Attributes in Clang — Clang 15.0.0git documentation

Use __attribute__((alloc_align()) on a function declaration to specify that the return value of the function (which must be a pointer type) is at ... «  Clangcommandlineargumentreference   ::   Contents   ::   DiagnosticflagsinClang  » AttributesinClang¶ Introduction AMDGPUAttributes amdgpu_flat_work_group_size amdgpu_num_sgpr amdgpu_num_vgpr amdgpu_waves_per_eu CallingConventions aarch64_sve_pcs aarch64_vector_pcs fastcall ms_abi pcs preserve_all preserve_most regcall regparm stdcall thiscall vectorcall ConsumedAnnotationChecking callable_when consumable param_typestate return_typestate set_typestate test_typestate CustomizingSwiftImport swift_async swift_async_error swift_async_name swift_attr swift_bridge swift_bridged swift_error swift_name swift_newtype swift_objc_members swift_private DeclarationAttributes Owner Pointer __single_inhertiance,__multiple_inheritance,__virtual_inheritance asm deprecated empty_bases enum_extensibility external_source_symbol flag_enum layout_version lto_visibility_public managed novtable ns_error_domain objc_boxable objc_direct objc_direct_members objc_non_runtime_protocol objc_nonlazy_class objc_runtime_name objc_runtime_visible objc_subclassing_restricted preferred_name randomize_layout,no_randomize_layout randomize_layout,no_randomize_layout selectany transparent_union trivial_abi using_if_exists weak FieldAttributes no_unique_address FunctionAttributes #pragmaompdeclaresimd #pragmaompdeclaretarget #pragmaompdeclarevariant SV_GroupIndex _Noreturn abi_tag acquire_capability,acquire_shared_capability alloc_align alloc_size allocator always_inline,__force_inline artificial assert_capability,assert_shared_capability assume assume_aligned availability btf_decl_tag callback carries_dependency cf_consumed cf_returns_not_retained cf_returns_retained cfi_canonical_jump_table clang::builtin_alias,clang_builtin_alias clang_arm_builtin_alias cmse_nonsecure_entry code_seg convergent cpu_dispatch cpu_specific diagnose_as_builtin diagnose_if disable_sanitizer_instrumentation disable_tail_calls enable_if enforce_tcb enforce_tcb_leaf error,warning exclude_from_explicit_instantiation export_name flatten force_align_arg_pointer format gnu_inline guard ifunc import_module import_name internal_linkage interrupt(ARM) interrupt(AVR) interrupt(MIPS) interrupt(RISCV) kernel lifetimebound long_call,far malloc micromips mig_server_routine min_vector_width minsize no_builtin no_caller_saved_registers no_profile_instrument_function no_sanitize no_sanitize_address,no_address_safety_analysis no_sanitize_memory no_sanitize_thread no_speculative_load_hardening no_split_stack no_stack_protector noalias nocf_check nodiscard,warn_unused_result noduplicate noinline nomicromips noreturn,_Noreturn not_tail_called nothrow ns_consumed ns_consumes_self ns_returns_autoreleased ns_returns_not_retained ns_returns_retained numthreads objc_method_family objc_requires_super optnone os_consumed os_consumes_this os_returns_not_retained os_returns_retained os_returns_retained_on_non_zero os_returns_retained_on_zero overloadable patchable_function_entry preserve_access_index reinitializes release_capability,release_shared_capability retain shader short_call,near signal speculative_load_hardening sycl_kernel target target_clones try_acquire_capability,try_acquire_shared_capability used xray_always_instrument,xray_never_instrument,xray_log_args xray_always_instrument,xray_never_instrument,xray_log_args zero_call_used_regs HandleAttributes acquire_handle release_handle use_handle NullabilityAttributes _Nonnull _Null_unspecified _Nullable _Nullable_result nonnull returns_nonnull OpenCLAddressSpaces [[clang::opencl_global_device]],[[clang::opencl_global_host]] [[clang::opencl_global_device]],[[clang::opencl_global_host]] __constant,constant,[[clang::opencl_constant]] __generic,generic,[[clang::opencl_generic]] __global,global,[[clang::opencl_global]] __local,local,[[clang::opencl_local]] __private,private,[[clang::opencl_private]] StatementAttributes #pragmaclangloop #pragmaunroll,#pragmanounroll __read_only,__write_only,__read_write(read_only,write_only,read_write) fallthrough intel_reqd_sub_group_size likelyandunlikely likelyandunlikely musttail nomerge opencl_unroll_hint suppress sycl_special_class TypeAttributes __ptr32 __ptr64 __sptr __uptr align_value annotate_type arm_sve_vector_bits btf_type_tag clang_arm_mve_strict_polymorphism cmse_nonsecure_call device_builtin_surface_type device_builtin_texture_type noderef objc_class_stub TypeSafetyChecking argument_with_type_tag pointer_with_type_tag type_tag_for_datatype Undocumented Alignas,align,alignas,aligned NSObject __kindof acquired_after acquired_before address_space alias amdgpu_kernel analyzer_noreturn annotate assert_exclusive_lock assert_shared_lock blocks capability,shared_capability cdecl cf_audited_transfer cf_unknown_transfer cleanup cold common const constant constructor consumable_auto_cast_state consumable_set_state_on_read destructor device exclusive_locks_required,requires_capability,requires_shared_capability,shared_locks_required exclusive_trylock_function ext_vector_type format_arg global guarded_by guarded_var host hot ibaction iboutlet iboutletcollection intel_ocl_bicc interrupt interrupt interrupt kernel launch_bounds lock_returned lockable locks_excluded matrix_type may_alias mips16 mode ms_struct naked neon_polyvector_type neon_vector_type no_instrument_function no_thread_safety_analysis nocommon nomips16 noreturn objc_arc_weak_reference_unavailable objc_bridge objc_bridge_mutable objc_bridge_related objc_designated_initializer objc_exception objc_gc objc_independent_class objc_ownership objc_precise_lifetime objc_protocol_requires_explicit_implementation objc_requires_property_definitions objc_returns_inner_pointer objc_root_class ownership_holds,ownership_returns,ownership_takes packed pascal pt_guarded_by pt_guarded_var pure reqd_work_group_size returns_twice scoped_lockable sentinel shared shared_trylock_function sysv_abi type_visibility unavailable uuid vec_type_hint vecreturn vector_size visibility warn_unused weak_import weakref work_group_size_hint VariableAttributes always_destroy called_once dllexport dllimport init_priority init_seg leaf loader_uninitialized maybe_unused,unused no_destroy nodebug noescape nosvm objc_externally_retained pass_object_size,pass_dynamic_object_size require_constant_initialization,constinit(C++20) section,__declspec(allocate) standalone_debug swift_async_context swift_context swift_error_result swift_indirect_result swiftasynccall swiftcall thread tls_model uninitialized Introduction¶ ThispageliststheattributescurrentlysupportedbyClang. AMDGPUAttributes¶ amdgpu_flat_work_group_size¶ SupportedSyntaxes¶ amdgpu_flat_work_group_size clang::amdgpu_flat_work_group_size           Yes Theflatwork-groupsizeisthenumberofwork-itemsinthework-groupsize specifiedwhenthekernelisdispatched.Itistheproductofthesizesofthe x,y,andzdimensionofthework-group. Clangsupportsthe __attribute__((amdgpu_flat_work_group_size(,)))attributeforthe AMDGPUtarget.Thisattributemaybeattachedtoakernelfunctiondefinition andisanoptimizationhint. parameterspecifiestheminimumflatwork-groupsize,and parameterspecifiesthemaximumflatwork-groupsize(mustbegreaterthan )towhichalldispatchesofthekernelwillconform.Passing0,0 as,impliesthedefaultbehavior(128,256). Ifspecified,theAMDGPUtargetbackendmightbeabletoproducebettermachine codeforbarriersandperformscratchpromotionbyestimatingavailablegroup segmentsize. Anerrorwillbegivenif: Specifiedvaluesviolatesubtargetspecifications; Specifiedvaluesarenotcompatiblewithvaluesprovidedthroughother attributes. amdgpu_num_sgpr¶ SupportedSyntaxes¶ amdgpu_num_sgpr clang::amdgpu_num_sgpr           Yes Clangsupportsthe__attribute__((amdgpu_num_sgpr()))and __attribute__((amdgpu_num_vgpr()))attributesfortheAMDGPU target.Theseattributesmaybeattachedtoakernelfunctiondefinitionandare anoptimizationhint. Iftheseattributesarespecified,thentheAMDGPUtargetbackendwillattempt tolimitthenumberofSGPRsand/orVGPRsusedtothespecifiedvalue(s).The numberofusedSGPRsand/orVGPRsmayfurtherberoundeduptosatisfythe allocationrequirementsorconstraintsofthesubtarget.Passing0as num_sgprand/ornum_vgprimpliesthedefaultbehavior(nolimits). TheseattributescanbeusedtotesttheAMDGPUtargetbackend.Itis recommendedthattheamdgpu_waves_per_euattributebeusedtocontrol resourcessuchasSGPRsandVGPRssinceitisawareofthelimitsfordifferent subtargets. Anerrorwillbegivenif: Specifiedvaluesviolatesubtargetspecifications; Specifiedvaluesarenotcompatiblewithvaluesprovidedthroughother attributes; TheAMDGPUtargetbackendisunabletocreatemachinecodethatcanmeetthe request. amdgpu_num_vgpr¶ SupportedSyntaxes¶ amdgpu_num_vgpr clang::amdgpu_num_vgpr           Yes Clangsupportsthe__attribute__((amdgpu_num_sgpr()))and __attribute__((amdgpu_num_vgpr()))attributesfortheAMDGPU target.Theseattributesmaybeattachedtoakernelfunctiondefinitionandare anoptimizationhint. Iftheseattributesarespecified,thentheAMDGPUtargetbackendwillattempt tolimitthenumberofSGPRsand/orVGPRsusedtothespecifiedvalue(s).The numberofusedSGPRsand/orVGPRsmayfurtherberoundeduptosatisfythe allocationrequirementsorconstraintsofthesubtarget.Passing0as num_sgprand/ornum_vgprimpliesthedefaultbehavior(nolimits). TheseattributescanbeusedtotesttheAMDGPUtargetbackend.Itis recommendedthattheamdgpu_waves_per_euattributebeusedtocontrol resourcessuchasSGPRsandVGPRssinceitisawareofthelimitsfordifferent subtargets. Anerrorwillbegivenif: Specifiedvaluesviolatesubtargetspecifications; Specifiedvaluesarenotcompatiblewithvaluesprovidedthroughother attributes; TheAMDGPUtargetbackendisunabletocreatemachinecodethatcanmeetthe request. amdgpu_waves_per_eu¶ SupportedSyntaxes¶ amdgpu_waves_per_eu clang::amdgpu_waves_per_eu           Yes Acomputeunit(CU)isresponsibleforexecutingthewavefrontsofawork-group. Itiscomposedofoneormoreexecutionunits(EU),whichareresponsiblefor executingthewavefronts.AnEUcanhaveenoughresourcestomaintainthestate ofmorethanoneexecutingwavefront.ThisallowsanEUtohidelatencyby switchingbetweenwavefrontsinasimilarwaytosymmetricmultithreadingona CPU.InordertoallowthestateformultiplewavefrontstofitonanEU,the resourcesusedbyasinglewavefronthavetobelimited.Forexample,thenumber ofSGPRsandVGPRs.Limitingsuchresourcescanallowgreaterlatencyhiding, butcanresultinhavingtospillsomeregisterstatetomemory. Clangsupportsthe__attribute__((amdgpu_waves_per_eu([,]))) attributefortheAMDGPUtarget.Thisattributemaybeattachedtoakernel functiondefinitionandisanoptimizationhint. parameterspecifiestherequestedminimumnumberofwavesperEU,and optionalparameterspecifiestherequestedmaximumnumberofwaves perEU(mustbegreaterthanifspecified).Ifisomitted, thenthereisnorestrictiononthemaximumnumberofwavesperEUotherthan theonedictatedbythehardwareforwhichthekerneliscompiled.Passing 0,0as,impliesthedefaultbehavior(nolimits). Ifspecified,thisattributeallowsanadvanceddevelopertotunethenumberof wavefrontsthatarecapableoffittingwithintheresourcesofanEU.TheAMDGPU targetbackendcanusethisinformationtolimitresources,suchasnumberof SGPRs,numberofVGPRs,sizeofavailablegroupandprivatememorysegments,in suchawaythatguaranteesthatatleastwavefrontsandatmost wavefrontsareabletofitwithintheresourcesofanEU.Requesting morewavefrontscanhidememorylatencybutlimitsavailableregisterswhich canresultinspilling.Requestingfewerwavefrontscanhelpreducecache thrashing,butcanreducememorylatencyhiding. ThisattributecontrolsthemachinecodegeneratedbytheAMDGPUtargetbackend toensureitiscapableofmeetingtherequestedvalues.However,whenthe kernelisexecuted,theremaybeotherreasonsthatpreventmeetingtherequest, forexample,theremaybewavefrontsfromotherkernelsexecutingontheEU. Anerrorwillbegivenif: Specifiedvaluesviolatesubtargetspecifications; Specifiedvaluesarenotcompatiblewithvaluesprovidedthroughother attributes; TheAMDGPUtargetbackendisunabletocreatemachinecodethatcanmeetthe request. CallingConventions¶ Clangsupportsseveraldifferentcallingconventions,dependingonthetarget platformandarchitecture.Thecallingconventionusedforafunctiondetermines howparametersarepassed,howresultsarereturnedtothecaller,andother low-leveldetailsofcallingafunction. aarch64_sve_pcs¶ SupportedSyntaxes¶ aarch64_sve_pcs clang::aarch64_sve_pcs clang::aarch64_sve_pcs           OnAArch64targets,thisattributechangesthecallingconventionofa functiontopreserveadditionalScalableVectorregistersandScalable Predicateregistersrelativetothedefaultcallingconventionusedfor AArch64. Thismeansitismoreefficienttocallsuchfunctionsfromcodethatperforms extensivescalablevectorandscalablepredicatecalculations,becausefewer liveSVEregistersneedtobesaved.Thispropertymakesitwell-suitedforSVE mathlibraryfunctions,whicharetypicallyleaffunctionsthatrequireasmall numberofregisters. However,usingthisattributealsomeansthatitismoreexpensivetocall afunctionthatadherestothedefaultcallingconventionfromwithinsuch afunction.Therefore,itisrecommendedthatthisattributeisonlyused forleaffunctions. Formoreinformation,seethedocumentationforaarch64_sve_pcsinthe ARMCLanguageExtension(ACLE)documentation. aarch64_vector_pcs¶ SupportedSyntaxes¶ aarch64_vector_pcs clang::aarch64_vector_pcs clang::aarch64_vector_pcs           OnAArch64targets,thisattributechangesthecallingconventionofa functiontopreserveadditionalfloating-pointandAdvancedSIMDregisters relativetothedefaultcallingconventionusedforAArch64. Thismeansitismoreefficienttocallsuchfunctionsfromcodethatperforms extensivefloating-pointandvectorcalculations,becausefewerliveSIMDandFP registersneedtobesaved.Thispropertymakesitwell-suitedfore.g. floating-pointorvectormathlibraryfunctions,whicharetypicallyleaf functionsthatrequireasmallnumberofregisters. However,usingthisattributealsomeansthatitismoreexpensivetocall afunctionthatadherestothedefaultcallingconventionfromwithinsuch afunction.Therefore,itisrecommendedthatthisattributeisonlyused forleaffunctions. Formoreinformation,seethedocumentationforaarch64_vector_pcson theArmDeveloperwebsite. fastcall¶ SupportedSyntaxes¶ fastcall gnu::fastcall gnu::fastcall   __fastcall_fastcall       On32-bitx86targets,thisattributechangesthecallingconventionofa functiontouseECXandEDXasregisterparametersandclearparametersoffof thestackonreturn.Thisconventiondoesnotsupportvariadiccallsor