GCC(1)

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

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🌸 NAME 🌸

gcc — GNU project C and C++ compiler

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🦋 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.

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💅 DESCRIPTION 💅

Okay bestie, so when you invoke GCC, it normally does preprocessing, compilation, assembly and linking. The "overall options" let you 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.

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.

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 gigibyte, and so on. Such arguments are designated by byte-size in the following text. Refer to the NIST, IEC, and other relevant national and international standards for the full listing and explanation of the binary and decimal byte size prefixes.

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🔥 OPTIONS 🔥

✨ Option Summary ✨

Here is a summary of all the options, grouped by type. Explanations are in the following sections.

🌈 Overall Options 🌈

• -c
• -S
• -E
• -o file
• -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
• -fplugin=file
• -fplugin-arg-name=arg
• -fdump-ada-spec[-slim]
• -fada-spec-parent=unit
• -fdump-go-spec=file

🍓 C Language Options 🍓

• -ansi
• -std=standard
• -fgnu89-inline
• -fpermitted-flt-eval-methods=standard
• -aux-info filename
• -fallow-parameterless-variadic-functions
• -fno-asm
• -fno-builtin
• -fno-builtin-function
• -fgimple
• -fhosted
• -ffreestanding
• -fopenacc
• -fopenacc-dim=geom
• -fopenmp
• -fopenmp-simd
• -fms-extensions
• -fplan9-extensions
• -fsso-struct=endianness
• -fallow-single-precision
• -fcond-mismatch
• -flax-vector-conversions
• -fsigned-bitfields
• -fsigned-char
• -funsigned-bitfields
• -funsigned-char

🧚 C++ Language Options 🧚

• -fabi-version=n
• -fno-access-control
• -faligned-new=n
• -fargs-in-order=n
• -fchar8_t
• -fcheck-new
• -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-implicit-templates
• -fno-implicit-inline-templates
• -fno-implement-inlines
• -fmodule-header[=kind]
• -fmodule-only
• -fmodules-ts
• -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
• -fpermissive
• -fno-pretty-templates
• -fno-rtti
• -fsized-deallocation
• -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
• -Wno-delete-incomplete
• -Wdelete-non-virtual-dtor
• -Wdeprecated-copy
• -Wdeprecated-copy-dtor
• -Wno-deprecated-enum-enum-conversion
• -Wno-deprecated-enum-float-conversion
• -Weffc++
• -Wno-exceptions
• -Wextra-semi
• -Wno-inaccessible-base
• -Wno-inherited-variadic-ctor
• -Wno-init-list-lifetime
• -Winvalid-imported-macros
• -Wno-invalid-offsetof
• -Wno-literal-suffix
• -Wno-mismatched-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
• -Wsign-promo
• -Wsized-deallocation
• -Wsuggest-final-methods
• -Wsuggest-final-types
• -Wsuggest-override
• -Wno-terminate
• -Wuseless-cast
• -Wno-vexing-parse
• -Wvirtual-inheritance
• -Wno-virtual-move-assign
• -Wvolatile
• -Wzero-as-null-pointer-constant

🎀 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

💜 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|json]
• -fno-diagnostics-show-option
• -fno-diagnostics-show-caret
• -fno-diagnostics-show-labels
• -fno-diagnostics-show-line-numbers
• -fno-diagnostics-show-cwe
• -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

✨ Warning Options ✨

Okay bestie, this is the section. The big one. The one everyone scrolls to. GCC has so many opinions about your code and these are the lil switches that decide which opinions get said out loud, and how loudly. 💅

You'll clock the -Wno- ones immediately — they're their own whole vibe. Every single flag below is here, in order, nothing skipped, no cap. 🫶

• -fsyntax-only
• -fmax-errors=n
• -Wpedantic
• -pedantic-errors
• -w
• -Wextra
• -Wall
• -Wabi=n
• -Waddress
• -Wno-address-of-packed-member
• -Waggregate-return
• -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
• -Wno-attributes
• -Wattribute-alias=n
• -Wno-attribute-alias
• -Wno-attribute-warning
• -Wbool-compare
• -Wbool-operation
• -Wno-builtin-declaration-mismatch
• -Wno-builtin-macro-redefined
• -Wc90-c99-compat
• -Wc99-c11-compat
• -Wc11-c2x-compat
• -Wc++-compat
• -Wc++11-compat
• -Wc++14-compat
• -Wc++17-compat
• -Wc++20-compat
• -Wcast-align
• -Wcast-align=strict
• -Wcast-function-type
• -Wcast-qual
• -Wchar-subscripts
• -Wclobbered
• -Wcomment
• -Wconversion
• -Wno-coverage-mismatch
• -Wno-cpp
• -Wdangling-else
• -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
• -Werror
• -Werror=*
• -Wexpansion-to-defined
• -Wfatal-errors
• -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
• -Wno-if-not-aligned
• -Wno-ignored-attributes
• -Wignored-qualifiers
• -Wno-incompatible-pointer-types
• -Wimplicit
• -Wimplicit-fallthrough
• -Wimplicit-fallthrough=n
• -Wno-implicit-function-declaration
• -Wno-implicit-int
• -Winit-self
• -Winline
• -Wno-int-conversion
• -Wint-in-bool-context
• -Wno-int-to-pointer-cast
• -Wno-invalid-memory-model
• -Winvalid-pch
• -Wjump-misses-init
• -Wlarger-than=byte-size
• -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
• -Wno-missing-profile
• -Wno-multichar
• -Wmultistatement-macros
• -Wnonnull
• -Wnonnull-compare
• -Wnormalized=[none|id|nfc|nfkc]
• -Wnull-dereference
• -Wno-odr
• -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-prio-ctor-dtor
• -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
• -Wsuggest-attribute=[pure|const|noreturn|format|malloc]
• -Wswitch
• -Wno-switch-bool
• -Wswitch-default
• -Wswitch-enum
• -Wno-switch-outside-range
• -Wno-switch-unreachable
• -Wsync-nand
• -Wsystem-headers
• -Wtautological-compare
• -Wtrampolines
• -Wtrigraphs
• -Wtsan
• -Wtype-limits
• -Wundef
• -Wuninitialized
• -Wunknown-pragmas
• -Wunsuffixed-float-constants
• -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
• -Wno-varargs
• -Wvariadic-macros
• -Wvector-operation-performance
• -Wvla
• -Wvla-larger-than=byte-size
• -Wno-vla-larger-than
• -Wvolatile-register-var
• -Wwrite-strings
• -Wzero-length-bounds

Not us reading that entire list like a grocery run 🛒 — hydrate, stretch, blink at something that isn't a terminal. You ate that. 💖

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🔬 Static Analyzer Options 🔬

Now THIS is the main character. 🎀 The static analyzer is the part of GCC that reads your code like a group chat it was added to against its will, and then tells you everything sus it noticed. These flags poke at it, tune it, and dump its inner monologue to files so you can stare at it.

• -fanalyzer
• -fanalyzer-call-summaries
• -fanalyzer-checker=name
• -fno-analyzer-feasibility
• -fanalyzer-fine-grained
• -fanalyzer-state-merge
• -fanalyzer-state-purge
• -fanalyzer-transitivity
• -fanalyzer-verbose-edges
• -fanalyzer-verbose-state-changes
• -fanalyzer-verbosity=level
• -fdump-analyzer
• -fdump-analyzer-stderr
• -fdump-analyzer-callgraph
• -fdump-analyzer-exploded-graph
• -fdump-analyzer-exploded-nodes
• -fdump-analyzer-exploded-nodes-2
• -fdump-analyzer-exploded-nodes-3
• -fdump-analyzer-feasibility
• -fdump-analyzer-json
• -fdump-analyzer-state-purge
• -fdump-analyzer-supergraph
• -Wno-analyzer-double-fclose
• -Wno-analyzer-double-free
• -Wno-analyzer-exposure-through-output-file
• -Wno-analyzer-file-leak
• -Wno-analyzer-free-of-non-heap
• -Wno-analyzer-malloc-leak
• -Wno-analyzer-mismatching-deallocation
• -Wno-analyzer-null-argument
• -Wno-analyzer-null-dereference
• -Wno-analyzer-possible-null-argument
• -Wno-analyzer-possible-null-dereference
• -Wno-analyzer-shift-count-negative
• -Wno-analyzer-shift-count-overflow
• -Wno-analyzer-stale-setjmp-buffer
• -Wno-analyzer-tainted-array-index
• -Wanalyzer-too-complex
• -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-write-to-const
• -Wno-analyzer-write-to-string-literal

The way those -Wno-analyzer-… names read like a haunted house tour of every possible disaster 💀 — it's giving "we've seen things."

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🍎 C and Objective-C-only Warning Options 🍎

These ones only show up for the C and Objective-C girlies. C++ crowd, this is not your section, love you though. 🌸

• -Wbad-function-cast
• -Wmissing-declarations
• -Wmissing-parameter-type
• -Wmissing-prototypes
• -Wnested-externs
• -Wold-style-declaration
• -Wold-style-definition
• -Wstrict-prototypes
• -Wtraditional
• -Wtraditional-conversion
• -Wdeclaration-after-statement
• -Wpointer-sign

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🐛 Debugging Options 🐛

And here's the debug drawer. 🦋 Where all the -g-prefixed stuff lives, plus a bunch of -f flags that tag along for the debug-info ride. If you've ever squinted at a backtrace and gone "who is she", this is the part of the toolbox that made that backtrace possible.

• -g
• -glevel
• -gdwarf
• -gdwarf-version
• -ggdb
• -grecord-gcc-switches
• -gno-record-gcc-switches
• -gstabs
• -gstabs+
• -gstrict-dwarf
• -gno-strict-dwarf
• -gas-loc-support
• -gno-as-loc-support
• -gas-locview-support
• -gno-as-locview-support
• -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
• -gxcoff
• -gxcoff+
• -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

Okay so we've now met every single one of the flags 💅 and there's still more page to go, so don't close the tab. ✨

🔥 OPTIMIZATION OPTIONS 🔥

okay bestie, this is THE section. the main character. the whole reason half of y'all opened this manpage at 2am. this is the full roster of optimization flags — every single one, and yes i mean every single one, plus the -O levels chillin' at the bottom of the list like the queens they are. no skipping, no "eh i'll just use -O2", we're reading the whole thing together like the devoted little compiler gremlins we are 💖

-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]]]] -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
-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 -fgcse  -fgcse-after-reload
-fgcse-las  -fgcse-lm  -fgraphite-identity -fgcse-sm
-fhoist-adjacent-loads  -fif-conversion -fif-conversion2
-findirect-inlining -finline-functions
-finline-functions-called-once  -finline-limit=n
-finline-small-functions -fipa-modref -fipa-cp  -fipa-cp-clone
-fipa-bit-cp  -fipa-vrp  -fipa-pta  -fipa-profile  -fipa-pure-const
-fipa-reference  -fipa-reference-addressable -fipa-stack-alignment
-fipa-icf  -fira-algorithm=algorithm -flive-patching=level
-fira-region=region  -fira-hoist-pressure -fira-loop-pressure
-fno-ira-share-save-slots -fno-ira-share-spill-slots
-fisolate-erroneous-paths-dereference
-fisolate-erroneous-paths-attribute -fivopts
-fkeep-inline-functions  -fkeep-static-functions
-fkeep-static-consts  -flimit-function-alignment
-flive-range-shrinkage -floop-block  -floop-interchange
-floop-strip-mine -floop-unroll-and-jam  -floop-nest-optimize
-floop-parallelize-all  -flra-remat  -flto  -flto-compression-level
-flto-partition=alg  -fmerge-all-constants -fmerge-constants
-fmodulo-sched  -fmodulo-sched-allow-regmoves -fmove-loop-invariants
-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-sibling-calls -fpartial-inlining  -fpeel-loops
-fpredictive-commoning -fprefetch-loop-arrays -fprofile-correction
-fprofile-use  -fprofile-use=path -fprofile-partial-training
-fprofile-values -fprofile-reorder-functions -freciprocal-math
-free  -frename-registers  -freorder-blocks
-freorder-blocks-algorithm=algorithm -freorder-blocks-and-partition
-freorder-functions -frerun-cse-after-loop
-freschedule-modulo-scheduled-loops -frounding-math
-fsave-optimization-record -fsched2-use-superblocks
-fsched-pressure -fsched-spec-load  -fsched-spec-load-dangerous
-fsched-stalled-insns-dep[=n]  -fsched-stalled-insns[=n]
-fsched-group-heuristic  -fsched-critical-path-heuristic
-fsched-spec-insn-heuristic  -fsched-rank-heuristic
-fsched-last-insn-heuristic  -fsched-dep-count-heuristic
-fschedule-fusion -fschedule-insns  -fschedule-insns2
-fsection-anchors -fselective-scheduling  -fselective-scheduling2
-fsel-sched-pipelining  -fsel-sched-pipelining-outer-loops
-fsemantic-interposition  -fshrink-wrap  -fshrink-wrap-separate
-fsignaling-nans -fsingle-precision-constant
-fsplit-ivs-in-unroller  -fsplit-loops -fsplit-paths
-fsplit-wide-types  -fsplit-wide-types-early  -fssa-backprop
-fssa-phiopt -fstdarg-opt  -fstore-merging  -fstrict-aliasing
-fthread-jumps  -ftracer  -ftree-bit-ccp -ftree-builtin-call-dce
-ftree-ccp  -ftree-ch -ftree-coalesce-vars  -ftree-copy-prop
-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  -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 -O  -O0  -O1  -O2  -O3  -Os  -Ofast  -Og

that's the whole lineup, no cap. if you counted and thought "there's no way that's all of them" — it is, and they're all here, in this order, exactly like this. 🫶

---

📊 PROGRAM INSTRUMENTATION OPTIONS 📊

next up: the instrumentation girlies. this is the family that's all about measuring and checking — profiling, coverage, sanitizers, stack protection, all that protective gear your program wears so it doesn't go out into the world naked. it's giving safety-first, and honestly? we love that for us. ✨

-p  -pg  -fprofile-arcs  --coverage  -ftest-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
-fasan-shadow-offset=number  -fsanitize-sections=s1,s2,...
-fsanitize-undefined-trap-on-error  -fbounds-check
-fcf-protection=[full|branch|return|none|check] -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
-fvtable-verify=[std|preinit|none] -fvtv-counts  -fvtv-debug
-finstrument-functions
-finstrument-functions-exclude-function-list=sym,sym,...
-finstrument-functions-exclude-file-list=file,file,...

note those ... at the end of the last two — they're part of the flag syntax, so yes, keep them. 💅

---

🧠 PREPROCESSOR OPTIONS 🧠

the preprocessor besties!! this is everything that happens before your code even gets to be real code — macros, includes, charsets, trigraphs, the whole pre-game. if you've ever yelled "WHERE DID THAT #define COME FROM" into the void, this section is your people. 🎀

-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  -flarge-source-files
-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

the single-letter -M-squad up in there is honestly iconic. ⭐

---

🔧 ASSEMBLER OPTIONS 🔧

tiny section, enormous energy. literally just two ways to hand something straight to the assembler. short but mighty, like a really good playlist with two songs. 🦋

-Wa,option  -Xassembler option

---

🔗 LINKER OPTIONS 🔗

linker o'clock!! this is where your object files get introduced to each other and become a whole executable. we got libraries, we got static vs shared, we got -Wl,option and -Xlinker option for passing the tea straight through, we got -z keyword doing whatever it does. also object-file-name is right there at the front with no dash, because sometimes you just name the file and that's the whole vibe. 😭

object-file-name  -fuse-ld=linker  -llibrary -nostartfiles
-nodefaultlibs  -nolibc  -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

look at all those -static-lib variants holding hands. a family. 🫶

---

📁 DIRECTORY OPTIONS 📁

where do things live, bestie? that's the whole question this section answers. include paths, library paths, plugin dirs, prefixes, sysroots — it's giving "let me show you around the neighborhood". 🏡

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

heads up: -I- really is spelled exactly like that, dash capital-i dash, and it is not a typo. do not come for me. 💀

---

🏗️ CODE GENERATION OPTIONS 🏗️

the ABI-and-codegen crowd. calling conventions, position independence, struct layout, visibility, the whole "how does this actually turn into instructions and a binary layout" situation. this is the deep-lore section and honestly it ate. 👑

-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  -ftrapv
-fwrapv -fvisibility=[default|internal|hidden|protected]
-fstrict-volatile-bitfields  -fsync-libcalls

-fpic vs -fPIC — yes, case matters, and yes, they're both listed, because they are not the same flag and i will not be merging them. ever. 🎀

---

🧪 DEVELOPER OPTIONS 🧪

okay so this is the compiler-dev section, aka the "you're either writing GCC or you're in way too deep" club. dumps, passes, reports, stats, timing, temp files, -Q, -print--anything. if you're here for fun, welcome, you're my kind of unhinged. if you're here because something broke — godspeed, bestie. 💜

-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]
-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]

-dletters is literally the flag named "letters" and honestly? that's the most honest naming in this entire document. 🙈

---

💻 MACHINE-DEPENDENT OPTIONS 💻

and finally (for this part, anyway) — machine-dependent options, where everything changes depending on what silicon you're targeting. right now we're looking at the AArch64 crew, and the list is chef's kiss. 🌸

AArch64 Options

-mabi=name  -mbig-endian  -mlittle-endian
-mgeneral-regs-only -mcmodel=tiny  -mcmodel=small  -mcmodel=large
-mstrict-align  -mno-strict-align -momit-leaf-frame-pointer
-mtls-dialect=desc  -mtls-dialect=traditional -mtls-size=size
-mfix-cortex-a53-835769  -mfix-cortex-a53-843419
-mlow-precision-recip-sqrt  -mlow-precision-sqrt
-mlow-precision-div -mpc-relative-literal-loads
-msign-return-address=scope -mbranch-protection=none|standard|pac-ret[+leaf +b-key]|bti
-mharden-sls=opts -march=name  -mcpu=name
-mtune=name -moverride=string  -mverbose-cost-dump
-mstack-protector-guard=guard -mstack-protector-guard-reg=sysreg
-mstack-protector-guard-offset=offset -mtrack-speculation
-moutline-atomics

those -mfix-cortex-a53-835769 and -mfix-cortex-a53-843419 names are just the erratum numbers, and yes, they're that specific on purpose. silicon is messy, bestie. 💅

and look at -mbranch-protection=none|standard|pac-ret[+leaf +b-key]|bti — that +b-key inside the brackets really does have a space in it. copy it exactly or it will not vibe with you. 🫶

---

🌸 Adapteva Epiphany Options 🌸

okay so if you're building for Adapteva Epiphany (yes she's niche, yes she's iconic) these are the knobs, bestie 💅

• -mhalf-reg-file
• -mprefer-short-insn-regs
• -mbranch-cost=num
• -mcmove
• -mnops=num
• -msoft-cmpsf
• -msplit-lohi
• -mpost-inc
• -mpost-modify
• -mstack-offset=num
• -mround-nearest
• -mlong-calls
• -mshort-calls
• -msmall16
• -mfp-mode=mode
• -mvect-double
• -max-vect-align=num
• -msplit-vecmove-early
• -m1reg-reg

---

✨ AMD GCN Options ✨

GPU girlies, this one's for you 🦋

• -march=gpu
• -mtune=gpu
• -mstack-size=bytes

---

🎀 ARC Options 🎀

the ARC list is LONG, like, novel long, but every single one is right here, no skips 🙈

• -mbarrel-shifter
• -mjli-always
• -mcpu=cpu
• -mA6
• -mARC600
• -mA7
• -mARC700
• -mdpfp
• -mdpfp-compact
• -mdpfp-fast
• -mno-dpfp-lrsr
• -mea
• -mno-mpy
• -mmul32x16
• -mmul64
• -matomic
• -mnorm
• -mspfp
• -mspfp-compact
• -mspfp-fast
• -msimd
• -msoft-float
• -mswap
• -mcrc
• -mdsp-packa
• -mdvbf
• -mlock
• -mmac-d16
• -mmac-24
• -mrtsc
• -mswape
• -mtelephony
• -mxy
• -misize
• -mannotate-align
• -marclinux
• -marclinux_prof
• -mlong-calls
• -mmedium-calls
• -msdata
• -mirq-ctrl-saved
• -mrgf-banked-regs
• -mlpc-width=width
• -G num
• -mvolatile-cache
• -mtp-regno=regno
• -malign-call
• -mauto-modify-reg
• -mbbit-peephole
• -mno-brcc
• -mcase-vector-pcrel
• -mcompact-casesi
• -mno-cond-exec
• -mearly-cbranchsi
• -mexpand-adddi
• -mindexed-loads
• -mlra
• -mlra-priority-none
• -mlra-priority-compact
• mlra-priority-noncompact
• -mmillicode
• -mmixed-code
• -mq-class
• -mRcq
• -mRcw
• -msize-level=level
• -mtune=cpu
• -mmultcost=num
• -mcode-density-frame
• -munalign-prob-threshold=probability
• -mmpy-option=multo
• -mdiv-rem
• -mcode-density
• -mll64
• -mfpu=fpu
• -mrf16
• -mbranch-index

---

💖 ARM Options 💖

ARM my beloved 🫶 so many flags, so much power

• -mapcs-frame
• -mno-apcs-frame
• -mabi=name
• -mapcs-stack-check
• -mno-apcs-stack-check
• -mapcs-reentrant
• -mno-apcs-reentrant
• -mgeneral-regs-only
• -msched-prolog
• -mno-sched-prolog
• -mlittle-endian
• -mbig-endian
• -mbe8
• -mbe32
• -mfloat-abi=name
• -mfp16-format=name
• -mthumb-interwork
• -mno-thumb-interwork
• -mcpu=name
• -march=name
• -mfpu=name
• -mtune=name
• -mprint-tune-info
• -mstructure-size-boundary=n
• -mabort-on-noreturn
• -mlong-calls
• -mno-long-calls
• -msingle-pic-base
• -mno-single-pic-base
• -mpic-register=reg
• -mnop-fun-dllimport
• -mpoke-function-name
• -mthumb
• -marm
• -mflip-thumb
• -mtpcs-frame
• -mtpcs-leaf-frame
• -mcaller-super-interworking
• -mcallee-super-interworking
• -mtp=name
• -mtls-dialect=dialect
• -mword-relocations
• -mfix-cortex-m3-ldrd
• -munaligned-access
• -mneon-for-64bits
• -mslow-flash-data
• -masm-syntax-unified
• -mrestrict-it
• -mverbose-cost-dump
• -mpure-code
• -mcmse
• -mfix-cmse-cve-2021-35465
• -mfdpic

---

🦋 AVR Options 🦋

tiny chip, big feelings ⭐

• -mmcu=mcu
• -mabsdata
• -maccumulate-args
• -mbranch-cost=cost
• -mcall-prologues
• -mgas-isr-prologues
• -mint8
• -mdouble=bits
• -mlong-double=bits
• -mn_flash=size
• -mno-interrupts
• -mmain-is-OS_task
• -mrelax
• -mrmw
• -mstrict-X
• -mtiny-stack
• -mfract-convert-truncate
• -mshort-calls
• -nodevicelib
• -nodevicespecs
• -Waddr-space-convert
• -Wmisspelled-isr

---

🎀 Blackfin Options 🎀

• -mcpu=cpu[-sirevision]
• -msim
• -momit-leaf-frame-pointer
• -mno-omit-leaf-frame-pointer
• -mspecld-anomaly
• -mno-specld-anomaly
• -mcsync-anomaly
• -mno-csync-anomaly
• -mlow-64k
• -mno-low64k
• -mstack-check-l1
• -mid-shared-library
• -mno-id-shared-library
• -mshared-library-id=n
• -mleaf-id-shared-library
• -mno-leaf-id-shared-library
• -msep-data
• -mno-sep-data
• -mlong-calls
• -mno-long-calls
• -mfast-fp
• -minline-plt
• -mmulticore
• -mcorea
• -mcoreb
• -msdram
• -micplb

---

✨ C6X Options ✨

• -mbig-endian
• -mlittle-endian
• -march=cpu
• -msim
• -msdata=sdata-type

---

💜 CRIS Options 💜

• -mcpu=cpu
• -march=cpu
• -mtune=cpu
• -mmax-stack-frame=n
• -metrax4
• -metrax100
• -mpdebug
• -mcc-init
• -mno-side-effects
• -mstack-align
• -mdata-align
• -mconst-align
• -m32-bit
• -m16-bit
• -m8-bit
• -mno-prologue-epilogue
• -melf
• -maout
• -sim
• -sim2
• -mmul-bug-workaround
• -mno-mul-bug-workaround

---

🌸 CR16 Options 🌸

• -mmac
• -mcr16cplus
• -mcr16c
• -msim
• -mint32
• -mbit-ops
• -mdata-model=model

---

🧚 C-SKY Options 🧚

• -march=arch
• -mcpu=cpu
• -mbig-endian
• -EB
• -mlittle-endian
• -EL
• -mhard-float
• -msoft-float
• -mfpu=fpu
• -mdouble-float
• -mfdivdu
• -mfloat-abi=name
• -melrw
• -mistack
• -mmp
• -mcp
• -mcache
• -msecurity
• -mtrust
• -mdsp
• -medsp
• -mvdsp
• -mdiv
• -msmart
• -mhigh-registers
• -manchor
• -mpushpop
• -mmultiple-stld
• -mconstpool
• -mstack-size
• -mccrt
• -mbranch-cost=n
• -mcse-cc
• -msched-prolog
• -msim

---

🍓 Darwin Options 🍓

macOS besties rise up 💅 and yeah, a couple of these show up twice in the list, that's just how she is

• -all_load
• -allowable_client
• -arch
• -arch_errors_fatal
• -arch_only
• -bind_at_load
• -bundle
• -bundle_loader
• -client_name
• -compatibility_version
• -current_version
• -dead_strip
• -dependency-file
• -dylib_file
• -dylinker_install_name
• -dynamic
• -dynamiclib
• -exported_symbols_list
• -filelist
• -flat_namespace
• -force_cpusubtype_ALL
• -force_flat_namespace
• -headerpad_max_install_names
• -iframework
• -image_base
• -init
• -install_name
• -keep_private_externs
• -multi_module
• -multiply_defined
• -multiply_defined_unused
• -noall_load
• -no_dead_strip_inits_and_terms
• -nofixprebinding
• -nomultidefs
• -noprebind
• -noseglinkedit
• -pagezero_size
• -prebind
• -prebind_all_twolevel_modules
• -private_bundle
• -read_only_relocs
• -sectalign
• -sectobjectsymbols
• -whyload
• -seg1addr
• -sectcreate
• -sectobjectsymbols
• -sectorder
• -segaddr
• -segs_read_only_addr
• -segs_read_write_addr
• -seg_addr_table
• -seg_addr_table_filename
• -seglinkedit
• -segprot
• -segs_read_only_addr
• -segs_read_write_addr
• -single_module
• -static
• -sub_library
• -sub_umbrella
• -twolevel_namespace
• -umbrella
• -undefined
• -unexported_symbols_list
• -weak_reference_mismatches
• -whatsloaded
• -F
• -gused
• -gfull
• -mmacosx-version-min=version
• -mkernel
• -mone-byte-bool

---

👑 DEC Alpha Options 👑

64-bit royalty, honestly

• -mno-fp-regs
• -msoft-float
• -mieee
• -mieee-with-inexact
• -mieee-conformant
• -mfp-trap-mode=mode
• -mfp-rounding-mode=mode
• -mtrap-precision=mode
• -mbuild-constants
• -mcpu=cpu-type
• -mtune=cpu-type
• -mbwx
• -mmax
• -mfix
• -mcix
• -mfloat-vax
• -mfloat-ieee
• -mexplicit-relocs
• -msmall-data
• -mlarge-data
• -msmall-text
• -mlarge-text
• -mmemory-latency=time

---

🌈 eBPF Options 🌈

kernel girlies, this is your moment 🫶

• -mbig-endian
• -mlittle-endian
• -mkernel=version
• -mframe-limit=bytes
• -mxbpf

---

💖 FR30 Options 💖

• -msmall-model
• -mno-lsim

---

⭐ FT32 Options ⭐

• -msim
• -mlra
• -mnodiv
• -mft32b
• -mcompress
• -mnopm

---

🔥 FRV Options 🔥

• -mgpr-32
• -mgpr-64
• -mfpr-32
• -mfpr-64
• -mhard-float
• -msoft-float
• -malloc-cc
• -mfixed-cc
• -mdword
• -mno-dword
• -mdouble
• -mno-double
• -mmedia
• -mno-media
• -mmuladd
• -mno-muladd
• -mfdpic
• -minline-plt
• -mgprel-ro
• -multilib-library-pic
• -mlinked-fp
• -mlong-calls
• -malign-labels
• -mlibrary-pic
• -macc-4
• -macc-8
• -mpack
• -mno-pack
• -mno-eflags
• -mcond-move
• -mno-cond-move
• -moptimize-membar
• -mno-optimize-membar
• -mscc
• -mno-scc
• -mcond-exec
• -mno-cond-exec
• -mvliw-branch
• -mno-vliw-branch
• -mmulti-cond-exec
• -mno-multi-cond-exec
• -mnested-cond-exec
• -mno-nested-cond-exec
• -mtomcat-stats
• -mTLS
• -mtls
• -mcpu=cpu

---

🐧 GNU/Linux Options 🐧

the distro-flavored ones, periodt

• -mglibc
• -muclibc
• -mmusl
• -mbionic
• -mandroid
• -tno-android-cc
• -tno-android-ld

---

🌸 H8/300 Options 🌸

• -mrelax
• -mh
• -ms
• -mn
• -mexr
• -mno-exr
• -mint32
• -malign-300

---

💜 HPPA Options 💜

• -march=architecture-type
• -mcaller-copies
• -mdisable-fpregs
• -mdisable-indexing
• -mfast-indirect-calls
• -mgas
• -mgnu-ld
• -mhp-ld
• -mfixed-range=register-range
• -mjump-in-delay
• -mlinker-opt
• -mlong-calls
• -mlong-load-store
• -mno-disable-fpregs
• -mno-disable-indexing
• -mno-fast-indirect-calls
• -mno-gas
• -mno-jump-in-delay
• -mno-long-load-store
• -mno-portable-runtime
• -mno-soft-float
• -mno-space-regs
• -msoft-float
• -mpa-risc-1-0
• -mpa-risc-1-1
• -mpa-risc-2-0
• -mportable-runtime
• -mschedule=cpu-type
• -mspace-regs
• -msio
• -mwsio
• -munix=unix-std
• -nolibdld
• -static
• -threads

---

🎀 IA-64 Options 🎀

Itanium bestie, you were so ahead of your time 😭 here's the full roster

• -mbig-endian
• -mlittle-endian
• -mgnu-as
• -mgnu-ld
• -mno-pic
• -mvolatile-asm-stop
• -mregister-names
• -msdata
• -mno-sdata
• -mconstant-gp
• -mauto-pic
• -mfused-madd
• -minline-float-divide-min-latency
• -minline-float-divide-max-throughput
• -mno-inline-float-divide
• -minline-int-divide-min-latency
• -minline-int-divide-max-throughput
• -mno-inline-int-divide
• -minline-sqrt-min-latency
• -minline-sqrt-max-throughput
• -mno-inline-sqrt
• -mdwarf2-asm
• -mearly-stop-bits
• -mfixed-range=register-range
• -mtls-size=tls-size
• -mtune=cpu-type
• -milp32
• -mlp64
• -msched-br-data-spec
• -msched-ar-data-spec
• -msched-control-spec
• -msched-br-in-data-spec
• -msched-ar-in-data-spec
• -msched-in-control-spec
• -msched-spec-ldc
• -msched-spec-control-ldc
• -msched-prefer-non-data-spec-insns
• -msched-prefer-non-control-spec-insns
• -msched-stop-bits-after-every-cycle
• -msched-count-spec-in-critical-path
• -msel-sched-dont-check-control-spec
• -msched-fp-mem-deps-zero-cost
• -msched-max-memory-insns-hard-limit
• -msched-max-memory-insns=max-insns

---

🦋 LM32 Options 🦋

• -mbarrel-shift-enabled
• -mdivide-enabled
• -mmultiply-enabled
• -msign-extend-enabled
• -muser-enabled

---

✨ M32R/D Options ✨

• -m32r2
• -m32rx
• -m32r
• -mdebug
• -malign-loops
• -mno-align-loops
• -missue-rate=number
• -mbranch-cost=number
• -mmodel=code-size-model-type
• -msdata=sdata-type
• -mno-flush-func
• -mflush-func=name
• -mno-flush-trap
• -mflush-trap=number
• -G num

---

🌸 M32C Options 🌸

• -mcpu=cpu
• -msim
• -memregs=number

---

💅 M680x0 Options 💅

m68k girlies, the classics ate and left no crumbs

• -march=arch
• -mcpu=cpu
• -mtune=tune
• -m68000
• -m68020
• -m68020-40
• -m68020-60
• -m68030
• -m68040
• -m68060
• -mcpu32
• -m5200
• -m5206e
• -m528x
• -m5307
• -m5407
• -mcfv4e
• -mbitfield
• -mno-bitfield
• -mc68000
• -mc68020
• -mnobitfield
• -mrtd
• -mno-rtd
• -mdiv
• -mno-div
• -mshort
• -mno-short
• -mhard-float
• -m68881
• -msoft-float
• -mpcrel
• -malign-int
• -mstrict-align
• -msep-data
• -mno-sep-data
• -mshared-library-id=n
• -mid-shared-library
• -mno-id-shared-library
• -mxgot
• -mno-xgot
• -mlong-jump-table-offsets

---

💜 MCore Options 💜

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

---

🌈 MeP Options 🌈

• -mabsdiff
• -mall-opts
• -maverage
• -mbased=n
• -mbitops
• -mc=n
• -mclip
• -mconfig=name
• -mcop
• -mcop32
• -mcop64
• -mivc2
• -mdc
• -mdiv
• -meb
• -mel
• -mio-volatile
• -ml
• -mleadz
• -mm
• -mminmax
• -mmult
• -mno-opts
• -mrepeat
• -ms
• -msatur
• -msdram
• -msim
• -msimnovec
• -mtf
• -mtiny=n

---

🧚 MicroBlaze Options 🧚

soft-core girlies, we see you 🫶

• -msoft-float
• -mhard-float
• -msmall-divides
• -mcpu=cpu
• -mmemcpy
• -mxl-soft-mul
• -mxl-soft-div
• -mxl-barrel-shift
• -mxl-pattern-compare
• -mxl-stack-check
• -mxl-gp-opt
• -mno-clearbss
• -mxl-multiply-high
• -mxl-float-convert
• -mxl-float-sqrt
• -mbig-endian
• -mlittle-endian
• -mxl-reorder
• -mxl-mode-app-model
• -mpic-data-is-text-relative
• -EL, -EB — endianness, pick your side bestie.
• -march=arch, -mtune=arch — target arch and tuning arch.
• -mips1, -mips2, -mips3, -mips4, -mips32, -mips32r2, -mips32r3, -mips32r5, -mips32r6, -mips64, -mips64r2, -mips64r3, -mips64r5, -mips64r6 — the whole ISA level ladder, pick a rung.
• -mips16, -mno-mips16, -mflip-mips16 — MIPS16 mode, off, or the flip.
• -minterlink-compressed, -mno-interlink-compressed, -minterlink-mips16, -mno-interlink-mips16 — interlinking the compressed and mips16 ways.
• -mabi=abi — set the ABI.
• -mabicalls, -mno-abicalls, -mshared, -mno-shared, -mplt, -mno-plt — abicalls, shared, and PLT toggles.
• -mxgot, -mno-xgot — big GOT energy, or not.
• -mgp32, -mgp64, -mfp32, -mfpxx, -mfp64 — register widths, general and float.
• -mhard-float, -msoft-float, -mno-float, -msingle-float, -mdouble-float, -modd-spreg, -mno-odd-spreg — float handling, the whole menu.
• -mabs=mode, -mnan=encoding — absolute value mode and NaN encoding.
• -mdsp, -mno-dsp, -mdspr2, -mno-dspr2, -mmcu, -mmno-mcu — DSP and MCU switches.
• -meva, -mno-eva, -mvirt, -mno-virt, -mxpa, -mno-xpa, -mcrc, -mno-crc, -mginv, -mno-ginv — EVA, virt, XPA, CRC, GINV.
• -mmicromips, -mno-micromips, -mmsa, -mno-msa — microMIPS and MSA.
• -mloongson-mmi, -mno-loongson-mmi, -mloongson-ext, -mno-loongson-ext, -mloongson-ext2, -mno-loongson-ext2 — the Loongson extensions.
• -mfpu=fpu-type — pick your FPU type.
• -msmartmips, -mno-smartmips, -mpaired-single, -mno-paired-single, -mdmx, -mno-mdmx, -mips3d, -mno-mips3d, -mmt, -mno-mt — SmartMIPS, paired-single, MDMX, MIPS-3D, MT.
• -mllsc, -mno-llsc, -mlong64, -mlong32, -msym32, -mno-sym32, -Gnum — LL/SC, long sizes, sym32, and -Gnum.
• -mlocal-sdata, -mno-local-sdata, -mextern-sdata, -mno-extern-sdata, -mgpopt, -mno-gopt — small-data and gp-opt toggles (yes, -mno-gopt, that's really how it's spelled).
• -membedded-data, -mno-embedded-data, -muninit-const-in-rodata, -mno-uninit-const-in-rodata, -mcode-readable=setting — embedded data and where your uninit consts live.
• -msplit-addresses, -mno-split-addresses, -mexplicit-relocs, -mno-explicit-relocs — split addresses and explicit relocs.
• -mcheck-zero-division, -mno-check-zero-division, -mdivide-traps, -mdivide-breaks — divide-by-zero handling, traps or breaks.
• -mload-store-pairs, -mno-load-store-pairs, -mmemcpy, -mno-memcpy, -mlong-calls, -mno-long-calls — load/store pairs, memcpy, long calls.
• -mmad, -mno-mad, -mimadd, -mno-imadd, -mfused-madd, -mno-fused-madd, -nocpp — MAD, IMADD, fused-madd, and -nocpp.
• -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 — the whole silicon-errata workaround fam.
• -mflush-func=func, -mno-flush-func, -mbranch-cost=num, -mbranch-likely, -mno-branch-likely, -mcompact-branches=policy — flush func, branch cost, branch-likely, compact branches.
• -mfp-exceptions, -mno-fp-exceptions, -mvr4130-align, -mno-vr4130-align, -msynci, -mno-synci — FP exceptions, VR4130 alignment, SYNCI.
• -mlxc1-sxc1, -mno-lxc1-sxc1, -mmadd4, -mno-madd4, -mrelax-pic-calls, -mno-relax-pic-calls, -mmcount-ra-address, -mframe-header-opt, -mno-frame-header-opt — LXC1/SXC1, MADD4, relaxed PIC calls, mcount RA address, frame header opt.

---

MMIX Options 🦋

• -mlibfuncs, -mno-libfuncs, -mepsilon, -mno-epsilon — libfuncs and epsilon.
• -mabi=gnu, -mabi=mmixware — pick your MMIX ABI flavor.
• -mzero-extend, -mknuthdiv, -mtoplevel-symbols, -melf — zero-extend, Knuth division, top-level symbols, ELF.
• -mbranch-predict, -mno-branch-predict, -mbase-addresses, -mno-base-addresses, -msingle-exit, -mno-single-exit — branch prediction, base addresses, single exit.

---

MN10300 Options 🌸

• -mmult-bug, -mno-mult-bug — multiply bug workaround, on or off.
• -mno-am33, -mam33, -mam33-2, -mam34 — AM33 family picks.
• -mtune=cpu-type, -mreturn-pointer-on-d0, -mno-crt0 — tuning, return pointer on d0, no crt0.
• -mrelax, -mliw, -msetlb — relax, LIW, setlb.

---

Moxie Options 🧚

• -meb, -mel — big or little endian.
• -mmul.x, -mno-crt0 — the mul.x option (yes, with a dot) and no crt0.

---

MSP430 Options 💜

• -msim, -masm-hex — simulator and hex assembly.
• -mmcu=, -mcpu= — MCU and CPU, both take a value.
• -mlarge, -msmall — big mode or small mode.
• -mrelax, -mwarn-mcu — relax and MCU warnings.
• -mcode-region=, -mdata-region= — code and data regions.
• -msilicon-errata=, -msilicon-errata-warn= — silicon errata and their warnings.
• -mhwmult=, -minrt, -mtiny-printf, -mmax-inline-shift= — hardware multiply, inrt, tiny printf, max inline shift.

---

NDS32 Options ✨

• -mbig-endian, -mlittle-endian — endianness again, obviously.
• -mreduced-regs, -mfull-regs — reduced or full registers.
• -mcmov, -mno-cmov, -mext-perf, -mno-ext-perf, -mext-perf2, -mno-ext-perf2, -mext-string, -mno-ext-string — cmov, perf extensions, perf2, string extensions.
• -mv3push, -mno-v3push, -m16bit, -mno-16bit — v3push and 16-bit mode.
• -misr-vector-size=num, -mcache-block-size=num — ISR vector size and cache block size.
• -march=arch, -mcmodel=code-model, -mctor-dtor, -mrelax — arch, code model, ctors/dtors, relax.

---

Nios II Options 🎀

• -G num — the G option, with a space this time.
• -mgpopt=option, -mgpopt, -mno-gpopt — gp-opt, three ways.
• -mgprel-sec=regexp, -mr0rel-sec=regexp — gp-relative and r0-relative sections.
• -mel, -meb, -mno-bypass-cache, -mbypass-cache — endian and cache bypass.
• -mno-cache-volatile, -mcache-volatile, -mno-fast-sw-div, -mfast-sw-div — volatile caching and fast software divide.
• -mhw-mul, -mno-hw-mul, -mhw-mulx, -mno-hw-mulx, -mno-hw-div, -mhw-div — hardware mul, mulx, div toggles.
• -mcustom-insn=N, -mno-custom-insn, -mcustom-fpu-cfg=name — custom instructions and custom FPU config.
• -mhal, -msmallc, -msys-crt0=name, -msys-lib=name — HAL, smallc, sys crt0, sys lib.
• -march=arch, -mbmx, -mno-bmx, -mcdx, -mno-cdx — arch, BMX, CDX.

---

Nvidia PTX Options 🌈

• -m64, -mmainkernel, -moptimize — 64-bit, mainkernel, and optimize. Short and sweet.

---

OpenRISC Options 🍓

• -mboard=name, -mnewlib — board name and newlib.
• -mhard-mul, -mhard-div, -msoft-mul, -msoft-div — hard or soft mul and div.
• -msoft-float, -mhard-float, -mdouble-float, -munordered-float — float handling.
• -mcmov, -mror, -mrori, -msext, -msfimm, -mshftimm — cmov, rotate, rotate-immediate, sign extend, and the immediate options.

---

PDP-11 Options 👑

• -mfpu, -msoft-float, -mac0, -mno-ac0 — FPU, soft float, AC0 toggles.
• -m40, -m45, -m10 — the classic models.
• -mint32, -mno-int16, -mint16, -mno-int32 — int sizes.
• -msplit, -munix-asm, -mdec-asm, -mgnu-asm, -mlra — split, the assembly dialects, and LRA.

---

picoChip Options 🧚

• -mae=ae_type, -mvliw-lookahead=N — AE type and VLIW lookahead.
• -msymbol-as-address, -mno-inefficient-warnings — symbol-as-address and the warning suppression.

---

PowerPC Options

See RS/6000 and PowerPC Options. (No, that's genuinely what it says — go look below, bestie.)

---

PRU Options 🌸

• -mmcu=mcu, -minrt, -mno-relax, -mloop, -mabi=variant — MCU, inrt, relax, loop, and ABI variant.

---

RISC-V Options 💖

• -mbranch-cost=N-instruction — branch cost in instructions.
• -mplt, -mno-plt, -mabi=ABI-string — PLT toggle and the ABI string.
• -mfdiv, -mno-fdiv, -mdiv, -mno-div — F div and div toggles.
• -march=ISA-string, -mtune=processor-string — ISA string and tuning processor.
• -mpreferred-stack-boundary=num, -msmall-data-limit=N-bytes — stack boundary and small data limit.
• -msave-restore, -mno-save-restore, -mshorten-memrefs, -mno-shorten-memrefs — save/restore and shortened memory refs.
• -mstrict-align, -mno-strict-align, -mcmodel=medlow, -mcmodel=medany — alignment and code models.
• -mexplicit-relocs, -mno-explicit-relocs, -mrelax, -mno-relax — explicit relocs and relax.
• -mriscv-attribute, -mmo-riscv-attribute, -malign-data=type — RISC-V attributes and data alignment.
• -mbig-endian, -mlittle-endian — endian, the eternal question.
• -mstack-protector-guard=guard, -mstack-protector-guard-reg=reg, -mstack-protector-guard-offset=offset — the stack protector guard trio.

---

RL78 Options 💅

• -msim, -mmul=none, -mmul=g13, -mmul=g14 — sim and the multiply variants.
• -mallregs, -mcpu=g10, -mcpu=g13, -mcpu=g14, -mg10, -mg13, -mg14 — all regs and the CPU picks (two spellings, both here).
• -m64bit-doubles, -m32bit-doubles, -msave-mduc-in-interrupts — double sizes and saving MDUC in interrupts.

---

RS/6000 and PowerPC Options 💜

This is the big one, bestie. Sit down.

• -mcpu=cpu-type, -mtune=cpu-type, -mcmodel=code-model — CPU, tuning, code model.
• -mpowerpc64, -maltivec, -mno-altivec — 64-bit PowerPC and AltiVec.
• -mpowerpc-gpopt, -mno-powerpc-gpopt, -mpowerpc-gfxopt, -mno-powerpc-gfxopt — the gp and gfx option sets.
• -mmfcrf, -mno-mfcrf, -mpopcntb, -mno-popcntb, -mpopcntd, -mno-popcntd, -mfprnd, -mno-fprnd, -mcmpb, -mno-cmpb, -mhard-dfp, -mno-hard-dfp — mfcrf, popcount, fprnd, cmpb, decimal floating point.
• -mfull-toc, -mminimal-toc, -mno-fp-in-toc, -mno-sum-in-toc — TOC variations.
• -m64, -m32, -mxl-compat, -mno-xl-compat, -mpe — 64/32, XL compat, PE.
• -malign-power, -malign-natural, -msoft-float, -mhard-float — alignment and float.
• -mmultiple, -mno-multiple, -mupdate, -mno-update — multiple and update.
• -mavoid-indexed-addresses, -mno-avoid-indexed-addresses, -mfused-madd, -mno-fused-madd — indexed addresses and fused-madd.
• -mbit-align, -mno-bit-align, -mstrict-align, -mno-strict-align — alignment toggles.
• -mrelocatable, -mno-relocatable, -mrelocatable-lib, -mno-relocatable-lib, -mtoc, -mno-toc — relocatable and TOC.
• -mlittle, -mlittle-endian, -mbig, -mbig-endian — endian, in two spellings each.
• -mdynamic-no-pic, -mswdiv, -msingle-pic-base — dynamic no-PIC, software div, single PIC base.
• -mprioritize-restricted-insns=priority, -msched-costly-dep=dependence_type, -minsert-sched-nops=scheme — scheduler knobs, all three.
• -mcall-aixdesc, -mcall-eabi, -mcall-freebsd, -mcall-linux, -mcall-netbsd, -mcall-openbsd, -mcall-sysv, -mcall-sysv-eabi, -mcall-sysv-noeabi — calling conventions, the whole tour.
• -mtraceback=traceback_type, -maix-struct-return, -msvr4-struct-return, -mabi=abi-type — traceback, struct returns, ABI type.
• -msecure-plt, -mbss-plt, -mlongcall, -mno-longcall, -mpltseq, -mno-pltseq — PLT styles and long calls.
• -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 — the inline limit crew.
• -misel, -mno-isel, -mvrsave, -mno-vrsave, -mmulhw, -mno-mulhw, -mdlmzb, -mno-dlmzb — isel, vrsave, mulhw, dlmzb.
• -mprototype, -mno-prototype, -msim, -mmvme, -mads, -myellowknife, -memb — prototype and the embedded targets.
• -msdata, -msdata=opt, -mreadonly-in-sdata, -mvxworks, -G num — sdata, readonly-in-sdata, VxWorks, and -G num.
• -mrecip, -mrecip=opt, -mno-recip, -mrecip-precision, -mno-recip-precision — reciprocal estimates.
• -mveclibabi=type, -mfriz, -mno-friz — vector lib ABI and friz.
• -mpointers-to-nested-functions, -mno-pointers-to-nested-functions, -msave-toc-indirect, -mno-save-toc-indirect — nested function pointers and saving TOC indirectly.
• -mpower8-fusion, -mno-mpower8-fusion, -mpower8-vector, -mno-power8-vector — POWER8 fusion and vector.
• -mcrypto, -mno-crypto, -mhtm, -mno-htm — crypto and HTM.
• -mquad-memory, -mno-quad-memory, -mquad-memory-atomic, -mno-quad-memory-atomic — quad memory and its atomic flavor.
• -mcompat-align-parm, -mno-compat-align-parm, -mfloat128, -mno-float128, -mfloat128-hardware, -mno-float128-hardware — param alignment and float128.
• -mgnu-attribute, -mno-gnu-attribute — GNU attributes.
• -mstack-protector-guard=guard, -mstack-protector-guard-reg=reg, -mstack-protector-guard-offset=offset — the stack protector guard trio, PowerPC edition.
• -mprefixed, -mno-prefixed, -mpcrel, -mno-pcrel — prefixed instructions and PC-relative.
• -mmma, -mno-mmma — the MMA toggle, both directions. 💅
• -mrop-protect, -mno-rop-protect, -mprivileged, -mno-privileged — ROP protection and privileged mode.

---

RX Options 🎀

• -m64bit-doubles, -m32bit-doubles, -fpu, -nofpu — double sizes and FPU toggles.
• -mcpu=, -mbig-endian-data, -mlittle-endian-data — CPU and data endianness.
• -msmall-data, -msim, -mno-sim — small data and sim.
• -mas100-syntax, -mno-as100-syntax, -mrelax — AS100 syntax and relax.
• -mmax-constant-size=, -mint-register=, -mpid — max constant size, int register, PID.
• -mallow-string-insns, -mno-allow-string-insns, -mjsr — string instructions and JSR.
• -mno-warn-multiple-fast-interrupts, -msave-acc-in-interrupts — fast interrupt warnings and saving ACC in interrupts.

---

S/390 and zSeries Options ✨

• -mtune=cpu-type, -march=cpu-type — tuning and arch.
• -mhard-float, -msoft-float, -mhard-dfp, -mno-hard-dfp — float and decimal float.
• -mlong-double-64, -mlong-double-128 — long double sizes.
• -mbackchain, -mno-backchain, -mpacked-stack, -mno-packed-stack — backchain and packed stack.
• -msmall-exec, -mno-small-exec, -mmvcle, -mno-mvcle — small exec and MVCLE.
• -m64, -m31, -mdebug, -mno-debug — 64/31 and debug.
• -mesa, -mzarch, -mhtm, -mvx, -mzvector — ESA, z/Architecture, HTM, vector extension, zvector.
• -mtpf-trace, -mno-tpf-trace, -mtpf-trace-skip, -mno-tpf-trace-skip — TPF trace toggles.
• -mfused-madd, -mno-fused-madd, -mwarn-framesize, -mwarn-dynamicstack — fused-madd and warning flags.
• -mstack-size, -mstack-guard, -mhotpatch=halfwords,halfwords — stack size, stack guard, and hotpatch.

---

Score Options 🦋

• -meb, -mel — endian.
• -mnhwloop, -muls, -mmac — nhwloop, uls, mac.
• -mscore5, -mscore5u, -mscore7, -mscore7d — the Score cores.

---

SH Options 💖

• -m1, -m2, -m2e, -m2a-nofpu, -m2a-single-only, -m2a-single, -m2a, -m3, -m3e — the early SH generations.
• -m4-nofpu, -m4-single-only, -m4-single, -m4, -m4a-nofpu, -m4a-single-only, -m4a-single, -m4a, -m4al — the whole SH4 lineup.
• -mb, -ml, -mdalign, -mrelax — endian, dalign, relax.
• -mbigtable, -mfmovd, -mrenesas, -mno-renesas, -mnomacsave — bigtable, fmovd, Renesas mode, no mac save.
• -mieee, -mno-ieee, -mbitops, -misize — IEEE, bitops, isize.
• -minline-ic_invalidate, -mpadstruct, -mprefergot, -musermode — inline IC invalidate, padstruct, prefer GOT, user mode.
• -multcost=number, -mdiv=strategy, -mdivsi3_libfunc=name, -mfixed-range=register-range — mult cost, div strategy, divsi3 libfunc, fixed register range.
• -maccumulate-outgoing-args, -matomic-model=atomic-model, -mbranch-cost=num — outgoing args, atomic model, branch cost.
• -mzdcbranch, -mno-zdcbranch, -mcbranch-force-delay-slot — ZDC branch and forced delay slot.
• -mfused-madd, -mno-fused-madd, -mfsca, -mno-fsca, -mfsrra, -mno-fsrra — fused-madd, FSCA, FSRRA.
• -mpretend-cmove, -mtas — pretend cmove and TAS.

---

Solaris 2 Options 🌸

• -mclear-hwcap, -mno-clear-hwcap — clear the hardware capability bits, or don't.
• -mimpure-text, -mno-impure-text — impure text toggle.
• -pthreads — threads, periodt.

---

SPARC Options 💜

• -mcpu=cpu-type, -mtune=cpu-type, -mcmodel=code-model, -mmemory-model=mem-model — CPU, tuning, code model, memory model.
• -m32, -m64 — 32 or 64.
• -mapp-regs, -mno-app-regs, -mfaster-structs, -mno-faster-structs, -mflat, -mno-flat — app regs, faster structs, flat.
• -mfpu, -mno-fpu, -mhard-float, -msoft-float — FPU and float handling.
• -mhard-quad-float, -msoft-quad-float — quad float, hard or soft.
• -mstack-bias, -mno-stack-bias, -mstd-struct-return, -mno-std-struct-return — stack bias and standard struct return.
• -munaligned-doubles, -mno-unaligned-doubles, -muser-mode, -mno-user-mode — unaligned doubles and user mode.
• -mv8plus, -mno-v8plus, -mvis, -mno-vis — V8+ and VIS.
• -mvis2, -mno-vis2, -mvis3, -mno-vis3, -mvis4, -mno-vis4, -mvis4b, -mno-vis4b — VIS2, VIS3, VIS4, VIS4B.
• -mcbcond, -mno-cbcond, -mfmaf, -mno-fmaf, -mfsmuld, -mno-fsmuld — cbcond, fmaf, fsmuld.
• -mpopc, -mno-popc, -msubxc, -mno-subxc — popc and subxc.
• -mfix-at697f, -mfix-ut699, -mfix-ut700, -mfix-gr712rc — the errata fix squad.
• -mlra, -mno-lra — LRA toggle.

---

System V Options 🍓

• -Qy, -Qn — the Q toggles.
• -YP,paths, -Ym,dir — path and dir options, comma and all.

---

TILE-Gx Options 🌈

• -mcpu=CPU, -m32, -m64 — CPU pick and bitness.
• -mbig-endian, -mlittle-endian, -mcmodel=code-model — endian and code model.

---

TILEPro Options 🧚

• -mcpu=cpu, -m32 — CPU and 32-bit. That's the whole list, bestie.

V850 Options — bestie this chip has a whole personality:

• -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, retro, lowkey a collectible:

• -mg, -mgnu, -munix

---

Visium Options — tiny list, big energy:

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

---

VMS Options — okay so:

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

---

VxWorks Options — embedded girlies rise up ✨:

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

---

x86 Options — okay this one is the main character, the list is genuinely giving encyclopedia, so buckle up 🎀:

• -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
• -mmmx
• -msse, -msse2, -msse3, -mssse3, -msse4.1, -msse4.2, -msse4
• -mavx, -mavx2
• -mavx512f, -mavx512pf, -mavx512er, -mavx512cd, -mavx512vl, -mavx512bw, -mavx512dq, -mavx512ifma, -mavx512vbmi
• -msha, -maes, -mpclmul, -mfsgsbase, -mrdrnd, -mf16c, -mfma, -mpconfig, -mwbnoinvd, -mptwrite, -mprefetchwt1, -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, -mforce-indirect-call, -mavx512vbmi2, -mavx512bf16, -menqcmd, -mvpclmulqdq, -mavx512bitalg, -mmovdiri, -mmovdir64b, -mavx512vpopcntdq, -mavx5124fmaps, -mavx512vnni, -mavx5124vnniw, -mprfchw, -mrdpid, -mrdseed, -msgx, -mavx512vp2intersect, -mserialize, -mtsxldtrk, -mamx-tile, -mamx-int8, -mamx-bf16, -muintr, -mhreset, -mavxvnni, -mcldemote, -mms-bitfields, -mno-align-stringops, -minline-all-stringops, -minline-stringops-dynamically, -mstringop-strategy=alg, -mkl, -mwidekl, -mmemcpy-strategy=strategy, -mmemset-strategy=strategy, -mpush-args, -maccumulate-outgoing-args, -m128bit-long-double, -m96bit-long-double, -mlong-double-64, -mlong-double-80, -mlong-double-128, -mregparm=num, -msseregparm, -mveclibabi=type, -mvect8-ret-in-mem, -mpc32, -mpc64, -mpc80, -mdaz-ftz, -mstackrealign, -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, -mindirect-branch=choice, -mfunction-return=choice, -mindirect-branch-register, -mharden-sls=choice, -mindirect-branch-cs-prefix, -mneeded

---

x86 Windows Options — windows besties this one's for you 🫶:

• -mconsole
• -mcygwin, -mno-cygwin
• -mdll
• -mnop-fun-dllimport
• -mthread
• -municode
• -mwin32
• -mwindows
• -fno-set-stack-executable

---

Xstormy16 Options — just the one, and honestly? Iconic for it:

• -msim

---

Xtensa Options — okay so:

• -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

---

zSeries Options — bestie these moved house, go read See S/390 and zSeries Options.

---

🎀 Options Controlling the Kind of Output 🎀

Okay so here's the tea on how compilation actually goes down, because it's not one single thing, it's a whole four-stage journey 💅:

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

...and it's always in that order, no skipping the queue.

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 newly compiled ones and the ones you passed in as input) into an executable file. That's the whole glow-up pipeline ✨.

For any given input file, the file name suffix determines what kind of compilation is done, so let's do a little vibe check on every suffix 👇

• file.c — C source code that must be preprocessed.
• file.i — C source code that should not be preprocessed.
• file.ii — C++ source code that should not be preprocessed.
• file.m — Objective-C source code. Heads up bestie: you must link with the libobjc library to make an Objective-C program work.
• file.mi — Objective-C source code that should not be preprocessed.
• file.mm
• file.M — Objective-C++ source code. Note that you must link with the libobjc library to make an Objective-C++ program work. And note that .M refers to a literal capital M 👑.
• file.mii — Objective-C++ source code that should not be preprocessed.
• file.h — C, C++, Objective-C or Objective-C++ header file to be turned into a precompiled header (that's the default), or C, C++ header file to be turned into an Ada spec (via the -fdump-ada-spec switch).
• file.cc
• file.cp
• file.cxx
• file.cpp
• file.CPP
• file.c++
• file.C — C++ source code that must be preprocessed. Note that in .cxx, the last two letters must both be literally x. Likewise, .C refers to a literal capital C. case-sensitive girlies stay winning 💅
• file.mm
• file.M — Objective-C++ source code that must be preprocessed.
• file.mii — Objective-C++ source code that should not be preprocessed.
• file.hh
• file.H
• file.hp
• file.hxx
• file.hpp
• file.HPP
• file.h++
• file.tcc — C++ header file to be turned into a precompiled header or Ada spec.
• file.f
• file.for
• file.ftn — Fixed form Fortran source code that should not be preprocessed.
• file.F
• file.FOR
• file.fpp
• file.FPP
• file.FTN — Fixed form Fortran source code that must be preprocessed (with the traditional preprocessor).
• file.f90
• file.f95
• file.f03
• file.f08 — Free form Fortran source code that should not be preprocessed.
• file.F90
• file.F95
• file.F03
• file.F08 — Free form Fortran source code that must be preprocessed (with the traditional preprocessor).
• file.go — Go source code.
• file.brig — BRIG files (binary representation of HSAIL).
• file.d — D source code.
• file.di — D interface file.
• file.dd — D documentation code (Ddoc).
• file.ads — Ada source code file that contains a library unit declaration (a declaration of a package, subprogram, or generic, or a generic instantiation), or a library unit renaming declaration (a package, generic, or subprogram renaming declaration). Such files are also called specs.
• file.adb — Ada source code file containing a library unit body (a subprogram or package body). Such files are also called bodies.
• file.s — Assembler code.
• file.S
• file.sx — Assembler code that must be preprocessed.
• other — An object file to be fed straight into linking. Any file name with no recognized suffix is treated this way.

---

You can also just say what language it is, out loud, with the -x option:

• -x language — Specify explicitly the language for the following input files (rather than letting the compiler choose a default based on the file name suffix). This option applies to all following input files until the next -x option. Possible values for language are:

        c  c-header  cpp-output
        c++  c++-header  c++-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
        d
        f77  f77-cpp-input f95  f95-cpp-input
        go
        brig

• -x none — Turn off any specification of a language, so that subsequent files are handled according to their file name suffixes (as they are if -x has not been used at all). We love a clean reset 🌸.

---

If you only want some of the stages of compilation, you can use -x (or filename suffixes) to tell gcc where to start, and one of the options -c, -S, or -E to say where gcc is to stop.

⚠️ Heads up: some combinations (for example, -x cpp-output -E) instruct gcc to do nothing at all. Literally nothing. A whole lot of vibes and zero output 😭.

• -c — Compile or assemble the source files, but do not link. The linking stage simply is not done. The ultimate output is in the form of an object file for each source file.

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

Unrecognized input files, not requiring compilation or assembly, are ignored.

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

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

Input files that don't require compilation are ignored.

• -E — Stop after the preprocessing stage; do not run the compiler proper. The output is in the form of preprocessed source code, which is sent to the standard output.

Input files that don't require preprocessing are ignored.

• -o file — Place the primary output in file file. This applies to whatever sort of output is being produced, whether it be an executable file, an object file, an assembler file or preprocessed C code.

If -o is not specified, the default is to put an executable file in a.out, the object file for source.suffix in source.o, its assembler file in source.s, a precompiled header file in source.suffix.gch, and all preprocessed C source on standard output. 💖

Okay so real talk: -o only names the primary output, but bestie it also messes with how auxiliary and dump outputs get named. See the examples below. Unless something overrides it, both aux outputs and dump outputs live in the same directory as the primary output. For aux outputs, the suffix of the input file gets swapped for the aux output file type's suffix; for dump outputs, the dump file's suffix gets appended to the input file's suffix. In compilation commands, the base name of both aux and dump outputs is the primary output's base name; in compile-and-link commands, the primary output name, minus the executable suffix, gets combined with the input file name. If they share the same base name (ignoring suffix), the combo is just that base name; otherwise they get concatenated with a dash between them. Periodt.

gcc -c foo.c ...

will use foo.o as the primary output, and drop aux outputs and dumps right next to it — e.g., aux file foo.dwo for -gsplit-dwarf, and dump file foo.c.???r.final for -fdump-rtl-final. <sparkles>✨</sparkles>

If a non-linker output file is explicitly specified, aux and dump files by default take the same base name:

gcc -c foo.c -o dir/foobar.o ...

will name aux outputs dir/foobar.* and dump outputs dir/foobar.c.*.

A linker output will instead prefix aux and dump outputs:

gcc foo.c bar.c -o dir/foobar ...

will generally name aux outputs dir/foobar-foo.* and dir/foobar-bar.*, and dump outputs dir/foobar-foo.c.* and dir/foobar-bar.c.*.

The one exception to the above is when the executable shares the base name with the single input:

gcc foo.c -o dir/foo ...

in which case aux outputs are named dir/foo.* and dump outputs named dir/foo.c.*.

The location and the names of auxiliary and dump outputs can be adjusted by the options -dumpbase, -dumpbase-ext, -dumpdir, -save-temps=cwd, and -save-temps=obj. iykyk

---

✨ -dumpbase dumpbase ✨

This option sets the base name for auxiliary and dump output files. It does not affect the name of the primary output file. Intermediate outputs, when preserved, are not regarded as primary outputs, but as auxiliary outputs:

gcc -save-temps -S foo.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 in save-foo.i, compiles to save-foo.s (now an intermediate, thus auxiliary output), and then assembles to the (implied) output file foo.o. slay

Absent this option, dump and aux files take their names from the input file, or from the (non-linker) output file, if one is explicitly specified: dump output files (e.g. those requested by -fdump-* options) with the input name suffix, and aux output files (those requested by other non-dump options, e.g. "-save-temps", "-gsplit-dwarf", "-fcallgraph-info") without it.

Similar suffix differentiation of dump and aux outputs can be attained for explicitly-given -dumpbase basename.suf by also specifying -dumpbase-ext .suf.

If dumpbase is explicitly specified with any directory component, any dumppfx specification (e.g. -dumpdir or -save-temps=*) is ignored, and instead of appending to it, dumpbase fully overrides it:

gcc foo.c -c -o dir/foo.o -dumpbase alt/foo \
  -dumpdir pfx- -save-temps=cwd ...

creates auxiliary and dump outputs named alt/foo.*, disregarding dir/ in -o, the ./ prefix implied by -save-temps=cwd, and pfx- in -dumpdir.

When -dumpbase is specified in a command that compiles multiple inputs, or that compiles and then links, it may be combined with dumppfx, as specified under -dumpdir. Then, each input file is compiled using the combined dumppfx, and default values for dumpbase and auxdropsuf are computed for each input file:

gcc foo.c bar.c -c -dumpbase main ...

creates foo.o and bar.o as primary outputs, and avoids overwriting the auxiliary and dump outputs by using the dumpbase as a prefix, creating auxiliary and dump outputs named main-foo.* and main-bar.*.

An empty string specified as dumpbase avoids the influence of the output basename in the naming of auxiliary and dump outputs during compilation, computing default values :

gcc -c foo.c -o dir/foobar.o -dumpbase " ...

will name aux outputs dir/foo.* and dump outputs dir/foo.c.*. Note how their basenames are taken from the input name, but the directory still defaults to that of the output.

The empty-string dumpbase does not prevent the use of the output basename for outputs during linking:

gcc foo.c bar.c -o dir/foobar -dumpbase " -flto ...

The compilation of the source files will name auxiliary outputs dir/foo.* and dir/bar.*, and dump outputs dir/foo.c.* and dir/bar.c.*. LTO recompilation during linking will use dir/foobar. as the prefix for dumps and auxiliary files.

---

💅 -dumpbase-ext auxdropsuf 💅

When forming the name of an auxiliary (but not a dump) output file, drop 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 is not 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 retain the suffix: dir/x-foo.c.*.

This option is disregarded if it does not 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. In case multiple such options are given, the last one prevails:

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. It does not matter that =obj is the default for -save-temps, nor that the output directory is implicitly the current directory. Dump outputs are named foo.c.*.

When compiling from multiple input files, if -dumpbase is specified, dumpbase, minus a auxdropsuf suffix, and a dash are appended to (or override, if containing any directory components) an explicit or defaulted dumppfx, so that each of the multiple compilations gets differently-named aux and dump outputs.

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

outputs auxiliary dumps to dir/pfx-main-foo.* and dir/pfx-main-bar.*, appending dumpbase- to dumppfx. Dump outputs retain the input file suffix: dir/pfx-main-foo.c.* and dir/pfx-main-bar.c.*, respectively. Contrast with the single-input compilation:

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

that, applying -dumpbase to a single source, does not compute and append a separate dumpbase per input file. Its auxiliary and dump outputs go in dir/pfx-main.*.

When compiling and then linking from multiple input files, a defaulted or explicitly specified dumppfx also undergoes the dumpbase- transformation above (e.g. the compilation of foo.c and bar.c above, but without -c). If neither -dumpdir nor -dumpbase are given, the linker output base name, minus auxdropsuf, if specified, or the executable suffix otherwise, plus a dash is appended to the default dumppfx instead. Note, however, that unlike earlier cases of linking:

gcc foo.c bar.c -dumpdir dir/pfx- -o main ...

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. no cap, that's the whole tea 💖

okay so let's sum this whole naming saga up — it's an intuitive but slightly imprecise data flow, and honestly same 💅:

• the primary output name gets chopped into a directory part and a basename part
• dumppfx gets set to the former (the directory part), unless it's overridden by -dumpdir or -save-temps=*
• dumpbase gets set to the latter (the basename part), unless it's overriden by -dumpbase
• if there are multiple inputs or linking, this dumpbase may get combined with dumppfx and taken from each input file
• auxiliary output names for each input are formed by combining dumppfx, dumpbase minus suffix, and the auxiliary output suffix; dump output names are only different in that the suffix from dumpbase is retained

When it comes to auxiliary and dump outputs created during LTO recompilation, a combination of dumppfx and dumpbase — as given, or as derived from the linker output name but not from inputs, even in cases in which this combination would not otherwise be used as such — is passed down with a trailing period replacing the compiler-added dash, if any, as a -dumpdir option to lto-wrapper; being involved in linking, this program does not normally get any -dumpbase and -dumpbase-ext, and it ignores them.

When running sub-compilers, lto-wrapper appends LTO stage names to the received dumppfx, ensures it contains a directory component so that it overrides any -dumpdir, and passes that as -dumpbase to sub-compilers.

---

• -v — Print (on standard error output) the commands executed to run the stages of compilation. Also prints the version number of the compiler driver program, and of the preprocessor, and of the compiler proper. three-for-one bestie 💖
• -### — 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. chef's kiss for scripting 🎀
• --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. receipts on receipts
• --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.

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

--help=target,undocumented

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

--help=warnings,^joined,^undocumented

The argument to --help= should not consist solely of inverted qualifiers. fr fr, don't do it

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 "what even are you" move 💅
• -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. 🫶
• -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 ....

• -ffile-prefix-map=old=new — When compiling files residing in directory old, record any references to them in the result of the compilation as if the files resided in directory new instead. Specifying this option is equivalent to specifying all the individual -f*-prefix-map options. This can be used to make reproducible builds that are location independent. See also -fmacro-prefix-map and -fdebug-prefix-map.
• -fplugin=name.so — Load the plugin code in file name.so, assumed to be a shared object to be dlopen'd by the compiler. The base name of the shared object file is used to identify the plugin for the purposes of argument parsing (See -fplugin-arg-name-key=value below). Each plugin should define the callback functions specified in the Plugins API.
• -fplugin-arg-name-key=value — Define an argument called key with a value of value for the plugin called name.
• -fdump-ada-spec[-slim] — For C and C++ source and include files, generate corresponding Ada specs.
• -fada-spec-parent=unit — In conjunction with -fdump-ada-spec[-slim] above, generate Ada specs as child units of parent unit.
• -fdump-go-spec=file — For input files in any language, generate corresponding Go declarations in file. This generates Go "const", "type", "var", and "func" declarations which may be a useful way to start writing a Go interface to code written in some other language. crossover episode energy ✨
• @file — Read command-line options from file. The options read are inserted in place of the original @file option. If file does not exist, or cannot be read, then the option will be treated literally, and not removed.

Options in file are separated by whitespace. A whitespace character may be included in an option by surrounding the entire option in either single or double quotes. Any character (including a backslash) may be included by prefixing the character to be included with a backslash. The file may itself contain additional @file options; any such options will be processed recursively. 🌀

---

✨ Compiling C++ Programs ✨

C++ source files conventionally use one of the suffixes .C, .cc, .cpp, .CPP, .c++, .cp, or .cxx; C++ header files often use .hh, .hpp, .H, or (for shared template code) .tcc; and preprocessed C++ files use the suffix .ii. GCC recognizes files with these names and compiles them as C++ programs even if you call the compiler the same way as for compiling C programs (usually with the name gcc). no cap, it just knows 💖

However, using gcc does not add the C++ library, bestie — that's the whole plot twist. g++ is the program that calls GCC and automatically links against the C++ library for you. It also treats .c, .h and .i files as C++ source files instead of C source files, unless -x is used. And it's lowkey perfect when you want to precompile a C header file with a .h extension for use in C++ compilations. On many systems, g++ is also installed under the name c++. 💅

When you compile C++ programs, you can specify a ton of the same command-line options you'd use for programs in any language, or the ones that are meaningful for C and its related languages, or the ones that are meaningful only for C++ programs. So many options. So much power. 🫶

---

🎀 Options Controlling C Dialect 🎀

These options control the dialect of C — or languages derived from C, like C++, Objective-C and Objective-C++ — that the compiler accepts:

• -ansi — In C mode this is equivalent to -std=c90. In C++ mode it's equivalent to -std=c++98.

This turns off certain features of GCC that are incompatible with ISO C90 (when compiling C code), or with standard C++ (when compiling C++ code) — like the asm and typeof keywords, plus predefined macros such as unix and vax that identify the type of system you're on. It also enables the undesirable and rarely used ISO trigraph feature. 😭 For the C compiler, it disables recognition of C++ style // comments as well as the inline keyword.

The alternate keywords __asm__, __extension__, __inline__ and __typeof__ keep working despite -ansi. You wouldn't want them in an ISO C program, obviously, but it's genuinely useful to put them in header files that might get included in compilations done with -ansi. Alternate predefined macros such as __unix__ and __vax__ are also available, with or without -ansi.

And no, -ansi does not cause non-ISO programs to be rejected gratuitously. For that drama you need -Wpedantic on top of -ansi.

The macro __STRICT_ANSI__ is predefined when the -ansi option is used. Some header files might notice this macro and refrain from declaring certain functions or defining certain macros that the ISO standard doesn't call for — this is to avoid interfering with any programs that might use those names for other things.

Functions that are normally built in but don't have semantics defined by ISO C (like alloca and ffs) are not built-in functions when -ansi is used.

• -std= — Determine the language standard. This option is currently only supported when compiling C or C++.

The compiler can accept several base standards, such as c90 or c++98, and GNU dialects of those standards, such as gnu90 or gnu++98. When a base standard is specified, the compiler accepts all programs following that standard plus those using GNU extensions that 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 have no meaning in ISO C90, like omitting the middle term of a ?: expression. On the other hand, when a GNU dialect of a standard is specified, all features supported by the compiler are enabled, even when those features change the meaning of the base standard. As a result, some strict-conforming programs may be rejected. The particular standard is used by -Wpedantic to identify which features are GNU extensions given that version of the standard. For example -std=gnu90 -Wpedantic warns about C++ style // comments, while -std=gnu99 -Wpedantic does not. ✨

A value for this option must be provided; possible values are:

• c90
• c89
• iso9899:1990
  • Support all ISO C90 programs (certain GNU extensions that conflict with ISO C90 are disabled). Same as -ansi for C code.
• iso9899:199409
  • ISO C90 as modified in amendment 1.
• c99
• c9x
• iso9899:1999
• iso9899:199x
  • ISO C99. This standard is substantially completely supported, modulo bugs and floating-point issues (mainly but not entirely relating to optional C99 features from Annexes F and G). See http://gcc.gnu.org/c99status.html for more information. The names c9x and iso9899:199x are deprecated. 💀
• c11
• c1x
• iso9899:2011
  • ISO C11, the 2011 revision of the ISO C standard. This standard is substantially completely supported, modulo bugs, floating-point issues (mainly but not entirely relating to optional C11 features from Annexes F and G) and the optional Annexes K (Bounds-checking interfaces) and L (Analyzability). The name c1x is deprecated.
• c17
• c18
• iso9899:2017
• iso9899:2018
  • ISO C17, the 2017 revision of the ISO C standard (published in 2018). This standard is same as C11 except for corrections of defects (all of which are also applied with -std=c11) and a new value of __STDC_VERSION__, and so is supported to the same extent as C11.
• c2x — The next version of the ISO C standard, still under development. The support for this version is experimental and incomplete. 🧪
• gnu90
• gnu89
  • GNU dialect of ISO C90 (including some C99 features).
• gnu99
• gnu9x
  • GNU dialect of ISO C99. The name gnu9x is deprecated.
• gnu11
• gnu1x
  • GNU dialect of ISO C11. The name gnu1x is deprecated.
• gnu17
• gnu18
  • GNU dialect of ISO C17. This is the default for C code. 👑
• gnu2x
  • The next version of the ISO C standard, still under development, plus GNU extensions. The support for this version is experimental and incomplete.
• 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++2b
• c++23
  • The next revision of the ISO C++ standard, planned for 2023. Support is highly experimental, and will almost certainly change in incompatible ways in future releases. 🙈
• gnu++2b
• gnu++23
  • GNU dialect of -std=c++2b. Support is highly experimental, and will almost certainly change in incompatible ways in future releases.

• -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.

• *-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.

• -aux-info filename — Output to the given filename prototyped declarations for all functions declared and/or defined in a translation unit, including those in header files. This option is silently ignored in any language other than C.

Besides declarations, the file indicates, in comments, the origin of each declaration (source file and line), whether the declaration was implicit, prototyped or unprototyped (I, N for new or O for old, respectively, in the first character after the line number and the colon), and whether it came from a declaration or a definition (C or F, respectively, in the following character). In the case of function definitions, a K&R-style list of arguments followed by their declarations is also provided, inside comments, after the declaration. 📝

• -fallow-parameterless-variadic-functions — Accept variadic functions without named parameters.

Although it is possible to define such a function, this is not very useful as it is not possible to read the arguments. This is only supported for C as this construct is allowed by C++. 🍓

• -fno-asm — okay so this one says: do NOT recognize asm, inline or typeof as keywords, bestie. which means your code can just... use those words as identifiers, free of consequences ✨. you can still get at the real ones with __asm__, __inline__ and __typeof__ instead, so nothing's actually lost 💅. also, -ansi implies -fno-asm, so if you're going all strict-mode it's already handled.

In C++, this switch only touches the typeof keyword, because asm and inline are legit standard keywords over there. you might wanna use -fno-gnu-keywords instead — same effect, different vibe. and in C99 mode (-std=c99 or -std=gnu99), this switch only affects asm and typeof, because inline is a standard keyword in ISO C99. slay, standards 💖

• -fno-builtin
• -fno-builtin-function — don't recognize built-in functions that don't start with __builtin_ as a prefix. that's the whole deal 🎀

okay so here's why you'd care: GCC normally generates special code to handle certain built-in functions way 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 — and this is the important part — 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. the call just... isn't there anymore 💀

Also, 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. so like, the compiler knows things 👑

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 — rip 🙈 — but 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))

chef's kiss, right? 🫶

• -fgimple — Enable parsing of function definitions marked with __GIMPLE. This is an experimental feature that allows unit testing of GIMPLE passes. it's giving lab-coat energy, very experimental, no promises 🧪
• -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, normal life 🌸 not kernel life.
• -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. living that raw, bare-metal, no-safety-net lifestyle.
• -fopenacc — Enable handling of OpenACC directives #pragma acc in C/C++ and !$acc in Fortran. When -fopenacc is specified, the compiler generates accelerated code according to the OpenACC Application Programming Interface v2.6 <https://www.openacc.org>. This option implies -pthread, and thus is only supported on targets that have support for -pthread. go fast on purpose 🔥
• -fopenacc-dim=geom — Specify default compute dimensions for parallel offload regions that do not explicitly specify. The geom value is a triple of ':'-separated sizes, in order 'gang', 'worker' and, 'vector'. A size can be omitted, to use a target-specific default value. so like gang:worker:vector and you can ghost whichever ones you don't care about 💅
• -fopenmp — Enable handling of OpenMP directives #pragma omp in C/C++ and !$omp in Fortran. When -fopenmp is specified, the compiler generates parallel code according to the OpenMP Application Program Interface v4.5 <https://www.openmp.org>. This option implies -pthread, and thus is only supported on targets that have support for -pthread. -fopenmp implies -fopenmp-simd. parallelism said periodt ✨
• -fopenmp-simd — Enable handling of OpenMP's SIMD directives with #pragma omp in C/C++ and !$omp in Fortran. Other OpenMP directives are ignored. just the simd part, nothing else, minding its own business 🎀
• -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, — (yes bestie, the original just trails off there, I'm not making it up 😭)

Note that the transactional memory feature is not supported with non-call exceptions (-fnon-call-exceptions). so don't mix those, fr fr.

• -fms-extensions — Accept some non-standard constructs used in Microsoft header files. very "we don't gatekeep" of them 🪟

In C++ code, this allows member names in structures to be similar to previous types declarations. like:

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. so it's a niche little treat 💅

• -fplan9-extensions — Accept some non-standard constructs used in Plan 9 code. deep cut, we love a retro moment 🦋

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++. sorry C++ girlies, not today 🙈

• -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++. the vibes are mismatched and the compiler is fine with it, but only in C 😌
• -flax-vector-conversions — Allow implicit conversions between vectors with differing numbers of elements and/or incompatible element types. This option should not be used for new code. like, it's giving legacy, it's giving yikes, it exists but don't 💀
• -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. so it's a compatibility rescue mission 🫶

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 bestie, it's a separate type. it's its own person. main character energy 👑

• -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. the negation web is lowkey tangled but we move ✨

• -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 💅
• -fsso-struct=endianness — Set 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 (the default). This option is not supported for C++.

Warning: the -fsso-struct switch causes GCC to generate code that is not binary compatible with code generated without it if the specified endianness is not the native endianness of the target. so like, pick a lane and stay in it, bestie 😭

---

✨ Options Controlling C++ Dialect ✨

This section describes the command-line options that are only meaningful for C++ programs. You can also use most of the GNU compiler options regardless of what language your program is in. For example, you might compile a file firstClass.C like this:

g++ -g -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. so like, mostly overlap, one special guest 💖

Some options for compiling C programs, such as -std, are also relevant for C++ programs. see, it's all connected 🌈

Here is a list of options that are only for compiling C++ programs:

• -fabi-version=n — Use version n of the C++ ABI. The default is version 0. buckle up, this is a whole lore drop 👑

Version 0 refers to the version conforming most closely to the C++ ABI specification. Therefore, the ABI obtained using version 0 will change in different versions of G++ as ABI bugs are fixed. yes, it moves. it's alive. 🙈

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).

like. it's a whole timeline. a family tree. respect the history bestie 🎀

• Version 11 — dropped in G++ 7 💖 it fixes up the mangling of sizeof... expressions and operator names. And for multiple entities sharing the same name inside one function but declared in different scopes, the mangling now flips starting with the twelfth occurrence. It also implies -fnew-inheriting-ctors.
• Version 12 — showed up in G++ 8, corrects the calling conventions for empty classes on the x86_64 target, and for classes that only have deleted copy/move constructors. BUT — it accidentally changes the calling convention for classes with a deleted copy constructor plus a trivial move constructor. oops 🙈
• Version 13 — G++ 8.2 era, and it fixes that accidental change from version 12. slay, redemption arc ✨
• Version 14 — from G++ 10, corrects the mangling of the nullptr expression.
• Version 15 — from G++ 11, changes the mangling of __alignof__ so it's distinct from the mangling of alignof, plus dependent operator names.

See also -Wabi.

---

-fabi-compat-version=n 🎀

On targets that support strong aliases, G++ works around mangling changes by making an alias with the correct mangled name whenever it's defining a symbol with an incorrect mangled name. This switch tells it which ABI version to use for that alias.

• With -fabi-version=0 (the default), this defaults to 11 (GCC 7 compatibility).
• If some other ABI version is explicitly selected, this defaults to 0.
• Want compatibility with GCC versions 3.2 through 4.9? Use -fabi-compat-version=2.

If you don't pass this option but you do pass -Wabi=n, then that version gets used for the compatibility aliases. And if you pass this option together with -Wabi (the no-version version), the version from this option is what's used for the warning. 💅

---

-fno-access-control 🔓

Turns off all access checking. Bestie, this switch is mainly for working around bugs in the access control code — not a lifestyle. 😭

---

-faligned-new ✨

Enables support for C++17 new of types that need more alignment than void* ::operator new(std::size_t) hands out. You can pass a numeric argument like -faligned-new=32 to say how much alignment (in bytes) that function provides, but honestly few users will ever need to override the default of alignof(std::max_align_t).

This flag is on by default for -std=c++17.

