GCC(1)

💖 NAME 💖

gcc — GNU project C and C++ compiler

okay so this is the GCC(1) manual bestie, and we're about to get so into it 💅✨

---

✨ SYNOPSIS ✨

    [-g] [-pg] [-Olevel]
    [-Wwarn...] [-Wpedantic]
    [-Idir...] [-Ldir...]
    [-Dmacro[=defn]...] [-Umacro]
    [-foption...] [-mmachine-option...]
    [-o outfile] [@file] infile...

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

---

🌸 DESCRIPTION 🌸

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

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. lowkey iconic that they just don't bother 🫶

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. no cap.

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. it's giving "choose your fighter" energy ⚔️

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. do not try to be cute and combine them, bestie, it will not slay 😭

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. so like, sometimes order is the moment and sometimes it's not 🎀

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

Some options take one or more arguments typically separated either by a space or by the equals sign (=) from the option name. Unless documented otherwise, an argument can be either numeric or a string. Numeric arguments must typically be small unsigned decimal or hexadecimal integers. Hexadecimal arguments must begin with the 0x prefix. Arguments to options that specify a size threshold of some sort may be arbitrarily large decimal or hexadecimal integers followed by a byte size suffix designating a multiple of bytes such as "kB" and "KiB" for kilobyte and kibibyte, respectively, "MB" and "MiB" for megabyte and mebibyte, "GB" and "GiB" for gigabyte and gigibyte, and so on. Such arguments are designated by byte-size in the following text. Refer to the NIST, IEC, and other relevant national and international standards for the full listing and explanation of the binary and decimal byte size prefixes. it's giving "units matter, bestie" 📏✨

---

🌈 OPTIONS 🌈

👑 Option Summary 👑

Here is a summary of all the options, grouped by type. Explanations are in the following sections. so like a lil appetizer before the main course 🍓

💖 Overall Options 💖

• -c
• -S
• -E
• -o file
• -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
• -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
• -Wabi-tag
• -Wcatch-value
• -Wcatch-value=n
• -Wno-class-conversion
• -Wclass-memaccess
• -Wcomma-subscript
• -Wconditionally-supported
• -Wno-conversion-null
• -Wctor-dtor-privacy
• -Wno-delete-incomplete
• -Wdelete-non-virtual-dtor
• -Wdeprecated-copy
• -Wdeprecated-copy-dtor
• -Weffc++
• -Wextra-semi
• -Wno-inaccessible-base
• -Wno-inherited-variadic-ctor
• -Wno-init-list-lifetime
• -Wno-invalid-offsetof
• -Wno-literal-suffix
• -Wmismatched-tags
• -Wmultiple-inheritance
• -Wnamespaces
• -Wnarrowing
• -Wnoexcept
• -Wnoexcept-type
• -Wnon-virtual-dtor
• -Wpessimizing-move
• -Wno-placement-new
• -Wplacement-new=n
• -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
• -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
• -Wselector
• -Wstrict-selector-match
• -Wundeclared-selector

🔥 Diagnostic Message Formatting Options 🔥

• -fmessage-length=n
• -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

🌸 Warning Options 🌸

Okay bestie, deep breath. This is the section. GCC has an honestly unhinged number of warning flags and we are going to look at every single one, no skips, no shortcuts, because that's just the kind of girl I am. 🎀

Starting with the general-purpose crew:

• -fsyntax-only
• -fmax-errors=n
• -Wpedantic
• -pedantic-errors
• -w
• -Wextra
• -Wall

Then it's alphabetical chaos all the way down, and honestly? iconic behaviour. 💅

• -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

the whole "are you speaking the right dialect of C++" saga, honestly a whole arc: ✅

• -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

And then the main character of the entire flag catalogue:

• -Werror
• -Werror=*
• -Wexpansion-to-defined
• -Wfatal-errors
• -Wfloat-conversion
• -Wfloat-equal

The format family, aka the ones that will humble you at 2am:

• -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-hsa
• -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]

Note the fancy one — -Wnormalized=[none|id|nfc|nfkc] takes one of those four values exactly, bestie, don't freestyle it. 🦋

• -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-pointer-div
• -Wsizeof-pointer-memaccess
• -Wstack-protector
• -Wstack-usage=byte-size
• -Wstrict-aliasing
• -Wstrict-aliasing=n
• -Wstrict-overflow
• -Wstrict-overflow=n
• -Wstring-compare
• -Wstringop-overflow=n
• -Wno-stringop-truncation
• -Wsuggest-attribute=[pure|const|noreturn|format|malloc]

Another pick-one, bestie — -Wsuggest-attribute=[pure|const|noreturn|format|malloc], those five options, periodt. ✨

• -Wswitch
• -Wno-switch-bool
• -Wswitch-default
• -Wswitch-enum
• -Wno-switch-outside-range
• -Wno-switch-unreachable
• -Wsync-nand
• -Wsystem-headers
• -Wtautological-compare
• -Wtrampolines
• -Wtrigraphs
• -Wtype-limits
• -Wundef
• -Wuninitialized
• -Wunknown-pragmas
• -Wunsuffixed-float-constants

And the whole unused saga, because apparently we're all just carrying dead code around like emotional baggage:

• -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

Okay phew, that was a journey. 🫶

---

🔍 Static Analyzer Options 🔍

This is where GCC stops just warning you and starts full-on investigating your code like it's a detective with a corkboard and red string. 🕵️‍♀️

• -fanalyzer
• -fanalyzer-call-summaries
• -fanalyzer-checker=name
• -fanalyzer-fine-grained
• -fanalyzer-state-merge
• -fanalyzer-state-purge
• -fanalyzer-transitivity
• -fanalyzer-verbose-edges
• -fanalyzer-verbose-state-changes
• -fanalyzer-verbosity=level

Then the dump squad — every single thing the analyzer can spill out for you:

• -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-state-purge
• -fdump-analyzer-supergraph

And the analyzer's own warning flags, all hanging out in their -Wno- era:

• -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-null-argument
• -Wno-analyzer-null-dereference
• -Wno-analyzer-possible-null-argument
• -Wno-analyzer-possible-null-dereference
• -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

---

🍓 C and Objective-C-only Warning Options 🍓

These ones are exclusive, bestie — C and Objective-C only, so if you're a C++ girlie these are not for you. 🙈

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

---

💀 Debugging Options 💀

Time to make your binary spill its secrets. 🌈

• -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
• -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

-gz[=type] — the =type bit is optional, so both -gz and -gz=type are a thing. 💖

And the debug-info-adjacent -f flags, the ones that lowkey run the whole backstage:

• -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

-femit-struct-debug-detailed[=spec-list] — the [=spec-list] part is optional, don't forget your brackets. 🎀

🚀 Optimization Options

okay bestie, deep breath, because this is THE list. this is the section where GCC keeps every single little optimization switch it owns, and it owns so many. one bullet per flag, exactly as written, because I refuse to lose a single one. let's go 💅

• -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-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
• --param name=value
• -O
• -O0
• -O1
• -O2
• -O3
• -Os
• -Ofast
• -Og

and that's the optimization roster, bestie. two hundred and thirty one of them and I counted. the -O family at the end is just sitting there like the popular kids at the lunch table 🍓

---

🧪 Program Instrumentation Options

the "we're watching you" collection. every one of these is in here, don't skip:

• -p
• -pg
• -fprofile-arcs
• --coverage
• -ftest-coverage
• -fprofile-abs-path
• -fprofile-dir=path
• -fprofile-generate
• -fprofile-generate=path
• -fprofile-note=path
• -fprofile-prefix-path=path
• -fprofile-update=method
• -fprofile-filter-files=regex
• -fprofile-exclude-files=regex
• -fprofile-reproducibility
• -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]
• -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 that -fsanitize-recover shows up twice, once bare and once with =style, and I am not merging them because I do not make the rules 💅

---

📝 Preprocessor Options

the before-the-compiler era. she's giving behind-the-scenes main character energy 🎀

• -Aquestion=answer
• -A-question[=answer]
• -C
• -CC
• -Dmacro[=defn]
• -dD
• -dI
• -dM
• -dN
• -dU
• -fdebug-cpp
• -fdirectives-only
• -fdollars-in-identifiers
• -fexec-charset=charset
• -fextended-identifiers
• -finput-charset=charset
• -fmacro-prefix-map=old=new
• -fmax-include-depth=depth
• -fno-canonical-system-headers
• -fpch-deps
• -fpch-preprocess
• -fpreprocessed
• -ftabstop=width
• -ftrack-macro-expansion
• -fwide-exec-charset=charset
• -fworking-directory
• -H
• -imacros file
• -include file
• -M
• -MD
• -MF
• -MG
• -MM
• -MMD
• -MP
• -MQ
• -MT
• -no-integrated-cpp
• -P
• -pthread
• -remap
• -traditional
• -traditional-cpp
• -trigraphs
• -Umacro
• -undef
• -Wp,option
• -Xpreprocessor option

---

🔧 Assembler Options

just two. short king section. we love a minimalist 👑

• -Wa,option
• -Xassembler option

---

🔗 Linker Options

the linker chapter, aka where the object files finally get a group chat.

• 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

yes, object-file-name is in there with no leading dash. that's not a typo, that's just what the list says. keep it. 🙈

---

📁 Directory Options

where to find things. the GPS of the compiler. 🗺️

• -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

---

🏗️ Code Generation Options

how the actual machine code gets shaped. the glow-up phase ✨

• -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
• -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

---

🛠️ Developer Options

the backstage pass section. this is for the people who debug the compiler while debugging their code. respect. 🫶

• -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]

---

🖥️ Machine-Dependent Options

different chips, different drama. this is where GCC says "okay but which CPU are we even talking about". 💀

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
• -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

<rainbow>-mbranch-protection=none|standard|pac-ret[+leaf +b-key]|bti</rainbow> is one single token, space and all. copy it exactly, don't get cute, I'm begging 🙏

Adapteva Epiphany Options

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

and that's where this part wraps, bestie — more machine-specific chaos incoming, don't you dare close the tab 🌸

✨ AMD GCN Options ✨

okay so AMD GCN bestie, this is the whole vibe check:

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

---

🎀 ARC Options 🎀

ARC? we love a long list. no skips, periodt:

• -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 girlies, this is the full menu, no crumbs:

• -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
• -mfdpic

---

🌸 AVR Options 🌸

AVR bestie, tiny chips big energy:

• -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 🖤

Blackfin? lowkey iconic, here's every flag:

• -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 ⭐

C6X bestie, short but slay:

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

---

💜 CRIS Options 💜

CRIS girlies, this is the whole list, no gatekeeping:

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

---

🌈 CR16 Options 🌈

CR16 bestie, we keeping it cute and complete:

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

---

🦄 C-SKY Options 🦄

C-SKY? okay main character architecture, here's the tea:

• -march=arch
• -mcpu=cpu
• -mbig-endian
• -EB
• -mlittle-endian
• -EL
• -mhard-float
• -msoft-float
• -mfpu=fpu
• -mdouble-float
• -mfdivdu
• -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

---

🍓 Darwin Options 🍓

Darwin bestie, this list is giving "long but worth it". every single one, even the repeats, we don't erase:

• -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 🌟

DEC Alpha bestie, old school but still ate:

• -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 🧚

eBPF girlies, kernel vibes only:

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

---

🎀 FR30 Options 🎀

FR30 bestie, short and sweet:

• -msmall-model
• -mno-lsim

---

✨ FT32 Options ✨

FT32? we love a compact queen:

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

---

🔥 FRV Options 🔥

FRV bestie, this list is long but we don't skip:

• -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 🐧

GNU/Linux bestie, libc choices are a vibe:

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

---

🌈 H8/300 Options 🌈

H8/300 bestie, retro but slay:

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

---

👑 HPPA Options 👑

HPPA queen, this list is giving royal treatment:

• -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 💀

IA-64 bestie, Itanium energy, we respect it:

• -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 🌸

LM32 bestie, small but mighty:

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

---

🦋 M32R/D Options 🦋

M32R/D girlies, this is the list, no cap:

• -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 🎀

M32C bestie, keeping it short:

• -mcpu=cpu
• -msim
• -memregs=number

---

🔥 M680x0 Options 🔥

M680x0 bestie, retro Motorola slay:

• -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 🌟

MCore bestie, here's every flag, periodt:

• -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 🧚

MeP bestie, this list is giving "compact but complete":

• -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 🌈

MicroBlaze bestie, last but never least:

• -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

✨ MIPS Options ✨

Bestie. Sit down. MIPS did not come to play — this is a whole vibe, an entire ecosystem of flags covering endianness, ISA levels, ABIs, float modes, DSP extensions, Loongson goodies and like, seventeen different "fix this specific chip" options. It's giving maximalist 💅 No skips, no edits, every single one is right here:

• -EL
• -EB
• -march=arch
• -mtune=arch
• -mips1
• -mips2
• -mips3
• -mips4
• -mips32
• -mips32r2
• -mips32r3
• -mips32r5
• -mips32r6
• -mips64
• -mips64r2
• -mips64r3
• -mips64r5
• -mips64r6
• -mips16
• -mno-mips16
• -mflip-mips16
• -minterlink-compressed
• -mno-interlink-compressed
• -minterlink-mips16
• -mno-interlink-mips16
• -mabi=abi
• -mabicalls
• -mno-abicalls
• -mshared
• -mno-shared
• -mplt
• -mno-plt
• -mxgot
• -mno-xgot
• -mgp32
• -mgp64
• -mfp32
• -mfpxx
• -mfp64
• -mhard-float
• -msoft-float
• -mno-float
• -msingle-float
• -mdouble-float
• -modd-spreg
• -mno-odd-spreg
• -mabs=mode
• -mnan=encoding
• -mdsp
• -mno-dsp
• -mdspr2
• -mno-dspr2
• -mmcu
• -mmno-mcu
• -meva
• -mno-eva
• -mvirt
• -mno-virt
• -mxpa
• -mno-xpa
• -mcrc
• -mno-crc
• -mginv
• -mno-ginv
• -mmicromips
• -mno-micromips
• -mmsa
• -mno-msa
• -mloongson-mmi
• -mno-loongson-mmi
• -mloongson-ext
• -mno-loongson-ext
• -mloongson-ext2
• -mno-loongson-ext2
• -mfpu=fpu-type
• -msmartmips
• -mno-smartmips
• -mpaired-single
• -mno-paired-single
• -mdmx
• -mno-mdmx
• -mips3d
• -mno-mips3d
• -mmt
• -mno-mt
• -mllsc
• -mno-llsc
• -mlong64
• -mlong32
• -msym32
• -mno-sym32
• -Gnum
• -mlocal-sdata
• -mno-local-sdata
• -mextern-sdata
• -mno-extern-sdata
• -mgpopt
• -mno-gopt
• -membedded-data
• -mno-embedded-data
• -muninit-const-in-rodata
• -mno-uninit-const-in-rodata
• -mcode-readable=setting
• -msplit-addresses
• -mno-split-addresses
• -mexplicit-relocs
• -mno-explicit-relocs
• -mcheck-zero-division
• -mno-check-zero-division
• -mdivide-traps
• -mdivide-breaks
• -mload-store-pairs
• -mno-load-store-pairs
• -mmemcpy
• -mno-memcpy
• -mlong-calls
• -mno-long-calls
• -mmad
• -mno-mad
• -mimadd
• -mno-imadd
• -mfused-madd
• -mno-fused-madd
• -nocpp
• -mfix-24k
• -mno-fix-24k
• -mfix-r4000
• -mno-fix-r4000
• -mfix-r4400
• -mno-fix-r4400
• -mfix-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
• -mflush-func=func
• -mno-flush-func
• -mbranch-cost=num
• -mbranch-likely
• -mno-branch-likely
• -mcompact-branches=policy
• -mfp-exceptions
• -mno-fp-exceptions
• -mvr4130-align
• -mno-vr4130-align
• -msynci
• -mno-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

The way that list just kept going and going 😭 she's committed, we respect her.

---

✨ MMIX Options ✨

Knuth's baby 💜 niche, elegant, kinda mysterious. Very main character energy:

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

---

✨ MN10300 Options ✨

Small, tidy, does her thing. ✨

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

---

✨ Moxie Options ✨

Tiny page, big presence. 🦋

• -meb
• -mel
• -mmul.x
• -mno-crt0

---

✨ MSP430 Options ✨

Microcontroller bestie 💖 lots of little knobs for a very small chip:

• -msim
• -masm-hex
• -mmcu=
• -mcpu=
• -mlarge
• -msmall
• -mrelax
• -mwarn-mcu
• -mcode-region=
• -mdata-region=
• -msilicon-errata=
• -msilicon-errata-warn=
• -mhwmult=
• -minrt
• -mtiny-printf

---

✨ NDS32 Options ✨

Endianness, register counts, extensions — a whole lil world. 🌸

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

---

✨ Nios II Options ✨

Look at all these custom instruction flags — she's customizable, she's bespoke, she's that girl 💅

• -G num
• -mgpopt=option
• -mgpopt
• -mno-gpopt
• -mgprel-sec=regexp
• -mr0rel-sec=regexp
• -mel
• -meb
• -mno-bypass-cache
• -mbypass-cache
• -mno-cache-volatile
• -mcache-volatile
• -mno-fast-sw-div
• -mfast-sw-div
• -mhw-mul
• -mno-hw-mul
• -mhw-mulx
• -mno-hw-mulx
• -mno-hw-div
• -mhw-div
• -mcustom-insn=N
• -mno-custom-insn
• -mcustom-fpu-cfg=name
• -mhal
• -msmallc
• -msys-crt0=name
• -msys-lib=name
• -march=arch
• -mbmx
• -mno-bmx
• -mcdx
• -mno-cdx

---

✨ Nvidia PTX Options ✨

Short and sweet, GPU-coded ✨

• -m32
• -m64
• -mmainkernel
• -moptimize

---

✨ OpenRISC Options ✨

Boards, libc flavours, mul/div in hardware or software, floats — she has range 🧚

• -mboard=name
• -mnewlib
• -mhard-mul
• -mhard-div
• -msoft-mul
• -msoft-div
• -msoft-float
• -mhard-float
• -mdouble-float
• -munordered-float
• -mcmov
• -mror
• -mrori
• -msext
• -msfimm
• -mshftimm

---

✨ PDP-11 Options ✨

Vintage, iconic, lowkey thrifted in the best way. 👑

• -mfpu
• -msoft-float
• -mac0
• -mno-ac0
• -m40
• -m45
• -m10
• -mint32
• -mno-int16
• -mint16
• -mno-int32
• -msplit
• -munix-asm
• -mdec-asm
• -mgnu-asm
• -mlra

---

✨ picoChip Options ✨

Four flags, zero notes, all vibe. 🎀

• -mae=ae_type
• -mvliw-lookahead=N
• -msymbol-as-address
• -mno-inefficient-warnings

---

✨ PowerPC Options ✨

Not me getting redirected 💀 she literally just says:

See RS/6000 and PowerPC Options.

---

✨ PRU Options ✨

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

---

✨ RISC-V Options ✨

The new girl and she is thriving ✨ so many options, so much self-knowledge:

• -mbranch-cost=N-instruction
• -mplt
• -mno-plt
• -mabi=ABI-string
• -mfdiv
• -mno-fdiv
• -mdiv
• -mno-div
• -march=ISA-string
• -mtune=processor-string
• -mpreferred-stack-boundary=num
• -msmall-data-limit=N-bytes
• -msave-restore
• -mno-save-restore
• -mstrict-align
• -mno-strict-align
• -mcmodel=medlow
• -mcmodel=medany
• -mexplicit-relocs
• -mno-explicit-relocs
• -mrelax
• -mno-relax
• -mriscv-attribute
• -mmo-riscv-attribute
• -malign-data=type

---

✨ RL78 Options ✨

Sim mode, mul modes, CPU flavours — organized, unbothered, moisturized. 💅

• -msim
• -mmul=none
• -mmul=g13
• -mmul=g14
• -mallregs
• -mcpu=g10
• -mcpu=g13
• -mcpu=g14
• -mg10
• -mg13
• -mg14
• -m64bit-doubles
• -m32bit-doubles
• -msave-mduc-in-interrupts

---

✨ RS/6000 and PowerPC Options ✨

THE big one. She is so back, she has a flag for every era of her life, and honestly? Iconic behaviour. Every single one, bestie:

• -mcpu=cpu-type
• -mtune=cpu-type
• -mcmodel=code-model
• -mpowerpc64
• -maltivec
• -mno-altivec
• -mpowerpc-gpopt
• -mno-powerpc-gpopt
• -mpowerpc-gfxopt
• -mno-powerpc-gfxopt
• -mmfcrf
• -mno-mfcrf
• -mpopcntb
• -mno-popcntb
• -mpopcntd
• -mno-popcntd
• -mfprnd
• -mno-fprnd
• -mcmpb
• -mno-cmpb
• -mhard-dfp
• -mno-hard-dfp
• -mfull-toc
• -mminimal-toc
• -mno-fp-in-toc
• -mno-sum-in-toc
• -m64
• -m32
• -mxl-compat
• -mno-xl-compat
• -mpe
• -malign-power
• -malign-natural
• -msoft-float
• -mhard-float
• -mmultiple
• -mno-multiple
• -mupdate
• -mno-update
• -mavoid-indexed-addresses
• -mno-avoid-indexed-addresses
• -mfused-madd
• -mno-fused-madd
• -mbit-align
• -mno-bit-align
• -mstrict-align
• -mno-strict-align
• -mrelocatable
• -mno-relocatable
• -mrelocatable-lib
• -mno-relocatable-lib
• -mtoc
• -mno-toc
• -mlittle
• -mlittle-endian
• -mbig
• -mbig-endian
• -mdynamic-no-pic
• -mswdiv
• -msingle-pic-base
• -mprioritize-restricted-insns=priority
• -msched-costly-dep=dependence_type
• -minsert-sched-nops=scheme
• -mcall-aixdesc
• -mcall-eabi
• -mcall-freebsd
• -mcall-linux
• -mcall-netbsd
• -mcall-openbsd
• -mcall-sysv
• -mcall-sysv-eabi
• -mcall-sysv-noeabi
• -mtraceback=traceback_type
• -maix-struct-return
• -msvr4-struct-return
• -mabi=abi-type
• -msecure-plt
• -mbss-plt
• -mlongcall
• -mno-longcall
• -mpltseq
• -mno-pltseq
• -mblock-move-inline-limit=num
• -mblock-compare-inline-limit=num
• -mblock-compare-inline-loop-limit=num
• -mstring-compare-inline-limit=num
• -misel
• -mno-isel
• -mvrsave
• -mno-vrsave
• -mmulhw
• -mno-mulhw
• -mdlmzb
• -mno-dlmzb
• -mprototype
• -mno-prototype
• -msim
• -mmvme
• -mads
• -myellowknife
• -memb
• -msdata
• -msdata=opt
• -mreadonly-in-sdata
• -mvxworks
• -G num
• -mrecip
• -mrecip=opt
• -mno-recip
• -mrecip-precision
• -mno-recip-precision
• -mveclibabi=type
• -mfriz
• -mno-friz
• -mpointers-to-nested-functions
• -mno-pointers-to-nested-functions
• -msave-toc-indirect
• -mno-save-toc-indirect
• -mpower8-fusion
• -mno-mpower8-fusion
• -mpower8-vector
• -mno-power8-vector
• -mcrypto
• -mno-crypto
• -mhtm
• -mno-htm
• -mquad-memory
• -mno-quad-memory
• -mquad-memory-atomic
• -mno-quad-memory-atomic
• -mcompat-align-parm
• -mno-compat-align-parm
• -mfloat128
• -mno-float128
• -mfloat128-hardware
• -mno-float128-hardware
• -mgnu-attribute
• -mno-gnu-attribute
• -mstack-protector-guard=guard
• -mstack-protector-guard-reg=reg
• -mstack-protector-guard-offset=offset
• -mprefixed
• -mno-prefixed
• -mpcrel
• -mno-pcrel
• -mmma
• -mno-mmma

She ate and left absolutely no crumbs. 😭

---

✨ RX Options ✨

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

---

✨ S/390 and zSeries Options ✨

Mainframe bestie, she's literally built different 💖 so many tuning knobs:

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

---

✨ Score Options ✨

• -meb
• -mel
• -mnhwloop
• -muls
• -mmac
• -mscore5
• -mscore5u
• -mscore7
• -mscore7d

---

✨ SH Options ✨

SuperH said "I'll take one of everything" and honestly, valid. 😌

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

---

✨ Solaris 2 Options ✨

Short list, big sun energy ☀️

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

---

✨ SPARC Options ✨

Legacy queen, never not relevant, always serving. 💅

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

---

✨ System V Options ✨

Just four, tiny but mighty. ⭐

• -Qy
• -Qn
• -YP,paths
• -Ym,dir

---

✨ TILE-Gx Options ✨

• -mcpu=CPU
• -m32
• -m64
• -mbig-endian
• -mlittle-endian
• -mcmodel=code-model

---

✨ TILEPro Options ✨

• -mcpu=cpu
• -m32

---

✨ V850 Options ✨

Renesas bestie with the longest "specific CPU variant" list you ever did see. 🌸

• -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 ✨

Closing us out with the original girlies: -mg, -mgnu, -munix. True vintage icons. 🦋

• -mg
• -mgnu
• -munix

Visium Options 💫

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

---

VMS Options 🌊

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

---

VxWorks Options 🛠️

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

---

x86 Options 👑

Bestie, sit down, hydrate, this list is long. Like, the way it just keeps going?? Every single one of these is a real flag, no cap:

-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 · -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 · -mcldemote · -mms-bitfields · -mno-align-stringops · -minline-all-stringops · -minline-stringops-dynamically · -mstringop-strategy=alg · -mmemcpy-strategy=strategy · -mmemset-strategy=strategy · -mpush-args · -maccumulate-outgoing-args · -m128bit-long-double · -m96bit-long-double · -mlong-double-64 · -mlong-double-80 · -mlong-double-128 · -mregparm=num · -msseregparm · -mveclibabi=type · -mvect8-ret-in-mem · -mpc32 · -mpc64 · -mpc80 · -mstackrealign · -momit-leaf-frame-pointer · -mno-red-zone · -mno-tls-direct-seg-refs · -mcmodel=code-model · -mabi=name · -maddress-mode=mode · -m32 · -m64 · -mx32 · -m16 · -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

---

x86 Windows Options 🪟

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

---

Xstormy16 Options ⛈️

-msim

---

Xtensa Options 🌪️

-mconst16 · -mno-const16 · -mfused-madd · -mno-fused-madd · -mforce-no-pic · -mserialize-volatile · -mno-serialize-volatile · -mtext-section-literals · -mno-text-section-literals · -mauto-litpools · -mno-auto-litpools · -mtarget-align · -mno-target-align · -mlongcalls · -mno-longcalls

---

zSeries Options 🌈

See S/390 and zSeries Options.

---

✨ Options Controlling the Kind of Output ✨

OK so here's the tea, bestie ☕ — a compile can go through up to four stages, and they always happen in this exact order:

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

GCC can preprocess and compile a bunch of files either into several assembler input files, or squish them into one assembler input file. Then every assembler input file produces an object file, and linking smooshes all those object files together — the ones you just compiled and the ones you handed it as input — into one executable file. Iconic behaviour, honestly. 👑

---

So like… which language is this file?? 🤔

For any given input file, the file name suffix decides what kind of compilation happens. Vibe check incoming:

• 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: 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. Also 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.
• 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 the language out loud (well, type it) with the -x option:

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

        c  c-header  cpp-output
        c++  c++-header  c++-cpp-output
        objective-c  objective-c-header  objective-c-cpp-output
        objective-c++ objective-c++-header objective-c++-cpp-output
        assembler  assembler-with-cpp
        ada
        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).

---

Stopping the train early 🚂✋

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.

Not me pointing out that some combos (for example, -x cpp-output -E) instruct gcc to do absolutely nothing at all. Literally nothing. Zero. Nada. 💀

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

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

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

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

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

Input files that don't require compilation are ignored.

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

Input files that don't require preprocessing are ignored.

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

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

• -v — Print (on standard error output) the commands executed to run the stages of compilation. Also print the version number of the compiler driver program and of the preprocessor and the compiler proper.
• -### — Like -v except the commands are not executed and arguments are quoted unless they contain only alphanumeric characters or "./-_". This is useful for shell scripts to capture the driver-generated command lines.
• --help — Print (on the standard output) a description of the command-line options understood by gcc. If the -v option is also specified then --help is also passed on to the various processes invoked by gcc, so that they can display the command-line options they accept. If the -Wextra option has also been specified (prior to the --help option), then command-line options that have no documentation associated with them are also displayed.
• --target-help — Print (on the standard output) a description of target-specific command-line options for each tool. For some targets extra target-specific information may also be printed.
• --help={class|[^]qualifier}[,...] — bestie, this one prints (straight to standard output) a whole description of every command-line option the compiler knows about that fits the classes and qualifiers you asked for. Here are the classes you can flex:
  • optimizers — shows you literally all the optimization options the compiler supports. ✨
  • warnings — shows you every option that controls the warning messages the compiler spits out.
  • target — shows target-specific options. BUT unlike the --target-help option, it does not show the target-specific options of the linker and assembler, because those tools just don't support the extended --help= syntax rn. 💀
  • params — shows you the values recognized by the --param option.
  • language — shows 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 the languages, you gotta select the common class instead.
  • common — shows the options that are common to all languages.

And these are the qualifiers you can throw in:

• undocumented — only shows the options that are undocumented. 🙈
• joined — shows options that take an argument appearing after an equal sign, all in one continuous piece of text, like --help=target.
• separate — shows options that take their argument as a separate word following the original option, like -o output-file.

So for example, to see all the undocumented target-specific switches the compiler supports, you'd do:

--help=target,undocumented

You can flip the sense of any qualifier by putting a ^ in front of it, so for example to show all the binary warning options (i.e. the ones that are either on or off and don't take an argument) that actually have a description, you'd go:

--help=warnings,^joined,^undocumented

Ok important: the argument to --help= must not be just a bunch of inverted qualifiers by themselves. The compiler will not be into that.

Combining several classes is possible too, though ngl it usually restricts the output so much that there's literally nothing to show. The one case where it does work, though, is when one of the classes is target. For example, to show all the target-specific optimization options:

--help=target,optimizers

Also! The --help= option can be repeated on the command line. Each successive use displays its requested class of options, skipping the ones that have already been shown. And if --help is also specified anywhere on the command line, then it takes precedence over any --help= option. 👑

One more thing: if the -Q option shows up on the command line before the --help= option, then the descriptive text that --help= displays changes. Instead of describing the displayed options, it gives you an indication of whether the option is enabled, disabled, or set to a specific value (assuming the compiler knows this at the point where the --help= option is used). 🌸

Here's 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 you can find out which optimizations are enabled at -O2 by using:

-Q -O2 --help=optimizers

Or 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 — displays the version number and copyrights of the invoked GCC. iconic. 💅
• -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 that returned an error indication. The C, C++, and Fortran front ends return 4 if an internal compiler error is encountered. 🫶
• -pipe — uses pipes instead of temporary files for communication between the various stages of compilation. Ngl this fails to work on some systems where the assembler can't read from a pipe; but the GNU assembler has no trouble at all. ✨
• -specs=file — processes file after the compiler reads in the standard specs file, in order to override the defaults that the gcc driver program uses when it's figuring out what switches to pass to cc1, cc1plus, as, ld, etc. You can specify more than one -specs=file on the command line, and they get processed in order, from left to right.
• -wrapper — invokes all subcommands under a wrapper program. The name of the wrapper program and its parameters get passed as a comma separated list.

gcc -c t.c -wrapper gdb,--args

This invokes all subprograms of gcc under gdb --args, so the invocation of cc1 is gdb --args cc1 ....

• -ffile-prefix-map=old=new — when compiling files that live in directory old, record any references to them in the result of the compilation as if the files lived 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 — loads 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 — defines an argument called key with a value of value for the plugin called name.
• -fdump-ada-spec[-slim] — for C and C++ source and include files, generates corresponding Ada specs. 💜
• -fada-spec-parent=unit — in conjunction with -fdump-ada-spec[-slim] above, generates Ada specs as child units of parent unit.
• -fdump-go-spec=file — for input files in any language, generates corresponding Go declarations in file. This generates Go "const", "type", "var", and "func" declarations which may be a useful way to start writing a Go interface to code written in some other language. 🌈
• @file — reads command-line options from file. The options read are inserted in place of the original @file option. If file doesn't exist, or can't be read, then the option is 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 get 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 exact same way as for compiling C programs (usually with the name gcc). ✨

However — and this is the part everyone forgets — the use of gcc does not add the C++ library. g++ is a program that calls GCC and automatically specifies linking against the C++ library. It treats .c, .h and .i files as C++ source files instead of C source files unless -x is used. This program is also useful when precompiling a C header file with a .h extension for use in C++ compilations. On many systems, g++ is also installed with the name c++. 💅

When you compile C++ programs, you may specify many of the same command-line options that you use for compiling programs in any language; or command-line options meaningful for C and related languages; or options that are meaningful only for C++ programs.

---

🌸 Options Controlling C Dialect 🌸

The following options control the dialect of C (or languages derived from C, such as C++, Objective-C and Objective-C++) that the compiler accepts:

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

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

The alternate keywords "__asm__", "__extension__", "__inline__" and "__typeof__" continue to work despite -ansi. You would not want to use them in an ISO C program, of course, but it's useful to put them in header files that might be included in compilations done with -ansi. Alternate predefined macros such as "__unix__" and "__vax__" are also available, with or without -ansi. 🦋

The -ansi option does not cause non-ISO programs to be rejected gratuitously. For that, -Wpedantic is required in addition to -ansi. 😭

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

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

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

Okay so here's the standards tea, bestie 🫖✨ — the compiler can handle a whole bunch of base standards, like c90 or c++98, and also the GNU dialects of those same standards, like gnu90 or gnu++98.

When you pick a base standard, the compiler takes all programs that follow that standard plus anything using GNU extensions that don't contradict it. For example, -std=c90 turns off certain GCC features that are incompatible with ISO C90 — like the asm and typeof keywords — but it does not turn off other GNU extensions that just don't have a meaning in ISO C90, like omitting the middle term of a ?: expression. iykyk 💅

On the other hand, when you specify a GNU dialect of a standard, every feature the compiler supports gets enabled — even the ones that change the meaning of the base standard. So yeah, some strict-conforming programs may get rejected. Not a bug, that's the deal 🙈

The particular standard you pick is what -Wpedantic uses to figure out which features count as GNU extensions for that version of the standard. For example, -std=gnu90 -Wpedantic warns about C++ style // comments, while -std=gnu99 -Wpedantic does not. Character development fr fr 🌸

You must provide a value for this option. The possible values are:

• c90, c89, iso9899:1990 — supports all ISO C90 programs, with certain GNU extensions that conflict with ISO C90 disabled. Same as -ansi for C code. the og 💖
• 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 the same as C11 except for corrections of defects (all of which are also applied with -std=c11) and a new value of __STDC_VERSION__, and so 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. 🧪 handle with care
• gnu90, gnu89 — GNU dialect of ISO C90 (including some C99 features). extra sprinkles
• 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. This is the default for C++ code. 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. The name gnu++1z is deprecated.
• c++20, c++2a — the next revision of the ISO C++ standard, planned for 2020. Support is highly experimental, and will almost certainly change in incompatible ways in future releases. 🧪⚠️
• gnu++20, gnu++2a — GNU dialect of -std=c++20. Support is highly experimental, and will almost certainly change in incompatible ways in future releases. 🧪⚠️

---

✨ -fgnu89-inline ✨

The option -fgnu89-inline tells GCC to use the traditional GNU semantics for inline functions when you're 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. handy!

✨ -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. so much lore 🦋

✨ -fallow-parameterless-variadic-functions ✨

Accept variadic functions without named parameters.

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

✨ -fno-asm ✨

Do not recognize asm, inline or typeof as a keyword, so that code can use these words as identifiers. You can use the keywords __asm__, __inline__ and __typeof__ instead. -ansi implies -fno-asm. 💅

In C++, this switch only affects the typeof keyword, since asm and inline are standard keywords. You may want to use the -fno-gnu-keywords flag instead, which has the same effect. In C99 mode (-std=c99 or -std=gnu99), this switch only affects the asm and typeof keywords, since inline is a standard keyword in ISO C99. context matters bestie

✨ -fno-builtin ✨

✨ -fno-builtin-function ✨

Don't recognize built-in functions that do not begin with __builtin_ as prefix.

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

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

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

✨ -fgimple ✨

Enable parsing of function definitions marked with __GIMPLE. This is an experimental feature that allows unit testing of GIMPLE passes. 🧪

✨ -fhosted ✨

Assert that compilation targets a hosted environment. This implies -fbuiltin. A hosted environment is one in which the entire standard library is available, and in which main has a return type of int. Examples are nearly everything except a kernel. 🌈 This is equivalent to -fno-freestanding.

✨ -ffreestanding ✨

Assert that compilation targets a freestanding environment. This implies -fno-builtin. A freestanding environment is one in which the standard library may not exist, and program startup may not necessarily be at main. The most obvious example is an OS kernel. 👑 This is equivalent to -fno-hosted.

• -fopenacc — bestie, this one turns on OpenACC handling. Like, it lets the compiler deal with #pragma acc directives in C/C++ and !$acc in Fortran 💖. When you slap -fopenacc on, the compiler generates accelerated code according to the OpenACC Application Programming Interface v2.6 (https://www.openacc.org). Heads up though: this option implies -pthread, so it's only supported on targets that actually have support for -pthread, no cap 🫶
• -fopenacc-dim=geom — sets the default compute dimensions for parallel offload regions that don't explicitly specify their own. The geom value is a triple of :-separated sizes, in this order: gang, worker, and vector. You can omit a size to just use a target-specific default value, lowkey convenient ✨
• -fopenmp — the OpenMP unlock 🔥. Enables handling of OpenMP directives #pragma omp in C/C++ and !$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). Just like its OpenACC cousin, this option implies -pthread, so it's only supported on targets that have support for -pthread. And plot twist: -fopenmp implies -fopenmp-simd too 💅
• -fopenmp-simd — this one's the SIMD-only slice 🍓. Enables handling of OpenMP's SIMD directives with #pragma omp in C/C++ and !$omp in Fortran. Other OpenMP directives are ignored, so it's giving "just the vectorized parts, thanks" ✨
• -fgnu-tm — when you specify this option, the compiler generates code for the Linux variant of Intel's current Transactional Memory ABI specification document (Revision 1.1, May 6 2009). Fr fr, this is an experimental feature whose interface may change in future versions of GCC as the official specification changes 😭. And please note: not all architectures are supported for this feature, so vibe check your target first.

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

Note that the transactional memory feature is not supported with non-call exceptions (-fnon-call-exceptions). Keep that in mind bestie 💀

• -fms-extensions — accepts some non-standard constructs used in Microsoft header files. In C++ code, this allows member names in structures to be similar to previous types declarations, 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 👑

• -fplan9-extensions — accepts some non-standard constructs used in Plan 9 code 🦋. This enables -fms-extensions, permits passing pointers to structures with anonymous fields to functions that expect pointers to elements of the type of the field, and permits referring to anonymous fields declared using a typedef. Big catch though: this is only supported for C, not C++, periodt.
• -fcond-mismatch — allows 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++, so don't come crying to me 🌸
• -flax-vector-conversions — allows implicit conversions between vectors with differing numbers of elements and/or incompatible element types. > This option should not be used for new code. It's giving legacy energy, iykyk 🙈
• -funsigned-char — lets the type char be unsigned, like unsigned char. Every kind of machine has a default for what char should be — 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 soooo many programs were 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. And listen: 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. Periodt 💅
• -fsigned-char — lets the type char be signed, like signed char. Note that this is equivalent to -fno-unsigned-char, which is the negative form of -funsigned-char. Likewise, the option -fno-signed-char is equivalent to -funsigned-char ✨
• -fsigned-bitfields, -funsigned-bitfields, -fno-signed-bitfields, -fno-unsigned-bitfields — these options control whether a bit-field is signed or unsigned when the declaration does not use either signed or unsigned. By default, such a bit-field is signed, because this is consistent: the basic integer types such as int are signed types 👑
• -fsso-struct=endianness — sets the default scalar storage order of structures and unions to the specified endianness. The accepted values are big-endian, little-endian and native for the native endianness of the target (which is the default). This option is not supported for C++, bestie.

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. Like, actually read this one 💀

---

✨ 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 🎀

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

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

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

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

Version 1 is the version of the C++ ABI that first appeared in G++ 3.2.

Version 2 is the version of the C++ ABI that first appeared in G++ 3.4, and was the default through G++ 4.9.

Version 3 corrects an error in mangling a constant address as a template argument.

Version 4, which first appeared in G++ 4.5, implements a standard mangling for vector types.

Version 5, which first appeared in G++ 4.6, corrects the mangling of attribute const/volatile on function pointer types, decltype of a plain decl, and use of a function parameter in the declaration of another parameter.

Version 6, which first appeared in G++ 4.7, corrects the promotion behavior of C++11 scoped enums and the mangling of template argument packs, const/static_cast, prefix ++ and --, and a class scope function used as a template argument.

Version 7, which first appeared in G++ 4.8, that treats nullptr_t as a builtin type and corrects the mangling of lambdas in default argument scope.

Version 8, which first appeared in G++ 4.9, corrects the substitution behavior of function types with function-cv-qualifiers.

Version 9, which first appeared in G++ 5.2, corrects the alignment of nullptr_t.

Version 10, which first appeared in G++ 6.1, adds mangling of attributes that affect type identity, such as ia32 calling convention attributes (e.g. stdcall).

Version 11, which first appeared in G++ 7, corrects the mangling of sizeof... expressions and operator names. For multiple entities with the same name within a function, that are declared in different scopes, the mangling now changes starting with the twelfth occurrence. It also implies -fnew-inheriting-ctors.

Version 12, which first appeared in G++ 8, corrects the calling conventions for empty classes on the x86_64 target and for classes with only deleted copy/move constructors. It accidentally changes the calling convention for classes with a deleted copy constructor and a trivial move constructor 🙈

Version 13, which first appeared in G++ 8.2, fixes the accidental change in version 12.

Version 14, which first appeared in G++ 10, corrects the mangling of the nullptr expression.

See also -Wabi. 💖

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

With -fabi-version=0 (the default), this defaults to 11 (GCC 7 compatibility). If another ABI version is explicitly selected, this defaults to 0. For compatibility with GCC versions 3.2 through 4.9, use -fabi-compat-version=2.

If this option is not provided but -Wabi=n is, that version is used for compatibility aliases. If this option is provided along with -Wabi (without the version), the version from this option is used for the warning. Slay, compatibility 🫶

• -fno-access-control — turns off all access checking. This switch is mainly useful for working around bugs in the access control code 💅
• -faligned-new — enables support for C++17 new of types that require more alignment than void* ::operator new(std::size_t) provides. A numeric argument such as -faligned-new=32 can be used to specify how much alignment (in bytes) is provided by that function, but few users will need to override the default of alignof(std::max_align_t). This flag is enabled by default for -std=c++17 ✨
• -fchar8_t, -fno-char8_t — enables support for char8_t as adopted for C++2a. This includes the addition of a new char8_t fundamental type, changes to the types of UTF-8 string and character literals, new signatures for user-defined literals, associated standard library updates, and new __cpp_char8_t and __cpp_lib_char8_t feature test macros 👑

This option enables functions to be overloaded for ordinary and UTF-8 strings:

int f(const char *);    // #1
int f(const char8_t *); // #2
int v1 = f("text");     // Calls #1
int v2 = f(u8"text");   // Calls #2

and introduces new signatures for user-defined literals:

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

okay so bestie, the type change for UTF-8 string and character literals? it's giving chaos — it introduces incompatibilities with ISO C++11 and later standards. like, watch this: the code below is well-formed under ISO C++11, but it's ill-formed when -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 — check that the pointer returned by operator new is non-null before we even try to modify the storage it allocated. ngl, this check is normally unnecessary, 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 all other cases, when operator new has a non-empty exception specification, memory exhaustion gets signalled by throwing std::bad_alloc. see also new (nothrow). ✨
• -fconcepts, -fconcepts-ts — below -std=c++2a, -fconcepts turns on support for the C++ Extensions for Concepts Technical Specification, ISO 19217 (2015). with -std=c++2a and above, Concepts are part of the language standard, so -fconcepts defaults to on. BUT — and this is the tea ☕ — the standard's spec of Concepts differs significantly from the TS, so some constructs that were allowed in the TS but didn't make it into the standard can still be enabled with -fconcepts-ts.
• -fconstexpr-depth=n — set the max 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 — set the max level of nested evaluation depth for C++11 constexpr functions that'll be cached to n. it's a heuristic that trades off compilation speed (when the cache dodges repeated calculations) against memory consumption (when the cache grows huge from highly recursive evaluations). the default is 8. very few users are likely to want to adjust it, but if your code does heavy constexpr calculations you might wanna experiment to find which value works best for you. 🧚
• -fconstexpr-loop-limit=n — set the max number of iterations for a loop in C++14 constexpr functions to n. you need a limit to detect infinite loops during constant expression evaluation. the default is 262144 (1<<18). 🔁
• -fconstexpr-ops-limit=n — set the max number of operations during a single constexpr evaluation. even when the number of iterations of a single loop is limited with the above limit, if there are several nested loops and each of them has many iterations but still smaller than the above limit — or if in a body of some loop, or even outside a loop, too many expressions need to be evaluated — the resulting constexpr evaluation might take too long. the default is 33554432 (1<<25). 😭
• -fcoroutines — enable support for the C++ coroutines extension (experimental). ✨
• -fno-elide-constructors — the C++ standard allows an implementation to omit 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 causes G++ to call trivial member functions which otherwise would be expanded inline. in C++17, the compiler is required to omit these temporaries, but this option still affects trivial member functions. 👑
• -fno-enforce-eh-specs — don't generate code to check for violation of exception specifications at run time. > this option violates the C++ standard, but it may be useful for reducing code size in production builds, much like defining NDEBUG. this does not give user code permission to throw exceptions in violation of the exception specifications; the compiler still optimizes based on the specifications, so throwing an unexpected exception results in undefined behavior at run time. 💀
• -fextern-tls-init, -fno-extern-tls-init — the C++11 and OpenMP standards allow thread_local and threadprivate variables to have dynamic (runtime) initialization. to support this, any use of such a variable goes through a wrapper function that performs any necessary initialization. when the use and definition of the variable are in the same translation unit, this overhead can be optimized away — but when the use is in a different translation unit there's significant overhead even if the variable doesn't actually need dynamic initialization. if the programmer can be sure that no use of the variable in a non-defining TU needs to trigger dynamic initialization (either because the variable is statically initialized, or a use of the variable in the defining TU will be executed before any uses in another TU), they can avoid this overhead with the -fno-extern-tls-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 — do not recognize typeof as a keyword, so that code can use this word as an identifier. you can use the keyword __typeof__ instead. this option is implied by the strict ISO C++ dialects: -ansi, -std=c++98, -std=c++11, etc.
• -fno-implicit-templates — never emit code for non-inline templates that are instantiated implicitly (i.e. by use); only emit code for explicit instantiations. if you use this option, you must take care to structure your code to include all the necessary explicit instantiations to avoid getting undefined symbols at link time. 🙈
• -fno-implicit-inline-templates — don't emit code for implicit instantiations of inline templates, either. the default is to handle inlines differently so that compiles with and without optimization need the same set of explicit instantiations.
• -fno-implement-inlines — to save space, do not emit out-of-line copies of inline functions controlled by #pragma implementation. > this causes linker errors if these functions are not inlined everywhere they are called.
• -fms-extensions — disable Wpedantic warnings about constructs used in MFC, such as implicit int and getting a pointer to member function via non-standard syntax.
• -fnew-inheriting-ctors — enable the P0136 adjustment to the semantics of C++11 constructor inheritance. this is part of C++17 but also considered to be a Defect Report against C++11 and C++14. this flag is enabled by default unless -fabi-version=10 or lower is specified. ✨
• -fnew-ttp-matching — enable the P0522 resolution to Core issue 150, template template parameters and default arguments: this allows a template with default template arguments as an argument for a template template parameter with fewer template parameters. this flag is enabled by default for -std=c++17. 💅
• -fno-nonansi-builtins — disable built-in declarations of functions that are not mandated by ANSI/ISO C. these include ffs, alloca, _exit, index, bzero, conjf, and other related functions.
• -fnothrow-opt — treat a throw() exception specification as if it were a noexcept specification, to reduce or eliminate the text size overhead relative to a function with no exception specification. if the function has local variables of types with non-trivial destructors, the exception specification actually makes the function smaller, because the EH cleanups for those variables can be optimized away. the semantic effect is that an exception thrown out of a function with such an exception specification results in a call to terminate rather than unexpected. ⭐
• -fno-operator-names — do not treat the operator name keywords and, bitand, bitor, compl, not, or and xor as synonyms as keywords.
• -fno-optional-diags — disable diagnostics that the standard says a compiler does not need to issue. currently, the only such diagnostic issued by G++ is the one for a name having multiple meanings within a class.
• -fpermissive — downgrade some diagnostics about nonconformant code from errors to warnings. thus, using -fpermissive allows some nonconforming code to compile. 🫶
• -fno-pretty-templates — when an error message refers to a specialization of a function template, the compiler normally prints the signature of the template followed by the template arguments and any typedefs or typenames in the signature (e.g. void f(T) [with T = int] rather than void f(int)) so that it's clear which template is involved. when an error message refers to a specialization of a class template, the compiler omits any template arguments that match the default template arguments for that template. if either of these behaviors makes it harder to understand the error message rather than easier, you can use -fno-pretty-templates to disable them.
• -fno-rtti — disable generation of information about every class with virtual functions for use by the C++ run-time type identification features (dynamic_cast and typeid). if you don't use those parts of the language, you can save some space by using this flag. note that exception handling uses the same information, but G++ generates it as needed. the dynamic_cast operator can still be used for casts that do not require run-time type information, i.e. casts to void * or to unambiguous base classes. > mixing code compiled with -frtti with that compiled with -fno-rtti may not work. for example, programs may fail to link if a class compiled with -fno-rtti is used as a base for a class compiled with -frtti. 💀
• -fsized-deallocation — bestie, this one turns on 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. It's giving very 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. And heads up: 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). Careful tho: this assumption may not be valid if the program uses a cast to convert an arbitrary integer value to the enumerated type. 🙈
• -fstrong-eval-order — evaluate member access, array subscripting, and shift expressions in left-to-right order, and evaluate assignment in right-to-left order, as adopted for C++17. Enabled by default with -std=c++17. And -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 — set the maximum number of template instantiation notes for a single warning or error to n. The default value is 10.
• -ftemplate-depth=n — set the maximum instantiation depth for template classes to n. A limit on the template instantiation depth is needed to detect endless recursions during template class instantiation. ANSI/ISO C++ conforming programs must not rely on a maximum depth greater than 17 (changed to 1024 in C++11). The default value is 900, as the compiler can run out of stack space before hitting 1024 in some situations. 😭
• -fno-threadsafe-statics — do not emit the extra code to use the routines specified in the C++ ABI for thread-safe initialization of local statics. You can use this option to reduce code size slightly in code that doesn't need to be thread-safe.
• -fuse-cxa-atexit — register destructors for objects with static storage duration with the __cxa_atexit function rather than the atexit function. This option is required for fully standards-compliant handling of static destructors, but only works if your C library supports __cxa_atexit. 🫶
• -fno-use-cxa-get-exception-ptr — don't use the __cxa_get_exception_ptr runtime routine. This causes std::uncaught_exception to be incorrect, but is necessary if the runtime routine is not available. yes, incorrect, we said it
• -fvisibility-inlines-hidden — this switch declares that the user does not attempt to compare pointers to inline functions or methods where the addresses of the two functions are taken in different shared objects.

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

The behavior of this switch is not quite the same as marking the methods as hidden directly, because it does not affect static variables local to the function or cause the compiler to deduce that the function is defined in only one shared object.

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

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

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

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

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

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

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

• -fno-weak — do not use weak symbol support, even if it is provided by the linker. By default, G++ uses weak symbols if they are available. This option exists only for testing, and should not be used by end-users; it results in inferior code and has no benefits. This option may be removed in a future release of G++. bestie do NOT touch this one 🙅‍♀️
• -fext-numeric-literals (C++ and Objective-C++ only) — accept imaginary, fixed-point, or machine-defined literal number suffixes as GNU extensions. When this option is turned off these suffixes are treated as C++11 user-defined literal numeric suffixes. This is on by default for all pre-C++11 dialects and all GNU dialects: -std=c++98, -std=gnu++98, -std=gnu++11, -std=gnu++14. This option is off by default for ISO C++11 onwards (-std=c++11, ...). ✨
• -nostdinc++ — do not search for header files in the standard directories specific to C++, but do still search the other standard directories. (This option is used when building the C++ library.) 💖

---

In addition, these warning options have meanings only for C++ programs:

• -Wabi-tag (C++ and Objective-C++ only) — warn when a type with an ABI tag is used in a context that does not have that ABI tag. See C++ Attributes for more information about ABI tags.
• -Wcomma-subscript (C++ and Objective-C++ only) — warn about uses of a comma expression within a subscripting expression. This usage was deprecated in C++2a. 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++2a.

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

• -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. Lowkey important, bestie.

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) — Okay so 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) — Warn when the destination of a call to a raw memory function such as "memset" or "memcpy" is an object of class type, and when writing into such an object might bypass the class non-trivial or deleted constructor or copy assignment, violate const-correctness or encapsulation, or corrupt virtual table pointers. Modifying the representation of such objects may violate invariants maintained by member functions of the class. For example, the call to "memset" below is undefined because it modifies a non-trivial class object and is, therefore, diagnosed. The safe way to either initialize or clear the storage of objects of such types is by using the appropriate constructor or assignment operator, if one is available.

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

The -Wclass-memaccess option is enabled by -Wall. Explicitly casting the pointer to the class object to "void *" or to a type that can be safely accessed by the raw memory function suppresses the warning. 🌸

• -Wnon-virtual-dtor (C++ and Objective-C++ only) — Warn when a class has virtual functions and an accessible non-virtual destructor itself or in an accessible polymorphic base class, in which case it is possible but unsafe to delete an instance of a derived class through a pointer to the class itself or base class. This warning is automatically enabled if -Weffc++ is specified. 👑
• -Wregister (C++ and Objective-C++ only) — Warn on uses of the "register" storage class specifier, except when it is part of the GNU Explicit Register Variables extension. The use of the "register" keyword as storage class specifier has been deprecated in C++11 and removed in C++17. Enabled by default with -std=c++17. 🎀
• -Wreorder (C++ and Objective-C++ only) — Warn when the order of member initializers given in the code does not match the order in which they must be executed. For instance:

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

The compiler rearranges the member initializers for "i" and "j" to match the declaration order of the members, emitting a warning to that effect. This warning is enabled by -Wall. 🔥

• -Wno-pessimizing-move (C++ and Objective-C++ only) — This warning warns when a call to "std::move" prevents copy elision. A typical scenario when copy elision can occur is when returning in a function with a class return type, when the expression being returned is the name of a non-volatile automatic object, and is not a function parameter, and has the same type as the function return type.

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

But in this example, the "std::move" call prevents copy elision.

This warning is enabled by -Wall. ✨

• -Wno-redundant-move (C++ and Objective-C++ only) — This warning warns about redundant calls to "std::move"; that is, when a move operation would have been performed even without the "std::move" call. This happens because the compiler is forced to treat the object as if it were an rvalue in certain situations such as returning a local variable, where copy elision isn't applicable. Consider:

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

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

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

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

This warning is enabled by -Wextra. 💜

• -Wredundant-tags (C++ and Objective-C++ only) — Warn about redundant class-key and enum-key in references to class types and enumerated types in contexts where the key can be eliminated without causing an ambiguity. For example:

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

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

struct foo;
void foo ();   // "hides" struct foo
void bar (struct foo&);  // no warning, keyword struct is necessary

• -Wno-subobject-linkage (C++ and Objective-C++ only) — Do not warn if a class type has a base or a field whose type uses the anonymous namespace or depends on a type with no linkage. If a type A depends on a type B with no or internal linkage, defining it in multiple translation units would be an ODR violation because the meaning of B is different in each translation unit. If A only appears in a single translation unit, the best way to silence the warning is to give it internal linkage by putting it in an anonymous namespace as well. The compiler doesn't give this warning for types defined in the main .C file, as those are unlikely to have multiple definitions. -Wsubobject-linkage is enabled by default. 🦋
• -Weffc++ (C++ and Objective-C++ only) — Warn about violations of the following style guidelines from Scott Meyers' Effective C++ series of books:
  • Define a copy constructor and an assignment operator for classes with dynamically-allocated memory.
  • Prefer initialization to assignment in constructors.
  • Have "operator=" return a reference to *this.
  • Don't try to return a reference when you must return an object.
  • Distinguish between prefix and postfix forms of increment and decrement operators.
  • Never overload "&&", "||", or ",".

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

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

• -Wstrict-null-sentinel (C++ and Objective-C++ only) — Warn about the use of an uncasted "NULL" as sentinel. When compiling only with GCC this is a valid sentinel, as "NULL" is defined to "__null". Although it is a null pointer constant rather than a null pointer, it is guaranteed to be of the same size as a pointer. But this use is not portable across different compilers. 🌈
• -Wno-non-template-friend (C++ and Objective-C++ only) — Disable warnings when non-template friend functions are declared within a template. In very old versions of GCC that predate implementation of the ISO standard, declarations such as friend int foo(int), where the name of the friend is an unqualified-id, could be interpreted as a particular specialization of a template function; the warning exists to diagnose compatibility problems, and is enabled by default. 🧚
• -Wold-style-cast (C++ and Objective-C++ only) — Warn if an old-style (C-style) cast to a non-void type is used within a C++ program. The new-style casts ("dynamic_cast", "static_cast", "reinterpret_cast", and "const_cast") are less vulnerable to unintended effects and much easier to search for. ✨
• -Woverloaded-virtual (C++ and Objective-C++ only) — Warn when a function declaration hides virtual functions from a base class. For example, in:

struct A {
  virtual void f();
};

struct B: public A {
  void f(int);
};

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

B* b;
b->f();

fails to compile. 👑

• -Wno-pmf-conversions (C++ and Objective-C++ only) — Disable the diagnostic for converting a bound pointer to member function to a plain pointer. 💖
• -Wsign-promo (C++ and Objective-C++ only) — okay so bestie, this one warns when overload resolution chooses a promotion from unsigned or enumerated type to a signed type, over a conversion to an unsigned type of the same size. Previous versions of G++ tried to preserve unsignedness, but the standard mandates the current behavior. 💅
• -Wtemplates (C++ and Objective-C++ only) — it’s giving warning energy when a primary template declaration is encountered. Some coding rules disallow templates, and this may be used to enforce that rule. The warning is inactive inside a system header file, such as the STL, so one can still use the STL. One may also instantiate or specialize templates. ✨
• -Wmismatched-tags (C++ and Objective-C++ only) — warns for declarations of structs, classes, and class templates and their specializations with a class-key that does not match either the definition or the first declaration if no definition is provided.

For example, the declaration of struct Object in the argument list of draw triggers the warning. To avoid it, either remove the redundant class-key struct or replace it with class to match its definition.

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

It is not wrong to declare a class with the class-key struct as the example above shows. The -Wmismatched-tags option is intended to help achieve a consistent style of class declarations. In code that is intended to be portable to Windows-based compilers the warning helps prevent unresolved references due to the difference in the mangling of symbols declared with different class-keys. The option can be used either on its own or in conjunction with -Wredundant-tags. 🫶

• -Wmultiple-inheritance (C++ and Objective-C++ only) — warns when a class is defined with multiple direct base classes. Some coding rules disallow multiple inheritance, and this may be used to enforce that rule. The warning is inactive inside a system header file, such as the STL, so one can still use the STL. One may also define classes that indirectly use multiple inheritance. 🌸
• -Wvirtual-inheritance — warns when a class is defined with a virtual direct base class. Some coding rules disallow multiple inheritance, and this may be used to enforce that rule. The warning is inactive inside a system header file, such as the STL, so one can still use the STL. One may also define classes that indirectly use virtual inheritance. 🦋
• -Wno-virtual-move-assign — suppresses warnings about inheriting from a virtual base with a non-trivial C++11 move assignment operator. No cap, this is dangerous because if the virtual base is reachable along more than one path, it is moved multiple times, which can mean both objects end up in the moved-from state. If the move assignment operator is written to avoid moving from a moved-from object, this warning can be disabled. 😭
• -Wnamespaces — warns when a namespace definition is opened. Some coding rules disallow namespaces, and this may be used to enforce that rule. The warning is inactive inside a system header file, such as the STL, so one can still use the STL. One may also use using directives and qualified names. 💜
• -Wno-terminate (C++ and Objective-C++ only) — disables the warning about a throw-expression that will immediately result in a call to terminate. 💀
• -Wno-class-conversion (C++ and Objective-C++ only) — do not warn when a conversion function converts an object to the same type, to a base class of that type, or to void; such a conversion function will never be called. 🙈
• -Wvolatile (C++ and Objective-C++ only) — warns about deprecated uses of the volatile qualifier. This includes postfix and prefix ++ and -- expressions of volatile-qualified types, using simple assignments where the left operand is a volatile-qualified non-class type for their value, compound assignments where the left operand is a volatile-qualified non-class type, volatile-qualified function return type, volatile-qualified parameter type, and structured bindings of a volatile-qualified type. This usage was deprecated in C++20.

Enabled by default with -std=c++2a. 🔥

• -Wzero-as-null-pointer-constant (C++ and Objective-C++ only) — warns when a literal 0 is used as null pointer constant. This can be useful to facilitate the conversion to nullptr in C++11. ⭐
• -Waligned-new — warns about a new-expression of a type that requires greater alignment than the alignof(std::max_align_t) but uses an allocation function without an explicit alignment parameter. This option is enabled by -Wall.

Normally this only warns about global allocation functions, but -Waligned-new=all also warns about class member allocation functions. 👑

• -Wno-placement-new, -Wplacement-new=n — warns about placement new expressions with undefined behavior, such as constructing an object in a buffer that is smaller than the type of the object. For example, the placement new expression below is diagnosed because it attempts to construct an array of 64 integers in a buffer only 64 bytes large.

char buf [64];
new (buf) int[64];

This warning is enabled by default.

• -Wplacement-new=1 — this is the default warning level of -Wplacement-new. At this level the warning is not issued for some strictly undefined constructs that GCC allows as extensions for compatibility with legacy code. For example, the following new expression is not diagnosed at this level even though it has undefined behavior according to the C++ standard because it writes past the end of the one-element array.

struct S { int n, a[1]; };
S *s = (S *)malloc (sizeof *s + 31 * sizeof s->a[0]);
new (s->a)int [32]();

• -Wplacement-new=2 — at this level, in addition to diagnosing all the same constructs as at level 1, a diagnostic is also issued for placement new expressions that construct an object in the last member of structure whose type is an array of a single element and whose size is less than the size of the object being constructed. While the previous example would be diagnosed, the following construct makes use of the flexible member array extension to avoid the warning at level 2.

struct S { int n, a[]; };
S *s = (S *)malloc (sizeof *s + 32 * sizeof s->a[0]);
new (s->a)int [32]();

• -Wcatch-value, -Wcatch-value=n (C++ and Objective-C++ only) — warns about catch handlers that do not catch via reference. With -Wcatch-value=1 (or -Wcatch-value for short) warn about polymorphic class types that are caught by value. With -Wcatch-value=2 warn about all class types that are caught by value. With -Wcatch-value=3 warn about all types that are not caught by reference. -Wcatch-value is enabled by -Wall. periodt. 🎀
• -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 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, however, offsetof typically gives meaningful results. This flag is for users who are aware that they are writing nonportable code and who have deliberately chosen to ignore the warning about it.

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

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

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

without a definition of the corresponding sized deallocation function

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

or vice versa. Enabled by -Wextra along with -fsized-deallocation. 🍓

• -Wsuggest-final-types — warns about types with virtual methods where code quality would be improved if the type were declared with the C++11 final specifier, or, if possible, declared in an anonymous namespace. This allows GCC to more aggressively devirtualize the polymorphic calls. This warning is more effective with link-time optimization, where the information about the class hierarchy graph is more complete. slay. 👑
• -Wsuggest-final-methods — warns about virtual methods where code quality would be improved if the method were declared with the C++11 final specifier, or, if possible, its type were declared in an anonymous namespace or with the final specifier. This warning is more effective with link-time optimization, where the information about the class hierarchy graph is more complete. It is recommended to first consider suggestions of -Wsuggest-final-types and then rebuild with new annotations. 💜
• -Wsuggest-override — warns about overriding virtual functions that are not marked with the override keyword. 🌟
• -Wuseless-cast (C++ and Objective-C++ only) — vibe check: warns when an expression is casted to its own type. 💅
• -Wno-conversion-null (C++ and Objective-C++ only) — bestie, do not warn when someone converts between NULL and a non-pointer type. And heads up: -Wconversion-null is enabled by default, so this is you actively turning the yelling off. 💅

---

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

(NOTE: This manual does not describe the Objective-C and Objective-C++ languages themselves. Like, we're here for the compiler flags, not a whole language tutorial, okay? Go read a book for that. 📚

Okay so this section is all about the command-line options that only make sense for Objective-C and Objective-C++ programs. ✨ But also — plot twist — you can still use most of the language-independent GNU compiler options too. 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; the other options you can use with any language GCC supports. Versatile king. 👑

Also note: since Objective-C is literally 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). Mix and match, it's giving crossover episode. 🦋

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

• -fconstant-string-class=class-name — use class-name as the name of the class to instantiate for each literal string 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, bestie, keep reading). And ooh — the -fconstant-cfstrings option, if it's also present, overrides the -fconstant-string-class setting and causes @"..." literals to be laid out as constant CoreFoundation strings. It's giving "actually no, I'm in charge now." 💅
• -fgnu-runtime — generate object code compatible with the standard GNU Objective-C runtime. This is the default for most types of systems. The classic, the staple, the moment. ✨
• -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. Conditional queen. 🎀
• -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 lets the runtime use more efficient entry points. Heads up though: this option is only available in conjunction with the NeXT runtime and ABI version 0 or 1. No exceptions, no cap. 🚫
• -fobjc-abi-version=n — use version n of the Objective-C ABI for the selected runtime. Currently this is only supported for the NeXT runtime. In that case:
  • Version 0 is the traditional (32-bit) ABI without support for properties and other Objective-C 2.0 additions.
  • Version 1 is the traditional (32-bit) ABI with support for properties and other Objective-C 2.0 additions.
  • Version 2 is the modern (64-bit) ABI.

If nothing is specified, the default is Version 0 on 32-bit target machines, and Version 2 on 64-bit target machines. Automatic glow-up based on your architecture, honestly iconic. 🌟

• -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 returns 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. Constructor squad up, destructor squad down. 😤

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. Timing is everything, fr fr. ⏰

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. Everyone else, sorry bestie. 😭

• -fobjc-direct-dispatch — allow fast jumps to the message dispatcher. On Darwin this is accomplished via the comm page. Zoom zoom, no traffic lights. 🔥
• -fobjc-exceptions — enable syntactic support for structured exception handling in Objective-C, similar to what C++ offers. This option is required to use the Objective-C keywords @try, @throw, @catch, @finally and @synchronized. Available with both the GNU runtime and the NeXT runtime — but not available in conjunction with the NeXT runtime on Mac OS X 10.2 and earlier. Gatekeeping your OS version, classic. 💀
• -fobjc-gc — enable garbage collection (GC) in Objective-C and Objective-C++ programs. Only available with the NeXT runtime; the GNU runtime has a different garbage collection implementation that does not require special compiler flags. Different path, same destination. 🧹
• -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. And currently this flag does literally nothing when the GNU runtime, or an older version of the NeXT runtime ABI, is used. It's giving decorative. 🙈
• -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. Super useful if you need to make sure your Objective-C code can be compiled with older versions of GCC. Retro-compatibility era, we love to see it. 🕰️
• -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. Living the hot-swap dream, no restart required. 🔄 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. Chaos-friendly debugging, we support it. ✨ And plot twist: the GNU runtime currently always retains calls to objc_get_class("...") regardless of command-line options. It does what it wants.
• -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 identity crisis between your variables. 🪞
• -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. Pick your vibe: public, protected, private, or package. 🔒
• -gen-decls — dump interface declarations for all classes seen in the source file to a file named sourcename.decl. Receipts, printed. 🧾
• -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. Silence is golden. 🤫
• -Wno-protocol (Objective-C and Objective-C++ only) — if a class is declared to implement a protocol, a warning is issued for every method in the protocol that is not implemented by the class. The default behavior is to issue a warning for every method not explicitly implemented in the class, even if a method implementation is inherited from the superclass. If you use the -Wno-protocol option, then methods inherited from the superclass are considered to be implemented, and no warning is issued for them. Inheritance counts now. Thanks, bestie. 🫶
• -Wselector (Objective-C and Objective-C++ only) — warn if multiple methods of different types for the same selector are found during compilation. The check is performed on the list of methods in the final stage of compilation. Additionally, a check is performed for each selector appearing in a @selector(...) expression, and a corresponding method for that selector has been found during compilation. BUT — because these checks scan the method table only at the end of compilation, these warnings are not produced if the final stage of compilation is not reached, for example because an error is found during compilation, or because the -fsyntax-only option is being used. So if you bail early, no warnings for you. That's just how it is. 💅
• -Wstrict-selector-match (Objective-C and Objective-C++ only) — warn if multiple methods with differing argument and/or return types are found for a given selector when attempting to send a message using this selector to a receiver of type id or Class. When this flag is off (which is the default behavior), the compiler omits such warnings if any differences found are confined to types that share the same size and alignment. Strict mode: activated. 🔍
• -Wundeclared-selector — Objective-C and Objective-C++ only, bestie 💅 Warns you when there's a @selector(...) expression floating around that refers to a selector nobody ever declared. So like… when is a selector "undeclared"? When no method with that name got declared before that @selector(...) expression — not explicitly in an @interface or @protocol declaration, and not implicitly in an @implementation section either. Rude, honestly.

The tea: this option always performs its checks the second it finds a @selector(...) expression, whereas -Wselector only does its checking in the final stage of compilation. It also lowkey enforces that whole coding-style-convention thing where methods and selectors have to be declared before you use them. Foreshadowing, but make it a compiler rule ✨

• -print-objc-runtime-info — Generates a C header describing the largest structure that is passed by value, if there even is one. That's it, that's the whole thing 🫶

---

🩺 Options to Control Diagnostic Messages Formatting 🩺

Okay so, traditionally, diagnostic messages have been formatted with absolutely zero awareness of the output device's aspect — like its width, and stuff. Iconic behaviour, honestly. But you can use the options below to take control of the formatting algorithm for diagnostics: how many characters per line, how often the source location info gets reported, all of it. 💅

Heads up though: some language front ends may simply not honor these options. Not everyone got the memo 😭

• -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 happens at all; every error message shows up on a single line. And that zero behaviour? That's the default for all front ends. No cap.

Also note — this option also affects how the #error and #warning pre-processor directives get displayed, plus the deprecated function/type/variable attribute. It does not, however, touch the pragma GCC warning and pragma GCC error pragmas. Those two said "not my problem" 🙈

• -fdiagnostics-show-location=once — Only meaningful in line-wrapping mode. Tells the diagnostic messages reporter to emit the source location information once. Meaning: if the message is too long to fit on one physical line and has to get wrapped, the source location won't be emitted as a prefix again and again and again on all the continuation lines. This is the default behaviour. One and done, periodt 💖
• -fdiagnostics-show-location=every-line — Only meaningful in line-wrapping mode. Tells the diagnostic messages reporter to emit the same source location information as a prefix for every physical line that results from breaking up a message that was too long to fit on a single line. Consistency queen ✨
• -fdiagnostics-color[=WHEN], -fno-diagnostics-color — Use colour in diagnostics. WHEN is never, always, or auto. The default depends on how the compiler was configured — it can be any of those WHEN options, or it can also be never if the GCC_COLORS environment variable isn't present in the environment, and auto otherwise. auto makes GCC use colour only when the standard error is a terminal, and when it's not executing in an emacs shell.