unprototypedfunctionsinC,andhasnoeffectonx86_64targets.Thiscalling conventionissupportedprimarilyforcompatibilitywithexistingcode.Users seekingregisterparametersshouldusetheregparmattribute,whichdoes notrequirecallee-cleanup.Seethedocumentationfor__fastcallonMSDN. ms_abi¶ SupportedSyntaxes¶ ms_abi gnu::ms_abi gnu::ms_abi           Onnon-Windowsx86_64targets,thisattributechangesthecallingconventionof afunctiontomatchthedefaultconventionusedonWindowsx86_64.This attributehasnoeffectonWindowstargetsornon-x86_64targets. pcs¶ SupportedSyntaxes¶ pcs gnu::pcs gnu::pcs           OnARMtargets,thisattributecanbeusedtoselectcallingconventions similartostdcallonx86.Validparametervaluesare“aapcs”and “aapcs-vfp”. preserve_all¶ SupportedSyntaxes¶ preserve_all clang::preserve_all clang::preserve_all           OnX86-64andAArch64targets,thisattributechangesthecallingconventionof afunction.Thepreserve_allcallingconventionattemptstomakethecode inthecallerevenlessintrusivethanthepreserve_mostcallingconvention. ThiscallingconventionalsobehavesidenticaltotheCcallingconvention onhowargumentsandreturnvaluesarepassed,butitusesadifferentsetof caller/callee-savedregisters.Thisremovestheburdenofsavingand recoveringalargeregistersetbeforeandafterthecallinthecaller.If theargumentsarepassedincallee-savedregisters,thentheywillbe preservedbythecalleeacrossthecall.Thisdoesn’tapplyforvalues returnedincallee-savedregisters. OnX86-64thecalleepreservesallgeneralpurposeregisters,exceptfor R11.R11canbeusedasascratchregister.Furthermoreitalsopreserves allfloating-pointregisters(XMMs/YMMs). Theideabehindthisconventionistosupportcallstoruntimefunctions thatdon’tneedtocallouttoanyotherfunctions. Thiscallingconvention,likethepreserve_mostcallingconvention,willbe usedbyafutureversionoftheObjective-Cruntimeandshouldbeconsidered experimentalatthistime. preserve_most¶ SupportedSyntaxes¶ preserve_most clang::preserve_most clang::preserve_most           OnX86-64andAArch64targets,thisattributechangesthecallingconventionof afunction.Thepreserve_mostcallingconventionattemptstomakethecode inthecallerasunintrusiveaspossible.Thisconventionbehavesidentically totheCcallingconventiononhowargumentsandreturnvaluesarepassed, butitusesadifferentsetofcaller/callee-savedregisters.Thisalleviates theburdenofsavingandrecoveringalargeregistersetbeforeandafterthe callinthecaller.Iftheargumentsarepassedincallee-savedregisters, thentheywillbepreservedbythecalleeacrossthecall.Thisdoesn’t applyforvaluesreturnedincallee-savedregisters. OnX86-64thecalleepreservesallgeneralpurposeregisters,exceptfor R11.R11canbeusedasascratchregister.Floating-pointregisters (XMMs/YMMs)arenotpreservedandneedtobesavedbythecaller. Theideabehindthisconventionistosupportcallstoruntimefunctions thathaveahotpathandacoldpath.Thehotpathisusuallyasmallpiece ofcodethatdoesn’tusemanyregisters.Thecoldpathmightneedtocalloutto anotherfunctionandthereforeonlyneedstopreservethecaller-saved registers,whichhaven’talreadybeensavedbythecaller.The preserve_mostcallingconventionisverysimilartothecoldcalling conventionintermsofcaller/callee-savedregisters,buttheyareusedfor differenttypesoffunctioncalls.coldccisforfunctioncallsthatare rarelyexecuted,whereaspreserve_mostfunctioncallsareintendedtobe onthehotpathanddefinitelyexecutedalot.Furthermorepreserve_most doesn’tpreventtheinlinerfrominliningthefunctioncall. ThiscallingconventionwillbeusedbyafutureversionoftheObjective-C runtimeandshouldthereforestillbeconsideredexperimentalatthistime. Althoughthisconventionwascreatedtooptimizecertainruntimecallsto theObjective-Cruntime,itisnotlimitedtothisruntimeandmightbeused byotherruntimesinthefuturetoo.Thecurrentimplementationonly supportsX86-64andAArch64,buttheintentionistosupportmorearchitectures inthefuture. regcall¶ SupportedSyntaxes¶ regcall gnu::regcall gnu::regcall   __regcall       Onx86targets,thisattributechangesthecallingconventionto __regcallconvention.Thisconventionaimstopassasmanyarguments aspossibleinregisters.Italsotriestoutilizeregistersforthe returnvaluewheneveritispossible. regparm¶ SupportedSyntaxes¶ regparm gnu::regparm gnu::regparm           On32-bitx86targets,theregparmattributecausesthecompilertopass thefirstthreeintegerparametersinEAX,EDX,andECXinsteadofonthe stack.Thisattributehasnoeffectonvariadicfunctions,andallparameters arepassedviathestackasnormal. stdcall¶ SupportedSyntaxes¶ stdcall gnu::stdcall gnu::stdcall   __stdcall_stdcall       On32-bitx86targets,thisattributechangesthecallingconventionofa functiontoclearparametersoffofthestackonreturn.Thisconventiondoes notsupportvariadiccallsorunprototypedfunctionsinC,andhasnoeffecton x86_64targets.ThiscallingconventionisusedwidelybytheWindowsAPIand COMapplications.Seethedocumentationfor__stdcallonMSDN. thiscall¶ SupportedSyntaxes¶ thiscall gnu::thiscall gnu::thiscall   __thiscall_thiscall       On32-bitx86targets,thisattributechangesthecallingconventionofa functiontouseECXforthefirstparameter(typicallytheimplicitthis parameterofC++methods)andclearparametersoffofthestackonreturn.This conventiondoesnotsupportvariadiccallsorunprototypedfunctionsinC,and hasnoeffectonx86_64targets.Seethedocumentationfor__thiscallon MSDN. vectorcall¶ SupportedSyntaxes¶ vectorcall clang::vectorcall clang::vectorcall   __vectorcall_vectorcall       On32-bitx86andx86_64targets,thisattributechangesthecalling conventionofafunctiontopassvectorparametersinSSEregisters. On32-bitx86targets,thiscallingconventionissimilarto__fastcall. ThefirsttwointegerparametersarepassedinECXandEDX.Subsequentinteger parametersarepassedinmemory,andcalleeclearsthestack.Onx86_64 targets,thecalleedoesnotclearthestack,andintegerparametersare passedinRCX,RDX,R8,andR9asisdoneforthedefaultWindowsx64calling convention. Onboth32-bitx86andx86_64targets,vectorandfloatingpointargumentsare passedinXMM0-XMM5.Homogeneousvectoraggregatesofuptofourelementsare passedinsequentialSSEregistersifenoughareavailable.IfAVXisenabled, 256bitvectorsarepassedinYMM0-YMM5.Anyvectororaggregatetypethat cannotbepassedinregistersforanyreasonispassedbyreference,which allowsthecallertoaligntheparametermemory. Seethedocumentationfor__vectorcallonMSDNformoredetails. ConsumedAnnotationChecking¶ Clangsupportsadditionalattributesforcheckingbasicresourcemanagement properties,specificallyforuniqueobjectsthathaveasingleowningreference. Thefollowingattributesarecurrentlysupported,althoughtheimplementation fortheseannotationsiscurrentlyindevelopmentandaresubjecttochange. callable_when¶ SupportedSyntaxes¶ callable_when clang::callable_when           Yes Use__attribute__((callable_when(...)))toindicatewhatstatesamethod maybecalledin.Validstatesareunconsumed,consumed,orunknown.Each argumenttothisattributemustbeaquotedstring.E.g.: __attribute__((callable_when("unconsumed","unknown"))) consumable¶ SupportedSyntaxes¶ consumable clang::consumable           Yes Eachclassthatusesanyofthetypestateannotationsmustfirstbemarked usingtheconsumableattribute.Failuretodosowillresultinawarning. Thisattributeacceptsasingleparameterthatmustbeoneofthefollowing: unknown,consumed,orunconsumed. param_typestate¶ SupportedSyntaxes¶ param_typestate clang::param_typestate           Yes Thisattributespecifiesexpectationsaboutfunctionparameters.Callstoan functionwithannotatedparameterswillissueawarningifthecorresponding argumentisn’tintheexpectedstate.Theattributeisalsousedtosetthe initialstateoftheparameterwhenanalyzingthefunction’sbody. return_typestate¶ SupportedSyntaxes¶ return_typestate clang::return_typestate           Yes Thereturn_typestateattributecanbeappliedtofunctionsorparameters. Whenappliedtoafunctiontheattributespecifiesthestateofthereturned value.Thefunction’sbodyischeckedtoensurethatitalwaysreturnsavalue inthespecifiedstate.Onthecallerside,valuesreturnedbytheannotated functionareinitializedtothegivenstate. Whenappliedtoafunctionparameteritmodifiesthestateofanargumentafter acalltothefunctionreturns.Thefunction’sbodyischeckedtoensurethat theparameterisintheexpectedstatebeforereturning. set_typestate¶ SupportedSyntaxes¶ set_typestate clang::set_typestate           Yes Annotatemethodsthattransitionanobjectintoanewstatewith __attribute__((set_typestate(new_state))).Thenewstatemustbe unconsumed,consumed,orunknown. test_typestate¶ SupportedSyntaxes¶ test_typestate clang::test_typestate           Yes Use__attribute__((test_typestate(tested_state)))toindicatethatamethod returnstrueiftheobjectisinthespecifiedstate.. CustomizingSwiftImport¶ ClangsupportsadditionalattributesforcustomizinghowAPIsareimportedinto Swift. swift_async¶ SupportedSyntaxes¶ swift_async clang::swift_async clang::swift_async         Yes Theswift_asyncattributespecifiesifandhowaparticularfunctionor Objective-Cmethodisimportedintoaswiftasyncmethod.Forinstance: @interfaceMyClass:NSObject -(void)notActuallyAsync:(int)p1withCompletionHandler:(void(^)())handler __attribute__((swift_async(none))); -(void)actuallyAsync:(int)p1callThisAsync:(void(^)())fun __attribute__((swift_async(swift_private,1))); @end Here,notActuallyAsync:withCompletionHandlerwouldhavebeenimportedas async(becauseit’slastparameter’sselectorpieceis withCompletionHandler)ifnotfortheswift_async(none)attribute. Conversely,actuallyAsync:callThisAsyncwouldn’thavebeenimportedas asyncifnotfortheswift_asyncattributebecauseitdoesn’tmatchthe namingconvention. Whenusingswift_asynctoenableimporting,thefirstargumenttothe attributeiseitherswift_privateornot_swift_privatetoindicate whetherthefunction/methodisprivatetothecurrentframework,andthesecond argumentistheindexofthecompletionhandlerparameter. swift_async_error¶ SupportedSyntaxes¶ swift_async_error clang::swift_async_error clang::swift_async_error         Yes Theswift_async_errorattributespecifieshowanerrorstatewillbe representedinaswiftasyncmethod.It’sabitanalogoustotheswift_error attributeforthegeneratedasyncmethod.Theswift_async_errorattribute canindicateavarietyofdifferentwaysofrepresentinganerror. __attribute__((swift_async_error(zero_argument,N))),specifiesthatthe asyncmethodisconsideredtohavefailediftheNthargumenttothe completionhandleriszero. __attribute__((swift_async_error(nonzero_argument,N))),specifiesthat theasyncmethodisconsideredtohavefailediftheNthargumenttothe completionhandlerisnon-zero. __attribute__((swift_async_error(nonnull_error))),specifiesthatthe asyncmethodisconsideredtohavefailediftheNSError*argumenttothe completionhandlerisnon-null. __attribute__((swift_async_error(none))),specifiesthattheasyncmethod cannotfail. Forinstance: @interfaceMyClass:NSObject -(void)asyncMethod:(void(^)(char,int,float))handler __attribute__((swift_async(swift_private,1))) __attribute__((swift_async_error(zero_argument,2))); @end Here,theswift_asyncattributespecifiesthathandleristhecompletion handlerforthismethod,andtheswift_async_errorattributespecifiesthat theintparameteristheonethatrepresentstheerror. swift_async_name¶ SupportedSyntaxes¶ swift_async_name             Yes Theswift_async_nameattributeprovidesthenameoftheasyncoverloadfor thegivendeclarationinSwift.Ifthisattributeisabsent,thenameis transformedaccordingtothealgorithmbuiltintotheSwiftcompiler. TheargumentisastringliteralthatcontainstheSwiftnameofthefunctionor method.ThenamemaybeacompoundSwiftname.Thefunctionormethodwithsuch anattributemusthavemorethanzeroparameters,asitslastparameteris assumedtobeacallbackthat’seliminatedintheSwiftasyncname. @interfaceURL +(void)loadContentsFrom:(URL*)urlcallback:(void(^)(NSData*))data__attribute__((__swift_async_name__("URL.loadContentsFrom(_:)"))) @end swift_attr¶ SupportedSyntaxes¶ swift_attr             Yes Theswift_attrprovidesaSwift-specificannotationforthedeclaration towhichtheattributeappertainsto.Itcanbeusedonanydeclaration inClang.ThiskindofannotationisignoredbyClangasitdoesn’thaveany semanticmeaninginlanguagessupportedbyClang.TheSwiftcompilercan interprettheseannotationsaccordingtoitsownruleswhenimportingCor Objective-Cdeclarations. swift_bridge¶ SupportedSyntaxes¶ swift_bridge               Theswift_bridgeattributeindicatesthatthedeclarationtowhichthe attributeappertainsisbridgedtothenamedSwifttype. __attribute__((__objc_root__)) @interfaceBase -(instancetype)init; @end __attribute__((__swift_bridge__("BridgedI"))) @interfaceI:Base @end Inthisexample,theObjective-CinterfaceIwillbemadeavailabletoSwift withthenameBridgedI.Itwouldbepossibleforthecompilertoreferto IstillinordertobridgethetypebacktoObjective-C. swift_bridged¶ SupportedSyntaxes¶ swift_bridged_typedef             Yes Theswift_bridged_typedefattributeindicatesthatwhenthetypedeftowhich theattributeappertainsisimportedintoSwift,itshouldrefertothebridged Swifttype(e.g.Swift’sString)ratherthantheObjective-Ctypeaswritten (e.g.NSString). @interfaceNSString; typedefNSString*AliasedString__attribute__((__swift_bridged_typedef__)); externvoidacceptsAliasedString(AliasedString_Nonnullparameter); Inthiscase,thefunctionacceptsAliasedStringwillbeimportedintoSwift asafunctionwhichacceptsaStringtypeparameter. swift_error¶ SupportedSyntaxes¶ swift_error             Yes Theswift_errorattributecontrolswhetheraparticularfunction(or Objective-Cmethod)isimportedintoSwiftasathrowingfunction,andifso, whichdynamicconventionituses. Alloftheseconventionsexceptnonerequirethefunctiontohaveanerror parameter.Currently,theerrorparameterisalwaysthelastparameteroftype NSError**orCFErrorRef*.Swiftwillremovetheerrorparameterfrom theimportedAPI.WhencallingtheAPI,Swiftwillalwayspassavalidaddress initializedtoanullpointer. swift_error(none)meansthatthefunctionshouldnotbeimportedas throwing.Theerrorparameterandresulttypewillbeimportednormally. swift_error(null_result)meansthatcallstothefunctionshouldbe consideredtohavethrowniftheyreturnanullvalue.Thereturntypemustbe apointertype,anditwillbeimportedintoSwiftwithanon-optionaltype. ThisisthedefaulterrorconventionforObjective-Cmethodsthatreturn pointers. swift_error(zero_result)meansthatcallstothefunctionshouldbe consideredtohavethrowniftheyreturnazeroresult.Thereturntypemustbe anintegraltype.IfthereturntypewouldhavebeenimportedasBool,it isinsteadimportedasVoid.Thisisthedefaulterrorconventionfor Objective-CmethodsthatreturnatypethatwouldbeimportedasBool. swift_error(nonzero_result)meansthatcallstothefunctionshouldbe consideredtohavethrowniftheyreturnanon-zeroresult.Thereturntypemust beanintegraltype.IfthereturntypewouldhavebeenimportedasBool, itisinsteadimportedasVoid. swift_error(nonnull_error)meansthatcallstothefunctionshouldbe