ability to combine multiply and add/subtract operations. Consistency queen 👑🦋
  • -mserialize-volatile, -mno-serialize-volatile — when this option is enabled, GCC inserts MEMW instructions before volatile memory references to guarantee sequential consistency. The default is -mserialize-volatile. Use -mno-serialize-volatile to omit the MEMW instructions 🙈
  • -mforce-no-pic — for targets, like GNU/Linux, where all user-mode Xtensa code must be position-independent code (PIC), this option disables PIC for compiling kernel code. Special little kernel-only pass 🔥
  • -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 ✨🎀
  • -mauto-litpools, -mno-auto-litpools — bestie, these two are all about how literal pools get treated 💅 The default is -mno-auto-litpools, and what that does is park your literals in a separate section of the output file — unless you go and use -mtext-section-literals, in which case, different story. But flip on -mauto-litpools and the assembler itself sprinkles the literals through the text section instead ✨ The compiler meanwhile does not emit explicit .literal directives, and it loads literals into registers with MOVI instructions rather than L32R, lowkey just handing the assembler the reins so it can do its relaxation thing and drop literals wherever it needs them 🫶 The payoff? The assembler is allowed to create several literal pools per function and assemble very big functions — which honestly may just not be possible with -mtext-section-literals. Iconic of it.
  • -mtarget-align, -mno-target-align — turn this on and GCC tells the assembler to auto-align instructions so branch penalties get reduced, at the cost of a lil' bit of code density 🙈 The assembler tries to widen density instructions to align branch targets and the instructions that follow call instructions. But if there aren't enough safe density instructions hanging around before a target to actually align it, no widening happens — it just doesn't force it, no cap. The default is -mtarget-align ⭐ Heads up tho: these options don't touch the treatment of auto-aligned instructions like LOOP, which the assembler always aligns, either by widening density instructions or by inserting NOP instructions. That one's non-negotiable 💀
  • -mlongcalls, -mno-longcalls — enable this and GCC instructs the assembler to translate direct calls into indirect calls, unless it can figure out that the target of a direct call is within the range the call instruction allows 🦋 This translation is typically what happens for calls to functions in other source files. Specifically — and this is the cute lil' mechanical detail — the assembler turns a direct CALL instruction into an L32R followed by a CALLX instruction 👑 The default is -mno-longcalls. You want this option in programs where the call target can potentially be out of range. And here's the thing: this option is implemented in the assembler, not the compiler, so the assembly code GCC generates still shows direct call instructions — like, look at the disassembled object code if you wanna see the actual instructions, fr fr. Also note the assembler uses an indirect call for every cross-file call, not just the ones that genuinely are out of range. Slightly extra of it, but okay 💅
  • -mabi=name — generate code for the ABI you name 🌸 Permissible values are: call0, windowed. The default ABI is chosen by the Xtensa core configuration, so it's giving "whatever your core feels like today" energy.
  • -mabi=call0 — with this version enabled, function parameters get passed in registers a2 through a7, registers a12 through a15 are caller-saved, and register a15 may be used as a frame pointer 🎀 Also, when this version of the ABI is enabled, the C preprocessor symbol __XTENSA_CALL0_ABI__ gets defined. Little treat for your preprocessor ✨
  • -mabi=windowed — with this one enabled, function parameters get passed in registers a10 through a15, and the called function rotates the register window by 8 registers on entry so that its arguments are found in registers a2 through a7 💫 Register a7 may be used as a frame pointer. Then the register window gets rotated 8 registers back upon return — round trip, slay 🌈 And when this version of the ABI is enabled, the C preprocessor symbol __XTENSA_WINDOWED_ABI__ is defined. Matching set, iykyk.
  • -mextra-l32r-costs=n — specify an extra cost of instruction RAM/ROM access for L32R instructions, measured in clock cycles ⏱️ This affects, when you're optimizing for speed, whether a constant gets loaded from a literal pool using L32R or synthesized from a small one with a couple of arithmetic instructions 🧚 The default value is 0.
  • -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 for cores that support both unaligned loads and stores in hardware, and -mstrict-align for all other cores. So, context-dependent, bestie.