And the aliases, because of course: -fdiagnostics-color and -fno-diagnostics-color are just aliases for -fdiagnostics-color=always and -fdiagnostics-color=never, respectively. Same energy, shorter name 🎀

The colours 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 get interpreted by your terminal or terminal emulator. (Go check the section in your text terminal's documentation for the permitted values and what they mean as character attributes.) Those substring values are integers in decimal representation and you can concatenate them 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
• 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 entirely. The supported capabilities are as follows, and yes, I'm listing every single one because we don't skip 💖

• 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. Clickable diagnostics, we love to see it 🦋

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, bestie, because there is a list.

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. Yes, the beep. You know the one 😭

At this time GCC tries to detect also a few terminals that are known to not implement the URL feature, and have bugs or at least had bugs in some versions that are still in use, where the URL escapes are likely to misbehave — i.e. print garbage on the screen 💀 That list is currently xfce4-terminal, certain known to be buggy gnome-terminal versions, the linux console, and mingw. This check can be skipped with the -fdiagnostics-urls=always.

• -fno-diagnostics-show-option — By default, every diagnostic emitted includes text indicating the command-line option that directly controls the diagnostic (if such an option is known to the diagnostic machinery). Specifying -fno-diagnostics-show-option suppresses that behaviour. So it stops telling you why it's yelling at you. Bold choice 🫶
• -fno-diagnostics-show-caret — By default, every diagnostic emitted includes the original source line and a caret ^ indicating the column. This option suppresses that info. The source line gets truncated to n characters if the -fmessage-length=n option is given. When the output is going to the terminal, the width is limited to the width given by the COLUMNS environment variable or, if that's not set, to the terminal width. No more lil pointer, sorry ✨
• -fno-diagnostics-show-labels — By default, when printing source code (via -fdiagnostics-show-caret), diagnostics can label ranges of source code with pertinent information, like the types of expressions:

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

This option suppresses the printing of these labels — in the example above, that's the vertical bars and the char * and long int text. Look at that diagram though. artistic 🌈

• -fno-diagnostics-show-cwe — Diagnostic messages can optionally have an associated CWE identifier (the docs point at https://cwe.mitre.org/index.html). GCC itself only provides such metadata for some of the -fanalyzer diagnostics. GCC plugins may also provide diagnostics with such metadata. By default, if this information is present, it gets printed with the diagnostic. This option suppresses the printing of this metadata. Extra receipts, gone 💅
• -fno-diagnostics-show-line-numbers — By default, when printing source code (via -fdiagnostics-show-caret), a left margin is printed, showing line numbers. This option suppresses that left margin. Unnumbered, unbothered 🌸
• -fdiagnostics-minimum-margin-width=width — Controls the minimum width of the left margin printed by -fdiagnostics-show-line-numbers. It defaults to 6. Six. That's the number 🎀
• -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"

Your IDE said thank you, actually 👑

The location is expressed as a half-open range, bestie — it's a count of bytes, and the count starts at byte 1 for the initial column 💅. So in that example up there, bytes 3 through 20 of line 45 of test.c are the ones getting swapped out for the given string:

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

The filename and the replacement string both escape backslash as \\, tab as \t, newline as \n, double quotes as \", and non-printable characters as octal (so like, vertical tab shows up as \013) ✨.

An empty replacement string means that range is getting removed entirely, periodt. And an empty range (like 45:3-45:3) means the string gets inserted at that position 🌸.

---

• -fdiagnostics-generate-patch — Prints fix-it hints to stderr in unified diff format, after all the diagnostics have been printed 🎀. Like, 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 🦋.

• -fdiagnostics-show-template-tree — In the C++ frontend, when it's printing diagnostics that show mismatching template types, like:

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

the -fdiagnostics-show-template-tree flag turns on printing a tree-like structure that shows the common parts and the differing parts of the types, like:

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

The parts that differ get highlighted with color (double and float in this case) 💖.

• -fno-elide-type — By default, when the C++ frontend prints diagnostics showing mismatching template types, common parts of the types are printed as [...] to simplify the error message, no cap. Like:

  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.

• -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, and inline-events is the default.

none means, literally, don't print diagnostic paths at all.

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

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 print the events "inline" within the source code 🌈. This view tries to consolidate the events into runs of sufficiently-close events, printing them as labelled ranges inside the source.

For example, those same events from above might get printed like:

  '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 gets 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 gives you extra info when printing control-flow paths that are associated with a diagnostic.

If this option is provided, then the stack depth gets printed for each run of events within -fdiagnostics-path-format=separate-events 💜.

This one is intended for use by GCC developers and plugin developers when debugging diagnostics that report interprocedural control flow 👑.

• -fno-show-column — Do not print column numbers in diagnostics 🚫. This may be necessary if diagnostics are being scanned by a program that doesn't understand column numbers, like dejagnu.
• -fdiagnostics-format=FORMAT — Select a different format for printing diagnostics. FORMAT is text or json. The default is text.

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

The JSON gets emitted as one line, without formatting; the examples below have been formatted for clarity, iykyk.

Diagnostics can have child diagnostics 🌸. Like, 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": {
                    "column": 3,
                    "file": "misleading-indentation.c",
                    "line": 15
                },
                "finish": {
                    "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": {
                            "column": 5,
                            "file": "misleading-indentation.c",
                            "line": 17
                        }
                    }
                ],
                "message": "...this statement, but the latter is ..."
            }
        ]
    },
    ...
]

where the note is a child of the warning 🫶.

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

A diagnostic can contain zero or more locations ✨. Each location has up to three positions within it: a caret position and optional start and finish positions. A location can also have an optional label string. 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}

bestie it literally has three locations. its primary location is at the "+" token, sitting pretty at column 23. and then it's got two secondary locations, describing the left-hand side and the right-hand side of the expression — and those two? they have labels. iykyk ✨

so it might get printed in JSON form like this:

{
    "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 + ..."
}

---

okay next up: fix-it hints 💅

if a diagnostic contains fix-it hints, it gets a "fixits" array. those are half-open intervals, just like the output of -fdiagnostics-parseable-fixits. no cap

for example, this diagnostic right here, 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 ..."
}

so like — the fix-it hint is suggesting you replace the text from "start" up to but not including "next" with the value of "string". got it? 🫶

• Deletions get expressed via an empty value for "string" (nothing there, bestie)
• Insertions happen by having "start" equal "next" ✨

---

and if the diagnostic has a path of control-flow events associated with 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 — these two are for representing interprocedural paths

the "function" represents the current function at that event, and the "depth" represents the stack depth relative to some baseline: the higher it is, the more frames are within the stack. stack go brrr 📚

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

"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!! 🎀 warnings are diagnostic messages that report constructions that are not inherently erroneous but that are risky, or that suggest there may have been an error. like a little "hey bestie… you sure?" ⚠️

the following language-independent options don't enable specific warnings — they just control the kinds of diagnostics produced by GCC. the vibes-controllers, if you will

• -fsyntax-only — check the code for syntax errors, but don't do anything beyond that. that's it, that's the whole job. just a lil syntax vibe check ✨
• -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. it literally gives up 💀 if n is 0 (the default), there is no limit on the number of error messages produced. and if -Wfatal-errors is also specified, then -Wfatal-errors takes precedence over this option. 👑
• -w — inhibit all warning messages. shhh. silence. no thoughts, no warnings 🙈
• -Werror — make all warnings into errors. glow-up, but make it hostile 💅
• -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. exceptions to the rule, periodt

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.) receipts in the message, love that for us 🧾

note that specifying -Werror=foo automatically implies -Wfoo. however, -Wno-error=foo does not imply anything. no free lunch 🍓

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

---

you can request many specific warnings with options beginning with -W, for example -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. the duality of girl 💅

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. it's giving cascading group chat, everyone's pulling everyone else in 💬

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. pragmas stay winning fr fr

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. backwards compatibility slay 💖

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. ngl some warnings need the optimization plot armor 🛡️

---

✨ -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. strict mode, no crumbs 👑

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. sorry bestie, we don't do that here 💀

-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. like... don't, actually 🫣

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. the plot twist nobody asked for 😭

A feature that reports <em>every</em> single failure to conform to ISO C might lowkey be useful in some instances, bestie, but fr fr it would require a ton of extra work and it'd be completely different from -Wpedantic. And no cap — we don't have plans to support such a feature in the near future. 🫶

Okay so, when the standard you pick with -std is a GNU extended dialect of C — think gnu90 or gnu99 — there's a base standard underneath it, aka the version of ISO C that the GNU dialect is built on. Warnings from -Wpedantic show up wherever the base standard requires them. And here's why that's the only way that makes sense: it would be nonsense to only warn about features that aren't in the GNU C dialect you picked, because by definition the GNU dialects of C include everything the compiler supports with the given option, so there would literally be nothing left to warn about. ✨

---

-pedantic-errors 💀

Give an error whenever the base standard (see -Wpedantic) requires a diagnostic — in some cases where there's undefined behavior at compile-time, and in some other cases that don't even prevent compilation of programs that are valid according to the standard.

Heads up: this is not equivalent to -Werror=pedantic, because there are errors enabled by this option that aren't enabled by that one, and vice versa. 🙈

-Wall 👑

This turns on all the warnings about constructions that some users consider questionable, and that are easy to avoid (or modify to prevent the warning), even when you're working with macros. It 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 — here's the whole squad, bestie:

• -Waddress
• -Warray-bounds=1 (only with -O2)
• -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++)
• -Wenum-conversion in C/ObjC;
• -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
• -Wreorder
• -Wrestrict
• -Wreturn-type
• -Wsequence-point
• -Wsign-compare (only in C++)
• -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
• -Wvolatile-register-var
• -Wzero-length-bounds

Not me about to tell you the tea though: some warning flags are not implied by -Wall. 🍓 Some of them warn about constructions users generally don't consider questionable, but which occasionally you might want to check for anyway. Others warn about constructions that are necessary or hard to avoid in some cases, and there's no simple way to modify the code to suppress the warning. Some of them get enabled by -Wextra, but many of them have to be enabled individually. iykyk. 💅

-Wextra 🌸

This enables some extra warning flags that aren't enabled by -Wall. (Fun fact: this option used to be called -W. The older name is still supported, but the newer name is way more descriptive.)

• -Wclobbered
• -Wcast-function-type
• -Wdeprecated-copy (C++ only)
• -Wempty-body
• -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++03 and in C99 and newer)
• -Wunused-parameter (only with -Wunused or -Wall)
• -Wunused-but-set-parameter (only with -Wunused or -Wall)

The option -Wextra also prints warning messages for the following cases:

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

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

Warn about code affected by ABI changes. This includes code that may not be compatible with the vendor-neutral C++ ABI as well as the psABI for the particular target.

Since G++ now defaults to updating the ABI with each major release, normally -Wabi warns only about C++ ABI compatibility problems if there's a check added later in a release series for an ABI issue discovered since the initial release. -Wabi warns about more things if an older ABI version is selected (with -fabi-version=n).

-Wabi can also be used with an explicit version number to warn about C++ ABI compatibility with a particular -fabi-version level, e.g. -Wabi=2 to warn about changes relative to -fabi-version=2.

If an explicit version number is provided and -fabi-compat-version is not specified, the version number from this option is used for compatibility aliases. If no explicit version number is provided with this option, but -fabi-compat-version is specified, that version number is used for C++ ABI warnings.

Although an effort has been made to warn about all such cases, there are probably some cases that are not warned about, even though G++ is generating incompatible code. There may also be cases where warnings are emitted even though the code that is generated is compatible. 😭

You should rewrite your code to avoid these warnings if you're concerned about the fact that code generated by G++ may not be binary compatible with code generated by other compilers.

Known incompatibilities in -fabi-version=2 (which was the default from GCC 3.4 to 4.9) include:

• A template with a non-type template parameter of reference type was mangled incorrectly:

extern int N;
template <int &> struct S {};
void n (S<N>) {2}

This was fixed in -fabi-version=3.

• SIMD vector types declared using "__attribute ((vector_size))" were mangled in a non-standard way that does not allow for overloading of functions taking vectors of different sizes.

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

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

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

• Scoped enumerators passed as arguments to a variadic function are promoted like unscoped enumerators, causing "va_arg" to complain. On most targets this does not actually affect the parameter passing ABI, as there is no way to pass an argument smaller than "int".

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

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

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

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

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

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

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

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

• Target-specific attributes that affect the identity of a type, such as ia32 calling conventions on a function type (stdcall, regparm, etc.), did not affect the mangled name, leading to name collisions when function pointers were used as template arguments.

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

This option also enables 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, this was not the case. For example:

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

"union U" is now always passed in memory.

-Wchar-subscripts ⭐

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

-Wno-coverage-mismatch 🔥

Warn 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 with -fprofile-use, the files with the profile feedback can fail to match the source file and GCC can't use the profile feedback information. By default, this warning is enabled and is treated as an error.

-Wno-coverage-mismatch can be used to disable the warning, or -Wno-error=coverage-mismatch can be used to disable the error. Disabling the error for this warning can result in poorly optimized code and is useful only in the case of very minor changes such as bug fixes to an existing code-base. Completely disabling the warning is not recommended. 💀

-Wno-cpp 💖

(C, Objective-C, C++, Objective-C++ and Fortran only) Suppress warning messages emitted by "#warning" directives.

• -Wdouble-promotion (C, C++, Objective-C and Objective-C++ only) — okay so this one yells at you when a float gets sneakily promoted to a double without you asking. Here's the tea: CPUs with a 32-bit "single-precision" floating-point unit do float in hardware but emulate double in software. So on a machine like that, doing math with double values is way more expensive because of all the software-emulation overhead. 💸

And bestie, it is so easy to accidentally do math in double, because floating-point literals are implicitly double. Like, look at this:

float area(float radius)
{
   return 3.14159 * radius * radius;
}

The compiler does that whole computation with double, because the floating-point literal is a double. caught in 4k 📸

• -Wduplicate-decl-specifier (C and Objective-C only) — warns you if a declaration has a duplicate const, volatile, restrict or _Atomic specifier. Like, pick one, babe. 🎀 This warning is enabled by -Wall.
• -Wformat / -Wformat=n — this is the big one. It checks calls to printf, scanf, etc. to make sure the arguments you supplied have types that actually fit the format string, and that the conversions in the format string make sense. ✨ This covers standard functions, plus others specified by format attributes, across the printf, scanf, strftime and strfmon families (heads up: strfmon is an X/Open extension, not in the C standard) — or other target-specific families. Which functions get checked without format attributes being specified depends on the standard version you selected, and those attribute-less checks get disabled by -ffreestanding or -fno-builtin.

Formats get checked against the format features supported by GNU libc version 2.2. That includes all ISO C90 and C99 features, plus stuff from the Single Unix Specification and some BSD and GNU extensions. Other library implementations may not support all these features, and GCC does not support warning about features that go beyond a particular library's limitations. But — if -Wpedantic is used with -Wformat, you do get warnings about format features not in the selected standard version (just not for strfmon formats, since those aren't in any version of the C standard).

• -Wformat=1 / -Wformat — -Wformat is equivalent to -Wformat=1, and -Wno-format is equivalent to -Wformat=0. Since -Wformat also checks for null format arguments for several functions, -Wformat also implies -Wnonnull. Some aspects of this level of format checking can be disabled by 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. maxed out 🔥

• -Wno-format-contains-nul — if -Wformat is specified, this tells it not to warn about format strings that contain NUL bytes.
• -Wno-format-extra-args — if -Wformat is specified, don't warn about excess arguments to a printf or scanf format function. The C standard says such arguments are ignored, so like... chill. 😌

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

• -Wformat-overflow / -Wformat-overflow=level — warns about calls to formatted input/output functions such as sprintf and vsprintf that might overflow the destination buffer. 🫣 When the exact number of bytes written by a format directive can't be determined at compile-time, it gets estimated based on heuristics that depend on the level argument and on optimization. Enabling optimization will in most cases improve the accuracy of the warning, but it may also result in false positives. iykyk.
  • -Wformat-overflow / -Wformat-overflow=1 — level 1 of -Wformat-overflow, enabled by -Wformat, employs a conservative approach that warns only about calls that most likely overflow the buffer. At this level, numeric arguments to format directives with unknown values are assumed to have the value of one, and strings of unknown length are assumed to be empty. Numeric arguments known to be bounded to a subrange of their type, or string arguments whose output is bounded either by their directive's precision or by a finite set of string literals, are assumed to take on the value within the range that results in the most bytes on output. For example, the call to sprintf below is diagnosed because even with both a and b equal to zero, the terminating NUL character ('\0') appended by the function to the destination buffer will be written past its end. Increasing the size of the buffer by a single byte is sufficient to avoid the warning, though it may not be sufficient to avoid the overflow. 👀

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

• -Wformat-overflow=2 — level 2 warns also about calls that might overflow the destination buffer given an argument of sufficient length or magnitude. At level 2, unknown numeric arguments are assumed to have the minimum representable value for signed types with a precision greater than 1, and the maximum representable value otherwise. Unknown string arguments whose length 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, the call in the example above is again diagnosed, but this time because with a equal to a 32-bit INT_MIN the first %i directive will write some of its digits beyond the end of the destination buffer. To make the call safe regardless of the values of the two variables, the size of the destination buffer must be increased to at least 34 bytes. GCC includes the minimum size of the buffer in an informational note following the warning. 💡

An alternative to increasing the size of the destination buffer is to constrain the range of formatted values. The maximum length of string arguments can be bounded by specifying the precision in the format directive. When numeric arguments of format directives can be assumed to be bounded by less than the precision of their type, choosing an appropriate length modifier to the format specifier will reduce the required buffer size. For example, if a and b in the example above can be assumed to be within the precision of the short int type, then using either the %hi format directive or casting the argument to short reduces the maximum required size of the buffer to 24 bytes.

void f (int a, int b)
{
  char buf [23];
  sprintf (buf, "a = %hi, b = %i\n", a, (short)b);
}

• -Wno-format-zero-length — if -Wformat is specified, don't warn about zero-length formats. The C standard says zero-length formats are allowed. empty but valid, slay ✨
• -Wformat-nonliteral — if -Wformat is specified, also warn if the format string is not a string literal and so 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 is not a string literal and there are no format arguments, as in printf (foo);. This may be a security hole if the format string came from untrusted input and contains %n. (This is currently a subset of what -Wformat-nonliteral warns about, but in future warnings may be added to -Wformat-security that are not included in -Wformat-nonliteral.)

bestie this one is not playing around. untrusted format strings + %n = 💀 territory.

• -Wformat-signedness — if -Wformat is specified, also warn if the format string requires an unsigned argument and the argument is signed, and vice versa. pick a lane bestie 🙈
• -Wformat-truncation / -Wformat-truncation=level — warns 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 gets estimated based on heuristics that depend on the level argument and on optimization. While enabling optimization will in most cases improve the accuracy of the warning, it may also result in false positives. Except as noted otherwise, the option uses the same logic as -Wformat-overflow.
  • -Wformat-truncation / -Wformat-truncation=1 — level 1 of -Wformat-truncation, enabled by -Wformat, employs a conservative approach that warns only about calls to bounded functions whose return value is unused and that will most likely result in output truncation.
  • -Wformat-truncation=2 — level 2 warns also about calls to bounded functions whose return value is used and that might result in truncation given an argument of sufficient length or magnitude. 😭
• -Wformat-y2k — if -Wformat is specified, also warn about strftime formats that may yield only a two-digit year. Y2K called, it wants its vibes back 🕰️
• -Wnonnull — warns 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 — warns when comparing an argument marked with the nonnull function attribute against null inside the function. Like, bestie, you already promised it wasn't null. 🤨

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

• -Wnull-dereference — ok so this one warns you when the compiler spots paths that would trigger erroneous or undefined behavior from dereferencing a null pointer. 💀 It's only active when -fdelete-null-pointer-checks is active, which gets enabled by optimizations on most targets. And ngl, how precise the warnings are depends on which optimization options you used. ✨
• -Winit-self (C, C++, Objective-C and Objective-C++ only) — warns about uninitialized variables that are literally initialized with themselves. 😭 Like bestie that's just chaos. Note this option can only be used with the -Wuninitialized option.

For example, GCC warns about i being uninitialized in the following snippet only when -Winit-self has been specified:

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

This warning is enabled by -Wall in C++. 🫶

• -Wno-implicit-int (C and Objective-C only) — this option controls warnings when a declaration does not specify a type. This warning is enabled by default 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. 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 — warns when a switch case falls through. For example:

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

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

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

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

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

C++17 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 is also possible to add a fallthrough comment to silence the warning. The whole body of the C or C++ style comment should match the given regular expressions listed below. The option argument n specifies what kind of comments are accepted:

• -Wimplicit-fallthrough=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:
  • -fallthrough
  • @fallthrough@
  • lint -fallthrough[ \t]*
  • [ \t.!]*(ELSE,? |INTENTIONAL(LY)? )?FALL(S | |-)?THR(OUGH|U)[ \t.!]*(-[^\n\r]*)?
  • [ \t.!]*(Else,? |Intentional(ly)? )?Fall((s | |-)[Tt]|t)hr(ough|u)[ \t.!]*(-[^\n\r]*)?
  • [ \t.!]*([Ee]lse,? |[Ii]ntentional(ly)? )?fall(s | |-)?thr(ough|u)[ \t.!]*(-[^\n\r]*)?
• -Wimplicit-fallthrough=4 case sensitively matches one of the following regular expressions:
  • -fallthrough
  • @fallthrough@
  • lint -fallthrough[ \t]*
  • [ \t]*FALLTHR(OUGH|U)[ \t]*
• -Wimplicit-fallthrough=5 doesn't recognize any comments as fallthrough comments, only attributes disable the warning.

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 if warnings triggered by the warn_if_not_aligned attribute should be issued. These warnings are enabled by default. 🦋
• -Wignored-qualifiers (C and C++ only) — warns if the return type of a function has a type qualifier such as const. For ISO C such a type qualifier has no effect, since the value returned by a function is not an lvalue. For C++, the warning is only emitted for scalar types or void. ISO C prohibits qualified void return types on function definitions, so such return types always receive a warning even without this option. 😭

This warning is also enabled by -Wextra.

• -Wno-ignored-attributes (C and C++ only) — this option controls warnings when an attribute is ignored. This is different from the -Wattributes option in that it warns whenever the compiler decides to drop an attribute, not that the attribute is either unknown, used in a wrong place, etc. This warning is enabled by default. ✨
• -Wmain — 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 is issued for if, else, while, and for clauses with a guarded statement that does not use braces, followed by an unguarded statement with the same indentation. 😳

In the following example, the call to bar is misleadingly indented as if it were guarded by the if conditional.

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

In the case of mixed tabs and spaces, the warning uses the -ftabstop= option to determine if the statements line up (defaulting to 8).

The warning is not issued for code involving multiline preprocessor logic such as the following example.

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

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

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

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

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

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 is missing one of the attributes.

template <class T>
T* __attribute__ ((malloc, alloc_size (1)))
allocate (size_t);

template <>
void* __attribute__ ((malloc))   // missing alloc_size
allocate<void> (size_t);

• -Wmissing-braces — warns if an aggregate or union initializer is not fully bracketed. In the following example, the initializer for a is not fully bracketed, but that for b is fully bracketed. 🎀

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

This warning is enabled by -Wall.

• -Wmissing-include-dirs (C, C++, Objective-C and Objective-C++ only) — warns if a user-supplied include directory does not exist. 🫶
• -Wno-missing-profile — bestie, this one is the vibe check for your feedback profiles 💅 it controls the warnings you get when feedback profiles go missing while you're out here using -fprofile-use.

So basically it diagnoses those cases where a brand-new function or a brand-new file got added between compiling with -fprofile-generate and compiling with -fprofile-use, and nobody regenerated the profiles. In those cases the profile feedback data files just… don't have any profile feedback info for the newly added function or file respectively. And also — the other scenario — if profile count data (.gcda) files get deleted, GCC literally cannot use any profile feedback information. 😭

In all these cases, warnings get issued to let you know that a profile generation step is due. Ignoring that warning can result in poorly optimized code, and we do not want that energy. -Wno-missing-profile can be used to disable the warning, but that is not recommended and should only be done when non-existent profile data is justified. Periodt.

---

• -Wmultistatement-macros — warns you about unsafe multiple statement macros that look like they're guarded by a clause such as if, else, for, switch, or while, when in reality only the first statement is actually guarded after the macro gets expanded. It's giving trap 🙈

For example:

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

will increment y unconditionally, not just when c holds. Sneaky.

This can usually be fixed 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 sitting in a place where a truth value is expected, or when operators are nested in a way that people's precedence brains keep getting confused about. 😵‍💫

Also warns if a comparison like x<=y<=z shows up; that's equivalent to (x<=y ? 1 : 0) <= z, which is a different interpretation from ordinary mathematical notation. Math girlies stay losing here fr fr.

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

For C++ 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. Undefined behavior is not the moment bestie, it is the villain origin story. 💀

The C and C++ standards define the order in which expressions in a C/C++ program are evaluated in terms of sequence points, which represent a partial ordering between the execution of parts of the program: those executed before the sequence point, and those executed after it. These occur after the evaluation of a full expression (one which is not part of a larger expression), after the evaluation of the first operand of a &&, ||, ? : or , (comma) operator, before a function is called (but after the evaluation of its arguments and the expression denoting the called function), and in certain other places. Other than as expressed by the sequence point rules, the order of evaluation of subexpressions of an expression is not specified. All these rules describe only a partial order rather than a total order, since, for example, if two functions are called within one expression with no sequence point between them, the order in which the functions are called is not specified. However, the standards committee have ruled that function calls do not overlap. 👑

It is not specified when between sequence points modifications to the values of objects take effect. Programs whose behavior depends on this have undefined behavior; the C and C++ standards specify that "Between the previous and next sequence point an object shall have its stored value modified at most once by the evaluation of an expression. Furthermore, the prior value shall be read only to determine the value to be stored.". If a program breaks these rules, the results on any particular implementation are entirely unpredictable. 🎲

Examples of code with undefined behavior are a = a++;, a[n] = b[n++] and a[i++] = i;. Some more complicated cases are not diagnosed by this option, and it may give an occasional false positive result, but in general it has been found fairly effective at detecting this sort of problem in programs.

The C++17 standard will define the order of evaluation of operands in more cases: in particular it requires that the right-hand side of an assignment be evaluated before the left-hand side, so the above examples are no longer undefined. But this option will still warn about them, to help people avoid writing code that is undefined in C and earlier revisions of C++. (C++17 really said "I'm fixing the timeline" and we stan. 💖)

The standard is worded confusingly, therefore there is some debate over the precise meaning of the sequence point rules in subtle cases. Links to discussions of the problem, including proposed formal definitions, may be found on the GCC readings page, at http://gcc.gnu.org/readings.html.

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

• -Wno-return-local-addr — do not warn about returning a pointer (or in C++, a reference) to a variable that goes out of scope after the function returns. Yeah bestie, that pointer is a ghost the second you get back. 👻
• -Wreturn-type — warns whenever a function is defined with a return type that defaults to int. Also warns about any return statement with no return value in a function whose return type is not void (falling off the end of the function body is considered returning without a value).

For C only, it warns about a return statement with an expression in a function whose return type is void, unless the expression type is also void. As a GNU extension, the 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 is not used. 😤

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

• -Wno-shift-count-negative — controls warnings if a shift count is negative. This warning is enabled by default.
• -Wno-shift-count-overflow — controls warnings if a shift count is greater than or equal to the bit width of the type. This warning is enabled by default.
• -Wshift-negative-value — warns if you left shift a negative value. This warning is enabled by -Wextra in C99 and C++11 modes (and newer).
• -Wno-shift-overflow, -Wshift-overflow=n — these options control warnings about left shift overflows. Two levels, two vibes:
  • -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++2A mode (and newer), as signed left shifts always wrap.
  • -Wshift-overflow=2 — this warning level also warns about left-shifting 1 into the sign bit, unless C++14 mode (or newer) is active.
• -Wswitch — warns whenever a switch statement has an index of enumerated type and lacks a case for one or more of the named codes of that enumeration. (The presence of a default label prevents this warning.) case labels outside the enumeration range also provoke warnings when this option is used (even if there is a default label). This warning is enabled by -Wall.
• -Wswitch-default — warns whenever a switch statement does not have a default case. Somebody forgot the safety net bestie. 🫶
• -Wswitch-enum — warns whenever a switch statement has an index of enumerated type and lacks a case for one or more of the named codes of that enumeration. case labels outside the enumeration range also provoke warnings when this option is used. The only difference between -Wswitch and this option is that this option gives a warning about an omitted enumeration code even if there is a default label.
• -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 is possible to suppress this warning by casting the controlling expression to a type other than bool. For example:

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

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

• -Wno-switch-outside-range — this option controls warnings when a switch case has a value that is 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) — warns when __sync_fetch_and_nand and __sync_nand_and_fetch built-in functions are used. These functions changed semantics in GCC 4.4. Glow-up era, but also, watch out. 💅
• -Wunused-but-set-parameter — warns 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 — bestie the tea is scalding ☕ warn whenever a local variable gets assigned to but is otherwise never used (declaration doesn't count, obviously). This one is enabled by -Wall.

To silence it, use the "unused" attribute.

It's also switched on by -Wunused, which itself is switched on by -Wall. it's giving "inherited drama" 💅

• -Wunused-function — yell about a static function that's declared but never defined, OR a non-inline static function that nobody calls. Enabled by -Wall. 👻 ghost code fr fr
• -Wunused-label — yell whenever a label is declared but never used. Enabled by -Wall.

To suppress it, use the "unused" attribute.

• -Wunused-local-typedefs (C, Objective-C, C++ and Objective-C++ only) — yell when a typedef defined locally inside a function just... sits there, unused. Enabled by -Wall. 🎀
• -Wunused-parameter — yell whenever a function parameter is unused aside from its declaration. the parameter is literally standing in the corner at the party 🧍

To suppress this warning use the "unused" attribute.

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

To suppress this warning use the "unused" attribute.

• -Wunused-const-variable
• -Wunused-const-variable=n — yell 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++. okay here's the lore: in C this declares variable storage, but in C++ it's not an error since const variables take the place of "#define"s.

To suppress this warning use the "unused" attribute.

• -Wunused-const-variable=1 — this is the warning level that -Wunused-variable enables for C. It only warns about unused static const variables defined in the main compilation unit, and never about static const variables declared in any header that got included. 🫶
• -Wunused-const-variable=2 — this level also warns about unused constant static variables sitting in headers (system headers excluded, don't worry). This is the level of plain -Wunused-const-variable and you have to explicitly ask for it, because in C++ this isn't an error and in C it might be way harder to clean up every single header you included. no cap, it's a whole cleanup arc 😭

• -Wunused-value — yell whenever a statement computes a result that is explicitly not used. To suppress it, cast the unused expression to "void". This covers an expression-statement, or the left-hand side of a comma expression that contains no side effects. Example time: an expression like "x[i,j]" causes a warning, while "x[(void)i,j]" does not. ✨

This warning is enabled by -Wall.

• -Wunused — all the above -Wunused options combined. the whole squad assembled 👑

Heads up though: to actually get a warning about an unused function parameter, you must either specify -Wextra -Wunused (and yes, -Wall implies -Wunused), or specify -Wunused-parameter separately. iykyk 💅

• -Wuninitialized — warn if an automatic variable is used without being initialized first. In C++, warn if a non-static reference or non-static "const" member shows up in a class that has no constructors. 🔥

If you want to warn about code that uses the uninitialized value of the variable in its own initializer, use the -Winit-self option.

These warnings happen for individual uninitialized elements of structure, union or array variables, as well as for variables that are uninitialized as a whole. They do not happen for variables or elements declared "volatile". And because these warnings depend on optimization, the exact variables or elements that get warned about depend on the precise optimization options and the version of GCC you're using. it's giving "results may vary" 🌸

Note that there may be no warning about a variable that's only used to compute a value that itself is never used, because such computations may get deleted by data flow analysis before the warnings are even printed. 💀

• -Wno-invalid-memory-model — this option controls warnings for invocations of __atomic Builtins, __sync Builtins, and the C11 atomic generic functions with a memory consistency argument that is either invalid for the operation or outside the range of values of the "memory_order" enumeration. For example, since the "__atomic_store" and "__atomic_store_n" built-ins are only defined for the relaxed, release, and sequentially consistent memory orders, the following code is diagnosed:

void store (int *i)
{
  __atomic_store_n (i, 0, memory_order_consume);
}

-Winvalid-memory-model is enabled by default. ⭐

• -Wmaybe-uninitialized — for an automatic (i.e. local) variable, if there exists a path from the function entry to a use of the variable that is initialized, but there also exist some other paths for which the variable is not initialized, the compiler emits a warning if it cannot prove those uninitialized paths are never executed at run time. schrödinger's variable 🦋

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

They're made optional because GCC may not be able to figure out when the code is correct even though it looks like it has 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. As a result, you may get a warning even when there is in fact no problem, because "longjmp" cannot actually be called at the place that would cause a problem. 😭

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

This warning is enabled by -Wall or -Wextra. ✨

• -Wunknown-pragmas — warn when a "#pragma" directive is encountered that GCC doesn't understand. If this command-line option is used, warnings are even issued for unknown pragmas in system header files. That is not the case if the warnings are only enabled by the -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 use of constructor and destructor attributes lets 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. don't touch the reserved zone bestie 🚫
• -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. The warning does not catch all cases, but it does attempt to catch the more common pitfalls. It's included in -Wall. It is equivalent to -Wstrict-aliasing=3. 🌈
• -Wstrict-aliasing=n — same deal: only active when -fstrict-aliasing is active. It warns about code that might break the strict aliasing rules the compiler is using for optimization. Higher levels mean higher accuracy (fewer false positives). Higher levels also mean more effort, kind of like how -O works. -Wstrict-aliasing is equivalent to -Wstrict-aliasing=3. ⭐
  • Level 1: most aggressive, quick, least accurate. Possibly useful when higher levels don't warn but -fstrict-aliasing still breaks the code, since it has very few false negatives. However, it has many false positives. Warns for all pointer conversions between possibly incompatible types, even if 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. yes, the optimization level, bestie — it's all connected 🌸

An optimization that assumes signed overflow doesn't happen is fully fine and safe if the values of the variables involved are, like, such that overflow never actually occurs. So yeah, this warning can super easily give you a false positive — a warning about code that isn't even a problem. 🙈 To help you focus on the stuff that actually matters, several warning levels are defined. And no warnings get issued at all for the use of undefined signed overflow when the compiler is estimating how many iterations a loop needs — especially when it's figuring out whether a loop will be executed at all. 💅

• -Wstrict-overflow=1 — Warns about cases that are both questionable and easy to avoid. Like, for example, the compiler simplifies x + 1 > x to 1. This level of -Wstrict-overflow is enabled by -Wall; the higher levels are not, and you have to explicitly ask for them.
• -Wstrict-overflow=2 — Also warns about other cases where a comparison gets 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. And -Wstrict-overflow with no level at all is the same as -Wstrict-overflow=2.
• -Wstrict-overflow=3 — Also warns about other cases where a comparison gets simplified. For example: x + 1 > 1 is simplified to x > 0.
• -Wstrict-overflow=4 — Also warns about other simplifications not covered by the cases above. For example: (x * 10) / 5 is simplified to x * 2.
• -Wstrict-overflow=5 — Also warns about cases where the compiler reduces the magnitude of a constant involved in a comparison. For example: x + 2 > y is simplified to x + 1 >= y. This one only gets reported at the highest warning level because this simplification applies to so many comparisons, so this level gives you a genuinely enormous number of false positives. 😭

---

✨ -Wstring-compare ✨

Warns for calls to strcmp and strncmp whose result is determined to be either zero or non-zero in tests for such equality, because the length of one argument is greater than the size of the array the other argument is stored in (or the bound, in the case of strncmp). Calls like this could totes be mistakes. For example, the call to strcmp below gets diagnosed because its result is necessarily non-zero no matter what's actually inside the array a.

extern char a[4];
void f (char *d)
{
  strcpy (d, "string");
  ...
  if (0 == strcmp (a, d))   // cannot be true
    puts ("a and d are the same");
}

-Wstring-compare is enabled by -Wextra. 💖

---

✨ -Wstringop-overflow ✨

• -Wstringop-overflow
• -Wstringop-overflow=type

Warns for 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 to perform to figure out the size of the destination. That argument is only meaningful for functions that operate on character arrays — not for raw memory functions like memcpy, which always make use of Object Size type-0. The option also warns for calls that specify a size in excess of the largest possible object, or at most SIZE_MAX / 2 bytes.

The option gives you the best results with optimization enabled, 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, the option only warns about a subset of the buffer overflows detected by the corresponding overflow checking built-ins. For example, the option issues a warning for the strcpy call below because it copies at least 5 characters (the string "blue" including the terminating NUL) into a buffer of size 4. 💀

enum Color { blue, purple, yellow };
const char* f (enum Color clr)
{
  static char buf [4];
  const char *str;
  switch (clr)
    {
      case blue: str = "blue"; break;
      case purple: str = "purple"; break;
      case yellow: str = "yellow"; break;
    }

  return strcpy (buf, str);   // warning here
}

Option -Wstringop-overflow=2 is enabled by default. ✨

• -Wstringop-overflow
• -Wstringop-overflow=1 — The -Wstringop-overflow=1 option uses type-zero Object Size Checking to determine the sizes of destination objects. This is the default setting of the option. At this setting the option does not warn for writes past the end of subobjects of larger objects accessed by pointers — unless the size of the largest surrounding object is known. When the destination could be one of several objects, it's assumed to be the largest one of them. On Linux systems, when optimization is enabled at this setting, the option warns for the same code as when the _FORTIFY_SOURCE macro is defined to a non-zero value.
• -Wstringop-overflow=2 — The -Wstringop-overflow=2 option uses type-one Object Size Checking to determine the sizes of destination objects. At this setting the option warns about overflows when writing to members of the largest complete objects whose exact size is known. However, it does not warn for excessive writes to the same members of unknown objects referenced by pointers, since they may point to arrays containing unknown numbers of elements.
• -Wstringop-overflow=3 — The -Wstringop-overflow=3 option uses type-two Object Size Checking to determine the sizes of destination objects. At this setting the option warns about overflowing the smallest object or data member. This is the most restrictive setting of the option, and it may result in warnings for safe code. 😬
• -Wstringop-overflow=4 — The -Wstringop-overflow=4 option uses type-three Object Size Checking to determine the sizes of destination objects. At this setting the option warns about overflowing any data members, and when the destination is one of several objects it uses the size of the largest of them to decide whether to issue a warning. Similarly to -Wstringop-overflow=3, this setting of the option may result in warnings for benign code.

---

✨ -Wno-stringop-truncation ✨

Do not warn for calls to bounded string manipulation functions like strncat, strncpy, and stpncpy that may 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. As a result, the copy of the source gets truncated, and so 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);
}

As another example, the following call to strncpy ends up copying to d just the characters preceding the terminating NUL, without appending the NUL to the end. Assuming the result of strncpy is necessarily a NUL-terminated string is a super common mistake, so the call is diagnosed. To avoid the warning when the result isn't expected to be NUL-terminated, call memcpy instead. 💅

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

In the example below, the call to strncpy specifies the size of the destination buffer as the bound. If the length of the source string is equal to or greater than this size, the result of the copy will not be NUL-terminated. So yeah, this call is also diagnosed. To avoid the warning, specify sizeof buf - 1 as the bound and set the last element of the buffer to NUL.

void copy (const char *s)
{
  char buf[80];
  strncpy (buf, s, sizeof buf);
  ...
}

In situations where a character array is meant to store a sequence of bytes with no terminating NUL, such an array can be annotated with attribute nonstring to avoid this warning. But heads up — arrays like that are not suitable arguments to functions that expect NUL-terminated strings. To help catch accidental misuses of these arrays, GCC issues warnings unless it can prove the use is safe. ⭐

---

✨ -Wsuggest-attribute=[pure|const|noreturn|format|cold|malloc] ✨

Warns for cases where adding an attribute might be beneficial. 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 attributes pure, const or noreturn or malloc. The compiler only warns for functions visible in other compilation units, or (in the case of pure and const) if it can't prove that the function returns normally. A function returns normally if it doesn't contain an infinite loop or return abnormally by throwing, calling abort or trapping. This analysis requires option -fipa-pure-const, which is enabled by default at -O and higher. Higher optimization levels improve the accuracy of the analysis. ✨

• -Wsuggest-attribute=format
• -Wmissing-format-attribute

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 that are used in assignment, initialization, parameter passing or return statements should have a corresponding format attribute in the resulting type. I.e. the left-hand side of the assignment or initialization, the type of the parameter variable, or the return type of the containing function respectively should also have a format attribute to avoid the warning. 🦋

GCC also gives you the side-eye for function definitions that might be candidates for format attributes. And again — bestie, listen — these are only possible candidates. GCC's whole guess is that a format attribute might be the vibe for any function that calls something like vprintf or vscanf, but that's not always the tea, and some functions that genuinely deserve format attributes might never get clocked at all. 🙈

• -Wsuggest-attribute=cold — Warns about functions that might be candidates for the cold attribute. This one's based on static detection and generally only fires on functions that always lead to a call to another cold function — think wrappers around C++ throw, or fatal-error-reporting functions that lead to abort. 💀
• -Walloc-zero — Warns about calls to allocation functions decorated with the alloc_size attribute that specify zero bytes, including calls to the built-in forms of aligned_alloc, alloca, calloc, malloc, and realloc. Why care? Because what these functions do with a zero size differs between implementations (and in the case of realloc it's been deprecated 😭), so relying on it can sneak in subtle portability bugs. Just avoid it, periodt.
• -Walloc-size-larger-than=byte-size — Warns about calls to functions decorated with the alloc_size attribute that try to allocate objects bigger 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. Want these warnings gone? Either pass a byte-size of SIZE_MAX or more, or use -Wno-alloc-size-larger-than.
• -Wno-alloc-size-larger-than — Turns off the -Walloc-size-larger-than= warnings. It's equivalent to -Walloc-size-larger-than=SIZE_MAX or larger.
• -Walloca — Warns on every use of alloca in the source. Every. Single. One. 👑
• -Walloca-larger-than=byte-size — Warns on calls to alloca with an integer argument whose value is either zero, or that isn't bounded by a controlling predicate limiting it to at most byte-size. It also yells for calls to alloca where the bound value is unknown. And heads up: arguments of non-integer types count as unbounded even if they look like they're constrained to the expected range. 🫠

A bounded case of alloca could look like this:

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

So 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, the compiler would emit a warning. 🔥

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

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.

And note, bestie, 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 one, n could be negative, which would cause a larger-than-expected argument to be implicitly cast into the alloca call. Sneaky.

This option also warns when alloca is used in a loop. 🌀

-Walloca-larger-than=PTRDIFF_MAX is enabled by default but is usually only effective when -ftree-vrp is active (default for -O2 and above).

See also -Wvla-larger-than=byte-size.

• -Wno-alloca-larger-than — Disables -Walloca-larger-than= warnings. Equivalent to -Walloca-larger-than=SIZE_MAX or larger.
• -Warith-conversion — Do warn about implicit conversions from arithmetic operations even when converting the operands to the same type couldn't change their values. This affects warnings 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 — Only active when -ftree-vrp is active (default for -O2 and above). It warns about subscripts to arrays that are always out of bounds. Enabled by -Wall.
  • -Warray-bounds=1 — This is the warning level of -Warray-bounds and it's enabled by -Wall. Higher levels are not, so you gotta ask for them explicitly.
  • -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. Lowkey this level may give you a larger number of false positives, and it's deactivated by default.
• -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, peep -Wmissing-attributes, 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.

• -Wbool-compare — Warns about a boolean expression being 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. It catches cases like

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

It does not warn when both branches contain just a null statement. This warning also fires 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.
• -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 an 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.
• -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's beyond the bounds of the enclosing object.

struct X { int a[0]; int b, c; };
struct X x;

void bad (void)
{
  x.a[0] = 0;   // -Wzero-length-bounds
  x.a[1] = 1;   // -Wzero-length-bounds
  x.a[2] = 2;   // -Warray-bounds
}

Option -Wzero-length-bounds is enabled by -Warray-bounds.

• -Wno-div-by-zero — okay so this one tells GCC like, don't clock the drama when you divide an integer by zero at compile time. But bestie, floating-point division by zero? 💅 Not warned about, ever — because that's actually a legit way of getting yourself infinities and NaNs. It's giving on purpose, not a mistake.
• -Wsystem-headers — this makes GCC spill the tea about constructs it finds in system header files. Normally those warnings are suppressed, because the assumption is they usually don't mean anything's actually wrong and they'd just make the compiler output harder to read. Using this command-line option tells GCC to emit warnings from system headers as if they occurred in user code. But — and this is the plot twist — using -Wall together with this option does not warn about unknown pragmas in system headers. For that, -Wunknown-pragmas must also be used. Two flags, one job. iykyk. ✨
• -Wtautological-compare — warns you when a self-comparison always evaluates to true or false. It catches all the little oopsies like this:

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

It also snitches on bitwise comparisons that always evaluate to true or false, like:

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

which will always be false. 💀 This warning is enabled by -Wall.

• -Wtrampolines — warns about trampolines generated for pointers to nested functions. And no, bestie, we're not talking about the bouncy kind 😭. A trampoline is a small piece of data or code that gets created at run time on the stack when the address of a nested function is taken, and it's used to call the nested function indirectly. For some targets, it's made up of data only, 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. That's the vibe check that matters here. 👑
• -Wfloat-equal — warns if floating-point values are used in equality comparisons. The idea behind this is that sometimes it's convenient (for the programmer, not for the CPU) to think of floating-point values as approximations to infinitely precise real numbers. If that's your situation, then you need to compute — by analyzing the code, or in some other way — the maximum or likely maximum error that the computation introduces, and allow for it when doing comparisons (and when producing output too, but that's a whole different problem, don't even get me started). In particular, instead of testing for equality, you should check whether the two values have ranges that overlap; and you do that with the relational operators. So like, equality comparisons here 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. Here's the whole list, no skipping:
  • Macro parameters that appear within string literals in the macro body. In traditional C, macro replacement takes place within string literals, but in ISO C it does not.
  • In traditional C, some preprocessor directives did not exist. Traditional preprocessors only considered a line to be a directive if the # appeared in column 1 on the line. Therefore -Wtraditional warns about directives that traditional C understands but ignores because the # does not appear as the first character on the line. It also suggests you hide directives like #pragma not understood by traditional C by indenting them. Some traditional implementations do not recognize #elif, so this option suggests avoiding it altogether.
  • A function-like macro that appears without arguments.
  • The unary plus operator.
  • The U integer constant suffix, or the F or L floating-point constant suffixes. (Traditional C does support the L suffix on integer constants.) Note, these suffixes appear in macros defined in the system headers of most modern systems, e.g. the _MIN/_MAX macros in <limits.h>. Use of these macros in user code might normally lead to spurious warnings, however GCC's integrated preprocessor has enough context to avoid warning in these cases.
  • A function declared external in one block and then used after the end of the block.
  • A switch statement has an operand of type long.
  • A non-static function declaration follows a static one. This construct is not accepted by some traditional C compilers.
  • The ISO type of an integer constant has a different width or signedness from its traditional type. This warning is only issued if the base of the constant is ten. I.e. hexadecimal or octal values, which typically represent bit patterns, are not warned about.
  • Usage of ISO string concatenation is detected.
  • Initialization of automatic aggregates.
  • Identifier conflicts with labels. Traditional C lacks a separate namespace for labels.
  • Initialization of unions. If the initializer is zero, the warning is omitted. This is done under the assumption that the zero initializer in user code appears conditioned on e.g. __STDC__ to avoid missing initializer warnings and relies on default initialization to zero in the traditional C case.
  • Conversions by prototypes between fixed/floating-point values and vice versa. The absence of these prototypes when compiling with traditional C causes serious problems. This is a subset of the possible conversion warnings; for the full set use -Wtraditional-conversion.
  • Use of ISO C style function definitions. This warning intentionally is not issued for prototype declarations or variadic functions because these ISO C features appear in your code when using libiberty's traditional C compatibility macros, PARAMS and VPARAMS. This warning is also bypassed for nested functions because that feature is already a GCC extension and thus not relevant to traditional C compatibility.
• -Wtraditional-conversion (C and Objective-C only) — warns if a prototype causes a type conversion that's different from what would happen to the same argument in the absence of a prototype. This includes conversions of fixed point to floating and vice versa, and conversions changing the width or signedness of a fixed-point argument — except when it's the same as the default promotion. ✨
• -Wdeclaration-after-statement (C and Objective-C only) — warns when a declaration is found after a statement in a block. This construct, known from C++, was introduced with ISO C99 and is by default allowed in GCC. It is not supported by ISO C90. 🫠
• -Wshadow — warns whenever a local variable or type declaration shadows another variable, parameter, type, class member (in C++), or instance variable (in Objective-C), or whenever a built-in function is shadowed. Note that in C++, the compiler warns if a local variable shadows an explicit typedef, but not if it shadows a struct/class/enum. If this warning is enabled, it includes also all instances of local shadowing. This means that -Wno-shadow=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. Chill vibes only.
• -Wshadow=global — warn for any shadowing. Same as -Wshadow.
• -Wshadow=local — warn when a local variable shadows another local variable or parameter.
• -Wshadow=compatible-local — warn when a local variable shadows another local variable or parameter whose type is compatible with that of the shadowing variable. In C++, type compatibility here means the type of the shadowing variable can be converted to that of the shadowed variable. The creation of this flag (in addition to -Wshadow=local) is based on the idea that when a local variable shadows another one of incompatible type, it's most likely intentional, not a bug or typo, as shown in the following example:

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

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

This warning is also enabled by -Wshadow=local.

• -Wlarger-than=byte-size — warns whenever an object is defined whose size exceeds byte-size. -Wlarger-than=PTRDIFF_MAX is enabled by default. Warnings controlled by the option can be disabled either by specifying byte-size of SIZE_MAX or more, or by -Wno-larger-than.
• -Wno-larger-than — disables -Wlarger-than= warnings. The option is equivalent to -Wlarger-than=SIZE_MAX or larger.
• -Wframe-larger-than=byte-size — warns if the size of a function frame exceeds byte-size. The computation done to determine the stack frame size is approximate and not conservative. The actual requirements may be somewhat greater than byte-size even if you do not get a warning. On top of that, any space allocated via alloca, variable-length arrays, or related constructs is not included by the compiler when determining whether or not to issue a warning. -Wframe-larger-than=PTRDIFF_MAX is enabled by default. Warnings controlled by the option can be disabled either by specifying byte-size of SIZE_MAX or more, or by -Wno-frame-larger-than.
• -Wno-frame-larger-than — disables -Wframe-larger-than= warnings. The option is equivalent to -Wframe-larger-than=SIZE_MAX or larger.
• -Wno-free-nonheap-object — do not warn when attempting to free an object that was not allocated on the heap. 🙈

---

• -Wstack-usage=byte-size — bestie, this one yells at you if a function's stack usage might go over byte-size. Heads up: the math it does to figure out stack usage is conservative, so it's guessing on the safe side. Anything allocated with alloca, variable-length arrays, or related stuff gets folded into the count when the compiler decides whether to warn you or not.

The message matches what -fstack-usage spits out, so they vibe together.

• If the stack usage is fully static but blows past the amount you gave, 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 on by default. You can kill these warnings either by giving a byte-size of SIZE_MAX or bigger, or by using -Wno-stack-usage.

• -Wno-stack-usage — turns off all the -Wstack-usage= warnings. Same energy as -Wstack-usage=SIZE_MAX or larger. ✨
• -Wunsafe-loop-optimizations — warns when a loop can't be optimized because the compiler can't assume anything about the bounds of the loop indices. And if you also pass -funsafe-loop-optimizations, it warns when the compiler does go ahead and make those assumptions. 🙈
• -Wno-pedantic-ms-format (MinGW targets only) — when you combine this with -Wformat and -pedantic without GNU extensions, it disables the warnings about non-ISO printf/scanf format width specifiers I32, I64, and I used on Windows targets. Those depend on the MS runtime, iykyk.
• -Wpointer-arith — warns about anything that leans on the "size of" a function type or of void. GNU C gives those types a size of 1 (for convenience when doing math with void * pointers and pointers to functions). In C++ it also warns when an arithmetic operation involves NULL. This warning is also switched on by -Wpedantic.
• -Wno-pointer-compare — don't warn when a pointer is compared with a zero character constant. Usually that means you meant to dereference the pointer, so like… 👀. For example:

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

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

This warning is enabled by default.

• -Wtype-limits — warns if a comparison is always true or always false because the data type's range is limited, but doesn't warn for constant expressions. Example: warn if an unsigned variable is compared against zero with < or >=. This warning is also enabled by -Wextra.
• -Wabsolute-value (C and Objective-C only) — warns on calls to standard functions that compute the absolute value of an argument when a better-suited standard function exists. Like, calling abs(3.14) triggers it, because the right function for a double is fabs. 💅 It also fires when the argument passed to such a function has an unsigned type. You can shush it with an explicit type cast, and it's also enabled by -Wextra.
• -Wcomment, -Wcomments — warns whenever a comment-start sequence /* shows up inside a /* comment, or whenever a backslash-newline shows up inside a // comment. Enabled by -Wall.
• -Wtrigraphs — warns if any trigraphs show up that might change what the program means. Trigraphs inside comments don't get warned about, except the ones that would form escaped newlines.

This option is implied by -Wall. And if -Wall isn't given, it's still on unless trigraphs are enabled. Want trigraph conversion without the warnings, but with the other -Wall warnings? Use -trigraphs -Wall -Wno-trigraphs.

• -Wundef — warns if an undefined identifier is evaluated in an #if directive. Those identifiers get replaced with zero, fyi.
• -Wexpansion-to-defined — warns whenever defined shows up in the expansion of a macro (including when the macro is expanded by an #if directive). That usage isn't portable, bestie. Also enabled by -Wpedantic and -Wextra.
• -Wunused-macros — warns about macros defined in the main file that never get used. A macro counts as used if it's expanded or tested for existence at least once. The preprocessor also warns if the macro hasn't been used by the time it's redefined or undefined.

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

Note: if a macro is actually used but only inside skipped conditional blocks, the preprocessor reports it as unused. To dodge the warning in that case, you could tighten up the macro's definition scope — like moving it into the first skipped block. Or you could give it a dummy use, something like:

#if defined the_macro_causing_the_warning
#endif

• -Wno-endif-labels — don't warn whenever an #else or an #endif is followed by text. This happens in older programs with code like:

#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 type that doesn't match. Example: 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. Like 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. Like 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. 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. Like omitting the string in _Static_assert, using [[]] syntax for attributes, using decimal floating-point types, and so on. 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++. Like, requesting an 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. Example: 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. Enabled by -Wall.
• -Wc++17-compat (C++ and Objective-C++ only) — warns about C++ constructs whose meaning differs between ISO C++ 2014 and ISO C++ 2017. Enabled by -Wall.
• -Wc++20-compat (C++ and Objective-C++ only) — warns about C++ constructs whose meaning differs between ISO C++ 2017 and ISO C++ 2020. Enabled by -Wall.
• -Wcast-qual — warns whenever a pointer is cast in a way that strips a type qualifier off the target type. Example: warn if a const char * gets cast to a plain char *.

It also warns when a cast introduces a type qualifier unsafely. Example: casting char ** to const char ** is unsafe, like this:

/* 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 so the target's required alignment goes up. 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 — warns whenever a pointer is cast so the target's required alignment goes up. Example: warn if a char * gets cast to an int *, regardless of the target machine. No exceptions, no mercy. 💀
• -Wcast-function-type — warns when a function pointer is cast to an incompatible function pointer. For casts involving function types with a variable argument list, only the types of the initial arguments that are provided get considered. Any pointer-type parameter matches any other pointer-type. Any benign differences in integral types are ignored, like int vs. long on ILP32 targets. Same deal for type qualifiers — ignored. The function type void (*) (void) is special and matches everything, which you can use to suppress this warning. For casts involving pointer-to-member types, this warning fires whenever the cast changes the pointer-to-member type. Enabled by -Wextra.
• -Wwrite-strings — when compiling C, this gives string constants the type const char[length], so copying the address of one into a non-const char * pointer produces a warning. These warnings help you catch, at compile time, code that might try to write into a string constant — but only if you've been super careful about using const in declarations and prototypes. Otherwise it's just a nuisance, which is exactly why -Wall doesn't request these warnings.

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

• -Wclobbered — warns for variables that might get changed by longjmp or vfork. Also enabled by -Wextra. 🫶
• -Wconversion — okay bestie, this one is nosy in the best way: it yells at you for implicit conversions that might actually change a value 💅. What counts? 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)). What does not count: explicit casts like abs ((int) x) and ui = (unsigned) -1, or when the value just isn't changed by the conversion, like in abs (2.0). Also, the whole "signed vs unsigned integers" complaint squad can be silenced with -Wno-sign-conversion.

For C++, it also warns about confusing overload resolution for user-defined conversions, plus conversions that never use a type conversion operator — that means conversions to void, the same type, a base class, or a reference to them. And heads up: in C++ the signed/unsigned integer warnings are off by default unless -Wsign-conversion is explicitly turned on.

Warnings about conversion from arithmetic on a small type back to that same type only show up with -Warith-conversion.

• -Wdangling-else — the "wait, which if does that else belong to??" warning 😭. Here's the cursed example:

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

In C/C++, every else branch belongs to the innermost possible if statement, which here is if (b). That is often not what the programmer expected, as the indentation in that example shows off. When this confusion is possible, GCC warns — but only if this flag is specified. To make the warning go away, add explicit braces around the innermost if statement so there's no way the else can belong to the enclosing if. 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__ show up, because they might prevent bit-wise-identical reproducible compilations 🫶.
• -Wempty-body — warns if there's an empty body in an if, else or do while statement. Also enabled by -Wextra.
• -Wno-endif-labels — do not warn about stray tokens after #else and #endif.
• -Wenum-compare — warns about comparing values of different enumerated types. In C++, enumerated type mismatches in conditional expressions also get diagnosed, and the warning is on by default there. In C, this warning is enabled by -Wall.
• -Wenum-conversion (C, Objective-C only) — warns when a value of enumerated type is implicitly converted to a different enumerated type. Enabled by -Wextra.
• -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. It only warns about variables that are initialized when they're declared. And btw this warning is only supported for C and Objective-C — in C++ this kind of branch is an error anyway, so it's not even a vibe check, it's a fail.

-Wjump-misses-init is included in -Wc++-compat. You can turn it off with the -Wno-jump-misses-init option.

• -Wsign-compare — warns when a comparison between signed and unsigned values could produce an incorrect result once the signed value gets converted to unsigned. In C++, this warning is also enabled by -Wall. In C, it's also enabled by -Wextra.
• -Wsign-conversion — warns about 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 also enabled by -Wconversion.
• -Wfloat-conversion — warns about implicit conversions that reduce the precision of a real value ✨. That includes conversions from real to integer, and from higher precision real to lower precision real values. This option is also enabled by -Wconversion.
• -Wno-scalar-storage-order — do not warn on suspicious constructs involving reverse scalar storage order.
• -Wsizeof-pointer-div — warns about 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 correctly with pointers 💀. It warns about stuff like sizeof (ptr) / sizeof (ptr[0]) if ptr is not an array but a pointer. Enabled by -Wall.
• -Wsizeof-pointer-memaccess — warns about suspicious length parameters to certain string and memory built-in functions when the argument uses sizeof 👀. It fires for example for memset (ptr, 0, sizeof (ptr)); if ptr is not an array but a pointer, and it suggests a possible fix. Also for memcpy (&foo, ptr, sizeof (&foo));.

-Wsizeof-pointer-memaccess also warns about calls to bounded string copy functions like strncat or strncpy that specify a sizeof expression of the source array as the bound. 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, so the call gets diagnosed.

        void make_file (const char *name)
        {
          char path[PATH_MAX];
          strncpy (path, name, sizeof path - 1);
          strncat (path, ".text", sizeof ".text");
          ...
        }

The -Wsizeof-pointer-memaccess option is enabled by -Wall.

• -Wno-sizeof-array-argument — do not warn when the sizeof operator is applied to a parameter that's declared as an array in a function definition. This warning is enabled by default for C and C++ programs.
• -Wmemset-elt-size — warns about suspicious calls to the memset built-in function when the first argument references an array, and the third argument is a number equal to the number of elements but not equal to the size of the array in memory. That means the user forgot a multiplication by the element size, bestie 🥲. Enabled by -Wall.
• -Wmemset-transposed-args — warns about 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 folds to zero, or a cast of zero to some type, it's far less likely the arguments were mistakenly transposed and no warning is emitted. Enabled by -Wall.
• -Waddress — warns about suspicious uses of memory addresses 🕵️. These include using the address of a function in a conditional expression, like void func(void); if (func), and comparisons against the memory address of a string literal, like if (x == "abc"). Such uses typically point to a programmer error: the address of a function always evaluates to true, so using it in a conditional usually means 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 mean the programmer intended to use strcmp. Enabled by -Wall.
• -Wno-address-of-packed-member — do not warn when the address of a packed member of a struct or union is taken, which usually results in an unaligned pointer value. This is enabled by default.
• -Wlogical-op — warns about suspicious uses of logical operators in expressions. That includes using logical operators in contexts where a bit-wise operator is likely expected. It also warns when the operands of a logical operator are the same:

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

• -Wlogical-not-parentheses — warns about logical not used on the left hand side operand of a comparison. It does not warn if the right operand is considered to be a boolean expression. Its whole purpose is to catch suspicious code like this:

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

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

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

This warning is enabled by -Wall.

• -Waggregate-return — warns if any functions that return structures or unions are defined or called. (In languages where you can return an array, this also elicits a warning.)
• -Wno-aggressive-loop-optimizations — warns if, in a loop with a constant number of iterations, the compiler detects undefined behavior in some statement during one or more of those iterations.
• -Wno-attributes — do not warn if an unexpected __attribute__ is used, like unrecognized attributes, function attributes applied to variables, etc. It 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 dodge the warning, include the appropriate header to bring the prototypes of built-in functions into scope 💅.

For example, the call to memset below gets 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 — okay so this one is giving “stop yelling at me about my built-in macros.” It says: do not warn if certain built-in macros are redefined. It suppresses warnings for redefinition of __TIMESTAMP__, __TIME__, __DATE__, __FILE__, and __BASE_FILE__. periodt ✨
• -Wstrict-prototypes (C and Objective-C only) — warn if a function is declared or defined without specifying the argument types. But bestie, an old-style function definition is permitted without a warning if it’s preceded by a declaration that specifies the argument types. 💅
• -Wold-style-declaration (C and Objective-C only) — warn for obsolescent usages, according to the C Standard, in a declaration. For example, warn if storage-class specifiers like static are not the first things in a declaration. This warning is also enabled by -Wextra. the more u know 🌟
• -Wold-style-definition (C and Objective-C only) — warn if an old-style function definition is used. A warning is given even if there is a previous prototype. A definition using () is not considered an old-style definition in C2X mode, because it is equivalent to (void) in that case, but is considered an old-style definition for older standards. standards said “it’s complicated” 😭
• -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. old school vibes, but make it warning 🎀

• -Wmissing-prototypes (C and Objective-C only) — warn if a global function is defined without a previous prototype declaration. This warning is issued even if the definition itself provides a prototype. Use this option to detect global functions that do not have a matching prototype declaration in a header file. This option is not valid for C++ because all function declarations provide prototypes and a non-matching declaration declares an overload rather than conflict with an earlier declaration. Use -Wmissing-declarations to detect missing declarations in C++. C++ girlies, this one is not for u 🙈
• -Wmissing-declarations — warn if a global function is defined without a previous declaration. Do so even if the definition itself provides a prototype. Use this option to detect global functions that are not declared in header files. In C, no warnings are issued for functions with previous non-prototype declarations; use -Wmissing-prototypes to detect missing prototypes. In C++, no warnings are issued for function templates, or for inline functions, or for functions in anonymous namespaces. header file accountability era 💖
• -Wmissing-field-initializers — warn if a structure’s initializer has some fields missing. For example, the following code causes such a warning, because x.h is implicitly zero:

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

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

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

In C this option does not warn 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. selective hearing, but for warnings 🍓

• -Wno-multichar — do not warn if a multicharacter constant ('FOOF') is used. Usually they indicate a typo in the user’s code, as they have implementation-defined values, and should not be used in portable code. typo energy detected 💀
• -Wnormalized=[none|id|nfc|nfkc] — in ISO C and ISO C++, two identifiers are different if they are different sequences of characters. However, sometimes when characters outside the basic ASCII character set are used, you can have two different character sequences that look the same. To avoid confusion, the ISO 10646 standard sets out some normalization rules which when applied ensure that two sequences that look the same are turned into the same sequence. GCC can warn you if you are using identifiers that have not been normalized; this option controls that warning.

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

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

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

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

• -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 -Werror flag. attribute said “warning” and GCC said “say less” 💙
• -Wno-deprecated — do not warn about usage of deprecated features. the deprecated stuff can stay unbothered ✨
• -Wno-deprecated-declarations — do not warn about uses of functions, variables, and types marked as deprecated by using the deprecated attribute. deprecated but make it quiet 🎀
• -Wno-overflow — do not warn about compile-time overflow in constant expressions. overflow? never heard of her 💅
• -Wno-odr — ngl the name is a whole mood — warn about One Definition Rule violations during link-time optimization. Enabled by default. yes, that’s what the manpage says 😭
• -Wopenmp-simd — warn if the vectorizer cost model overrides the OpenMP simd directive set by user. The -fsimd-cost-model=unlimited option can be used to relax the cost model. simd drama, but make it optimized 🔥
• -Woverride-init (C and Objective-C only) — warn if an initialized field without side effects is overridden when using designated initializers. This warning is included in -Wextra. To get other -Wextra warnings without this one, use -Wextra -Wno-override-init. designated initializer said “actually, I’m overriding that” 🙈
• -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. side effects said “I’m still here” 💖
• -Wpacked — warn if a structure is given the packed attribute, but the packed attribute has no effect on the layout or size of the structure. Such structures may be mis-aligned for little benefit. For instance, in this code, the variable f.x in struct bar is misaligned even though struct bar does not itself have the packed attribute:

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

packed but not really packing? it’s giving pointless 💀

• -Wnopacked-bitfield-compat — the 4.1, 4.2 and 4.3 series of GCC ignore the packed attribute on bit-fields of type char. This was fixed in GCC 4.4 but the change can lead to differences in the structure layout. GCC informs you when the offset of such a field has changed in GCC 4.4. For example there is no longer a 4-bit padding between field a and b in this structure:

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

This warning is enabled by default. Use -Wno-packed-bitfield-compat to disable this warning. GCC 4.4 said “new layout, who dis?” 🦋

• -Wpacked-not-aligned (C, C++, Objective-C and Objective-C++ only) — warn if a structure field with explicitly specified alignment in a packed struct or union is misaligned. For example, a warning will be issued on struct S, like, warning: alignment 1 of 'struct S' is less than 8, in this code:

struct __attribute__ ((aligned (8))) S8 { char a[8]; };
struct __attribute__ ((packed)) S {
  struct S8 s8;
};

This warning is enabled by -Wall. alignment said “I’m not like other fields” ⭐

• -Wpadded — warn if padding is included in a structure, either to align an element of the structure or to align the whole structure. Sometimes when this happens it is possible to rearrange the fields of the structure to reduce the padding and so make the structure smaller. padding is lowkey stealing ur bytes 💅
• -Wredundant-decls — warn if anything is declared more than once in the same scope, even in cases where multiple declaration is valid and changes nothing. redundant? in this economy? 😭
• -Wrestrict — warn when an object referenced by a restrict-qualified parameter (or, in C++, a __restrict-qualified parameter) is aliased by another argument, or when copies between such objects overlap. For example, the call to the strcpy function below attempts to truncate the string by replacing its initial characters with the last four. However, because the call writes the terminating NUL into a[4], the copies overlap and the call is diagnosed.

void foo (void)
{
  char a[] = "abcd1234";
  strcpy (a, a + 4);
  ...
}

The -Wrestrict option detects some instances of simple overlap even without optimization but works best at -O2 and above. It is included in -Wall. overlap said “we’re not just friends” 🔥

• -Wnested-externs (C and Objective-C only) — warn if an extern declaration is encountered within a function. extern inside a function? bestie, why 🎀
• -Winline — bestie, this one yells at you if a function you declared inline just... doesn't get inlined. Rude, honestly. But heads up: even with this flag on, the compiler stays quiet about failed inlines in system headers. It's not gonna snitch on the standard library.

The compiler uses a whole bunch of heuristics to decide whether to inline something — like, it looks at how big the function is, and how much inlining has already happened in the current function. Which means the tiniest little change in your source can make these warnings appear or vanish. It's giving chaos, fr fr.

• -Wint-in-bool-context — warns when you use integer values somewhere a boolean was expected, and it looks suspicious. Like conditional expressions (?:) using non-boolean integer constants in a boolean context, e.g. if (a <= b ? 2 : 3). Or left-shifting signed integers in boolean context, like for (a = 0; 1 << a; a++);. Same energy for all kinds of multiplications, no matter the data type. This one is enabled by -Wall. 👑
• -Wno-int-to-pointer-cast — suppresses warnings when you cast to a pointer type from an integer of a different size. And in C++, casting to a pointer type of smaller size is straight up an error. Note that Wint-to-pointer-cast is enabled by default. iykyk
• -Wno-pointer-to-int-cast (C and Objective-C only) — suppresses warnings from casting from a pointer to an integer type of a different size. C and Obj-C only, bestie, don't try this elsewhere.
• -Winvalid-pch — warns when a precompiled header is sitting in the search path but can't actually be used. It's giving found it but couldn't use it. 💀
• -Wlong-long — warns if the long long type gets used. Enabled by either -Wpedantic or -Wtraditional in ISO C90 and C++98 modes. Wanna silence it? Use -Wno-long-long.
• -Wvariadic-macros — warns if variadic macros show up in ISO C90 mode, or if the GNU alternate syntax is used in ISO C99 mode. Enabled by either -Wpedantic or -Wtraditional. To shut it up, use -Wno-variadic-macros.
• -Wno-varargs — stops warning about questionable usage of the macros used to handle variable arguments, like va_start. These warnings are enabled by default, so this flag is your off switch.
• -Wvector-operation-performance — warns if a vector operation isn't implemented via the SIMD capabilities of the architecture. Mainly useful for performance tuning. Here's the tea on how a vector operation can be implemented:
  • "piecewise" — the scalar operation is performed on every vector element.
  • "in parallel" — the vector operation is implemented using scalars of wider type, which normally is more performance efficient.
  • "as a single scalar" — the vector fits into a scalar type.
• -Wvla — warns if a variable-length array is used in the code. And -Wno-vla prevents the -Wpedantic warning about variable-length arrays. 🎀
• -Wvla-larger-than=byte-size — with this on, the compiler warns for declarations of variable-length arrays whose size is either unbounded, or bounded by an argument that allows the array size to exceed byte-size bytes. Similar to how -Walloca-larger-than=byte-size works, but for variable-length arrays.

Lowkey important: GCC may optimize small variable-length arrays of a known value into plain arrays, so this warning may not get triggered for those.

Also, -Wvla-larger-than=PTRDIFF_MAX is enabled by default, but it's typically only effective when -ftree-vrp is active (which is default for -O2 and above).