consideredtohavethrowniftheyleaveanon-nullerrorintheerrorparameter. Thereturntypeisleftunmodified. swift_name¶ SupportedSyntaxes¶ swift_name               Theswift_nameattributeprovidesthenameofthedeclarationinSwift.If thisattributeisabsent,thenameistransformedaccordingtothealgorithm builtintotheSwiftcompiler. TheargumentisastringliteralthatcontainstheSwiftnameofthefunction, variable,ortype.Whenrenamingafunction,thenamemaybeacompoundSwift name.Foratype,enumconstant,property,orvariabledeclaration,thename mustbeasimpleorqualifiedidentifier. @interfaceURL -(void)initWithString:(NSString*)s__attribute__((__swift_name__("URL.init(_:)"))) @end void__attribute__((__swift_name__("squareRoot()")))sqrt(doublev){ } swift_newtype¶ SupportedSyntaxes¶ swift_newtypeswift_wrapper             Yes Theswift_newtypeattributeindicatesthatthetypedeftowhichthe attributeappertainsisimportedasanewSwifttypeofthetypedef’sname. Previously,theattributewasspeltswift_wrapper.Whilethebehaviourof theattributeisidenticalwitheitherspelling,swift_wrapperis deprecated,onlyexistsforcompatibilitypurposes,andshouldnotbeusedin newcode. swift_newtype(struct)meansthataSwiftstructwillbecreatedforthis typedef. swift_newtype(enum)meansthataSwiftenumwillbecreatedforthis typedef. //ImportUIFontTextStyleasanenumtype,withenumeratedvaluesbeing //constants. typedefNSString*UIFontTextStyle__attribute__((__swift_newtype__(enum))); //ImportUIFontDescriptorFeatureKeyasastructuretype,withenumerated //valuesbeingmembersofthetypestructure. typedefNSString*UIFontDescriptorFeatureKey__attribute__((__swift_newtype__(struct))); swift_objc_members¶ SupportedSyntaxes¶ swift_objc_members             Yes ThisattributeindicatesthatSwiftsubclassesandmembersofSwiftextensions ofthisclasswillbeimplicitlymarkedwiththe@objcMembersSwift attribute,exposingthembacktoObjective-C. swift_private¶ SupportedSyntaxes¶ swift_private               Declarationsmarkedwiththeswift_privateattributearehiddenfromthe frameworkclientbutarestillmadeavailableforusewithintheframeworkor SwiftSDKoverlay. Thepurposeofthisattributeistopermitamoreidomaticimplementationof declarationsinSwiftwhilehidingthenon-idiomaticone. DeclarationAttributes¶ Owner¶ SupportedSyntaxes¶   gsl::Owner           Yes Note Thisattributeisexperimentalanditseffectonanalysisissubjecttochangein afutureversionofclang. Theattribute[[gsl::Owner(T)]]appliestostructsandclassesthatownan objectoftypeT: class[[gsl::Owner(int)]]IntOwner{ private: intvalue; public: int*getInt(){return&value;} }; TheargumentTisoptionalandisignored. Thisattributemaybeusedbyanalysistoolsandhasnoeffectoncode generation.Avoidargumentmeansthattheclasscanownanytype. SeePointerforanexample. Pointer¶ SupportedSyntaxes¶   gsl::Pointer           Yes Note Thisattributeisexperimentalanditseffectonanalysisissubjecttochangein afutureversionofclang. Theattribute[[gsl::Pointer(T)]]appliestostructsandclassesthatbehave likepointerstoanobjectoftypeT: class[[gsl::Pointer(int)]]IntPointer{ private: int*valuePointer; public: int*getInt(){return&valuePointer;} }; TheargumentTisoptionalandisignored. Thisattributemaybeusedbyanalysistoolsandhasnoeffectoncode generation.Avoidargumentmeansthatthepointercanpointtoanytype. Example: Whenconstructinganinstanceofaclassannotatedlikethis(aPointer)from aninstanceofaclassannotatedwith[[gsl::Owner]](anOwner), thentheanalysiswillconsiderthePointertopointinsidetheOwner. WhentheOwner’slifetimeends,itwillconsiderthePointertobedangling. intf(){ IntPointerP; if(true){ IntOwnerO(7); P=IntPointer(O);//P"pointsinto"O }//Pisdangling returnP.get();//error:UsingadanglingPointer. } __single_inhertiance,__multiple_inheritance,__virtual_inheritance¶ SupportedSyntaxes¶         __single_inheritance__multiple_inheritance__virtual_inheritance__unspecified_inheritance       Thiscollectionofkeywordsisenabledunder-fms-extensionsandcontrols thepointer-to-memberrepresentationusedon*-*-win32targets. The*-*-win32targetsutilizeapointer-to-memberrepresentationwhich variesinsizeandalignmentdependingonthedefinitionoftheunderlying class. However,thisisproblematicwhenaforwarddeclarationisonlyavailableand nodefinitionhasbeenmadeyet.Insuchcases,Clangisforcedtoutilizethe mostgeneralrepresentationthatisavailabletoit. Thesekeywordsmakeitpossibletouseapointer-to-memberrepresentationother thanthemostgeneraloneregardlessofwhetherornotthedefinitionwillever bepresentinthecurrenttranslationunit. Thisfamilyofkeywordsbelongbetweentheclass-keyandclass-name: struct__single_inheritanceS; intS::*i; structS{}; Thiskeywordcanbeappliedtoclasstemplatesbutonlyhasaneffectwhenused onfullspecializations: templatestruct__single_inheritanceA;//warning:inheritancemodelignoredonprimarytemplate templatestruct__multiple_inheritanceA;//warning:inheritancemodelignoredonpartialspecialization template<>struct__single_inheritanceA; Notethatchoosinganinheritancemodellessgeneralthanstrictlynecessaryis anerror: struct__multiple_inheritanceS;//error:inheritancemodeldoesnotmatchdefinition intS::*i; structS{}; asm¶ SupportedSyntaxes¶         asm__asm__       Thisattributecanbeusedonafunctionorvariabletospecifyitssymbolname. Onsometargets,allCsymbolsareprefixedbydefaultwithasinglecharacter, typically_.Thiswasdonehistoricallytodistinguishthemfromsymbols usedbyotherlanguages.(ThisprefixisalsoaddedtothestandardItanium C++ABIprefixon“mangled”symbolnames,sothate.g.onsuchtargetsthetrue symbolnameforaC++variabledeclaredasintcppvar;wouldbe __Z6cppvar;notethetwounderscores.)Thisprefixisnotaddedtothe symbolnamesspecifiedbytheasmattribute;programmerswishingtomatcha Csymbolnamemustcompensateforthis. Forexample,considerthefollowingCcode: intvar1asm("altvar")=1;//"altvar"insymboltable. intvar2=1;//"_var2"insymboltable. voidfunc1(void)asm("altfunc"); voidfunc1(void){}//"altfunc"insymboltable. voidfunc2(void){}//"_func2"insymboltable. Clang’simplementationofthisattributeiscompatiblewithGCC’s,documentedhere. Whileitispossibletousethisattributetonameaspecialsymbolused internallybythecompiler,suchasanLLVMintrinsic,thisisneither recommendednorsupportedandmaycausethecompilertocrashormiscompile. Userswhowishtogainaccesstointrinsicbehaviorarestronglyencouragedto requestnewbuiltinfunctions. deprecated¶ SupportedSyntaxes¶ deprecated gnu::deprecateddeprecated gnu::deprecateddeprecated deprecated         Thedeprecatedattributecanbeappliedtoafunction,avariable,ora type.Thisisusefulwhenidentifyingfunctions,variables,ortypesthatare expectedtoberemovedinafutureversionofaprogram. Considerthefunctiondeclarationforahypotheticalfunctionf: voidf(void)__attribute__((deprecated("message","replacement"))); Whenspelledas__attribute__((deprecated)),thedeprecatedattributecanhave twooptionalstringarguments.Thefirstoneisthemessagetodisplaywhen emittingthewarning;thesecondoneenablesthecompilertoprovideaFix-It toreplacethedeprecatednamewithanewname.Otherwise,whenspelledas [[gnu::deprecated]]or[[deprecated]],theattributecanhaveoneoptional stringargumentwhichisthemessagetodisplaywhenemittingthewarning. empty_bases¶ SupportedSyntaxes¶       empty_bases         Theempty_basesattributepermitsthecompilertoutilizethe empty-base-optimizationmorefrequently. Thisattributeonlyappliestostruct,class,anduniontypes. ItisonlysupportedwhenusingtheMicrosoftC++ABI. enum_extensibility¶ SupportedSyntaxes¶ enum_extensibility clang::enum_extensibility clang::enum_extensibility         Yes Attributeenum_extensibilityisusedtodistinguishbetweenenumdefinitions thatareextensibleandthosethatarenot.Theattributecantakeeither closedoropenasanargument.closedindicatesavariableofthe enumtypetakesavaluethatcorrespondstooneoftheenumeratorslistedinthe enumdefinitionor,whentheenumisannotatedwithflag_enum,avaluethat canbeconstructedusingvaluescorrespondingtotheenumerators.open indicatesavariableoftheenumtypecantakeanyvaluesallowedbythe standardandinstructsclangtobemorelenientwhenissuingwarnings. enum__attribute__((enum_extensibility(closed)))ClosedEnum{ A0,A1 }; enum__attribute__((enum_extensibility(open)))OpenEnum{ B0,B1 }; enum__attribute__((enum_extensibility(closed),flag_enum))ClosedFlagEnum{ C0=1<<0,C1=1<<1 }; enum__attribute__((enum_extensibility(open),flag_enum))OpenFlagEnum{ D0=1<<0,D1=1<<1 }; voidfoo1(){ enumClosedEnumce; enumOpenEnumoe; enumClosedFlagEnumcfe; enumOpenFlagEnumofe; ce=A1;//nowarnings ce=100;//warningissued oe=B1;//nowarnings oe=100;//nowarnings cfe=C0|C1;//nowarnings cfe=C0|C1|4;//warningissued ofe=D0|D1;//nowarnings ofe=D0|D1|4;//nowarnings } external_source_symbol¶ SupportedSyntaxes¶ external_source_symbol clang::external_source_symbol clang::external_source_symbol         Yes Theexternal_source_symbolattributespecifiesthatadeclarationoriginates fromanexternalsourceanddescribesthenatureofthatsource. ThefactthatClangiscapableofrecognizingdeclarationsthatweredefined externallycanbeusedtoprovidebettertoolingsupportformixed-language projectsorprojectsthatrelyonauto-generatedcode.Forinstance,anIDEthat usesClangandthatsupportsmixed-languageprojectscanusethisattributeto provideacorrect‘jump-to-definition’feature.Foraconcreteexample, consideraprotocolthat’sdefinedinaSwiftfile: @objcpublicprotocolSwiftProtocol{ funcmethod() } ThisprotocolcanbeusedfromObjective-Ccodebyincludingaheaderfilethat wasgeneratedbytheSwiftcompiler.Thedeclarationsinthatheadercanuse theexternal_source_symbolattributetomakeClangawareofthefact thatSwiftProtocolactuallyoriginatesfromaSwiftmodule: __attribute__((external_source_symbol(language="Swift",defined_in="module"))) @protocolSwiftProtocol @required -(void)method; @end Consequently,when‘jump-to-definition’isperformedatalocationthat referencesSwiftProtocol,theIDEcanjumptotheoriginaldefinitionin theSwiftsourcefileratherthanjumpingtotheObjective-Cdeclarationinthe auto-generatedheaderfile. Theexternal_source_symbolattributeisacomma-separatedlistthatincludes clausesthatdescribetheoriginandthenatureoftheparticulardeclaration. Thoseclausescanbe: language=string-literal Thenameofthesourcelanguageinwhichthisdeclarationwasdefined. defined_in=string-literal Thenameofthesourcecontainerinwhichthedeclarationwasdefined.The exactdefinitionofsourcecontainerislanguage-specific,e.g.Swift’s sourcecontainersaremodules,sodefined_inshouldspecifytheSwift modulename. generated_declaration Thisdeclarationwasautomaticallygeneratedbysometool. Theclausescanbespecifiedinanyorder.Theclausesthatarelistedaboveare alloptional,buttheattributehastohaveatleastoneclause. flag_enum¶ SupportedSyntaxes¶ flag_enum clang::flag_enum clang::flag_enum         Yes Thisattributecanbeaddedtoanenumeratortosignaltothecompilerthatit isintendedtobeusedasaflagtype.Thiswillcausethecompilertoassume thattherangeofthetypeincludesallofthevaluesthatyoucangetby manipulatingbitsoftheenumeratorwhenissuingwarnings. layout_version¶ SupportedSyntaxes¶       layout_version         Thelayout_versionattributerequeststhatthecompilerutilizetheclass layoutrulesofaparticularcompilerversion. Thisattributeonlyappliestostruct,class,anduniontypes. ItisonlysupportedwhenusingtheMicrosoftC++ABI. lto_visibility_public¶ SupportedSyntaxes¶ lto_visibility_public clang::lto_visibility_public clang::lto_visibility_public         Yes SeeLTOVisibility. managed¶ SupportedSyntaxes¶ managed     __managed__       Yes The__managed__attributecanbeappliedtoaglobalvariabledeclarationinHIP. Amanagedvariableisemittedasanundefinedglobalsymbolinthedevicebinaryandis registeredby__hipRegisterManagedVariableininitfunctions.TheHIPruntimeallocates managedmemoryandusesittodefinethesymbolwhenloadingthedevicebinary. Amanagedvariablecanbeaccessedinbothdeviceandhostcode. novtable¶ SupportedSyntaxes¶       novtable         Thisattributecanbeaddedtoaclassdeclarationordefinitiontosignalto thecompilerthatconstructorsanddestructorswillnotreferencethevirtual functiontable.ItisonlysupportedwhenusingtheMicrosoftC++ABI. ns_error_domain¶ SupportedSyntaxes¶ ns_error_domain             Yes InCocoaframeworksinObjective-C,onecangrouprelatederrorcodesinenums andcategorizetheseenumswitherrordomains. Thens_error_domainattributeindicatesaglobalNSStringor CFStringconstantrepresentingtheerrordomainthatanerrorcodebelongs to.Forpointeruniquenessandcodesizethisisaconstantsymbol,nota literal. Thedomainanderrorcodeneedtobeusedtogether.Thens_error_domain attributelinkserrorcodestotheirdomainatthesourcelevel. Thismetadataisusefulfordocumentationpurposes,forstaticanalysis,andfor improvinginteroperabilitybetweenObjective-CandSwift.Itisnotusedfor codegenerationinObjective-C. Forexample: #defineNS_ERROR_ENUM(_type,_name,_domain)\ enum_name:_type_name;enum__attribute__((ns_error_domain(_domain)))_name:_type externNSString*constMyErrorDomain; typedefNS_ERROR_ENUM(unsignedchar,MyErrorEnum,MyErrorDomain){ MyErrFirst, MyErrSecond, }; objc_boxable¶ SupportedSyntaxes¶ objc_boxable clang::objc_boxable clang::objc_boxable         Yes Structsandunionsmarkedwiththeobjc_boxableattributecanbeused withtheObjective-Cboxedexpressionsyntax,@(...). Usage:__attribute__((objc_boxable)).Thisattribute canonlybeplacedonadeclarationofatrivially-copyablestructorunion: struct__attribute__((objc_boxable))some_struct{ inti; }; union__attribute__((objc_boxable))some_union{ inti; floatf; }; typedefstruct__attribute__((objc_boxable))_some_structsome_struct; //... some_structss; NSValue*boxed=@(ss); objc_direct¶ SupportedSyntaxes¶ objc_direct clang::objc_direct clang::objc_direct         Yes Theobjc_directattributecanbeusedtomarkanObjective-Cmethodas beingdirect.Adirectmethodistreatedstaticallylikeanordinarymethod, butdynamicallyitbehavesmorelikeaCfunction.Thislowerssomeofthecosts associatedwiththemethodbutalsosacrificessomeoftheordinarycapabilities ofObjective-Cmethods. Amessagesendofadirectmethodcallstheimplementationdirectly,asifit wereaCfunction,ratherthanusingordinaryObjective-Cmethoddispatch.This issubstantiallyfasterandpotentiallyallowstheimplementationtobeinlined, butitalsomeansthemethodcannotbeoverriddeninsubclassesorreplaced dynamically,asordinaryObjective-Cmethodscan. Furthermore,adirectmethodisnotlistedintheclass’smethodlists.This substantiallyreducesthecode-sizeoverheadofthemethodbutalsomeansit cannotbecalleddynamicallyusingordinaryObjective-Cmethoddispatchatall; inparticular,thismeansthatitcannotoverrideasuperclassmethodorsatisfy aprotocolrequirement. Becauseadirectmethodcannotbeoverridden,itisanerrortoperform