🦋 zSeries Options 🦋

These are listed under

🌸 ENVIRONMENT 🌸

okay so this whole section is about the env vars that lowkey run the show behind the scenes with GCC. 💅 Some of them work by specifying directories or prefixes to use when GCC is out here searching for various kinds of files. Others are just there to specify other aspects of the compilation environment. Either way, they matter.

💡 tiny but crucial note before you fall in love with any of these: you can also specify places to search using options such as -B, -I and -L. Those take precedence over places specified using environment variables, and the environment variables in turn take precedence over those specified by the configuration of GCC. so yeah — command line first, env vars second, config last. the hierarchy ate. 👑

LANG, LC_CTYPE, LC_MESSAGES, LC_ALL

these four besties control the way GCC uses localization information, which is the whole reason GCC can work with different national conventions. 🌍 GCC inspects the locale categories LC_CTYPE and LC_MESSAGES if it has been configured to do so. You can set these locale categories to any value supported by your installation. A typical value is en_GB.UTF-8, for English in the United Kingdom encoded in UTF-8. cute, love that for us.

  • LC_CTYPE — specifies character classification. GCC uses it to determine the character boundaries in a string, which is needed for some multibyte encodings that contain quote and escape characters that are otherwise interpreted as a string end or escape. 😭 genuinely important.
  • LC_MESSAGES — specifies the language to use in diagnostic messages. so your errors can be multilingual, slay.
  • LC_ALL — if this one is set, it overrides the value of LC_CTYPE and LC_MESSAGES. Otherwise, LC_CTYPE and LC_MESSAGES default to the value of the LANG environment variable.
  • and if none of these variables are set, GCC defaults to traditional C English behavior. no cap. 🫶

TMPDIR

if TMPDIR is set, it specifies the directory to use for temporary files. 🗂️ GCC uses temporary files to hold the output of one stage of compilation that's going to be used 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, honestly.

GCC_COMPARE_DEBUG

setting GCC_COMPARE_DEBUG is nearly equivalent to passing -fcompare-debug to the compiler driver. See the documentation of this option for more details. that's it, that's the vibe. ✨

GCC_EXEC_PREFIX

if GCC_EXEC_PREFIX is set, it specifies a prefix to use in the names of the subprograms executed by the compiler. No slash is added when this prefix is combined with the name of a subprogram, but you can specify a prefix that ends with a slash if you wish. you do you. 💅

  • if GCC_EXEC_PREFIX is not set, GCC attempts to figure out an appropriate prefix to use based on the pathname it is invoked with.
  • if GCC cannot find the subprogram using the specified prefix, it tries looking in the usual places for the subprogram. it's persistent, we love that. 🔥
  • the default value of GCC_EXEC_PREFIX is prefix/lib/gcc/ where prefix is the prefix to the installed compiler. In many cases prefix is the value of "prefix" when you ran the configure script.
  • other prefixes specified with -B take precedence over this prefix.
  • this prefix is also used for finding files such as crt0.o that are used for linking.
  • in addition, the prefix is used in an unusual way in finding the directories to search for header files. 👀 For each of the standard directories whose name normally begins with /usr/local/lib/gcc (more precisely, with the value of GCC_INCLUDE_DIR), GCC tries replacing that beginning with the specified prefix to produce an alternate directory name. Thus, with -Bfoo/, GCC searches foo/bar just before it searches the standard directory /usr/local/lib/bar. If a standard directory begins with the configured prefix then the value of prefix is replaced by GCC_EXEC_PREFIX when looking for header files. like, it's a whole remix. 🎧

COMPILER_PATH

the value of COMPILER_PATH is a colon-separated list of directories, much like PATH. GCC tries the directories thus specified when searching for subprograms, if it cannot find the subprograms using GCC_EXEC_PREFIX. backup plan behavior, honestly iconic. 💜