See also -Walloca-larger-than=byte-size. ✨

• -Wno-vla-larger-than — disables -Wvla-larger-than= warnings. Equivalent to -Wvla-larger-than=SIZE_MAX or larger.
• -Wvolatile-register-var — warns if a register variable is declared volatile. And the way that the volatile modifier does not inhibit all optimizations that may eliminate reads and/or writes to register variables — so yeah, it's not the shield you think it is. Enabled by -Wall.
• -Wdisabled-optimization — warns if a requested optimization pass gets disabled. Ngl, this doesn't generally mean anything's wrong with your code; it just means GCC's optimizers can't handle the code effectively. Often the problem is your code is too big or too complex — GCC refuses to optimize programs when the optimization itself would likely take an inordinate amount of time. It's protecting its peace. 🫶
• -Wpointer-sign (C and Objective-C only) — warns for pointer argument passing or assignment with different signedness. Only supported for C and Objective-C. It's implied by -Wall and by -Wpedantic, which you can disable with -Wno-pointer-sign.
• -Wstack-protector — only active when -fstack-protector is active. It warns about functions that are not protected against stack smashing. 💀
• -Woverlength-strings — warns about string constants longer than the "minimum maximum" length specified in the C standard. Modern compilers generally allow string constants way longer than the standard's minimum limit, but very portable programs should avoid using longer strings.

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

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

• -Wunsuffixed-float-constants (C and Objective-C only) — issues a warning for any floating constant that doesn't have a suffix. When used together with -Wsystem-headers, it warns about such constants in system header files too. This can be useful when preparing code to use with the FLOAT_CONST_DECIMAL64 pragma from the decimal floating-point extension to C99. 🍓
• -Wno-lto-type-mismatch — during link-time optimization, don't warn about type mismatches in global declarations from different compilation units. Requires -flto to be enabled. Enabled by default.
• -Wno-designated-init (C and Objective-C only) — suppresses warnings when a positional initializer is used to initialize a structure that's been marked with the designated_init attribute.
• -Wno-hsa — don't warn when HSAIL can't be emitted for the compiled function or OpenMP construct. These warnings are enabled by default.

---

✨ Options That Control Static Analysis ✨

• -fanalyzer — this enables a static analysis of program flow that looks for "interesting" interprocedural paths through the code, and issues warnings for problems found on them. 👑

Heads up bestie: this analysis is much more expensive than other GCC warnings. It's giving main character energy but it costs.

Enabling this option effectively enables the following warnings:

-Wanalyzer-double-fclose -Wanalyzer-double-free
-Wanalyzer-exposure-through-output-file -Wanalyzer-file-leak
-Wanalyzer-free-of-non-heap -Wanalyzer-malloc-leak
-Wanalyzer-possible-null-argument
-Wanalyzer-possible-null-dereference -Wanalyzer-null-argument
-Wanalyzer-null-dereference -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

This option is only available if GCC was configured with analyzer support enabled. No analyzer support, no analysis.

• -Wanalyzer-too-complex — if -fanalyzer is enabled, the analyzer uses various heuristics to try to explore the control flow and data flow in the program, but these can be defeated by sufficiently complicated code.

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

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

This diagnostic warns for paths through the code in which a FILE * can have fclose called on it more than once. 🫶

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

This diagnostic warns for paths through the code in which a pointer can have free called on it more than once.

• -Wno-analyzer-exposure-through-output-file — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-exposure-through-output-file to disable it.

This diagnostic warns for paths through the code in which a security-sensitive value is written to an output file (such as writing a password to a log file). Not the password in the log file, bestie!! 😭

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

This diagnostic warns for paths through the code in which a <stdio.h> FILE * stream object is leaked.

• -Wno-analyzer-free-of-non-heap — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-free-of-non-heap to disable it.

This diagnostic warns for paths through the code in which free is called on a non-heap pointer (e.g. an on-stack buffer, or a global). 💀

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

This diagnostic warns for paths through the code in which a pointer allocated via malloc is leaked.

• -Wno-analyzer-possible-null-argument — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-possible-null-argument to disable it.

This diagnostic warns for paths through the code in which a possibly-NULL value is passed to a function argument marked with __attribute__((nonnull)) as requiring a non-NULL value.

• -Wno-analyzer-possible-null-dereference — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-possible-null-dereference to disable it.

This diagnostic warns for paths through the code in which a possibly-NULL value is dereferenced. 🙈

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

This diagnostic warns for paths through the code in which a value known to be NULL is passed to a function argument marked with __attribute__((nonnull)) as requiring a non-NULL value.

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

…which brings us to the end of that diagnostic: this one yells at you for any path through the code where a value that's known to be NULL gets dereferenced. like bestie, you literally checked it and then used it anyway? 💀

---

More analyzer warnings (the -Wno- family, because they're on by default when the analyzer's vibing)

• -Wno-analyzer-stale-setjmp-buffer — this one needs -fanalyzer to even exist (that's what turns it on), so use -Wno-analyzer-stale-setjmp-buffer to switch it off. It flags paths where longjmp gets called to rewind to a jmp_buf that belongs to a setjmp call in a function that has already returned. Here's the tea: when setjmp runs on a jmp_buf to record a rewind location, it saves the stack frame. That frame goes invalid the second the function containing the setjmp call returns. So rewinding to it via longjmp would reference a stack frame that doesn't exist anymore — and that's giving crash, or honestly, worse 😭
• -Wno-analyzer-tainted-array-index — needs both -fanalyzer and -fanalyzer-checker=taint to be enabled; use -Wno-analyzer-tainted-array-index to turn it off. It warns about paths where a value that could be under an attacker's control gets used as the index of an array access without being sanitized. 🕵️♀️
• -Wno-analyzer-unsafe-call-within-signal-handler — requires -fanalyzer, which enables it; disable with -Wno-analyzer-unsafe-call-within-signal-handler. Flags paths where a function known to be async-signal-unsafe (like fprintf) gets called from inside a signal handler. 🙈
• -Wno-analyzer-use-after-free — requires -fanalyzer, which enables it; disable with -Wno-analyzer-use-after-free. Warns about paths where a pointer is used after free was called on it. classic.
• -Wno-analyzer-use-of-pointer-in-stale-stack-frame — requires -fanalyzer, which enables it; disable with -Wno-analyzer-use-of-pointer-in-stale-stack-frame. Warns about paths where a pointer gets dereferenced that points to a variable in a stale stack frame. 💀

---

The parameters that actually steer the exploration, bestie: --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 — simplifies interprocedural analysis by computing the effect of certain calls instead of exploring, like, every single path through the function from callsite to each possible return. If enabled, call summaries only get used for functions with more than one call site that are sufficiently complicated (as per --param analyzer-min-snodes-for-call-summary=value).
• -fanalyzer-checker=name — restricts the analyzer to run just the named checker, and enables it. Some checkers are disabled by default (even with -fanalyzer), like the taint checker that implements -Wanalyzer-tainted-array-index, and this option is what you need to turn them on. 🔓
• -fanalyzer-fine-grained — this option is intended for analyzer developers. Internally the analyzer builds an "exploded graph" that combines control flow graphs with data flow information. By default, an edge in this graph can contain the effects of a run of multiple statements within a basic block. With -fanalyzer-fine-grained, each statement gets its own edge. ✂️
• -fanalyzer-show-duplicate-count — also for analyzer developers: if multiple diagnostics have been detected as being duplicates of each other, it emits a note when reporting the best diagnostic, giving the number of additional diagnostics that were suppressed by the deduplication logic. 💅
• -fno-analyzer-state-merge — analyzer-developer option. By default the analyzer tries to simplify analysis by merging sufficiently similar states at each program point as it builds its "exploded graph". With -fno-analyzer-state-merge that merging can be suppressed, for debugging state-handling issues.
• -fno-analyzer-state-purge — analyzer-developer option. By default the analyzer tries 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 — enables transitivity of constraints within the analyzer.
• -fanalyzer-verbose-edges — analyzer-developer option. It enables more verbose, lower-level detail in the descriptions of control flow within diagnostic paths.
• -fanalyzer-verbose-state-changes — analyzer-developer option. It enables more verbose, lower-level detail in the descriptions of events relating to state machines within diagnostic paths.
• -fanalyzer-verbosity=level — controls the complexity of the control flow paths that are emitted for analyzer diagnostics. The level can be one of:
  0. At this level, interprocedural call and return events are displayed, along with the most pertinent state-change events relating to a diagnostic. For example, for a double-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.
• -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. 😅
• -fdump-analyzer-state-purge — as per -fdump-analyzer-supergraph, dump a representation of the "supergraph" suitable for viewing with GraphViz, but annotate the graph with information on what state will be purged at each node. The graph is written 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. that's it, that's the whole vibe.

GCC lets you use -g with -O. But heads up — 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're 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 -Og may result in a better debugging experience. lowkey the move. 💅

• -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. 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 4. DWARF Version 5 is only experimental. ⚗️

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

Version 4 may require GDB 7.0 and -fvar-tracking-assignments for maximum benefit.

GCC no longer supports DWARF Version 1, which is substantially different than Version 2 and later. For historical reasons, some other DWARF-related options such as -fno-dwarf2-cfi-asm retain a reference to DWARF Version 2 in their names, but apply to all currently-supported versions of DWARF. (old name, new vibes, we love that for it)

• -gstabs — bestie, this one produces debugging information in stabs format (if that's even supported), and it comes without the GDB extensions. This is the format DBX uses on most BSD systems. 💅 But real talk: on MIPS, Alpha and System V Release 4 systems this option produces stabs debugging output that DBX does not understand. 🙈 And on System V Release 4 systems, this option requires the GNU assembler.
• -gstabs+ — same stabs format (if supported), but now with GNU extensions that only the GNU debugger (GDB) gets. The use of these extensions is likely to make other debuggers crash or refuse to read the program. 😭 So like, know your audience.
• -gxcoff — produces debugging information in XCOFF format (if supported). This is the format the DBX debugger uses on IBM RS/6000 systems. 🖥️
• -gxcoff+ — XCOFF format (if supported) with 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. 💀 double trouble fr fr.
• -gvms — produces debugging information in Alpha/VMS debug format (if supported). This is the format DEBUG uses on Alpha/VMS systems. 🫶

---

• -glevel, -ggdblevel, -gstabslevel, -gxcofflevel, -gvmslevel — request debugging information, and also use level to say how much info you want. The default level is 2.

So here's the level tea, bestie:

• Level 0 produces no debug information at all. Which means -g0 negates -g. Not me undoing my own work 💀.
• 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, periodt.

Heads up: -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. no symbol left behind 💖
• -femit-class-debug-always — instead of emitting debugging information for a C++ class in only one object file, emit it in all object files using the class. This option should be used only with debuggers that are unable to handle the way GCC normally emits debugging information for classes, because using this option increases the size of debugging information by as much as a factor of two. 🫠 yeah, that's a big oof.
• -fno-merge-debug-strings — direct the linker to not merge together strings in the debugging information that are identical in different object files. Merging is not supported by all assemblers or linkers. Merging decreases the size of the debug information in the output file at the cost of increasing link processing time. Merging is enabled by default. ⚖️ classic tradeoff moment.
• -fdebug-prefix-map=old=new — when compiling files residing in directory old, record debugging information describing them as if the files resided in directory new instead. This can be used to replace a build-time path with an install-time path in the debug info. It can also be used to change an absolute path to a relative path by using . for new. This can give more reproducible builds, which are location independent, but may require an extra command to tell GDB where to find the source files. See also -ffile-prefix-map. 🗺️ path glow-up fr.
• -fvar-tracking — run variable tracking pass. It computes where variables are stored at each position in code. Better debugging information is then generated (if the debugging information format supports this information).

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

• -fvar-tracking-assignments — annotate assignments to user variables early in the compilation and attempt to carry the annotations over throughout the compilation all the way to the end, in an attempt to improve debug information while optimizing. Use of -gdwarf-4 is recommended along with it. 📝

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

• -gsplit-dwarf — separate as much DWARF debugging information as possible into a separate output file with the extension .dwo. This option allows the build system to avoid linking files with debug information. To be useful, this option requires a debugger capable of reading .dwo files. 🔪 split it bestie.
• -gdescribe-dies — add description attributes to some DWARF DIEs that have no name attribute, such as artificial variables, external references and call site parameter DIEs. 💬
• -gpubnames — generate DWARF ".debug_pubnames" and ".debug_pubtypes" sections.
• -ggnu-pubnames — generate ".debug_pubnames" and ".debug_pubtypes" sections in a format suitable for conversion into a GDB index. This option is only useful with a linker that can produce GDB index version 7. 🔢
• -fdebug-types-section — when using DWARF Version 4 or higher, type DIEs can be put into their own ".debug_types" section instead of making them part of the ".debug_info" section. It is more efficient to put them in a separate comdat section since the linker can then remove duplicates. But not all DWARF consumers support ".debug_types" sections yet and on some objects ".debug_types" produces larger instead of smaller debugging information. 😬 sometimes the flex backfires.
• -grecord-gcc-switches, -gno-record-gcc-switches — this switch causes the command-line options used to invoke the compiler that may affect code generation to be appended to the DW_AT_producer attribute in DWARF debugging information. The options are concatenated with spaces separating them from each other and from the compiler version. It is enabled by default. See also -frecord-gcc-switches for another way of storing compiler options into the object file. 📼 receipts, but for your build.
• -gstrict-dwarf — disallow using extensions of later DWARF standard version than selected with -gdwarf-version. On most targets using non-conflicting DWARF extensions from later standard versions is allowed. 🚧 stay in your lane.
• -gno-strict-dwarf — allow using extensions of later DWARF standard version than selected with -gdwarf-version. ✨ live a little.
• -gas-loc-support — inform the compiler that the assembler supports ".loc" directives. It may then use them for the assembler to generate DWARF2+ line number tables.

This is generally desirable, because assembler-generated line-number tables are a lot more compact than those the compiler can generate itself. 🎀

This option will be enabled by default if, at GCC configure time, the assembler was found to support such directives.

• -gno-as-loc-support — force GCC to generate DWARF2+ line number tables internally, if DWARF2+ line number tables are to be generated. fine, I'll do it myself 🙄
• -gas-locview-support — inform the compiler that the assembler supports "view" assignment and reset assertion checking in ".loc" directives.

This option will be enabled by default if, at GCC configure time, the assembler was found to support them.

• -gno-as-locview-support — force GCC to assign view numbers internally, if -gvariable-location-views are explicitly requested.
• -gcolumn-info, -gno-column-info — emit location column information into DWARF debugging information, rather than just file and line. This option is enabled by default. 📍 not just where, but where where.
• -gstatement-frontiers, -gno-statement-frontiers — this option causes GCC to create markers in the internal representation at the beginning of statements, and to keep them roughly in place throughout compilation, using them to guide the output of "is_stmt" markers in the line number table. This is enabled by default when compiling with optimization (-Os, -O, -O2, ...), and outputting DWARF 2 debug information at the normal level. 🚩
• -gvariable-location-views, -gvariable-location-views=incompat5, -gno-variable-location-views — augment variable location lists with progressive view numbers implied from the line number table. This enables debug information consumers to inspect state at certain points of the program, even if no instructions associated with the corresponding source locations are present at that point. If the assembler lacks support for view numbers in line number tables, this will cause the compiler to emit the line number table, which generally makes them somewhat less compact. The augmented line number tables and location lists are fully backward-compatible, so they can be consumed by debug information consumers that are not aware of these augmentations, but they won't derive any benefit from them either.

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

There is a proposed representation for view numbers that is not backward compatible with the location list format introduced in DWARF 5, that can be enabled with -gvariable-location-views=incompat5. This option may be removed in the future, is only provided as a reference implementation of the proposed representation. Debug information consumers are not expected to support this extended format, and they would be rendered unable to decode location lists using it. 💀 so like, use at your own risk bestie.

• -ginternal-reset-location-views, -gno-internal-reset-location-views — bestie this one tries to figure out which location views can be left out of the location view lists entirely. The catch? It needs the compiler to have very accurate insn length estimates, which like… isn't always a thing, and it may silently generate incorrect view lists when you're using an assembler that doesn't support location view lists. The GNU assembler will flag any such oopsie as a "view number mismatch". Only turned on for ports that define a reliable estimation function. so specific 💅
• -ginline-points, -gno-inline-points — generate extended debug info for inlined functions. Location view tracking markers get inserted at inlined entry points so address and view numbers can be computed and output in the debug info. You can enable this independently of location views — but then the view numbers won't actually be output 🫠 — and it can only be enabled along with statement frontiers. Also it's only on by default if location views are enabled.
• -gz[=type] — produce compressed debug sections in DWARF format, if that's supported. If you don't pass type, the default depends on what the assembler and linker you're using can actually do. type can 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 just gets rejected 🚫. Otherwise, if the assembler doesn't support them, -gz is silently ignored when producing object files. sneaky but ok

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

This one substantially shrinks your debugging info size, but at significant potential loss in type information to the debugger 😭. Peep -femit-struct-debug-reduced for a less aggressive option, and -femit-struct-debug-detailed if you want the full control panel.

This option works only with DWARF debug output. DWARF girlies rise

• -femit-struct-debug-reduced — emit debug info for struct-like types only when the base name of the compilation source file matches the base name of the file where the type is defined — unless the struct is a template or defined in a system header. So like, slightly chiller than the baseonly one.

This significantly reduces debugging info size, with some potential loss in type information to the debugger. See -femit-struct-debug-baseonly for a more aggressive option, and -femit-struct-debug-detailed for more detailed control.

This option works only with DWARF debug output. ✨

• -femit-struct-debug-detailed[=spec-list] — specify exactly which struct-like types the compiler generates debug info for. The whole intent is to reduce duplicate struct debug info between different object files within the same program. deduplication queen

This option is the detailed version of -femit-struct-debug-reduced and -femit-struct-debug-baseonly, and honestly it serves most needs.

A specification has the 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 arise through pointers to structs — that is, when use of an incomplete struct is valid, the use is indirect. An example is struct one direct; struct two * indirect;.

The optional second word limits the spec to ordinary structs (ord:) or generic structs (gen:). Generic structs are… a bit complicated to explain ngl. For C++, these are non-explicit specializations of template classes, or non-template classes within the above. Other programming languages have generics too, but -femit-struct-debug-detailed does not yet implement them. coming soon maybe? not our call

The third word specifies the source files for those structs the compiler should emit debug info for. The values none and any have their normal meaning. The value base means the base of the name of the file where the type declaration appears must match the base of the name of the main compilation file. In practice, this means when compiling foo.c, debug info is generated for types declared in that file and foo.h, but not other header files. The value sys means those types satisfying base or declared in system or compiler headers.

You may need to experiment to determine the best settings for your application. trial and error era fr fr

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

This option works only with DWARF debug output.

• -fno-dwarf2-cfi-asm — emit DWARF unwind info as a compiler-generated ".eh_frame" section instead of using GAS ".cfi_*" directives. different route, same destination vibes
• -fno-eliminate-unused-debug-types — normally, when producing DWARF output, GCC avoids producing debug symbol output for types that are nowhere used in the source file being compiled. But sometimes it's actually useful to have GCC emit debugging info for all types declared in a compilation unit, regardless of whether they're used in it — for example, if in the debugger you want to cast a value to a type that isn't actually used in your program (but is declared). More often than not though? This results in a significant amount of wasted space 💀.

---

Options That Control Optimization 🚀

These options control various sorts of optimizations. the glow-up section

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. predictable, honest, lowkey iconic

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. Tradeoffs, bestie. Always tradeoffs.

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 knows things 👀

Not all optimizations are controlled directly by a flag. Only optimizations that have a flag are listed in this section.

Most optimizations are completely disabled at -O0 or if an -O level is not set on the command line, even if individual optimization flags are specified. Similarly, -Og suppresses many optimization passes.

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. receipts, always get the receipts

-O, -O1

Optimize. Optimizing compilation takes somewhat more time, and a lot more memory for a large function. 😮‍💨

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

-O turns on the following optimization flags:

-fauto-inc-dec -fbranch-count-reg -fcombine-stack-adjustments
-fcompare-elim -fcprop-registers -fdce -fdefer-pop -fdelayed-branch
-fdse -fforward-propagate -fguess-branch-probability -fif-conversion
-fif-conversion2 -finline-functions-called-once -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. she's giving more, and she's charging more time for it

-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. do not skip this, bestie, it's there for a reason

-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. the chill baseline, no notes

-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 and efficient, the dream 💖

• -Ofast — bestie, this one literally says "standards? never met her." It disables strict standards compliance entirely. -Ofast turns on all the -O3 optimizations, plus a whole bunch that are not valid for all standard-compliant programs (so like, be careful, okay?). It flips on -ffast-math, -fallow-store-data-races, and the Fortran-specific -fstack-arrays (unless you specified -fmax-stack-var-size, in which case that one's off the table), and -fno-protect-parens. 💅
• -Og — okay so this is the debugging glow-up. -Og is the optimization level you want for your standard edit-compile-debug cycle, giving you a reasonable level of optimization while keeping compilation fast and the debugging experience good. It's a better pick than -O0 for producing debuggable code, because some compiler passes that collect debug info are actually disabled at -O0. The way -O0 just throws away your debug info... tragic. 😭

So like -O0, -Og completely disables a bunch of optimization passes, which means individual options that control those passes have no effect (rip to those flags, they're just sitting there doing nothing 🫶). Otherwise, -Og enables all -O1 optimization flags except for the ones that might mess 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's effective. Last one wins, periodt. 👑

---

Options of the form -fflag specify machine-independent flags. Most flags have both a positive and a negative form; the negative form of -ffoo is -fno-foo. In the table below, only one of the forms is listed — the one you'd typically use. You can figure out the other form by either removing the no- or adding it. iykyk ✨

The following options control specific optimizations. They're either activated by the -O options or related to ones that are. You can use the following flags in those rare cases when you want to "fine-tune" the optimizations being performed. It's giving ✨ custom order ✨ energy.

• -fno-defer-pop — for machines that have to pop arguments after a function call, this makes them always pop the arguments as soon as each function returns. At levels -O1 and higher, -fdefer-pop is the default; that lets the compiler let arguments pile up on the stack for several function calls and pop them all at once. Stacked up and snatched together, we love efficiency. 🎀
• -fforward-propagate — performs a forward propagation pass on RTL. The pass tries to combine two instructions and checks whether the result can be simplified. If loop unrolling is active, two passes get performed, and the second one is scheduled after loop unrolling.

This option is enabled by default at optimization levels -O, -O2, -O3, -Os.

• *-ffp-contract=style* — okay so:
  • -ffp-contract=off disables floating-point expression contraction.
  • -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. This is currently not implemented and is treated equal to -ffp-contract=off. Not me catching the compiler in 4K saying "yeah we just didn't do that one." 💀

The default is -ffp-contract=fast.

• -fomit-frame-pointer — omits the frame pointer in functions that don't need one. This avoids the instructions to save, set up and restore the frame pointer; on many targets it also makes an extra register available. Free real estate, honestly. 🌸

On some targets this flag has no effect, because the standard calling sequence always uses a frame pointer, so it can't be omitted. Sorry bestie, no frame pointer freedom there. 🙈

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 — does 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. Special little exception, we love her. 💖

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

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

• -findirect-inlining — inlines also indirect calls that are discovered to be known at compile time thanks to previous inlining. This option 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 isn't output as assembler code in its own right.

Enabled at levels -O1, -O2, -O3 and -Os, but not -Og. Og said "not on my watch." 💅

• -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 makes 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 allows coarse control of this limit. n is the size of functions that can be inlined, in number of pseudo instructions.

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

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

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

Note: there may be no value to -finline-limit that results in default behavior.

Note: pseudo instruction represents, in this particular context, an abstract measurement of a function's size. In no way does it represent a count of assembly instructions, and as such its exact meaning might change from one release to another. So don't @ it too hard, bestie. 🦋

• -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, emits 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++, it emits any and all inline functions into the object file.
• -fkeep-static-functions — emits static functions into the object file, even if the function is never used. She's keeping them anyway, whether you like it or not. 🎀
• -fkeep-static-consts — emits variables declared static const when optimization isn't turned on, even if the variables aren't referenced.

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

• -fmerge-constants — attempts to merge identical constants (string constants and floating-point constants) across compilation units.

This option is the default for optimized compilation if the assembler and linker support it. Use -fno-merge-constants to inhibit this behavior.

Enabled at levels -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. 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. Okay so that's a real warning, not a vibe — heads up. ⚠️

• -fmodulo-sched — performs swing modulo scheduling immediately before the first scheduling pass. This pass looks at innermost loops and reorders their instructions by overlapping different iterations.
• -fmodulo-sched-allow-regmoves — performs more aggressive SMS-based modulo scheduling with register moves allowed. By setting this flag, certain anti-dependences edges get 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. She's not cleaning up after the other passes' mess, ngl. 💀

The default is -fbranch-count-reg at -O1 and higher, except for -Og.

• -fno-function-cse — bestie, this one says do not stash function addresses in registers. Instead, every single instruction that calls a constant function has to spell out that function's address explicitly, right there, no shortcuts 💅 The tea is it gives you less efficient code, but some strange hacks that alter the assembler output can get confused by the optimizations performed when this option is <em>not</em> used. The default is -ffunction-cse ✨
• -fno-zero-initialized-in-bss — if the target supports a BSS section, GCC by default drops variables that are initialized to zero into BSS, and that lowkey saves space in the resulting code 🎀 This option turns that behavior off, because some programs explicitly rely on variables going to the data section — like, so 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 whether a jump branches to a location where another comparison subsumed by the first is found 👀 If so, the first branch gets 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 you're using a type that occupies multiple registers, such as long long on a 32-bit system, this splits the registers apart and allocates them independently 🧚 This normally generates better code for those types, but heads up, it may make debugging more difficult 😭 Enabled at levels -O, -O2, -O3, -Os.
• -fsplit-wide-types-early — fully splits wide types early, instead of very late. This option has no effect unless -fsplit-wide-types is turned on. This is the default on some targets ✨
• -fcse-follow-jumps — in common subexpression elimination (CSE), this scans through jump instructions when the target of the jump is not reached by any other path 🌸 For example, when CSE encounters an if statement with an else clause, CSE follows the jump when the condition tested is false. Enabled at levels -O2, -O3, -Os.
• -fcse-skip-blocks — similar to -fcse-follow-jumps, but this one makes CSE follow jumps that conditionally skip over blocks 💫 When CSE encounters a simple if statement with no else clause, -fcse-skip-blocks causes CSE to follow the jump around the body of the if. Enabled at levels -O2, -O3, -Os.
• -frerun-cse-after-loop — re-runs common subexpression elimination after loop optimizations are performed 🔁 Enabled at levels -O2, -O3, -Os.
• -fgcse — performs a global common subexpression elimination pass. This pass also performs global constant and copy propagation 👑

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

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

• -fgcse-lm — when -fgcse-lm is enabled, global common subexpression elimination attempts to move loads that are only killed by stores into themselves 🌀 This allows a loop containing a load/store sequence to be changed to a load outside the loop, and a copy/store within the loop. Enabled by default when -fgcse is enabled 💖
• -fgcse-sm — when -fgcse-sm is enabled, a store motion pass is run after global common subexpression elimination 🌈 This pass attempts to move stores out of loops. When used in conjunction with -fgcse-lm, loops containing a load/store sequence can be changed to a load before the loop and a store after the loop. Not enabled at any optimization level.
• -fgcse-las — when -fgcse-las is enabled, the global common subexpression elimination pass eliminates redundant loads that come after stores to the same memory location (both partial and full redundancies) 🍓 Not enabled at any optimization level.
• -fgcse-after-reload — when -fgcse-after-reload is enabled, a redundant load elimination pass is performed after reload. The purpose of this pass is to clean up redundant spilling 🧹 Enabled by -fprofile-use and -fauto-profile ✨
• -faggressive-loop-optimizations — this option tells the loop optimizer to use language constraints to derive bounds for the number of iterations of a loop 👀 This assumes that loop code does not invoke undefined behavior by for example causing signed integer overflows or out-of-bound array accesses ⚠️ The bounds for the number of iterations of a loop are used to guide loop unrolling and peeling and loop exit test optimizations. This option is enabled by default.
• -funconstrained-commons — this option 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 — performs the cross-jumping transformation. This transformation unifies equivalent code and saves code size 💅 The resulting code may or may not perform better than without cross-jumping. Enabled at levels -O2, -O3, -Os.
• -fauto-inc-dec — combines increments or decrements of addresses with memory accesses 🔗 This pass is always skipped on architectures that do not have instructions to support this. Enabled by default at -O and higher on architectures that support this.
• -fdce — performs dead code elimination (DCE) on RTL 💀 Enabled by default at -O and higher.
• -fdse — performs dead store elimination (DSE) on RTL 🗑️ Enabled by default at -O and higher.
• -fif-conversion — attempts to transform conditional jumps into branch-less equivalents 🦋 This includes use of conditional moves, min, max, set flags and abs instructions, and some tricks doable by standard arithmetics. The use of conditional execution on chips where it is available is controlled by -fif-conversion2. Enabled at levels -O, -O2, -O3, -Os, but not with -Og 👑
• -fif-conversion2 — uses 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 — okay so the C++ ABI requires multiple entry points for constructors and destructors: one for a base subobject, one for a complete object, and one for a virtual destructor that 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 — assumes 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. In addition, other optimization passes in GCC use this flag to control global dataflow analyses that eliminate useless checks for null pointers; these assume that 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 however that in some environments this assumption is not true ⚠️ Use -fno-delete-null-pointer-checks to disable this optimization for programs that depend on that behavior.

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 — attempts 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 — attempts to convert calls to virtual functions to speculative direct calls 🔮 Based on the analysis of the type inheritance graph, determine for a given call the set of likely targets. If the set is small, preferably of size 1, change the call into a conditional deciding between direct and indirect calls. The speculative calls enable more optimizations, such as inlining. When they seem useless after further optimization, they are converted back into original form 🔁
• -fdevirtualize-at-ltrans — streams extra information needed for aggressive devirtualization when running the link-time optimizer in local transformation mode 📡 This option enables more devirtualization but significantly increases the size of streamed data. For this reason it is disabled by default.
• -fexpensive-optimizations — performs a number of minor optimizations that are relatively expensive 💸 Enabled at levels -O2, -O3, -Os.
• -free — attempts to remove redundant extension instructions ✂️ This is especially helpful for the x86-64 architecture, which implicitly zero-extends in 64-bit registers after writing to their lower 32-bit half. Enabled for Alpha, AArch64 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 — attempts to decrease register pressure through register live range shrinkage 🎈 This is helpful for fast processors with small or moderate size register sets 💅
• -fira-algorithm=algorithm — okay so this picks the coloring algorithm for the integrated register allocator (IRA, the icon). algorithm can be priority, which is Chow's priority coloring, or CB, which is Chaitin-Briggs coloring. Lowkey important tea: Chaitin-Briggs is not implemented for all architectures, but on the targets that do support it, it's the default because it just generates better code. periodt.
• -fira-region=region — chooses the regions IRA works with. region should be one of these besties:
  • all — use all loops as register allocation regions. This can give the best results for machines with a small and/or irregular register set.
  • mixed — use all loops except loops with small register pressure as the regions. This value usually gives the best results in most cases and for most architectures, and it's 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 it's 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 usually results in smaller code, but it can slow the compiler down. 🐌

It's 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 usually results in generation of faster and smaller code on machines with large register files (>= 32 registers), but yeah it can slow the compiler down.

It's enabled at level -O3 for some targets.

• -fno-ira-share-save-slots — disables sharing of stack slots used for saving call-used hard registers living through a call. Each hard register gets its own separate stack slot, and as a result function stack frames are larger. it's giving spacious but at what cost 💀
• -fno-ira-share-spill-slots — disables sharing of stack slots allocated for pseudo-registers. Each pseudo-register that doesn't get a hard register gets a separate stack slot, and as a result function stack frames are larger.
• -flra-remat — enables CFG-sensitive rematerialization in LRA. Instead of loading values of spilled pseudos, LRA tries to rematerialize (recalculate) values if it's profitable. ✨

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

• -fdelayed-branch — if supported for the target machine, attempts to reorder instructions to exploit instruction slots available after delayed branch instructions.

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

• -fschedule-insns — if supported for the target machine, attempts to reorder instructions to eliminate execution stalls due to required data being unavailable. This helps machines that have slow floating point or memory load instructions by allowing other instructions to be issued until the result of the load or floating-point instruction is required. 🧚

Enabled at levels -O2, -O3.

• -fschedule-insns2 — similar to -fschedule-insns, but requests an additional pass of instruction scheduling after register allocation has been done. This is especially useful on machines with a relatively small number of registers and where memory load instructions take more than one cycle.

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

• -fno-sched-interblock — disables instruction scheduling across basic blocks, which is normally enabled when scheduling 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. This only makes sense when scheduling before register allocation is enabled, i.e. with -fschedule-insns or at -O2 or higher. Using this option can improve the generated code and decrease its size by preventing register pressure increase above the number of available hard registers and the subsequent spills in register allocation. 💅
• -fsched-spec-load — allows speculative motion of some load instructions. This only makes sense when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.
• -fsched-spec-load-dangerous — allows speculative motion of more load instructions. This only makes sense when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher. yes bestie, the flag literally says dangerous, it's right there in the name 😭
• -fsched-stalled-insns, -fsched-stalled-insns=n — defines how many insns (if any) can be moved prematurely from the queue of stalled insns into the ready list during the second scheduling pass. -fno-sched-stalled-insns means no insns are moved prematurely, -fsched-stalled-insns=0 means there's no limit on how many queued insns can be moved prematurely. And -fsched-stalled-insns without a value is equivalent to -fsched-stalled-insns=1.
• -fsched-stalled-insns-dep, -fsched-stalled-insns-dep=n — defines how many insn groups (cycles) are examined for a dependency on a stalled insn that's a candidate for premature removal from the queue of stalled insns. This only has an effect during the second scheduling pass, and only if -fsched-stalled-insns is used. -fno-sched-stalled-insns-dep is equivalent to -fsched-stalled-insns-dep=0. And -fsched-stalled-insns-dep without a value is equivalent to -fsched-stalled-insns-dep=1.
• -fsched2-use-superblocks — when scheduling after register allocation, use superblock scheduling. This allows motion across basic block boundaries, resulting in faster schedules. Heads up though: this option is experimental, because not all machine descriptions used by GCC model the CPU closely enough to avoid unreliable results from the algorithm. 🙈

This only makes sense when scheduling after register allocation, i.e. with -fschedule-insns2 or at -O2 or higher.

• -fsched-group-heuristic — enables the group heuristic in the scheduler. This heuristic favors the instruction that belongs to a schedule group. It's enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.
• -fsched-critical-path-heuristic — enables the critical-path heuristic in the scheduler. This heuristic favors instructions on the critical path. Enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.
• -fsched-spec-insn-heuristic — enables the speculative instruction heuristic in the scheduler. This heuristic favors speculative instructions with greater dependency weakness. Enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.
• -fsched-rank-heuristic — enables the rank heuristic in the scheduler. This heuristic favors the instruction belonging to a basic block with greater size or frequency. Enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.
• -fsched-last-insn-heuristic — enables the last-instruction heuristic in the scheduler. This heuristic favors the instruction that's less dependent on the last instruction scheduled. Enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.
• -fsched-dep-count-heuristic — enables the dependent-count heuristic in the scheduler. This heuristic favors the instruction that has more instructions depending on it. Enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.
• -freschedule-modulo-scheduled-loops — modulo scheduling is performed before traditional scheduling. If a loop is modulo scheduled, later scheduling passes may change its schedule. Use this option to control that behavior. 🌀
• -fselective-scheduling — schedule instructions using the selective scheduling algorithm. Selective scheduling runs instead of the first scheduler pass.
• -fselective-scheduling2 — schedule instructions using the selective scheduling algorithm. Selective scheduling runs instead of the second scheduler pass.
• -fsel-sched-pipelining — enables software pipelining of innermost loops during selective scheduling. This option has no effect unless one of -fselective-scheduling or -fselective-scheduling2 is turned on.
• -fsel-sched-pipelining-outer-loops — when pipelining loops during selective scheduling, also pipeline outer loops. This option has no effect unless -fsel-sched-pipelining is turned on.
• -fsemantic-interposition — okay this one's a whole essay so buckle up 🫶. Some object formats, like ELF, allow interposing of symbols by the dynamic linker. This means that for symbols exported from the DSO, the compiler can't perform interprocedural propagation, inlining and other optimizations in anticipation that the function or variable in question may change. While this feature is useful, for example, to rewrite memory allocation functions by a debugging implementation, it's expensive in terms of code quality. With -fno-semantic-interposition the compiler assumes that if interposition happens for functions, the overwriting function will have precisely the same semantics (and side effects). Similarly, if interposition happens for variables, the constructor of the variable will be the same. The flag has no effect for functions explicitly declared inline (where it's never allowed for interposition to change semantics) and for symbols explicitly declared weak.
• -fshrink-wrap — emit function prologues only before the parts of the function that need it, rather than at the top of the function. This flag is enabled by default at -O and higher. ✨
• -fshrink-wrap-separate — shrink-wrap separate parts of the prologue and epilogue separately, so those parts are only executed when needed. This option is on by default, but has no effect unless -fshrink-wrap is also turned on and the target supports this.
• -fcaller-saves — bestie, this one lets values get allocated into registers that function calls like to clobber, by emitting extra instructions to save and restore those registers around such calls. 💅 It only does this when it looks like it'll actually produce better code. On certain machines this is always enabled by default — 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 goes looking for ways to combine them. Enabled by default at -O1 and higher. ✨
• -fipa-ra — uses caller-save registers for allocation if those registers aren't used by any called function. In that case there's no need to save and restore them around calls. This is only possible if the called functions are part of the same compilation unit as the current function and they're compiled before it. Enabled at levels -O2, -O3, -Os, however the option is disabled if the generated code will be instrumented for profiling (-p, or -pg) or if the callee's register usage can't be known exactly (this happens on targets that don't expose prologues and epilogues in RTL). 🫶
• -fconserve-stack — attempts to minimize stack usage. The compiler tries to use less stack space, even if that makes the program slower. This option implies setting the large-stack-frame parameter to 100 and the large-stack-frame-growth parameter to 400. 🌸
• -ftree-reassoc — performs reassociation on trees. Enabled by default at -O and higher.
• -fcode-hoisting — performs code hoisting. Code hoisting tries to move the evaluation of expressions executed on all paths to the function exit as early as possible. This is especially useful as a code size optimization, but it often helps for code speed as well. Enabled by default at -O2 and higher. 👑
• -ftree-pre — performs partial redundancy elimination (PRE) on trees. Enabled by default at -O2 and -O3.
• -ftree-partial-pre — makes partial redundancy elimination (PRE) more aggressive. Enabled by default at -O3. 🔥
• -ftree-forwprop — performs forward propagation on trees. Enabled by default at -O and higher.
• -ftree-fre — performs full redundancy elimination (FRE) on trees. The difference between FRE and PRE is that FRE only considers expressions that are computed on all paths leading to the redundant computation. This analysis is faster than PRE, though it exposes fewer redundancies. Enabled by default at -O and higher.
• -ftree-phiprop — performs hoisting of loads from conditional pointers on trees. Enabled by default at -O and higher.
• -fhoist-adjacent-loads — speculatively hoists loads from both branches of an if-then-else if the loads are from adjacent locations in the same structure and the target architecture has a conditional move instruction. Enabled by default at -O2 and higher. ✨
• -ftree-copy-prop — performs copy propagation on trees. This pass eliminates unnecessary copy operations. Enabled by default at -O and higher.
• -fipa-pure-const — discovers which functions are pure or constant. Enabled by default at -O and higher.
• -fipa-reference — discovers which static variables do not escape the compilation unit. Enabled by default at -O and higher.
• -fipa-reference-addressable — discovers read-only, write-only and non-addressable static variables. Enabled by default at -O and higher. 🦋
• -fipa-stack-alignment — reduces stack alignment on call sites if possible. Enabled by default.
• -fipa-pta — performs interprocedural pointer analysis and interprocedural modification and reference analysis. Heads up though: this option can cause excessive memory and compile-time usage on large compilation units. It is not enabled by default at any optimization level. 😭
• -fipa-profile — performs interprocedural profile propagation. The functions called only from cold functions are marked as cold. Also functions executed once (such as "cold", "noreturn", static constructors or destructors) are identified. Cold functions and loop less parts of functions executed once are then optimized for size. Enabled by default at -O and higher.
• -fipa-cp — performs interprocedural constant propagation. This optimization analyzes the program to determine when values passed to functions are constants and then optimizes accordingly. It can substantially increase performance if the application has constants passed to functions. Enabled by default at -O2, -Os and -O3. It is also enabled by -fprofile-use and -fauto-profile. 💖
• -fipa-cp-clone — performs function cloning to make interprocedural constant propagation stronger. When enabled, interprocedural constant propagation performs function cloning when an externally visible function can be called with constant arguments. Because this optimization can create multiple copies of functions, it may significantly increase code size (see --param ipa-cp-unit-growth=value). Enabled by default at -O3. It is also enabled by -fprofile-use and -fauto-profile.
• -fipa-bit-cp — when enabled, performs interprocedural bitwise constant propagation. Enabled by default at -O2 and by -fprofile-use and -fauto-profile. It requires that -fipa-cp is enabled. 🔗
• -fipa-vrp — when enabled, performs interprocedural propagation of value ranges. Enabled by default at -O2. It requires that -fipa-cp is enabled.
• -fipa-icf — performs Identical Code Folding for functions and read-only variables. The optimization reduces code size and may disturb unwind stacks by replacing a function by an equivalent one with a different name. The optimization works more effectively with link-time optimization enabled.

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

Enabled by default at -O2 and -Os.

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

If the compiler's optimization uses a function's body or information extracted from its body to optimize/change another function, the latter is called an impacted function of the former. If a function is patched, its impacted functions should be patched too.

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

Usually, the more IPA optimizations enabled, the larger the number of impacted functions for each function. In order to control the number of impacted functions and more easily compute the list of impacted functions, IPA optimizations can be partially enabled at two different levels.

The level argument should be one of the following:

• inline-clone — only enable inlining and cloning optimizations, which includes inlining, cloning, interprocedural scalar replacement of aggregates and partial inlining. As a result, when patching a function, all its callers and its clones' callers are impacted, therefore need to be patched as well.

-flive-patching=inline-clone disables the following optimization flags: -fwhole-program -fipa-pta -fipa-reference -fipa-ra -fipa-icf -fipa-icf-functions -fipa-icf-variables -fipa-bit-cp -fipa-vrp -fipa-pure-const -fipa-reference-addressable -fipa-stack-alignment

• inline-only-static — only enable inlining of static functions. As a result, when patching a static function, all its callers are impacted and so need to be patched as well.

In addition to all the flags that -flive-patching=inline-clone disables, -flive-patching=inline-only-static disables the following additional optimization flags: -fipa-cp-clone -fipa-sra -fpartial-inlining -fipa-cp

When -flive-patching is specified without any value, the default value is inline-clone.

This flag is disabled by default.

Note that -flive-patching is not supported with link-time optimization (-flto). 🙈

• -fisolate-erroneous-paths-dereference — detects paths that trigger erroneous or undefined behavior due to dereferencing a null pointer. It isolates those paths from the main control flow and turns the statement with erroneous or undefined behavior into a trap. Enabled 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 due to a null value being used in a way forbidden by a "returns_nonnull" or "nonnull" attribute. Isolates those paths from the main control flow and turns the statement with erroneous or undefined behavior into a trap. This is not currently enabled, but may be enabled by -O2 in the future. ⭐
• -ftree-sink — performs forward store motion on trees. Enabled by default at -O and higher.
• -ftree-bit-ccp — performs sparse conditional bit constant propagation on trees and propagates pointer alignment information. This pass only operates on local scalar variables and is enabled by default at -O1 and higher, except for -Og. It requires that -ftree-ccp is enabled.
• -ftree-ccp — performs sparse conditional constant propagation (CCP) on trees. This pass only operates on local scalar variables and is enabled by default at -O and higher.
• -fssa-backprop — propagates information about uses of a value up the definition chain in order to simplify the definitions. For example, this pass strips sign operations if the sign of a value never matters. Enabled by default at -O and higher. 🌈
• -fssa-phiopt — bestie, this one does pattern matching on SSA PHI nodes to optimize conditional code ✨ it's giving "spot the vibe, rewrite the branch". It's on by default at -O1 and higher, except for -Og (sorry -Og girlies, not today 💅).
• -ftree-switch-conversion — takes the simple initializations inside a switch and turns them into initializations from a scalar array 🎀 on by default at -O2 and higher.
• -ftree-tail-merge — hunts for identical code sequences 👀 and when it finds a match, replaces one of them 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 — performs dead code elimination (DCE) on trees 💀 (like, literally dead, no cap). 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. Read that twice bestie, the -Os clause matters.
• -ffinite-loops — assume that a loop with an exit will eventually take that exit and not loop forever 🌈 this lets the compiler remove loops that otherwise have no side-effects, and it does not consider eventual endless looping to be a side-effect. Enabled by default at -O2 for C++ with -std=c++11 or higher.
• -ftree-dominator-opts — a whole lil glow-up routine of simple scalar cleanups: constant/copy propagation, redundancy elimination, range propagation and expression simplification, all based on a dominator tree traversal ✨ it also does jump threading (to reduce jumps to jumps). On by default at -O and higher.
• -ftree-dse — performs dead store elimination (DSE) on trees. A dead store is a store into a memory location that gets overwritten later by another store with no intervening loads — so the earlier store can just be deleted 🙈 on by default at -O and higher.
• -ftree-ch — performs loop header copying on trees 🌸 this is beneficial because it increases the effectiveness of code motion optimizations, and it also saves one jump. On by default at -O and higher. Not enabled for -Os, since it usually increases code size (size queens stay winning, but at a cost 💅).
• -ftree-loop-optimize — performs loop optimizations on trees 🔁 on by default at -O and higher.
• -ftree-loop-linear
• -floop-strip-mine
• -floop-block — these three do loop nest optimizations, and they're all the 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 🦋.
• -fgraphite-identity — enables the identity transformation for graphite. For every SCoP we generate the polyhedral representation and transform it back to gimple ✨ using -fgraphite-identity we can check the costs or benefits of the GIMPLE -> GRAPHITE -> GIMPLE transformation. Some minimal optimizations are also performed by the code generator isl, like index splitting and dead code elimination in loops 👑.
• -floop-nest-optimize — enables the isl based loop nest optimizer. It's a generic loop nest optimizer based on the Pluto optimization algorithms. It calculates a loop structure optimized for data-locality and parallelism 💖. Heads up: this option is experimental — girl is still in her testing era.
• -floop-parallelize-all — uses the Graphite data dependence analysis to identify loops that can be parallelized 🌈 and parallelizes all the loops that can be analyzed to not contain loop carried dependences — without checking that it's actually profitable to parallelize them. No vibe check on profitability, just send it 😭.
• -ftree-coalesce-vars — while transforming the program out of the SSA representation, it attempts to reduce copying by coalescing versions of different user-defined variables, instead of just compiler temporaries 🎀. Warning bestie: this may severely limit your ability to debug an optimized program compiled with -fno-var-tracking-assignments. In the negated form, this flag prevents SSA coalescing of user variables. Enabled by default if optimization is enabled, and it does very little otherwise.
• -ftree-loop-if-convert — attempts 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 them better. Enabled by default if vectorization is enabled.
• -ftree-loop-distribution — performs 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 — performs 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 — performs loop interchange outside of graphite 🔄. This flag can improve cache performance on loop nests 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 — applies unroll and jam transformations on feasible loops 🍓. In a loop nest this unrolls the outer loop by some factor and fuses the resulting multiple inner loops. This flag is enabled by default at -O3. It is also enabled by -fprofile-use and -fauto-profile.
• -ftree-loop-im — performs loop invariant motion on trees 🌈. This pass moves only invariants that are hard to handle at RTL level (function calls, operations that expand to nontrivial sequences of insns). With -funswitch-loops it also moves operands of conditions that are invariant out of the loop, so that we can use just trivial invariantness analysis in loop unswitching. The pass also includes store motion 👑.
• -ftree-loop-ivcanon — creates a canonical counter for the number of iterations in loops for which determining the number of iterations requires complicated analysis 🤯. Later optimizations then may determine the number easily. Useful especially in connection with unrolling.
• -ftree-scev-cprop — performs final value replacement ✨. If a variable is modified in a loop in such a way that its value when exiting the loop can be determined using only its initial value and the number of loop iterations, it replaces 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 — performs induction variable optimizations (strength reduction, induction variable merging and induction variable elimination) on trees 🔥.
• -ftree-parallelize-loops=n — parallelizes loops, i.e., splits their iteration space to run in n threads 🧵. This is only possible for loops whose iterations are independent and can be arbitrarily reordered. The optimization is only profitable on multiprocessor machines, for loops that are CPU-intensive, rather than constrained e.g. by memory bandwidth. This option implies -pthread, and thus is only supported on targets that have support for -pthread 🫶.
• -ftree-pta — performs function-local points-to analysis on trees 👀. This flag is enabled by default at -O1 and higher, except for -Og.
• -ftree-sra — performs scalar replacement of aggregates ✨. This pass replaces structure references with scalars to prevent committing structures to memory too early. This flag is enabled by default at -O1 and higher, except for -Og.
• -fstore-merging — performs merging of narrow stores to consecutive memory addresses 🎀. This pass merges contiguous stores of immediate values narrower than a word into fewer wider stores to reduce the number of instructions. Enabled by default at -O2 and higher as well as -Os 💅.
• -ftree-ter — performs temporary expression replacement during the SSA->normal phase 🔁. Single use/single def temporaries are replaced at their use location with their defining expression. This results in non-GIMPLE code, but gives the expanders much more complex trees to work on, resulting in better RTL generation 🌸. Enabled by default at -O and higher.
• -ftree-slsr — performs straight-line strength reduction on trees 💪. This recognizes related expressions involving multiplications and replaces them by less expensive calculations when possible. Enabled by default at -O and higher.
• -ftree-vectorize — performs vectorization on trees 🌈. This flag enables -ftree-loop-vectorize and -ftree-slp-vectorize if not explicitly specified.
• -ftree-loop-vectorize — performs loop vectorization on trees 🔥. This flag is enabled by default at -O3 and by -ftree-vectorize, -fprofile-use, and -fauto-profile.
• -ftree-slp-vectorize — does basic block vectorization on trees, bestie ✨ It's on by default at -O3, and also gets switched on by -ftree-vectorize, -fprofile-use, and -fauto-profile.
• -fvect-cost-model=model — swaps out the cost model that vectorization uses 💅 The model you pass has to be one of unlimited, dynamic or cheap. With unlimited, the vectorized code-path is just assumed to be profitable, periodt. With dynamic, a runtime check guards the vectorized code-path so it only fires for iteration counts that'll probably run faster than the original scalar loop. The cheap model says nope to vectorizing loops where it'd be cost-prohibitive — like when you'd need runtime checks for data dependence or alignment — but other than that it's literally the same as dynamic. Which one you get by default depends on your other optimization flags, and it's either dynamic or cheap.
• -fsimd-cost-model=model — same energy but for vectorization of loops marked with the OpenMP simd directive 🌸 The model argument should be one of unlimited, dynamic, cheap. All the model values mean exactly what they mean in -fvect-cost-model, and by default whatever cost model you set with -fvect-cost-model is the one that gets used.
• -ftree-vrp — performs Value Range Propagation on trees 🦋 It's like the constant propagation pass but, instead of actual values, it propagates ranges of values. This lets the optimizers yeet unnecessary range checks like array bound checks and null pointer checks. It's enabled by default at -O2 and higher. Heads up though: null pointer check elimination only happens if -fdelete-null-pointer-checks is enabled.
• -fsplit-paths — splits paths that lead to loop backedges ✨ This can level up dead code elimination and common subexpression elimination. Enabled by default at -O3 and above.
• -fsplit-ivs-in-unroller — lets the values of induction variables in later iterations of the unrolled loop be expressed using the value from the first iteration 👑 This breaks up those long dependency chains, so the scheduling passes run way more efficiently.

A combo of -fweb and CSE is often enough to get the same effect. But ngl, that's not reliable when the loop body is more complicated than a single basic block. It also doesn't work at all on some architectures because of restrictions in the CSE pass.

This optimization is enabled by default 💅

• -fvariable-expansion-in-unroller — with this option the compiler makes multiple copies of some local variables when it unrolls a loop, which can give you superior code 🌈

This optimization is enabled by default for PowerPC targets, but disabled by default everywhere else.

• -fpartial-inlining — inlines parts of functions 🎀 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 ♻️

This option is 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 💖

Fr fr, this option may generate better or worse code — the results are highly dependent on the structure of the loops inside your source code.

Disabled at level -Os.

• -fno-printf-return-value — do not substitute constants for the known return value of formatted output functions like 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 pinning down 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 snprintf) 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 gives its best results with -O2 and above. It works in tandem with the -Wformat-overflow and -Wformat-truncation options. And the -fprintf-return-value option is enabled by default 💅

• -fno-peephole
• -fno-peephole2 — disable any machine-specific peephole optimizations 🦋 The difference between -fno-peephole and -fno-peephole2 is in how they're implemented in the compiler; some targets use one, some use the other, and a few use both.

-fpeephole is enabled by default. -fpeephole2 is enabled at levels -O2, -O3, -Os.

• -fno-guess-branch-probability — do not guess branch probabilities using heuristics, bestie 🚫

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 that 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 — reorders basic blocks inside the compiled function to cut down the number of taken branches and improve code locality 👑

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

• -freorder-blocks-algorithm=algorithm — uses the specified algorithm for basic block reordering ✨ The algorithm argument can be simple, which doesn't increase code size (except sometimes due to secondary effects like alignment), or stc, the "software trace cache" algorithm, which tries to put all the often-executed code together and minimizes 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 — on top of reordering basic blocks in the compiled function (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 when there's exception handling or unwind tables (on targets using setjump/longjump or a target-specific scheme), for linkonce sections, for functions with a user-defined section attribute, and on any architecture that doesn't support named sections. When -fsplit-stack is used, this option isn't enabled by default (to avoid linker errors), but it may be enabled explicitly (if you're using a working linker).

Enabled for x86 at levels -O2, -O3, -Os.

• -freorder-functions — reorders functions in the object file to improve code locality 🌸 This is implemented using special subsections: .text.hot for the most frequently executed functions, and .text.unlikely for the unlikely-executed ones. The reordering is done by the linker, so the object file format has to support named sections and the linker has to place them in a reasonable way.

This option isn't effective unless you either provide profile feedback (see -fprofile-arcs for the deets) or manually annotate functions with the hot or cold attributes.

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

• -fstrict-aliasing — lets the compiler assume the strictest aliasing rules that apply 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 to never live 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 extra 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 super 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 — aligns the start of functions to the next power-of-two greater than or equal to n, skipping up to m-1 bytes 🎀 This makes sure 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 isn't specified, it defaults to n.

Examples: -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, and -falign-functions=32:7 aligns to the next 32-byte boundary only if this can be done by skipping 6 bytes or less.

• -falign-functions=n:m:n2:m2 (the second pair, continuing bestie) — so like, that second set of n2:m2 values lets you set a secondary alignment. So -falign-functions=64:7:32:3 means: snap to the next 64-byte boundary if you can do it by skipping 6 bytes or less, and if you can't, snap to the next 32-byte boundary if that's doable by skipping 2 bytes or less. And if you don't give m2? It just defaults to n2. Easy.
• Some assemblers only vibe with this flag when n is a power of two — in that case it gets rounded up. ✨
• -fno-align-functions and -falign-functions=1 are the same thing, periodt, and both mean functions just... aren't aligned.
• If n isn't specified or it's zero, you get a machine-dependent default. The biggest n you're allowed to use is 65536.
• Enabled at levels -O2, -O3. 🔥

---

• -flimit-function-alignment — turn this on and the compiler tries to not overalign functions for no reason. It tells the assembler to align by whatever -falign-functions says, but like, not to skip more bytes than the function is actually big. 💅

---

• -falign-labels
• -falign-labels=n
• -falign-labels=n:m
• -falign-labels=n:m:n2
• -falign-labels=n:m:n2:m2 — aligns every branch target to a power-of-two boundary, slay.
• The parameters here are analogous to the -falign-functions option. -fno-align-labels and -falign-labels=1 are equivalent and mean labels just don't get aligned, no cap.
• If -falign-loops or -falign-jumps apply and they're bigger than this value, then their values get used instead. Sorry bestie, they win.
• If n isn't specified or is zero, you get a machine-dependent default that is very likely to be 1, meaning no alignment at all. The max 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 — aligns loops to a power-of-two boundary. And if those loops run a ton of times, it fully makes up for all the dummy padding instructions you had to execute. worth it.
• If -falign-labels is greater than this value, its value is used instead.
• Parameters are analogous to -falign-functions. -fno-align-loops and -falign-loops=1 are equivalent and mean loops aren't aligned. The max allowed n value is 65536.
• If n isn't specified or is zero, machine-dependent default, you know the drill.
• Enabled at levels -O2, -O3. ⭐

---

• -falign-jumps
• -falign-jumps=n
• -falign-jumps=n:m
• -falign-jumps=n:m:n2
• -falign-jumps=n:m:n2:m2 — aligns branch targets to a power-of-two boundary, but specifically for branch targets that can only be reached by jumping. In that case, no dummy operations need to be executed. icon behaviour.
• If -falign-labels is greater than this value, then its value is used instead.
• Parameters are analogous to -falign-functions. -fno-align-jumps and -falign-jumps=1 are equivalent and mean loops are not aligned. (yes it says loops, that's what the page says, we don't question it 💀)
• If n isn't specified or is zero, machine-dependent default. Max allowed n value is 65536.
• Enabled at levels -O2, -O3. 🦋

---

• -fno-allocation-dce — do not remove unused C++ allocations during dead code elimination. leave them alone bestie.

---

• -fallow-store-data-races — lets the compiler go ahead and introduce brand new data races on stores. yeah. that's the whole thing. 😭
• Enabled at level -Ofast.

---

• -funit-at-a-time — this one is just kept around for compatibility reasons, lowkey. -funit-at-a-time does literally nothing, while -fno-unit-at-a-time implies -fno-toplevel-reorder and -fno-section-anchors.
• Enabled by default.

---

• -fno-toplevel-reorder — don't reorder top-level functions, variables, and asm statements. Output them in the exact same order they appear in the input file. When this is on, unreferenced static variables are not removed. This option exists to support existing code that relies on a particular ordering. For new code, it's better to use attributes when you can. 💜
• -ftoplevel-reorder is the default at -O1 and higher, and also at -O0 if -fsection-anchors is explicitly requested. Also, -fno-toplevel-reorder implies -fno-section-anchors.

---

• -fweb — constructs webs the way they're commonly used for register allocation, and assigns each web its own individual pseudo register. This lets the register allocation pass work on pseudos directly, but it also strengthens several other optimization passes, like CSE, the loop optimizer, and the trivial dead code remover. However, it can make debugging impossible, since variables no longer stay in a "home register". 🙈
• Enabled by default with -funroll-loops.

---

• -fwhole-program — assumes that the current compilation unit is the whole program being compiled. All public functions and variables, except main and those merged by the attribute externally_visible, become static functions and get optimized way more aggressively by the interprocedural optimizers. main character energy fr fr.

This option should not be used in combination with -flto. Instead, relying on a linker plugin should give you safer and more precise information. 🫶

---

• -flto[=n] — okay this one's a big deal, sit down bestie. This option runs the standard link-time optimizer. When you invoke it with source code, it generates GIMPLE (one of GCC's internal representations) and writes it into special ELF sections in the object file. Then when the object files get linked together, all the function bodies are read from those ELF sections and instantiated as if they had been part of the same translation unit. iconic.
• To use the link-time optimizer, -flto and your optimization options should be specified at compile time and during the final link. It's recommended that you compile all the files participating in the same link with the same options, and also specify those options at link time. 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. Which means, for example, that the inliner can inline functions in bar.o into functions in foo.o and vice-versa. 🔥
• Another (simpler) way to enable link-time optimization is:

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

• The above generates bytecode for foo.c and bar.c, merges them together into a single GIMPLE representation and optimizes them as usual to produce myprog.
• The important thing to keep in mind is that to enable link-time optimizations you need to use the GCC driver to perform the link step. GCC automatically performs link-time optimization if any of the objects involved were compiled with the -flto command-line option. You can always override that automatic decision 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 it's 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 isn't available, -fwhole-program should be used to let the compiler make these assumptions, which leads to more aggressive optimization decisions.
• When a file is compiled with -flto without -fuse-linker-plugin, the generated object file is larger than a regular object file, because it contains GIMPLE bytecodes and the usual final code (see -ffat-lto-objects). This means object files with LTO information can be linked as normal object files; if -fno-lto is passed to the linker, no interprocedural optimizations get applied. Note that when -fno-fat-lto-objects is enabled the compile stage is faster, but you cannot perform a regular, non-LTO link on them. 💀
• When producing the final binary, GCC only applies link-time optimizations to those files that contain bytecode. 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. flexible queen.
• 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 don't 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's generally ineffective to specify an optimization level option only at link time and not at compile time, for two reasons. First, compiling without optimization suppresses compiler passes that gather information needed for effective optimization at link time. Second, some early optimization passes can be performed only at compile time and not at link time. 😭
• There are some code generation flags preserved by GCC when generating bytecodes, since 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: -fPIC, -fpic, -fpie, -fcommon, -fexceptions, -fnon-call-exceptions, -fgnu-tm and all the -m target flags.
• Certain ABI-changing flags are required to match in all compilation units, and trying to override this at link time with a conflicting value gets 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. ✨

ok so quick heads-up before we dive in: diagnostic options like -Wstringop-overflow get passed through to the link stage, and their setting matches whatever the compile-step had, at function granularity. This only matters for diagnostics emitted during optimization, bestie — but here's the sneaky part: code transforms like inlining can make warnings turn on or off for regions of code that aren't consistent with the setting at compile time. 💀 So yeah. It vibes like that.

When you need to pass options to the assembler via -Wa or -Xassembler, make sure you either compile those translation units with -fno-lto, or consistently use the same assembler options on all translation units. The other option? Specify assembler options at LTO link time. Pick one and commit, no cap. ✨

To get debug info generation going, you need to supply -g at compile time. If any of the input files at link time were built with debug info generation enabled, the link will enable debug info generation too. But any elaborate 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 runs into 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, btw), a non-fatal diagnostic may be issued. The behavior is still undefined at run time. Similar diagnostics may pop up for other languages. So like... it warned you, but it did not save you. 😭

Another LTO feature? 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 the final link is done with g++ to get the C++ runtime libraries, and -lgfortran is added to get the Fortran runtime libraries. In general, when mixing languages in LTO mode, you should use the same link command options as when mixing languages in a regular (non-LTO) compilation. Consistency is the moment. 💅

If object files containing GIMPLE bytecode are stored in a library archive — say libfoo.a — you can extract and use them in an LTO link if you're using a linker with plugin support. To create static libraries suitable for LTO, use gcc-ar and gcc-ranlib instead of ar and ranlib; to show the symbols of object files with GIMPLE bytecode, use gcc-nm. Those commands require that ar, ranlib and nm have been compiled with plugin support. At link time, use the flag -fuse-linker-plugin to 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 so they become part of the aggregated GIMPLE image that gets optimized. iconic behavior honestly. ✨

If you are not using a linker with plugin support and/or you 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. 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 doesn't require any symbols to be exported, you can combine -flto and -fwhole-program to let the interprocedural optimizers use more aggressive assumptions, which may lead to improved optimization opportunities. Use of -fwhole-program isn't needed when the linker plugin is active (see -fuse-linker-plugin).

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

Link-time optimization also doesn't work well with generation of debugging information on systems other than those using a combination of ELF and DWARF. Just so you know. 🌸

If you specify the optional n, the optimization and code generation done at link time is executed in parallel using n parallel jobs, by utilizing an installed make program. The environment variable MAKE may be used to override the program used. 🧚

You can also specify -flto=jobserver to use GNU make's job server mode to determine the number of parallel jobs. This is useful when the Makefile calling GCC is already executing in parallel. You must prepend a + to the command recipe in the parent Makefile for this to work. This option likely only works if MAKE is GNU make. Even without the option value, GCC tries to automatically detect a running GNU make's job server. 👑

Use -flto=auto to use GNU make's job server, if available, or otherwise fall back to autodetection of the number of CPU threads present in your system. adaptive queen energy. 💜

---

• -flto-partition=alg — picks the partitioning algorithm used by the link-time optimizer. The value is either 1to1 (partitioning that mirrors the original source files), or balanced (partitioning into equally sized chunks whenever possible), or max (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 — sets the level of compression used for the intermediate language written to LTO object files, and it's only meaningful in conjunction with LTO mode (-flto). Valid values are 0 (no compression) to 9 (maximum compression). Values outside this range get clamped to either 0 or 9. If you don't give the option, 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). auto-slaying, as a treat. ✨

• -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 it 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 a 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 because of 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 the train run are optimized aggressively for size rather than speed. In some cases it's not practical to train all possible hot paths in the program (for example, the program may contain functions specific for a given hardware, and training may not cover all hardware configurations the 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 the train run isn't representative — but also leads to significantly bigger code. pick your fighter. 💅
• -fprofile-use
• -fprofile-use=path — enable profile feedback-directed optimizations, and the following optimizations, many of which are generally profitable only with profile feedback available:
• -fbranch-probabilities -fprofile-values -funroll-loops -fpeel-loops -ftracer -fvpt -finline-functions -fipa-cp -fipa-cp-clone -fipa-bit-cp -fpredictive-commoning -fsplit-loops -funswitch-loops -fgcse-after-reload -ftree-loop-vectorize -ftree-slp-vectorize -fvect-cost-model=dynamic -ftree-loop-distribute-patterns -fprofile-reorder-functions

Okay bestie, before you can even use this option you have to generate profiling information first. Like, you can't skip that step, it's the whole point. 💅

By default, GCC throws an error message at you if the feedback profiles don't match the source code. Rude, honestly. But you can turn that error into just a warning by using -Wno-error=coverage-mismatch. ⚠️ Heads up though: this may result in poorly optimized code. And also, by default GCC also emits a warning message if the feedback profiles don't exist at all (peep -Wmissing-profile).

If path is specified, GCC looks at the path to find the profile feedback data files. See -fprofile-dir.

---

🦋 -fauto-profile / -fauto-profile=path 🦋

Enable sampling-based feedback-directed optimizations, plus the following optimizations — many of which are only generally profitable when profile feedback is 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 just ✨omit it✨, it defaults to fbdata.afdo in the current directory.

Producing an AutoFDO profile data file requires running your program with the perf utility on a supported GNU/Linux target system. For more info, see <https://perf.wiki.kernel.org/>.

E.g.

perf record -e br_inst_retired:near_taken -b -o perf.data \
    -- your_program

Then use the create_gcov tool to convert the raw profile data to a format that can be used by GCC. You must also supply the unstripped binary for your program to this tool. See <https://github.com/google/autofdo>.

E.g.

create_gcov --binary=your_program.unstripped --profile=perf.data \
    --gcov=profile.afdo

---

🌸 floating-point vibes 🌸

The following options control compiler behavior regarding floating-point arithmetic. These options trade off between speed and correctness. All must be specifically enabled — they don't just show up on their own, you gotta ask. 🙈

-ffloat-store

Do not store floating-point variables in registers, and inhibit other options that might change whether a floating-point value is taken from a register or memory.

This option prevents undesirable excess precision on machines such as the 68000 where the floating registers (of the 68881) keep more precision than a "double" is supposed to have. Same energy for the x86 architecture. For most programs, the excess precision does only good, but a few programs rely on the precise definition of IEEE floating point. Use -ffloat-store for such programs, after modifying them to store all pertinent intermediate computations into variables. 💾

-fexcess-precision=style

This option allows further control over excess precision on machines where floating-point operations occur 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 that operations may be carried out in a wider precision than the types specified in the source if that would result in faster code, and it is unpredictable when rounding to the types specified in the source code takes place. 😭 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 the 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". There is therefore no reason for the compiler to consider the possibility that it might, and -fno-math-errno is the default. 🍎

-funsafe-math-optimizations

Allow optimizations for floating-point arithmetic that (a) assume that arguments and results are valid and (b) may violate IEEE or ANSI standards. When used at link time, it may include libraries or startup files that change the default FPU control word or other similar optimizations.

This option is not turned on by any -O option since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that do not require the guarantees of these specifications. Enables -fno-signed-zeros, -fno-trapping-math, -fassociative-math and -freciprocal-math.

The default is -fno-unsafe-math-optimizations.

-fassociative-math

Allow re-association of operands in series of floating-point operations. This violates the ISO C and C++ language standard by possibly changing computation result. NOTE: re-ordering may change the sign of zero as well as ignore NaNs and inhibit or create underflow or overflow (and thus cannot be used on code that relies on rounding behavior like "(x + 2**52) - 2**52". May also reorder floating-point comparisons and thus may not be used when ordered comparisons are required. This option requires that both -fno-signed-zeros and -fno-trapping-math be in effect. Moreover, it doesn't make much sense with -frounding-math. For Fortran the option is automatically enabled when both -fno-signed-zeros and -fno-trapping-math are in effect.

The default is -fno-associative-math.

-freciprocal-math

Allow the reciprocal of a value to be used instead of dividing by the value if this enables optimizations. For example "x / y" can be replaced with "x * (1/y)", which is useful if "(1/y)" is subject to common subexpression elimination. Note that this loses precision and increases the number of flops operating on the value. 💀

The default is -fno-reciprocal-math.

-ffinite-math-only

Allow optimizations for floating-point arithmetic that assume that arguments and results are not NaNs or +-Infs.

This option is not turned on by any -O option since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that do not require the guarantees of these specifications.

The default is -fno-finite-math-only.

-fno-signed-zeros

Allow optimizations for floating-point arithmetic that ignore the signedness of zero. IEEE arithmetic specifies the behavior of distinct +0.0 and -0.0 values, which then prohibits simplification of expressions such as x+0.0 or 0.0*x (even with -ffinite-math-only). This option implies that the sign of a zero result isn't significant.

The default is -fsigned-zeros.

-fno-trapping-math

Compile code assuming that floating-point operations cannot generate user-visible traps. These traps include division by zero, overflow, underflow, inexact result and invalid operation. This option requires that -fno-signaling-nans be in effect. Setting this option may allow faster code if one relies on "non-stop" IEEE arithmetic, for example.

This option should never be turned on by any -O option since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. 😱

The default is -ftrapping-math.

-frounding-math

Disable transformations and optimizations that assume default floating-point rounding behavior. This is round-to-zero for all floating point to integer conversions, and round-to-nearest for all other arithmetic truncations. This option should be specified for programs that change the FP rounding mode dynamically, or that may be executed with a non-default rounding mode. This option disables constant folding of floating-point expressions at compile time (which may be affected by rounding mode) and arithmetic transformations that are unsafe in the presence of sign-dependent rounding modes.

The default is -fno-rounding-math.

This option is experimental and does not currently guarantee to disable all GCC optimizations that are affected by rounding mode. Future versions of GCC may provide finer control of this setting using C99's "FENV_ACCESS" pragma. This command-line option will be used to specify the default state for "FENV_ACCESS". 🧪

Okay so continuing the option dump, bestie — these next ones are all about floating-point chaos, and then we slide straight into the optimization girlies that no -O level turns on by itself. Some of these are literally experimental and can produce broken code, so read the fine print, fr fr. 💅

• -fsignaling-nans — compile code assuming that IEEE signaling NaNs may generate user-visible traps during floating-point operations. Setting this option disables optimizations that may change the number of exceptions visible with signaling NaNs. This option implies -ftrapping-math.

This option causes the preprocessor macro __SUPPORT_SNAN__ to be defined. ✨

The default is -fno-signaling-nans.