asupermessagesendofone. Althoughamessagesendofadirectmethodcausesthemethodtobecalled directlyasifitwereaCfunction,itstillobeysObjective-Csemanticsinother ways: Ifthereceiverisnil,themessagesenddoesnothingandreturnsthezerovalue forthereturntype. Amessagesendofadirectclassmethodwillcausetheclasstobeinitialized, includingcallingthe+initializemethodifpresent. Theimplicit_cmdparametercontainingthemethod’sselectorisstilldefined. Inordertominimizecode-sizecosts,theimplementationwillnotemitareference totheselectoriftheparameterisunusedwithinthemethod. Symbolsfordirectmethodimplementationsareimplicitlygivenhidden visibility,meaningthattheycanonlybecalledwithinthesamelinkageunit. Itisanerrortodoanyofthefollowing: declareadirectmethodinaprotocol, declareanoverrideofadirectmethodwithamethodinasubclass, declareanoverrideofanon-directmethodwithadirectmethodinasubclass, declareamethodwithdifferentdirectnessindifferentclassinterfaces,or implementanon-directmethod(asdeclaredinanyclassinterface)withadirectmethod. Ifanyoftheseruleswouldbeviolatedifeverymethoddefinedinan @implementationwithinasinglelinkageunitweredeclaredinan appropriateclassinterface,theprogramisill-formedwithnodiagnostic required.Ifaviolationofthisruleisnotdiagnosed,behaviorremains well-defined;thisparagraphissimplyreservingtherighttodiagnosesuch conflictsinthefuture,nottotreatthemasundefinedbehavior. Additionally,Clangwillwarnaboutany@selectorexpressionthat namesaselectorthatisonlyknowntobeusedfordirectmethods. Forthepurposeoftheserules,a“classinterface”includesaclass’sprimary @interfaceblock,itsclassextensions,itscategories,itsdeclaredprotocols, andalltheclassinterfacesofitssuperclasses. AnObjective-Cpropertycanbedeclaredwiththedirectproperty attribute.Ifadirectpropertydeclarationcausesanimplicitdeclarationof agetterorsettermethod(thatis,ifthegivenmethodisnotexplicitly declaredelsewhere),themethodisdeclaredtobedirect. Someprogrammersmaywishtomakemanymethodsdirectatonce.Inorder tosimplifythis,theobjc_direct_membersattributeisprovided;seeits documentationformoreinformation. objc_direct_members¶ SupportedSyntaxes¶ objc_direct_members clang::objc_direct_members clang::objc_direct_members         Yes Theobjc_direct_membersattributecanbeplacedonanObjective-C @interfaceor@implementationtomarkthatmethodsdeclared thereinshouldbeconsidereddirectbydefault.Seethedocumentation forobjc_directformoreinformationaboutdirectmethods. Whenobjc_direct_membersisplacedonan@interfaceblock,every methodintheblockisconsideredtobedeclaredasdirect.Thisincludesany implicitmethoddeclarationsintroducedbypropertydeclarations.Ifthemethod redeclaresanon-directmethod,thedeclarationisill-formed,exactlyasifthe methodwasannotatedwiththeobjc_directattribute. Whenobjc_direct_membersisplacedonan@implementationblock, methodsdefinedintheblockareconsideredtobedeclaredasdirectunless theyhavebeenpreviouslydeclaredasnon-directinanyinterfaceoftheclass. Thisincludestheimplicitmethoddefinitionsintroducedbysynthesized properties,includingauto-synthesizedproperties. objc_non_runtime_protocol¶ SupportedSyntaxes¶ objc_non_runtime_protocol clang::objc_non_runtime_protocol clang::objc_non_runtime_protocol         Yes Theobjc_non_runtime_protocolattributecanbeusedtomarkthatan Objective-Cprotocolisonlyusedduringstatictype-checkinganddoesn’tneed toberepresenteddynamically.Thisavoidsseveralsmallcode-sizeandrun-time overheadsassociatedwithhandlingtheprotocol’smetadata.Anon-runtime protocolcannotbeusedastheoperandofa@protocolexpression,and dynamicattemptstofinditwithobjc_getProtocolwillfail. Ifanon-runtimeprotocolinheritsfromanyordinaryprotocols,classesand derivedprotocolsthatdeclareconformancetothenon-runtimeprotocolwill dynamicallylisttheirconformancetothosebareprotocols. objc_nonlazy_class¶ SupportedSyntaxes¶ objc_nonlazy_class clang::objc_nonlazy_class clang::objc_nonlazy_class         Yes ThisattributecanbeaddedtoanObjective-C@interfaceor @implementationdeclarationtoaddtheclasstothelistofnon-lazily initializedclasses.Anon-lazyclasswillbeinitializedeagerlywhenthe Objective-Cruntimeisloaded.Thisisrequiredforcertainsystemclasseswhich haveinstancesallocatedinnon-standardways,suchastheclassesforblocks andconstantstrings.Addingthisattributeisessentiallyequivalentto providingatrivial+loadmethodbutavoidsthe(fairlysmall)load-time overheadsassociatedwithdefiningandcallingsuchamethod. objc_runtime_name¶ SupportedSyntaxes¶ objc_runtime_name clang::objc_runtime_name clang::objc_runtime_name         Yes Bydefault,theObjective-Cinterfaceorprotocolidentifierisused inthemetadatanameforthatobject.Theobjc_runtime_name attributeallowsannotatedinterfacesorprotocolstousethe specifiedstringargumentintheobject’smetadatanameinsteadofthe defaultname. Usage:__attribute__((objc_runtime_name("MyLocalName"))).Thisattribute canonlybeplacedbeforean@protocolor@interfacedeclaration: __attribute__((objc_runtime_name("MyLocalName"))) @interfaceMessage @end objc_runtime_visible¶ SupportedSyntaxes¶ objc_runtime_visible clang::objc_runtime_visible clang::objc_runtime_visible         Yes ThisattributespecifiesthattheObjective-Cclasstowhichitappliesis visibletotheObjective-Cruntimebutnottothelinker.Classesannotated withthisattributecannotbesubclassedandcannothavecategoriesdefinedfor them. objc_subclassing_restricted¶ SupportedSyntaxes¶ objc_subclassing_restricted clang::objc_subclassing_restricted clang::objc_subclassing_restricted         Yes ThisattributecanbeaddedtoanObjective-C@interfacedeclarationto ensurethatthisclasscannotbesubclassed. preferred_name¶ SupportedSyntaxes¶ preferred_name clang::preferred_name             Thepreferred_nameattributecanbeappliedtoaclasstemplate,and specifiesapreferredwayofnamingaspecializationofthetemplate.The preferrednamewillbeusedwheneverthecorrespondingtemplatespecialization wouldotherwisebeprintedinadiagnosticorsimilarcontext. Thepreferrednamemustbeatypedefortypealiasdeclarationthatreferstoa specializationoftheclasstemplate(notincludinganytypequalifiers).In generalthisrequiresthetemplatetobedeclaredatleasttwice.Forexample: templatestructbasic_string; usingstring=basic_string; usingwstring=basic_string; templatestruct[[clang::preferred_name(string), clang::preferred_name(wstring)]]basic_string{ //... }; randomize_layout,no_randomize_layout¶ SupportedSyntaxes¶ no_randomize_layout gnu::no_randomize_layout gnu::no_randomize_layout         Yes Theattributerandomize_layout,whenattachedtoaCstructure,selectsit forstructurelayoutfieldrandomization;acompile-timehardeningtechnique.A “seed”value,isspecifiedviathe-frandomize-layout-seed=commandlineflag. Forexample: SEED=`od-An-tx8-N32/dev/urandom|tr-d'\n'` make...CFLAGS="-frandomize-layout-seed=$SEED"... Youcanalsosupplytheseedinafilewith-frandomize-layout-seed-file=. Forexample: od-An-tx8-N32/dev/urandom|tr-d'\n'>/tmp/seed_file.txt make...CFLAGS="-frandomize-layout-seed-file=/tmp/seed_file.txt"... Therandomizationisdeterministicbasedforagivenseed,sotheentire programshouldbecompiledwiththesameseed,butkeeptheseedsafe otherwise. Theattributeno_randomize_layout,whenattachedtoaCstructure, instructsthecompilerthatthisstructureshouldnothaveitsfieldlayout randomized. randomize_layout,no_randomize_layout¶ SupportedSyntaxes¶ randomize_layout gnu::randomize_layout gnu::randomize_layout         Yes Theattributerandomize_layout,whenattachedtoaCstructure,selectsit forstructurelayoutfieldrandomization;acompile-timehardeningtechnique.A “seed”value,isspecifiedviathe-frandomize-layout-seed=commandlineflag. Forexample: SEED=`od-An-tx8-N32/dev/urandom|tr-d'\n'` make...CFLAGS="-frandomize-layout-seed=$SEED"... Youcanalsosupplytheseedinafilewith-frandomize-layout-seed-file=. Forexample: od-An-tx8-N32/dev/urandom|tr-d'\n'>/tmp/seed_file.txt make...CFLAGS="-frandomize-layout-seed-file=/tmp/seed_file.txt"... Therandomizationisdeterministicbasedforagivenseed,sotheentire programshouldbecompiledwiththesameseed,butkeeptheseedsafe otherwise. Theattributeno_randomize_layout,whenattachedtoaCstructure, instructsthecompilerthatthisstructureshouldnothaveitsfieldlayout randomized. selectany¶ SupportedSyntaxes¶ selectany gnu::selectany gnu::selectany selectany         Thisattributeappertainstoaglobalsymbol,causingittohaveaweak definition( linkonce ),allowingthelinkertoselectanydefinition. Formoreinformationsee gccdocumentation ormsvcdocumentation. transparent_union¶ SupportedSyntaxes¶ transparent_union gnu::transparent_union gnu::transparent_union           Thisattributecanbeappliedtoauniontochangethebehaviorofcallsto functionsthathaveanargumentwithatransparentuniontype.Thecompiler behaviorischangedinthefollowingmanner: Avaluewhosetypeisanymemberofthetransparentunioncanbepassedasan argumentwithouttheneedtocastthatvalue. Theargumentispassedtothefunctionusingthecallingconventionofthe firstmemberofthetransparentunion.Consequently,allthemembersofthe transparentunionshouldhavethesamecallingconventionasitsfirstmember. TransparentunionsarenotsupportedinC++. trivial_abi¶ SupportedSyntaxes¶ trivial_abi clang::trivial_abi           Yes Thetrivial_abiattributecanbeappliedtoaC++class,struct,orunion. ItinstructsthecompilertopassandreturnthetypeusingtheCABIforthe underlyingtypewhenthetypewouldotherwisebeconsiderednon-trivialforthe purposeofcalls. Aclassannotatedwithtrivial_abicanhavenon-trivialdestructorsor copy/moveconstructorswithoutautomaticallybecomingnon-trivialforthe purposesofcalls.Forexample: //Aistrivialforthepurposesofcallsbecause``trivial_abi``makesthe //user-providedspecialfunctionstrivial. struct__attribute__((trivial_abi))A{ ~A(); A(constA&); A(A&&); intx; }; //B'sdestructorandcopy/moveconstructorareconsideredtrivialforthe //purposeofcallsbecauseAistrivial. structB{ Aa; }; Ifatypeistrivialforthepurposesofcalls,hasanon-trivialdestructor, andispassedasanargumentbyvalue,theconventionisthatthecalleewill destroytheobjectbeforereturning. Ifatypeistrivialforthepurposeofcalls,itisassumedtobetrivially relocatableforthepurposeof__is_trivially_relocatable. Attributetrivial_abihasnoeffectinthefollowingcases: Theclassdirectlydeclaresavirtualbaseorvirtualmethods. Copyconstructorsandmoveconstructorsoftheclassarealldeleted. Theclasshasabaseclassthatisnon-trivialforthepurposesofcalls. Theclasshasanon-staticdatamemberwhosetypeisnon-trivialforthe purposesofcalls,whichincludes: classesthatarenon-trivialforthepurposesofcalls __weak-qualifiedtypesinObjective-C++ arraysofanyoftheabove using_if_exists¶ SupportedSyntaxes¶ using_if_exists clang::using_if_exists             Theusing_if_existsattributeappliestoausing-declaration.Itallows programmerstoimportadeclarationthatpotentiallydoesnotexist,instead deferringanyerrorstothepointofuse.Forinstance: namespaceempty_namespace{}; __attribute__((using_if_exists)) usingempty_namespace::does_not_exist;//noerror! does_not_existx;//error:useofunresolved'using_if_exists' TheC++spellingoftheattribte([[clang::using_if_exists]])isalso supportedasaclangextension,sinceISOC++doesn’tsupportattributesinthis position.Iftheentityreferredtobytheusing-declarationisfoundbyname lookup,theattributehasnoeffect.Thisattributeisusefulforlibraries (primarily,libc++)thatwishtoredeclareasetofdeclarationsinanother namespace,whentheavailabilityofthosedeclarationsisdifficultor impossibletodetectatcompiletimewiththepreprocessor. weak¶ SupportedSyntaxes¶ weak gnu::weak gnu::weak         Yes Insupportedoutputformatstheweakattributecanbeusedto specifythatavariableorfunctionshouldbeemittedasasymbolwith weak(ifadefinition)orextern_weak(ifadeclarationofan externalsymbol)linkage. Ifthereisanon-weakdefinitionofthesymbolthelinkerwillselect thatovertheweak.Theymusthavesametypeandalignment(variables mustalsohavethesamesize),butmayhaveadifferentvalue. Iftherearemultipleweakdefinitionsofsamesymbol,butnonon-weak definition,theyshouldhavesametype,size,alignmentandvalue,the linkerwillselectoneofthem(seealsoselectanyattribute). Iftheweakattributeisappliedtoaconstqualifiedvariable definitionthatvariableisnolongerconsideracompiletimeconstant asitsvaluecanchangeduringlinking(ordynamiclinking).This meansthatitcane.gnolongerbepartofaninitializerexpression. constintANSWER__attribute__((weak))=42; /*Thisfunctionmaybereplacedlink-time*/ __attribute__((weak))voiddebug_log(constchar*msg) { fprintf(stderr,"DEBUG:%s\n",msg); } intmain(intargc,constchar**argv) { debug_log("Startingup..."); /*Thismayprintsomethingelsethan"6*7=42", ifthereisanon-weakdefinitionof"ANSWER"in anobjectlinkedin*/ printf("6*7=%d\n",ANSWER); return0; } Ifanexternaldeclarationismarkedweakandthatsymboldoesnot existduringlinking(possiblydynamic)theaddressofthesymbolwill evaluatetoNULL. voidmay_not_exist(void)__attribute__((weak)); intmain(intargc,constchar**argv) { if(may_not_exist){ may_not_exist(); }else{ printf("Functiondidnotexist\n"); } return0; } FieldAttributes¶ no_unique_address¶ SupportedSyntaxes¶   no_unique_address             Theno_unique_addressattributeallowstailpaddinginanon-staticdata membertooverlapothermembersoftheenclosingclass(andinthespecial casewhenthetypeisempty,permitsittofullyoverlapothermembers). Thefieldislaidoutasifabaseclasswereencounteredatthecorresponding pointwithintheclass(exceptthatitdoesnotshareavptrwiththeenclosing object). Exampleusage: templatestructmy_vector{ T*p; [[no_unique_address]]Allocalloc; //... }; static_assert(sizeof(my_vector>)==sizeof(int*)); [[no_unique_address]]isastandardC++20attribute.Clangsupportsitsuse inC++11onwards. FunctionAttributes¶ #pragmaompdeclaresimd¶ SupportedSyntaxes¶           ompdeclaresimd     Thedeclaresimdconstructcanbeappliedtoafunctiontoenablethecreation ofoneormoreversionsthatcanprocessmultipleargumentsusingSIMD instructionsfromasingleinvocationinaSIMDloop.Thedeclaresimd directiveisadeclarativedirective.Theremaybemultipledeclaresimd directivesforafunction.Theuseofadeclaresimdconstructonafunction enablesthecreationofSIMDversionsoftheassociatedfunctionthatcanbe usedtoprocessmultipleargumentsfromasingleinvocationfromaSIMDloop concurrently. Thesyntaxofthedeclaresimdconstructisasfollows: #pragmaompdeclaresimd[clause[[,]clause]...]new-line [#pragmaompdeclaresimd[clause[[,]clause]...]new-line] [...] functiondefinitionordeclaration whereclauseisoneofthefollowing: simdlen(length) linear(argument-list[:constant-linear-step]) aligned(argument-list[:alignment]) uniform(argument-list) inbranch notinbranch #pragmaompdeclaretarget¶ SupportedSyntaxes¶           ompdeclaretarget     Thedeclaretargetdirectivespecifiesthatvariablesandfunctionsaremapped