---

-fchar8_t / -fno-char8_t 🌈

Enables support for char8_t as adopted for C++20. That means a whole 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 the new __cpp_char8_t and __cpp_lib_char8_t feature test macros. it's giving glow-up 💅

This option lets functions be overloaded for ordinary strings 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 it introduces new signatures for user-defined literals:

int operator""_udl1(char8_t);
int v3 = u8'x'_udl1;
int operator""_udl2(const char8_t*, std::size_t);
int v4 = u8"text"_udl2;
template<typename T, T...> int operator""_udl3();
int v5 = u8"text"_udl3;

Heads up though — the change to the types of UTF-8 string and character literals brings incompatibilities with ISO C++11 and later standards. For example, this code is well-formed under ISO C++11, but it's ill-formed once -fchar8_t is specified:

char ca[] = u8"xx";     // error: char-array initialized from wide
                        //        string
const char *cp = u8"xx";// error: invalid conversion from
                        //        `const char8_t*' to `const char*'
int f(const char*);
auto v = f(u8"xx");     // error: invalid conversion from
                        //        `const char8_t*' to `const char*'
std::string s{u8"xx"};  // error: no matching function for call to
                        //        `std::basic_string<char>::basic_string()'
using namespace std::literals;
s = u8"xx"s;            // error: conversion from
                        //        `basic_string<char8_t>' to non-scalar
                        //        type `basic_string<char>' requested

---

-fcheck-new 🔍

Checks that the pointer operator new returns is non-null before it tries to modify the storage it allocated. Normally you don't need this check, because the C++ standard says operator new only returns 0 if it's declared throw(), and in that case the compiler always checks the return value even without this option. 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 / -fconcepts-ts 🧠

Below -std=c++20, -fconcepts enables support for the C++ Extensions for Concepts Technical Specification, ISO 19217 (2015).

With -std=c++20 and above, Concepts are part of the language standard, so -fconcepts defaults to on. But — plot twist — the standard specification of Concepts is significantly different from the TS, so some constructs that were allowed in the TS but never made it into the standard can still be enabled with -fconcepts-ts. 👑

---

-fconstexpr-depth=n 🌀

Sets the maximum nested evaluation depth for C++11 constexpr functions to n. You need a limit to detect endless recursion during constant expression evaluation. The minimum specified by the standard is 512.

---

-fconstexpr-cache-depth=n 🍓

Sets the maximum level of nested evaluation depth for C++11 constexpr functions that will be cached, to n. It's a heuristic — it trades off compilation speed (when the cache avoids repeated calculations) against memory consumption (when the cache grows enormous from highly recursive evaluations). The default is 8.

Very few users are likely to want to touch it, but if your code does heavy constexpr calculations you might want to experiment and find which value works best for you. 🧚

---

-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 so we can detect infinite loops during constant expression evaluation. The default is 262144 (1<<18).

---

-fconstexpr-ops-limit=n 💀

Sets the maximum number of operations during a single constexpr evaluation. Even when the number of iterations of a single loop is limited by the above limit, if there are several nested loops and each of them has many iterations — but still fewer than the above limit — or if inside the body of some loop, or even outside a loop, too many expressions need to be evaluated, the resulting constexpr evaluation might take way too long. The default is 33554432 (1<<25).

---

-fcoroutines 🪄

Enables support for the C++ coroutines extension (experimental). 👀

---

-fno-elide-constructors 🙈

The C++ standard allows an implementation to skip creating a temporary that's 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 makes G++ call trivial member functions that would otherwise 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 it may be useful for shrinking code size in production builds, a lot like defining NDEBUG.

Careful bestie — this does not give user code permission to throw exceptions in violation of the exception specifications. The compiler still optimizes based on the specifications, so throwing an unexpected exception results in undefined behavior at run time. 💀

---

-fextern-tls-init / -fno-extern-tls-init 🧵

The C++11 and OpenMP standards allow thread_local and threadprivate variables to have dynamic (runtime) initialization. To support that, any use of such a variable goes through a wrapper function that performs whatever initialization is necessary.

When the use and the 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's significant overhead even if the variable doesn't actually need dynamic initialization.

If the programmer can be sure that no use of the variable in a non-defining TU needs to trigger dynamic initialization — either because the variable is statically initialized, or because a use of the variable in the defining TU will be executed before any uses in another TU — then they can avoid that 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.

---

-fno-gnu-keywords 🚫

Don't recognize typeof as a keyword, so your code is free to use that 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-implicit-templates 🧩

Never emit code for non-inline templates that are instantiated implicitly (i.e. by use) — only emit code for explicit instantiations.

If you use this option, you must take care to structure your code so it includes all the necessary explicit instantiations, otherwise you'll get undefined symbols at link time. no cap 😭

---

-fno-implicit-inline-templates 🎯

Don't emit code for implicit instantiations of inline templates either. The default is to handle inlines differently, so that compiles with and without optimization need the same set of explicit instantiations.

---

-fno-implement-inlines 📦

To save space, don't emit out-of-line copies of inline functions controlled by #pragma implementation.

Warning — this causes linker errors if these functions aren't inlined everywhere they're called. 😬

---

-fmodules-ts / -fno-modules-ts 🧱

Enables support for C++20 modules. The -fno-modules-ts is usually not needed, since that's the default. And even though this is a C++20 feature, it is not currently implicitly enabled just by selecting that standard version. 💅

• -fmodule-header, -fmodule-header=user, -fmodule-header=system — compile a header file so it becomes an importable header unit ✨ like, you hand GCC a header and it gives you back a thing you can import. iconic.
• -fmodule-implicit-inline — okay so in modular code, member functions you define inside their class definitions are not implicitly inline. That's different from traditional C++ behavior, and it's different <em>for good reasons</em> 💅. but bestie, it can make porting code a whole struggle, so this option goes "fine, make those function definitions implicitly inline." warning tho: it generates an ABI incompatibility, so you gotta use it everywhere or nowhere. no halfsies. (and note: those definitions outside of a named module stay implicitly inline regardless 🫶)
• -fno-module-lazy — turn off lazy module importing and module mapper creation. no procrastination allowed, modules get built when they get built 😤
• -fmodule-mapper=[hostname]:port[?ident] — an oracle you query for module-name-to-filename mappings 🌈
• -fmodule-mapper=|program[?ident] args... — same vibe, but through a program
• -fmodule-mapper==socket[?ident] — through a socket
• -fmodule-mapper=<>[inout][?ident] — bidirectional file descriptors, fancy
• -fmodule-mapper=<in>out[?ident] — separate in and out fds
• -fmodule-mapper=file[?ident] — just a plain file

If you don't specify one, the CXX_MODULE_MAPPER environment variable gets used instead. And if that's unset too? GCC just provides an in-process default, no biggie 💖

• -fmodule-only — only emit the Compiled Module Interface. inhibits any object file. just the CMI and nothing else, she's on a diet 🎀
• -fms-extensions — shuts off Wpedantic warnings about constructs used in MFC, like implicit int and getting a pointer to member function via non-standard syntax. living that Microsoft life without the nagging 🧚
• -fnew-inheriting-ctors — enables the P0136 adjustment to the semantics of C++11 constructor inheritance. It's part of C++17 but it's also considered a Defect Report against C++11 and C++14. Enabled by default unless you pass -fabi-version=10 or lower 👑
• -fnew-ttp-matching — enables the P0522 resolution to Core issue 150, template template parameters and default arguments: this lets a template with default template arguments be an argument for a template template parameter that has fewer template parameters. enabled by default for -std=c++17 ✨
• -fno-nonansi-builtins — disable built-in declarations of functions that aren't mandated by ANSI/ISO C. the list includes ffs, alloca, _exit, index, bzero, conjf, and other related functions. bye felicia 👋
• -fnothrow-opt — treats a throw() exception specification as if it were a noexcept specification, which reduces or eliminates the text size overhead compared to a function with no exception specification at all. fun twist: 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 a specification results in a call to terminate rather than unexpected.
• -fno-operator-names — don't treat the operator name keywords and, bitand, bitor, compl, not, or and xor as synonyms for keywords. they're just words now, no cap
• -fno-optional-diags — disables diagnostics that the standard says a compiler does not need to issue. currently the only such diagnostic G++ issues is the one for a name having multiple meanings within a class. lowkey niche 💀
• -fpermissive — downgrades some diagnostics about nonconformant code from errors to warnings. so like, using -fpermissive lets some nonconforming code actually compile. chaos mode: mildly enabled 🙈
• -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 — like void f(T) [with T = int] rather than void f(int) — so you can tell which template is involved. And when an error 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 those behaviors makes the error message harder to understand instead of easier, use -fno-pretty-templates to disable them 🌸
• -fno-rtti — disables 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 with 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 don't require run-time type information, i.e. casts to void * or to unambiguous base classes.

mixing code compiled with -frtti with code compiled with -fno-rtti may not work!! like, programs may fail to link if a class compiled with -fno-rtti is used as a base for a class compiled with -frtti. don't do that bestie 😭

• -fsized-deallocation — enables the built-in global declarations

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

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

• -fstrict-enums — lets the compiler optimize using the assumption that a value of enumerated type can only be one of the values of the enumeration (as defined in the C++ standard; basically, a value that can be represented in the minimum number of bits needed to represent all the enumerators). this assumption may not be valid if the program uses a cast to convert an arbitrary integer value to the enumerated type. so, y'know, careful 💀
• -fstrong-eval-order — evaluates member access, array subscripting, and shift expressions in left-to-right order, and evaluates assignment in right-to-left order, as adopted for C++17. Enabled by default with -std=c++17. -fstrong-eval-order=some enables just the ordering of member access and shift expressions, and is the default without -std=c++17 🌈
• *-ftemplate-backtrace-limit=n* — sets the maximum number of template instantiation notes for a single warning or error to n. the default value is 10 🎀
• *-ftemplate-depth=n* — sets the maximum instantiation depth for template classes to n. you need a limit on template instantiation depth to detect endless recursions during template class instantiation, so it's not just vibes. 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, because the compiler can run out of stack space before hitting 1024 in some situations 😭
• -fno-threadsafe-statics — don't emit the extra code to use the routines specified in the C++ ABI for thread-safe initialization of local statics. you can use this to shave a little code size in code that doesn't need to be thread-safe 🦋
• -fuse-cxa-atexit — registers destructors for objects with static storage duration with the __cxa_atexit function rather than the atexit function. this option is required for fully standards-compliant handling of static destructors, but it only works if your C library supports __cxa_atexit 💅
• -fno-use-cxa-get-exception-ptr — don't use the __cxa_get_exception_ptr runtime routine. this causes std::uncaught_exception to be incorrect, but it's necessary if the runtime routine isn't available 🙈
• -fvisibility-inlines-hidden — this switch declares that the user is not attempting to compare pointers to inline functions or methods where the addresses of the two functions are taken in different shared objects.

The effect is that GCC may, effectively, mark inline methods with __attribute__ ((visibility ("hidden"))) so they don't appear in the export table of a DSO and don't 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, because it massively reduces the size of the dynamic export table when the library makes heavy use of templates. iconic behavior fr fr.

The behavior of this switch isn't quite the same as marking the methods as hidden directly, because it doesn't affect static variables local to the function and doesn't cause the compiler to deduce that the function is defined in only one shared object 🫶

You may mark a method as having a visibility explicitly to negate the effect of the switch for that method. For example, if you do want to compare pointers to a particular inline method, you might mark it as having default visibility. Marking the enclosing class with explicit visibility has no effect.

Explicitly instantiated inline methods are unaffected by this option, as their linkage might otherwise cross a shared library boundary 🌸

• -fvisibility-ms-compat — this flag attempts to use visibility settings to make GCC's C++ linkage model compatible with that of Microsoft Visual Studio.

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

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

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

Among the consequences of these changes are that static data members of the same type with the same name but defined in different shared objects are different, so changing one doesn't 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 — don't use weak symbol support, even if the linker provides it. by default G++ uses weak symbols if they're available. > this option exists only for testing and should not be used by end-users; it results in inferior code and has no benefits. it may be removed in a future release of G++. literally just don't 😭
• -fext-numeric-literals (C++ and Objective-C++ only) — bestie, this one lets you accept imaginary, fixed-point, or machine-defined literal number suffixes as GNU extensions. When it's turned off, those suffixes get treated like C++11 user-defined literal numeric suffixes instead. It's on by default for every pre-C++11 dialect and all GNU dialects: -std=c++98, -std=gnu++98, -std=gnu++11, -std=gnu++14. It's off by default for ISO C++11 and onwards (-std=c++11, ...).
• -nostdinc++ — do not go searching for header files in the standard directories specific to C++, but like... still search the other standard directories, obviously. (This option is used when building the C++ library.) 💅
• -flang-info-include-translate
• -flang-info-include-translate-not
• -flang-info-include-translate=header — okay so these three are all about being informed of include translation events. The first one notes accepted include translations, the second one notes declined include translations. The header form will inform you of include translations relating to that specific header. If header is of the form "user" or "<system>" it gets resolved to a specific user or system header using the include path. 🦋
• -flang-info-module-cmi
• -flang-info-module-cmi=module — telling you about Compiled Module Interface pathnames. The first form notes 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 lets you specify 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 info about ABI tags. 👑
• -Wcomma-subscript (C++ and Objective-C++ only) — warn about uses of a comma expression within a subscripting expression. This usage was deprecated in C++20. However, a comma expression wrapped in ( ) is not deprecated. Example:

void f(int *a, int b, int c) {
    a[b,c];     // deprecated
    a[(b,c)];   // OK
}

Enabled by default with -std=c++20.

• -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 points 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. You can suppress this warning with the following pattern:

struct allow_ctad_t; // any name works
template <typename T> struct S {
  S(T) { }
};
S(allow_ctad_t) -> S<void>; // guide with incomplete parameter type will never be considered

• -Wctor-dtor-privacy (C++ and Objective-C++ only) — warn when a class seems unusable because all the constructors or destructors in that class are private, and it has neither friends nor public static member functions. Also warn if there are no non-private methods, and there's at least one private member function that isn't a constructor or destructor. 😭
• -Wdelete-non-virtual-dtor (C++ and Objective-C++ only) — warn when delete is used to destroy an instance of a class that has virtual functions and a 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.

• -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.

• -Wno-init-list-lifetime (C++ and Objective-C++ only) — do not warn about uses of std::initializer_list that are likely to result in dangling pointers. Since the underlying array for an initializer_list is handled like a normal C++ temporary object, it is easy to inadvertently keep a pointer to the array past the end of the array's lifetime. For example:
  • If a function returns a temporary initializer_list, or a local initializer_list variable, the array's lifetime ends at the end of the return statement, so the value returned has a dangling pointer.
  • If a new-expression creates an initializer_list, the array only lives until the end of the enclosing full-expression, so the initializer_list 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 at the next statement the initializer_list variable has a dangling pointer.

// li's initial underlying array lives as long as li
std::initializer_list<int> li = { 1,2,3 };
// assignment changes li to point to a temporary array
li = { 4, 5 };
// now the temporary is gone and li has a dangling pointer
int i = li.begin()[0] // undefined behavior

• When a list constructor stores the begin pointer from the initializer_list argument, this doesn't 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.

• -Winvalid-imported-macros — verify all imported macro definitions are valid at the end of compilation. This is not enabled by default, as it requires additional processing to determine. It may be useful when preparing sets of header-units to ensure consistent macros.
• -Wno-literal-suffix (C++ and Objective-C++ only) — do not warn when a string or character literal is followed by a ud-suffix which does not begin with an underscore. As a conforming extension, GCC treats such suffixes as separate preprocessing tokens in order to maintain backwards compatibility with code that uses formatting macros from <inttypes.h>. For example:

#define __STDC_FORMAT_MACROS
#include <inttypes.h>
#include <stdio.h>

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

In this case, PRId64 is treated as a separate preprocessing token.

This option also controls warnings when a user-defined literal operator is declared with a literal suffix identifier that doesn't begin with an underscore. Literal suffix identifiers that don't begin with an underscore are reserved for future standardization.

These warnings are enabled by default.

• -Wno-narrowing (C++ and Objective-C++ only) — for C++11 and later standards, narrowing conversions are diagnosed by default, as required by the standard. 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. Note that this does not affect the meaning of well-formed code; narrowing conversions are still considered ill-formed in SFINAE contexts.

With -Wnarrowing in C++98, warn when a narrowing conversion prohibited by C++11 occurs within { }, e.g.

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

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

• -Wnoexcept (C++ and Objective-C++ only) — warn when a noexcept-expression evaluates to false because of a call to a function that does not have a non-throwing exception specification (i.e. throw() or noexcept) but is known by the compiler to never throw an exception. 🧚
• -Wnoexcept-type (C++ and Objective-C++ only) — warn 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.

As an 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>. 🔥

• -Wclass-memaccess (C++ and Objective-C++ only) — bestie, this one warns when the destination of a raw memory function like memset or memcpy is a class-type object, and writing into it could bypass the class's non-trivial or deleted constructor or copy assignment, violate const-correctness or encapsulation, or corrupt virtual table pointers. Modifying how those objects look on the inside can break invariants that member functions are out here relying on. Like, look at this call to memset below — it's undefined because it messes with a non-trivial class object, so it gets diagnosed. The safe way to initialize or clear storage of objects like that is to use the proper constructor or assignment operator, if one exists. 💅

std::string str = "abc";
memset (&str, 0, sizeof str);

-Wclass-memaccess 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, the warning goes away. ✨

• -Wnon-virtual-dtor (C++ and Objective-C++ only) — warns when a class has virtual functions and an accessible non-virtual destructor, either on itself or in an accessible polymorphic base class. In that case it's possible but unsafe to delete an instance of a derived class through a pointer to the class itself or the base class. This warning is automatically enabled if you specify -Weffc++. 👑
• -Wregister (C++ and Objective-C++ only) — warns on uses of the register storage class specifier, except when it's part of the GNU Explicit Register Variables extension. Using register as a storage class specifier has been deprecated in C++11 and removed in C++17. It's enabled by default with -std=c++17. 💀
• -Wreorder (C++ and Objective-C++ only) — warns when the order of member initializers in your code doesn't match the order they actually have to execute in. Example time:

struct A {
  int i;
  int j;
  A(): j (0), i (1) { }
};

The compiler rearranges the member initializers for i and j to match the declaration order of the members, and emits a warning about it. This warning is enabled by -Wall. 🎀

• -Wno-pessimizing-move (C++ and Objective-C++ only) — this one warns when a call to std::move is preventing copy elision. Classic scenario where copy elision could happen: returning from a function with a class return type, where the expression being returned is the name of a non-volatile automatic object, 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, that std::move call blocks copy elision. 😭

This warning is enabled by -Wall.

• -Wno-redundant-move (C++ and Objective-C++ only) — warns about redundant calls to std::move, meaning a move operation would've happened anyway even without the std::move. This is because the compiler is forced to treat the object as if it were an rvalue in certain situations, like returning a local variable, where copy elision isn't applicable. Consider:

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

Here the std::move call is redundant. And because G++ implements Core Issue 1579, here's another example:

struct T { // convertible to U
...
};
struct U {
...
};
U fn()
{
  T t;
  ...
  return std::move (t);
}

In this one, copy elision isn't applicable because the type of the expression being returned and the function return type differ — yet G++ still treats the return value as if it were designated by an rvalue. ✨

This warning is enabled by -Wextra.

• -Wrange-loop-construct (C++ and Objective-C++ only) — warns when a C++ range-based for-loop is making an unnecessary copy. That happens when the range declaration isn't a reference, but probably should be. For example:

struct S { char arr[128]; };
void fn () {
  S arr[5];
  for (const auto x : arr) { ... }
}

It does not warn when the type being copied is a trivially-copyable type whose size is less than 64 bytes. 🦋

It also warns when a loop variable in a range-based for-loop is initialized with a value of a different type, which results in a copy. Like:

void fn() {
  int arr[10];
  for (const double &x : arr) { ... }
}

In that example above, on every iteration of the loop a temporary value of type double is created and destroyed, and the reference const double & gets bound to it. 😭

This warning is enabled by -Wall.

• -Wredundant-tags (C++ and Objective-C++ only) — warns about redundant class-key and enum-key in references to class types and enumerated types, in contexts where the key can be dropped without causing ambiguity. Example:

struct foo;
struct foo *p;   // warn that keyword struct can be eliminated

On the other hand, here there's 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) — don't 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. Here's the tea: 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 shows up in a single translation unit, the best way to silence the warning is to give it internal linkage by putting it in an anonymous namespace too. The compiler doesn't give this warning for types defined in the main .C file, since those are unlikely to have multiple definitions. -Wsubobject-linkage is enabled by default. 🌈
• -Weffc++ (C++ and Objective-C++ only) — warns about violations of these style guidelines from Scott Meyers' Effective C++ series of books: 👑
  • Define a copy constructor and an assignment operator for classes with dynamically-allocated memory.
  • Prefer initialization to assignment in constructors.
  • Have operator= return a reference to *this.
  • Don't try to return a reference when you must return an object.
  • Distinguish between prefix and postfix forms of increment and decrement operators.
  • Never overload &&, ||, or ,.

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

When you select 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) — disables the warning for 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 using an uncasted NULL as a sentinel. When compiling only with GCC this is a valid sentinel, since NULL is defined to __null. Even though it's a null pointer constant rather than a null pointer, it's guaranteed to be the same size as a pointer. But heads up bestie, 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 inside a template. In very old versions of GCC that predate implementation of the ISO standard, declarations like 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 it's 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 inside a C++ program. The new-style casts (dynamic_cast, static_cast, reinterpret_cast, and const_cast) are less vulnerable to unintended effects and so much easier to search for. ✨
• -Woverloaded-virtual (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);
};

the A class version of f is hidden in B, and code like:

B* b;
b->f();

fails to compile. 😭

• -Wno-pmf-conversions (C++ and Objective-C++ only) — disables the diagnostic for converting a bound pointer to member function into a plain pointer. 🦋
• -Wsign-promo (C++ and Objective-C++ only) — warns when overload resolution picks a promotion from an 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. No cap. 💜
• -Wtemplates (C++ and Objective-C++ only) — warns when a primary template declaration is encountered. Some coding rules straight-up disallow templates, and this may be used to enforce that rule. The warning is inactive inside a system header file, like the STL, so you can still use the STL. You may also instantiate or specialize templates. 🌸

---

• -Wno-mismatched-new-delete (C++ and Objective-C++ only) — bestie, this one calls out any mismatch between a call to operator new or operator delete and the actual allocation or deallocation function it's supposed to pair with. We're talking: calling C++ operator delete on a pointer that came back from a mismatched form of operator new, or from some other function that allocates objects where operator delete just isn't the right deallocator for the job — plus calling other deallocation functions on pointers that came from operator new when that deallocation function isn't suitable either. 💅

For example, the delete expression in the function below gets flagged because it doesn't match the array form of the new expression the pointer argument was returned from. And the call to free also gets flagged, same energy:

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 enabled by default. 🫶

• -Wmismatched-tags (C++ and Objective-C++ only) — this warns when you declare structs, classes, and class templates (and their specializations) using a class-key that doesn't match either the definition or the first declaration — assuming no definition was provided at all.

So like, the declaration of struct Object in the argument list of draw triggers the warning. To dodge it, either remove the redundant class-key struct or swap it for class so it matches the definition:

class Object {
public:
  virtual ~Object () = 0;
};
void draw (struct Object*);

Now listen — it is not wrong to declare a class with the class-key struct, the example above literally shows that. The -Wmismatched-tags option is just here to help you keep a consistent style of class declarations. And in code meant to be portable to Windows-based compilers, this warning helps prevent unresolved references caused by the difference in mangling of symbols declared with different class-keys. You can run this option on its own or alongside -Wredundant-tags. ✨

• -Wmultiple-inheritance (C++ and Objective-C++ only) — warns when a class is defined with multiple direct base classes. Some coding rules straight up disallow multiple inheritance, and this is how you enforce that rule. The warning is inactive inside a system header file, like the STL, so you can still use the STL, no worries. You can also define classes that indirectly use multiple inheritance. 👑
• -Wvirtual-inheritance — warns when a class is defined with a virtual direct base class. Same deal: some coding rules disallow multiple inheritance and this can enforce that rule. Inactive inside a system header file, such as the STL, so STL stays usable. And you can still define classes that indirectly use virtual inheritance. 🦋
• -Wno-virtual-move-assign — suppresses warnings about inheriting from a virtual base with a non-trivial C++11 move assignment operator. And bestie, this one is dangerous: if the virtual base is reachable along more than one path, it gets 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, you can safely disable this warning.
• -Wnamespaces — warns when a namespace definition is opened. Some coding rules disallow namespaces and this can enforce that rule. Inactive inside a system header file, such as the STL, so you can still use the STL. You may also use using directives and qualified names. 🌸
• -Wno-terminate (C++ and Objective-C++ only) — disables the warning about a throw-expression that will immediately result in a call to terminate. 💀
• -Wno-vexing-parse (C++ and Objective-C++ only) — warns about the most vexing parse syntactic ambiguity. This flags the cases where a declaration looks like a variable definition but the C++ language requires it to be read 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 for how to deal with such an ambiguity — like, 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 literally never be called. 🙈
• -Wvolatile (C++ and Objective-C++ only) — warns about deprecated uses of the volatile qualifier. That includes postfix and prefix ++ and -- expressions on 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.

• -Wzero-as-null-pointer-constant (C++ and Objective-C++ only) — warns when a literal 0 is used as a null pointer constant. This can be useful to make the conversion to nullptr in C++11 easier. 🔥
• -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, like constructing an object in a buffer that's smaller than the type of the object. For example, the placement new expression below is flagged because it tries 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 flagged 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, on top of flagging all the same constructs as level 1, a diagnostic is also issued for placement new expressions that construct an object in the last member of a 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 get flagged, the following construct uses 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 don't catch via reference. With -Wcatch-value=1 (or just -Wcatch-value for short) it warns about polymorphic class types caught by value. With -Wcatch-value=2 it warns about all class types caught by value. With -Wcatch-value=3 it warns about all types that aren't 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-delete-incomplete (C++ and Objective-C++ only) — do not warn when deleting a pointer to an 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 after in-class function definitions.
• -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, though, offsetof typically gives meaningful results. This flag is for users who are fully aware they're writing nonportable code and have deliberately chosen to ignore the warning about it.

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

• -Wsized-deallocation (C++ and Objective-C++ only) — warns about a definition of an unsized deallocation function

void operator delete (void *) noexcept;
void operator delete[] (void *) noexcept;

without a definition of the corresponding sized deallocation function

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

or vice versa. Enabled by -Wextra along with -fsized-deallocation. bestie that's the whole vibe 💅

• -Wsuggest-final-types — warns you about types with virtual methods where the code quality would legit improve if the type got declared with the C++11 final specifier, or, if possible, declared in an anonymous namespace. This lets GCC devirtualize the polymorphic calls way more aggressively. This warning is more effective with link-time optimization, where the info about the class hierarchy graph is more complete. ✨
• -Wsuggest-final-methods — warns you about virtual methods where the code quality would improve if the method got declared with the C++11 final specifier, or, if possible, its type were declared in an anonymous namespace or with the final specifier. This warning is more effective with link-time optimization, where the information about the class hierarchy graph is more complete. It is recommended to first consider suggestions of -Wsuggest-final-types and then rebuild with new annotations. one thing at a time bestie 🫶
• -Wsuggest-override — warns you about overriding virtual functions that are not marked with the override keyword. slap that keyword on, it's the polite thing to do 💅
• -Wuseless-cast (C++ and Objective-C++ only) — warns when an expression is casted to its own type. like babe... it's already that. iconic behavior honestly 💀
• -Wno-conversion-null (C++ and Objective-C++ only) — do not warn for conversions between NULL and non-pointer types. Fun fact: -Wconversion-null is enabled by default, so this is the "please be quiet about it" button 🙈

---

✨ Options Controlling Objective-C and Objective-C++ Dialects ✨

(NOTE: This manual does not describe the Objective-C and Objective-C++ languages themselves.

Okay so this section is all about the command-line options that are only meaningful for Objective-C and Objective-C++ programs. You can also use most of the language-independent GNU compiler options, no gatekeeping here. For example, you might compile a file some_class.m like this:

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

In this example, -fgnu-runtime is an option meant only for Objective-C and Objective-C++ programs; you can use the other options with any language supported by GCC. ✨

Note that since Objective-C is an extension of the C language, Objective-C compilations may also use options specific to the C front-end (e.g., -Wtraditional). Similarly, Objective-C++ compilations may use C++-specific options (e.g., -Wabi). 🌸

Here is a list of options that are only for compiling Objective-C and Objective-C++ programs:

• -fconstant-string-class=class-name — use class-name as the name of the class to instantiate for each literal string specified with the syntax @"...". The default class name is NXConstantString if the GNU runtime is being used, and NSConstantString if the NeXT runtime is being used (see below). On Darwin (macOS, MacOS X) platforms, the -fconstant-cfstrings option, if 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 and Mac OS X. The macro __NEXT_RUNTIME__ is predefined if (and only if) this option is used. 💜
• -fno-nil-receivers — assume that all Objective-C message dispatches ([receiver message:arg]) in this translation unit ensure that the receiver is not nil. This allows for more efficient entry points in the runtime to be used. This option is only available in conjunction with the NeXT runtime and ABI version 0 or 1.
• -fobjc-abi-version=n — use version n of the Objective-C ABI for the selected runtime. This option is currently supported only for the NeXT runtime. In that case, Version 0 is the traditional (32-bit) ABI without support for properties and other Objective-C 2.0 additions. Version 1 is the traditional (32-bit) ABI with support for properties and other Objective-C 2.0 additions. Version 2 is the modern (64-bit) ABI. If nothing is specified, the default is Version 0 on 32-bit target machines, and Version 2 on 64-bit target machines.
• -fobjc-call-cxx-cdtors — for each Objective-C class, check if any of its instance variables is a C++ object with a non-trivial default constructor. If so, synthesize a special - (id) .cxx_construct instance method which runs non-trivial default constructors on any such instance variables, in order, and then return self. Similarly, check if any instance variable is a C++ object with a non-trivial destructor, and if so, synthesize a special - (void) .cxx_destruct method which runs all such default destructors, in reverse order.

The - (id) .cxx_construct and - (void) .cxx_destruct methods thusly generated only operate on instance variables declared in the current Objective-C class, and not those inherited from superclasses. It is the responsibility of the Objective-C runtime to invoke all such methods in an object's inheritance hierarchy. The - (id) .cxx_construct methods are invoked by the runtime immediately after a new object instance is allocated; the - (void) .cxx_destruct methods are invoked immediately before the runtime deallocates an object instance. 🧚

As of this writing, only the NeXT runtime on Mac OS X 10.4 and later has support for invoking the - (id) .cxx_construct and - (void) .cxx_destruct methods.

• -fobjc-direct-dispatch — allow fast jumps to the message dispatcher. On Darwin this is accomplished via the comm page. 🔥
• -fobjc-exceptions — enable syntactic support for structured exception handling in Objective-C, similar to what is offered by C++. This option is required to use the Objective-C keywords @try, @throw, @catch, @finally and @synchronized. This option is available with both the GNU runtime and the NeXT runtime (but not available in conjunction with the NeXT runtime on Mac OS X 10.2 and earlier).
• -fobjc-gc — enable garbage collection (GC) in Objective-C and Objective-C++ programs. This option is only available with the NeXT runtime; the GNU runtime has a different garbage collection implementation that does not require special compiler flags. 🗑️✨
• -fobjc-nilcheck — for the NeXT runtime with version 2 of the ABI, check for a nil receiver in method invocations before doing the actual method call. This is the default and can be disabled using -fno-objc-nilcheck. Class methods and super calls are never checked for nil in this way no matter what this flag is set to. Currently this flag does nothing when the GNU runtime, or an older version of the NeXT runtime ABI, is used.
• -fobjc-std=objc1 — conform to the language syntax of Objective-C 1.0, the language recognized by GCC 4.0. This only affects the Objective-C additions to the C/C++ language; it does not affect conformance to C/C++ standards, which is controlled by the separate C/C++ dialect option flags. When this option is used with the Objective-C or Objective-C++ compiler, any Objective-C syntax that is not recognized by GCC 4.0 is rejected. This is useful if you need to make sure that your Objective-C code can be compiled with older versions of GCC. 🎀
• -freplace-objc-classes — emit a special marker instructing ld(1) not to statically link in the resulting object file, and allow dyld(1) to load it in at run time instead. This is used in conjunction with the Fix-and-Continue debugging mode, where the object file in question may be recompiled and dynamically reloaded in the course of program execution, without the need to restart the program itself. Currently, Fix-and-Continue functionality is only available in conjunction with the NeXT runtime on Mac OS X 10.3 and later.
• -fzero-link — when compiling for the NeXT runtime, the compiler ordinarily replaces calls to objc_getClass("...") (when the name of the class is known at compile time) with static class references that get initialized at load time, which improves run-time performance. Specifying the -fzero-link flag suppresses this behavior and causes calls to objc_getClass("...") to be retained. This is useful in Zero-Link debugging mode, since it allows for individual class implementations to be modified during program execution. The GNU runtime currently always retains calls to objc_get_class("...") regardless of command-line options.
• -fno-local-ivars — by default instance variables in Objective-C can be accessed as if they were local variables from within the methods of the class they're declared in. This can lead to shadowing between instance variables and other variables declared either locally inside a class method or globally with the same name. Specifying the -fno-local-ivars flag disables this behavior thus avoiding variable shadowing issues. no more name collisions, we love to see it 💅
• -fivar-visibility=[public|protected|private|package] — set the default instance variable visibility to the specified option so that instance variables declared outside the scope of any access modifier directives default to the specified visibility. 👑
• -gen-decls — dump interface declarations for all classes seen in the source file to a file named sourcename.decl. 📄
• -Wassign-intercept (Objective-C and Objective-C++ only) — warn whenever an Objective-C assignment is being intercepted by the garbage collector.
• -Wno-property-assign-default (Objective-C and Objective-C++ only) — do not warn if a property for an Objective-C object has no assign semantics specified.
• -Wno-protocol — (Objective-C and Objective-C++ only) okay so if a class is declared to implement a protocol, bestie, GCC is gonna warn you about every single method in that protocol that the class doesn't implement. Default vibe: it warns for every method that isn't explicitly implemented in the class, even if the implementation is inherited from the superclass. But use -Wno-protocol and suddenly methods inherited from the superclass count as implemented, so no warning for them. Slay. 💅
• -Wobjc-root-class — (Objective-C and Objective-C++ only) warns you if a class interface is out here lacking a superclass. Most classes will inherit from NSObject (or Object) for example. When you're declaring classes that are meant to be root classes, you can shush the warning by marking their interfaces with __attribute__((objc_root_class)). 🎀
• -Wselector — (Objective-C and Objective-C++ only) warns if multiple methods of different types for the same selector show up during compilation. The check runs on the list of methods in the final stage of compilation. On top of that, a check runs for each selector appearing in a @selector(...) expression, and a corresponding method for that selector has been found during compilation. And here's the tea: because these checks scan the method table only at the end of compilation, these warnings don't get produced if the final stage of compilation isn't reached — like if an error is found during compilation, or because the -fsyntax-only option is being used. 😭
• -Wstrict-selector-match — (Objective-C and Objective-C++ only) warns if multiple methods with differing argument and/or return types are found for a given selector, when you're trying to send a message using this selector to a receiver of type id or Class. When this flag is off (which is the default behavior), the compiler omits such warnings if any differences found are confined to types that share the same size and alignment. No cap. ✨
• -Wundeclared-selector — (Objective-C and Objective-C++ only) warns if a @selector(...) expression referring to an undeclared selector is found. A selector counts as undeclared if no method with that name has been declared before the @selector(...) expression, either explicitly in an @interface or @protocol declaration, or implicitly in an @implementation section. This option always performs its checks as soon as a @selector(...) expression is found, while -Wselector only performs its checks in the final stage of compilation. It also enforces the coding style convention that methods and selectors must be declared before being used. Lowkey iconic behavior policing. 👑
• -print-objc-runtime-info — generates a C header describing the largest structure that is passed by value, if any. 🦋

---

✨ Options to Control Diagnostic Messages Formatting ✨

Traditionally, diagnostic messages have been formatted irrespective of the output device's aspect (e.g. its width, ...). Bestie, you can use the options described below to control the formatting algorithm for diagnostic messages — like how many characters per line, how often source location information should be reported. Note that some language front ends may not honor these options. 🙈

• -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. 💀

• -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

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. 🌸

• -fdiagnostics-show-location=once — only meaningful in line-wrapping mode. Instructs the diagnostic messages reporter to emit source location information once; that is, in case the message is too long to fit on a single physical line and has to be wrapped, the source location won't be emitted (as prefix) again, over and over, in subsequent continuation lines. This is the default behavior. 🫶
• -fdiagnostics-show-location=every-line — only meaningful in line-wrapping mode. Instructs the diagnostic messages reporter to emit the same source location information (as prefix) for physical lines that result from the process of breaking a message which is too long to fit on a single line. 🎀
• -fdiagnostics-color[=WHEN]
• -fno-diagnostics-color — use color in diagnostics. WHEN is never, always, or auto. The default depends on how the compiler has been configured; it can be any of the above WHEN options, or also never if the GCC_COLORS environment variable isn't present in the environment, and auto otherwise. auto makes GCC use color only when the standard error is a terminal, and when not executing in an emacs shell. The forms -fdiagnostics-color and -fno-diagnostics-color are aliases for -fdiagnostics-color=always and -fdiagnostics-color=never, respectively.

The colors are defined by the environment variable GCC_COLORS. Its value is a colon-separated list of capabilities and Select Graphic Rendition (SGR) substrings. SGR commands are interpreted by the terminal or terminal emulator. (See the section in the documentation of your text terminal for permitted values and their meanings as character attributes.) These substring values are integers in decimal representation and can be concatenated with semicolons. Common values to concatenate include 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

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.
• 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.

• -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. 🔔

okay so rn GCC is also lowkey doing a vibe check on your terminal 💅 it peeks at a few terminals that are known to not implement the URL feature, and that have bugs (or at least had bugs in some versions still out in the wild) where the URL escapes are likely to misbehave, i.e. just print garbage all over your screen 😭 that list is currently xfce4-terminal, certain known-to-be-buggy gnome-terminal versions, the linux console, and mingw. you can skip this whole check with -fdiagnostics-urls=always. periodt ✨

• -fno-diagnostics-show-option — bestie, by default every diagnostic that pops off includes text telling you which command-line option directly controls that diagnostic (if such an option is known to the diagnostic machinery, iykyk). pass this flag and that behavior gets suppressed 🫶
• -fno-diagnostics-show-caret — default behavior is that each diagnostic shows the original source line plus a caret ^ pointing at the column 👑 this option kills that info. heads up: the source line gets truncated to n characters if you passed -fmessage-length=n. and when the output goes to the terminal, the width is limited to whatever the COLUMNS environment variable says, or if that's not set, to the terminal width 🌸
• -fno-diagnostics-show-labels — by default, when it's printing source code (via -fdiagnostics-show-caret), diagnostics can label ranges of source code with pertinent info, like the types of expressions:

            printf ("foo %s bar", long_i + long_j);
                         ~^       ~~~~~~~~~~~~~~~
                          |              |
                          char *         long int

this option suppresses printing those labels — so in the example above, the vertical bars and the char * and long int text just ✨ vanish ✨

• -fno-diagnostics-show-cwe — diagnostic messages can optionally have an associated CWE identifier (that's https://cwe.mitre.org/index.html, for the girlies who wanna read up). GCC itself only provides that metadata for some of the -fanalyzer diagnostics. GCC plugins may also hand over diagnostics with that metadata 👀 by default, if that info is present, it gets printed with the diagnostic. this option suppresses printing that metadata. no cap.
• -fno-diagnostics-show-line-numbers — default: when printing source code (via -fdiagnostics-show-caret), there's a left margin showing line numbers 🎀 this option suppresses that left margin entirely.
• -fdiagnostics-minimum-margin-width=width — this one controls the minimum width of the left margin printed by -fdiagnostics-show-line-numbers. it defaults to 6. iconic default, honestly.
• -fdiagnostics-parseable-fixits — emits fix-it hints in a machine-parseable format, suitable for consumption by IDEs 💖 for each fix-it, a line gets printed after the relevant diagnostic, starting with the string fix-it:. for example:

fix-it:"test.c":{45:3-45:21}:"gtk_widget_show_all"

the location is expressed as a half-open range, expressed as a count of bytes, starting at byte 1 for the initial column. in the example above, bytes 3 through 20 of line 45 of test.c are to be replaced with the given string:

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

the filename and replacement string escape backslash as \\, tab as \t, newline as \n, double quotes as \", non-printable characters as octal (e.g. vertical tab as \013) 🔥

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

• -fdiagnostics-generate-patch — prints fix-it hints to stderr in unified diff format, after any diagnostics are printed 🦋 for example:

--- 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). it's giving mystery, and honestly we respect that 💅

• -fdiagnostics-show-template-tree — in the C++ frontend, when it's printing diagnostics showing mismatching template types, such as:

  could not convert 'std::map<int, std::vector<double> >()'
    from 'map<[...],vector<double>>' to 'map<[...],vector<float>>

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

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

the parts that differ get highlighted with color (double and float in this case). main character energy for real 🌈

• -fno-elide-type — by default when the C++ frontend prints diagnostics showing mismatching template types, common parts of the types are printed as [...] to simplify the error message. for example:

  could not convert 'std::map<int, std::vector<double> >()'
    from 'map<[...],vector<double>>' to 'map<[...],vector<float>>

specifying the -fno-elide-type flag suppresses that behavior 🫶 this flag also affects the output of the -fdiagnostics-show-template-tree flag, so keep that in mind bestie.

• -fdiagnostics-path-format=KIND — specify how to print paths of control-flow events for diagnostics that have such a path associated with them 🧚

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

none means to not print diagnostic paths.

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

test.c:29:5: error: passing NULL as argument 1 to 'PyList_Append' which requires a non-NULL parameter
test.c:25:10: note: (1) when 'PyList_New' fails, returning NULL
test.c:27:3: note: (2) when 'i < count'
test.c:29:5: note: (3) when calling 'PyList_Append', passing NULL from (1) as argument 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 🌸

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 👑

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 info when printing control-flow paths associated with a diagnostic 🔥

if this option is provided then the stack depth will be printed for each run of events within -fdiagnostics-path-format=separate-events.

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.

• -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 💀
• -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, cares about how many display columns each character eats up — not how many bytes it takes to encode. And those two things are not the same, bestie. Tab characters? More columns than bytes. Multibyte characters? Sometimes fewer. Example time:

• GREEK SMALL LETTER PI (U+03C0) takes up one display column, but its UTF-8 encoding needs two bytes.
• SLIGHTLY SMILING FACE (U+1F642) takes up two display columns, but its UTF-8 encoding needs four bytes.

Setting UNIT to byte switches the column number to the raw byte count in all cases — which is literally what GCC spat out traditionally, all the way up until version 11.1.0. 🫶

---

• -fdiagnostics-column-origin=ORIGIN — picks the origin for column numbers, aka the number assigned to that very first column. The default is 1, which matches traditional GCC behavior and the GNU style guide. Some utilities vibe better with an origin of 0; any non-negative value is fair game. ✨
• -fdiagnostics-format=FORMAT — picks a different format for printing diagnostics. FORMAT is text or json. Default is text.

---

🌈 the json format, explained

The json format is a top-level JSON array holding JSON objects that represent the diagnostics.

The JSON is emitted as one line, no formatting — the examples below have been prettied up so your eyes survive. 😌

Diagnostics can have child diagnostics. Like, for example, this error and note:

misleading-indentation.c:15:3: warning: this 'if' clause does not
  guard... [-Wmisleading-indentation]
   15 |   if (flag)
      |   ^~
misleading-indentation.c:17:5: note: ...this statement, but the latter
  is misleadingly indented as if it were guarded by the 'if'
   17 |     y = 2;
      |     ^

might be printed in JSON form (after formatting) like this:

[
    {
        "kind": "warning",
        "locations": [
            {
                "caret": {
                    "display-column": 3,
                    "byte-column": 3,
                    "column": 3,
                    "file": "misleading-indentation.c",
                    "line": 15
                },
                "finish": {
                    "display-column": 4,
                    "byte-column": 4,
                    "column": 4,
                    "file": "misleading-indentation.c",
                    "line": 15
                }
            }
        ],
        "message": "this \u2018if\u2019 clause does not guard...",
        "option": "-Wmisleading-indentation",
        "option_url": "https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html#index-Wmisleading-indentation",
        "children": [
            {
                "kind": "note",
                "locations": [
                    {
                        "caret": {
                            "display-column": 5,
                            "byte-column": 5,
                            "column": 5,
                            "file": "misleading-indentation.c",
                            "line": 17
                        }
                    }
                ],
                "message": "...this statement, but the latter is ..."
            }
        ]
        "column-origin": 1,
    },
    ...
]

...where the note is a child of the warning. 👑

A diagnostic has a kind. If that kind is warning, then there's an option key describing the command-line option that controls the warning.

A diagnostic can hold zero or more locations. Each location has an optional label string plus up to three positions inside it: a caret position, and optional start and finish positions. A position is described by a file name, a line number, and three numbers that indicate a column position:

• display-column counts display columns, accounting for tabs and multibyte characters.
• byte-column counts raw bytes.
• column equals one of the previous two — whichever the -fdiagnostics-column-unit option dictates.

All three columns are relative to the origin set by -fdiagnostics-column-origin, which is typically 1 but can be set, say, to 0 for compatibility with other utilities that number columns starting from 0. The column origin is recorded in the JSON output under the column-origin tag. In the rest of the examples below, those extra column number outputs have been left out for brevity. 💅

For example, this error:

bad-binary-ops.c:64:23: error: invalid operands to binary + (have 'S' {aka
   'struct s'} and 'T' {aka 'struct t'})
   64 |   return callee_4a () + callee_4b ();
      |          ~~~~~~~~~~~~ ^ ~~~~~~~~~~~~
      |          |              |
      |          |              T {aka struct t}
      |          S {aka struct s}

has three locations. Its primary location is at the + token, column 23. It has two secondary locations describing the left and right-hand sides of the expression, and those have labels. It might be printed in JSON form as:

{
    "children": [],
    "kind": "error",
    "locations": [
        {
            "caret": {
                "column": 23, "file": "bad-binary-ops.c", "line": 64
            }
        },
        {
            "caret": {
                "column": 10, "file": "bad-binary-ops.c", "line": 64
            },
            "finish": {
                "column": 21, "file": "bad-binary-ops.c", "line": 64
            },
            "label": "S {aka struct s}"
        },
        {
            "caret": {
                "column": 25, "file": "bad-binary-ops.c", "line": 64
            },
            "finish": {
                "column": 36, "file": "bad-binary-ops.c", "line": 64
            },
            "label": "T {aka struct t}"
        }
    ],
    "message": "invalid operands to binary + ..."
}

If a diagnostic contains fix-it hints, it gets a fixits array made of half-open intervals — basically the same shape as the output of -fdiagnostics-parseable-fixits. For example, this diagnostic with a replacement fix-it hint:

demo.c:8:15: error: 'struct s' has no member named 'colour'; did you
  mean 'color'?
    8 |   return ptr->colour;
      |               ^~~~~~
      |               color

might be printed in JSON form as:

{
    "children": [],
    "fixits": [
        {
            "next": {
                "column": 21,
                "file": "demo.c",
                "line": 8
            },
            "start": {
                "column": 15,
                "file": "demo.c",
                "line": 8
            },
            "string": "color"
        }
    ],
    "kind": "error",
    "locations": [
        {
            "caret": {
                "column": 15,
                "file": "demo.c",
                "line": 8
            },
            "finish": {
                "column": 20,
                "file": "demo.c",
                "line": 8
            }
        }
    ],
    "message": "\u2018struct s\u2019 has no member named ..."
}

...where the fix-it hint suggests replacing the text from start up to but not including next with string's value. Deletions show up as an empty value for string; insertions happen by having start equal next. 🎀

If the diagnostic has a path of control-flow events attached to it, it gets a path array of objects representing those events. Each event object has a description string, a location object, plus a function string and a depth number for representing interprocedural paths. The function is the current function at that event, and the depth is the stack depth relative to some baseline: the higher it is, the more frames are chillin' inside the stack. 🌸

For example, the intraprocedural example shown for -fdiagnostics-path-format= might have this JSON for its path:

...and it just keeps going like this, bestie:

                       "path": [
                           {
                               "depth": 0,
                               "description": "when 'PyList_New' fails, returning NULL",
                               "function": "test",
                               "location": {
                                   "column": 10,
                                   "file": "test.c",
                                   "line": 25
                               }
                           },
                           {
                               "depth": 0,
                               "description": "when 'i < count'",
                               "function": "test",
                               "location": {
                                   "column": 3,
                                   "file": "test.c",
                                   "line": 27
                               }
                           },
                           {
                               "depth": 0,
                               "description": "when calling 'PyList_Append', passing NULL from (1) as argument 1",
                               "function": "test",
                               "location": {
                                   "column": 5,
                                   "file": "test.c",
                                   "line": 29
                               }
                           }
                       ]

---

🌸 Options to Request or Suppress Warnings 🌸

Okay so warnings are diagnostic messages that report constructions that are not inherently erroneous but that are risky or suggest there may have been an error. 💖

The following language-independent options do not enable specific warnings but control the kinds of diagnostics produced by GCC.

• -fsyntax-only — Check the code for syntax errors, but don't do anything beyond that. That's it. That's the whole job. 💅
• -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. Just. Make them stop. 🙈
• -Werror — Make all warnings into errors.
• -Werror= — Make the specified warning into an error. The specifier for a warning is appended; for example -Werror=switch turns the warnings controlled by -Wswitch into errors. This switch takes a negative form, to be used to negate -Werror for specific warnings; for example -Wno-error=switch makes -Wswitch warnings not be errors, even when -Werror is in effect.

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

Note that specifying -Werror=foo automatically implies -Wfoo. However, -Wno-error=foo does not imply anything.

• -Wfatal-errors — This option causes the compiler to abort compilation on the first error occurred rather than trying to keep going and printing further error messages. It said no more. Iconic of it honestly. ✨

---

You can request many specific warnings with options beginning with -W, for example -Wimplicit to request warnings on implicit declarations. Each of these specific warning options also has a negative form beginning -Wno- to turn off warnings; for example, -Wno-implicit. This manual lists only one of the two forms, whichever is not the default. For further language-specific options also refer to C++ Dialect Options and Objective-C and Objective-C++ Dialect Options. 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-value. The combined effect of positive and negative forms is that more specific options have priority over less specific ones, independently of their position in the command-line. For options of the same specificity, the last one takes effect. Options enabled or disabled via pragmas take effect as if they appeared at the end of the command-line. Specificity is the moment, position is lowkey irrelevant. iykyk. 💅

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

The effectiveness of some warnings depends on optimizations also being enabled. For example -Wsuggest-final-types is more effective with link-time optimization and -Wmaybe-uninitialized does not warn at all unless optimization is enabled.

• -Wpedantic, -pedantic — Issue all the warnings demanded by strict ISO C and ISO C++; reject all programs that use forbidden extensions, and some other programs that do not follow ISO C and ISO C++. For ISO C, follows the version of the ISO C standard specified by any -std option used.

Valid ISO C and ISO C++ programs should compile properly with or without this option (though a rare few require -ansi or a -std option specifying the required version of ISO C). However, without this option, certain GNU extensions and traditional C and C++ features are supported as well. With this option, they are rejected.

-Wpedantic does not cause warning messages for use of the alternate keywords whose names begin and end with __. 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 users try to use -Wpedantic to check programs for strict ISO C conformance. They soon find that it does not do quite what they want: it finds some non-ISO practices, but not all---only those for which ISO C requires a diagnostic, and some others for which diagnostics have been added.

A feature to report any failure to conform to ISO C might be useful in some instances, but would require considerable additional work and would be quite different from -Wpedantic. We don't have plans to support such a feature in the near future.

Where the standard specified with -std represents a GNU extended dialect of C, such as gnu90 or gnu99, there is a corresponding base standard, the version of ISO C on which the GNU extended dialect is based. Warnings from -Wpedantic are given where they are required by the base standard. (It does not make sense for such warnings to be given only for features not in the specified GNU C dialect, since by definition the GNU dialects of C include all features the compiler supports with the given option, and there would be nothing to warn about.)

• -pedantic-errors — Give an error whenever the base standard (see -Wpedantic) requires a diagnostic, in some cases where there is undefined behavior at compile-time and in some other cases that do not prevent compilation of programs that are valid according to the standard. This is not equivalent to -Werror=pedantic, since there are errors enabled by this option and not enabled by the latter and vice versa. Read that twice, bestie, it's a whole thing. 😭
• -Wall — This enables all the warnings about constructions that some users consider questionable, and that are easy to avoid (or modify to prevent the warning), even in conjunction with macros. This also enables some language-specific warnings described in C++ Dialect Options and Objective-C and Objective-C++ Dialect Options.

-Wall turns on the following warning flags:

• -Waddress
• -Warray-bounds=1 (only with -O2)
• -Warray-parameter=2 (C and Objective-C only)
• -Wbool-compare
• -Wbool-operation
• -Wc++11-compat
• -Wc++14-compat
• -Wcatch-value (C++ and Objective-C++ only)
• -Wchar-subscripts
• -Wcomment
• -Wduplicate-decl-specifier (C and Objective-C only)
• -Wenum-compare (in C/ObjC; this is on by default in C++)
• -Wformat
• -Wformat-overflow
• -Wformat-truncation
• -Wint-in-bool-context
• -Wimplicit (C and Objective-C only)
• -Wimplicit-int (C and Objective-C only)
• -Wimplicit-function-declaration (C and Objective-C only)
• -Winit-self (only for C++)
• -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++)
• -Wmissing-attributes
• -Wmissing-braces (only for C/ObjC)
• -Wmultistatement-macros
• -Wnarrowing (only for C++)
• -Wnonnull
• -Wnonnull-compare
• -Wopenmp-simd
• -Wparentheses
• -Wpessimizing-move (only for C++)
• -Wpointer-sign
• -Wrange-loop-construct (only for C++)
• -Wreorder
• -Wrestrict
• -Wreturn-type
• -Wsequence-point
• -Wsign-compare (only in C++)
• -Wsizeof-array-div
• -Wsizeof-pointer-div
• -Wsizeof-pointer-memaccess
• -Wstrict-aliasing
• -Wstrict-overflow=1
• -Wswitch
• -Wtautological-compare
• -Wtrigraphs
• -Wuninitialized
• -Wunknown-pragmas
• -Wunused-function
• -Wunused-label
• -Wunused-value
• -Wunused-variable
• -Wvla-parameter (C and Objective-C only)
• -Wvolatile-register-var
• -Wzero-length-bounds

Note that some warning flags are not implied by -Wall. Some of them warn about constructions that users generally do not consider questionable, but which occasionally you might wish to check for; others warn about constructions that are necessary or hard to avoid in some cases, and there is no simple way to modify the code to suppress the warning. Some of them are enabled by -Wextra but many of them must be enabled individually.

• -Wextra — This enables some extra warning flags that are not enabled by -Wall. (This option used to be called -W. The older name is still supported, but the newer name is more descriptive.) glow-up confirmed 💅
  • -Wclobbered
  • -Wcast-function-type
  • -Wdeprecated-copy (C++ only)
  • -Wempty-body
  • -Wenum-conversion (C only)
  • -Wignored-qualifiers
  • -Wimplicit-fallthrough=3
  • -Wmissing-field-initializers
  • -Wmissing-parameter-type (C only)
  • -Wold-style-declaration (C only)
  • -Woverride-init
  • -Wsign-compare (C only)
  • -Wstring-compare
  • -Wredundant-move (only for C++)
  • -Wtype-limits
  • -Wuninitialized
  • -Wshift-negative-value (in C++11 to C++17 and in C99 and newer)
  • -Wunused-parameter (only with -Wunused or -Wall)
  • -Wunused-but-set-parameter (only with -Wunused or -Wall)

So yeah, -Wextra also pops off with warnings for these little situations:

• A pointer gets compared against integer zero using <, <=, >, or >=. 🫠
• (C++ only) An enumerator and a non-enumerator both show up in one conditional expression.
• (C++ only) Ambiguous virtual bases. messy.
• (C++ only) Subscripting an array that was declared register.
• (C++ only) Taking the address of a variable that was declared register.
• (C++ only) A base class doesn't get initialized in the copy constructor of a derived class.

---

• -Wabi (C, Objective-C, C++ and Objective-C++ only) — warns you about code that's been affected by ABI changes, bestie. That means code that might not play nice with the vendor-neutral C++ ABI, and code that might clash with the psABI for whatever target you're on. 😭

So here's the tea: since G++ now defaults to updating the ABI with every major release, -Wabi normally only warns about C++ ABI compatibility problems if a check got added later in a release series for an ABI issue that was discovered after the initial release. But if you pick an older ABI version (with -fabi-version=n), -Wabi warns about way more stuff.

You can also hand -Wabi an explicit version number to warn about C++ ABI compatibility with a specific -fabi-version level — like -Wabi=2 warns about changes relative to -fabi-version=2.

If you give an explicit version number and you don't specify -fabi-compat-version, then the version number from this option gets used for compatibility aliases. And if you don't give an explicit version number with this option, but you do specify -fabi-compat-version, then that version number is what gets used for C++ ABI warnings.

Real talk though: even though people tried to warn about all of these cases, there are probably still some that slip through unwarned even while G++ is out here generating incompatible code. And there might also be cases where warnings fire even though the generated code is actually compatible. 🙈

If you're stressed about G++ code maybe not being binary compatible with code from other compilers, you should rewrite your code to dodge these warnings.

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 got 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)) got mangled in a non-standard way that doesn't allow for overloading functions that take vectors of different sizes.

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

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

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

• Scoped enumerators passed as arguments to a variadic function get promoted like unscoped enumerators, which makes va_arg complain. On most targets this doesn't actually affect the parameter passing ABI, since there's no way to pass an argument smaller than int.

Also, the ABI changed the mangling of template argument packs, const_cast, static_cast, prefix increment/decrement, and a class scope function used as a template argument.

These issues were corrected in -fabi-version=6.

• Lambdas in default argument scope got mangled incorrectly, and the ABI changed the mangling of nullptr_t.

These issues were corrected in -fabi-version=7.

• When mangling a function type with function-cv-qualifiers, the un-qualified function type got incorrectly treated as a substitution candidate.

This was fixed in -fabi-version=8, the default for GCC 5.1.

• decltype(nullptr) incorrectly had an alignment of 1, which led to unaligned accesses. Note that this did not affect the ABI of a function with a nullptr_t parameter, because parameters have a minimum alignment.

This was fixed in -fabi-version=9, the default for GCC 5.2.

• Target-specific attributes that affect the identity of a type — like ia32 calling conventions on a function type (stdcall, regparm, etc.) — didn't affect the mangled name, which led to name collisions when function pointers were used as template arguments.

This was fixed in -fabi-version=10, the default for GCC 6.1.

This option also turns on warnings about psABI-related changes. The known psABI changes at this point include:

• For SysV/x86-64, unions with long double members are passed in memory as specified in psABI. Prior to GCC 4.4, that was not the case. For example:

union U {
  long double ld;
  int i;
};

union U is now always passed in memory.

• -Wchar-subscripts — warns you if an array subscript has type char. This is a super common oopsie because programmers keep forgetting 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 can't use the profile feedback info. By default this warning is enabled and treated as an error. -Wno-coverage-mismatch disables the warning, or -Wno-error=coverage-mismatch disables the error. Turning off the error for this warning can give you poorly optimized code and is only useful for very minor changes like bug fixes to an existing codebase. Fully disabling the warning? Not recommended, bestie. 🚫
• -Wno-cpp (C, Objective-C, C++, Objective-C++ and Fortran only) — suppresses warning messages emitted by #warning directives.
• -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 why that matters: CPUs with a 32-bit "single-precision" floating-point unit implement float in hardware, but emulate double in software. On a machine like that, doing computations with double values is way more expensive because of all the overhead that software emulation needs. 💸

It's so 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 does the entire computation with double, because the floating-point literal is a double.

• -Wduplicate-decl-specifier (C and Objective-C only) — warns you if a declaration has a duplicate const, volatile, restrict or _Atomic specifier. Enabled by -Wall. ✨
• -Wformat
• -Wformat=n — checks calls to printf and scanf, etc., to make sure the arguments you supplied have types appropriate for the format string you specified, and that the conversions in the format string actually make sense. This covers standard functions, plus others specified by format attributes, across the printf, scanf, strftime and strfmon families (strfmon is 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 get disabled by -ffreestanding or -fno-builtin.

The formats are checked against the format features supported by GNU libc version 2.2. That includes all ISO C90 and C99 features, plus features from the Single Unix Specification and some BSD and GNU extensions. Other library implementations might not support all these features; GCC doesn't support warning about features that go beyond a particular library's limitations. However, if -Wpedantic is used with -Wformat, you get warnings about format features that aren't in the selected standard version (but not for strfmon formats, since those aren't 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 with these options: -Wno-format-contains-nul, -Wno-format-extra-args, and -Wno-format-zero-length. -Wformat is enabled by -Wall.

• -Wformat=2

Enables -Wformat plus extra format checks. Currently equivalent to -Wformat -Wformat-nonliteral -Wformat-security -Wformat-y2k.

• -Wno-format-contains-nul — if -Wformat is specified, this stops it from warning about format strings that contain NUL bytes.
• -Wno-format-extra-args — if -Wformat is specified, this stops it from warning about excess arguments to a printf or scanf format function. The C standard says those arguments get ignored anyway.

Where the unused arguments sit between used arguments that are specified with $ operand number specifications, warnings are normally still given, because the implementation couldn't know what type to pass to va_arg to skip over the unused arguments. However, for scanf formats, this option suppresses the warning if the unused arguments are all pointers, since the Single Unix Specification says those unused arguments are allowed.

• -Wformat-overflow
• -Wformat-overflow=level — warns about calls to formatted input/output functions like sprintf and vsprintf that might overflow the destination buffer. When the exact number of bytes written by a format directive can't be figured out at compile-time, it gets estimated using heuristics that depend on the level argument and on optimization. Turning on optimization will, in most cases, make the warning more accurate — but it might also give you false positives. 😬
• -Wformat-overflow, -Wformat-overflow=1 — okay so level 1 of -Wformat-overflow, which you get automatically when -Wformat is on, is the cautious girly. She only calls out the calls that are like, almost definitely gonna overflow the buffer. At this level, numeric arguments to format directives with unknown values get assumed to be one, and strings of unknown length get 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 produces the most bytes on output. So for example, the sprintf call below gets diagnosed, because even with both a and b equal to zero, the terminating NUL character ('\0') that the function appends to the destination buffer is gonna get written past its end. Bumping the buffer size up by a single byte is enough to dodge the warning, though it may not be enough to dodge the overflow. 💅

void f (int a, int b)
{
  char buf [13];
  sprintf (buf, "a = %i, b = %i\n", a, b);
}

• -Wformat-overflow=2 — level 2 is level 1 but she's also clocking 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 can't be assumed 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, that call in the example above is again diagnosed, but this time it's because with a equal to a 32-bit INT_MIN, the first %i directive is gonna write some of its digits past the end of the destination buffer. To make the call safe no matter what the two variables are, the destination buffer size has to go up to at least 34 bytes. GCC drops the minimum size of the buffer into an informational note right after the warning, bestie, it literally tells you.

Another way to handle it instead of enlarging 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. And when numeric arguments of format directives can be assumed to be bounded by less than the precision of their type, picking an appropriate length modifier for the format specifier shrinks 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 drops the maximum required buffer size 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, don't warn about zero-length formats. The C standard says zero-length formats are allowed, so like, chill.
• -Wformat-nonliteral — if -Wformat is specified, also warn if the format string isn't a string literal and therefore can't 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 isn't a string literal and there are no format arguments, as in printf (foo);. That can be a straight-up 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 aren't included in -Wformat-nonliteral.) 🚨
• -Wformat-signedness — if -Wformat is specified, also warn if the format string requires an unsigned argument and the argument is signed, and vice versa.
• -Wformat-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 can't be determined at compile-time, it's estimated based on heuristics that depend on the level argument and on optimization. Turning optimization on will in most cases improve the accuracy of the warning, but it may also result in false positives. Except as noted otherwise, this option uses the same logic as -Wformat-overflow.
  • -Wformat-truncation, -Wformat-truncation=1 — level 1 of -Wformat-truncation, enabled by -Wformat, is the conservative one again: she only warns about calls to bounded functions whose return value is unused and that will most likely result in output truncation.
  • -Wformat-truncation=2 — level 2 also warns 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. it's giving year 2000 energy 🫠
• -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. How precise the warnings are depends on the optimization options used.
• -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 doesn't specify a type. This warning is enabled by default in C99 and later dialects of C, and also by -Wall.
• -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 in C99 and later dialects of C, and also by -Wall. The warning is made into 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.
• -Wimplicit-fallthrough — -Wimplicit-fallthrough is the same as -Wimplicit-fallthrough=3, and -Wno-implicit-fallthrough is the same as -Wimplicit-fallthrough=0.
• -Wimplicit-fallthrough=n — warn when a switch case falls through. For example:

switch (cond)
  {
  case 1:
    a = 1;
    break;
  case 2:
    a = 2;
  case 3:
    a = 3;
    break;
  }

This warning doesn't fire when the last statement of a case can't fall through, e.g. when there's a return statement or a call to a function declared with the noreturn attribute. -Wimplicit-fallthrough= also takes control flow statements, such as ifs, into account, and only warns when appropriate. E.g.

switch (cond)
  {
  case 1:
    if (i > 3) {
      bar (5);
      break;
    } else if (i < 1) {
      bar (0);
    } else
      return;
  default:
    ...
  }

Since there are occasions where a switch case fall through is actually desirable, GCC provides an attribute, __attribute__ ((fallthrough)), that's meant to be used along with a null statement to suppress this warning that would normally occur:

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

C++17 provides a standard way to suppress the -Wimplicit-fallthrough warning using [[fallthrough]]; instead of the GNU attribute. In C++11 or C++14 users can use [[gnu::fallthrough]];, which is a GNU extension. Instead of these attributes, it'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.
• -Wimplicit-fallthrough=1 matches .* regular expression, any comment is used as fallthrough comment.
• -Wimplicit-fallthrough=2 case insensitively matches .*falls?[ \t-]*thr(ough|u).* regular expression.
• -Wimplicit-fallthrough=3 case sensitively matches one of the following regular expressions:

So like, level 3 is the chill one — it matches any of these regexes and it does it case-insensitively:

• -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 — bestie this one is picky, it matches case sensitively against one of these:
• -fallthrough
• @fallthrough@
• lint -fallthrough[ \t]*
• [ \t]*FALLTHR(OUGH|U)[ \t]*
• -Wimplicit-fallthrough=5 — doesn't recognize any comments as fallthrough comments, periodt. Only attributes disable the warning.

The comment needs to be followed after optional whitespace and other comments by case or default keywords, or by a user label that precedes some case or default label.

switch (cond)
  {
  case 1:
    bar (0);
    /* FALLTHRU */
  default:
    ...
  }

The -Wimplicit-fallthrough=3 warning is enabled by -Wextra. 💅

---

• -Wno-if-not-aligned (C, C++, Objective-C and Objective-C++ only) — controls whether warnings triggered by the warn_if_not_aligned attribute get 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 like const. For ISO C that type qualifier has no effect, since the value returned by a function isn't an lvalue. For C++, the warning only fires for scalar types or void. And ISO C prohibits qualified void return types on function definitions, so those return types always get a warning even without this option. 💀

This warning is also enabled by -Wextra.

• -Wno-ignored-attributes (C and C++ only) — this one controls warnings when an attribute is ignored. Not the same as -Wattributes, bestie — this warns whenever the compiler decides to drop an attribute, not that the attribute is unknown or used in a wrong place or whatever. 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. 👑
• -Wmisleading-indentation (C and C++ only) — warns when the indentation of the code does not reflect the block structure. Specifically, a warning fires for if, else, while, and for clauses with a guarded statement that doesn't use braces, followed by an unguarded statement with the same indentation.

In this next example, the call to bar is misleadingly indented as if it were guarded by the if conditional. the way that... no. it's just a lie. 🙈

  if (some_condition ())
    foo ();
    bar ();  /* Gotcha: this is not guarded by the "if".  */

With mixed tabs and spaces, the warning uses the -ftabstop= option to figure out if the statements line up (defaulting to 8).

The warning is not issued for code involving multiline preprocessor logic, like this:

  if (flagA)
    foo (0);
#if SOME_CONDITION_THAT_DOES_NOT_HOLD
  if (flagB)
#endif
    foo (1);

Also not issued after a #line directive, since that typically means 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, it fires 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 fires 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..

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 isn't fully bracketed. In this example, the initializer for a isn't fully bracketed, but the one for b is. 🌸

int a[2][2] = { 0, 1, 2, 3 };
int b[2][2] = { { 0, 1 }, { 2, 3 } };

This warning is enabled by -Wall.

• -Wmissing-include-dirs (C, C++, Objective-C and Objective-C++ only) — warns if a user-supplied include directory doesn't exist. 🦋
• -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 don't 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 can't 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.

Not me warning you that ignoring this one lowkey tanks your optimization. Do with that what you will. 💀

• -Wno-mismatched-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
}

In C++, the related option -Wmismatched-new-delete diagnoses mismatches involving either operator new or operator delete.

Option -Wmismatched-dealloc is enabled by default. 💅

• -Wmultistatement-macros — warns about unsafe multiple statement macros that look like they're guarded by a clause such as if, else, for, switch, or while, but where only the first statement is actually guarded after the macro is expanded. the betrayal. 🙈

For example:

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

will increment y unconditionally, not just when c holds. You can usually fix this 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 — warns if parentheses are omitted in certain contexts, like when there's an assignment in a context where a truth value is expected, or when operators are nested whose precedence people often get confused about. 🤔

Also warns if a comparison like x<=y<=z appears — that's equivalent to (x<=y ? 1 : 0) <= z, which is a different interpretation from ordinary mathematical notation. (math girlies, beware.)

Also warns for dangerous uses of the GNU extension to ?: with the omitted middle operand. When the condition in the ?: operator is a boolean expression, the omitted value is always 1. Often programmers expect it to be a value computed inside the conditional expression instead. 😭

For C++ this also warns for some cases of unnecessary parentheses in declarations, which can indicate an attempt at a function call instead of a declaration:

{
  // Declares a local variable called mymutex.
  std::unique_lock<std::mutex> (mymutex);
  // User meant std::unique_lock<std::mutex> lock (mymutex);
}

This warning is enabled by -Wall.

• -Wsequence-point — warns about code that may have undefined semantics because of violations of sequence point rules in the C and C++ standards.

Okay bestie, "undefined semantics" is not a vibe, it's a warning that your code might just do whatever it wants. Take it seriously. 🔥

so like, imagine two points in ur code: stuff that happens before the sequence point and stuff that happens after. That's literally all a sequence point is, bestie — a lil checkpoint. And these checkpoints show up in specific spots: after a full expression gets evaluated (a full expression = one that's not nested inside a bigger expression), after the first operand of &&, ||, ? : or , (the comma operator) gets evaluated, before a function gets called (but after its arguments and the expression naming the function are evaluated), and in a few other places too. 💅

Now here's the tea: apart from what the sequence point rules say, the order that subexpressions inside an expression get evaluated is not specified. And all of this is only a partial order, not a total order — like, if two functions get called in the same expression with no sequence point between them, nobody can tell you which one runs first. However, the standards committee did rule that function calls don't overlap. So there's that. ✨

Also not specified: when modifications to object values actually take effect between sequence points. Programs that depend on this? Undefined behavior. The C and C++ standards say, and I quote: "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 ur program breaks these rules, the results on any particular implementation are entirely unpredictable. 😭

Examples of undefined behavior code: a = a++;, a[n] = b[n++] and a[i++] = i;. Some more complicated cases don't get diagnosed by this option, and it might occasionally throw a false positive, but ngl in general it's been found fairly effective at catching this kind of problem in programs.

The C++17 standard is gonna define evaluation order in more cases: specifically it requires the right-hand side of an assignment be evaluated before the left-hand side, so the examples above stop being 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 (shocking, I know), so there's some debate over the precise meaning of the sequence point rules in subtle cases. Links to discussions of the problem, including proposed formal definitions, may be found on the GCC readings page, at <http://gcc.gnu.org/readings.html>.

This warning is enabled by -Wall for C and C++. ⭐

• -Wno-return-local-addr — do not warn about returning a pointer (or in C++, a reference) to a variable that goes out of scope after the function returns. Basically turning off the "bestie that's a dangling pointer" alarm. 🙈
• -Wreturn-type — warn whenever a function is defined with a return type that defaults to int. Also warn about any return statement with no return value in a function whose return type isn't void (falling off the end of the function body counts as returning without a value, fyi).

For C only, warn 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.

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 isn't 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 — warn if left shifting 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 — warn whenever a switch statement has an index of enumerated type and lacks a case for one or more of the named codes of that enumeration. (The presence of a default label prevents this warning, btw.) case labels outside the enumeration range also provoke warnings when this option is used (even if there's a default label). This warning is enabled by -Wall.
• -Wswitch-default — warn whenever a switch statement does not have a default case. Like bestie where's ur safety net 🥺
• -Wswitch-enum — warn whenever a switch statement has an index of enumerated type and lacks a case for one or more of the named codes of that enumeration. case labels outside the enumeration range also provoke warnings when this option is used. The only difference between -Wswitch and this option is that this option gives a warning about an omitted enumeration code even if there's a default label. She's thorough like that. 💅
• -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. It's possible to suppress this warning by casting the controlling expression to a type other than bool. For example:

switch ((int) (a == 4))
  {
  ...
  }

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

• -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. For example:

switch (cond)
  {
   i = 15;
  ...
   case 5:
  ...
  }

-Wswitch-unreachable does not warn if the statement between the controlling expression and the first case label is just a declaration:

switch (cond)
  {
   int i;
  ...
   case 5:
   i = 5;
  ...
  }

This warning is enabled by default for C and C++ programs. ✨

• -Wsync-nand (C and C++ only) — warn when __sync_fetch_and_nand and __sync_nand_and_fetch built-in functions are used. These functions changed semantics in GCC 4.4. so like, heads up. 🚨
• -Wunused-but-set-parameter — warn whenever a function parameter is assigned to, but otherwise unused (aside from its declaration).

To suppress this warning use the unused attribute.

This warning is also enabled by -Wunused together with -Wextra.

• -Wunused-but-set-variable — warn whenever a local variable is assigned to, but otherwise unused (aside from its declaration). This warning is enabled by -Wall.

To suppress this warning use the unused attribute.

This warning is also enabled by -Wunused, which is enabled by -Wall.

• -Wunused-function — warn whenever a static function is declared but not defined, or a non-inline static function is unused. This warning is enabled by -Wall. 👑
• -Wunused-label — warn whenever a label is declared but not used. This warning is enabled by -Wall.

To suppress this warning use the unused attribute.

• -Wunused-local-typedefs (C, Objective-C, C++ and Objective-C++ only) — warn when a typedef locally defined in a function is not used. This warning is enabled by -Wall.
• -Wunused-parameter — warn whenever a function parameter is unused aside from its declaration.

To suppress this warning use the unused attribute.

• -Wno-unused-result — do not warn if a caller of a function marked with attribute warn_unused_result does not use its return value. The default is -Wunused-result.
• -Wunused-variable — warn whenever a local or static variable is unused aside from its declaration. This option implies -Wunused-const-variable=1 for C, but not for C++. This warning is enabled by -Wall.

To suppress this warning use the unused attribute.

• -Wunused-const-variable, -Wunused-const-variable=n — warn whenever a constant static variable is unused aside from its declaration. -Wunused-const-variable=1 is enabled by -Wunused-variable for C, but not for C++. In C this declares variable storage, but in C++ this is not an error since const variables take the place of #defines.

To suppress this warning use the unused attribute.

• -Wunused-const-variable=1 — this is the warning level that's enabled by -Wunused-variable for C. It warns only about unused static const variables defined in the main compilation unit, but not about static const variables declared in any header included.
• -Wunused-const-variable=2 — this warning level also warns for unused constant static variables in headers (excluding system headers). This is the warning level of -Wunused-const-variable and must be explicitly requested, since in C++ this isn't an error and in C it might be harder to clean up all headers included. 💅

• -Wunused-value — bestie, this one yells at you whenever a statement computes a result and then just… doesn't use it 💀. If you genuinely meant to throw it away, cast that unused expression to void and the warning shuts up. This catches expression-statements, and the left-hand side of a comma expression that has no side effects. So like, x[i,j] gets a warning but x[(void)i,j] is chill ✨.

This warning is enabled by -Wall.

• -Wunused — the whole girlboss bundle: all of the above -Wunused options combined 👑.

Lowkey important: to get a warning about an unused function parameter, you gotta either pass -Wextra -Wunused (heads up, -Wall implies -Wunused), or just say -Wunused-parameter by itself.

• -Wuninitialized — warns you when an object with automatic or allocated storage duration gets used before it was ever initialized 😭. In C++, it also warns if a non-static reference or a non-static const member shows up in a class with no constructors.

Also: passing a pointer (or in C++, a reference) to an uninitialized object into a const-qualified argument of a built-in function that's known to read the object — yeah, that gets diagnosed by this warning too. (For ordinary functions it's -Wmaybe-uninitialized that shows up instead.)

Wanna get warned about code that uses the uninitialized value of a variable in its own initializer? That's -Winit-self's job.

These warnings fire for individual uninitialized elements of structure, union or array variables, and for whole variables that are uninitialized. They do not fire for variables or elements declared volatile. And because these warnings depend on optimization, exactly which variables or elements get warned about depends on the precise optimization options and the version of GCC you're running. No cap.

Note: there may be no warning at all about a variable that's only used to compute a value that itself is never used, because data flow analysis can delete those computations before the warnings even get printed. Sneaky.

• -Wno-invalid-memory-model — this option controls warnings for invocations of __atomic Builtins, __sync Builtins, and the C11 atomic generic functions when the memory consistency argument is either invalid for the operation or outside the range of values of the memory_order enumeration. Example time: since __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's a path from function entry to a use of the object that is initialized, but there are also some other paths where the object isn't initialized, the compiler emits a warning when it can't prove those uninitialized paths never run at runtime. It's giving "we're not sure but we're worried" energy 🙈.

Also, passing a pointer (or in C++, a reference) to an uninitialized object to a const-qualified function argument gets diagnosed by this warning too. (-Wuninitialized is what fires for built-in functions known to read the object.) If you annotate the function with attribute access (none), that says the argument isn't used to access the object — which avoids the warning. Slay.

These warnings are only possible in optimizing compilation, because otherwise GCC doesn't keep track of the state of variables. Periodt.

They're made optional because GCC may not be able to tell when the code is correct even though it looks like an error. Here's one example of how that can happen:

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

If the value of y is always 1, 2 or 3, then x is always initialized — but GCC doesn't know that 😤. To suppress the warning, you need to provide a default case with assert(0) or similar code.

This option also warns when a non-volatile automatic variable might be changed by a call to longjmp. The compiler only sees the calls to setjmp. It cannot know where longjmp will be called; in fact, a signal handler could call it at any point in the code. So you may get a warning even when there's actually no problem, because longjmp can't in fact be called at the place that would cause a problem.

Some spurious warnings can be avoided if you declare all the functions you use that never return as noreturn.

This warning is enabled by -Wall or -Wextra.

• -Wunknown-pragmas — warns 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 does not happen 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 conflicts between pragmas. See also -Wunknown-pragmas. 💅
• -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 to control its 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. 👑
• -Wstrict-aliasing — this option is only active when -fstrict-aliasing is active. It warns about code that might break the strict aliasing rules the compiler is using for optimization. It doesn't catch all cases, but it does try to catch the more common pitfalls. It's included in -Wall. It is equivalent to -Wstrict-aliasing=3.
• -Wstrict-aliasing=n — also 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 the way -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 never dereferenced. Runs in the front end only.
  • Level 2: Aggressive, quick, not too precise. May still have many false positives (not as many as level 1 though), and few false negatives (but possibly more than level 1). Unlike level 1, it only warns when an address is taken. Warns about incomplete types. Runs in the front end only.
  • Level 3 (default for -Wstrict-aliasing): Should have very few false positives and few false negatives. Slightly slower than levels 1 or 2 when optimization is enabled. Takes care of the common pun+dereference pattern in the front end: *(int*)&some_float. If optimization is enabled, it also runs in the back end, where it deals with multiple statement cases using flow-sensitive points-to information. Only warns when the converted pointer is dereferenced. Does not warn about incomplete types.
• -Wstrict-overflow
• -Wstrict-overflow=n — this option is only active when signed overflow is undefined. It warns about cases where the compiler optimizes based on the assumption that signed overflow does not occur 🔥. Note that it does not warn about all cases where the code might overflow: it only warns about cases where the compiler implements some optimization. So this warning depends on the optimization level.

An optimization that assumes signed overflow doesn't occur is perfectly safe if the values of the variables involved are such that overflow never actually happens. So this warning can easily give a false positive: a warning about code that isn't actually a problem. To help focus on important issues, several warning levels are defined. No warnings are issued for the use of undefined signed overflow when estimating how many iterations a loop requires — in particular when determining whether a loop will be executed at all. 💀

• -Wstrict-overflow=1 — Warn about cases that are both questionable and easy to avoid. For example, the compiler simplifies x + 1 > x to 1. This level of -Wstrict-overflow is enabled by -Wall; higher levels are not, and must be explicitly requested.
• -Wstrict-overflow=2 — Also warn about other cases where a comparison is simplified to a constant. For example: abs (x) >= 0. This can only be simplified when signed integer overflow is undefined, because abs (INT_MIN) overflows to INT_MIN, which is less than zero. -Wstrict-overflow (with no level) is the same as -Wstrict-overflow=2.
• -Wstrict-overflow=3 — Also warn about other cases where a comparison is simplified. For example: x + 1 > 1 is simplified to x > 0.
• -Wstrict-overflow=4 — Also warn about other simplifications not covered by the above cases. For example: (x * 10) / 5 is simplified to x * 2.
• -Wstrict-overflow=5 — Also warn about cases where the compiler reduces the magnitude of a constant involved in a comparison. For example: x + 2 > y is simplified to x + 1 >= y. This is reported only at the highest warning level because this simplification applies to many comparisons, so this warning level gives a very large number of false positives. 🌈

• -Wstring-compare — ok so this one is lowkey the tattletale of the warning family 💅 It clocks calls to strcmp and strncmp where the result is literally already decided to be either zero or non-zero in an equality test, all because one argument's length is bigger than the size of the array the other argument lives in (or the bound, if we're talking strncmp). Basically: these calls are probably mistakes, bestie. The strcmp below gets called out because its result is necessarily non-zero no matter what's actually sitting 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");
}

Oh and fun fact: -Wstring-compare is enabled by -Wextra ✨

• -Wno-stringop-overflow
• -Wstringop-overflow
• -Wstringop-overflow=type — this trio is the buffer overflow alarm 🚨 It warns about calls to string manipulation functions like memcpy and strcpy that are determined to overflow the destination buffer. The optional argument is one greater than the type of Object Size Checking used to figure out the size of the destination. Heads up though: that argument only means anything for functions that work on character arrays — not for raw memory functions like memcpy, which always use Object Size type-0, no exceptions. The option also yells about calls that specify a size bigger than the largest possible object, or at least SIZE_MAX / 2 bytes. It works its best magic with optimization on, but it can still catch 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. Either way, it only warns about a subset of the buffer overflows that the matching overflow-checking built-ins detect. For example, it warns on the strcpy call below because it copies at least 5 characters (the string "blue", terminating NUL included) 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
}

And psst: -Wstringop-overflow=2 is enabled by default 👑

• -Wstringop-overflow
• -Wstringop-overflow=1 — this one uses type-zero Object Size Checking to figure out the sizes of destination objects. At this setting it does not warn about writes past the end of subobjects of larger objects accessed by pointers, unless the size of the biggest surrounding object is known. When the destination could be one of several objects, it just assumes it's the largest one of them. And on Linux systems, when optimization is enabled 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 uses type-one Object Size Checking to figure out the sizes of destination objects. At this setting it warns about overflows when writing to members of the largest complete objects whose exact size is known. But it does not warn about excessive writes to those same members of unknown objects referenced by pointers, since those pointers might be pointing at arrays containing unknown numbers of elements. This is the default setting of the option, bestie 💖
• -Wstringop-overflow=3 — this one uses type-two Object Size Checking to figure out the sizes of destination objects. At this setting it warns about overflowing the smallest object or data member. This is the most restrictive setting of the option, and heads up: it may result in warnings for safe code 😭
• -Wstringop-overflow=4 — this one uses type-three Object Size Checking to figure out the sizes of destination objects. At this setting it warns about overflowing any data members, and when the destination is one of several objects it uses the size of the largest of them to decide whether to issue a warning. And just like -Wstringop-overflow=3, this setting may result in warnings for benign code 🙈

• -Wno-stringop-overread — this one warns for calls to string manipulation functions like memchr, or strcpy, that are determined to read past the end of the source sequence. Reading off the edge of the world, basically 💀

Also important: -Wstringop-overread is enabled by default.

• -Wno-stringop-truncation — do not warn for calls to bounded string manipulation functions like strncat, strncpy, and stpncpy that might either truncate the copied string or leave the destination unchanged.