Okay so real talk: this option is experimental and does not currently guarantee to disable all GCC optimizations that affect signaling NaN behavior. 🙈

• -fno-fp-int-builtin-inexact — don't allow the built-in functions ceil, floor, round and trunc, and their float and long double variants, to generate code that raises the "inexact" floating-point exception for noninteger arguments. ISO C99 and C11 allow these functions to raise the "inexact" exception, but ISO/IEC TS 18661-1:2014 — the C bindings to IEEE 754-2008, as integrated into ISO C2X — does not allow these functions to do so. 😤

The default is -ffp-int-builtin-inexact, allowing the exception to be raised, unless C2X or a later C standard is selected. This option does nothing unless -ftrapping-math is in effect.

Even if -fno-fp-int-builtin-inexact is used, if the functions generate a call to a library function then the "inexact" exception may be raised if the library implementation does not follow TS 18661. 💀

• -fsingle-precision-constant — treat floating-point constants as single precision instead of implicitly converting them to double-precision constants.
• -fcx-limited-range — when enabled, this option states that a range reduction step is not needed when performing complex division. Also, there is no checking whether the result of a complex multiplication or division is NaN + I*NaN, with an attempt to rescue the situation in that case. The default is -fno-cx-limited-range, but it is enabled by -ffast-math.

This option controls the default setting of the ISO C99 CX_LIMITED_RANGE pragma. Nevertheless, the option applies to all languages. 🌈

• -fcx-fortran-rules — complex multiplication and division follow Fortran rules. Range reduction is done as part of complex division, but there is no checking whether the result of a complex multiplication or division is NaN + I*NaN, with an attempt to rescue the situation in that case.

The default is -fno-cx-fortran-rules.

---

The following options control optimizations that may improve performance, but are not enabled by any -O options. And this section includes experimental options that may produce broken code. So vibe check before you use them, 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 the number of times each branch was taken. When a program compiled with -fprofile-arcs exits, it saves arc execution counts to a file called sourcename.gcda for each source file. The information in this data file is very dependent on the structure of the generated code, so you must use the same source code and the same optimization options for both compilations. 👀

With -fbranch-probabilities, GCC puts a REG_BR_PROB note on each JUMP_INSN and CALL_INSN. These can be used to improve optimization. Currently, they are only used in one place: in reorg.c, instead of guessing which path a branch is most likely to take, the REG_BR_PROB values are used to exactly determine which path is taken more often.

Enabled by -fprofile-use and -fauto-profile.

• -fprofile-values — if combined with -fprofile-arcs, it adds code so that some data about values of expressions in the program is gathered.

With -fbranch-probabilities, it reads back the data gathered from profiling values of expressions for usage in optimizations.

Enabled by -fprofile-generate, -fprofile-use, and -fauto-profile.

• -fprofile-reorder-functions — function reordering based on profile instrumentation collects first time of execution of a function and orders these functions in ascending order.

Enabled with -fprofile-use.

• -fvpt — if combined with -fprofile-arcs, this option instructs the compiler to add code to gather information about values of expressions.

With -fbranch-probabilities, it reads back the data gathered and actually performs the optimizations based on them. Currently the optimizations include specialization of division operations using the knowledge about the value of the denominator.

Enabled with -fprofile-use and -fauto-profile.

• -frename-registers — attempt to avoid false dependencies in scheduled code by making use of registers left over after register allocation. This optimization most benefits processors with lots of registers. Depending on the debug information format adopted by the target, however, it can make debugging impossible, since variables no longer stay in a "home register". 😭

Enabled by default with -funroll-loops.

• -fschedule-fusion — performs a target dependent pass over the instruction stream to schedule instructions of same type together because target machine can execute them more efficiently if they are adjacent to each other in the instruction flow.

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

• -ftracer — perform tail duplication to enlarge superblock size. This transformation simplifies the control flow of the function allowing other optimizations to do a better job.

Enabled by -fprofile-use and -fauto-profile.

• -funroll-loops — unroll loops whose number of iterations can be determined at compile time or upon entry to the loop. -funroll-loops implies -frerun-cse-after-loop, -fweb and -frename-registers. It also turns on complete loop peeling (i.e. complete removal of loops with a small constant number of iterations). This option makes code larger, and may or may not make it run faster.

Enabled by -fprofile-use and -fauto-profile.

• -funroll-all-loops — unroll all loops, even if their number of iterations is uncertain when the loop is entered. This usually makes programs run more slowly. -funroll-all-loops implies the same options as -funroll-loops.
• -fpeel-loops — peels loops for which there is enough information that they do not roll much (from profile feedback or static analysis). It also turns on complete loop peeling (i.e. complete removal of loops with small constant number of iterations).

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

• -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 — split a loop into two if it contains a condition that's always true for one side of the iteration space and false for the other.

Enabled by -fprofile-use and -fauto-profile.

• -funswitch-loops — move branches with loop invariant conditions out of the loop, with duplicates of the loop on both branches (modified according to result of the condition).

Enabled by -fprofile-use and -fauto-profile.

• -fversion-loops-for-strides — if a loop iterates over an array with a variable stride, create another version of the loop that assumes the stride is always one. For example:

for (int i = 0; i < n; ++i)
  x[i * stride] = ...;

becomes:

if (stride == 1)
  for (int i = 0; i < n; ++i)
    x[i] = ...;
else
  for (int i = 0; i < n; ++i)
    x[i * stride] = ...;

This is particularly useful for assumed-shape arrays in Fortran where (for example) it allows better vectorization assuming contiguous accesses. This flag is enabled by default at -O3. It is also enabled by -fprofile-use and -fauto-profile.

• -ffunction-sections, -fdata-sections — place each function or data item into its own section in the output file if the target supports arbitrary sections. The name of the function or the name of the data item determines the section's name in the output file. 🎀

Use these options on systems where the linker can perform optimizations to improve locality of reference in the instruction space. Most systems using the ELF object format have linkers with such optimizations. On AIX, the linker rearranges sections (CSECTs) based on the call graph. The performance impact varies.

Together with a linker garbage collection (linker --gc-sections option) these options may lead to smaller statically-linked executables (after stripping).

On ELF/DWARF systems these options do not degenerate the quality of the debug information. There could be issues with other object files/debug info formats.

Only use these options when there are significant benefits from doing so. When you specify these options, the assembler and linker create larger object and executable files and are also slower. These options affect code generation. They prevent optimizations by the compiler and assembler using relative locations inside a translation unit since the locations are unknown until link time. An example of such an optimization is relaxing calls to short call instructions.

• -fstdarg-opt — optimize the prologue of variadic argument functions with respect to usage of those arguments.
• -fsection-anchors — try to reduce the number of symbolic address calculations by using shared "anchor" symbols to address nearby objects. This transformation can help to reduce the number of GOT entries and GOT accesses on some targets.

For example, the implementation of the following function foo:

static int a, b, c;
int foo (void) { return a + b + c; }

usually calculates the addresses of all three variables, but if you compile it with -fsection-anchors, it accesses the variables from a common anchor point instead. The effect is similar to the following pseudocode (which isn't valid C):

int foo (void)
{
  register int *xr = &x;
  return xr[&a - &x] + xr[&b - &x] + xr[&c - &x];
}

Not all targets support this option. 🦋

• --param name=value — okay so GCC has a bunch of internal constants that control how much optimization happens. For example, GCC does not inline functions that contain more than a certain number of instructions, and that certain number is one of these constants. You can control some of these constants on the command line using the --param option. 💅

⚠️ The names of specific parameters, and the meaning of the values, are tied to the internals of the compiler, and are subject to change without notice in future releases. So like, don't hardcode them into your soul, bestie. 😭

In order to get the 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 probability lower than this threshold (in percent), then it is considered well predictable. 🎀
• max-rtl-if-conversion-insns — RTL if-conversion tries to remove conditional branches around a block and replace them with conditionally executed instructions. This parameter gives the maximum number of instructions in a block which should be considered for if-conversion. The compiler will also use other heuristics to decide whether if-conversion is likely to be profitable. 🔥
• max-rtl-if-conversion-predictable-cost, max-rtl-if-conversion-unpredictable-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. ⭐
• 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. 🧚
• max-delay-slot-live-search — When trying to fill delay slots, the maximum number of instructions to consider when searching for a block with valid live register information. Increasing this arbitrarily chosen value means more aggressive optimization, increasing the compilation time. This parameter should be removed when the delay slot code is rewritten to maintain the control-flow graph. 💜
• max-gcse-memory — The approximate maximum amount of memory that can be allocated in order to perform the global common subexpression elimination optimization. If more memory than specified is required, the optimization is not done. 🌈
• max-gcse-insertion-ratio — If the ratio of expression insertions to deletions is larger than this value for any expression, then RTL PRE inserts or removes the expression and thus leaves partially redundant computations in the instruction stream. 💀
• max-pending-list-length — The maximum number of pending dependencies scheduling allows before flushing the current state and starting over. Large functions with few branches or calls can create excessively large lists which needlessly consume memory and resources. 🙈
• max-modulo-backtrack-attempts — The maximum number of backtrack attempts the scheduler should make when modulo scheduling a loop. Larger values can exponentially increase compilation time. 🍓
• max-inline-insns-single — Several parameters control the tree inliner used in GCC. This number sets the maximum number of instructions (counted in GCC's internal representation) in a single function that the tree inliner considers for inlining. This only affects functions declared inline and methods implemented in a class declaration (C++). 👑
• 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). 🎀
• 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 opportunities exposed by inlining. 💅
• 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). 💖
• 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. 🌸
• 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. 🧚
• 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. 🌈
• 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. 💀
• 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. 🙈
• 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. 👑
• max-inline-insns-recursive, max-inline-insns-recursive-auto — Specifies the maximum number of instructions an out-of-line copy of a self-recursive inline function can grow into by performing recursive inlining.

--param max-inline-insns-recursive applies to functions declared inline. For functions not declared inline, recursive inlining happens only when -finline-functions (included in -O3) is enabled; --param max-inline-insns-recursive-auto applies instead. 🎀

• max-inline-recursive-depth, max-inline-recursive-depth-auto — Specifies the maximum recursion depth used for recursive inlining. ✨

okay so real quick, --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 — bestie, recursive inlining is only profitable for functions that have deep recursion on average. It can hurt functions with little recursion depth by increasing the prologue size or making the function body more complex for other optimizers. When profile feedback is available (see -fprofile-generate), the actual recursion depth can be guessed from the probability that the 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 the early inliner can make. Basically 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 a C++ inline function with comdat visibility is shared across multiple compilation units. 🦋
• profile-func-internal-id — A parameter to control whether to use function internal id in profile database lookup. If the value is 0, the compiler uses an id based on 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., 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-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 minimum threshold for probability of semi-invariant condition statement to trigger loop split. 🔀
• iv-consider-all-candidates-bound — Bound on number of candidates for induction variables, below which all candidates are considered for each use in induction variable optimizations. If there are more candidates than this, only the most relevant ones are considered to avoid quadratic time complexity. 🧮
• iv-max-considered-uses — The induction variable optimizations give up on loops that contain more induction variable uses. 🛑
• iv-always-prune-cand-set-bound — If the number of candidates in the set is smaller than this value, always try to remove unnecessary ivs from the set when adding a new one. 🧹
• 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 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 — The number of most executed permilles, ranging from 0 to 1000, of the profiled execution of the entire program to which the execution count of a basic block must be part of in order to be considered hot. The default is 990, which means that a basic block is considered hot if its execution count contributes to the upper 990 permilles, or 99.0%, of the profiled execution of the entire program. 0 means that it is never considered hot. Used in LTO mode. 🌡️
• hot-bb-frequency-fraction — The denominator n of fraction 1/n of the execution frequency of the entry block of a function that a basic block of this function needs to at least have in order to be considered hot. The default is 1000, which means that a basic block is considered hot in a function if it is executed more frequently than 1/1000 of the frequency of the entry block of the function. 0 means that it is never considered hot. ⚡
• unlikely-bb-count-fraction — The denominator n of fraction 1/n of the number of profiled runs of the entire program below which the execution count of a basic block must be in order for the basic block to be considered unlikely executed. The default is 20, which means that a basic block is considered unlikely executed if it is executed in fewer than 1/20, or 5%, of the runs of the program. 0 means that it is always considered unlikely executed. 🥶
• max-predicted-iterations — The maximum number of loop iterations we predict statically. This is useful in cases where a function contains a single loop with known bound and another loop with unknown bound. The known number of iterations is predicted correctly, while the unknown number of iterations average to roughly 10. This means that the loop without bounds appears artificially cold relative to the other one. 🔮
• builtin-expect-probability — this one controls the probability that the expression is gonna have the value you specified. it takes a percentage, so 0 ... 100, as its input. 💫
• builtin-string-cmp-inline-length — the max length of a constant string for a builtin string cmp call that's eligible for inlining. so like, if the string is longer than this, no inlining for it, bestie. ✂️
• align-threshold — pick the fraction of the maximal frequency of executions of a basic block in a function to align that basic block. ✨
• align-loop-iterations — a loop that's expected to iterate at least the selected number of iterations gets aligned. 👑
• tracer-dynamic-coverage, tracer-dynamic-coverage-feedback — this value limits superblock formation once the given percentage of executed instructions is covered. it's literally there to cap unnecessary code size expansion. 🛑

the tracer-dynamic-coverage-feedback parameter is only used when profile feedback is available, because the real profiles (as opposed to the statically estimated ones) are way less balanced, which lets the threshold be a larger value.

• tracer-max-code-growth — stop tail duplication once code growth has hit the given percentage. ngl this is a rather artificial limit, since most of the duplicates get eliminated later in cross jumping, so it may be set to much higher values than is the actual desired code growth. 🙈
• 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 a probability lower than this threshold.

similarly to tracer-dynamic-coverage, there are two parameters provided. 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) in order to make 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 please, that last bit is the important part: too big = vulnerable. no cap.

• stack-clash-protection-probe-interval — stack clash protection involves 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 a value larger than the operating system provided guard will leave code vulnerable to stack clash style attacks. 🔥
• max-cse-path-length — the max number of basic blocks on a path that CSE considers. 🦋
• max-cse-insns — the max number of instructions CSE processes before it flushes. 🚽
• ggc-min-expand — okay so GCC uses a garbage collector to manage its own memory allocation. this parameter specifies the minimum percentage by which the garbage collector's heap should be allowed to expand between collections. tuning this may improve compilation speed; it has no effect on code generation. ✨

the default is 30% + 70% * (RAM/1GB) with an upper bound of 100% when RAM >= 1GB. if getrlimit is available, the notion of "RAM" is the smallest of actual RAM and RLIMIT_DATA or RLIMIT_AS. if GCC is not able to calculate RAM on a particular platform, the lower bound of 30% is used. setting this parameter and ggc-min-heapsize to zero causes a full collection to occur at every opportunity. this is extremely slow, but can be useful for debugging. 💀

• ggc-min-heapsize — minimum size of the garbage collector's heap before it begins bothering to collect garbage. the first collection occurs after the heap expands by ggc-min-expand% beyond ggc-min-heapsize. again, tuning this may improve compilation speed, and has no effect on code generation. 🌸

the default is the smaller of RAM/8, RLIMIT_RSS, or a limit that tries to ensure that RLIMIT_DATA or RLIMIT_AS are not exceeded, but with a lower bound of 4096 (four megabytes) and an upper bound of 131072 (128 megabytes). if GCC is not able to calculate RAM on a particular platform, the lower bound is used. setting this parameter very large effectively disables garbage collection. setting this parameter and ggc-min-expand to zero causes a full collection to occur at every opportunity. 🧚

• max-reload-search-insns — the max number of instructions reload should look backward for an equivalent register. increasing values mean more aggressive optimization, making the compilation time increase with probably slightly better performance. 📈
• max-cselib-memory-locations — the max number of memory locations cselib should take into account. increasing values mean more aggressive optimization, making the compilation time increase with probably slightly better performance. 📈
• max-sched-ready-insns — the max 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 the compilation time increase with probably little benefit. 🥲
• max-sched-region-blocks — the max number of blocks in a region to be considered for interblock scheduling. 🧩
• max-pipeline-region-blocks — the max number of blocks in a region to be considered for pipelining in the selective scheduler. 🧩
• max-sched-region-insns — the max number of insns in a region to be considered for interblock scheduling. 🧩
• max-pipeline-region-insns — the max 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 max number of iterations through CFG to extend regions. a value of 0 disables region extensions. 🚫
• max-sched-insn-conflict-delay — the max 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 store and load targeting same memory locations. 🕐
• selsched-max-lookahead — the max size of the lookahead window of selective scheduling. it's a depth of search for available instructions. 🔍
• selsched-max-sched-times — the max number of times that an instruction is scheduled during selective scheduling. this is the limit on the number of iterations through which the instruction may be pipelined. 🔁
• selsched-insns-to-rename — the max 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 max size measured as number of RTLs that can be recorded in an expression in combiner for a pseudo register as last known value of that register. 📝
• max-combine-insns — the max number of instructions the RTL combiner tries to combine. 🧬
• integer-share-limit — small integer constants can use a shared data structure, reducing the compiler's memory usage and increasing its speed. this sets the max 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. 🛡️
• min-size-for-stack-sharing — the minimum size of variables taking part in stack slot sharing when not optimizing. 🫶
• max-jump-thread-duplication-stmts — max number of statements allowed in a block that needs to be duplicated when threading jumps. 🧵
• max-fields-for-field-sensitive — max number of fields in a structure treated in a field sensitive manner during pointer analysis. 🔬
• prefetch-latency — estimate on average number of instructions that are executed before prefetch finishes. the distance prefetched ahead is proportional to this constant. increasing this number may also lead to less streams being prefetched (see simultaneous-prefetches). 🚀
• simultaneous-prefetches — max number of prefetches that can run at the same time. ⚡
• l1-cache-line-size — the size of cache line in L1 data cache, in bytes. 📏
• l1-cache-size — the size of L1 data cache, in kilobytes. 💾
• l2-cache-size — the size of L2 data cache, in kilobytes. 💾
• prefetch-dynamic-strides — whether the loop array prefetch pass should issue software prefetch hints for strides that are non-constant. in some cases this may be beneficial, though the fact the stride is non-constant may make it hard to predict when there is clear benefit to issuing these hints. 🤔

set to 1 if the prefetch hints should be issued for non-constant strides. set to 0 if prefetch hints should be issued only for strides that are known to be constant and below prefetch-minimum-stride.

• prefetch-minimum-stride — minimum constant stride, in bytes, to start using prefetch hints for. if the stride is less than this threshold, prefetch hints will not be issued. ✋

---

okay so this setting? lowkey iconic for any processor that's got hardware prefetchers, because when hardware prefetchers and software prefetchers are both doing their thing they can literally beef with each other. 💀 if your hardware prefetchers have a maximum stride they can handle, drop that value here so your software prefetchers can actually shine.

A value of -1 means we don't have a threshold, so prefetch hints can be issued for any constant stride. no cap.

And heads up bestie: this setting is only useful for strides that are known and constant. if your stride is out here being unpredictable, this one's not for you. 🦋

• loop-interchange-max-num-stmts — the max number of stmts in a loop that can be interchanged.
• loop-interchange-stride-ratio — the minimum ratio between the stride of two loops for interchange to actually be profitable. it's giving "is this even worth it" energy. 💅
• min-insn-to-prefetch-ratio — the minimum ratio between the number of instructions and the number of prefetches for prefetching to get enabled in a loop.
• prefetch-min-insn-to-mem-ratio — the minimum ratio between the number of instructions and the number of memory references for prefetching to get enabled in a loop.
• use-canonical-types — should the compiler use the "canonical" type system? bestie, it should always be 1, because 1 uses a way more efficient internal mechanism for comparing types in C++ and Objective-C++. BUT if bugs in the canonical type system are making your compilation fail, set this to 0 to disable canonical types. 🫶
• switch-conversion-max-branch-ratio — switch initialization conversion refuses to create arrays that are bigger than switch-conversion-max-branch-ratio times the number of branches in the switch.
• max-partial-antic-length — the max length of the partial antic set computed during the tree partial redundancy elimination optimization (-ftree-pre) when optimizing at -O3 and above. for some sorts of source code the enhanced partial redundancy elimination can straight up run away and eat all the memory available on the host machine 😭. this parameter caps the length of the sets that get computed, which stops that runaway behavior. setting it to 0 allows an unlimited set length (good luck with that).
• rpo-vn-max-loop-depth — max loop depth that gets value-numbered optimistically. when the limit hits, the innermost rpo-vn-max-loop-depth loops and the outermost loop in the loop nest get value-numbered optimistically, and the remaining ones don't.
• sccvn-max-alias-queries-per-access — max number of alias-oracle queries we perform when looking for redundancies for loads and stores. if this limit gets hit, the search is aborted and the load or store is not considered redundant. the number of queries is algorithmically limited to the number of stores on all paths from the load to the function entry. 🔥
• ira-max-loops-num — IRA uses regional register allocation by default. if a function has more loops than the number given here, only at most that given number of the most frequently-executed loops form regions for regional register allocation.
• ira-max-conflict-table-size — even though IRA uses a sophisticated algorithm to compress the conflict table, that table can still require excessive amounts of memory for huge functions. if the conflict table for a function could be more than the size in MB given by this parameter, the register allocator instead uses a faster, simpler, and lower-quality algorithm that doesn't require building a pseudo-register conflict table.
• ira-loop-reserved-regs — IRA can be used to evaluate more accurate register pressure in loops, for decisions about moving loop invariants (see -O3). the number of available registers reserved for some other purposes is given by this parameter. the default is the best found from numerous experiments. ✨
• lra-inheritance-ebb-probability-cutoff — LRA tries to reuse values reloaded in registers in subsequent insns. this optimization is called inheritance. EBB is used as the region to do this optimization. this parameter defines a minimal fall-through edge probability in percentage used to add BB to 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 the amount of needed compile-time memory, when loops are very large. loops with more basic blocks than this parameter won't have loop invariant motion optimization performed on them. 🫶
• 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 are considered for data dependence analysis. these large loops aren't handled by the optimizations that use loop data dependencies.
• max-vartrack-size — sets a max number of hash table slots to use during variable tracking dataflow analysis of any function. if this limit is exceeded with variable tracking at assignments enabled, analysis for that function is retried without it, after removing all debug insns from the function. if the limit is exceeded even without debug insns, var tracking analysis is completely disabled for that function. setting the parameter to zero makes it unlimited. 😭
• max-vartrack-expr-depth — sets a max number of recursion levels when attempting to map variable names or debug temporaries to value expressions. this trades compilation time for more complete debug information. if you set it too low, value expressions that are available and could be represented in debug information may end up not being used; setting it higher may enable the compiler to find more complex debug expressions, but compile time and memory use may grow. it's giving tradeoff energy. 💅
• max-debug-marker-count — sets a threshold on the number of debug markers (like begin stmt markers) to avoid complexity explosion at inlining or expanding to RTL. if a function has more such gimple stmts than the set limit, those stmts get dropped from the inlined copy of a 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, but debug insns may get (non-overlapping) uids above it if the reserved range gets exhausted.
• ipa-sra-ptr-growth-factor — IPA-SRA replaces a pointer to an aggregate with one or more new parameters only when their cumulative size is less than or equal to ipa-sra-ptr-growth-factor times the size of the original pointer parameter.
• ipa-sra-max-replacements — max pieces of an aggregate that IPA-SRA tracks. as a consequence, it's also the max 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) aim to replace scalar parts of aggregates with uses of independent scalar variables. these parameters control the max size, in storage units, of aggregate that's considered for replacement when compiling for speed (sra-max-scalarization-size-Ospeed) or size (sra-max-scalarization-size-Osize) respectively. 🌸
• sra-max-propagations — the max number of artificial accesses that Scalar Replacement of Aggregates (SRA) will track, per one local variable, in order to 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 are saved with the logging functions as opposed to save/restore code sequence pairs. this one only applies when you're using -fgnu-tm.
• graphite-max-nb-scop-params — to avoid exponential effects in the Graphite loop transforms, the number of parameters in a Static Control Part (SCoP) is bounded. a value of zero can be used to lift the bound. a variable whose value is unknown at compilation time and defined outside a SCoP is a parameter of the SCoP. 🦋
• loop-block-tile-size — loop blocking or strip mining transforms, enabled with -floop-block or -floop-strip-mine, strip mine each loop in the loop nest by a given number of iterations. the strip length can be changed using the loop-block-tile-size parameter.
• ipa-cp-value-list-size — IPA-CP attempts to track all possible values and types passed to a function's parameter in order to propagate them and perform devirtualization. ipa-cp-value-list-size is the max number of values and types it stores per one formal parameter of a function.
• ipa-cp-eval-threshold — IPA-CP calculates its own score of cloning profitability heuristics and performs those cloning opportunities with scores that exceed ipa-cp-eval-threshold.
• ipa-cp-max-recursive-depth — max depth of recursive cloning for a self-recursive function.
• ipa-cp-min-recursive-probability — recursive cloning only happens when the probability of the call being executed exceeds this parameter.
• ipa-cp-recursion-penalty — percentage penalty recursive functions receive when they're evaluated for cloning.
• ipa-cp-single-call-penalty — percentage penalty functions containing a single call to another function receive when they're evaluated for cloning.
• ipa-max-agg-items — IPA-CP is also capable of propagating a number of scalar values passed in an aggregate. ipa-max-agg-items controls the max number of such values per one parameter. 💖

💖 ipa-cp-loop-hint-bonus

okay so picture this: IPA-CP is looking at a cloning candidate and realizes that cloning it would make the number of iterations of a loop known — like, actually countable. When that happens, it adds a bonus of ipa-cp-loop-hint-bonus to that candidate's profitability score. ✨ It's basically a lil tip jar for being helpful, and honestly? deserved.

ipa-max-aa-steps

While IPA-CP is deep in its analysis of function bodies, it uses alias analysis to keep track of the values that function parameters point to. But bestie, it is not about to spend forever analyzing some giant function — so once it has examined ipa-max-aa-steps statements that modify memory, it gives up and considers all memory clobbered. 😭 Walking away is a form of self-care.

ipa-max-switch-predicate-bounds

This one is the maximal number of boundary endpoints of the case ranges of a switch statement. If a switch exceeds this limit, IPA-CP will not construct the cloning cost predicate for the default case of that switch — and that predicate is literally the thing it uses to estimate cloning benefit. Sorry default case, no math for you. 🙈

ipa-max-param-expr-ops

IPA-CP will analyze a conditional statement that references some function parameter, so it can estimate the benefit of cloning upon 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 a complicated one and just... is not handled by IPA analysis. Too much going on, it's giving chaos. 💀

lto-partitions

Specifies the desired number of partitions produced during WHOPR compilation. Pro tip: the number of partitions should exceed the number of CPUs used for compilation. 👑

lto-min-partition

Size of the minimal partition for WHOPR, measured in estimated instructions. This exists so we don't waste effort splitting very small programs into a million tiny partitions. Preventing expenses, periodt. 🎀

lto-max-partition

Size of the max partition for WHOPR, also in estimated instructions. It gives an upper bound for the individual size of a partition. Meant to be used only with balanced partitioning, so don't go rogue with it. 🦋

lto-max-streaming-parallelism

The maximal number of parallel processes used for LTO streaming. That's it, that's the whole vibe. 🌈

cxx-max-namespaces-for-diagnostic-help

The maximum number of namespaces to consult for suggestions when C++ name lookup fails for an identifier. Yes bestie, the compiler is trying to help you find your typo, and this is how hard it's willing to look. 🔍

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. Larger numbers = more aggressive statement sinking, no cap. A small positive adjustment gets applied for statements with memory operands, because those are even more profitable to sink. 💅

max-stores-to-sink

The maximum number of conditional store pairs that can be sunk. Heads up: set this to 0 if either vectorization (-ftree-vectorize) or if-conversion (-ftree-loop-if-convert) is disabled. 🚨

case-values-threshold

The smallest number of different values for which it is best to use a jump-table instead of a tree of conditional branches. If the value is 0, use the default for the machine. Let the machine decide, queen. 👑

jump-table-max-growth-ratio-for-size

The maximum code size growth ratio when expanding into a jump table, in percent. This parameter is used when optimizing for size. 📏

jump-table-max-growth-ratio-for-speed

The maximum code size growth ratio when expanding into a jump table, in percent. This parameter is used when optimizing for speed. 🏎️ Same number, different agenda.

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 that are used by default. So like, you're the boss now. 💪

sched-pressure-algorithm

Choose between the two available implementations of -fsched-pressure:

• Algorithm 1 is the original implementation, and it is the more likely one to prevent instructions from being reordered.
• Algorithm 2 was designed as a compromise between the relatively conservative approach taken by algorithm 1 and the rather aggressive approach taken by the default scheduler. It relies more heavily on having a regular register file and accurate register pressure classes.

See haifa-sched.c in the GCC sources for more details. The default choice depends on the target. 🎯

max-slsr-cand-scan

Sets the maximum number of existing candidates that are considered when seeking a basis for a new straight-line strength reduction candidate. Shopping around, but only so much. 🛍️

asan-globals

Enables buffer overflow detection for global objects. This kind of protection is enabled by default if you're using the -fsanitize=address option. To disable global objects protection, use --param asan-globals=0. 🌍

asan-stack

Enables buffer overflow detection for stack objects. This kind of protection is enabled by default when using -fsanitize=address. To disable stack protection, use the --param asan-stack=0 option. 📚

asan-instrument-reads

Enables buffer overflow detection for memory reads. This kind of protection is enabled by default when using -fsanitize=address. To disable memory reads protection, use --param asan-instrument-reads=0. 👀

asan-instrument-writes

Enables buffer overflow detection for memory writes. This kind of protection is enabled by default when using -fsanitize=address. To disable memory writes protection, use the --param asan-instrument-writes=0 option. ✍️

asan-memintrin

Enables detection for built-in functions. This kind of protection is enabled by default when using -fsanitize=address. To disable built-in functions protection, use --param asan-memintrin=0. 🧰

asan-use-after-return

Enables detection of use-after-return. This kind of protection is enabled by default when using the -fsanitize=address option. To disable it, use --param asan-use-after-return=0. 👻

Note: by default the check is disabled at run time. To enable it, add "detect_stack_use_after_return=1" to the environment variable ASAN_OPTIONS.

asan-instrumentation-with-call-threshold

If the number of memory accesses in 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. 📞

use-after-scope-direct-emission-threshold

If the size of a local variable in bytes is smaller than or equal to this number, directly poison (or unpoison) shadow memory instead of using run-time callbacks. ✨ Small enough to just do it yourself.

max-fsm-thread-path-insns

The maximum number of instructions to copy when duplicating blocks on a finite state automaton jump thread path. ✂️

max-fsm-thread-length

The maximum number of basic blocks on a finite state automaton jump thread path. 🧱

max-fsm-thread-paths

The maximum number of new jump thread paths to create for a finite state automaton. 🛤️

parloops-chunk-size

Chunk size of the omp schedule for loops parallelized by parloops. 🍰

parloops-schedule

Schedule type of the omp schedule for loops parallelized by parloops (static, dynamic, guided, auto, runtime). Pick your fighter. 💅

parloops-min-per-thread

The minimum number of iterations per thread of an innermost parallelized loop for which the parallelized variant is preferred over the single threaded one. 📝 Note that for a parallelized loop nest, the minimum number of iterations of the outermost loop per thread is two.

max-ssa-name-query-depth

Maximum depth of recursion when querying properties of SSA names in things like fold routines. One level of recursion corresponds to following a use-def chain. 🔗 Down the rabbit hole we go.

hsa-gen-debug-stores

Enables emission of special debug stores within HSA kernels, which are then read and reported by the libgomp plugin. Generation of these stores is disabled by default, so use --param hsa-gen-debug-stores=1 to enable it. 🐛

max-speculative-devirt-maydefs

The maximum number of may-defs we analyze when looking for a must-def that specifies the dynamic type of an object invoking a virtual call we might be able to devirtualize speculatively. Say that five times fast, bestie. 🌀

max-vrp-switch-assertions

The maximum number of assertions to add along the default edge of a switch statement during VRP. 📢

unroll-jam-min-percent

The minimum percentage of memory references that must be optimized away for the unroll-and-jam transformation to be considered profitable. 🍓 Gotta be worth it.

unroll-jam-max-unroll

The maximum number of times the outer loop should be unrolled by the unroll-and-jam transformation. 🔁

max-rtl-if-conversion-unpredictable-cost

Maximum permissible cost for the sequence that would be generated by the RTL if-conversion pass for a branch that is considered unpredictable. 💸

max-variable-expansions-in-unroller

If -fvariable-expansion-in-unroller is used, 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 a split region (in percent relative to the 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

Enables loop epilogue vectorization using a smaller vector size. 🦋

slp-max-insns-in-bb

Maximum number of instructions in a basic block to be considered for SLP vectorization. 🧮

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 that the swing modulo scheduler is willing to consider. if your loop's average count doesn't clear this bar, it's simply not in the group chat 💅
• sms-dfa-history — how many cycles the swing modulo scheduler looks back on when it's checking conflicts using DFA.
• max-inline-insns-recursive-auto — the max 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 is on.
• sms-max-ii-factor — a factor for tuning the upper bound that the swing modulo scheduler uses when scheduling a loop.
• lra-max-considered-reload-pseudos — the max number of reload pseudos considered while 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 — a bound on the cost of an expression used to compute the number of iterations.
• max-isl-operations — maximum number of isl operations; 0 means unlimited. no cap, fr fr 🧢
• 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. this one's 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 that 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. it's the number of lookahead cycles the model looks into; at ' ' only enable instruction sorting heuristic.
• loop-versioning-max-inner-insns — the maximum number of instructions an inner loop can have before the loop versioning pass decides it's just too big to copy. 🙈
• loop-versioning-max-outer-insns — the maximum number of instructions an outer loop can have before the loop versioning pass decides it's too big to copy — discounting any instructions in inner loops that directly benefit from versioning.
• ssa-name-def-chain-limit — the maximum number of SSA_NAME assignments to follow when determining a property of a variable, like its value. this caps the number of iterations or recursive calls GCC performs when optimizing certain statements, or when determining their validity before it hands out 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 it terminates analysis of that point.
• 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-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.
• gimple-fe-computed-hot-bb-threshold — the number of executions of a basic block that counts as hot. this parameter is used only in GIMPLE FE.
• analyzer-bb-explosion-factor — the maximum number of 'after supernode' exploded nodes within the analyzer per supernode, before terminating analysis.

The following choices of name are available on AArch64 targets:

• aarch64-sve-compare-costs — when vectorizing for SVE, consider using "unpacked" vectors for smaller elements and let the cost model pick the cheapest approach. also use 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.

• aarch64-float-recp-precision — the number of Newton iterations for calculating the reciprocal for float type. the precision of division is proportional to this param when division approximation is enabled. the default value is 1.
• aarch64-double-recp-precision — the number of Newton iterations for calculating the reciprocal for double type. the precision of division is proportional to this param when division approximation is enabled. the default value is 2.

---

🎀 Program Instrumentation Options 🎀

okay so this is the glow-up section. GCC supports a bunch of command-line options that control adding run-time instrumentation to the code it normally generates. like, one purpose of instrumentation is collecting profiling statistics so you can find program hot spots, do code coverage analysis, or run 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 — plus deliberately hostile attacks such as stack smashing or C++ vtable hijacking. and there's also a general hook that can be used to implement other forms of tracing or function-level instrumentation for debug or program analysis purposes. 🦋

---

• -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. not every function needs main character energy, and that's okay 💅

• -fprofile-arcs — add code so that program flow arcs get instrumented. during execution the program records how many times each branch and call is executed, and how many times it is taken or returns. on targets that support constructors with priority support, profiling properly handles constructors, destructors, and C++ constructors (and destructors) of classes which are used as a type of a global variable.

when the compiled program exits it saves this data to a file called auxname.gcda for each source file. 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. it's 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. iconic behavior, honestly ✨

• -ftest-coverage — bestie this one makes 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. Go scroll back to the -fprofile-arcs option above for the full description of auxname plus the instructions on how to generate test coverage data — it's all there. And lowkey important: coverage data matches the source files way more closely if you do not optimize 💅.
• -fprofile-abs-path — automatically converts relative source file names into absolute path names inside the .gcno files 🫶. Why do we care? Because it lets gcov find the correct sources in projects where compilations happen with different working directories. it's giving "I know exactly where I came from" energy.
• -fprofile-dir=path — sets 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 work. By default, GCC uses the current directory as path, so the profile data file appears in the same directory as the object file. And to prevent the file name from 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 the 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 — turns on the options usually used for instrumenting an application to produce a profile that's useful for later recompilation with profile feedback based optimization 🔥. Heads up: you must use -fprofile-generate both when compiling and when linking your program, no skipping steps bestie.

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-note=path — if path is specified, GCC saves the .gcno file into the path location ✨. But watch out: if you combine this option with multiple source files, the .gcno file will be overwritten 😭.
• -fprofile-prefix-path=path — this one can be used in combination with profile-generate=profile_dir and profile-use=profile_dir to inform GCC where the base directory of the built source tree is 🌸. By default, profile_dir will contain files with mangled absolute paths of all object files in the built project. That's not desirable when the directory used to build the instrumented binary differs from the directory used to build the binary optimized with profile feedback, because then the profile data will not be found during the optimized build. In such setups, -fprofile-prefix-path=path with path pointing to the base directory of the build can be used to strip the irrelevant part of the path and keep all file names relative to the main build directory. chef's kiss path hygiene 👑
• -fprofile-update=method — alters the update method for an application instrumented for profile feedback based optimization. The method argument should be one of single, atomic or prefer-atomic. The first one is useful for single-threaded applications, while the second one prevents profile corruption by emitting thread-safe code 🛡️.

Warning: when an application does not properly join all threads (or creates a detached thread), a profile file can still be corrupted 💀.

Using prefer-atomic would be transformed either to atomic, when supported by a target, or to single otherwise. And the GCC driver automatically selects prefer-atomic when -pthread is present in the command line.

• -fprofile-filter-files=regex — instruments only functions from files where the names match any regular expression (separated by a semi-colon) 🎀.

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 — instruments only functions from files where the names do not match all the regular expressions (separated by a semi-colon) 🦋.

For example, -fprofile-exclude-files=/usr/* will prevent instrumentation of all files that are located in the /usr/ folder.

• -fprofile-reproducible — 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-reproducibility=serial the profile gathered by -fprofile-generate is reproducible provided the trained program behaves the same at each invocation of the train run, it is not multi-threaded, and profile data streaming is always done in the same order. Note that profile streaming happens at the end of the program run but also before fork function is invoked.

Note that it's quite common that execution counts of some part of programs depends, for example, on the length of temporary file names or memory space randomization (which may affect hash-table collision rate) 🙈. Such non-reproducible parts of programs may be annotated by the no_instrument_function function attribute. gcov-dump with -l can be used to dump gathered data and verify that they are indeed reproducible.

With -fprofile-reproducibility=parallel-runs the collected profile stays reproducible regardless of the order of streaming of the data into gcda files 🌈. This setting makes it possible to run multiple instances of an instrumented program in parallel (such as with make -j). This reduces the quality of the gathered data, in particular of indirect call profiling. trade-offs, we love to see them ✨.

• -fsanitize=address — enables AddressSanitizer, a fast memory error detector 💥. Memory access instructions are instrumented to detect out-of-bounds and use-after-free bugs. The option enables -fsanitize-address-use-after-scope. See <https://github.com/google/sanitizers/wiki/AddressSanitizer> for more details. The run-time behavior can be influenced using the ASAN_OPTIONS environment variable. When set to help=1, the available options are shown at startup of the instrumented program. See <https://github.com/google/sanitizers/wiki/AddressSanitizerFlags#run-time-flags> for a list of supported options. The option cannot be combined with -fsanitize=thread 🚫.
• -fsanitize=kernel-address — enables AddressSanitizer for the Linux kernel 🐧. See <https://github.com/google/kasan/wiki> for more details.
• -fsanitize=pointer-compare — instruments the comparison operation (<, <=, >, >=) with pointer operands. The option must be combined with either -fsanitize=kernel-address or -fsanitize=address. The option cannot be combined with -fsanitize=thread. Note: by default the check is disabled at run time. To enable it, add detect_invalid_pointer_pairs=2 to the environment variable ASAN_OPTIONS. Using detect_invalid_pointer_pairs=1 detects invalid operation only when both pointers are non-null 🔍.
• -fsanitize=pointer-subtract — instruments subtraction with pointer operands. The option must be combined with either -fsanitize=kernel-address or -fsanitize=address. The option cannot be combined with -fsanitize=thread. Note: by default the check is disabled at run time. To enable it, add detect_invalid_pointer_pairs=2 to the environment variable ASAN_OPTIONS. Using detect_invalid_pointer_pairs=1 detects invalid operation only when both pointers are non-null 🔍.
• -fsanitize=thread — enables ThreadSanitizer, a fast data race detector 🏁. Memory access instructions are instrumented to detect data race bugs. See <https://github.com/google/sanitizers/wiki#threadsanitizer> for more details. The run-time behavior can be influenced using the TSAN_OPTIONS environment variable; see <https://github.com/google/sanitizers/wiki/ThreadSanitizerFlags> for a list of supported options. The 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 using the LSAN_OPTIONS environment variable. The option cannot be combined with -fsanitize=thread 🚫.
• -fsanitize=undefined — enables UndefinedBehaviorSanitizer, a fast undefined behavior detector 😱. Various computations are instrumented to detect undefined behavior at runtime. Current suboptions are:
  • -fsanitize=shift — this option enables checking that the result of a shift operation is not undefined. Note that what exactly is considered undefined differs slightly between C and C++, as well as between ISO C90 and C99, etc. This option has two suboptions, -fsanitize=shift-base and -fsanitize=shift-exponent.
  • -fsanitize=shift-exponent — this option enables checking that the second argument of a shift operation is not negative and is smaller than the precision of the promoted first argument.
  • -fsanitize=shift-base — if the second argument of a shift operation is within range, check that the result of a shift operation is not undefined. Note that what exactly is considered undefined differs slightly between C and C++, as well as between ISO C90 and C99, etc.
  • -fsanitize=integer-divide-by-zero — 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 — this option instructs the compiler to check that the size of a variable length array is positive 🌸.
• -fsanitize=null — bestie this one is all about pointer checking 💅 when your app is built with it on, it'll throw an error message the moment it tries to dereference a NULL pointer, or if a reference (yes, even an rvalue reference) gets bound to a NULL pointer, or if a method gets invoked on an object that a NULL pointer is pointing at. caught in 4k fr.
• -fsanitize=return — return statement checking, slay ✨ programs built with this on will throw an error message when the end of a non-void function gets reached without actually returning a value. heads up: this one works in C++ only.
• -fsanitize=signed-integer-overflow — signed integer overflow checking, no cap. we check that the result of "+", "*", and both unary and binary "-" does not overflow in the signed arithmetics. note tho, integer promotion rules must be taken into account. that is, the following is not an overflow:

signed char a = SCHAR_MAX;
a++;

• -fsanitize=bounds — instrumentation of array bounds, periodt. various out of bounds accesses get detected. that said, flexible array members, flexible array member-like arrays, and initializers of variables with static storage are not instrumented 🙈
• -fsanitize=bounds-strict — the strict version of array bounds instrumentation 🔥 most out of bounds accesses get detected, including flexible array members and flexible array member-like arrays. initializers of variables with static storage are still not instrumented.
• -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 💫
• -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. okay so unlike other similar options, -fsanitize=float-divide-by-zero is not enabled by -fsanitize=undefined, bc floating-point division by zero can legit be a way of obtaining infinities and NaNs. it's giving valid actually 🌈
• -fsanitize=float-cast-overflow — floating-point type to integer conversion checking. we check that the result of the conversion doesn't overflow. and like the one above, -fsanitize=float-cast-overflow is not enabled by -fsanitize=undefined. also important: 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.
• -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 👀
• -fsanitize=bool — instrumentation of loads from bool. if a value other than 0/1 gets loaded, a run-time error is issued 💀
• -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, to verify the referenced object has the correct dynamic type. vibe check on ur dynamic types fr 💅
• -fsanitize=pointer-overflow — instrumentation of pointer arithmetics. if the pointer arithmetics overflows, a run-time error is issued.
• -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 😭

while -ftrapv causes traps for signed overflows to be emitted, -fsanitize=undefined gives a diagnostic message instead. currently this works only for the C family of languages 🫶

---

💖 -fno-sanitize=all 💖

this option disables all previously enabled sanitizers. lowkey important: -fsanitize=all is not allowed, as some sanitizers cannot be used together. nope, not a thing bestie 💀

✨ -fasan-shadow-offset=number ✨

this option forces GCC to use a custom shadow offset in AddressSanitizer checks. it's useful for experimenting with different shadow memory layouts in Kernel AddressSanitizer 🧚

🌸 -fsanitize-sections=s1,s2,... 🌸

sanitize global variables in selected user-defined sections. si may contain wildcards.

💅 -fsanitize-recover[=opts] 💅

-fsanitize-recover= controls error recovery mode for 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. this means multiple runtime errors can be reported in a single program run, and the exit code of the program may indicate success even when errors have been reported 👑

the -fno-sanitize-recover= option can be used to alter this behavior: only the first detected error is reported and the program then exits with a non-zero exit code. so like, one and done.

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 is 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 🦋

the -fsanitize-undefined-trap-on-error option instructs the compiler to report undefined behavior using "__builtin_trap" rather than a "libubsan" library routine. the advantage of this is that the "libubsan" library is not needed and is not linked in, so this is usable even in freestanding environments 🌈 iconic tbh.

⭐ -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. that's a whole lotta trace functions bestie but we love her 💖

💜 -fcf-protection=[full|branch|return|none] 💜

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 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).

🌸 -fstack-protector 🌸

emit extra code to check for buffer overflows, such as stack smashing attacks. this is done by adding a guard variable to functions with vulnerable objects. this includes functions that call "alloca", and functions with buffers larger than or equal to 8 bytes. the guards are initialized when a function is entered and then checked when the function exits. if a guard check fails, an error message is printed and the program exits 💀 only variables that are actually allocated on the stack are considered — optimized away variables or variables allocated in registers don't count. not them getting left out bestie 😭

👑 -fstack-protector-all 👑

like -fstack-protector except that all functions are protected. no one gets left behind 💅✨

• -fstack-protector-strong — okay so this is like -fstack-protector but it protects even more functions: specifically the ones that have local array definitions, or that reference local frame addresses. 🛡️ Tiny detail though — only variables that are actually allocated on the stack count. Variables the optimizer yeeted away, or ones living in registers? They don't count, bestie, they're not invited. 💅
• -fstack-protector-explicit — again like -fstack-protector, but this one is picky: it only protects functions that carry the "stack_protect" attribute. Exclusive guest list energy. 🎀
• -fstack-check — generates code that verifies you're not stepping past the boundary of the stack. 🚧 You should pass this flag if you're running in a multi-threaded environment — but in a single-threaded one you only rarely need it, because on nearly all systems stack overflow gets detected automatically when there's just one stack. Lucky you.

Heads up: this switch does not actually perform the checking itself. The operating system or the language runtime has to do that. What this switch does is generate the code so that they can see the stack being extended. 👀

You can also pass a string parameter: no means no checking, generic forces the use of old-style checking, and specific uses 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 it comes with these drawbacks, and we're listing them:

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 the compiler issues a warning.
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. But it is not generally sufficient to protect against stack-clash attacks. For those you want -fstack-clash-protection. 🫶

• -fstack-clash-protection — generates code that prevents stack clash style attacks. 🔥 When this is enabled, the compiler only allocates one page of stack space at a time, and each page gets accessed immediately after allocation. So it stops allocations from jumping over any stack guard page the operating system provides. Iconic, honestly.

Most targets don't fully support stack clash protection. But on those targets, -fstack-clash-protection will still protect dynamic stack allocations. It may also give you limited protection for static stack allocations if the target supports -fstack-check=specific.

• -fstack-limit-register=reg
• -fstack-limit-symbol=sym
• -fno-stack-limit — these generate code ensuring the stack doesn't grow past a certain value: either the value of a register or the address of a symbol. If a bigger stack is required, a signal gets raised at run time. ⚠️ For most targets the signal is raised before the stack overruns the boundary, so you can 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 can't allocate any more memory at all. This is most useful when running threaded programs, because you no longer have to calculate a good stack size for each thread. As of now this is only implemented for the x86 targets running GNU/Linux. 🐧

When code compiled with -fsplit-stack calls code compiled without it, there may not be much stack space available for that latter code to run. If compiling all the code — library code included — with -fsplit-stack isn't an option, then the linker can fix up these calls so that the code compiled without -fsplit-stack always has a large stack. Support for this is implemented in the gold linker in GNU binutils release 2.21 and later. ✨

• -fvtable-verify=[std|preinit|none] — this one is only available when compiling C++ code. 👑 It turns on (or off, if you use -fvtable-verify=none) the security feature that verifies at run time, for every virtual call, that the vtable pointer the call goes through is valid for the type of the object, and hasn't been corrupted or overwritten. If an invalid vtable pointer gets detected at run time, an error is reported and execution of the program is immediately halted. No mercy.

This option causes run-time data structures to be built at program startup, and those get used for verifying 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 shows up multiple times on the command line with different values, none takes highest priority over both std and preinit; and preinit takes priority over std. 📋

• -fvtv-debug — when used together with -fvtable-verify=std or -fvtable-verify=preinit, it causes debug versions of the runtime functions for the vtable verification feature to be called. This flag also makes the compiler log info 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, make sure you delete any existing one first. 🗑️

• -fvtv-counts — this is a debugging flag. 🐛 When used 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 info for each compilation unit. The compiler writes this 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 that info to vtv_class_set_sizes.log in the same directory. 📊

Note: this feature appends data to the log files. To get fresh log files, make sure you delete any existing ones. 🗑️

• -finstrument-functions — generates instrumentation calls for entry and exit to functions. 🎯 Just after function entry and just before function exit, these profiling functions get called with the address of the current function and its call site. (On some platforms, "__builtin_return_address" doesn't 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 inside other functions. The profiling calls indicate where, conceptually, the inline function is entered and exited. This means that addressable versions of such functions have to be available. If all your uses of a function get 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. (That's 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 can be given the attribute "no_instrument_function", and then this instrumentation doesn't happen for it. You could use this for, say, the profiling functions listed above, high-priority interrupt routines, and any functions from which the profiling functions can't safely be called — maybe signal handlers, if the profiling routines generate output or allocate memory. 🚫

• -finstrument-functions-exclude-file-list=file,file,... — sets the list of functions excluded from instrumentation (peep the description of -finstrument-functions). If the file containing a function definition matches one of file, then that function is not instrumented. The match is done on substrings: if the file parameter is a substring of the file name, it counts as a match. 🔎

For example:

-finstrument-functions-exclude-file-list=/bits/stl,include/sys

excludes any inline function defined in files whose pathnames contain /bits/stl or include/sys.

If, for some reason, you want to include letter , in one of sym, write ,. For example, -finstrument-functions-exclude-file-list=',,tmp' (note the single quote surrounding the option). 🧚

• -finstrument-functions-exclude-function-list=sym,sym,... — okay so this is basically the sibling of -finstrument-functions-exclude-file-list, except instead of files it's a whole list of function names you're telling GCC to please not instrument 💅. The name that gets matched is the user-visible one, like vector<int> blah(const vector<int> &), not the internal mangled one (looking at you, _Z4blahRSt6vectorIiSaIiEE 🙈). And the matching? It's substring matching, bestie — if the sym param shows up anywhere inside the function name, that's a match, no cap. Oh and for C99 and C++ extended identifiers, you gotta hand over the function name in UTF-8, not with universal character names. Periodt.
• -fpatchable-function-entry=N[,M] — this one drops N NOPs right at the start of every function, with the function entry point sitting before the Mth NOP. If you leave M out, it defaults to 0, so the function entry points to the address right at the first NOP ✨. Those NOP instructions are reserving extra space for you — space you can later patch with whatever instrumentation you want at run time, provided the code segment is writable. The amount of space is controlled indirectly through the number of NOPs; the actual NOP instruction used is whatever the internal GCC back-end interface gen_nop emits. Heads up: this behavior is target-specific and can also depend on the architecture variant and/or other compilation options 👑

For run-time identification, the starting addresses of these areas — which correspond to their respective function entries minus M — get collected in the __patchable_function_entries section of the resulting binary. Iconic.

Ngl, note this: the value of __attribute__ ((patchable_function_entry (N,M))) takes precedence over the command-line option -fpatchable-function-entry=N,M. So you can use it to grow the area size or nuke it entirely on a single function. And if N=0, no pad location is recorded. If N=0, no pad location is recorded. (Say it twice, it matters.)

The NOP instructions get inserted at — and maybe before, depending on M — the function entry address, even before the prologue. Yeah, before. Even then. 😭

---

✨ Options Controlling the Preprocessor ✨

These are the options that run the show for the C preprocessor, which does its little pass over every C source file before the real compilation even starts. Main character energy, honestly.

If you use the -E option, nothing happens except preprocessing. That's it, that's the whole vibe. And some of these options only make sense alongside -E, because they make the preprocessor output unsuitable for actual compilation. So like, don't do that unless you meant to 🫶.

On top of everything listed here, there are a bunch of options controlling search paths for include files, documented over in Directory Options. And options controlling preprocessor diagnostics? Those live in Warning Options.

• -D name — predefine name as a macro, with definition 1. Simple as that.
• -D name=definition — the contents of definition get tokenized and processed as if they showed up 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 the shell's quoting syntax to protect characters like spaces that actually mean something in shell syntax.

Wanna define a function-like macro on the command line? Write its argument list with surrounding parentheses before the equals sign (if there is one). Parentheses are meaningful to most shells, so quote the option. With sh and csh, this works:

-D'name(args...)=definition'

And listen — -D and -U options are processed in the order they're given on the command line. All -imacros file and -include file options are processed after all -D and -U options. Order matters, bestie 🎀.

• -U name — cancel any previous definition of name, whether it was built in or handed to you by a -D option. Bye boo 👋.
• -include file — process file as if #include "file" appeared as the very first line of the primary source file. BUT — plot twist — the first directory searched for file is the preprocessor's working directory, not the directory containing the main source file. If it's not there, it gets searched for in the remainder of the #include "..." search chain as normal.

If you give multiple -include options, the files are included in the order they appear on the command line. Yes, order. Always order.

• -imacros file — exactly like -include, except any output produced by scanning file is thrown away. The macros it defines stay defined though. So you can snag all the macros from a header without also processing its declarations. Chefs kiss 💖.

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 stay defined though, they're not going anywhere.
• -pthread — define the additional macros required for using the POSIX threads library. Use this option consistently for both compilation and linking, okay? Consistency is the vibe. 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 the ones coming from -include or -imacros command-line options.

Unless you specify it explicitly (with -MT or -MQ), the object file name is the source file name with any suffix replaced by the object file suffix, and with any leading directory parts removed. If there are lots of included files, the rule gets split into several lines using \-newline. And the rule has no commands. Zero. None.

Also, this option does not suppress the preprocessor's debug output, like -dM. To avoid mixing that debug output with your dependency rules, explicitly specify the dependency output file with -MF, or use an environment variable like DEPENDENCIES_OUTPUT. Debug output still goes to the regular output stream as normal, sorry not sorry.

Passing -M to the driver implies -E, and suppresses warnings with an implicit -w.

• -MM — like -M, but it does not mention header files found in system header directories, nor header files included (directly or indirectly) from such a header.

Which means: the choice of angle brackets or double quotes in an #include directive does not by itself determine whether that header shows up in -MM dependency output. The way that works is lowkey subtle, iykyk.

• -MF file — when used with -M or -MM, specifies a file to write the dependencies to. If no -MF switch is given, the preprocessor sends the rules to the same place it would send preprocessed output.

When used with the driver options -MD or -MMD, -MF overrides the default dependency output file.

And if file is -, then the dependencies are written to stdout. Classic.

• -MG — together with an option like -M that requests 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, since a missing header file renders that useless anyway 🤷.

This feature is used in automatic updating of makefiles. Slay.

• -MP — this tells CPP to add a phony target for each dependency other than the main file, making each one depend on nothing. These dummy rules work around the errors make throws at you if you remove header files without updating the Makefile to match. The classic struggle.

Typical output looks like this:

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 like .c, and appends the platform's usual object suffix. That result is your target.

An -MT option sets the target to be exactly the string you specify. Want multiple targets? Pass them as a single argument to -MT, or use multiple -MT options. Your call 💅.

For example, -MT '$(objpfx)foo.o' might give:

$(objpfx)foo.o: foo.c

• -MQ target — same as -MT, but it quotes any characters which are special to Make. So -MQ '$(objpfx)foo.o' gives:

$$(objpfx)foo.o: foo.c

And the default target is automatically quoted, as if it were given with -MQ.

• -MD — -MD is equivalent to -M -MF file, except that -E is not implied. The driver determines file based on whether an -o option is given. If it is, the driver uses its argument but with a suffix of .d; otherwise it takes the name of the input file, removes any directory components and suffix, and applies a .d suffix.

If -MD is used together 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. Same flag, different meaning, we love a plot twist 😭.

And since -E is not implied, -MD can be used to generate a dependency output file as a side effect of the compilation process. Multitasking queen.

• -MMD — like -MD, except it mentions only user header files, not system header files.
• -fpreprocessed — tells the preprocessor that the input file has already been preprocessed. This suppresses things like macro expansion, trigraph conversion, escaped newline splicing, and processing of most directives. The preprocessor still recognizes and removes comments, so you can pass a file preprocessed with -C to the compiler without problems 🫶. In this mode the integrated preprocessor is little more than a tokenizer for the front ends.

-fpreprocessed is implicit if the input file has one of the extensions .i, .ii or .mi. Those are the extensions GCC uses for preprocessed files created by -save-temps.

• -fdirectives-only — when preprocessing, handle directives, but do not expand macros.

The option's behavior depends on the -E and -fpreprocessed options. Vibe check:

With -E, preprocessing is limited to handling directives such as #define, #ifdef, and #error. Other preprocessor operations — like macro expansion and trigraph conversion — are not performed. In addition, the -dD option is implicitly enabled.

With -fpreprocessed, predefinition of command line and most builtin macros is disabled. Macros like __LINE__, which are contextually dependent, are handled normally. This enables compilation of files previously preprocessed with -E -fdirectives-only. Chef's kiss, no notes 💜.

Okay bestie, real talk: when you've got both -E and -fpreprocessed in the mix, the rules for -fpreprocessed take precedence. That means you can fully preprocess files you already preprocessed with -E -fdirectives-only. Slay. 💅 No cap.

---

• -fdollars-in-identifiers — Accept $ in identifiers. It's giving chaotic variable names, but GCC said yes. 💸
• -fextended-identifiers — Accept universal character names and extended characters in identifiers. This option is enabled by default for C99 (and later C standard versions) and C++. Iconic. ✨
• -fno-canonical-system-headers — When preprocessing, do not shorten system header paths with canonicalization. Lowkey a niche one. 🫶
• -fmax-include-depth=depth — Set the maximum depth of the nested #include. The default is 200. So like, 200 layers deep. Don't go too wild. 😭
• -ftabstop=width — Set the distance between tab stops. This helps the preprocessor report correct column numbers in warnings or errors, even if tabs appear on the line. If the value is less than 1 or greater than 100, the option is ignored. The default is 8. Tabs said respect my spacing. 💅
• -ftrack-macro-expansion[=level] — Track locations of tokens across macro expansions. This allows the compiler to emit diagnostic about the current macro expansion stack when a compilation error occurs in a macro expansion. Using this option makes the preprocessor and the compiler consume more memory. The level parameter can be used to choose the level of precision of token location tracking thus decreasing the memory consumption if necessary. Value 0 of level de-activates this option. Value 1 tracks tokens locations in a degraded mode for the sake of minimal memory overhead. In this mode all tokens resulting from the expansion of an argument of a function-like macro have the same location. Value 2 tracks tokens locations completely. This value is the most memory hungry. When this option is given no argument, the default parameter value is 2.

Note that -ftrack-macro-expansion=2 is activated by default. Ngl, this one is a memory eater. 🫠

• -fmacro-prefix-map=old=new — When preprocessing files residing in directory old, expand the __FILE__ and __BASE_FILE__ macros as if the files resided in directory new instead. This can be used to change an absolute path to a relative path by using . for new which can result in more reproducible builds that are location independent. This option also affects __builtin_FILE() during compilation. See also -ffile-prefix-map. Reproducible builds are the main character. 👑
• -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. Slay encodings. 🌈
• -fwide-exec-charset=charset — Set the wide execution character set, used for wide string and character constants. The default is UTF-32 or UTF-16, whichever corresponds to the width of wchar_t. As with -fexec-charset, charset can be any encoding supported by the system's iconv library routine; however, you will have problems with encodings that do not fit exactly in wchar_t. It's giving square peg, round hole. 😭
• -finput-charset=charset — Set the input character set, used for translation from the character set of the input file to the source character set used by GCC. If the locale does not specify, or GCC cannot get this information from the locale, the default is UTF-8. This can be overridden by either the locale or this command-line option. Currently the command-line option takes precedence if there's a conflict. charset can be any encoding supported by the system's iconv library routine. Locale vs CLI? CLI ate. 💅
• -fpch-deps — When using precompiled headers, this flag causes the dependency-output flags to also list the files from the precompiled header's dependencies. If not specified, 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. PCH said I'm independent. 🦋
• -fpch-preprocess — This option allows use of a precompiled header together with -E. It inserts a special #pragma, #pragma GCC pch_preprocess "filename" in the output to mark the place where the precompiled header was found, and its filename. When -fpreprocessed is in use, GCC recognizes this #pragma and loads the PCH.

This option is off by default, because the resulting preprocessed output is only really suitable as input to GCC. It is switched on by -save-temps.

You should not write this #pragma in your own code, but it is safe to edit the filename if the PCH file is available in a different location. The filename may be absolute or it may be relative to GCC's current directory. So like, don't use this for human eyes. 🙈

• -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 enabled, the preprocessor emits, after the initial linemarker, a second linemarker with the current working directory followed by two slashes. GCC uses this directory, when it's present in the preprocessed input, as the directory emitted as the current working directory in some debugging information formats. This option is implicitly enabled if debugging information is enabled, but this can be inhibited with the negated form -fno-working-directory. If the -P flag is present in the command line, this option has no effect, since no #line directives are emitted whatsoever. So -P said not today. 💀
• -A predicate=answer — Make an assertion with the predicate predicate and answer answer. This form is preferred to the older form -A predicate(answer), which is still supported, because it does not use shell special characters. Shell-proof, periodt. 🐚
• -A -predicate=answer — Cancel an assertion with the predicate predicate and answer answer. Bye felicia. 👋
• -C — Do not discard comments. All comments are passed through to the output file, except for comments in processed directives, which are deleted along with the directive.

You should be prepared for side effects when using -C; it causes the preprocessor to treat comments as tokens in their own right. For example, comments appearing at the start of what would be a directive line have the effect of turning that line into an ordinary source line, since the first token on the line is no longer a #. Comments said we're tokens now. 💅

• -CC — Do not discard comments, including during macro expansion. This is like -C, except that comments contained within macros are also passed through to the output file where the macro is expanded.

In addition to the side effects of the -C option, the -CC option causes all C++-style comments inside a macro to be converted to C-style comments. This is to prevent later use of that macro from inadvertently commenting out the remainder of the source line.

The -CC option is generally used to support lint comments. Lint said thanks bestie. 🧹

• -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. Not C code? No linemarkers for you. 🚫
• -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. Retro vibes, but only with -E. 🕰️

• -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. Trigraphs are giving cursed keyboard energy. 😭

• -remap — Enable special code to work around file systems which only permit very short file names, such as MS-DOS. MS-DOS said 8.3 or bust. 💾
• -H — Print the name of each header file used, in addition to other normal activities. Each name is indented to show how deep in the #include stack it is. Precompiled header files are also printed, even if they are found to be invalid; an invalid precompiled header file is printed with ...x and a valid one with ...! . Header detective mode. 🔍
• -dletters — 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. Undefined behavior? We don't do that here. 💀
  • -dM Instead of the normal output, generate a list of #define directives for all the macros defined during the execution of the preprocessor, including predefined macros. This gives you a way of finding out what is predefined in your version of the preprocessor. Assuming you have no file foo.h, the command

touch foo.h; cpp -dM foo.h

shows all the predefined macros.

If you use -dM without the -E option, -dM is interpreted as a synonym for -fdump-rtl-mach. Macro peek behind the curtain. 👀

• -dD Like -dM except in two respects: it does not include the predefined macros, and it outputs both the #define directives and the result of preprocessing. Both kinds of output go to the standard output file. Double duty. ✌️
• -dN Like -dD, but emit only the macro names, not their expansions. Names only, no spoilers. 🤫
• -dI Output #include directives in addition to the result of preprocessing. Includes said hey. 👋
• -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. Only the used ones get the spotlight. 🌟

• -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. Debugging GCC? You're built different. 🐛

• -Wp,option — bestie, this one is the sneaky back door 🚪. It lets you bypass the compiler driver entirely and yeet option straight through to the preprocessor. If option has commas in it, it gets split into multiple options right at the commas, no questions asked. BUT. Here's the tea 🍵: a ton of options get modified, translated or interpreted by the compiler driver before they ever reach the preprocessor, and -Wp forcibly skips all of that. The preprocessor's direct interface is undocumented and subject to change, so whenever possible you should avoid -Wp and just let the driver handle the options instead. iykyk 💅
• -Xpreprocessor option — pass option as an option to the preprocessor. This is your girl for supplying system-specific preprocessor options that GCC doesn't recognize ✨.

Heads up 🫶: if you want to pass an option that takes an argument, you gotta use -Xpreprocessor twice — once for the option, once for the argument.

• -no-integrated-cpp — perform preprocessing as a separate pass before compilation. By default GCC does preprocessing as an integrated part of input tokenization and parsing (all cozy in one go 🛋️). If you provide this option, the appropriate language front end (cc1, cc1plus, or cc1obj for C, C++, and Objective-C, respectively) gets invoked twice — once for preprocessing only, and once for actual compilation of the preprocessed input. This can be useful alongside the -B or -wrapper options to specify an alternate preprocessor or do extra processing of the program source between normal preprocessing and compilation. Iconic, honestly 👑

---

🌸 Passing Options to the Assembler 🌸

Okay so you can pass options to the assembler too. Slay.

• -Wa,option — pass option as an option to the assembler. If option contains commas, it is split into multiple options at the commas. Simple as 🎀
• -Xassembler option — pass option as an option to the assembler. Use this to supply system-specific assembler options that GCC doesn't recognize 💫

Same rule, bestie: if you want to pass an option that takes an argument, you must use -Xassembler twice — once for the option and once for the argument.

---

🦋 Options for Linking 🦋

These options come into play when the compiler links object files into an executable output file. They are meaningless if the compiler isn't doing a link step. Like, no link step, no vibe check. 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, not the name — sneaky 👀.) 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. Fr fr, don't do it 🙈
• -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. Let's break down your type options, bestie:
  • 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 building 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 the 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 the object files in an incremental link cannot be used for link-time optimization, the linker plugin issues a warning and uses nolto-rel. To maintain whole program optimization, it is recommended to link such objects into a 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. We love options ✨
• -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 — lowkey a relic 🫠.)

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.

Big one, bestie 💖: 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, no cap 🧚

• -lobjc — you need this special case of the -l option in order to link an Objective-C or Objective-C++ program.
• -nostartfiles — do not use the standard system startup files when linking. The standard system libraries are used normally, unless -nostdlib, -nolibc, or -nodefaultlibs is used.
• -nodefaultlibs — do not use the standard system libraries when linking. Only the libraries you specify are passed to the linker, and options specifying linkage of the system libraries, such as -static-libgcc or -shared-libgcc, are ignored. The standard startup files are used normally, unless -nostartfiles is used.

The compiler may generate calls to memcmp, memset, memcpy and memmove. These entries are usually resolved by entries in libc. These entry points should be supplied through some other mechanism when this option is specified. Don't say I didn't warn you 🙈

• -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. Bare metal vibes 🦋
• -nostdlib — do not use the standard system startup files or libraries when linking. No startup files, and only the libraries you specify are passed to the linker, and options specifying linkage of the system libraries, such as -static-libgcc or -shared-libgcc, are ignored.

The compiler may generate calls to memcmp, memset, memcpy and memmove. These entries are usually resolved by entries in libc. These entry points should be supplied through some other mechanism when this option is specified.

One of the standard libraries bypassed by -nostdlib and -nodefaultlibs is libgcc.a, a library of internal subroutines which GCC uses to overcome shortcomings of particular machines, or special needs for some languages.

In most cases, you need libgcc.a even when you want to avoid other standard libraries. In other words, when you specify -nostdlib or -nodefaultlibs you should usually specify -lgcc as well. This ensures that you have no unresolved references to internal GCC library subroutines. (An example of such an internal subroutine is __main, used to ensure C++ constructors are called.) 💅

• -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.
• -no-pie — don't produce a dynamically linked position independent executable. Simple as 🎀
• -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 — okay so bestie, this one links you against the POSIX threads library 🧵. It's supported on GNU/Linux targets, most other Unix derivatives, and also on x86 Cygwin and MinGW targets. Lowkey important: on some targets this option also sets flags for the preprocessor, so you gotta use it consistently for both compilation and linking. Like, don't do half and half, that's giving chaos.
• -r — produces a relocatable object as output ✨. It's also known as partial linking. Slay.
• -rdynamic — passes the flag -export-dynamic to the ELF linker, on targets that support it. This tells the linker to add all symbols, not just the used ones, to the dynamic symbol table 💅. You need this for some uses of dlopen or to allow obtaining backtraces from within a program. So if you're out here wanting stack traces, this is your girl.
• -s — removes all symbol table and relocation information from the executable 💀. Gone. Poof. Not there anymore.
• -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. Ngl, it's a whole personality change.
• -shared — produces a shared object which can then be linked with other objects to form an executable 🦋. Not all systems support this option. And for predictable results — listen closely bestie — you must also specify the same set of options used for compilation (-fpic, -fPIC, or model suboptions) when you specify this linker option.[1] Consistency is the moment.
• -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. (So like, they just sit there being decorative 🙈.)

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. Periodt.

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. Main character optimization energy honestly 👑

However — and this is the important bit — 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 shortcuts bestie.

• -static-libasan — when the -fsanitize=address option is used to link a program, the GCC driver automatically links against libasan 🫶. If libasan is available as a shared library, and the -static option is not used, then this links against the shared version of libasan. The -static-libasan option directs the GCC driver to link libasan statically, without necessarily linking other libraries statically.
• -static-libtsan — when the -fsanitize=thread option is used to link a program, the GCC driver automatically links against libtsan 🧵. If libtsan is available as a shared library, and the -static option is not used, then this links against the shared version of libtsan. The -static-libtsan option directs the GCC driver to link libtsan statically, without necessarily linking other libraries statically.
• -static-liblsan — when the -fsanitize=leak option is used to link a program, the GCC driver automatically links against liblsan 💧. If liblsan is available as a shared library, and the -static option is not used, then this links against the shared version of liblsan. The -static-liblsan option directs the GCC driver to link liblsan statically, without necessarily linking other libraries statically.
• -static-libubsan — when the -fsanitize=undefined option is used to link a program, the GCC driver automatically links against libubsan 🙈. If libubsan is available as a shared library, and the -static option is not used, then this links against the shared version of libubsan. The -static-libubsan option directs the GCC driver to link libubsan statically, without necessarily linking other libraries statically.
• -static-libstdc++ — when the g++ program is used to link a C++ program, it normally automatically links against libstdc++ 💜. If libstdc++ is available as a shared library, and the -static option is not used, then this links against the shared version of libstdc++. That is normally fine. However, it is sometimes useful to freeze the version of libstdc++ used by the program without going all the way to a fully static link 🧊. The -static-libstdc++ option directs the g++ driver to link libstdc++ statically, without necessarily linking other libraries statically.
• -symbolic — binds references to global symbols when building a shared object 🌈. Warns about any unresolved references (unless overridden by the link editor option -Xlinker -z -Xlinker defs). Only a few systems support this option. Rare girlie.
• -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 — passes 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. Classic gotcha, don't fall for it bestie 😭.