toadeviceforOpenMPoffloadmechanism. Thesyntaxofthedeclaretargetdirectiveisasfollows: #pragmaompdeclaretargetnew-line declarations-definition-seq #pragmaompenddeclaretargetnew-line or #pragmaompdeclaretarget(extended-list)new-line or #pragmaompdeclaretargetclause[[,]clause...]new-line whereclauseisoneofthefollowing: to(extended-list) link(list) device_type(host|nohost|any) #pragmaompdeclarevariant¶ SupportedSyntaxes¶           ompdeclarevariant     Thedeclarevariantdirectivedeclaresaspecializedvariantofabase functionandspecifiesthecontextinwhichthatspecializedvariantisused. Thedeclarevariantdirectiveisadeclarativedirective. Thesyntaxofthedeclarevariantconstructisasfollows: #pragmaompdeclarevariant(variant-func-id)clausenew-line [#pragmaompdeclarevariant(variant-func-id)clausenew-line] [...] functiondefinitionordeclaration whereclauseisoneofthefollowing: match(context-selector-specification) andwherevariant-func-idisthenameofafunctionvariantthatiseithera baselanguageidentifieror,forC++,atemplate-id. Clangprovidesthefollowingcontextselectorextensions,usedvia implementation={extension(EXTENSION)}: match_all match_any match_none disable_implicit_base allow_templates Thematchextensionschangewhentheentirecontextselectorisconsidereda matchforanOpenMPcontext.Thedefaultisall,withnonenotraitinthe selectorisallowedtobeintheOpenMPcontext,withanyasingletraitin boththeselectorandOpenMPcontextissufficient.Onlyasinglematch extensiontraitisallowedpercontextselector. Thedisableextensionsremovedefaulteffectsofthebegindeclarevariant appliedtoadefinition.Ifdisable_implicit_baseisgiven,wewillnot introduceanimplicitbasefunctionforavariantifnobasefunctionwas found.Thevariantisstillgeneratedbutwillneverbecalled,duetothe absenceofabasefunctionandconsequentlycallstoabasefunction. Theallowextensionschangewhenthebegindeclarevarianteffectis appliedtoadefinition.Ifallow_templatesisgiven,templatefunction definitionsareconsideredasspecializationsofexistingorassumedtemplate declarationswiththesamename.Thetemplateparametersforthebasefunctions areusedtoinstantiatethespecialization. SV_GroupIndex¶ SupportedSyntaxes¶             SV_GroupIndex   TheSV_GroupIndexsemantic,whenappliedtoaninputparameter,specifiesa databindingtomapthegroupindextothespecifiedparameter.Thisattribute isonlysupportedincomputeshaders. Thefulldocumentationisavailablehere:https://docs.microsoft.com/en-us/windows/win32/direct3dhlsl/sv-groupindex _Noreturn¶ SupportedSyntaxes¶         _Noreturn       Afunctiondeclaredas_Noreturnshallnotreturntoitscaller.The compilerwillgenerateadiagnosticforafunctiondeclaredas_Noreturn thatappearstobecapableofreturningtoitscaller.Despitebeingatype specifier,the_Noreturnattributecannotbespecifiedonafunction pointertype. abi_tag¶ SupportedSyntaxes¶ abi_tag gnu::abi_tag           Yes Theabi_tagattributecanbeappliedtoafunction,variable,classor inlinenamespacedeclarationtomodifythemanglednameoftheentity.Itgives theabilitytodistinguishbetweendifferentversionsofthesameentitybut withdifferentABIversionssupported.Forexample,anewerversionofaclass couldhaveadifferentsetofdatamembersandthushaveadifferentsize.Using theabi_tagattribute,itispossibletohavedifferentmanglednamesfor aglobalvariableoftheclasstype.Therefore,theoldcodecouldkeepusing theoldmanglednameandthenewcodewillusethenewmanglednamewithtags. acquire_capability,acquire_shared_capability¶ SupportedSyntaxes¶ acquire_capabilityacquire_shared_capabilityexclusive_lock_functionshared_lock_function clang::acquire_capabilityclang::acquire_shared_capability             Marksafunctionasacquiringacapability. alloc_align¶ SupportedSyntaxes¶ alloc_align gnu::alloc_align gnu::alloc_align           Use__attribute__((alloc_align())onafunction declarationtospecifythatthereturnvalueofthefunction(whichmustbea pointertype)isatleastasalignedasthevalueoftheindicatedparameter.The parameterisgivenbyitsindexinthelistofformalparameters;thefirst parameterhasindex1unlessthefunctionisaC++non-staticmemberfunction, inwhichcasethefirstparameterhasindex2toaccountfortheimplicitthis parameter. //Thereturnedpointerhasthealignmentspecifiedbythefirstparameter. void*a(size_talign)__attribute__((alloc_align(1))); //Thereturnedpointerhasthealignmentspecifiedbythesecondparameter. void*b(void*v,size_talign)__attribute__((alloc_align(2))); //Thereturnedpointerhasthealignmentspecifiedbythesecondvisible //parameter,howeveritmustbeadjustedfortheimplicit'this'parameter. void*Foo::b(void*v,size_talign)__attribute__((alloc_align(3))); Notethatthisattributemerelyinformsthecompilerthatafunctionalways returnsasufficientlyalignedpointer.Itdoesnotcausethecompilerto emitcodetoenforcethatalignment.Thebehaviorisundefinedifthereturned pointerisnotsufficientlyaligned. alloc_size¶ SupportedSyntaxes¶ alloc_size gnu::alloc_size gnu::alloc_size           Thealloc_sizeattributecanbeplacedonfunctionsthatreturnpointersin ordertohinttothecompilerhowmanybytesofmemorywillbeavailableatthe returnedpointer.alloc_sizetakesoneortwoarguments. alloc_size(N)impliesthatargumentnumberNequalsthenumberof availablebytesatthereturnedpointer. alloc_size(N,M)impliesthattheproductofargumentnumberNand argumentnumberMequalsthenumberofavailablebytesatthereturned pointer. Argumentnumbersare1-based. Anexampleofhowtousealloc_size void*my_malloc(inta)__attribute__((alloc_size(1))); void*my_calloc(inta,intb)__attribute__((alloc_size(1,2))); intmain(){ void*constp=my_malloc(100); assert(__builtin_object_size(p,0)==100); void*consta=my_calloc(20,5); assert(__builtin_object_size(a,0)==100); } Note ThisattributeworksdifferentlyinclangthanitdoesinGCC. Specifically,clangwillonlytraceconstpointers(asabove);wegiveup onpointersthatarenotmarkedasconst.Inthevastmajorityofcases, thisisunimportant,becauseLLVMhassupportforthealloc_size attribute.However,thismaycausemildlyunintuitivebehaviorwhenusedwith otherattributes,suchasenable_if. allocator¶ SupportedSyntaxes¶       allocator         The__declspec(allocator)attributeisappliedtofunctionsthatallocate memory,suchasoperatornewinC++.WhenCodeViewdebuginformationisemitted (enabledbyclang-gcodevieworclang-cl/Z7),Clangwillattemptto recordthecodeoffsetofheapallocationcallsitesinthedebuginfo.Itwill alsorecordthetypebeingallocatedusingsomelocalheuristics.TheVisual Studiodebuggerusesthisinformationtoprofilememoryusage. Thisattributedoesnotaffectoptimizationsinanyway,unlikeGCC’s __attribute__((malloc)). always_inline,__force_inline¶ SupportedSyntaxes¶ always_inline gnu::always_inlineclang::always_inline gnu::always_inlineclang::always_inline   __forceinline     Yes Inliningheuristicsaredisabledandinliningisalwaysattemptedregardlessof optimizationlevel. [[clang::always_inline]]spellingcanbeusedasastatementattribute;other spellingsoftheattributearenotsupportedonstatements.Ifastatementis marked[[clang::always_inline]]andcontainscalls,thecompilerattempts toinlinethosecalls. intexample(void){ inti; [[clang::always_inline]]foo();//attemptstoinlinefoo [[clang::always_inline]]i=bar();//attemptstoinlinebar [[clang::always_inline]]returnf(42,baz(bar()));//attemptstoinlineeverything } Adeclarationstatement,whichisastatement,isnotastatementthatcanhavean attributeassociatedwithit(theattributeappliestothedeclaration,notthe statementinthatcase).Sothisusecasewillnotwork: intexample(void){ [[clang::always_inline]]inti=bar(); returni; } Thisattributedoesnotguaranteethatinlinesubstitutionactuallyoccurs. Note:applyingthisattributetoacoroutineatthe-O0optimizationlevel hasnoeffect;otheroptimizationlevelsmayonlypartiallyinlineandresultina diagnostic. SeealsotheMicrosoftDocsonInlineFunctions,theGCCCommonFunction Attributedocs,andtheGCCInlinedocs. artificial¶ SupportedSyntaxes¶ artificial gnu::artificial gnu::artificial           Theartificialattributecanbeappliedtoaninlinefunction.Ifsucha functionisinlined,theattributeindicatesthatdebuggersshouldassociate theresultinginstructionswiththecallsite,ratherthanwiththe correspondinglinewithintheinlinedcallee. assert_capability,assert_shared_capability¶ SupportedSyntaxes¶ assert_capabilityassert_shared_capability clang::assert_capabilityclang::assert_shared_capability             Marksafunctionthatdynamicallytestswhetheracapabilityisheld,andhalts theprogramifitisnotheld. assume¶ SupportedSyntaxes¶ assume clang::assume clang::assume         Yes Clangsupportsthe__attribute__((assume("assumption")))attributeto provideadditionalinformationtotheoptimizer.Thestring-literal,here “assumption”,willbeattachedtothefunctiondeclarationsuchthatlater analysisandoptimizationpassescanassumethe“assumption”tohold. Thisissimilarto__builtin_assumebut insteadofanexpressionthatcanbeassumedtobenon-zero,theassumptionis expressedasastringanditholdsfortheentirefunction. Afunctioncanhavemultipleassumeattributesandtheypropagatefromprior declarationstolaterdefinitions.Multipleassumptionsareaggregatedintoa singlecommaseparatedstring.Thus,onecanprovidemultipleassumptionsvia acommaseparatedstring,i.a., __attribute__((assume("assumption1,assumption2"))). WhileLLVMpluginsmightprovidemoreassumptionstrings,thedefaultLLVM optimizationpassesareawareofthefollowingassumptions: "omp_no_openmp" "omp_no_openmp_routines" "omp_no_parallelism" TheOpenMPstandarddefinesthemeaningofOpenMPassumptions(“omp_XYZ”is spelled“XYZ”intheOpenMP5.1Standard). assume_aligned¶ SupportedSyntaxes¶ assume_aligned gnu::assume_aligned gnu::assume_aligned         Yes Use__attribute__((assume_aligned([,]))onafunction declarationtospecifythatthereturnvalueofthefunction(whichmustbea pointertype)hasthespecifiedoffset,inbytes,fromanaddresswiththe specifiedalignment.Theoffsetistakentobezeroifomitted. //Thereturnedpointervaluehas32-bytealignment. void*a()__attribute__((assume_aligned(32))); //Thereturnedpointervalueis4bytesgreaterthananaddresshaving //32-bytealignment. void*b()__attribute__((assume_aligned(32,4))); Notethatthisattributeprovidesinformationtothecompilerregardinga conditionthatthecodealreadyensuresistrue.Itdoesnotcausethecompiler toenforcetheprovidedalignmentassumption. availability¶ SupportedSyntaxes¶ availability clang::availability clang::availability         Yes Theavailabilityattributecanbeplacedondeclarationstodescribethe lifecycleofthatdeclarationrelativetooperatingsystemversions.Consider thefunctiondeclarationforahypotheticalfunctionf: voidf(void)__attribute__((availability(macos,introduced=10.4,deprecated=10.6,obsoleted=10.7))); TheavailabilityattributestatesthatfwasintroducedinmacOS10.4, deprecatedinmacOS10.6,andobsoletedinmacOS10.7.Thisinformation isusedbyClangtodeterminewhenitissafetousef:forexample,if ClangisinstructedtocompilecodeformacOS10.5,acalltof() succeeds.IfClangisinstructedtocompilecodeformacOS10.6,thecall succeedsbutClangemitsawarningspecifyingthatthefunctionisdeprecated. Finally,ifClangisinstructedtocompilecodeformacOS10.7,thecall failsbecausef()isnolongeravailable. Theavailabilityattributeisacomma-separatedliststartingwiththe platformnameandthenincludingclausesspecifyingimportantmilestonesinthe declaration’slifetime(inanyorder)alongwithadditionalinformation.Those clausescanbe: introduced=version Thefirstversioninwhichthisdeclarationwasintroduced. deprecated=version Thefirstversioninwhichthisdeclarationwasdeprecated,meaningthat usersshouldmigrateawayfromthisAPI. obsoleted=version Thefirstversioninwhichthisdeclarationwasobsoleted,meaningthatit wasremovedcompletelyandcannolongerbeused. unavailable Thisdeclarationisneveravailableonthisplatform. message=string-literal AdditionalmessagetextthatClangwillprovidewhenemittingawarningor erroraboutuseofadeprecatedorobsoleteddeclaration.Usefultodirect userstoreplacementAPIs. replacement=string-literal AdditionalmessagetextthatClangwillusetoprovideFix-Itwhenemitting awarningaboutuseofadeprecateddeclaration.TheFix-Itwillreplace thedeprecateddeclarationwiththenewdeclarationspecified. Multipleavailabilityattributescanbeplacedonadeclaration,whichmay correspondtodifferentplatforms.Formostplatforms,theavailability attributewiththeplatformcorrespondingtothetargetplatformwillbeused; anyotherswillbeignored.However,theavailabilityforwatchOSand tvOScanbeimplicitlyinferredfromaniOSavailabilityattribute. Anyexplicitavailabilityattributesforthoseplatformsarestillpreferredover theimplicitlyinferredavailabilityattributes.Ifnoavailabilityattribute specifiesavailabilityforthecurrenttargetplatform,theavailability attributesareignored.Supportedplatformsare: ios Apple’siOSoperatingsystem.Theminimumdeploymenttargetisspecifiedby the-mios-version-min=*version*or-miphoneos-version-min=*version* command-linearguments. macos Apple’smacOSoperatingsystem.Theminimumdeploymenttargetis specifiedbythe-mmacosx-version-min=*version*command-lineargument. macosxissupportedforbackward-compatibilityreasons,butitis deprecated. tvos Apple’stvOSoperatingsystem.Theminimumdeploymenttargetisspecifiedby the-mtvos-version-min=*version*command-lineargument. watchos Apple’swatchOSoperatingsystem.Theminimumdeploymenttargetisspecifiedby the-mwatchos-version-min=*version*command-lineargument. driverkit Apple’sDriverKituserspacekernelextensions.Theminimumdeploymenttarget isspecifiedaspartofthetriple. Adeclarationcantypicallybeusedevenwhendeployingbacktoaplatform versionpriortowhenthedeclarationwasintroduced.Whenthishappens,the declarationisweaklylinked, asiftheweak_importattributewereaddedtothedeclaration.A weakly-linkeddeclarationmayormaynotbepresentarun-time,andaprogram candeterminewhetherthedeclarationispresentbycheckingwhetherthe addressofthatdeclarationisnon-NULL. TheflagstrictdisallowsusingAPIwhendeployingbacktoa platformversionpriortowhenthedeclarationwasintroduced.An attempttousesuchAPIbeforeitsintroductioncausesaharderror. Weakly-linkingisalmostalwaysabetterAPIchoice,sinceitallows userstoqueryavailabilityatruntime. Iftherearemultipledeclarationsofthesameentity,theavailability attributesmusteithermatchonaper-platformbasisorlater declarationsmustnothaveavailabilityattributesforthat platform.Forexample: voidg(void)__attribute__((availability(macos,introduced=10.4))); voidg(void)__attribute__((availability(macos,introduced=10.4)));//okay,matches voidg(void)__attribute__((availability(ios,introduced=4.0)));//okay,addsanewplatform voidg(void);//okay,inheritsbothmacosandiosavailabilityfromabove. voidg(void)__attribute__((availability(macos,introduced=10.5)));//error:mismatch