In this next example, the call to strncat specifies a bound that's less than the length of the source string. So the copy of the source gets truncated, and therefore the call is diagnosed. To dodge the warning, use bufsize - strlen (buf) - 1) as the bound:

void append (char *buf, size_t bufsize)
{
  strncat (buf, ".txt", 3);
}

And as another example, the following call to strncpy copies to d just the characters before 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 gets diagnosed. If you don't expect the result to be NUL-terminated and wanna skip the warning, just call memcpy instead:

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

And in this example, the call to strncpy specifies the size of the destination buffer as the bound. If the source string's length is equal to or greater than that size, the result of the copy won't be NUL-terminated. So yeah, this 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);
  ...
}

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

• -Wsuggest-attribute=[pure|const|noreturn|format|cold|malloc] — warns you about cases where adding an attribute might be a glow-up ✨ The attributes currently supported are listed below.
  • -Wsuggest-attribute=pure
  • -Wsuggest-attribute=const
  • -Wsuggest-attribute=noreturn
  • -Wmissing-noreturn
  • -Wsuggest-attribute=malloc — warns about functions that might be candidates for the pure, const, noreturn or malloc attributes. 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 or return abnormally by throwing, calling abort or trapping. This analysis needs option -fipa-pure-const, which is enabled by default at -O and higher. Higher optimization levels make the analysis more accurate, so like… crank it up for the best tea ☕
  • -Wsuggest-attribute=format
  • -Wmissing-format-attribute — warns 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, ok? 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 where format attributes would be appropriate may not get detected at all. It's giving ✨vibes-based heuristics✨

• -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 which always lead to a call to another cold function — like wrappers of C++ throw, or fatal error reporting functions leading to abort 🥶

• -Walloc-zero — warns about calls to allocation functions decorated with attribute alloc_size that specify zero bytes, including calls to the built-in forms of the functions aligned_alloc, alloca, calloc, malloc, and realloc. Why care? Because the behavior of these functions when called with a zero size differs among implementations (and in the case of realloc it's been deprecated), so relying on it may result in subtle portability bugs and should be avoided, fr fr 🚫
• -Walloc-size-larger-than=byte-size — warns about calls to functions decorated with attribute alloc_size that try 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. -Walloc-size-larger-than=PTRDIFF_MAX is enabled by default. And the warnings controlled by this option can be disabled either by specifying a byte-size of SIZE_MAX or more, or by -Wno-alloc-size-larger-than 💜
• -Wno-alloc-size-larger-than — turns off the -Walloc-size-larger-than= warnings. It's literally the same thing as -Walloc-size-larger-than=SIZE_MAX or bigger. so basically it's the "i said what i said" button 💅
• -Walloca — this one yells at you for every single use of alloca anywhere in your source. all of them. no hiding. 👑
• -Walloca-larger-than=byte-size — ok so this is the picky one 🎀 it warns you when someone calls alloca with an integer argument whose value is either zero, or not bounded by a controlling predicate that caps it at byte-size. it also warns on alloca calls where the bound value is just unknown bestie. and here's the tea: arguments of non-integer types count as unbounded even if they look like they're constrained to the expected range. looks can be deceiving fr fr 🙈

so like, a bounded case of alloca could be:

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

in that example, passing -Walloca-larger-than=1000 would not issue a warning, because the call to alloca is known to be at most 1000 bytes. but if you passed -Walloca-larger-than=500 instead? the compiler would emit a warning. caught. 💀

unbounded uses, on the other hand, are uses of alloca with no controlling predicate constraining its integer argument. like:

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.

and note (this is the part that gets people 😭) that even seemingly correct code involving signed integers could cause a warning:

void func (signed int n)
{
  if (n < 500)
    {
      p = alloca (n);
      f (p);
    }
}

in that example, n could be negative, which causes a larger than expected argument to be implicitly cast into the alloca call. sneaky little negative number. not on my watch ✋

this option also warns when alloca is used in a loop. (loop alloca is giving chaos energy, we don't stan.)

-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 — disables the -Walloca-larger-than= warnings. it's equivalent to -Walloca-larger-than=SIZE_MAX or larger. same vibe, just louder threshold 🫶
• -Warith-conversion — do warn about implicit conversions from arithmetic operations even when converting the operands to the same type can't change their values. this one messes with the warnings you get from -Wconversion, -Wfloat-conversion, and -Wsign-conversion. ✨

void f (char c, int i)
{
  c = c + i; // warns with B<-Wconversion>
  c = c + 1; // only warns with B<-Warith-conversion>
}

• -Warray-bounds, -Warray-bounds=n — this one is only active when -ftree-vrp is active (default for -O2 and above). it warns about subscripts to arrays that are always out of bounds. this warning is enabled by -Wall. 🎯
  • -Warray-bounds=1 — this is the warning level of -Warray-bounds and it's enabled by -Wall. higher levels are not, and you have to explicitly ask for them. no free upgrades bestie 💅
  • -Warray-bounds=2 — this level also warns about out of bounds access for arrays at the end of a struct and for arrays accessed through pointers. heads up tho: this level may give a larger number of false positives and is deactivated by default. drama alert 🚨
• -Warray-parameter, -Warray-parameter=n — warns about redeclarations of functions involving arguments of array or pointer types of inconsistent kinds or forms, and it enables the detection of out-of-bounds accesses to those parameters by warnings like -Warray-bounds. 👀

if the first function declaration uses the array 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 by warnings such as -Warray-bounds. at level 1 the warning diagnoses inconsistencies involving array parameters declared using the T[static N] form.

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 with the keyword static specifies that the array argument must have at least four elements.

void f (int[static 4]);
void f (int[]);           // warning (inconsistent array form)

void g (void)
{
  int *p = (int *)malloc (4);
  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. and the -Wvla-parameter option triggers warnings for similar inconsistencies involving Variable Length Array arguments. 🌸

• -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. it diagnoses incompatibilities between the type of the alias declaration and that of its target. those 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. your call bestie 💅

• -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 — warns about suspicious operations on expressions of a boolean type. for instance, bitwise negation of a boolean is very likely a bug in the program. for C, this warning also warns about incrementing or decrementing a boolean, which rarely makes sense. (in C++, decrementing a boolean is always invalid. incrementing a boolean is invalid in C++17, and deprecated otherwise.)

this warning is enabled by -Wall. 🌈

• -Wduplicated-branches — warns when an if-else has identical branches. this warning detects cases like

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

it doesn't warn when both branches contain just a null statement. this warning also warns for conditional operators:

int i = x ? *p : *p;

• -Wduplicated-cond — warns about duplicated conditions in an if-else-if chain. for instance, warn for the following code:

if (p->q != NULL) { ... }
else if (p->q != NULL) { ... }

• -Wframe-address — warns when __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. shhh, we don't talk about it 🤫
• -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's 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. iykyk 💅
• -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. 👑
• -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. see

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. bestie that's the whole lore, it comes free with its parent 💖

---

• -Wno-div-by-zero — okay so this one says do not warn about integer division by zero that happens at compile time. And floating-point division by zero? never warned about, periodt, because dividing floats by zero is a legit way to get infinities and NaNs 🌈 no cap, that's just how floats work.
• -Wsystem-headers — this makes GCC print warnings for stuff it finds in system header files. Normally those warnings are suppressed, because the assumption is they usually don't mean real problems and they'd just make the compiler output harder to read 🙈. Passing this command-line option tells GCC to emit warnings from system headers as if they happened in your code. But heads up: using -Wall together with this option does not warn about unknown pragmas in system headers — for that you also need -Wunknown-pragmas 💅
• -Wtautological-compare — warns when a self-comparison always evaluates to true or false. It catches little mistakes like:

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

It also warns about bitwise comparisons that always come out true or false, for example:

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

which will always be false. This warning is enabled by -Wall. it's giving "caught in 4k" energy 🔥

• -Wtrampolines — warns about trampolines generated for pointers to nested functions. A trampoline is a tiny piece of data or code created at run time on the stack when you take the address of a nested function, and it's used to call that nested function indirectly. For some targets it's made up of data only, so it needs no special treatment 🎀. But for most targets it's made up of code, which means the stack has to be made executable for the program to work properly — and you already know how that goes 😭
• -Wfloat-equal — warns if floating-point values are used in equality comparisons. The idea here is that sometimes it's convenient (for the programmer) to think of floating-point values as approximations of infinitely precise real numbers. If that's your vibe, then you need to figure out — by analyzing the code, or some other way — the maximum or likely maximum error the computation introduces, and allow for it when doing comparisons (and when producing output, but that's a whole different problem bestie 💀). In particular, instead of testing for equality you should check whether the two values have ranges that overlap; that's done with the relational operators. So like... equality comparisons are probably a mistake. 🫶
• -Wtraditional (C and Objective-C only) — warns about certain constructs that behave differently in traditional C versus ISO C. It also warns about ISO C constructs that have no traditional C equivalent, and/or problematic constructs that should be avoided. The list is long, so buckle up:
  • Macro parameters that appear within string literals in the macro body. In traditional C, macro replacement takes place inside 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. So -Wtraditional warns about directives that traditional C understands but ignores because the # isn't the first character on the line. It also suggests you hide directives like #pragma that traditional C doesn't understand by indenting them. Some traditional implementations don't 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 that these suffixes show up in macros defined in the system headers of most modern systems, e.g. the _MIN/_MAX macros in <limits.h>. Using those macros in user code might normally lead to spurious warnings, however GCC's integrated preprocessor has enough context to avoid warning in these cases.
  • A function declared external in one block and then used after the end of the block.
  • A switch statement has an operand of type long.
  • A non-static function declaration follows a static one. This construct is not accepted by some traditional C compilers.
  • The ISO type of an integer constant has a different width or signedness from its traditional type. This warning is only issued if the base of the constant is ten. I.e. hexadecimal or octal values, which typically represent bit patterns, are not warned about.
  • Usage of ISO string concatenation is detected.
  • Initialization of automatic aggregates.
  • Identifier conflicts with labels. Traditional C lacks a separate namespace for labels.
  • Initialization of unions. If the initializer is zero, the warning is omitted. This is done under the assumption that the zero initializer in user code appears conditioned on e.g. __STDC__ to avoid missing initializer warnings, and relies on default initialization to zero in the traditional C case.
  • Conversions by prototypes between fixed/floating-point values and vice versa. The absence of these prototypes when compiling with traditional C causes serious problems. This is a subset of the possible conversion warnings; for the full set use -Wtraditional-conversion.
  • Use of ISO C style function definitions. This warning intentionally is not issued for prototype declarations or variadic functions because these ISO C features appear in your code when you're 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) — warns if a prototype causes a type conversion that's different from what would happen to the same argument if there were no prototype. This includes conversions of fixed point to floating and vice versa, plus conversions that change the width or signedness of a fixed-point argument — except when it's the same as the default promotion. ✨
• -Wdeclaration-after-statement (C and Objective-C only) — warns when a declaration shows up after a statement in a block. This construct, which you'd know from C++, was introduced with ISO C99 and is allowed in GCC by default. It is not supported by ISO C90. 👑
• -Wshadow — warns whenever a local variable or type declaration shadows another variable, parameter, type, class member (in C++), or instance variable (in Objective-C), or whenever a built-in function is shadowed. Note that in C++, the compiler warns if a local variable shadows an explicit typedef, but not if it shadows a struct/class/enum. If this warning is enabled, it also includes all instances of local shadowing — which means -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. that's it, that's the whole option 🎀
• -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 (on top of -Wshadow=local) is based on the idea that when a local variable shadows another one of incompatible type, it's most likely intentional, not a bug or a typo — like in this example:

for (SomeIterator i = SomeObj.begin(); i != SomeObj.end(); ++i)
{
  for (int i = 0; i < N; ++i)
  {
    ...
  }
  ...
}

Since the two variables 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. Using 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 this option can be disabled either by specifying a byte-size of SIZE_MAX or more, or by -Wno-larger-than.

It also warns 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 — disables -Wlarger-than= warnings. The option is equivalent to -Wlarger-than=SIZE_MAX or larger. bye felicia to those warnings 💖
• -Wframe-larger-than=byte-size — bestie, this one yells at you when a function's frame size goes over byte-size. But hear me out: the math it does to figure out the stack frame size is approximate and not conservative, so the real requirement could genuinely be bigger than byte-size and it'd still stay silent. 🙈 Also, anything you allocate with alloca, variable-length arrays, or related constructs is not counted by the compiler when it decides whether to warn. -Wframe-larger-than=PTRDIFF_MAX is enabled by default. You can shut these warnings off by passing a byte-size of SIZE_MAX or more, or by using -Wno-frame-larger-than.
• -Wno-frame-larger-than — turns off -Wframe-larger-than= warnings entirely. It's literally equivalent to -Wframe-larger-than=SIZE_MAX or larger. 💅
• -Wno-free-nonheap-object — warns you when you try to deallocate something that either wasn't allocated on the heap, or was allocated via a pointer that did not come back from a prior call to the matching allocation function. Like, example: the call to stpcpy hands you a pointer to the terminating nul character, not to the beginning of the object — so the free call below gets diagnosed. 😭

void f (char *p)
{
  p = stpcpy (p, "abc");
  // ...
  free (p);   // warning
}

-Wfree-nonheap-object is enabled by default.

• -Wstack-usage=byte-size — warns if a function's stack usage might exceed byte-size. The computation here is conservative (the opposite energy of the frame one, iconic 💅). Any space allocated via alloca, variable-length arrays, or related constructs is included by the compiler when it decides whether to warn.

The message matches 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 a byte-size of SIZE_MAX or more, or by -Wno-stack-usage.

• -Wno-stack-usage — disables -Wstack-usage= warnings. Equivalent to -Wstack-usage=SIZE_MAX or larger. 🫶
• -Wunsafe-loop-optimizations — warns if a loop can't be optimized because the compiler can't assume anything about the bounds of the loop indices. With -funsafe-loop-optimizations, it warns if the compiler does make such assumptions. 👀
• -Wno-pedantic-ms-format (MinGW targets only) — when you combine it with -Wformat and -pedantic without GNU extensions, this disables the warnings about non-ISO printf / scanf format width specifiers I32, I64, and I used on Windows targets — the ones that depend on the MS runtime.
• -Wpointer-arith — warns about anything that depends on the "size of" a function type or of void. GNU C gives these types a size of 1, just for convenience when doing calculations with void * pointers and pointers to functions. In C++, it also warns when an arithmetic operation involves NULL. This warning is also enabled by -Wpedantic.
• -Wno-pointer-compare — do not warn if a pointer is compared with a zero character constant. That pattern usually means the pointer was meant to be dereferenced. For example:

const char *p = foo ();
if (p == '\0')
  return 42;

Heads up: the code above is invalid in C++11. 💀

This warning is enabled by default.

• -Wtsan — warns about unsupported features in ThreadSanitizer.

ThreadSanitizer does not support std::atomic_thread_fence and can report false positives. 😭

This warning is enabled by default.

• -Wtype-limits — warns if a comparison is always true or always false because of the limited range of the data type — but it does not warn for constant expressions. So like, it'll warn if an unsigned variable gets compared against zero with < or >=. This warning is also enabled by -Wextra.
• -Wabsolute-value (C and Objective-C only) — warns for calls to standard functions that compute the absolute value of an argument when a more appropriate standard function exists. Example: calling abs(3.14) triggers the warning, because the correct function for the absolute value of a double argument is fabs. ✨ It also fires when the argument in a call to such a function has an unsigned type. You can suppress this warning with an explicit type cast, and it's also enabled by -Wextra.
• -Wcomment —
• -Wcomments — warns whenever a comment-start sequence /* appears inside a /* comment, or whenever a backslash-newline shows up in a // comment. This warning is enabled by -Wall.
• -Wtrigraphs — warns if any trigraphs are encountered that might change the meaning of the program. Trigraphs inside comments are not warned about — except the ones that would form escaped newlines.

This option is implied by -Wall. If -Wall isn't given, this option is still enabled unless trigraphs are enabled. To get trigraph conversion without the warnings, but still get the other -Wall warnings, use -trigraphs -Wall -Wno-trigraphs.

• -Wundef — warns if an undefined identifier is evaluated in an #if directive. Such identifiers get replaced with zero. 🌸
• -Wexpansion-to-defined — warns whenever defined is encountered in the expansion of a macro (including when the macro is expanded by an #if directive). Such usage is not portable. This warning is also enabled by -Wpedantic and -Wextra.
• -Wunused-macros — warns about macros defined in the main file that are unused. 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 gets redefined or undefined.

Built-in macros, macros defined on the command line, and macros defined in include files are not warned about. 💅

Note: if a macro is actually used, but only used in skipped conditional blocks, the preprocessor reports it as unused. To dodge the warning in that case, you could improve the scope of the macro's definition — for example, move it into the first skipped block. Or, alternatively, give it a dummy use with something like:

#if defined the_macro_causing_the_warning
#endif

• -Wno-endif-labels — do not warn whenever an #else or an #endif is followed by text. This sometimes happens in older programs with code shaped like this:

#if FOO
...
#else FOO
...
#endif FOO

That second and third FOO should be in comments. 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. 😬
• -Wc90-c99-compat (C and Objective-C only) — warns about features that aren't in ISO C90 but are in ISO C99. For instance: variable length arrays, the long long type, the 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 that aren't in ISO C99 but are in ISO C11. For instance: anonymous structures and unions, the _Atomic type qualifier, the _Thread_local storage-class specifier, the _Alignas specifier, the Alignof operator, the _Generic keyword, and so on. This option is independent of the standards mode. Warnings are disabled in the expression that follows __extension__.
• -Wc11-c2x-compat (C and Objective-C only) — warns about features that aren't in ISO C11 but are in ISO C2X. For instance: omitting the string in _Static_assert, use of [[]] syntax for attributes, use of decimal floating-point types, and so on. This option is independent of the standards mode. Warnings are disabled in the expression that follows __extension__.
• -Wc++-compat (C and Objective-C only) — warns about ISO C constructs that fall 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 in ISO C++ 1998 that are keywords in ISO C++ 2011. This warning turns on -Wnarrowing and is enabled by -Wall.
• -Wc++14-compat (C++ and Objective-C++ only) — warns about C++ constructs whose meaning differs between ISO C++ 2011 and ISO C++ 2014. This warning is 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. This warning is 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. This warning is enabled by -Wall.
• -Wcast-qual — warns whenever a pointer is cast in a way that removes a type qualifier from the target type. Example: warn if a const char * gets cast to an ordinary char *.

It also warns when a cast 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. Example: warn if a char * gets cast to an int * on machines where integers can only be accessed at two- or four-byte boundaries. ⚠️
• -Wcast-align=strict — bestie, this one yells whenever a pointer gets cast and the required alignment of the target goes up. Like, warn if a char * gets cast to an int * no matter what machine you're on. Every target. No exceptions. 💅
• -Wcast-function-type — warns when a function pointer gets cast to an incompatible function pointer. Okay so the rules: if the cast involves function types with a variable argument list, only the types of the initial arguments that are actually provided get 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. Type qualifiers get ignored too. 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.
• -Wwrite-strings — when compiling C, this 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, at compile time, code that can 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. This is why we did not make -Wall request these warnings. 😌 When compiling C++, warn about the deprecated conversion from string literals to char *. This warning is enabled by default for C++ programs.
• -Wclobbered — warns for variables that might be changed by longjmp or vfork. This warning is also enabled by -Wextra.
• -Wconversion — warns for implicit conversions that may alter a value. This includes conversions between real and integer, like abs (x) when x is double; conversions between signed and unsigned, like unsigned ui = -1; and conversions to smaller types, like sqrtf (M_PI). Do not warn for explicit casts like abs ((int) x) and ui = (unsigned) -1, or if the value is not changed by the conversion like in abs (2.0). Warnings about conversions between signed and unsigned integers can be disabled by using -Wno-sign-conversion. For C++, also warn for confusing overload resolution for user-defined conversions; and conversions that never use a type conversion operator: conversions to void, the same type, a base class or a reference to them. Warnings about conversions between signed and unsigned integers are disabled by default in C++ unless -Wsign-conversion is explicitly enabled. Warnings about conversion from arithmetic on a small type back to that type are only given with -Warith-conversion.
• -Wdangling-else — warns about constructions where there might be confusion about which if statement an else branch belongs to. Here's an example of that exact chaos:

{
  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). That's often not what the programmer expected, as illustrated by the indentation they chose. 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.

• -Wdate-time — warns when the macros __TIME__, __DATE__ or __TIMESTAMP__ are encountered, since they might prevent bit-wise-identical reproducible compilations. 🕰️
• -Wempty-body — warns if an empty body shows up in an if, else or do while statement. This warning is also enabled by -Wextra.
• -Wno-endif-labels — do not warn about stray tokens after #else and #endif.
• -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.
• -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.
• -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 are declared. This warning is only supported for C and Objective-C; in C++ this sort of branch is an error in any case. -Wjump-misses-init is included in -Wc++-compat. It can be disabled with the -Wno-jump-misses-init option.
• -Wsign-compare — 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.
• -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.
• -Wfloat-conversion — warns for implicit conversions that reduce the precision of a real value. This includes conversions from real to integer, and from higher precision real to lower precision real values. This option is also enabled by -Wconversion.
• -Wno-scalar-storage-order — do not warn on suspicious constructs involving reverse scalar storage order.
• -Wsizeof-array-div — warns about divisions of two sizeof operators when the first one is applied to an array and the divisor does not equal the size of the array element. In that case, the computation will not yield the number of elements in the array, which is probably what the user wanted. This warning 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. This warning 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));. -Wsizeof-pointer-memaccess also warns about calls to bounded string copy functions like strncat or strncpy that specify as the bound a sizeof expression of the source array. 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.

• -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's a big ol' sign that the user omitted a multiplication by the element size. This warning is enabled by -Wall.
• -Wmemset-transposed-args — warns for suspicious calls to the memset built-in function where the second argument is not zero and the third argument is zero. For example, the call memset (buf, sizeof buf, 0) is diagnosed because memset (buf, 0, sizeof buf) was meant instead. The diagnostic is only emitted if the third argument is a literal zero. Otherwise, if it's an expression that is folded to zero, or a cast of zero to some type, it's far less likely that the arguments have been mistakenly transposed and no warning is emitted. This warning is enabled by -Wall.
• -Waddress — warns about suspicious uses of memory addresses. These include using the address of a function in a conditional expression, such as void func(void); if (func), and comparisons against the memory address of a string literal, such as if (x == "abc"). Such uses typically indicate a programmer error: the address of a function always evaluates to true, so their use in a conditional usually indicates that the programmer forgot the parentheses in a function call; and comparisons against string literals result in unspecified behavior and are not portable in C, so they usually indicate that the programmer intended to use strcmp. This warning is enabled by -Wall.
• -Wno-address-of-packed-member — do not warn when the address of packed member of 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 to be expected. Also warns when the operands of a logical operator are the same:

                   extern int a;
                   if (a < 0 && a < 0) { ... }

• -Wlogical-not-parentheses — okay so this one is so nosy in the best way 💅 it warns you when a logical ! gets 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, bestie — it's literally just here to catch shady-looking code like this:

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

Good news though — you can silence the warning by wrapping the LHS into parentheses, like:

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

This warning is 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. no cap.)
• -Wno-aggressive-loop-optimizations — warns if, in a loop with a constant number of iterations, the compiler detects undefined behavior in some statement during one or more of the iterations. 💀
• -Wno-attributes — do not warn if an unexpected "__attribute__" is used — like unrecognized attributes, function attributes applied to variables, etc. Heads up though: this does not stop errors for incorrect use of supported 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. When -Wextra is specified, 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 avoid the warning, include the appropriate header to bring the prototypes of built-in functions into scope. ✨

For example, the call to "memset" below is diagnosed by the warning 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__". 🦋
• -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 — so like, you get a pass if you declared it properly first, bestie.)
• -Wold-style-declaration (C and Objective-C only) — warns for obsolescent usages, according to the C Standard, in a declaration. For example, warn if storage-class specifiers like "static" are not the first things in a declaration. This warning is also enabled by -Wextra. 👑
• -Wold-style-definition (C and Objective-C only) — warns if an old-style function definition is used. A warning is given even if there's a previous prototype. A definition using () is not considered an old-style definition in C2X mode, because it is equivalent to (void) in that case — but it is considered an old-style definition for older standards. 😭
• -Wmissing-parameter-type (C and Objective-C only) — a function parameter is declared without a type specifier in K&R-style functions:

        void foo(bar) { }

This warning is also enabled by -Wextra. 🌸

• -Wmissing-prototypes (C and Objective-C only) — warns if a global function is defined without a previous prototype declaration. This warning is issued even if the definition itself provides a prototype. Use this option to detect global functions that don't have a matching prototype declaration in a header file. Important: 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-declarations — warns if a global function is defined without a previous declaration. Does so even if the definition itself provides a prototype. Use this option to detect global functions that aren't declared in header files. In C, no warnings are issued for functions with previous non-prototype declarations — use -Wmissing-prototypes to detect missing prototypes instead. In C++, no warnings are issued for function templates, or for inline functions, or for functions in anonymous namespaces. 🎀
• -Wmissing-field-initializers — warns if a structure's initializer has some fields missing. For example, this code causes such a warning, because "x.h" is implicitly zero:

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

This option does not warn about designated initializers, so the following modification doesn't trigger a warning:

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

In C, this option does not warn about the universal zero initializer { 0 }:

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

Likewise, in C++, this option does not warn about the empty { } initializer, for example:

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

This warning is included in -Wextra. To get other -Wextra warnings without this one, use -Wextra -Wno-missing-field-initializers. 🫶

• -Wno-multichar — do not warn if a multicharacter constant ('FOOF') is used. Usually they indicate a typo in the user's code, because they have implementation-defined values, and should not be used in portable code. fr fr. 💀
• -Wnormalized=[none|id|nfc|nfkc] — okay buckle up bestie, this one's a whole journey 🌈 In ISO C and ISO C++, two identifiers are different if they are different sequences of characters. However, sometimes when characters outside the basic ASCII character set are used, you can have two different character sequences that look the same. To avoid confusion, the ISO 10646 standard sets out some normalization rules which, when applied, ensure that two sequences that look the same are turned into the same sequence. GCC can warn you if you'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 is not in the ISO 10646 "C" normalized form, NFC. NFC is the recommended form for most uses. It is equivalent to -Wnormalized.

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

You can switch the warning off for all characters by writing -Wnormalized=none or -Wno-normalized. You should only do this if you are using some other normalization scheme (like "D"), because otherwise you can easily create bugs that are literally impossible to see. 😭

Some characters in ISO 10646 have distinct meanings but look identical in some fonts or display methodologies, especially once formatting has been applied. For instance "\u207F", "SUPERSCRIPT LATIN SMALL LETTER N", displays just like a regular "n" that has been placed in a superscript. ISO 10646 defines the NFKC normalization scheme to convert all these into a standard form as well, and GCC warns if your code is not in NFKC if you use -Wnormalized=nfkc. This warning is comparable to warning about every identifier that contains the letter O because it might be confused with the digit 0 — and so it is not the default, but it may be useful as a local coding convention if the programming environment cannot be fixed to display these characters distinctly. ✨

• -Wno-attribute-warning — do not warn about usage of functions declared with "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. 💅
• -Wno-deprecated-declarations — do not warn about uses of functions, variables, and types marked as deprecated by using the "deprecated" attribute. 🙈
• -Wno-overflow — do not warn about compile-time overflow in constant expressions. 🔥
• -Wno-odr — warns about One Definition Rule violations during link-time optimization. Enabled by default. 👑
• -Wopenmp-simd — warns 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) — warns if an initialized field without side effects is overridden when using designated initializers.

This warning is included in -Wextra. To get other -Wextra warnings without this one, use -Wextra -Wno-override-init. 🫶

• -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 — warns if a structure is given the packed attribute, but the packed attribute has no effect on the layout or size of the structure. Such structures may be mis-aligned for little benefit. For instance, in this code, the variable "f.x" in "struct bar" is misaligned even though "struct bar" does not itself have the packed attribute:

        struct foo {
          int x;
          char a, b, c, d;
        } __attribute__((packed));
        struct bar {
          char z;
          struct foo f;
        };

• -Wnopacked-bitfield-compat — okay bestie, story time 📖✨. The 4.1, 4.2 and 4.3 GCC eras literally ignored the packed attribute on bit-fields of type char. Rude, honestly. It got fixed in GCC 4.4, but that glow-up changed the structure layout, so GCC will inform you when the offset of such a field has changed in GCC 4.4. Like, there's no longer a 4-bit padding between field a and b in this lil struct:

struct foo
{
  char a:4;
  char b:8;
} __attribute__ ((packed));

This warning is on by default. Use -Wno-packed-bitfield-compat to turn it off. 💅

• -Wpacked-not-aligned (C, C++, Objective-C and Objective-C++ only) — warns you when a structure field with explicitly specified alignment inside a packed struct or union ends up 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 — warns you if padding got slipped into a structure, either to align an element of the structure or to align the whole structure. Sometimes when this happens you can literally rearrange the fields to reduce the padding and make the structure smaller. slay 🔥
• -Wredundant-decls — warns you if anything is declared more than once in the same scope, even when the multiple declaration is valid and changes nothing. like girl we heard you the first time 💀
• -Wrestrict — warns 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 is included in -Wall. ✨

• -Wnested-externs (C and Objective-C only) — warns if an extern declaration shows up inside a function. not the vibe 🙈
• -Winline — warns if a function that is declared as inline cannot be inlined. Even with this option, the compiler does not warn about failures to inline functions declared in system headers.

The compiler uses a variety of heuristics to determine whether or not to inline a function. For example, the compiler takes into account the size of the function being inlined and the amount of inlining that has already been done in the current function. Therefore, seemingly insignificant changes in the source program can cause the warnings produced by -Winline to appear or disappear. chaotic, but that's the algorithm's main character energy 🦋

• -Wint-in-bool-context — warns for suspicious use of integer values where boolean values are expected, such as conditional expressions (?:) using non-boolean integer constants in boolean context, like if (a <= b ? 2 : 3). Or left shifting of signed integers in boolean context, like for (a = 0; 1 << a; a++);. Likewise for all kinds of multiplications regardless of the data type. This warning is enabled by -Wall. ⭐
• -Wno-int-to-pointer-cast — 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 an error. 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 found in the search path but cannot be used. it's giving "found but unbothered" 😭
• -Wlong-long — warns if long long type is used. This is enabled by either -Wpedantic or -Wtraditional in ISO C90 and C++98 modes. To inhibit the warning messages, use -Wno-long-long.
• -Wvariadic-macros — warns if variadic macros are used in ISO C90 mode, or if the GNU alternate syntax is used in ISO C99 mode. This is enabled by either -Wpedantic or -Wtraditional. To inhibit the warning messages, use -Wno-variadic-macros.
• -Wno-varargs — do not 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 vector operation is not implemented via SIMD capabilities of the architecture. Mainly useful for the performance tuning 💅. Vector operation can be implemented piecewise, which means that the scalar operation is performed on every vector element; in parallel, which means that the vector operation is implemented using scalars of wider type, which normally is more performance efficient; and as a single scalar, which means that vector fits into a scalar type.
• -Wvla — warns if a variable-length array is used in the code. -Wno-vla prevents the -Wpedantic warning of the variable-length array.
• -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. This is 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.

-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 the 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. In addition, 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]);
void f (int, int[]);     // warning: argument 2 previously declared as a VLA

void g (int n)
{
    if (n > 4)
      return;
    int a[n];
    f (sizeof a, a);     // warning: access to a by f may be out of bounds
  ...
}

-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. 😤
• -Wdisabled-optimization — warns if a requested optimization pass is disabled. This warning does not generally indicate that there is anything wrong with your code; it merely indicates that GCC's optimizers are unable to handle the code effectively. Often, the problem is that your code is too big or too complex; GCC refuses to optimize programs when the optimization itself is likely to take inordinate amounts of time. not a you problem, it's a compiler-bandwidth problem 💀
• -Wpointer-sign (C and Objective-C only) — warns for pointer argument passing or assignment with different signedness. This option is only supported for C and Objective-C. It is implied by -Wall and by -Wpedantic, which can be disabled with -Wno-pointer-sign.
• -Wstack-protector — this option is only active when -fstack-protector is active. It warns about functions that are not protected against stack smashing. protect ya neck 🛡️
• -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 that are much longer than the standard's minimum limit, but very portable programs should avoid using longer strings.

The limit applies after string constant concatenation, and does not count the trailing NUL. In C90, the limit was 509 characters; in C99, it was raised to 4095. C++98 does not specify a normative minimum maximum, so we do not diagnose overlength strings in C++.

This option is implied by -Wpedantic, and can be disabled with -Wno-overlength-strings.

• -Wunsuffixed-float-constants (C and Objective-C only) — 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 the link-time optimization, do not 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 — this option enables an static analysis of program flow which looks for interesting interprocedural paths through the code, and issues warnings for problems found on them. it's the compiler putting on its detective hat and reading your code like a group chat 🔍👀

okay so this analysis is <strong>so much more expensive</strong> than the other GCC warnings, bestie — like, she is doing the MOST and she will take her sweet time doing it 💅

Turning this option on effectively turns on all of these, no cap:

-Wanalyzer-double-fclose -Wanalyzer-double-free
-Wanalyzer-exposure-through-output-file -Wanalyzer-file-leak
-Wanalyzer-free-of-non-heap -Wanalyzer-malloc-leak
-Wanalyzer-mismatching-deallocation
-Wanalyzer-possible-null-argument
-Wanalyzer-possible-null-dereference -Wanalyzer-null-argument
-Wanalyzer-null-dereference -Wanalyzer-shift-count-negative
-Wanalyzer-shift-count-overflow -Wanalyzer-stale-setjmp-buffer
-Wanalyzer-tainted-array-index
-Wanalyzer-unsafe-call-within-signal-handler
-Wanalyzer-use-after-free
-Wanalyzer-use-of-pointer-in-stale-stack-frame
-Wanalyzer-write-to-const -Wanalyzer-write-to-string-literal

heads up bestie: this option is <strong>only available if GCC was configured with analyzer support enabled</strong>. no analyzer support, no analyzer warnings. periodt.

---

🫶 the analyzer options, one by one 🫶

• -Wanalyzer-too-complex — if -fanalyzer is on, the analyzer busts out all her little heuristics to try to explore the control flow and data flow in your program, but sufficiently complicated code can just… defeat her 😭 by default the analysis <em>silently</em> stops when the code is too complicated for her to fully explore and she hits an internal limit. -Wanalyzer-too-complex is the one that actually <strong>warns you</strong> when that happens.
• -Wno-analyzer-double-fclose — this warning <strong>requires -fanalyzer</strong>, which enables it; use -Wno-analyzer-double-fclose to turn it off. This diagnostic warns about paths through the code where a "FILE *" can have fclose called on it <strong>more than once</strong>. that's a no from me ✋
• -Wno-analyzer-double-free — this warning <strong>requires -fanalyzer</strong>, which enables it; use -Wno-analyzer-double-free to disable it. Warns about paths where a pointer can have a deallocator called on it more than once — either free, or a deallocator referenced by attribute malloc.
• -Wno-analyzer-exposure-through-output-file — this warning <strong>requires -fanalyzer</strong>, which enables it; use -Wno-analyzer-exposure-through-output-file to disable it. Warns about paths where a security-sensitive value gets written to an output file — think writing a password to a log file 🙈 <span style="color:pink">(the manpage is literally saying: not the password in the log, babes)</span>
• -Wno-analyzer-file-leak — this warning <strong>requires -fanalyzer</strong>, which enables it; use -Wno-analyzer-file-leak to disable it. Warns about paths where a "<stdio.h>" "FILE *" stream object is leaked. she's just… gone. 💀
• -Wno-analyzer-free-of-non-heap — this warning <strong>requires -fanalyzer</strong>, which enables it; use -Wno-analyzer-free-of-non-heap to disable it. Warns about paths where free is called on a non-heap pointer — like an on-stack buffer, or a global. girl what are you doing 😭
• -Wno-analyzer-malloc-leak — this warning <strong>requires -fanalyzer</strong>, which enables it; use -Wno-analyzer-malloc-leak to disable it. Warns about paths where a pointer allocated via an allocator is leaked: either malloc, or a function marked with attribute malloc.
• -Wno-analyzer-mismatching-deallocation — this warning <strong>requires -fanalyzer</strong>, which enables it; use -Wno-analyzer-mismatching-deallocation to disable it. Warns about paths where the <strong>wrong deallocation function</strong> is called on a pointer value, based on which function was used to allocate that pointer value. The diagnostic will warn about mismatches between free, scalar delete and vector delete[], and those marked as allocator/deallocator pairs using attribute malloc. it's giving "matching set or nothing" ✨
• -Wno-analyzer-possible-null-argument — this warning <strong>requires -fanalyzer</strong>, which enables it; use -Wno-analyzer-possible-null-argument to disable it. Warns about paths where a possibly-NULL value is passed to a function argument marked with __attribute__((nonnull)) as requiring a non-NULL value.
• -Wno-analyzer-possible-null-dereference — this warning <strong>requires -fanalyzer</strong>, which enables it; use -Wno-analyzer-possible-null-dereference to disable it. Warns about paths where a possibly-NULL value is dereferenced.
• -Wno-analyzer-null-argument — this warning <strong>requires -fanalyzer</strong>, which enables it; use -Wno-analyzer-null-argument to disable it. Warns about paths where a value <em>known</em> to be NULL is passed to a function argument marked with __attribute__((nonnull)) as requiring a non-NULL value.
• -Wno-analyzer-null-dereference — this warning <strong>requires -fanalyzer</strong>, which enables it; use -Wno-analyzer-null-dereference to disable it. Warns about paths where a value <em>known</em> to be NULL is dereferenced.
• -Wno-analyzer-shift-count-negative — this warning <strong>requires -fanalyzer</strong>, which enables it; use -Wno-analyzer-shift-count-negative to disable it. Warns about paths where a shift is attempted with a <strong>negative count</strong>. It's analogous to the -Wshift-count-negative diagnostic implemented in the C/C++ front ends, but it's implemented based on analyzing <strong>interprocedural paths</strong>, rather than merely parsing the syntax tree. However, the analyzer does not prioritize detection of such paths, so <strong>false negatives are more likely</strong> relative to other warnings.
• -Wno-analyzer-shift-count-overflow — this warning <strong>requires -fanalyzer</strong>, which enables it; use -Wno-analyzer-shift-count-overflow to disable it. Warns about paths where a shift is attempted with a count <strong>greater than or equal to the precision of the operand's type</strong>. It's analogous to the -Wshift-count-overflow diagnostic implemented in the C/C++ front ends, but it's implemented based on analyzing <strong>interprocedural paths</strong>, rather than merely parsing the syntax tree. However, the analyzer does not prioritize detection of such paths, so <strong>false negatives are more likely</strong> relative to other warnings.
• -Wno-analyzer-stale-setjmp-buffer — this warning <strong>requires -fanalyzer</strong>, which enables it; use -Wno-analyzer-stale-setjmp-buffer to disable it. Warns about paths where longjmp is called to rewind to a jmp_buf relating to a setjmp call in a function that has <strong>already returned</strong>.

When setjmp is called on a jmp_buf to record a rewind location, it records the stack frame. That stack frame becomes invalid the moment the function containing the setjmp call returns. Trying to rewind to it via longjmp would reference a stack frame that <strong>no longer exists</strong>, and likely lead to a crash (or worse). or worse is doing a LOT of heavy lifting in that sentence and I respect it 💀

• -Wno-analyzer-tainted-array-index — this warning <strong>requires both -fanalyzer and -fanalyzer-checker=taint</strong> to enable it; use -Wno-analyzer-tainted-array-index to disable it. Warns about paths where a value that could be under an attacker's control is used as the <strong>index of an array access</strong> without being sanitized. 🚨
• -Wno-analyzer-unsafe-call-within-signal-handler — this warning <strong>requires -fanalyzer</strong>, which enables it; use -Wno-analyzer-unsafe-call-within-signal-handler to disable it. Warns about paths where a function known to be <strong>async-signal-unsafe</strong> (such as fprintf) is called from a signal handler.
• -Wno-analyzer-use-after-free — this warning <strong>requires -fanalyzer</strong>, which enables it; use -Wno-analyzer-use-after-free to disable it. Warns about paths where a pointer is used <strong>after</strong> a deallocator is called on it: either free, or a deallocator referenced by attribute malloc.
• -Wno-analyzer-use-of-pointer-in-stale-stack-frame — this warning <strong>requires -fanalyzer</strong>, which enables it; use -Wno-analyzer-use-of-pointer-in-stale-stack-frame to disable it. Warns about paths where a pointer is dereferenced that points to a variable in a <strong>stale stack frame</strong>.
• -Wno-analyzer-write-to-const — this warning <strong>requires -fanalyzer</strong>, which enables it; use -Wno-analyzer-write-to-const to disable it. Warns about paths where the analyzer detects an attempt to write through a pointer to a const object. However, the analyzer does not prioritize detection of such paths, so <strong>false negatives are more likely</strong> relative to other warnings.
• -Wno-analyzer-write-to-string-literal — this warning <strong>requires -fanalyzer</strong>, which enables it; use -Wno-analyzer-write-to-string-literal to disable it. Warns about paths where the analyzer detects an attempt to write through a pointer to a <strong>string literal</strong>. However, the analyzer does not prioritize detection of such paths, so <strong>false negatives are more likely</strong> relative to other warnings.

---

the pertinent parameters for controlling the exploration are: --param analyzer-bb-explosion-factor=value, --param analyzer-max-enodes-per-program-point=value, --param analyzer-max-recursion-depth=value, and --param analyzer-min-snodes-for-call-summary=value.

The following options control the analyzer. 👑

• -fanalyzer-call-summaries — simplify interprocedural analysis by <strong>computing the effect of certain calls</strong>, rather than exploring all paths through the function from callsite to each possible return. If enabled, call summaries are only used for functions with <strong>more than one call site</strong>, and that are sufficiently complicated (as per --param analyzer-min-snodes-for-call-summary=value). efficiency queen fr fr ✨
• -fanalyzer-checker=name — restrict the analyzer to run <strong>just the named checker</strong>, and enable it. Some checkers are disabled by default (even with -fanalyzer), such as the taint checker that implements -Wanalyzer-tainted-array-index, and <strong>this option is required to enable them</strong>. so like… she's shy, you gotta invite her personally 🎀
• -fno-analyzer-feasibility — this option is intended for <strong>analyzer developers</strong>. By default the analyzer verifies that there is a feasible control flow path for each diagnostic it emits: that the conditions that hold are <strong>not mutually exclusive</strong>. Diagnostics for which no feasible path can be found are <strong>rejected</strong>. This filtering can be suppressed with -fno-analyzer-feasibility, for debugging issues in this code.
• -fanalyzer-fine-grained — this option is intended for <strong>analyzer developers</strong>.

Internally the analyzer builds an "exploded graph" that combines control flow graphs with data flow information. iconic architecture fr 💅

By default, an edge in this graph can contain the effects of a run of multiple statements within a basic block. With -fanalyzer-fine-grained, each statement gets its own edge. ✨

• -fanalyzer-show-duplicate-count — ngl this one is for analyzer developers 👩‍💻. If multiple diagnostics got 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. receipts on the receipts 🧾
• -fno-analyzer-state-merge — bestie this option is intended for analyzer developers.

By default the analyzer attempts to simplify analysis by merging sufficiently similar states at each program point as it builds its "exploded graph". With -fno-analyzer-state-merge this merging can be suppressed, for debugging state-handling issues. 🔍

• -fno-analyzer-state-purge — yeah this one? also for analyzer developers 🫶

By default the analyzer attempts to simplify analysis by purging aspects of state at a program point that appear to no longer be relevant, e.g. the values of locals that aren't accessed later in the function and which aren't relevant to leak analysis.

With -fno-analyzer-state-purge this purging of state can be suppressed, for debugging state-handling issues. 🧹

• -fanalyzer-transitivity — this option enables transitivity of constraints within the analyzer. constraints knowing constraints, it's giving social network 💖
• -fanalyzer-verbose-edges — intended for analyzer developers. It enables more verbose, lower-level detail in the descriptions of control flow within diagnostic paths. 🛣️
• -fanalyzer-verbose-state-changes — also for analyzer developers. It enables more verbose, lower-level detail in the descriptions of events relating to state machines within diagnostic paths. 🔄
• -fanalyzer-verbosity=level — this one controls the complexity of the control flow paths that are emitted for analyzer diagnostics. vibe check on how much it yaps 📢

The level can be one of:

0. At this level, interprocedural call and return events are displayed, along with the most pertinent state-change events relating to a diagnostic. For example, for a double-"free" diagnostic, both calls to "free" will be shown.
1. As per the previous level, but also show events for the entry to each function.
2. As per the previous level, but also show events relating to control flow that are significant to triggering the issue (e.g. "true path taken" at a conditional).

This level is the default. ⭐

3. As per the previous level, but show all control flow events, not just significant ones. 💀
4. This level is intended for analyzer developers; it adds various other events intended for debugging the analyzer.

• -fdump-analyzer — dumps internal details about what the analyzer is doing to file.analyzer.txt. This option is overridden by -fdump-analyzer-stderr. 📄
• -fdump-analyzer-stderr — dumps internal details about what the analyzer is doing to stderr. This option overrides -fdump-analyzer. stderr said move over 🫣
• -fdump-analyzer-callgraph — dumps a representation of the call graph suitable for viewing with GraphViz to file.callgraph.dot. 🌐
• -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. 🎨
• -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. 😭 your filesystem will feel it
• -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 and file.*.tg.dot. 🧭
• -fdump-analyzer-json — dumps a compressed JSON representation of analyzer internals to file.analyzer.json.gz. The precise format is subject to change. don't get attached to it bestie 🍓
• -fdump-analyzer-state-purge — as per -fdump-analyzer-supergraph, dumps a representation of the "supergraph" suitable for viewing with GraphViz, but annotates the graph with information on what state will be purged at each node. The graph is written to file.state-purge.dot. 🧼
• -fdump-analyzer-supergraph — dumps representations of the "supergraph" suitable for viewing with GraphViz to file.supergraph.dot and to file.supergraph-eg.dot. These show 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" and diagnostics associated with them. 🕸️

---

🐛 Options for Debugging Your Program 🐛

To tell GCC to emit extra information for use by a debugger, in almost all cases you need only to add -g to your other options. one flag, so simple, we love her 💖

GCC allows you to use -g with -O. but hear me out — the shortcuts taken by optimized code may occasionally be surprising: some variables you declared may not exist at all; flow of control may briefly move where you did not expect it; some statements may not be executed because they compute constant results or their values are already at hand; some statements may execute in different places because they have been moved out of loops. 😵‍💫 Nevertheless it is possible to debug optimized output. This makes it reasonable to use the optimizer for programs that might have bugs.

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. 👑

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

On most systems that use stabs format, -g 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. not a drill, other debuggers may simply give up 💀 If you want to control for certain whether to generate the extra information, use -gstabs+, -gstabs, -gxcoff+, -gxcoff, or -gvms (see below).

• -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. ✨
• -gdwarf
• -gdwarf-version — produces 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/Mac OS X, 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. ⬆️

GCC no longer supports DWARF Version 1, which is substantially different than Version 2 and later. gone, discontinued, no longer with us 🪦 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. 🙈

• -gstabs — produces debugging information in stabs format (if that is supported), without GDB extensions. This is the format used by DBX on most BSD systems. On MIPS, Alpha and System V Release 4 systems this option produces stabs debugging output that is not understood by DBX. On System V Release 4 systems this option requires the GNU assembler. 🧐
• -gstabs+ — produces debugging information in stabs format (if that is supported), using GNU extensions understood only by the GNU debugger (GDB). The use of these extensions is likely to make other debuggers crash or refuse to read the program. periodt, consider yourself warned 😭
• -gxcoff — produces debugging information in XCOFF format (if that is supported). This is the format used by the DBX debugger on IBM RS/6000 systems. 💜
• -gxcoff+ — produces debugging information in XCOFF format (if that is supported), using GNU extensions understood only by the GNU debugger (GDB). The use of these extensions is likely to make other debuggers crash or refuse to read the program, and may cause assemblers other than the GNU assembler (GAS) to fail with an error. 🫠
• -gvms — produces debugging information in Alpha/VMS debug format (if that is supported). This is the format used by DEBUG on Alpha/VMS systems. 🌸
• -glevel
• -ggdblevel
• -gstabslevel
• -gxcofflevel
• -gvmslevel — request debugging information and also use level to specify how much information. The default level is 2. ⭐

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

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

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

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. 🎀

• -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. 🦋
• -femit-class-debug-always — okay so normally, for a C++ class, GCC puts the debugging info in just one object file. This flag says "nah, put it in all the object files that use the class." 💅

Use this only with debuggers that can't handle the way GCC normally emits class debugging info, because bestie… it can blow up the size of your debugging info by as much as a factor of two. 😭

• -fno-merge-debug-strings — tells the linker to not merge together strings in the debugging info that are identical across different object files.

Fair warning: merging isn't supported by all assemblers or linkers. When it does work, merging shrinks the debug info in the output file, at the cost of making link processing take longer. And it's enabled by default, so this flag is the "turn it off" vibe. ✨

• -fdebug-prefix-map=old=new — when you're compiling files that live in directory old, this records the debugging info as if they actually lived in directory new instead. Iconic for swapping a build-time path for an install-time path in the debug info. 🦋

You can also flip an absolute path to a relative one by using . for new. This can give you more reproducible builds that are location independent, but it may require an extra command to tell GDB where to find the source files. See also -ffile-prefix-map.

• -fvar-tracking — runs the variable tracking pass, which computes where variables are stored at each position in the code. Then better debugging info gets generated (as long as the debugging info format supports that kind of info). 💖

It's enabled by default when you compile with optimization (-Os, -O, -O2, ...), debugging information (-g), and the debug info format supports it.

• -fvar-tracking-assignments — annotates assignments to user variables early in compilation and tries to carry those annotations allll the way through to the end, in an attempt to improve debug info while optimizing. Using -gdwarf-4 along with it is recommended. 🎀

It can be enabled even if var-tracking is disabled, in which case the annotations get created and maintained, but discarded at the end. By default this flag is enabled together with -fvar-tracking, except when selective scheduling is enabled.

• -gsplit-dwarf — if DWARF debugging info is enabled, this separates as much debugging info as possible into a separate output file with the extension .dwo. Lets the build system avoid linking files that have debug info in them. 🧚

To actually be useful, this option requires a debugger capable of reading .dwo files. No cap.

• -gdwarf32
• -gdwarf64 — if DWARF debugging info is enabled, -gdwarf32 selects the 32-bit DWARF format and -gdwarf64 selects the 64-bit DWARF format. The default is target specific, but on most targets it's -gdwarf32.

The 32-bit DWARF format is smaller, but it can't support more than 2GiB of debug info in any of the DWARF debug info sections. The 64-bit DWARF format allows larger debug info and might not be well supported by all consumers yet. 🙈

• -gdescribe-dies — adds description attributes to some DWARF DIEs that have no name attribute, like artificial variables, external references and call site parameter DIEs. ✨
• -gpubnames — generates the DWARF .debug_pubnames and .debug_pubtypes sections.
• -ggnu-pubnames — generates .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 you're using DWARF Version 4 or higher, type DIEs can be put into their own .debug_types section instead of being part of the .debug_info section. It's more efficient to put them in a separate comdat section, since the linker can then remove duplicates. 💅

But plot twist: not all DWARF consumers support .debug_types sections yet, and on some objects .debug_types produces larger instead of smaller debugging info. 😭

• -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 info. The options are concatenated with spaces separating them from each other and from the compiler version. It's enabled by default. See also -frecord-gcc-switches for another way of storing compiler options into the object file.
• -gstrict-dwarf — disallows using extensions of a later DWARF standard version than the one selected with -gdwarf-version. On most targets, using non-conflicting DWARF extensions from later standard versions is allowed.
• -gno-strict-dwarf — allows using extensions of a later DWARF standard version than selected with -gdwarf-version. (Yep, it's the flip side of the one above. 🦋)
• -gas-loc-support — informs the compiler that the assembler supports .loc directives. It may then use them so the assembler generates DWARF2+ line number tables.

This is generally desirable, because assembler-generated line-number tables are a lot more compact than the ones 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 — forces GCC to generate DWARF2+ line number tables internally, if DWARF2+ line number tables are to be generated.
• -gas-locview-support — informs the compiler that the assembler supports "view" assignment and reset assertion checking in .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 — forces GCC to assign view numbers internally, if -gvariable-location-views is explicitly requested.
• -gcolumn-info
• -gno-column-info — emits location column information into DWARF debugging info, rather than just file and line. This option is enabled by default.
• -gstatement-frontiers
• -gno-statement-frontiers — 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. Enabled by default when compiling with optimization (-Os, -O, -O2, ...), and outputting DWARF 2 debug info at the normal level. ✨
• -gvariable-location-views
• -gvariable-location-views=incompat5
• -gno-variable-location-views — augments variable location lists with progressive view numbers implied from the line number table. This lets debug info consumers inspect state at certain points of the program, even if no instructions associated with the corresponding source locations are present at that point. If the assembler lacks support for view numbers in line number tables, this causes the compiler to emit the line number table, which generally makes them somewhat less compact. 🎀

The augmented line number tables and location lists are fully backward-compatible, so they can be consumed by debug info consumers that aren't aware of these augmentations — but those consumers won't derive any benefit from them either.

This is enabled by default when outputting DWARF 2 debug info at the normal level, as long as there's assembler support, -fvar-tracking-assignments is enabled and -gstrict-dwarf is not. When assembler support isn't available, this may still be enabled, but it will force GCC to output internal line number tables, and if -ginternal-reset-location-views isn't enabled, that will most certainly lead to silently mismatching location views. 💀

There's a proposed representation for view numbers that is not backward compatible with the location list format introduced in DWARF 5, which can be enabled with -gvariable-location-views=incompat5. This option may be removed in the future and is only provided as a reference implementation of the proposed representation. Debug info consumers aren't expected to support this extended format, and they'd be rendered unable to decode location lists using it.

• -ginternal-reset-location-views
• -gno-internal-reset-location-views — attempts 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 doesn't 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 — generates extended debug info 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 info. 🦋

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's only enabled by default if location views are enabled.

• -gz[=type] — produces compressed debug sections in DWARF format, if that's supported. If type isn't given, the default type depends on the capabilities of the assembler and linker used. type may be one of:
  • none — don't compress debug sections
  • zlib — use zlib compression in ELF gABI format
  • zlib-gnu — use zlib compression in traditional GNU format

If the linker doesn't support writing compressed debug sections, the option is rejected. Otherwise, if the assembler doesn't support them, -gz is silently ignored when producing object files. 💅

• -femit-struct-debug-baseonly — emits debug info for struct-like types only when the base name of the compilation source file matches the base name of the file in which the struct is defined.

This option substantially reduces the size of debugging info, but at significant potential loss in type information to the debugger. See -femit-struct-debug-reduced for a less aggressive option, and see -femit-struct-debug-detailed for more detailed control.

Heads up bestie: this option works only with DWARF debug output. 🔥

• -femit-struct-debug-reduced — bestie, this one's a glow-up for your debug info size 💅 it only emits debug info for struct-ish types when the base name of the compilation source file matches the base name of the file where that type is defined — unless the struct is a template or it lives in a system header.

The way this significantly shrinks your debugging info… iconic. But there's a tradeoff, and I'm not gonna lie to you: you can lose some type information as far as the debugger is concerned. 🙈

Want something more aggressive? Go see -femit-struct-debug-baseonly. Want the whole detailed menu? -femit-struct-debug-detailed is right there.

Heads up: this option works only with DWARF debug output. No DWARF, no vibes. ✨

• -femit-struct-debug-detailed[=spec-list] — okay so THIS one lets you pick exactly which struct-like types get debug info generated. The whole intent? Cutting down on duplicate struct debug info between different object files in the same program. Teamwork, but make it efficient 🫶

It's the detailed version of -femit-struct-debug-reduced and -femit-struct-debug-baseonly — those two serve most needs, but this one is for when you want full control, main character energy only 👑

A specification has this syntax:

[dir:|ind:][ord:|gen:](any|sys|base|none)

The optional first word limits the spec to structs that are used directly (dir:) or used indirectly (ind:). A struct type is used directly when it's the type of a variable or member. Indirect uses show up through pointers to structs — like, when using an incomplete struct is valid, that use is indirect. Example time:

struct one direct; struct two * indirect;

The optional second word limits the spec to ordinary structs (ord:) or generic structs (gen:). Generic structs are… a bit of a saga to explain, ngl. For C++, these are non-explicit specializations of template classes, or non-template classes within the above. Other languages have generics too, but -femit-struct-debug-detailed doesn't implement them yet. 😭

The third word says which source files those structs have to come from for the compiler to emit debug info. The values none and any mean the normal thing. base means the base of the name of the file where the type declaration appears has to match the base of the name of the main compilation file — so practically, when you're compiling foo.c, debug info gets generated for types declared in that file and foo.h, but not other headers. And sys means those types satisfying base or declared in system or compiler headers.

Real talk: you may need to experiment to figure out the best settings for your application. Science is messy, it's fine 🔥

The default is -femit-struct-debug-detailed=all.

And again, bestie: this option works only with DWARF debug output. 🦋

• -fno-dwarf2-cfi-asm — emits DWARF unwind info as a compiler-generated .eh_frame section instead of using GAS .cfi_* directives. Different route, same destination 🚗
• -fno-eliminate-unused-debug-types — normally, when you're producing DWARF output, GCC avoids spitting out debug symbol output for types that are nowhere used in the source file being compiled. Efficient! But sometimes you actually want GCC to emit debugging information for all types declared in a compilation unit, regardless of whether they're used in that compilation unit — like when you're in the debugger and you want to cast a value to a type that isn't actually used in your program (but is declared). Very valid. 🙌

More often than not, though? This results in a significant amount of wasted space. So like… choose wisely 💀

---

🚀 OPTIONS THAT CONTROL OPTIMIZATION 🚀

These options control various sorts of optimizations. The whole vibe shift of your binary, basically.

Without any optimization option, the compiler's goal is to reduce the cost of compilation and to make debugging produce the expected results. Statements are independent: if you stop the program with a breakpoint between statements, you can then assign a new value to any variable or change the program counter to any other statement in the function and get exactly the results you expect from the source code. No surprises, no chaos, just truth ✨

Turning on optimization flags makes the compiler attempt to improve the performance and/or code size at the expense of compilation time and possibly the ability to debug the program. That's the trade, periodt 💅

The compiler performs optimization based on the knowledge it has of the program. Compiling multiple files at once to a single output file mode allows the compiler to use information gained from all of the files when compiling each of them. It's giving collaborative 🫶

Not all optimizations are controlled directly by a flag. Only optimizations that have a flag are listed in this section. Sorry to the flagless ones, they're just doing their thing backstage 🎭

Most optimizations are completely disabled at -O0 or if an -O level is not set on the command line, even if individual optimization flags are specified. Similarly, -Og suppresses many optimization passes. Like, you can ask for the individual thing and it still won't happen. Tragic but true 😭

Depending on the target and how GCC was configured, a slightly different set of optimizations may be enabled at each -O level than those listed here. You can invoke GCC with -Q --help=optimizers to find out the exact set of optimizations that are enabled at each level. Go check, bestie, don't just trust the list 📋

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

With -O, the compiler tries to reduce code size and execution time, without performing any optimizations that take a great deal of compilation time. Modest. Tasteful. Respectable. 🎀

-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 -fmerge-constants -fmove-loop-invariants
-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 do not involve a space-speed tradeoff. As compared to -O, this option increases both compilation time and the performance of the generated code. Glow up unlocked 🌈

-O2 turns on all optimization flags specified by -O. It also turns on the following optimization flags:

-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-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-pre
-ftree-switch-conversion  -ftree-tail-merge -ftree-vrp

Please note the warning under -fgcse about invoking -O2 on programs that use computed gotos. Not a drill ⚠️

NOTE: In Ubuntu 8.10 and later versions, -D_FORTIFY_SOURCE=2, in Ubuntu 24.04 and later versions, -D_FORTIFY_SOURCE=3, is set by default, and is activated when -O is set to 2 or higher. This enables additional compile-time and run-time checks for several libc functions. To disable, specify either -U_FORTIFY_SOURCE or -D_FORTIFY_SOURCE=0. 💖

• -O3 — Optimize yet more. -O3 turns on all optimizations specified by -O2 and also turns on the following optimization flags:

-fgcse-after-reload -fipa-cp-clone -floop-interchange
-floop-unroll-and-jam -fpeel-loops -fpredictive-commoning
-fsplit-loops -fsplit-paths -ftree-loop-distribution
-ftree-loop-vectorize -ftree-partial-pre -ftree-slp-vectorize
-funswitch-loops -fvect-cost-model -fvect-cost-model=dynamic
-fversion-loops-for-strides

• -O0 — Reduce compilation time and make debugging produce the expected results. This is the default. Chill mode, zero stress 🧘
• -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. Smol is beautiful 🍓

• -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.

Rules? We don't know her. But bestie, know what you're signing up for — "not valid for all standard-compliant programs" is doing a LOT of heavy lifting there 💀

• -Og — Optimize debugging experience. -Og should be the optimization level of choice for the standard edit-compile-debug cycle, offering a reasonable level of optimization while maintaining fast compilation and a good debugging experience. It's a better choice than -O0 for producing debuggable code because some compiler passes that collect debug information are disabled at -O0. The bestie who's got your back in the debugger 🫶

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  -fssa-phiopt -ftree-bit-ccp  -ftree-dse
-ftree-pta  -ftree-sra

If you use multiple -O options, with or without level numbers, the last such option is the one that is effective. Last one wins, no take-backs 👑

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. So like, you can always flip it yourself, iykyk 💅

The following options control specific optimizations. They are either activated by -O options or are related to ones that are. You can use the following flags in the rare cases when "fine-tuning" of optimizations to be performed is desired. Rare cases only, bestie — don't be out here micro-managing for no reason 🌸

• -fno-defer-pop — okay so for the machines that have to pop arguments after a function call, this makes them pop the args the second each function returns 🫡. At -O1 and higher, -fdefer-pop is the default instead, which lets the compiler just let arguments pile up on the stack across several function calls and pop them all in one go. Efficient, lowkey iconic ✨
• -fforward-propagate — runs a forward propagation pass on RTL. The pass tries to smoosh two instructions together and checks if the result can be simplified 💅. If loop unrolling is active, two passes happen and the second one is scheduled after loop unrolling. Enabled by default at optimization levels -O, -O2, -O3, -Os 👑
• -ffp-contract=style — the floating-point expression contraction menu, bestie:
  • -ffp-contract=off — disables floating-point expression contraction, periodt.
  • -ffp-contract=fast — enables floating-point expression contraction, like 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. Currently this is not implemented and treated equal to -ffp-contract=off. 🙈

The default is -ffp-contract=fast.

• -fomit-frame-pointer — omits the frame pointer in functions that don't need one. This dodges all the instructions to save, set up and restore the frame pointer, and on many targets it also frees up an extra register 🎀. On some targets this flag literally does nothing because the standard calling sequence always uses a frame pointer, so it can't be omitted — no cap. Also 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 -O and higher.
• -foptimize-sibling-calls — optimizes sibling and tail recursive calls. Enabled at levels -O2, -O3, -Os 💖
• -foptimize-strlen — optimizes various standard C string functions (e.g. strlen, strchr or strcpy) and their _FORTIFY_SOURCE counterparts into faster alternatives 🦋. Enabled at levels -O2, -O3.
• -fno-inline — do not expand any functions inline, apart from those marked with the always_inline attribute. This is the default when not optimizing. Single functions can be exempted from inlining by marking them with the noinline attribute 💅
• -finline-small-functions — 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 indirect calls too, the ones that get discovered to be known at compile time thanks to previous inlining 🧚. This option only has any effect 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're not declared inline. The compiler heuristically decides which functions are worth integrating this way 🤔. If all calls to a given function are integrated, and the function is declared static, then the function is normally not output as assembler code in its own right. Enabled at levels -O2, -O3, -Os. Also enabled by -fprofile-use and -fauto-profile ✨
• -finline-functions-called-once — considers all static functions called once for inlining into their caller, even if they're 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 and 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. 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 🔄. Enabled at levels -O2, -O3 and -Os.
• -finline-limit=n — by default, GCC limits the size of functions that can be inlined. This flag lets you have 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. so like, don't @ 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 — emit static functions into the object file, even if the function is never used 💅
• -fkeep-static-consts — emit variables declared static const when optimization isn't turned on, even if the variables aren't referenced 🌸. GCC enables this option by default. If you want to force the compiler to check if a variable is referenced, regardless of whether or not optimization is turned on, use the -fno-keep-static-consts option 👑
• -fmerge-constants — 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 -O, -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.

Warning: languages like C or C++ require each variable, including multiple instances of the same variable in recursive calls, to have distinct locations, so using this option results in non-conforming behavior 💀 not it, 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. iconic scheduling behavior fr fr.
• -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.
• -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 -O2, -O3, -Os ✨
• -fsplit-wide-types — when using a type that occupies multiple registers, such as long long on a 32-bit system, split the registers apart and allocate them independently 🍰. This normally generates better code for those types, but may make debugging more difficult 👀. Enabled at levels -O, -O2, -O3, -Os.
• -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 — bestie, in common subexpression elimination (CSE for short 💅), this makes CSE scan through jump instructions whenever the target of the jump isn't reached by any other path. Like, when CSE bumps into an if statement that has an else clause, CSE follows the jump when the condition being tested is false. Slay. Enabled at levels -O2, -O3, -Os.
• -fcse-skip-blocks — this one is giving similar to -fcse-follow-jumps, but it makes CSE follow jumps that conditionally skip over blocks. So when CSE encounters a simple if statement with no else clause, -fcse-skip-blocks makes CSE follow the jump around the body of the if. Enabled at levels -O2, -O3, -Os.
• -frerun-cse-after-loop — re-run common subexpression elimination after loop optimizations have been performed. Yes, a re-run, because sometimes you gotta do it twice ✨ Enabled at levels -O2, -O3, -Os.
• -fgcse — perform a global common subexpression elimination pass. This pass also does global constant and copy propagation, so it's kind of doing the most. Enabled at levels -O2, -O3, -Os.

Note: when compiling a program that uses computed gotos (a GCC extension, iykyk), you may get better run-time performance if you disable the global common subexpression elimination pass by adding -fno-gcse to the command line.

• -fgcse-lm — when this is enabled, global common subexpression elimination attempts to move loads that are only killed by stores into themselves. This lets a loop containing a load/store sequence get changed to a load outside the loop, plus a copy/store within the loop. Enabled by default when -fgcse is enabled.
• -fgcse-sm — when this is enabled, a store motion pass runs after global common subexpression elimination. This pass tries to move stores out of loops. When you use it together with -fgcse-lm, loops containing a load/store sequence can be changed to a load before the loop and a store after the loop. 🌈 Not enabled at any optimization level.
• -fgcse-las — when this is enabled, the global common subexpression elimination pass eliminates redundant loads that come after stores to the same memory location (both partial and full redundancies, no crumbs left 💅). Not enabled at any optimization level.
• -fgcse-after-reload — when this is enabled, a redundant load elimination pass is performed after reload. The whole purpose of this pass is to clean up redundant spilling. Enabled by -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 that loop code does not invoke undefined behavior, by for example causing signed integer overflows or out-of-bound array accesses. The bounds for the number of iterations of a loop are used to guide loop unrolling and peeling and loop exit test optimizations. This option is enabled by default.
• -funconstrained-commons — this 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.
• -fcrossjumping — perform cross-jumping transformation. This transformation unifies equivalent code and saves code size. The resulting code may or may not perform better than without cross-jumping, so, vibe check pending 😭 Enabled at levels -O2, -O3, -Os.
• -fauto-inc-dec — combine increments or decrements of addresses with memory accesses. This pass is always skipped on architectures that don't have instructions to support this. Enabled by default at -O and higher on architectures that support this.
• -fdce — perform dead code elimination (DCE) on RTL. Enabled by default at -O and higher.
• -fdse — perform dead store elimination (DSE) on RTL. Enabled by default at -O and higher.
• -fif-conversion — attempt to transform conditional jumps into branch-less equivalents. This includes use of conditional moves, min, max, set flags and abs instructions, and some tricks doable by standard arithmetics. The use of conditional execution on chips where it's available is controlled by -fif-conversion2. Enabled at levels -O, -O2, -O3, -Os, but not with -Og.
• -fif-conversion2 — use conditional execution (where available) to transform conditional jumps into branch-less equivalents. Enabled at levels -O, -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, which means two copies of the function 😭 With this option, the base and complete variants are changed to be thunks that call a common implementation. Enabled by -Os.
• -fdelete-null-pointer-checks — assume that programs cannot safely dereference null pointers, and that no code or data element resides at address zero. This option enables simple constant folding optimizations at all optimization levels. Plus, other optimization passes in GCC use this flag to control global dataflow analyses that eliminate useless checks for null pointers; these assume that a memory access to address zero always results in a trap, so that if a pointer is checked after it has already been dereferenced, it cannot be null.

Note though that in some environments this assumption is not true. Use -fno-delete-null-pointer-checks to disable this optimization for programs that depend on that behavior.

This option is enabled by default on most targets. On Nios II ELF, it defaults to off. On AVR, CR16, and MSP430, this option is completely disabled. Passes that use the dataflow information are enabled independently at different optimization levels.

• -fdevirtualize — attempt to convert calls to virtual functions to direct calls. This is done both within a procedure and interprocedurally as part of indirect inlining (-findirect-inlining) and interprocedural constant propagation (-fipa-cp). Enabled at levels -O2, -O3, -Os.
• -fdevirtualize-speculatively — attempt to convert calls to virtual functions to speculative direct calls. Based on the analysis of the type inheritance graph, determine for a given call the set of likely targets. If the set is small, preferably of size 1, change the call into a conditional deciding between direct and indirect calls. The speculative calls enable more optimizations, such as inlining. When they seem useless after further optimization, they are converted back into original form. Not me getting transformed and then un-transformed 💀
• -fdevirtualize-at-ltrans — stream extra information needed for aggressive devirtualization when running the link-time optimizer in local transformation mode. This option enables more devirtualization but significantly increases the size of streamed data. For this reason it is disabled by default.
• -fexpensive-optimizations — perform a number of minor optimizations that are relatively expensive. Enabled at levels -O2, -O3, -Os.
• -free — attempt to remove redundant extension instructions. This is especially helpful for the x86-64 architecture, which implicitly zero-extends in 64-bit registers after writing to their lower 32-bit half. Enabled for Alpha, AArch64 and x86 at levels -O2, -O3, -Os.
• -fno-lifetime-dse — in C++ the value of an object is only affected by changes within its lifetime: when the constructor begins, the object has an indeterminate value, and any changes during the lifetime of the object are dead when the object is destroyed. Normally dead store elimination will take advantage of this; if your code relies on the value of the object storage persisting beyond the lifetime of the object, you can use this flag to disable this optimization. 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. -flifetime-dse=0 is equivalent to -fno-lifetime-dse.
• -flive-range-shrinkage — attempt to decrease register pressure through register live range shrinkage. This is helpful for fast processors with small or moderate size register sets. 🧚
• -fira-algorithm=algorithm — use the specified coloring algorithm for the integrated register allocator. The algorithm argument can be priority, which specifies Chow's priority coloring, or CB, which specifies Chaitin-Briggs coloring. Chaitin-Briggs coloring is not implemented for all architectures, but for those targets that do support it, it is the default because it generates better code. 👑
• -fira-region=region — use specified regions for the integrated register allocator. The region argument should be one of the following:
  • all — use all loops as register allocation regions. This can give the best results for machines with a small and/or irregular register set.
  • mixed — use all loops except for loops with small register pressure as the regions. This value usually gives the best results in most cases and for most architectures, and is enabled by default when compiling with optimization for speed (-O, -O2, ...).
  • one — use all functions as a single region. This typically results in the smallest code size, and is enabled by default for -Os or -O0.
• -fira-hoist-pressure — use IRA to evaluate register pressure in the code hoisting pass for decisions to hoist expressions. This option usually results in smaller code, but it can slow the compiler down. This option is enabled at level -Os for all targets.
• -fira-loop-pressure — use IRA to evaluate register pressure in loops for decisions to move loop invariants. This option usually results in generation of faster and smaller code on machines with large register files (>= 32 registers), but it can slow the compiler down. This option is enabled at level -O3 for some targets.

Okay continuing the register-allocation + scheduling saga, bestie, and these next ones are so specific, but you asked for the full tour so here we go 💅

• -fno-ira-share-save-slots — turns OFF the sharing of stack slots that get used for saving call-used hard registers that live through a call. So every single hard register gets its own separate stack slot, which means… yeah, function stack frames get bigger. 🙈
• -fno-ira-share-spill-slots — same energy but for pseudo-registers. Turns off sharing of stack slots allocated for pseudos, so every pseudo-register that doesn't land a hard register gets its own private stack slot. Result: bigger function stack frames again. 💀
• -flra-remat — enables CFG-sensitive rematerialization in LRA. Instead of reloading values of spilled pseudos, LRA tries to rematerialize them (aka recalculate) when that's actually profitable. Enabled at levels -O2, -O3, -Os.
• -fdelayed-branch — if the target machine supports it, this tries to reorder instructions so it can exploit the instruction slots sitting there after delayed branch instructions. Enabled at levels -O, -O2, -O3, -Os, but NOT at -Og.
• -fschedule-insns — if the target machine supports it, attempt to reorder instructions so you stop stalling waiting on data that isn't ready yet. It's a lifesaver for machines with slow floating point or memory load instructions, because it lets other instructions get issued while you're still waiting on that load or FP result. Enabled at levels -O2, -O3.
• -fschedule-insns2 — basically -fschedule-insns but it asks for an extra pass of instruction scheduling after register allocation is done. Especially useful on machines with a fairly small number of registers, and where memory load instructions take more than one cycle. Enabled at levels -O2, -O3, -Os.
• -fno-sched-interblock — disables instruction scheduling across basic blocks. That scheduling is normally enabled when scheduling happens before register allocation, i.e. with -fschedule-insns or at -O2 or higher. 🚫
• -fno-sched-spec — disables speculative motion of non-load instructions, which is normally enabled when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.
• -fsched-pressure — enables register-pressure-sensitive insn scheduling before register allocation. Only makes sense when scheduling before register allocation is actually on, i.e. with -fschedule-insns or at -O2 or higher. Using it can improve the generated code and shrink it, by stopping register pressure from climbing above the number of available hard registers (and the spills that come after that in register allocation). 🫶
• -fsched-spec-load — allows speculative motion of some load instructions. Again, only makes sense when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.
• -fsched-spec-load-dangerous — allows speculative motion of more load instructions. Yes bestie, it literally says dangerous in the name. Same rules: 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 nothing gets moved prematurely, and -fsched-stalled-insns=0 means there's no limit on how many queued insns can be moved prematurely. -fsched-stalled-insns with no value at all 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. -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: this option is experimental, because not all machine descriptions used by GCC model the CPU closely enough to avoid unreliable results from the algorithm. Only makes sense when scheduling after register allocation, i.e. with -fschedule-insns2 or at -O2 or higher.
• -fsched-group-heuristic — turns on the group heuristic in the scheduler. It favors the instruction that belongs to a schedule group. On by default whenever scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.
• -fsched-critical-path-heuristic — turns on the critical-path heuristic. It favors instructions sitting on the critical path. On by default whenever scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.
• -fsched-spec-insn-heuristic — turns on the speculative instruction heuristic. It favors speculative instructions with greater dependency weakness. On by default whenever scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.
• -fsched-rank-heuristic — turns on the rank heuristic. It favors the instruction belonging to a basic block with greater size or frequency. On by default whenever scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.
• -fsched-last-insn-heuristic — turns on the last-instruction heuristic. It favors the instruction that's less dependent on the last instruction that got scheduled. On by default whenever scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.
• -fsched-dep-count-heuristic — turns on the dependent-count heuristic. It favors the instruction that has more instructions depending on it. On by default whenever scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.

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• -freschedule-modulo-scheduled-loops — modulo scheduling happens before traditional scheduling, and if a loop gets 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, but this time it runs instead of the second scheduler pass.
• -fsel-sched-pipelining — enables software pipelining of innermost loops during selective scheduling. This option does nothing unless one of -fselective-scheduling or -fselective-scheduling2 is turned on.
• -fsel-sched-pipelining-outer-loops — when pipelining loops during selective scheduling, also pipeline the outer loops. This one does nothing unless -fsel-sched-pipelining is turned on.
• -fsemantic-interposition — okay so some object formats, like ELF, let the dynamic linker interpose symbols. What that means 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. That feature is useful, like, for example to rewrite memory allocation functions with a debugging implementation — but it is expensive in terms of code quality. With -fno-semantic-interposition, the compiler assumes that if interposition happens for functions, the overwriting function will have precisely the same semantics (and side effects). Same deal for variables: if interposition happens, 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 — emit function prologues only before the parts of the function that actually need them, instead of dumping them at the top of the function. Enabled by default at -O and higher. 🎀
• -fshrink-wrap-separate — shrink-wrap the separate parts of the prologue and epilogue separately, so those parts only get executed when they're actually needed. This is on by default, but it has no effect unless -fshrink-wrap is also turned on and the target supports this.
• -fcaller-saves — enables allocating values to registers that get clobbered by function calls, by emitting extra instructions that save and restore those registers around such calls. That allocation only happens when it looks like it'll produce better code. This option is always enabled by default on certain machines — usually the ones that have no call-preserved registers to use instead. Enabled at levels -O2, -O3, -Os.
• -fcombine-stack-adjustments — tracks stack adjustments (pushes and pops) and stack memory references, then tries to find ways to combine them. Enabled by default at -O1 and higher.
• -fipa-ra — use caller save registers for allocation if those registers aren't used by any called function. In that case there's no need to save and restore them around calls. This is only possible if the called functions are part of the same compilation unit as the current function, and they get compiled before it. Enabled at levels -O2, -O3, -Os — but heads up, the option is disabled if the generated code is going to be instrumented for profiling (-p, or -pg), or if the callee's register usage can't be known exactly (this happens on targets that don't expose prologues and epilogues in RTL).
• -fconserve-stack — try to minimize stack usage. The compiler will attempt to use less stack space, even if that makes the program slower. This option implies setting the large-stack-frame parameter to 100 and the large-stack-frame-growth parameter to 400.
• -ftree-reassoc — perform reassociation on trees. Enabled by default at -O and higher.
• -fcode-hoisting — perform code hoisting. Code hoisting tries to move the evaluation of expressions that get executed on all paths to the function exit as early as possible. It's especially useful as a code size optimization, but honestly it often helps with code speed too. Enabled by default at -O2 and higher. ✨
• -ftree-pre — does partial redundancy elimination (PRE) on trees 🌳 bestie it's hunting down computations that are partially redundant and yeeting them. On by default at -O2 and -O3 ✨
• -ftree-partial-pre — makes that PRE thing more aggressive 💅 like it's not playing around anymore. On by default at -O3.
• -ftree-forwprop — forward propagation on trees, go off 🌸 on by default at -O and higher.
• -ftree-fre — full redundancy elimination (FRE) on trees. Now the tea ☕: FRE vs PRE is that FRE only looks at expressions that get computed on all the paths leading to the redundant computation. So it's a faster analysis than PRE, but ngl it exposes fewer redundancies 🙈. On by default at -O and higher.
• -ftree-phiprop — hoisting of loads from conditional pointers on trees 🦋 this pass is on by default at -O and higher.
• -fhoist-adjacent-loads — speculatively hoists loads from both branches of an if-then-else, but only if the loads are from adjacent locations in the same structure and the target architecture has a conditional move instruction 👑 on by default at -O2 and higher.
• -ftree-copy-prop — copy propagation on trees, which just eliminates unnecessary copy operations, so iconic of it 💖 on by default at -O and higher.
• -fipa-pure-const — sniffs out which functions are pure or constant 🔍 enabled by default at -O and higher.
• -fipa-reference — figures out which static variables do not escape the compilation unit. On by default at -O and higher.
• -fipa-reference-addressable — discovers read-only, write-only and non-addressable static variables 🎀 on by default at -O and higher.
• -fipa-stack-alignment — reduces stack alignment on call sites when possible. Enabled by default.
• -fipa-pta — interprocedural pointer analysis plus interprocedural modification and reference analysis 🧚 heads up though: this one can cause excessive memory and compile-time usage on large compilation units 😭 it is not enabled by default at any optimization level.
• -fipa-profile — interprocedural profile propagation. Functions called only from cold functions get marked cold ❄️ and functions executed once (like "cold", "noreturn", static constructors or destructors) get identified too. Then cold functions and the loop-less parts of functions executed once get optimized for size. Enabled by default at -O and higher.
• -fipa-modref — interprocedural mod/ref analysis. It analyzes the side effects of functions (memory locations that are modified or referenced) and enables better optimization across the function call boundary 🫶 on by default at -O and higher.
• -fipa-cp — interprocedural constant propagation. It analyzes the program to figure out when values passed to functions are constants, then optimizes accordingly. This can substantially increase performance if your app passes constants to functions 🔥 on by default at -O2, -Os and -O3, and also enabled by -fprofile-use and -fauto-profile.
• -fipa-cp-clone — function cloning to make interprocedural constant propagation stronger 💪 when enabled, it clones functions when an externally visible function can be called with constant arguments. Warning tho: because this can create multiple copies of functions, it may significantly increase code size (see --param ipa-cp-unit-growth=value). On by default at -O3, and also enabled by -fprofile-use and -fauto-profile.
• -fipa-bit-cp — when enabled, does interprocedural bitwise constant propagation ✨ on by default at -O2 and by -fprofile-use and -fauto-profile. It requires that -fipa-cp is enabled.
• -fipa-vrp — when enabled, does interprocedural propagation of value ranges 🌈 on by default at -O2. It requires that -fipa-cp is enabled.
• -fipa-icf — Identical Code Folding for functions and read-only variables 👯 this optimization reduces code size and may disturb unwind stacks by replacing a function with an equivalent one that has a different name. It works more effectively with link-time optimization enabled.

So even though the behavior is similar to the Gold Linker's ICF optimization, GCC ICF works on different levels, and thus the optimizations are not the same — there are equivalences found only by GCC and equivalences found only by Gold.

This flag is enabled by default at -O2 and -Os.

• -flive-patching=level — controls GCC's optimizations to produce output suitable for live-patching 💉

So here's the concept: if the compiler's optimization uses a function's body (or info extracted from its body) to optimize/change another function, that other one is called an impacted function of the first. And if a function gets patched, its impacted functions should be patched too.

The impacted functions are determined by the compiler's interprocedural optimizations. For example, a caller is impacted when inlining a function into its caller, cloning a function and changing its caller to call this new clone, or extracting a function's pureness/constness information to optimize its direct or indirect callers, etc.

Usually the more IPA optimizations you enable, the larger the number of impacted functions for each function. So to control the number of impacted functions and make it easier to compute the list of impacted functions, IPA optimizations can be partially enabled at two different levels.

The level argument should be one of these:

• inline-clone — only enables inlining and cloning optimizations, which includes inlining, cloning, interprocedural scalar replacement of aggregates and partial inlining. As a result, when you patch a function, all its callers and its clones' callers are impacted, so they need to be patched too.

-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 you patch a static function, all its callers are impacted and so need to be patched too.