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* — passes 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. So many ways to say the same thing, iconic ✨.
• -u symbol — pretends 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. Stack 'em up.
• -z keyword — -z is passed directly on to the linker along with the keyword keyword. See the section in the documentation of your linker for permitted values and their meanings 📖.

🎀 Options for Directory Search 🎀

These options specify directories to search for header files, for libraries and for parts of the compiler 💖:

• -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. She's retired, let her rest 🪦.

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 — Set prefix as the prefix for subsequent -iwithprefix options. If the prefix represents a directory, bestie, you gotta include the final /. 🎀
• -iwithprefix dir — Append dir to the prefix specified previously with -iprefix, and add the resulting directory to the include search path. This one puts it where -idirafter would. ✨
• -iwithprefixbefore dir — Append dir to the prefix specified previously with -iprefix, and add the resulting directory to the include search path. This one puts it in the same place -I would. 🫶
• -isysroot dir — This option is like the --sysroot option, but applies only to header files (except for Darwin targets, where it applies to both header files and libraries). See the --sysroot option for more information. 🍓
• -imultilib dir — Use dir as a subdirectory of the directory containing target-specific C++ headers. 💜
• -nostdinc — Do not search the standard system directories for header files. Only the directories explicitly specified with -I, -iquote, -isystem, and/or -idirafter options (and the directory of the current file, if appropriate) are searched. 👑
• -nostdinc++ — Do not search for header files in the C++-specific standard directories, but do still search the other standard directories. (This option is used when building the C++ library.) 🌈
• -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 — This option instructs the compiler to assume that signed arithmetic overflow of addition, subtraction and multiplication wraps around using twos-complement representation. This flag enables some optimizations and disables others. The options -ftrapv and -fwrapv override each other, so using -ftrapv -fwrapv on the command-line results in -fwrapv being effective. Note that only active options override, so using -ftrapv -fwrapv -fno-wrapv on the command-line results in -ftrapv being effective. 💅
• -fwrapv-pointer — This option instructs the compiler to assume that pointer arithmetic overflow on addition and subtraction wraps around using twos-complement representation. This flag disables some optimizations which assume pointer overflow is invalid. 🦋
• -fstrict-overflow — This option implies -fno-wrapv -fno-wrapv-pointer and when negated implies -fwrapv -fwrapv-pointer. 🌸
• -fexceptions — Enable exception handling. Generates extra code needed to propagate exceptions. For some targets, this implies GCC generates frame unwind information for all functions, which can produce significant data size overhead, although it does not affect execution. If you do not specify this option, GCC enables it by default for languages like C++ that normally require exception handling, and disables it for languages like C that do not normally require it. However, you may need to enable this option when compiling C code that needs to interoperate properly with exception handlers written in C++. You may also wish to disable this option if you are compiling older C++ programs that don't use exception handling. ✨
• -fnon-call-exceptions — Generate code that allows trapping instructions to throw exceptions. Note that this requires platform-specific runtime support that does not exist everywhere. Moreover, it only allows trapping instructions to throw exceptions, i.e. memory references or floating-point instructions. It does not allow exceptions to be thrown from arbitrary signal handlers such as "SIGALRM". 💀
• -fdelete-dead-exceptions — okay so bestie, this one tells GCC: "if an instruction can throw an exception but literally contributes nothing else to how the program runs, feel free to yeet it entirely" 💀. Dead exceptions? Gone. This is on by default for the Ada front end, because the Ada language spec says that's allowed. And the optimization passes that actually do the dead-exception removing get enabled independently at different -O levels — so it's not one switch flipping everything, iykyk.
• -funwind-tables — it's giving -fexceptions's chill cousin 🌸. Same energy, except this one only generates the static data that's needed, and does not touch the generated code in any other way. And real talk: you normally don't need to turn this on yourself. A language processor that needs this handling enables it on your behalf, so let it cook 🫶.
• -fasynchronous-unwind-tables — generate the unwind table in DWARF format, if the target machine supports it. And the slay here is that the table is exact at each instruction boundary, which means it can be used for stack unwinding from asynchronous events — think debugger or garbage collector just showing up uninvited mid-instruction 😭✨.
• -fno-gnu-unique — okay so on systems with a recent GNU assembler and C library, the C++ compiler uses the STB_GNU_UNIQUE binding to make sure definitions of template static data members and static local variables in inline functions are unique, even when RTLD_LOCAL is in the picture. Why do we care? Because otherwise you get problems when a library is used by two different RTLD_LOCAL plugins, each depending on a definition in one of them, and then they disagree with each other about the binding of the symbol. Drama 💅.

But — and this is the tradeoff — it causes dlclose to be ignored for the affected DSOs. So if your program relies on reinitializing a DSO via dlclose and dlopen, you can use -fno-gnu-unique to fix that.

• -fpcc-struct-return — return short struct and union values in memory like the longer ones do, instead of in registers. Is it less efficient? Yes bestie 😭. But the glow-up is that it allows intercallability between GCC-compiled files and files compiled with other compilers — particularly the Portable C Compiler (pcc).

The precise convention for returning structures in memory depends on the target configuration macros.

And "short" structures and unions here means 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. For small structures this is way more efficient than -fpcc-struct-return, periodt ✨.

If you specify neither -fpcc-struct-return nor -freg-struct-return, GCC defaults to whichever convention is standard for the target. And if there's no standard convention? GCC defaults to -fpcc-struct-return — except on targets where GCC is the principal compiler. In those cases we get to choose the standard, and we chose the more efficient register return alternative 👑.

Warning: code compiled with the -freg-struct-return switch is not binary compatible with code compiled with the -fpcc-struct-return switch. Use it to conform to a non-default application binary interface.

• -fshort-enums — allocate to an enum type only as many bytes as it actually needs for the declared range of possible values 🎀. Specifically, the enum type becomes equivalent to the smallest integer type that has enough room.

Warning: the -fshort-enums switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface.

• -fshort-wchar — override the underlying type for wchar_t to be short unsigned int instead of the default for the target 🦋. Super 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 where global variables defined without an initializer go — the C standard calls these tentative definitions 💅. And bestie, tentative definitions are a whole different thing from declaring a variable with the extern keyword, because those declarations do not allocate storage.

The default is -fno-common, which says: the compiler places uninitialized global variables in the BSS section of the object file. Downside? It inhibits the linker from merging tentative definitions, so you get a multiple-definition error if the same variable is accidentally defined in more than one compilation unit 😭.

The -fcommon option places uninitialized global variables in a common block instead. That lets the linker resolve all tentative definitions of the same variable in different compilation units to the same object, or to a non-tentative definition. Lowkey it's inconsistent with C++, and on many targets it implies a speed and code size penalty on global variable references. It's mainly useful for getting legacy code to link without errors.

• -fno-ident — ignore the #ident directive. That's it, that's the whole vibe 🙈.
• -finhibit-size-directive — don't output a .size assembler directive, or anything else that would cause trouble if the function gets split in the middle and the two halves end up placed at locations far apart in memory 💀. This option is used when compiling crtstuff.c — and bestie, you should not need it for anything else. Fr fr.
• -fverbose-asm — put extra commentary info into the generated assembly code so it's actually readable 🌸. This is generally only of use to people who genuinely need to read the generated assembly (maybe while debugging the compiler itself, no cap).

-fno-verbose-asm, the default, omits that extra information — handy when you're 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

And just so you know — these comments are intended for humans rather than machines, so the precise format of the comments is subject to change 🙈. Don't go parsing them, bestie.

• -frecord-gcc-switches — this switch causes the command line used to invoke the compiler to be recorded into the object file that's being created 🎀. Heads up: it's only implemented on some targets, and the exact format of the recording is target and binary file format dependent — but usually it takes the form of a section containing ASCII text. It's related to -fverbose-asm, except that switch only records info in the assembler output file as comments, so it never reaches the object file. See also -grecord-gcc-switches for another way of storing compiler options into the object file ✨.
• -fpic — generate position-independent code (PIC) suitable for use in a shared library, if it'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 no, the dynamic loader is not part of GCC, it's part of the operating system, okay? 💅

If the GOT size for the linked executable exceeds a machine-specific maximum size, you get an error message from the linker saying -fpic does not work — in that case, recompile with -fPIC instead. (These maximums are 8k on the SPARC, 28k on AArch64, and 32k on the m68k and RS/6000. The x86 has no such limit 😌.)

Position-independent code requires special support, so it works only on certain machines. For the x86, GCC supports PIC for System V but not for the Sun 386i. And code generated for the IBM RS/6000 is always position-independent.

When this flag is set, the macros __pic__ and __PIC__ are defined to 1.

• -fPIC — if supported for the target machine, emit position-independent code, suitable for dynamic linking and avoiding any limit on the size of the global offset table 🔥. This option 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 — okay so these are like -fpic and -fPIC, but the generated position-independent code can only be linked into executables. Usually you use them to compile code that's gonna be linked with the -pie GCC option. Both -fpie and -fPIE define the macros __pie__ and __PIE__. Those macros have the value 1 for -fpie and 2 for -fPIE. 💅
• -fno-plt — do not use the PLT for external function calls in position-independent code. Instead, load the callee address at call sites from the GOT and branch to it. This leads to more efficient code by eliminating PLT stubs and exposing GOT loads to optimizations. On architectures like 32-bit x86 where PLT stubs expect the GOT pointer in a specific register, this gives the compiler 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 it would be more efficient than other code generation strategies. This option is useful with -fpic or -fPIC for building code that forms part of a dynamic linker and cannot reference the address of a jump table. On some targets, jump tables don't require a GOT and this option isn't needed.
• -ffixed-reg — treat the register named reg as a fixed register; generated code should never refer to it (except maybe as a stack pointer, frame pointer, or in some other fixed role). reg must be the name of a register. The register names accepted are machine-specific and are defined in the REGISTER_NAMES macro in the machine description macro file. This flag does not have a negative form, because it specifies a three-way choice.
• -fcall-used-reg — treat the register named reg as an allocable register that is clobbered by function calls. It may be allocated for temporaries or variables that do not live across a call. Functions compiled this way do not save and restore the register reg. It is an error to use this flag with the frame pointer or stack pointer. Use of this flag for other registers that have fixed pervasive roles in the machine's execution model produces disastrous results. 💀 This flag does not have a negative form, because it specifies a three-way choice.
• -fcall-saved-reg — treat the register named reg as an allocable register saved by functions. It may be allocated even for temporaries or variables that live across a call. Functions compiled this way save and restore the register reg if they use it. It is an error to use this flag with the frame pointer or stack pointer. Use of this flag for other registers that have fixed pervasive roles in the machine's execution model produces disastrous results. 💀 A different sort of disaster results from use of this flag for a register in which function values may be returned. This flag does not have a negative form, because it specifies a three-way choice.
• -fpack-struct[=n] — without a value, pack all structure members together without holes. When a value is specified (it 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 is created at run time on the stack when the address of a nested function is taken, and is used to call the nested function indirectly. Therefore, it requires the stack to be made executable in order for the program to work properly. -fno-trampolines is enabled by default on a language-by-language basis to let the compiler avoid generating them, if it computes that this is safe, and replace them with descriptors. Descriptors are made up of data only, but the generated code must be prepared to deal with them. As of this writing, -fno-trampolines is enabled by default only for Ada. Moreover, code compiled with -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 to marking things hidden when the default is public is to make the default hidden and mark things public. This is the norm with DLLs on Windows and with -fvisibility=hidden and __attribute__ ((visibility("default"))) instead of __declspec(dllexport) you get almost identical semantics with identical syntax. This is a great boon to those working with cross-platform projects. For those adding visibility support to existing code, you may find #pragma GCC visibility of use. This works by you enclosing the declarations you wish to set visibility for with (for example) #pragma GCC visibility push(hidden) and #pragma GCC visibility pop. Bear in mind that symbol visibility should be viewed as part of the API interface contract and thus all new code should always specify visibility when it is not the default; i.e., declarations only for use within the local DSO should always be marked explicitly as hidden as so to avoid PLT indirection overheads---making this abundantly clear also aids readability and self-documentation of the code. Note that due to ISO C++ specification requirements, operator new and operator delete must always be of default visibility. Be aware that headers from outside your project, in particular system headers and headers from any other library you use, may not be expecting to be compiled with visibility other than the default. You may need to explicitly say #pragma GCC visibility push(default) before including any such headers. extern declarations are not affected by -fvisibility, so a lot of code can be recompiled with -fvisibility=hidden with no modifications. However, this means that calls to extern functions with no explicit visibility use the PLT, so it is more effective to use __attribute ((visibility)) and/or #pragma GCC visibility to tell the compiler which extern declarations should be treated as hidden. Note that -fvisibility does affect C++ vague linkage entities. This means that, for instance, an exception class that is be thrown between DSOs must be explicitly marked with default visibility so that the type_info nodes are unified between the DSOs. An overview of these techniques, their benefits and how to use them is at <http://gcc.gnu.org/wiki/Visibility>.
• -fstrict-volatile-bitfields — this option should be used if accesses to volatile bit-fields (or other structure fields, although the compiler usually honors those types anyway) should use a single access of the width of the field's type, aligned to a natural alignment if possible. For example, targets with memory-mapped peripheral registers might require all such accesses to be 16 bits wide; with this flag you can declare all peripheral bit-fields as unsigned short (assuming short is 16 bits on these targets) to force GCC to use 16-bit accesses instead of, perhaps, a more efficient 32-bit access. If this option is disabled, the compiler uses the most efficient instruction. In the previous example, that might be a 32-bit load instruction, even though that accesses bytes that do not contain any portion of the bit-field, or memory-mapped registers unrelated to the one being updated.

In some cases — like when the packed attribute gets slapped onto a structure field — it's lowkey impossible to access that field with a single read or write that's correctly aligned for the target machine 😭. And when that happens, GCC does not just wing it. It falls back to generating multiple accesses instead of code that would fault or truncate the result at run time. that's the self-control we love to see ✨

Note: thanks to the restrictions of the C/C++11 memory model, write accesses are not allowed to touch non bit-field members 🚫. So the move is: define all bits of the field's type as bit-field members. fr fr, do it.

The default value of this option is determined by the application binary interface for the target processor. (iykyk)

• -fsync-libcalls — this one controls whether any out-of-line instance of the __sync family of functions is allowed to be used to implement the C++11 __atomic family of functions 👀

The default value is enabled, so honestly the only useful form of this option is -fno-sync-libcalls 💅. This option is used in the implementation of the libatomic runtime library.

---

🛠️ GCC Developer Options 🛠️

This bit is all about the command-line options that are primarily of interest to GCC developers — the ones that support compiler testing, digging into compiler bugs, and investigating compile-time performance problems 🧚. That includes options that produce debug dumps at various points in the compilation, options that print statistics like memory use and execution time, and options that print info about GCC's configuration, like where it goes searching for libraries. You should rarely need any of these for ordinary compilation and linking tasks — they're giving backstage-pass energy, not everyday-user energy 💖.

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, a phase letter, and a pass name. The base dump file name is the name of the output file produced by the compiler if it's explicitly specified and not an executable; otherwise it's 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're executed earlier 👑. The phase letter is one of i (inter-procedural analysis), l (language-specific), r (RTL), or t (tree). The files get 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 extra per-node and/or per-edge info if you also hand it 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 the times specified by letters. This is what you use 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 all the possible letters for use in pass and letters, and what they mean:

• -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 — these 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 — these 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 — these 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 — these 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 — these enable dumping after the two forward propagation passes.
• -fdump-rtl-gcse1, -fdump-rtl-gcse2 — these 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 — this one 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 — these enable dumping after the basic block scheduling passes.
• -fdump-rtl-ree — Dump after sign/zero extension elimination.
• -fdump-rtl-seqabstr — Dump after common sequence discovery.
• -fdump-rtl-shorten — Dump after shortening branches.
• -fdump-rtl-sibling — Dump after sibling call optimizations.
• -fdump-rtl-split1, -fdump-rtl-split2, -fdump-rtl-split3, -fdump-rtl-split4, -fdump-rtl-split5 — these options enable dumping after five rounds of instruction splitting. five rounds bestie, she's committed 💅
• -fdump-rtl-sms — Dump after modulo scheduling. This pass is only run on some architectures.
• -fdump-rtl-stack — Dump after conversion from GCC's "flat register file" registers to the x87's stack-like registers. This pass is only run on x86 variants.
• -fdump-rtl-subreg1, -fdump-rtl-subreg2 — these enable dumping after the two subreg expansion passes.
• -fdump-rtl-unshare — Dump after all rtl has been unshared.
• -fdump-rtl-vartrack — Dump after variable tracking.
• -fdump-rtl-vregs — Dump after converting virtual registers to hard registers.
• -fdump-rtl-web — Dump after live range splitting.
• -fdump-rtl-regclass, -fdump-rtl-subregs_of_mode_init, -fdump-rtl-subregs_of_mode_finish, -fdump-rtl-dfinit, -fdump-rtl-dfinish — these dumps are defined but always produce empty files 💀 iconic behaviour honestly, they're just there vibing.
• -da, -fdump-rtl-all — produce all the dumps listed above 🔥
• -dA — annotate the assembler output with miscellaneous debugging information.
• -dD — dump all macro definitions, at the end of preprocessing, in addition to normal output.
• -dH — produce a core dump whenever an error occurs 😭
• -dp — annotate the assembler output with a comment indicating which pattern and alternative is used. The length and cost of each instruction are also printed.
• -dP — dump the RTL in the assembler output as a comment before each instruction. Also turns on -dp annotation.
• -dx — just generate RTL for a function instead of compiling it. Usually used with -fdump-rtl-expand.

• -fdump-debug — dump debugging information generated during the debug generation phase 🐛
• -fdump-earlydebug — dump debugging information generated during the early debug generation phase 🌸
• -fdump-noaddr — when doing debugging dumps, suppress address output 🙈. This makes it way more feasible to use diff on debugging dumps for compiler invocations with different compiler binaries and/or different text / bss / data / heap / stack / dso start locations. genuinely so handy.
• -freport-bug — collect and dump debug information into a temporary file if an internal compiler error (ICE) occurs 🧊.
• -fdump-unnumbered — bestie, when you're out here doing debugging dumps, this one is like "numbers? addresses? never heard of her." It suppresses instruction numbers and address output. Why would you want that? So you can actually use diff on debugging dumps for compiler invocations with different options — especially comparing with and without -g. It's giving clean comparison energy. ✨
• -fdump-unnumbered-links — when you're doing debugging dumps (peep the -d option above, bestie), this one suppresses instruction numbers for the links to the previous and next instructions in a sequence. Very niche, very specific, and honestly? We respect it. 💅
• -fdump-ipa-switch
• -fdump-ipa-switch-options — okay so this controls the dumping at various stages of inter-procedural analysis language tree to a file. The file name? Generated by appending a switch-specific suffix to the source file name, and the file is created in the same directory as the output file. The following dumps are possible:
  • all — Enables all inter-procedural analysis dumps. Every last one. The whole buffet. 🍓
  • 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 get the tea AND the spill. 🫖

By default, the dump will contain messages about successful optimizations (equivalent to -optimized) together with low-level details about the analysis.

• -fdump-lang-all
• -fdump-lang-switch
• -fdump-lang-switch-options
• -fdump-lang-switch-options=filename — this controls 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.
  • raw — Dump the raw internal tree data. This option is applicable to C++ only.
• -fdump-passes — prints on stderr the list of optimization passes that are turned on and off by the current command-line options. It's literally a little receipt of what's active and what's not. 🧾 So useful for the girlies who need to know.
• -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 as it changes according to the environment and source file. Its primary use is for tying up a dump file with a debug environment.
  • asmname — If "DECL_ASSEMBLER_NAME" has been set for a given decl, use that in the dump instead of "DECL_NAME". Its primary use is ease of use working backward from mangled names in the assembly file.
  • slim — When dumping front-end intermediate representations, inhibit dumping of members of a scope or body of a function merely because that scope has been reached. Only dump such items when they are directly reachable by some other path.

When dumping pretty-printed trees, this option inhibits dumping the bodies of control structures.

When dumping RTL, print the RTL in slim (condensed) form instead of the default LISP-like representation.

• raw — Print a raw representation of the tree. By default, trees are pretty-printed into a C-like representation.
• details — Enable more detailed dumps (not honored by every dump option). Also include information from the optimization passes.
• stats — Enable dumping various statistics about the pass (not honored by every dump option).
• blocks — Enable showing basic block boundaries (disabled in raw dumps).
• graph — For each of the other indicated dump files (-fdump-rtl-pass), dump a representation of the control flow graph suitable for viewing with GraphViz to file.passid.pass.dot. Each function in the file is pretty-printed as a subgraph, so that GraphViz can render them all in a single plot.

This option currently only works for RTL dumps, and the RTL is always dumped in slim form.

• vops — Enable showing virtual operands for every statement.
• lineno — Enable showing line numbers for statements.
• uid — Enable showing the unique ID ("DECL_UID") for each variable.
• verbose — Enable showing the tree dump for each statement.
• eh — Enable showing the EH region number holding each statement.
• scev — Enable showing scalar evolution analysis details.
• optimized — Enable showing optimization information (only available in certain passes).
• missed — Enable showing missed optimization information (only available in certain passes).
• note — Enable other detailed optimization information (only available in certain passes).
• 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 composed of three components separated by a period: the name of the source file GCC was invoked to compile, a numeric suffix indicating the pass number followed by the letter 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 in a file named myfile.c.038t.evrp in the current working directory. Note that the numeric codes are not stable and may change from one version of GCC to another. So don't get attached, bestie. 😭

• -fopt-info
• -fopt-info-options
• -fopt-info-options=filename — Controls optimization dumps from various optimization passes. If the -options form is used, options is a list of - separated option keywords to select the dump details and optimizations.

The options can be divided into three groups:

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.

The options from each group can be freely mixed as they are non-overlapping. However, in case of any conflicts, the later options override the earlier options on the command line. Later wins. Always. Periodt. 💅

The following options control which kinds of messages should be emitted:

• optimized — Print information when an optimization is successfully applied. It is up to a pass to decide which information is relevant. For example, the vectorizer passes print the source location of loops which are successfully vectorized.
• missed — Print information about missed optimizations. Individual passes control which information to include in the output.
• note — Print verbose information about optimizations, such as certain transformations, more detailed messages about decisions etc.
• all — Print detailed optimization information. This includes optimized, missed, and note.

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. 👑 For the compiler girlies who want the deep lore.

One or more of the following option keywords can be used to describe a group of optimizations:

• ipa — Enable dumps from all interprocedural optimizations.
• loop — Enable dumps from all loop optimizations.
• inline — Enable dumps from all inlining optimizations.
• omp — Enable dumps from all OMP (Offloading and Multi Processing) optimizations.
• vec — Enable dumps from all vectorization optimizations.
• optall — Enable dumps from all optimizations. This is a superset of the optimization groups listed above.

If options is omitted, it defaults to optimized-optall, which means to dump messages about successful optimizations from all the passes, omitting messages that are treated as "internals".

If the filename is provided, then the dumps from all the applicable optimizations are concatenated into the filename. Otherwise the dump is output onto stderr. Though multiple -fopt-info options are accepted, only one of them can include a filename. If other filenames are provided then all but the first such option are ignored. Bestie. Only the first one counts. The rest are just background noise. 🙈

Note that the output filename is overwritten in case of multiple translation units. If a combined output from multiple translation units is desired, stderr should be used instead. So if you want everything in one place, stderr is your girl. 💖

Slay, back to the -fopt-info gossip 💅 okay so here's how it all plays out irl:

In this next example the optimization info just goes straight to stderr:

        gcc -O3 -fopt-info

Then this one:

        gcc -O3 -fopt-info-missed=missed.all

drops the missed-optimization report from all the passes into missed.all. And this one:

        gcc -O2 -ftree-vectorize -fopt-info-vec-missed

prints the tea on missed optimization opportunities from the vectorization passes over on stderr. Not me pointing out that -fopt-info-vec-missed is literally equivalent to -fopt-info-missed-vec — the order of the optimization group names and message types listed after -fopt-info does not matter at all, bestie. 🫶

As another example:

        gcc -O3 -fopt-info-inline-optimized-missed=inline.txt

outputs info about missed optimizations plus optimized locations from all the inlining passes into inline.txt.

And finally, consider this:

        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 😭 because only one output file is allowed. In this case only the first option takes effect and the subsequent options are ignored. So you only get vec.miss, containing the vectorizer's dumps about missed opportunities. Only one. Periodt.

---

• -fsave-optimization-record — writes 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 inside that compressed JSON file is subject to change. No cap.

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 these extra deets for every 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. Nested, like your group chats. 🧚

• -fsched-verbose=n — on targets that use instruction scheduling, this option controls the amount of debugging output the scheduler prints to the dump files.

For n greater than zero, -fsched-verbose outputs the same information as -fdump-rtl-sched1 and -fdump-rtl-sched2. For n greater than one, it also outputs basic block probabilities, detailed ready list information and unit/insn info. For n greater than two, it includes RTL at abort point, control-flow and regions info. And for n over four, -fsched-verbose also includes dependence info. the way it just keeps giving more and more… 👑

• -fenable-kind-pass
• -fdisable-kind-pass=range-list

This is a 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. (so like… devs only, bestie, stay in your lane 💅)

• -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, verbatim, no edits:

        # 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. fr fr, don't reuse it.

• -save-temps
• -save-temps=cwd — store the usual "temporary" intermediate files permanently; place them in the current directory and name them based on the source file. So compiling foo.c with -c -save-temps produces files foo.i and foo.s, as well as foo.o. This creates a preprocessed foo.i output file even though the compiler now normally uses an integrated preprocessor. keeping the receipts, iconic behavior 🎀

When used in combination with the -x command-line option, -save-temps is sensible enough to avoid over writing an input source file with the same extension as an intermediate file. The corresponding intermediate file may be obtained by renaming the source file before using -save-temps.

If you invoke GCC in parallel, compiling several different source files that share a common base name in different subdirectories or the same source file compiled for multiple output destinations, it is likely that the different parallel compilers will interfere with each other, and overwrite the temporary files. chaos, actually. For instance:

        gcc -save-temps -o outdir1/foo.o indir1/foo.c&
        gcc -save-temps -o outdir2/foo.o indir2/foo.c&

may result in foo.i and foo.o being written to simultaneously by both compilers. 😭💀

• -save-temps=obj — store the usual "temporary" intermediate files permanently. If the -o option is used, the temporary files are based on the object file. If the -o option is not used, the -save-temps=obj switch behaves like -save-temps.

For example:

        gcc -save-temps=obj -c foo.c
        gcc -save-temps=obj -c bar.c -o dir/xbar.o
        gcc -save-temps=obj foobar.c -o dir2/yfoobar

creates foo.i, foo.s, dir/xbar.i, dir/xbar.s, dir2/yfoobar.i, dir2/yfoobar.s, and dir2/yfoobar.o.

• -time[=file] — report the CPU time taken by each subprocess in the compilation sequence. For C source files, this is the compiler proper and assembler (plus the linker if linking is done).

Without the specification of an output file, the output looks like this:

        # cc1 0.12 0.01
        # as 0.00 0.01

The first number on each line is the "user time", that is time spent executing the program itself. The second number is "system time", time spent executing operating system routines on behalf of the program. Both numbers are in seconds. ⏱️

With the specification of an output file, the output is appended to the named file, and it looks like this:

        0.12 0.01 cc1 <options>
        0.00 0.01 as <options>

The "user time" and the "system time" are moved before the program name, and the options passed to the program are displayed, so that one can later tell what file was being compiled, and with which options. receipts on receipts, we love an audit trail 💖

• -fdump-final-insns[=file] — dump the final internal representation (RTL) to file. If the optional argument is omitted (or if file is .), the name of the dump file is determined by appending .gkd to the compilation output file name.
• -fcompare-debug[=opts] — if no error occurs during compilation, run the compiler a second time, adding opts and -fcompare-debug-second to the arguments passed to the second compilation. Dump the final internal representation in both compilations, and print an error if they differ. 👀

If the equal sign is omitted, the default -gtoggle is used.

GCC_COMPARE_DEBUG is that one env var that just does things 💅 — if it's defined, non-empty, and nonzero, she implicitly enables -fcompare-debug for you. And if you set GCC_COMPARE_DEBUG to a string that starts with a dash, then that string is what gets used for opts; otherwise the default -gtoggle is used. Simple as that, bestie. ✨

-fcompare-debug=, with the equal sign but without opts? Yeah that's literally equivalent to -fno-compare-debug 🫶 — it disables the dumping of the final representation and the second compilation, which means even GCC_COMPARE_DEBUG can't take effect. Not even a little bit. She's blocked, periodt.

Okay so if you wanna 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 — like, actual compilation, not preprocessing, not assembly, not linking. And if you only want a warning instead? Setting GCC_COMPARE_DEBUG to -w%n-fcompare-debug not overridden will do. 🌸

• -fcompare-debug-second — this one gets implicitly passed to the compiler for the second compilation that -fcompare-debug requests, along with options to silence warnings, and it omits 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 as to contain the ".gk" additional extension during the second compilation, so they don't overwrite the ones generated by the first. 💖

And when this option gets passed to the compiler driver? It causes the first compilation to be skipped, which makes it useful for little other than debugging the compiler proper. 💀

• -gtoggle — turn off generation of debug info, if leaving out this option generates it, or turn it on at level 2 otherwise. The position of this argument in the command line does not matter; it takes effect after all other options are processed, and it does so only once, no matter how many times it's given. This is mainly intended to be used with -fcompare-debug. 👑
• -fvar-tracking-assignments-toggle — toggles -fvar-tracking-assignments, in the same way that -gtoggle toggles -g. Cute, right?
• -Q — makes the compiler print out each function name as it is compiled, and print some statistics about each pass when it finishes.
• -ftime-report — makes the compiler print some statistics about the time consumed by each pass when it finishes.
• -ftime-report-details — record the time consumed by infrastructure parts separately for each pass.
• -fira-verbose=n — control the verbosity of the dump file for the integrated register allocator. The default value is 5. If the value n is greater or equal to 10, the dump output is sent to stderr using the same format as n minus 10.
• -flto-report — prints a report with internal details on the workings of the link-time optimizer. The contents of this report vary from version to version. It is meant to be useful to GCC developers when processing object files in LTO mode (via -flto). Disabled by default.
• -flto-report-wpa — like -flto-report, but only print for the WPA phase of link-time optimization.
• -fmem-report — makes the compiler print some statistics about permanent memory allocation when it finishes.
• -fmem-report-wpa — makes the compiler print some statistics about permanent memory allocation for the WPA phase only.
• -fpre-ipa-mem-report, -fpost-ipa-mem-report — makes the compiler print some statistics about permanent memory allocation before or after interprocedural optimization.
• -fprofile-report — makes the compiler print some statistics about consistency of the (estimated) profile and effect of individual passes.
• -fstack-usage — makes the compiler output stack usage information for the program, on a per-function basis. The filename for the dump is made by appending .su to the auxname. auxname is generated from the name of the output file, if explicitly specified and it is not an executable, otherwise it is the basename of the source file. An entry is made up of three fields:
  • The name of the function.
  • A number of bytes.
  • One or more qualifiers: "static", "dynamic", "bounded".

The qualifier "static" means that the function manipulates the stack statically: a fixed number of bytes are allocated for the frame on function entry and released on function exit; no stack adjustments are otherwise made in the function. The second field is this fixed number of bytes. 🦋

The qualifier "dynamic" means that the function manipulates the stack dynamically: in addition to the static allocation described above, stack adjustments are made in the body of the function, for example to push/pop arguments around function calls. If the qualifier "bounded" is also present, the amount of these adjustments is bounded at compile time and the second field is an upper bound of the total amount of stack used by the function. If it is not present, the amount of these adjustments is not bounded at compile time and the second field only represents the bounded part. 🎀

• -fstats — emit statistics about front-end processing at the end of the compilation. This option is supported only by the C++ front end, and the information is generally only useful to the G++ development team.
• -fdbg-cnt-list — print the name and the counter upper bound for all debug counters.
• -fdbg-cnt=counter-value-list — set the internal debug counter lower and upper bound. counter-value-list is a comma-separated list of name:lower_bound1-upper_bound1 [:lower_bound2-upper_bound2...] tuples which sets the name of the counter and list of closed intervals. The lower_bound is optional and is zero initialized if not set. For example, with -fdbg-cnt=dce:2-4:10-11,tail_call:10, dbg_cnt(dce) returns true only for second, third, fourth, tenth and eleventh invocation. For dbg_cnt(tail_call) true is returned for first 10 invocations.
• -print-file-name=library — print the full absolute name of the library file library that would be used when linking — and don't do anything else. With this option, GCC does not compile or link anything; it just prints the file name.
• -print-multi-directory — print the directory name corresponding to the multilib selected by any other switches present in the command line. This directory is supposed to exist in GCC_EXEC_PREFIX.
• -print-multi-lib — print the mapping from multilib directory names to compiler switches that enable them. The directory name is separated from the switches by ;, and each switch starts with an @ instead of the -, without spaces between multiple switches. This is supposed to ease shell processing.
• -print-multi-os-directory — print the path to OS libraries for the selected multilib, relative to some lib subdirectory. If OS libraries are present in the lib subdirectory and no multilibs are used, this is usually just ., if OS libraries are present in libsuffix sibling directories this prints e.g. ../lib64, ../lib or ../lib32, or if OS libraries are present in lib/subdir subdirectories it prints e.g. amd64, sparcv9 or ev6.
• -print-multiarch — print the path to OS libraries for the selected multiarch, relative to some lib subdirectory.
• -print-prog-name=program — like -print-file-name, but searches for a program such as cpp.
• -print-libgcc-file-name — same as -print-file-name=libgcc.a. This is useful when you use -nostdlib or -nodefaultlibs but you do want to link with libgcc.a. You can do:

gcc -nostdlib <files>... `gcc -print-libgcc-file-name`

• -print-search-dirs — print the name of the configured installation directory and a list of program and library directories gcc searches — and don't do anything else. This is useful when gcc prints the error message installation problem, cannot exec cpp0: No such file or directory. To resolve this you either need to put cpp0 and the other compiler components where gcc expects to find them, or you can set the environment variable GCC_EXEC_PREFIX to the directory where you installed them. Don't forget the trailing /. 🫶
• -print-sysroot — print the target sysroot directory that is used during compilation. This is the target sysroot specified either at configure time or using the --sysroot option, possibly with an extra suffix that depends on compilation options. If no target sysroot is specified, the option prints nothing.
• -print-sysroot-headers-suffix — print the suffix added to the target sysroot when searching for headers, or give an error if the compiler is not configured with such a suffix — and don't do anything else.
• -dumpmachine — print the compiler's target machine (for example, i686-pc-linux-gnu) — and don't do anything else.
• -dumpversion — print the compiler version (for example, 3.0, 6.3.0 or 7) — and don't do anything else. This is the compiler version used in filesystem paths and specs. Depending on how the compiler has been configured it can be just a single number (major version), two numbers separated by a dot (major and minor version) or three numbers separated by dots (major, minor and patchlevel version).
• -dumpfullversion — print the full compiler version — and don't do anything else. The output is always three numbers separated by dots, major, minor and patchlevel version.
• -dumpspecs — print the compiler's built-in specs — and don't do anything else. (This is used when GCC itself is being built.) 💅

---

🦋 Machine-Dependent Options 🦋

Each target machine supported by GCC can have its own options — for example, to allow you to compile for a particular processor variant or ABI, or to control optimizations specific to that machine. By convention, the names of machine-specific options start with -m. 🌈

Some configurations of the compiler also support additional target-specific options, usually for compatibility with other compilers on the same platform.

🌸 AArch64 Options 🌸

These options are defined for AArch64 implementations:

• -mabi=name — generate code for the specified data model. Permissible values are ilp32 for SysV-like data model where int, long int and pointers are 32 bits, and lp64 for SysV-like data model where int is 32 bits, but long int and pointers are 64 bits. ⭐

The default depends on the specific target configuration. And bestie, listen: the LP64 and ILP32 ABIs are not link-compatible 😭 you have to compile your entire program with the same ABI and link against a compatible set of libraries. No mixing. None. Zero. That's the vibe check and you will fail it.

• -mbig-endian — generate big-endian code 🐘 This is the default when GCC is configured for an aarch64_be-*-* target.
• -mgeneral-regs-only — generate code that uses only the general-purpose registers. This stops the compiler from touching floating-point and Advanced SIMD registers, but heads up: it puts no restrictions on the assembler whatsoever. The assembler is doing its own thing, living its own life 💅
• -mlittle-endian — generate little-endian code 🐭 This is the default when GCC is configured for an aarch64-*-* target but not an aarch64_be-*-* target.
• -mcmodel=tiny — generate code for the tiny code model 🧚 The program and its statically defined symbols must be within 1MB of each other. Programs can be statically or dynamically linked.
• -mcmodel=small — generate code for the small code model ✨ The program and its statically defined symbols must be within 4GB of each other. Programs can be statically or dynamically linked. This is the default code model, the main character of code models, no cap.
• -mcmodel=large — generate code for the large code model 💪 It makes no assumptions about addresses and sizes of sections. Programs can be statically linked only. And the plot twist: -mcmodel=large is incompatible with -mabi=ilp32, -fpic and -fPIC. They do not sit together, they do not pass the vibe check, do not try it 😭
• -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 (omitting it) 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 the 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 🐧 iykyk.

• -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 the 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 🐧

bestie yes it's listed twice 💀 that's literally what the page does, we are not fixing it, we keep every crumb.

• -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, so if your binutils is ancient it's giving ✨ expired ✨.
• -mfix-cortex-a53-835769
• -mno-fix-cortex-a53-835769 — enable or disable the workaround for the ARM Cortex-A53 erratum number 835769 🩹 This involves inserting a NOP instruction between memory instructions and 64-bit integer multiply-accumulate instructions.
• -mfix-cortex-a53-843419
• -mno-fix-cortex-a53-843419 — enable or disable the workaround for the ARM Cortex-A53 erratum number 843419 🩹 This erratum workaround is made at link time, and this will only pass the corresponding flag to the linker. So it's lowkey the linker's problem now, not the compiler's 🙈
• -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 the precision of reciprocal square root results to about 16 bits for single precision and to 32 bits for double precision. So like... vibes over accuracy, fr fr.
• -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 the 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 🦋 (yes bestie, the lore connects).
• -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 the precision of division results to about 16 bits for single precision and to 32 bits for double precision.
• -mtrack-speculation
• -mno-track-speculation — enable or disable generation of additional code to track speculative execution through conditional branches 👀 The tracking state can then be used by the compiler when expanding calls to "__builtin_speculation_safe_copy" to permit a more efficient code sequence to be generated. Very spy-thriller of it, honestly.
• -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 using a later revision, e.g. -march=armv8.1-a or -march=armv8-a+lse, the ARMv8.1-Atomics instructions will be used directly. The same applies when using -mcpu= when the selected cpu supports the lse feature. This option is on by default ✨

• -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:

• 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
• armv8.4-a : Armv8.4-A : armv8.3-a, +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

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.

• -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, ares, exynos-m1, emag, falkor, neoverse-e1,neoverse-n1,qdf24xx, saphira, phecda, xgene1, vulcan, octeontx, octeontx81, octeontx83, octeontx2, octeontx2t98, octeontx2t96 octeontx2t93, octeontx2f95, octeontx2f95n, octeontx2f95mm 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 native 🌈 (yes the spacing in that list is chaotic, that's canon, we're not touching it).

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 👑 two brains, one chip.

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.

Where none of -mtune=, -mcpu= or -march= are specified, the code is tuned to perform well across a range of target processors.

This option cannot be suffixed by feature modifiers 🚫 don't even try, she doesn't accept them.

• -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.

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. The specific one you typed last wins, basically. Main character energy ✨

• -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. Like, this is a backstage pass, not a public tour 🎀

• -mverbose-cost-dump — enable verbose cost model dumping in the debug dump files 📊 This option is provided for use in debugging the compiler 🐛
• -mpc-relative-literal-loads, -mno-pc-relative-literal-loads — turns PC-relative literal loads on or off. When it's on, literal pools get grabbed with a single instruction and dropped in right after every function 💫 — which means each function is capped at a max size of 1MB. This one is on by default for -mcmodel=tiny.

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• -msign-return-address=scope — picks which function scope gets return address signing 🎀. The allowed values are none (return address signing off, periodt), non-leaf (pointer signing for functions that aren't leaf functions), and all (pointer signing for literally every function). Default is none. Heads up bestie: this option has been deprecated by -mbranch-protection.
• -mbranch-protection=none|standard|pac-ret[+leaf+b-key]|bti — selects which branch protection features you want 🌈. none is the default and switches off all types of branch protection. standard switches on all types of branch protection features; if a feature has extra tuning options, standard sets it to its standard level. pac-ret[+leaf] turns on return address signing at 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 leaf argument extends the signing to cover leaf functions too. The optional b-key argument signs the functions with the B-key instead of the A-key. And bti turns on the branch target identification mechanism 🔥.
• -msve-vector-bits=bits — says how many bits live in an SVE vector register ✨. This option only does anything 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 lets GCC make assumptions about the vector length when that helps with optimization 🧚. 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 🙈.

The default is -msve-vector-bits=scalable, which produces vector-length agnostic code 💅.

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🌟 -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. 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. On by default for all possible values for options -march and -mcpu.
• simd — enable Advanced SIMD instructions. This also enables floating-point instructions. 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. On by default for -march=armv8.1-a.
• rdma — enable Round Double Multiply Accumulate instructions. 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. 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 it's 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 one 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. 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. Only to enable the extension at the assembler level and does not affect code generation. Enabled by default for -march=armv8.5-a.
• ssbs — enable the Armv8-a Speculative Store Bypass Safe instruction. Only to enable the extension at the assembler level and does not affect code generation. Enabled by default for -march=armv8.5-a.
• predres — enable the Armv8-a Execution and Data Prediction Restriction instructions. Only to enable the extension at the assembler level and does not affect code generation. 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. 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 — enable brain half-precision floating-point instructions. This also enables Advanced SIMD and floating-point instructions. Enabled by default for -march=armv8.6-a. Use of this option with architectures prior to Armv8.2-A is not supported.

Feature crypto implies aes, sha2, and simd, which implies fp. Conversely, nofp implies nosimd, which implies nocrypto, noaes and nosha2. 🦋

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🫶 Adapteva Epiphany Options 🫶

These -m options are defined for Adapteva Epiphany:

• -mhalf-reg-file — don't allocate any register in the range r32...r63. That lets code run on hardware variants that lack these registers.
• -mprefer-short-insn-regs — preferentially allocate registers that allow short instruction generation. This can result in an increased instruction count, so it may either reduce or increase overall code size 📈📉.
• -mbranch-cost=num — set the cost of branches to roughly num "simple" instructions. This cost is only a heuristic and is not guaranteed to produce consistent results across releases.
• -mcmove — enable the generation of conditional moves.
• -mnops=num — emit num NOPs before every other generated instruction.
• -mno-soft-cmpsf — for single-precision floating-point comparisons, emit an fsub instruction and test the flags. This is faster than a software comparison, but 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.
• -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. Also note 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 whether a different stack offset would give you better code, but to actually use a different stack offset to build working programs, it's recommended to configure the toolchain with the appropriate --with-stack-offset=num option.
• -mno-round-nearest — make the scheduler assume that the rounding mode has been set to truncating. The default is -mround-nearest.
• -mlong-calls — if not otherwise specified by an attribute, assume all calls might be beyond the offset range of the b / bl instructions, and therefore load the function address into a register before performing an (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 — sets the whole vibe of the floating-point unit, bestie. Like, it picks which FP mode is given and expected when a function gets called and when it bounces back. If you line this up with whatever mode you mostly need right at function start, your program gets smaller and faster because it's not doing unnecessary mode switches. periodt.

mode can be any one of these:

• caller — whatever mode the function was entered with is valid, and it's kept or restored when the function returns, and also when it calls other functions. Use this for compiling libraries or other compilation units you might wanna drop into different programs with different prevailing FPU modes — the convenience of one single object file outweighs the size and speed overhead of any extra mode switching that might be needed, compared to what a more specific prevailing FPU mode choice would need. 💅
• truncate — the mode used for floating-point calculations with truncating (i.e. round towards zero) rounding mode. That includes conversion from floating point to integer.
• round-nearest — the mode used for floating-point calculations with round-to-nearest-or-even rounding mode.
• int — the mode used to do integer calculations in the FPU, e.g. integer multiply, or integer multiply-and-accumulate.

The default is -mfp-mode=caller.

• -mno-split-lohi, -mno-postinc, -mno-postmodify — code generation tweaks that disable, respectively: splitting of 32-bit loads, generation of post-increment addresses, and generation of post-modify addresses. The defaults are msplit-lohi, -mpost-inc, and -mpost-modify.
• -mnovect-double — change the preferred SIMD mode to SImode. The default is -mvect-double, which uses DImode as the preferred SIMD mode.
• -max-vect-align=num — the maximum alignment for SIMD vector mode types. num may be 4 or 8. The default is 8. Heads up though: this is an ABI change, even though a bunch of library function interfaces are unaffected if they don't use SIMD vector modes anywhere that affects the size and/or alignment of the relevant types. 😭
• -msplit-vecmove-early — split vector moves into single word moves before reload. In theory this can give better register allocation, but so far the reverse seems to be generally the case. 🙈
• -m1reg-reg — specify a register to hold the constant -1, which makes loading small negative constants and certain bitmasks faster. Allowable values for reg are r43 and r63, which specify use of that register as a fixed register, and none, which means no register is used for this purpose. The default is -m1reg-none.

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AMD GCN Options 🎀

These options are defined specifically for the AMD GCN port.

• -march=gpu, -mtune=gpu — set the architecture type or the tuning for gpu. The 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).
• -mstack-size=bytes — specify how many bytes of stack space get requested for each GPU thread (wave-front). Beware bestie, there may be many threads and only limited memory available. The size of the stack allocation can also affect run-time performance. The default is 32KB when using OpenACC or OpenMP, and 1MB otherwise.

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ARC Options 🦋

The following options control the architecture variant that code is being compiled for:

• -mbarrel-shifter — generate instructions supported by barrel shifter. This is the default unless -mcpu=ARC601 or -mcpu=ARCEM is in effect.
• -mjli-always — force calling a function using jli_s instruction. This option is valid only for the ARCv2 architecture.
• -mcpu=cpu — set the architecture type, register usage, and instruction scheduling parameters for cpu. There are also shortcut alias options available for backward compatibility and convenience. The 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 — do not generate "mpy"-family instructions for ARC700. This option is deprecated. 👀
• -mmul32x16 — generate 32x16-bit multiply and multiply-accumulate instructions.
• -mmul64 — generate "mul64" and "mulu64" instructions. Only valid for -mcpu=ARC600.
• -mnorm — generate "norm" instructions. This is the default if -mcpu=ARC700 is in effect.
• -mspfp, -mspfp-compact — generate single-precision FPX instructions, tuned for the compact implementation.
• -mspfp-fast — generate single-precision FPX instructions, tuned for the fast implementation.
• -msimd — enable generation of ARC SIMD instructions via target-specific builtins. Only valid for -mcpu=ARC700.
• -msoft-float — this option is ignored; it's provided for compatibility purposes only. Software floating-point code is emitted by default, and this default can be overridden by the FPX options: -mspfp, -mspfp-compact, or -mspfp-fast for single precision, and -mdpfp, -mdpfp-compact, or -mdpfp-fast for double precision.
• -mswap — generate "swap" instructions.
• -matomic — 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 the 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. The 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 — turns on double-precision floating-point hardware extensions that work via double-precision assist instructions ✨ and, while we're at it, the single-precision floating-point, square-root and divide extensions get enabled too. Only available for ARC EM, bestie.
• fpuda_fma — same double-precision-assist deal 💅, but this time the single-precision floating-point and fused multiply-and-add hardware extensions come along for the ride. Still ARC EM only.
• fpuda_all — the whole double-precision-assist buffet 🍓: all single-precision floating-point hardware extensions are enabled as well. And yep, ARC EM only.
• fpus_div — enables single-precision floating-point, square-root and divide hardware extensions. That's it, that's the vibe 🎀.
• fpud_div — enables double-precision floating-point, square-root and divide hardware extensions, and it includes fpus_div too. Not available for ARC EM.
• fpus_fma — enables single-precision floating-point and fused multiply-and-add hardware extensions ✨.
• fpud_fma — double-precision floating-point and fused multiply-and-add hardware extensions, and it includes 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 — tells the compiler which general-purpose registers your processor just automatically saves and restores on interrupt entry and exit, no cap 🙈. register-range is written as two registers with a dash between them. The range always starts with r0, and the upper limit is the fp register. blink and lp_count are optional. This one is only valid for ARC EM and ARC HS cores.
• -mrgf-banked-regs=number — says how many registers get replicated into a second register bank when a fast interrupt hits. Fast interrupts are the ones with the highest priority level, P0 ⚡. They only save the PC and STATUS32 registers, so no memory transactions happen during the interrupt entry and exit sequences. Reach for this option when you're using fast interrupts on an ARC V2 family processor. Allowed values: 4, 8, 16, and 32.
• -mlpc-width=width — sets the width of the lp_count register. Valid width values are 8, 16, 20, 24, 28 and 32 bits, and the default is locked in at 32 bits. If the width is under 32, the compiler won't try to transform your loops into the zero-delay loop mechanism unless it knows the lp_count register can hold the loop-counter value you need. Depending on what width you picked, the compiler and the run-time library might still use the loop mechanism for other stuff. This option defines the macro __ARC_LPC_WIDTH__ with the value of width 💜.
• -mrf16 — tells the compiler to generate code for a 16-entry register file. It defines the __ARC_RF16__ preprocessor macro 🎀.
• -mbranch-index — enables use of bi or bih instructions to implement jump tables 🦋.

---

The following options are passed through to the assembler, and they also define preprocessor macro symbols, iykyk.

• -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 — annotates assembler instructions with estimated addresses 🎀.
• -mannotate-align — explains what alignment considerations led to the decision to make an instruction short or long. (The compiler is basically narrating its own choices, we love a self-aware icon 💅.)

---

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 one is enabled by default in tool chains built for arc-linux-uclibc and arceb-linux-uclibc targets when profiling is requested 🦋.

---

The following options control the semantics of generated code:

• -mlong-calls — generates calls as register indirect calls, which gives you access to the full 32-bit address range 🌈.
• -mmedium-calls — don't use less than a 25-bit addressing range for calls, which is the offset available for an unconditional branch-and-link instruction. Conditional execution of function calls gets suppressed so the 25-bit range can be used instead of the 21-bit range with conditional branch-and-link. This is the default for tool chains built for arc-linux-uclibc and arceb-linux-uclibc targets.
• -G num — puts definitions of externally-visible data into a small data section if that data is no bigger than num bytes. The default value of num is 4 for any ARC configuration, or 8 when we have double load/store operations 🍓.
• -mno-sdata — don't generate sdata references. This is the default for tool chains built for arc-linux-uclibc and arceb-linux-uclibc targets.
• -mvolatile-cache — uses ordinarily cached memory accesses for volatile references. This is the default.
• -mno-volatile-cache — enables cache bypass for volatile references 👀.

---

The following options fine tune code generation:

• -malign-call — does alignment optimizations for call instructions.
• -mauto-modify-reg — enables the use of pre/post modify with register displacement ✨.
• -mbbit-peephole — enables bbit peephole2.
• -mno-brcc — disables a target-specific pass in arc_reorg that generates compare-and-branch (brcc) instructions. It has no effect on generation of these instructions driven by the combiner pass.
• -mcase-vector-pcrel — uses PC-relative switch case tables to enable case table shortening. This is the default for -Os.
• -mcompact-casesi — enables the compact casesi pattern. This is the default for -Os, and it's only available for ARCv1 cores. This option is deprecated.
• -mno-cond-exec — disables the ARCompact-specific pass that generates conditional execution instructions.

Okay so here's the lore 📖: because of delay slot scheduling and the interactions between operand numbers, literal sizes, instruction lengths, and the support for conditional execution, the target-independent pass that generates conditional execution is often lacking. So the ARC port keeps a special pass around that tries to find more conditional execution generation opportunities after register allocation, branch shortening, and delay slot scheduling have already been done. This pass generally — but not always — improves performance and code size, at the cost of extra compilation time, which is literally why there's an option to switch it off. If you have a problem with call instructions exceeding their allowable offset range because they're conditionalized, you should consider using -mmedium-calls instead 💅.

• -mearly-cbranchsi — enables pre-reload use of the cbranchsi pattern.
• -mexpand-adddi — expands adddi3 and subdi3 at RTL generation time into add.f, adc etc. This option is deprecated.
• -mindexed-loads — enables the use of indexed loads. Heads up, this can be problematic, because some optimizers then assume that indexed stores exist, which is not the case 😭.
• -mlra — enables 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 a call to a special function in libgcc; this is referred to as a millicode call. Since these calls can pose performance issues, and/or cause linking issues when linking in a nonstandard way, this option exists to turn millicode call generation on or off 🎀.
• -mcode-density-frame — lets the compiler emit enter and leave instructions. These instructions are only valid for CPUs with the code-density feature 👑.
• -mmixed-code — tweaks register allocation to help 16-bit instruction generation. Generally this decreases the average instruction size while increasing the instruction count. (Smaller but more, it's giving quantity-over-size 💀.)
• -mq-class — this option is deprecated. Enables q instruction alternatives. This is the default for -Os.
• -mRcq — turns on Rcq constraint handling. Most short code generation depends on this, so like… yeah, she matters. And she's the default, bestie. Slay. 💅
• -mRcw — enables Rcw constraint handling. Most ccfsm condexec mostly depends on this. Also the default. We love a low-effort icon. ✨
• -msize-level=level — fine-tunes size optimization when it comes 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 — on top of that, alignment of loops and of code after barriers are dropped.
  • 3 — on top of that, optional data alignment is dropped, and the option Os is enabled.

This defaults to 3 when -Os is in effect. Otherwise, if you don't set it at all, the behavior is equivalent to level 1. 🎀

• -mtune=cpu — sets the instruction scheduling parameters for cpu, overriding anything 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 — the cost to assume for a multiply instruction, where 4 is equal to a normal instruction.
• -munalign-prob-threshold=probability — sets the probability threshold for unaligning branches. When you're tuning for ARC700 and optimizing for speed, branches without a filled delay slot are preferably emitted unaligned and long, unless profiling indicates that the probability of the branch being taken is below probability. The default is (REG_BR_PROB_BASE/2), i.e. 5000.

The following options are kept around for backward compatibility, but they're deprecated now and will be removed in a future release. So like, don't get attached. 😭

• -margonaut — obsolete FPX.
• -mbig-endian, -EB — compile code for big-endian targets. Using these is deprecated now. 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. Using these is deprecated now. 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 — the 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. The permissible values are: apcs-gnu, atpcs, aapcs, aapcs-linux and iwmmxt.
• -mapcs-frame — generate a stack frame that's compliant with the ARM Procedure Call Standard for all functions, even when it's not strictly necessary for the code to execute correctly. If you specify -fomit-frame-pointer alongside this option, the stack frames won't be generated for leaf functions. The default is -mno-apcs-frame. This option is deprecated. 💀
• -mapcs — this is just a synonym for -mapcs-frame, and it's deprecated too.
• -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 can't be reliably used inside one program. The default is -mno-thumb-interwork, since slightly larger code gets generated when -mthumb-interwork is specified. In AAPCS configurations this option is meaningless. 🙈
• -mno-sched-prolog — stops instructions in the function prologue from being reordered, and stops those instructions from being merged with instructions in the function's body. What that means: every function starts with a recognizable set of instructions (or actually one of a choice from a small set of different function prologues), and that info 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. The permissible values are: soft, softfp and hard.
  • soft makes GCC 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 the generation of floating-point instructions and uses FPU-specific calling conventions.

The default depends on the specific target configuration. Heads up: the hard-float and soft-float ABIs are not link-compatible — you must compile your entire program with the same ABI, and link with a compatible set of libraries. No mixing, bestie. 🫶

• -mgeneral-regs-only — generate code that uses only the general-purpose registers. This stops the compiler from using floating-point and Advanced SIMD registers, but it 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, these select between the BE8 and BE32 formats. The option has no effect for little-endian images and is ignored. The default depends 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 figure out what kind of instructions it can emit when generating assembly code. You can use this option in conjunction with, or instead of, the -mcpu= option.

The permissible names are: armv4t, armv5t, armv5te, armv6, armv6j, armv6k, armv6kz, armv6t2, armv6z, armv6zk, armv7, armv7-a, armv7ve, armv8-a, armv8.1-a, armv8.2-a, armv8.3-a, armv8.4-a, armv8.5-a, armv8.6-a, 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, the following architectures, which lack support for the Thumb execution state, are recognized but their support is deprecated: armv4.

A lot of these architectures support extensions. You add them by appending +extension to the architecture name. Extension options are processed in order and capabilities accumulate. An extension will also enable any necessary base extensions it depends on. For example, the +crypto extension will always enable the +simd extension. The exception to this additive construction is for extensions prefixed with +no...: those extensions disable the specified option and any other extensions that might depend on that extension being present.

For example, -march=armv7-a+simd+nofp+vfpv4 is equivalent to writing -march=armv7-a+vfpv4, since the +simd option is entirely disabled by the +nofp option that follows it.

Most extension names are generically named, but their effect depends on the architecture you apply them to. For example, the +simd option can be applied to both armv7-a and armv8-a architectures, but it enables the original ARMv7-A Advanced SIMD (Neon) extensions for armv7-a and the ARMv8-A variant for armv8-a.

The list below lays out the supported extensions for each architecture. Architectures that aren't mentioned don't 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 not supported by the base ARMv7-M architecture, but is compatible with both the ARMv7-A and ARMv7-R architectures.
• +nofp — disable the floating-point instructions.

armv7-a

• +mp — the multiprocessing extension.
• +sec — the security extension.
• +fp — the VFPv3 floating-point instructions, with 16 double-precision registers. The extension +vfpv3-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 + VFPv3 floating-point instructions, but make it half-precision era: it brings the half-precision floating-point conversion operations too. 💖
• +neon-vfpv4 — the Advanced SIMD (Neon) v2 + VFPv4 floating-point instructions. newer, fancier, still iconic.
• +nosimd — turns the Advanced SIMD instructions OFF. heads up bestie, it does not disable floating point. 🙈
• +nofp — disables the floating-point and the Advanced SIMD instructions. both. gone. 💀

armv7ve

okay so this one is the extended version of the ARMv7-A architecture, and it comes with support for virtualization. periodt.

• +fp — the VFPv4 floating-point instructions, with 16 double-precision registers. btw you can use the extension +vfpv4-d16 as an alias for this one. 🎀
• +simd — the Advanced SIMD (Neon) v2 and the VFPv4 floating-point instructions. and yup, +neon-vfpv4 works as an alias for this. ✨
• +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 plus the half-precision floating-point conversion operations. 🦋
• +vfpv3-fp16 — the VFPv3 floating-point instructions, with 32 double-precision registers plus 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. also, +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 — disables the Advanced SIMD instructions (and nope, it does not disable floating point).
• +nofp — disables the floating-point and Advanced SIMD instructions.

armv8-a

• +crc — the Cyclic Redundancy Check (CRC) instructions. slay.
• +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 one 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

(these two share the exact same extension list, so bestie, same energy for both 💖)

• +fp16 — the half-precision floating-point data processing instructions. this also enables the Advanced SIMD and floating-point instructions.
• +fp16fml — the half-precision floating-point fmla extension. this also enables the half-precision floating-point extension and the 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. that's a lot of enabling, and honestly? she ate.
• +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 as well as the Dot Product extension.
• +crypto — the cryptographic instructions. this also enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension.
• +nocrypto — disable the cryptographic extension.
• +nofp — disable the floating-point, Advanced SIMD and cryptographic instructions.
• +i8mm — 8-bit Integer Matrix Multiply instructions. this also enables Advanced SIMD and floating-point instructions.
• +bf16 — Brain half-precision floating-point instructions. this also enables Advanced SIMD and floating-point instructions. 🦋

armv8.6-a

• +fp16 — the half-precision floating-point data processing instructions. this also enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension and the half-precision floating-point fmla extension.
• +simd — the ARMv8.3-A Advanced SIMD and floating-point instructions as well as the Dot Product extension.
• +crypto — the cryptographic instructions. this also enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension.
• +nocrypto — disable the cryptographic extension.
• +nofp — disable the floating-point, Advanced SIMD and cryptographic instructions.
• +i8mm — 8-bit Integer Matrix Multiply instructions. this also enables Advanced SIMD and floating-point instructions.
• +bf16 — Brain half-precision floating-point instructions. this also enables Advanced SIMD and floating-point instructions. 💜

armv7-r

• +fp.sp — the single-precision VFPv3 floating-point instructions. the extension +vfpv3xd can be used as an alias for this extension. 🎀
• +fp — the VFPv3 floating-point instructions with 16 double-precision registers. the extension +vfpv3-d16 can be used as an alias for this extension.
• +vfpv3xd-d16-fp16 — the single-precision VFPv3 floating-point instructions with 16 double-precision registers and the half-precision floating-point conversion operations.
• +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 — turns on the Custom Datapath Extension (CDE) on whichever coprocessors you name, using the numbers in the range 0 to 7. 🎀

armv8-m.main

• +dsp — the DSP instructions. ✨
• +nodsp — nope, turn the DSP extension off. 🙈
• +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 the coprocessors you pick, by the numbers, 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. all of them. gone. 💀

-march=native makes the compiler auto-detect the architecture of the machine it's being built on. heads up bestie: right now this only works on GNU/Linux, and not every architecture is even recognized. if the auto-detect flops, the option just does nothing. no error, no drama, it's giving nothing. 🫶

---

• -mtune=name — this one picks the name of the target ARM processor that GCC should tune its code performance for. for some ARM implementations you get better performance just by setting this. the 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, 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-m1.small-multiply, cortex-m0.small-multiply, cortex-m0plus.small-multiply, exynos-m1, marvell-pj4, neoverse-n1, xscale, iwmmxt, iwmmxt2, ep9312, fa526, fa626, fa606te, fa626te, fmp626, fa726te, xgene1. that's a lot of names, i know, but they're all real. 👑

bonus round: this option can also tell GCC to tune the code for a big.LITTLE system. the permissible names for that 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 for a whole blend of processors inside architecture arch. the goal is code that runs well on the most popular processors of the moment, balancing the optimizations that help some CPUs in the range against avoiding the performance pitfalls of the others. lowkey, the effects of this option can change in future GCC versions, because CPU models come and go. 🌈

-mtune accepts the exact same extension options as -mcpu — but here's the thing, those extension options do not affect the tuning of the generated code. they're just vibing.

-mtune=native makes the compiler auto-detect the CPU of the build computer. same deal as before: right now it's GNU/Linux only, not every architecture is recognized, and if the auto-detect fails the option does absolutely nothing. ✨

---

• -mcpu=name[+extension...] — this specifies the name of the target ARM processor. GCC takes that name and derives two things from it: the name of the target ARM architecture (like as if you'd written -march), and the ARM processor type to tune for performance (like as if you'd written -mtune). and if you use this together with -march or -mtune, those explicit options win — they take precedence over the matching part of this one. 😌

lots of the supported CPUs implement optional architectural extensions. where that's the case, those architectural extensions are normally enabled by default. but if there are implementations out there that lack the extension, you can use the extension syntax to switch off the ones that got omitted. for floating-point and Advanced SIMD (Neon) instructions you also have to factor in the settings of -mfloat-abi and -mfpu: floating-point and Advanced SIMD instructions only get used if -mfloat-abi is not set to soft, and any -mfpu setting other than auto overrides the available floating-point and SIMD extension instructions. it's giving layers. 🧅

for example, cortex-a9 shows up 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 disable just the SIMD, or both the SIMD and the floating-point instructions respectively. slay. 💅

the permissible names for this option are the same as the ones for -mtune.

these 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.
• +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 the floating-point) instructions on generic-armv7-a, cortex-a5, cortex-a7 and cortex-a9.
• +crypto — enables the cryptographic instructions on cortex-a32, cortex-a35, cortex-a53, cortex-a55, cortex-a57, cortex-a72, cortex-a73, cortex-a75, exynos-m1, xgene1, cortex-a57.cortex-a53, cortex-a72.cortex-a53, cortex-a73.cortex-a35, cortex-a73.cortex-a53 and cortex-a75.cortex-a55. 🔐

also, the generic-armv7-a pseudo target defaults to VFPv3 with 16 double-precision registers. it supports these extension options: mp, sec, vfpv3-d16, vfpv3, vfpv3-d16-fp16, vfpv3-fp16, vfpv4-d16, vfpv4, neon, neon-vfpv3, neon-fp16, neon-vfpv4. the meanings are the same as for the extensions to -march=armv7-a.

-mcpu=generic-arch is also permissible, and it's equivalent to -march=arch -mtune=generic-arch. check out -mtune above for the details. 🌸

-mcpu=native makes the compiler auto-detect the CPU of the build computer. at present this feature is only supported on GNU/Linux, and not all architectures are recognized. if the auto-detect is unsuccessful the option has no effect. 🙈

---

• -mfpu=name — specifies what floating-point hardware (or hardware emulation) is available on the target. the 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. fyi neon is an alias for neon-vfpv3, and vfp is an alias for vfpv2. 👑

the setting auto is the default and it's special — it makes the compiler pick the floating-point and Advanced SIMD instructions based on your -mcpu and -march settings.

if the floating-point hardware you selected includes the NEON extension (like -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 using NEON instructions may lead to a loss of precision. it's giving risky. ⚠️

you can also set the fpu name at function level using the target("fpu=") function attributes or pragmas. cute. 🎀

---

• -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, and in that case the __fp16 type simply isn't defined at all. 💜

---

• -mstructure-size-boundary=n — the sizes of all structures and unions get 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 between 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 it 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. so like, pick one and commit, bestie. 💅

this option is deprecated. 🚫

---

• -mabort-on-noreturn — generate a call to the function abort at the end of a noreturn function. it gets executed if the function tries to return anyway. 😭

---

• -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 that register. you need this switch if the target function lies outside of the 64-megabyte addressing range of the offset-based version of the subroutine call instruction. so, far away function? this is your girl. 🦋

Even if this switch is on, not every single function call becomes a long call, bestie. The heuristic is: static functions, functions carrying the "short_call" attribute, functions inside the scope of a "#pragma no_long_calls" directive, and functions whose definitions have already been compiled inside the current compilation unit — those stay short. The exceptions (so the ones that always go long) are weak function definitions, functions with the "long_call" attribute or the "section" attribute, and functions inside the scope of a "#pragma long_calls" directive.

This feature is not enabled by default, okurrr. Specifying -mno-long-calls restores the default behavior, and so does wrapping your function calls in the scope of a "#pragma long_calls_off" directive. Note that these switches do literally nothing about how the compiler generates code for calling functions via function pointers — that's a whole separate vibe. 🦋

• -msingle-pic-base — treat the register used for PIC addressing as read-only, instead of loading it up in the prologue of every single function. The runtime system is the one responsible for initializing this register with an appropriate value before execution begins. Teamwork, fr fr. 🫶
• *-mpic-register=reg* — name the register used for PIC addressing. For the standard PIC base case, the default is any suitable register determined by the compiler (she chooses). For the single PIC base case, the default is R9 if the target is EABI based or stack-checking is enabled; otherwise the default is R10. 👑
• -mpic-data-is-text-relative — assume the displacement between the text and data segments is fixed at static link time. That lets the compiler use PC-relative addressing operations to reach data it knows lives in the data segment. For non-VxWorks RTP targets, this option is enabled by default. When you disable it on those targets, it will enable -msingle-pic-base by default. Sneaky little chain reaction. ✨
• -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 there's a function name embedded immediately preceding this location, and it has length "((pc[-3]) & 0xff000000)". Detective work but make it assembly. 🔍

• -mthumb, -marm — pick between generating code that executes in ARM state and code that executes in Thumb state. For most configurations the default is code that executes in ARM state, but you can change that default by configuring GCC with the --with-mode=state configure option.

You can also override the ARM/Thumb mode for each individual function using the "target("thumb")" and "target("arm")" function attributes, or pragmas. Per-function glow-ups, we love to see it. 💅

• -mflip-thumb — switch ARM/Thumb modes on alternating functions. This option exists purely for regression testing of mixed Thumb/ARM code generation and is not intended for ordinary use in compiling code. It's a lab experiment, not a lifestyle. 🧪
• -mtpcs-frame — generate a stack frame 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 compliant with the Thumb Procedure Call Standard for all leaf functions. (Again: a leaf function is one that doesn't call any other functions.) The default is -mno-apcs-leaf-frame. 😭 (yes, the original really does pair a tpcs option with an apcs default — noted, not fixing it, not my business)
• -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. That way these functions can be called from non-interworking code. This option is not valid in AAPCS configurations, because interworking is already enabled by default there. 🚫
• -mcaller-super-interworking — lets calls via function pointers (including virtual functions) execute correctly no matter whether the target code was compiled for interworking or not. There's a small overhead in the cost of executing a function pointer if this option is enabled. This option is also not valid in AAPCS configurations, because interworking is enabled by default. Same energy, same restriction. ⭐
• *-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. Let her pick, she's got taste. 🌸
• *-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 gives better performance for shared libraries. The GNU descriptor scheme is compatible with the original scheme, but it does require new assembler, linker and library support. Initial and local exec TLS models are unaffected by this option and always use the original scheme. 📚
• -mword-relocations — only 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 whenever -fpic or -fPIC is specified. This option conflicts with -mslow-flash-data. ⚠️
• -mfix-cortex-m3-ldrd — some Cortex-M3 cores can cause data corruption when "ldrd" instructions with overlapping destination and base registers are used. This option avoids generating those instructions. It's enabled by default when -mcpu=cortex-m3 is specified. Crisis averted, you're welcome. 💀
• -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 isn't enabled, then words in packed data structures are accessed a byte at a time.

The ARM attribute "Tag_CPU_unaligned_access" is set in the generated object file to either true or false, depending on the setting of this option. If unaligned access is enabled, then the preprocessor symbol "__ARM_FEATURE_UNALIGNED" is also defined. 🏷️

• -mneon-for-64bits — this option is deprecated and has no effect. It's giving ghost. 👻
• -mslow-flash-data — assume loading data from flash is slower than fetching instructions. So literal loads get minimized for better performance. This option is only supported when compiling for ARMv7 M-profile and is off by default. It conflicts with -mword-relocations. 🔥
• -masm-syntax-unified — 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 — however, this may change in future releases of GCC. Divided syntax should be considered deprecated. 👀
• -mrestrict-it — restricts generation of IT blocks to conform to the rules of ARMv8-A. IT blocks can only contain a single 16-bit instruction from a select set of instructions. This option is on by default for ARMv8-A Thumb mode. 🎀
• -mprint-tune-info — print CPU tuning information as a comment in the assembler file. 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. 🧪
• -mverbose-cost-dump — enable 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. Additionally, 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 can be found at <http://infocenter.arm.com/help/topic/com.arm.doc.ecm0359818/ECM0359818_armv8m_security_extensions_reqs_on_dev_tools_1_0.pdf>. Secure girl summer. 🛡️
• -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.

Note that static linking is not supported, because it would still involve the dynamic linker when the program self-relocates. If that behavior is acceptable to you, 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:

• *-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@tie{}KiB of program memory. mcu@tie{}= "attiny22", "attiny26", "at90s2313", "at90s2323", "at90s2333", "at90s2343", "at90s4414", "at90s4433", "at90s4434", "at90c8534", "at90s8515", "at90s8535". 💖

"avr25" 🎀

Bestie, these are the "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" 🦋

The "Classic" girlies with 16 KiB up to 64 KiB of program memory. mcu = "at76c711", "at43usb355".