Whenonemethodoverridesanother,theoverridingmethodcanbemorewidelyavailablethantheoverriddenmethod,e.g.,: @interfaceA -(id)method__attribute__((availability(macos,introduced=10.4))); -(id)method2__attribute__((availability(macos,introduced=10.4))); @end @interfaceB:A -(id)method__attribute__((availability(macos,introduced=10.3)));//okay:methodmovedintobaseclasslater -(id)method__attribute__((availability(macos,introduced=10.5)));//error:thismethodwasavailableviathebaseclassin10.4 @end StartingwiththemacOS10.12SDK,theAPI_AVAILABLEmacrofrom cansimplifythespelling: @interfaceA -(id)methodAPI_AVAILABLE(macos(10.11))); -(id)otherMethodAPI_AVAILABLE(macos(10.11),ios(11.0)); @end Availabilityattributescanalsobeappliedusinga#pragmaclangattribute. Anyexplicitavailabilityattributewhoseplatformcorrespondstothetarget platformisappliedtoadeclarationregardlessoftheavailabilityattributes specifiedinthepragma.Forexample,inthecodebelow, hasExplicitAvailabilityAttributewillusethemacOSavailability attributethatisspecifiedwiththedeclaration,whereas getsThePragmaAvailabilityAttributewillusethemacOSavailability attributethatisappliedbythepragma. #pragmaclangattributepush(__attribute__((availability(macOS,introduced=10.12))),apply_to=function) voidgetsThePragmaAvailabilityAttribute(void); voidhasExplicitAvailabilityAttribute(void)__attribute__((availability(macos,introduced=10.4))); #pragmaclangattributepop ForplatformslikewatchOSandtvOS,whoseavailabilityattributescan beimplicitlyinferredfromaniOSavailabilityattribute,thelogicis slightlymorecomplex.Theexplicitandthepragma-appliedavailability attributeswhoseplatformcorrespondstothetargetplatformareappliedas describedinthepreviousparagraph.However,theimplicitlyinferredattributes areappliedtoadeclarationonlywhenthereisnoexplicitorpragma-applied availabilityattributewhoseplatformcorrespondstothetargetplatform.For example,thefunctionbelowwillreceivethetvOSavailabilityfromthe pragmaratherthanusingtheinferrediOSavailabilityfromthedeclaration: #pragmaclangattributepush(__attribute__((availability(tvOS,introduced=12.0))),apply_to=function) voidgetsThePragmaTVOSAvailabilityAttribute(void)__attribute__((availability(iOS,introduced=11.0))); #pragmaclangattributepop Thecompilerisalsoabletoapplyimplicitlyinferredattributesfromapragma aswell.Forexample,whentargetingtvOS,thefunctionbelowwillreceive atvOSavailabilityattributethatisimplicitlyinferredfromtheiOS availabilityattributeappliedbythepragma: #pragmaclangattributepush(__attribute__((availability(iOS,introduced=12.0))),apply_to=function) voidinfersTVOSAvailabilityFromPragma(void); #pragmaclangattributepop Theimplicitattributesthatareinferredfromexplicitlyspecifiedattributes whoseplatformcorrespondstothetargetplatformareappliedtothedeclaration evenifthereisanavailabilityattributethatcanbeinferredfromapragma. Forexample,thefunctionbelowwillreceivethetvOS,introduced=11.0 availabilitythatisinferredfromtheattributeonthedeclarationratherthan inferringavailabilityfromthepragma: #pragmaclangattributepush(__attribute__((availability(iOS,unavailable))),apply_to=function) voidinfersTVOSAvailabilityFromAttributeNextToDeclaration(void) __attribute__((availability(iOS,introduced=11.0))); #pragmaclangattributepop Alsoseethedocumentationfor@available btf_decl_tag¶ SupportedSyntaxes¶ btf_decl_tag clang::btf_decl_tag clang::btf_decl_tag         Yes Clangsupportsthe__attribute__((btf_decl_tag("ARGUMENT")))attributefor alltargets.Thisattributemaybeattachedtoastruct/union,struct/union field,function,functionparameter,variableortypedefdeclaration.If-gis specified,theARGUMENTinfowillbepreservedinIRandbeemittedto dwarf.ForBPFtargets,theARGUMENTinfowillbeemittedto.BTFELF sectiontoo. callback¶ SupportedSyntaxes¶ callback clang::callback clang::callback         Yes Thecallbackattributespecifiesthattheannotatedfunctionmayinvokethe specifiedcallbackzeroormoretimes.Thecallback,aswellasthepassed arguments,areidentifiedbytheirparameternameorposition(startingwith 1!)intheannotatedfunction.Thefirstpositionintheattributeidentifies thecallbackcallee,thefollowingpositionsdeclaredescribeitsarguments. Thecallbackcalleeisrequiredtobecallablewiththenumber,andorder,of thespecifiedarguments.Theindex0,ortheidentifierthis,isusedto representanimplicit“this”pointerinclassmethods.Ifthereisnoimplicit “this”pointeritshallnotbereferenced.Theindex‘-1’,orthename“__”, representsanunknowncallbackcalleeargument.Thiscanbeavaluewhichis notpresentinthedeclaredparameterlist,oronethatis,butispotentially inspected,captured,ormodified.Parameternamesandindicescanbemixedin thecallbackattribute. Thecallbackattribute,whichisdirectlytranslatedtocallback metadata,makethe connectionbetweenthecalltotheannotatedfunctionandthecallbackcallee. Thiscanenableinterproceduraloptimizationswhichwereotherwiseimpossible. Ifafunctionparameterismentionedinthecallbackattribute,throughits position,itisundefinedifthatparameterisusedforanythingotherthanthe actualcallback.Inspected,captured,ormodifiedparametersshallnotbe listedinthecallbackmetadata. Exampleencodingsforthecallbackperformedbypthread_createareshown below.Theexplicitattributeannotationindicatesthatthethirdparameter (start_routine)iscalledzeroormoretimesbythepthread_createfunction, andthatthefourthparameter(arg)ispassedalong.Notethatthecallback behaviorofpthread_createisautomaticallyrecognizedbyClang.Inaddition, thedeclarationsof__kmpc_fork_teamsand__kmpc_fork_call,generatedfor #pragmaomptargetteamsand#pragmaompparallel,respectively,arealso automaticallyrecognizedasbrokerfunctions.Furtherfunctionsmightbeadded inthefuture. __attribute__((callback(start_routine,arg))) intpthread_create(pthread_t*thread,constpthread_attr_t*attr, void*(*start_routine)(void*),void*arg); __attribute__((callback(3,4))) intpthread_create(pthread_t*thread,constpthread_attr_t*attr, void*(*start_routine)(void*),void*arg); carries_dependency¶ SupportedSyntaxes¶ carries_dependency carries_dependency           Yes Thecarries_dependencyattributespecifiesdependencypropagationintoand outoffunctions. WhenspecifiedonafunctionorObjective-Cmethod,thecarries_dependency attributemeansthatthereturnvaluecarriesadependencyoutofthefunction, sothattheimplementationneednotconstrainorderinguponreturnfromthat function.Implementationsofthefunctionanditscallermaychoosetopreserve dependenciesinsteadofemittingmemoryorderinginstructionssuchasfences. Note,thisattributedoesnotchangethemeaningoftheprogram,butmayresult ingenerationofmoreefficientcode. cf_consumed¶ SupportedSyntaxes¶ cf_consumed clang::cf_consumed clang::cf_consumed         Yes ThebehaviorofafunctionwithrespecttoreferencecountingforFoundation (Objective-C),CoreFoundation(C)andOSObject(C++)isdeterminedbyanaming convention(e.g.functionsstartingwith“get”areassumedtoreturnat +0). Itcanbeoverriddenusingafamilyofthefollowingattributes.In Objective-C,theannotation__attribute__((ns_returns_retained))appliedto afunctioncommunicatesthattheobjectisreturnedat+1,andthecaller isresponsibleforfreeingit. Similarly,theannotation__attribute__((ns_returns_not_retained)) specifiesthattheobjectisreturnedat+0andtheownershipremainswith thecallee. Theannotation__attribute__((ns_consumes_self))specifiesthat theObjective-Cmethodcallconsumesthereferencetoself,e.g.by attachingittoasuppliedparameter. Additionally,parameterscanhaveanannotation __attribute__((ns_consumed)),whichspecifiesthatpassinganownedobject asthatparametereffectivelytransferstheownership,andthecallerisno longerresponsibleforit. TheseattributesaffectcodegenerationwheninteractingwithARCcode,and theyareusedbytheClangStaticAnalyzer. InCprogramsusingCoreFoundation,asimilarsetofattributes: __attribute__((cf_returns_not_retained)), __attribute__((cf_returns_retained))and__attribute__((cf_consumed)) havethesamerespectivesemanticswhenappliedtoCoreFoundationobjects. TheseattributesaffectcodegenerationwheninteractingwithARCcode,and theyareusedbytheClangStaticAnalyzer. Finally,inC++interactingwithXNUkernel(objectsinheritingfromOSObject), thesameattributefamilyispresent: __attribute__((os_returns_not_retained)), __attribute__((os_returns_retained))and__attribute__((os_consumed)), withthesamerespectivesemantics. Similarto__attribute__((ns_consumes_self)), __attribute__((os_consumes_this))specifiesthatthemethodcallconsumes thereferenceto“this”(e.g.,whenattachingittoadifferentobjectsupplied asaparameter). Outparameters(parametersthefunctionismeanttowriteinto, eitherviapointers-to-pointersorreferences-to-pointers) maybeannotatedwith__attribute__((os_returns_retained)) or__attribute__((os_returns_not_retained))whichspecifiesthattheobject writtenintotheoutparametershould(orrespectivelyshouldnot)bereleased afteruse. Sinceoftenoutparametersmayormaynotbewrittendependingontheexit codeofthefunction, annotations__attribute__((os_returns_retained_on_zero)) and__attribute__((os_returns_retained_on_non_zero))specifythat anoutparameterat+1iswrittenifandonlyifthefunctionreturnsazero (respectivelynon-zero)errorcode. Observethatreturn-code-dependentoutparameterannotationsareonly availableforretainedoutparameters,asnon-retainedobjectdonothavetobe releasedbythecallee. TheseattributesareonlyusedbytheClangStaticAnalyzer. ThefamilyofattributesX_returns_X_retainedcanbeaddedtofunctions, C++methods,andObjective-Cmethodsandproperties. AttributesX_consumedcanbeaddedtoparametersofmethods,functions, andObjective-Cmethods. cf_returns_not_retained¶ SupportedSyntaxes¶ cf_returns_not_retained clang::cf_returns_not_retained clang::cf_returns_not_retained           ThebehaviorofafunctionwithrespecttoreferencecountingforFoundation (Objective-C),CoreFoundation(C)andOSObject(C++)isdeterminedbyanaming convention(e.g.functionsstartingwith“get”areassumedtoreturnat +0). Itcanbeoverriddenusingafamilyofthefollowingattributes.In Objective-C,theannotation__attribute__((ns_returns_retained))appliedto afunctioncommunicatesthattheobjectisreturnedat+1,andthecaller isresponsibleforfreeingit. Similarly,theannotation__attribute__((ns_returns_not_retained)) specifiesthattheobjectisreturnedat+0andtheownershipremainswith thecallee. Theannotation__attribute__((ns_consumes_self))specifiesthat theObjective-Cmethodcallconsumesthereferencetoself,e.g.by attachingittoasuppliedparameter. Additionally,parameterscanhaveanannotation __attribute__((ns_consumed)),whichspecifiesthatpassinganownedobject asthatparametereffectivelytransferstheownership,andthecallerisno longerresponsibleforit. TheseattributesaffectcodegenerationwheninteractingwithARCcode,and theyareusedbytheClangStaticAnalyzer. InCprogramsusingCoreFoundation,asimilarsetofattributes: __attribute__((cf_returns_not_retained)), __attribute__((cf_returns_retained))and__attribute__((cf_consumed)) havethesamerespectivesemanticswhenappliedtoCoreFoundationobjects. TheseattributesaffectcodegenerationwheninteractingwithARCcode,and theyareusedbytheClangStaticAnalyzer. Finally,inC++interactingwithXNUkernel(objectsinheritingfromOSObject), thesameattributefamilyispresent: __attribute__((os_returns_not_retained)), __attribute__((os_returns_retained))and__attribute__((os_consumed)), withthesamerespectivesemantics. Similarto__attribute__((ns_consumes_self)), __attribute__((os_consumes_this))specifiesthatthemethodcallconsumes thereferenceto“this”(e.g.,whenattachingittoadifferentobjectsupplied asaparameter). Outparameters(parametersthefunctionismeanttowriteinto, eitherviapointers-to-pointersorreferences-to-pointers) maybeannotatedwith__attribute__((os_returns_retained)) or__attribute__((os_returns_not_retained))whichspecifiesthattheobject writtenintotheoutparametershould(orrespectivelyshouldnot)bereleased afteruse. Sinceoftenoutparametersmayormaynotbewrittendependingontheexit codeofthefunction, annotations__attribute__((os_returns_retained_on_zero)) and__attribute__((os_returns_retained_on_non_zero))specifythat anoutparameterat+1iswrittenifandonlyifthefunctionreturnsazero (respectivelynon-zero)errorcode. Observethatreturn-code-dependentoutparameterannotationsareonly availableforretainedoutparameters,asnon-retainedobjectdonothavetobe releasedbythecallee. TheseattributesareonlyusedbytheClangStaticAnalyzer. ThefamilyofattributesX_returns_X_retainedcanbeaddedtofunctions, C++methods,andObjective-Cmethodsandproperties. AttributesX_consumedcanbeaddedtoparametersofmethods,functions, andObjective-Cmethods. cf_returns_retained¶ SupportedSyntaxes¶ cf_returns_retained clang::cf_returns_retained clang::cf_returns_retained           ThebehaviorofafunctionwithrespecttoreferencecountingforFoundation (Objective-C),CoreFoundation(C)andOSObject(C++)isdeterminedbyanaming convention(e.g.functionsstartingwith“get”areassumedtoreturnat +0). Itcanbeoverriddenusingafamilyofthefollowingattributes.In Objective-C,theannotation__attribute__((ns_returns_retained))appliedto afunctioncommunicatesthattheobjectisreturnedat+1,andthecaller isresponsibleforfreeingit. Similarly,theannotation__attribute__((ns_returns_not_retained)) specifiesthattheobjectisreturnedat+0andtheownershipremainswith thecallee. Theannotation__attribute__((ns_consumes_self))specifiesthat theObjective-Cmethodcallconsumesthereferencetoself,e.g.by attachingittoasuppliedparameter. Additionally,parameterscanhaveanannotation __attribute__((ns_consumed)),whichspecifiesthatpassinganownedobject asthatparametereffectivelytransferstheownership,andthecallerisno longerresponsibleforit. TheseattributesaffectcodegenerationwheninteractingwithARCcode,and theyareusedbytheClangStaticAnalyzer. InCprogramsusingCoreFoundation,asimilarsetofattributes: __attribute__((cf_returns_not_retained)), __attribute__((cf_returns_retained))and__attribute__((cf_consumed)) havethesamerespectivesemanticswhenappliedtoCoreFoundationobjects. TheseattributesaffectcodegenerationwheninteractingwithARCcode,and theyareusedbytheClangStaticAnalyzer. Finally,inC++interactingwithXNUkernel(objectsinheritingfromOSObject), thesameattributefamilyispresent: __attribute__((os_returns_not_retained)), __attribute__((os_returns_retained))and__attribute__((os_consumed)), withthesamerespectivesemantics. Similarto__attribute__((ns_consumes_self)), __attribute__((os_consumes_this))specifiesthatthemethodcallconsumes thereferenceto“this”(e.g.,whenattachingittoadifferentobjectsupplied asaparameter). Outparameters(parametersthefunctionismeanttowriteinto, eitherviapointers-to-pointersorreferences-to-pointers) maybeannotatedwith__attribute__((os_returns_retained)) or__attribute__((os_returns_not_retained))whichspecifiesthattheobject