In addition to all the flags that -flive-patching=inline-clone disables, -flive-patching=inline-only-static disables these extra ones: -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.

bestie listen: -flive-patching is not supported with link-time optimization (-flto) 💀

• -fisolate-erroneous-paths-dereference — detects paths that trigger erroneous or undefined behavior by 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. On by default at -O2 and higher, and it depends on -fdelete-null-pointer-checks also being enabled.
• -fisolate-erroneous-paths-attribute — detects paths that trigger erroneous or undefined behavior because a null value is 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 — forward store motion on trees 🌊 on by default at -O and higher.
• -ftree-bit-ccp — sparse conditional bit constant propagation on trees, plus it propagates pointer alignment information 📐 this pass only operates on local scalar variables and is on by default at -O1 and higher, except for -Og. It requires that -ftree-ccp is enabled.
• -ftree-ccp — sparse conditional constant propagation (CCP) on trees ✨ this pass only operates on local scalar variables and is on by default at -O and higher.
• -fssa-backprop — propagates info about uses of a value up the definition chain to simplify the definitions 🔁 for example, it strips sign operations if the sign of a value never matters. On by default at -O and higher.
• -fssa-phiopt — pattern matching on SSA PHI nodes to optimize conditional code 🧩 on by default at -O1 and higher, except for -Og.
• -ftree-switch-conversion — converts simple initializations in a switch into initializations from a scalar array 🔀 on by default at -O2 and higher.
• -ftree-tail-merge — looks for identical code sequences, and when it finds them, replaces one with a jump to the other 👯 this optimization is known as tail merging or cross jumping. On by default at -O2 and higher. The compilation time in this pass can be limited using the max-tail-merge-comparisons parameter and the max-tail-merge-iterations parameter.
• -ftree-dce — dead code elimination (DCE) on trees 💀 on by default at -O and higher.
• -ftree-builtin-call-dce — conditional dead code elimination (DCE) for calls to built-in functions that may set errno but are otherwise free of side effects. On by default at -O2 and higher if -Os is not also specified.
• -ffinite-loops — assumes that a loop with an exit will eventually take the exit and not loop forever ♾️ this lets the compiler remove loops that otherwise have no side-effects, since it's not counting eventual endless looping as a side effect. This option is enabled by default at -O2 for C++ with -std=c++11 or higher.
• -ftree-dominator-opts — does a whole lil variety of simple scalar cleanups (constant/copy propagation, redundancy elimination, range propagation and expression simplification) based on a dominator tree traversal 🌳 it also performs jump threading (to reduce jumps to jumps) 🪡 on by default at -O and higher.
• -ftree-dse — bestie, this does dead store elimination (DSE) on trees 🌳. So what's a "dead store"? It's when you write into a memory location and then literally nobody reads it before some other store overwrites it. That first store? Wasted. Deleted. Gone. It's giving nothing, so we cut it 💅. This flag is on by default at -O and higher.
• -ftree-ch — loop header copying on trees 🎀. Why do we love her? Because she makes code motion optimizations hit harder, and she also saves you one whole jump. Enabled by default at -O and higher ✨. But heads up: she is not enabled for -Os, because she usually makes your code bigger, and -Os said "we're being skinny, actually" 🫶.
• -ftree-loop-optimize — perform loop optimizations on trees. On by default at -O and higher, no cap 🧚.
• -ftree-loop-linear, -floop-strip-mine, -floop-block — perform 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. Like, you can't just want it, you gotta build for it 💀.
• -fgraphite-identity — enable the identity transformation for graphite. For every SCoP we generate the polyhedral representation and then 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. It's basically a vibe check on your own pipeline 👑
• -floop-nest-optimize — enable the isl based loop nest optimizer 🌈. This is a generic loop nest optimizer based on the Pluto optimization algorithms. It calculates a loop structure optimized for data-locality and parallelism. This option is experimental ⚠️ — so treat her like a new friend, cute but unpredictable.
• -floop-parallelize-all — use the Graphite data dependence analysis to identify loops that can be parallelized 🚀. It parallelizes all the loops that can be analyzed to not contain loop carried dependences — and it does not check that it's profitable to parallelize them. She said "we're parallelizing, idc if it helps" 💀.
• -ftree-coalesce-vars — while transforming the program out of the SSA representation, attempt to reduce copying by coalescing versions of different user-defined variables, instead of just compiler temporaries 🫶. Fair warning: 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 — attempt to transform conditional jumps in the innermost loops to branch-less equivalents ✨. The intent is to remove control-flow from the innermost loops so the vectorization pass can handle those loops better. Enabled by default if vectorization is enabled.
• -ftree-loop-distribution — perform loop distribution. This flag can improve cache performance on big loop bodies and allow further loop optimizations, like parallelization or vectorization, to take place 👑. For example, the loop

        DO I = 1, N
          A(I) = B(I) + C
          D(I) = E(I) * F
        ENDDO

is transformed to

        DO I = 1, N
           A(I) = B(I) + C
        ENDDO
        DO I = 1, N
           D(I) = E(I) * F
        ENDDO

This flag is enabled by default at -O3. It is also enabled by -fprofile-use and -fauto-profile.

• -ftree-loop-distribute-patterns — perform loop distribution of patterns that can be code generated with calls to a library 📚. This flag is enabled by default at -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 🌸. This flag is enabled by default at -O3. It is also enabled by -fprofile-use and -fauto-profile.

• -floop-interchange — perform 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];

This flag is enabled by default at -O3. It is also enabled by -fprofile-use and -fauto-profile.

• -floop-unroll-and-jam — apply 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. This flag is enabled by default at -O3. It is also enabled by -fprofile-use and -fauto-profile.
• -ftree-loop-im — perform loop invariant motion on trees. This pass moves only invariants that are hard to handle at RTL level (function calls, operations that expand to nontrivial sequences of insns) 🧠. With -funswitch-loops it also moves operands of conditions that are invariant out of the loop, so that we can use just trivial invariantness analysis in loop unswitching. The pass also includes store motion.
• -ftree-loop-ivcanon — create a canonical counter for number of iterations in loops for which determining number of iterations requires complicated analysis 🎀. Later optimizations then may determine the number easily. Useful especially in connection with unrolling.
• -ftree-scev-cprop — perform 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 is sufficiently cheap. This reduces data dependencies and may allow further simplifications. Enabled by default at -O and higher.
• -fivopts — perform induction variable optimizations (strength reduction, induction variable merging and induction variable elimination) on trees 💪.
• *-ftree-parallelize-loops=n*— parallelize loops, i.e., split their iteration space to run in *n* threads 🧵. This is only possible for loops whose iterations are independent and can be arbitrarily reordered. The optimization is only profitable on multiprocessor machines, for loops that are CPU-intensive, rather than constrained e.g. by memory bandwidth. This option implies-pthread, and thus is only supported on targets that have support for -pthread`.
• -ftree-pta — perform function-local points-to analysis on trees 🔍. This flag is enabled by default at -O1 and higher, except for -Og.
• -ftree-sra — perform scalar replacement of aggregates 🫶. This pass replaces structure references with scalars to prevent committing structures to memory too early. This flag is enabled by default at -O1 and higher, except for -Og.
• -fstore-merging — perform 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. This is enabled by default at -O2 and higher as well as -Os.
• -ftree-ter — perform temporary expression replacement during the SSA->normal phase. Single use/single def temporaries are replaced at their use location with their defining expression ✨. This results in non-GIMPLE code, but gives the expanders much more complex trees to work on resulting in better RTL generation. This is enabled by default at -O and higher.
• -ftree-slsr — perform straight-line strength reduction on trees 💅. This recognizes related expressions involving multiplications and replaces them by less expensive calculations when possible. This is enabled by default at -O and higher.
• -ftree-vectorize — perform vectorization on trees 🚀. This flag enables -ftree-loop-vectorize and -ftree-slp-vectorize if not explicitly specified.
• -ftree-loop-vectorize — perform loop vectorization on trees 🌈. This flag is enabled by default at -O3 and by -ftree-vectorize, -fprofile-use, and -fauto-profile.
• -ftree-slp-vectorize — perform basic block vectorization on trees ⭐. This flag is enabled by default at -O3 and by -ftree-vectorize, -fprofile-use, and -fauto-profile.
• *-fvect-cost-model=model* — alter 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, while with the dynamic model a runtime check guards the vectorized code-path to enable it only for iteration counts that will likely execute faster than when executing the original scalar loop. The cheap model disables vectorization of loops where doing so would be cost prohibitive — for example due to required runtime checks for data dependence or alignment — but otherwise is equal to the dynamic model. The very-cheap model only allows vectorization if the vector code would entirely replace the scalar code that is being vectorized. For example, if each iteration of a vectorized loop would only be able to handle exactly four iterations of the scalar loop, the very-cheap model would only allow vectorization if the scalar iteration count is known to be a multiple of four 👑.

The default cost model depends on other optimization flags and is either dynamic or cheap.

• *-fsimd-cost-model=model* — alter the cost model used for vectorization of loops marked with the OpenMP 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 — ok so this does Value Range Propagation on trees 🌳 and it's giving cousin-of-constant-propagation energy, except instead of yeeting actual values around, we're propagating ranges of values. why do we care? because it lets the optimizers straight-up delete unnecessary range checks like array bound checks and null pointer checks 💅 and no cap that's free speed. it's enabled by default at -O2 and higher ✨. one lil caveat bestie: null pointer check elimination is only done if -fdelete-null-pointer-checks is enabled.
• -fsplit-paths — splits paths that lead to loop backedges 🔁 which can improve dead code elimination and common subexpression elimination. enabled by default at -O3 and above 🔥.
• -fsplit-ivs-in-unroller — this one lets values of induction variables in later iterations of the unrolled loop be expressed using the value from the first iteration 🌀. it breaks those long dependency chains, which makes the scheduling passes way more efficient, periodt.

so like, a combo of -fweb and CSE is often enough to get the same effect. but that's not reliable when the loop body is more complicated than a single basic block 😭. and it literally does not work at all on some architectures because of restrictions in the CSE pass. rip.

this optimization is enabled by default 💖.

• -fvariable-expansion-in-unroller — with this, the compiler makes 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 everywhere else 🙈.

• -fpartial-inlining — inline parts of functions 🧩. heads up tho: this option only does anything 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) that were performed in previous iterations of loops 🔄.

enabled at level -O3. it's also enabled by -fprofile-use and -fauto-profile 🌸.

• -fprefetch-loop-arrays — if the target machine supports it, this generates instructions to prefetch memory, improving the performance of loops that access large arrays 📚.

fair warning bestie: 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 the known return value of formatted output functions such as sprintf, snprintf, vsprintf, and vsnprintf (but not printf or fprintf 😤). this transformation lets GCC optimize or even eliminate branches based on the known return value of these functions when they're 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 🤝. and -fprintf-return-value 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're implemented in the compiler; some targets use one, some use the other, a few use both. chaotic but valid ✨.

-fpeephole is enabled by default. -fpeephole2 is 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 aren't 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 the effects of __builtin_expect are easier to understand 👀.

it's also possible to specify the expected probability of the expression with the __builtin_expect_with_probability built-in function 💫.

the default is -fguess-branch-probability at levels -O, -O2, -O3, -Os.

• -freorder-blocks — reorder basic blocks in the compiled function in order to reduce the number of taken branches and improve code locality 🧱.

enabled at levels -O, -O2, -O3, -Os.

• -freorder-blocks-algorithm=algorithm — use 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 -O, -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 the number of taken branches, this 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 — reorder functions in the object file in order to improve code locality 🗂️. this is implemented by using special subsections .text.hot for most frequently executed functions and .text.unlikely for unlikely executed functions. reordering is done by the linker, so the object file format must support named sections and the 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.

• -fstrict-aliasing — allows the compiler to assume the strictest aliasing rules applicable to the language being compiled 😈. for C (and C++), this activates optimizations based on the type of expressions. in particular, an object of one type is assumed never to reside at the same address as an object of a different type, unless the types are almost the same. for example, an unsigned int can alias an int, but not a void* or a double. a character type may alias any other type 🌈.

pay special attention to code like this bestie:

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.

• -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.

if m is not specified, it defaults to n.

examples, bestie ✨: -falign-functions=32 aligns functions to the next 32-byte boundary, -falign-functions=24 aligns to the next 32-byte boundary only if 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 allows you to 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 — if this option is 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 🎯.

Okay so real talk, the -falign-labels params work exactly like the -falign-functions ones — same energy, different target, bestie. 💅

-fno-align-labels and -falign-labels=1 are literally the same thing and both mean labels do not get aligned, periodt.

If -falign-loops or -falign-jumps apply and their value is bigger than this one? Their value wins, no discussion. 🫶

And if n isn't specified or is zero, GCC falls back to a machine-dependent default that is very likely to be 1, which means no alignment at all. The maximum allowed n 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. 🔁

So here's the tea: if the loops get executed many times, that totally makes up for the cost of executing the dummy padding instructions. Math-ing out.

If -falign-labels is greater than this value, then its value gets used instead.

The parameters of this option are analogous to the -falign-functions option. And -fno-align-loops and -falign-loops=1 are equivalent — both mean loops are not aligned, okay? 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, specifically for branch targets where the targets can only be reached by jumping. 🎯

And the slay here is that in this case, no dummy operations need be executed. Not a single one. We love efficiency.

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. (Yes, it says loops. We're quoting the manual, don't come for me. 😭)

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. ✨

---

• -fno-allocation-dce — do not remove unused C++ allocations in dead code elimination. 🗑️ Basically you're telling GCC "leave my unused allocations ALONE" and it's like okay bestie, your funeral, they stay.

---

• -fallow-store-data-races — let the compiler do optimizations that may introduce new data races on stores, without it having to prove 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. 🔥

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• -funit-at-a-time — this option is left for compatibility reasons. -funit-at-a-time has no effect, while -fno-unit-at-a-time implies -fno-toplevel-reorder and -fno-section-anchors. It's giving legacy relic, honestly. 🏺

Enabled by default.

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• -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. (Word. Take the note, bestie.)

-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.

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• -fweb — constructs webs as commonly used for register allocation purposes and assigns each web its own 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.

But ngl, it can make debugging impossible, since variables no longer stay in a "home register". 💀 Rip debugger, you will be missed.

Enabled by default with -funroll-loops.

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• -fwhole-program — assume that the current compilation unit represents the whole program being compiled. 👑

All public functions and variables, with the exception of main and those merged by attribute externally_visible, become static functions and in effect are optimized more aggressively by interprocedural optimizers.

This option should not be used in combination with -flto. Instead relying on a linker plugin should provide safer and more precise information. 🚨

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• -flto[=n] — the main character. 💅 This option runs the standard link-time optimizer. When invoked with source code, it generates GIMPLE (one of GCC's internal representations) and writes it to special ELF sections in the object file. When the object files are linked together, all the function bodies are read from these ELF sections and instantiated as if they had been part of the same translation unit. 🧠✨

To use the link-time optimizer, -flto and optimization options should be specified at compile time and during the final link. It is recommended that you compile all the files participating in the same link with the same options and also specify those options at link time. For example:

gcc -c -O2 -flto foo.c
gcc -c -O2 -flto bar.c
gcc -o myprog -flto -O2 foo.o bar.o

The first two invocations to GCC save a bytecode representation of GIMPLE into special ELF sections inside foo.o and bar.o. The final invocation reads the GIMPLE bytecode from foo.o and bar.o, merges the two files into a single internal image, and compiles the result as usual. Since both foo.o and bar.o are merged into a single image, this causes all the interprocedural analyses and optimizations in GCC to work across the two files as if they were a single one. This means, for example, that the inliner is able to inline functions in bar.o into functions in foo.o and vice-versa. 🤝 It's giving "we're all one file in this house".

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 that object files with LTO information can be linked as normal object files; if -fno-lto is passed to the linker, no interprocedural optimizations are applied. Note that when -fno-fat-lto-objects is enabled the compile stage is faster but you cannot perform a regular, non-LTO link on them. 😭

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.

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. Two reasons, bestie. Two. ✌️

There are some code generation flags preserved by GCC when generating bytecodes, as they need to be used during the final link. Currently, the following options and their settings are taken from the first object file that explicitly specifies them: -fcommon, -fexceptions, -fnon-call-exceptions, -fgnu-tm and all the -m target flags.

The following options -fPIC, -fpic, -fpie and -fPIE are combined based on the following scheme:

-fPIC + -fpic = -fpic
-fPIC + -fno-pic = -fno-pic
-fpic/-fPIC + (no option) = (no option)
-fPIC + -fPIE = -fPIE
-fpic + -fPIE = -fpie
-fPIC/-fpic + -fpie = -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 like -Wstringop-overflow? Yeah, those get passed straight through to the link stage, and their setting matches whatever the compile-step had, at function granularity. Lowkey important caveat though: this only matters for diagnostics that get emitted during optimization. And also — code transforms like inlining can lead to warnings being enabled or disabled for regions of code that aren't consistent with the setting at compile time. 💅

When you need to pass options to the assembler via -Wa or -Xassembler, bestie, make sure you either compile those translation units with -fno-lto, or you consistently use the same assembler options on all translation units. No mixing, it's giving chaos. You can alternatively also specify assembler options at LTO link time. ✨

To enable debug info generation you gotta supply -g at compile time. And if any of the input files at link time were built with debug info generation enabled, the link will enable debug info generation too. But — and this is the part people always miss 😭 — 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 across different translation units is discouraged.

If LTO encounters objects with C linkage declared with incompatible types in separate translation units that are being linked together (that's undefined behavior according to ISO C99 6.2.7), a non-fatal diagnostic may be issued. But the behavior is still undefined at run time. Similar diagnostics may be raised for other languages. So like. Don't do that. 🙈

---

Another feature of LTO is that you can 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 you're mixing languages in LTO mode, you should use the same link command options as when mixing languages in a regular (non-LTO) compilation. Consistency is the vibe. 👑

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. It's giving teamwork. 🫶

If you are not using a linker with plugin support and/or don't enable the linker plugin, then the objects inside libfoo.a are extracted and linked as usual, but they do not participate in the LTO optimization process. In order to make a static library suitable for both LTO optimization and usual linkage, compile its object files with -flto -ffat-lto-objects.

Link-time optimizations do not require the presence of the whole program to operate. If the program does not require any symbols to be exported, it is possible to combine -flto and -fwhole-program to allow the interprocedural optimizers to use more aggressive assumptions, which may lead to improved optimization opportunities. Use of -fwhole-program is not needed when linker plugin is active (see -fuse-linker-plugin).

The current implementation of LTO makes no attempt to generate bytecode that is portable between different types of hosts. The bytecode files are versioned and there is a strict version check, so bytecode files generated in one version of GCC do not work with an older or newer version of GCC. Periodt.

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. slay. 💅

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 — Specify the partitioning algorithm used by the link-time optimizer. The value is either 1to1 to specify a partitioning mirroring the original source files, or balanced to specify partitioning into equally sized chunks (whenever possible), or max to create a new partition for every symbol where possible. Specifying none as an algorithm disables partitioning and streaming completely. The default value is balanced. While 1to1 can be used as a workaround for various code ordering issues, the max partitioning is intended for internal testing only. The value one specifies that exactly one partition should be used, while the value none bypasses partitioning and executes the link-time optimization step directly from the WPA phase. ✨
• -flto-compression-level=n — This option specifies the level of compression used for intermediate language written to LTO object files, and is only meaningful in conjunction with LTO mode (-flto). GCC currently supports two LTO compression algorithms. For zstd, valid values are 0 (no compression) to 19 (maximum compression), while zlib supports values from 0 to 9. Values outside this range are clamped to either minimum or maximum of the supported values. If the option is not given, a default balanced compression setting is used.
• -fuse-linker-plugin — Enables the use of a linker plugin during link-time optimization. This option relies on plugin support in the linker, which is available in gold or in GNU ld 2.21 or newer. 🌈

This option enables the extraction of object files with GIMPLE bytecode out of library archives. This improves the quality of optimization by exposing more code to the link-time optimizer. This information specifies what symbols can be accessed externally (by non-LTO object or during dynamic linking). Resulting code quality improvements on binaries (and shared libraries that use hidden visibility) are similar to -fwhole-program. See -flto for a description of the effect of this flag and how to use it.

This option is enabled by default when LTO support in GCC is enabled and GCC was configured for use with a linker supporting plugins (GNU ld 2.21 or newer or gold).

• -ffat-lto-objects — Fat LTO objects are object files that contain both the intermediate language and the object code. This makes them usable for both LTO linking and normal linking. This option is effective only when compiling with -flto and is ignored at link time. 🍓

-fno-fat-lto-objects improves compilation time over plain LTO, but requires the complete toolchain to be aware of LTO. It requires a linker with linker plugin support for basic functionality. Additionally, nm, ar and ranlib need to support linker plugins to allow a full-featured build environment (capable of building static libraries etc). GCC provides the gcc-ar, gcc-nm, gcc-ranlib wrappers to pass the right options to these tools. With non fat LTO, makefiles need to be modified to use them. 😮‍💨

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).

The default is -fno-fat-lto-objects on targets with linker plugin support.

• -fcompare-elim — After register allocation and post-register allocation instruction splitting, identify arithmetic instructions that compute processor flags similar to a comparison operation based on that arithmetic. If possible, eliminate the explicit comparison operation. 👑

This pass only applies to certain targets that cannot explicitly represent the comparison operation before register allocation is complete.

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

• -fcprop-registers — After register allocation and post-register allocation instruction splitting, perform a copy-propagation pass to try to reduce scheduling dependencies and occasionally eliminate the copy.

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

• -fprofile-correction — Profiles collected using an instrumented binary for multi-threaded programs may be inconsistent due to missed counter updates. When this option is specified, GCC uses heuristics to correct or smooth out such inconsistencies. By default, GCC emits an error message when an inconsistent profile is detected. 😭

This option is enabled by -fauto-profile.

• -fprofile-partial-training — With "-fprofile-use" all portions of programs not executed during train run are optimized agressively 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 trianing may not cover all hardware configurations program is run on.) With "-fprofile-partial-training" profile feedback will be 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. 🦋
• -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:

okay so these are the optimizations that come along for the ride when you use profile feedback — the whole squad, no cap:

• -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

girlies, bestie, listen — before you can use this option, you must first generate profiling information. It is literally a prerequisite. No profile data, no party. 💅

By default, GCC throws an error at you if the feedback profiles don't match the source code. Rude but fair. You can turn that error into a lil warning by using -Wno-error=coverage-mismatch — but heads up, that may result in poorly optimized code. Also, by default GCC drops a warning too if the feedback profiles don't exist at all (peep -Wmissing-profile for that one).

If path is specified, GCC goes looking at that path to find the profile feedback data files. See -fprofile-dir for the deets. 🫶

---

💫 -fauto-profile 💫

💫 -fauto-profile=path 💫

Turn on sampling-based feedback-directed optimizations, plus all of these following optimizations — and honestly a lot of them are only generally profitable when you've got 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 you omit it, it defaults to fbdata.afdo in the current directory. 🎀

Making an AutoFDO profile data file means running your program with the perf utility on a supported GNU/Linux target system. Want more info? <https://perf.wiki.kernel.org/>

E.g.

perf record -e br_inst_retired:near_taken -b -o perf.data \
    -- your_program

Then you use the create_gcov tool to convert that raw profile data into a format GCC can actually use. You've also gotta hand this tool the unstripped binary for your program. See <https://github.com/google/autofdo>.

E.g.

create_gcov --binary=your_program.unstripped --profile=perf.data \
    --gcov=profile.afdo

---

And now… the floating-point corner of the internet 💖 The following options control how the compiler behaves around floating-point arithmetic. These options are all about trading off between speed and correctness. And — important — all of them must be specifically enabled. They do not just show up on their own, bestie.

---

• -ffloat-store — do not store floating-point variables in registers, and shut down other options that might change whether a floating-point value gets taken from a register or from memory.

This one stops that undesirable excess precision on machines like the 68000, where the floating registers (of the 68881) keep more precision than a double is supposed to have. Same story for the x86 architecture. For most programs, that extra precision does only good — but a few programs rely on the precise definition of IEEE floating point, and for those you want -ffloat-store, after modifying them so all pertinent intermediate computations get stored into variables. 🧚

• -fexcess-precision=style — this gives you even more control over excess precision on machines where floating-point operations happen in a format with more precision or range than the IEEE standard and interchange floating-point types.

By default, -fexcess-precision=fast is in effect; this means operations may be carried out in a wider precision than the types specified in the source if that would result in faster code, and it is unpredictable when rounding to the types specified in the source code takes place. When compiling C, if -fexcess-precision=standard is specified then excess precision follows the rules specified in ISO C99; in particular, both casts and assignments cause values to be rounded to their semantic types (whereas -ffloat-store only affects assignments). This option is enabled by default for C if a strict conformance option such as -std=c99 is used. -ffast-math enables -fexcess-precision=fast by default regardless of whether a strict conformance option is used.

-fexcess-precision=standard is not implemented for languages other than C. On the x86, it has no effect if -mfpmath=sse or -mfpmath=sse+387 is specified; in the former case, IEEE semantics apply without excess precision, and in the latter, rounding is unpredictable. 😭

• -ffast-math — sets all these options: -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. ✨

This option is not turned on by any -O option besides -Ofast, since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that do not require the guarantees of these specifications.

• -fno-math-errno — do not set errno after calling math functions that are executed with a single instruction, e.g., sqrt. A program that relies on IEEE exceptions for math error handling may want to use this flag for speed while maintaining IEEE arithmetic compatibility.

This option is not turned on by any -O option since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that do not require the guarantees of these specifications.

The default is -fmath-errno.

On Darwin systems, the math library never sets errno. So there's literally no reason for the compiler to consider the possibility that it might — and -fno-math-errno is the default there. 🍓

• -funsafe-math-optimizations — allow optimizations for floating-point arithmetic that (a) assume that arguments and results are valid and (b) may violate IEEE or ANSI standards. When used at link time, it may include libraries or startup files that change the default FPU control word or other similar optimizations. unsafe is right there in the name, bestie, we been knew

This option is not turned on by any -O option since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that do not require the guarantees of these specifications. It enables -fno-signed-zeros, -fno-trapping-math, -fassociative-math and -freciprocal-math.

The default is -fno-unsafe-math-optimizations.

• -fassociative-math — allow re-association of operands in series of floating-point operations. This violates the ISO C and C++ language standard by possibly changing computation result.

NOTE: re-ordering may change the sign of zero as well as ignore NaNs and inhibit or create underflow or overflow (and thus cannot be used on code that relies on rounding behavior like (x + 2**52) - 2**52). May also reorder floating-point comparisons and thus may not be used when ordered comparisons are required. 💀

This option requires that both -fno-signed-zeros and -fno-trapping-math be in effect. Moreover, it doesn't make much sense with -frounding-math. For Fortran the option is automatically enabled when both -fno-signed-zeros and -fno-trapping-math are in effect.

The default is -fno-associative-math.

• -freciprocal-math — allow the reciprocal of a value to be used instead of dividing by the value if this enables optimizations. For example x / y can be replaced with x * (1/y), which is useful if (1/y) is subject to common subexpression elimination. Note that this loses precision and increases the number of flops operating on the value. 🙈

The default is -fno-reciprocal-math.

• -ffinite-math-only — allow optimizations for floating-point arithmetic that assume that arguments and results are not NaNs or +-Infs. Living in denial, but make it fast. 💅

This option is not turned on by any -O option since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that do not require the guarantees of these specifications.

The default is -fno-finite-math-only.

• -fno-signed-zeros — allow optimizations for floating-point arithmetic that ignore the signedness of zero. IEEE arithmetic specifies the behavior of distinct +0.0 and -0.0 values, which then prohibits simplification of expressions such as x+0.0 or 0.0*x (even with -ffinite-math-only). This option implies that the sign of a zero result isn't significant. plus zero, minus zero, who's to say honestly

The default is -fsigned-zeros.

• -fno-trapping-math — compile code assuming that floating-point operations cannot generate user-visible traps. These traps include division by zero, overflow, underflow, inexact result and invalid operation. This option requires that -fno-signaling-nans be in effect. Setting this option may allow faster code if one relies on "non-stop" IEEE arithmetic, for example.

This option should never be turned on by any -O option since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions.

The default is -ftrapping-math.

• -frounding-math — disable transformations and optimizations that assume default floating-point rounding behavior. This is round-to-zero for all floating point to integer conversions, and round-to-nearest for all other arithmetic truncations. This option should be specified for programs that change the FP rounding mode dynamically, or that may be executed with a non-default rounding mode. This option disables constant folding of floating-point expressions at compile time (which may be affected by rounding mode) and arithmetic transformations that are unsafe in the presence of sign-dependent rounding modes.

The default is -fno-rounding-math.

This option is experimental and does not currently guarantee to disable all GCC optimizations that are affected by rounding mode. Future versions of GCC may provide finer control of this setting using C99's FENV_ACCESS pragma. This command-line option will be used to specify the default state for FENV_ACCESS. 🌈

• -fsignaling-nans — bestie, this one tells the compiler to just assume that IEEE signaling NaNs can throw user-visible traps mid floating-point op. Turning it on kills off all the optimizations that might mess with how many exceptions you actually see when signaling NaNs are in play. Oh, and it quietly drags -ftrapping-math along with it 💅

Also it defines the preprocessor macro __SUPPORT_SNAN__ for you, slay.

The default is -fno-signaling-nans.

Heads up: this option is experimental, and rn it does not guarantee it disables every single GCC optimization that touches signaling NaN behavior. So like, don't bet your whole life on it, okay? 😭

• -fno-fp-int-builtin-inexact — this stops the built-in functions ceil, floor, round and trunc (plus all their float and long double variants) from generating code that raises the inexact floating-point exception when you hand them noninteger arguments. ISO C99 and C11 are fine with these functions raising inexact, but ISO/IEC TS 18661-1:2014 — the C bindings to IEEE 754-2008, which got folded into ISO C2X — says nope, they can't do that.

Default is -ffp-int-builtin-inexact, which lets the exception be raised, unless C2X or a newer C standard is selected. Lowkey important: this option does absolutely nothing unless -ftrapping-math is in effect.

And even with -fno-fp-int-builtin-inexact on, if those functions end up generating a call to a library function, the inexact exception can still get raised if the library implementation doesn't follow TS 18661. Fr fr, watch out for that 🫶

• -fsingle-precision-constant — treats floating-point constants as single precision instead of sneakily converting them into double-precision constants. No more surprise upgrades ✨
• -fcx-limited-range — when you turn this on, you're basically declaring that no range reduction step is needed when doing complex division. Also, it skips checking whether a complex multiplication or division result is NaN + I*NaN and skips the whole attempt to rescue that situation. The default is -fno-cx-limited-range, but -ffast-math enables it.

This option controls the default setting of the ISO C99 CX_LIMITED_RANGE pragma. But like, it applies to all languages anyway, so jot that down 📝

• -fcx-fortran-rules — complex multiplication and division follow Fortran rules. Range reduction is done as part of complex division, but there's no checking whether a complex multiplication or division result is NaN + I*NaN, and no attempt to rescue that situation.

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. So like… vibe check before you use these, bestie 💀

• -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 how many times each branch got taken. When a program compiled with -fprofile-arcs exits, it saves arc execution counts to a file called sourcename.gcda for each source file. That data file is super dependent on the structure of the generated code, so you have to use the exact same source code and the same optimization options for both compilations. No switching things up between rounds 🙈

With -fbranch-probabilities, GCC puts a REG_BR_PROB note on each JUMP_INSN and CALL_INSN. These can be used to improve optimization. Currently they're only used in one place: in reorg.c, instead of guessing which path a branch is most likely to take, the REG_BR_PROB values are used to exactly determine which path is taken more often. Iconic, honestly.

Enabled by -fprofile-use and -fauto-profile.

• -fprofile-values — if you combine it with -fprofile-arcs, it adds code so that some data about values of expressions in the program gets gathered up.

With -fbranch-probabilities, it reads back that gathered data about expression values for use in optimizations.

Enabled by -fprofile-generate, -fprofile-use, and -fauto-profile.

• -fprofile-reorder-functions — function reordering based on profile instrumentation collects the first time of execution of a function and orders those functions in ascending order.

Enabled with -fprofile-use.

• -fvpt — if combined with -fprofile-arcs, this option tells the compiler to add code that gathers info about the values of expressions.

With -fbranch-probabilities, it reads that gathered data back and actually performs the optimizations based on it. Currently the optimizations include specialization of division operations using the knowledge about the value of the denominator. Math girlies stay winning 🔥

Enabled with -fprofile-use and -fauto-profile.

• -frename-registers — attempts 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. But depending on the debug information format adopted by the target, it can make debugging impossible, since variables no longer stay in a "home register". So like, trade-offs, bestie 😭

Enabled by default with -funroll-loops.

• -fschedule-fusion — performs a target dependent pass over the instruction stream to schedule instructions of the same type together, because the target machine can execute them more efficiently when they're adjacent to each other in the instruction flow. It's giving organized ✨

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

• -ftracer — performs tail duplication to enlarge superblock size. This transformation simplifies the control flow of the function, letting other optimizations do a better job.

Enabled by -fprofile-use and -fauto-profile.

• -funroll-loops — unrolls 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. So like, it might eat, or it might flop, we simply don't know 💅

Enabled by -fprofile-use and -fauto-profile.

• -funroll-all-loops — unrolls 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's enough info that they don't 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.

• -fmove-loop-invariants — enables the loop invariant motion pass in the RTL loop optimizer. Enabled at level -O1 and higher, except for -Og.
• -fsplit-loops — splits 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.

Enabled by -fprofile-use and -fauto-profile.

• -funswitch-loops — moves branches with loop invariant conditions out of the loop, with duplicates of the loop on both branches (modified according to the 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, this creates another version of the loop that assumes the stride is always one. For example:

for (int i = 0; i < n; ++i)
  x[i * stride] = ...;

becomes:

if (stride == 1)
  for (int i = 0; i < n; ++i)
    x[i] = ...;
else
  for (int i = 0; i < n; ++i)
    x[i * stride] = ...;

This is particularly useful for assumed-shape arrays in Fortran where (for example) it allows better vectorization assuming contiguous accesses. This flag is enabled by default at -O3. It is also enabled by -fprofile-use and -fauto-profile. Vectorization slay 🌈

• -ffunction-sections
• -fdata-sections — place each function or data item into its own section in the output file if the target supports arbitrary sections. The name of the function or the name of the data item determines the section's name in the output file.

Use these options on systems where the linker can perform optimizations to improve locality of reference in the instruction space. Most systems using the ELF object format have linkers with such optimizations. On AIX, the linker rearranges sections (CSECTs) based on the call graph. The performance impact varies.

Together with a linker garbage collection (linker --gc-sections option) these options may lead to smaller statically-linked executables (after stripping).

On ELF/DWARF systems these options do not degenerate the quality of the debug information. There could be issues with other object files/debug info formats.

Only use these options when there are significant benefits from doing so. When you specify these options, the assembler and linker create larger object and executable files and are also slower. These options affect code generation. They prevent optimizations by the compiler and assembler using relative locations inside a translation unit, since the locations are unknown until link time. An example of such an optimization is relaxing calls to short call instructions. So like… know what you're doing first, okay? 👑

• -fstdarg-opt — optimizes the prologue of variadic argument functions with respect to usage of those arguments.

NOTE: In Ubuntu 14.10 and later versions, -fstack-protector-strong is enabled by default for C, C++, ObjC, ObjC++, if none of -fno-stack-protector, -nostdlib, nor -ffreestanding are found.

• -fsection-anchors — tries to reduce the number of symbolic address calculations by using shared "anchor" symbols to address nearby objects. This transformation can help reduce the number of GOT entries and GOT accesses on some targets. Very demure, very mindful 🎀

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];
}

quick heads up tho — not all targets support this option. so like, vibe check your target first bestie 💀

• -fzero-call-used-regs=choice — zeroes out the call-used registers when a function returns, and it's giving security main character energy ✨ it either mitigates Return-Oriented Programming (ROP) attacks or stops info from leaking out through registers. two birds, one slay.

The possible values of choice are the exact same ones 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 🌈

okay so this one is a whole mood. in some places, GCC uses various constants to control the amount of optimization that is done. like, for example, GCC does not inline functions that contain more than a certain number of instructions. and you can control some of these constants on the command line using the --param option. iconic behavior honestly 💅

real talk tho — 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. so don't get too attached bestie, they might ghost you 😭

In order to get minimal, maximal and default value of a parameter, one can use --help=param -Q options. 🦋

In each case, the value is an integer. The following choices of name are recognized for all targets:

• predictable-branch-outcome — when a branch is predicted to be taken with a probability lower than this threshold (in percent), then it's considered well predictable. periodt.
• 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. it's giving big brain decisions.
• max-rtl-if-conversion-predictable-cost — RTL if-conversion will try to remove conditional branches around a block and replace them with conditionally executed instructions. These parameters give the maximum permissible cost for the sequence that would be generated by if-conversion, depending on whether the branch is statically determined to be predictable or not. The units for this parameter are the same as those for the GCC internal seq_cost metric. The compiler will try to provide a reasonable default for this parameter using the BRANCH_COST target macro. 🎀
• max-crossjump-edges — the maximum number of incoming edges to consider for cross-jumping. The algorithm used by -fcrossjumping is O(N^2) in the number of edges incoming to each block. Increasing values mean more aggressive optimization, making the compilation time increase with probably small improvement in executable size. tradeoffs, bestie, tradeoffs 😮‍💨
• min-crossjump-insns — the minimum number of instructions that must be matched at the end of two blocks before cross-jumping is performed on them. This value is ignored in the case where all instructions in the block being cross-jumped from are matched.
• 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.
• max-goto-duplication-insns — the maximum number of instructions to duplicate to a block that jumps to a computed goto. To avoid O(N^2) behavior in a number of passes, GCC factors computed gotos early in the compilation process, and unfactors them as late as possible. Only computed jumps at the end of a basic blocks with no more than max-goto-duplication-insns are unfactored.
• max-delay-slot-insn-search — the maximum number of instructions to consider when looking for an instruction to fill a delay slot. If more than this arbitrary number of instructions are searched, the time savings from filling the delay slot are minimal, so stop searching. Increasing values mean more aggressive optimization, making the compilation time increase with probably small improvement in execution time. diminishing returns arc fr fr.
• max-delay-slot-live-search — when trying to fill delay slots, the maximum number of instructions to consider when searching for a block with valid live register information. Increasing this arbitrarily chosen value means more aggressive optimization, increasing the compilation time. This parameter should be removed when the delay slot code is rewritten to maintain the control-flow graph. noted for the future, besties.
• 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 not done. it simply says no 💅
• 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.
• max-pending-list-length — the maximum number of pending dependencies scheduling allows before flushing the current state and starting over. Large functions with few branches or calls can create excessively large lists which needlessly consume memory and resources. memory glow-up denied 🙈
• 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. exponentially?? okay diva, calm down 😭
• max-inline-insns-single — several parameters control the tree inliner used in GCC. This number sets the maximum number of instructions (counted in GCC's internal representation) in a single function that the tree inliner considers for inlining. This only affects functions declared inline and methods implemented in a class declaration (C++). 🎀
• max-inline-insns-auto — when you use -finline-functions (included in -O3), a lot of functions that would otherwise not be considered for inlining by the compiler are investigated. To those functions, a different (more restrictive) limit compared to functions declared inline can be applied (--param max-inline-insns-auto). strict queen energy ✨
• max-inline-insns-small — this is bound applied to calls which are considered relevant with -finline-small-functions.
• max-inline-insns-size — this is bound applied to calls which are optimized for size. Small growth may be desirable to anticipate optimization oppurtunities exposed by inlining. (and yes bestie, that's how it's spelled in the source, we don't fix it, we respect it 💖)
• uninlined-function-insns — number of instructions accounted by inliner for function overhead such as function prologue and epilogue.
• uninlined-function-time — extra time accounted by inliner for function overhead such as time needed to execute function prologue and epilogue.
• 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). the vibes said yes so we scale up 🌈
• uninlined-thunk-insns, uninlined-thunk-time — same as --param uninlined-function-insns and --param uninlined-function-time but applied to function thunks. 🦋
• 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. speed slay overrides the rules 💅
• 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. no cap, the back end gets dramatic.
• large-function-growth — specifies maximal growth of large function caused by inlining in percents. For example, 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. context matters, always has, always will ✨
• lazy-modules — maximum number of concurrently open C++ module files when lazy loading. so real for this 😴
• inline-unit-growth — specifies maximal overall growth of the compilation unit caused by inlining. For example, parameter value 20 limits unit growth to 1.2 times the original size. Cold functions (either marked cold via an attribute or by profile feedback) are not accounted into the unit size. cold girlies don't count toward the budget ❄️
• ipa-cp-unit-growth — specifies maximal overall growth of the compilation unit caused by interprocedural constant propagation. For example, 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. everyone needs a threshold bestie.
• large-stack-frame — the limit specifying large stack frames. While inlining the algorithm is trying to not grow past this limit too much. 🎀
• large-stack-frame-growth — specifies maximal growth of large stack frames caused by inlining in percents. For example, parameter value 1000 limits large stack frame growth to 11 times the original size. eleven times?? that's not a frame that's a skyscraper 🏙️
• 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. recursion going recursive, we love a self-referential queen 🔁

So like, --param max-inline-insns-recursive is the one that applies to functions declared inline, bestie. For functions not declared inline, recursive inlining only goes down when -finline-functions (which is included in -O3) is enabled; then --param max-inline-insns-recursive-auto takes over instead. 💅

• max-inline-recursive-depth, max-inline-recursive-depth-auto — sets the maximum recursion depth used for recursive inlining. --param max-inline-recursive-depth applies to functions declared inline. For functions not declared inline, recursive inlining happens only when -finline-functions (included in -O3) is enabled; --param max-inline-recursive-depth-auto applies instead. 🌀
• min-inline-recursive-probability — recursive inlining is profitable only for functions that have deep recursion on average, and it can hurt functions with little recursion depth by increasing prologue size or the complexity of the function body for other optimizers. When profile feedback is available (see -fprofile-generate), the actual recursion depth can be guessed from the probability that a 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 — specify the growth that the early inliner can make. In effect, it increases the amount of inlining for code that has a large abstraction penalty. 🌱
• max-early-inliner-iterations — limit of iterations of the early inliner. This basically bounds the number of nested indirect calls the early inliner can resolve. Deeper chains are still handled by late inlining. 🔁
• comdat-sharing-probability — probability (in percent) that C++ inline functions with comdat visibility are shared across multiple compilation units. 🤝
• modref-max-bases, modref-max-refs, modref-max-accesses — specifies the maximal number of base pointers, references and accesses stored for a single function by mod/ref analysis. 📚
• modref-max-tests — specifies the maximum number of tests the alias oracle can perform to disambiguate memory locations using the mod/ref information. This parameter ought to be bigger than --param modref-max-bases and --param modref-max-refs. 🧪
• modref-max-depth — specifies the maximum depth of the DFS walk used by modref escape analysis. Setting it to 0 disables the analysis completely. 🕳️
• modref-max-escape-points — specifies the maximum number of escape points tracked by modref per SSA-name. 🎯
• profile-func-internal-id — a parameter to control whether to use the function internal id in profile database lookup. If the value is 0, the compiler uses an id that is 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. The number of iterations after vectorization needs to be greater than the value specified by this option to allow vectorization. 🧮
• gcse-cost-distance-ratio — scaling factor in the calculation of the maximum distance an expression can be moved by GCSE optimizations. This is currently supported only in the code hoisting pass. The bigger the ratio, the more aggressive code hoisting is with simple expressions, i.e., the expressions that have cost less than gcse-unrestricted-cost. Specifying 0 disables hoisting of simple expressions. 🏗️
• gcse-unrestricted-cost — cost, roughly measured as the cost of a single typical machine instruction, at which GCSE optimizations do not constrain the distance an expression can travel. This is currently supported only in the code hoisting pass. The lesser the cost, the more aggressive code hoisting is. Specifying 0 allows all expressions to travel unrestricted distances. 🛫
• max-hoist-depth — the depth of search in the dominator tree for expressions to hoist. This is used to avoid quadratic behavior in the hoisting algorithm. The value of 0 does not limit the search, but may slow down compilation of huge functions. 🌳
• max-tail-merge-comparisons — the maximum amount of similar bbs to compare a bb with. This is used to avoid quadratic behavior in tree tail merging. 🧷
• max-tail-merge-iterations — the maximum amount of iterations of the pass over the function. This is used to limit compilation time in tree tail merging. ⏳
• store-merging-allow-unaligned — allow the store merging pass to introduce unaligned stores if it is 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 in the attempt 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 that a loop may have to be unrolled. If a loop is unrolled, this parameter also determines how many times the loop code is unrolled. 🔄
• max-average-unrolled-insns — the maximum number of instructions biased by probabilities of their execution that a loop may have to be unrolled. If a loop is unrolled, this parameter also determines how many times the loop code is unrolled. 📊
• max-unroll-times — the maximum number of unrollings of a single loop. 🔂
• max-peeled-insns — the maximum number of instructions that a loop may have to be peeled. If a loop is peeled, this parameter also determines how many times the loop code is peeled. 🍌
• max-peel-times — the maximum number of peelings of a single loop. 🍊
• max-peel-branches — the maximum number of branches on the hot path through the peeled sequence. 🌶️
• max-completely-peeled-insns — the maximum number of insns of a completely peeled loop. 🧅
• max-completely-peel-times — the maximum number of iterations of a loop to be suitable for complete peeling. ⏱️
• max-completely-peel-loop-nest-depth — the maximum depth of a loop nest suitable for complete peeling. 🪆
• max-unswitch-insns — the maximum number of insns of an unswitched loop. 🔀
• max-unswitch-level — the maximum number of branches unswitched in a single loop. 🎚️
• lim-expensive — the minimum cost of an expensive expression in the loop invariant motion. 💰
• 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 loop split. 🧩
• iv-consider-all-candidates-bound — 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 are considered to avoid quadratic time complexity. 🧠
• iv-max-considered-uses — the induction variable optimizations give up on loops that contain more induction variable uses. 🛑
• iv-always-prune-cand-set-bound — if the number of candidates in the set is smaller than this value, always try to remove unnecessary ivs from the set when adding a new one. ✂️
• 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. 🗃️
• 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. 🐌
• scev-max-expr-complexity — bound on the complexity of the expressions in the scalar evolutions analyzer. Complex expressions slow the analyzer. 🌀
• max-tree-if-conversion-phi-args — maximum number of arguments in a PHI supported by TREE if conversion unless the loop is marked with simd pragma. 🧬
• vect-max-version-for-alignment-checks — the maximum number of run-time checks that can be performed when doing loop versioning for alignment in the vectorizer. 📐
• vect-max-version-for-alias-checks — the maximum number of run-time checks that can be performed when doing loop versioning for alias in the vectorizer. 🪞
• vect-max-peeling-for-alignment — the maximum number of loop peels to enhance access alignment for vectorizer. Value -1 means no limit. ♾️
• max-iterations-to-track — the maximum number of iterations of a loop the brute-force algorithm for analysis of the number of iterations of the loop tries to evaluate. 🔎
• hot-bb-count-fraction — the denominator n of 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 in order to be considered hot. The default is 10000, which means that a basic block is considered hot if its execution count is greater than 1/10000 of the maximal execution count. 0 means that it is never considered hot. Used in non-LTO mode. 🔥
• hot-bb-count-ws-permille — bestie this one is about how many permilles (so like, thousandths, ranging from 0 to 1000) of the profiled execution of the entire program a basic block's execution count has to be part of for it to count as hot 🔥. Default is 990, which means a basic block is hot if its execution count contributes to the upper 990 permilles — that's 99.0% — of the profiled execution of the whole program. Set it to 0 and it is never considered hot, periodt. Only used in LTO mode 💅
• hot-bb-frequency-fraction — the denominator n of the fraction 1/n of the entry block's execution frequency that a basic block of that function needs to at least have to be considered hot 👑. Default is 1000, meaning a basic block is hot in a function if it runs more frequently than 1/1000 of the frequency of that function's entry block. 0 = never considered hot, no cap.
• unlikely-bb-count-fraction — the denominator n of the fraction 1/n of the number of profiled runs of the whole program, below which a basic block's execution count must be for that block to be considered unlikely executed 🙈. Default is 20, so a basic block counts as unlikely executed if it runs in fewer than 1/20 — that's 5% — of the program's runs. 0 means it's always considered unlikely executed.
• max-predicted-iterations — the maximum number of loop iterations we predict statically ✨. This is giving 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 looks artificially cold next to the other one 😭
• builtin-expect-probability — controls the probability of the expression having the specified value. Takes a percentage as input, so like 0 ... 100 🎀
• 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 — pick the fraction of the maximal frequency of executions of a basic block in a function, and that's what aligns the 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 only used when profile feedback is available. Real profiles (as opposed to statically estimated ones) are much less balanced, which lets the threshold be a larger value 🌸

• tracer-max-code-growth — stop tail duplication once code growth has reached the given percentage. Lowkey an artificial limit, since most of the duplicates get eliminated later in cross jumping, so it may be set to way higher values than the code growth you actually want 😭
• tracer-min-branch-ratio — stop 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 — stop forward growth if the best edge has probability lower than this threshold ✂️

Same energy as tracer-dynamic-coverage: two parameters, because why not. tracer-min-branch-probability-feedback is used for compilation with profile feedback, and tracer-min-branch-probability for compilation without. The value for compilation with profile feedback needs to be more conservative (higher) to keep the tracer effective 👑

• stack-clash-protection-guard-size — specify 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 a value larger than the operating system provided guard will leave code vulnerable to stack clash style attacks 😭

bestie. that last sentence is a whole warning, okay? do not go bigger than the OS guard. that's the vibes.

• stack-clash-protection-probe-interval — stack clash protection means probing stack space as it's allocated. 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 will leave 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. This parameter specifies the minimum percentage by which the garbage collector's heap should be allowed to expand between collections. Tuning it may improve compilation speed; it has no effect on code generation ✨

The default is 30% + 70% * (RAM/1GB) with an upper bound of 100% when RAM >= 1GB. If getrlimit is available, the notion of "RAM" is the smallest of actual RAM and RLIMIT_DATA or RLIMIT_AS. If GCC 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 — which is extremely slow, but can be useful for debugging 💀

• ggc-min-heapsize — minimum size of the garbage collector's heap before it begins bothering to collect garbage. The first collection occurs after the heap expands by ggc-min-expand% beyond ggc-min-heapsize. Again, tuning this may improve compilation speed, and has no effect on code generation 🎀

The default is the smaller of RAM/8, RLIMIT_RSS, or a limit that tries to ensure RLIMIT_DATA or RLIMIT_AS are not exceeded, but with a lower bound of 4096 (four megabytes) and an upper bound of 131072 (128 megabytes). If GCC 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, which makes compilation time go up with probably slightly better performance 📈
• 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 🥲
• 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 are 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 a 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 👑
• sms-min-sc — the minimum value of stage count that swing modulo scheduler generates 🎛️
• max-last-value-rtl — the maximum size, measured as number of RTLs, that can be recorded in an expression in combiner for a pseudo register as the last known value of that register 📝
• max-combine-insns — the maximum number of instructions the RTL combiner tries to combine 🧬
• 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 💖
• ssp-buffer-size — the minimum size of buffers (i.e. arrays) that receive stack smashing protection when -fstack-protection is used 🛡️

Okay quick footnote on that last one, bestie — this default was "8" before Ubuntu 10.10, and now it's "4", and the whole reason is to bump up the number of functions that get protected by the stack protector. More armor, more slay. 💅

• min-size-for-stack-sharing — the smallest size a variable can be and still get to take part in stack slot sharing when you're not optimizing. Tiny vars don't get an invite to that party.
• max-jump-thread-duplication-stmts — the max number of statements allowed in a block that needs to be duplicated when we're threading jumps. Past that, no duplicating. 🙈
• max-fields-for-field-sensitive — the max number of fields a struct can have and still get treated in a field sensitive way during pointer analysis.
• prefetch-latency — your estimate of how many instructions, on average, get executed before a prefetch actually finishes. The distance we prefetch ahead is proportional to this constant. Heads up though: cranking this up can also mean fewer streams get prefetched (peep simultaneous-prefetches).
• simultaneous-prefetches — the max number of prefetches that are allowed to be running at the same time. Parallel queen behavior. 👑
• l1-cache-line-size — the size of a cache line in the L1 data cache, in bytes.
• l1-cache-size — the size of the L1 data cache, in kilobytes.
• l2-cache-size — the size of the L2 data cache, in kilobytes.
• prefetch-dynamic-strides — should the loop array prefetch pass drop software prefetch hints for strides that aren't constant? Sometimes that's a win, but ngl, if the stride isn't constant it can be hard to predict whether there's a clear benefit to issuing those hints at all.

Set it to 1 if hints should go out for non-constant strides. Set it to 0 if hints should only go out for strides that are known to be constant and below prefetch-minimum-stride.

• prefetch-minimum-stride — the minimum constant stride, in bytes, before we start using prefetch hints for it. If the stride is smaller than this threshold, no hints. Periodt.

This one's a lifesaver for processors with hardware prefetchers, because otherwise the hardware prefetchers and the software prefetchers can beef with each other. If your hardware prefetchers have a max stride they can handle, put that number here to get better use out of the software prefetchers. 🫶

A value of -1 means we have no threshold at all, so prefetch hints can be issued for any constant stride.

And reminder: this setting is only useful for strides that are known and constant.

• loop-interchange-max-num-stmts — the max number of stmts in a loop for it to be interchanged.
• loop-interchange-stride-ratio — the minimum ratio between the strides of two loops before interchanging them 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? This should always be 1, which uses a more efficient internal mechanism for comparing types in C++ and Objective-C++. However, if bugs in the canonical type system are causing compilation failures, set this value to 0 to disable canonical types. > Bug workaround, not a lifestyle choice, bestie.
• 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. That's the cap. 🎀
• max-partial-antic-length — the maximum length of the partial antic set computed during the tree partial redundancy elimination optimization (-ftree-pre) when you're optimizing at -O3 and above. For some kinds of source code, the enhanced partial redundancy elimination optimization can just run away and eat all the memory available on the host machine. 😭 This parameter puts a limit on the length of the sets that get computed, which stops the runaway behavior. Setting it to 0 allows an unlimited set length — do that at your own risk.
• 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 rest of them don't.
• sccvn-max-alias-queries-per-access — the max number of alias-oracle queries we do when hunting for redundancies in loads and stores. If we hit this limit, the search gets aborted and that load or store just isn't 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 does regional register allocation by default. If a function has more loops than the number you give here, then only at most that given number of the most frequently-executed loops form regions for regional register allocation.
• ira-max-conflict-table-size — IRA uses a fancy algorithm to compress the conflict table, but that table can still eat an excessive amount of memory for huge functions. If the conflict table for a function could be bigger than the size in MB given by this parameter, the register allocator switches to a faster, simpler, lower-quality algorithm that doesn't need to build a pseudo-register conflict table. 💀
• ira-loop-reserved-regs — IRA can be used to evaluate more accurate register pressure in loops, so it can decide whether to move loop invariants (see -O3). The number of available registers reserved for some other purposes is given by this parameter. The default is the best value found from numerous experiments — trust the science. 🔥
• lra-inheritance-ebb-probability-cutoff — LRA tries to reuse values that got reloaded into registers in subsequent insns. That optimization is called inheritance, and 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 very expensive, both in compilation time and in how much compile-time memory it needs, especially with giant loops. Loops with more basic blocks than this parameter won't have loop invariant motion optimization performed on them. Sorry bestie, too big. 🚫
• loop-max-datarefs-for-datadeps — building data dependencies is expensive for very large loops. This parameter limits the number of data references in loops that get considered for data dependence analysis. These large loops just aren't handled by the optimizations that use loop data dependencies.
• max-vartrack-size — sets a maximum number of hash table slots to use during variable tracking dataflow analysis of any function. If this limit gets exceeded while variable tracking at assignments is enabled, the analysis for that function is retried without it, after removing all debug insns from the function. If the limit is exceeded even without debug insns, var tracking analysis is completely disabled for that function. Setting the parameter to zero makes it unlimited.
• max-vartrack-expr-depth — sets a maximum number of recursion levels when we're trying to map variable names or debug temporaries to value expressions. This is a straight-up trade: compilation time in exchange for more complete debug information. If you set this too low, value expressions that are available and could have been represented in debug info might just end up unused. Set it higher and the compiler may find more complex debug expressions, but compile time and memory use may grow. 💸
• max-debug-marker-count — sets a threshold on the number of debug markers (like begin stmt markers) so we don't get a complexity explosion at inlining or expansion 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.
• min-nondebug-insn-uid — use uids starting at this parameter for nondebug insns. The range below the parameter is reserved exclusively for debug insns created by -fvar-tracking-assignments, though debug insns may get (non-overlapping) uids above it if the reserved range runs out.
• ipa-sra-ptr-growth-factor — IPA-SRA only replaces a pointer to an aggregate with one or more new parameters when their cumulative size is less than or equal to ipa-sra-ptr-growth-factor times the size of the original pointer parameter.
• ipa-sra-max-replacements — the maximum number of pieces of an aggregate that IPA-SRA tracks. Which, as a consequence, is also the maximum number of replacements of a formal parameter. ✨
• sra-max-scalarization-size-Ospeed, sra-max-scalarization-size-Osize — the two Scalar Reduction of Aggregates passes (SRA and IPA-SRA) are all about replacing the scalar parts of aggregates with uses of independent scalar variables. These parameters control the maximum size, in storage units, of an aggregate that gets considered for replacement when you're compiling for speed (sra-max-scalarization-size-Ospeed) or for size (sra-max-scalarization-size-Osize), respectively. Pick your fighter. 🦋
• sra-max-propagations — the maximum number of artificial accesses that Scalar Replacement of Aggregates (SRA) will track, per one local variable, so it can facilitate copy propagation.
• tm-max-aggregate-size — when making copies of thread-local variables in a transaction, this parameter specifies the size in bytes after which variables get saved with the logging functions instead of save/restore code sequence pairs. And fyi, this option only applies when you're using -fgnu-tm. 🧚

---

• graphite-max-nb-scop-params — so Graphite's loop transforms can go ✨exponential✨ if you let them, and we simply do not do that here. This caps how many parameters a Static Control Part (SCoP) can have. Wanna lift the cap? Set it to 0 and go wild. (Quick refresher: a parameter of a SCoP is any variable whose value isn't known at compile time and gets defined outside the SCoP. iykyk 💅)
• loop-block-tile-size — loop blocking and strip mining (the transforms you turn on with -floop-block or -floop-strip-mine) strip-mine every loop in the nest by a set number of iterations. This param is literally how you change that strip length. periodt.
• ipa-jump-function-lookups — says how many statements get visited while doing jump function offset discovery. that's it, that's the whole vibe.
• ipa-cp-value-list-size — okay so IPA-CP is out here trying to track all the possible values and types passed into a function's parameter so it can propagate them and do devirtualization 💖. This param is the max number of values and types it'll store per one formal parameter of a function. not infinite, bestie, there's a budget.
• ipa-cp-eval-threshold — IPA-CP scores its own cloning profitability heuristics, and it only actually does the cloning when a candidate's score beats ipa-cp-eval-threshold. no free clones, the math has to math. 🔥
• ipa-cp-max-recursive-depth — the maximum depth of recursive cloning for a self-recursive function. yes, recursion gets a depth limit, we're being responsible.
• ipa-cp-min-recursive-probability — recursive cloning only happens when the probability of the call actually being executed exceeds this parameter. low odds? no clone. simple as.
• ipa-cp-recursion-penalty — the percentage penalty recursive functions get slapped with when they're being evaluated for cloning. 💀 sorry recursive girlies.
• ipa-cp-single-call-penalty — same energy, different target: the percentage penalty that functions containing a single call to another function receive when they're evaluated for cloning.
• ipa-max-agg-items — IPA-CP can also propagate a bunch of scalar values that got passed inside an aggregate 🫶. 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 ipa-cp-loop-hint-bonus to that candidate's profitability score. we love a glow-up bonus ✨
• ipa-max-loop-predicates — the max number of different predicates IPA will use to describe when loops in a function have known properties.
• ipa-max-aa-steps — during function body analysis, IPA-CP uses alias analysis to track values pointed to by function parameters. But it's not about to spend forever analyzing giant functions, so after examining ipa-max-aa-steps statements that modify memory, it gives up and just considers all memory clobbered. dramatic? maybe. efficient? absolutely. 💅
• ipa-max-switch-predicate-bounds — the maximal number of boundary endpoints of case ranges of a switch statement. If a switch blows past this limit, IPA-CP will not construct a cloning cost predicate for the default case (that predicate is what's used to estimate cloning benefit). no predicate for you, default case.
• ipa-max-param-expr-ops — IPA-CP will analyze a conditional statement that references some function parameter to estimate the cloning benefit for a certain constant value. But if the number of operations in a parameter expression exceeds ipa-max-param-expr-ops, that expression gets treated as complicated and IPA just won't handle it. too much math, we're out. 😭
• lto-partitions — sets the desired number of partitions produced during WHOPR compilation. Pro tip from the manpage itself: the number of partitions should exceed the number of CPUs used for compilation. 🧚
• lto-min-partition — the size of the minimal partition for WHOPR (measured in estimated instructions). This is what stops you from paying the expense of splitting a tiny program into way too many partitions.
• lto-max-partition — the size of the max partition for WHOPR (again, in estimated instructions), giving an upper bound on the individual size of a partition. Meant to be used only with balanced partitioning. don't get cute with it. 🌸
• lto-max-streaming-parallelism — the maximal number of parallel processes used for LTO streaming.
• cxx-max-namespaces-for-diagnostic-help — the max number of namespaces to consult for suggestions when C++ name lookup fails for an identifier. your "did you mean" gets a budget too. 💖
• sink-frequency-threshold — the maximum relative execution frequency (in percents) of the target block relative to a statement's original block, which is what allows statement sinking of a statement. Bigger numbers = more aggressive statement sinking. And heads up: a small positive adjustment gets applied for statements with memory operands, since those are even more profitable to sink. 👑
• max-stores-to-sink — the 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. fr fr.
• 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, it uses the default for the machine.
• jump-table-max-growth-ratio-for-size — the maximum code size growth ratio when expanding into a jump table, in percent. This one's used when you're optimizing for size.
• jump-table-max-growth-ratio-for-speed — the maximum code size growth ratio when expanding into a jump table, in percent. And this one's used when you're optimizing for speed. same name energy, different goal, keep them straight bestie.
• tree-reassoc-width — sets the maximum number of instructions executed in parallel in a reassociated tree. If it has a non-zero value, this parameter overrides the target-dependent heuristics used by default.
• sched-pressure-algorithm — choose between the two available implementations of -fsched-pressure:
  • Algorithm 1 is the original implementation, and it's the more likely one to prevent instructions from being reordered.
  • Algorithm 2 was designed as a compromise between algorithm 1's relatively conservative approach and the rather aggressive approach taken by the default scheduler. It relies more heavily on having a regular register file and accurate register pressure classes.

For more details, see haifa-sched.c in the GCC sources. 🔍

The default choice depends on the target.

• max-slsr-cand-scan — sets the maximum number of existing candidates that get considered when seeking a basis for a new straight-line strength reduction candidate.
• asan-globals — enable buffer overflow detection for global objects. This kind of protection is on by default if you're using -fsanitize=address. To turn off global objects protection, use --param asan-globals=0.
• asan-stack — enable buffer overflow detection for stack objects. On by default when using -fsanitize=address. To disable stack protection, use --param asan-stack=0. 🫶
• asan-instrument-reads — enable buffer overflow detection for memory reads. On by default when using -fsanitize=address. To disable memory reads protection, use --param asan-instrument-reads=0.
• asan-instrument-writes — enable buffer overflow detection for memory writes. On by default when using -fsanitize=address. To disable memory writes protection, use --param asan-instrument-writes=0.
• asan-memintrin — enable detection for built-in functions. On by default when using -fsanitize=address. To disable built-in functions protection, use --param asan-memintrin=0.
• asan-use-after-return — enable detection of use-after-return. On by default when using -fsanitize=address. To disable it, use --param asan-use-after-return=0.

⚠️ Note: by default this check is disabled at run time. To enable it, add detect_stack_use_after_return=1 to the environment variable ASAN_OPTIONS. don't skip this step, it literally will not work otherwise. 😭

• asan-instrumentation-with-call-threshold — if the number of memory accesses in the function being instrumented is greater than 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.
• hwasan-instrument-stack — enable hwasan instrumentation of statically sized stack-allocated variables. This kind of instrumentation is on by default when using -fsanitize=hwaddress, and off 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, this decides tags for stack variables using a deterministic sequence that begins at a random tag for each frame. With this parameter unset, tags are chosen using the same sequence but beginning from 1. It's 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 — enable hwasan instrumentation of dynamically sized stack-allocated variables. On by default when using -fsanitize=hwaddress, and off 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 — enable hwasan checks on memory reads. Instrumentation of reads is on by default for both -fsanitize=hwaddress and -fsanitize=kernel-hwaddress. To disable checking memory reads, use --param hwasan-instrument-reads=0. 🌈

okay bestie we're still deep in the --param list, do not panic, just breathe 💖 these are all little tuning knobs for GCC's internals and we're getting through them one by one, no cap.

• hwasan-instrument-writes — turns on hwasan checks for memory writes. This instrumentation of writes is on by default for both -fsanitize=hwaddress and -fsanitize=kernel-hwaddress, so if you wanna stop it from checking memory writes you hit it with --param hwasan-instrument-writes=0.
• hwasan-instrument-mem-intrinsics — turns on hwasan instrumentation of builtin functions. Also on by default for both -fsanitize=hwaddress and -fsanitize=kernel-hwaddress. Wanna kill the instrumentation of builtin functions? --param hwasan-instrument-mem-intrinsics=0. periodt.
• use-after-scope-direct-emission-threshold — if a local variable's size in bytes is smaller than or equal to this number, GCC just directly poisons (or unpoisons) shadow memory instead of bothering with run-time callbacks. shortcut queen behavior ✨
• 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 — the max number of instructions to copy when duplicating blocks on a finite state automaton jump thread path.
• max-fsm-thread-length — the max number of basic blocks on a finite state automaton jump thread path.
• max-fsm-thread-paths — the max number of new jump thread paths to create for a finite state automaton.

---

• 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. Your options are 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. Heads up: for a parallelized loop nest, the minimum number of iterations of the outermost loop per thread is two.
• max-ssa-name-query-depth — max depth of recursion when querying properties of SSA names in things like fold routines. One level of recursion = following a use-def chain. iykyk 💅
• max-speculative-devirt-maydefs — the maximum number of may-defs we analyze when we're looking for a must-def that specifies the dynamic type of an object invoking a virtual call we might be able to devirtualize speculatively.
• max-vrp-switch-assertions — the maximum number of assertions to add along the default edge of a switch statement during VRP.
• evrp-mode — specifies the mode Early VRP should operate in.

---

• unroll-jam-min-percent — the minimum percentage of memory references that have to be optimized away before the unroll-and-jam transformation counts as 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's considered unpredictable.
• max-variable-expansions-in-unroller — if -fvariable-expansion-in-unroller is used, this is the maximum number of times that an individual variable will be expanded during loop unrolling.
• tracer-min-branch-probability-feedback — stop forward growth if the probability of the best edge is less than this threshold (in percent). Used when profile feedback is available.
• 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 the 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 a loop header duplicated by the copy loop headers pass.
• vect-epilogues-nomask — enable loop epilogue vectorization using a 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. Lowkey important: this parameter only has an effect on targets that support partial vector loads and stores.
• avoid-fma-max-bits — maximum number of bits for which we avoid creating FMAs.
• 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.
• max-inline-insns-recursive-auto — the maximum number of instructions a non-inline function can grow to via recursive inlining.
• graphite-allow-codegen-errors — whether codegen errors should be ICEs when -fchecking.
• sms-max-ii-factor — a factor for tuning the upper bound that the 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 — maximum depth of sqrt chains to use when synthesizing exponentiation by a real constant.
• max-dse-active-local-stores — maximum number of active local stores in RTL dead store elimination.
• asan-instrument-allocas — enable asan allocas/VLAs protection.
• max-iterations-computation-cost — bound on the cost of an expression to compute the number of iterations.
• max-isl-operations — maximum number of isl operations, 0 means unlimited.
• graphite-max-arrays-per-scop — maximum number of arrays per scop.
• max-vartrack-reverse-op-size — max size of loc list for which reverse ops should be added.
• tracer-dynamic-coverage-feedback — the percentage of function, weighted by execution frequency, that must be covered by trace formation. Used when profile feedback is available.
• max-inline-recursive-depth-auto — the maximum depth of recursive inlining for non-inline functions.

---

• fsm-scale-path-stmts — scale factor to apply to the number of statements in a threading path when comparing to the number of (scaled) blocks.
• fsm-maximum-phi-arguments — maximum number of arguments a PHI may have before the FSM threader will not try to thread through its block.
• uninit-control-dep-attempts — maximum number of nested calls to search for control dependencies during uninitialized variable analysis.
• sra-max-scalarization-size-Osize — maximum size, in storage units, of an aggregate which should be considered for scalarization when compiling for size.
• fsm-scale-path-blocks — scale factor to apply to the number of blocks in a threading path when comparing to the number of (scaled) statements.
• sched-autopref-queue-depth — hardware autoprefetcher scheduler model control flag. Number of lookahead cycles the model looks into; at ' ' only enable instruction sorting heuristic. (yes bestie that's literally a single space in quotes, iconic 🫶)
• loop-versioning-max-inner-insns — the maximum number of instructions that an inner loop can have before the loop versioning pass considers it too big to copy.
• loop-versioning-max-outer-insns — the maximum number of instructions that an outer loop can have before the loop versioning pass considers it too big to copy, discounting any instructions in inner loops that directly benefit from versioning.
• ssa-name-def-chain-limit — the maximum number of SSA_NAME assignments to follow in determining a property of a variable such as its value. This limits the number of iterations or recursive calls GCC performs when optimizing certain statements or when determining their validity prior to issuing diagnostics.

---

• store-merging-max-size — maximum size of a single store merging region in bytes.
• hash-table-verification-limit — the number of elements for which hash table verification is done for each searched element.
• max-find-base-term-values — maximum number of VALUEs handled during a single find_base_term call.
• analyzer-max-enodes-per-program-point — the maximum number of exploded nodes per program point within the analyzer, before terminating analysis of that point.
• analyzer-max-constraints — the maximum number of constraints per state.
• analyzer-min-snodes-for-call-summary — the minimum number of supernodes within a function for the analyzer to consider summarizing its effects at call sites.
• analyzer-max-enodes-for-full-dump — the maximum depth of exploded nodes that should appear in a dot dump before switching to a less verbose format.
• analyzer-max-recursion-depth — the maximum number of times a callsite can appear in a call stack within the analyzer, before terminating analysis of a call that would recurse deeper.
• analyzer-max-svalue-depth — the maximum depth of a symbolic value, before approximating the value as unknown.
• analyzer-max-infeasible-edges — the maximum 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 is considered hot. This parameter is used only in GIMPLE FE.

okay and we are STILL not done, the param list just keeps going, stay with me 🌈

• analyzer-bb-explosion-factor — the max number of 'after supernode' exploded nodes chillin' inside the analyzer per supernode before it's like "okay bestie we're done here" and terminates the analysis. 💀
• ranger-logical-depth — the maximum depth of logical expression evaluation that ranger will look through when it's evaluating outgoing edge ranges. iykyk. 🔍
• openacc-kernels — picks the mode for how OpenACC kernels constructs get handled. So with --param=openacc-kernels=decompose, OpenACC kernels constructs get broken down into parts, a whole sequence of compute constructs, and each one is then handled individually. Heads up tho: this is work in progress fr. With --param=openacc-kernels=parloops, OpenACC kernels constructs are handled by the parloops pass, en bloc. That one's the current default. ✨

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🦋 AArch64 target params, bestie 🦋

The following choices of name are available on AArch64 targets:

• aarch64-sve-compare-costs — when you're vectorizing for SVE, this says consider using "unpacked" vectors for smaller elements and use the cost model to pick the cheapest approach. It also uses the cost model to choose between SVE and Advanced SIMD vectorization.

Using unpacked vectors includes storing smaller elements in larger containers and accessing elements with extending loads and truncating stores. slay.

• 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 propotional to this param when division approximation is enabled. The default value is 2. (yes, "propotional" — that's literally how it's spelled in the source, iconic typo behavior, we keep it exactly 💅)
• aarch64-autovec-preference — forces an ISA selection strategy for auto-vectorization. Accepts values from 0 to 4, inclusive.
  • 0 — use the default heuristics.
  • 1 — use only Advanced SIMD for auto-vectorization.
  • 2 — use only SVE for auto-vectorization.
  • 3 — use both Advanced SIMD and SVE. Prefer Advanced SIMD when the costs are deemed equal.
  • 4 — use both Advanced SIMD and SVE. Prefer SVE when the costs are deemed equal.

The default value is 0.

• aarch64-loop-vect-issue-rate-niters — 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. 🧚

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🎀 Program Instrumentation Options 🎀

GCC supports a number of command-line options that control adding run-time instrumentation to the code it normally generates. 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. There is also a general hook which can be used to implement other forms of tracing or function-level instrumentation for debug or program analysis purposes.

Okay so like, this whole section is GCC being nosy on purpose — and that's the point. 👑

• -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.

• -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). 💖

• --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.
  • For profile-directed optimizations, compile the source files again with the same optimization and code generation options plus -fbranch-probabilities.
  • For test coverage analysis, use gcov to produce human readable information from the .gcno and .gcda files. Refer to the gcov documentation for further information.

With -fprofile-arcs, for each function of your program GCC creates a program flow graph, then finds a spanning tree for the graph. Only arcs that are not on the spanning tree have to be instrumented: the compiler adds code to count the number of times that these arcs are executed. When an arc is the only exit or only entrance to a block, the instrumentation code can be added to the block; otherwise, a new basic block must be created to hold the instrumentation code. 🌸

• -ftest-coverage — produce a notes file that the gcov code-coverage utility can use to show program coverage. Each source file's note file is called auxname.gcno. Refer to the -fprofile-arcs option above for a description of auxname and instructions on how to generate test coverage data. Coverage data matches the source files more closely if you do not optimize. (ngl, optimize anyway if you must, but know the tradeoff bestie.)
• -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.
• -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 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.

The following options are enabled: -fprofile-arcs, -fprofile-values, -finline-functions, and -fipa-bit-cp.

If path is specified, GCC looks at the path to find the profile feedback data files. See -fprofile-dir.

To optimize the program based on the collected profile information, use -fprofile-use.

• -fprofile-info-section
• -fprofile-info-section=name — register the profile information in the specified section instead of using a constructor/destructor. The section name is name if it is specified, otherwise the section name defaults to ".gcov_info". A pointer to the profile information generated by -fprofile-arcs or -ftest-coverage is placed in the specified section for each translation unit. This option disables the profile information registration through a constructor and it disables the profile information processing through a destructor. This option is not intended to be used in hosted environments such as GNU/Linux. It targets systems with limited resources which do not support constructors and destructors. The linker could collect the input sections in a continuous memory block and define start and end symbols. The runtime support could dump the profiling information registered in this linker set during program termination to a serial line for example. 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 = .);
                     }

---

• -fprofile-note=path — bestie, if you give it a path, GCC drops the .gcno file right there in that location. But heads up 🚨 if you pair this option with multiple source files, the .gcno file gets overwritten. One file, one survivor. 💀
• -fprofile-prefix-path=path — this one's a little team player 🫶 you use it together with profile-generate=profile_dir and profile-use=profile_dir to tell GCC where the base directory of your built source tree actually lives. Here's the tea ☕ by default, profile_dir is full of files with mangled absolute paths of every object file in the built project. That's lowkey a problem when the directory you built the instrumented binary in is different from the directory you build the profile-feedback-optimized binary in — because then the profile data just… won't be found during the optimized build. 😭 So in those setups, you whip out -fprofile-prefix-path=path with path pointing at the base directory of the build, and it strips off the irrelevant part of the path so all the file names stay relative to the main build directory. Slay. ✨
• -fprofile-update=method — changes the update method for an app that's been instrumented for profile feedback based optimization. The method argument should be one of single, atomic, or prefer-atomic. single is the vibe for single-threaded applications, while atomic stops profile corruption by emitting thread-safe code. 🧵

Warning: when an application doesn't properly join all threads (or goes and creates a detached thread 💀), a profile file can still be corrupted. No cap.

Using prefer-atomic gets transformed into either atomic — when the target supports it — or into single otherwise. And the GCC driver automatically picks prefer-atomic when -pthread shows up on the command line. 👑

• -fprofile-filter-files=regex — instruments only functions from files whose name matches any of the regular expressions (separated by semi-colons). So for example, -fprofile-filter-files=main\.c;module.*\.c will instrument only main.c and all C files starting with 'module'. 💅
• -fprofile-exclude-files=regex — the opposite twin 🪞 instruments only functions from files whose name does not match any of the regular expressions (separated by semi-colons). Example time: -fprofile-exclude-files=/usr/.* will prevent instrumentation of all files located in the /usr/ folder. Bye bestie ✌️
• -fprofile-reproducible=[multithreaded|parallel-runs|serial] — controls the level of reproducibility of the profile gathered by -fprofile-generate. This makes it possible to rebuild a program with the same outcome, which is useful — for example — for distribution packages. 📦

With -fprofile-reproducible=serial, the profile gathered by -fprofile-generate is reproducible provided the trained program behaves the same at each invocation of the train run, it's not multi-threaded, and profile data streaming is always done in the same order. Note that profile streaming happens at the end of the program run, but also before fork is invoked. 🌱

Also note: it's quite common for execution counts of some part of programs to depend on stuff like the length of temporary file names or memory space randomization (which may affect hash-table collision rate). That non-reproducible part of your program can be annotated with the no_instrument_function function attribute. And gcov-dump with -l can dump the gathered data so you can verify it's actually reproducible. 🔍

With -fprofile-reproducible=parallel-runs, the collected profile stays reproducible regardless of the order the data gets streamed into gcda files. This setting lets you run multiple instances of the instrumented program in parallel (like with make -j 🏃♀️🏃♀️). It does reduce the quality of the gathered data though — in particular of indirect call profiling. Trade-offs, bestie.

• -fsanitize=address — turns on AddressSanitizer, a fast memory error detector. 🚨 Memory access instructions get 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 with the ASAN_OPTIONS environment variable — set it to help=1 and all the available options show up at startup of the instrumented program. 👀 See <https://github.com/google/sanitizers/wiki/AddressSanitizerFlags#run-time-flags> for a list of supported options. This option cannot be combined with -fsanitize=thread or -fsanitize=hwaddress. And ngl, the only target -fsanitize=hwaddress is currently supported on is AArch64.
• -fsanitize=kernel-address — enables AddressSanitizer for the Linux kernel. 🐧 See <https://github.com/google/kasan> for more details.
• -fsanitize=hwaddress — enables Hardware-assisted AddressSanitizer, which uses a hardware ability to ignore the top byte of a pointer so it can detect memory errors with low memory overhead. ✨ Memory access instructions get instrumented to detect out-of-bounds and use-after-free bugs. The option enables -fsanitize-address-use-after-scope. See <https://clang.llvm.org/docs/HardwareAssistedAddressSanitizerDesign.html> for more details. Run-time behavior can be influenced with the HWASAN_OPTIONS environment variable — set it to help=1 and the available options are shown at startup of the instrumented program. This option cannot be combined with -fsanitize=thread or -fsanitize=address, and is currently only available on AArch64.
• -fsanitize=kernel-hwaddress — enables Hardware-assisted AddressSanitizer for compilation of the Linux kernel. 🐧 Similar to -fsanitize=kernel-address but using an alternate instrumentation method, and similar to -fsanitize=hwaddress but with instrumentation differences necessary for compiling the Linux kernel. Those differences exist 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 -fsanitize=hwaddress. Instrumenting the stack and alloca calls are not on by default, but they're 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 operations (<, <=, >, >=) with pointer operands. 🫱🫲 This option must be combined with either -fsanitize=kernel-address or -fsanitize=address. It cannot be combined with -fsanitize=thread. Note: by default the check is disabled at run time — to enable it, add detect_invalid_pointer_pairs=2 to the environment variable ASAN_OPTIONS. Using detect_invalid_pointer_pairs=1 detects an invalid operation only when both pointers are non-null. 🧐
• -fsanitize=pointer-subtract — instruments subtraction with pointer operands. ➖ This option must be combined with either -fsanitize=kernel-address or -fsanitize=address. It cannot be combined with -fsanitize=thread. Note: by default the check is disabled at run time — to enable it, add detect_invalid_pointer_pairs=2 to the environment variable ASAN_OPTIONS. Using detect_invalid_pointer_pairs=1 detects an invalid operation only when both pointers are non-null. 🧐
• -fsanitize=thread — enables ThreadSanitizer, a fast data race detector. 🧵💥 Memory access instructions get instrumented to detect data race bugs. See <https://github.com/google/sanitizers/wiki#threadsanitizer> for more details. The run-time behavior can be influenced with the TSAN_OPTIONS environment variable; see <https://github.com/google/sanitizers/wiki/ThreadSanitizerFlags> for a list of supported options. This option cannot be combined with -fsanitize=address, -fsanitize=leak.

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, and 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 with the LSAN_OPTIONS environment variable. This option cannot be combined with -fsanitize=thread.
• -fsanitize=undefined — enables UndefinedBehaviorSanitizer, a fast undefined behavior detector. 👻 Various computations get 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 with the UBSAN_OPTIONS environment variable. Current suboptions are:
  • -fsanitize=shift — 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 — 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 — detects integer division by zero as well as INT_MIN / -1 division. 🚫➗
  • -fsanitize=unreachable — with this option, the compiler turns the __builtin_unreachable call into a diagnostics message call instead. When reaching the __builtin_unreachable call, the behavior is undefined. ⚠️
  • -fsanitize=vla-bound — instructs the compiler to check that the size of a variable length array is positive. 📐
• -fsanitize=null — bestie this one is the pointer patrol 👮‍♀️ it turns on pointer checking. So like, an app built with this on will throw an error message at you when it tries to dereference a NULL pointer, OR when a reference (and yes, possibly an rvalue reference 👀) gets bound to a NULL pointer, OR when a method gets invoked on an object that's being pointed at by a NULL pointer. Basically the three ways NULL tries to ruin your day.
• -fsanitize=return — return statement checking, periodt. Programs built with this turned on will yell at you when the end of a non-void function gets reached without actually returning a value 😭 heads up tho: this one works in C++ only.
• -fsanitize=signed-integer-overflow — signed integer overflow checking, and the way this works is we check that the result of +, *, and both unary and binary - does not overflow in the signed arithmetics. ⚠️ Note: integer promotion rules must be taken into account. So like, this next thing is not an overflow:

signed char a = SCHAR_MAX;
a++;

• -fsanitize=bounds — turns on instrumentation of array bounds 💅 various out of bounds accesses get detected. Not instrumented tho: flexible array members, flexible array member-like arrays, and initializers of variables with static storage.
• -fsanitize=bounds-strict — the stricter version, lowkey the overachiever of the bounds family ✨ most out of bounds accesses get detected, including flexible array members and flexible array member-like arrays. Still not instrumented: initializers of variables with static storage.
• -fsanitize=alignment — checking the alignment of pointers when they get dereferenced, or when a reference is bound to an insufficiently aligned target, or when a method or constructor is invoked on an insufficiently aligned object. Alignment really said "vibe check" 📐
• -fsanitize=object-size — instrumentation of memory references using the __builtin_object_size function. Various out of bounds pointer accesses get detected 🎯
• -fsanitize=float-divide-by-zero — detects floating-point division by zero. And here's the tea 🫖 unlike the other similar options, -fsanitize=float-divide-by-zero is not enabled by -fsanitize=undefined, because floating-point division by zero can be a legitimate way of obtaining infinities and NaNs. It's not a bug, it's a lifestyle.
• -fsanitize=float-cast-overflow — turns on floating-point type to integer conversion checking. We check that the result of the conversion does not overflow. Same energy as above: unlike the other similar options, -fsanitize=float-cast-overflow is not enabled by -fsanitize=undefined. Also heads up, this option does not work well with FE_INVALID exceptions enabled 💀
• -fsanitize=nonnull-attribute — instrumentation of calls, checking whether null values are not passed to arguments marked as requiring a non-null value by the nonnull function attribute. Like, the attribute said non-null, so we're holding you to it 💅
• -fsanitize=returns-nonnull-attribute — instrumentation of return statements in functions marked with the returns_nonnull function attribute, to detect returning of null values from such functions. Caught in 4K 🙈
• -fsanitize=bool — instrumentation of loads from bool. If a value other than 0/1 gets loaded, a run-time error is issued. There's only two bools bestie, pick one.
• -fsanitize=enum — instrumentation of loads from an enum type. If a value outside the range of values for the enum type gets loaded, a run-time error is issued 🚨
• -fsanitize=vptr — instrumentation of C++ member function calls, member accesses and some conversions between pointers to base and derived classes, all to verify the referenced object has the correct dynamic type. Very "who do you think you are" of it 👑
• -fsanitize=pointer-overflow — instrumentation of pointer arithmetics. If the pointer arithmetics overflows, a run-time error is issued. Math is mathing and it's not supposed to be 📉
• -fsanitize=builtin — instrumentation of arguments to selected builtin functions. If an invalid value gets passed to such arguments, 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. Sir, that's not a valid input 🫠

Okay so real quick, comparing the two: while -ftrapv causes traps for signed overflows to be emitted, -fsanitize=undefined gives a diagnostic message. This currently works only for the C family of languages.

• -fno-sanitize=all — this one disables all previously enabled sanitizers. And nope, -fsanitize=all is not allowed, because some sanitizers cannot be used together. They're not friends like that 💀
• -fasan-shadow-offset=number — forces GCC to use a custom shadow offset in AddressSanitizer checks. Useful for experimenting with different shadow memory layouts in Kernel AddressSanitizer 🔬
• -fsanitize-sections=s1,s2,... — sanitize global variables in selected user-defined sections. And psst, si may contain wildcards 🃏
• -fsanitize-recover[=opts] — -fsanitize-recover= controls error recovery mode for sanitizers mentioned in the comma-separated list of opts. Enabling this option for a sanitizer component causes it to attempt to continue running the program as if no error happened. 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. 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.