"avr31" ✨

"Classic" devices with 128 KiB of program memory, living large. 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" 🌸

The "Enhanced" era 💖 — 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. This list is enormous, so buckle up, bestie — 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 a 3-byte PC — meaning more than 128 KiB of program memory, okay, that's main character energy fr fr. 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 (she's transparent, she's showing off 😌). 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 — she has range 💅. mcu = "atxmega64a1", "atxmega64a1u".

"avrxmega6" 😭

"XMEGA" devices with more than 128 KiB of program memory. mcu = "atxmega128a3", "atxmega128a3u", "atxmega128b1", "atxmega128b3", "atxmega128c3", "atxmega128d3", "atxmega128d4", "atxmega192a3", "atxmega192a3u", "atxmega192c3", "atxmega192d3", "atxmega256a3", "atxmega256a3b", "atxmega256a3bu", "atxmega256a3u", "atxmega256c3", "atxmega256d3", "atxmega384c3", "atxmega384d3".

"avrxmega7" 🫶

"XMEGA" devices with more than 128 KiB of program memory and more than 64 KiB of RAM. mcu = "atxmega128a1", "atxmega128a1u", "atxmega128a4u".

"avrtiny" 🐣

"TINY" Tiny core devices with 512 B up to 4 KiB of program memory. So small, so cute, so valid. mcu = "attiny4", "attiny5", "attiny9", "attiny10", "attiny20", "attiny40".

"avr1" 🧍

This ISA is implemented by the minimal AVR core and supported for assembler only — no compiler vibes here, bestie. mcu = "attiny11", "attiny12", "attiny15", "attiny28", "at90s1200".

---

-mabsdata 🌟

Assume that all data in static storage can be accessed by LDS / STS instructions. Heads up: 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 function prologue/epilogue. Without this option, outgoing arguments get pushed before calling a function and popped afterwards.

Popping the arguments after the function call can be expensive on AVR, so accumulating the stack space might lead to smaller executables, because arguments need not be removed from the stack after such a function call.

This option can lead to reduced code size for functions that perform several calls to functions that get their arguments on the stack — like calls to printf-like functions.

-mbranch-cost=cost 💸

Set the branch costs for conditional branch instructions to cost. Reasonable values for cost are small, non-negative integers. The default branch cost is 0.

-mcall-prologues 🎀

Function prologues/epilogues are expanded as calls to appropriate subroutines. Code size is smaller. Periodt.

-mdouble=bits

-mlong-double=bits 🔢

Set the size (in bits) of the double or long double type, respectively. Possible values for bits are 32 and 64. Whether or not a specific value for bits is allowed depends on the --with-double= and --with-long-double= configure options (https://gcc.gnu.org/install/configure.html#avr), and the same applies for the default values of the options.

-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 no_gccisr function attribute (AVR Function Attributes). This feature is activated per default if optimization is on — but not with -Og (see the Optimize Options section) — and if GNU Binutils support PR21683 (https://sourceware.org/PR21683).

-mint8 🎱

Assume int to be 8-bit integer. This affects the sizes of all types: a char is 1 byte, an int is 1 byte, a long is 2 bytes, and long long is 4 bytes. Please note that this option does not conform to the C standards, but it results in smaller code size. Trade-offs, bestie, trade-offs.

-mmain-is-OS_task 🫡

Do not save registers in main. The effect is the same like attaching attribute OS_task (AVR Function Attributes) 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. She's small but she's not safe, choose wisely.

-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 either — the toolchain's got you 🫶.

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 then the compiler knows if the device or architecture has an 8-bit stack pointer and thus no "SPH" register or not. iykyk 💅

• -mstrict-X — use the address register "X" in a way proposed by the hardware. Meaning: "X" only gets used in indirect, post-increment or pre-decrement addressing. That's it, that's the whole personality.

Without this option, "X" may be used in the same way as "Y" or "Z", which then gets emulated by extra instructions. For example, loading a value with "X+const" addressing with a small non-negative "const < 64" into a register Rn is performed as:

adiw r26, const   ; X += const
ld   <Rn>, X        ; <Rn> = *X
sbiw r26, const   ; X -= const

• -mtiny-stack — only change the lower 8@tie{}bits of the stack pointer. baby stack energy 🎀
• -mfract-convert-truncate — allow truncation instead of rounding towards zero for fractional fixed-point types. picky picky
• -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. Which means: you have to supply your very own private device specs file via -specs=path-to-specs-file. And there's no more need for the option -mmcu=mcu. 💅

This option can also serve as a replacement for the older way of specifying custom device-specs files 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. nostalgia era fr fr 🦋

• -Waddr-space-convert — warn about conversions between address spaces when the resulting address space is not contained in the incoming address space. caught in 4k 💀
• -Wmisspelled-isr — warn if the ISR is misspelled, i.e. without the __vector prefix. Enabled by default, so it's already looking over your shoulder bestie.

---

🦋 "EIND" and Devices with More Than 128 Ki Bytes of Flash 🦋

Pointers in this implementation are 16@tie{}bits wide. The address of a function or label is represented as a word address, so that indirect jumps and calls can target any code address in the range of 64@tie{}Ki words.

In order to facilitate indirect jump on devices with more than 128@tie{}Ki bytes of program memory space, there's a special function register called "EIND" that serves as the most significant part of the target address when "EICALL" or "EIJMP" instructions are used. she's the main character of this section honestly 👑

Indirect jumps and calls on these devices are handled as follows by the compiler, and they're subject to some limitations:

• The compiler never sets "EIND".
• The compiler uses "EIND" implicitly in "EICALL"/"EIJMP" instructions, or might read "EIND" directly in order to emulate an indirect call/jump by means of a "RET" instruction.
• The compiler assumes that "EIND" never changes during the startup code or during the application. In particular, "EIND" is not saved/restored in function or interrupt service routine prologue/epilogue.
• For indirect calls to functions and computed goto, the linker generates stubs. Stubs are jump pads, sometimes also called trampolines. So the indirect call/jump jumps to such a stub. The stub contains a direct jump to the desired address. 🎀
• Linker relaxation must be turned on so that the linker generates the stubs correctly in all situations. See the compiler option -mrelax and the linker option --relax. There are corner cases where the linker is supposed to generate stubs but aborts without relaxation and without a helpful error message. the audacity 😭
• 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 starts 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. so iconic of it tbh

#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. pick your fighter 💅

---

🌈 Handling of the "RAMPD", "RAMPX", "RAMPY" and "RAMPZ" Special Function Registers 🌈

Some AVR devices support memories larger than the 64@tie{}KiB range that can be accessed with 16-bit pointers. To access memory locations outside this 64@tie{}KiB range, the content of a "RAMP" register is used as the high part of the address: the "X", "Y", "Z" address register is concatenated with the "RAMPX", "RAMPY", "RAMPZ" special function register, respectively, to get a wide address. Similarly, "RAMPD" is used together with direct addressing. teamwork makes the dream work 🫶

• The startup code initializes the "RAMP" special function registers with zero.
• If an AVR Named Address Spaces, named address space other than generic or "__flash" is used, then "RAMPZ" is set as needed before the operation.
• If the device supports RAM larger than 64@tie{}KiB and the compiler needs to change "RAMPZ" to accomplish an operation, "RAMPZ" is reset to zero after the operation. so polite of it ✨
• If the device comes with a specific "RAMP" register, the ISR prologue/epilogue saves/restores that SFR and initializes it with zero in case the ISR code might (implicitly) use it.
• RAM larger than 64@tie{}KiB is not supported by GCC for AVR targets. If you use inline assembler to read from locations outside the 16-bit address range and change one of the "RAMP" registers, you must reset it to zero after the access. that's on you bestie, no cap 💀

---

💜 AVR Built-in Macros 💜

GCC defines several built-in macros so that user code can test for the presence or absence of features. Almost any of the following built-in macros are deduced from device capabilities and thus triggered by the -mmcu= command-line option.

For even more AVR-specific built-in macros see AVR Named Address Spaces and AVR Built-in Functions.

• "__AVR_ARCH__" — a build-in macro that resolves to a decimal number that identifies the architecture and depends on the -mmcu=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. case sensitivity girlies rise

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. slay 💅
• "__AVR_HAVE_LPMX__" — okay so bestie, this macro means the device has the "LPM Rn,Z" and "LPM Rn,Z+" instructions. slay ✨
• "__AVR_HAVE_MUL__" — the device has a hardware multiplier. periodt 💅
• "__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. iykyk
• "__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. main character energy 👑
• "__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. lowkey important
• "__AVR_HAVE_8BIT_SP__", "__AVR_HAVE_16BIT_SP__" — the stack pointer (SP) register is treated as 8-bit respectively 16-bit register by the compiler. The definition of these macros is affected by -mtiny-stack. noted 🫶
• "__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. the way that works 💜
• "__AVR_HAVE_RAMPD__", "__AVR_HAVE_RAMPX__", "__AVR_HAVE_RAMPY__", "__AVR_HAVE_RAMPZ__" — the device has the "RAMPD", "RAMPX", "RAMPY", "RAMPZ" special function register, respectively. periodt
• "__NO_INTERRUPTS__" — this macro reflects the -mno-interrupts command-line option. don't @ me
• "__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. the more you know 🌈
• "__AVR_ISA_RMW__" — the device has Read-Modify-Write instructions (XCH, LAC, LAS and LAT). ate
• "__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. literally 🦋
• "__AVR_SHORT_CALLS__" — the -mshort-calls command line option is set. okurrr
• "__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_*". iconic behavior 🌟
• "__WITH_AVRLIBC__" — the compiler is configured to be used together with AVR-Libc. See the --with-avrlibc configure option. bestie note
• "__HAVE_DOUBLE_MULTILIB__" — defined if -mdouble= acts as a multilib option. got it
• "__HAVE_DOUBLE32__", "__HAVE_DOUBLE64__" — defined if the compiler supports 32-bit double resp. 64-bit double. The actual layout is specified by option -mdouble=. slay
• "__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__". pro tip
• "__HAVE_LONG_DOUBLE32__", "__HAVE_LONG_DOUBLE64__", "__HAVE_LONG_DOUBLE_MULTILIB__", "__DEFAULT_LONG_DOUBLE__" — same as above, but for "long double" instead of "double". periodt
• "__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. the details matter
• "__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). obsessed

💖 Blackfin Options 💖

• -mcpu=cpu[-sirevision] — okay so this 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. ngl, important 😭

• -msim — specifies that the program will be run on the simulator. This causes the simulator BSP provided by libgloss to be linked in. This option has effect only for bfin-elf toolchain. Certain other options, such as -mid-shared-library and -mfdpic, imply -msim. periodt
• -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. slay
• -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. the way that works
• -mno-specld-anomaly — don't generate extra code to prevent speculative loads from occurring. got it
• -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. noted
• -mno-csync-anomaly — don't generate extra code to prevent CSYNC or SSYNC instructions from occurring too soon after a conditional branch. okay
• -mlow64k — when enabled, the compiler is free to take advantage of the knowledge that the entire program fits into the low 64k of memory. lowkey useful
• -mno-low64k — assume that the program is arbitrarily large. This is the default. default vibe
• -mstack-check-l1 — do stack checking using information placed into L1 scratchpad memory by the uClinux kernel. the details
• -mid-shared-library — generate code that supports shared libraries via the library ID method. This allows for execute in place and shared libraries in an environment without virtual memory management. This option implies -fPIC. With a bfin-elf target, this option implies -msim. iconic
• -mno-id-shared-library — generate code that doesn't assume ID-based shared libraries are being used. This is the default. default
• -mleaf-id-shared-library — generate code that supports shared libraries via the library ID method, but assumes that this library or executable won't link against any other ID shared libraries. That allows the compiler to use faster code for jumps and calls. faster? slay
• -mno-leaf-id-shared-library — do not assume that the code being compiled won't link against any ID shared libraries. Slower code is generated for jump and call insns. insns, periodt
• -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. iykyk
• -msep-data — generate code that allows the data segment to be located in a different area of memory from the text segment. This allows for execute in place in an environment without virtual memory management by eliminating relocations against the text section. main character energy
• -mno-sep-data — generate code that assumes that the data segment follows the text segment. This is the default. default
• -mlong-calls, -mno-long-calls — tells the compiler to perform function calls by first loading the address of the function into a register and then performing a subroutine call on this register. This switch is needed if the target function lies outside of the 24-bit addressing range of the offset-based version of subroutine call instruction.

This feature is not enabled by default. Specifying -mno-long-calls restores the default behavior. Note these switches have no effect on how the compiler generates code to handle function calls via function pointers. the way that works

• -mfast-fp — link with the fast floating-point library. This library relaxes some of the IEEE floating-point standard's rules for checking inputs against Not-a-Number (NAN), in the interest of performance. performance queen
• -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. noted
• -mmulticore — build a standalone application for multicore Blackfin processors. This option causes proper start files and link scripts supporting multicore to be used, and defines the macro "__BFIN_MULTICORE". It can only be used with -mcpu=bf561[-sirevision].

This option can be used with -mcorea or -mcoreb, which selects the one-application-per-core programming model. Without -mcorea or -mcoreb, the single-application/dual-core programming model is used. In this model, the main function of Core B should be named as "coreb_main".

If this option is not used, the single-core application programming model is used. multicore slay 💖

• -mcorea — bestie, this builds a standalone application for Core A of BF561 when you're doing the whole one-application-per-core programming model thing. Proper start files and link scripts get used to support Core A, and the macro __BFIN_COREA gets defined. 🎀 Heads up though: this option can only be used in conjunction with -mmulticore. It's giving package deal, fr fr.
• -mcoreb — same vibe, different core: builds a standalone application for Core B of BF561 under the one-application-per-core model. Proper start files and link scripts support Core B, and the macro __BFIN_COREB is defined. 💅 When you use this one, coreb_main should be used instead of main — not me calling out your muscle memory but like, yes, that's the rule. Also only valid with -mmulticore. Periodt.
• -msdram — builds a standalone application for SDRAM. Proper start files and link scripts are used to put the application into SDRAM, and the macro __BFIN_SDRAM is defined. ✨ Important: the loader should initialize SDRAM before loading the application. No skipping steps, bestie.
• -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. Different defaults for different contexts, iykyk.

---

C6X Options 💖

• -march=name — specifies the name of the target architecture. GCC uses this name to figure out what kind of instructions it can emit when generating assembly code. Permissible names are: c62x, c64x, c64x+, c67x, c67x+, c674x. ✨
• -mbig-endian — generate code for a big-endian target. 🦋
• -mlittle-endian — generate code for a little-endian target. This is the default. 🌸
• -msim — choose startup files and linker script suitable for the simulator. 👑
• -msdata=default — put small global and static data in the .neardata section, which is pointed to by register B14. Put small uninitialized global and static data in the .bss section, which is adjacent to the .neardata section. Put small read-only data into the .rodata section. The corresponding sections used for large pieces of data are .fardata, .far and .const. 💅
• -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. All in, no half measures. 🔥
• -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. Niche but iconic.

• -march=architecture-type
• -mcpu=architecture-type — generate code for the specified architecture. The choices for architecture-type are v3, v8 and v10 for respectively ETRAX 4, ETRAX 100, and ETRAX 100 LX. Default is v0 except for cris-axis-linux-gnu, where the default is v10. 💜
• -mtune=architecture-type — tune to architecture-type everything applicable about the generated code, except for the ABI and the set of available instructions. The choices for architecture-type are the same as for -march=architecture-type. 🧚
• -mmax-stack-frame=n — warn when the stack frame of a function exceeds n bytes. ⚠️
• -metrax4
• -metrax100 — these options are synonyms for -march=v3 and -march=v8 respectively. Shortcuts, slay.
• -mmul-bug-workaround
• -mno-mul-bug-workaround — work around a bug in the muls and mulu instructions for CPU models where it applies. This option is active by default. 🐛
• -mpdebug — enable CRIS-specific verbose debug-related information in the assembly code. This option also has the effect of turning off the #NO_APP formatted-code indicator to the assembler at the beginning of the assembly file. 🙈
• -mcc-init — do not use condition-code results from previous instruction; always emit compare and test instructions before use of condition codes. Safety first, bestie.
• -mno-side-effects — do not emit instructions with side effects in addressing modes other than post-increment. 🦋
• -mstack-align
• -mno-stack-align
• -mdata-align
• -mno-data-align
• -mconst-align
• -mno-const-align — these options (and their no- counterparts) arrange (eliminate arrangements) for the stack frame, individual data and constants to be aligned for the maximum single data access size for the chosen CPU model. The default is to arrange for 32-bit alignment. ABI details such as structure layout are not affected by these options. 💅
• -m32-bit
• -m16-bit
• -m8-bit — similar to the stack-, data- and const-align options above, these options arrange for stack frame, writable data and constants to all be 32-bit, 16-bit or 8-bit aligned. The default is 32-bit alignment. ✨
• -mno-prologue-epilogue
• -mprologue-epilogue — with -mno-prologue-epilogue, the normal function prologue and epilogue which set up the stack frame are omitted and no return instructions or return sequences are generated in the code. ⚠️ Use this option only together with visual inspection of the compiled code: no warnings or errors are generated when call-saved registers must be saved, or storage for local variables needs to be allocated. Like, genuinely, look at the output yourself, bestie. No cap.
• -mno-gotplt
• -mgotplt — with -fpic and -fPIC, don't generate (do generate) instruction sequences that load addresses for functions from the PLT part of the GOT rather than (traditional on other architectures) calls to the PLT. The default is -mgotplt. 🌸
• -melf — legacy no-op option only recognized with the cris-axis-elf and cris-axis-linux-gnu targets. It's giving nostalgia. 👑
• -mlinux — legacy no-op option only recognized with the cris-axis-linux-gnu target. 🫶
• -sim — this option, recognized for the cris-axis-elf, arranges to link with input-output functions from a simulator library. Code, initialized data and zero-initialized data are allocated consecutively. 🧚
• -sim2 — like -sim, but pass linker options to locate initialized data at 0x40000000 and zero-initialized data at 0x80000000. 💖

---

CR16 Options ⭐

These options are defined specifically for the CR16 ports. Very specific, very demure.

• -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 in compatible with simulator. Applicable to ELF compiler only. 🔥
• -mint32 — choose integer type as 32-bit wide. 💅
• -mbit-ops — generates sbit/cbit instructions for bit manipulations. ✨
• -mdata-model=model — choose a data model. The choices for model are near, far or medium. medium is default. However, far is not valid with -mcr16c, as the CR16C architecture does not support the far data model. ⚠️

---

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 — specify the C-SKY target processor. Valid values for cpu are: ck801, ck801t, ck802, ck802t, ck802j, ck803, ck803h, ck803t, ck803ht, ck803f, ck803fh, ck803e, ck803eh, ck803et, ck803eht, ck803ef, ck803efh, ck803ft, ck803eft, ck803efht, ck803r1, ck803hr1, ck803tr1, ck803htr1, ck803fr1, ck803fhr1, ck803er1, ck803ehr1, ck803etr1, ck803ehtr1, ck803efr1, ck803efhr1, ck803ftr1, ck803eftr1, ck803efhtr1, ck803s, ck803st, ck803se, ck803sf, ck803sef, ck803seft, ck807e, ck807ef, ck807, ck807f, ck810e, ck810et, ck810ef, ck810eft, ck810, ck810v, ck810f, ck810t, ck810fv, ck810tv, ck810ft, and ck810ftv. 😭 that is a lot of options and I respect every single one of them.
• -mbig-endian
• -EB
• -mlittle-endian
• -EL — select big- or little-endian code. The default is little-endian. 🌸
• -mhard-float
• -msoft-float — select hardware or software floating-point implementations. The default is soft float. 🧚
• -mdouble-float
• -mno-double-float — when -mhard-float is in effect, enable generation of double-precision float instructions. This is the default except when compiling for CK803. ✨
• -mfdivdu
• -mno-fdivdu — when -mhard-float is in effect, enable generation of frecipd, fsqrtd, and fdivd instructions. This is the default except when compiling for CK803. 💅
• -mfpu=fpu — select the floating-point processor. This option 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. This option defaults to on for CK801 and off otherwise. 🔥
• -mistack
• -mno-istack — enable interrupt stack instructions; the default is off. The -mistack option is required to handle the interrupt and isr function attributes. ⚠️
• -mmp — enable multiprocessor instructions; the default is off. 🦋
• -mcp — enable coprocessor instructions; the default is off. 💜
• -mcache — enable coprocessor instructions; the default is off. 🎀 (yes bestie, the original says coprocessor here too — I'm not fixing it, I'm reporting it fr fr)
• -msecurity — enable C-SKY security instructions; the default is off. 🔒
• -mtrust — enable C-SKY trust instructions; the default is off. 🫶
• -mdsp
• -medsp
• -mvdsp — enable C-SKY DSP, Enhanced DSP, or Vector DSP instructions, respectively. All of these options 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 to allow use of 16-bit instructions. This option is ignored for CK801 where this is the required behavior, and it defaults to on for CK802. For other targets, the default is off. 💅
• -mhigh-registers
• -mno-high-registers — generate code using the high registers numbered 16-31. This option is not supported on CK801, CK802, or CK803, and is enabled by default for other processors. 🌈
• -manchor
• -mno-anchor — generate code using global anchor symbol addresses. 🎀
• -mpushpop
• -mno-pushpop — generate code using push and pop instructions. This option defaults to on. 🫶
• -mmultiple-stld
• -mstm
• -mno-multiple-stld
• -mno-stm — generate code using stm and ldm instructions. This option isn't supported on CK801 but is enabled by default on other processors. 👑
• -mconstpool, -mno-constpool — build the constant pools inside the compiler instead of making the assembler do it later. This one is the default, and it's straight-up required for correct code generation on CK801 and CK802 — on other processors it's optional. ✨
• -mstack-size, -mno-stack-size — emit .stack_size directives for every single function in the assembly output. This one is off by default. 🫶
• -mccrt, -mno-ccrt — generate code for the C-SKY compiler runtime instead of libgcc. Also off by default. 💅
• -mbranch-cost=n — set the branch costs to roughly n instructions. Default is 1.
• -msched-prolog, -mno-sched-prolog — allow scheduling of function prologue and epilogue sequences. ⚠️ Heads up bestie: using this can give you code that is not compliant with the C-SKY V2 ABI prologue requirements, and it cannot be debugged or backtraced. It is disabled by default. Not worth the drama, fr fr.

---

🍎 Darwin Options 🍎

These ones are defined for all architectures running the Darwin operating system. 🌈

FSF GCC on Darwin does not create "fat" object files — it creates an object file for the single architecture that GCC was built to target. Apple's GCC on Darwin does create "fat" files if multiple -arch options are used; it pulls that off by running the compiler or linker multiple times and joining the results together with lipo. 👑

The subtype of the file created (like ppc7400 or ppc970 or i686) is determined by the flags that specify the ISA that GCC is targeting, like -mcpu or -march. The -force_cpusubtype_ALL option can be used to override this. 💫

The Darwin tools vary in their behavior when presented with an ISA mismatch, and it's giving chaos:

• The assembler, as, only permits instructions to be used that are valid for the subtype of the file it is generating, so you cannot put 64-bit instructions in a ppc750 object file.
• The linker for shared libraries, /usr/bin/libtool, fails and prints an error if asked to create a shared library with a less restrictive subtype than its input files (for instance, trying to put a ppc970 object file in a ppc7400 library).
• The linker for executables, ld, quietly gives the executable the most restrictive subtype of any of its input files. 🙈
• -Fdir — add the framework directory dir to the head of the list of directories to be searched for header files. These directories are interleaved with those specified by -I options and are scanned in a left-to-right order. 🔍

A framework directory is a directory with frameworks in it. A framework is a directory with a Headers and/or PrivateHeaders directory contained directly in it that ends in .framework. The name of a framework is the name of this directory excluding the .framework. Headers associated with the framework are found in one of those two directories, with Headers being searched first. A subframework is a framework directory that is in a framework's Frameworks directory. Includes of subframework headers can only appear in a header of a framework that contains the subframework, or in a sibling subframework header. Two subframeworks are siblings if they occur in the same framework. A subframework should not have the same name as a framework; a warning is issued if this is violated. Currently a subframework cannot have subframeworks; in the future, the mechanism may be extended to support this. The standard frameworks can be found in /System/Library/Frameworks and /Library/Frameworks. An example include looks like #include <Framework/header.h>, where Framework denotes the name of the framework and header.h is found in the PrivateHeaders or Headers directory. 🎀

• -iframeworkdir — like -F except the directory is treated as a system directory. The main difference between this -iframework and -F is that with -iframework the compiler does not warn about constructs contained within header files found via dir. This option is valid only for the C family of languages. ✨
• -gused — emit debugging information for symbols that are used. For stabs debugging format, this enables -feliminate-unused-debug-symbols. This is by default ON. 💖
• -gfull — emit debugging information for all symbols and types. 🌟
• -mmacosx-version-min=version — the earliest version of MacOS X that this executable will run on is version. Typical values of version include 10.1, 10.2, and 10.3.9.

If the compiler was built to use the system's headers by default, then the default for this option is the system version on which the compiler is running, otherwise the default is to make choices that are compatible with as many systems and code bases as possible. 🦋

• -mkernel — enable kernel development mode. The -mkernel option sets -static, -fno-common, -fno-use-cxa-atexit, -fno-exceptions, -fno-non-call-exceptions, -fapple-kext, -fno-weak and -fno-rtti where applicable. This mode also sets -mno-altivec, -msoft-float, -fno-builtin and -mlong-branch for PowerPC targets. 🛠️
• -mone-byte-bool — override the defaults for bool so that sizeof(bool)==1. By default sizeof(bool) is 4 when compiling for Darwin/PowerPC and 1 when compiling for Darwin/x86, so this option has no effect on x86. 👀

⚠️ Warning: The -mone-byte-bool switch causes GCC to generate code that is not binary compatible with code generated without that switch. Using this switch may require recompiling all other modules in a program, including system libraries. Use this switch to conform to a non-default data model. 💀

• -mfix-and-continue, -ffix-and-continue, -findirect-data — generate code suitable for fast turnaround development, such as to allow GDB to dynamically load .o files into already-running programs. -findirect-data and -ffix-and-continue are provided for backwards compatibility. 🫶
• -all_load — loads all members of static archive libraries. See man ld(1) for more information.
• -arch_errors_fatal — cause the errors having to do with files that have the wrong architecture to be fatal. 😤
• -bind_at_load — causes the output file to be marked such that the dynamic linker will bind all undefined references when the file is loaded or launched.
• -bundle — produce a Mach-o bundle format file. See man ld(1) for more information.
• -bundle_loader executable — this option specifies the executable that will load the build output file being linked. See man ld(1) for more information.
• -dynamiclib — when passed this option, GCC produces a dynamic library instead of an executable when linking, using the Darwin libtool command. 📚
• -force_cpusubtype_ALL — this causes GCC's output file to have the ALL subtype, instead of one controlled by the -mcpu or -march option.

And then there's this whole iconic roster of options that get passed straight to the Darwin linker — the Darwin linker man page describes them in detail, so go read that for the deets. Not me listing every single one though, because bestie, there are a lot: 💅

• -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

---

💜 DEC Alpha Options 💜

These -m options are defined for the DEC Alpha implementations: 🌸

• -mno-soft-float, -msoft-float — use (do not use) the hardware floating-point instructions for floating-point operations. When -msoft-float is specified, functions in libgcc.a are used to perform floating-point operations. Unless they are replaced by routines that emulate the floating-point operations, or compiled in such a way as to call such emulation routines, these routines issue floating-point operations. If you are compiling for an Alpha without floating-point operations, you must ensure that the library is built so as not to call them. 😭

Note that Alpha implementations without floating-point operations are required to have floating-point registers.

• -mfp-reg, -mno-fp-regs — generate code that uses (does not use) the floating-point register set. -mno-fp-regs implies -msoft-float. If the floating-point register set is not used, floating-point operands are passed in integer registers as if they were integers and floating-point results are passed in $0 instead of $f0. This is a non-standard calling sequence, so any function with a floating-point argument or return value called by code compiled with -mno-fp-regs must also be compiled with that option. 🫠

A typical use of this option is building a kernel that does not use, and hence need not save and restore, any floating-point registers. 🔥

• -mieee — the Alpha architecture implements floating-point hardware optimized for maximum performance. It is mostly compliant with the IEEE floating-point standard. However, for full compliance, software assistance is required. This option generates code fully IEEE-compliant code except that the inexact-flag is not maintained (see below). If this option is turned on, the preprocessor macro _IEEE_FP is defined during compilation. The resulting code is less efficient but is able to correctly support denormalized numbers and exceptional IEEE values such as not-a-number and plus/minus infinity. Other Alpha compilers call this option -ieee_with_no_inexact. ⭐
• -mieee-with-inexact — okay so this is literally -mieee but the generated code ALSO keeps the IEEE inexact-flag alive and well 🫶. Flip this on and your generated code implements fully-compliant IEEE math, no cap. On top of _IEEE_FP you also get _IEEE_FP_EXACT defined as a preprocessor macro ✨. Heads up bestie: on some Alpha implementations the resulting code can run significantly slower than the default code, and since there's very little code out there that even depends on the inexact-flag, you should normally not specify this option. Other Alpha compilers call this one -ieee_with_inexact.
• -mfp-trap-mode=trap-mode — this one controls which floating-point related traps get enabled. Other Alpha compilers call it -fptm trap-mode. The trap mode can be set to one of four values:
  • n — the default (normal) setting. The only traps enabled are the ones that can't be disabled in software (e.g., division by zero trap).
  • u — in addition to the traps enabled by n, underflow traps are enabled as well.
  • su — like u, but the instructions are marked to be safe for software completion (peep the Alpha architecture manual for the details).
  • sui — like su, but inexact traps are enabled as well.
• -mfp-rounding-mode=rounding-mode — selects the IEEE rounding mode. Other Alpha compilers call this option -fprm rounding-mode. The rounding-mode can be one of:
  • n — Normal IEEE rounding mode. Floating-point numbers are rounded towards the nearest machine number, or towards the even machine number in case of a tie.
  • m — Round towards minus infinity.
  • c — Chopped rounding mode. Floating-point numbers are rounded towards zero.
  • d — Dynamic rounding mode. A field in the floating-point control register (fpcr, see Alpha architecture reference manual) controls the rounding mode in effect. The C library initializes this register for rounding towards plus infinity. So unless your program modifies the fpcr, d corresponds to round towards plus infinity.
• -mtrap-precision=trap-precision — in the Alpha architecture, floating-point traps are imprecise 😭 which means without software assistance it's literally impossible to recover from a floating trap, and program execution normally needs to be terminated. GCC can generate code that helps operating system trap handlers figure out the exact location that caused a floating-point trap 💅. Depending on what your application needs, you can pick different levels of precision:
  • p — Program precision. This option is the default and means a trap handler can only identify which program caused a floating-point exception.
  • f — Function precision. The trap handler can determine the function that caused a floating-point exception.
  • i — Instruction precision. The trap handler can determine the exact instruction that caused a floating-point exception.
  • Other Alpha compilers provide the equivalent options called -scope_safe and -resumption_safe.
• -mieee-conformant — marks the generated code as IEEE conformant 👑. You must not use this option unless you also specify -mtrap-precision=i and either -mfp-trap-mode=su or -mfp-trap-mode=sui. Its only effect is to emit the line .eflag 48 in the function prologue of the generated assembly file.
• -mbuild-constants — normally GCC examines a 32- or 64-bit integer constant to see if it can construct it from smaller constants in two or three instructions. If it can't, it outputs the constant as a literal and generates code to load it from the data segment at run time. Use this option to require GCC to construct all integer constants using code, even if it takes more instructions (the maximum is six). You typically use this option to build a shared library dynamic loader — itself a shared library, it must relocate itself in memory before it can find the variables and constants in its own data segment 🌀.
• -mbwx, -mno-bwx, -mcix, -mno-cix, -mfix, -mno-fix, -mmax, -mno-max — these indicate whether GCC should generate code to use the optional BWX, CIX, FIX and MAX instruction sets. The default is to use the instruction sets supported by the CPU type specified via -mcpu= option, or that of the CPU on which GCC was built if none is specified.
• -mfloat-vax, -mfloat-ieee — generate code that uses (does not use) VAX F and G floating-point arithmetic instead of IEEE single and double precision.
• -mexplicit-relocs, -mno-explicit-relocs — older Alpha assemblers gave you 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 is built and sets the default accordingly.
• -msmall-data, -mlarge-data — when -mexplicit-relocs is in effect, static data is accessed via gp-relative relocations. When -msmall-data is used, objects 8 bytes long or smaller are placed in a small data area (the .sdata and .sbss sections) and are accessed via 16-bit relocations off of the $gp register. This limits the size of the small data area to 64KB, but lets the variables be directly accessed via a single instruction 👑.

The default is -mlarge-data. With this option the data area is limited to just below 2GB. Programs that require more than 2GB of data must use malloc or mmap to allocate the data in the heap instead of in the program's data segment.

When generating code for shared libraries, -fpic implies -msmall-data and -fPIC implies -mlarge-data.

• -msmall-text, -mlarge-text — when -msmall-text is used, the compiler assumes that the code of the entire program (or shared library) fits in 4MB, and is thus reachable with a branch instruction. When -msmall-data is used, the compiler can assume that all local symbols share the same $gp value, and thus reduce the number of instructions required for a function call from 4 to 1 🎀.

The default is -mlarge-text.

• -mcpu=cpu_type — set the instruction set and instruction scheduling parameters for machine type cpu_type. You can specify either the EV style name or the corresponding chip number. GCC supports scheduling parameters for the EV4, EV5 and EV6 family of processors and chooses the default values for the instruction set from the processor you specify. If you don't specify a processor type, GCC defaults to the processor on which the compiler was built.

Supported values for cpu_type are:

• ev4
• ev45
• 21064 — Schedules as an EV4 and has no instruction set extensions.
• ev5
• 21164 — Schedules as an EV5 and has no instruction set extensions.
• ev56
• 21164a — Schedules as an EV5 and supports the BWX extension.
• pca56
• 21164pc
• 21164PC — Schedules as an EV5 and supports the BWX and MAX extensions.
• ev6
• 21264 — Schedules as an EV6 and supports the BWX, FIX, and MAX extensions.
• ev67
• 21264a — Schedules as an EV6 and supports the BWX, CIX, FIX, and MAX extensions.

Native toolchains also support the value native, which selects the best architecture option for the host processor. -mcpu=native has no effect if GCC does not recognize the processor.

• -mtune=cpu_type — set only the instruction scheduling parameters for machine type cpu_type. The instruction set is not changed, periodt.

Native toolchains also support the value native, which selects the best architecture option for the host processor. -mtune=native has no effect if GCC does not recognize the processor.

• -mmemory-latency=time — sets the latency the scheduler should assume for typical memory references as seen by the application. This number is highly dependent on the memory access patterns used by the application and the size of the external cache on the machine 🧠.

Valid options for time are:

• number — A decimal number representing clock cycles.
• L1
• L2
• L3
• main — The compiler contains estimates of the number of clock cycles for "typical" EV4 & EV5 hardware for the Level 1, 2 & 3 caches (also called Dcache, Scache, and Bcache), as well as to main memory. Note that L3 is only valid for EV5.

---

💫 eBPF Options 💫

• -mframe-limit=bytes — this specifies the hard limit for frame sizes, in bytes. Currently, the value that can be specified 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 💅.

---

🌸 FR30 Options 🌸

These options are defined specifically for the FR30 port.

• -msmall-model — use the small address space model. This can produce smaller code, but it does assume that all symbolic values and addresses fit into a 20-bit range 👀.
• -mno-lsim — assume that runtime support has been provided and so there is no need to include the simulator library (libsim.a) on the linker command line.

---

🦋 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.

bestie listen 😭 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 — turns on Local Register Allocation ✨ heads up tho, this is still experimental for FT32, so by default the compiler just uses standard reload instead.
• -mnodiv — do not use div and mod instructions, periodt.
• -mft32b — unlock the extended instructions of the FT32B processor 💅
• -mcompress — compress all the code using the Ft32B code compression scheme 🗜️✨
• -mnopm — don't generate code that reads program memory, bestie.

---

🦋 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's known to be in read-only sections ✨ It's enabled by default, except for -fpic or -fpie: even though it may help make the global offset table smaller, it trades 1 instruction for 4. With -fPIC or -fPIE, it trades 3 instructions for 4, one of which may be shared by multiple symbols, and it avoids the need for a GOT entry for the referenced symbol, so it's more likely to be a win. If it 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, iykyk 🙈
• -mlinked-fp — follow the EABI requirement of always creating a frame pointer whenever a stack frame is allocated. This option is enabled by default and can be disabled with -mno-linked-fp.
• -mlong-calls — use indirect addressing to call functions outside the current compilation unit. This lets the functions be placed anywhere within the 32-bit address space 🌈
• -malign-labels — try to align labels to an 8-byte boundary by inserting NOPs into the previous packet. This option only does anything when VLIW packing is enabled. It doesn't create new packets; it merely adds NOPs to existing ones 🎀
• -mlibrary-pic — generate position-independent EABI code.
• -macc-4 — use only the first four media accumulator registers.
• -macc-8 — use all eight media accumulator registers 💅
• -mpack — pack VLIW instructions.
• -mno-pack — do not pack VLIW instructions.
• -mno-eflags — do not mark ABI switches in e_flags.
• -mcond-move — enable the use of conditional-move instructions (default).

This switch is mainly for debugging the compiler and will likely be removed in a future version 💀

• -mno-cond-move — disable the use of conditional-move instructions.

This switch is mainly for debugging the compiler and will likely be removed in a future version 💀

• -mscc — enable the use of conditional set instructions (default).

This switch is mainly for debugging the compiler and will likely be removed in a future version 💀

• -mno-scc — disable the use of conditional set instructions.

This switch is mainly for debugging the compiler and will likely be removed in a future version 💀

• -mcond-exec — enable the use of conditional execution (default).

This switch is mainly for debugging the compiler and will likely be removed in a future version 💀

• -mno-cond-exec — disable the use of conditional execution.

This switch is mainly for debugging the compiler and will likely be removed in a future version 💀

• -mvliw-branch — run a pass to pack branches into VLIW instructions (default).

This switch is mainly for debugging the compiler and will likely be removed in a future version 💀

• -mno-vliw-branch — do not run a pass to pack branches into VLIW instructions.

This switch is mainly for debugging the compiler and will likely be removed in a future version 💀

• -mmulti-cond-exec — enable optimization of && and || in conditional execution (default).

This switch is mainly for debugging the compiler and will likely be removed in a future version 💀

• -mno-multi-cond-exec — disable optimization of && and || in conditional execution.

This switch is mainly for debugging the compiler and will likely be removed in a future version 💀

• -mnested-cond-exec — enable nested conditional execution optimizations (default).

This switch is mainly for debugging the compiler and will likely be removed in a future version 💀

• -mno-nested-cond-exec — disable nested conditional execution optimizations.

This switch is mainly for debugging the compiler and will likely be removed in a future version 💀

• -moptimize-membar — this switch removes redundant membar instructions from the compiler-generated code. It's enabled by default ✨
• -mno-optimize-membar — this switch disables the automatic removal of redundant membar instructions from the generated code.
• -mtomcat-stats — cause gas to print out tomcat statistics 🐱📊
• *-mcpu=cpu* — select the processor type for which to generate code. Possible values are frv, fr550, tomcat, fr500, fr450, fr405, fr400, fr300 and simple 👑

---

🐧 GNU/Linux Options 🐧

These -m options are defined for GNU/Linux targets, bestie:

• -mglibc — use the GNU C library. This is the default except on *-*-linux-*uclibc*, *-*-linux-*musl* and *-*-linux-*android* targets.
• -muclibc — use uClibc C library. This is the default on *-*-linux-*uclibc* targets.
• -mmusl — use the musl C library. This is the default on *-*-linux-*musl* targets.
• -mbionic — use Bionic C library. This is the default on *-*-linux-*android* targets 🤖
• -mandroid — compile code compatible with Android platform. This is the default on *-*-linux-*android* targets.

When compiling, this option enables -mbionic, -fPIC, -fno-exceptions and -fno-rtti by default. When linking, this option makes the GCC driver pass Android-specific options to the linker. Finally, this option causes the preprocessor macro __ANDROID__ to be defined ✨

• -tno-android-cc — disable compilation effects of -mandroid, i.e., do not enable -mbionic, -fPIC, -fno-exceptions and -fno-rtti by default.
• -tno-android-ld — disable linking effects of -mandroid, i.e., pass standard Linux linking options to the linker.

---

💜 H8/300 Options 💜

These -m options are defined for the H8/300 implementations:

• -mrelax — shorten some address references at link time, when possible; uses the linker option -relax.
• -mh — generate code for the H8/300H.
• -ms — generate code for the H8S.
• -mn — generate code for the H8S and H8/300H in the normal mode. This switch must be used either with -mh or -ms.
• -ms2600 — generate code for the H8S/2600. This switch must be used with -ms.
• -mexr — extended registers are stored on stack before execution of function with monitor attribute. Default option is -mexr. This option is valid only for H8S targets.
• -mno-exr — extended registers are not stored on stack before execution of function with monitor attribute. Default option is -mno-exr. This option is valid only for H8S targets.
• -mint32 — make int data 32 bits by default.
• -malign-300 — on the H8/300H and H8S, use the same alignment rules as for the H8/300. The default for the H8/300H and H8S is to align longs and floats on 4-byte boundaries. -malign-300 causes them to be aligned on 2-byte boundaries. This option has no effect on the H8/300.

---

🌸 HPPA Options 🌸

These -m options are defined for the HPPA family of computers:

• -march=architecture-type — generate code for the specified architecture. The choices for architecture-type are 1.0 for PA 1.0, 1.1 for PA 1.1, and 2.0 for PA 2.0 processors. Refer to /usr/lib/sched.models on an HP-UX system to determine the proper architecture option for your machine. Code compiled for lower numbered architectures runs on higher numbered architectures, but **not the other way around 💀
• -mpa-risc-1-0, -mpa-risc-1-1, -mpa-risc-2-0 — synonyms for -march=1.0, -march=1.1, and -march=2.0 respectively.
• -mcaller-copies — the caller copies function arguments passed by hidden reference. Use this one with care, bestie, because it's 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.
• -mjump-in-delay — this option is ignored and provided for compatibility purposes only 🙈
• -mdisable-fpregs — bestie, this one says floating-point registers cannot be used in any manner. It’s necessary for compiling kernels that perform lazy context switching of floating-point registers. But if you use this option and then attempt to perform floating-point operations, the compiler aborts. 💀
• -mdisable-indexing — stops 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 lets GCC generate faster indirect calls and use unscaled index address modes. Such code is suitable for level 0 PA systems and kernels. 🫶
• -mfast-indirect-calls — generate code that assumes calls never cross space boundaries. This lets GCC emit code that performs faster indirect calls. Warning: this option does not work in the presence of shared libraries or nested functions. 🚫
• -mfixed-range=register-range — generate code treating the given register range as fixed registers. A fixed register is one that the register allocator cannot use. This is useful when compiling kernel code. A register range is specified as two registers separated by a dash. Multiple register ranges can be specified separated by a comma. 🎀
• -mlong-load-store — generate 3-instruction load and store sequences as sometimes required by the HP-UX 10 linker. This is equivalent to the +k option to the HP compilers. ✨
• -mportable-runtime — use the portable calling conventions proposed by HP for ELF systems. 💖
• -mgas — enable the use of assembler directives only GAS understands. 👑
• -mschedule=cpu-type — schedule code according to the constraints for the machine type cpu-type. The choices for cpu-type are 700 7100, 7100LC, 7200, 7300, and 8000. Refer to /usr/lib/sched.models on an HP-UX system to determine the proper scheduling option for your machine. The default scheduling is 8000. ⭐
• -mlinker-opt — enable the optimization pass in the HP-UX linker. Note this makes symbolic debugging impossible. It also triggers a bug in the HP-UX 8 and HP-UX 9 linkers in which they give bogus error messages when linking some programs. 😭
• -msoft-float — generate output containing library calls for floating point.

Warning: the requisite libraries are not available for all HPPA targets. Normally the facilities of the machine’s usual C compiler are used, but this cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation.

-msoft-float changes the calling convention in the output file; therefore, it is only useful if you compile all of a program with this option. In particular, you need to compile libgcc.a, the library that comes with GCC, with -msoft-float in order for this to work.

• -msio — generate the predefine, "_SIO", for server IO. The default is -mwsio. This generates the predefines, "__hp9000s700", "__hp9000s700__" and "_WSIO", for workstation IO. These options are available under HP-UX and HI-UX. 🦋
• -mgnu-ld — use options specific to GNU ld. This passes -shared to ld when building a shared library. It is the default when GCC is configured, explicitly or implicitly, with the GNU linker. This option does not affect which ld is called; it only changes what parameters are passed to that ld. The ld that is called is determined by the --with-ld configure option, GCC’s program search path, and finally by the user’s PATH. The linker used by GCC can be printed using 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 gcc -print-prog-name=ld. This option is only available on the 64-bit HP-UX GCC, i.e. configured with hppa*64*-*-hpux*. 🔥
• -mlong-calls — generate code that uses long call sequences. This ensures that a call is always able to reach linker generated stubs. The default is to generate long calls only when the distance from the call site to the beginning of the function or translation unit, as the case may be, exceeds a predefined limit set by the branch type being used. The limits for normal calls are 7,600,000 and 240,000 bytes, respectively for the PA 2.0 and PA 1.X architectures. Sibcalls are always limited at 240,000 bytes.

Distances are measured from the beginning of functions when using the -ffunction-sections option, or when using the -mgas and -mno-portable-runtime options together under HP-UX with the SOM linker.

It is normally not desirable to use this option as it degrades performance. However, it may be useful in large applications, particularly when partial linking is used to build the application.

The types of long calls used depends on the capabilities of the assembler and linker, and the type of code being generated. The impact on systems that support long absolute calls, and long pic symbol-difference or pc-relative calls should be relatively small. However, an indirect call is used on 32-bit ELF systems in pic code and it is quite long. 😭

• -munix=unix-std — generate compiler predefines and select a startfile for the specified UNIX standard. The choices for unix-std are 93, 95 and 98. 93 is supported on all HP-UX versions. 95 is available on HP-UX 10.10 and later. 98 is available on HP-UX 11.11 and later. The default values are 93 for HP-UX 10.00, 95 for HP-UX 10.10 though to 11.00, and 98 for HP-UX 11.11 and later.

-munix=93 provides the same predefines as GCC 3.3 and 3.4. -munix=95 provides additional predefines for "XOPEN_UNIX" and "_XOPEN_SOURCE_EXTENDED", and the startfile unix95.o. -munix=98 provides additional predefines for "_XOPEN_UNIX", "_XOPEN_SOURCE_EXTENDED", "_INCLUDE__STDC_A1_SOURCE" and "_INCLUDE_XOPEN_SOURCE_500", and the startfile unix98.o.

It is important to note that this option changes the interfaces for various library routines. It also affects the operational behavior of the C library. Thus, extreme care is needed in using this option.

Library code that is intended to operate with more than one UNIX standard must test, set and restore the variable "__xpg4_extended_mask" as appropriate. Most GNU software doesn’t provide this capability. ⚠️

• -nolibdld — suppress the generation of link options to search libdld.sl when the -static option is specified on HP-UX 10 and later. 🚫
• -static — the HP-UX implementation of setlocale in libc has a dependency on libdld.sl. There isn’t an archive version of libdld.sl. Thus, when the -static option is specified, special link options are needed to resolve this dependency.

On HP-UX 10 and later, the GCC driver adds the necessary options to link with libdld.sl when the -static option is specified. This causes the resulting binary to be dynamic. On the 64-bit port, the linkers generate dynamic binaries by default in any case. The -nolibdld option can be used to prevent the GCC driver from adding these link options. 💅

• -threads — add support for multithreading with the dce thread library under HP-UX. This option sets flags for both the preprocessor and linker. 🧵

---

🌈 IA-64 Options 🌈

These are the -m options defined for the Intel IA-64 architecture. 👑

• -mbig-endian — generate code for a big-endian target. This is the default for HP-UX. 💖
• -mlittle-endian — generate code for a little-endian target. This is the default for AIX5 and GNU/Linux. 🦋
• -mgnu-as, -mno-gnu-as — generate (or don’t) code for the GNU assembler. This is the default. ✨
• -mgnu-ld, -mno-gnu-ld — generate (or don’t) code for the GNU linker. This is the default. 🔥
• -mno-pic — generate code that does not use a global pointer register. The result is not position independent code, and violates the IA-64 ABI. 💀
• -mvolatile-asm-stop, -mno-volatile-asm-stop — generate (or don’t) a stop bit immediately before and after volatile asm statements. ⭐
• -mregister-names, -mno-register-names — generate (or don’t) in, loc, and out register names for the stacked registers. This may make assembler output more readable. 🎀
• -mno-sdata, -msdata — disable (or enable) optimizations that use the small data section. This may be useful for working around optimizer bugs. 🐛
• -mconstant-gp — generate code that uses a single constant global pointer value. This is useful when compiling kernel code. 👑
• -mauto-pic — generate code that is self-relocatable. This implies -mconstant-gp. This is useful when compiling firmware code. 💾
• -minline-float-divide-min-latency — generate code for inline divides of floating-point values using the minimum latency algorithm. ⚡
• -minline-float-divide-max-throughput — generate code for inline divides of floating-point values using the maximum throughput algorithm. 🚀
• -mno-inline-float-divide — do not generate inline code for divides of floating-point values. 🚫
• -minline-int-divide-min-latency — generate code for inline divides of integer values using the minimum latency algorithm. ⚡
• -minline-int-divide-max-throughput — generate code for inline divides of integer values using the maximum throughput algorithm. 🚀
• -mno-inline-int-divide — do not generate inline code for divides of integer values. 🚫
• -minline-sqrt-min-latency — generate code for inline square roots using the minimum latency algorithm. ⚡
• -minline-sqrt-max-throughput — generate code for inline square roots using the maximum throughput algorithm. 🚀
• -mno-inline-sqrt — do not generate inline code for sqrt. 🚫
• -mfused-madd, -mno-fused-madd — ok so this is a do/don't situation: do (or 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. Lowkey useful when you're 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. It's giving "maybe slay, maybe not" 💅
• -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 the 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, iykyk.
• -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. (your moment to shine, bestie 💅)

---

🍓 M32C Options 🍓

• -mcpu=name — select the CPU for which code is generated. name may be one of r8c for the R8C/Tiny series, m16c for the M16C (up to /60) series, m32cm for the M16C/80 series, or m32c for the M32C/80 series.
• -msim — specifies that the program will be run on the simulator. This causes an alternate runtime library to be linked in which supports, for example, file I/O.

You must not use this option when generating programs that will run on real hardware; you must provide your own runtime library for whatever I/O functions are needed. 🚨

• -memregs=number — specifies the number of memory-based pseudo-registers GCC uses during code generation. These pseudo-registers are used like real registers, so there is a tradeoff between GCC's ability to fit the code into available registers, and the performance penalty of using memory instead of registers.

Note that all modules in a program must be compiled with the same value for this option. Because of that, you must not use this option with GCC's default runtime libraries. 😭

---

🌈 M32R/D Options 🌈

These -m options are defined for Renesas M32R/D architectures:

• -m32r2 — generate code for the M32R/2.
• -m32rx — generate code for the M32R/X.
• -m32r — generate code for the M32R. This is the default. 👑
• -mmodel=small — assume all objects live in the lower 16MB of memory (so that their addresses can be loaded with the "ld24" instruction), and assume all subroutines are reachable with the "bl" instruction. This is the default.

The addressability of a particular object can be set with the "model" attribute.

• -mmodel=medium — assume objects may be anywhere in the 32-bit address space (the compiler generates "seth/add3" instructions to load their addresses), and assume all subroutines are reachable with the "bl" instruction.
• -mmodel=large — assume objects may be anywhere in the 32-bit address space (the compiler generates "seth/add3" instructions to load their addresses), and assume subroutines may not be reachable with the "bl" instruction (the compiler generates the much slower "seth/add3/jl" instruction sequence). 🐌
• -msdata=none — disable use of the small data area. Variables are put into one of ".data", ".bss", or ".rodata" (unless the "section" attribute has been specified). This is the default.

The small data area consists of sections ".sdata" and ".sbss". Objects may be explicitly put in the small data area with the "section" attribute using one of these sections.

• -msdata=sdata — put small global and static data in the small data area, but do not generate special code to reference them.
• -msdata=use — put small global and static data in the small data area, and generate special instructions to reference them.
• -G num — put global and static objects less than or equal to num bytes into the small data or BSS sections instead of the normal data or BSS sections. The default value of num is 8. The -msdata option must be set to one of sdata or use for this option to have any effect.

All modules should be compiled with the same -G num value. Compiling with different values of num may or may not work; if it doesn't the linker gives an error message---incorrect code is not generated. 💀

• -mdebug — makes the M32R-specific code in the compiler display some statistics that might help in debugging programs.
• -malign-loops — align all loops to a 32-byte boundary. ✨
• -mno-align-loops — do not enforce a 32-byte alignment for loops. This is the default.
• -missue-rate=number — issue number instructions per cycle. number can only be 1 or 2.
• -mbranch-cost=number — number can only be 1 or 2. If it is 1 then branches are preferred over conditional code, if it is 2, then the opposite applies.
• -mflush-trap=number — specifies the trap number to use to flush the cache. The default is 12. Valid numbers are between 0 and 15 inclusive.
• -mno-flush-trap — specifies that the cache cannot be flushed by using a trap.
• -mflush-func=name — specifies the name of the operating system function to call to flush the cache. The default is _flush_cache, but a function call is only used if a trap is not available.
• -mno-flush-func — indicates that there is no OS function for flushing the cache. 🙈

---

🎀 M680x0 Options 🎀

These are the -m options defined for M680x0 and ColdFire processors. The default settings depend on which architecture was selected when the compiler was configured; the defaults for the most common choices are given below.

• -march=arch — bestie, this one says: make code for one specific M680x0 or ColdFire instruction set architecture. 🎯

The allowed arch values for M680x0 are: 68000, 68010, 68020, 68030, 68040, 68060 and cpu32. For ColdFire, they're picked by Freescale's ISA classification, and the allowed ones are: isaa, isaaplus, isab and isac.

GCC defines a macro called __mcfarch__ whenever it's generating code for a ColdFire target, and the arch inside that macro is one of the -march arguments from right up there. 👆

When you use -march and -mtune together, you get code that runs on a whole family of similar processors but is optimized for one particular microarchitecture. It's giving range with a favourite. 💅

• -mcpu=cpu — generate code for one specific M680x0 or ColdFire processor.

The M680x0 cpus are: 68000, 68010, 68020, 68030, 68040, 68060, 68302, 68332 and cpu32. The ColdFire cpus are in this table, which also sorts 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. And any other combo of -mcpu and -march? Rejected, periodt. 🚫

GCC defines the macro __mcf_cpu_cpu when the ColdFire target cpu is selected. It also defines __mcf_family_family, where family is whatever the table above says. ✨

• -mtune=tune — tune the code for a particular microarchitecture, but only within the constraints that -march and -mcpu already set. 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 do -mtune=68020-40 for code that needs to run relatively well on 68020, 68030 and 68040 targets. And -mtune=68020-60 is the same vibe but it includes 68060 targets too. These two options pick the exact same tuning decisions as -m68020-40 and -m68020-60 respectively. 👯

GCC defines the macros __mcarch and __mcarch__ when it's tuning for 680x0 architecture arch. It also defines mcarch, unless -ansi or a non-GNU -std option is used. And if GCC is tuning for a range of architectures, like with -mtune=68020-40 or -mtune=68020-60, it defines the macros for every architecture in that range. All of them. No crumbs left. 🍓

GCC also defines the macro __muarch__ when tuning for ColdFire microarchitecture uarch, where uarch is one of the arguments listed above.

• -m68000, -mc68000 — generate output for a 68000. This is the default when the compiler is configured for 68000-based systems, and it's equivalent to -march=68000.

Use this one for microcontrollers with a 68000 or EC000 core, including the 68008, 68302, 68306, 68307, 68322, 68328 and 68356. 💖

• -m68010 — generate output for a 68010. Default when the compiler is configured for 68010-based systems, and equivalent to -march=68010.
• -m68020, -mc68020 — generate output for a 68020. Default when the compiler is configured for 68020-based systems, and equivalent to -march=68020.
• -m68030 — generate output for a 68030. Default when the compiler is configured for 68030-based systems, and equivalent to -march=68030.
• -m68040 — generate output for a 68040. Default when the compiler is configured for 68040-based systems, and equivalent to -march=68040.

Heads up: this option inhibits the use of 68881/68882 instructions that have to be emulated by software on the 68040. So use it if your 68040 doesn't have the code to emulate those instructions. 🫶

• -m68060 — generate output for a 68060. Default when the compiler is configured for 68060-based systems, and equivalent to -march=68060.

This one inhibits the use of 68020 and 68881/68882 instructions that have to be emulated by software on the 68060. Use it if your 68060 doesn't have code to emulate those instructions. Same energy as above, different chip. 💀

• -mcpu32 — generate output for a CPU32. Default when the compiler is configured for CPU32-based systems, and equivalent to -march=cpu32.

Use it 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's equivalent to -mcpu=5206, and is now deprecated in favor of that option. 🥀

Use it for a microcontroller with a 5200 core, including the MCF5202, MCF5203, MCF5204 and MCF5206.

• -m5206e — generate output for a 5206e ColdFire CPU. This option is now deprecated in favor of the equivalent -mcpu=5206e.
• -m528x — generate output for a member of the ColdFire 528X family. This option is now deprecated in favor of the equivalent -mcpu=528x.
• -m5307 — generate output for a ColdFire 5307 CPU. This option is now deprecated in favor of the equivalent -mcpu=5307.
• -m5407 — generate output for a ColdFire 5407 CPU. This option is now deprecated in favor of the equivalent -mcpu=5407.
• -mcfv4e — generate output for a ColdFire V4e family CPU (e.g. 547x/548x). This includes use of hardware floating-point instructions. It's equivalent to -mcpu=547x, and is now deprecated in favor of that option.
• -m68020-40 — generate output for a 68040, but without using any of the new instructions. The result is code that can run relatively efficiently on either a 68020/68881 or a 68030 or a 68040. Note that the generated code does use the 68881 instructions that are emulated on the 68040.

This option is equivalent to -march=68020 -mtune=68020-40. ✨

• -m68020-60 — generate output for a 68060, without using any of the new instructions. That gives you code that can run relatively efficiently on either a 68020/68881 or a 68030 or a 68040. And the generated code does use the 68881 instructions that are emulated on the 68060.

This option is equivalent to -march=68020 -mtune=68020-60.

• -mhard-float, -m68881 — generate floating-point instructions. This is the default for 68020 and above, and for ColdFire devices that have an FPU. It defines the macro __HAVE_68881__ on M680x0 targets and __mcffpu__ on ColdFire targets. 🌈
• -msoft-float — do not generate floating-point instructions; use library calls instead. This is the default for 68000, 68010, and 68832 targets. It's also the default for ColdFire devices that have no FPU.
• -mdiv, -mno-div — generate (or don't 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 comes from the target CPU (either the default CPU, or the one you named with -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. On top of that, parameters passed on the stack are also aligned to a 16-bit boundary, even on targets whose API mandates promotion to 32-bit.
• -mno-short — do not consider type int to be 16 bits wide. This is the default.
• -mnobitfield, -mno-bitfield — do not use the bit-field instructions. The -m68000, -mcpu32 and -m5200 options imply -mnobitfield.
• -mbitfield — do use the bit-field instructions. The -m68020 option implies -mbitfield. This is the default if you use a configuration designed for a 68020. 💅
• -mrtd — use a different function-calling convention, in which functions that take a fixed number of arguments return with the rtd instruction, which pops their arguments while returning. This saves one instruction in the caller, since there's 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. Not a drill, bestie.

In addition, seriously incorrect code results if you call a function with too many arguments. (Normally, extra arguments are harmlessly ignored.)

The rtd instruction is supported by the 68010, 68020, 68030, 68040, 68060 and CPU32 processors, but not by the 68000 or 5200.

The default is -mno-rtd.

• -malign-int, -mno-align-int — control whether GCC aligns int, long, long long, float, double, and long double variables on a 32-bit boundary (-malign-int) or a 16-bit boundary (-mno-align-int). Aligning variables on 32-bit boundaries produces code that runs somewhat faster on processors with 32-bit busses, at the expense of more memory.

Warning: if you use the -malign-int switch, GCC aligns structures containing the above types differently than most published application binary interface specifications for the m68k. 🙈

Use the pc-relative addressing mode of the 68000 directly, instead of using a global offset table. At present, this option implies -fpic, allowing at most a 16-bit offset for pc-relative addressing. -fPIC is not presently supported with -mpcrel, though this could be supported for 68020 and higher processors.

• -mno-strict-align, -mstrict-align — do not (do) assume that unaligned memory references are handled by the system. 🦋
• -msep-data — bestie this one lets your data segment live in a totally different area of memory from your text segment 🦋 which means you get execute-in-place even in an environment with no virtual memory management at all. It implies -fPIC, periodt.
• -mno-sep-data — the opposite vibe: assumes the data segment just follows the text segment like a loyal little shadow. This is the default. 💅
• -mid-shared-library — generates code that supports shared libraries via the library ID method. Execute-in-place and shared libraries in an environment with no virtual memory management? Iconic. This option implies -fPIC. ✨
• -mno-id-shared-library — generates code that does not assume ID-based shared libraries are in play. Also the default. 🌸
• -mshared-library-id=n — tells GCC the identification number of the ID-based shared library you're compiling. Give it 0 and you get more compact code; give it any other value and you force that number to be allocated to the current library — but ngl, that's no more space- or time-efficient than just leaving the option out. 🙈
• -mxgot
• -mno-xgot — when you're generating position-independent code for ColdFire, this generates code that still works if the GOT has more than 8192 entries. Downside: that code is bigger and slower than the normal stuff. On M680x0 processors you don't need this at all — -fPIC suffices, no cap. 💀

So here's the tea: GCC normally loads values from the GOT with a single instruction. Super efficient, love that for us — but it only works if the GOT is smaller than about 64k. Anything bigger and the linker hits you with an error like:

relocation truncated to fit: R_68K_GOT16O foobar

If that happens, recompile your code with -mxgot and it should then handle very large GOTs. But code generated with -mxgot is less efficient, since fetching the value of a global symbol takes 4 instructions. 😭

Also note: some linkers — including newer versions of the GNU linker — can create multiple GOTs and sort GOT entries. If you've got one of those, you should only need -mxgot when compiling a single object file that accesses more than 8192 GOT entries. Very few do, fr fr.

Heads up: these options have no effect unless GCC is generating position-independent code. 🫶

• -mlong-jump-table-offsets — uses 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. Let's get into it 🎀

• -mhardlit
• -mno-hardlit — inlines constants right into the code stream if it can be done in two instructions or less. Snappy. 🔥
• -mdiv
• -mno-div — uses the divide instruction. (Enabled by default). 💅
• -mrelax-immediate
• -mno-relax-immediate — allows arbitrary-sized immediates in bit operations. 🌈
• -mwide-bitfields
• -mno-wide-bitfields — always treats bit-fields as int-sized. 👑
• -m4byte-functions
• -mno-4byte-functions — forces all functions to be aligned to a 4-byte boundary. 🎯
• -mcallgraph-data
• -mno-callgraph-data — emits callgraph information. 🦋
• -mslow-bytes
• -mno-slow-bytes — prefers word access when reading byte quantities. 🍓
• -mlittle-endian
• -mbig-endian — generates code for a little-endian target. 🌸
• -m210
• -m340 — generates code for the 210 processor. ✨
• -mno-lsim — assumes runtime support has already been provided, so it omits the simulator library (libsim.a) from the linker command line. 🫶
• -mstack-increment=size — sets the max amount for a single stack increment operation. Big values can speed up programs that have functions needing a lot of stack space — but they can also trigger a segmentation fault if the stack gets extended too much 😭. The default value is 0x1000. 💀

---

💖 MeP Options 💖

• -mabsdiff — enables the abs instruction, which is the absolute difference between two registers. 🎀
• -mall-opts — enables all the optional instructions — average, multiply, divide, bit operations, leading zero, absolute difference, min/max, clip, and saturation. The full set, no skips. 👑
• -maverage — enables the ave instruction, which computes the average of two registers. ✨
• -mbased=n — variables of size n bytes or smaller get placed in the .based section by default. Based variables use the $tp register as a base register, and there's a 128-byte limit to the .based section. 🌸
• -mbitops — enables the bit operation instructions — bit test (btstm), set (bsetm), clear (bclrm), invert (bnotm), and test-and-set (tas). 💅
• -mc=name — selects which section constant data is placed in. name may be tiny, near, or far. 🦋
• -mclip — enables the clip instruction. Quick note: -mclip is not useful unless you also provide -mminmax. 🙈
• -mconfig=name — selects one of the built-in core configurations. Each MeP chip has one or more modules in it; each module has a core CPU plus a whole variety of coprocessors, optional instructions, and peripherals. The MeP-Integrator tool — not part of GCC — provides these configurations through this option; using this option is the same as using all the corresponding command-line options. The default configuration is default. ⭐
• -mcop — enables the coprocessor instructions. By default this is a 32-bit coprocessor. Note that the coprocessor is normally enabled via the -mconfig= option. 🔥
• -mcop32 — enables the 32-bit coprocessor's instructions. ✨
• -mcop64 — enables the 64-bit coprocessor's instructions. ✨
• -mivc2 — enables IVC2 scheduling. IVC2 is a 64-bit VLIW coprocessor. 🌈
• -mdc — causes constant variables to be placed in the .near section. 🎀
• -mdiv — enables the div and divu instructions. 💅
• -meb — generate big-endian code. 🌸
• -mel — generate little-endian code. 🌸
• -mio-volatile — tells the compiler that any variable marked with the io attribute is to be considered volatile. 🫶
• -ml — causes variables to be assigned to the .far section by default. 🦋
• -mleadz — enables the leadz (leading zero) instruction. ⭐
• -mm — causes variables to be assigned to the .near section by default. 🎀
• -mminmax — enables the min and max instructions. 👑
• -mmult — enables the multiplication and multiply-accumulate instructions. 🔥
• -mno-opts — disables all the optional instructions that -mall-opts turns on. 💀
• -mrepeat — enables the repeat and erepeat instructions, used for low-overhead looping. 🌀
• -ms — causes all variables to default to the .tiny section. Note there's a 65536-byte limit to this section, and accesses to these variables use the %gp base register. 🙈
• -msatur — enables the saturation instructions. Note the compiler does not currently generate these itself, but this option is included for compatibility with other tools, like as. 🌸
• -msdram — links the SDRAM-based runtime instead of the default ROM-based runtime. 🦋
• -msim — links the simulator run-time libraries. ✨
• -msimnovec — links the simulator runtime libraries, excluding built-in support for reset and exception vectors and tables. 💅
• -mtf — causes all functions to default to the .far section. Without this option, functions default to the .near section. 🎀
• -mtiny=n — variables that are n bytes or smaller get allocated to the .tiny section. These variables use the $gp base register. The default for this option is 4, but heads up — there's a 65536-byte limit to the .tiny section. ⭐

---

🦋 MicroBlaze Options 🦋

• -msoft-float — use software emulation for floating point (default). 🌸
• -mhard-float — use hardware floating-point instructions. 🔥
• -mmemcpy — do not optimize block moves, use memcpy. 💅
• -mno-clearbss — > ⚠️ this option is deprecated. Use -fno-zero-initialized-in-bss instead. 💀
• -mcpu=cpu-type — use features of, and schedule code for, the given CPU. Supported values are in the format vX.YY.Z, where X is a major version, YY is the minor version, and Z is compatibility code. Example values are v3.00.a, v4.00.b, v5.00.a, v5.00.b, v6.00.a. 👑
• -mxl-soft-mul — use software multiply emulation (default). 🌸
• -mxl-soft-div — use software emulation for divides (default). 🌸
• -mxl-barrel-shift — use the hardware barrel shifter. ✨
• -mxl-pattern-compare — use pattern compare instructions. 🎀
• -msmall-divides — use table lookup optimization for small signed integer divisions. 🦋
• -mxl-stack-check — > ⚠️ this option is deprecated. Use -fstack-check instead. 💀
• -mxl-gp-opt — use GP-relative .sdata/.sbss sections. 💅
• -mxl-multiply-high — use multiply high instructions for the high part of a 32x32 multiply. 🔥
• -mxl-float-convert — use hardware floating-point conversion instructions. ✨
• -mxl-float-sqrt — use the hardware floating-point square root instruction. 🌈
• -mbig-endian — generate code for a big-endian target. 🌸
• -mlittle-endian — generate code for a little-endian target. 🌸
• -mxl-reorder — use reorder instructions (swap and byte reversed load/store). 🌀
• -mxl-mode-app-model — select application model app-model. The valid models are:
  • executable — normal executable (default), uses startup code crt0.o. 🎀
    • -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 the GOT, since PC-relative addressing is not supported. 🦋
  • xmdstub — for use with Xilinx Microprocessor Debugger (XMD) based software intrusive debug agent called xmdstub. This uses startup file crt1.o and sets the start address of the program to 0x800. 💅
  • bootstrap — for applications that are loaded using a bootloader. This model uses startup file crt2.o, which does not contain a processor reset vector handler. This is suitable for transferring control on a processor reset to the bootloader rather than the application. ✨
  • novectors — for applications that do not require any of the MicroBlaze vectors. This option may be useful for applications running within a monitoring application. This model uses crt3.o as a startup file. 🌸

Also: option -xl-mode-app-model is a deprecated alias for -mxl-mode-app-model. 💀

---

👑 MIPS Options 👑

• -EB — generate big-endian code. 🌈
• -EL — generate little-endian code. This is the default for mips*el-*-* configurations. 🦋
• -march=arch — generate code that actually runs on arch, bestie, and arch can be either the name of a generic MIPS ISA or the name of a specific processor. The ISA names are: mips1, mips2, mips3, mips4, mips32, mips32r2, mips32r3, mips32r5, mips32r6, mips64, mips64r2, mips64r3, mips64r5 and mips64r6. And the processor names are, in all their glory:

4kc, 4km, 4kp, 4ksc, 4kec, 4kem, 4kep, 4ksd,
5kc, 5kf, 20kc, 24kc, 24kf2_1, 24kf1_1, 24kec, 24kef2_1, 24kef1_1,
34kc, 34kf2_1, 34kf1_1, 34kn, 74kc, 74kf2_1, 74kf1_1, 74kf3_2,
1004kc, 1004kf2_1, 1004kf1_1, i6400, i6500, interaptiv, loongson2e,
loongson2f, loongson3a, gs464, gs464e, gs264e, m4k, m14k, m14kc,
m14ke, m14kec, m5100, m5101, octeon, octeon+, octeon2, octeon3,
orion, p5600, p6600, r2000, r3000, r3900, r4000, r4400, r4600,
r4650, r4700, r5900, r6000, r8000, rm7000, rm9000, r10000, r12000,
r14000, r16000, sb1, sr71000, vr4100, vr4111, vr4120, vr4130,
vr4300, vr5000, vr5400, vr5500, xlr and xlp

The special value from-abi selects the most compatible architecture for the selected ABI — that is, mips1 for 32-bit ABIs and mips3 for 64-bit ABIs. 💅

The native Linux/GNU toolchain also supports the value native, which selects the best architecture option for the host processor. Heads up tho: -march=native has no effect if GCC does not recognize the processor. 😭

In processor names, a final 000 can be abbreviated as k (for example, -march=r2k). Prefixes are optional, and vr may be written r. ✨

Names of the form nf2_1 refer to processors with FPUs clocked at half the rate of the core, names of the form nf1_1 refer to processors with FPUs clocked at the same rate as the core, and names of the form nf3_2 refer to processors with FPUs clocked a ratio of 3:2 with respect to the core. For compatibility reasons, nf is accepted as a synonym for nf2_1 while nx and bfx are accepted as synonyms for nf1_1. iykyk 🧚

GCC defines two macros based on the value of this option. The first is _MIPS_ARCH, which gives the name of target architecture, as a string. The second has the form _MIPS_ARCH_foo, where foo is the capitalized value of _MIPS_ARCH. For example, -march=r2000 sets _MIPS_ARCH to "r2000" and defines the macro _MIPS_ARCH_R2000. 👑

Note that the _MIPS_ARCH macro uses the processor names given above. In other words, it has the full prefix and does not abbreviate 000 as k. In the case of from-abi, the macro names the resolved architecture (either "mips1" or "mips3"). It names the default architecture when no -march option is given.