writtenintotheoutparametershould(orrespectivelyshouldnot)bereleased afteruse. Sinceoftenoutparametersmayormaynotbewrittendependingontheexit codeofthefunction, annotations__attribute__((os_returns_retained_on_zero)) and__attribute__((os_returns_retained_on_non_zero))specifythat anoutparameterat+1iswrittenifandonlyifthefunctionreturnsazero (respectivelynon-zero)errorcode. Observethatreturn-code-dependentoutparameterannotationsareonly availableforretainedoutparameters,asnon-retainedobjectdonothavetobe releasedbythecallee. TheseattributesareonlyusedbytheClangStaticAnalyzer. ThefamilyofattributesX_returns_X_retainedcanbeaddedtofunctions, C++methods,andObjective-Cmethodsandproperties. AttributesX_consumedcanbeaddedtoparametersofmethods,functions, andObjective-Cmethods. cfi_canonical_jump_table¶ SupportedSyntaxes¶ cfi_canonical_jump_table clang::cfi_canonical_jump_table clang::cfi_canonical_jump_table         Yes Use__attribute__((cfi_canonical_jump_table))onafunctiondeclarationto makethefunction’sCFIjumptablecanonical.SeetheCFIdocumentationformoredetails. clang::builtin_alias,clang_builtin_alias¶ SupportedSyntaxes¶ clang_builtin_alias clang::builtin_alias clang::builtin_alias         Yes ThisattributeisusedintheimplementationoftheCintrinsics. ItallowstheCintrinsicfunctionstobedeclaredusingthenamesdefined intargetbuiltins,andstillberecognizedasclangbuiltinsequivalenttothe underlyingname.Forexample,riscv_vector.hdeclaresthefunctionvadd with__attribute__((clang_builtin_alias(__builtin_rvv_vadd_vv_i8m1))). Thisensuresthatbothfunctionsarerecognizedasthatclangbuiltin, andinthelattercase,thechoiceofwhichbuiltintoidentifythe functionascanbedeferreduntilafteroverloadresolution. ThisattributecanonlybeusedtosetupthealiasesforcertainARM/RISC-V Cintrinsicfunctions;itisintendedforuseonlyinsidearm_*.hand riscv_*.handisnotageneralmechanismfordeclaringarbitraryaliases forclangbuiltinfunctions. clang_arm_builtin_alias¶ SupportedSyntaxes¶ __clang_arm_builtin_alias clang::__clang_arm_builtin_alias clang::__clang_arm_builtin_alias         Yes ThisattributeisusedintheimplementationoftheACLEintrinsics. Itallowstheintrinsicfunctionsto bedeclaredusingthenamesdefinedinACLE,andstillberecognized asclangbuiltinsequivalenttotheunderlyingname.Forexample, arm_mve.hdeclaresthefunctionvaddq_u32with __attribute__((__clang_arm_mve_alias(__builtin_arm_mve_vaddq_u32))), andsimilarly,oneofthetype-overloadeddeclarationsofvaddq willhavethesameattribute.Thisensuresthatbothfunctionsare recognizedasthatclangbuiltin,andinthelattercase,thechoice ofwhichbuiltintoidentifythefunctionascanbedeferreduntil afteroverloadresolution. ThisattributecanonlybeusedtosetupthealiasesforcertainArm intrinsicfunctions;itisintendedforuseonlyinsidearm_*.h andisnotageneralmechanismfordeclaringarbitraryaliasesfor clangbuiltinfunctions. Inordertoavoidduplicatingtheattributedefinitionsforsimilar purposeforotherarchitecture,thereisageneralformforthe attributeclang_builtin_alias. cmse_nonsecure_entry¶ SupportedSyntaxes¶ cmse_nonsecure_entry             Yes Thisattributedeclaresafunctionthatcanbecalledfromnon-securestate,or fromsecurestate.Enteringfromandreturningtonon-securestatewouldswitch toandfromsecurestate,respectively,andpreventflowofinformation tonon-securestate,exceptviareturnvalues.SeeARMv8-MSecurityExtensions: RequirementsonDevelopmentTools-EngineeringSpecificationDocumentationformoreinformation. code_seg¶ SupportedSyntaxes¶       code_seg         The__declspec(code_seg)attributeenablestheplacementofcodeintoseparate namedsegmentsthatcanbepagedorlockedinmemoryindividually.Thisattribute isusedtocontroltheplacementofinstantiatedtemplatesandcompiler-generated code.Seethedocumentationfor__declspec(code_seg)onMSDN. convergent¶ SupportedSyntaxes¶ convergent clang::convergent clang::convergent         Yes Theconvergentattributecanbeplacedonafunctiondeclaration.Itis translatedintotheLLVMconvergentattribute,whichindicatesthatthecall instructionsofafunctionwiththisattributecannotbemadecontrol-dependent onanyadditionalvalues. InlanguagesdesignedforSPMD/SIMTprogrammingmodel,e.g.OpenCLorCUDA, thecallinstructionsofafunctionwiththisattributemustbeexecutedby allworkitemsorthreadsinaworkgrouporsubgroup. Thisattributeisdifferentfromnoduplicatebecauseitallowsduplicating functioncallsifitcanbeprovedthattheduplicatedfunctioncallsare notmadecontrol-dependentonanyadditionalvalues,e.g.,unrollingaloop executedbyallworkitems. Sampleusage: voidconvfunc(void)__attribute__((convergent)); //SettingitasaC++11attributeisalsovalidinaC++program. //voidconvfunc(void)[[clang::convergent]]; cpu_dispatch¶ SupportedSyntaxes¶ cpu_dispatch clang::cpu_dispatch clang::cpu_dispatch cpu_dispatch       Yes Thecpu_specificandcpu_dispatchattributesareusedtodefineand resolvemultiversionedfunctions.Thisformofmultiversioningprovidesa mechanismfordeclaringversionsacrosstranslationunitsandmanually specifyingtheresolvedfunctionlist.AspecifiedCPUdefinesasetofminimum featuresthatarerequiredforthefunctiontobecalled.Theresultofthisis thatfutureprocessorsexecutethemostrestrictiveversionofthefunctionthe newprocessorcanexecute. Inaddition,unliketheICCimplementationofthisfeature,theselectionofthe versiondoesnotconsiderthemanufacturerormicroarchitectureoftheprocessor. Ittestssolelythelistoffeaturesthatarebothsupportedbythespecified processorandpresentinthecompiler-rtlibrary.Thiscanbesurprisingattimes, astheruntimeprocessormaybefromacompletelydifferentmanufacturer,aslong asitsupportsthesamefeatureset. Thiscanadditionallybesurprising,assomeprocessorsareindistringuishablefrom othersbasedonthelistoftestablefeatures.Whenthishappens,thevariant isselectedinanunspecifiedmanner. Functionversionsaredefinedwithcpu_specific,whichtakesoneormoreCPU namesasaparameter.Forexample: //Declaresanddefinestheivybridgeversionofsingle_cpu. __attribute__((cpu_specific(ivybridge))) voidsingle_cpu(void){} //Declaresanddefinestheatomversionofsingle_cpu. __attribute__((cpu_specific(atom))) voidsingle_cpu(void){} //Declaresanddefinesboththeivybridgeandatomversionofmulti_cpu. __attribute__((cpu_specific(ivybridge,atom))) voidmulti_cpu(void){} Adispatching(orresolving)functioncanbedeclaredanywhereinaproject’s sourcecodewithcpu_dispatch.ThisattributetakesoneormoreCPUnames asaparameter(likecpu_specific).Functionsmarkedwithcpu_dispatch arenotexpectedtobedefined,onlydeclared.Ifsuchamarkedfunctionhasa definition,anysideeffectsofthefunctionareignored;trivialfunction bodiesarepermissibleforICCcompatibility. //Createsaresolverforsingle_cpuabove. __attribute__((cpu_dispatch(ivybridge,atom))) voidsingle_cpu(void){} //Createsaresolverformulti_cpu,butaddsa3rdversiondefinedinanother //translationunit. __attribute__((cpu_dispatch(ivybridge,atom,sandybridge))) voidmulti_cpu(void){} Notethatitispossibletohavearesolvingfunctionthatdispatchesbasedon moreorfeweroptionsthanarepresentintheprogram.Specifyingfewerwill resultintheomittedoptionsnotbeingconsideredduringresolution.Specifying aversionforresolutionthatisn’tdefinedintheprogramwillresultina linkingfailure. ItisalsopossibletospecifyaCPUnameofgenericwhichwillberesolved iftheexecutingprocessordoesn’tsatisfythefeaturesrequiredintheCPU name.Thebehaviorofaprogramexecutingonaprocessorthatdoesn’tsatisfy anyoptionofamultiversionedfunctionisundefined. cpu_specific¶ SupportedSyntaxes¶ cpu_specific clang::cpu_specific clang::cpu_specific cpu_specific       Yes Thecpu_specificandcpu_dispatchattributesareusedtodefineand resolvemultiversionedfunctions.Thisformofmultiversioningprovidesa mechanismfordeclaringversionsacrosstranslationunitsandmanually specifyingtheresolvedfunctionlist.AspecifiedCPUdefinesasetofminimum featuresthatarerequiredforthefunctiontobecalled.Theresultofthisis thatfutureprocessorsexecutethemostrestrictiveversionofthefunctionthe newprocessorcanexecute. Inaddition,unliketheICCimplementationofthisfeature,theselectionofthe versiondoesnotconsiderthemanufacturerormicroarchitectureoftheprocessor. Ittestssolelythelistoffeaturesthatarebothsupportedbythespecified processorandpresentinthecompiler-rtlibrary.Thiscanbesurprisingattimes, astheruntimeprocessormaybefromacompletelydifferentmanufacturer,aslong asitsupportsthesamefeatureset. Thiscanadditionallybesurprising,assomeprocessorsareindistringuishablefrom othersbasedonthelistoftestablefeatures.Whenthishappens,thevariant isselectedinanunspecifiedmanner. Functionversionsaredefinedwithcpu_specific,whichtakesoneormoreCPU namesasaparameter.Forexample: //Declaresanddefinestheivybridgeversionofsingle_cpu. __attribute__((cpu_specific(ivybridge))) voidsingle_cpu(void){} //Declaresanddefinestheatomversionofsingle_cpu. __attribute__((cpu_specific(atom))) voidsingle_cpu(void){} //Declaresanddefinesboththeivybridgeandatomversionofmulti_cpu. __attribute__((cpu_specific(ivybridge,atom))) voidmulti_cpu(void){} Adispatching(orresolving)functioncanbedeclaredanywhereinaproject’s sourcecodewithcpu_dispatch.ThisattributetakesoneormoreCPUnames asaparameter(likecpu_specific).Functionsmarkedwithcpu_dispatch arenotexpectedtobedefined,onlydeclared.Ifsuchamarkedfunctionhasa definition,anysideeffectsofthefunctionareignored;trivialfunction bodiesarepermissibleforICCcompatibility. //Createsaresolverforsingle_cpuabove. __attribute__((cpu_dispatch(ivybridge,atom))) voidsingle_cpu(void){} //Createsaresolverformulti_cpu,butaddsa3rdversiondefinedinanother //translationunit. __attribute__((cpu_dispatch(ivybridge,atom,sandybridge))) voidmulti_cpu(void){} Notethatitispossibletohavearesolvingfunctionthatdispatchesbasedon moreorfeweroptionsthanarepresentintheprogram.Specifyingfewerwill resultintheomittedoptionsnotbeingconsideredduringresolution.Specifying aversionforresolutionthatisn’tdefinedintheprogramwillresultina linkingfailure. ItisalsopossibletospecifyaCPUnameofgenericwhichwillberesolved iftheexecutingprocessordoesn’tsatisfythefeaturesrequiredintheCPU name.Thebehaviorofaprogramexecutingonaprocessorthatdoesn’tsatisfy anyoptionofamultiversionedfunctionisundefined. diagnose_as_builtin¶ SupportedSyntaxes¶ diagnose_as_builtin clang::diagnose_as_builtin clang::diagnose_as_builtin         Yes Thediagnose_as_builtinattributeindicatesthatFortifydiagnosticsareto beappliedtothedeclaredfunctionasifitwerethefunctionspecifiedbythe attribute.Thebuiltinfunctionwhosediagnosticsaretobemimickedshouldbe given.Inaddition,theorderinwhichargumentsshouldbeappliedmustalso begiven. Forexample,theattributecanbeusedasfollows. __attribute__((diagnose_as_builtin(__builtin_memset,3,2,1))) void*mymemset(intn,intc,void*s){ //... } Thisindicatesthatcallstomymemsetshouldbediagnosedasiftheywere callsto__builtin_memset.Thearguments3,2,1indicatebyindexthe orderinwhichargumentsofmymemsetshouldbeappliedto __builtin_memset.Thethirdargumentshouldbeappliedfirst,thenthe second,andthenthefirst.Thus(whenFortifywarningsareenabled)thecall mymemset(n,c,s)willdiagnoseoverflowsasifitwerethecall __builtin_memset(s,c,n). Forvariadicfunctions,thevariadicargumentsmustcomeinthesameorderas theywouldtothebuiltinfunction,afterallnormalarguments.Forinstance, todiagnoseanewfunctionasifitweresscanf,wecanusetheattributeas follows. __attribute__((diagnose_as_builtin(sscanf,1,2))) intmysscanf(constchar*str,constchar*format,...){ //... } Thenthecallmysscanf(“abcdef”,“%4s%4s”,buf1,buf2)willbediagnosedas ifitwerethecallsscanf(“abcdef”,“%4s%4s”,buf1,buf2). Thisattributecannotbeappliedtonon-staticmemberfunctions. diagnose_if¶ SupportedSyntaxes¶ diagnose_if               Thediagnose_ifattributecanbeplacedonfunctiondeclarationstoemit warningsorerrorsatcompile-timeifcallstotheattributedfunctionmeet certainuser-definedcriteria.Forexample: intabs(inta) __attribute__((diagnose_if(a>=0,"Redundantabscall","warning"))); intmust_abs(inta) __attribute__((diagnose_if(a>=0,"Redundantabscall","error"))); intval=abs(1);//warning:Redundantabscall intval2=must_abs(1);//error:Redundantabscall intval3=abs(val); intval4=must_abs(val);//Becauserun-timechecksarenotemittedfor //diagnose_ifattributes,thisexecuteswithout //issue. diagnose_ifiscloselyrelatedtoenable_if,withafewkeydifferences: Overloadresolutionisnotawareofdiagnose_ifattributes:they’re consideredonlyafterweselectthebestcandidatefromagivencandidateset. Functiondeclarationsthatdifferonlyintheirdiagnose_ifattributesare consideredtoberedeclarationsofthesamefunction(notoverloads). Iftheconditionprovidedtodiagnose_ifcannotbeevaluated,no diagnosticwillbeemitted. Otherwise,diagnose_ifisessentiallythelogicalnegationofenable_if. Asaresultofbulletnumbertwo,diagnose_ifattributeswillstackonthe samefunction.Forexample: intfoo()__attribute__((diagnose_if(1,"diag1","warning"))); intfoo()__attribute__((diagnose_if(1,"diag2","warning"))); intbar=foo();//warning:diag1 //warning:diag2 int(*fooptr)(void)=foo;//warning:diag1 //warning:diag2 constexprintsupportsAPILevel(intN){returnN<5;} intbaz(inta) __attribute__((diagnose_if(!supportsAPILevel(10), "UpgradetoAPIlevel10tousebaz","error"))); intbaz(inta) __attribute__((diagnose_if(!a,"0isnotrecommended.","warning"))); int(*bazptr)(int)=baz;//error:UpgradetoAPIlevel10tousebaz intv=baz(0);//error:UpgradetoAPIlevel10tousebaz Queryforthisfeaturewith__has_attribute(diagnose_if). disable_sanitizer_instrumentation¶ SupportedSyntaxes¶ disable_sanitizer_instrumentation clang::disable_sanitizer_instrumentation clang::disable_sanitizer_instrumentation         Yes Usethedisable_sanitizer_instrumentationattributeonafunction, Objective-Cmethod,orglobalvariable,tospecifythatnosanitizer instrumentationshouldbeapplied. Thisisnotthesameas__attribute__((no_sanitize(...))),whichdepending onthetoolmaystillinsertinstrumentationtopreventfalsepositivereports. disable_tail_calls¶ SupportedSyntaxes¶ disable_tail_calls clang::disable_tail_calls clang::disable_tail_calls         Yes Thedisable_tail_callsattributeinstructsthebackendtonotperformtail calloptimizationinsidethemarkedfunction. Forexample: intcallee(int); intfoo(inta)__attribute__((disable_tail_calls)){ returncallee(a);//Thiscallisnottail-calloptimized. } Markingvirtualfunctionsasdisable_tail_callsislegal. intcallee(int); classBase{ public: [[clang::disable_tail_calls]]virtualintfoo1(){ returncallee();//Thiscallisnottail-calloptimized. } }; classDerived1:publicBase{ public: intfoo1()override{ returncallee();//Thiscallistail-calloptimized. } }; enable_if¶ SupportedSyntaxes¶ enable_if             Yes Note Somefeaturesofthisattributeareexperimental.Themeaningof multipleenable_ifattributesonasingledeclarationissubjecttochangein afutureversionofclang.Also,theABIisnotstandardizedandthename manglingmaychangeinfutureversions.Toavoidthat,useasmlabels. Theenable_ifattributecanbeplacedonfunctiondeclarationstocontrol whichoverloadisselectedbasedonthevaluesofthefunction’sarguments. Whencombinedwiththeoverloadableattribute,thisfeatureisalso availableinC. intisdigit(intc); intisdigit(intc)__attribute__((enable_if(c<=-1||c>255,"chosenwhen'c'isoutofrange")))__attribute__((unavailable("'c'musthavethevalueofanunsignedcharorEOF"))); voidfoo(charc){ isdigit(c); isdigit(10); isdigit(-10);//resultsinacompile-timeerror. } Theenable_ifattributetakestwoarguments,thefirstisanexpressionwritten intermsofthefunctionparameters,thesecondisastringexplainingwhythis overloadcandidatecouldnotbeselectedtobedisplayedindiagnostics.The expressionispartofthefunctionsignatureforthepurposesofdetermining whetheritisaredeclaration(followingtherulesusedwhendetermining whetheraC++templatespecializationisODR-equivalent),butisnotpartof thetype. Theenable_ifexpressionisevaluatedasifitwerethebodyofa bool-returningconstexprfunctiondeclaredwiththeargumentsofthefunction itisbeingappliedto,thencalledwiththeparametersatthecallsite.Ifthe resultisfalseorcouldnotbedeterminedthroughconstantexpression evaluation,thenthisoverloadwillnotbechosenandtheprovidedstringmay beusedinadiagnosticifthecompilefailsasaresult. Becausetheenable_ifexpressionisanunevaluatedcontext,therearenoglobal statechanges,northeabilitytopassinformationfromtheenable_if expressiontothefunctionbody.Forexample,supposewewantcallsto strnlen(strbuf,maxlen)toresolvetostrnlen_chk(strbuf,maxlen,sizeof strbuf)onlyifthesizeofstrbufcanbedetermined: __attribute__((always_inline)) staticinlinesize_tstrnlen(constchar*s,size_tmaxlen) __attribute__((overloadable)) __attribute__((enable_if(__builtin_object_size(s,0)!