⚠️ 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 🧹
• -fsanitize-undefined-trap-on-error — this option instructs the compiler to report undefined behavior using __builtin_trap rather than a libubsan library routine. The advantage? The libubsan library is not needed and is not linked in, so this is usable even in freestanding environments 🏕️ iconic for the low-resource girlies.
• -fsanitize-coverage=trace-pc — enable coverage-guided fuzzing code instrumentation. Inserts a call to __sanitizer_cov_trace_pc into every basic block 🐛
• -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, __sanitizer_cov_trace_cmpf or __sanitizer_cov_trace_cmpd for float or double comparisons, and __sanitizer_cov_trace_switch for switch statements. Okay that was a lot of names but we listed every single one bestie 💖
• -fcf-protection=[full|branch|return|none|check] — enable code instrumentation of control-flow transfers 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 such threats as Return-oriented Programming (ROP), and similarly call/jmp-oriented programming (COP/JOP).

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 value none turns off instrumentation.

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 macro __CET__ is defined when -fcf-protection is used. The first bit of __CET__ is set to 1 for the value branch and the second bit of __CET__ is set to 1 for the return ✨

You can also use the nocf_check attribute to identify which functions and calls should be skipped from instrumentation.

Currently the x86 GNU/Linux target provides an implementation based on Intel Control-flow Enforcement Technology (CET) which works for i686 processor or newer 🖥️

NOTE: In Ubuntu 19.10 and later versions, -fcf-protection is enabled by default for C, C++, ObjC, ObjC++, if none of -fno-cf-protection nor -fcf-protection=* are found. So like, surprise, it's already on 🌈

• -fstack-protector — bestie, this one adds extra code to sniff out buffer overflows, like those stack smashing attacks 💀. How? It drops a lil' guard variable into functions that have vulnerable objects. That means functions that call alloca, plus functions with buffers that are bigger than or equal to 8 bytes. The guards get initialized when a function is entered, then checked when it exits. Guard check fails? An error message gets printed and the program exits, no cap. Only variables that are actually allocated on the stack count — optimized-away variables or variables chilling in registers do not count, periodt.
• -fstack-protector-all — same vibe as -fstack-protector except every single function gets protected. All of them. Everyone eats. 💅
• -fstack-protector-strong — like -fstack-protector but it scoops up extra functions to protect — the ones with local array definitions, or the ones that reference local frame addresses. And same rule as before: only variables actually allocated on the stack are considered, optimized-away variables or register variables don't count. ✨
• -fstack-protector-explicit — like -fstack-protector, but it only protects the functions that carry the stack_protect attribute. Very exclusive. Invite only. 🎀
• -fstack-check — this generates code to verify you're not stepping past the boundary of the stack 🦋. You should specify this flag if you're running in an environment with multiple threads, but you only rarely need it in a single-threaded environment, because stack overflow is automatically detected on nearly all systems when there's only one stack.

Heads up: this switch does not actually cause checking to happen. The operating system or the language runtime has to do that part. What the switch does is generate code so that they can see the stack being extended. 👀

You can also pass a string parameter: no means no checking, generic means force the use of old-style checking, and specific means use the best checking method — which is equivalent to bare -fstack-check.

Old-style checking is a generic mechanism that needs no specific target support in the compiler, but omg it comes with drawbacks:

1. Modified allocation strategy for large objects: they're always allocated dynamically if their size exceeds a fixed threshold. Note this may change the semantics of some code. 😭
2. Fixed limit on the size of the static frame of functions: when it's topped by a particular function, stack checking is not reliable and a warning is issued by the compiler.
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 — detecting infinite recursion and stack overflows. specific is an excellent choice when compiling Ada code, fr fr. It is not generally sufficient to protect against stack-clash attacks. To protect against those you want -fstack-clash-protection. 👑

• -fstack-clash-protection — generates code to prevent stack clash style attacks. When it's on, the compiler only allocates one page of stack space at a time, and each page is accessed immediately after allocation. So it stops allocations from jumping over any stack guard page the operating system set up. Obsessed with the logic tbh. 🫶

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.

NOTE: In Ubuntu 19.10 and later versions, -fstack-clash-protection is enabled by default for C, C++, ObjC, ObjC++, unless -fno-stack-clash-protection is found. 🍓

• -fstack-limit-register=reg, -fstack-limit-symbol=sym, -fno-stack-limit — these generate code to make sure the stack does not grow beyond a certain value, either the value of a register or the address of a symbol. Need a bigger stack than that? A signal is raised at run time, simple as. For most targets the signal is raised before the stack overruns the boundary, so you can actually catch the signal without taking special precautions. 🌈

For instance, if the stack starts at absolute address 0x80000000 and grows downwards, you can use the flags -fstack-limit-symbol=__stack_limit and -Wl,--defsym,__stack_limit=0x7ffe0000 to enforce a stack limit of 128KB. Note that this may only work with the GNU linker.

You can locally override stack limit checking by using the no_stack_limit function attribute. ✨

• -fsplit-stack — generates code that automatically splits the stack before it overflows. The resulting program has a discontiguous stack which can only overflow if the program is unable to allocate any more memory. This is most useful when running threaded programs, since it's no longer necessary to calculate a good stack size to use for each thread. Currently this is only implemented for the x86 targets running GNU/Linux. 💅

When code compiled with -fsplit-stack calls code compiled without -fsplit-stack, there may not be much stack space available for the latter code to run. If compiling all code — including library code — with -fsplit-stack 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. 👑

• -fvtable-verify=[std|preinit|none] — this one is only available when compiling C++ code, bestie. 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 is detected at run time, an error is reported and execution of the program is immediately halted. 💀

This option causes run-time data structures to be built at program startup, and those get used for verifying the vtable pointers. The options std and preinit control the timing of when these data structures are built. In both cases the data structures are built before execution reaches main. Using -fvtable-verify=std causes the data structures to be built after shared libraries have been loaded and initialized. -fvtable-verify=preinit causes them to be built before shared libraries have been loaded and initialized. 🦋

If this option appears multiple times on the command line with different values specified, none takes highest priority over both std and preinit; preinit takes priority over std. Ranking system, iykyk. 🎀

• -fvtv-debug — when used in conjunction with -fvtable-verify=std or -fvtable-verify=preinit, this causes debug versions of the runtime functions for the vtable verification feature to be called. This flag also makes the compiler log information about which vtable pointers it finds for each class. That info gets 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. 🌸

Note: This feature appends data to the log file. If you want a fresh log file, be sure to delete any existing one. 🙈

• -fvtv-counts — this is a debugging flag. When used in conjunction with -fvtable-verify=std or -fvtable-verify=preinit, it makes the compiler keep track of the total number of virtual calls it encounters and the number of verifications it inserts. It also counts the number of calls to certain run-time library functions that it inserts, and logs this information for each compilation unit. The compiler writes this info 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 information to vtv_class_set_sizes.log in the same directory. 📊

Note: This feature appends data to the log files. To get fresh log files, be sure to delete any existing ones. 🫶

• -finstrument-functions — generates instrumentation calls for entry and exit to functions. Just after function entry and just before function exit, the following profiling functions are called with the address of the current function and its call site. (On some platforms, __builtin_return_address does not work beyond the current function, so the call site information may not be available to the profiling functions otherwise.) 👀

void __cyg_profile_func_enter (void *this_fn,
                               void *call_site);
void __cyg_profile_func_exit  (void *this_fn,
                               void *call_site);

The first argument is the address of the start of the current function, which may be looked up exactly in the symbol table. ⭐

This instrumentation is also done for functions expanded inline in other functions. The profiling calls indicate where, conceptually, the inline function is entered and exited. This means that addressable versions of such functions must be available. If all your uses of a function are expanded inline, this may mean an additional expansion of code size. If you use extern inline in your C code, an addressable version of such functions must be provided. (This is normally the case anyway, but if you get lucky and the optimizer always expands the functions inline, you might have gotten away without providing static copies.) 🍓

A function may be given the attribute no_instrument_function, in which case this instrumentation is not done. This can be used, for example, for the profiling functions listed above, high-priority interrupt routines, and any functions from which the profiling functions cannot safely be called (perhaps signal handlers, if the profiling routines generate output or allocate memory). 💜

• -finstrument-functions-exclude-file-list=file,file,... — okay so this sets up your exclusion list for -finstrument-functions (go read that one if you skipped it 💅). If the file that holds a function definition matches any of the file entries, that function just... doesn't get instrumented. And the match is a substring match: if file is a substring of the file name, congrats, it's a match. ✨

Like, for example:

-finstrument-functions-exclude-file-list=/bits/stl,include/sys

this excludes any inline function defined in files whose pathnames contain /bits/stl or include/sys. Slay. 🎀

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,... — literally the same energy as -finstrument-functions-exclude-file-list but for function names instead of files. The function name you match against is its user-visible name, like vector<int> blah(const vector<int> &), not the internal mangled name (e.g., _Z4blahRSt6vectorIiSaIiEE) — she doesn't want the mangled one, bestie. 🙈 The match is substring-based: if sym is a substring of the function name, it counts as a match. And for C99 and C++ extended identifiers, the function name must be given in UTF-8, not using universal character names. No cap. 💜
• -fpatchable-function-entry=N[,M] — this one 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. Those NOP instructions are reserving extra space for you to patch in literally any instrumentation you want at run time — provided that the code segment is writable, so like, vibe check that first. 🔥 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. It's giving very "depends on the situation" of it. 😭

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. You can use this 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.

The maximum value of N and M is 65535. Don't test fate, she will humble you. 💀

---

✨ Options Controlling the Preprocessor ✨

These options control the C preprocessor, which is run on each C source file before actual compilation. She goes first, always. 👑

If you use the -E option, nothing is done except preprocessing. Periodt. Some of these options only make sense together with -E because they cause the preprocessor output to be unsuitable for actual compilation — like, it's not for compiling, it's for looking. 🦋

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. Go peek at those too, iykyk.

• -D name — predefine name as a macro, with definition 1. Simple. Iconic. 🌸
• -D name=definition — the contents of definition are tokenized and processed as if they appeared during translation phase three in a #define directive. In particular, the definition is truncated by embedded newline characters. 💅

If you're invoking the preprocessor from a shell or shell-like program, you may need to use the shell's quoting syntax to protect characters such as spaces that have a meaning in the shell syntax. Shells are dramatic like that.

If you wish to define a function-like macro on the command line, write its argument list with surrounding parentheses before the equals sign (if any). Parentheses are meaningful to most shells, so you should quote the option. With sh and csh, -D'name(args...)=definition' works. 🫶

-D and -U options are processed in the order they are given on the command line. All -imacros file and -include file options are processed after all -D and -U options. Order matters, bestie.

• -U name — cancel any previous definition of name, either built in or provided with a -D option. Not me undefining your whole macro. 😭
• -include file — process file as if #include "file" appeared as the first line of the primary source file. However! The first directory searched for file is the preprocessor's working directory instead of the directory containing the main source file. If it's not found there, it's searched for in the remainder of the #include "..." search chain as normal. Sneaky but make it fashion. ✨

If multiple -include options are given, the files are included in the order they appear on the command line.

• -imacros file — exactly like -include, except that any output produced by scanning file is thrown away. 🗑️ But the macros it defines remain defined. This lets you acquire all the macros from a header without also processing its declarations — grab the vibes, leave the baggage. 💖

All files specified by -imacros are processed before all files specified by -include.

• -undef — do not predefine any system-specific or GCC-specific macros. The standard predefined macros remain defined, though. She's not that undef. 🌈
• -pthread — define additional macros required for using the POSIX threads library. You should use this option consistently for both compilation and linking, okay? Both. Or it's giving chaos. 💅 This option is supported on GNU/Linux targets, most other Unix derivatives, and also on x86 Cygwin and MinGW targets. 👑
• -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. Very main character, very dependency-graph. 🔥

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, the rule is split into several lines using \-newline. The rule has no commands.

This option does not suppress the preprocessor's debug output, such as -dM. To avoid mixing such debug output with the dependency rules you should explicitly specify the dependency output file with -MF, or use an environment variable like DEPENDENCIES_OUTPUT. Debug output is still sent to the regular output stream as normal.

Passing -M to the driver implies -E, and suppresses warnings with an implicit -w. ✨

• -MM — 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. System headers stay unbothered. 🙈

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.

• -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.

If file is -, then the dependencies are written to stdout. 👑

• -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. No point, so she skips it. 💅

This feature is used in automatic updating of makefiles.

• -Mno-modules — disable dependency generation for compiled module interfaces. Bye. 🦋
• -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. She's doing damage control, respectfully. 😭

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.

For example, -MT '$(objpfx)foo.o' might give

$(objpfx)foo.o: foo.c

• -MQ target — same as -MT, but it quotes any characters which are special to Make. -MQ '$(objpfx)foo.o' gives

$$(objpfx)foo.o: foo.c

The default target is automatically quoted, as if it were given with -MQ. She quotes herself, it's called self-love. 💅

• -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 is everything, 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 icon behavior. ✨

• -MMD — like -MD except mention only user header files, not system header files. 💖
• -fpreprocessed — tells the preprocessor bestie that the file it's getting has already been through the preprocessor 💅. So it suppresses stuff like macro expansion, trigraph conversion, escaped newline splicing, and processing of most directives. It still recognizes and removes comments though, so you can hand it a file preprocessed with -C with zero problems. In this mode the integrated preprocessor is basically just a tokenizer for the front ends, and that's it.

-fpreprocessed is implicit if your input file has one of these extensions: .i, .ii or .mi. Those are literally the extensions GCC uses for preprocessed files created by -save-temps.

• -fdirectives-only — when preprocessing, handle directives but do not expand macros.

The behavior depends on -E and -fpreprocessed, so listen up:

• With -E, preprocessing is limited to handling directives like "#define", "#ifdef", and "#error". Other preprocessor operations, like macro expansion and trigraph conversion, are not performed. Also, -dD is implicitly enabled.
• With -fpreprocessed, predefinition of command line and most builtin macros is disabled. Macros like "__LINE__", which are contextually dependent, get handled normally. This is what lets you compile files previously preprocessed with "-E -fdirectives-only".
• With both -E and -fpreprocessed, the rules for -fpreprocessed take precedence. That enables full preprocessing of files previously preprocessed with "-E -fdirectives-only".

• -fdollars-in-identifiers — accept $ in identifiers. iconic, honestly.
• -fextended-identifiers — accept universal character names and extended characters in identifiers. This one is enabled by default for C99 (and later C standard versions) and C++. 🦋
• -fno-canonical-system-headers — when preprocessing, do not shorten system header paths with canonicalization.
• *-fmax-include-depth=depth* — set the maximum depth of the nested #include. The default is 200.
• -ftabstop=width — set the distance between tab stops. This helps the preprocessor report correct column numbers in warnings or errors, even when tabs appear on the line. If the value is less than 1 or greater than 100, the option is **ignored. The default is 8.
• -ftrack-macro-expansion[=level] — track locations of tokens across macro expansions. This lets the compiler emit diagnostics about the current macro expansion stack when a compilation error happens inside a macro expansion. Heads up though: using this option makes the preprocessor and the compiler consume more memory. 😭

The level parameter chooses how precise the token location tracking is, so you can dial memory consumption down if you need to:

• Value 0 de-activates this option.
• Value 1 tracks token locations in a degraded mode, for 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.

Note that "-ftrack-macro-expansion=2" is activated by default.

• *-fmacro-prefix-map=old*=new — when preprocessing files living in directory old, expand the "__FILE__" and "__BASE_FILE__" macros as if the files lived in directory new instead. You can use this to change an absolute path into a relative path by using . for new, which can give you more reproducible builds that are location independent ✨. This option also affects "__builtin_FILE()" during compilation. See also -ffile-prefix-map.
• *-fexec-charset=charset* — set the execution character set, used for string and character constants. The default is UTF-8. charset can be any encoding supported by the system's "iconv" library routine.
• -fwide-exec-charset=charset — set the wide execution character set, used for wide string and character constants. The default is one of UTF-32BE, UTF-32LE, UTF-16BE, or UTF-16LE — whichever corresponds to the width of "wchar_t" and the big-endian or little-endian byte order being used for code generation. Same as -fexec-charset, charset can be any encoding supported by the system's "iconv" library routine. **However, you will have problems with encodings that don't fit exactly in "wchar_t". 💀
• *-finput-charset=charset* — set the input character set, used for translation from the character set of the input file to the source character set used by GCC. If the locale doesn't specify it, or GCC can't get this info from the locale, the default is UTF-8. Either the locale or this command-line option can override it — 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 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 is listed, and not the files that were used to create it — because those files are not consulted when a precompiled header is used.
• -fpch-preprocess — this option lets you use a precompiled header together with -E. It inserts a special "#pragma", "#pragma GCC pch_preprocess "filename"" into the output to mark the place 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 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 — enable generation of linemarkers in the preprocessor output that let the compiler know the current working directory at the time of preprocessing. When this option is on, 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.
• -A predicate=answer — make an assertion with the predicate predicate and answer answer. This form is preferred over the older form -A predicate(answer), which is still supported, because it doesn't use shell special characters. 🐚
• -A - predicate=answer — cancel an assertion with the predicate predicate and answer answer.
• -C — do not discard comments. All comments are passed through to the output file, except for comments in processed directives, which are deleted along with the directive.

Be prepared for side effects when using -C; it causes the preprocessor to treat comments as tokens in their own right. For example, comments appearing at the start of what would be a directive line have the effect of turning that line into an ordinary source line, since the first token on the line is no longer a #. 😳

• -CC — do not discard comments, including during macro expansion. This is like -C, except that comments contained within macros are also passed through to the output file where the macro is expanded.

On top of all the side effects of -C, the -CC option causes all C++-style comments inside a macro to be converted to C-style comments. This is to prevent later use of that macro from inadvertently commenting out the remainder of the source line. 🙈

The -CC option is generally used to support lint comments.

• -P — inhibit generation of linemarkers in the output from the preprocessor. This might be useful when running the preprocessor on something that is not C code, and will be sent to a program which might be confused by the linemarkers.
• -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 details.

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.

• -trigraphs — support ISO C trigraphs. These are three-character sequences, all starting with ??, that are defined by ISO C to stand for single characters. For example, ??/ stands for \, so '??/n' is a character constant for a newline.

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. throwback fr fr 💾
• -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 ...! .
• -d letters — says to make debugging dumps during compilation as specified by letters. The flags documented here are those relevant to the preprocessor. Other letters are interpreted by the compiler proper, or reserved for future versions of GCC, and so are silently ignored. If you specify letters whose behavior conflicts, the result is undefined. 💀
  • -dM — 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

...and boom, that right there shows you every single predefined macro bestie, no cap 💖

okay but real talk — if you use -dM without the -E option? It gets interpreted as a synonym for -fdump-rtl-mach. So like, don't do that unless that's what you actually want. 🙈

• -dD — like -dM except in two ways: it does not include the predefined macros, and it outputs both the #define directives and the result of preprocessing. Both kinds of output go to the standard output file. slay ✨
• -dN — like -dD, but it emits only the macro names, not their expansions. minimal queen behavior 💅
• -dI — outputs #include directives in addition to the result of preprocessing. extra. on purpose. 🎀
• -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. it's giving "only show up when you're needed" energy fr fr 🦋
• -fdebug-cpp — this option is only useful for debugging GCC. when used from CPP or with -E, it dumps debugging information about location maps. every token in the output is preceded by the dump of the map its location belongs to. 📍

when used from GCC without -E, this option has no effect. literally nothing. 💀

• -Wp,option — you can use -Wp,option to bypass the compiler driver and pass option directly through to the preprocessor. if option contains commas, it is split into multiple options at the commas. however, many options are modified, translated or interpreted by the compiler driver before being passed to the preprocessor, and -Wp forcibly bypasses this phase.

the preprocessor's direct interface is undocumented and subject to change, so whenever possible you should avoid using -Wp and let the driver handle the options instead. like bestie pls just let the driver do its job 😭

• -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. it's a whole thing. 🫶

• -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. ✨

• -flarge-source-files — adjust GCC to expect large source files, at the expense of slower compilation and higher memory usage. big file, big vibes, big cost 💸

specifically, GCC normally tracks both column numbers and line numbers within source files and it normally prints both of these numbers in diagnostics. however, once it has processed a certain number of source lines, it stops tracking column numbers and only tracks line numbers. this means that diagnostics for later lines do not include column numbers. it also means that options like -Wmisleading-indentation cease to work at that point, although the compiler prints a note if this happens. passing -flarge-source-files significantly increases the number of source lines that GCC can process before it stops tracking columns. 📏

---

🌸 Passing Options to the Assembler 🌸

you can pass options to the assembler. yes you. go off 💅

• -Wa,option — pass option as an option to the assembler. if option contains commas, it is split into multiple options at the commas. 🎀
• -Xassembler option — pass option as an option to the assembler. you can use this to supply system-specific assembler options that GCC does not recognize. 🌈

if you want to pass an option that takes an argument, you must use -Xassembler twice, once for the option and once for the argument. same rule as before, bestie 🫶

---

👑 Options for Linking 👑

these options come into play when the compiler links object files into an executable output file. they are meaningless if the compiler is not doing a link step. like, no link, no relevance, periodt 💀

• object-file-name — a file name that does not end in a special recognized suffix is considered to name an object file or library. (object files are distinguished from libraries by the linker according to the file contents.) if linking is done, these object files are used as input to the linker. 📦
• -c
• -S
• -E — if any of these options is used, then the linker is not run, and object file names should not be used as arguments. noted. 🚫
• -flinker-output=type — this option controls code generation of the link-time optimizer. by default the linker output is automatically determined by the linker plugin. for debugging the compiler and if incremental linking with a non-LTO object file is desired, 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 to build shared libraries without position-independent code on architectures where this is 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. 😭

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. 🧚

• -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. like, it works but... 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. 🎀

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 bestie, always has 😤

• -lobjc — you need this special case of the -l option in order to link an Objective-C or Objective-C++ program. special lil exception just for objc 💖
• -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 whole "do it yourself" package fr fr 💅

The compiler may generate calls to memcmp, memset, memcpy and memmove. Normally these get resolved by entries in libc, okay? But when you've specified this option, bestie, those entry points have to be supplied through some other mechanism. No free rides. 💅

One of the standard libraries that -nostdlib and -nodefaultlibs bypass is libgcc.a — a whole library of internal subroutines that GCC uses to overcome the shortcomings of particular machines, or special needs for some languages. It's giving "the compiler's emotional support library," honestly. 🫶

In most cases you actually need libgcc.a even when you're trying to avoid other standard libraries. So lowkey, when you specify -nostdlib or -nodefaultlibs you should usually specify -lgcc as well. This ensures 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.) Don't forget it or the linker will be like "who??" 💀

---

• -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. Consistency is the vibe, fr fr. ✨
• -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. Threads said "I'm here for both phases of your life," periodt. 🧵
• -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. Very main character energy for debugging. 🔥
• -s Remove all symbol table and relocation information from the executable. Bye bye, extra bytes. 🦋
• -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. G++ is out here making good decisions for you, and honestly? Slay. 👑

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. No cap, this part matters. 🚨

• -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. Freeze it in place, like a screenshot. 📸
• -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. Two separate trips, bestie, the linker is not sharing a plate with you. 🙈

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.

NOTE: In Ubuntu 8.10 and later versions, for LDFLAGS, the option -Wl,-z,relro is used. To disable, use -Wl,-z,norelro.

• -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. It's giving "manifesting a symbol into existence." ✨
• -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. We're not gonna explain them here, go read your linker's lore. 📖

---

🌸 Options for Directory Search 🌸

These options specify directories to search for header files, for libraries and for parts of the compiler. Basically: "where do I even look for things," the eternal question. 💭

• -I dir — -iquote dir — -isystem dir — -idirafter dir Add the directory dir to the list of directories to be searched for header files during preprocessing. If dir begins with = or $SYSROOT, then the = or $SYSROOT is replaced by the sysroot prefix; see --sysroot and -isysroot.

Directories specified with -iquote apply only to the quote form of the directive, #include "file". Directories specified with -I, -isystem, or -idirafter apply to lookup for both the #include "file" and #include <file> directives.

You can specify any number or combination of these options on the command line to search for header files in several directories. The lookup order is as follows:

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, -iwithprefixbefore dir — Append dir to the prefix specified previously with -iprefix, and add the resulting directory to the include search path. -iwithprefixbefore puts it in the same place -I would; -iwithprefix puts it where -idirafter would. 🌸
• -isysroot dir — This option is like the --sysroot option, but applies only to header files (except for Darwin targets, where it applies to both header files and libraries). See the --sysroot option for more information. 🍓
• -imultilib dir — Use dir as a subdirectory of the directory containing target-specific C++ headers. 💜
• -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.) 👑
• -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. 🌈

The compiler driver program runs one or more of the subprograms cpp, cc1, as and ld. It tries prefix as a prefix for each program it tries to run, both with and without machine/version/ for the corresponding target machine and compiler version.

For each subprogram to be run, the compiler driver first tries the -B prefix, if any. If that name is not found, or if -B is not specified, the driver tries two standard prefixes, /usr/lib/gcc/ and /usr/local/lib/gcc/. If neither of those results in a file name that is found, the unmodified program name is searched for using the directories specified in your PATH environment variable.

The compiler checks to see if the path provided by -B refers to a directory, and if necessary it adds a directory separator character at the end of the path.

-B prefixes that effectively specify directory names also apply to libraries in the linker, because the compiler translates these options into -L options for the linker. They also apply to include files in the preprocessor, because the compiler translates these options into -isystem options for the preprocessor. In this case, the compiler appends include to the prefix.

The runtime support file libgcc.a can also be searched for using the -B prefix, if needed. If it is not found there, the two standard prefixes above are tried, and that is all. The file is left out of the link if it is not found by those means.

Another way to specify a prefix much like the -B prefix is to use the environment variable GCC_EXEC_PREFIX.

As a special kludge, if the path provided by -B is [dir/]stageN/, where N is a number in the range 0 to 9, then it is replaced by [dir/]include. This is to help with boot-strapping the compiler. 😭

• -no-canonical-prefixes — 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. ✨

If you use both this option and the -isysroot option, then the --sysroot option applies to libraries, but the -isysroot option applies to header files.

The GNU linker (beginning with version 2.16) has the necessary support for this option. If your linker does not support this option, the header file aspect of --sysroot still works, but the library aspect does not. 💅

• --no-sysroot-suffix — For some targets, a suffix is added to the root directory specified with --sysroot, depending on the other options used, so that headers may for example be found in dir/suffix/usr/include instead of dir/usr/include. This option disables the addition of such a suffix. 🌸

✨ Options for Code Generation Conventions ✨

These machine-independent options control the interface conventions used in code generation. 🎀

Most of them have both positive and negative forms; the negative form of -ffoo is -fno-foo. In the table below, only one of the forms is listed---the one that is not the default. You can figure out the other form by either removing no- or adding it. 👑

• -fstack-reuse=reuse-level — This option controls stack space reuse for user declared local/auto variables and compiler generated temporaries. reuse_level can be all, named_vars, or none. all enables stack reuse for all local variables and temporaries, named_vars enables the reuse only for user defined local variables with names, and none disables stack reuse completely. The default value is all. The option is needed when the program extends the lifetime of a scoped local variable or a compiler generated temporary beyond the end point defined by the language. When a lifetime of a variable ends, and if the variable lives in memory, the optimizing compiler has the freedom to reuse its stack space with other temporaries or scoped local variables whose live range does not overlap with it. Legacy code extending local lifetime is likely to break with the stack reuse optimization. 😭

For example,

                    int *p;
                    {
                      int local1;

                      p = &local1;
                      local1 = 10;
                      ....
                    }
                    {
                       int local2;
                       local2 = 20;
                       ...
                    }

                    if (*p == 10)  // out of scope use of local1
                      {

                      }

Another example:

                    struct A
                    {
                        A(int k) : i(k), j(k) { }
                        int i;
                        int j;
                    };

                    A *ap;

                    void foo(const A& ar)
                    {
                       ap = &ar;
                    }

                    void bar()
                    {
                       foo(A(10)); // temp object's lifetime ends when foo returns

                       {
                         A a(20);
                         ....
                       }
                       ap->i+= 10;  // ap references out of scope temp whose space
                                    // is reused with a. What is the value of ap->i?
                    }

The lifetime of a compiler generated temporary is well defined by the C++ standard. When a lifetime of a temporary ends, and if the temporary lives in memory, the optimizing compiler has the freedom to reuse its stack space with other temporaries or scoped local variables whose live range does not overlap with it. However some of the legacy code relies on the behavior of older compilers in which temporaries' stack space is not reused, the aggressive stack reuse can lead to runtime errors. This option is used to control the temporary stack reuse optimization. 💖

• -ftrapv — This option generates traps for signed overflow on addition, subtraction, multiplication operations. 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 — bestie, this one tells the compiler to just assume that signed arithmetic overflow from addition, subtraction and multiplication wraps around using twos-complement representation. It turns some optimizations ON and some OFF, it's giving chaos but make it legal. And here's the ✨drama✨: -ftrapv and -fwrapv override each other, so putting -ftrapv -fwrapv on the command-line means -fwrapv wins. Only active options override though, so -ftrapv -fwrapv -fno-wrapv on the command-line ends up with -ftrapv being the effective one. iykyk 💅
• -fwrapv-pointer — same energy but for pointers 🫶. This tells the compiler to assume that pointer arithmetic overflow on addition and subtraction wraps around using twos-complement representation. It disables some optimizations that assume pointer overflow is invalid.
• -fstrict-overflow — this one implies -fno-wrapv -fno-wrapv-pointer, and when you negate it, it implies -fwrapv -fwrapv-pointer. Package deal, no cap.
• -fexceptions — enable exception handling ✨. It generates the extra code you need to propagate exceptions. For some targets this implies GCC generates frame unwind information for all functions, which can produce significant data size overhead — though lowkey it does not affect execution. If you don't 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 don't normally require it. But bestie, you may need to enable it when compiling C code that needs to interoperate properly with exception handlers written in C++. You may also want to disable it if you're compiling older C++ programs that don't use exception handling. 🎀
• -fnon-call-exceptions — generate code that allows trapping instructions to throw exceptions 👑. Heads up though: this requires platform-specific runtime support that does not exist everywhere 😭. And it only allows trapping instructions to throw exceptions, i.e. memory references or floating-point instructions. It does not allow exceptions to be thrown from arbitrary signal handlers such as "SIGALRM".
• -fdelete-dead-exceptions — consider that instructions that may throw exceptions but don't otherwise contribute to the execution of the program can be optimized away 💀. This option is enabled by default for the Ada compiler, as permitted by the Ada language specification. Optimization passes that cause dead exceptions to be removed are enabled independently at different optimization levels.
• -funwind-tables — similar to -fexceptions, except 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. She's a background queen fr fr.
• -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). Iconic precision, we love to see it.
• -fno-gnu-unique — okay so 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 — plot twist 😭 — this causes "dlclose" to be ignored for affected DSOs. So if your program relies on reinitialization of a DSO via "dlclose" and "dlopen", you can use -fno-gnu-unique.
• -fpcc-struct-return — return "short" "struct" and "union" values in memory like longer ones, rather than in registers. This convention is less efficient, but it has the advantage of allowing intercallability between GCC-compiled files and files compiled with other compilers, particularly the Portable C Compiler (pcc) 🫶.

The precise convention for returning structures in memory depends on the target configuration macros.

Short structures and unions are those whose size and alignment match that of some integer type.

Warning: code compiled with the -fpcc-struct-return switch is not binary compatible with code compiled with the -freg-struct-return switch. Use it to conform to a non-default application binary interface. 🚨

• -freg-struct-return — return "struct" and "union" values in registers when possible. This is more efficient for small structures than -fpcc-struct-return ✨.

If you specify neither -fpcc-struct-return nor -freg-struct-return, GCC defaults to whichever convention is standard for the target. If there is no standard convention, GCC defaults to -fpcc-struct-return, except on targets where GCC is the principal compiler. In those cases, we can choose the standard, and we chose the more efficient register return alternative. Main character decision tbh 👑

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. She's frugal, she's efficient, she ate 💅.

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. Okay bestie stay with me, this is the lore 📖:

The default is -fno-common, which specifies that the compiler places uninitialized global variables in the BSS section of the object file. This inhibits the merging of tentative definitions by the linker so you get a multiple-definition error if the same variable is accidentally defined in more than one compilation unit. 🙈

The -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. Short, sweet, does exactly what it says on the tin 💖.
• -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. Like, fr, leave it alone unless you know what you're doing 🫣.
• -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). Niche queen behavior 👑.

-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. So don't go writing a parser for them bestie, they WILL change on you 😭💀.

• -frecord-gcc-switches — bestie, this one literally takes the command line you used to invoke the compiler and records it into the object file you're creating ✨. Now the tea: it's only implemented on some targets, and the exact format of the recording depends on the target and the binary file format — but usually it shows up as a section containing ASCII text. It's related to the -fverbose-asm switch, except that one only records info in the assembler output file as comments, so it never reaches the object file. Also check -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 that's supported for the target machine 💅. Code like this accesses all constant addresses through a global offset table (GOT). The dynamic loader resolves the GOT entries when the program starts — and heads up, the dynamic loader is not part of GCC, it's part of the operating system. If the GOT size for the linked executable exceeds a machine-specific maximum size, you get an error message from the linker saying -fpic doesn't work; in that case, recompile with -fPIC instead. (Those 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, so it only works on certain machines, periodt. 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 it's 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 actually 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 are like -fpic and -fPIC, except the generated position-independent code can only be linked into executables 🎀. Usually you use these to compile code that'll be linked using the -pie GCC option.

Both -fpie and -fPIE 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 gives you more efficient code by eliminating PLT stubs and exposing GOT loads to optimizations 🔥. On architectures like 32-bit x86, where PLT stubs expect the GOT pointer in a specific register, this hands the compiler way more register allocation freedom. 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 that would be more efficient than other code generation strategies 🙈. This one's useful in conjunction with -fpic or -fPIC when you're building code that forms part of a dynamic linker and cannot reference the address of a jump table. On some targets, jump tables don't require a GOT and this option isn't needed at all.
• -fno-bit-tests — Do not use bit tests for switch statements, even where that would be more efficient than other code generation strategies 💀.
• -ffixed-reg — Treat the register named reg as a fixed register; generated code should never refer to it (except perhaps as a stack pointer, frame pointer, or in some other fixed role).

reg must be the name of a register. The register names accepted are machine-specific and are defined in the "REGISTER_NAMES" macro in the machine description macro file.

This flag does not have a negative form, because it specifies a three-way choice. 😤

• -fcall-used-reg — Treat the register named reg as an allocable register that 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.

It is an error to use this flag with the frame pointer or stack pointer. Use of this flag for other registers that have fixed pervasive roles in the machine's execution model produces disastrous results. 💀

This flag does not have a negative form, because it specifies a three-way choice.

• -fcall-saved-reg — Treat the register named reg as an allocable register saved by functions. It may be allocated even for temporaries or variables that live across a call. Functions compiled this way save and restore the register reg if they use it.

It is an error to use this flag with the frame pointer or stack pointer. Use of this flag for other registers that have fixed pervasive roles in the machine's execution model produces disastrous results.

And a different sort of disaster results from using this flag for a register in which function values may be returned 😭.

This flag does not have a negative form, because it specifies a three-way choice.

• -fpack-struct[=n] — Without a value specified, pack all structure members together without holes 🫶. When a value is specified (which must be a small power of two), pack structure members according to this value, representing the maximum alignment (that is, objects with default alignment requirements larger than this are output potentially unaligned at the next fitting location.

Warning: the -fpack-struct switch causes GCC to generate code that is not binary compatible with code generated without that switch. Additionally, it makes the code suboptimal. Use it to conform to a non-default application binary interface. 🙈

• -fleading-underscore — This option and its counterpart, -fno-leading-underscore, forcibly change the way C symbols are represented in the object file. One use is to help link with legacy assembly code 🦋.

Warning: the -fleading-underscore switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface. Not all targets provide complete support for this switch. 😬

• -ftls-model=model — Alter the thread-local storage model to be used ✨. The model argument should be one of global-dynamic, local-dynamic, initial-exec or local-exec. Note that the choice is subject to optimization: the compiler may use a more efficient model for symbols not visible outside of the translation unit, or if -fpic is not given on the command line.

The default without -fpic is initial-exec; with -fpic the default is global-dynamic. 💅

• -ftrampolines — For targets that normally need trampolines for nested functions, always generate them instead of using descriptors. Otherwise, for targets that don't need them — like, for example, HP-PA or IA-64 — do nothing 🎀.

A trampoline is a small piece of code that's 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. Therefore, it requires the stack to be made executable in order for the program to work properly.

-fno-trampolines is enabled by default on a language by language basis to let the compiler avoid generating them, if it computes that this is safe, and replace them with descriptors. Descriptors are made up of data only, but the generated code must be prepared to deal with them. As of this writing, -fno-trampolines is enabled by default only for Ada.

Moreover, code compiled with -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. 😭

• -fvisibility=[default|internal|hidden|protected] — Set the default ELF image symbol visibility to the specified option — all symbols are marked with this unless overridden within the code 👑. Using this feature can very substantially improve linking and load times of shared object libraries, produce more optimized code, provide near-perfect API export and prevent symbol clashes. It is strongly recommended that you use this in any shared objects you distribute.

Despite the nomenclature, default always means public; i.e., available to be linked against from outside the shared object. protected and internal are pretty useless in real-world usage, so the only other commonly used option is hidden. The default if -fvisibility isn't specified is default, i.e., make every symbol public.

A good explanation of the benefits offered by ensuring ELF symbols have the correct visibility is given by "How To Write Shared Libraries" by Ulrich Drepper (which can be found at https://www.akkadia.org/drepper/) — however a superior solution made possible by this option, instead of marking things hidden when the default is public, is to make the default hidden and mark things public. This is the norm with DLLs on Windows, and with -fvisibility=hidden and "__attribute__ ((visibility("default")))" instead of "__declspec(dllexport)" you get almost identical semantics with identical syntax. This is a great boon to those working with cross-platform projects. 💖

Okay so boom — still deep in visibility land bestie, and then we pivot to the dev-nerd corner. Stay with me 💅

If you're retrofitting visibility support into code that already exists, you might find #pragma GCC visibility your new bestie. How it works: you wrap the declarations you want to set visibility for in #pragma GCC visibility push(hidden) and #pragma GCC visibility pop. But like, listen — symbol visibility is part of the API interface contract, okay? So all new code should always specify visibility when it's not the default. Declarations that are only for use inside the local DSO should always be marked explicitly as hidden, so you dodge PLT indirection overheads — and making that abundantly clear also helps readability and self-documentation of the code. Obsessed with clarity, honestly. 🫶

Heads up: due to ISO C++ specification requirements, operator new and operator delete must always be of default visibility. No exceptions, fr fr.

Also be aware that headers from outside your project — especially 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. Little polite heads-up for the compiler, y'know? 🎀

extern declarations are not affected by -fvisibility, so a lot of code can be recompiled with -fvisibility=hidden with zero modifications. Slay. But — and this is the catch — that means calls to extern functions with no explicit visibility use the PLT, so it's more effective to use __attribute ((visibility)) and/or #pragma GCC visibility to tell the compiler which extern declarations should be treated as hidden. Tell it who's who. 👑

Note that -fvisibility does affect C++ vague linkage entities. So 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. Otherwise it's giving chaos 😭

An overview of these techniques, their benefits and how to use them lives at <http://gcc.gnu.org/wiki/Visibility>.

---

-fstrict-volatile-bitfields

Use this one 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. Example time: targets with memory-mapped peripheral registers might require all such accesses to be 16 bits wide. With this flag you can declare all peripheral bit-fields as unsigned short (assuming short is 16 bits on these targets) to force GCC to use 16-bit accesses instead of, perhaps, a more efficient 32-bit access. ✨

If this option is disabled, the compiler uses the most efficient instruction. In the previous example, that might be a 32-bit load instruction — even though that accesses bytes that don't contain any portion of the bit-field, or memory-mapped registers unrelated to the one being updated. Rude of it, honestly. 💀

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. Graceful. Iconic. 🌸

Note: due to restrictions of the C/C++11 memory model, write accesses are not allowed to touch non bit-field members. So it's recommended to define all bits of the field's type as bit-field members. Do that. 💅

The default value of this option is determined by the application binary interface for the target processor.

-fsync-libcalls

This option controls whether any out-of-line instance of the __sync family of functions may be used to implement the C++11 __atomic family of functions.

The default value of this option is enabled, so the only useful form of the option is -fno-sync-libcalls. This option is used in the implementation of the libatomic runtime library. iykyk 🧚

---

GCC Developer Options ✨

This section is 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. Basically the backstage pass. 🎟️ It 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 info about GCC's configuration, like where it searches for libraries. For ordinary compilation and linking tasks you should rarely need any of these. They're the deep-lore options, 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 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 info is generated in the common VCG format. It can be decorated with additional, per-node and/or per-edge information if you additionally specify a list of comma-separated markers. When the su marker is specified, the callgraph gets decorated with stack usage information — it's equivalent to -fstack-usage. When the da marker is specified, the callgraph gets 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 — buckle up bestie, it's a whole list 📋:

• -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.

---

and yes bestie, there's more where that came from… keep scrolling 💖

• -fdump-rtl-ree — dumps right after sign/zero extension elimination. slay.
• -fdump-rtl-seqabstr — dumps after common sequence discovery 💅
• -fdump-rtl-shorten — dumps after shortening branches ✨
• -fdump-rtl-sibling — dumps after sibling call optimizations 🫶
• -fdump-rtl-split1, -fdump-rtl-split2, -fdump-rtl-split3, -fdump-rtl-split4, -fdump-rtl-split5 — these five enable dumping after five rounds of instruction splitting. yes, five. the compiler really does split that many times. iconic behavior honestly.
• -fdump-rtl-sms — dumps after modulo scheduling. heads up tho: this pass only runs on some architectures, so don't come for it if your target doesn't do it 💀
• -fdump-rtl-stack — dumps after the conversion from GCC's "flat register file" registers over to the x87's stack-like registers. lowkey only runs on x86 variants, iykyk.
• -fdump-rtl-subreg1, -fdump-rtl-subreg2 — -fdump-rtl-subreg1 and -fdump-rtl-subreg2 enable dumping after the two subreg expansion passes 🌸
• -fdump-rtl-unshare — dumps after all rtl has been unshared. periodt.
• -fdump-rtl-vartrack — dumps after variable tracking 🔍
• -fdump-rtl-vregs — dumps after converting virtual registers to hard registers ✨
• -fdump-rtl-web — dumps 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 — bestie these dumps are defined but always produce empty files. they exist. they do nothing. it's giving ghost 👻. not a bug, just how it is.
• -da, -fdump-rtl-all — produce all the dumps listed above. all of them. every single one. go off 💅
• -dA — annotate the assembler output with miscellaneous debugging information 🎀
• -dD — dump all macro definitions, at the end of preprocessing, on top of the normal output 🌸
• -dH — produce a core dump whenever an error occurs 💀 dramatic but ok
• -dp — annotate the assembler output with a comment showing which pattern and alternative is used. the length and cost of each instruction get printed too ✨
• -dP — dump the RTL in the assembler output as a comment before each instruction. also turns on -dp annotation, so you get both, no cap 🫶
• -dx — just generate RTL for a function instead of compiling it. usually used with -fdump-rtl-expand 🦋
• -fdump-debug — dump debugging information generated during the debug generation phase 👑
• -fdump-earlydebug — dump debugging information generated during the early debug generation phase ⭐
• -fdump-noaddr — when doing debugging dumps, suppress address output. why would you want that? so it's actually feasible to run diff on debugging dumps from compiler invocations with different compiler binaries and/or different text / bss / data / heap / stack / dso start locations. literally the diff-enabler 🔥
• -freport-bug — collect and dump debug information into a temporary file if an internal compiler error (ICE) occurs 🧊
• -fdump-unnumbered — when doing debugging dumps, suppress instruction numbers and address output. makes it way more feasible to use diff on debugging dumps for compiler invocations with different options, in particular with and without -g ✨
• -fdump-unnumbered-links — when doing debugging dumps (see the -d option above), suppress instruction numbers for the links to the previous and next instructions in a sequence 🎀
• -fdump-ipa-switch, -fdump-ipa-switch-options — control the dumping at various stages of inter-procedural analysis language tree to a file. the file name is generated by appending a switch specific suffix to the source file name, and the file gets created in the same directory as the output file. the following dumps are possible:
  • all — enables all inter-procedural analysis dumps 💖
  • cgraph — dumps information about call-graph optimization, unused function removal, and inlining decisions 👑
  • inline — dump after function inlining ✨

additionally, the options -optimized, -missed, -note, and -all can be provided, with the same meaning as for -fopt-info, defaulting to -optimized 💅

for example, -fdump-ipa-inline-optimized-missed will emit information on callsites that were inlined, along with callsites that were not inlined. so you see both the ate and the didn't-eat, fr fr.

by default, the dump will contain messages about successful optimizations (equivalent to -optimized) together with low-level details about the analysis 🌸

• -fdump-lang — dump language-specific information. the file name is made by appending .lang to the source file name 🦋
• -fdump-lang-all, -fdump-lang-switch, -fdump-lang-switch-options, -fdump-lang-switch-options=filename — control the dumping of language-specific information. the options and filename portions behave as described in the -fdump-tree option. the following switch values are accepted:
  • all — enable all language-specific dumps 🌈
  • class — dump class hierarchy information. virtual table information is emitted unless slim is specified. this option is applicable to C++ only 💜
  • module — dump module information. 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 — print on stderr the list of optimization passes that are turned on and off by the current command-line options 🔍
• -fdump-statistics-option — enable and control dumping of pass statistics in a separate file. the file name is generated by appending a suffix ending in .statistics to the source file name, and the file is created in the same directory as the output file. if the -option form is used, -stats causes counters to be summed over the whole compilation unit while -details dumps every event as the passes generate them. the default with no option is to sum counters for each function compiled 🎀
• -fdump-tree-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; those that are not meaningful are ignored. the following options are available:
  • address — print the address of each node. usually this is not meaningful since it changes according to the environment and source file. its primary use is for tying up a dump file with a debug environment 🦋
  • asmname — if "DECL_ASSEMBLER_NAME" has been set for a given decl, use that in the dump instead of "DECL_NAME". its primary use is ease of use working backward from mangled names in the assembly file ✨
  • slim — when dumping front-end intermediate representations, inhibit dumping of members of a scope or body of a function merely because that scope has been reached. only dump such items when they are directly reachable by some other path.

when dumping pretty-printed trees, this option inhibits dumping the bodies of control structures.

when dumping RTL, print the RTL in slim (condensed) form instead of the default LISP-like representation 💅

• raw — print a raw representation of the tree. by default, trees are pretty-printed into a C-like representation 🌸
• details — enable more detailed dumps (not honored by every dump option). also include information from the optimization passes 🔍
• stats — enable dumping various statistics about the pass (not honored by every dump option) 🎀
• blocks — enable showing basic block boundaries (disabled in raw dumps) 🧱
• graph — for each of the other indicated dump files (-fdump-rtl-pass), dump a representation of the control flow graph suitable for viewing with GraphViz to file.passid.pass.dot. each function in the file is pretty-printed as a subgraph, so that GraphViz can render them all in a single plot.

this option currently only works for RTL dumps, and the RTL is always dumped in slim form 👑

• vops — enable showing virtual operands for every statement 💜
• lineno — enable showing line numbers for statements 🔢
• uid — enable showing the unique ID ("DECL_UID") for each variable ⭐
• verbose — enable showing the tree dump for each statement 🗣️
• eh — enable showing the EH region number holding each statement 🌈
• scev — enable showing scalar evolution analysis details ✨
• optimized — enable showing optimization information (only available in certain passes) 🔥
• missed — enable showing missed optimization information (only available in certain passes) 🙈
• note — enable other detailed optimization information (only available in certain passes) 📝
• all — turn on all options, except raw, slim, verbose and lineno 🌟
• optall — turn on all optimization options, i.e., optimized, missed, and note 💖

to determine what tree dumps are available or find the dump for a pass of interest follow the steps below 👇

1. invoke GCC with -fdump-passes and in the stderr output look for a code that corresponds to the pass you are interested in. for example, the codes "tree-evrp", "tree-vrp1", and "tree-vrp2" correspond to the three Value Range Propagation passes. the number at the end distinguishes distinct invocations of the same pass 🧚
2. to enable the creation of the dump file, append the pass code to the -fdump- option prefix and invoke GCC with it. for example, to enable the dump from the Early Value Range Propagation pass, invoke GCC with the -fdump-tree-evrp option. optionally, you may specify the name of the dump file. if you don't specify one, GCC creates 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 chillin' in a file named myfile.c.038t.evrp in the current working directory. Heads up though bestie: those numeric codes are not stable and may change from one version of GCC to another. 😭

---

-fopt-info, -fopt-info-options, -fopt-info-options=filename

Okay so this one controls optimization dumps from, like, all the various optimization passes. If you use the -options form, then options is a list of - separated option keywords that pick the dump details and optimizations. 💅

The options split into three groups, and no they do not overlap:

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 any group since they're non-overlapping. But if there's a conflict? The later options override the earlier options on the command line. Periodt. 👑

The following options control which kinds of messages should be emitted:

• optimized — Print info when an optimization is successfully applied. Whether something counts as relevant is up to the pass, okay? For example, the vectorizer passes print the source location of loops which are successfully vectorized. ✨
• missed — Print info about missed optimizations. Individual passes decide what info goes in the output. 🙈
• note — Print verbose info about optimizations, like certain transformations, more detailed messages about decisions, etc.
• all — Print detailed optimization information. This includes optimized, missed, and note.

The following option controls the dump verbosity:

• internals — By default only "high-level" messages are emitted. This option enables additional, more detailed, messages, which are likely to only be of interest to GCC developers. iykyk 🧚

One or more of the following option keywords can be used to describe a group of optimizations:

• ipa — Enable dumps from all interprocedural optimizations.
• loop — Enable dumps from all loop optimizations.
• inline — Enable dumps from all inlining optimizations.
• omp — Enable dumps from all OMP (Offloading and Multi Processing) optimizations.
• vec — Enable dumps from all vectorization optimizations.
• optall — Enable dumps from all optimizations. This is a superset of the optimization groups listed above. 👑

If options is omitted, it defaults to optimized-optall, which means dumping messages about successful optimizations from all the passes, while omitting messages that are treated as "internals".

If the filename is provided, then the dumps from all the applicable optimizations get concatenated into the filename. Otherwise the dump goes out onto stderr. And like, even though multiple -fopt-info options are accepted, only one of them can include a filename. If other filenames are provided then all but the first such option are ignored. 💀

Note that the output filename is overwritten in case of multiple translation units. If you want a combined output from multiple translation units, stderr should be used instead. 😭

In the following example, the optimization info is output to stderr:

gcc -O3 -fopt-info

This example:

gcc -O3 -fopt-info-missed=missed.all

outputs a 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. 🌈

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. So only vec.miss is produced, and it contains dumps from the vectorizer about missed opportunities. That's the way that it is bestie 🫶

---

-fsave-optimization-record

Write a SRCFILE.opt-record.json.gz file detailing what optimizations were performed, for those optimizations that support -fopt-info. 🎀

This option is experimental and the format of the data within the compressed JSON file is subject to change.

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, plus the following extra data for each message:

• the execution count of the code being optimized, along with metadata about whether this was from actual profile data, or just an estimate, allowing consumers to prioritize messages by code hotness, 🔥
• the function name of the code being optimized, where applicable,
• the "inlining chain" for the code being optimized, so that when a function is inlined into several different places (which might themselves be inlined), the reader can distinguish between the copies,
• objects identifying those parts of the message that refer to expressions, statements or symbol-table nodes, which of these categories they are, and, when available, their source code location,
• the GCC pass that emitted the message, and
• the location in GCC's own code from which the message was emitted.

Additionally, some messages are logically nested within other messages, reflecting implementation details of the optimization passes. 🦋

---

-fsched-verbose=n

On targets that use instruction scheduling, this option controls the amount of debugging output the scheduler prints to the dump files. 👀

For n greater than zero, -fsched-verbose outputs the same information as -fdump-rtl-sched1 and -fdump-rtl-sched2. For n greater than one, it also outputs basic block probabilities, detailed ready list information and unit/insn info. For n greater than two, it includes RTL at abort point, control-flow and regions info. And for n over four, -fsched-verbose also includes dependence info. 🧠

---

-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, fr fr. 💅

• -fdisable-ipa-pass — Disable IPA pass pass. pass is the pass name. If the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from 1.
• -fdisable-rtl-pass, -fdisable-rtl-pass=range-list — Disable RTL pass pass. pass is the pass name. If the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from 1. range-list is a comma-separated list of function ranges or assembler names. Each range is a number pair separated by a colon. The range is inclusive in both ends. If the range is trivial, the number pair can be simplified as a single number. If the function's call graph node's uid falls within one of the specified ranges, the pass is disabled for that function. The uid is shown in the function header of a dump file, and the pass names can be dumped by using option -fdump-passes. 🔍
• -fdisable-tree-pass, -fdisable-tree-pass=range-list — Disable tree pass pass. See -fdisable-rtl for the description of option arguments.
• -fenable-ipa-pass — Enable IPA pass pass. pass is the pass name. If the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from 1.
• -fenable-rtl-pass, -fenable-rtl-pass=range-list — Enable RTL pass pass. See -fdisable-rtl for option argument description and examples.
• -fenable-tree-pass, -fenable-tree-pass=range-list — Enable tree pass pass. See -fdisable-rtl for the description of option arguments.

Here are some examples showing uses of these options. 📚

        # disable ccp1 for all functions
           -fdisable-tree-ccp1
        # disable complete unroll for function whose cgraph node uid is 1
           -fenable-tree-cunroll=1
        # disable gcse2 for functions at the following ranges [1,1],
        # [300,400], and [400,1000]
        # disable gcse2 for functions foo and foo2
           -fdisable-rtl-gcse2=foo,foo2
        # disable early inlining
           -fdisable-tree-einline
        # disable ipa inlining
           -fdisable-ipa-inline
        # enable tree full unroll
           -fenable-tree-unroll

---

-fchecking, -fchecking=n

Enable internal consistency checking. The default depends on the compiler configuration. -fchecking=2 enables further internal consistency checking that might affect code generation. ⚠️

---

-frandom-seed=string

This option provides a seed that GCC uses in place of random numbers in generating certain symbol names that have to be different in every compiled file. It's also used to place unique stamps in coverage data files and the object files that produce them. You can use the -frandom-seed option to produce reproducibly identical object files. 🌱✨

The string can either be a number (decimal, octal or hex) or an arbitrary string (in which case it's converted to a number by computing CRC32). 🎲

The string should be different for every file you compile. no copy-paste energy, bestie. 💅

• -save-temps — okay so this one keeps the usual "temporary" intermediate files forever instead of yeeting them, and names them as auxiliary output files, exactly like -dumpbase and -dumpdir say. ✨

When you pair it with the -x command-line option, -save-temps is sensible enough to not overwrite an input source file that has the same extension as an intermediate file. If you want that corresponding intermediate file anyway, just rename the source file first, then use -save-temps. 🎀

• -save-temps=cwd — literally equivalent to -save-temps -dumpdir ./. that's it. 🫶
• -save-temps=obj — equivalent to -save-temps -dumpdir outdir/, where outdir/ is the directory of the output file you specified after the -o option, directory separators and all. and if you didn't use -o? then -save-temps=obj just behaves like -save-temps=cwd. cute. 💖
• -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 happening). ⏱️

If you don't specify an output file, the output looks like this:

        # cc1 0.12 0.01
        # as 0.00 0.01

First number on each line = "user time", aka time spent executing the program itself. Second number = "system time", aka time spent executing operating system routines on behalf of the program. both in seconds, no cap. 🔥

If you do specify an output file, the output gets appended to that named file, and it looks like this:

        0.12 0.01 cc1 <options>
        0.00 0.01 as <options>

So now the "user time" and "system time" get moved before the program name, and the options passed to the program are displayed too — so later you can tell what file was being compiled and with which options. iconic. 👑

• -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 name is determined by appending .gkd to the dump base name, see -dumpbase. 🌸
• -fcompare-debug[=opts] — if no error happens during compilation, it 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. 🦋

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. and 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.

To verify full coverage during -fcompare-debug testing, set GCC_COMPARE_DEBUG to say -fcompare-debug-not-overridden, which GCC rejects as an invalid option in any actual compilation (rather than preprocessing, assembly or linking). and if you just want a warning instead? setting GCC_COMPARE_DEBUG to -w%n-fcompare-debug not overridden will do. 💜

• -fcompare-debug-second — this option is implicitly passed to the compiler for the second compilation requested by -fcompare-debug, along with options to silence warnings, and omitting other options that would cause the compiler to produce output to files or to standard output as a side effect. dump files and preserved temporary files get renamed so they contain the .gk additional extension during that second compilation, so nothing overwrites what the first one generated. 🎀

When this option is passed to the compiler driver, it causes the first compilation to be skipped, which makes it useful for little other than debugging the compiler proper.

• -gtoggle — turns off generation of debug info if leaving out this option generates 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 does so only once, no matter how many times you give it. this is mainly intended to be used with -fcompare-debug. 🌈
• -fvar-tracking-assignments-toggle — toggles -fvar-tracking-assignments, in the same way that -gtoggle toggles -g. 💅
• -Q — makes the compiler print out each function name as it's compiled, and print some statistics about each pass when it finishes. 📊
• -ftime-report — makes the compiler print some statistics about the time consumed by each pass when it finishes. ⏳
• -ftime-report-details — records the time consumed by infrastructure parts separately for each pass. 🧚
• -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. 🔍
• -flto-report — prints a report with internal details on the workings of the link-time optimizer. the contents of this report vary from version to version. it's meant to be useful to GCC developers when processing object files in LTO mode (via -flto). 🛠️

Disabled by default.

• -flto-report-wpa — like -flto-report, but only prints for the WPA phase of link-time optimization. 💜
• -fmem-report — makes the compiler print some statistics about permanent memory allocation when it finishes. 🧠
• -fmem-report-wpa — makes the compiler print some statistics about permanent memory allocation for the WPA phase only. ✨
• -fpre-ipa-mem-report
• -fpost-ipa-mem-report — makes the compiler print some statistics about permanent memory allocation before or after interprocedural optimization. 💖
• -fprofile-report — makes the compiler print some statistics about consistency of the (estimated) profile and effect of individual passes. 📈
• -fstack-usage — makes the compiler output stack usage information for the program, on a per-function basis. the filename for the dump is made by appending .su to the auxname. auxname is generated from the name of the output file, if it's explicitly specified and it's not an executable, otherwise it's the basename of the source file. an entry is made up of three fields: 🥞
  • The name of the function.
  • A number of bytes.
  • One or more qualifiers: "static", "dynamic", "bounded".

The qualifier "static" means the function manipulates the stack statically: a fixed number of bytes are allocated for the frame on function entry and released on function exit; no stack adjustments are otherwise made in the function. the second field is this fixed number of bytes. 🎯

The qualifier "dynamic" means the function manipulates the stack dynamically: in addition to the static allocation described above, stack adjustments are made in the body of the function, for example to push/pop arguments around function calls. if the qualifier "bounded" is also present, the amount of these adjustments is bounded at compile time and the second field is an upper bound of the total amount of stack used by the function. if it is not present, the amount of these adjustments is not bounded at compile time and the second field only represents the bounded part. 💀

• -fstats — emits statistics about front-end processing at the end of the compilation. this option is supported only by the C++ front end, and the information is generally only useful to the G++ development team. 👑
• -fdbg-cnt-list — prints the name and the counter upper bound for all debug counters. 🔢
• -fdbg-cnt=counter-value-list — sets 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 the second, third, fourth, tenth and eleventh invocation. for dbg_cnt(tail_call) true is returned for the first 10 invocations. 🧮
• -print-file-name=library — prints the full absolute name of the library file library that would be used when linking — and does nothing else. with this option, GCC does not compile or link anything; it just prints the file name. 📝
• -print-multi-directory — prints 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 — prints 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 -, with no spaces between multiple switches. this is supposed to ease shell processing. 🐚
• -print-multi-os-directory — prints 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 — prints 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 — prints the name of the configured installation directory and a list of program and library directories gcc searches — and does nothing else. 🗺️

This is useful</p> when gcc hits you with the whole installation problem, cannot exec cpp0: No such file or directory situation. Bestie, to fix this you've got two choices: either put cpp0 and the rest of the compiler components exactly where gcc expects to find them, or set the environment variable GCC_EXEC_PREFIX to the directory where you installed them. And do not forget the trailing /. The slash is not optional, she is doing her job. 🎀

• -print-sysroot — prints the target sysroot directory that gets used during compilation. That's the target sysroot you specified either at configure time or with the --sysroot option, possibly with an extra suffix that depends on your compilation options. If no target sysroot is specified, this option prints absolutely nothing. Silence. Iconic of her. 💅
• -print-sysroot-headers-suffix — prints the suffix that gets added to the target sysroot when searching for headers, or throws an error if the compiler was not configured with such a suffix — and it does nothing else. No extra tasks, no side quests. Periodt.
• -dumpmachine — prints the compiler's target machine (like i686-pc-linux-gnu) — and does not do anything else. Just the machine, bestie. 🦋
• -dumpversion — prints the compiler version (for example 3.0, 6.3.0 or 7) — and don't you dare expect it to do anything else. This is the compiler version used in filesystem paths and specs. Depending on how the compiler was configured, it can be just a single number (major version), two numbers separated by a dot (major and minor version), or three numbers separated by dots (major, minor and patchlevel version). So like, it might be short, it might be long, vibe check your own build. ✨
• -dumpfullversion — prints the full compiler version — and, you guessed it, nothing else. This output is always three numbers separated by dots: major, minor and patchlevel version. No surprises, no drama. 👑
• -dumpspecs — prints the compiler's built-in specs — and does nothing else whatsoever. (This one gets used when GCC itself is being built, iykyk.) 🧚

---

🌈 Machine-Dependent Options 🌈

Every target machine that GCC supports can have its own little options — like, to let you compile for a specific processor variant or ABI, or to control optimizations that are specific to that machine. By convention, machine-specific option names start with -m. 💖

Some configurations of the compiler also support extra target-specific options, usually for compatibility with other compilers living on the same platform. We love a flexible queen.

🌸 AArch64 Options 🌸

These options are defined for AArch64 implementations:

• -mabi=name — generates code for the specified data model. Permissible values are ilp32 for a SysV-like data model where int, long int and pointers are 32 bits, and lp64 for a 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. Heads up though: 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. No mixing, bestie, that's a mess. 😭

• -mbig-endian — generates big-endian code. This is the default when GCC is configured for an aarch64_be-*-* target.
• -mgeneral-regs-only — generates code that uses only the general-purpose registers. This prevents the compiler from using floating-point and Advanced SIMD registers, but it will not impose any restrictions on the assembler. She's just asking the compiler to chill.
• -mlittle-endian — generates little-endian code. This is the default when GCC is configured for an aarch64-*-* target, but not an aarch64_be-*-* target.
• -mcmodel=tiny — generates 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. Tiny but mighty. 🎀
• -mcmodel=small — generates 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, so it's giving main character energy by default. ✨
• -mcmodel=large — generates code for the large code model. This makes no assumptions about addresses and sizes of sections. Programs can be statically linked only. And heads up: the -mcmodel=large option is incompatible with -mabi=ilp32, -fpic and -fPIC. Do not try it, she will not cooperate. 💀
• -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 a 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, because this is entirely for use within the Linux kernel. Very niche, very specific, we respect it. 🫶

• -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 the bit size of immediate TLS offsets. Valid values are 12, 24, 32, 48. This option requires binutils 2.26 or newer. Don't skip the version check, bestie. 💅
• -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. Little band-aid, big job. 🩹
• -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. So the linker does the actual fixing, she's just the messenger.
• -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. Faster but fuzzier, trade-offs are real. ⚡
• -mlow-precision-sqrt
• -mno-low-precision-sqrt — enable or disable the 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 square root results to about 16 bits for single precision and to 32 bits for double precision. If enabled, it implies -mlow-precision-recip-sqrt. Package deal, no takebacks.
• -mlow-precision-div
• -mno-low-precision-div — enable or disable the division approximation. This option only has an effect if -ffast-math or -funsafe-math-optimizations is used as well. Enabling this reduces precision of division results to about 16 bits for single precision and to 32 bits for double precision.
• -mtrack-speculation
• -mno-track-speculation — enable or disable generation of additional code to track speculative execution through conditional branches. That tracking state can then be used by the compiler when expanding calls to "__builtin_speculation_safe_copy" to permit a more efficient code sequence to be generated. 👀
• -moutline-atomics
• -mno-outline-atomics — enable or disable calls to out-of-line helpers to implement atomic operations. These helpers will, at runtime, determine if the LSE instructions from ARMv8.1-A can be used; if not, they will use the load/store-exclusive instructions that are present in the base ARMv8.0 ISA.

This option is only applicable when compiling for the base ARMv8.0 instruction set. If you're using a later revision, e.g. -march=armv8.1-a or -march=armv8-a+lse, the ARMv8.1-Atomics instructions will be used directly. Same story when using -mcpu= and the selected cpu supports the lse feature. This option is on by default. ✨

• -march=name — specify the name of the target architecture and, optionally, one or more feature modifiers. This option has the form -march=arch{+[no]feature}*.

The table below summarizes the permissible values for arch and the features that they enable by default:

arch value : Architecture : Includes by default
armv8-a : Armv8-A : +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
armv8.4-a : Armv8.4-A : armv8.3-a, +flagm, +fp16fml, +dotprod
armv8.5-a : Armv8.5-A : armv8.4-a, +sb, +ssbs, +predres
armv8.6-a : Armv8.6-A : armv8.5-a, +bf16, +i8mm
armv8-r : Armv8-R : armv8-r

The value native is available on native AArch64 GNU/Linux and causes the compiler to pick the architecture of the host system. This option has no effect if the compiler is unable to recognize the architecture of the host system,

The permissible values for feature are listed in the sub-section on aarch64-feature-modifiers,,-march and -mcpu Feature Modifiers. Where conflicting feature modifiers are specified, the right-most feature is used.

GCC uses name to determine what kind of instructions it can emit when generating assembly code. If -march is specified without either of -mtune or -mcpu also being specified, the code is tuned to perform well across a range of target processors implementing the target architecture. Versatile queen. 👑

• -mtune=name — specify the name of the target processor for which GCC should tune the performance of the code. Permissible values for this option are: generic, cortex-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, neoverse-512tvb, neoverse-e1, neoverse-n1, neoverse-n2, neoverse-v1,neoverse-v2, grace, qdf24xx, saphira, phecda, xgene1, vulcan, octeontx, octeontx81, octeontx83, octeontx2, octeontx2t98, octeontx2t96 octeontx2t93, octeontx2f95, octeontx2f95n, octeontx2f95mm, a64fx, 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-x1, ampere1, ampere1a, cobalt-100 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 specify that GCC should tune for a big.LITTLE system.

The value neoverse-512tvb specifies that 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, the option tells GCC to tune for Neoverse cores that can execute 4 128-bit Advanced SIMD arithmetic instructions a cycle and that can execute an equivalent number of SVE arithmetic instructions per cycle (2 for 256-bit SVE, 4 for 128-bit SVE). This is more general than tuning for a specific core like Neoverse V1 but is more specific than the default tuning described below.

Additionally on native AArch64 GNU/Linux systems the value native tunes performance to the host system. This option has no effect if the compiler is unable to recognize the processor of the host system. 😭 like bestie it literally cannot vibe-check your CPU, so it just gives up.

Where none of -mtune=, -mcpu= or -march= are specified, the code is tuned to perform well across a range of target processors. ✨ jack of all trades energy ✨

This option cannot be suffixed by feature modifiers. no extra toppings, periodt.

-mcpu=name

Specify the name of the target processor, optionally suffixed by one or more feature modifiers. This option has the form -mcpu=cpu{+[no]feature}*, where the permissible values for cpu are the same as those available for -mtune. The permissible values for feature are documented in the sub-section on aarch64-feature-modifiers,,-march and -mcpu Feature Modifiers. Where conflicting feature modifiers are specified, the right-most feature is used. last one wins, that's just the rules 💅

GCC uses name to determine what kind of instructions it can emit when generating assembly code (as if by -march) and to determine the target processor for which to tune for performance (as if by -mtune). Where this option is used in conjunction with -march or -mtune, those options take precedence over the appropriate part of this option.

-mcpu=neoverse-512tvb is special in that it does not refer to a specific core, but instead 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. Unless overridden by -mtune, -mcpu=neoverse-512tvb tunes code in the same way as for -mtune=neoverse-512tvb.

---

-moverride=string

Override tuning decisions made by the back-end in response to a -mtune= switch. The syntax, semantics, and accepted values for string in this option are not guaranteed to be consistent across releases. 🫠

This option is only intended to be useful when developing GCC. it's giving "devs only, everyone else please step back" 🚧

---

-mverbose-cost-dump

Enable verbose cost model dumping in the debug dump files. This option is provided for use in debugging the compiler. 🔍 very "peek behind the curtain" of it all.

---

-mpc-relative-literal-loads

-mno-pc-relative-literal-loads

Enable or disable PC-relative literal loads. With this option literal pools are accessed using a single instruction and emitted after each function. This limits the maximum size of functions to 1MB. This is enabled by default for -mcmodel=tiny. ✨ one instruction, so efficient, so cute ✨

---

-msign-return-address=scope

Select the function scope on which return address signing will be applied. Permissible values are:

• none — which disables return address signing
• non-leaf — which enables pointer signing for functions which are not leaf functions
• all — which enables pointer signing for all functions

The default value is none.

This option has been deprecated by -mbranch-protection. 💀 it's retired bestie, move on

---

-mbranch-protection=none|standard|pac-ret[+leaf+b-key]|bti

Select the 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. If a feature has additional tuning options, then 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 can be used to extend the signing to include leaf functions. The optional argument b-key can be used to sign the functions with the B-key instead of the A-key.
• bti turns on branch target identification mechanism.

---

-mharden-sls=opts

Enable compiler hardening against straight line speculation (SLS). opts is a comma-separated list of the following options:

• retbr
• blr

In addition, -mharden-sls=all enables all SLS hardening while -mharden-sls=none disables all SLS hardening. 🛡️ security era, we love to see it

---

-msve-vector-bits=bits

Specify the number of bits in an SVE vector register. This option only has an effect when SVE is enabled.

GCC supports two forms of SVE code generation: "vector-length agnostic" output that works with any size of vector register and "vector-length specific" output that allows GCC to make assumptions about the vector length when it is useful for optimization reasons. The possible values of bits are: scalable, 128, 256, 512, 1024 and 2048.

• Specifying scalable selects vector-length agnostic output.
• At present -msve-vector-bits=128 also generates vector-length agnostic output for big-endian targets.
• All other values generate vector-length specific code.

The behavior of these values may change in future releases and no value except scalable should be relied on for producing code that is portable across different hardware SVE vector lengths. fr fr, if you're not using scalable don't come crying to me later 😭

The default is -msve-vector-bits=scalable, which produces vector-length agnostic code. 👑

---

🌈 -march and -mcpu Feature Modifiers 🌈

Feature modifiers used with -march and -mcpu can be any of the following and their inverses nofeature:

• crc — Enable CRC extension. This is on by default for -march=armv8.1-a.
• crypto — Enable Crypto extension. This also enables Advanced SIMD and floating-point instructions.
• fp — Enable floating-point instructions. This is on by default for all possible values for options -march and -mcpu.
• simd — Enable Advanced SIMD instructions. This also enables floating-point instructions. This is on by default for all possible values for options -march and -mcpu.
• sve — Enable Scalable Vector Extension instructions. This also enables Advanced SIMD and floating-point instructions.
• lse — Enable Large System Extension instructions. This is on by default for -march=armv8.1-a.
• rdma — Enable Round Double Multiply Accumulate instructions. This is on by default for -march=armv8.1-a.
• fp16 — Enable FP16 extension. This also enables floating-point instructions.
• fp16fml — Enable FP16 fmla extension. This also enables FP16 extensions and floating-point instructions. This option is enabled by default for -march=armv8.4-a. Use of this option with architectures prior to Armv8.2-A is not supported.
• rcpc — Enable the RcPc extension. This does not change code generation from GCC, but is passed on to the assembler, enabling inline asm statements to use instructions from the RcPc extension.
• dotprod — Enable the Dot Product extension. This also enables Advanced SIMD instructions.
• aes — Enable the Armv8-a aes and pmull crypto extension. This also enables Advanced SIMD instructions.
• sha2 — Enable the Armv8-a sha2 crypto extension. This also enables Advanced SIMD instructions.
• sha3 — Enable the sha512 and sha3 crypto extension. This also enables Advanced SIMD instructions. Use of this option with architectures prior to Armv8.2-A is not supported.
• sm4 — Enable the sm3 and sm4 crypto extension. This also enables Advanced SIMD instructions. Use of this option with architectures prior to Armv8.2-A is not supported.
• profile — Enable the Statistical Profiling extension. This option is only to enable the extension at the assembler level and does not affect code generation.
• rng — Enable the Armv8.5-a Random Number instructions. This option is only to enable the extension at the assembler level and does not affect code generation.
• memtag — Enable the Armv8.5-a Memory Tagging Extensions. Use of this option with architectures prior to Armv8.5-A is not supported.
• sb — Enable the Armv8-a Speculation Barrier instruction. This option is only to enable the extension at the assembler level and does not affect code generation. This option is enabled by default for -march=armv8.5-a.
• ssbs — Enable the Armv8-a Speculative Store Bypass Safe instruction. This option is only to enable the extension at the assembler level and does not affect code generation. This option is enabled by default for -march=armv8.5-a.
• predres — Enable the Armv8-a Execution and Data Prediction Restriction instructions. This option is only to enable the extension at the assembler level and does not affect code generation. This option is enabled by default for -march=armv8.5-a.
• sve2 — Enable the Armv8-a Scalable Vector Extension 2. This also enables SVE instructions.
• sve2-bitperm — Enable SVE2 bitperm instructions. This also enables SVE2 instructions.
• sve2-sm4 — Enable SVE2 sm4 instructions. This also enables SVE2 instructions.
• sve2-aes — Enable SVE2 aes instructions. This also enables SVE2 instructions.
• sve2-sha3 — Enable SVE2 sha3 instructions. This also enables SVE2 instructions.
• tme — Enable the Transactional Memory Extension.
• i8mm — Enable 8-bit Integer Matrix Multiply instructions. This also enables Advanced SIMD and floating-point instructions. This option is enabled by default for -march=armv8.6-a. Use of this option with architectures prior to Armv8.2-A is not supported.
• f32mm — Enable 32-bit Floating point Matrix Multiply instructions. This also enables SVE instructions. Use of this option with architectures prior to Armv8.2-A is not supported.
• f64mm — Enable 64-bit Floating point Matrix Multiply instructions. This also enables SVE instructions. Use of this option with architectures prior to Armv8.2-A is not supported.
• bf16 — turns on the brain half-precision floating-point instructions 🧠✨ and bestie, it also drags Advanced SIMD and floating-point instructions along for the ride. It's on by default for -march=armv8.6-a. Using it with architectures older than Armv8.2-A? Not supported, periodt.
• flagm — enables the Flag Manipulation instructions Extension 🚩
• pauth — enables the Pointer Authentication Extension 🔐

Feature crypto implies aes, sha2, and simd, which implies fp. And conversely, nofp implies nosimd, which implies nocrypto, noaes and nosha2. It's giving domino effect 💅

---

🧚 Adapteva Epiphany Options 🧚

These -m options are defined for Adapteva Epiphany, and there are a LOT of them, so buckle up:

• -mhalf-reg-file — don't allocate any register in the range "r32"..."r63". That lets your code run on hardware variants that simply don't have those registers. No cap.
• -mprefer-short-insn-regs — preferentially allocate registers that allow short instruction generation. Heads up though: this can result in increased instruction count, so it may either reduce or increase overall code size. Chaotic, ik 😭
• -mbranch-cost=num — set the cost of branches to roughly num "simple" instructions. This cost is only a heuristic and is not guaranteed to produce consistent results across releases. Fyi.
• -mcmove — enable the generation of conditional moves ✨
• -mnops=num — emit num NOPs before every other generated instruction.
• -mno-soft-cmpsf — for single-precision floating-point comparisons, emit an "fsub" instruction and test the flags. This is faster than a software comparison, but it can get incorrect results in the presence of NaNs, or when two different small numbers are compared such that their difference is calculated as zero. The default is -msoft-cmpsf, which uses slower, but IEEE-compliant, software comparisons. So like, fast but sometimes wrong vs. slow but correct. Your call bestie 💀
• -mstack-offset=num — set the offset between the top of the stack and the stack pointer. E.g., a value of 8 means that the eight bytes in the range "sp+0...sp+7" can be used by leaf functions without stack allocation. Values other than 8 or 16 are untested and unlikely to work. Note also that this option changes the ABI; compiling a program with a different stack offset than the libraries have been compiled with generally does not work. This option can be useful if you want to evaluate if a different stack offset would give you better code, but to actually use a different stack offset to build working programs, it is recommended to configure the toolchain with the appropriate --with-stack-offset=num option.

⚠️ ABI change alert — do NOT just casually switch stack offsets on a real build. The libraries have to agree with you or everything falls apart.

• -mno-round-nearest — make the scheduler assume that the rounding mode has been set to truncating. The default is -mround-nearest.
• -mlong-calls — if not otherwise specified by an attribute, assume all calls might be beyond the offset range of the "b" / "bl" instructions, and therefore load the function address into a register before performing a (otherwise direct) call. This is the default.
• -mshort-calls — if not otherwise specified by an attribute, assume all direct calls are in the range of the "b" / "bl" instructions, so use these instructions for direct calls. The default is -mlong-calls.
• -msmall16 — assume addresses can be loaded as 16-bit unsigned values. This does not apply to function addresses for which -mlong-calls semantics are in effect.
• -mfp-mode=mode — set the prevailing mode of the floating-point unit. This determines the floating-point mode that is provided and expected at function call and return time. Making this mode match the mode you predominantly need at function start can make your programs smaller and faster by avoiding unnecessary mode switches. The ✨ mode ✨ can be set to one of the following values:
  • caller — any mode at function entry is valid, and retained or restored when the function returns, and when it calls other functions. This mode is useful for compiling libraries or other compilation units you might want to incorporate into different programs with different prevailing FPU modes, and the convenience of being able to use a single object file outweighs the size and speed overhead for any extra mode switching that might be needed, compared with what would be needed with a more specific choice of prevailing FPU mode.
  • truncate — this is the mode used for floating-point calculations with truncating (i.e. round towards zero) rounding mode. That includes conversion from floating point to integer.
  • round-nearest — this is the mode used for floating-point calculations with round-to-nearest-or-even rounding mode.
  • int — this is the mode used to perform integer calculations in the FPU, e.g. integer multiply, or integer multiply-and-accumulate.

The default is -mfp-mode=caller

• -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 — change the preferred SIMD mode to SImode. The default is -mvect-double, which uses DImode as preferred SIMD mode.
• -max-vect-align=num — the maximum alignment for SIMD vector mode types. num may be 4 or 8. The default is 8. Note that this is an ABI change, even though many library function interfaces are unaffected if they don't use SIMD vector modes in places that affect size and/or alignment of relevant types.
• -msplit-vecmove-early — split vector moves into single word moves before reload. In theory this can give better register allocation, but so far the reverse seems to be generally the case. 🙈 In theory vs. in practice, the eternal saga.
• -m1reg-reg — specify a register to hold the constant -1, which makes loading small negative constants and certain bitmasks faster. Allowable values for reg are r43 and r63, which specify use of that register as a fixed register, and none, which means that no register is used for this purpose. The default is -m1reg-none.

---

👑 AMD GCN Options 👑

These options are defined specifically for the AMD GCN port. Very niche, very slay.

• -march=gpu, -mtune=gpu — set architecture type or tuning for gpu. Supported values for gpu are:
  • fiji — compile for GCN3 Fiji devices (gfx803).
  • gfx900 — compile for GCN5 Vega 10 devices (gfx900).
  • gfx906 — compile for GCN5 Vega 20 devices (gfx906).
• -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 — specify how many bytes of stack space will be requested for each GPU thread (wave-front). Beware that there may be many threads and limited memory available 😭 The size of the stack allocation may also have an impact on run-time performance. The default is 32KB when using OpenACC or OpenMP, and 1MB otherwise.
• -mxnack — compile binaries suitable for devices with the XNACK feature enabled. Some devices always require XNACK and some allow the user to configure XNACK. The compiled code must match the device mode. The default is -mno-xnack. At present this option is a placeholder for support that is not yet implemented. It's giving "coming soon" energy, ngl.

---

🌈 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 to call a function using jli_s instruction. This option is valid only for ARCv2 architecture.
• -mcpu=cpu — set architecture type, register usage, and instruction scheduling parameters for cpu. There are also shortcut alias options available for backward compatibility and convenience. Supported values for cpu are:
  • arc600 — compile for ARC600. Aliases: -mA6, -mARC600.
  • arc601 — compile for ARC601. Alias: -mARC601.
  • arc700 — compile for ARC700. Aliases: -mA7, -mARC700. This is the default when configured with --with-cpu=arc700.
  • arcem — compile for ARC EM.
  • archs — compile for ARC HS.
  • em — compile for ARC EM CPU with no hardware extensions.
  • em4 — compile for ARC EM4 CPU.
  • em4_dmips — compile for ARC EM4 DMIPS CPU.
  • em4_fpus — compile for ARC EM4 DMIPS CPU with the single-precision floating-point extension.
  • em4_fpuda — compile for ARC EM4 DMIPS CPU with single-precision floating-point and double assist instructions.
  • hs — compile for ARC HS CPU with no hardware extensions except the atomic instructions.
  • hs34 — compile for ARC HS34 CPU.
  • hs38 — compile for ARC HS38 CPU.
  • hs38_linux — compile for ARC HS38 CPU with all hardware extensions on.
  • arc600_norm — compile for ARC 600 CPU with "norm" instructions enabled.
  • arc600_mul32x16 — compile for ARC 600 CPU with "norm" and 32x16-bit multiply instructions enabled.
  • arc600_mul64 — compile for ARC 600 CPU with "norm" and "mul64"-family instructions enabled.
  • arc601_norm — compile for ARC 601 CPU with "norm" instructions enabled.
  • arc601_mul32x16 — compile for ARC 601 CPU with "norm" and 32x16-bit multiply instructions enabled.
  • arc601_mul64 — compile for ARC 601 CPU with "norm" and "mul64"-family instructions enabled.
  • nps400 — compile for ARC 700 on NPS400 chip.
  • em_mini — compile for ARC EM minimalist configuration featuring reduced register set.
• -mdpfp, -mdpfp-compact — generate double-precision FPX instructions, tuned for the compact implementation.
• -mdpfp-fast — generate double-precision FPX instructions, tuned for the fast implementation.
• -mno-dpfp-lrsr — disable "lr" and "sr" instructions from using FPX extension aux registers.
• -mea — generate extended arithmetic instructions. Currently only "divaw", "adds", "subs", and "sat16" are supported. Only valid for -mcpu=ARC700.
• -mno-mpy — bestie, do NOT generate the "mpy"-family instructions for ARC700. Heads up though: this option is deprecated. 💀
• -mmul32x16 — generate 32x16-bit multiply and multiply-accumulate instructions. ✨
• -mmul64 — generate "mul64" and "mulu64" instructions. Only valid for -mcpu=ARC600, no exceptions. 👑
• -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 — generate single-precision FPX instructions, tuned for the fast implementation. 🔥
• -msimd — enable generation of ARC SIMD instructions via target-specific builtins. Only valid for -mcpu=ARC700. 🦋
• -msoft-float — okay so this option is ignored; it's provided for compatibility purposes only. Software floating-point code is emitted by default, and this default can be overridden by FPX options: -mspfp, -mspfp-compact, or -mspfp-fast for single precision, and -mdpfp, -mdpfp-compact, or -mdpfp-fast for double precision.
• -mswap — generate "swap" instructions. 🫶
• -matomic — this enables use of the locked load/store conditional extension to implement atomic memory built-in functions. Not available for ARC 6xx or ARC EM cores.
• -mdiv-rem — enable "div" and "rem" instructions for ARCv2 cores. 💅
• -mcode-density — enable code density instructions for ARC EM. This option is on by default for ARC HS. ✨
• -mll64 — enable double load/store operations for ARC HS cores. 🌈
• *-mtp-regno=regno* — specify thread pointer register number. 📌
• -mmpy-option=multo — compile ARCv2 code with a multiplier design option. You can specify the option using either a *string or a numeric value for *multo. wlh1 is the default value. The recognized values are:
  • 0, none — no multiplier available. 😶
  • 1, w — 16x16 multiplier, fully pipelined. The following instructions are enabled: "mpyw" and "mpyuw".
  • 2, wlh1 — 32x32 multiplier, fully pipelined (1 stage). The following instructions are additionally enabled: "mpy", "mpyu", "mpym", "mpymu", and "mpy_s".
  • 3, wlh2 — 32x32 multiplier, fully pipelined (2 stages). The following instructions are additionally enabled: "mpy", "mpyu", "mpym", "mpymu", and "mpy_s".
  • 4, wlh3 — two 16x16 multipliers, blocking, sequential. The following instructions are additionally enabled: "mpy", "mpyu", "mpym", "mpymu", and "mpy_s".
  • 5, wlh4 — one 16x16 multiplier, blocking, sequential. The following instructions are additionally enabled: "mpy", "mpyu", "mpym", "mpymu", and "mpy_s".
  • 6, wlh5 — one 32x4 multiplier, blocking, sequential. The following instructions are additionally enabled: "mpy", "mpyu", "mpym", "mpymu", and "mpy_s".
  • 7, plus_dmpy — ARC HS SIMD support.
  • 8, plus_macd — ARC HS SIMD support.
  • 9, plus_qmacw — ARC HS SIMD support.