• -mtune=arch — optimize for arch. Among other things, this option controls the way instructions are scheduled, and the perceived cost of arithmetic operations. The list of arch values is the same as for -march. 🌸

When this option is not used, GCC optimizes for the processor specified by -march. By using -march and -mtune together, it is possible to generate code that runs on a family of processors, but optimize the code for one particular member of that family. iconic behavior fr fr 💖

-mtune defines the macros _MIPS_TUNE and _MIPS_TUNE_foo, which work in the same way as the -march ones described above.

• -mips1 — equivalent to -march=mips1.
• -mips2 — equivalent to -march=mips2.
• -mips3 — equivalent to -march=mips3.
• -mips4 — equivalent to -march=mips4.
• -mips32 — equivalent to -march=mips32.
• -mips32r3 — equivalent to -march=mips32r3.
• -mips32r5 — equivalent to -march=mips32r5.
• -mips32r6 — equivalent to -march=mips32r6.
• -mips64 — equivalent to -march=mips64.
• -mips64r2 — equivalent to -march=mips64r2.
• -mips64r3 — equivalent to -march=mips64r3.
• -mips64r5 — equivalent to -march=mips64r5.
• -mips64r6 — equivalent to -march=mips64r6.
• -mips16
• -mno-mips16 — generate (do not generate) MIPS16 code. If GCC is targeting a MIPS32 or MIPS64 architecture, it makes use of the MIPS16e ASE. 🎀

MIPS16 code generation can also be controlled on a per-function basis by means of "mips16" and "nomips16" attributes.

• -mflip-mips16 — generate MIPS16 code on alternating functions. This option is provided for regression testing of mixed MIPS16/non-MIPS16 code generation, and is not intended for ordinary use in compiling user code. like, do not ship this bestie 🙈
• -minterlink-compressed
• -mno-interlink-compressed — require (do not require) that code using the standard (uncompressed) MIPS ISA be link-compatible with MIPS16 and microMIPS code, and vice versa. 💫

For example, code using the standard ISA encoding cannot jump directly to MIPS16 or microMIPS code; it must either use a call or an indirect jump. -minterlink-compressed therefore disables direct jumps unless GCC knows that the target of the jump is not compressed.

• -minterlink-mips16
• -mno-interlink-mips16 — aliases of -minterlink-compressed and -mno-interlink-compressed. These options predate the microMIPS ASE and are retained for backwards compatibility. vintage but make it ✨compatible✨
• -mabi=32
• -mabi=o64
• -mabi=n32
• -mabi=64
• -mabi=eabi — generate code for the given ABI.

Note that the EABI has a 32-bit and a 64-bit variant. GCC normally generates 64-bit code when you select a 64-bit architecture, but you can use -mgp32 to get 32-bit code instead.

For information about the O64 ABI, see http://gcc.gnu.org/projects/mipso64-abi.html.

GCC supports a variant of the o32 ABI in which floating-point registers are 64 rather than 32 bits wide. You can select this combination with -mabi=32 -mfp64. This ABI relies on the mthc1 and mfhc1 instructions and is therefore only supported for MIPS32R2, MIPS32R3 and MIPS32R5 processors. 🫶

The register assignments for arguments and return values remain the same, but each scalar value is passed in a single 64-bit register rather than a pair of 32-bit registers. For example, scalar floating-point values are returned in $f0 only, not a $f0/$f1 pair. The set of call-saved registers also remains the same in that the even-numbered double-precision registers are saved. no cap

Two additional variants of the o32 ABI are supported to enable a transition from 32-bit to 64-bit registers. These are FPXX (-mfpxx) and FP64A (-mfp64 -mno-odd-spreg). The FPXX extension mandates that all code must execute correctly when run using 32-bit or 64-bit registers. The code can be interlinked with either FP32 or FP64, but not both. The FP64A extension is similar to the FP64 extension but forbids the use of odd-numbered single-precision registers. This can be used in conjunction with the "FRE" mode of FPUs in MIPS32R5 processors and allows both FP32 and FP64A code to interlink and run in the same process without changing FPU modes. the way that works?? 💅

• -mabicalls
• -mno-abicalls — generate (do not generate) code that is suitable for SVR4-style dynamic objects. -mabicalls is the default for SVR4-based systems. 🌈
• -mshared
• -mno-shared — generate (do not generate) code that is fully position-independent, and that can therefore be linked into shared libraries. This option only affects -mabicalls.

All -mabicalls code has traditionally been position-independent, regardless of options like -fPIC and -fpic. However, as an extension, the GNU toolchain allows executables to use absolute accesses for locally-binding symbols. It can also use shorter GP initialization sequences and generate direct calls to locally-defined functions. This mode is selected by -mno-shared. 👑

-mno-shared depends on binutils 2.16 or higher and generates objects that can only be linked by the GNU linker. However, the option does not affect the ABI of the final executable; it only affects the ABI of relocatable objects. Using -mno-shared generally makes executables both smaller and quicker. ate that 🔥

-mshared is the default.

• -mplt
• -mno-plt — assume (do not assume) that the static and dynamic linkers support PLTs and copy relocations. This option only affects -mno-shared -mabicalls. For the n64 ABI, this option has no effect without -msym32. 💀

You can make -mplt the default by configuring GCC with --with-mips-plt. The default is -mno-plt otherwise.

• -mxgot
• -mno-xgot — lift (do not lift) the usual restrictions on the size of the global offset table. 🦋

GCC normally uses a single instruction to load values from the GOT. While this is relatively efficient, it only works if the GOT is smaller than about 64k. Anything larger causes the linker to report an error such as:

relocation truncated to fit: R_MIPS_GOT16 foobar

If this happens, you should recompile your code with -mxgot. This works with very large GOTs, although the code is also less efficient, since it takes three instructions to fetch the value of a global symbol. 😭

Note that some linkers can create multiple GOTs. If you have such a linker, you should only need to use -mxgot when a single object file accesses more than 64k's worth of GOT entries. Very few do.

These options have no effect unless GCC is generating position independent code.

• -mgp32 — assume that general-purpose registers are 32 bits wide.
• -mgp64 — assume that general-purpose registers are 64 bits wide.
• -mfp32 — assume that floating-point registers are 32 bits wide.
• -mfp64 — assume that floating-point registers are 64 bits wide.
• -mfpxx — do not assume the width of floating-point registers.
• -mhard-float — use floating-point coprocessor instructions.
• -msoft-float — do not use floating-point coprocessor instructions. Implement floating-point calculations using library calls instead.
• -mno-float — equivalent to -msoft-float, but additionally asserts that the program being compiled does not perform any floating-point operations. This option is presently supported only by some bare-metal MIPS configurations, where it may select a special set of libraries that lack all floating-point support (including, for example, the floating-point "printf" formats). If code compiled with -mno-float accidentally contains floating-point operations, it is likely to suffer a link-time or run-time failure. be so careful bestie 🫶
• -msingle-float — assume that the floating-point coprocessor only supports single-precision operations.
• -mdouble-float — bestie, we're assuming the floating-point coprocessor can do double-precision ops. This is the default. periodt ✨
• -modd-spreg / -mno-odd-spreg — turn on (or off) using odd-numbered single-precision floating-point registers for the o32 ABI 🎀. This is the default for processors that are known to support these registers. But lowkey, when you're using the o32 FPXX ABI, -mno-odd-spreg gets set by default. no cap.
• -mabs=2008 / -mabs=legacy — these two control how we treat that special not-a-number (NaN) IEEE 754 floating-point data when it meets the "abs.fmt" and "neg.fmt" machine instructions 👀.

By default — or whenever you use -mabs=legacy — the legacy treatment is what you get. In that case these instructions count as arithmetic, so they get avoided wherever correct operation is required and the input operand might be a NaN. Instead, GCC uses a longer sequence of instructions that manually manipulates the sign bit of the floating-point datum — unless you also passed -ffinite-math-only.

The -mabs=2008 option selects the IEEE 754-2008 treatment 💅. Now these instructions count as non-arithmetic, so they operate correctly in all cases, including — and especially — when the input operand is a NaN. So these instructions are always used for the respective operations. slay.

• -mnan=2008 / -mnan=legacy — these control the encoding of the special not-a-number (NaN) IEEE 754 floating-point data 🌸.

The -mnan=legacy option selects the legacy encoding. In this case quiet NaNs (qNaNs) are denoted by the first bit of their trailing significand field being 0, whereas signaling NaNs (sNaNs) are denoted by the first bit of their trailing significand field being 1.

The -mnan=2008 option selects the IEEE 754-2008 encoding ✨. Here it flips: qNaNs are denoted by the first bit of their trailing significand field being 1, whereas sNaNs are denoted by the first bit of their trailing significand field being 0.

The default is -mnan=legacy unless GCC has been configured with --with-nan=2008.

• -mllsc / -mno-llsc — use (or don't use) ll, sc, and sync instructions to implement atomic memory built-in functions 🧚. When neither option is specified, GCC uses the instructions if the target architecture supports them.

-mllsc is useful if the runtime environment can emulate the instructions, and -mno-llsc can be useful when you're 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 the deets.

• -mdsp / -mno-dsp — use (or don't use) revision 1 of the MIPS DSP ASE 💖.

This option defines the preprocessor macro "__mips_dsp". It also defines "__mips_dsp_rev" to 1.

• -mdspr2 / -mno-dspr2 — use (or don't use) revision 2 of the MIPS DSP ASE 🌈.

This option defines the preprocessor macros "__mips_dsp" and "__mips_dspr2". It also defines "__mips_dsp_rev" to 2.

• -msmartmips / -mno-smartmips — use (or don't use) the MIPS SmartMIPS ASE ✨.
• -mpaired-single / -mno-paired-single — use (or don't use) paired-single floating-point instructions 🎀.

This option requires hardware floating-point support to be enabled. not negotiable, bestie.

• -mdmx / -mno-mdmx — use (or don't use) MIPS Digital Media Extension instructions 💜. This option can only be used when generating 64-bit code, and it requires hardware floating-point support to be enabled.
• -mips3d / -mno-mips3d — use (or don't use) the MIPS-3D ASE 🦋. The option -mips3d implies -mpaired-single.
• -mmicromips / -mno-micromips — generate (or don't generate) microMIPS code 👑.

MicroMIPS code generation can also be controlled on a per-function basis by means of "micromips" and "nomicromips" attributes. iykyk.

• -mmt / -mno-mt — use (or don't use) MT Multithreading instructions 🔥.
• -mmcu / -mno-mcu — use (or don't use) the MIPS MCU ASE instructions 🌸.
• -meva / -mno-eva — use (or don't use) the MIPS Enhanced Virtual Addressing instructions ⭐.
• -mvirt / -mno-virt — use (or don't use) the MIPS Virtualization (VZ) instructions 🧚.
• -mxpa / -mno-xpa — use (or don't use) the MIPS eXtended Physical Address (XPA) instructions 💖.
• -mcrc / -mno-crc — use (or don't use) the MIPS Cyclic Redundancy Check (CRC) instructions 💅.
• -mginv / -mno-ginv — use (or don't use) the MIPS Global INValidate (GINV) instructions ✨.
• -mloongson-mmi / -mno-loongson-mmi — use (or don't use) the MIPS Loongson MultiMedia extensions Instructions (MMI) 🌈.
• -mloongson-ext / -mno-loongson-ext — use (or don't use) the MIPS Loongson EXTensions (EXT) instructions 🦋.
• -mloongson-ext2 / -mno-loongson-ext2 — use (or don't use) the MIPS Loongson EXTensions r2 (EXT2) instructions 💜.
• -mlong64 — force "long" types to be 64 bits wide 👑. See -mlong32 for an explanation of the default and the way that the pointer size is determined.
• -mlong32 — force "long", "int", and pointer types to be 32 bits wide 🎀.

The default size of "int"s, "long"s and pointers depends on the ABI. All the supported ABIs use 32-bit "int"s. The n64 ABI uses 64-bit "long"s, as does the 64-bit EABI; the others use 32-bit "long"s. Pointers are the same size as "long"s, or the same size as integer registers, whichever is smaller. fr fr.

• -msym32 / -mno-sym32 — assume (or don't assume) that all symbols have 32-bit values, regardless of the selected ABI 🌸. This option is useful in combination with -mabi=64 and -mno-abicalls because it lets GCC generate shorter and faster references to symbolic addresses. ate.
• -G num — put definitions of externally-visible data in a small data section if that data is no bigger than num bytes 💖. Then GCC can generate more efficient accesses to the data; see -mgpopt for the details.

The default -G option depends on the configuration.

• -mlocal-sdata / -mno-local-sdata — extend (or don't extend) the -G behavior to local data too, like static variables in C ✨. -mlocal-sdata is the default for all configurations.

If the linker starts complaining that an application is using too much small data, you might want to try rebuilding the less performance-critical parts with -mno-local-sdata. You might also want to build large libraries with -mno-local-sdata, so that the libraries leave more room for the main program 🫶.

• -mextern-sdata / -mno-extern-sdata — assume (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 if you compile a module Mod with -mextern-sdata -G num -mgpopt, and Mod references a variable Var that is no bigger than num bytes, you must make sure that Var is placed in a small data section. If Var is defined by another module, you must either compile that module with a high-enough -G setting or attach a "section" attribute to Var's definition. If Var is common, you must link the application with a high-enough -G setting.

The easiest way of satisfying these restrictions is to compile and link every module with the same -G option. However, you may wish to build a library that supports several different small data limits. You can do this by compiling the library with the highest supported -G setting and additionally using -mno-extern-sdata to stop the library from making assumptions about externally-defined data. iconic behavior honestly.

• -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.)

-mno-gpopt implies -mno-local-sdata and -mno-extern-sdata.

• -membedded-data / -mno-embedded-data — allocate variables to the read-only data section first if possible, then next in the small data section if possible, otherwise in data 🌸. This gives slightly slower code than the default, but it reduces the amount of RAM required when executing, and thus may be preferred for some embedded systems.
• -muninit-const-in-rodata / -mno-uninit-const-in-rodata — put uninitialized "const" variables in the read-only data section ✨. This option is only meaningful in conjunction with -membedded-data.
• -mcode-readable=setting — specify whether GCC may generate code that reads from executable sections 👀. There are three possible settings:
  • -mcode-readable=yes — instructions may freely access executable sections. This is the default setting.
  • -mcode-readable=pcrel — MIPS16 PC-relative load instructions can access executable sections, but other instructions must not do so. This option is useful on 4KSc and 4KSd processors when the code TLBs have the Read Inhibit bit set. It's also useful on processors that can be configured to have a dual instruction/data SRAM interface and that, like the M4K, automatically redirect PC-relative loads to the instruction RAM 💖.
  • -mcode-readable=no — instructions must not access executable sections. This option can be useful on targets that are configured to have a dual instruction/data SRAM interface but that (unlike the M4K) do not automatically redirect PC-relative loads to the instruction RAM.
• -msplit-addresses / -mno-split-addresses — enable (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.
• -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 — okay so MIPS systems check for division by zero in one of two ways: they either generate a conditional trap or a break instruction. Bestie, the tea is that traps give you smaller code, but they're only supported on MIPS II and later 😤. And like… some versions of the Linux kernel have a bug that stops the trap from generating the proper signal (SIGFPE), which is lowkey a problem. So you 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. And if you want zero division checks at all, you can completely disable them with -mno-check-zero-division. ✨ gone, deleted, no more ✨

• -mload-store-pairs, -mno-load-store-pairs — enable (or disable) an optimization that pairs up consecutive load or store instructions so you get load/store bonding 🫶. This one is enabled by default, but — important — it only actually does anything when the selected architecture is known to support bonding. So like, it's on, but it's not on on, yk? vibe check passed conditionally
• -mmemcpy, -mno-memcpy — force (or don't force) the use of memcpy for non-trivial block moves. The default is -mno-memcpy, which lets GCC inline most constant-sized copies. inlining my beloved
• -mlong-calls, -mno-long-calls — disable (or don't disable) use of the jal instruction. Calling functions with jal is more efficient, but it requires that the caller and callee both live in the same 256 megabyte segment. 256 MB bestie, that's the limit, no exceptions

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. slay
• -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 said "not today" 😭
• -mfused-madd, -mno-fused-madd — enable (or disable) use of the floating-point multiply-accumulate instructions, when they're available. The default is -mfused-madd.

⚠️ warning time ⚠️ 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. So like, R8000 is doing its own thing main character energy fr fr

• -nocpp — tell the MIPS assembler to not run its preprocessor over user assembler files (the ones with a .s suffix) when assembling them. preprocessing? we don't know her
• -mfix-24k, -mno-fix-24k — work around the 24K E48 errata (that's "lost data on stores during refill", yikes). The workarounds are implemented by the assembler rather than by GCC. not our problem, it's the assembler's problem 💅
• -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. deadlock?? bestie RUN

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 instead always schedule a nop instruction there. So here's the lore: 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. okay strategy
• -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.) 💅

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• -mr10k-cache-barrier=setting — specify whether GCC should insert cache barriers to avoid the side effects of speculation on R10K processors. buckle up, this one's a whole saga

In common with many processors, the R10K tries to predict the outcome of a conditional branch and speculatively executes instructions from the "taken" branch. It later aborts these instructions if the predicted outcome is wrong. However — and this is the plot twist — on the R10K, even aborted instructions can have side effects. 😭 the audacity

This problem only affects kernel stores and, depending on the system, kernel loads. As an example: a speculatively-executed store may load the target memory into cache and mark the cache line as dirty, even if the store itself is later aborted. If a DMA operation writes to the same area of memory before the "dirty" line is flushed, the cached data overwrites the DMA-ed data. 💀 catastrophic See the R10K processor manual for a full description, including other potential problems.

One workaround is to insert cache barrier instructions before every memory access that might be speculatively executed and that might have side effects even if aborted. -mr10k-cache-barrier=setting controls GCC's implementation of this workaround. It assumes that aborted accesses to any byte in the following regions does not have side effects:

1. the memory occupied by the current function's stack frame;
2. the memory occupied by an incoming stack argument;
3. the memory occupied by an object with a link-time-constant address.

It is the kernel's responsibility to ensure that speculative accesses to these regions are indeed safe. not GCC's problem, that's on the kernel bestie

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. mutual respect, iykyk

The option has three forms:

• -mr10k-cache-barrier=load-store — insert a cache barrier before a load or store that might be speculatively executed and that might have side effects even if aborted.
• -mr10k-cache-barrier=store — insert a cache barrier before a store that might be speculatively executed and that might have side effects even if aborted.
• -mr10k-cache-barrier=none — disable the insertion of cache barriers. This is the default setting.

• -mflush-func=func, -mno-flush-func — specifies the function to call to flush the I and D caches, or to not call any such function. If called, the function must take the same arguments as the common _flush_func, that is: the address of the memory range for which the cache is being flushed, the size of the memory range, and the number 3 (to flush both caches). The default depends on the target GCC was configured for, but commonly is either _flush_func or __cpu_flush. ✨ mystery default ✨
• mbranch-cost=num — set the cost of branches to roughly num "simple" instructions. This cost is only a heuristic and is not guaranteed to produce consistent results across releases. A zero cost redundantly selects the default, which is based on the -mtune setting. it's giving "approximate"
• -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. An exception is the MIPS32 and MIPS64 architectures and processors that implement those architectures — for those, Branch Likely instructions are not generated by default, because the MIPS32 and MIPS64 architectures specifically deprecate their use. deprecated, periodt
• -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. never ever
  • The -mcompact-branches=always option ensures that a compact branch instruction will be generated if available. If a compact branch instruction is not available, a delay slot form of the branch will be used instead.
  • This option is supported from MIPS Release 6 onwards. release 6 girlies rise
  • 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. optimal literally meaning optimal ✨
• -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. 🫶

So like, picture the SB-1: if FP exceptions are off and we're spitting out 64-bit code, bestie, we get to use both FP pipes 💅. Any other situation? Only one FP pipe for you. Tragic but true.

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• -mvr4130-align, -mno-vr4130-align — the VR4130 pipeline is two-way superscalar, but it can literally only issue two instructions together if the first one is 8-byte aligned. Turn this on and GCC lines up pairs of instructions it thinks should run in parallel. ✨ Heads up tho: this only does anything when you're optimizing for the VR4130. Usually it makes your code faster, but at the cost of making it bigger. It's on by default at optimization level -O3.
• -msynci, -mno-synci — turn on (or off) generation of synci instructions on architectures that support them. If enabled, the synci instructions get generated whenever __builtin___clear_cache is compiled. 🧚 Default is -mno-synci, but you can override that default by configuring GCC with --with-synci. When you're compiling for single processor systems it's generally safe to use synci. But on lots of multi-core (SMP) systems?? It does not invalidate the instruction caches on all the cores and may lead to undefined behavior. 😭 So like, be careful.
• -mrelax-pic-calls, -mno-relax-pic-calls — try to turn PIC calls that normally get 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. And with -mno-explicit-relocs, this optimization can be performed by the assembler and the linker all on their own, no help from the compiler needed. 🫶
• -mmcount-ra-address, -mno-mcount-ra-address — emit (or don't emit) code that lets _mcount modify the calling function's return address. When it's 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. Then _mcount can modify the return address by doing both of these:
  • returning the new address in register $31, and
  • storing the new address in *ra-address, if ra-address is nonnull.

Default is -mno-mcount-ra-address.

• -mframe-header-opt, -mno-frame-header-opt — enable (or disable) frame header optimization in the o32 ABI. When you're using the o32 ABI, calling functions allocate 16 bytes on the stack so the called function can write out register arguments. When this is enabled, the optimization suppresses that frame header allocation if it can figure out it's unused. It's off by default at all optimization levels. 🙈
• -mlxc1-sxc1, -mno-lxc1-sxc1 — when applicable, enable (or disable) generation of lwxc1, swxc1, ldxc1, sdxc1 instructions. Enabled by default. 💖
• -mmadd4, -mno-madd4 — when applicable, enable (or disable) generation of 4-operand madd.s, madd.d and related instructions. Enabled by default. 🔥

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🦋 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 (inside the called function) are seen as registers $0 and up, as opposed to the GNU ABI which uses global registers $231 and up.
• -mzero-extend, -mno-zero-extend — when reading data from memory in sizes shorter than 64 bits, use (or don't use) zero-extending load instructions by default, rather than sign-extending ones.
• -mknuthdiv, -mno-knuthdiv — make the result of a division that yields 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, and the latter is almost exclusively the one people use. 💅
• -mtoplevel-symbols, -mno-toplevel-symbols — prepend (or don't prepend) a : to all global symbols, so the assembly code can be used with the PREFIX assembly directive.
• -melf — generate an executable in the ELF format, rather than the default mmo format used by the mmix simulator.
• -mbranch-predict, -mno-branch-predict — use (or don't use) the probable-branch instructions, when static branch prediction indicates a probable branch.
• -mbase-addresses, -mno-base-addresses — generate (or don't generate) code that uses base addresses. Using a base address automatically generates a request (handled by the assembler and the linker) for a constant to be set up in a global register. That register is used for one or more base address requests within the range 0 to 255 from the value held in the register. This generally leads to short and fast code, but the number of different data items that can be addressed is limited. Which means a program that uses lots of static data may need -mno-base-addresses. 😬
• -msingle-exit, -mno-single-exit — force (or don't force) generated code to have a single exit point in each function.

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🌸 MN10300 OPTIONS 🌸

These -m options are defined for Matsushita MN10300 architectures:

• -mmult-bug — generate code to avoid bugs in the multiply instructions for the MN10300 processors. This is the default.
• -mno-mult-bug — do not generate code to avoid bugs in the multiply instructions for the MN10300 processors.
• -mam33 — generate code using features specific to the AM33 processor.
• -mno-am33 — do not generate code using features specific to the AM33 processor. This is the default.
• -mam33-2 — generate code using features specific to the AM33/2.0 processor.
• -mam34 — generate code using features specific to the AM34 processor.
• -mtune=cpu-type — use the timing characteristics of the indicated CPU type when scheduling instructions. This does not change the targeted processor type. The CPU type has to be one of mn10300, am33, am33-2 or am34.
• -mreturn-pointer-on-d0 — when generating a function that returns a pointer, return the pointer in both a0 and d0. Otherwise the pointer is returned only in a0, and attempts to call such functions without a prototype result in errors. Note that this option is on by default; use -mno-return-pointer-on-d0 to disable it. 🫶
• -mno-crt0 — do not link in the C run-time initialization object file.
• -mrelax — tell the linker it should perform a relaxation optimization pass to shorten branches, calls and absolute memory addresses. This option only does anything when used on the command line for the final link step. ⚠️ Heads up: this option makes symbolic debugging impossible.
• -mliw — allow the compiler to generate Long Instruction Word instructions if the target is the AM33 or later. This is the default. This option defines the preprocessor macro __LIW__.
• -mno-liw — do not allow the compiler to generate Long Instruction Word instructions. This option defines the preprocessor macro __NO_LIW__.
• -msetlb — allow the compiler to generate the SETLB and Lcc instructions if the target is the AM33 or later. This is the default. This option defines the preprocessor macro __SETLB__.
• -mno-setlb — do not allow the compiler to generate SETLB or Lcc instructions. This option defines the preprocessor macro __NO_SETLB__.

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✨ MOXIE OPTIONS ✨

• -meb — generate big-endian code. This is the default for moxie-*-* configurations.
• -mel — generate little-endian code.
• -mmul.x — generate mul.x and umul.x instructions. This is the default for moxiebox-*-* configurations.
• -mno-crt0 — do not link in the C run-time initialization object file.

---

🎀 MSP430 OPTIONS 🎀

These options are defined for the MSP430:

• -masm-hex — force assembly output to always use hex constants. Normally those constants are signed decimals, but this option exists for testsuite and/or aesthetic purposes. 💅
• -mmcu= — select the MCU to target. This is used to create a C preprocessor symbol based on the MCU name, converted to upper case and pre- and post-fixed with __. That in turn gets used by the msp430.h header file to select an MCU-specific supplementary header file.

The option also sets the ISA to use. If the MCU name is one that's known to only support the 430 ISA then that one gets selected, otherwise the 430X ISA is selected. A generic MCU name of msp430 can also be used to select the 430 ISA. Similarly the generic msp430x MCU name selects the 430X ISA.

On top of that, 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 causes the linker to search for a script called xxx.ld. ✨

The ISA and hardware multiply supported for the different MCUs is hard-coded into GCC. However, an external devices.csv file can be used to extend device support beyond the ones that have been hard-coded. 👑

GCC searches for the devices.csv file using the following methods in the given precedence order, where the first method takes precedence over the second, which takes precedence over the third:

• Include path specified with -I and -L — devices.csv 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 value of the global environment variable MSP430_GCC_INCLUDE_DIR to the full path to the directory containing devices.csv, and GCC will search that directory for devices.csv. If devices.csv is found, this directory will also be 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

okay so LAST resort bestie — GCC is gonna peek at msp430-elf{,bare}/include/devices from the toolchain root directory. 💅

and like… this directory does not exist in a default installation, so don't go looking for it and getting confused. but IF the user created it and copied devices.csv in there, then the MCU data gets read. ✨

same deal as before, queen: this directory also gets registered as an include path and a linker library path.

If none of the above search methods find devices.csv, then the hard-coded MCU data is used. that's the fallback, periodt. 🫶

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• -mwarn-mcu, -mno-warn-mcu — turns warnings about conflicts ON or OFF, specifically the beef between the MCU name you gave with -mmcu and the ISA set by -mcpu and/or the hardware multiply support set by -mhwmult. it also toggles warnings about unrecognized MCU names. this one is on by default. 👑
• -mcpu= — specifies the ISA to use. accepted values are msp430, msp430x and msp430xv2. ⚠️ this option is deprecated, so like… don't. use -mmcu= to select the ISA instead. it's giving old news.
• -msim — link to the simulator runtime libraries and linker script. this overrides any scripts that would've been selected by the -mmcu= option. 😭
• -mlarge — use large-model addressing (20-bit pointers, 32-bit size_t).
• -msmall — use small-model addressing (16-bit pointers, 16-bit size_t).
• -mrelax — this option gets passed to the assembler and linker, and it lets the linker do certain optimizations that literally cannot happen until the final link. no cap. 🔥
• mhwmult= — describes the type of hardware multiply supported by the target. accepted values:
  • none — no hardware multiply
  • 16bit — the original 16-bit-only multiply supported by early MCUs
  • 32bit — the 16/32-bit multiply supported by later MCUs
  • f5series — the 16/32-bit multiply supported by F5-series MCUs
  • auto — can also be given! this tells GCC to deduce the hardware multiply support based upon the MCU name provided by the -mmcu option. if no -mmcu option is specified or if the MCU name is not recognized, then no hardware multiply support is assumed. auto is the default setting. ✨

Hardware multiplies are normally performed by calling a library routine — this saves space in the generated code. 📦 but when you're compiling at -O3 or higher, the hardware multiplier gets invoked inline instead. bigger, but faster code. it's giving main character energy. 💅

The hardware multiply routines disable interrupts whilst running and restore the previous interrupt state when they finish. so they're 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 is sent to write.

This option requires Newlib Nano IO, so GCC must be configured with --enable-newlib-nano-formatted-io. 🫶

• -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= — passes on a request to the assembler to enable the fixes for the named silicon errata. 🔧
• -msilicon-errata-warn= — passes on a request to the assembler to enable warning messages when a silicon errata might need to be applied.
• -mwarn-devices-csv, -mno-warn-devices-csv — warn if devices.csv is not found or there are problems parsing it (default: on). 🙈

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NDS32 Options

These options are defined for NDS32 implementations: 💜

• -mbig-endian — generate code in big-endian mode.
• -mlittle-endian — generate code in little-endian mode.
• -mreduced-regs — use reduced-set registers for register allocation.
• -mfull-regs — use full-set registers for register allocation.
• -mcmov — generate conditional move instructions.
• -mno-cmov — do not generate conditional move instructions.
• -mext-perf — generate performance extension instructions.
• -mno-ext-perf — do not generate performance extension instructions.
• -mext-perf2 — generate performance extension 2 instructions.
• -mno-ext-perf2 — do not generate performance extension 2 instructions.
• -mext-string — generate string extension instructions.
• -mno-ext-string — do not generate string extension instructions.
• -mv3push — generate v3 push25/pop25 instructions.
• -mno-v3push — do not generate v3 push25/pop25 instructions.
• -m16-bit — generate 16-bit instructions.
• -mno-16-bit — do not generate 16-bit instructions.
• *-misr-vector-size=num* — specify the size of each interrupt vector, which must be 4 or 16. 📐
• *-mcache-block-size=num* — specify the size of each cache block, which must be a power of 2 between 4 and 512.
• *-march=arch* — specify the name of the target architecture.
• *-mcmodel=code-model* — set the code model to one of:
  • small — all the data and read-only data segments must be within 512KB addressing space. the text segment must be within 16MB addressing space.
  • medium — the data segment must be within 512KB while the read-only data segment can be within 4GB addressing space. the text segment should be still within 16MB addressing space.
  • large — all the text and data segments can be within 4GB addressing space. 🔥
• -mctor-dtor — enable constructor/destructor feature.
• -mrelax — guide linker to relax instructions.

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Nios II Options

These are the options defined for the Altera Nios II processor. 🦋

• -G num — put global and static objects less than or equal to num bytes into the small data or BSS sections instead of the normal data or BSS sections. the default value of num is 8.
• -mgpopt=option, -mgpopt, -mno-gpopt — generate (do not generate) GP-relative accesses. the following option names are recognized:
  • none — do not generate GP-relative accesses.
  • local — generate GP-relative accesses for small data objects that are not external, 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 — as for local, but also generate GP-relative accesses for small data objects that are external, weak, or common. ⚠️ if you use this option, you must ensure that all parts of your program (including libraries) are compiled with the same -G setting.
  • data — generate GP-relative accesses for all data objects in the program. ⚠️ if you use this option, the entire data and BSS segments of your program must fit in 64K of memory and you must use an appropriate linker script to allocate them within the addressable range of the global pointer.
  • all — generate GP-relative addresses for function pointers as well as data pointers. ⚠️ if you use this option, the entire text, data, and BSS segments of your program must fit in 64K of memory and you must use an appropriate linker script to allocate them within the 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. 🙈

• -mel, -meb — generate little-endian (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 — bestie, when you turn this on, volatile memory accesses dodge the cache entirely by using the I/O variants of the load and store instructions. Default? Not to bypass. Periodt.
• -mno-fast-sw-div, -mfast-sw-div — this one says "no table-based fast divide for tiny numbers, thank u next". 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 — toggle whether the compiler emits the "mul", "mulx" and "div" family of instructions. The default is to emit "mul" and not emit "div" and "mulx". That's just how it is. 🫶
• -mbmx, -mno-bmx, -mcdx, -mno-cdx — turn generation of Nios II R2 BMX (bit manipulation) and CDX (code density) instructions on or off. Enabling these also requires -march=r2. Since these are optional extensions to the R2 architecture, the default is not to emit them. ✨
• -mcustom-insn=N, -mno-custom-insn — okay so each -mcustom-insn=N option enables use of a custom instruction with encoding N when generating code that uses insn. Like, 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. 👑

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.

---

And now the plot twist, bestie — 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. no cap, it handles all of that for you 🧚

• fwrx — write src1 into the least significant half of X and src2 into the most significant half of X.
• fwry — write src1 into Y.
• frdxhi, frdxlo — read the most or least (respectively) significant half of X and store it in dest.
• frdy — read the value of Y and store it into dest.

Note that you can gain more local control over generation of Nios II custom instructions by using the target("custom-insn=N") and target("no-custom-insn") function attributes or pragmas.

• -mcustom-fpu-cfg=name — this one enables a predefined, named set of custom instruction encodings (peep -mcustom-insn above 👆). Currently, the following sets are defined:

-mcustom-fpu-cfg=60-1 is equivalent to: -mcustom-fmuls=252
-mcustom-fadds=253 -mcustom-fsubs=254 -fsingle-precision-constant

-mcustom-fpu-cfg=60-2 is equivalent to: -mcustom-fmuls=252
-mcustom-fadds=253 -mcustom-fsubs=254 -mcustom-fdivs=255
-fsingle-precision-constant

-mcustom-fpu-cfg=72-3 is equivalent to: -mcustom-floatus=243
-mcustom-fixsi=244 -mcustom-floatis=245 -mcustom-fcmpgts=246
-mcustom-fcmples=249 -mcustom-fcmpeqs=250 -mcustom-fcmpnes=251
-mcustom-fmuls=252 -mcustom-fadds=253 -mcustom-fsubs=254
-mcustom-fdivs=255 -fsingle-precision-constant

Custom instruction assignments given by individual -mcustom-insn= options override those given by -mcustom-fpu-cfg=, regardless of the order of the options on the command line. That's the rule, we don't make it. 💅

Note that you can gain more local control over selection of a FPU configuration by using the target("custom-fpu-cfg=name") function attribute or pragma.

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These additional -m options are available for the Altera Nios II ELF (bare-metal) target:

• -mhal — link with HAL BSP. This suppresses linking with the GCC-provided C runtime startup and termination code, and is typically used in conjunction with -msys-crt0= to specify the location of the alternate startup code provided by the HAL BSP.
• -msmallc — link with a limited version of the C library, -lsmallc, rather than Newlib.
• -msys-crt0=startfile — startfile is the file name of the startfile (crt0) to use when linking. This option is only useful in conjunction with -mhal.
• -msys-lib=systemlib — systemlib is the library name of the library that provides low-level system calls required by the C library, e.g. read and write. This option is typically used to link with a library provided by a HAL BSP.

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🌈 Nvidia PTX Options 🌈

These options are defined for Nvidia PTX:

• -m32, -m64 — generate code for 32-bit or 64-bit ABI.
• -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_30.
• -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.
• -mgomp — generate code for use in OpenMP offloading: enables -msoft-stack and -muniform-simt options, and selects corresponding multilib variant.

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🌸 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.
• -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 — okay so when -mhard-float is the vibe, this turns on generation of unordered floating point compare and set flag (lf.sfun*) instructions 💅. By default, functions from libgcc are the ones doing the unordered floating point compare and set flag ops. slay.
• -mcmov — enables generation of conditional move (l.cmov) instructions ✨. Default behaviour? The equivalent gets generated using set and branch instead. no cap.
• -mror — turns on generation of rotate right (l.ror) instructions 🌀. By default, functions from libgcc handle rotate right operations.
• -mrori — enables generation of rotate right with immediate (l.rori) instructions 💫. By default, libgcc functions handle rotate right with immediate operations.
• -msext — enables generation of sign extension (l.ext*) instructions 🫶. Default is memory loads doing the sign extension.
• -msfimm — enables generation of compare and set flag with immediate (l.sf*i) instructions 🔥. By default, extra instructions get generated to store the immediate to a register first.
• -mshftimm — enables generation of shift with immediate (l.srai, l.srli, l.slli) instructions ⭐. By default, extra instructions get generated to store the immediate to a register first.

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🌈 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 the one in use.

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🍓 picoChip Options 🍓

These -m options are defined for picoChip implementations:

• -mae=ae_type — set the instruction set, register set, and instruction scheduling parameters for array element type ae_type. Supported values for ae_type are ANY, MUL, and MAC.
  • -mae=ANY selects a completely generic AE type. Code generated with this option runs on any of the other AE types. The code is not as efficient as it would be if compiled for a specific AE type, and some types of operation (e.g., multiplication) do not work properly on all types of AE.
  • -mae=MUL selects a MUL AE type. This is the most useful AE type for compiled code, and is the default.
  • -mae=MAC selects a DSP-style MAC AE. Code compiled with this option may suffer from poor performance of byte (char) manipulation, since the DSP AE does not provide hardware support for byte load/stores.
• -msymbol-as-address — enable the compiler to directly use a symbol name as an address in a load/store instruction, without first loading it into a register 👑. Typically, the use of this option generates larger programs, which run faster than when the option isn't used. However, the results vary from program to program, so it is left as a user option, rather than being permanently enabled.
• -mno-inefficient-warnings — disables warnings about the generation of inefficient code 💀. These warnings can be generated, for example, when compiling code that performs byte-level memory operations on the MAC AE type. The MAC AE has no hardware support for byte-level memory operations, so all byte load/stores must be synthesized from word load/store operations. This is inefficient and a warning is generated to indicate that you should rewrite the code to avoid byte operations, or to target an AE type that has the necessary hardware support. This option disables these warnings.

---

🌸 PowerPC Options 🌸

These are listed under

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💜 PRU Options 💜

These command-line options are defined for PRU target:

• -minrt — link with a minimum runtime environment, with no support for static initializers and constructors 🎀. Using this option can significantly reduce the size of the final ELF binary. Beware that the compiler could still generate code with static initializers and constructors. It is up to the programmer to ensure that the source program will not use those features.
• -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. These are the differences:

Function Pointer Size

TI ABI specifies that function (code) pointers are 16-bit, whereas GNU supports only 32-bit data and code pointers.

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:

• -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.
• -mtune=processor-string — optimize the output for the given processor, specified by microarchitecture name. Permissible values for this option are: rocket, sifive-3-series, sifive-5-series, sifive-7-series, and size.

When -mtune= is not specified, the default is rocket.

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. fr fr, do not skip this.

• -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.
• -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 — bestie this one makes code for the medium-any code model 💖 and the rules are: your program and all its statically defined symbols have to live inside any single 2 GiB address range. Doesn't matter if you're statically or dynamically linked, both work. ✨
• -mexplicit-relocs, -mno-exlicit-relocs — turn ON (or OFF) the use of assembler relocation operators when you're dealing with symbolic addresses 🎀. The alternative vibe is to use assembler macros instead, and heads up: that may limit optimization 😭.
• -mrelax, -mno-relax — this is the "take advantage of linker relaxations" switch, and it exists to reduce the number of instructions needed to materialize symbol addresses 🔥. And the default? take advantage of linker relaxations, obviously, we love efficiency in this house 💅.
• -memit-attribute, -mno-emit-attribute — emit (or don't emit, we're not judging) a RISC-V attribute that records extra information into ELF objects 🦋. Real talk though: this feature needs at least binutils 2.32, no cap.
• -malign-data=*type* — controls how GCC aligns variables and constants of array, structure, or union types 🌸. The supported values for type are:
  • xlen — uses x register width as the alignment value
  • natural — uses natural alignment

And the default is xlen, periodt. 👑

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🍓 RL78 Options 🍓

• -msim — links in additional target libraries so your stuff can actually operate inside a simulator 🧚.
• -mmul=none, -mmul=g10, -mmul=g13, -mmul=g14, -mmul=rl78 — tells GCC what kind of hardware multiplication and division support to use ✨.
  • none is the simplest — software for both multiplication and division — and it's the default.
  • g13 is for the hardware multiply/divide peripheral found on the RL78/G13 (S2 core) targets.
  • g14 selects the multiplication and division instructions supported by the RL78/G14 (S3 core) parts.
  • rl78 is an alias for g14.
  • mg10 is an alias for none.

Also, a C preprocessor macro gets defined based on this setting 🎀. Possible values are: __RL78_MUL_NONE__, __RL78_MUL_G13__, or __RL78_MUL_G14__.

• -mcpu=g10, -mcpu=g13, -mcpu=g14, -mcpu=rl78 — specifies which RL78 core to target 💖. The default is the G14 core, also known as an S3 core or just RL78. The G13 (S2) core doesn't have multiply or divide instructions — it uses a hardware peripheral for those instead. The G10 (S1) core has no register banks, so it uses a different calling convention.

If you set this option, it also picks the type of hardware multiply support to use, unless you override it with an explicit -mmul=none on the command line 🌈. So specifying -mcpu=g13 enables the G13 hardware multiply peripheral, and specifying -mcpu=g10 disables hardware multiplications altogether.

Note: even though the RL78/G14 core is the default target, specifying -mcpu=g14 or -mcpu=rl78 on the command line does change the behavior of the toolchain, because it also enables G14 hardware multiply support. If you don't specify these on the command line, then software multiplication routines get used even though the code targets the RL78 core. This is for backwards compatibility with older toolchains that didn't have hardware multiply and divide support 😭.

Also, a C preprocessor macro gets defined based on this setting too 🦋. Possible values are: __RL78_G10__, __RL78_G13__, or __RL78_G14__.

• -mg10, -mg13, -mg14, -mrl78 — these are aliases for the corresponding -mcpu= option 💅. They exist purely for backwards compatibility, iconic behavior honestly.
• -mallregs — lets the compiler use all of the available registers 🌸. By default r24..r31 are reserved for use in interrupt handlers, but with this option enabled those registers can be used in ordinary functions as well. Slay.
• -m64bit-doubles, -m32bit-doubles — makes the double data type be 64 bits (-m64bit-doubles) or 32 bits (-m32bit-doubles) in size ✨. The default is -m32bit-doubles.
• -msave-mduc-in-interrupts, -mno-save-mduc-in-interrupts — says that interrupt handler functions should preserve the MDUC registers 🎀. This is only necessary if normal code might use the MDUC registers — for example because it performs multiplication and division operations. The default is to ignore the MDUC registers since that makes interrupt handlers faster 💅. Real talk: you need to pass the target option -mg13 for this to work, because this feature is only available on the G13 target (S2 core). Also, the MDUC registers only get saved if the interrupt handler performs a multiplication or division operation, or if it calls another function.

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👑 IBM RS/6000 and PowerPC Options 👑

These -m options are defined for the IBM RS/6000 and PowerPC:

• -mpowerpc-gpopt, -mno-powerpc-gpopt, -mpowerpc-gfxopt, -mno-powerpc-gfxopt, -mpowerpc64, -mno-powerpc64, -mmfcrf, -mno-mfcrf, -mpopcntb, -mno-popcntb, -mpopcntd, -mno-popcntd, -mfprnd, -mno-fprnd, -mcmpb, -mno-cmpb, -mhard-dfp, -mno-hard-dfp — you use these options to specify which instructions are available on the processor you're using 🦋. The default value of these options is determined when configuring GCC. Specifying the -mcpu=cpu_type overrides the specification of these options. And ngl, we recommend you use the -mcpu=cpu_type option rather than the options listed above 👀.

Specifying -mpowerpc-gpopt allows GCC to use the optional PowerPC architecture instructions in the General Purpose group, including floating-point square root ✨. Specifying -mpowerpc-gfxopt allows GCC to use the optional PowerPC architecture instructions in the Graphics group, including floating-point select.

The -mmfcrf option allows GCC to generate the move from condition register field instruction implemented on the POWER4 processor and other processors that support the PowerPC V2.01 architecture 🌸. The -mpopcntb option allows GCC to generate the popcount and double-precision FP reciprocal estimate instruction implemented on the POWER5 processor and other processors that support the PowerPC V2.02 architecture. The -mpopcntd option allows GCC to generate the popcount instruction implemented on the POWER7 processor and other processors that support the PowerPC V2.06 architecture. The -mfprnd option allows GCC to generate the FP round to integer instructions implemented on the POWER5+ processor and other processors that support the PowerPC V2.03 architecture. The -mcmpb option allows GCC to generate the compare bytes instruction implemented on the POWER6 processor and other processors that support the PowerPC V2.05 architecture. The -mhard-dfp option allows GCC to generate the decimal floating-point instructions implemented on some POWER processors 🔥.

The -mpowerpc64 option allows GCC to generate the additional 64-bit instructions that are found in the full PowerPC64 architecture, and to treat GPRs as 64-bit, doubleword quantities 💖. GCC defaults to -mno-powerpc64.

• -mcpu=cpu_type — sets the architecture type, register usage, and instruction scheduling parameters for machine type cpu_type 👑. The supported values for cpu_type are:

401, 403, 405, 405fp, 440, 440fp, 464, 464fp, 476, 476fp, 505,
601, 602, 603, 603e, 604, 604e, 620, 630, 740, 7400, 7450, 750, 801,
821, 823, 860, 970, 8540, a2, e300c2, e300c3, e500mc, e500mc64,
e5500, e6500, ec603e, G3, G4, G5, titan, power3, power4, power5,
power5+, power6, power6x, power7, power8, power9, future, powerpc,
powerpc64, powerpc64le, rs64, and native.

-mcpu=powerpc, -mcpu=powerpc64, and -mcpu=powerpc64le specify pure 32-bit PowerPC (either endian), 64-bit big endian PowerPC, and 64-bit little endian PowerPC architecture machine types, with an appropriate, generic processor model assumed for scheduling purposes 🎀.

Specifying native as cpu type detects and selects the architecture option that corresponds to the host processor of the system performing the compilation 🦋. Heads up: -mcpu=native has no effect if GCC doesn't recognize the processor 😭.

The other options specify a specific processor. Code generated under those options runs best on that processor, and may not run at all on others 👀.

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

The particular options set for any particular CPU varies between compiler versions, depending on what setting seems to produce optimal code for that CPU 💅. It doesn't necessarily reflect the actual hardware's capabilities. If you wish to set an individual option to a particular value, you may specify it after the -mcpu option, like -mcpu=970 -mno-altivec ✨.

On AIX, the -maltivec and -mpowerpc64 options are not enabled or disabled by the -mcpu option at present, because AIX does not have full support for these options 🌸. You may still enable or disable them individually if you're sure it'll work in your environment.

• -mtune=cpu_type — 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 both are specified, the code generated uses the architecture and registers set by -mcpu, but the scheduling parameters set by -mtune 💖.
• -mcmodel=small — generate PowerPC64 code for the small model: the TOC is limited to 64k 🌈.
• -mcmodel=medium — generate PowerPC64 code for the medium model: the TOC and other static data may be up to a total of 4G in size 🦋. This is the default for 64-bit Linux.
• -mcmodel=large — generate PowerPC64 code for the large model: the TOC may be up to 4G in size 🌸. Other data and code is only limited by the 64-bit address space. Big model energy, honestly.
• -maltivec, -mno-altivec — generate code that uses (or 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 👑.

Okay so, the way that -maltivec handles element order? It's giving "match the target, bestie." The element order for AltiVec intrinsics like vec_splat, vec_extract, and vec_insert matches the array element order that corresponds to the endianness of the target. So element zero identifies the leftmost element in a vector register when you're targeting a big-endian platform, and it identifies the rightmost element in a vector register when you're targeting a little-endian platform. ✨

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• -mvrsave, -mno-vrsave — generate VRSAVE instructions when generating AltiVec code. 💅
• -msecure-plt — generate code that allows ld and ld.so to build executables and shared libraries with non-executable .plt and .got sections. This is a PowerPC 32-bit SYSV ABI option.
• -mbss-plt — generate code that uses a BSS .plt section that ld.so fills in, and requires .plt and .got sections that are both writable and executable. This is a PowerPC 32-bit SYSV ABI option. (Yes, writable and executable, we're being brave today 🫶)
• -misel, -mno-isel — this switch enables or disables the generation of ISEL instructions.
• -mvsx, -mno-vsx — generate code that uses (or does not use) vector/scalar (VSX) instructions, and also enable the use of built-in functions that allow more direct access to the VSX instruction set. 🦋
• -mcrypto, -mno-crypto — enable the use (disable) of the built-in functions that allow direct access to the cryptographic instructions that were added in version 2.07 of the PowerPC ISA.
• -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 the manpage says "does not keeps," and honestly? iconic, we're leaving it exactly as it is 💀)
• -mpower8-vector, -mno-power8-vector — generate code that uses (does not use) the vector and scalar instructions that were added in version 2.07 of the PowerPC ISA. Also enable the use of built-in functions that allow more direct access to the vector instructions.
• -mquad-memory, -mno-quad-memory — generate code that uses (does not use) the non-atomic quad word memory instructions. The -mquad-memory option requires use of 64-bit mode. 👑
• -mquad-memory-atomic, -mno-quad-memory-atomic — generate code that uses (does not use) the atomic quad word memory instructions. The -mquad-memory-atomic option requires use of 64-bit mode.
• -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, -mcpu=power7, -mcpu=power8), or -mcpu=power9 must be enabled to use the IEEE 128-bit floating point support. The IEEE 128-bit floating point support only works on PowerPC Linux systems. 🌸

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 — turn ON (or OFF) codegen for the string-search dlmzb instruction on the IBM 405, 440, 464 and 476 processors. And bestie, it's on by default when you're targeting those chips, so if you're not trying to use it you gotta say so 💅
• -mno-bit-align, -mbit-align — on System V.4 and embedded PowerPC systems, do NOT (or DO) force structs and unions that contain bit-fields to align to the base type of the bit-field.

Example time, because this one's a whole vibe: by default, a struct containing nothing but 8 unsigned bit-fields of length 1 gets aligned to a 4-byte boundary and is 4 bytes big. Use -mno-bit-align and it snaps 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 (or DO) assume unaligned memory references get handled by the system.
• -mrelocatable, -mno-relocatable — generate code that allows (or doesn't allow) a static executable to be relocated to a different address at run time. A simple embedded PowerPC system loader should relocate the entire contents of .got2 plus the 4-byte locations listed in the .fixup section — which is a table of 32-bit addresses that this option generates. For any of this to work, every object linked together must be compiled with -mrelocatable or -mrelocatable-lib. Also, -mrelocatable code aligns the stack to an 8-byte boundary.
• -mrelocatable-lib, -mno-relocatable-lib — like -mrelocatable, -mrelocatable-lib generates a .fixup section so static executables can be relocated at run time, BUT -mrelocatable-lib doesn't use the smaller stack alignment of -mrelocatable. The cute part: 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 (or DO) assume that register 2 holds a pointer to a global area pointing at 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. -mlittle-endian is the same thing as -mlittle, no cap.
• -mbig, -mbig-endian — on System V.4 and embedded PowerPC systems, compile code for the processor in big-endian mode. -mbig-endian is the same as -mbig, periodt.
• -mdynamic-no-pic — on Darwin and Mac OS X systems, compile code so it is not relocatable, but its external references are relocatable. The resulting code is suitable for applications, but not for shared libraries. 😭
• -msingle-pic-base — treat the register used for PIC addressing as read-only instead of loading it in the prologue of every function. The runtime system is the one responsible for initializing this register with an appropriate value before execution begins.
• -mprioritize-restricted-insns=priority — controls the priority assigned to dispatch-slot restricted instructions during the second scheduling pass. The argument priority takes the value 0, 1, or 2 to assign no, highest, or second-highest (respectively) priority to dispatch-slot restricted instructions.
• -msched-costly-dep=dependence_type — controls which dependences the target considers costly during instruction scheduling. The argument dependence_type takes one of these values:
  • no — no dependence is costly.
  • all — all dependences are costly.
  • true_store_to_load — a true dependence from store to load is costly.
  • store_to_load — any dependence from store to load is costly.
  • number — any dependence whose latency is greater than or equal to number is costly.
• -minsert-sched-nops=scheme — controls which NOP insertion scheme gets used during the second scheduling pass. The argument scheme takes one of these values:
  • no — don't insert NOPs.
  • pad — pad with NOPs any dispatch group that has vacant issue slots, according to the scheduler's grouping.
  • regroup_exact — insert NOPs to force costly dependent insns into separate groups. Insert exactly as many NOPs as needed to force an insn into a new group, according to the estimated processor grouping.
  • number — insert NOPs to force costly dependent insns into separate groups. Insert number NOPs to force an insn into a new group.
• -mcall-sysv — on System V.4 and embedded PowerPC systems, compile code using calling conventions that stick to the March 1995 draft of the System V Application Binary Interface, PowerPC processor supplement. This is the default unless you configured GCC using powerpc-*-eabiaix.
• -mcall-sysv-eabi, -mcall-eabi — specify both -mcall-sysv and -meabi.
• -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 — picks the type of traceback table. Valid values for traceback_type are full, part, and no.
• -maix-struct-return — return all structures in memory, exactly as the AIX ABI specifies.
• -msvr4-struct-return — return structures smaller than 8 bytes in registers, exactly as the SVR4 ABI specifies.
• -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.
• -mabi=ibmlongdouble — change the current ABI to use IBM extended-precision long double. This is not likely to work if your system defaults to using IEEE extended-precision long double. And 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. This is not likely to work if your system defaults to using IBM extended-precision long double. And if you change the long double type away from IBM extended-precision, the compiler will throw a warning unless you use the -Wno-psabi option. Requires -mlong-double-128 to be enabled.
• -mabi=elfv1 — change the current ABI to use the ELFv1 ABI. This is the default ABI for big-endian PowerPC 64-bit Linux. Overriding the default ABI requires special system support and is likely to fail in spectacular ways. 💀
• -mabi=elfv2 — change the current ABI to use the ELFv2 ABI. This is the default ABI for little-endian PowerPC 64-bit Linux. Overriding the default ABI requires special system support and is likely to fail in spectacular ways. 💀
• -mgnu-attribute, -mno-gnu-attribute — emit .gnu_attribute assembly directives to set tag/value pairs in a .gnu.attributes section that specify ABI variations in function parameters or return values.
• -mprototype, -mno-prototype — on System V.4 and embedded PowerPC systems, assume that all calls to variable argument functions are properly prototyped. Otherwise the compiler has to insert an instruction before every non-prototyped call to set or clear bit 6 of the condition code register (CR), which is how it indicates whether floating-point values are passed in the floating-point registers in case the function takes variable arguments. With -mprototype, only calls to prototyped variable argument functions set or clear that bit.
• -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. (yes, yellowknife, iconic name, we love her 🍓)
• -mvxworks — on System V.4 and embedded PowerPC systems, specify that you're compiling for a VxWorks system.
• -memb — on embedded PowerPC systems, set the PPC_EMB bit in the ELF flags header to indicate that eabi extended relocations are used.
• -meabi, -mno-eabi — on System V.4 and embedded PowerPC systems, do (or do not) adhere to the Embedded Applications Binary Interface (EABI), which is a set of modifications to the System V.4 specifications. 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 is pointed to by register r2. Put small initialized non-const global and static data in the .sdata section, which is pointed to by register r13. Put small uninitialized global and static data in the .sbss section, which sits right next to .sdata. Heads up bestie: -msdata=eabi is incompatible with the -mrelocatable option. Also, -msdata=eabi sets the -memb option. 🔥
• -msdata=sysv — bestie, on System V.4 and embedded PowerPC systems this drops small global and static data into the .sdata section, which is pointed to by register r13 💅. Small uninitialized global and static data goes in .sbss, sitting right next to .sdata. Heads up though: -msdata=sysv is incompatible with -mrelocatable, so don't try to serve both at once.
• -msdata=default, -msdata — on System V.4 and embedded PowerPC systems, if -meabi is in play, this compiles code exactly like -msdata=eabi; otherwise it compiles code like -msdata=sysv. Basically it reads the room first 🫶.
• -msdata=data — on System V.4 and embedded PowerPC systems, small global data goes in .sdata and small uninitialized global data goes in .sbss. But — plot twist — it does not use register r13 to address that small data 🙈. This is the default behavior unless some other -msdata option gets picked.
• -msdata=none, -mno-sdata — on embedded PowerPC systems, every initialized global and static item goes in .data, and all the uninitialized stuff goes in .bss. Simple. Clean. No small-data drama ✨.
• -mreadonly-in-sdata — puts read-only objects in the .sdata section too. And yes, this is the default, so it's already the vibe 👑.
• -mblock-move-inline-limit=num — inlines every block move (think calls to memcpy, or structure copies) that's less than or equal to num bytes. The minimum num is 32 bytes on 32-bit targets and 64 bytes on 64-bit targets. The default is target-specific, iykyk.
• -mblock-compare-inline-limit=num — generates non-looping inline code for every block compare (like calls to memcmp, or structure compares) that's less than or equal to num bytes. If num is 0, then all inline expansion of block compare — both non-loop and loop — is disabled 💀. Default is target-specific.
• -mblock-compare-inline-loop-limit=num — generates an inline expansion using loop code for every block compare that's 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 handle the rest of the block. Default? You guessed it: target-specific 🌸.
• -mstring-compare-inline-limit=num — compares at most num string bytes with inline code. If the difference or the end of string isn't found by the end of that inline compare, a call to strcmp or strncmp takes care of the rest of the comparison. The default is 64 bytes.
• -G num — on embedded PowerPC systems, global and static items less than or equal to num bytes get dropped 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 along to the linker, and — important — all modules should be compiled with the same -G num value, fr fr. Consistency is the moment.
• -mregnames, -mno-regnames — on System V.4 and embedded PowerPC systems, do (or do not) emit register names in the assembly language output using symbolic forms. Symbolic slay vs. plain, your call 💜.
• -mlongcall, -mno-longcall — by default this assumes all calls are far away, so a longer and more expensive calling sequence is required. That's needed for calls farther than 32 megabytes (33,554,432 bytes) from the current location. A short call gets generated if the compiler knows the call can't be that far away. This setting can be overridden by the shortcall function attribute, or by #pragma longcall(0).

Some linkers can detect out-of-range calls and generate glue code on the fly. On those systems, long calls are unnecessary and generate slower code. As of this writing, the AIX linker can do this, and so can the GNU linker for PowerPC/64. It's planned to add this feature to the GNU linker for 32-bit PowerPC systems as well.

On PowerPC64 ELFv2 and 32-bit PowerPC systems with newer GNU linkers, GCC can generate long calls using an inline PLT call sequence (see -mpltseq). PowerPC with -mbss-plt and PowerPC64 ELFv1 (big-endian) do not support inline PLT calls 🙅.

On Darwin/PPC systems, #pragma longcall generates jbsr callee, L42, plus a branch island (glue code). The two target addresses represent the callee and the branch island. The Darwin/PPC linker prefers the first address and generates a bl callee if the PPC bl instruction reaches the callee directly; otherwise, the linker generates bl L42 to call the branch island. The branch island is appended to the body of the calling function; it computes the full 32-bit address of the callee and jumps to it.

On Mach-O (Darwin) systems, this option directs the compiler to emit glue for every direct call, and the Darwin linker decides whether to use or discard it.

In the future, GCC may ignore all longcall specifications when the linker is known to generate glue. Foreshadowing ✨.

• -mpltseq, -mno-pltseq — implement (or do not 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. Also: -mpltseq code and -mbss-plt 32-bit PowerPC relocatable objects may not be linked together.
• -mtls-markers, -mno-tls-markers — mark (or do not 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 use of the reciprocal estimate and reciprocal square root estimate instructions, with extra Newton-Raphson steps to bump up precision, instead of doing a divide or a square root and divide for floating-point arguments. You should use -ffast-math when using -mrecip — or at least -funsafe-math-optimizations, -ffinite-math-only, -freciprocal-math and -fno-trapping-math. Note that while the throughput of the sequence is generally higher than the throughput of the non-reciprocal instruction, the precision of the sequence can be decreased by up to 2 ulp for reciprocal square roots (i.e. the inverse of 1.0 equals 0.99999994) 😭.
• -mrecip=opt — this one controls which reciprocal estimate instructions may be used. opt is a comma-separated list of options, which may be preceded by a ! to invert the option:
  • all — enable all estimate instructions.
  • default — enable the default instructions, equivalent to -mrecip.
  • none — disable all estimate instructions, equivalent to -mno-recip.
  • div — enable the reciprocal approximation instructions for both single and double precision.
  • divf — enable the single-precision reciprocal approximation instructions.
  • divd — enable the double-precision reciprocal approximation instructions.
  • rsqrt — enable the reciprocal square root approximation instructions for both single and double precision.
  • rsqrtf — enable the single-precision reciprocal square root approximation instructions.
  • rsqrtd — enable the double-precision reciprocal square root approximation instructions.

So, for example, -mrecip=all,!rsqrtd enables all of the reciprocal estimate instructions, except for the FRSQRTE, XSRSQRTEDP, and XVRSQRTEDP instructions which handle the double-precision reciprocal square root calculations. Chef's kiss selection 💅.

• -mrecip-precision, -mno-recip-precision — assume (or do not assume) that the reciprocal estimate instructions provide higher-precision estimates than is mandated by the PowerPC ABI. Selecting -mcpu=power6, -mcpu=power7 or -mcpu=power8 automatically selects -mrecip-precision. The double-precision square root estimate instructions are not generated by default on low-precision machines, since they do not provide an estimate that converges after three steps. Math said no 🚫.
• -mveclibabi=type — specifies the ABI type to use for vectorizing intrinsics using an external library. The only type supported at present is mass, which specifies using 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 — don't forget or it's giving undefined reference energy 😭.
• -mfriz, -mno-friz — generate (or do not generate) the friz instruction when the -funsafe-math-optimizations option is used to optimize rounding of floating-point values to 64-bit integer and back to floating point. Caveat: the friz instruction does not return the same value if the floating-point number is too large to fit in an integer. Know the limits, bestie.
• -mpointers-to-nested-functions, -mno-pointers-to-nested-functions — generate (or do not generate) code to load up the static chain register (r11) when calling through a pointer on AIX and 64-bit Linux systems where a function pointer points to a 3-word descriptor giving the function address, 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. And a big one: you cannot call through pointers to nested functions, or pointers to functions compiled in other languages that use the static chain, if you use -mno-pointers-to-nested-functions 💀.
• -msave-toc-indirect, -mno-save-toc-indirect — ok so these two decide whether we generate (or don't generate 💅) code that saves the TOC value into that reserved stack slot up in the function prologue, but ONLY when the function calls through a pointer, and only on AIX and 64-bit Linux systems. And if we're NOT saving the TOC value in the prologue? It gets saved right before the call through the pointer instead. Oh and bestie, -mno-save-toc-indirect is the default, so that's the vibe we ship with. ✨
• -mcompat-align-parm, -mno-compat-align-parm — generate (or don't generate 🫶) code that passes structure parameters with a maximum alignment of 64 bits, purely for compatibility with older versions of GCC.

So here's the lore: older versions of GCC (prior to 4.9.0) incorrectly did not align a structure parameter on a 128-bit boundary when that structure contained a member requiring 128-bit alignment. That got fixed in more recent versions of GCC. So if you need code that plays nice with functions compiled by those older GCCs, this option is your girl. 💖

And -mno-compat-align-parm 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. The supported locations are global for a global canary, or tls for a per-thread canary living in the TLS block (that's the default with GNU libc version 2.4 or later, no cap 🧚).

When you pick that second one, -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset additionally tell it which register to use as the base register for reading the canary, and from what offset off of that base register. The default for those two is as specified in the relevant ABI, periodt.

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 🙈) pc-relative addressing when the option -mcpu=future is being used. Heads up though: -mpcrel requires that the medium code model (-mcmodel=medium) and prefixed addressing (-mprefixed) options are enabled. It's giving "you can't sit with us unless you bring your friends." 💅
• -mprefixed, -mno-prefixed — generate (or don't ✨) addressing modes that use prefixed load and store instructions, when the option -mcpu=future is used.
• -mmma, -mno-mma — generate (or don't 💀) the MMA instructions when the option -mcpu=future is used.

---

🦋 RX Options 🦋

These command-line options are defined for RX targets, bestie, buckle up:

• -m64bit-doubles, -m32bit-doubles — make the double data type be 64 bits (-m64bit-doubles) or 32 bits (-m32bit-doubles) in size. The default is -m32bit-doubles. And here's why that's the default: RX floating-point hardware only works on 32-bit values. That's the whole reason, iykyk. 👑
• -fpu, -nofpu — enables (-fpu) or disables (-nofpu) the use of RX floating-point hardware. The default is enabled for the RX600 series and disabled for the RX200 series.

But ngl, floating-point instructions are only generated for 32-bit floating-point values, so the FPU hardware is not used for doubles if you've got the -m64bit-doubles option on.

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 literally in the name, bestie. 😭

• -mcpu=name — selects the type of RX CPU to be targeted. Currently three types are supported: the generic RX600 and RX200 series hardware, and the specific RX610 CPU. The default is RX600.

The only difference between RX600 and RX610 is that the RX610 does not support the MVTIPL instruction. That's it, that's the tea. 🍓

The RX200 series does not have a hardware floating-point unit, so -nofpu is enabled by default when this type is selected.

• -mbig-endian-data, -mlittle-endian-data — store data (but not code, ok?) in the big-endian format. The default is -mlittle-endian-data, i.e. to store data in the little-endian format. 🎀
• -msmall-data-limit=N — specifies the maximum size in bytes of global and static variables that can be placed into the small data area. Using the small data area can lead to smaller and faster code, BUT the size of the area is limited and it is up to the programmer to make sure the area does not overflow. Also, when the small data area is used, one of the RX's registers (usually r13) is reserved for pointing at this area, so it's no longer available for the compiler to use. That could result in slower and/or larger code if variables get pushed onto the stack instead of being held in this register. Tradeoffs, bestie. 😭

Note: common variables (the ones that have not been initialized) and constants are not placed into the small data area, because they're assigned to other sections in the output executable.

The default value is zero, which disables this feature. And note that this feature is not enabled by default at higher optimization levels (-O2 etc) because of the potentially detrimental effects of reserving a register. It's up to the programmer to experiment and discover whether this feature actually benefits 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 — specifies the maximum size, in bytes, of a constant that can be used as an operand in an RX instruction. So even though 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 used in instructions. Constants that are too big get placed into a constant pool instead, and referenced via register indirection. 💅

The value N can be between 0 and 4. A value of 0 (the default) or 4 means 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 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, since that 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 on 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. So yes, it's totally possible for the small data area register to end up being r8 if both -mint-register=4 and -mpid are specified on the command line. Register hunger games, fr fr. 👑

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 while compiling a file. The default is to issue a warning for each extra fast interrupt handler found, since the RX only supports one such interrupt. Like, one (1) is the limit, bestie. 💀
• -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 you're 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. That's on you, bestie. 🫶

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. And note that -mno-jsr does not mean "don't use JSR" — instead it means that any type of branch may be used. Read it twice, it's tricky. 🌸

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 has been restricted via the -ffixed-reg or -mint-register command-line options. 👑

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💜 S/390 and zSeries Options 💜

Okay bestie, these are the <code>-m</code> options for the <strong>S/390 and zSeries</strong> architecture, and they're honestly giving mainframe royalty energy 👑💅

• -mhard-float, -msoft-float — do (or don't) use the hardware floating-point instructions and registers for floating-point operations. If you pass -msoft-float, functions in libgcc.a are what perform your floating-point operations. If you pass -mhard-float, the compiler generates IEEE floating-point instructions instead. This is the default. ✨
• -mhard-dfp, -mno-hard-dfp — do (or don't) use the hardware decimal-floating-point instructions for decimal-floating-point operations. With -mno-hard-dfp, functions in libgcc.a handle your decimal-floating-point operations. With -mhard-dfp, the compiler generates decimal-floating-point hardware instructions. This is the default for -march=z9-ec or higher. 🌸
• -mlong-double-64, -mlong-double-128 — these switches control the size of the long double type. A size of 64 bits makes the long double type equivalent to the double type. This is the default.
• -mbackchain, -mno-backchain — store (or don't store) the address of the caller's frame as backchain pointer into the callee's stack frame. You might need a backchain so that debugging tools that don't understand DWARF call frame information can still work. When -mno-packed-stack is in effect, the backchain pointer is stored at the bottom of the stack frame; when -mpacked-stack is in effect, the backchain is placed into the topmost word of the 96/160 byte register save area. 🫶

In general, code compiled with -mbackchain is call-compatible with code compiled with -mmo-backchain; however, use of the backchain for debugging purposes usually requires that the whole binary is built with -mbackchain. Heads up 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. When -mno-packed-stack is specified, the compiler uses the all fields of the 96/160 byte register save area only for their default purpose; unused fields still take up stack space. When -mpacked-stack is specified, register save slots are densely packed at the top of the register save area; unused space is reused for other purposes, allowing for more efficient use of the available stack space. But when -mbackchain is also in effect, the topmost word of the save area is always used to store the backchain, and the return address register is always saved two words below the backchain. 💖

As long as the stack frame backchain is not used, code generated with -mpacked-stack is call-compatible with code generated with -mno-packed-stack. Ngl, note that some non-FSF releases of GCC 2.95 for S/390 or zSeries generated code that uses the stack frame backchain at run time, not just for debugging purposes. Such code is not call-compatible with code compiled with -mpacked-stack. Also, note that the combination of -mbackchain, -mpacked-stack and -mhard-float is not supported. In order to build a linux kernel use -msoft-float.

The default is to not use the packed stack layout.

• -msmall-exec, -mno-small-exec — generate (or do not generate) code using the bras instruction to do subroutine calls. This only works reliably if the total executable size does not exceed 64k. The default is to use the basr instruction instead, which does not have this limitation. 🦋
• -m64, -m31 — when -m31 is specified, generate code compliant to the GNU/Linux for S/390 ABI. When -m64 is specified, generate code compliant to the GNU/Linux for zSeries ABI. This allows GCC in particular to generate 64-bit instructions. For the s390 targets, the default is -m31, while the s390x targets default to -m64. ✨
• -mzarch, -mesa — when -mzarch is specified, generate code using the instructions available on z/Architecture. When -mesa is specified, generate code using the instructions available on ESA/390. Note that -mesa is not possible with -m64. When generating code compliant to the GNU/Linux for S/390 ABI, the default is -mesa. When generating code compliant to the GNU/Linux for zSeries ABI, the default is -mzarch.
• -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, 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.
• -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 function label is prepended with the given number of two-byte NOP instructions (pre-halfwords, maximum 1000000). After the label, 2 * post-halfwords bytes are appended, using the largest NOP like instructions the architecture allows (maximum 1000000). ✨

If both arguments are zero, hotpatching is disabled.

This option can be overridden for individual functions with the hotpatch attribute.