LIBRARY_PATH

the value of LIBRARY_PATH is a colon-separated list of directories, much like PATH. When configured as a native compiler, GCC tries the directories thus specified when searching for special linker files, if it cannot find them using GCC_EXEC_PREFIX. Linking using GCC also uses these directories when searching for ordinary libraries for the -l option (but directories specified with -L come first). ok so -L is the main character and LIBRARY_PATH is the supportive bestie. 🫶

LANG

(yes, LANG again — she does double duty, we stan a multitasker 💅) This variable is used to pass locale information to the compiler. One way in which this information is used is to determine the character set to be used when character literals, string literals and comments are parsed in C and C++. When the compiler is configured to allow multibyte characters, the following values for LANG are recognized:

C-JIS

Recognize JIS characters. ✨

C-SJIS

Recognize SJIS characters. ✨

C-EUCJP

Recognize EUCJP characters. ✨

if LANG is not defined, or if it has some other value, then the compiler uses "mblen" and "mbtowc" as defined by the default locale to recognize and translate multibyte characters. default behavior, no notes. 😌

GCC_EXTRA_DIAGNOSTIC_OUTPUT

if GCC_EXTRA_DIAGNOSTIC_OUTPUT is set to one of the following values, then additional text will be emitted to stderr when fix-it hints are emitted. 🛠️ -fdiagnostics-parseable-fixits and -fno-diagnostics-parseable-fixits take precedence over this environment variable, just so you know who's in charge.

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. numbers stay numbers, bestie. 🔢

fixits-v2

As "fixits-v1", but columns are expressed as display columns, as per -fdiagnostics-column-unit=display. glow-up edition. ✨

some additional environment variables affect the behavior of the preprocessor. 🎀

CPATH, C_INCLUDE_PATH, CPLUS_INCLUDE_PATH, OBJC_INCLUDE_PATH

each variable's value is a list of directories separated by a special character, much like PATH, in which to look for header files. 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. 🌍

  • 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. universal icon. 👑
  • the remaining environment variables apply only when preprocessing the particular language indicated. Each specifies a list of directories to be searched as if specified with -isystem, but after any paths given with -isystem options on the command line.

in all these variables, an empty element instructs the compiler to search its current working directory. Empty elements can appear at the beginning or end of a path. For instance, if the value of CPATH is :/special/include, that has the same effect as -I. -I/special/include. see? empty string doing real work. 💀

DEPENDENCIES_OUTPUT

if this variable is set, its value specifies how to output dependencies for Make based on the non-system header files processed by the compiler. System header files are ignored in the dependency output. 🚫

the value of DEPENDENCIES_OUTPUT can be just a file name, in which case the Make rules are written to that file, guessing the target name from the source file name. Or the value can have the form file target, in which case the rules are written to file file using target as the target name. two modes, both valid, pick your fighter. 💅

in other words, this environment variable is equivalent to combining the options -MM and -MF, with an optional -MT switch too. so convenient tbh. ✨

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. so close to her sister but with opinions. 😌

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. 🕰️ reproducible builds girlies rise.

  • 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. please don't just make one up, bestie. 🙈

🐛 BUGS 🐛

for instructions on reporting bugs, see <https://gcc.gnu.org/bugs/>. go tell them everything, they want to know. 🫶

📎 FOOTNOTES 📎

  1. On some systems, gcc -shared needs to build supplementary stub code for constructors to work. On multi-libbed systems, gcc -shared must select the correct support libraries to link against. Failing to supply the correct flags may lead to subtle defects. Supplying them in cases where they are not necessary is innocuous. -shared suppresses the addition of startup code to alter the floating-point environment as done with -ffast-math, -Ofast or -funsafe-math-optimizations on some targets. 😭 read this one twice, it's the sneaky one.

👀 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, honestly. 🫶

💖 AUTHOR 💖

See the Info entry for gcc, or <https://gcc.gnu.org/onlinedocs/gcc/Contributors.html>, for contributors to GCC. every single one of them a legend. 👑

Copyright (c) 1988-2025 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 the whole page, bestie!! 🌈 you made it to the end of GCC(1) — this edition is gcc-15.2.0, dated 2025-08-08, and it is, as always, that girl. go compile something iconic. 💅✨🦋

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