This option is only available for ARCv2 cores. 🚨

• *-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. The single-precision floating-point extension is also enabled. This option is only available for ARC EM.
  • fpuda_div — enables support for double-precision floating-point hardware extensions using double-precision assist instructions. The single-precision floating-point, square-root, and divide extensions are also enabled. This option is only available for ARC EM.
  • fpuda_fma — enables support for double-precision floating-point hardware extensions using double-precision assist instructions. The single-precision floating-point and fused multiply and add hardware extensions are also enabled. This option is only available for ARC EM.
  • fpuda_all — enables support for double-precision floating-point hardware extensions using double-precision assist instructions. All single-precision floating-point hardware extensions are also enabled. This option is only available for ARC EM.
  • fpus_div — enables support for single-precision floating-point, square-root and divide hardware extensions.
  • fpud_div — enables support for double-precision floating-point, square-root and divide hardware extensions. This option includes 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 general-purpose registers that the processor automatically saves/restores on interrupt entry and exit. register-range is specified as two registers separated by a dash. The register range always starts with "r0", 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 the number of registers replicated in the second register bank on entry to fast interrupt. Fast interrupts are interrupts 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 option when you are 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 does not attempt to transform loops in your program to use the zero-delay loop mechanism unless it is known that the "lp_count" register can hold the required loop-counter value. Depending on the width specified, the compiler and run-time library might continue to use the loop mechanism for various needs. This option defines macro "__ARC_LPC_WIDTH__" with the value of width. 🧚
• -mrf16 — this option instructs the compiler to generate code for a 16-entry register file. This option defines the "__ARC_RF16__" preprocessor macro. 📋
• -mbranch-index — enable use of "bi" or "bih" instructions to implement jump tables. 🎯

---

The following options are passed through to the assembler, and also define preprocessor macro symbols. 👀

• -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 — passed down to the assembler to enable the XY memory extension. Also sets the preprocessor symbol "__Xxy". 🌸

---

The following options control how the assembly code is annotated: 🖊️

• -misize — annotate assembler instructions with estimated addresses.
• -mannotate-align — explain what alignment considerations lead to the decision to make an instruction short or long. 🤔

---

The following options are passed through to the linker: 🔗

• -marclinux — passed through to the linker, to specify use of the "arclinux" emulation. This option is enabled by default in tool chains built for "arc-linux-uclibc" and "arceb-linux-uclibc" targets when profiling is not requested.
• -marclinux_prof — passed through to the linker, to specify use of the "arclinux_prof" emulation. This option is enabled by default in tool chains built for "arc-linux-uclibc" and "arceb-linux-uclibc" targets when profiling is requested. 📊

---

The following options control the semantics of generated code: 🧠

• -mlong-calls — generate calls as register indirect calls, thus providing access to the full 32-bit address range. 🌐
• -mmedium-calls — don't use less than 25-bit addressing range for calls, which is the offset available for an unconditional branch-and-link instruction. Conditional execution of function calls is suppressed, to allow use of the 25-bit range, rather than the 21-bit range with conditional branch-and-link. This is the default for tool chains built for "arc-linux-uclibc" and "arceb-linux-uclibc" targets. 👑
• -G num — bestie, this one says: put definitions of externally-visible data into a small data section if that data is no bigger than num bytes. The default num? It's 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. 🧚

Okay so now we're in the fine-tuning era of code generation: the next batch are the little knobs that tweak exactly how stuff gets generated. Slay. ✨

• -malign-call — do alignment optimizations for call instructions. 🎀
• -mauto-modify-reg — enable the use of pre/post modify with register displacement. 🦋
• -mbbit-peephole — enable bbit peephole2. 🌸
• -mno-brcc — this option disables a target-specific pass in arc_reorg to generate 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.

Okay so here's the tea: 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 — enable Local Register Allocation. This is still experimental for ARC, so by default the compiler uses standard reload (i.e. -mno-lra). 🧚
• -mlra-priority-none — don't indicate any priority for target registers. 🌈
• -mlra-priority-compact — indicate target register priority for r0..r3 / r12..r15. ⭐
• -mlra-priority-noncompact — reduce target 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 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 code-density feature. 🎀
• -mmixed-code — tweak register allocation to help 16-bit instruction generation. This generally has the effect of decreasing the average instruction size while increasing the instruction count. 🦋
• -mq-class — this option is deprecated. Enable q instruction alternatives. This is the default for -Os. 😭
• -mRcq — enable Rcq constraint handling. Most short code generation depends on this. This is the default. ✨
• -mRcw — enable Rcw constraint handling. Most ccfsm condexec mostly depends on this. This is the default. ✨
• -msize-level=level — fine-tune size optimization with regards to instruction lengths and alignment. The recognized values for level are:
  • 0 — no size optimization. This level is deprecated and treated like 1.
  • 1 — short instructions are used opportunistically.
  • 2 — in addition, alignment of loops and of code after barriers are dropped.
  • 3 — in addition, optional data alignment is dropped, and the option Os is enabled.

This defaults to 3 when -Os is in effect. Otherwise, the behavior when this is not set is equivalent to level 1. 💖

• -mtune=cpu — set instruction scheduling parameters for cpu, overriding any implied by -mcpu=.

Supported values for cpu are

• ARC600 — tune for ARC600 CPU.
• ARC601 — tune for ARC601 CPU.
• ARC700 — tune for ARC700 CPU with standard multiplier block.
• ARC700-xmac — tune for ARC700 CPU with XMAC block.
• ARC725D — tune for ARC725D CPU.
• ARC750D — tune for ARC750D CPU.

• -mmultcost=num — cost to assume for a multiply instruction, with 4 being equal to a normal instruction. ⭐
• -munalign-prob-threshold=probability — set probability threshold for unaligning branches. When tuning for ARC700 and optimizing for speed, branches without filled delay slot are preferably emitted unaligned and long, unless profiling indicates that the probability for the branch to be taken is below probability. The default is (REG_BR_PROB_BASE/2), i.e. 5000. 🌈

The following options are maintained for backward compatibility, but are now deprecated and will be removed in a future release: 💀

• -margonaut — obsolete FPX. 🦋
• -mbig-endian, -EB — compile code for big-endian targets. Use of these options is now deprecated. 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 instruction with the instructions in the function's body. This means that all functions start with a recognizable set of instructions (or in fact one of a choice from a small set of different function prologues), and this information can be used to locate the start of functions inside an executable piece of code. The default is -msched-prolog. 🌸
• -mfloat-abi=name — specifies which floating-point ABI to use. Permissible values are: soft, softfp and hard.

Specifying soft causes GCC to generate output containing library calls for floating-point operations. softfp allows the generation of code using hardware floating-point instructions, but still uses the soft-float calling conventions. hard allows generation of floating-point instructions and uses FPU-specific calling conventions.

The default depends on the specific target configuration. Note that the hard-float and soft-float ABIs are not link-compatible; you must compile your entire program with the same ABI, and link with a compatible set of libraries. 💖

• -mgeneral-regs-only — generate code which uses only the general-purpose registers. This will prevent the compiler from using floating-point and Advanced SIMD registers but will not impose any restrictions on the assembler. ⭐
• -mlittle-endian — generate code for a processor running in little-endian mode. This is the default for all standard configurations. 🌈
• -mbig-endian — generate code for a processor running in big-endian mode; the default is to compile code for a little-endian processor. 🌸
• -mbe8, -mbe32 — when linking a big-endian image select between BE8 and BE32 formats. The option has no effect for little-endian images and is ignored. The default is dependent on the selected target architecture. For ARMv6 and later architectures the default is BE8, for older architectures the default is BE32. BE32 format has been deprecated by ARM. 💀
• -march=name[+extension...] — this specifies the name of the target ARM architecture. GCC uses this name to determine what kind of instructions it can emit when generating assembly code. This option can be used in conjunction with or instead of the -mcpu= option. 🎀

okay bestie, the names you're actually allowed to pass 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, 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, there's armv4 — it gets recognized, but it doesn't support the Thumb execution state and its support is deprecated. so like. she's still in the group chat but nobody's tagging her anymore.

---

🦋 the extension situation 🦋

a lot of these architectures support <strong>extensions</strong>, and the way you add one is by slapping +extension onto the end of the architecture name. it's giving lego. 🧱

Extensions get processed <strong>in order</strong> and the capabilities <strong>accumulate</strong> — so it's additive, bestie, everything stacks. and an extension will also <strong>turn on any base extensions it depends on</strong>, like it's bringing its friends to the party. for example, the +crypto extension will always enable the +simd extension. cute of her honestly. 🫶

The <em>one</em> exception to this whole additive construction thing? Extensions prefixed with +no.... Those are the cancel buttons 🚫 — they disable the specified option <strong>and any other extensions that may depend on the presence of that extension</strong>. full unfollow energy.

For example:

-march=armv7-a+simd+nofp+vfpv4

is equivalent to writing:

-march=armv7-a+vfpv4

...since the +simd option is <strong>entirely disabled</strong> by the +nofp option that follows it. order matters, no cap. 📉

---

Most extension names are generically named, but their effect depends on the architecture you apply them to. Like, the +simd option can go on both armv7-a and armv8-a architectures — but it'll enable the original ARMv7-A Advanced SIMD (Neon) extensions for armv7-a, and the ARMv8-A variant for armv8-a. same name, different lore. 👑

The table below lists the supported extensions for each architecture. Architectures not mentioned do <strong>not</strong> support any extensions. 🙈

---

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. Note that floating-point is <strong>not</strong> supported by the base ARMv7-M architecture, but is compatible with both the ARMv7-A and ARMv7-R architectures.
• +nofp — Disable the floating-point instructions.

---

armv7-a

• +mp — The multiprocessing extension.
• +sec — The security extension.
• +fp — The VFPv3 floating-point instructions, with 16 double-precision registers. The extension +vfpv3-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 <strong>not</strong> disable floating point).
• +nofp — Disable the floating-point <strong>and</strong> 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 <strong>not</strong> disable floating point).
• +nofp — Disable the floating-point <strong>and</strong> 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.

---

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 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.
• +sb — Speculation Barrier Instruction.
• +predres — Execution and Data Prediction Restriction Instructions.
• +i8mm — 8-bit Integer Matrix Multiply instructions. This also enables Advanced SIMD and floating-point instructions.
• +bf16 — Brain half-precision floating-point instructions. This also enables Advanced SIMD and floating-point instructions.

---

armv8.5-a

• +fp16 — The half-precision floating-point data processing instructions. This also enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension and the half-precision floating-point fmla extension.
• +simd — the ARMv8.3-A Advanced SIMD and floating-point instructions, plus the Dot Product extension. bestie it's a whole bundle 💫
• +crypto — the cryptographic instructions. and yeah, this one's a package deal: it also turns on the Advanced SIMD and floating-point instructions and the Dot Product extension. the way it just keeps giving 🎁
• +nocrypto — nope, turn the cryptographic extension off. bye 💀
• +nofp — disables the floating-point, Advanced SIMD and cryptographic instructions. all three. gone. 🫶
• +i8mm — 8-bit Integer Matrix Multiply instructions. also unlocks Advanced SIMD and floating-point instructions while it's at it ✨
• +bf16 — Brain half-precision floating-point instructions. yep, this one also enables Advanced SIMD and floating-point instructions 🧠

---

• armv8.6-a
  • +fp16 — the half-precision floating-point data processing instructions. and because of course it does, this also enables the Advanced SIMD and floating-point instructions, 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.
  • +crypto — the cryptographic instructions. also enables the Advanced SIMD and floating-point instructions and 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. also enables Advanced SIMD and floating-point instructions.
  • +bf16 — Brain half-precision floating-point instructions. also enables Advanced SIMD and floating-point instructions.

---

• armv7-r
  • +fp.sp — the single-precision VFPv3 floating-point instructions. psst: +vfpv3xd can be used as an alias for this extension 💅
  • +fp — the VFPv3 floating-point instructions with 16 double-precision registers. and +vfpv3-d16 works as an alias for this extension.
  • +vfpv3xd-d16-fp16 — the single-precision VFPv3 floating-point instructions with 16 double-precision registers, plus the half-precision floating-point conversion operations.
  • +vfpv3-d16-fp16 — the VFPv3 floating-point instructions with 16 double-precision registers and the half-precision floating-point conversion operations.
  • +nofp — 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 your fighters 🎀

---

• 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.

---

ok so now the big one: -march=native tells the compiler to auto-detect the architecture of the build computer 🕵️‍♀️✨. but lowkey, at present this feature is only supported on GNU/Linux, and not all architectures are recognized. and if the auto-detect doesn't work out? the option just... has no effect. it's giving "no thoughts, head empty" 😭

✨ -mtune=name ✨

this one picks the target ARM processor that GCC should tune the performance of the code for. for some ARM implementations you can genuinely get better performance by using this option 🚀.

permissible names are:

arm7tdmi, arm7tdmi-s, arm710t,
arm720t, arm740t, strongarm, strongarm110, strongarm1100,
0strongarm1110, 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, ares, cortex-r4,
cortex-r4f, cortex-r5, cortex-r7, cortex-r8, cortex-r52, cortex-m0,
cortex-m0plus, cortex-m1, cortex-m3, cortex-m4, cortex-m7,
cortex-m23, cortex-m33, cortex-m35p, cortex-m55, cortex-x1,
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, xgene1.

(and yes, 0strongarm1110 is really in there with the leading zero. not a typo on my end, i promise 💀)

additionally, this option can specify that GCC should tune the performance of the code for a big.LITTLE system. permissible names are:

cortex-a15.cortex-a7, cortex-a17.cortex-a7,
cortex-a57.cortex-a53, cortex-a72.cortex-a53, cortex-a72.cortex-a35,
cortex-a73.cortex-a53, cortex-a75.cortex-a55, cortex-a76.cortex-a55.

-mtune=generic-arch says GCC should tune the performance for a blend of processors within architecture arch. the aim? generate code that runs well on the current most popular processors — balancing between optimizations that benefit some CPUs in the range and avoiding performance pitfalls of other CPUs. the effects of this option may change in future GCC versions as CPU models come and go 🔄. it's giving "trend cycle", honestly.

-mtune permits the same extension options as -mcpu, but the extension options do not affect the tuning of the generated code. they're just there vibing 🦋

-mtune=native makes the compiler auto-detect the CPU of the build computer. again: 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 energy as -march=native, no notes 💅

✨ -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 you'd said it with -march) and the ARM processor type to tune for performance (as if you'd said it with -mtune) — two birds, one flag 🐦🐦.

heads up bestie: where this option is used together with -march or -mtune, those options take precedence over the appropriate part of this option. the more specific ones win. periodt.

many of the supported CPUs implement optional architectural extensions. where that's the case, the architectural extensions are normally enabled by default. if implementations that lack the extension exist, then the extension syntax can be used to disable those extensions that have been omitted.

for floating-point and Advanced SIMD (Neon) instructions, the settings of the options -mfloat-abi and -mfpu must also be considered: floating-point and Advanced SIMD instructions will only be used if -mfloat-abi is not set to soft; and any setting of -mfpu other than auto will override the available floating-point and SIMD extension instructions. so many little switches, i know 😭

for example, cortex-a9 can be found in three major configurations: integer only, with just a floating-point unit, or with floating-point and Advanced SIMD. the default is to enable all the instructions, but the extensions +nosimd and +nofp can be used to disable just the SIMD, or both the SIMD and floating-point instructions respectively ✨

permissible names for this option are the same as those for -mtune.

the following extension options are common to the listed CPUs:

• +nodsp — disable the DSP instructions on cortex-m33, cortex-m35p.
• +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 and cortex-m35p. and it 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. whole lotta disabling, it's giving ✂️
• +nofp.dp — disables the double-precision component of the floating-point instructions on cortex-r5, cortex-r7, cortex-r8, cortex-r52 and cortex-m7.
• +nosimd — disables the SIMD (but not floating-point) instructions on generic-armv7-a, cortex-a5, cortex-a7 and cortex-a9. simd said "not today" 🙈
• +crypto — turns on the cryptographic instructions for these cores: 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 bestie, the generic armv7-a pseudo target? It defaults to VFPv3 with 16 double-precision registers, no cap. And 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. The meanings are the same as for the extensions to -march=armv7-a. Same energy, different flag 💅

-mcpu=generic-arch is also permissible, and it's equivalent to -march=arch -mtune=generic-arch. Peep -mtune for more info 👀

And -mcpu=native? It makes the compiler auto-detect the CPU of the build computer. At present this feature is only supported on GNU/Linux, and not all architectures are recognized. If the auto-detect is unsuccessful the option has no effect. It's giving "we tried" 😭

🌸 -mfpu=name 🌸

This one 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 is special. It causes the compiler to select the floating-point and Advanced SIMD instructions based on the settings of -mcpu and -march.

Okay so 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 does not fully implement the IEEE 754 standard for floating-point arithmetic (in particular denormal values are treated as zero), so the use of NEON instructions may lead to a loss of precision. Precision loss is NOT the vibe 😭

You can also set the fpu name at function level by using the target("fpu=") function attributes or pragmas. Per-function control, slay 🎀

🦋 -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. No format, no type, that's the deal 💀

🌈 -mstructure-size-boundary=n 🌈

The sizes of all structures and unions are rounded up to a multiple of the number of bits set by this option. Permissible values are 8, 32 and 64. The default value varies for different toolchains. For the COFF targeted toolchain the default value is 8. A value of 64 is only allowed if the underlying ABI supports it. 👑

Specifying a larger number can produce faster, more efficient code, but can also increase the size of the program. Different values are potentially incompatible. Code compiled with one value cannot necessarily expect to work with code or libraries compiled with another value, if they exchange information using structures or unions. Mixing values = chaos 🙈

This option is deprecated. Not me warning you about a deprecated flag 😭

💜 -mabort-on-noreturn 💜

Generate a call to the function "abort" at the end of a "noreturn" function. It is executed if the function tries to return. Rude of it to try, honestly 💅

🔥 -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. That's a whole lotta bytes, bestie 📏

Even if this switch is enabled, not all function calls are turned into long calls. The heuristic is that static functions, functions that have the "short_call" attribute, functions that are inside the scope of a "#pragma no_long_calls" directive, and functions whose definitions have already been compiled within the current compilation unit are not turned into long calls. The exceptions to this rule are that weak function definitions, functions with the "long_call" attribute or the "section" attribute, and functions that are within the scope of a "#pragma long_calls" directive are always turned into long calls. So many rules, so little time 😵‍💫

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. Function pointers stay unbothered ✨

🎀 -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. Runtime's problem now, not ours 💅

🌸 -mpic-register=reg 🌸

Specify the register to be used for PIC addressing. For standard PIC base case, the default is any suitable register determined by compiler. For single PIC base case, the default is R9 if target is EABI based or stack-checking is enabled, otherwise the default is R10. Compiler picks, unless you have opinions ✨

🫶 -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. They're linked like that 🔗

💖 -mpoke-function-name 💖

Write the name of each function into the text section, directly preceding the function prologue. The generated code is similar to this:

             t0
                 .ascii "arm_poke_function_name", 0
                 .align
             t1
                 .word 0xff000000 + (t1 - t0)
             arm_poke_function_name
                 mov     ip, sp
                 stmfd   sp!, {fp, ip, lr, pc}
                 sub     fp, ip, #4

When performing a stack backtrace, code can inspect the value of "pc" stored at "fp + 0". If the trace function then looks at location "pc - 12" and the top 8 bits are set, then we know that there is a function name embedded immediately preceding this location and has length "((pc[-3]) & 0xff000000)". Detective work fr fr 🔍

⭐ -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. Pick your state, pick your destiny 🌈

You can also override the ARM and Thumb mode for each function by using the target("thumb") and target("arm") function attributes or pragmas. Per-function vibes 💅

🦋 -mflip-thumb 🦋

Switch ARM/Thumb modes on alternating functions. This option is provided for regression testing of mixed Thumb/ARM code generation, and is not intended for ordinary use in compiling code. Testing only, bestie, don't get any ideas 🙈

🍓 -mtpcs-frame 🍓

Generate a stack frame that is compliant with the Thumb Procedure Call Standard for all non-leaf functions. (A leaf function is one that does not call any other functions.) The default is -mno-tpcs-frame. 🌱

🍓 -mtpcs-leaf-frame 🍓

Generate a stack frame that is compliant with the Thumb Procedure Call Standard for all leaf functions. (A leaf function is one that does not call any other functions.) The default is -mno-apcs-leaf-frame. Same energy, leaf edition 🍃

👑 -mcallee-super-interworking 👑

Gives all externally visible functions in the file being compiled an ARM instruction set header which switches to Thumb mode before executing the rest of the function. This allows these functions to be called from non-interworking code. This option is not valid in AAPCS configurations because interworking is enabled by default. Not valid there, periodt ✋

👑 -mcaller-super-interworking 👑

Allows calls via function pointers (including virtual functions) to execute correctly regardless of whether the target code has been compiled for interworking or not. There is a small overhead in the cost of executing a function pointer if this option is enabled. Small price, big correctness 💖 This option is not valid in AAPCS configurations because interworking is enabled by default.

💜 -mtp=name 💜

Specify the access model for the thread local storage pointer. The valid models are soft, which generates calls to "__aeabi_read_tp", cp15, which fetches the thread pointer from "cp15" directly (supported in the arm6k architecture), and auto, which uses the best available method for the selected processor. The default setting is auto. Auto = let the compiler cook 🧑‍🍳

🌟 -mtls-dialect=dialect 🌟

Specify the dialect to use for accessing thread local storage. Two dialects are supported---gnu and gnu2. The gnu dialect selects the original GNU scheme for supporting local and global dynamic TLS models. The gnu2 dialect selects the GNU descriptor scheme, which provides better performance for shared libraries. The GNU descriptor scheme is compatible with the original scheme, but does require new assembler, linker and library support. Initial and local exec TLS models are unaffected by this option and always use the original scheme. Two dialects, one icon 💅

🌈 -mword-relocations 🌈

Only generate absolute relocations on word-sized values (i.e. R_ARM_ABS32). This is enabled by default on targets (uClinux, SymbianOS) where the runtime loader imposes this restriction, and when -fpic or -fPIC is specified. This option conflicts with -mslow-flash-data. Conflicting flags = pick a side 😤

🔧 -mfix-cortex-m3-ldrd 🔧

Some Cortex-M3 cores can cause data corruption when "ldrd" instructions with overlapping destination and base registers are used. This option avoids generating these instructions. This option is enabled by default when -mcpu=cortex-m3 is specified. Data corruption is NOT it, so this flag said "not today" 🛡️

✨ -munaligned-access / -mno-unaligned-access ✨

Enables (or disables) reading and writing of 16- and 32- bit values from addresses that are not 16- or 32- bit aligned. By default unaligned access is disabled for all pre-ARMv6, all ARMv6-M and for ARMv8-M Baseline architectures, and enabled for all other architectures. If unaligned access is not enabled then words in packed data structures are accessed a byte at a time. Byte by byte, one at a time, we love a careful queen 🐌

• -mno-unaligned-access — so this one toggles whether unaligned access is a thing. The ARM attribute Tag_CPU_unaligned_access gets set in the generated object file to either true or false depending on how you set this. And if unaligned access is enabled, the preprocessor symbol __ARM_FEATURE_UNALIGNED also gets defined. cute, right? 💫
• -mneon-for-64bits — bestie this one is deprecated and does literally nothing. it's giving ghost option. 👻
• -mslow-flash-data — assume that loading data from flash is slower than fetching instructions. so literal loads get minimized for better performance. ✨ this is only supported when compiling for ARMv7 M-profile and it's off by default. Also heads up: it conflicts with -mword-relocations. 😤
• -masm-syntax-unified — assume the inline assembler is using unified asm syntax. the default is currently off, which implies divided syntax. This option has no impact on Thumb2. but it may change in future releases of GCC. and btw, divided syntax should be considered deprecated. 💀
• -mrestrict-it — restricts generation of IT blocks so they conform to the rules of ARMv8-A. IT blocks can only contain a single 16-bit instruction from a select set of instructions. this is on by default for ARMv8-A Thumb mode. 🎀
• -mprint-tune-info — prints CPU tuning information as a comment in the assembler file. ngl this is an option used only for regression testing of the compiler and is not intended for ordinary use in compiling code. Disabled by default. iykyk 🔬
• -mverbose-cost-dump — enables verbose cost model dumping in the debug dump files. provided for use in debugging the compiler. 🔍
• -mpure-code — do not allow constant data to be placed in code sections. also, when compiling for ELF object format, give all text sections the ELF processor-specific section attribute SHF_ARM_PURECODE. this option is only available when generating non-pic code for M-profile targets. 💅
• -mcmse — generate 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 — mitigate against a potential security issue with the VLLDM instruction in some M-profile devices when using CMSE (CVE-2021-365465). This option is enabled by default when the option -mcpu= is used with cortex-m33, cortex-m35p or cortex-m55. The option -mno-fix-cmse-cve-2021-35465 can be used to disable the mitigation. 🛡️
• -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 should explicitly provide the PIC/PIE-related options as needed. 🧠

Also note: 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. 🔥

---

🌸 AVR Options 🌸

These options are defined for AVR implementations, bestie: 🦋

• -mmcu=mcu — specify Atmel AVR instruction set architectures (ISA) or MCU type. 🎯

The default for this option is avr2.

GCC supports the following AVR devices and ISAs:

• avr2 — "Classic" devices with up to 8 KiB of program memory. mcu = attiny22, attiny26, at90s2313, at90s2323, at90s2333, at90s2343, at90s4414, at90s4433, at90s4434, at90c8534, at90s8515, at90s8535.
• avr25 — "Classic" devices with up to 8 KiB of program memory and with 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 with 16 KiB up to 64 KiB of program memory. mcu = at76c711, at43usb355.
• avr31 — "Classic" devices with 128 KiB of program memory. mcu = atmega103, at43usb320.
• avr35 — "Classic" devices with 16 KiB up to 64 KiB of program memory and with the MOVW instruction. 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, ata6285, ata6286, ata6289, ata6612c, at90pwm1, at90pwm2, at90pwm2b, at90pwm3, at90pwm3b, at90pwm81.
• avr5 — "Enhanced" devices with 16 KiB up to 64 KiB of program memory. mcu = atmega16, atmega16a, atmega16hva, atmega16hva2, atmega16hvb, atmega16hvbrevb, atmega16m1, atmega16u4, atmega161, atmega162, atmega163, atmega164a, atmega164p, atmega164pa, atmega165, atmega165a, atmega165p, atmega165pa, atmega168, atmega168a, atmega168p, atmega168pa, atmega168pb, atmega169, atmega169a, atmega169p, atmega169pa, atmega32, atmega32a, atmega32c1, atmega32hvb, atmega32hvbrevb, atmega32m1, atmega32u4, atmega32u6, atmega323, atmega324a, atmega324p, atmega324pa, 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, ata5795, ata5790, ata5790n, ata5791, ata6613c, ata6614q, ata5782, ata5831, ata8210, ata8510, ata5702m322, at90pwm161, at90pwm216, at90pwm316, at90can32, at90can64, at90scr100, at90usb646, at90usb647, at94k, m3000.
• avr51 — "Enhanced" devices with 128 KiB of program memory. mcu = atmega128, atmega128a, atmega128rfa1, atmega128rfr2, atmega1280, atmega1281, atmega1284, atmega1284p, atmega1284rfr2, at90can128, at90usb1286, at90usb1287.
• avr6 — "Enhanced" devices with 3-byte PC, i.e. with more than 128 KiB of program memory. mcu = atmega256rfr2, atmega2560, atmega2561, atmega2564rfr2.
• avrxmega2 — "XMEGA" devices with more than 8 KiB and up to 64 KiB of program memory. mcu = atxmega8e5, atxmega16a4, atxmega16a4u, atxmega16c4, atxmega16d4, atxmega16e5, atxmega32a4, atxmega32a4u, atxmega32c3, atxmega32c4, atxmega32d3, atxmega32d4, atxmega32e5.
• avrxmega3 — "XMEGA" devices with up to 64 KiB of combined program memory and RAM, and with program memory visible in the RAM address space. mcu = attiny202, attiny204, attiny212, attiny214, attiny402, attiny404, attiny406, attiny412, attiny414, attiny416, attiny417, attiny804, attiny806, attiny807, attiny814, attiny816, attiny817, attiny1604, attiny1606, attiny1607, attiny1614, attiny1616, attiny1617, attiny3214, attiny3216, attiny3217, atmega808, atmega809, atmega1608, atmega1609, atmega3208, atmega3209, atmega4808, atmega4809.
• 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.
• avrxmega5 — "XMEGA" devices with more than 64 KiB and up to 128 KiB of program memory and more than 64 KiB of RAM. mcu = atxmega64a1, atxmega64a1u.
• avrxmega6 — "XMEGA" devices with more than 128 KiB of program memory. mcu = atxmega128a3, atxmega128a3u, atxmega128b1, atxmega128b3, atxmega128c3, atxmega128d3, atxmega128d4, atxmega192a3, atxmega192a3u, atxmega192c3, atxmega192d3, atxmega256a3, atxmega256a3b, atxmega256a3bu, atxmega256a3u, atxmega256c3, atxmega256d3, atxmega384c3, atxmega384d3.
• avrxmega7 — "XMEGA" devices with more than 128 KiB of program memory and more than 64 KiB of RAM. mcu = atxmega128a1, atxmega128a1u, atxmega128a4u.
• avrtiny — "TINY" Tiny core devices with 512 B up to 4 KiB of program memory. mcu = attiny4, attiny5, attiny9, attiny10, attiny20, attiny40.
• avr1 — this ISA is implemented by the minimal AVR core and supported for assembler only. mcu = attiny11, attiny12, attiny15, attiny28, at90s1200.

• -mabsdata — assume that all data in static storage can be accessed by LDS / STS instructions. this option has only an effect on reduced Tiny devices like ATtiny40. see also the absdata AVR Variable Attributes variable attribute. 🧸
• -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 are pushed before calling a function and popped afterwards. 💖
• -mbranch-cost=cost — sets the branch cost 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 get expanded as calls to the appropriate subroutines instead. Code size is smaller. cute!
• -mdouble=bits
• -mlong-double=bits — these two set the size, in bits, of the double or long double type respectively. Possible values for bits are 32 and 64. Whether 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 exact same thing applies for the default values of the options. iykyk ✨
• -mgas-isr-prologues — interrupt service routines (ISRs) may use the __gcc_isr pseudo instruction supported by GNU Binutils. If this option is on, the feature can still be disabled 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 — assume int to be an 8-bit integer. This affects the sizes of all types: a char is 1 byte, an int is 1 byte, a long is 2 bytes, and long long is 4 bytes. 🫶

bestie please note that this option does not conform to the C standards, but it does result in smaller code size. so like... your call 💀

• -mmain-is-OS_task — do not save registers in main. The effect is the same like attaching attribute AVR Function Attributes OS_task to main. It is activated per default if optimization is on.
• -mn-flash=num — assume that the flash memory has a size of num times 64 KiB.
• -mno-interrupts — generated code is not compatible with hardware interrupts. Code size is smaller. (yes that's the tradeoff, no we're not sorry)
• -mrelax — try to replace CALL resp. JMP instruction by the shorter RCALL resp. RJMP instruction if applicable. Setting -mrelax just adds the --mlink-relax option to the assembler's command line and the --relax option to the linker's command line.

Jump relaxing is performed by the linker because jump offsets are not known before code is located. Therefore, the assembler code generated by the compiler is the same, but the instructions in the executable may differ from instructions in the assembler code. 🤯

Relaxing must be turned on if linker stubs are needed, see the section on EIND and linker stubs below. 👀

• -mrmw — assume that the device supports the Read-Modify-Write instructions XCH, LAC, LAS and LAT.
• -mshort-calls — assume that RJMP and RCALL can target the whole program memory.

This option is used internally for multilib selection. It is not an optimization option, and you don't need to set it by hand.

• -msp8 — treat the stack pointer register as an 8-bit register, i.e. assume the high byte of the stack pointer is zero. In general, you don't need to set this option by hand.

This option is used internally by the compiler to select and build multilibs for architectures avr2 and avr25. These architectures mix devices with and without SPH. For any setting other than -mmcu=avr2 or -mmcu=avr25 the compiler driver adds or removes this option from the compiler proper's command line, because the compiler then knows if the device or architecture has an 8-bit stack pointer and thus no SPH register or not.

• -mstrict-X — use address register X in a way proposed by the hardware. This means that X is only used in indirect, post-increment or pre-decrement addressing.

Without this option, the X register may be used in the same way as Y or Z which then is emulated by additional instructions. For example, loading a value with X+const addressing with a small non-negative const < 64 to a register Rn is performed as

        adiw r26, const   ; X += const
        ld   <Rn>, X        ; <Rn> = *X
        sbiw r26, const   ; X -= const

• -mtiny-stack — only change the lower 8 bits of the stack pointer.
• -mfract-convert-truncate — allow to use truncation instead of rounding towards zero for fractional fixed-point types.
• -nodevicelib — don't link against AVR-LibC's device specific library lib<mcu>.a.
• -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. This means that the user has to supply her private device specs file by means of -specs=path-to-specs-file. There is 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 __vector prefix. Enabled by default.

---

✨ 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 word address so that indirect jumps and calls can target any code address in the range of 64 Ki words.

In order to facilitate indirect jump on devices with more than 128 Ki bytes of program memory space, there is a special function register called EIND that serves as 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. Thus, the indirect call/jump jumps to such a stub. The stub contains a direct jump to the desired address.
• Linker relaxation must be turned on so that the linker generates the stubs correctly in all situations. See the compiler option -mrelax and the linker option --relax. There are corner cases where the linker is supposed to generate stubs but aborts without relaxation and without a helpful error message. 💀
• The default linker script is arranged for code with EIND = 0. If code is supposed to work for a setup with EIND != 0, a custom linker script has to be used in order to place the sections whose name start with .trampolines into the segment where EIND points to.
• The startup code from libgcc never sets EIND. Notice that startup code is a blend of code from libgcc and AVR-LibC. For the impact of AVR-LibC on EIND, see the AVR-LibC user manual (http://nongnu.org/avr-libc/user-manual/).
• It is legitimate for user-specific startup code to set up EIND early, for example by means of initialization code located in section .init3. Such code runs prior to general startup code that initializes RAM and calls constructors, but after the bit of startup code from AVR-LibC that sets EIND to the segment where the vector table is located.

        #include <avr/io.h>

        static void
        __attribute__((section(".init3"),naked,used,no_instrument_function))
        init3_set_eind (void)
        {
          __asm volatile ("ldi r24,pm_hh8(__trampolines_start)\n\t"
                          "out %i0,r24" :: "n" (&EIND) : "r24","memory");
        }

The __trampolines_start symbol is defined in the linker script.

• Stubs are generated automatically by the linker if the following two conditions are met:
  • The address of a label is taken by means of the gs modifier (short for generate stubs) like so:

        LDI r24, lo8(gs(<func>))
        LDI r25, hi8(gs(<func>))

• The final location of that label is in a code segment outside the segment where the stubs are located.

• The compiler emits such gs modifiers for code labels in the following situations:
  • Taking address of a function or code label.
  • Computed goto.
  • If prologue-save function is used, see -mcall-prologues command-line option.
  • Switch/case dispatch tables. If you do not want such dispatch tables you can specify the -fno-jump-tables command-line option.
  • C and C++ constructors/destructors called during startup/shutdown.
  • If the tools hit a gs() modifier explained above.
• Jumping to non-symbolic addresses like so is not supported:

        int main (void)
        {
            /* Call function at word address 0x2 */
            return ((int(*)(void)) 0x2)();
        }

Instead, a stub has to be set up, i.e. the function has to be called through a symbol (func_4 in the example):

        int main (void)
        {
            extern int func_4 (void);

            /* Call function at byte address 0x4 */
            return func_4();
        }

and the application be linked with -Wl,--defsym,func_4=0x4. Alternatively, func_4 can be defined in the linker script. 💅

Handling of the RAMPD, RAMPX, RAMPY and RAMPZ Special Function Registers

Some AVR devices have memories bigger than the 64 KiB range that 16-bit pointers can even reach, bestie. So to poke at memory outside that 64 KiB zone, 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 gets used together with direct addressing. It's giving ✨address glow-up✨.

• The startup code initializes the RAMP special function registers with zero. 🌱
• If a AVR Named Address Spaces, named address space other than generic or __flash is used, then RAMPZ is set as needed before the operation. 🔧
• If the device supports RAM larger than 64 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. If you use inline assembler to read from locations outside the 16-bit address range and change one of the RAMP registers, you must reset it to zero after the access. No exceptions, bestie, that's the rule 💅

---

✨ AVR Built-in Macros ✨

GCC defines several built-in macros so your code can vibe-check for the presence or absence of features. Almost any of the following built-in macros are deduced from device capabilities and thus triggered by the -mmcu= command-line option. 🔍

For even more AVR-specific built-in macros see AVR Named Address Spaces and AVR Built-in Functions.

• __AVR_ARCH__ — a build-in macro that resolves to a decimal number identifying the architecture, and it 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_XMEGA__ — the device / architecture belongs to the XMEGA family of devices. 🌈
• __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_MOVW__ — the device has the MOVW instruction to perform 16-bit register-register moves.
• __AVR_HAVE_LPMX__ — the device has the LPM Rn,Z and LPM Rn,Z+ instructions.
• __AVR_HAVE_MUL__ — the device has a hardware multiplier. ✖️
• __AVR_HAVE_JMP_CALL__ — the device has the JMP and CALL instructions. This is the case for devices with 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 an 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 is 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 is no need to use address spaces like __flash or features like attribute progmem and pgm_read_*. 🎉
• __WITH_AVRLIBC__ — the compiler is configured to be used together with AVR-Libc. See the --with-avrlibc configure option.
• __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).

---

🖤 Blackfin Options 🖤

• -mcpu=cpu[-sirevision] — specifies the name of the target Blackfin processor. Currently, cpu can be one of bf512, bf514, bf516, bf518, bf522, bf523, bf524, bf525, bf526, bf527, bf531, bf532, bf533, bf534, bf536, bf537, bf538, bf539, bf542, bf544, bf547, bf548, bf549, bf542m, bf544m, bf547m, bf548m, bf549m, bf561, bf592.

The optional sirevision specifies the silicon revision of the target Blackfin processor. Any workarounds available for the targeted silicon revision are enabled. If sirevision is none, no workarounds are enabled. If sirevision is any, all workarounds for the targeted processor are enabled. The __SILICON_REVISION__ macro is defined to two hexadecimal digits representing the major and minor numbers in the silicon revision. If sirevision is none, the __SILICON_REVISION__ is not defined. If sirevision is any, the __SILICON_REVISION__ is defined to be 0xffff. If this optional sirevision is not used, GCC assumes the latest known silicon revision of the targeted Blackfin processor. 🔬

GCC defines a preprocessor macro for the specified cpu. For the bfin-elf toolchain, this option causes the hardware BSP provided by libgloss to be linked in if -msim is not given.

Without this option, bf532 is used as the processor by default.

Note that support for bf561 is incomplete. For bf561, only the preprocessor macro is defined. 💀

• -msim — specifies that the program will be run on the simulator. This causes the simulator BSP provided by libgloss to be linked in. This option has effect only for the bfin-elf toolchain. Certain other options, such as -mid-shared-library and -mfdpic, imply -msim. 🖥️
• -momit-leaf-frame-pointer — don't keep the frame pointer in a register for leaf functions. This avoids the instructions to save, set up and restore frame pointers and makes an extra register available in leaf functions. 🍃
• -mspecld-anomaly — when enabled, the compiler ensures that the generated code does not contain speculative loads after jump instructions. If this option is used, __WORKAROUND_SPECULATIVE_LOADS is defined.
• -mno-specld-anomaly — don't generate extra code to prevent speculative loads from occurring.
• -mcsync-anomaly — when enabled, the compiler ensures that the generated code does not contain CSYNC or SSYNC instructions too soon after conditional branches. If this option is used, __WORKAROUND_SPECULATIVE_SYNCS is defined.
• -mno-csync-anomaly — don't generate extra code to prevent CSYNC or SSYNC instructions from occurring too soon after a conditional branch.
• -mlow64k — when enabled, the compiler is free to take advantage of the knowledge that the entire program fits into the low 64k of memory. 🗂️
• -mno-low64k — bestie, this one just says "assume the program is arbitrarily large" 😌 that's literally it. And yeah, it's the default. No cap.
• -mstack-check-l1 — do your stack checking using info that the uClinux kernel dropped into L1 scratchpad memory 🧠✨
• -mid-shared-library — generate code that supports shared libraries through the library ID method 🦋 this is what lets you do execute in place and shared libraries in an environment with no virtual memory management. Heads up tho: this option implies -fPIC 💅 and if you're targeting bfin-elf, it also implies -msim.
• -mno-id-shared-library — generate code that does not assume ID-based shared libraries are being used. This is the default 🎀
• -mleaf-id-shared-library — same library-ID-method shared library support as above, BUT it assumes this library or executable won't link against any other ID shared libraries 👀 which means the compiler gets to use faster code for jumps and calls. slay.
• -mno-leaf-id-shared-library — do not assume the code you're compiling won't link against ID shared libraries. Result: slower code for jump and call insns 😭 it's giving cautious.
• -mshared-library-id=n — sets the identification number of the ID-based shared library you're compiling 🏷️ setting the value to 0 generates more compact code; setting any other value forces that number onto the current library but is no more space- or time-efficient than just omitting this option entirely. so like… do with that what you will 💀
• -msep-data — generate code that lets the data segment live in a different area of memory than the text segment 🌈 this enables execute in place in an environment without virtual memory management, by eliminating relocations against the text section. iconic.
• -mno-sep-data — generate code that assumes the data segment follows the text segment. This is the default 🎀
• -mlong-calls, -mno-long-calls — tells the compiler to do function calls by first loading the address of the function into a register, and then doing a subroutine call on that register 🫶 you need this switch when the target function sits outside the 24-bit addressing range of the offset-based version of the subroutine call instruction. It's not enabled by default, and specifying -mno-long-calls restores the default behavior. ⚠️ Important: these switches have no effect on how the compiler generates code to handle function calls via function pointers. periodt.
• -mfast-fp — link with the fast floating-point library 🍓 this library relaxes some of the IEEE floating-point standard's rules for checking inputs against Not-a-Number (NAN), all in the interest of performance. speed over strictness, iykyk.
• -minline-plt — enable inlining of PLT entries in function calls to functions that aren't known to bind locally ✨ note: it has no effect without -mfdpic.
• -mmulticore — build a standalone application for multicore Blackfin processors 🧚 this option makes proper start files and link scripts that support multicore get used, and it defines the macro "__BFIN_MULTICORE". It can only be used with -mcpu=bf561[-sirevision].

This option can be used together 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. simple as that.

• -mcorea — build a standalone application for Core A of BF561 when you're using the one-application-per-core programming model 🌸 proper start files and link scripts get used to support Core A, and the macro "__BFIN_COREA" is defined. This option can only be used in conjunction with -mmulticore.
• -mcoreb — build a standalone application for Core B of BF561 when using the one-application-per-core programming model 💜 proper start files and link scripts support Core B, and the macro "__BFIN_COREB" is defined. When this option is used, "coreb_main" should be used instead of "main". Again: only with -mmulticore.
• -msdram — build a standalone application for SDRAM ⭐ proper start files and link scripts put the application into SDRAM, and the macro "__BFIN_SDRAM" is defined. Note bestie: the loader should initialize SDRAM before loading the application.
• -micplb — assume that ICPLBs are enabled at run time 👑 this has an effect on certain anomaly workarounds. For Linux targets, the default is to assume ICPLBs are enabled; for standalone applications, the default is off.

---

✨ C6X Options ✨

• -march=name — 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 one is the default.
• -msim — choose startup files and linker script suitable for the simulator 🎮
• -msdata=default — put small global and static data in the ".neardata" section, which is pointed to by register "B14" 🫶 put small uninitialized global and static data in the ".bss" section, which is adjacent to the ".neardata" section. Put small read-only data into the ".rodata" section. The corresponding sections used for large pieces of data are ".fardata", ".far" and ".const".
• -msdata=all — put all data, not just small objects, into the sections reserved for small data, and use addressing relative to the "B14" register to access them 💖
• -msdata=none — make no use of the sections reserved for small data, and use absolute addresses to access all data 🦋 put all initialized global and static data in the ".fardata" section, and all uninitialized data in the ".far" section. Put all constant data into the ".const" section.

---

✨ CRIS Options ✨

These options are defined specifically for the CRIS ports 🌸

• -march=architecture-type, -mcpu=architecture-type — generate code for the specified architecture 🧚 the choices for architecture-type are v3, v8 and v10 for respectively ETRAX 4, ETRAX 100, and ETRAX 100 LX. Default is v0.
• -mtune=architecture-type — tune to architecture-type everything applicable about the generated code, except for the ABI and the set of available instructions ✨ the choices for architecture-type are the same as for -march=architecture-type.
• -mmax-stack-frame=n — warn when the stack frame of a function exceeds n bytes 🚨
• -metrax4, -metrax100 — these are synonyms for -march=v3 and -march=v8 respectively 💅
• -mmul-bug-workaround, -mno-mul-bug-workaround — work around a bug in the "muls" and "mulu" instructions for CPU models where it applies 🐛 this option is active by default.
• -mpdebug — enable CRIS-specific verbose debug-related information in the assembly code 🔍 this option also has the effect of turning off the #NO_APP formatted-code indicator to the assembler at the beginning of the assembly file.
• -mcc-init — do not use condition-code results from previous instructions; always emit compare and test instructions before use of condition codes 🎀
• -mno-side-effects — do not emit instructions with side effects in addressing modes other than post-increment 🙈
• -mstack-align, -mno-stack-align, -mdata-align, -mno-data-align, -mconst-align, -mno-const-align — these options (the no- versions) arrange (eliminate arrangements) for the stack frame, individual data and constants to be aligned for the maximum single data access size for the chosen CPU model 👑 the default is to arrange for 32-bit alignment. Note bestie: ABI details such as structure layout are not affected by these options.
• -m32-bit, -m16-bit, -m8-bit — similar to the stack-, data- and const-align options above 💖 these arrange for stack frame, writable data and constants to all be 32-bit, 16-bit or 8-bit aligned. The default is 32-bit alignment.
• -mno-prologue-epilogue, -mprologue-epilogue — with -mno-prologue-epilogue, the normal function prologue and epilogue that set up the stack frame are omitted, and no return instructions or return sequences are generated in the code 😭

⚠️ Use this option only together with visual inspection of the compiled code: no warnings or errors are generated when call-saved registers must be saved, or when storage for local variables needs to be allocated. fr fr, be careful.

• -melf — legacy no-op option 💀 it does nothing.
• -sim — this option arranges to link with input-output functions from a simulator library 🌈 code, initialized data and zero-initialized data are allocated consecutively.
• -sim2 — like -sim, but it passes linker options to locate initialized data at 0x40000000 and zero-initialized data at 0x80000000 🎯

---

✨ CR16 Options ✨

These options are defined specifically for the CR16 ports 🦋

• -mmac — enable the use of multiply-accumulate instructions ✖️➕ disabled by default.
• -mcr16cplus, -mcr16c — generate code for CR16C or CR16C+ architecture 💫 CR16C+ architecture is the default.
• -msim — links the library libsim.a, which is compatible with the simulator 🎮 applicable to ELF compiler only.
• -mint32 — choose integer type as 32-bit wide 🍓
• -mbit-ops — generates "sbit"/"cbit" instructions for bit manipulations ✨
• -mdata-model=model — choose a data model 📦 the choices for model are near, far or medium. medium is the default. However ⚠️ far is not valid with -mcr16c, since the CR16C architecture does not support the far data model.

---

✨ C-SKY Options ✨

GCC supports these options when compiling for C-SKY V2 processors 💜

• -march=arch — specify the C-SKY target architecture 👑 valid values for arch are: ck801, ck802, ck803, ck807, and ck810. The default is ck810.
• -mcpu=cpu — picks your C-SKY target processor, bestie. Here's the whole guest list of valid cpu values, no skips:

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.

• -mbig-endian, -EB, -mlittle-endian, -EL — choose big- or little-endian code. The default? little-endian, obviously. 🦋
• -mfloat-abi=name — says which floating-point ABI we're running with. Permissible values: soft, softfp, and hard.
  • soft makes GCC spit out output that contains library calls for floating-point operations.
  • softfp lets it generate code with hardware floating-point instructions but still uses the soft-float calling conventions.
  • hard generates floating-point instructions and uses FPU-specific calling conventions.

The default depends on the specific target configuration, so don't come for me. Also note: the hard-float and soft-float ABIs are not link-compatible — you have to compile your whole program with the same ABI and link with a compatible set of libraries. No mixing, periodt.

• -mhard-float, -msoft-float — pick hardware or software floating-point implementations. Default is soft float.
• -mdouble-float, -mno-double-float — while -mhard-float is in effect, turn on generation of double-precision float instructions. This is the default except when compiling for CK803. 🙈
• -mfdivdu, -mno-fdivdu — while -mhard-float is in effect, turn on generation of frecipd, fsqrtd, and fdivd instructions. Default except when compiling for CK803.
• -mfpu=fpu — select the floating-point processor. This one can only be used with -mhard-float. 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, so don't forget it.
• -mmp — enable multiprocessor instructions; default is off.
• -mcp — enable coprocessor instructions; default is off.
• -mcache — enable coprocessor instructions; default is off.
• -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 we can use 16-bit instructions. This option is ignored for CK801 (where that's the required behavior anyway) and it defaults to on for CK802. For other targets, the default is off.
• -mhigh-registers, -mno-high-registers — generate code using the high registers numbered 16-31. Not supported on CK801, CK802, or CK803, and it's enabled by default for the other processors.
• -manchor, -mno-anchor — generate code using global anchor symbol addresses.
• -mpushpop, -mno-pushpop — generate code using push and pop instructions. Defaults to on.
• -mmultiple-stld, -mstm, -mno-multiple-stld, -mno-stm — generate code using stm and ldm instructions. Not supported on CK801, but it's enabled by default on the other processors.
• -mconstpool, -mno-constpool — create constant pools in the compiler instead of deferring it to the assembler. This is the default and it's required for correct code generation on CK801 and CK802, and it's optional on the other processors.
• -mstack-size, -mno-stack-size — emit .stack_size directives for each function in the assembly output. Defaults to off.
• -mccrt, -mno-ccrt — generate code for the C-SKY compiler runtime instead of libgcc. Defaults to off.
• -mbranch-cost=n — set the branch costs to roughly n instructions. Default is 1.
• -msched-prolog, -mno-sched-prolog — permit scheduling of function prologue and epilogue sequences.

Bestie, be careful: using this option can result in code that is not compliant with the C-SKY V2 ABI prologue requirements, and that code cannot be debugged or backtraced. It's disabled by default for a reason. 💀

• -msim — links the library libsemi.a, which is compatible with the simulator. Applicable to ELF compiler only.

---

🍎 Darwin Options 🍎

These options are defined for all architectures running the Darwin operating system. Buckle up, this is a whole vibe. 🌈

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 this 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 they get an ISA mismatch, which is honestly so on brand:

• The assembler, as, only permits instructions that are valid for the subtype of the file it's generating — so you can't put 64-bit instructions in a ppc750 object file.
• The linker for shared libraries, /usr/bin/libtool, fails and prints an error if it's 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 searched for header files. These directories are interleaved with the ones 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 live in one of those two directories, with Headers being searched first. A subframework is a framework directory that's inside 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 the 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 — this 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, like letting GDB 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 — marks the output file so the dynamic linker binds every single undefined reference the moment the file gets loaded or launched. no lazy vibes, everything's locked in on arrival ✨
• -bundle — produces a Mach-o bundle format file. peep man ld(1) for the full tea 🫖
• -bundle_loader executable — tells the linker which executable is gonna load the build output file being linked. again, man ld(1) has the deets.
• -dynamiclib — pass this and GCC makes a dynamic library instead of an executable when linking, using the Darwin libtool command. it's giving library energy 💅
• -force_cpusubtype_ALL — makes GCC's output file have the ALL subtype instead of whatever subtype -mcpu or -march was controlling. not it being indecisive, it's just forcing the vibe.

---

okay bestie, here comes a big block. these next ones all get handed straight off to the Darwin linker, and the Darwin linker man page describes each one in detail. so like, I'm not gonna fake deep-dive them, I'm just gonna list them exactly how they come 👑

• -allowable_client client_name
• -client_name
• -compatibility_version
• -current_version
• -dead_strip
• -dependency-file
• -dylib_file
• -dylinker_install_name
• -dynamic
• -exported_symbols_list
• -filelist
• -flat_namespace
• -force_flat_namespace
• -headerpad_max_install_names
• -image_base
• -init
• -install_name
• -keep_private_externs
• -multi_module
• -multiply_defined
• -multiply_defined_unused
• -noall_load
• -no_dead_strip_inits_and_terms
• -nofixprebinding
• -nomultidefs
• -noprebind
• -noseglinkedit
• -pagezero_size
• -prebind
• -prebind_all_twolevel_modules
• -private_bundle
• -read_only_relocs
• -sectalign
• -sectobjectsymbols
• -whyload
• -seg1addr
• -sectcreate
• -sectobjectsymbols
• -sectorder
• -segaddr
• -segs_read_only_addr
• -segs_read_write_addr
• -seg_addr_table
• -seg_addr_table_filename
• -seglinkedit
• -segprot
• -segs_read_only_addr
• -segs_read_write_addr
• -single_module
• -static
• -sub_library
• -sub_umbrella
• -twolevel_namespace
• -umbrella
• -undefined
• -unexported_symbols_list
• -weak_reference_mismatches
• -whatsloaded

fr fr, all of the above get passed to the Darwin linker. the Darwin linker man page describes them in detail. that's the whole vibe. 🦋

---

🌸 DEC Alpha Options 🌸

these -m options are the ones defined for the DEC Alpha implementations. buckle up, we're going retro 💜

• -mno-soft-float, -msoft-float — use (or don't 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 those routines get replaced by routines that emulate the floating-point operations, or compiled in such a way as to call such emulation routines, those routines still issue floating-point operations. so if you're compiling for an Alpha without floating-point operations, you must make sure the library is built so it doesn't call them.

also, heads up: Alpha implementations without floating-point operations are required to have floating-point registers. yeah. that's just how it is 😭

• -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 isn't used, floating-point operands get passed in integer registers as if they were integers, and floating-point results get passed in $0 instead of $f0. this is a non-standard calling sequence, so any function with a floating-point argument or return value that's called by code compiled with -mno-fp-regs must also be compiled with that option. no mixing, bestie 🙈

a typical use of this option is building a kernel that doesn't use — and therefore doesn't need to save and restore — any floating-point registers. iconic minimalism 💅

• -mieee — the Alpha architecture implements floating-point hardware optimized for maximum performance, and it's mostly compliant with the IEEE floating-point standard. but for full compliance, software assistance is required. this option generates fully IEEE-compliant code, except that the inexact-flag is not maintained (see below). turn it on and the preprocessor macro _IEEE_FP gets defined during compilation. the resulting code is less efficient but can correctly support denormalized numbers and exceptional IEEE values like not-a-number and plus/minus infinity. other Alpha compilers call this option -ieee_with_no_inexact.
• -mieee-with-inexact — like -mieee, except the generated code also maintains the IEEE inexact-flag. turn this on and the generated code implements fully-compliant IEEE math. on top of _IEEE_FP, _IEEE_FP_EXACT also gets defined as a preprocessor macro. on some Alpha implementations the resulting code may execute significantly slower than the default code. and since there's very little code that actually depends on the inexact-flag, you should normally not specify this option. other Alpha compilers call this option -ieee_with_inexact.
• *-mfp-trap-mode=trap-mode* — this 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 — this is the default (normal) setting. the only traps enabled are the ones that can't be disabled in software (e.g., division by zero trap).
  • u — on top of the traps enabled by n, underflow traps are enabled as well.
  • su — like u, but the instructions are marked to be safe for software completion (peep the Alpha architecture manual for details).
  • sui — like su, but inexact traps are enabled as well.
• *-mfp-rounding-mode=rounding-mode* — selects the IEEE rounding mode. other Alpha compilers call this option -fprm rounding-mode. the rounding-mode can be one of:
  • n — normal IEEE rounding mode. floating-point numbers 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 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. meaning: without software assistance it's impossible to recover from a floating trap, and program execution normally needs to be terminated 💀 GCC can generate code that assists operating system trap handlers in figuring out the exact location that caused a floating-point trap. depending on what an application needs, you can pick different levels of precision:
  • p — program precision. this is the default, and it means a trap handler can only identify which program caused a floating-point exception.
  • f — function precision. the trap handler can determine the function that caused a floating-point exception.
  • i — instruction precision. the trap handler can determine the exact instruction that caused a floating-point exception.

other Alpha compilers provide the equivalent options called -scope_safe and -resumption_safe.

• -mieee-conformant — marks the generated code as IEEE conformant. ⚠️ you must not use this option unless you also specify -mtrap-precision=i and either -mfp-trap-mode=su or -mfp-trap-mode=sui. its only effect is to emit the line .eflag 48 in the function prologue of the generated assembly file. that's it, that's the whole glow-up ✨
• -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 to build a shared library dynamic loader — itself a shared library, it has to relocate itself in memory before it can find the variables and constants in its own data segment. main character energy, honestly 👑
• -mbwx, -mno-bwx, -mcix, -mno-cix, -mfix, -mno-fix, -mmax, -mno-max — these say 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. iykyk 💫
• -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 had 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. with -msmall-data, 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 lets the variables be directly accessed via a single instruction. slay, efficiency 🎀

the default is -mlarge-data. with that 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 — bestie, with -msmall-text the compiler straight up assumes the code of the entire program (or shared library) fits in 4MB, so a branch instruction can reach it. 🌸 And when -msmall-data is used, the compiler gets to assume that all local symbols share the same $gp value, which shrinks the number of instructions required for a function call from 4 to 1. obsessed.

The default is -mlarge-text. 💖

• -mcpu=cpu_type — sets the instruction set and the instruction scheduling parameters for machine type cpu_type. You can specify either the EV style name or the corresponding chip number, whichever vibe you're on. 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. And if you don't specify a processor type at all? 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. Heads up though: -mcpu=native has no effect if GCC does not recognize the processor. 😭

• -mtune=cpu_type — this one sets only the instruction scheduling parameters for machine type cpu_type. The instruction set does not change. periodt. ✨

Native toolchains also support the value native, which selects the best architecture option for the host processor. Same deal as above: -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. And ngl, this number is highly dependent on the memory access patterns used by the application and the size of the external cache on the machine. so real. 🧚

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. 🌈
• -mkernel=version — this specifies the minimum version of the kernel that will run the compiled program. GCC uses this version to determine which instructions to use, what kernel helpers to allow, etc. Currently, version can be one of 4.0, 4.1, 4.2, 4.3, 4.4, 4.5, 4.6, 4.7, 4.8, 4.9, 4.10, 4.11, 4.12, 4.13, 4.14, 4.15, 4.16, 4.17, 4.18, 4.19, 4.20, 5.0, 5.1, 5.2, latest and native. 💖
• -mbig-endian — generate code for a big-endian target.
• -mlittle-endian — generate code for a little-endian target. This is the default. 🎀
• -mxbpf — generate code for an expanded version of BPF, which relaxes some of the restrictions imposed by the BPF architecture:
  • Save and restore callee-saved registers at function entry and exit, respectively.

---

🦋 FR30 Options 🦋

These options are defined specifically for the FR30 port. iykyk.

• -msmall-model — use the small address space model. This can produce smaller code, but it does assume that all symbolic values and addresses fit into a 20-bit range.
• -mno-lsim — assume that runtime support has been provided and so there is no need to include the simulator library (libsim.a) on the linker command line.

---

🍓 FT32 Options 🍓

These options are defined specifically for the FT32 port.

• -msim — specifies that the program will be run on the simulator. This causes an alternate runtime startup and library to be linked. > You must not use this option when generating programs that will run on real hardware; you must provide your own runtime library for whatever I/O functions are needed.
• -mlra — enable Local Register Allocation. This is still experimental for FT32, so by default the compiler uses standard reload.
• -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. With a bfin-elf target, this option implies -msim. 💫
• -minline-plt — enable inlining of PLT entries in function calls to functions that are not known to bind locally. It has no effect without -mfdpic. It's enabled by default if optimizing for speed and compiling for shared libraries (i.e., -fPIC or -fpic), or when an optimization option such as -O3 or above is present in the command line.
• -mTLS — assume a large TLS segment when generating thread-local code.
• -mtls — do not assume a large TLS segment when generating thread-local code.
• -mgprel-ro — enable the use of "GPREL" relocations in the FDPIC ABI for data that is known to be in read-only sections. It's enabled by default, except for -fpic or -fpie: even though it may help make the global offset table smaller, it trades 1 instruction for 4. With -fPIC or -fPIE, it trades 3 instructions for 4, one of which may be shared by multiple symbols, and it avoids the need for a GOT entry for the referenced symbol, so it's more likely to be a win. If it is not, -mno-gprel-ro can be used to disable it. 🧚
• -multilib-library-pic — link with the (library, not FD) pic libraries. It's implied by -mlibrary-pic, as well as by -fPIC and -fpic without -mfdpic. You should never have to use it explicitly.
• -mlinked-fp — follow the EABI requirement of always creating a frame pointer whenever a stack frame is allocated. This option is enabled by default and can be disabled with -mno-linked-fp.
• -mlong-calls — use indirect addressing to call functions outside the current compilation unit. This allows the functions to be placed anywhere within the 32-bit address space.
• -malign-labels — try to align labels to an 8-byte boundary by inserting NOPs into the previous packet. This option only has an effect when VLIW packing is enabled. It doesn't create new packets; it merely adds NOPs to existing ones. 💅
• -mlibrary-pic — generate position-independent EABI code.
• -macc-4 — use only the first four media accumulator registers.
• -macc-8 — use all eight media accumulator registers. ✨
• -mpack — pack VLIW instructions.
• -mno-pack — do not pack VLIW instructions.
• -mno-eflags — do not mark ABI switches in e_flags.
• -mcond-move — enable the use of conditional-move instructions (default). This switch is mainly for debugging the compiler and will likely be removed in a future version.
• -mno-cond-move — disable the use of conditional-move instructions. This switch is mainly for debugging the compiler and will likely be removed in a future version.
• -mscc — enable the use of conditional set instructions (default). This switch is mainly for debugging the compiler and will likely be removed in a future version.
• -mno-scc — disable the use of conditional set instructions. This switch is mainly for debugging the compiler and will likely be removed in a future version.
• -mcond-exec — enable the use of conditional execution (default). This switch is mainly for debugging the compiler and will likely be removed in a future version.
• -mno-cond-exec — disable the use of conditional execution. This switch is mainly for debugging the compiler and will likely be removed in a future version.
• -mvliw-branch — run a pass to pack branches into VLIW instructions (default). This switch is mainly for debugging the compiler and will likely be removed in a future version.
• -mno-vliw-branch — do not run a pass to pack branches into VLIW instructions. This switch is mainly for debugging the compiler and will likely be removed in a future version.
• -mmulti-cond-exec — enable optimization of "&&" and "||" in conditional execution (default). This switch is mainly for debugging the compiler and will likely be removed in a future version.
• -mno-multi-cond-exec — turns OFF the optimization of && and || inside conditional execution.

bestie this switch is basically just for debugging the compiler itself, and it's probably getting yeeted in a future version 💀

• -mnested-cond-exec — turns ON nested conditional execution optimizations (this is the default, periodt).

also just for debugging the compiler, also probably getting removed later, don't get attached 🙈

• -mno-nested-cond-exec — turns OFF nested conditional execution optimizations.

same deal, debugging-only energy, likely gone in a future version, no cap

• -moptimize-membar — this one strips redundant membar instructions out of the compiler-generated code. It's enabled by default, slay ✨
• -mno-optimize-membar — disables that automatic removal of redundant membar instructions from the generated code. bye optimization 💅
• -mtomcat-stats — makes gas print out tomcat statistics. cute.
• *-mcpu=cpu* — picks the processor type you're generating code for. The possible values are frv, fr550, tomcat, fr500, fr450, fr405, fr400, fr300 and simple. pick your fighter 💖

---

🌈 GNU/Linux Options 🌈

these -m options are defined for GNU/Linux targets, iykyk:

• -mglibc — use the GNU C library. This is the default except on *-*-linux-*uclibc*, *-*-linux-*musl* and *-*-linux-*android* targets.
• -muclibc — use uClibc C library. This is the default on *-*-linux-*uclibc* targets.
• -mmusl — use the musl C library. This is the default on *-*-linux-*musl* targets.
• -mbionic — use Bionic C library. This is the default on *-*-linux-*android* targets.
• -mandroid — compile code compatible with the Android platform. This is the default on *-*-linux-*android* targets.

when compiling, this option enables -mbionic, -fPIC, -fno-exceptions and -fno-rtti by default. When linking, this option makes the GCC driver pass Android-specific options to the linker. And finally, this option causes the preprocessor macro __ANDROID__ to be defined. three-for-one deal fr fr 🫶

• -tno-android-cc — disable the compilation effects of -mandroid, i.e., do not enable -mbionic, -fPIC, -fno-exceptions and -fno-rtti by default.
• -tno-android-ld — disable the linking effects of -mandroid, i.e., pass standard Linux linking options to the linker.

---

🌸 H8/300 Options 🌸

these -m options are defined for the H8/300 implementations:

• -mrelax — shorten some address references at link time, when possible; uses the linker option -relax.
• -mh — generate code for the H8/300H.
• -ms — generate code for the H8S.
• -mn — generate code for the H8S and H8/300H in the normal mode. This switch must be used either with -mh or -ms. no solo runs bestie.
• -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 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 monitor attribute. The default option is -mno-exr. This option is valid only for H8S targets.
• -mint32 — make int data 32 bits by default.
• -malign-300 — on the H8/300H and H8S, use the same alignment rules as for the H8/300. The default for the H8/300H and H8S is to align longs and floats on 4-byte boundaries. -malign-300 causes them to be aligned on 2-byte boundaries. This option has no effect on the H8/300. noted 📝

---

👑 HPPA Options 👑

these -m options are defined for the HPPA family of computers:

• -march=architecture-type — generate code for the specified architecture. The choices for architecture-type are 1.0 for PA 1.0, 1.1 for PA 1.1, and 2.0 for PA 2.0 processors. Refer to /usr/lib/sched.models on an HP-UX system to determine the proper architecture option for your machine. Code compiled for lower numbered architectures runs on higher numbered architectures, but **not the other way around. one-way street energy 🦋
• -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 💜

• -mcaller-copies — the caller copies function arguments passed by hidden reference. This option should be used with care as 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. read the fine print bestie 😭
• -mjump-in-delay — this option is ignored and provided for compatibility purposes only. it's giving decorative.
• -mdisable-fpregs — prevent floating-point registers from being used in any manner. This is necessary for compiling kernels that perform lazy context switching of floating-point registers. If you use this option and then attempt to perform floating-point operations, the compiler aborts. so like, don't do that 💀
• -mdisable-indexing — prevent the compiler from using indexing address modes. This avoids some rather obscure problems when compiling MIG generated code under MACH.
• -mno-space-regs — generate code that assumes the target has no space registers. This allows GCC to generate faster indirect calls and use unscaled index address modes.

such code is suitable for level 0 PA systems and kernels. niche but iconic ✨

• -mfast-indirect-calls — generate code that assumes calls never cross space boundaries. This allows GCC to emit code that performs faster indirect calls.

heads up: this option does not work in the presence of shared libraries or nested functions. no cap, it just breaks.

• -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. stack 'em up 🎀
• -mlong-load-store — generate 3-instruction load and store sequences as sometimes required by the HP-UX 10 linker. This is equivalent to the +k option to the HP compilers.
• -mportable-runtime — use the portable calling conventions proposed by HP for ELF systems.
• -mgas — enable the use of assembler directives only GAS understands.
• *-mschedule=cpu-type* — schedule code according to the constraints for the machine type cpu-type. The choices for cpu-type are 700, 7100, 7100LC, 7200, 7300 and 8000. Refer to /usr/lib/sched.models on an HP-UX system to determine the proper scheduling option for your machine. The default scheduling is 8000. 🗓️
• -mlinker-opt — enable the optimization pass in the HP-UX linker. Note this makes symbolic debugging impossible. It also triggers a bug in the HP-UX 8 and HP-UX 9 linkers in which they give bogus error messages when linking some programs. yikes, proceed with caution ⚠️
• -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. all-or-nothing situation fr fr 💅

• -msio — generate the predefine, _SIO, for server IO. The default is -mwsio. This generates the predefines, __hp9000s700, __hp9000s700__ and _WSIO, for workstation IO. These options are available under HP-UX and HI-UX.
• -mgnu-ld — use options specific to GNU ld. This passes -shared to ld when building a shared library. It is the default when GCC is configured, explicitly or implicitly, with the GNU linker. This option does not affect which ld is called; it only changes what parameters are passed to that ld. The ld that is called is determined by the --with-ld configure option, GCC's program search path, and finally by the user's PATH. The linker used by GCC can be printed using which gcc -print-prog-name=ld. This option is only available on the 64-bit HP-UX GCC, i.e. configured with hppa*64*-*-hpux*. 🧚
• -mhp-ld — use options specific to HP ld. This passes -b to ld when building a shared library and passes +Accept TypeMismatch to ld on all links. It is the default when GCC is configured, explicitly or implicitly, with the HP linker. This option does not affect which ld is called; it only changes what parameters are passed to that ld. The ld that is called is determined by the --with-ld configure option, GCC's program search path, and finally by the user's PATH. The linker used by GCC can be printed using which gcc -print-prog-name=ld. This option is only available on the 64-bit HP-UX GCC, i.e. configured with hppa*64*-*-hpux*. twin energy with -mgnu-ld honestly 🌈
• -mlong-calls — generate code that uses long call sequences. This ensures that a call is always able to reach linker generated stubs. The default is to generate long calls only when the distance from the call site to the beginning of the function or translation unit, as the case may be, exceeds a predefined limit set by the branch type being used. The limits for normal calls are 7,600,000 and 240,000 bytes, respectively for the PA 2.0 and PA 1.X architectures. Sibcalls are always limited at 240,000 bytes.

distances are measured from the beginning of functions when using the -ffunction-sections option, or when using the -mgas and -mno-portable-runtime options together under HP-UX with the SOM linker.

it is normally not desirable to use this option as it degrades performance. However, it may be useful in large applications, particularly when partial linking is used to build the application.

the types of long calls used depends on the capabilities of the assembler and linker, and the type of code being generated. The impact on systems that support long absolute calls, and long pic symbol-difference or pc-relative calls should be relatively small. However, an indirect call is used on 32-bit ELF systems in pic code and it is quite long. 💀 long indeed.

• -munix= unix-std — bestie this one generates compiler predefines AND picks a startfile for whichever UNIX standard you name. The choices for unix-std are 93, 95 and 98. 93 is supported on all HP-UX versions. 95 is available on HP-UX 10.10 and later. 98 is available on HP-UX 11.11 and later. And the default values? 93 for HP-UX 10.00, 95 for HP-UX 10.10 through to 11.00, and 98 for HP-UX 11.11 and later. ✨
  • -munix=93 provides the same predefines as GCC 3.3 and 3.4.
  • -munix=95 provides additional predefines for XOPEN_UNIX and _XOPEN_SOURCE_EXTENDED, and the startfile unix95.o.
  • -munix=98 provides additional predefines for _XOPEN_UNIX, _XOPEN_SOURCE_EXTENDED, _INCLUDE__STDC_A1_SOURCE and _INCLUDE_XOPEN_SOURCE_500, and the startfile unix98.o.

okay so this is the part where I need you to actually pay attention 💀 It is important to note that this option changes the interfaces for various library routines. It also affects the operational behavior of the C library. Thus, extreme care is needed in using this option. No cap.

Library code that is intended to operate with more than one UNIX standard must test, set and restore the variable __xpg4_extended_mask as appropriate. Most GNU software doesn't provide this capability. 🙈

• -nolibdld — suppresses the generation of link options to search libdld.sl when the -static option is specified on HP-UX 10 and later.
• -static — okay so the HP-UX implementation of setlocale in libc has a dependency on libdld.sl. There isn't an archive version of libdld.sl. Thus, when the -static option is specified, special link options are needed to resolve this dependency. 🫶

On HP-UX 10 and later, the GCC driver adds the necessary options to link with libdld.sl when the -static option is specified. This causes the resulting binary to be dynamic. On the 64-bit port, the linkers generate dynamic binaries by default in any case. The -nolibdld option can be used to prevent the GCC driver from adding these link options.

• -threads — adds support for multithreading with the dce thread library under HP-UX. This option sets flags for both the preprocessor and linker.

---

👑 IA-64 Options 👑

These are the -m options defined for the Intel IA-64 architecture. it's giving very specific 💅

• -mbig-endian — generate code for a big-endian target. This is the default for HP-UX.
• -mlittle-endian — generate code for a little-endian target. This is the default for AIX5 and GNU/Linux.
• -mgnu-as, -mno-gnu-as — generate (or don't) code for the GNU assembler. This is the default.
• -mgnu-ld, -mno-gnu-ld — generate (or don't) code for the GNU linker. This is the default.
• -mno-pic — generate code that does not use a global pointer register. The result is not position independent code, and violates the IA-64 ABI. ⚠️
• -mvolatile-asm-stop, -mno-volatile-asm-stop — generate (or don't) a stop bit immediately before and after volatile asm statements.
• -mregister-names, -mno-register-names — generate (or don't) in, loc, and out register names for the stacked registers. This may make assembler output more readable. ✨
• -mno-sdata, -msdata — disable (or enable) optimizations that use the small data section. This may be useful for working around optimizer bugs.
• -mconstant-gp — generate code that uses a single constant global pointer value. This is useful when compiling kernel code.
• -mauto-pic — generate code that is self-relocatable. This implies -mconstant-gp. This is useful when compiling firmware code.
• -minline-float-divide-min-latency — generate code for inline divides of floating-point values using the minimum latency algorithm.
• -minline-float-divide-max-throughput — generate code for inline divides of floating-point values using the maximum throughput algorithm.
• -mno-inline-float-divide — do not generate inline code for divides of floating-point values.
• -minline-int-divide-min-latency — generate code for inline divides of integer values using the minimum latency algorithm.
• -minline-int-divide-max-throughput — generate code for inline divides of integer values using the maximum throughput algorithm.
• -mno-inline-int-divide — do not generate inline code for divides of integer values.
• -minline-sqrt-min-latency — generate code for inline square roots using the minimum latency algorithm.
• -minline-sqrt-max-throughput — generate code for inline square roots using the maximum throughput algorithm.
• -mno-inline-sqrt — do not generate inline code for sqrt.
• -mfused-madd, -mno-fused-madd — do (don't) generate code that uses the fused multiply/add or multiply/subtract instructions. The default is to use these instructions.
• -mno-dwarf2-asm, -mdwarf2-asm — don't (or do) generate assembler code for the DWARF line number debugging info. This may be useful when not using the GNU assembler.
• -mearly-stop-bits, -mno-early-stop-bits — allow stop bits to be placed earlier than immediately preceding the instruction that triggered the stop bit. This can improve instruction scheduling, but does not always do so.
• -mfixed-range= register-range — generate code treating the given register range as fixed registers. A fixed register is one that the register allocator cannot use. This is useful when compiling kernel code. A register range is specified as two registers separated by a dash. Multiple register ranges can be specified separated by a comma.
• -mtls-size= tls-size — specify bit size of immediate TLS offsets. Valid values are 14, 22, and 64.
• -mtune= cpu-type — tune the instruction scheduling for a particular CPU. Valid values are itanium, itanium1, merced, itanium2, and mckinley.
• -milp32, -mlp64 — generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long and pointer to 32 bits. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits. These are HP-UX specific flags.
• -mno-sched-br-data-spec, -msched-br-data-spec — (dis/en)able data speculative scheduling before reload. This results in generation of ld.a instructions and the corresponding check instructions (ld.c / chk.a). The default 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 the 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 the 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 instructions priorities. This makes the use of the 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 that non-memory operations are preferred when the limit is reached, but memory operations may still be scheduled.

---

🌈 LM32 Options 🌈

These -m options are defined for the LatticeMico32 architecture:

• -mbarrel-shift-enabled — enable barrel-shift instructions.
• -mdivide-enabled — enable divide and modulus instructions.
• -mmultiply-enabled — enable multiply instructions.
• -msign-extend-enabled — enable sign extend instructions.
• -muser-enabled — enable user-defined instructions.

---

🦋 M32C Options 🦋

• -mcpu=name — picks the CPU that code gets generated for. name can be one of r8c for the R8C/Tiny series, m16c for the M16C (up to /60) series, m32cm for the M16C/80 series, or m32c for the M32C/80 series. ✨
• -msim — says the program is gonna run on the simulator 🎮. This makes an alternate runtime library get linked in that supports stuff like file I/O. Bestie, do NOT use this when you're building programs that will run on real hardware — you gotta provide your own runtime library for whatever I/O functions you need. 💅
• -memregs=number — sets how many memory-based pseudo-registers GCC uses during code generation. These pseudo-registers get used like real registers, so there's a tradeoff 🔄 between GCC's ability to fit the 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 Renesas M32R/D architectures:

• -m32r2 — generate code for the M32R/2. 💖
• -m32rx — generate code for the M32R/X.
• -m32r — generate code for the M32R. This is the default.
• -mmodel=small — assume all objects live in the lower 16MB of memory (so their addresses can be loaded with the ld24 instruction), and assume all subroutines are reachable with the bl instruction. This is the default.

The addressability of a particular object can be set with the model attribute.

• -mmodel=medium — assume objects may be anywhere in the 32-bit address space (the compiler generates seth/add3 instructions to load their addresses), and assume all subroutines are reachable with the bl instruction.
• -mmodel=large — assume objects may be anywhere in the 32-bit address space (the compiler generates seth/add3 instructions to load their addresses), and assume subroutines may not be reachable with the bl instruction (the compiler generates the much slower seth/add3/jl instruction sequence). 🐌
• -msdata=none — disable use of the small data area. Variables 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 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 less than or equal to num bytes into the small data or BSS sections instead of the normal data or BSS sections. The default value of num is 8. The -msdata option must be set to one of sdata or use for this option to have any effect.

All modules should be compiled with the same -G num value. Compiling with different values of num may or may not work; if it doesn't, the linker gives an error message—incorrect code is not generated.

• -mdebug — makes the M32R-specific code in the compiler display some statistics that might help in debugging programs. 🔍
• -malign-loops — align all loops to a 32-byte boundary.
• -mno-align-loops — do not enforce a 32-byte alignment for loops. This is the default.
• -missue-rate=number — issue number instructions per cycle. number can only be 1 or 2.
• -mbranch-cost=number — number can only be 1 or 2. If it'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 is generating code for a ColdFire target. The arch in this macro is one of the -march arguments given above.

When used together, -march and -mtune select code that runs on a family of similar processors but that is optimized for a particular microarchitecture. ✨

• -mcpu=cpu — generate code for a specific M680x0 or ColdFire processor. The M680x0 cpus are: 68000, 68010, 68020, 68030, 68040, 68060, 68302, 68332 and cpu32. The ColdFire cpus are given by the table below, which also classifies the CPUs into families:

Family : -mcpu arguments
51 : 51 51ac 51ag 51cn 51em 51je 51jf 51jg 51jm 51mm 51qe 51qm
5206 : 5202 5204 5206
5206e : 5206e
5208 : 5207 5208
5211a : 5210a 5211a
5213 : 5211 5212 5213
5216 : 5214 5216
52235 : 52230 52231 52232 52233 52234 52235
5225 : 5224 5225
52259 : 52252 52254 52255 52256 52258 52259
5235 : 5232 5233 5234 5235 523x
5249 : 5249
5250 : 5250
5271 : 5270 5271
5272 : 5272
5275 : 5274 5275
5282 : 5280 5281 5282 528x
53017 : 53011 53012 53013 53014 53015 53016 53017
5307 : 5307
5329 : 5327 5328 5329 532x
5373 : 5372 5373 537x
5407 : 5407
5475 : 5470 5471 5472 5473 5474 5475 547x 5480 5481 5482 5483 5484 5485

-mcpu=cpu overrides -march=arch if arch is compatible with cpu. Other combinations of -mcpu and -march are rejected. 💀

GCC defines the macro __mcf_cpu_cpu when ColdFire target cpu is selected. It also defines __mcf_family_family, where the value of family is given by the table above.

• -mtune=tune — tune the code for a particular microarchitecture within the constraints set by -march and -mcpu. The M680x0 microarchitectures are: 68000, 68010, 68020, 68030, 68040, 68060 and cpu32. The ColdFire microarchitectures are: cfv1, cfv2, cfv3, cfv4 and cfv4e.

You can also use -mtune=68020-40 for code that needs to run relatively well on 68020, 68030 and 68040 targets. -mtune=68020-60 is similar but includes 68060 targets as well. These two options select the same tuning decisions as -m68020-40 and -m68020-60 respectively.

GCC defines the macros __mcarch and __mcarch__ when tuning for 680x0 architecture arch. It also defines mcarch unless either -ansi or a non-GNU -std option is used. If GCC is tuning for a range of architectures, as selected by -mtune=68020-40 or -mtune=68020-60, it defines the macros for every architecture in the range.

GCC also defines the macro __muarch__ when tuning for ColdFire microarchitecture uarch, where uarch is one of the arguments given above.

• -m68000
• -mc68000 — generate output for a 68000. This is the default when the compiler is configured for 68000-based systems. It is equivalent to -march=68000.

Use this option for microcontrollers with a 68000 or EC000 core, including the 68008, 68302, 68306, 68307, 68322, 68328 and 68356.

• -m68010 — generate output for a 68010. This is the default when the compiler is configured for 68010-based systems. It is equivalent to -march=68010.
• -m68020
• -mc68020 — generate output for a 68020. This is the default when the compiler is configured for 68020-based systems. It is equivalent to -march=68020.
• -m68030 — generate output for a 68030. This is the default when the compiler is configured for 68030-based systems. It is equivalent to -march=68030.
• -m68040 — generate output for a 68040. This is the default when the compiler is configured for 68040-based systems. It is equivalent to -march=68040.

This option inhibits the use of 68881/68882 instructions that have to be emulated by software on the 68040. Use this option if your 68040 does not have code to emulate those instructions. 🛑

• -m68060 — generate output for a 68060. This is the default when the compiler is configured for 68060-based systems. It is equivalent to -march=68060.

This option inhibits the use of 68020 and 68881/68882 instructions that have to be emulated by software on the 68060. Use this option if your 68060 does not have code to emulate those instructions.

• -mcpu32 — generate output for a CPU32. This is the default when the compiler is configured for CPU32-based systems. It is equivalent to -march=cpu32.

Use this option for microcontrollers with a CPU32 or CPU32+ core, including the 68330, 68331, 68332, 68333, 68334, 68336, 68340, 68341, 68349 and 68360.

• -m5200 — generate output for a 520X ColdFire CPU. This is the default when the compiler is configured for 520X-based systems. It is equivalent to -mcpu=5206, and is now deprecated in favor of that option.

Use this option for microcontroller with a 5200 core, including the MCF5202, MCF5203, MCF5204 and MCF5206.

• -m5206e — generate output for a 5206e ColdFire CPU. The option is now deprecated in favor of the equivalent -mcpu=5206e.
• -m528x — generate output for a member of the ColdFire 528X family. The option is now deprecated in favor of the equivalent -mcpu=528x.
• -m5307 — generate output for a ColdFire 5307 CPU. The option is now deprecated in favor of the equivalent -mcpu=5307. 🫶
• -m5407 — Generate output for a ColdFire 5407 CPU. The option is now deprecated in favor of the equivalent -mcpu=5407. ngl, deprecated era 💀
• -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. slay

• -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. macros are lowkey iconic 💖
• -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. no FPU? no problem, library calls carry 🫶
• -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. int but make it smol 🎀
• -mno-short — Do not consider type int to be 16 bits wide. This is the default. regular int energy ✨
• -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. so like, don't unless you know bestie 💅

• -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.