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🎀 Score Options 🎀

These options are defined for Score implementations, and yes they're short and sweet bestie:

• -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 — bestie, this one says: the target architecture is SCORE5. That's it, that's the whole vibe. ✨
• -mscore5u — target architecture = SCORE5U. Slay. 💅
• -mscore7 — target architecture = SCORE7, and this is the default. The main character of the SCORE lineup fr. 👑
• -mscore7d — target architecture = SCORE7D. Also iconic. 🦋

🎀 SH Options 🎀

Okay so these -m options are the ones defined for the SH implementations. Buckle up, there's a lot of them, and we're listing every single one, no skipping. 💖

• -m1 — Generate code for the SH1.
• -m2 — Generate code for the SH2.
• -m2e — Generate code for the SH2e.
• -m2a-nofpu — Generate code for the SH2a without FPU, or for a SH2a-FPU in such a way that the floating-point unit is not used.
• -m2a-single-only — Generate code for the SH2a-FPU, in such a way that no double-precision floating-point operations are used.
• -m2a-single — Generate code for the SH2a-FPU assuming the floating-point unit is in single-precision mode by default.
• -m2a — Generate code for the SH2a-FPU assuming the floating-point unit is in double-precision mode by default.
• -m3 — Generate code for the SH3.
• -m3e — Generate code for the SH3e.
• -m4-nofpu — Generate code for the SH4 without a floating-point unit.
• -m4-single-only — Generate code for the SH4 with a floating-point unit that only supports single-precision arithmetic.
• -m4-single — Generate code for the SH4 assuming the floating-point unit is in single-precision mode by default.
• -m4 — Generate code for the SH4.
• -m4-100 — Generate code for SH4-100.
• -m4-100-nofpu — Generate code for SH4-100 in such a way that the floating-point unit is not used.
• -m4-100-single — Generate code for SH4-100 assuming the floating-point unit is in single-precision mode by default.
• -m4-100-single-only — Generate code for SH4-100 in such a way that no double-precision floating-point operations are used.
• -m4-200 — Generate code for SH4-200.
• -m4-200-nofpu — Generate code for SH4-200 in such a way that the floating-point unit is not used.
• -m4-200-single — Generate code for SH4-200 assuming the floating-point unit is in single-precision mode by default.
• -m4-200-single-only — Generate code for SH4-200 in such a way that no double-precision floating-point operations are used.
• -m4-300 — Generate code for SH4-300.
• -m4-300-nofpu — Generate code for SH4-300 in such a way that the floating-point unit is not used.
• -m4-300-single — Generate code for SH4-300 in such a way that no double-precision floating-point operations are used.
• -m4-300-single-only — Generate code for SH4-300 in such a way that no double-precision floating-point operations are used.
• -m4-340 — Generate code for SH4-340 (no MMU, no FPU).
• -m4-500 — Generate code for SH4-500 (no FPU). Passes -isa=sh4-nofpu to the assembler.
• -m4a-nofpu — Generate code for the SH4al-dsp, or for a SH4a in such a way that the floating-point unit is not used.
• -m4a-single-only — Generate code for the SH4a, in such a way that no double-precision floating-point operations are used.
• -m4a-single — Generate code for the SH4a assuming the floating-point unit is in single-precision mode by default.
• -m4a — Generate code for the SH4a.
• -m4al — Same as -m4a-nofpu, except that it implicitly passes -dsp to the assembler. GCC doesn't generate any DSP instructions at the moment.
• -mb — Compile code for the processor in big-endian mode.
• -ml — Compile code for the processor in little-endian mode.
• -mdalign — Align doubles at 64-bit boundaries. 👀 Heads up bestie: this changes the calling conventions, and thus some functions from the standard C library do not work unless you recompile it first with -mdalign.
• -mrelax — Shorten some address references at link time, when possible; uses the linker option -relax.
• -mbigtable — Use 32-bit offsets in switch tables. The default is to use 16-bit offsets.
• -mbitops — Enable the use of bit manipulation instructions on SH2A.
• -mfmovd — Enable the use of the instruction fmovd. Check -mdalign for alignment constraints.
• -mrenesas — Comply with the calling conventions defined by Renesas.
• -mno-renesas — Comply with the calling conventions defined for GCC before the Renesas conventions were available. This option is the default for all targets of the SH toolchain.
• -mnomacsave — Mark the MAC register as call-clobbered, even if -mrenesas is given.
• -mieee, -mno-ieee — Control the IEEE compliance of floating-point comparisons, which affects the handling of cases where the result of a comparison is unordered. By default -mieee is implicitly enabled. If -ffinite-math-only is enabled, -mno-ieee is implicitly set, which results in faster floating-point greater-equal and less-equal comparisons. The implicit settings can be overridden by specifying either -mieee or -mno-ieee. 🌸
• -minline-ic_invalidate — Inline code to invalidate instruction cache entries after setting up nested function trampolines. This option has no effect if -musermode is in effect and the selected code generation option (e.g. -m4) does not allow the use of the icbi instruction. If the selected code generation option does not allow the use of the icbi instruction, and -musermode is not in effect, the inlined code manipulates the instruction cache address array directly with an associative write. This not only requires privileged mode at run time, but it also fails if the cache line had been mapped via the TLB and has become unmapped.
• -misize — Dump instruction size and location in the assembly code.
• -mpadstruct — This option is deprecated. It pads structures to multiple of 4 bytes, which is incompatible with the SH ABI.
• -matomic-model=model — Sets the model of atomic operations and additional parameters as a comma separated list. For details on the atomic built-in functions see __atomic Builtins. The following models and parameters are supported:
  • none — Disable compiler generated atomic sequences and emit library calls for atomic operations. This is the default if the target is not sh*-*-linux*.
  • soft-gusa — Generate GNU/Linux compatible gUSA software atomic sequences for the atomic built-in functions. The generated atomic sequences require additional support from the interrupt/exception handling code of the system and are only suitable for SH3* and SH4* single-core systems. This option is enabled by default when the target is sh*-*-linux* and SH3* or SH4*. When the target is SH4A, this option also partially utilizes the hardware atomic instructions movli.l and movco.l to create more efficient code, unless strict is specified.
  • soft-tcb — Generate software atomic sequences that use a variable in the thread control block. This is a variation of the gUSA sequences which can also be used on SH1* and SH2* targets. The generated atomic sequences require additional support from the interrupt/exception handling code of the system and are only suitable for single-core systems. When using this model, the gbr-offset= parameter has to be specified as well.
  • soft-imask — Generate software atomic sequences that temporarily disable interrupts by setting SR.IMASK = 1111. This model works only when the program runs in privileged mode and is only suitable for single-core systems. Additional support from the interrupt/exception handling code of the system is not required. This model is enabled by default when the target is sh*-*-linux* and SH1* or SH2*.
  • hard-llcs — Generate hardware atomic sequences using the movli.l and movco.l instructions only. This is only available on SH4A and is suitable for multi-core systems. Since the hardware instructions support only 32 bit atomic variables, access to 8 or 16 bit variables is emulated with 32 bit accesses. Code compiled with this option is also compatible with other software atomic model interrupt/exception handling systems if executed on an SH4A system. Additional support from the interrupt/exception handling code of the system is not required for this model.
  • gbr-offset= — This parameter specifies the offset in bytes of the variable in the thread control block structure that should be used by the generated atomic sequences when the soft-tcb model has been selected. For other models this parameter is ignored. The specified value must be an integer multiple of four and in the range 0-1020.
  • strict — This parameter prevents mixed usage of multiple atomic models, even if they are compatible, and makes the compiler generate atomic sequences of the specified model only.
• -mtas — Generate the tas.b opcode for __atomic_test_and_set. 👀 Notice that depending on the particular hardware and software configuration this can degrade overall performance due to the operand cache line flushes that are implied by the tas.b instruction. On multi-core SH4A processors the tas.b instruction must be used with caution since it can result in data corruption for certain cache configurations.
• -mprefergot — When generating position-independent code, emit function calls using the Global Offset Table instead of the Procedure Linkage Table.
• -musermode, -mno-usermode — Don't allow (allow) the compiler generating privileged mode code. Specifying -musermode also implies -mno-inline-ic_invalidate if the inlined code would not work in user mode. -musermode is the default when the target is sh*-*-linux*. If the target is SH1* or SH2*, -musermode has no effect, since there is no user mode.
• -multcost=number — Set the cost to assume for a multiply insn.
• -mdiv=strategy — Set the division strategy to be used for integer division operations. strategy can be one of:
  • call-div1 — Calls a library function that uses the single-step division instruction div1 to perform the operation. Division by zero calculates an unspecified result and does not trap. This is the default except for SH4, SH2A and SHcompact.
• call-fp — calls a library function that performs the operation in double precision floating point 💅. Division by zero causes a floating-point exception. This is the default for SHcompact with FPU. Specifying this for targets that do not have a double precision FPU defaults to "call-div1".
• call-table — calls a library function that uses a lookup table for small divisors and the "div1" instruction with case distinction for larger divisors. Division by zero calculates an unspecified result and does not trap. This is the default for SH4. Specifying this for targets that do not have dynamic shift instructions defaults to "call-div1".

When a division strategy has not been specified, the default strategy is selected based on the current target. For SH2A, the default strategy is to use the "divs" and "divu" instructions instead of library function calls. periodt ✨

• -maccumulate-outgoing-args — Reserve space once for outgoing arguments in the function prologue rather than around each call. Generally beneficial for performance and size. Also needed for unwinding to avoid changing the stack frame around conditional code.
• -mdivsi3_libfunc=name — Set the name of the library function used for 32-bit signed division to name. This only affects the name used in the call division strategies, and the compiler still expects the same sets of input/output/clobbered registers as if this option were not present.
• -mfixed-range=register-range — Generate code treating the given register range as fixed registers. A fixed register is one that the register allocator cannot use. This is useful when compiling kernel code. A register range is specified as two registers separated by a dash. Multiple register ranges can be specified separated by a comma.
• -mbranch-cost=num — Assume num to be the cost for a branch instruction. Higher numbers make the compiler try to generate more branch-free code if possible. If not specified the value is selected depending on the processor type that is being compiled for.
• -mzdcbranch, -mno-zdcbranch — Assume (do not assume) that zero displacement conditional branch instructions "bt" and "bf" are fast. If -mzdcbranch is specified, the compiler prefers zero displacement branch code sequences. This is enabled by default when generating code for SH4 and SH4A. It can be explicitly disabled by specifying -mno-zdcbranch.
• -mcbranch-force-delay-slot — Force the usage of delay slots for conditional branches, which stuffs the delay slot with a "nop" if a suitable instruction cannot be found. By default this option is disabled. It can be enabled to work around hardware bugs as found in the original SH7055.
• -mfused-madd, -mno-fused-madd — Generate code that uses (does not use) the floating-point multiply and accumulate instructions. These instructions are generated by default if hardware floating point is used. The machine-dependent -mfused-madd option is now mapped to the machine-independent -ffp-contract=fast option, and -mno-fused-madd is mapped to -ffp-contract=off.
• -mfsca, -mno-fsca — Allow or disallow the compiler to emit the "fsca" instruction for sine and cosine approximations. The option -mfsca must be used in combination with -funsafe-math-optimizations. It is enabled by default when generating code for SH4A. Using -mno-fsca disables sine and cosine approximations even if -funsafe-math-optimizations is in effect.
• -mfsrra, -mno-fsrra — Allow or disallow the compiler to emit the "fsrra" instruction for reciprocal square root approximations. The option -mfsrra must be used in combination with -funsafe-math-optimizations and -ffinite-math-only. It is enabled by default when generating code for SH4A. Using -mno-fsrra disables reciprocal square root approximations even if -funsafe-math-optimizations and -ffinite-math-only are in effect.
• -mpretend-cmove — Prefer zero-displacement conditional branches for conditional move instruction patterns. This can result in faster code on the SH4 processor.
• -mfdpic — Generate code using the FDPIC ABI.

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☀️ Solaris 2 Options ☀️

These -m options are supported on Solaris 2:

• -mclear-hwcap — -mclear-hwcap tells the compiler to remove the hardware capabilities generated by the Solaris assembler. This is only necessary when object files use ISA extensions not supported by the current machine, but check at runtime whether or not to use them.
• -mimpure-text — -mimpure-text, used in addition to -shared, tells the compiler to not pass -z text to the linker when linking a shared object. Using this option, you can link position-dependent code into a shared object.

-mimpure-text suppresses the "relocations remain against allocatable but non-writable sections" linker error message. However, the necessary relocations trigger copy-on-write, and the shared object is not actually shared across processes. Instead of using -mimpure-text, you should compile all source code with -fpic or -fPIC.

These switches are supported in addition to the above on Solaris 2:

• -pthreads — This is a synonym for -pthread.

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✨ 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, 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 get used for the values that pick an architecture and not a specific implementation. Those are v7, v8, sparclite, sparclet, v9. 💫

Here's the whole guest list of supported architectures and the implementations each one brings along:

• v7 — cypress, leon3v7
• v8 — supersparc, hypersparc, leon, leon3
• sparclite — f930, f934, sparclite86x
• sparclet — tsc701
• v9 — ultrasparc, ultrasparc3, niagara, niagara2, niagara3, niagara4, niagara7, m8

Okay so by default (unless somebody configured it otherwise, which, fair), GCC spits out code for the V7 variant of the SPARC architecture. Add -mcpu=cypress and the compiler also optimizes for the Cypress CY7C602 chip, the one living in the SPARCStation/SPARCServer 3xx series. It's also the right call for the older SPARCStation 1, 2, IPX, that whole era. 🕰️

With -mcpu=v8, GCC generates code for the V8 variant. The only glow-up from V7 is that the compiler now emits the integer multiply and integer divide instructions — the ones that exist in SPARC-V8 but not SPARC-V7. And with -mcpu=supersparc, it also optimizes for the SuperSPARC chip, as seen in the SPARCStation 10, 1000 and 2000 series. ✨

With -mcpu=sparclite, GCC generates code for the SPARClite variant. This adds the integer multiply, integer divide step and scan ("ffs") instructions that SPARClite has and SPARC-V7 does not. With -mcpu=f930, it also optimizes for the Fujitsu MB86930 chip — the original SPARClite, no FPU, we respect her. With -mcpu=f934, it also optimizes for the Fujitsu MB86934 chip, the more recent SPARClite with an FPU.

With -mcpu=sparclet, GCC generates code for the SPARClet variant. This adds the integer multiply, multiply/accumulate, integer divide step and scan ("ffs") instructions that exist in SPARClet but not in SPARC-V7. With -mcpu=tsc701, it also optimizes for the TEMIC SPARClet chip. 🎀

With -mcpu=v9, GCC generates code for the V9 variant. This adds 64-bit integer and floating-point move instructions, 3 additional floating-point condition code registers, and conditional move instructions. Then it's chip-specific bestie hours:

• -mcpu=ultrasparc — also optimizes for the Sun UltraSPARC I/II/IIi chips.
• -mcpu=ultrasparc3 — also optimizes for the Sun UltraSPARC III/III+/IIIi/IIIi+/IV/IV+ chips.
• -mcpu=niagara — also optimizes for Sun UltraSPARC T1 chips.
• -mcpu=niagara2 — also optimizes for Sun UltraSPARC T2 chips.
• -mcpu=niagara3 — also optimizes for Sun UltraSPARC T3 chips.
• -mcpu=niagara4 — also optimizes for Sun UltraSPARC T4 chips.
• -mcpu=niagara7 — also optimizes for Oracle SPARC M7 chips.
• -mcpu=m8 — also optimizes for Oracle M8 chips. 👑

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🌸 Options (continued) 🌸

• -mtune=cpu_type — Sets the instruction scheduling parameters for machine type cpu_type, but does not set the instruction set or register set that -mcpu=cpu_type does. Same values work here as for -mcpu=cpu_type, but the only useful ones are the ones that pick a particular CPU implementation: cypress, supersparc, hypersparc, leon, leon3, leon3v7, f930, f934, sparclite86x, tsc701, ultrasparc, ultrasparc3, niagara, niagara2, niagara3, niagara4, niagara7 and m8. And with native Solaris and GNU/Linux toolchains, native can be used too. 🦋
• -mv8plus, -mno-v8plus — With -mv8plus, GCC generates code for the SPARC-V8+ ABI. The difference from the V8 ABI? The global and out registers are treated as 64 bits wide. This is on by default on Solaris in 32-bit mode for all SPARC-V9 processors.
• -mvis, -mno-vis — With -mvis, GCC generates code that takes advantage of the UltraSPARC Visual Instruction Set extensions. 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 architecture versions V8 or V9 with an FPU — except -mcpu=leon, she's the exception. 🙈
• -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 — Enable the documented workaround for the single erratum of the Atmel AT697F processor (which corresponds to erratum #13 of the AT697E processor).
• -mfix-ut699 — Enable the documented workarounds for the floating-point errata and the data cache nullify errata of the UT699 processor.
• -mfix-ut700 — Enable the documented workaround for the back-to-back store errata of the UT699E/UT700 processor.
• -mfix-gr712rc — Enable the documented workaround for the back-to-back store errata of the GR712RC processor.

---

These -m options are supported in addition to all of the above on SPARC-V9 processors in 64-bit environments:

• -m32, -m64 — Generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long and pointer to 32 bits. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits.
• -mcmodel=which — Set the code model to one of:
  • medlow — The Medium/Low code model: 64-bit addresses, programs must be linked in the low 32 bits of memory. Programs can be statically or dynamically linked.
  • medmid — The Medium/Middle code model: 64-bit addresses, programs must be linked in the low 44 bits of memory, the text and data segments must be less than 2GB in size and the data segment must be located within 2GB of the text segment.
  • medany — The Medium/Anywhere code model: 64-bit addresses, programs may be linked anywhere in memory, the text and data segments must be less than 2GB in size and the data segment must be located within 2GB of the text segment.
  • embmedany — The Medium/Anywhere code model for embedded systems: 64-bit addresses, the text and data segments must be less than 2GB in size, both starting anywhere in memory (determined at link time). The global register %g4 points to the base of the data segment. Programs are statically linked and PIC is not supported. 🫶
• -mmemory-model=mem-model — Set the memory model in force on the processor to one of:
  • default — The default memory model for the processor and operating system.
  • rmo — Relaxed Memory Order
  • pso — Partial Store Order
  • tso — Total Store Order
  • sc — Sequential Consistency

These memory models are formally defined in Appendix D of the SPARC-V9 architecture manual, as set in the processor's "PSTATE.MM" field. 📖

• -mstack-bias, -mno-stack-bias — With -mstack-bias, GCC assumes that the stack pointer, and frame pointer if present, are offset by -2047 which must be added back when making stack frame references. This is the default in 64-bit mode. Otherwise, assume no such offset is present.

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💜 Options for System V 💜

These extra 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.

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⭐ TILE-Gx Options ⭐

These -m options are supported on the TILE-Gx:

---

• -mcmodel=small — okay so this one generates 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. no cap.
• -mcmodel=large — generate code for the large model ✨. There is no limitation on call distance, pc-relative addresses, or absolute addresses. it's giving unbothered.
• -mcpu=name — selects the type of CPU to be targeted. Currently the only supported type is tilegx. periodt.
• -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. bestie pick your fighter.
• -mbig-endian, -mlittle-endian — generate code in big/little endian mode, respectively. slay.

---

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. lowkey pointless but iconic.

---

V850 Options 💜

These -m options are defined for V850 implementations:

• -mlong-calls, -mno-long-calls — treat all calls as being far away (near). If calls are assumed to be far away, the compiler always loads the function's address into a register, and calls indirect through the pointer. 🫶
• -mno-ep, -mep — do not optimize (do optimize) basic blocks that use the same index pointer 4 or more times to copy pointer into the "ep" register, and use the shorter "sld" and "sst" instructions. The -mep option is on by default if you optimize. 💅
• -mno-prolog-function, -mprolog-function — do not use (do use) external functions to save and restore registers at the prologue and epilogue of a function. The external functions are slower, but use less code space if more than one function saves the same number of registers. The -mprolog-function option is on by default if you optimize. ✨
• -mspace — try to make the code as small as possible. At present, this just turns on the -mep and -mprolog-function options. tiny but mighty.
• -mtda=n — put static or global variables whose size is n bytes or less into the tiny data area that register "ep" points to. The tiny data area can hold up to 256 bytes in total (128 bytes for byte references). 🎀
• -msda=n — put static or global variables whose size is n bytes or less into the small data area that register "gp" points to. The small data area can hold up to 64 kilobytes. 🌸
• -mzda=n — put static or global variables whose size is n bytes or less into the first 32 kilobytes of memory. 🔥
• -mv850 — specify that the target processor is the V850. 💜
• -mv850e3v5 — specify that the target processor is the V850E3V5. The preprocessor constant "__v850e3v5__" is defined if this option is used. ✨
• -mv850e2v4 — specify that the target processor is the V850E3V5. This is an alias for the -mv850e3v5 option. 👑
• -mv850e2v3 — specify that the target processor is the V850E2V3. The preprocessor constant "__v850e2v3__" is defined if this option is used. 💅
• -mv850e2 — specify that the target processor is the V850E2. The preprocessor constant "__v850e2__" is defined if this option is used. 🦋
• -mv850e1 — specify that the target processor is the V850E1. The preprocessor constants "__v850e1__" and "__v850e__" are defined if this option is used. 🌈
• -mv850es — specify that the target processor is the V850ES. This is an alias for the -mv850e1 option. 💖
• -mv850e — specify that the target processor is the V850E. The preprocessor constant "__v850e__" is defined if this option is used. ✨

If neither -mv850 nor -mv850e nor -mv850e1 nor -mv850e2 nor -mv850e2v3 nor -mv850e3v5 are defined then a default target processor is chosen and the relevant __v850*__ preprocessor constant is defined.

The preprocessor constants "__v850" and "__v851__" are always defined, regardless of which processor variant is the target. no cap.

• -mdisable-callt, -mno-disable-callt — this option suppresses generation of the "CALLT" instruction for the v850e, v850e1, v850e2, v850e2v3 and v850e3v5 flavors of the v850 architecture.

This option is enabled by default when the RH850 ABI is in use (see -mrh850-abi), and disabled by default when the GCC ABI is in use. If "CALLT" instructions are being generated then the C preprocessor symbol "__V850_CALLT__" is defined. 💅

• -mrelax, -mno-relax — pass on (or do not pass on) the -mrelax command-line option to the assembler. 🫶
• -mlong-jumps, -mno-long-jumps — disable (or re-enable) the generation of PC-relative jump instructions. ✨
• -msoft-float, -mhard-float — disable (or re-enable) the generation of hardware floating point instructions. This option is only significant when the target architecture is V850E2V3 or higher. If hardware floating point instructions are being generated then the C preprocessor symbol "__FPU_OK__" is defined, otherwise the symbol "__NO_FPU__" is defined. 🔥
• -mloop — enables the use of the e3v5 LOOP instruction. The use of this instruction is not enabled by default when the e3v5 architecture is selected because its use is still experimental. 🙈
• -mrh850-abi, -mghs — enables support for the RH850 version of the V850 ABI. This is the default. With this version of the ABI the following rules apply:
  • Integer sized structures and unions are returned via a memory pointer rather than a register.
  • Large structures and unions (more than 8 bytes in size) are passed by value.
  • Functions are aligned to 16-bit boundaries.
  • The -m8byte-align command-line option is supported.
  • The -mdisable-callt command-line option is enabled by default. The -mno-disable-callt command-line option is not supported.

When this version of the ABI is enabled the C preprocessor symbol "__V850_RH850_ABI__" is defined. 💖

• -mgcc-abi — enables support for the old GCC version of the V850 ABI. With this version of the ABI the following rules apply:
  • Integer sized structures and unions are returned in register "r10".
  • Large structures and unions (more than 8 bytes in size) are passed by reference.
  • Functions are aligned to 32-bit boundaries, unless optimizing for size.
  • The -m8byte-align command-line option is not supported.
  • The -mdisable-callt command-line option is supported but not enabled by default.

When this version of the ABI is enabled the C preprocessor symbol "__V850_GCC_ABI__" is defined. 🌈

• -m8byte-align, -mno-8byte-align — enables support for "double" and "long long" types to be aligned on 8-byte boundaries. The default is to restrict the alignment of all objects to at most 4-bytes. When -m8byte-align is in effect the C preprocessor symbol "__V850_8BYTE_ALIGN__" is defined. 🎀
• -mbig-switch — generate code suitable for big switch tables. Use this option only if the assembler/linker complain about out of range branches within a switch table. 💀
• -mapp-regs — this option causes r2 and r5 to be used in the code generated by the compiler. This setting is the default. ✨
• -mno-app-regs — this option causes r2 and r5 to be treated as fixed registers. 🫶

---

VAX Options 🧚

These -m options are defined for the VAX:

• -munix — do not output certain jump instructions ("aobleq" and so on) that the Unix assembler for the VAX cannot handle across long ranges. 💜
• -mgnu — do output those jump instructions, on the assumption that the GNU assembler is being used. ✨
• -mg — output code for G-format floating-point numbers instead of D-format. 🌸

---

Visium Options 🦋

• -mdebug — a program which performs file I/O and is destined to run on an MCM target should be linked with this option. It causes the libraries libc.a and libdebug.a to be linked. The program should be run on the target under the control of the GDB remote debugging stub. 💅
• -msim — a program which performs file I/O and is destined to run on the simulator should be linked with option. This causes libraries libc.a and libsim.a to be linked. (yes bestie, “with option” — we don’t make the rules) 😭
• -mfpu, -mhard-float — generate code containing floating-point instructions. This is the default. 🔥
• -mno-fpu, -msoft-float — generate code containing library calls for floating-point.

-msoft-float changes the calling convention in the output file; therefore, it is only useful if you compile all of a program with this option. In particular, you need to compile libgcc.a, the library that comes with GCC, with -msoft-float in order for this to work. ⚠️

• -mcpu=cpu_type — set the instruction set, register set, and instruction scheduling parameters for machine type cpu_type. Supported values for cpu_type are mcm, gr5 and gr6.

mcm is a synonym of gr5 present for backward compatibility.

By default (unless configured otherwise), GCC generates code for the GR5 variant of the Visium architecture.

With -mcpu=gr6, GCC generates code for the GR6 variant of the Visium architecture. The only difference from GR5 code is that the compiler will generate block move instructions. 👑

• -mtune=cpu_type — set the instruction scheduling parameters for machine type cpu_type, but do not set the instruction set or register set that the option -mcpu=cpu_type would. ✨
• -msv-mode — generate code for the supervisor mode, where there are no restrictions on the access to general registers. This is the default. 💖
• -muser-mode — generate code for the user mode, where the access to some general registers is forbidden: on the GR5, registers r24 to r31 cannot be accessed in this mode; on the GR6, only registers r29 to r31 are affected. 🌈

---

VMS Options 💜

These -m options are defined for the VMS implementations:

• -mvms-return-codes — return VMS condition codes from "main". The default is to return POSIX-style condition (e.g. error) codes. 🫶
• -mdebug-main=prefix — flag the first routine whose name starts with prefix as the main routine for the debugger. 🎀
• -mmalloc64 — default to 64-bit memory allocation routines. 💅
• -mpointer-size=size — sets the default size of pointers, bestie. Your options for size are 32 or short for 32-bit pointers, 64 or long for 64-bit pointers, and no if you wanna support only 32-bit pointers. Heads up tho: that last one disables pragma pointer_size. ✨

---

🦋 VxWorks Options 🦋

The options in this section are defined for all VxWorks targets, no cap. Any options that are specific to your target hardware? Those live with the other options for that target, iykyk.

• -mrtp — GCC can generate code for both VxWorks kernels and real time processes (RTPs). This option switches from the former to the latter 💅 and it also defines the preprocessor macro __RTP__.
• -non-static — link an RTP executable against shared libraries instead of static libraries. The options -static and -shared can also be used for RTPs; -static is the default.
• -Bstatic and -Bdynamic — these options get passed down to the linker. They're defined for compatibility with Diab.
• -Xbind-lazy — enable lazy binding of function calls. This one is equivalent to -Wl,-z,now and is defined for compatibility with Diab.
• -Xbind-now — disable lazy binding of function calls. This is the default, and it's defined for compatibility with Diab.

---

👑 x86 Options 👑

These -m options are defined for the x86 family of computers. It's giving legendary lineage fr fr.

• -march=cpu-type — generate instructions for the machine type cpu-type. And listen, this is not the same energy as -mtune=cpu-type, which merely tunes the generated code for the specified cpu-type. -march=cpu-type lets GCC generate code that might not run at all on processors other than the one you indicated 😭. Also: specifying -march=cpu-type implies -mtune=cpu-type.

The choices for cpu-type are:

• native — this selects the CPU to generate code for at compilation time by determining the processor type of the compiling machine. Using -march=native enables all instruction subsets supported by the local machine (hence the result might not run on different machines, periodt). Using -mtune=native produces code optimized for the local machine under the constraints of the selected instruction set.
• x86-64 — a generic CPU with 64-bit extensions.
• i386 — original Intel i386 CPU.
• i486 — Intel i486 CPU. (No scheduling is implemented for this chip, so don't come for it.)
• i586, pentium — Intel Pentium CPU with no MMX support.
• lakemont — Intel Lakemont MCU, based on Intel Pentium CPU.
• pentium-mmx — Intel Pentium MMX CPU, based on Pentium core with MMX instruction set support.
• pentiumpro — Intel Pentium Pro CPU.
• i686 — when used with -march, the Pentium Pro instruction set is used, so the code runs on all i686 family chips. When used with -mtune, it has the same meaning as generic.
• pentium2 — Intel Pentium II CPU, based on Pentium Pro core with MMX instruction set support.
• pentium3, pentium3m — Intel Pentium III CPU, based on Pentium Pro core with MMX and SSE instruction set support.
• pentium-m — Intel Pentium M; low-power version of Intel Pentium III CPU with MMX, SSE and SSE2 instruction set support. Used by Centrino notebooks, iconic.
• pentium4, pentium4m — Intel Pentium 4 CPU with MMX, SSE and SSE2 instruction set support.
• prescott — improved version of Intel Pentium 4 CPU with MMX, SSE, SSE2 and SSE3 instruction set support.
• nocona — improved version of Intel Pentium 4 CPU with 64-bit extensions, MMX, SSE, SSE2 and SSE3 instruction set support.
• core2 — Intel Core 2 CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support.
• nehalem — Intel Nehalem CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2 and POPCNT instruction set support.
• westmere — Intel Westmere CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES and PCLMUL instruction set support.
• sandybridge — Intel Sandy Bridge CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AES and PCLMUL instruction set support.
• ivybridge — Intel Ivy Bridge CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AES, PCLMUL, FSGSBASE, RDRND and F16C instruction set support.
• haswell — Intel Haswell CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2 and F16C instruction set support.
• broadwell — Intel Broadwell CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX and PREFETCHW instruction set support.
• 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, 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, PCLMUL, RDRND, XSAVE, 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, PCLMUL, RDRND, XSAVE, 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, PCLMUL, RDRND, XSAVE, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, UMIP, GFNI-SSE, CLWB and ENCLV instruction set support.
• knl — Intel Knight's Landing CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, AVX512F, AVX512PF, AVX512ER and AVX512CD instruction set support.
• 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, 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 and AVX512VP2INTERSECT instruction set support.
• k6 — AMD K6 CPU with MMX instruction set support.
• k6-2, k6-3 — improved versions of AMD K6 CPU with MMX and 3DNow! instruction set support.
• athlon, athlon-tbird — AMD Athlon CPU with MMX, 3dNOW!, enhanced 3DNow! and SSE prefetch instructions support.

• athlon-4, athlon-xp, athlon-mp — the glow-up era of AMD Athlon, bestie 💅 packing MMX, 3DNow!, enhanced 3DNow! and the full SSE instruction set. she's got range.
• k8, opteron, athlon64, athlon-fx — processors built on the AMD K8 core, and yes they speak x86-64 ✨. this covers the AMD Opteron, Athlon 64 and Athlon 64 FX. (she supersets MMX, SSE, SSE2, 3DNow!, enhanced 3DNow! and the 64-bit instruction set extensions — no cap.)
• k8-sse3, opteron-sse3, athlon64-sse3 — improved versions of the AMD K8 cores, now with SSE3 support. same girl, new trick 🦋.
• amdfam10, barcelona — CPUs on AMD Family 10h cores with x86-64 support. (supersets MMX, SSE, SSE2, SSE3, SSE4A, 3DNow!, enhanced 3DNow!, ABM and the 64-bit instruction set extensions.)
• bdver1 — CPUs on AMD Family 15h cores with x86-64 support. (supersets FMA4, AVX, XOP, LWP, AES, PCLMUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and the 64-bit instruction set extensions.)
• bdver2 — AMD Family 15h core based CPUs with x86-64 support. (supersets BMI, TBM, F16C, FMA, FMA4, AVX, XOP, LWP, AES, PCLMUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and the 64-bit instruction set extensions.)
• bdver3 — AMD Family 15h core based CPUs with x86-64 support. (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 the 64-bit instruction set extensions.)
• bdver4 — AMD Family 15h core based CPUs with x86-64 support. (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 the 64-bit instruction set extensions.)
• znver1 — AMD Family 17h core based CPUs with x86-64 support. (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 the 64-bit instruction set extensions.)
• znver2 — AMD Family 17h core based CPUs with x86-64 support. (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 the 64-bit instruction set extensions.)
• btver1 — CPUs on AMD Family 14h cores with x86-64 support. (supersets MMX, SSE, SSE2, SSE3, SSSE3, SSE4A, CX16, ABM and the 64-bit instruction set extensions.)
• btver2 — CPUs on AMD Family 16h cores with x86-64 support. this one includes MOVBE, F16C, BMI, AVX, PCLMUL, AES, SSE4.2, SSE4.1, CX16, ABM, SSE4A, SSSE3, SSE3, SSE2, SSE, MMX and the 64-bit instruction set extensions.
• winchip-c6 — IDT WinChip C6 CPU, handled exactly like i486 but with MMX support added on top.
• winchip2 — IDT WinChip 2 CPU, handled like i486 with MMX and 3DNow! support added.
• c3 — VIA C3 CPU with MMX and 3DNow! support. (No scheduling is implemented for this chip. 🙈)
• c3-2 — VIA C3-2 (Nehemiah/C5XL) CPU with MMX and SSE support. (No scheduling is implemented for this chip.)
• c7 — VIA C7 (Esther) CPU with MMX, SSE, SSE2 and SSE3 support. (No scheduling is implemented for this chip.)
• samuel-2 — VIA Eden Samuel 2 CPU with MMX and 3DNow! support. (No scheduling is implemented for this chip.)
• nehemiah — VIA Eden Nehemiah CPU with MMX and SSE support. (No scheduling is implemented for this chip.)
• esther — VIA Eden Esther CPU with MMX, SSE, SSE2 and SSE3 support. (No scheduling is implemented for this chip.)
• eden-x2 — VIA Eden X2 CPU with x86-64, MMX, SSE, SSE2 and SSE3 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 support. (No scheduling is implemented for this chip.)
• nano — generic VIA Nano CPU with x86-64, MMX, SSE, SSE2, SSE3 and SSSE3 support. (No scheduling is implemented for this chip.)
• nano-1000 — VIA Nano 1xxx CPU with x86-64, MMX, SSE, SSE2, SSE3 and SSSE3 support. (No scheduling is implemented for this chip.)
• nano-2000 — VIA Nano 2xxx CPU with x86-64, MMX, SSE, SSE2, SSE3 and SSSE3 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 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 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 support. (No scheduling is implemented for this chip.)
• geode — AMD Geode embedded processor with MMX and 3DNow! support.

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• -mtune=cpu-type — tunes everything applicable about the generated code to cpu-type, except for the ABI and the set of available instructions 👑. Picking a specific cpu-type schedules things appropriately for that particular chip, but heads up: the compiler does not generate any code that can't run on the default machine type unless you also pass a -march=cpu-type option. so like, if GCC is configured for i686-pc-linux-gnu, then -mtune=pentium4 generates code tuned for Pentium 4 that still runs on i686 machines. bestie it's just a vibe, not a whole architecture change ✨.

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 your code will run on, use the corresponding -mtune or -march option instead of -mtune=generic. but if you have no idea what CPU your users will have, then yeah, use this one.

As new processors get deployed in the marketplace, the behavior of this option will change 💀. so if you upgrade to a newer version of GCC, code generation controlled by this option changes to reflect the processors that are most common at the time that version of GCC was released.

There is no -march=generic option because -march indicates the instruction set the compiler can use, and there is no generic instruction set applicable to all processors. meanwhile -mtune indicates the processor (or, in this case, collection of processors) the code is optimized for. different question, different answer, iykyk.

• intel — produce code optimized for the most current Intel processors, which are Haswell and Silvermont for this version of GCC. if you know the CPU your code will run on, use the corresponding -mtune or -march option instead of -mtune=intel. but if you want your application to perform better on both Haswell and Silvermont, then use this one 🫶.

As new Intel processors get deployed in the marketplace, the behavior of this option will change 😭. so if you upgrade to a newer version of GCC, code generation controlled by this option changes to reflect the most current Intel processors at the time that version of GCC was 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. -mtune meanwhile indicates the processor (or, in this case, collection of processors) the code is optimized for. periodt.

• -mcpu=cpu-type — a deprecated synonym for -mtune. she's giving vintage, but like, the "please stop using me" kind of vintage 🌸.
• -mfpmath=unit — generate floating-point arithmetic for the selected unit 💖. the choices for unit are:
  • 387 — use the standard 387 floating-point coprocessor, which is 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, which means slightly different results compared to most other chips. see -ffloat-store for a more detailed description.

This is the default choice for non-Darwin x86-32 targets.

• sse — use the scalar floating-point instructions present in the SSE instruction set. this instruction set is supported by Pentium III and newer chips, and on the AMD side by the Athlon-4, Athlon XP and Athlon MP chips. the earlier version of the SSE instruction set supports only single-precision arithmetic, so 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 the -march=cpu-type, -msse or -msse2 switches to enable SSE extensions and make this option actually do anything. for the x86-64 compiler, these extensions are enabled by default.

The resulting code should be considerably faster in the majority of cases and avoids the numerical instability problems of 387 code, but ⚠️ it may break some existing code that expects temporaries to be 80 bits.

fr fr, that last one is the "read before you ship" warning. she's fast, she's pretty, she might rearrange your floating-point numbers behind your back 🎀.

This is the default choice for the x86-64 compiler, the Darwin x86-32 targets, and also the default for x86-32 targets that have the SSE2 instruction set when -ffast-math is switched on. ✨

• sse,387
• sse+387
• both — try to use both instruction sets at the same time 💅. This literally doubles the registers you have to play with, and on chips where 387 and SSE have separate execution units, it doubles the execution resources too. But bestie, use this one with care — it's still experimental, because the GCC register allocator doesn't model separate functional units well, so your performance ends up unstable. It's giving chaos, not consistency. 😭

---

OPTIONS (the x86 part, continued) 🌈

• -masm=dialect — spit out assembly instructions in whichever dialect you pick. It also decides which dialect gets used for basic asm and extended asm. The supported choices, in dialect order, are att or intel. Default is att. And heads up: Darwin does not support intel 🙈.
• -mieee-fp / -mno-ieee-fp — turn IEEE floating-point comparisons on or off. These are the ones that correctly handle the case where a comparison result is unordered. 🌸
• -m80387 / -mhard-float — generate output that contains 80387 instructions for floating point. 💖
• -mno-80387 / -msoft-float — generate output that contains library calls for floating point instead.

Warning: the libraries you need are not part of GCC. Normally you'd lean on the facilities of the machine's usual C compiler, but that can't be done directly in cross-compilation. You have to make your own arrangements to supply suitable library functions for cross-compilation. 👑

On machines where a function returns floating-point results in the 80387 register stack, some floating-point opcodes may still get emitted even when -msoft-float is used. 🦋

• -mno-fp-ret-in-387 — don't use the FPU registers for function return values.

The usual calling convention has functions return values of types float and double in an FPU register, even if there's no FPU at all — the whole idea being that the operating system should emulate an FPU for you. 🧚

The option -mno-fp-ret-in-387 makes those values come back in ordinary CPU registers instead.

• -mno-fancy-math-387 — some 387 emulators just do not support the sin, cos and sqrt instructions for the 387. So specify this option and GCC will avoid generating them. It gets overridden when -march says the target CPU always has an FPU, because then the instruction doesn't need emulating anyway. And ngl — those instructions aren't 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 makes code that runs somewhat faster on a Pentium, but it costs you more memory. 💀

On x86-64, -malign-double is enabled by default.

Warning: if you use the -malign-double switch, structures containing the above types get aligned differently than the published application binary interface specifications for x86-32, and they are not binary compatible with structures in code compiled without that switch. 🚨

• -m96bit-long-double / -m128bit-long-double — these switches control the size of the long double type. The x86-32 application binary interface specifies the size as 96 bits, so -m96bit-long-double is the default in 32-bit mode. 🌸

Modern architectures (Pentium and newer) would rather have long double aligned to an 8- or 16-byte boundary. But in arrays or structures that conform to the ABI, that's just not possible. So specifying -m128bit-long-double aligns long double to a 16-byte boundary by padding the long double with an additional 32-bit zero. 💅

In the x86-64 compiler, -m128bit-long-double is the default choice, since its ABI specifies that long double is aligned on a 16-byte boundary.

Also notice: neither of these options gives you any extra precision beyond the x87 standard of 80 bits for a long double. ✨

Warning: if you override the default value for your target ABI, that changes the size of structures and arrays containing long double variables, and it modifies the function calling convention for functions taking long double. So they are not binary-compatible with code compiled without that switch. 🫶

• -mlong-double-64 / -mlong-double-80 / -mlong-double-128 — these switches control the size of the long double type. A size of 64 bits makes the long double type equivalent to the double type, and that's the default for the 32-bit Bionic C library. A size of 128 bits makes the long double type equivalent to the __float128 type, and that's the default for the 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, and it modifies 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. The supported values for type are: compat, which uses an increased alignment value compatible with GCC 4.8 and earlier; abi, which uses the alignment value as specified by the psABI; and cacheline, which uses an 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 get placed in the large data section. This value must be the same across all objects linked into the binary, and it defaults to 65535. 👑
• -mrtd — use a different function-calling convention, where functions taking a fixed number of arguments return with the ret num instruction, which pops their arguments while returning. That saves one instruction in the caller, since there's no need to pop the arguments there. 💅

You can say that an individual function is called with this calling sequence by using 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 can't use it if you need to call libraries compiled with the Unix compiler. 🚨

Also, you must provide function prototypes for all functions that take a variable number of arguments (including printf) — otherwise incorrect code gets generated for calls to those functions. 😭

And on top of that, seriously incorrect code results if you call a function with too many arguments. (Normally, extra arguments are harmlessly ignored. Not here bestie.) 💀

• -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 behaviour 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. That includes the system libraries and the startup modules. 🫶

• -msseregparm — use SSE register passing conventions for float and double arguments and return values. You can control this behaviour 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. That includes the system libraries and the 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 up until version 12. Only use this option if you need to stay compatible with existing code produced by those previous compiler versions, or by older versions of GCC. 🙈
• -mpc32 / -mpc64 / -mpc80 — set 80387 floating-point precision to 32, 64 or 80 bits. When -mpc32 is specified, the significands of results of floating-point operations are rounded to 24 bits (single precision); -mpc64 rounds the significands of results of floating-point operations to 53 bits (double precision); and -mpc80 rounds the significands of results of floating-point operations to 64 bits (extended double precision), which is the default. When this option is used, floating-point operations in higher precisions are not available to the programmer without setting the FPU control word explicitly. ⭐

Setting the rounding of floating-point operations to less than the default 80 bits can speed some programs by 2% or more. But note that some mathematical libraries 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. 😭

• -mstackrealign — realign the stack at entry. On the x86, the -mstackrealign option generates an alternate prologue and epilogue that realigns the run-time stack if necessary. This supports mixing legacy code that keeps 4-byte stack alignment with modern code that keeps 16-byte stack alignment for SSE compatibility. See also the attribute force_align_arg_pointer, which applies to individual functions. 💖
• -mpreferred-stack-boundary=num — attempt to keep the stack boundary aligned to a 2 raised to num byte boundary. If -mpreferred-stack-boundary is not specified, the default is 4 (16 bytes or 128 bits). 🦋

Warning, bestie, listen up: when you're 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. But the x86-64 ABI requires 16 byte stack alignment, so this is ABI incompatible, and it's only meant to be used in a controlled environment where stack space is a serious limitation.

The way this goes wrong 💀: this option leads to wrong code when functions compiled with 16 byte stack alignment (like functions from a standard library, hello) get called with a misaligned stack. In that case, SSE instructions may lead to misaligned memory access traps. Also — and this is the part that really hurts — 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. Yes, that includes the system libraries and the startup modules. No cap, all of them.

---

• -mincoming-stack-boundary=num — assume the incoming stack is aligned to a 2 raised to num byte boundary. If -mincoming-stack-boundary is not specified, the one specified by -mpreferred-stack-boundary is used.

Okay so on Pentium and Pentium Pro, double and long double values should be aligned to an 8-byte boundary (peep -malign-double) or they suffer significant run time performance penalties. On Pentium III, the Streaming SIMD Extension (SSE) data type __m128 may not work properly if it is not 16-byte aligned. iykyk 💅

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 like, calling a function compiled with a higher preferred stack boundary from a function compiled with a lower preferred stack boundary most likely misaligns the stack. ✨ It is recommended that libraries that use callbacks always use the default setting.

This extra alignment does consume extra stack space, and generally increases code size. Code that is sensitive to stack space usage, such as embedded systems and operating system kernels, may want to reduce the preferred alignment to -mpreferred-stack-boundary=2.

---

✨ The extended instruction set switch squad ✨

Okay bestie, this is the iconic list. Buckle up 🫶:

• -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
• -mavx512vpopcntdq
• -mavx512vp2intersect
• -mavx5124fmaps
• -mavx512vnni
• -mavx5124vnniw
• -mcldemote

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, or CLDEMOTE extended instruction sets. Each one has a corresponding -mno- option to disable use of these instructions. Balance, bestie. ⚖️

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. She's versatile like that. 💅

These options enable GCC to use these extended instructions in generated code, even without -mfpmath=sse. Applications that perform run-time CPU detection must compile separate files for each supported architecture, using the appropriate flags. In particular, the file containing the CPU detection code should be compiled without these options. 🎀

---

• -mdump-tune-features — this option instructs GCC to dump the names of the x86 performance tuning features and default settings. The names can be used in -mtune-ctrl=feature-list. Dump it, bestie, get the receipts.
• -mtune-ctrl=feature-list — this option is used to do fine grain control of x86 code generation features. feature-list is a comma separated list of feature names. See also -mdump-tune-features. When specified, the feature is turned on if it is not preceded with ^, otherwise, it is turned off. Note: -mtune-ctrl=feature-list is intended to be used by GCC developers. Using it may lead to code paths not covered by testing and can potentially result in compiler ICEs or runtime errors. You have been warned, it's giving "do not touch unless you ARE the compiler team". 💀
• -mno-default — this option instructs GCC to turn off all tunable features. See also -mtune-ctrl=feature-list and -mdump-tune-features. Everything off, chaos mode, we love a clean slate. ✨
• -mcld — this option instructs GCC to emit a cld instruction in the prologue of functions that use string instructions. String instructions depend on the DF flag to select between autoincrement or autodecrement mode. While the ABI specifies the DF flag to be cleared on function entry, some operating systems violate this specification by not clearing the DF flag in their exception dispatchers. The exception handler can be invoked with the DF flag set, which leads to wrong direction mode when string instructions are used. This option can be enabled by default on 32-bit x86 targets by configuring GCC with the --enable-cld configure option. Generation of cld instructions can be suppressed with the -mno-cld compiler option in this case. Got it? Got it. 🫶
• -mvzeroupper — this option instructs GCC to emit a vzeroupper instruction before a transfer of control flow out of the function to minimize the AVX to SSE transition penalty as well as remove unnecessary zeroupper intrinsics. Performance self-care, honestly. 🌸
• -mprefer-avx128 — this option instructs GCC to use 128-bit AVX instructions instead of 256-bit AVX instructions in the auto-vectorizer. Tiny but mighty. 🦋
• -mprefer-vector-width=opt — this option instructs GCC to use opt-bit vector width in instructions instead of default on the selected platform.
  • none — no extra limitations applied to GCC other than defined by the selected platform.
  • 128 — prefer 128-bit vector width for instructions.
  • 256 — prefer 256-bit vector width for instructions.
  • 512 — prefer 512-bit vector width for instructions.
• -mcx16 — this option enables GCC to generate CMPXCHG16B instructions in 64-bit code to implement compare-and-exchange operations on 16-byte aligned 128-bit objects. This is 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. Icons only. 👑
• -msahf — this option enables generation of SAHF instructions in 64-bit code. Early Intel Pentium 4 CPUs with Intel 64 support, prior to the introduction of Pentium 4 G1 step in December 2005, lacked the LAHF and SAHF instructions which are supported by AMD64. These are load and store instructions, respectively, for certain status flags. In 64-bit mode, the SAHF instruction is used to optimize fmod, drem, and remainder built-in functions; see Other Builtins for details. 🎀
• -mmovbe — this option enables use of the movbe instruction to implement __builtin_bswap32 and __builtin_bswap64. Swapped and slaying. 🔥
• -mshstk — the -mshstk option enables shadow stack built-in functions from x86 Control-flow Enforcement Technology (CET). Security but make it cute. 🛡️
• -mcrc32 — this option enables built-in functions __builtin_ia32_crc32qi, __builtin_ia32_crc32hi, __builtin_ia32_crc32si and __builtin_ia32_crc32di to generate the crc32 machine instruction. ✨
• -mrecip — this option enables use of RCPSS and RSQRTSS instructions (and their vectorized variants RCPPS and RSQRTPS) with an additional Newton-Raphson step to increase precision instead of DIVSS and SQRTSS (and their vectorized variants) for single-precision floating-point arguments. These instructions are generated only when -funsafe-math-optimizations is enabled together with -ffinite-math-only and -fno-trapping-math. Note that while the throughput of the sequence is higher than the throughput of the non-reciprocal instruction, the precision of the sequence can be decreased by up to 2 ulp (i.e. the inverse of 1.0 equals 0.99999994). Not exact, bestie, just fast.

Note that GCC implements 1.0f/sqrtf(x) in terms of RSQRTSS (or RSQRTPS) already with -ffast-math (or the above option combination), and doesn't need -mrecip.

Also note that GCC emits the above sequence with additional Newton-Raphson step for vectorized single-float division and vectorized sqrtf(x) already with -ffast-math (or the above option combination), and doesn't need -mrecip. Double note, double slay. 💅

• -mrecip=opt — this option controls which reciprocal estimate instructions may be used. opt is a comma-separated list of options, which may be preceded by a ! to invert the option:
  • all — enable all estimate instructions.
  • default — enable the default instructions, equivalent to -mrecip.
  • none — disable all estimate instructions, equivalent to -mno-recip.
  • div — enable the approximation for scalar division.
  • vec-div — enable the approximation for vectorized division.
  • sqrt — enable the approximation for scalar square root.
• vec-sqrt — turns on the approximation for vectorized square root ✨

So like, for example, -mrecip=all,!sqrt enables all of the reciprocal approximations, except for square root. Bestie math 💅

---

More x86 options, let's GO 🎀

• -mveclibabi=type — says which ABI type to use for vectorizing intrinsics via 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 library, no vibes 🫶

GCC currently emits calls to vmldExp2, vmldLn2, vmldLog102, vmldPow2, vmldTanh2, vmldTan2, vmldAtan2, vmldAtanh2, vmldCbrt2, vmldSinh2, vmldSin2, vmldAsinh2, vmldAsin2, vmldCosh2, vmldCos2, vmldAcosh2, vmldAcos2, vmlsExp4, vmlsLn4, vmlsLog104, vmlsPow4, vmlsTanh4, vmlsTan4, vmlsAtan4, vmlsAtanh4, vmlsCbrt4, vmlsSinh4, vmlsSin4, vmlsAsinh4, vmlsAsin4, vmlsCosh4, vmlsCos4, vmlsAcosh4 and vmlsAcos4 for corresponding function type when -mveclibabi=svml is used, and __vrd2_sin, __vrd2_cos, __vrd2_exp, __vrd2_log, __vrd2_log2, __vrd2_log10, __vrs4_sinf, __vrs4_cosf, __vrs4_expf, __vrs4_logf, __vrs4_log2f, __vrs4_log10f and __vrs4_powf for the corresponding function type when -mveclibabi=acml is used. Iconic function-name energy, fr fr 🔥

• -mabi=name — generates code for the specified calling convention. Permissible values are sysv for the ABI used on GNU/Linux and other systems, and ms for the Microsoft ABI. The default is to use the Microsoft ABI when targeting Microsoft Windows, and the SysV ABI on all other systems. You can control this behavior for specific functions by using the function attributes ms_abi and sysv_abi. Pick your fighter 💖
• -mforce-indirect-call — forces 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. It's giving ✨precision✨
• -mmanual-endbr — inserts an 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. You're the one holding the keys to the ENDBR door 🔑
• -mcall-ms2sysv-xlogues — okay so because of differences in 64-bit ABIs, any Microsoft ABI function that calls a System V ABI function has to consider RSI, RDI and XMM6-15 as clobbered 😭 By default, the code for saving and restoring these registers is emitted inline, which gives you fairly lengthy prologues and epilogues. Using -mcall-ms2sysv-xlogues emits prologues and epilogues that use stubs in the static portion of libgcc to do these saves and restores instead — so you get reduced function size, at the cost of a few extra instructions. Trade-off queen 👑
• -mtls-dialect=type — generates 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. Highkey efficient, highkey risky 🦋
• -mpush-args, -mno-push-args — use PUSH operations to store outgoing parameters. This method is shorter and usually equally fast as the method using SUB/MOV operations, and it's enabled by default. In some cases, disabling it may improve performance because of improved scheduling and reduced dependencies. Not me flipping the default and getting gains 💅
• -maccumulate-outgoing-args — if enabled, the maximum amount of space required for outgoing arguments is computed in the function prologue. This is faster on most modern CPUs, because of reduced dependencies, improved scheduling, and reduced stack usage when the preferred stack boundary is not equal to 2. The drawback is a notable increase in code size. This switch implies -mno-push-args. Big brain move, bigger binary 🙈
• -mthreads — supports 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. Cleanup era 🧹✨
• -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. The padding and alignment of members of structures, and whether a bit-field can straddle a storage-unit boundary, are determined by these rules:

1. Structure members are stored sequentially in the order in which they are declared: the first member has the lowest memory address and the last member the highest.
2. Every data object has an alignment requirement. The alignment requirement for all data except structures, unions, and arrays is either the size of the object or the current packing size (specified with either the aligned attribute or the pack pragma), whichever is less. For structures, unions, and arrays, the alignment requirement is the largest alignment requirement of its members. Every object is allocated an offset so that:

offset % alignment_requirement == 0

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 following non-bit-field members are ignored:

struct
 {
   char foo;
   long : 0;
   char bar;
 } t5;

Here, t5 takes up 2 bytes.

• -mno-align-stringops — do not align the destination of inlined string operations. This switch reduces code size and improves performance in case the destination is already aligned, but GCC doesn't know about it. Surprise! It was aligned the whole time 😭
• -minline-all-stringops — by default GCC inlines string operations only when the destination is known to be aligned to at least a 4-byte boundary. This enables more inlining and increases code size, but may improve performance of code that depends on fast memcpy and memset for short lengths. The option enables inline expansion of strlen for all pointer alignments. All the inlining, all the time 🌈
• -minline-stringops-dynamically — for string operations of unknown size, use run-time checks with inline code for small blocks and a library call for large blocks. Small block? Inline. Big block? Library. Vibe check passed ✅
• -mstringop-strategy=alg — override the internal decision heuristic for the particular algorithm to use for inlining string operations. The allowed values for alg are:
  • rep_byte
  • rep_4byte
  • rep_8byte

Expand using i386 rep prefix of the specified size.

• byte_loop
• loop
• unrolled_loop

Expand into an inline loop.

• libcall

Always use a library call.

Pick your algorithm, bestie, they're all valid 💜

• -mmemcpy-strategy=strategy — okay so GCC has ✨vibes✨ about whether to inline __builtin_memcpy and which inline algo to use when it knows how big the copy is. This option lets you override that internal decision heuristic and be the main character yourself. Your strategy is a comma-separated list of alg:max_size:dest_align triplets 💅
  • alg is one of the algos you'd name in -mstringop-strategy
  • max_size = the max byte size for which inline algorithm alg is allowed
  • for the last triplet, max_size must be -1 (no cap, bestie)
  • the max_size values across the triplets must be listed in increasing order 🔥
  • the minimal byte size for alg is 0 for the first triplet, and max_size + 1 of the preceding range for everything after
• -mmemset-strategy=strategy — literally the same energy as -mmemcpy-strategy=, except it's here to control __builtin_memset expansion instead 🧚
• -momit-leaf-frame-pointer — don't keep the frame pointer in a register for leaf functions. This skips the instructions that save, set up, and restore frame pointers, and frees up an extra register in leaf functions 🎀 the option -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 — tells the assembler it should encode SSE instructions with the VEX prefix 💅 the option -mavx turns this on by default.
• -mfentry, -mno-fentry — if profiling is active (-pg), put the profiling counter call before the prologue 👑 Note: on x86 architectures the attribute ms_hook_prologue isn't possible at the moment for -mfentry and -pg 🙈
• -mrecord-mcount, -mno-record-mcount — if profiling is active (-pg), generate a __mcount_loc section that holds pointers to each profiling call 🍓 this is useful for automatically patching and out calls.
• -mnop-mcount, -mno-nop-mcount — if profiling is active (-pg), generate the calls to the profiling functions as NOPs ✨ this is useful when they should be patched in later dynamically. It's likely only useful together with -mrecord-mcount, fr fr.
• -minstrument-return=type — instruments function exit in -pg -mfentry instrumented functions with a call to a specified function 🎀 this only instruments true returns ending with ret, but not sibling calls ending with jump. Valid types are:
  • none — don't instrument at all
  • call — generate a call to __return__
  • nop5 — generate a 5 byte nop
• -mrecord-return, -mno-record-return — generate a __return_loc section pointing to all return instrumentation code 💜
• -mfentry-name=name — sets the name of the __fentry__ symbol called at function entry for -pg -mfentry functions 🌸
• -mfentry-section=name — sets 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 be used to 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, breathe.

• -m8bit-idiv, -mno-8bit-idiv — on some processors, like Intel Atom, 8-bit unsigned integer divide is much faster than 32-bit/64-bit integer divide 🔥 this option generates a run-time check. If both dividend and divisor are within range 0 to 255, 8-bit unsigned integer divide is used instead of 32-bit/64-bit integer divide ✨
• -mavx256-split-unaligned-load, -mavx256-split-unaligned-store — split 32-byte AVX unaligned load and store 💅
• -mstack-protector-guard=guard, -mstack-protector-guard-reg=reg, -mstack-protector-guard-offset=offset — 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 has effect when -fstack-protector or -fstack-protector-all is specified 👑

With the latter choice, the options -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset furthermore specify which segment register (%fs or %gs) to use as 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. Your options are:
  • keep — leaves indirect call and jump unmodified (the default)
  • thunk — converts indirect call and jump to call and return thunk
  • thunk-inline — converts indirect call and jump to inlined call and return thunk
  • thunk-extern — converts indirect call and jump to external call and return thunk provided in a separate object file

You can control this behavior for a specific function by using the function attribute indirect_branch ✨

Note that -mcmodel=large is incompatible with -mindirect-branch=thunk and -mindirect-branch=thunk-extern, since the thunk function may not be reachable in the large code model 😬

Note that -mindirect-branch=thunk-extern is compatible with -fcf-protection=branch, since the external thunk can be made to enable control-flow check ✅

• -mfunction-return=choice — convert function return with choice 🌸 the default is keep, which keeps function return unmodified. Your options are:
  • keep — leaves function return unmodified (the default)
  • thunk — converts function return to call and return thunk
  • thunk-inline — converts function return to inlined call and return thunk
  • thunk-extern — converts function return to external call and return thunk provided in a separate object file

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 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 💅

---

These -m switches are supported in addition to 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 ✨
  • -m32 sets int, long, and pointer types to 32 bits, and generates code that runs on any i386 system.
  • -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.
  • -m16 is the same as -m32, except that it outputs the .code16gcc assembly directive at the beginning of the assembly output so that the binary can run in 16-bit mode 🧚
  • -miamcu generates code which conforms to Intel MCU psABI. It requires the -m32 option to be turned on 👀
• -mno-red-zone — do not use a so-called "red zone" for x86-64 code 🚫 the red zone is mandated by the x86-64 ABI; it's a 128-byte area beyond the location of the stack pointer that is not modified by signal or interrupt handlers, and therefore can be used for temporary data without adjusting the stack pointer. The flag -mno-red-zone disables this red zone, periodt.
• -mcmodel=small — generate code for the small code model: the program and its symbols must be linked in the lower 2 GB of the address space 🎀 pointers are 64 bits. Programs can be statically or dynamically linked. This is the default code model 👑
• -mcmodel=kernel — generate code for the kernel code model 🌈 the kernel runs in the negative 2 GB of the address space. This model has to be used for Linux kernel code.
• -mcmodel=medium — generate code for the medium model: the program is linked in the lower 2 GB of the address space ✨ small symbols are also placed there. Symbols with sizes larger than -mlarge-data-threshold are put into large data or BSS sections and can be located above 2GB. Programs can be statically or dynamically linked.
• -mcmodel=large — generate code for the large model 💜 this model makes no assumptions about addresses and sizes of sections.
• -maddress-mode=long — generate code for long address mode 🔥 this is only supported for 64-bit and x32 environments. It's the default address mode for 64-bit environments.
• -maddress-mode=short — generate code for short address mode 🌸 this is only supported for 32-bit and x32 environments. It's the default address mode for 32-bit and x32 environments.

---

🪟 x86 Windows Options 🪟

These additional options are available for Microsoft Windows targets 💅

• -mconsole — specifies that a console application is to be generated, by instructing the linker to set the PE header subsystem type required for console applications ✨ this option is available for Cygwin and MinGW targets and is enabled by default on those targets 👑
• -mdll — available for Cygwin and MinGW targets 🎀 specifies that a DLL—a dynamic link library—is to be generated, enabling the selection of the required runtime startup object and entry point.
• -mnop-fun-dllimport — available for Cygwin and MinGW targets 🌈 specifies that the dllimport attribute should be ignored.
• -mthread — available for MinGW targets 🦋 specifies that MinGW-specific thread support is to be used.

okay bestie we're in the target-specific options zone now and it's giving very "every chip has opinions" energy 💅 buckle up.

---

🌈 Windows-adjacent Target Options 🌈

• -municode — bestie this one's only for MinGW-w64 targets. It does two things: it makes the UNICODE preprocessor macro get predefined, and it picks runtime startup code that's Unicode-capable. So like, your program wakes up already knowing about Unicode. 🫶
• -mwin32 — available for Cygwin and MinGW targets. This says "hey preprocessor, set the typical Microsoft Windows predefined macros," but plot twist — it does not influence which runtime library or startup code gets chosen. It's just the macro vibe, not the runtime vibe. ✨
• -mwindows — also for Cygwin and MinGW. This one means a GUI application is gonna be generated, and it does that by telling the linker to set the PE header subsystem type appropriately. No terminal window, all windows. 🪟💖
• -fno-set-stack-executable — for MinGW targets only. It specifies that the executable flag for the stack used by nested functions isn't set. Why would you ever want that? Because it's necessary for binaries running in kernel mode of Microsoft Windows — down there the User32 API (the thing that's normally used to set executable privileges) just isn't available. So no User32, no exec flag, periodt. 💀
• -fwritable-relocated-rdata — available for MinGW and Cygwin targets. It says relocated-data that lives in a read-only section gets put into the .data section instead. This is necessary for older runtimes that don't support modification of .rdata sections for pseudo-relocation. Older runtime, be so fr, we're helping you out. 🙈
• -mpe-aligned-commons — for Cygwin and MinGW targets. It says the GNU extension to the PE file format — the one that permits correct alignment of COMMON variables — should be used when generating code. And it's enabled by default if GCC detects that the target assembler found during configuration supports the feature. Auto-detected, we love a self-aware compiler. 👑

See also under x86 Options for standard options. iykyk.

---

🎀 Xstormy16 Options 🎀

These options are defined for Xstormy16:

• -msim — choose startup files and linker script suitable for the simulator. That's it, that's the whole menu, short and iconic. ⭐

---

🦋 Xtensa Options 🦋

These options are supported for Xtensa targets:

• -mconst16
• -mno-const16

Enable or disable use of CONST16 instructions for loading constant values. Ngl, the CONST16 instruction is currently not a standard option from Tensilica. When it's enabled, CONST16 instructions are always used in place of the standard L32R instructions. And here's the tea: the use of CONST16 is enabled by default only if the L32R instruction is not available. So it's the understudy that only goes on when the main character can't perform. 🎭

• -mfused-madd
• -mno-fused-madd

Enable or disable use of fused multiply/add and multiply/subtract instructions in the floating-point option. Important: this has no effect if the floating-point option is not also enabled. Disabling fused multiply/add and multiply/subtract instructions forces the compiler to use separate instructions for the multiply and add/subtract operations. Why would you do that? Because in some cases you need strict IEEE 754-compliant results: the fused multiply add/subtract instructions do not round the intermediate result, so they produce results with more bits of precision than the IEEE standard specifies. 😱 And disabling them also ensures the program output isn't sensitive to the compiler's ability to combine multiply and add/subtract operations. Determinism girlies, this one's for you. 💜

• -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. Volatile said "in order, please," and GCC said "say less." 🔥

• -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 code plays by its own rules, fr fr. 🧚

• -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. That's nice because it 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. Cozy, close-knit literal family. 🏠

• -mauto-litpools
• -mno-auto-litpools

These options control the treatment of literal pools too. 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. The 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 the assembler to create several literal pools per function and assemble very big functions, which may not be possible with -mtext-section-literals. Big function rights! 🌸

• -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. Also note 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 is the main character and always shows up aligned. 💅

• -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.

Real talk: 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. And note that the assembler uses an indirect call for every cross-file call, not just those that really are out of range. No cap, it's all of them. 😭

---

👑 zSeries Options 👑

These are listed under

🌸 ENVIRONMENT 🌸

okay bestie this section is all about the environment variables that change how GCC does its thing 💅 some of them work by telling GCC which directories or prefixes to use when it's out here searching for files, and some of them just straight-up describe other parts of the compilation vibe.

ngl you can ALSO specify search spots with options like -B, -I and -L. those take precedence over the environment-variable spots, which in turn take precedence over whatever the configuration of GCC said. order matters, bestie, no cap.

---

the locale girlies 🎀

• LANG
• LC_CTYPE
• LC_MESSAGES
• LC_ALL

these control the way GCC uses localization info so it can play nice with different national conventions. GCC peeks at the locale categories LC_CTYPE and LC_MESSAGES if it was configured to do that. these locale categories can be set to literally any value your installation supports. a typical one 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 figure out the character boundaries in a string — this is needed for some multibyte encodings that contain quote and escape characters that would otherwise get read as a string end or an 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. and if none of these variables are set? GCC defaults to traditional C English behavior. iconic default behavior honestly 💅

---

temp files & debug vibes ✨

• TMPDIR — if TMPDIR is set, it specifies the directory to use for temporary files. GCC uses temp files to hold the output of one stage of compilation that's meant to be the input to the next stage: for example, the output of the preprocessor, which is the input to the compiler proper. pipeline girlies rise 🫶
• GCC_COMPARE_DEBUG — setting GCC_COMPARE_DEBUG is nearly equivalent to passing -fcompare-debug to the compiler driver. see the documentation of that option for more details, bestie.

---

finding the subprograms 🔍

• 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 gets added when this prefix is combined with the name of a subprogram, but you can specify a prefix that ends with a slash if you want to 💅

if GCC_EXEC_PREFIX is not set, GCC tries to figure out an appropriate prefix based on the pathname it was invoked with.

if GCC can't find the subprogram using the specified prefix, it goes looking in the usual places for it.

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 a very unusual way when finding directories to search for header files. for each of the standard directories whose name normally begins with /usr/local/lib/gcc (more precisely, with the value of GCC_INCLUDE_DIR), GCC tries replacing that beginning with the specified prefix to produce an alternate directory name. so, with -Bfoo/, GCC searches foo/bar just before it searches the standard directory /usr/local/lib/bar. and if a standard directory begins with the configured prefix, then the value of prefix gets replaced by GCC_EXEC_PREFIX when looking for header files. the way that this works is lowkey unhinged but okay 🦋

• COMPILER_PATH — the value of COMPILER_PATH is a colon-separated list of directories, much like PATH. GCC tries the directories you listed when searching for subprograms, if it can't find the subprograms using GCC_EXEC_PREFIX.
• LIBRARY_PATH — the value of LIBRARY_PATH is a colon-separated list of directories, much like PATH. when configured as a native compiler, GCC tries the directories you listed when searching for special linker files, if it can't find them using GCC_EXEC_PREFIX. linking with GCC also uses these directories when searching for ordinary libraries for the -l option (but directories specified with -L come first, periodt).

---

LANG, round two 👑

• LANG — this variable is used to pass locale information to the compiler. one way this info gets used is to determine the character set to use when character literals, string literals and comments are parsed in C and C++. when the compiler is configured to allow multibyte characters, these 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.

---

preprocessor-coded environment variables 🌈

some additional environment variables affect the behavior of the preprocessor, bestie.

• CPATH
• C_INCLUDE_PATH
• CPLUS_INCLUDE_PATH
• OBJC_INCLUDE_PATH

each variable's value is a list of directories separated by a special character, much like PATH, in which to look for header files. that special character, "PATH_SEPARATOR", is target-dependent and determined at GCC build time. for Microsoft Windows-based targets it's a semicolon, and for almost all other targets it's a colon.

CPATH specifies a list of directories to be searched as if specified with -I, but after any paths given with -I options on the command line. this environment variable is used regardless of which language is being preprocessed.

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. so cute, so useful 💖

---

make dependency girlies 🍓

• 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 get 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. sneaky little difference, iykyk 😌

---

reproducible timestamps ⭐

• 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. obsessed with reproducibility ngl 🧚

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 @command{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.

---

💀 BUGS 💀

for instructions on reporting bugs, see <https://gcc.gnu.org/bugs/>. go tell the GCC girlies what happened, bestie, they want to know 🫶

---

📝 FOOTNOTES 📝

1. on some systems, gcc -shared needs to build supplementary stub code for constructors to work. on multi-libbed systems, gcc -shared must select the correct support libraries to link against. failing to supply the correct flags may lead to subtle defects. supplying them in cases where they are not necessary is innocuous. so like... when in doubt, the flags won't hurt you, but skipping them might 👀

---

🔥 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.

the whole friend group 💅 go read them, they're all iconic in their own way.

---

👑 AUTHOR 👑

see the Info entry for gcc, or <http://gcc.gnu.org/onlinedocs/gcc/Contributors.html>, for contributors to GCC.

---

🎀 COPYRIGHT 🎀

Copyright (c) 1988-2020 Free Software Foundation, Inc.

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with the Invariant Sections being "GNU General Public License" and "Funding Free Software", the Front-Cover texts being (a) (see below), and with the Back-Cover Texts being (b) (see below). A copy of the license is included in the gfdl(7) man page.

(a) The FSF's Front-Cover Text is:

A GNU Manual

(b) The FSF's Back-Cover Text is:

You have freedom to copy and modify this GNU Manual, like GNU
software.  Copies published by the Free Software Foundation raise
funds for GNU development.

---

and that's a wrap on the whole GCC(1) page, bestie!! 🦋✨ you just read all 106 parts, you're literally the main character of manpage reading rn. this one was version gcc-10.2.0, dated 2020-07-23, and it's the GCC(1) manual — go forth and compile something gorgeous 💖💅