And here's a lil paragraph that's just floating around, no bullet, but it's important: 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. iykyk 🦋

• -mno-strict-align, -mstrict-align — Do not (do) assume that unaligned memory references are handled by the system. vibe check: strict or not strict? 💅
• -msep-data — Generate code that allows the data segment to be located in a different area of memory from the text segment. This allows for execute-in-place in an environment without virtual memory management. This option implies -fPIC.
• -mno-sep-data — Generate code that assumes that the data segment follows the text segment. This is the default.
• -mid-shared-library — Generate code that supports shared libraries via the library ID method. This allows for execute-in-place and shared libraries in an environment without virtual memory management. This option implies -fPIC.
• -mno-id-shared-library — Generate code that doesn't assume ID-based shared libraries are being used. This is the default.
• -mshared-library-id=n — Specifies the identification number of the ID-based shared library being compiled. Specifying a value of 0 generates more compact code; specifying other values forces the allocation of that number to the current library, but is no more space- or time-efficient than omitting this option. 0 is the main character here 💅
• -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. so like, context matters bestie 💅

• -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. ngl, M*Core is giving retro chic 🎀

• -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). periodt 💅
• -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. little-endian bestie? or big? 💅
• -m210, -m340 — Generate code for the 210 processor.
• -mno-lsim — Assume that runtime support has been provided and so omit the simulator library (libsim.a) from the linker command line.
• -mstack-increment=size — Set the maximum amount for a single stack increment operation. Large values can increase the speed of programs that contain functions that need a large amount of stack space, but they can also trigger a segmentation fault if the stack is extended too much. The default value is 0x1000. careful, bestie, segfault is not the vibe 😭

---

🦋 MeP Options 🦋

• -mabsdiff — Enables the abs instruction, which is the absolute difference between two registers.
• -mall-opts — Enables all the optional instructions---average, multiply, divide, bit operations, leading zero, absolute difference, min/max, clip, and saturation. all the things! ✨
• -maverage — Enables the ave instruction, which computes the average of two registers.
• -mbased=n — Variables of size n bytes or smaller are placed in the .based section by default. Based variables use the $tp register as a base register, and there is a 128-byte limit to the .based section. 128-byte limit? tiny but mighty 🎀
• -mbitops — Enables the bit operation instructions---bit test (btstm), set (bsetm), clear (bclrm), invert (bnotm), and test-and-set (tas).
• -mc=name — Selects which section constant data is placed in. name may be tiny, near, or far.
• -mclip — Enables the clip instruction. Note that -mclip is not useful unless you also provide -mminmax. teamwork makes the dream work 💅
• -mconfig=name — Selects one of the built-in core configurations. Each MeP chip has one or more modules in it; each module has a core CPU and a variety of coprocessors, optional instructions, and peripherals. The MeP-Integrator tool, not part of GCC, provides these configurations through this option; using this option is the same as using all the corresponding command-line options. The default configuration is default.
• -mcop — Enables the coprocessor instructions. By default, this is a 32-bit coprocessor. Note that the coprocessor is normally enabled via the -mconfig= option.
• -mcop32 — Enables the 32-bit coprocessor's instructions.
• -mcop64 — Enables the 64-bit coprocessor's instructions.
• -mivc2 — Enables IVC2 scheduling. IVC2 is a 64-bit VLIW coprocessor. 64-bit VLIW? fancy 💅
• -mdc — Causes constant variables to be placed in the .near section.
• -mdiv — Enables the div and divu instructions.
• -meb — Generate big-endian code. big-endian era 🦋
• -mel — makes little-endian code. tiny bytes, little vibes 💅
• -mio-volatile — tells the compiler that literally ANY variable wearing the "io" attribute is to be considered volatile. no exceptions, bestie 🫶
• -ml — causes variables to be assigned to the .far section by default. they're going the distance ✨
• -mleadz — enables the "leadz" (leading zero) instruction. she counts from the front 💖
• -mm — causes variables to be assigned to the .near section by default. cozy, close, right here 🎀
• -mminmax — enables the "min" and "max" instructions. pick your fighter 🔥
• -mmult — enables the multiplication and multiply-accumulate instructions. she does math 💅
• -mno-opts — disables ALL the optional instructions that -mall-opts turned on. the reverse-uno card 💀
• -mrepeat — enables the "repeat" and "erepeat" instructions, used for low-overhead looping. say it again for the people in the back 🎤
• -ms — causes all variables to default to the .tiny section. > heads up: there's a 65536-byte limit to this section. Accesses to these variables use the %gp base register. smol section, big rules 🦋
• -msatur — enables the saturation instructions. > note that the compiler does not currently generate these itself, but this option is included for compatibility with other tools, like "as". it's giving "we support it but we don't use it" 🙈
• -msdram — link the SDRAM-based runtime instead of the default ROM-based runtime. glow up your runtime 🧚
• -msim — link the simulator run-time libraries. fake it till you make it ✨
• -msimnovec — link the simulator runtime libraries, excluding built-in support for reset and exception vectors and tables. sim mode, but make it minimal 💜
• -mtf — causes all functions to default to the .far section. without this option, functions default to the .near section. functions going long-distance 🌈
• -mtiny=n — variables that are n bytes or smaller get allocated to the .tiny section. these variables use the $gp base register. the default for this option is 4, but bestie, note that there's a **65536-byte limit to the .tiny section. ⭐

---

🌸 MicroBlaze Options 🌸

• -msoft-float — use software emulation for floating point (this is the default). pretending fr fr 💅
• -mhard-float — use hardware floating-point instructions. the real thing, no cap 🔥
• -mmemcpy — do not optimize block moves, just use "memcpy". let the library handle it 🫶
• -mno-clearbss — this option is deprecated. use -fno-zero-initialized-in-bss instead. it's giving "old and busted, use the new one" 💀
• *-mcpu=cpu-type* — use features of, and schedule code for, the given CPU. supported values are in the format vX.YY.Z, where X is a major version, YY is the minor version, and Z is compatibility code. example values are v3.00.a, v4.00.b, v5.00.a, v5.00.b, v6.00.a. pick your CPU like it's a personality test 👑
• -mxl-soft-mul — use software multiply emulation (default). soft launch energy ✨
• -mxl-soft-div — use software emulation for divides (default). same vibe, different operation 💖
• -mxl-barrel-shift — use the hardware barrel shifter. shifting but make it hardware 🎀
• -mxl-pattern-compare — use pattern compare instructions. spot the pattern bestie 🦋
• -msmall-divides — use table lookup optimization for small signed integer divisions. tiny divisions, big optimization ✨
• -mxl-stack-check — this option is deprecated. use -fstack-check instead. legacy behavior, please migrate 🙈
• -mxl-gp-opt — use GP-relative .sdata/.sbss sections. gp-relative? slay 💅
• -mxl-multiply-high — use multiply high instructions for the high part of a 32x32 multiply. only the high part, main character energy 👑
• -mxl-float-convert — use hardware floating-point conversion instructions. converting, but with hardware 🔥
• -mxl-float-sqrt — use the hardware floating-point square root instruction. √ but make it fast ⭐
• -mbig-endian — generate code for a big-endian target. big bytes first 🌈
• -mlittle-endian — generate code for a little-endian target. tiny bytes first 💜
• -mxl-reorder — use reorder instructions (swap and byte reversed load/store). rearrange, she's iconic 💅
• *-mxl-mode-app-model* — select application model app-model. valid models are:
  • executable — normal executable (default), uses startup code crt0.o. the classic 🎀
  • -mpic-data-is-text-relative — assume that the displacement between the text and data segments is fixed at static link time. this allows data to be referenced by offset from the start of the text address instead of GOT, since PC-relative addressing is not supported. 🦋
  • 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. ✨

option -xl-mode-app-model is a deprecated alias for -mxl-mode-app-model. 💀

---

👑 MIPS Options 👑

• -EB — generate big-endian code. big energy 🔥
• -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). a whole roster, bestie 🌈

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 *n*f2_1 refer to processors with FPUs clocked at half the rate of the core, names of the form *n*f1_1 refer to processors with FPUs clocked at the same rate as the core, and names of the form *n*f3_2 refer to processors with FPUs clocked at a ratio of 3:2 with respect to the core. for compatibility reasons, *n*f is accepted as a synonym for *n*f2_1 while *n*x and *n*bfx are accepted as synonyms for *n*f1_1. 💅

GCC defines two macros based on the value of this option. the first is _MIPS_ARCH, which gives the name of the target architecture, as a string. the second has the form _MIPS_ARCH_foo, where foo is the capitalized value of _MIPS_ARCH. for example, -march=r2000 sets _MIPS_ARCH to "r2000" and defines the macro _MIPS_ARCH_R2000. 🎀

note that the _MIPS_ARCH macro uses the processor names given above. in other words, it has the full prefix and does not abbreviate 000 as k. in the case of from-abi, the macro names the resolved architecture (either "mips1" or "mips3"). it names the default architecture when no -march option is given. 💖

• *-mtune=arch* — optimize for arch. among other things, this option controls the way instructions are scheduled, and the perceived cost of arithmetic operations. the list of arch values is the same as for -march. 👑

when this option is not used, GCC optimizes for the processor specified by -march. by using -march and -mtune together, it is possible to generate code that runs on a family of processors, but optimize the code for one particular member of that family. so versatile, we love to see it 🌸

-mtune defines the macros _MIPS_TUNE and _MIPS_TUNE_foo, which work in the same way as the -march ones described above. ⭐

• -mips1 — equivalent to -march=mips1.
• -mips2 — equivalent to -march=mips2.
• -mips3 — equivalent to -march=mips3.
• -mips4 — equivalent to -march=mips4.
• -mips32 — equivalent to -march=mips32.
• -mips32r3 — equivalent to -march=mips32r3.
• -mips32r5 — equivalent to -march=mips32r5.
• -mips32r6 — equivalent to -march=mips32r6.
• -mips64 — equivalent to -march=mips64.
• -mips64r2 — equivalent to -march=mips64r2.
• -mips64r3 — equivalent to -march=mips64r3.
• -mips64r5 — equivalent to -march=mips64r5.
• -mips64r6 — equivalent to -march=mips64r6.
• -mips16, -mno-mips16 — generate (do not generate) MIPS16 code. if GCC is targeting a MIPS32 or MIPS64 architecture, it makes use of the MIPS16e ASE. 🦋

MIPS16 code generation can also be controlled on a per-function basis by means of "mips16" and "nomips16" attributes. per-function control? she's flexible 💅

• -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. > bestie, this is a testing thing, don't ship it 💀
• -minterlink-compressed, -mno-interlink-compressed — require (do not require) that code using the standard (uncompressed) MIPS ISA be link-compatible with MIPS16 and microMIPS code, and vice versa. 🔗

for example, code using the standard ISA encoding cannot jump directly to MIPS16 or microMIPS code; it must either use a call or an indirect jump. -minterlink-compressed therefore disables direct jumps unless GCC knows that the target of the jump is not compressed. the details matter, iykyk ✨

• -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. old but respected 🎀
• -mabi=32, -mabi=o64, -mabi=n32, -mabi=64, -mabi=eabi — generate code for the given ABI. 💖

note that the EABI has a 32-bit and a 64-bit variant. GCC normally generates 64-bit code when you select a 64-bit architecture, but you can use -mgp32 to get 32-bit code instead. 🌈

for information about the O64 ABI, see <http://gcc.gnu.org/projects/mipso64-abi.html>. ⭐

GCC supports a variant of the o32 ABI in which floating-point registers are 64 rather than 32 bits wide. you can select this combination with -mabi=32 -mfp64. this ABI relies on the "mthc1" and "mfhc1" instructions and is therefore only supported for MIPS32R2, MIPS32R3 and MIPS32R5 processors. 🫶

okay so the register assignments for arguments and return values? literally unchanged, bestie. same energy. BUT — and this is the glow-up — each scalar value now gets passed in a single 64-bit register instead of a pair of 32-bit registers. so like, scalar floating-point values come back in $f0 only, not a $f0/$f1 pair. 💅 and the set of call-saved registers also stays the same, in that the even-numbered double-precision registers</> are saved. no notes.

now, there are two extra variants of the o32 ABI, and they exist purely to make the 32-bit → 64-bit register transition not a whole ordeal. meet FPXX (-mfpxx) and FP64A (-mfp64 -mno-odd-spreg) 🎀

the FPXX extension has a whole mandate: 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. pick a lane, gorgeous. 🚦

the FP64A extension is basically FP64's cousin except it forbids the use of odd-numbered single-precision registers. it can be used in conjunction with the "FRE" mode of FPUs in MIPS32R5 processors, and it lets both FP32 and FP64A code interlink and run in the same process without changing FPU modes. iconic behaviour, honestly. ✨

---

• -mabicalls, -mno-abicalls — generate (do not generate) code that's suitable for SVR4-style dynamic objects. -mabicalls is the default for SVR4-based systems. 🌸
• -mshared, -mno-shared — generate (do not generate) code that is fully position-independent, aka the kind that can be linked into shared libraries. heads up: this option only affects -mabicalls.

so, all -mabicalls code has traditionally been position-independent, no matter what options like -fPIC and -fpic say. 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 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 — and this is important — the option does not affect the ABI of the final executable; it only affects the ABI of relocatable objects. using -mno-shared generally makes executables both smaller and quicker. slay.

-mshared is the default.

• -mplt, -mno-plt — assume (do not assume) that the static and dynamic linkers support PLTs and copy relocations. this option only affects -mno-shared -mabicalls. and for the n64 ABI, it has no effect without -msym32. 💀

you can make -mplt the default by configuring GCC with --with-mips-plt. otherwise the default is -mno-plt.

• -mxgot, -mno-xgot — lift (do not lift) the usual restrictions on the size of the global offset table.

GCC normally uses a single instruction to load values from the GOT. relatively efficient, yes, but it only works if the GOT is smaller than about 64k. anything bigger and the linker starts screaming at you with an error like:

        relocation truncated to fit: R_MIPS_GOT16 foobar

if that happens, recompile your code with -mxgot. it works with very large GOTs, though the code is also less efficient, since it takes three instructions to fetch the value of a global symbol. 😭

note that some linkers can create multiple GOTs. if you've got one of those, you should only need -mxgot when a single object file accesses more than 64k's worth of GOT entries. very few do. 🦋

these options have no effect unless GCC is generating position independent code.

• -mgp32 — assume that general-purpose registers are 32 bits wide.
• -mgp64 — assume that general-purpose registers are 64 bits wide.
• -mfp32 — assume that floating-point registers are 32 bits wide.
• -mfp64 — assume that floating-point registers are 64 bits wide.
• -mfpxx — do not assume the width of floating-point registers. mystery registers. we simply don't know her. 🙈
• -mhard-float — use floating-point coprocessor instructions.
• -msoft-float — do not use floating-point coprocessor instructions. implement floating-point calculations using library calls instead.
• -mno-float — equivalent to -msoft-float, but it 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. not a vibe. be careful.
• -msingle-float — assume that the floating-point coprocessor only supports single-precision operations.
• -mdouble-float — assume that the floating-point coprocessor supports double-precision operations. this is the default. ✨
• -modd-spreg, -mno-odd-spreg — enable the use of odd-numbered single-precision floating-point registers for the o32 ABI. this is the default for processors that are known to support these registers. when using the o32 FPXX ABI, -mno-odd-spreg is set by default.
• -mabs=2008, -mabs=legacy — these 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. instead, a longer sequence of instructions that manipulate the sign bit of floating-point datum manually is used — 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. so these instructions are always used for the respective operations. she's confident. 💅

• -mnan=2008, -mnan=legacy — these control the encoding of the special not-a-number (NaN) IEEE 754 floating-point data.

-mnan=legacy 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. 🔢

-mnan=2008 selects the IEEE 754-2008 encoding. in this case qNaNs are denoted by the first bit of their trailing significand field being 1, whereas sNaNs are denoted by the first bit of their trailing significand field being 0. yes, they're swapped. no, we don't make the rules. 😭

the default is -mnan=legacy unless GCC has been configured with --with-nan=2008.

• -mllsc, -mno-llsc — use (do not use) ll, sc, and sync instructions to implement atomic memory built-in functions. when neither option is specified, GCC uses the instructions if the target architecture supports them.

-mllsc is useful if the runtime environment can emulate the instructions, and -mno-llsc can be useful when compiling for nonstandard ISAs. you can make either option the default by configuring GCC with --with-llsc and --without-llsc respectively. --with-llsc is the default for some configurations; see the installation documentation for details. 📚

• -mdsp, -mno-dsp — use (do not use) revision 1 of the MIPS DSP ASE.

this option defines the preprocessor macro __mips_dsp. it also defines __mips_dsp_rev to 1.

• -mdspr2, -mno-dspr2 — use (do not use) revision 2 of the MIPS DSP ASE.

this option defines the preprocessor macros __mips_dsp and __mips_dspr2. it also defines __mips_dsp_rev to 2.

• -msmartmips, -mno-smartmips — use (do not use) the MIPS SmartMIPS ASE.
• -mpaired-single, -mno-paired-single — use (do not use) paired-single floating-point instructions.

this option requires hardware floating-point support to be enabled. 🔥

• -mdmx, -mno-mdmx — use (do not use) MIPS Digital Media Extension instructions. this option can only be used when generating 64-bit code and requires hardware floating-point support to be enabled.
• -mips3d, -mno-mips3d — use (do not use) the MIPS-3D ASE. the option -mips3d implies -mpaired-single. 🫶
• -mmicromips, -mno-micromips — generate (do not generate) microMIPS code.

microMIPS code generation can also be controlled on a per-function basis by means of micromips and nomicromips attributes.

• -mmt, -mno-mt — use (do not use) MT Multithreading instructions.
• -mmcu, -mno-mcu — use (do not use) the MIPS MCU ASE instructions.
• -meva, -mno-eva — use (do not use) the MIPS Enhanced Virtual Addressing instructions.
• -mvirt, -mno-virt — use (do not use) the MIPS Virtualization (VZ) instructions.
• -mxpa, -mno-xpa — use (do not use) the MIPS eXtended Physical Address (XPA) instructions.
• -mcrc, -mno-crc — use (do not use) the MIPS Cyclic Redundancy Check (CRC) instructions.
• -mginv, -mno-ginv — use (do not use) the MIPS Global INValidate (GINV) instructions.
• -mloongson-mmi, -mno-loongson-mmi — use (do not use) the MIPS Loongson MultiMedia extensions Instructions (MMI).
• -mloongson-ext, -mno-loongson-ext — use (do not use) the MIPS Loongson EXTensions (EXT) instructions.
• -mloongson-ext2, -mno-loongson-ext2 — use (do not use) the MIPS Loongson EXTensions r2 (EXT2) instructions. 🍓
• -mlong64 — 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 ints, longs and pointers depends on the ABI. all the supported ABIs use 32-bit ints. the n64 ABI uses 64-bit longs, as does the 64-bit EABI; the others use 32-bit longs. pointers are the same size as longs, or the same size as integer registers, whichever is smaller. 💅

• -msym32, -mno-sym32 — assume (do not assume) that all symbols have 32-bit values, regardless of the selected ABI. this option is useful in combination with -mabi=64 and -mno-abicalls, because it allows GCC to generate shorter and faster references to symbolic addresses. ⭐
• -G num — bestie, this one puts the definitions of externally-visible data into a small data section if that data is no bigger than num bytes. Why do we care? Because then GCC can generate way more efficient accesses to it — peep -mgpopt for the deets. 💅

The default -G option depends on the configuration. (iykyk, configs be like that.)

• -mlocal-sdata, -mno-local-sdata — extend (or don't extend) the -G behavior to local data too, like static variables in C. -mlocal-sdata is the default for all configurations, no cap. ✨

If the linker starts complaining that your app is using too much small data, you might wanna try rebuilding the less performance-critical parts with -mno-local-sdata. You might also wanna build large libraries with -mno-local-sdata so those libraries leave more room for the main program. 🫶

• -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 here's the tea 🍓: if you compile a module Mod with -mextern-sdata -G num -mgpopt, and Mod references a variable Var that is no bigger than num bytes, you must make sure that Var is placed in a small data section. If Var is defined by another module, you must either compile that module with a high-enough -G setting or attach a "section" attribute to Var's definition. And if Var is common, you must link the application with a high-enough -G setting.

The easiest way of satisfying these restrictions is to compile and link every module with the same -G option. But maybe you wanna build a library that supports several different small data limits — iconic, love that for you. You can do this by compiling the library with the highest supported -G setting and additionally using -mno-extern-sdata to stop the library from making assumptions about externally-defined data. 🌈

• -mgpopt, -mno-gpopt — use (or don't use) GP-relative accesses for symbols that are known to be in a small data section; see -G, -mlocal-sdata and -mextern-sdata. -mgpopt is the default for all configurations. 💖

-mno-gpopt is useful for cases where the $gp register might not hold the value of "_gp". For example, if the code is part of a library that might be used in a boot monitor, programs that call boot monitor routines pass an unknown value in $gp. (In such situations, the boot monitor itself is usually compiled with -G0.)

Heads up bestie: -mno-gpopt implies -mno-local-sdata and -mno-extern-sdata. ⚠️

• -membedded-data, -mno-embedded-data — allocate variables to the read-only data section first if possible, then next in the small data section if possible, otherwise in data. This gives slightly slower code than the default, but reduces the amount of RAM required when executing, and thus may be preferred for some embedded systems. 🧚
• -muninit-const-in-rodata, -mno-uninit-const-in-rodata — put uninitialized "const" variables in the read-only data section. This option is only meaningful in conjunction with -membedded-data. 👑
• *-mcode-readable=setting* — specify whether GCC may generate code that reads from executable sections. There are three possible settings:
  • -mcode-readable=yes — instructions may freely access executable sections. This is the default setting.
  • -mcode-readable=pcrel — MIPS16 PC-relative load instructions can access executable sections, but other instructions must not do so. This option is useful on 4KSc and 4KSd processors when the code TLBs have the Read Inhibit bit set. It is also useful on processors that can be configured to have a dual instruction/data SRAM interface and that, like the M4K, automatically redirect PC-relative loads to the instruction RAM.
  • -mcode-readable=no — instructions must not access executable sections. This option can be useful on targets that are configured to have a dual instruction/data SRAM interface but that (unlike the M4K) do not automatically redirect PC-relative loads to the instruction RAM.
• -msplit-addresses, -mno-split-addresses — enable (or disable) use of the %hi() and %lo() assembler relocation operators. This option has been superseded by -mexplicit-relocs but is retained for backwards compatibility. (vintage, but make it work 💀)
• -mexplicit-relocs, -mno-explicit-relocs — use (or don't use) assembler relocation operators when dealing with symbolic addresses. The alternative, selected by -mno-explicit-relocs, is to use assembler macros instead.

-mexplicit-relocs is the default if GCC was configured to use an assembler that supports relocation operators.

• -mcheck-zero-division, -mno-check-zero-division — trap (or don't 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 (or 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. 🔥
• -mmemcpy, -mno-memcpy — force (or don't 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 (or don't 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 (or disable) use of the "mad", "madu" and "mul" instructions, as provided by the R4650 ISA.
• -mimadd, -mno-imadd — enable (or 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. (74k catching strays fr fr 💀)
• -mfused-madd, -mno-fused-madd — enable (or 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.

Okay so real talk — the R10K, like a bunch of other processors out there, tries to predict which way a conditional branch is gonna go, and then it speculatively executes instructions from the "taken" branch. Later on it aborts those instructions if the prediction turned out wrong. Cute concept, right? Except — and this is the plot twist — on the R10K, even the aborted instructions can still have side effects. 💀

This problem only affects kernel stores and, depending on the system, kernel loads. Like, say a speculatively-executed store loads the target memory into cache and marks that cache line as dirty, even though the store itself gets aborted later. If a DMA operation writes to that same area of memory before the "dirty" line gets flushed, then the cached data just overwrites the DMA-ed data. Bye data. 😭 See the R10K processor manual for a full description, including other potential problems.

One workaround: insert cache barrier instructions before every memory access that might be speculatively executed and that might have side effects even if aborted. -mr10k-cache-barrier=setting controls GCC's implementation of this workaround. It assumes that aborted accesses to any byte in the following regions 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 make sure that speculative accesses to these regions are actually safe. No pressure or anything. 🫶

If the input program contains a function declaration such as:

void foo (void);

then the implementation of "foo" must allow "j foo" and "jal foo" to be executed speculatively. GCC honors this restriction for functions it compiles itself. It expects non-GCC functions (such as hand-written assembly code) to do the same.

The option has three forms:

• -mr10k-cache-barrier=load-store — Insert a cache barrier before a load or store that might be speculatively executed and that might have side effects even if aborted.
• -mr10k-cache-barrier=store — Insert a cache barrier before a store that might be speculatively executed and that might have side effects even if aborted.
• -mr10k-cache-barrier=none — Disable the insertion of cache barriers. This is the default setting.

---

• -mflush-func=func, -mno-flush-func — Specifies the function to call to flush the I and D caches, or to not call any such function at all. If it is called, the function has to take the same arguments as the common _flush_func, that is: the address of the memory range for which the cache is being flushed, the size of the memory range, and the number 3 (to flush both caches). The default depends on the target GCC was configured for, but commonly it's either _flush_func or __cpu_flush.
• mbranch-cost=num — Set the cost of branches to roughly num "simple" instructions. Heads up bestie, 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're supported by the selected architecture. One exception though: for the MIPS32 and MIPS64 architectures and the processors that implement those architectures, Branch Likely instructions are not generated by default, because MIPS32 and MIPS64 specifically deprecate their use. 🚫
• -mcompact-branches=never, -mcompact-branches=optimal, -mcompact-branches=always — These options control which form of branches will be generated. The default is -mcompact-branches=optimal.
  • The -mcompact-branches=never option ensures that compact branch instructions will never be generated.
  • The -mcompact-branches=always option ensures that a compact branch instruction will be generated if available. If a compact branch instruction isn't available, a delay slot form of the branch will be used instead.
  • This option is supported from MIPS Release 6 onwards.
  • The -mcompact-branches=optimal option 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, we can only use one FP pipe.
• -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. 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, on many multi-core (SMP) systems, it does not invalidate the instruction caches on all cores and may lead to undefined behavior. 😬
• -mrelax-pic-calls, -mno-relax-pic-calls — Try to turn PIC calls that are normally dispatched via register $25 into direct calls. This is only possible if the linker can resolve the destination at link time and if the destination is within range for a direct call.
  • -mrelax-pic-calls is the default if GCC was configured to use an assembler and a linker that support the .reloc assembly directive and -mexplicit-relocs is in effect. With -mno-explicit-relocs, this optimization can be performed by the assembler and the linker alone without help from the compiler.
• -mmcount-ra-address, -mno-mcount-ra-address — Emit (do not emit) code that allows _mcount to modify the calling function's return address. When enabled, this option extends the usual _mcount interface with a new ra-address parameter, which has type intptr_t * and is passed in register $12. _mcount can then modify the return address by doing both of the following:
  • Returning the new address in register $31.
  • Storing the new address in *ra-address, if ra-address is nonnull.
  • The default is -mno-mcount-ra-address.
• -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's unused.
  • This optimization is off by default at all optimization levels.
• -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.
• -mepsilon, -mno-epsilon — Generate floating-point comparison instructions that compare with respect to the rE epsilon register.
• -mabi=mmixware, -mabi=gnu — Generate code that passes function parameters and return values that (in the called function) are seen as registers $0 and up, as opposed to the GNU ABI which uses global registers $231 and up.
• -mzero-extend, -mno-zero-extend — When reading data from memory in sizes shorter than 64 bits, use (do not use) zero-extending load instructions by default, rather than sign-extending ones.
• -mknuthdiv, -mno-knuthdiv — Make the result of a division yielding a remainder have the same sign as the divisor. With the default, -mno-knuthdiv, the sign of the remainder follows the sign of the dividend. Both methods are arithmetically valid, the latter being almost exclusively used.
• -mtoplevel-symbols, -mno-toplevel-symbols — Prepend (do not prepend) a : to all global symbols, so the assembly code can be used with the PREFIX assembly directive.
• -melf — Generate an executable in the ELF format, rather than the default mmo format used by the mmix simulator.
• -mbranch-predict, -mno-branch-predict — Use (do not use) the probable-branch instructions, when static branch prediction indicates a probable branch.
• -mbase-addresses, -mno-base-addresses — Generate (do not generate) code that uses base addresses. Using a base address automatically generates a request (handled by the assembler and the linker) for a constant to be set up in a global register. The register is used for one or more base address requests within the range 0 to 255 from the value held in the register. This generally leads to short and fast code, but the number of different data items that can be addressed is limited. Which means that a program that uses lots of static data may require -mno-base-addresses.
• -msingle-exit, -mno-single-exit — Force (do not force) generated code to have a single exit point in each function.

---

🎀 MN10300 Options 🎀

These -m options are defined for Matsushita MN10300 architectures:

• -mmult-bug — Generate code to avoid bugs in the multiply instructions for the MN10300 processors. This is the default.
• -mno-mult-bug — bestie, this one says don't generate code that dodges the buggy multiply instructions on MN10300 processors. like, no workarounds, we're rawdogging the hardware 💀
• -mam33 — generate code using the features that are specific to the AM33 processor. slay ✨
• -mno-am33 — do not generate code using AM33-specific features. this is the default, periodt.
• -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 given cpu-type when scheduling instructions. ngl this does *not change the targeted processor type, it's just the vibe of the timing. the *cpu-type has to be one of mn10300, am33, am33-2 or am34, no substitutions bestie.
• -mreturn-pointer-on-d0 — when generating a function that returns a pointer, return the pointer in both a0 and d0. otherwise the pointer only comes back in a0, and trying to call such functions without a prototype gives you errors 😭. note that this option is on by default, so use -mno-return-pointer-on-d0 to turn it off.
• -mno-crt0 — do not link in the C run-time initialization object file. (yes this one shows up more than once in this page, it's that girl 💅)
• -mrelax — tell the linker it should do a relaxation optimization pass to shorten branches, calls and absolute memory addresses. heads up: this only does anything when you use it on the command line for the final link step.

this option makes symbolic debugging impossible. like, choose your fighter.

• -mliw — let the compiler generate Long Instruction Word instructions if the target is AM33 or later. this is the default. this option defines the preprocessor macro __LIW__.
• -mno-liw — do not let the compiler generate Long Instruction Word instructions. this option defines the preprocessor macro __NO_LIW__.
• -msetlb — let the compiler generate the SETLB and Lcc instructions if the target is AM33 or later. this is the default. this option defines the preprocessor macro __SETLB__.
• -mno-setlb — do not let the compiler 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. (she's back 🎀)

---

🌸 MSP430 Options 🌸

these options are defined for the MSP430:

• -masm-hex — force assembly output to always use hex constants. normally these constants are signed decimals, but this option exists for testsuite and/or aesthetic purposes 💅. aesthetic purposes. iconic.
• -mmcu= — select the MCU to target. this gets used to create a C preprocessor symbol based on the MCU name, converted to upper case and pre- and post-fixed with __. that symbol then gets used by the msp430.h header file to pick an MCU-specific supplementary header file.

the option also sets the ISA to use. if the MCU name is one that's known to only support the 430 ISA then that's what gets selected, otherwise the 430X ISA is selected. you can also use the generic MCU name msp430 to select the 430 ISA. similarly the generic msp430x MCU name selects the 430X ISA.

in addition, an MCU-specific linker script gets added to the linker command line. the script's name is the name of the MCU with .ld appended. so specifying -mmcu=xxx on the gcc command line defines the C preprocessor symbol __XXX__ and makes the linker search for a script called xxx.ld.

the ISA and hardware multiply supported for the different MCUs is hard-coded into GCC. but bestie, an external devices.csv file can be used to extend device support beyond the hard-coded ones ✨

GCC searches for the devices.csv file using these methods, in this precedence order — the first takes precedence over the second, which takes precedence over the third:

• Include path specified with -I and -L — devices.csv will be 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 — set the global environment variable MSP430_GCC_INCLUDE_DIR to the full path of the directory containing devices.csv, and GCC will search that directory for devices.csv. if devices.csv is found, this directory also gets 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 will examine msp430-elf{,bare}/include/devices from the toolchain root directory. this directory does not exist in a default installation, but if you created it and copied devices.csv there, then the MCU data will be read. same as above, this directory also gets 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. no cap.

• -mwarn-mcu
• -mno-warn-mcu — these enable or disable 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 instead.
• -msim — link to the simulator runtime libraries and linker script. this 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 gets passed to the assembler and linker, and lets the linker perform certain optimizations that can't 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. you can also give a value of auto. that tells GCC to deduce the hardware multiply support based on the MCU name provided by the -mmcu option. if no -mmcu option is specified, or if the MCU name isn't 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. but when compiling at -O3 or higher, the hardware multiplier is invoked inline. this makes for bigger, but faster code 🔥

the hardware multiply routines disable interrupts while 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 they 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's 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 that 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 can't 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 you pass for this option gets divided by 2 and the resulting value is used instead.

• -mcode-region=
• -mdata-region= — these options tell the compiler where to place functions and data that don't 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 the 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 isn't found or there are problems 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 — nope, don't generate performance extension 2 instructions. that's it, that's the whole vibe 💅
• -mext-string — okay generate string extension instructions ✨
• -mno-ext-string — and do NOT generate string extension instructions 🙅
• -mv3push — generate v3 push25/pop25 instructions 🫶
• -mno-v3push — skip those v3 push25/pop25 instructions, we're not doing that
• -m16-bit — generate 16-bit instructions ✨
• -mno-16-bit — no 16-bit instructions, not today 💀
• -misr-vector-size=num — set how big each interrupt vector is. lowkey it must be 4 or 16, no in-betweens bestie
• -mcache-block-size=num — set the size of each cache block. it has to be a power of 2, somewhere between 4 and 512 🎀
• -march=arch — tell it the name of the target architecture 👑
• -mcmodel=code-model — pick your code model, here are the options:
  • small — all the data and read-only data segments gotta live within 512KB addressing space. the text segment must be within 16MB addressing space.
  • medium — the data segment must be within 512KB, but the read-only data segment gets to stretch out within 4GB addressing space. text segment should still be within 16MB addressing space.
  • large — literally everything, text and data segments, can be within 4GB addressing space. main character energy fr fr 🌈
• -mctor-dtor — enable the constructor/destructor feature ✨
• -mrelax — guide the linker to relax instructions 🧘

---

🌸 Nios II Options 🌸

these are the options defined for the Altera Nios II processor. okay so let's get into it 💖

• -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 ⭐
• -mgpopt=option
• -mgpopt
• -mno-gpopt — generate (or don't generate) GP-relative accesses. here are the option names it recognizes:
  • none — do not generate GP-relative accesses.
  • local — generate GP-relative accesses for small data objects that are not external, weak, or uninitialized common symbols. also use GP-relative addressing for objects that have been explicitly placed in a small data section via a "section" attribute.
  • global — same as local, but ALSO generate GP-relative accesses for small data objects that are external, weak, or common. > heads up: if you use this option, you must ensure that all parts of your program (including libraries) are compiled with the same -G setting. no cap.
  • data — generate GP-relative accesses for all data objects in the program. if you use this option, the entire data and BSS segments of your program must fit in 64K of memory, AND you must use an appropriate linker script to allocate them within the addressable range of the global pointer. big ask, ik 😭
  • all — generate GP-relative addresses for function pointers as well as data pointers. if you use this option, the entire text, data, and BSS segments of your program must fit in 64K of memory and you must use an appropriate linker script to allocate them within the addressable range of the global pointer.

-mgpopt is equivalent to -mgpopt=local, and -mno-gpopt is equivalent to -mgpopt=none.

the default is -mgpopt — except when -fpic or -fPIC is specified to generate position-independent code. note that the Nios II ABI does not permit GP-relative accesses from shared libraries.

you may need to specify -mno-gpopt explicitly when building programs that include large amounts of small data, including large GOT data sections. in this case, the 16-bit offset for GP-relative addressing may not be large enough to allow access to the entire small data section. 💀

• -mgprel-sec=regexp — this option specifies additional section names that can be accessed via GP-relative addressing. it's most useful in conjunction with "section" attributes on variable declarations and a custom linker script. the regexp is a POSIX Extended Regular Expression ✨

this option does not affect the behavior of the -G option, and the specified sections are in addition to the standard ".sdata" and ".sbss" small-data sections that are recognized by -mgpopt.

• -mr0rel-sec=regexp — this option specifies names of sections that can be accessed via a 16-bit offset from "r0"; that is, in the low 32K or high 32K of the 32-bit address space. it's most useful in conjunction with "section" attributes on variable declarations and a custom linker script. the regexp is a POSIX Extended Regular Expression 🎀

in contrast to the use of GP-relative addressing for small data, zero-based addressing is never generated by default and there are no conventional section names used in standard linker scripts for sections in the low or high areas of memory. periodt.

• -mel
• -meb — generate little-endian (that's the default) or big-endian (experimental 👀) code, respectively.
• -march=arch — this specifies the name of the target Nios II architecture. GCC uses this name to determine what kind of instructions it can emit when generating assembly code. permissible names are: r1, r2.

the preprocessor macro "__nios2_arch__" is available to programs, with value 1 or 2, indicating the targeted ISA level.

• -mbypass-cache
• -mno-bypass-cache — force all load and store instructions to always bypass cache by using I/O variants of the instructions. the default is not to bypass the cache.
• -mno-cache-volatile
• -mcache-volatile — volatile memory access bypasses the cache using the I/O variants of the load and store instructions. the default is not to bypass the cache.
• -mno-fast-sw-div
• -mfast-sw-div — do not use table-based fast divide for small numbers. the default is to use the fast divide at -O3 and above.
• -mno-hw-mul
• -mhw-mul
• -mno-hw-mulx
• -mhw-mulx
• -mno-hw-div
• -mhw-div — enable or disable emitting the "mul", "mulx" and "div" family of instructions by the compiler. the default is to emit "mul" and not emit "div" and "mulx" 💅
• -mbmx
• -mno-bmx
• -mcdx
• -mno-cdx — enable or disable generation of Nios II R2 BMX (bit manipulation) and CDX (code density) instructions. enabling these instructions also requires -march=r2. since these instructions are optional extensions to the R2 architecture, the default is not to emit them 🦋
• -mcustom-insn=N
• -mno-custom-insn — each -mcustom-insn=N option enables use of a custom instruction with encoding N when generating code that uses insn. for example, -mcustom-fadds=253 generates custom instruction 253 for single-precision floating-point add operations instead of the default behavior of using a library call. iconic ngl 🔥

the following values of insn are supported. except as otherwise noted, floating-point operations are expected to be implemented with normal IEEE 754 semantics and correspond directly to the C operators or the equivalent GCC built-in functions.

Single-precision floating point:

• fadds, fsubs, fdivs, fmuls — binary arithmetic operations.
• fnegs — unary negation.
• fabss — unary absolute value.
• fcmpeqs, fcmpges, fcmpgts, fcmples, fcmplts, fcmpnes — comparison operations.
• fmins, fmaxs — floating-point minimum and maximum. these instructions are only generated if -ffinite-math-only is specified.
• fsqrts — unary square root operation.
• fcoss, fsins, ftans, fatans, fexps, flogs — floating-point trigonometric and exponential functions. these instructions are only generated if -funsafe-math-optimizations is also specified.

Double-precision floating point:

• faddd, fsubd, fdivd, fmuld — binary arithmetic operations.
• fnegd — unary negation.
• fabsd — unary absolute value.
• fcmpeqd, fcmpged, fcmpgtd, fcmpled, fcmpltd, fcmpned — comparison operations.
• fmind, fmaxd — double-precision minimum and maximum. these instructions are only generated if -ffinite-math-only is specified.
• fsqrtd — unary square root operation.
• fcosd, fsind, ftand, fatand, fexpd, flogd — double-precision trigonometric and exponential functions. these instructions are only generated if -funsafe-math-optimizations is also specified.

Conversions:

• fextsd — conversion from single precision to double precision.
• ftruncds — conversion from double precision to single precision.
• fixsi, fixsu, fixdi, fixdu — conversion from floating point to signed or unsigned integer types, with truncation towards zero.
• round — conversion from single-precision floating point to signed integer, rounding to the nearest integer and ties away from zero. this corresponds to the "__builtin_lroundf" function when -fno-math-errno is used.
• floatis, floatus, floatid, floatud — conversion from signed or unsigned integer types to floating-point types.

in addition, all of the following transfer instructions for internal registers X and Y must be provided to use any of the double-precision floating-point instructions. custom instructions taking two double-precision source operands expect the first operand in the 64-bit register X. the other operand (or only operand of a unary operation) is given to the custom arithmetic instruction with the least significant half in source register src1 and the most significant half in src2. a custom instruction that returns a double-precision result returns the most significant 32 bits in the destination register and the other half in 32-bit register Y. GCC automatically generates the necessary code sequences to write register X and/or read register Y when double-precision floating-point instructions are used. okay that's a lot, but you got this bestie 🫶

• fwrx — write src1 into the least significant half of X and src2 into the most significant half of X.
• fwry — write src1 into Y.
• frdxhi, frdxlo — read the most or least (respectively) significant half of X and store it in dest.
• frdy — read the value of Y and store it into dest.

note that you can gain more local control over generation of Nios II custom instructions by using the target("custom-insn=N") and target("no-custom-insn") function attributes or pragmas. 💖

• -mcustom-fpu-cfg=name — okay so this one lets you turn on a whole predefined, named bundle of custom instruction encodings in one go (peep -mcustom-insn above, bestie, it's the same family 💅). Right now these are the sets that exist:
  • -mcustom-fpu-cfg=60-1 is equivalent to: -mcustom-fmuls=252 -mcustom-fadds=253 -mcustom-fsubs=254 -fsingle-precision-constant
  • -mcustom-fpu-cfg=60-2 is equivalent to: -mcustom-fmuls=252 -mcustom-fadds=253 -mcustom-fsubs=254 -mcustom-fdivs=255 -fsingle-precision-constant
  • -mcustom-fpu-cfg=72-3 is equivalent to: -mcustom-floatus=243 -mcustom-fixsi=244 -mcustom-floatis=245 -mcustom-fcmpgts=246 -mcustom-fcmples=249 -mcustom-fcmpeqs=250 -mcustom-fcmpnes=251 -mcustom-fmuls=252 -mcustom-fadds=253 -mcustom-fsubs=254 -mcustom-fdivs=255 -fsingle-precision-constant
  • -mcustom-fpu-cfg=fph2 is equivalent to: -mcustom-fabss=224 -mcustom-fnegs=225 -mcustom-fcmpnes=226 -mcustom-fcmpeqs=227 -mcustom-fcmpges=228 -mcustom-fcmpgts=229 -mcustom-fcmples=230 -mcustom-fcmplts=231 -mcustom-fmaxs=232 -mcustom-fmins=233 -mcustom-round=248 -mcustom-fixsi=249 -mcustom-floatis=250 -mcustom-fsqrts=251 -mcustom-fmuls=252 -mcustom-fadds=253 -mcustom-fsubs=254 -mcustom-fdivs=255

Custom instruction assignments given by individual -mcustom-insn= options override those given by -mcustom-fpu-cfg=, regardless of the order of the options on the command line. Like, order does not save you here, no cap. 😭

Also — you can gain more local control over selection of a FPU configuration by using the target("custom-fpu-cfg=name") function attribute or pragma. Fancy, right? 🎀

The name fph2 is an abbreviation for Nios II Floating Point Hardware 2 Component. And please note that the custom instructions enabled by -mcustom-fmins=233 and -mcustom-fmaxs=234 are only generated if -ffinite-math-only is specified. The custom instruction enabled by -mcustom-round=248 is only generated if -fno-math-errno is specified. And in contrast to the other configurations, -fsingle-precision-constant is not set. So like, this one's different, okay? 🙈

---

These additional -m options are available for the Altera Nios II ELF (bare-metal) target:

• -mhal — link with HAL BSP. This suppresses linking with the GCC-provided C runtime startup and termination code, and is typically used in conjunction with -msys-crt0= to specify the location of the alternate startup code provided by the HAL BSP. It's giving ✨bare metal✨.
• -msmallc — link with a limited version of the C library, -lsmallc, rather than Newlib.
• -msys-crt0=startfile — startfile is the file name of the startfile (crt0) to use when linking. This option is only useful in conjunction with -mhal.
• -msys-lib=systemlib — systemlib is the library name of the library that provides low-level system calls required by the C library, e.g. "read" and "write". This option is typically used to link with a library provided by a HAL BSP.

---

🌈 Nvidia PTX Options 🌈

These options are defined for Nvidia PTX:

• -m64 — ignored, but preserved for backward compatibility. Only 64-bit ABI is supported. So it's literally just vibing there for the old heads 💀
• -misa=ISA-string — generate code for given the specified PTX ISA (e.g. sm_35). ISA strings must be lower-case. Valid ISA strings include sm_30 and sm_35. The default ISA is sm_35. Lowercase only, bestie, don't test me 🔥
• -mmainkernel — link in code for a __main kernel. This is for stand-alone instead of offloading execution.
• -moptimize — apply partitioned execution optimizations. This is the default when any level of optimization is selected. 👑
• -msoft-stack — generate code that does not use ".local" memory directly for stack storage. Instead, a per-warp stack pointer is maintained explicitly. This enables variable-length stack allocation (with variable-length arrays or "alloca"), and when global memory is used for underlying storage, makes it possible to access automatic variables from other threads, or with atomic instructions. This code generation variant is used for OpenMP offloading, but the option is exposed on its own for the purpose of testing the compiler; to generate code suitable for linking into programs using OpenMP offloading, use option -mgomp.
• -muniform-simt — switch to code generation variant that allows to execute all threads in each warp, while maintaining memory state and side effects as if only one thread in each warp was active outside of OpenMP SIMD regions. All atomic operations and calls to runtime (malloc, free, vprintf) are conditionally executed (iff current lane index equals the master lane index), and the register being assigned is copied via a shuffle instruction from the master lane. Outside of SIMD regions lane 0 is the master; inside, each thread sees itself as the master. Shared memory 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. Okay that was a lot, but every word of it is load-bearing 🫶
• -mgomp — generate code for use in OpenMP offloading: enables -msoft-stack and -muniform-simt options, and selects corresponding multilib variant.

---

🦋 OpenRISC Options 🦋

These options are defined for OpenRISC:

• -mboard=name — configure a board specific runtime. This will be passed to the linker for newlib board library linking. The default is "or1ksim".
• -mnewlib — this option is ignored; it is for compatibility purposes only. This used to select linker and preprocessor options for use with newlib. Rip old flag, you're still iconic tho 💅
• -msoft-div
• -mhard-div — select software or hardware divide ("l.div", "l.divu") instructions. This default is hardware divide.
• -msoft-mul
• -mhard-mul — select software or hardware multiply ("l.mul", "l.muli") instructions. This default is hardware multiply.
• -msoft-float
• -mhard-float — select software or hardware for floating point operations. The default is software.
• -mdouble-float — when -mhard-float is selected, 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, 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 — enable generation of conditional move ("l.cmov") instructions. By default the equivalent will be generated using set and branch.
• -mror — enable generation of rotate right ("l.ror") instructions. By default functions from libgcc are used to perform rotate right operations.
• -mrori — enable generation of rotate right with immediate ("l.rori") instructions. By default functions from libgcc are used to perform rotate right with immediate operations.
• -msext — enable generation of sign extension ("l.ext*") instructions. By default memory loads are used to perform sign extension.
• -msfimm — enable 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 — enable 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.

---

💜 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.

---

🍓 picoChip Options 🍓

These -m options are defined for picoChip implementations:

• -mae=ae_type — set the instruction set, register set, and instruction scheduling parameters for array element type ae_type. Supported values for ae_type are ANY, MUL, and MAC.
  • -mae=ANY selects a completely generic AE type. Code generated with this option runs on any of the other AE types. The code is not as efficient as it would be if compiled for a specific AE type, and some types of operation (e.g., multiplication) do not work properly on all types of AE. It's giving "works everywhere, slays nowhere" 😭
  • -mae=MUL selects a MUL AE type. This is the most useful AE type for compiled code, and is the default. 👑
  • -mae=MAC selects a DSP-style MAC AE. Code compiled with this option may suffer from poor performance of byte (char) manipulation, since the DSP AE does not provide hardware support for byte load/stores.
• -msymbol-as-address — enable the compiler to directly use a symbol name as an address in a load/store instruction, without first loading it into a register. Typically, the use of this option generates larger programs, which run faster than when the option isn't used. However, the results vary from program to program, so it is left as a user option, rather than being permanently enabled. Your call, bestie 🎀
• -mno-inefficient-warnings — disables warnings about the generation of inefficient code. These warnings can be generated, for example, when compiling code that performs byte-level memory operations on the MAC AE type. The MAC AE has no hardware support for byte-level memory operations, so all byte load/stores must be synthesized from word load/store operations. This is inefficient and a warning is generated to indicate that you should rewrite the code to avoid byte operations, or to target an AE type that has the necessary hardware support. This option disables these warnings.

---

👑 PowerPC Options 👑

These are listed under

💖 PRU Options 💖

okay bestie, these are the command-line options for the PRU target, and there are only a few so you can absolutely hold all of them in your head 🧠✨

• -minrt — link with a minimum runtime environment, with zero support for static initializers and constructors. Using this can significantly shrink the size of the final ELF binary 🔥. But beware, babe: the compiler could still generate code with static initializers and constructors, so it is up to the programmer to make sure the source program won't use those features. No cap.
• -mmcu=mcu — specify the PRU MCU variant to use. Check Newlib for the exact list of supported MCUs 👑
• -mno-relax — make 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's the tea on the differences ☕:

Function Pointer Size

TI ABI says function (code) pointers are 16-bit, whereas GNU supports only 32-bit data and code pointers. 😭

Optional Return Value Pointer

Function return values larger than 64 bits are passed by using a hidden pointer as the first argument of the function. TI ABI, though, mandates that the pointer can be NULL in case the caller is not using the returned value. GNU always passes and expects a valid return value pointer.

The current -mabi=ti implementation simply raises a compile error when any of the above code constructs is detected 🚨. As a consequence the standard C library cannot be built and it is omitted when linking with -mabi=ti.

Relaxation is a GNU feature and for safety reasons is disabled when using -mabi=ti. The TI toolchain does not emit relocations for QBBx instructions, so the GNU linker cannot adjust them when shortening adjacent LDI32 pseudo instructions.

---

🌈 RISC-V Options 🌈

These command-line options are defined for RISC-V targets, and there are a lot of them, so grab a snack bestie 🍓

• -mbranch-cost=n — set 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 — specify integer and floating-point calling convention. ABI-string contains two parts: the size of integer types and the registers used for floating-point types. For example -march=rv64ifd -mabi=lp64d means that long and pointers are 64-bit (implicitly defining int to be 32-bit), and that floating-point values up to 64 bits wide are passed in F registers. Contrast this with -march=rv64ifd -mabi=lp64f, which still allows the compiler to generate code that uses the F and D extensions but only allows floating-point values up to 32 bits long to be passed in registers; or -march=rv64ifd -mabi=lp64, in which no floating-point arguments will be passed in registers. 💅

The default for this argument is system dependent, users who want a specific calling convention should specify one explicitly. The valid calling conventions are: ilp32, ilp32f, ilp32d, lp64, lp64f, and lp64d. Some calling conventions are impossible to implement on some ISAs: for example, -march=rv32if -mabi=ilp32d is invalid because the ABI requires 64-bit values be passed in F registers, but F registers are only 32 bits wide. There is also the ilp32e ABI that can only be used with the rv32e architecture. This ABI is not well specified at present, and is 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.
• -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.
• -march=ISA-string — generate code for given RISC-V ISA (e.g. rv64im). ISA strings must be lower-case. Examples include rv64i, rv32g, rv32e, and rv32imaf.

When -march= is not specified, use the setting from -mcpu.

If both -march and -mcpu= are not specified, the default for this argument is system dependent, users who want a specific architecture extensions should specify one explicitly.

• -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, and sifive-u74.
• -mtune=processor-string — optimize the output for the given processor, specified by microarchitecture or particular CPU name. Permissible values for this option are: rocket, sifive-3-series, sifive-5-series, sifive-7-series, size, 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. ✨

• -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. Don't play yourself 💀

• -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.
• -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.
• -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.
• -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.
• -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.
• -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.
• -memit-attribute, -mno-emit-attribute — emit (do not emit) RISC-V attribute to record extra information into ELF objects. This feature requires at least binutils 2.32.
• -malign-data=type — control how GCC aligns variables and constants of array, structure, or union types. Supported values for type are xlen which uses x register width as the alignment value, and natural which uses natural alignment. xlen is the default.
• -mbig-endian — generate big-endian code. This is the default when GCC is configured for a riscv64be-*-* or riscv32be-*-* target.
• -mlittle-endian — generate little-endian code. This is the default when GCC is configured for a riscv64-*-* or riscv32-*-* but not a riscv64be-*-* or riscv32be-*-* target.
• -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 tls for per-thread canary in the TLS block.

With the latter choice the options -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset furthermore specify which register to use as base register for reading the canary, and from what offset from that base register. There is no default register or offset as this is entirely for use within the Linux kernel. 🐧

---

🦋 RL78 Options 🦋

• -msim — links in additional target libraries to support operation within a simulator.
• -mmul=none, -mmul=g10, -mmul=g13, -mmul=g14, -mmul=rl78 — specifies the type of hardware multiplication and division support to be used. The simplest is "none", which uses software for both multiplication and division. This is the default. The "g13" value is for the hardware multiply/divide peripheral found on the RL78/G13 (S2 core) targets. The "g14" value selects the use of the multiplication and division instructions supported by the RL78/G14 (S3 core) parts. The value "rl78" is an alias for "g14" and the value "mg10" is an alias for "none".

In addition a C preprocessor macro is defined, based upon the setting of this option. Possible values are: __RL78_MUL_NONE__, __RL78_MUL_G13__ or __RL78_MUL_G14__. 💜

• -mcpu=g10, -mcpu=g13, -mcpu=g14, -mcpu=rl78 — specifies the RL78 core to target. The default is the G14 core, also known as an S3 core or just RL78. The G13 or S2 core does not have multiply or divide instructions, instead it uses a hardware peripheral for these operations. The G10 or S1 core does not have register banks, so it uses a different calling convention.

If you set this one, it also picks what kind of hardware multiply support you get — unless you go and override it with an explicit -mmul=none on the command line. So basically: saying -mcpu=g13 turns on the G13 hardware multiply peripheral, and saying -mcpu=g10 straight up kills hardware multiplication entirely. 💀

Heads up bestie, even though RL78/G14 is the default target, actually typing -mcpu=g14 or -mcpu=rl78 on the command line does change what the toolchain does, because it switches on G14 hardware multiply support. And if you don't pass those options? Then software multiplication routines get used — even though the code is targeting the RL78 core. Why? Backwards compatibility with older toolchains that didn't have hardware multiply and divide support. Nostalgia, but make it compiler flags. 🙈

Also, a C preprocessor macro gets defined based on whatever you set this option to. The possible values are: __RL78_G10__, __RL78_G13__ or __RL78_G14__.

• -mg10, -mg13, -mg14, -mrl78 — these are just aliases for the matching -mcpu= option. They're here for backwards compatibility, iconic of them. 💅
• -mallregs — lets the compiler use all the available registers. Normally, r24..r31 are reserved for interrupt handlers. Turn this on and those registers become fair game in ordinary functions too. 👑
• -m64bit-doubles, -m32bit-doubles — sets the double data type to be 64 bits (-m64bit-doubles) or 32 bits (-m32bit-doubles). The default? -m32bit-doubles. no cap.
• -msave-mduc-in-interrupts, -mno-save-mduc-in-interrupts — says interrupt handler functions should preserve the MDUC registers. You only actually need this if normal code might be using the MDUC registers — say, because it's doing multiplication and division operations. The default is to ignore the MDUC registers, because that makes the interrupt handlers faster. To make this work you gotta pass the target option -mg13, since this feature only exists on the G13 target (S2 core). And the MDUC registers only get saved if the interrupt handler does a multiplication or division operation, or calls another function. ✨

---

🌈 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 to say which instructions are available on the processor you're using. The default value of these options is decided when GCC gets configured. Specifying -mcpu=cpu_type overrides whatever you set for these. And honestly? We recommend you just use -mcpu=cpu_type instead of the pile of options listed above. 🫶

So, specifying -mpowerpc-gpopt lets GCC use the optional PowerPC architecture instructions in the General Purpose group — floating-point square root included. Specifying -mpowerpc-gfxopt lets GCC use the optional PowerPC architecture instructions in the Graphics group, floating-point select included. slay.

The -mmfcrf option lets GCC generate the move from condition register field instruction, which is implemented on the POWER4 processor and other processors that support the PowerPC V2.01 architecture. The -mpopcntb option lets GCC 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 lets GCC generate the popcount instruction implemented on the POWER7 processor and other processors that support the PowerPC V2.06 architecture. The -mfprnd option lets GCC 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 lets GCC generate the compare bytes instruction implemented on the POWER6 processor and other processors that support the PowerPC V2.05 architecture. The -mhard-dfp option lets GCC generate the decimal floating-point instructions implemented on some POWER processors. 🔥

The -mpowerpc64 option lets GCC generate the extra 64-bit instructions found in the full PowerPC64 architecture and treat GPRs as 64-bit, doubleword quantities. GCC defaults to -mno-powerpc64.

• -mcpu=cpu_type — sets the architecture type, register usage, and instruction scheduling parameters for machine type cpu_type. Supported values for cpu_type are 401, 403, 405, 405fp, 440, 440fp, 464, 464fp, 476, 476fp, 505, 601, 602, 603, 603e, 604, 604e, 620, 630, 740, 7400, 7450, 750, 801, 821, 823, 860, 970, 8540, a2, e300c2, e300c3, e500mc, e500mc64, e5500, e6500, ec603e, G3, G4, G5, titan, power3, power4, power5, power5+, power6, power6x, power7, power8, power9, power10, powerpc, powerpc64, powerpc64le, rs64, and native. that's a whole alphabet bestie 😭

-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 matches the host processor of the system doing the compilation. But -mcpu=native does nothing if GCC doesn't recognize the processor. 🙈

The other options each specify a specific processor. Code generated under those options runs best on that processor, and might not run at all on others. 💀

The -mcpu options automatically enable or disable the following options:

-maltivec  -mfprnd  -mhard-float  -mmfcrf  -mmultiple -mpopcntb
-mpopcntd  -mpowerpc64 -mpowerpc-gpopt  -mpowerpc-gfxopt -mmulhw
-mdlmzb  -mmfpgpr  -mvsx -mcrypto  -mhtm  -mpower8-fusion
-mpower8-vector -mquad-memory  -mquad-memory-atomic  -mfloat128
-mfloat128-hardware -mprefixed -mpcrel -mmma -mrop-protect

The exact options set for any particular CPU varies between compiler versions, depending on what setting seems to produce optimal code for that CPU — and ngl, it doesn't necessarily reflect the actual hardware's capabilities. If you want to set an individual option to a specific 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. The same values for cpu_type are used for -mtune as for -mcpu. If you specify both, the code generated uses the architecture and registers set by -mcpu, but the scheduling parameters set by -mtune.
• -mcmodel=small — generates PowerPC64 code for the small model: the TOC is limited to 64k.
• -mcmodel=medium — generates PowerPC64 code for the medium model: the TOC and other static data may be up to a total of 4G in size. This is the default for 64-bit Linux.
• -mcmodel=large — generates PowerPC64 code for the large model: the TOC may be up to 4G in size. Other data and code is only limited by the 64-bit address space.
• -maltivec, -mno-altivec — generate code that uses (does not use) AltiVec instructions, and also enable the use of built-in functions that allow more direct access to the AltiVec instruction set. You may also need to set -mabi=altivec to adjust the current ABI with AltiVec ABI enhancements. 🦋

When -maltivec is used, 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. Which means: 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. iykyk.

• -mvrsave, -mno-vrsave — generate VRSAVE instructions when generating AltiVec code.
• -msecure-plt — generates code that allows ld and ld.so to build executables and shared libraries with non-executable .plt and .got sections. This is a PowerPC 32-bit SYSV ABI option. 🔒
• -mbss-plt — 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.
• -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 (disable) the use of the built-in functions that allow direct access to the cryptographic instructions that were added in version 2.07 of the PowerPC ISA. 🔐
• -mhtm, -mno-htm — enable (disable) the use of the built-in functions that allow direct access to the Hardware Transactional Memory (HTM) instructions that were added in version 2.07 of the PowerPC ISA.
• -mpower8-fusion, -mno-power8-fusion — generate code that keeps (does not keeps) some integer operations adjacent so that the instructions can be fused together on power8 and later processors. (yes, "does not keeps", that's the manpage's own grammar, we don't fix icons 💅)
• -mpower8-vector, -mno-power8-vector — generate code that uses (does not use) the vector and scalar instructions that were added in version 2.07 of the PowerPC ISA. Also enables the use of built-in functions that allow more direct access to the vector instructions.
• -mquad-memory, -mno-quad-memory — generate code that uses (does not use) the non-atomic quad word memory instructions. And heads up: the -mquad-memory option requires use of 64-bit mode. 🎀
• -mquad-memory-atomic, -mno-quad-memory-atomic — okay bestie 💖 this duo is all about atomic quad word memory instructions. -mquad-memory-atomic makes GCC generate code that uses them, and -mno-quad-memory-atomic says nope, not doing that and does not use them. Heads up: -mquad-memory-atomic requires use of 64-bit mode. No cap. ✨
• -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 (-mpower9-vector or -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 — modify generation of the TOC (Table Of Contents), which is created for every executable file. The -mfull-toc option is selected by default. In that case, GCC allocates at least one TOC entry for each unique non-automatic variable reference in your program. GCC also places floating-point constants in the TOC. However, only 16,384 entries are available in the TOC. If you receive a linker error message that saying you have overflowed the available TOC space, you can reduce the amount of TOC space used with the -mno-fp-in-toc and -mno-sum-in-toc options. -mno-fp-in-toc prevents GCC from putting floating-point constants in the TOC and -mno-sum-in-toc forces GCC to generate code to calculate the sum of an address and a constant at run time instead of putting that sum into the TOC. You may specify one or both of these options. Each causes GCC to produce very slightly slower and larger code at the expense of conserving TOC space. If you still run out of space in the TOC even when you specify both of these options, specify -mminimal-toc instead. This option causes GCC to make only one TOC entry for every file. When you specify this option, GCC produces code that is slower and larger but which uses extremely little TOC space. You may wish to use this option only on files that contain less frequently-executed code. 🫶
• -maix64, -maix32 — enable 64-bit AIX ABI and calling convention: 64-bit pointers, 64-bit "long" type, and the infrastructure needed to support them. Specifying -maix64 implies -mpowerpc64, while -maix32 disables the 64-bit ABI and implies -mno-powerpc64. GCC defaults to -maix32. 👑
• -mxl-compat, -mno-xl-compat — produce code that conforms more closely to IBM XL compiler semantics when using AIX-compatible ABI. Pass floating-point arguments to prototyped functions beyond the register save area (RSA) on the stack in addition to argument FPRs. Do not assume that most significant double in 128-bit long double value is properly rounded when comparing values and converting to double. Use XL symbol names for long double support routines. The AIX calling convention was extended but not initially documented to handle an obscure K&R C case of calling a function that takes the address of its arguments with fewer arguments than declared. IBM XL compilers access floating-point arguments that do not fit in the RSA from the stack when a subroutine is compiled without optimization. Because always storing floating-point arguments on the stack is inefficient and rarely needed, this option is not enabled by default and only is necessary when calling subroutines compiled by IBM XL compilers without optimization. 😭
• -mpe — support IBM RS/6000 SP Parallel Environment (PE). Link an application written to use message passing with special startup code to enable the application to run. The system must have PE installed in the standard location (/usr/lpp/ppe.poe/), or the specs file must be overridden with the -specs= option to specify the appropriate directory location. The Parallel Environment does not support threads, so the -mpe option and the -pthread option are incompatible. 💀
• -malign-natural, -malign-power — on AIX, 32-bit Darwin, and 64-bit PowerPC GNU/Linux, the option -malign-natural overrides the ABI-defined alignment of larger types, such as floating-point doubles, on their natural size-based boundary. The option -malign-power instructs GCC to follow the ABI-specified alignment rules. GCC defaults to the standard alignment defined in the ABI. On 64-bit Darwin, natural alignment is the default, and -malign-power is not supported. 🦋
• -msoft-float, -mhard-float — generate code that does not use (uses) the floating-point register set. Software floating-point emulation is provided if you use the -msoft-float option, and pass the option to GCC when linking. 🌸
• -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. 💀
• -mavoid-indexed-addresses, -mno-avoid-indexed-addresses — generate code that tries to avoid (not avoid) the use of indexed load or store instructions. These instructions can incur a performance penalty on Power6 processors in certain situations, such as when stepping through large arrays that cross a 16M boundary. This option is enabled by default when targeting Power6 and disabled otherwise. 🔥
• -mfused-madd, -mno-fused-madd — generate code that uses (does not use) the floating-point multiply and accumulate instructions. These instructions are generated by default if hardware floating point is used. The machine-dependent -mfused-madd option is now mapped to the machine-independent -ffp-contract=fast option, and -mno-fused-madd is mapped to -ffp-contract=off. ✨
• -mmulhw, -mno-mulhw — generate code that uses (does not use) the half-word multiply and multiply-accumulate instructions on the IBM 405, 440, 464 and 476 processors. These instructions are generated by default when targeting those processors. 💅
• -mdlmzb, -mno-dlmzb — generate code that uses (does not use) the string-search dlmzb instruction on the IBM 405, 440, 464 and 476 processors. This instruction is generated by default when targeting those processors. 🍓
• -mno-bit-align, -mbit-align — on System V.4 and embedded PowerPC systems do not (do) force structures and unions that contain bit-fields to be aligned to the base type of the bit-field. For example, by default a structure containing nothing but 8 "unsigned" bit-fields of length 1 is aligned to a 4-byte boundary and has a size of 4 bytes. By using -mno-bit-align, the structure is aligned to a 1-byte boundary and is 1 byte in size. 🎀
• -mno-strict-align, -mstrict-align — on System V.4 and embedded PowerPC systems do not (do) assume that unaligned memory references are handled by the system. 🌈
• -mrelocatable, -mno-relocatable — generate code that allows (does not allow) a static executable to be relocated to a different address at run time. A simple embedded PowerPC system loader should relocate the entire contents of ".got2" and 4-byte locations listed in the ".fixup" section, a table of 32-bit addresses generated by this option. For this to work, all objects linked together must be compiled with -mrelocatable or -mrelocatable-lib. -mrelocatable code aligns the stack to an 8-byte boundary. 👑
• -mrelocatable-lib, -mno-relocatable-lib — like -mrelocatable, -mrelocatable-lib generates a ".fixup" section to allow static executables to be relocated at run time, but -mrelocatable-lib does not use the smaller stack alignment of -mrelocatable. Objects compiled with -mrelocatable-lib may be linked with objects compiled with any combination of the -mrelocatable options. 🧚
• -mno-toc, -mtoc — on System V.4 and embedded PowerPC systems do not (do) assume that register 2 contains a pointer to a global area pointing to the addresses used in the program. 💜
• -mlittle, -mlittle-endian — on System V.4 and embedded PowerPC systems compile code for the processor in little-endian mode. The -mlittle-endian option is the same as -mlittle. 🌸
• -mbig, -mbig-endian — on System V.4 and embedded PowerPC systems compile code for the processor in big-endian mode. The -mbig-endian option is the same as -mbig. 🦋
• -mdynamic-no-pic — on Darwin and Mac OS X systems, compile code so that it is not relocatable, but that its external references are relocatable. The resulting code is suitable for applications, but not shared libraries. 🫶
• -msingle-pic-base — okay so for PIC addressing, this makes the register it uses read-only instead of loading it fresh in the prologue of every single function. ✨ Your runtime system has to be the responsible one here and initialize that register with a good value before execution even starts. no cap.
• -mprioritize-restricted-insns=priority — this one controls the priority given to dispatch-slot restricted instructions during the second scheduling pass. priority can be 0, 1, or 2, which give no, highest, or second-highest priority (respectively) to those dispatch-slot restricted instructions. 💅
• -msched-costly-dep=dependence_type — controls which dependences the target treats as costly during instruction scheduling. dependence_type takes one of these:
  • no — no dependence is costly. 🙅
  • all — every dependence is 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 whose latency is greater than or equal to number is costly.
• -minsert-sched-nops=scheme — picks which NOP insertion scheme gets used during the second scheduling pass. scheme takes one of these:
  • 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, using exactly as many NOPs as needed to force an insn into a new group, according to the estimated processor grouping.
  • number — insert NOPs to force costly dependent insns into separate groups, using number NOPs to force an insn into a new group.
• -mcall-sysv — on System V.4 and embedded PowerPC systems, compile code using calling conventions that follow the March 1995 draft of the System V Application Binary Interface, PowerPC processor supplement. This is the default unless you configured GCC with powerpc-*-eabiaix. 👑
• -mcall-sysv-eabi
• -mcall-eabi — specify both -mcall-sysv and -meabi. 🫶
• -mcall-sysv-noeabi — specify both -mcall-sysv and -mno-eabi.
• -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 — pick 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 an 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. Bestie, this is not likely to work if your system defaults to using IEEE extended-precision long double. 😭 If you change the long double type away from IEEE extended-precision, the compiler will throw 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. Also not likely to work if your system defaults to IBM extended-precision long double. If you change the long double type away from IBM extended-precision, the compiler warns you unless you use -Wno-psabi. 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 needs special system support and is likely to fail in spectacular ways. iconic, honestly.
• -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 also needs special system support and is likewise likely to fail in spectacular ways. 💀
• -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 all calls to variable argument functions are properly prototyped. Otherwise, the compiler has to insert an instruction before every non-prototyped call to set or clear bit 6 of the condition code register ("CR") to indicate whether floating-point values are passed in the floating-point registers, in case the function takes variable arguments. With -mprototype, only calls to prototyped variable argument functions set or clear the bit. 🎀
• -msim — on embedded PowerPC systems, assume 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 the startup module is called crt0.o and the standard C libraries are libmvme.a and libc.a.
• -mads — on embedded PowerPC systems, assume the startup module is called crt0.o and the standard C libraries are libads.a and libc.a.
• -myellowknife — on embedded PowerPC systems, assume the startup module is called crt0.o and the standard C libraries are libyk.a and libc.a. 🔪✨
• -mvxworks — on System V.4 and embedded PowerPC systems, specify that you're compiling for a VxWorks system.
• -memb — on embedded PowerPC systems, set the "PPC_EMB" bit in the ELF flags header to say that eabi extended relocations are used.
• -meabi
• -mno-eabi — on System V.4 and embedded PowerPC systems, do (or do not) adhere to the Embedded Applications Binary Interface (EABI), which is a set of modifications to the System V.4 specs. Picking -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. Picking -mno-eabi means the stack is aligned to a 16-byte boundary, no EABI initialization function is called from "main", and -msdata only uses "r13" to point to a single small data area. -meabi is on by default if you configured GCC using one of the powerpc*-*-eabi* options.
• -msdata=eabi — on System V.4 and embedded PowerPC systems, put small initialized "const" global and static data in the .sdata2 section, which register "r2" points to. Put small initialized non-"const" global and static data in the .sdata section, which register "r13" points to. Put small uninitialized global and static data in the .sbss section, which sits right next to the .sdata section. 🫶 Heads up: -msdata=eabi is incompatible with the -mrelocatable option. Also, -msdata=eabi sets the -memb option.
• -msdata=sysv — on System V.4 and embedded PowerPC systems, put small global and static data in the .sdata section, which register "r13" points to. Put small uninitialized global and static data in the .sbss section, adjacent to .sdata. And ngl, -msdata=sysv is incompatible with -mrelocatable too.
• -msdata=default
• -msdata — on System V.4 and embedded PowerPC systems, if -meabi is used, compile code the same as -msdata=eabi; otherwise compile the same as -msdata=sysv.
• -msdata=data — on System V.4 and embedded PowerPC systems, put small global data in the .sdata section and small uninitialized global data in the .sbss section. But do not use register "r13" to address small data. This is the default behavior unless other -msdata options are used.
• -msdata=none
• -mno-sdata — on embedded PowerPC systems, put all initialized global and static data in the .data section, and all uninitialized data in the .bss section.
• -mreadonly-in-sdata — put read-only objects in the .sdata section as well. This is the default. ✨
• -mblock-move-inline-limit=num — inline all block moves (like calls to "memcpy" or structure copies) that are less than or equal to num bytes. The minimum value for num is 32 bytes on 32-bit targets and 64 bytes on 64-bit targets. The default value is target-specific. 💅
• -mblock-compare-inline-limit=num — generate non-looping inline code for all block compares (like calls to "memcmp" or structure compares) that are less than or equal to num bytes. If num is 0, all inline expansion (non-loop and loop) of block compare is disabled. The default value is target-specific.
• -mblock-compare-inline-loop-limit=num — generate an inline expansion using loop code for all block compares that are less than or equal to num bytes, but greater than the limit for non-loop inline block compare expansion. If the block length isn't constant, at most num bytes get compared before "memcmp" is called to compare the remainder of the block. The default value is target-specific. 🦋
• -mstring-compare-inline-limit=num — compare at most num string bytes with inline code. If the difference or end of string isn't found by the end of the inline compare, a call to "strcmp" or "strncmp" handles the rest of the comparison. The default is 64 bytes.
• -G num — on embedded PowerPC systems, put global and static items that are less than or equal to num bytes into the small data or BSS sections instead of the normal data or BSS section. By default, num is 8. The -G num switch also gets passed to the linker, so all modules should be compiled with the same -G num value. periodt. 👑
• -mregnames, -mno-regnames — on System V.4 and embedded PowerPC systems, do (or don't) spit out register names in the assembly output using symbolic forms. cute little readable asm moment 💅
• -mlongcall, -mno-longcall — bestie, by default GCC assumes every call is far away, so it busts out a longer and way more expensive calling sequence. This is needed for calls more than 32 megabytes (33,554,432 bytes) away from where you're standing rn. If the compiler knows the call can't possibly be that far, it'll do a short call instead. You can override this whole vibe with the "shortcall" function attribute, or with #pragma longcall(0).

Some linkers can spot out-of-range calls and generate glue code on the fly. On those systems long calls are just unnecessary and make slower code — not the flex you think it is. As of this writing, the AIX linker can do this, and so can the GNU linker for PowerPC/64. There's a plan 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. nope 🙅

On Darwin/PPC systems, #pragma longcall generates "jbsr callee, L42", plus a branch island (glue code). Those two target addresses represent the callee and the branch island. The Darwin/PPC linker prefers the first address and generates a "bl callee" if the PPC "bl" instruction reaches the callee directly; otherwise the linker generates "bl L42" to call the branch island. The branch island gets appended to the body of the calling function; it computes the full 32-bit address of the callee and jumps to it. iconique 💫

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 yeet it.

In the future, GCC may just ignore all longcall specifications when the linker is known to generate glue. so stay tuned ig

• -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. Heads up: -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 a function call with the argument setup instructions for TLS optimization, which in turn lets GCC schedule the sequence better. teamwork makes the dream work 🫶
• -mrecip, -mno-recip — this enables the reciprocal estimate and reciprocal square root estimate instructions, with extra Newton-Raphson steps to bump up the precision, instead of doing a divide or a square root and divide for floating-point arguments. You should use -ffast-math when you use -mrecip (or at least -funsafe-math-optimizations, -ffinite-math-only, -freciprocal-math and -fno-trapping-math). Note that while the throughput of this sequence is generally 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) for reciprocal square roots. so like… fast but slightly wrong, you know? 😭
• -mrecip=opt — this one controls which reciprocal estimate instructions are allowed. opt is a comma-separated list of options, and any of them can be preceded by a "!" to invert it:
  • all — enable all estimate instructions. go big or go home 💖
  • 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. see what they did there ✨

• -mrecip-precision, -mno-recip-precision — assume (or don't assume) that the reciprocal estimate instructions provide higher-precision estimates than the PowerPC ABI demands. 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. lowkey not it 🙈
• -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. The MASS libraries must be specified at link time. that's a lot of names bestie, they ate tho 💅
• -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 integer and back to floating point. The "friz" instruction does not return the same value if the floating-point number is too large to fit in an integer. so, y'know, caveat emptor 🫠
• -mpointers-to-nested-functions, -mno-pointers-to-nested-functions — generate (or don't generate) code to load up the static chain register ("r11") when calling through a pointer on AIX and 64-bit Linux systems where a function pointer points to a 3-word descriptor giving the function address, the TOC value to be loaded in register "r2", and the static chain value to be loaded in register "r11". The -mpointers-to-nested-functions is on by default. You cannot call through pointers to nested functions, or pointers to functions compiled in other languages that use the static chain, if you use -mno-pointers-to-nested-functions. no cap 🚫
• -msave-toc-indirect, -mno-save-toc-indirect — generate (or don't generate) code to save the TOC value in the reserved stack location in the function prologue if the function calls through a pointer on AIX and 64-bit Linux systems. If the TOC value isn't saved in the prologue, it gets saved right before the call through the pointer. The -mno-save-toc-indirect option is the default.
• -mcompat-align-parm, -mno-compat-align-parm — generate (or don't generate) code to pass structure parameters with a maximum alignment of 64 bits, for compatibility with older versions of GCC.

Older versions of GCC (prior to 4.9.0) incorrectly did not align a structure parameter on a 128-bit boundary when that structure contained a member requiring 128-bit alignment. This is corrected in more recent versions of GCC. This option may be used to generate code that's compatible with functions compiled with older versions of GCC. nostalgia core 🥲

The -mno-compat-align-parm option is the default.

• -mstack-protector-guard=guard
• -mstack-protector-guard-reg=reg
• -mstack-protector-guard-offset=offset
• -mstack-protector-guard-symbol=symbol — generate stack protection code using a canary at guard. Supported locations are global for a global canary, or tls for a per-thread canary in the TLS block (the default with GNU libc version 2.4 or later). 🐤

With that latter choice, the options -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset furthermore specify which register to use as the base register for reading the canary, and from what offset from that base register. The default for those is as specified in the relevant ABI. And -mstack-protector-guard-symbol=symbol overrides the offset with a symbol reference to a canary in the TLS block.

• -mpcrel, -mno-pcrel — generate (or don't generate) pc-relative addressing. The -mpcrel option requires that the medium code model (-mcmodel=medium) and prefixed addressing (-mprefixed) options are enabled. package deal bestie 📦
• -mprefixed, -mno-prefixed — generate (or don'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 — generate (or don't generate) the MMA instructions. The -mma option requires that the option -mcpu=power10 (or later) is enabled.
• -mrop-protect, -mno-rop-protect — generate (or don't generate) ROP protection instructions when the target processor supports them. Currently this option disables the shrink-wrap optimization (-fshrink-wrap). security era 🔥
• -mprivileged, -mno-privileged — generate (or don't generate) code that will run in privileged state. very main character of it 👑
• -mblock-ops-unaligned-vsx, -mno-block-ops-unaligned-vsx — generate (or don't generate) unaligned vsx loads and stores for inline expansion of "memcpy" and "memmove".

---

🌸 RX Options 🌸

These command-line options are defined for RX targets:

• -m64bit-doubles / -m32bit-doubles — bestie, this is literally just "how big is a double?" 💖 -m64bit-doubles makes the double data type 64 bits, -m32bit-doubles makes it 32 bits. The default is -m32bit-doubles. And lowkey there's a reason: RX floating-point hardware only works on 32-bit values, which is exactly why the default is -m32bit-doubles. no cap.
• -fpu / -nofpu — turns the use of RX floating-point hardware ON (-fpu) or OFF (-nofpu). ✨ Default is enabled for the RX600 series and disabled for the RX200 series. That said, floating-point instructions are only generated for 32-bit floating-point values, so the FPU hardware is just… not used for doubles if you went and used the -m64bit-doubles option. 🙈

Note: if the -fpu option is enabled then -funsafe-math-optimizations is also enabled automatically. Why? Because the RX FPU instructions are themselves unsafe. it's giving risky behavior fr.

• -mcpu=name — picks which type of RX CPU you're targeting. Currently three types are supported: the generic RX600 and RX200 series hardware, and the specific RX610 CPU. Default is RX600. 🎀 The only difference between RX600 and RX610 is that the RX610 does not support the MVTIPL instruction. And the RX200 series doesn't have a hardware floating-point unit at all, so -nofpu is enabled by default when this type is selected.
• -mbig-endian-data / -mlittle-endian-data — store data (but not code, bestie, read it again 👀) in the big-endian format. Default is -mlittle-endian-data, i.e. storing data in the little-endian format.
• -msmall-data-limit=N — sets the maximum size in bytes of global and static variables that can go into the small data area. Using the small data area can lead to smaller and faster code 🔥 but the size of the area is limited, and it is up to the programmer to make sure the area does not overflow. Also, when the small data area is used, one of the RX's registers (usually r13) is reserved for pointing to this area, so it's no longer available for use by the compiler. That could result in slower and/or larger code if variables get pushed onto the stack instead of being held in that register. 💀

Note: common variables (variables that have not been initialized) and constants are not placed into the small data area, as they are assigned to other sections in the output executable.

The default value is zero, which disables this feature. Note, this feature is not enabled by default with higher optimization levels (-O2 etc) because of the potentially detrimental effects of reserving a register. It is up to the programmer to experiment and discover whether this feature is of benefit to their program. See the description of the -mpid option for a description of how the actual register to hold the small data area pointer is chosen. 🫶

• -msim / -mno-sim — use the simulator runtime. The default is to use the libgloss board-specific runtime.
• -mas100-syntax / -mno-as100-syntax — when generating assembler output, use a syntax that's compatible with Renesas's AS100 assembler. This syntax can also be handled by the GAS assembler, but it has some restrictions, so it is not generated by default. 💅
• -mmax-constant-size=N — sets the maximum size, in bytes, of a constant that can be used as an operand in an 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. So in some circumstances it can be beneficial to restrict the size of constants that are used in instructions. Constants that are too big are instead placed into a constant pool and referenced via register indirection. The value N can be between 0 and 4. A value of 0 (the default) or 4 means that constants of any size are allowed. ⭐
• -mrelax — enable linker relaxation. Linker relaxation is a process whereby the linker attempts to reduce the size of a program by finding shorter versions of various instructions. Disabled by default. ✨
• -mint-register=N — specify the number of registers to reserve for fast interrupt handler functions. The value N can be between 0 and 4. A value of 1 means that register r13 is reserved for the exclusive use of fast interrupt handlers. A value of 2 reserves r13 and r12. A value of 3 reserves r13, r12 and r11, and a value of 4 reserves r13 through r10. A value of 0, the default, does not reserve any registers. 🎀
• -msave-acc-in-interrupts — specifies that interrupt handler functions should preserve the accumulator register. This is only necessary if normal code might use the accumulator register, for example because it performs 64-bit multiplications. The default is to ignore the accumulator, as this makes the interrupt handlers faster. 🔥
• -mpid / -mno-pid — enables the generation of position independent data. When enabled, any access to constant data is done via an offset from a base address held in a register. This allows the location of constant data to be determined at run time without requiring the executable to be relocated, which is a benefit to embedded applications with tight memory constraints. Data that can be modified is not affected by this option. 🦋

Note, using this feature reserves a register, usually r13, for the constant data base address. This can result in slower and/or larger code, especially in complicated functions. 😭

The actual register chosen to hold the constant data base address depends upon whether the -msmall-data-limit and/or the -mint-register command-line options are enabled. Starting with register r13 and proceeding downwards, registers are allocated first to satisfy the requirements of -mint-register, then -mpid and finally -msmall-data-limit. Thus it is possible for the small data area register to be r8 if both -mint-register=4 and -mpid are specified on the command line. 🤯

By default this feature is not enabled. The default can be restored via the -mno-pid command-line option.

• -mno-warn-multiple-fast-interrupts / -mwarn-multiple-fast-interrupts — prevents GCC from issuing a warning message if it finds more than one fast interrupt handler when it is compiling a file. The default is to issue a warning for each extra fast interrupt handler found, as the RX only supports one such interrupt. 👑
• -mallow-string-insns / -mno-allow-string-insns — enables or disables the use of the string manipulation instructions SMOVF, SCMPU, SMOVB, SMOVU, SUNTIL, SWHILE and also the RMPA instruction. These instructions may prefetch data, which is not safe to do if accessing an I/O register. (See section 12.2.7 of the RX62N Group User's Manual for more information.) 🚨

The default is to allow these instructions, but it is not possible for GCC to reliably detect all circumstances where a string instruction might be used to access an I/O register, so their use cannot be disabled automatically. Instead it is reliant upon the programmer to use the -mno-allow-string-insns option if their program accesses I/O space.

When the instructions are enabled GCC defines the C preprocessor symbol __RX_ALLOW_STRING_INSNS__, otherwise it defines the symbol __RX_DISALLOW_STRING_INSNS__.

• -mjsr / -mno-jsr — use only (or not only) JSR instructions to access functions. This option can be used when code size exceeds the range of BSR instructions. Note that -mno-jsr does not mean to not use JSR but instead means that 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. This attribute indicates a function intended to process fast interrupts. GCC ensures that it only uses the registers r10, r11, r12 and/or r13 and only provided that the normal use of the corresponding registers have been restricted via the -ffixed-reg or -mint-register command-line options. 💖

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🌈 S/390 and zSeries Options 🌈

These are the -m options defined for the S/390 and zSeries architecture. let's get into it 💅

• -mhard-float / -msoft-float — use (do not use) the hardware floating-point instructions and registers for floating-point operations. When -msoft-float is specified, functions in libgcc.a are used to perform floating-point operations. When -mhard-float is specified, the compiler generates IEEE floating-point instructions. This is the default. ✨
• -mhard-dfp / -mno-hard-dfp — use (do not use) the hardware decimal-floating-point instructions for decimal-floating-point operations. When -mno-hard-dfp is specified, functions in libgcc.a are used to perform decimal-floating-point operations. When -mhard-dfp is specified, the compiler generates decimal-floating-point hardware instructions. This is the default for -march=z9-ec or higher. 🔥
• -mlong-double-64 / -mlong-double-128 — these switches control the size of the long double type. A size of 64 bits makes the long double type equivalent to the double type. This is the default. 🎀
• -mbackchain / -mno-backchain — store (do not store) the address of the caller's frame as backchain pointer into the callee's stack frame. A backchain may be needed to allow debugging using tools that do not understand DWARF call frame information. When -mno-packed-stack is in effect, the backchain pointer is stored at the bottom of the stack frame; when -mpacked-stack is in effect, the backchain is placed into the topmost word of the 96/160 byte register save area. 🦋

In general, code compiled with -mbackchain is call-compatible with code compiled with -mno-backchain; however, use of the backchain for debugging purposes usually requires that the whole binary is built with -mbackchain. Note that the combination of -mbackchain, -mpacked-stack and -mhard-float is not supported. In order to build a linux kernel use -msoft-float. 🫶

The default is to not maintain the backchain.

• -mpacked-stack, -mno-packed-stack — use (or don't use 💅) the packed stack layout. With -mno-packed-stack, the compiler uses all the fields of the 96/160 byte register save area only for their default purpose — and yeah, unused fields still eat up stack space, no escaping that. With -mpacked-stack, register save slots get densely packed at the top of the register save area, unused space gets reused for other stuff, and you get way more efficient use of the stack space you've got. BUT bestie, listen: 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. That's just how it is.

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 🫶. Lowkey important note though: some non-FSF releases of GCC 2.95 for S/390 or zSeries generated code that uses the stack frame backchain at run time, not just for debugging purposes. That kind of code is not call-compatible with code compiled with -mpacked-stack. Also — and this is the part where you go 😭 — 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) code using the "bras" instruction to do subroutine calls. This only works reliably if the total executable size does not exceed 64k, so it's giving "small and cute only". The default is to use the "basr" instruction instead, which does not have this limitation ✨.
• -m64, -m31 — when -m31 is specified, generate code compliant to the GNU/Linux for S/390 ABI. When -m64 is specified, generate code compliant to the GNU/Linux for zSeries ABI. This 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 — when -mzarch is specified, generate code using the instructions available on z/Architecture. When -mesa is specified, generate code using the instructions available on ESA/390. Note that -mesa is not possible with -m64 🚫. When generating code compliant to the GNU/Linux for S/390 ABI, the default is -mesa. When generating code compliant to the GNU/Linux for zSeries ABI, the default is -mzarch.
• -mhtm, -mno-htm — the -mhtm option 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 — when -mvx is specified, 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 will be added to mark the resulting binary with the ABI used. -mvx is enabled by default when using -march=z13.
• -mzvector, -mno-zvector — the -mzvector option 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 do not generate) code using the "mvcle" instruction to perform block moves. When -mno-mvcle is specified, use a "mvc" loop instead. This is the default unless optimizing for size.
• -mdebug, -mno-debug — print (or do not print) additional debug information when compiling. The default is to not print debug information 🙈.
• *-march=cpu-type* — generate code that runs on cpu-type, which is the name of a system representing a certain processor type. Possible values for cpu-type are z900/arch5, z990/arch6, z9-109, z9-ec/arch7, z10/arch8, z196/arch9, zEC12, z13/arch11, z14/arch12, z15/arch13, z16/arch14, and native.

The default is -march=z900.

Specifying native as cpu type can be used to select the best architecture option for the host processor. -march=native has no effect if GCC does not recognize the processor.

• *-mtune=cpu-type* — tune to cpu-type everything applicable about the generated code, except for the ABI and the set of available instructions. The list of cpu-type values is the same as for -march. The default is the value used for -march.
• -mtpf-trace, -mno-tpf-trace — generate code that adds (does not add) in TPF OS specific branches to trace routines in the operating system. This option is off by default, even when compiling for the TPF OS.
• -mtpf-trace-skip, -mno-tpf-trace-skip — generate code that changes (does not change) the default branch targets enabled by -mtpf-trace to point to specialized trace routines providing the ability of selectively skipping function trace entries for the TPF OS. This option is off by default, even when compiling for the TPF OS and specifying -mtpf-trace.
• -mfused-madd, -mno-fused-madd — generate code that uses (does not use) the floating-point multiply and accumulate instructions. These instructions are generated by default if hardware floating point is used.
• *-mwarn-framesize=framesize* — emit a warning if the current function exceeds the given frame size. Because this is a compile-time check it doesn't need to be a real problem when the program runs. It is intended to identify functions that most probably cause a stack overflow 🫠. It is useful to be used in an environment with limited stack size e.g. the linux kernel.
• -mwarn-dynamicstack — emit a warning if the function calls "alloca" or uses dynamically-sized arrays. This is generally a bad idea with a limited stack size, fr fr.
• -mstack-guard=stack-guard, -mstack-size=stack-size — if these options are provided the S/390 back end emits additional instructions in the function prologue that trigger a trap if the stack size is stack-guard bytes above the stack-size (remember that the stack on S/390 grows *downward ⬇️). If the *stack-guard option is omitted the smallest power of 2 larger than the frame size of the compiled function is chosen. These options are intended to be used to help debugging stack overflow problems. The additionally emitted code causes only little overhead and hence can also be used in production-like systems without greater performance degradation. The given values have to be exact powers of 2 and stack-size has to be greater than stack-guard without exceeding 64k. In order to be efficient the extra code makes the assumption that the stack starts at an address aligned to the value given by stack-size. The stack-guard option can only be used in conjunction with stack-size.
• *-mhotpatch=pre-halfwords,post-halfwords* — if the hotpatch option is enabled, a "hot-patching" function prologue is generated for all functions in the compilation unit. The funtion label is prepended with the given number of two-byte NOP instructions (pre-halfwords, maximum 1000000). After the label, 2 * post-halfwords bytes are appended, using the largest NOP like instructions the architecture allows (maximum 1000000).

If both arguments are zero, hotpatching is disabled.

This option can be overridden for individual functions with the "hotpatch" attribute.

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🌸 Score Options 🌸

These options are defined for Score implementations:

• -meb — compile code for big-endian mode. This is the default.
• -mel — compile code for little-endian mode.
• -mnhwloop — disable generation of "bcnz" instructions.
• -muls — enable generation of unaligned load and store instructions.
• -mmac — enable the use of multiply-accumulate instructions. Disabled by default.
• -mscore5 — specify the SCORE5 as the target architecture.
• -mscore5u — specify the SCORE5U of the target architecture.
• -mscore7 — specify the SCORE7 as the target architecture. This is the default.
• -mscore7d — specify the SCORE7D as the target architecture.

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🦋 SH Options 🦋

These -m options are defined for the SH implementations:

• -m1 — generate code for the SH1.
• -m2 — generate code for the SH2.
• -m2e — generate code for the SH2e.
• -m2a-nofpu — generate code for the SH2a without FPU, or for a SH2a-FPU in such a way that the floating-point unit is not used.
• -m2a-single-only — generate code for the SH2a-FPU, in such a way that no double-precision floating-point operations are used.
• -m2a-single — generate code for the SH2a-FPU assuming the floating-point unit is in single-precision mode by default.
• -m2a — generate code for the SH2a-FPU assuming the floating-point unit is in double-precision mode by default.
• -m3 — generate code for the SH3.
• -m3e — generate code for the SH3e.
• -m4-nofpu — generate code for the SH4 without a floating-point unit.
• -m4-single-only — generate code for the SH4 with a floating-point unit that only supports single-precision arithmetic.
• -m4-single — generate code for the SH4 assuming the floating-point unit is in single-precision mode by default.
• -m4 — generate code for the SH4.
• -m4-100 — generate code for SH4-100.
• -m4-100-nofpu — generate code for SH4-100 in such a way that the floating-point unit is not used.
• -m4-100-single — generate code for SH4-100 assuming the floating-point unit is in single-precision mode by default.
• -m4-100-single-only — generate code for SH4-100 in such a way that no double-precision floating-point operations are used.
• -m4-200 — generate code for SH4-200.
• -m4-200-nofpu — Bestie, this one makes code for the SH4-200 that just straight up does not use the floating-point unit. No FPU vibes at all. 💅
• -m4-200-single — Generate code for SH4-200 assuming the FPU is in single-precision mode by default. So like, it's giving single precision energy. ✨
• -m4-200-single-only — Code for SH4-200 where no double-precision floating-point operations are used. Double precision? Never heard of her. 🙈
• -m4-300 — Generate code for SH4-300. That's it. Simple. 🎀
• -m4-300-nofpu — SH4-300 but the FPU is not used. It's giving no-FPU realness. 💀
• -m4-300-single — Generate code for SH4-300 in such a way that no double-precision floating-point operations are used. Same as the next one, fr fr. ✨
• -m4-300-single-only — Okay so this is exactly like -m4-300-single up there. No double-precision floating-point operations. That's the vibe. 💅
• -m4-340 — Code for SH4-340. And just so you know: no MMU, no FPU. It's giving bare-metal energy. 🔥
• -m4-500 — SH4-500 code (no FPU). And it passes -isa=sh4-nofpu to the assembler. Slay. 💅
• -m4a-nofpu — For the SH4al-dsp, or a SH4a where the floating-point unit is not used. No floating-point unit? We don't know her. 🙈
• -m4a-single-only — SH4a code, but no double-precision floating-point operations. Single precision only, bestie. ✨
• -m4a-single — SH4a assuming the FPU is in single-precision mode by default. So it's giving single precision by default. 💖
• -m4a — Just generate code for the SH4a. Simple. 🎀
• -m4al — Same as -m4a-nofpu, but it implicitly passes -dsp to the assembler. And heads up: GCC doesn't generate any DSP instructions at the moment. So like, it's giving DSP but not really. 😭
• -mb — Compile code for the processor in big-endian mode. Big-endian energy. 💅
• -ml — Compile code for the processor in little-endian mode. Little-endian vibes. ✨
• -mdalign — Align doubles at 64-bit boundaries. But bestie, listen: this changes the calling conventions, so some functions from the standard C library will NOT work unless you recompile it first with -mdalign. It's giving compatibility issues. 💀
• -mrelax — Shorten some address references at link time when possible. It uses the linker option -relax. Slay. 💅
• -mbigtable — Use 32-bit offsets in switch tables. The default is to use 16-bit offsets. So this is the big table energy. 🔥
• -mbitops — Enable bit manipulation instructions on SH2A. Bit ops, periodt. 💅
• -mfmovd — Enable the use of the instruction fmovd. And check -mdalign for alignment constraints. Don't forget. 🙈
• -mrenesas — Comply with the calling conventions defined by Renesas. Renesas said so. 💅
• -mno-renesas — Comply with the calling conventions defined for GCC before the Renesas conventions were available. This is the default for all targets of the SH toolchain. So it's the OG vibe. ✨
• -mnomacsave — Mark the MAC register as call-clobbered, even if -mrenesas is given. So it's like, not saving MAC. 💀
• -mieee, -mno-ieee — Control the IEEE compliance of floating-point comparisons. This affects how cases where the result of a comparison is unordered are handled. By default, -mieee is implicitly enabled. If -ffinite-math-only is enabled, -mno-ieee is implicitly set, which gives faster floating-point greater-equal and less-equal comparisons. You can override the implicit settings by specifying either -mieee or -mno-ieee. It's giving IEEE realness. 💅
• -minline-ic_invalidate — Inline code to invalidate instruction cache entries after setting up nested function trampolines. But heads up: this option has no effect if -musermode is in effect and the selected code generation option (e.g. -m4) does not allow the use of the icbi instruction. If the selected code generation option does not allow the use of the icbi instruction, and -musermode is not in effect, the inlined code manipulates the instruction cache address array directly with an associative write. This not only requires privileged mode at run time, but it also fails if the cache line had been mapped via the TLB and has become unmapped. So like, be careful. 💀
• -misize — Dump instruction size and location in the assembly code. For when you need to see the receipts. 🔍
• -mpadstruct — This option is deprecated. It pads structures to multiple of 4 bytes, which is incompatible with the SH ABI. So like, don't use it. It's giving deprecated energy. 🚫
• -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*. So like, no compiler-generated atomics. 💅
  • 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. So like, gUSA software atomic sequences. 💖
  • 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. So like, thread control block atomic sequences. ✨
  • 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*. So like, disable interrupts. 💀
  • 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. So like, hardware atomic sequences. 🔥
  • 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. So like, offset in bytes. 📏
  • 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. So like, no mixing. Periodt. 💅
• -mtas — Generate the tas.b opcode for __atomic_test_and_set. But heads up: depending on the particular hardware and software configuration this can degrade overall performance due to the operand cache line flushes that are implied by the tas.b instruction. On multi-core SH4A processors the tas.b instruction must be used with caution since it can result in data corruption for certain cache configurations. So like, be careful. 💀
• -mprefergot — When generating position-independent code, emit function calls using the Global Offset Table instead of the Procedure Linkage Table. GOT > PLT, apparently. 💅
• -musermode, -mno-usermode — Don't allow (or allow) the compiler to generate privileged mode code. Specifying -musermode also implies -mno-inline-ic_invalidate if the inlined code would not work in user mode. -musermode is the default when the target is sh*-*-linux*. If the target is SH1* or SH2*, -musermode has no effect, since there is no user mode. So like, user mode vibes. ✨
• -multcost=number — Set the cost to assume for a multiply insn. So like, how expensive is a multiply? 💸
• -mdiv=strategy — Set the division strategy to be used for integer division operations. strategy can be one of:
  • call-div1 — Calls a library function that uses the single-step division instruction div1 to perform the operation. Division by zero calculates an unspecified result and does not trap. This is the default except for SH4, SH2A and SHcompact. So like, call-div1. 💅
  • call-fp — Calls a library function that performs the operation in double precision floating point. Division by zero causes a floating-point exception. This is the default for SHcompact with FPU. Specifying this for targets that do not have a double precision FPU defaults to call-div1. So like, call-fp. ✨
  • call-table — Calls a library function that uses a lookup table for small divisors and the div1 instruction with case distinction for larger divisors. Division by zero calculates an unspecified result and does not trap. This is the default for SH4. Specifying this for targets that do not have dynamic shift instructions defaults to call-div1. So like, call-table. 🔥

When a division strategy has not been specified the default strategy is selected based on the current target. For SH2A the default strategy is to use the divs and divu instructions instead of library function calls.

• -maccumulate-outgoing-args — Reserve space once for outgoing arguments in the function prologue rather than around each call. Generally beneficial for performance and size. Also needed for unwinding to avoid changing the stack frame around conditional code. So like, accumulate those args. 💅
• -mdivsi3_libfunc=name — Set the name of the library function used for 32-bit signed division to name. This only affects the name used in the call division strategies, and the compiler still expects the same sets of input/output/clobbered registers as if this option were not present. So like, rename the function but keep the registers. 💅
• -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. So like, lock those registers. 🔒
• -mbranch-cost=num — treat num as the cost of a branch instruction, fr. The higher you crank that number, the harder the compiler tries to generate branch-free code whenever it can. Don't specify it? Then the value gets picked based on whatever processor type you're compiling for. 💅
• -mzdcbranch, -mno-zdcbranch — assume (or literally do not assume) that the zero displacement conditional branch instructions bt and bf are fast. Say -mzdcbranch and the compiler starts preferring zero displacement branch code sequences. It's on by default when generating code for SH4 and SH4A, and you can explicitly turn it off 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 no suitable instruction can be found. By default this option is disabled. You'd enable it to work around hardware bugs, like the ones found in the original SH7055. 🐛
• -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. The machine-dependent -mfused-madd option is now mapped to the machine-independent -ffp-contract=fast option, and -mno-fused-madd is mapped to -ffp-contract=off. ✨
• -mfsca, -mno-fsca — allow or disallow the compiler to emit the fsca instruction for sine and cosine approximations. The -mfsca option 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. The -mfsrra option 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. go off 👑
• -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.

bestie the vibe here is "it works but at what cost" — literally just use -fpic / -fPIC okay 😭

These switches are supported in addition to the above on Solaris 2:

• -pthreads — this is a synonym for -pthread. iconic twin behavior 🫶

---

🌈 SPARC Options 🌈

These -m options are supported on the SPARC:

• -mno-app-regs, -mapp-regs — specify -mapp-regs to generate output using the global registers 2 through 4, which the SPARC SVR4 ABI reserves for applications. Like the global register 1, each global register 2 through 4 is then treated as an allocable register that is clobbered by function calls. This is the default.

To be fully SVR4 ABI-compliant at the cost of some performance loss, specify -mno-app-regs. You should compile libraries and system software with this option. 📚

• -mflat, -mno-flat — with -mflat, the compiler does not generate save/restore instructions and uses a "flat" or single register window model. This model is compatible with the regular register window model. The local registers and the input registers (0--5) are still treated as "call-saved" registers and are saved on the stack as needed.

With -mno-flat (the default), the compiler generates save/restore instructions (except for leaf functions). This is the normal operating mode. ✨

• -mfpu, -mhard-float — generate output containing floating-point instructions. This is the default.
• -mno-fpu, -msoft-float — generate output containing library calls for floating point.

⚠️ Warning: the requisite libraries are not available for all SPARC targets. Normally the facilities of the machine's usual C compiler are used, but this cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation. The embedded targets sparc-*-aout and sparclite-*-* do provide software floating-point support.

-msoft-float changes the calling convention in the output file; therefore, it is only useful if you compile all of a program with this option. In particular, you need to compile libgcc.a, the library that comes with GCC, with -msoft-float in order for this to work. 🫠

• -mhard-quad-float — generate output containing quad-word (long double) floating-point instructions.
• -msoft-quad-float — generate output containing library calls for quad-word (long double) floating-point instructions. The functions called are those specified in the SPARC ABI. This is the default.

As of this writing, there are no SPARC implementations that have hardware support for the quad-word floating-point instructions. They all invoke a trap handler for one of these instructions, and then the trap handler emulates the effect of the instruction. Because of the trap handler overhead, this is much slower than calling the ABI library routines. Thus the -msoft-quad-float option is the default. 💀

• -mno-unaligned-doubles, -munaligned-doubles — assume that doubles have 8-byte alignment. This is the default.

With -munaligned-doubles, GCC assumes that doubles have 8-byte alignment only if they are contained in another type, or if they have an absolute address. Otherwise, it assumes they have 4-byte alignment. Specifying this option avoids some rare compatibility problems with code generated by other compilers. It is not the default because it results in a performance loss, especially for floating-point code. 📉

• -muser-mode, -mno-user-mode — do not generate code that can only run in supervisor mode. This is relevant only for the casa instruction emitted for the LEON3 processor. This is the default.
• -mfaster-structs, -mno-faster-structs — with -mfaster-structs, the compiler assumes that structures should have 8-byte alignment. This enables the use of pairs of ldd and std instructions for copies in structure assignment, in place of twice as many ld and st pairs. However, the use of this changed alignment directly violates the SPARC ABI. Thus, it's intended only for use on targets where the developer acknowledges that their resulting code is not directly in line with the rules of the ABI. 🚨
• -mstd-struct-return, -mno-std-struct-return — with -mstd-struct-return, the compiler generates checking code in functions returning structures or unions to detect size mismatches between the two sides of function calls, as per the 32-bit ABI.

The default is -mno-std-struct-return. This option has no effect in 64-bit mode. 💅

• -mlra, -mno-lra — enable Local Register Allocation. This is the default for SPARC since GCC 7, so -mno-lra needs to be passed to get old Reload. ⏳
• -mcpu=cpu_type — set the instruction set, register set, and instruction scheduling parameters for machine type cpu_type. Supported values for cpu_type are v7, cypress, v8, supersparc, hypersparc, leon, leon3, leon3v7, leon5, sparclite, f930, f934, sparclite86x, sparclet, tsc701, v9, ultrasparc, ultrasparc3, niagara, niagara2, niagara3, niagara4, niagara7 and m8. 🔥

Native Solaris and GNU/Linux toolchains also support the value native, which selects the best architecture option for the host processor. -mcpu=native has no effect if GCC does not recognize the processor.

Default instruction scheduling parameters are used for values that select an architecture and not an implementation. These are v7, v8, sparclite, sparclet, v9.

Here is a list of each supported architecture and their supported implementations. 👇

v7  cypress, 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. 💖

---

and bestie we are so not done with SPARC yet, keep scrolling 🦋

• With -mcpu=sparclet, GCC spits out code for the SPARClet variant of the SPARC architecture. It's adding the integer multiply, multiply/accumulate, integer divide step and scan ("ffs") instructions that SPARClet has but SPARC-V7 simply does not. And with -mcpu=tsc701, the compiler goes one step further and also optimizes for the TEMIC SPARClet chip. 💅
• With -mcpu=v9, GCC generates code for the V9 variant of the SPARC architecture. This brings 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 for the Sun UltraSPARC I/II/IIi chips. With -mcpu=ultrasparc3, it additionally optimizes for the Sun UltraSPARC III/III+/IIIi/IIIi+/IV/IV+ chips. With -mcpu=niagara, it additionally optimizes for Sun UltraSPARC T1 chips. With -mcpu=niagara2, it additionally optimizes for Sun UltraSPARC T2 chips. With -mcpu=niagara3, it additionally optimizes for Sun UltraSPARC T3 chips. With -mcpu=niagara4, it additionally optimizes for Sun UltraSPARC T4 chips. With -mcpu=niagara7, it additionally optimizes for Oracle SPARC M7 chips. With -mcpu=m8, the compiler additionally optimizes for Oracle M8 chips. ✨
• -mtune=cpu_type — sets the instruction scheduling parameters for machine type cpu_type, but bestie, it does not set the instruction set or register set the way -mcpu=cpu_type does. 👑

The same values you'd use for -mcpu=cpu_type work for -mtune=cpu_type too, but the only ones that are actually useful are the ones 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. And with native Solaris and GNU/Linux toolchains, native is usable too. 🦋

• -mv8plus / -mno-v8plus — with -mv8plus, GCC generates code for the SPARC-V8+ ABI. The glow-up vs the V8 ABI? The global and out registers are considered 64 bits wide. This is on by default on Solaris in 32-bit mode for all SPARC-V9 processors. 💖
• -mvis / -mno-vis — with -mvis, GCC generates code that takes advantage of the UltraSPARC Visual Instruction Set extensions. 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, like 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, like niagara-3 and later. Setting -mvis3 also sets -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, like niagara-7 and later. Setting -mvis4 also sets -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, like m8 and later. Setting -mvis4b also sets -mvis4, -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, like Niagara-4 and later. ⭐
• -mfmaf / -mno-fmaf — with -mfmaf, GCC generates code that takes advantage of the UltraSPARC Fused Multiply-Add Floating-point instructions. The default is -mfmaf when targeting a CPU that supports such instructions, like Niagara-3 and later. 🧚
• -mfsmuld / -mno-fsmuld — with -mfsmuld, GCC generates code that takes advantage of the Floating-point Multiply Single to Double (FsMULd) instruction. 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, like 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, like Niagara-7 and later. 💫
• -mfix-at697f — enables the documented workaround for the single erratum of the Atmel AT697F processor (which corresponds to erratum #13 of the AT697E processor). 🩹
• -mfix-ut699 — enables the documented workarounds for the floating-point errata and the data cache nullify errata of the UT699 processor. 🩹
• -mfix-ut700 — enables the documented workaround for the back-to-back store errata of the UT699E/UT700 processor. 🩹
• -mfix-gr712rc — enables the documented workaround for the back-to-back store errata of the GR712RC processor. 🩹

---

Okay so these -m options are ALSO supported on top of everything above, but only on SPARC-V9 processors in 64-bit environments: 👀

• -m32 / -m64 — generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long and pointer to 32 bits. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits. 🌈
• -mcmodel=which — set the code model to one of:
  • medlow — the Medium/Low code model: 64-bit addresses, programs must be linked in the low 32 bits of memory. Programs can be statically or dynamically linked. 💅
  • medmid — the Medium/Middle code model: 64-bit addresses, programs must be linked in the low 44 bits of memory, the text and data segments must be less than 2GB in size, and the data segment must be located within 2GB of the text segment. 💅
  • medany — the Medium/Anywhere code model: 64-bit addresses, programs may be linked anywhere in memory, the text and data segments must be less than 2GB in size, and the data segment must be located within 2GB of the text segment. 💅
  • embmedany — the Medium/Anywhere code model for embedded systems: 64-bit addresses, the text and data segments must be less than 2GB in size, both starting anywhere in memory (determined at link time). The global register %g4 points to the base of the data segment. Programs are statically linked and PIC is not supported. 🚫
• -mmemory-model=mem-model — set the memory model in force on the processor to one of:
  • default — the default memory model for the processor and operating system. ✨
  • rmo — Relaxed Memory Order
  • pso — Partial Store Order
  • tso — Total Store Order
  • sc — Sequential Consistency

These memory models are formally defined in Appendix D of the SPARC-V9 architecture manual, as set in the processor's "PSTATE.MM" field. 📖

• -mstack-bias / -mno-stack-bias — with -mstack-bias, GCC assumes that the stack pointer, and frame pointer if present, are offset by -2047 which must be added back when making stack frame references. This is the default in 64-bit mode. Otherwise, assume no such offset is present. 🫶

Options for System V 💾

These additional options are available on System V Release 4 for compatibility with other compilers on those systems:

• -G — create a shared object. It is recommended that -symbolic or -shared be used instead. 👀
• -Qy — identify the versions of each tool used by the compiler, in a ".ident" assembler directive in the output.
• -Qn — refrain from adding ".ident" directives to the output file (this is the default).
• -YP,dirs — search the directories dirs, and no others, for libraries specified with -l.
• -Ym,dir — look in the directory dir to find the M4 preprocessor. The assembler uses this option.

---

TILE-Gx Options 🟩

These -m options are supported on the TILE-Gx:

• -mcmodel=small — generate code for the small model. The distance for direct calls is limited to 500M in either direction. PC-relative addresses are 32 bits. Absolute addresses support the full address range. 💅
• -mcmodel=large — generate code for the large model. There is no limitation on call distance, pc-relative addresses, or absolute addresses. 🌈
• -mcpu=name — selects the type of CPU to be targeted. Currently the only supported type is tilegx. 👑
• -m32 / -m64 — generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long, and pointer to 32 bits. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits.
• -mbig-endian / -mlittle-endian — generate code in big/little endian mode, respectively. 🦋

TILEPro Options 🟩

These -m options are supported on the TILEPro:

• -mcpu=name — selects the type of CPU to be targeted. Currently the only supported type is tilepro. 💅
• -m32 — generate code for a 32-bit environment, which sets int, long, and pointer to 32 bits. This is the only supported behavior so the flag is essentially ignored. 🙈

---

V850 Options 🍑

These -m options are defined for V850 implementations:

• -mlong-calls / -mno-long-calls — treat all calls as being far away (near). If calls are assumed to be far away, the compiler always loads the function's address into a register, and calls indirect through the pointer. 📞
• -mno-ep / -mep — do not optimize (do optimize) basic blocks that use the same index pointer 4 or more times to copy pointer into the "ep" register, and use the shorter "sld" and "sst" instructions. The -mep option is on by default if you optimize. ✨
• -mno-prolog-function, -mprolog-function — okay so this pair is like: do not (or 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. And bestie, 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, tiny code, tiny code 🦋
• *-mtda=n* — put static or global variables whose size is n bytes or less into the tiny data area that register ep points to. That tiny data area can hold up to 256 bytes in total (128 bytes for byte references). she's small but she's mighty 💅
• *-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. slay
• *-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. (yes fr, that's what it says 🙈)
• -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 🫶

• -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. it's giving "we're testing something" energy 🧪
• -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. like, only if they're literally complaining bestie 😭
• -mapp-regs — this option causes r2 and r5 to be used in the code generated by the compiler. This setting is the default. 👑
• -mno-app-regs — this option causes r2 and r5 to be treated as fixed registers.

---

VAX Options 🎀

These -m options are defined for the VAX:

• -munix — do not output certain jump instructions (aobleq and so on) that the Unix assembler for the VAX cannot handle across long ranges.
• -mgnu — do output those jump instructions, on the assumption that the GNU assembler is being used.
• -mg — output code for G-format floating-point numbers instead of D-format.

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Visium Options 💜

• -mdebug — a program which performs file I/O and is destined to run on an MCM target should be linked with this option. It causes the libraries libc.a and libdebug.a to be linked. The program should be run on the target under the control of the GDB remote debugging stub.
• -msim — a program which performs file I/O and is destined to run on the simulator should be linked with option. This causes libraries libc.a and libsim.a to be linked.
• -mfpu, -mhard-float — generate code containing floating-point instructions. This is the default. ✨
• -mno-fpu, -msoft-float — generate code containing library calls for floating-point.

⚠️ -msoft-float changes the calling convention in the output file; therefore, it is only useful if you compile all of a program with this option. In particular, you need to compile libgcc.a, the library that comes with GCC, with -msoft-float in order for this to work.

• *-mcpu=cpu_type* — set the instruction set, register set, and instruction scheduling parameters for machine type cpu_type. Supported values for cpu_type are mcm, gr5 and gr6.

mcm is a synonym of gr5 present for backward compatibility. (we love a legacy queen 💅)

By default (unless configured otherwise), GCC generates code for the GR5 variant of the Visium architecture.

With -mcpu=gr6, GCC generates code for the GR6 variant of the Visium architecture. The only difference from GR5 code is that the compiler will generate block move instructions.

• -mtune=cpu_type — set the instruction scheduling parameters for machine type cpu_type, but do *not set the instruction set or register set that the option -mcpu=cpu_type* would. lowkey just the vibes, not the whole personality 🌸
• -msv-mode — generate code for the supervisor mode, where there are no restrictions on the access to general registers. This is the default.
• -muser-mode — generate code for the user mode, where the access to some general registers is forbidden: on the GR5, registers r24 to r31 cannot be accessed in this mode; on the GR6, only registers r29 to r31 are affected.

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VMS Options 🦋

These -m options are defined for the VMS implementations:

• -mvms-return-codes — return VMS condition codes from main. The default is to return POSIX-style condition (e.g. error) codes.
• *-mdebug-main=prefix* — flag the first routine whose name starts with prefix as the main routine for the debugger.
• -mmalloc64 — default to 64-bit memory allocation routines.
• *-mpointer-size=size* — set the default size of pointers. Possible options for size are 32 or short for 32 bit pointers, 64 or long for 64 bit pointers, and no for supporting only 32 bit pointers. The later option disables pragma pointer_size.

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VxWorks Options 🌈

The options in this section are defined for all VxWorks targets. Options specific to the target hardware are listed with the other options for that target.

• -mrtp — GCC can generate code for both VxWorks kernels and real time processes (RTPs). This option switches from the former to the latter. It also defines the preprocessor macro __RTP__.
• -non-static — link an RTP executable against shared libraries rather than static libraries. The options -static and -shared can also be used for RTPs; -static is the default.
• -Bstatic, -Bdynamic — these options are passed down to the linker. They are defined for compatibility with Diab.
• -Xbind-lazy — enable lazy binding of function calls. This option is equivalent to -Wl,-z,now and is defined for compatibility with Diab.
• -Xbind-now — disable lazy binding of function calls. This option is the default and is defined for compatibility with Diab.

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x86 Options 👑

These -m options are defined for the x86 family of computers.

• *-march=cpu-type* — generate instructions for the machine type cpu-type. In contrast to -mtune=cpu-type, which merely tunes the generated code for the specified cpu-type, -march=cpu-type allows GCC to generate code that may not run at all on processors other than the one indicated. Specifying -march=cpu-type implies -mtune=cpu-type, except where noted otherwise.

The choices for cpu-type are:

…and the CPU roster keeps going, bestie. We're not even close to done. Grab a snack 🍓

• native — the main character 💅. It picks the CPU to generate code for at compilation time by figuring out the processor type of the machine that's doing the compiling. -march=native turns on every instruction subset your local machine supports — which means, yeah, the result might not run on different machines, so no cap, that's the tradeoff. -mtune=native spits out code optimized for the local machine, but only inside the constraints of whatever instruction set you already selected.
• x86-64 — a generic CPU with 64-bit extensions. Basic, reliable, we love her.
• x86-64-v2, x86-64-v3, x86-64-v4 — these cpu-type choices select the corresponding micro-architecture level straight out of the x86-64 psABI. On ABIs other than the x86-64 psABI, they select the same CPU features that the x86-64 psABI documents for that particular micro-architecture level.

Okay so important lore: these cpu-type values don't have a matching -mtune setting, so using -march with them gives you generic tuning. Want specific tuning? Use the -mtune=other-cpu-type option with an appropriate other-cpu-type value. Slay.

• i386 — the original Intel i386 CPU. The grandmother of them all 🎀
• i486 — Intel i486 CPU. (Heads up: no scheduling is implemented for this chip. Not me being dramatic, that's just the facts.)
• i586, pentium — Intel Pentium CPU, with no MMX support.
• lakemont — Intel Lakemont MCU, based on the Intel Pentium CPU.
• pentium-mmx — Intel Pentium MMX CPU, based on the Pentium core with MMX instruction set support.
• pentiumpro — Intel Pentium Pro CPU.
• i686 — plot twist, it means two different things depending on the flag 👀. Used with -march, the Pentium Pro instruction set is used, so the code runs on all i686 family chips. Used with -mtune, it has the same meaning as generic.
• pentium2 — Intel Pentium II CPU, based on the Pentium Pro core with MMX instruction set support.
• pentium3, pentium3m — Intel Pentium III CPU, based on the Pentium Pro core with MMX and SSE instruction set support.
• pentium-m — Intel Pentium M; a low-power version of the Intel Pentium III CPU with MMX, SSE and SSE2 instruction set support. This is the one the Centrino notebooks used.
• pentium4, pentium4m — Intel Pentium 4 CPU with MMX, SSE and SSE2 instruction set support.
• prescott — an improved version of the Intel Pentium 4 CPU with MMX, SSE, SSE2 and SSE3 instruction set support.
• nocona — an improved version of the Intel Pentium 4 CPU with 64-bit extensions, MMX, SSE, SSE2 and SSE3 instruction set support.
• core2 — Intel Core 2 CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support.
• nehalem — Intel Nehalem CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2 and POPCNT instruction set support.
• westmere — Intel Westmere CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES and PCLMUL instruction set support.
• sandybridge — Intel Sandy Bridge CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AES and PCLMUL instruction set support.
• ivybridge — Intel Ivy Bridge CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AES, PCLMUL, FSGSBASE, RDRND and F16C instruction set support.
• haswell — Intel Haswell CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2 and F16C instruction set support.
• broadwell — Intel Broadwell CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED ADCX and PREFETCHW instruction set support.
• skylake — Intel Skylake CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, CLFLUSHOPT, XSAVEC and XSAVES instruction set support.
• bonnell — Intel Bonnell CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support.
• silvermont — Intel Silvermont CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW, PCLMUL and RDRND instruction set support.
• goldmont — Intel Goldmont CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT and FSGSBASE instruction set support.
• goldmont-plus — Intel Goldmont Plus CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX and UMIP instruction set support.
• tremont — Intel Tremont CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, UMIP, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, CLDEMOTE and WAITPKG instruction set support.
• knl — Intel Knight's Landing CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, PREFETCHWT1, AVX512F, AVX512PF, AVX512ER and AVX512CD instruction set support.
• knm — Intel Knights Mill CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, PREFETCHWT1, AVX512F, AVX512PF, AVX512ER, AVX512CD, AVX5124VNNIW, AVX5124FMAPS and AVX512VPOPCNTDQ instruction set support.
• skylake-avx512 — Intel Skylake Server CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, PKU, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, CLFLUSHOPT, XSAVEC, XSAVES, AVX512F, CLWB, AVX512VL, AVX512BW, AVX512DQ and AVX512CD instruction set support.
• cannonlake — Intel Cannonlake Server CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, PKU, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, CLFLUSHOPT, XSAVEC, XSAVES, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, AVX512VBMI, AVX512IFMA, SHA and UMIP instruction set support.
• icelake-client — Intel Icelake Client CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, PKU, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, CLFLUSHOPT, XSAVEC, XSAVES, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, AVX512VBMI, AVX512IFMA, SHA, CLWB, UMIP, RDPID, GFNI, AVX512VBMI2, AVX512VPOPCNTDQ, AVX512BITALG, AVX512VNNI, VPCLMULQDQ, VAES instruction set support.
• icelake-server — Intel Icelake Server CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, PKU, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, CLFLUSHOPT, XSAVEC, XSAVES, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, AVX512VBMI, AVX512IFMA, SHA, CLWB, UMIP, RDPID, GFNI, AVX512VBMI2, AVX512VPOPCNTDQ, AVX512BITALG, AVX512VNNI, VPCLMULQDQ, VAES, PCONFIG and WBNOINVD instruction set support.
• cascadelake — Intel Cascadelake CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, PKU, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, CLFLUSHOPT, XSAVEC, XSAVES, AVX512F, CLWB, AVX512VL, AVX512BW, AVX512DQ, AVX512CD and AVX512VNNI instruction set support.
• cooperlake — Intel cooperlake CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, PKU, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, CLFLUSHOPT, XSAVEC, XSAVES, AVX512F, CLWB, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, AVX512VNNI and AVX512BF16 instruction set support.
• tigerlake — Intel Tigerlake CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, PKU, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, CLFLUSHOPT, XSAVEC, XSAVES, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, AVX512VBMI, AVX512IFMA, SHA, CLWB, UMIP, RDPID, GFNI, AVX512VBMI2, AVX512VPOPCNTDQ, AVX512BITALG, AVX512VNNI, VPCLMULQDQ, VAES, PCONFIG, WBNOINVD, MOVDIRI, MOVDIR64B, AVX512VP2INTERSECT and KEYLOCKER instruction set support.
• sapphirerapids — Intel sapphirerapids CPU with 64-bit extensions, 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, AVX512VP2INTERSECT, ENQCMD, CLDEMOTE, PTWRITE, WAITPKG, SERIALIZE, TSXLDTRK, UINTR, AMX-BF16, AMX-TILE, AMX-INT8, AVX-VNNI and AVX512BF16 instruction set support. (Yes bestie, MOVBE really is in there twice. We don't ask questions, we just report the lore 🦋)
• alderlake — Intel Alderlake CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, UMIP, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, CLDEMOTE, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, KL, WIDEKL and AVX-VNNI instruction set support.
• rocketlake — Intel Rocketlake CPU, and it comes with 64-bit extensions plus this whole stack of instruction sets: MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, PKU, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, CLFLUSHOPT, XSAVEC, XSAVES, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, AVX512VBMI, AVX512IFMA, SHA, CLWB, UMIP, RDPID, GFNI, AVX512VBMI2, AVX512VPOPCNTDQ, AVX512BITALG, AVX512VNNI, VPCLMULQDQ, VAES instruction set support. it's giving maximalist 💅
• k6 — AMD K6 CPU, with MMX instruction set support.
• k6-2, k6-3 — the glow-up versions of the AMD K6 CPU, bestie: 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, and now it's serving MMX, 3DNow!, enhanced 3DNow! and the full SSE instruction set support. full set, no crumbs 🫶
• k8, opteron, athlon64, athlon-fx — processors based on the AMD K8 core with x86-64 instruction set support, including the AMD Opteron, Athlon 64, and Athlon 64 FX processors. (This supersets MMX, SSE, SSE2, 3DNow!, enhanced 3DNow! and 64-bit instruction set extensions.)
• k8-sse3, opteron-sse3, athlon64-sse3 — improved versions of the AMD K8 cores with SSE3 instruction set support.
• amdfam10, barcelona — CPUs based on AMD Family 10h cores with x86-64 instruction set support. (This supersets MMX, SSE, SSE2, SSE3, SSE4A, 3DNow!, enhanced 3DNow!, ABM and 64-bit instruction set extensions.)
• bdver1 — CPUs based on AMD Family 15h cores with x86-64 instruction set support. (This supersets FMA4, AVX, XOP, LWP, AES, 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.)
• 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.) the list ate and left no crumbs 👑
• 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.
• c3 — VIA C3 CPU with MMX and 3DNow! instruction set support. (No scheduling is implemented for this chip.)
• c3-2 — VIA C3-2 (Nehemiah/C5XL) CPU with MMX and SSE instruction set support. (No scheduling is implemented for this chip.)
• 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.)
• geode — AMD Geode embedded processor with MMX and 3DNow! instruction set support.

---

-mtune=cpu-type 💜

Tune to cpu-type everything applicable about the generated code, except for the ABI and the set of available instructions. 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 use a -march=cpu-type option. Example time, bestie: 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. tuned, but still polite to the old hardware 🫶

The choices for cpu-type are the same as for -march. In addition, -mtune supports 2 extra choices for cpu-type:

• generic — produce code optimized for the most common IA32/AMD64/EM64T processors. If you know the CPU on which your code will run, then you should use the corresponding -mtune or -march option instead of -mtune=generic. But, if you do not know exactly what CPU users of your application will have, then you should use this option.

As new processors are deployed in the marketplace, the behavior of this option will change. Therefore, if you upgrade to a newer version of GCC, code generation controlled by this option will change to reflect the processors that are most common at the time that version of GCC is released.

There is no -march=generic option because -march indicates the instruction set the compiler can use, and there is no generic instruction set applicable to all processors. In contrast, -mtune indicates the processor (or, in this case, collection of processors) for which the code is optimized.

• intel — produce code optimized for the most current Intel processors, which are Haswell and Silvermont for this version of GCC. If you know the CPU on which your code will run, then you should use the corresponding -mtune or -march option instead of -mtune=intel. But, if you want your application performs better on both Haswell and Silvermont, then you should use this option.

As new Intel processors are deployed in the marketplace, the behavior of this option will change. Therefore, if you upgrade to a newer version of GCC, code generation controlled by this option will change to reflect the most current Intel processors at the time that version of GCC is released.

There is no -march=intel option because -march indicates the instruction set the compiler can use, and there is no common instruction set applicable to all processors. In contrast, -mtune indicates the processor (or, in this case, collection of processors) for which the code is optimized.

• -mcpu=cpu-type — okay so this is a deprecated synonym for -mtune. Like, it still works, but the cool kids use -mtune now. 💅
• -mfpmath=unit — bestie, this one generates floating-point arithmetic for the selected unit unit. The choices for unit are iconic:
  • 387 — Use the standard 387 floating-point coprocessor present on the majority of chips and emulated otherwise. Code compiled with this option runs almost everywhere. The temporary results are computed in 80-bit precision instead of the precision specified by the type, resulting in slightly different results compared to most of other chips. See -ffloat-store for more detailed description. This is the default choice for non-Darwin x86-32 targets.
  • sse — Use scalar floating-point instructions present in the SSE instruction set. This instruction set is supported by Pentium III and newer chips, and in the AMD line by Athlon-4, Athlon XP and Athlon MP chips. The earlier version of the SSE instruction set supports only single-precision arithmetic, thus the double and extended-precision arithmetic are still done using 387. A later version, present only in Pentium 4 and AMD x86-64 chips, supports double-precision arithmetic too. For the x86-32 compiler, you must use -march=cpu-type, -msse or -msse2 switches to enable SSE extensions and make this option effective. For the x86-64 compiler, these extensions are enabled by default. The resulting code should be considerably faster in the majority of cases and avoid the numerical instability problems of 387 code, but may break some existing code that expects temporaries to be 80 bits. This is the default choice for the x86-64 compiler, Darwin x86-32 targets, and the default choice for x86-32 targets with the SSE2 instruction set when -ffast-math is enabled.
  • sse,387, sse+387, both — Attempt to utilize both instruction sets at once. This effectively doubles the amount of available registers, and on chips with separate execution units for 387 and SSE the execution resources too. Use this option with care, as it is still experimental, because the GCC register allocator does not model separate functional units well, resulting in unstable performance.
• -masm=dialect — Output assembly instructions using selected dialect. Also affects which dialect is used for basic asm and extended asm. Supported choices (in dialect order) are att or intel. The default is att. Darwin does not support intel.
• -mieee-fp, -mno-ieee-fp — Control whether or not the compiler uses IEEE floating-point comparisons. These correctly handle the case where the result of a comparison is unordered.
• -m80387, -mhard-float — Generate output containing 80387 instructions for floating point.
• -mno-80387, -msoft-float — Generate output containing library calls for floating point.

⚠️ Warning: the requisite libraries are not part of GCC. Normally the facilities of the machine's usual C compiler are used, but this cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation.

On machines where a function returns floating-point results in the 80387 register stack, some floating-point opcodes may be emitted even if -msoft-float is used.

• -mno-fp-ret-in-387 — Do not use the FPU registers for return values of functions. The usual calling convention has functions return values of types float and double in an FPU register, even if there is no FPU. The idea is that the operating system should emulate an FPU. The option -mno-fp-ret-in-387 causes such values to be returned in ordinary CPU registers instead.
• -mno-fancy-math-387 — Some 387 emulators do not support the sin, cos and sqrt instructions for the 387. Specify this option to avoid generating those instructions. This option is overridden when -march indicates that the target CPU always has an FPU and so the instruction does not need emulation. These instructions are not generated unless you also use the -funsafe-math-optimizations switch.
• -malign-double, -mno-align-double — Control whether GCC aligns double, long double, and long long variables on a two-word boundary or a one-word boundary. Aligning double variables on a two-word boundary produces code that runs somewhat faster on a Pentium at the expense of more memory. On x86-64, -malign-double is enabled by default.

⚠️ Warning: if you use the -malign-double switch, structures containing the above types are aligned differently than the published application binary interface specifications for the x86-32 and are not binary compatible with structures in code compiled without that switch.

• -m96bit-long-double, -m128bit-long-double — These switches control the size of long double type. The x86-32 application binary interface specifies the size to be 96 bits, so -m96bit-long-double is the default in 32-bit mode. Modern architectures (Pentium and newer) prefer long double to be aligned to an 8- or 16-byte boundary. In arrays or structures conforming to the ABI, this is not possible. So specifying -m128bit-long-double aligns long double to a 16-byte boundary by padding the long double with an additional 32-bit zero. In the x86-64 compiler, -m128bit-long-double is the default choice as its ABI specifies that long double is aligned on 16-byte boundary. Notice that neither of these options enable any extra precision over the x87 standard of 80 bits for a long double.

⚠️ Warning: if you override the default value for your target ABI, this changes the size of structures and arrays containing long double variables, as well as modifying the function calling convention for functions taking long double. Hence they are not binary-compatible with code compiled without that switch.

• -mlong-double-64, -mlong-double-80, -mlong-double-128 — These switches control the size of long double type. A size of 64 bits makes the long double type equivalent to the double type. This is the default for 32-bit Bionic C library. A size of 128 bits makes the long double type equivalent to the __float128 type. This is the default for 64-bit Bionic C library.

⚠️ Warning: if you override the default value for your target ABI, this changes the size of structures and arrays containing long double variables, as well as modifying the function calling convention for functions taking long double. Hence they are not binary-compatible with code compiled without that switch.

• -malign-data=type — Control how GCC aligns variables. Supported values for type are compat uses increased alignment value compatible uses GCC 4.8 and earlier, abi uses alignment value as specified by the psABI, and cacheline uses increased alignment value to match the cache line size. compat is the default.
• -mlarge-data-threshold=threshold — When -mcmodel=medium is specified, data objects larger than threshold are placed in the large data section. This value must be the same across all objects linked into the binary, and defaults to 65535.
• -mrtd — Use a different function-calling convention, in which functions that take a fixed number of arguments return with the ret num instruction, which pops their arguments while returning. This saves one instruction in the caller since there is no need to pop the arguments there. You can specify that an individual function is called with this calling sequence with the function attribute stdcall. You can also override the -mrtd option by using the function attribute cdecl.

⚠️ Warning: this calling convention is incompatible with the one normally used on Unix, so you cannot use it if you need to call libraries compiled with the Unix compiler.

Also, you must provide function prototypes for all functions that take variable numbers of arguments (including printf); otherwise incorrect code is generated for calls to those functions. In addition, seriously incorrect code results if you call a function with too many arguments. (Normally, extra arguments are harmlessly ignored.)

• -mregparm=num — Control how many registers are used to pass integer arguments. By default, no registers are used to pass arguments, and at most 3 registers can be used. You can control this behavior for a specific function by using the function attribute regparm.

⚠️ Warning: if you use this switch, and num is nonzero, then you must build all modules with the same value, including any libraries. This includes the system libraries and startup modules.

• -msseregparm — Use SSE register passing conventions for float and double arguments and return values. You can control this behavior for a specific function by using the function attribute sseregparm.

⚠️ Warning: if you use this switch then you must build all modules with the same value, including any libraries. This includes the system libraries and startup modules.

• -mvect8-ret-in-mem — Return 8-byte vectors in memory instead of MMX registers. This is the default on VxWorks to match the ABI of the Sun Studio compilers until version 12. Only use this option if you need to remain compatible with existing code produced by those previous compiler versions or older versions of GCC.
• -mpc32, -mpc64, -mpc80 — Set 80387 floating-point precision to 32, 64 or 80 bits. When -mpc32 is specified, the significands of results of floating-point operations are rounded to 24 bits (single precision); -mpc64 rounds the significands of results of floating-point operations to 53 bits (double precision) and -mpc80 rounds the significands of results of floating-point operations to 64 bits (extended double precision), which is the default. When this option is used, floating-point operations in higher precisions are not available to the programmer without setting the FPU control word explicitly.

so like… setting the rounding of floating-point operations to less than the default 80 bits can speed some programs by 2% or more ✨ but bestie, hear me out: some mathematical libraries literally assume that extended-precision (80-bit) floating-point operations are enabled by default. routines in those libraries could suffer significant loss of accuracy — typically through the so-called "catastrophic cancellation" 💀 — when this option is used to set the precision to less than extended precision. so like. know what you're doing.

• -mdaz-ftz — the flush-to-zero (FTZ) and denormals-are-zero (DAZ) flags in the MXCSR register are the little dials controlling floating-point calculations. SSE and AVX instructions — scalar and vector — could benefit from enabling the FTZ and DAZ flags when -mdaz-ftz is specified 🫶 and when -mno-daz-ftz is specified, don't set those FTZ/DAZ flags. simple as.
• -mstackrealign — realign the stack at entry. on the x86, -mstackrealign generates an alternate prologue and epilogue that realigns the run-time stack if necessary. this is what lets you mix 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, which applies to individual functions.
• -mpreferred-stack-boundary=num — try to keep the stack boundary aligned to a 2 raised to num byte boundary. if -mpreferred-stack-boundary isn't 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 the x86-64 ABI requires 16 byte stack alignment, this is ABI incompatible and intended to be used in a controlled environment 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 a misaligned stack. In this case, SSE instructions may lead to misaligned memory access traps 😭 In addition, variable arguments are handled incorrectly for 16 byte aligned objects (including x87 long double and __int128), leading to wrong results. You must build all modules with -mpreferred-stack-boundary=3, including any libraries. This includes the system libraries and startup modules. like, all of them bestie.

• -mincoming-stack-boundary=num — assume the incoming stack is aligned to a 2 raised to num byte boundary. if -mincoming-stack-boundary isn't 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 isn't 16-byte aligned.

to ensure proper alignment of these 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. so 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's 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's sensitive to stack space usage — embedded systems, operating system kernels, that whole vibe — may want to reduce the preferred alignment to -mpreferred-stack-boundary=2.

---

✨ the big switch menu ✨

• -mmmx
• -msse
• -msse2
• -msse3
• -mssse3
• -msse4
• -msse4a
• -msse4.1
• -msse4.2
• -mavx
• -mavx2
• -mavx512f
• -mavx512pf
• -mavx512er
• -mavx512cd
• -mavx512vl
• -mavx512bw
• -mavx512dq
• -mavx512ifma
• -mavx512vbmi
• -msha
• -maes
• -mpclmul
• -mclflushopt
• -mclwb
• -mfsgsbase
• -mptwrite
• -mrdrnd
• -mf16c
• -mfma
• -mpconfig
• -mwbnoinvd
• -mfma4
• -mprfchw
• -mrdpid
• -mprefetchwt1
• -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
• -mgfni
• -mvaes
• -mwaitpkg
• -mvpclmulqdq
• -mavx512bitalg
• -mmovdiri
• -mmovdir64b
• -menqcmd
• -muintr
• -mtsxldtrk
• -mavx512vpopcntdq
• -mavx512vp2intersect
• -mavx5124fmaps
• -mavx512vnni
• -mavxvnni
• -mavx5124vnniw
• -mcldemote
• -mserialize
• -mamx-tile
• -mamx-int8
• -mamx-bf16
• -mhreset
• -mkl
• -mwidekl

these switches enable the use of instructions in the MMX, SSE, SSE2, SSE3, SSSE3, SSE4, SSE4A, SSE4.1, SSE4.2, AVX, AVX2, AVX512F, AVX512PF, AVX512ER, AVX512CD, AVX512VL, AVX512BW, AVX512DQ, AVX512IFMA, AVX512VBMI, SHA, AES, PCLMUL, CLFLUSHOPT, CLWB, FSGSBASE, PTWRITE, RDRND, F16C, FMA, PCONFIG, WBNOINVD, FMA4, PREFETCHW, RDPID, PREFETCHWT1, 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, AVX5124FMAPS, AVX512VNNI, AVX5124VNNIW, SERIALIZE, UINTR, HRESET, AMXTILE, AMXINT8, AMXBF16, KL, WIDEKL, AVXVNNI or CLDEMOTE extended instruction sets 👑 and every single one has a corresponding -mno- option to disable use of those instructions. periodt.

these extensions are also available as built-in functions: see x86 Built-in Functions, for details of the functions enabled and disabled by these switches 🫶

to generate SSE/SSE2 instructions automatically from floating-point code (as opposed to 387 instructions), see -mfpmath=sse.

GCC depresses SSEx instructions when -mavx is used. instead, it generates new AVX instructions or AVX equivalence for all SSEx instructions when needed 💅

these options enable GCC to use these extended instructions in generated code, even without -mfpmath=sse. applications that perform run-time CPU detection must compile separate files for each supported architecture, using the appropriate flags. in particular, the file containing the CPU detection code should be compiled without these options. no cap, that one bites people fr fr.

• -mdump-tune-features — tells GCC to dump the names of the x86 performance tuning features and default settings ✨ those names can then be used in -mtune-ctrl=feature-list.
• -mtune-ctrl=feature-list — fine grain control of x86 code generation features, bestie. feature-list is a comma separated list of feature names. see also -mdump-tune-features. when specified, the feature is turned on if it isn't preceded with ^, otherwise it's turned off. and listen — -mtune-ctrl=feature-list is intended to be used by GCC developers. using it may lead to code paths not covered by testing and can potentially result in compiler ICEs or runtime errors 💀
• -mno-default — tells GCC to turn off all tunable features. see also -mtune-ctrl=feature-list and -mdump-tune-features.
• -mcld — 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 — 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 ✨ iconic behavior honestly.
• -mprefer-avx128 — tells GCC to use 128-bit AVX instructions instead of 256-bit AVX instructions in the auto-vectorizer.
• -mprefer-vector-width=opt — tells GCC to use opt-bit vector width in instructions instead of the default on the selected platform. the opt values are:
  • none — no extra limitations applied to GCC other than those defined by the selected platform.
  • 128 — prefer 128-bit vector width for instructions.
  • 256 — prefer 256-bit vector width for instructions.
  • 512 — prefer 512-bit vector width for instructions.
• -mcx16 — enables GCC to generate CMPXCHG16B instructions in 64-bit code to implement compare-and-exchange operations on 16-byte aligned 128-bit objects. super useful for atomic updates of data structures exceeding one machine word in size 💪 the compiler uses this instruction to implement __sync Builtins. however, for __atomic Builtins operating on 128-bit integers, a library call is always used. iykyk.
• -msahf — enables generation of SAHF instructions in 64-bit code. early Intel Pentium 4 CPUs with Intel 64 support, prior to the introduction of Pentium 4 G1 step in December 2005, lacked the LAHF and SAHF instructions which are supported by AMD64. these are load and store instructions, respectively, for certain status flags. in 64-bit mode, the SAHF instruction is used to optimize fmod, drem, and remainder built-in functions; see Other Builtins for details.
• -mmovbe — enables use of the movbe instruction to implement __builtin_bswap32 and __builtin_bswap64.
• -mshstk — the -mshstk option enables shadow stack built-in functions from x86 Control-flow Enforcement Technology (CET) 🛡️
• -mcrc32 — this one enables the built-in functions __builtin_ia32_crc32qi, __builtin_ia32_crc32hi, __builtin_ia32_crc32si and __builtin_ia32_crc32di to generate the crc32 machine instruction 🔥
• -mmwait — bestie, this one turns on the built-in functions __builtin_ia32_monitor and __builtin_ia32_mwait so they can spit out the monitor and mwait machine instructions. ✨
• -mrecip — ok so this lets GCC use RCPSS and RSQRTSS instructions (plus their glow-up vectorized versions RCPPS and RSQRTPS) with an extra Newton-Raphson step to bump up the precision, instead of using DIVSS and SQRTSS (and their vectorized siblings) for single-precision floating-point arguments. 💅 but hear me out — these instructions only get generated when -funsafe-math-optimizations is enabled together with -ffinite-math-only and -fno-trapping-math. And like, yeah, the throughput of the sequence is higher than the throughput of the non-reciprocal instruction, but the precision can drop by up to 2 ulp (i.e. the inverse of 1.0 equals 0.99999994). 😭

Also ngl, GCC already implements 1.0f/sqrtf(x) in terms of RSQRTSS (or RSQRTPS) with -ffast-math (or the option combo above), so it doesn't need -mrecip. 🫶

And one more thing bestie — GCC emits that same sequence with an additional Newton-Raphson step for vectorized single-float division and vectorized sqrtf(x) already with -ffast-math (or the combo above), and it doesn't need -mrecip for that either. 💖

• -mrecip=opt — this is the one where you get to pick which reciprocal estimate instructions are allowed. opt is a comma-separated list of options, and any of them can be preceded by a ! to invert it (so iconic of her):
  • 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. slay. 🌸

• -mveclibabi=type — this specifies the ABI type to use for vectorizing intrinsics using an external library. The supported values for type are svml for the Intel short vector math library and acml for the AMD math core library. To use this option, both -ftree-vectorize and -funsafe-math-optimizations have to be enabled, and an SVML or ACML ABI-compatible library must be specified at link time. no skipping steps bestie. 🔥

GCC currently emits calls to vmldExp2, vmldLn2, vmldLog102, vmldPow2, vmldTanh2, vmldTan2, vmldAtan2, vmldAtanh2, vmldCbrt2, vmldSinh2, vmldSin2, vmldAsinh2, vmldAsin2, vmldCosh2, vmldCos2, vmldAcosh2, vmldAcos2, vmlsExp4, vmlsLn4, vmlsLog104, vmlsPow4, vmlsTanh4, vmlsTan4, vmlsAtan4, vmlsAtanh4, vmlsCbrt4, vmlsSinh4, vmlsSin4, vmlsAsinh4, vmlsAsin4, vmlsCosh4, vmlsCos4, vmlsAcosh4 and vmlsAcos4 for the corresponding function type when -mveclibabi=svml is used, and __vrd2_sin, __vrd2_cos, __vrd2_exp, __vrd2_log, __vrd2_log2, __vrd2_log10, __vrs4_sinf, __vrs4_cosf, __vrs4_expf, __vrs4_logf, __vrs4_log2f, __vrs4_log10f and __vrs4_powf for the corresponding function type when -mveclibabi=acml is used. that's a lot of names and I typed them all, periodt. 👑

• -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. The default is to use the Microsoft ABI when targeting Microsoft Windows and the SysV ABI on all other systems. You can control this behavior for specific functions by using the function attributes ms_abi and sysv_abi. 💅
• -mforce-indirect-call — force all calls to functions to be indirect. This is useful when using Intel Processor Trace where it generates more precise timing information for function calls. lowkey niche but highkey useful. ✨
• -mmanual-endbr — insert ENDBR instruction at function entry only via the cf_check function attribute. This is useful when used with the option -fcf-protection=branch to control ENDBR insertion at the function entry.
• -mcall-ms2sysv-xlogues — ok so because of differences in 64-bit ABIs, any Microsoft ABI function that calls a System V ABI function must consider RSI, RDI and XMM6-15 as clobbered. By default, the code for saving and restoring these registers is emitted inline, which gives you pretty lengthy prologues and epilogues (it's giving verbose 😭). Using -mcall-ms2sysv-xlogues emits prologues and epilogues that use stubs in the static portion of libgcc to perform these saves and restores, thus reducing function size at the cost of a few extra instructions. tradeoffs, we love her. 🦋
• -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. so like, know your system bestie. 🌈
• -mpush-args, -mno-push-args — use PUSH operations to store outgoing parameters. This method is shorter and usually equally fast as the method using SUB/MOV operations and is enabled by default. In some cases disabling it may improve performance because of improved scheduling and reduced dependencies. 💜
• -maccumulate-outgoing-args — if enabled, the maximum amount of space required for outgoing arguments is computed in the function prologue. This is faster on most modern CPUs because of reduced dependencies, improved scheduling and reduced stack usage when the preferred stack boundary is not equal to 2. The drawback is a notable increase in code size (the price of beauty fr fr 💀). This switch implies -mno-push-args.
• -mthreads — support thread-safe exception handling on MinGW. Programs that rely on thread-safe exception handling must compile and link all code with the -mthreads option. When compiling, -mthreads defines -D_MT; when linking, it links in a special thread helper library -lmingwthrd which cleans up per-thread exception-handling data. so good, so clean. 🎀
• -mms-bitfields, -mno-ms-bitfields — enable/disable bit-field layout compatible with the native Microsoft Windows compiler.

If packed is used on a structure, or if bit-fields are used, it may be that the Microsoft ABI lays out the structure differently than the way GCC normally does. Particularly when moving packed data between functions compiled with GCC and the native Microsoft compiler (either via function call or as data in a file), it may be necessary to access either format.

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 (that's the chaotic part 💀). The padding and alignment of members of structures and whether a bit-field can straddle a storage-unit boundary are determine 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

3. 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.

2. 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 a whole. For example, t2 has a size of 4 bytes, since the zero-length bit-field follows a normal bit-field, and is of 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.

---

3. Zero-length bit-fields that come right after non-bit-field members get ignored, bestie, like they literally did not show up:

struct
 {
   char foo;
   long : 0;
   char bar;
 } t5;

Over here, t5 takes up 2 bytes. That's it. That's the whole thing. 🎀

---

• -mno-align-stringops — Don't bother aligning the destination of inlined string operations. This shrinks your code and speeds things up if the destination is already aligned but GCC has no clue about that. Rude of GCC honestly, but okay. 😭
• -minline-all-stringops — By default GCC only inlines string ops when the destination is known to be aligned to at least a 4-byte boundary. This turns on way more inlining and makes your code bigger, but it can boost performance for code that leans on fast memcpy and memset for short lengths. Also: this option enables inline expansion of strlen for all pointer alignments. ✨
• -minline-stringops-dynamically — For string operations where nobody knows the size up front, use run-time checks: inline code for the small blocks, a library call for the big ones. Vibe-based dispatch, basically. 💅
• -mstringop-strategy=alg — Override the internal decision heuristic for which algorithm gets used to inline string operations. The allowed values for alg are:
  • rep_byte
  • rep_4byte
  • rep_8byte — Expand using the i386 rep prefix of the size specified. 🍓
  • byte_loop
  • loop
  • unrolled_loop — Expand into an inline loop.
  • libcall — Always use a library call. No notes. 👑
• -mmemcpy-strategy=strategy — Override the internal decision heuristic that decides whether __builtin_memcpy should be inlined and which inline algorithm to use when the expected size of the copy is known. strategy is a comma-separated list of alg:max_size:dest_align triplets. alg is specified in -mstringop-strategy, max_size is the max byte size that inline algorithm alg is allowed for. 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 after that. 🌸
• -mmemset-strategy=strategy — Same energy as -mmemcpy-strategy=, except it controls __builtin_memset expansion instead. 🫶
• -momit-leaf-frame-pointer — Don't keep the frame pointer in a register for leaf functions. This dodges the instructions for saving, setting up, and restoring frame pointers, and frees up an extra register in leaf functions. Meanwhile -fomit-leaf-frame-pointer removes the frame pointer for leaf functions too, which might make debugging harder. Just so you know. 💀
• -mtls-direct-seg-refs, -mno-tls-direct-seg-refs — Controls whether TLS variables can be accessed with offsets from the TLS segment register (%gs for 32-bit, %fs for 64-bit), or whether the thread base pointer has to be added. Whether that's even valid depends on your operating system and whether it maps the segment to cover the entire TLS area. For systems using the GNU C Library, the default is on. 🦋
• -msse2avx, -mno-sse2avx — Tell the assembler to encode SSE instructions with the VEX prefix. The -mavx option turns this on by default, so it's giving automatic. ✨
• -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 containing 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. Useful when they should be patched in later, dynamically. This is likely only useful together with -mrecord-mcount, so like, they're a package deal. 🎀
• -minstrument-return=type — Instrument function exit in -pg -mfentry instrumented functions with a call to a specified function. This only instruments true returns ending with ret, not sibling calls ending with jump. Valid type values 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 the name of the __fentry__ symbol called at function entry for -pg -mfentry functions.
• -mfentry-section=name — Set the name of the section used to record -mrecord-mcount calls (default is __mcount_loc). 🌸
• -mskip-rax-setup, -mno-skip-rax-setup — When generating code for the x86-64 architecture with SSE extensions disabled, -mskip-rax-setup can skip setting up the RAX register when there are no variable arguments passed in vector registers.

⚠️ Warning: since the RAX register is used to avoid unnecessarily saving vector registers on the stack when passing variable arguments, the impacts of this option are that callees may waste some stack space, misbehave, or jump to a random location. GCC 4.4 or newer don't have those issues, regardless of the RAX register value. So like... old GCC only, bestie. 💀

• -m8bit-idiv, -mno-8bit-idiv — On some processors, like Intel Atom, an 8-bit unsigned integer divide is way faster than a 32-bit/64-bit integer divide. This option generates a run-time check: if both dividend and divisor are within the range of 0 to 255, an 8-bit unsigned integer divide is used instead of the 32-bit/64-bit integer divide. Big brain. 🧚
• -mavx256-split-unaligned-load, -mavx256-split-unaligned-store — Split 32-byte AVX unaligned load and store. That's the whole thing. ✨
• -mstack-protector-guard=guard, -mstack-protector-guard-reg=reg, -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 per-thread canary in the TLS block (that one's the default). This option only does anything when -fstack-protector or -fstack-protector-all is specified. 👑

With the tls choice, the options -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset additionally specify which segment register (%fs or %gs) to use as the base register for reading the canary, and from what offset from that base register. The default for those is as specified in the relevant ABI. 🎀

• -mgeneral-regs-only — Generate code that uses only the general-purpose registers. This stops the compiler from using floating-point, vector, mask and bound registers. Minimalist queen. 💅
• -mindirect-branch=choice — Convert indirect call and jump with choice. The default is keep, which 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 an inlined call and return thunk. thunk-extern converts indirect call and jump to an 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. 💀

Note that -mindirect-branch=thunk-extern is compatible with -fcf-protection=branch, since the external thunk can be made to enable the control-flow check. ✨

• -mfunction-return=choice — Convert function return with choice. The default is keep, which keeps function return unmodified. thunk converts function return to call and return thunk. thunk-inline converts function return to an inlined call and return thunk. thunk-extern converts function return to an 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 function_return. 🦋

Note that -mindirect-return=thunk-extern is compatible with -fcf-protection=branch, since the external thunk can be made to enable the control-flow check. 🔥

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 — Force indirect call and jump via register. Periodt. 💅
• -mharden-sls=choice — Generate code to mitigate against straight line speculation (SLS) with choice. The default is none, which disables all SLS hardening. return enables SLS hardening for function returns. indirect-jmp enables SLS hardening for indirect jumps. all enables all SLS hardening. Pick your fighter. 👑
• -mindirect-branch-cs-prefix — Add a CS prefix to call and jmp to indirect thunk with branch target in r8-r15 registers, so that the call and jmp instruction length is 6 bytes to allow them to be replaced with lfence; call *%r8-r15 or lfence; jmp *%r8-r15 at run-time. So precise. So iconic. 🎀

---

These -m switches are supported in addition to all the above on x86-64 processors in 64-bit environments. ✨

• -m32, -m64, -mx32, -m16, -miamcu — Generate code for a 16-bit, 32-bit or 64-bit environment. Let's break it down, bestie:
  • -m32 sets int, long, and pointer types to 32 bits, and generates code that runs in 32-bit mode. 🌸
  • -m64 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. 🦋
  • -mx32 sets int, long, and pointer types to 32 bits, and generates code for the x86-64 architecture. Yes, all three at 32 bits, that's the whole point. 💅
  • -m16 is the same as -m32, except that it outputs the .code16gcc assembly directive at the beginning of the assembly output, so the binary can run in 16-bit mode. 🍓
  • -miamcu generates code which conforms to the Intel MCU psABI. It requires the -m32 option to be turned on. No -m32, no -miamcu, them's the rules. 🎀
• -mno-red-zone — Do not use a so-called "red zone" for x86-64 code. The red zone is mandated by the x86-64 ABI — it's a 128-byte area beyond the location of the stack pointer that isn't modified by signal or interrupt handlers, and therefore can be used for temporary data without adjusting the stack pointer. The flag -mno-red-zone disables this red zone. 🔥
• -mcmodel=small — bestie this is the small code model and it's the default one 💖 like the one you get if you don't say anything. Your program AND all its symbols have to be linked down in the lower 2 GB of the address space, okay? Pointers are still 64 bits tho. Programs can be statically or dynamically linked, no drama either way. ✨
• -mcmodel=kernel — kernel code model time 👑 the kernel runs in the negative 2 GB of the address space. This model has to be used for Linux kernel code, periodt, no exceptions fr fr.
• -mcmodel=medium — the medium model 🌸 the program is linked in the lower 2 GB of the address space, and small symbols also live there. But symbols with sizes larger than -mlarge-data-threshold get put into large data or BSS sections and can be located above 2GB. Programs can be statically or dynamically linked 💅
• -mcmodel=large — the large model 🦋 this one makes no assumptions about addresses and sizes of sections. It's giving unbothered. It's giving free spirit.
• -maddress-mode=long — long address mode ✨ only supported for 64-bit and x32 environments, and it's the default address mode for 64-bit environments.
• -maddress-mode=short — short address mode 🎀 only supported for 32-bit and x32 environments, and it's the default address mode for 32-bit and x32 environments.
• -mneeded, -mno-needed — these emit the GNU_PROPERTY_X86_ISA_1_NEEDED GNU property for Linux target, to indicate the micro-architecture ISA level required to execute the binary. iykyk 🔥

---

🪟 x86 Windows Options 💻

These additional options are available for Microsoft Windows targets, bestie:

• -mconsole — says a console application is to be generated, by instructing the linker to set the PE header subsystem type required for console applications. Available for Cygwin and MinGW targets and it's enabled by default on those targets. Slay 💅
• -mdll — available for Cygwin and MinGW targets. It specifies that a DLL---a dynamic link library---is to be generated, which enables the selection of the required runtime startup object and entry point. Dynamic link library core 🌈
• -mnop-fun-dllimport — available for Cygwin and MinGW targets. It specifies that the "dllimport" attribute should be ignored. Not me ignoring it 🙈
• -mthread — available for MinGW targets. It specifies that MinGW-specific thread support is to be used. Threads thread-ing 🧵✨
• -municode — available for MinGW-w64 targets. It causes the "UNICODE" preprocessor macro to be predefined, and chooses Unicode-capable runtime startup code. Unicode ate 💖
• -mwin32 — available for Cygwin and MinGW targets. It specifies that the typical Microsoft Windows predefined macros are to be set in the pre-processor, but it does not influence the choice of runtime library/startup code. Important distinction bestie!!
• -mwindows — available for Cygwin and MinGW targets. It specifies that a GUI application is to be generated by instructing the linker to set the PE header subsystem type appropriately.
• -fno-set-stack-executable — available for MinGW targets. It specifies that the executable flag for the stack used by nested functions isn't set. This is necessary for binaries running in kernel mode of Microsoft Windows, as there the User32 API, which is used to set executable privileges, isn't available. Lowkey a whole situation 😭
• -fwritable-relocated-rdata — available for MinGW and Cygwin targets. It specifies that relocated-data in read-only section is put into the ".data" section. This is a necessary for older runtimes not supporting modification of ".rdata" sections for pseudo-relocation.
• -mpe-aligned-commons — available for Cygwin and MinGW targets. It specifies that the GNU extension to the PE file format that permits the correct alignment of COMMON variables should be used when generating code. It is enabled by default if GCC detects that the target assembler found during configuration supports the feature. Vibe check passed ✅

See also under x86 Options for standard options.

---

🌩️ Xstormy16 Options 🌩️

These options are defined for Xstormy16:

• -msim — choose startup files and linker script suitable for the simulator. Simulator vibes only 🎀

---

🦋 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. Conditional default, we love her 💅
• -mfused-madd, -mno-fused-madd — enable or disable use of fused multiply/add and multiply/subtract instructions in the floating-point option. This has no effect if the floating-point option is not also enabled. Disabling fused multiply/add and multiply/subtract instructions forces the compiler to use separate instructions for the multiply and add/subtract operations. This may be desirable in some cases where strict IEEE 754-compliant results are required: the fused multiply add/subtract instructions do not round the intermediate result, thereby producing results with more bits of precision than specified by the IEEE standard. Disabling fused multiply add/subtract instructions also ensures that the program output is not sensitive to the compiler's ability to combine multiply and add/subtract operations. So much lore in one option 😭
• -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. Default behavior, bestie ✨
• -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. Kernel mode unlocked 🔓
• -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. Organized AND efficient 🎀
• -mauto-litpools, -mno-auto-litpools — these options control the treatment of literal pools. The default is -mno-auto-litpools, which places literals in a separate section in the output file unless -mtext-section-literals is used. With -mauto-litpools the literals are interspersed in the text section by the assembler. Compiler does not produce explicit ".literal" directives and loads literals into registers with "MOVI" instructions instead of "L32R" to let the assembler do relaxation and place literals as necessary. This option allows assembler to create several literal pools per function and assemble very big functions, which may not be possible with -mtext-section-literals. Big function energy 🔥
• -mtarget-align, -mno-target-align — when this option is enabled, GCC instructs the assembler to automatically align instructions to reduce branch penalties at the expense of some code density. The assembler attempts to widen density instructions to align branch targets and the instructions following call instructions. If there are not enough preceding safe density instructions to align a target, no widening is performed. The default is -mtarget-align. These options do not affect the treatment of auto-aligned instructions like "LOOP", which the assembler always aligns, either by widening density instructions or by inserting NOP instructions. "LOOP" does what it wants, periodt 💅
• -mlongcalls, -mno-longcalls — when this option is enabled, GCC instructs the assembler to translate direct calls to indirect calls unless it can determine that the target of a direct call is in the range allowed by the call instruction. This translation typically occurs for calls to functions in other source files. Specifically, the assembler translates a direct "CALL" instruction into an "L32R" followed by a "CALLX" instruction. The default is -mno-longcalls. This option should be used in programs where the call target can potentially be out of range. This option is implemented in the assembler, not the compiler, so the assembly code generated by GCC still shows direct call instructions---look at the disassembled object code to see the actual instructions. Note that the assembler uses an indirect call for every cross-file call, not just those that really are out of range. Sneaky little detail, don't forget it 🙈
• -mabi=name — generate code for the specified ABI. Permissible values are: call0, windowed. Default ABI is chosen by the Xtensa core configuration.
• -mabi=call0 — when this option is enabled function parameters are passed in registers "a2" through "a7", registers "a12" through "a15" are caller-saved, and register "a15" may be used as a frame pointer. When this version of the ABI is enabled the C preprocessor symbol "__XTENSA_CALL0_ABI__" is defined. Preprocessor symbol unlocked ✨
• -mabi=windowed — when this option is enabled function parameters are passed in registers "a10" through "a15", and called function rotates 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. Register window is rotated 8 registers back upon return. When this version of the ABI is enabled the C preprocessor symbol "__XTENSA_WINDOWED_ABI__" is defined. Rotating windows, she's flexible like that 🦋

---

👑 zSeries Options 👑

These are listed under

💖 ENVIRONMENT 💖

okay bestie, this is the part where GCC reads the room before it even starts compiling. Some env vars are like "where do I look for files", some are like "what language are we speaking", and some are just little switches that change the vibe. Also important: you can specify places to search using options such as -B, -I and -L. Those take precedence over places specified using environment variables, which in turn take precedence over those specified by the configuration of GCC. So the chain is: options > env vars > config. periodt.

LANG, LC_CTYPE, LC_MESSAGES, LC_ALL

These environment variables control the way that GCC uses localization information, which lets GCC work with different national conventions. GCC inspects the locale categories LC_CTYPE and LC_MESSAGES if it has been configured to do so. These locale categories can be set to any value supported by your installation. A typical value is en_GB.UTF-8 for English in the United Kingdom encoded in UTF-8.

The LC_CTYPE environment variable specifies character classification. GCC uses it to determine the character boundaries in a string; this is needed for some multibyte encodings that contain quote and escape characters that are otherwise interpreted as a string end or escape.

The LC_MESSAGES environment variable specifies the language to use in diagnostic messages.

If the LC_ALL environment variable is set, it overrides the value of LC_CTYPE and LC_MESSAGES; otherwise, LC_CTYPE and LC_MESSAGES default to the value of the LANG environment variable. If none of these variables are set, GCC defaults to traditional C English behavior.

TMPDIR

If TMPDIR is set, it specifies the directory to use for temporary files. GCC uses temporary files to hold the output of one stage of compilation which is to be used as input to the next stage: for example, the output of the preprocessor, which is the input to the compiler proper. temp file pipeline moment.

GCC_COMPARE_DEBUG

Setting GCC_COMPARE_DEBUG is nearly equivalent to passing -fcompare-debug to the compiler driver. See the documentation of this option for more details.

GCC_EXEC_PREFIX

If GCC_EXEC_PREFIX is set, it specifies a prefix to use in the names of the subprograms executed by the compiler. No slash is added when this prefix is combined with the name of a subprogram, but you can specify a prefix that ends with a slash if you wish.

If GCC_EXEC_PREFIX is not set, GCC attempts to figure out an appropriate prefix to use based on the pathname it is invoked with.

If GCC cannot find the subprogram using the specified prefix, it tries looking in the usual places for the subprogram.

The default value of GCC_EXEC_PREFIX is prefix/lib/gcc/ where prefix is the prefix to the installed compiler. In many cases prefix is the value of "prefix" when you ran the configure script.

Other prefixes specified with -B take precedence over this prefix.

This prefix is also used for finding files such as crt0.o that are used for linking.

In addition, the prefix is used in an unusual way in finding the directories to search for header files. For each of the standard directories whose name normally begins with /usr/local/lib/gcc (more precisely, with the value of GCC_INCLUDE_DIR), GCC tries replacing that beginning with the specified prefix to produce an alternate directory name. Thus, with -Bfoo/, GCC searches foo/bar just before it searches the standard directory /usr/local/lib/bar. If a standard directory begins with the configured prefix then the value of prefix is replaced by GCC_EXEC_PREFIX when looking for header files.

COMPILER_PATH

The value of COMPILER_PATH is a colon-separated list of directories, much like PATH. GCC tries the directories thus specified when searching for subprograms, if it cannot find the subprograms using GCC_EXEC_PREFIX.

LIBRARY_PATH

The value of LIBRARY_PATH is a colon-separated list of directories, much like PATH. When configured as a native compiler, GCC tries the directories thus specified when searching for special linker files, if it 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). linker girlies know.

LANG

This variable is used to pass locale information to the compiler. One way in which this information is used is to determine the character set to be used when character literals, string literals and comments are parsed in C and C++. When the compiler is configured to allow multibyte characters, the following values for LANG are recognized:

• C-JIS — Recognize JIS characters.
• C-SJIS — Recognize SJIS characters.
• C-EUCJP — Recognize EUCJP characters.

If LANG is not defined, or if it has some other value, then the compiler uses mblen and mbtowc as defined by the default locale to recognize and translate multibyte characters.

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.

• fixits-v1 — Emit parseable fix-it hints, equivalent to -fdiagnostics-parseable-fixits. In particular, columns are expressed as a count of bytes, starting at byte 1 for the initial column.
• fixits-v2 — As fixits-v1, but columns are expressed as display columns, as per -fdiagnostics-column-unit=display.

Some additional environment variables affect the behavior of the preprocessor.

CPATH, C_INCLUDE_PATH, CPLUS_INCLUDE_PATH, OBJC_INCLUDE_PATH

Each variable's value is a list of directories separated by a special character, much like PATH, in which to look for header files. The special character, PATH_SEPARATOR, is target-dependent and determined at GCC build time. For Microsoft Windows-based targets it is a semicolon, and for almost all other targets it is a colon.

CPATH specifies a list of directories to be searched as if specified with -I, but after any paths given with -I options on the command line. This environment variable is used regardless of which language is being preprocessed.

The remaining environment variables apply only when preprocessing the particular language indicated. Each specifies a list of directories to be searched as if specified with -isystem, but after any paths given with -isystem options on the command line.

In all these variables, an empty element instructs the compiler to search its current working directory. Empty elements can appear at the beginning or end of a path. For instance, if the value of CPATH is :/special/include, that has the same effect as -I. -I/special/include.

DEPENDENCIES_OUTPUT

If this variable is set, its value specifies how to output dependencies for Make based on the non-system header files processed by the compiler. System header files are ignored in the dependency output.

The value of DEPENDENCIES_OUTPUT can be just a file name, in which case the Make rules are written to that file, guessing the target name from the source file name. Or the value can have the form file target, in which case the rules are written to file file using target as the target name.

In other words, this environment variable is equivalent to combining the options -MM and -MF, with an optional -MT switch too.

SUNPRO_DEPENDENCIES

This variable is the same as DEPENDENCIES_OUTPUT (see above), except that system header files are not ignored, so it implies -M rather than -MM. However, the dependence on the main input file is omitted.

SOURCE_DATE_EPOCH

If this variable is set, its value specifies a UNIX timestamp to be used in replacement of the current date and time in the __DATE__ and __TIME__ macros, so that the embedded timestamps become reproducible.

The value of SOURCE_DATE_EPOCH must be a UNIX timestamp, defined as the number of seconds (excluding leap seconds) since 01 Jan 1970 00:00:00 represented in ASCII; identical to the output of date +%s on GNU/Linux and other systems that support the %s extension in the date command.

The value should be a known timestamp such as the last modification time of the source or package and it should be set by the build process. reproducible build besties, rise.

🐛 BUGS 🐛

For instructions on reporting bugs, see https://gcc.gnu.org/bugs/.

📎 FOOTNOTES 📎

1. On some systems, gcc -shared needs to build supplementary stub code for constructors to work. On multi-libbed systems, gcc -shared must select the correct support libraries to link against. Failing to supply the correct flags may lead to subtle defects. Supplying them in cases where they are not necessary is innocuous.

👀 SEE ALSO 👀

gpl(7), gfdl(7), fsf-funding(7), cpp(1), gcov(1), as(1), ld(1), gdb(1), dbx(1) and the Info entries for gcc, cpp, as, ld, binutils and gdb.

✍️ AUTHOR ✍️

See the Info entry for gcc, or http://gcc.gnu.org/onlinedocs/gcc/Contributors.html, for contributors to GCC.

©️ COPYRIGHT ©️

Copyright (c) 1988-2021 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 thing, bestie. this page is gcc-11, dated 2024-07-19, and it's the GCC(1) man page. you read the entire manual. iconic behavior. 💅✨🦋