=-1))), "chosenwhenthebuffersizeisknownbut'maxlen'isnot"))) { returnstrnlen_chk(s,maxlen,__builtin_object_size(s,0)); } Multipleenable_ifattributesmaybeappliedtoasingledeclaration.Inthis case,theenable_ifexpressionsareevaluatedfromlefttorightinthe followingmanner.First,thecandidateswhoseenable_ifexpressionsevaluateto falseorcannotbeevaluatedarediscarded.Iftheremainingcandidatesdonot shareODR-equivalentenable_ifexpressions,theoverloadresolutionis ambiguous.Otherwise,enable_ifoverloadresolutioncontinueswiththenext enable_ifattributeonthecandidatesthathavenotbeendiscardedandhave remainingenable_ifattributes.Inthisway,wepickthemostspecific overloadoutofanumberofviableoverloadsusingenable_if. voidf()__attribute__((enable_if(true,"")));//#1 voidf()__attribute__((enable_if(true,"")))__attribute__((enable_if(true,"")));//#2 voidg(inti,intj)__attribute__((enable_if(i,"")));//#1 voidg(inti,intj)__attribute__((enable_if(j,"")))__attribute__((enable_if(true)));//#2 Inthisexample,acalltof()isalwaysresolvedto#2,asthefirstenable_if expressionisODR-equivalentforbothdeclarations,but#1doesnothaveanother enable_ifexpressiontocontinueevaluating,sothenextroundofevaluationhas onlyasinglecandidate.Inacalltog(1,1),thecallisambiguouseventhough #2hasmoreenable_ifattributes,becausethefirstenable_ifexpressionsare notODR-equivalent. Queryforthisfeaturewith__has_attribute(enable_if). Notethatfunctionswithoneormoreenable_ifattributesmaynothave theiraddresstaken,unlessalloftheconditionsspecifiedbysaid enable_ifareconstantsthatevaluatetotrue.Forexample: constintTrueConstant=1; constintFalseConstant=0; intf(inta)__attribute__((enable_if(a>0,""))); intg(inta)__attribute__((enable_if(a==0||a!=0,""))); inth(inta)__attribute__((enable_if(1,""))); inti(inta)__attribute__((enable_if(TrueConstant,""))); intj(inta)__attribute__((enable_if(FalseConstant,""))); voidfn(){ int(*ptr)(int); ptr=&f;//error:'a>0'isnotalwaystrue ptr=&g;//error:'a==0||a!=0'isnotatruthyconstant ptr=&h;//OK:1isatruthyconstant ptr=&i;//OK:'TrueConstant'isatruthyconstant ptr=&j;//error:'FalseConstant'isaconstant,butnottruthy } Becauseenable_ifevaluationhappensduringoverloadresolution, enable_ifmaygiveunintuitiveresultswhenusedwithtemplates,depending onwhenoverloadsareresolved.Intheexamplebelow,clangwillemita diagnosticaboutnoviableoverloadsforfooinbar,butnotinbaz: doublefoo(inti)__attribute__((enable_if(i>0,""))); void*foo(inti)__attribute__((enable_if(i<=0,""))); template autobar(){returnfoo(I);} template autobaz(){returnfoo(T::number);} structWithNumber{constexprstaticintnumber=1;}; voidcallThem(){ bar(); baz(); } Thisisbecause,inbar,fooisresolvedpriortotemplate instantiation,sothevalueforIisn’tknown(thus,bothenable_if conditionsforfoofail).However,inbaz,fooisresolvedduring templateinstantiation,sothevalueforT::numberisknown. enforce_tcb¶ SupportedSyntaxes¶ enforce_tcb clang::enforce_tcb clang::enforce_tcb         Yes Theenforce_tcbattributecanbeplacedonfunctionstoenforcethata trustedcomputebase(TCB)doesnotcalloutoftheTCB.Thisgeneratesa warningeverytimeafunctionnotmarkedwithanenforce_tcbattributeis calledfromafunctionwiththeenforce_tcbattribute.Afunctionmaybea partofmultipleTCBs.Invocationsthroughfunctionpointersarecurrently notchecked.BuiltinsareconsideredtoapartofeveryTCB. enforce_tcb(Name)indicatesthatthisfunctionisapartoftheTCBnamedName enforce_tcb_leaf¶ SupportedSyntaxes¶ enforce_tcb_leaf clang::enforce_tcb_leaf clang::enforce_tcb_leaf         Yes Theenforce_tcb_leafattributesatisfiestherequirementenforcedby enforce_tcbforthemarkedfunctiontobeinthenamedTCBbutdoesnot continuetocheckthefunctionscalledfromwithintheleaffunction. enforce_tcb_leaf(Name)indicatesthatthisfunctionisapartoftheTCBnamedName error,warning¶ SupportedSyntaxes¶ errorwarning gnu::errorgnu::warning gnu::errorgnu::warning         Yes Theerrorandwarningfunctionattributescanbeusedtospecifya customdiagnostictobeemittedwhenacalltosuchafunctionisnot eliminatedviaoptimizations.Thiscanbeusedtocreatecompiletime assertionsthatdependonoptimizations,whileprovidingdiagnostics pointingtopreciselocationsofthecallsiteinthesource. __attribute__((warning("ohno")))voiddontcall(); voidfoo(){ if(someCompileTimeAssertionThatsTrue) dontcall();//Warning dontcall();//Warning if(someCompileTimeAssertionThatsFalse) dontcall();//NoWarning sizeof(dontcall());//NoWarning } exclude_from_explicit_instantiation¶ SupportedSyntaxes¶ exclude_from_explicit_instantiation clang::exclude_from_explicit_instantiation clang::exclude_from_explicit_instantiation         Yes Theexclude_from_explicit_instantiationattributeopts-outamemberofa classtemplatefrombeingpartofexplicittemplateinstantiationsofthat classtemplate.Thismeansthatanexplicitinstantiationwillnotinstantiate membersoftheclasstemplatemarkedwiththeattribute,butalsothatcode whereanexterntemplatedeclarationoftheenclosingclasstemplateisvisible willnottakeforgrantedthatanexternalinstantiationoftheclasstemplate wouldprovidethosemembers(whichwouldotherwisebealinkerror,sincethe explicitinstantiationwon’tprovidethosemembers).Forexample,let’ssaywe don’twantthedata()methodtobepartoflibc++’sABI.Tomakesureit isnotexportedfromthedylib,wegiveithiddenvisibility: //in template classbasic_string{ public: __attribute__((__visibility__("hidden"))) constvalue_type*data()constnoexcept{...} }; templateclassbasic_string; Sinceanexplicittemplateinstantiationdeclarationforbasic_string isprovided,thecompilerisfreetoassumethatbasic_string::data() willbeprovidedbyanothertranslationunit,anditisfreetoproducean externalcalltothisfunction.However,sincedata()hashiddenvisibility andtheexplicittemplateinstantiationisprovidedinasharedlibrary(as opposedtosimplyanothertranslationunit),basic_string::data() won’tbefoundandalinkerrorwillensue.Thishappensbecausethecompiler assumesthatbasic_string::data()ispartoftheexplicittemplate instantiationdeclaration,whenitreallyisn’t.Totellthecompilerthat data()isnotpartoftheexplicittemplateinstantiationdeclaration,the exclude_from_explicit_instantiationattributecanbeused: //in template classbasic_string{ public: __attribute__((__visibility__("hidden"))) __attribute__((exclude_from_explicit_instantiation)) constvalue_type*data()constnoexcept{...} }; templateclassbasic_string; Now,thecompilerwon’tassumethatbasic_string::data()isprovided externallydespitetherebeinganexplicittemplateinstantiationdeclaration: thecompilerwillimplicitlyinstantiatebasic_string::data()inthe TUswhereitisused. Thisattributecanbeusedonstaticandnon-staticmemberfunctionsofclass templates,staticdatamembersofclasstemplatesandmemberclassesofclass templates. export_name¶ SupportedSyntaxes¶ export_name clang::export_name clang::export_name         Yes Clangsupportsthe__attribute__((export_name())) attributefortheWebAssemblytarget.Thisattributemaybeattachedtoa functiondeclaration,whereitmodifieshowthesymbolistobeexported fromthelinkedWebAssembly. WebAssemblyfunctionsareexportedviastringname.Bydefaultwhenasymbol isexported,theexportnameforC/C++symbolsarethesameastheirC/C++ symbolnames.Thisattributecanbeusedtooverridethedefaultbehavior,and requestaspecificstringnamebeusedinstead. flatten¶ SupportedSyntaxes¶ flatten gnu::flatten gnu::flatten         Yes Theflattenattributecausescallswithintheattributedfunctionto beinlinedunlessitisimpossibletodoso,forexampleifthebodyofthe calleeisunavailableorifthecalleehasthenoinlineattribute. force_align_arg_pointer¶ SupportedSyntaxes¶ force_align_arg_pointer gnu::force_align_arg_pointer gnu::force_align_arg_pointer           Usethisattributetoforcestackalignment. Legacyx86codeuses4-bytestackalignment.NeweralignedSSEinstructions (like‘movaps’)thatworkwiththestackrequireoperandstobe16-bytealigned. Thisattributerealignsthestackinthefunctionprologuetomakesurethe stackcanbeusedwithSSEinstructions. Notethatthex86_64ABIforces16-bytestackalignmentatthecallsite. Becauseofthis,‘force_align_arg_pointer’isnotneededonx86_64,exceptin rarecaseswherethecallerdoesnotalignthestackproperly(e.g.flow jumpsfromi386archcode). __attribute__((force_align_arg_pointer)) voidf(){ ... } format¶ SupportedSyntaxes¶ format gnu::format gnu::format           Clangsupportstheformatattribute,whichindicatesthatthefunction acceptsaprintforscanf-likeformatstringandcorresponding argumentsorava_listthatcontainsthesearguments. PleaseseeGCCdocumentationaboutformatattributetofinddetails aboutattributesyntax. Clangimplementstwokindsofcheckswiththisattribute. Clangchecksthatthefunctionwiththeformatattributeiscalledwith aformatstringthatusesformatspecifiersthatareallowed,andthat argumentsmatchtheformatstring.Thisisthe-Wformatwarning,itis onbydefault. Clangchecksthattheformatstringargumentisaliteralstring.Thisis the-Wformat-nonliteralwarning,itisoffbydefault. ClangimplementsthismostlythesamewayasGCC,butthereisadifference forfunctionsthatacceptava_listargument(forexample,vprintf). GCCdoesnotemit-Wformat-nonliteralwarningforcallstosuch functions.Clangdoesnotwarniftheformatstringcomesfromafunction parameter,wherethefunctionisannotatedwithacompatibleattribute, otherwiseitwarns.Forexample: __attribute__((__format__(__scanf__,1,3))) voidfoo(constchar*s,char*buf,...){ va_listap; va_start(ap,buf); vprintf(s,ap);//warning:formatstringisnotastringliteral } Inthiscasewewarnbecausescontainsaformatstringfora scanf-likefunction,butitispassedtoaprintf-likefunction. Iftheattributeisremoved,clangstillwarns,becausetheformatstringis notastringliteral. Anotherexample: __attribute__((__format__(__printf__,1,3))) voidfoo(constchar*s,char*buf,...){ va_listap; va_start(ap,buf); vprintf(s,ap);//warning } InthiscaseClangdoesnotwarnbecausetheformatstringsand thecorrespondingargumentsareannotated.Iftheargumentsare incorrect,thecalleroffoowillreceiveawarning. gnu_inline¶ SupportedSyntaxes¶ gnu_inline gnu::gnu_inline gnu::gnu_inline         Yes Thegnu_inlinechangesthemeaningofexterninlinetouseGNUinline semantics,meaning: Ifanydeclarationthatisdeclaredinlineisnotdeclaredextern, thentheinlinekeywordisjustahint.Inparticular,anout-of-line definitionisstillemittedforafunctionwithexternallinkage,evenifall callsitesareinlined,unlikeinC99andC++inlinesemantics. Ifalldeclarationsthataredeclaredinlinearealsodeclared extern,thenthefunctionbodyispresentonlyforinliningandno out-of-lineversionisemitted. Someimportantconsequences:staticinlineemitsanout-of-line versionifneeded,aplaininlinedefinitionemitsanout-of-lineversion always,andanexterninlinedefinition(inaheader)followedbya (non-extern)inlinedeclarationinasourcefileemitsanout-of-line versionofthefunctioninthatsourcefilebutprovidesthefunctionbodyfor inliningtoallincludersoftheheader. Either__GNUC_GNU_INLINE__(GNUinlinesemantics)or __GNUC_STDC_INLINE__(C99semantics)willbedefined(theyaremutually exclusive).If__GNUC_STDC_INLINE__isdefined,thenthegnu_inline functionattributecanbeusedtogetGNUinlinesemanticsonaperfunction basis.If__GNUC_GNU_INLINE__isdefined,thenthetranslationunitis alreadybeingcompiledwithGNUinlinesemanticsastheimplieddefault.Itis unspecifiedwhichmacroisdefinedinaC++compilation. GNUinlinesemanticsarethedefaultbehaviorwith-std=gnu89, -std=c89,-std=c94,or-fgnu89-inline. guard¶ SupportedSyntaxes¶       guard         CodecanindicateCFGchecksarenotwantedwiththe__declspec(guard(nocf)) attribute.ThisdirectsthecompilertonotinsertanyCFGchecksfortheentire function.Thisapproachistypicallyusedonlysparinglyinspecificsituations wheretheprogrammerhasmanuallyinserted“CFG-equivalent”protection.The programmerknowsthattheyarecallingthroughsomeread-onlyfunctiontable whoseaddressisobtainedthroughread-onlymemoryreferencesandforwhichthe indexismaskedtothefunctiontablelimit.Thisapproachmayalsobeapplied tosmallwrapperfunctionsthatarenotinlinedandthatdonothingmorethan makeacallthroughafunctionpointer.Sinceincorrectusageofthisdirective cancompromisethesecurityofCFG,theprogrammermustbeverycarefulusing thedirective.Typically,thisusageislimitedtoverysmallfunctionsthat onlycallonefunction. ControlFlowGuarddocumentation ifunc¶ SupportedSyntaxes¶ ifunc gnu::ifunc gnu::ifunc         Yes __attribute__((ifunc("resolver")))isusedtomarkthattheaddressofa declarationshouldberesolvedatruntimebycallingaresolverfunction. Thesymbolnameoftheresolverfunctionisgiveninquotes.Afunctionwith thisname(aftermangling)mustbedefinedinthecurrenttranslationunit;it maybestatic.Theresolverfunctionshouldreturnapointer. Theifuncattributemayonlybeusedonafunctiondeclaration.Afunction declarationwithanifuncattributeisconsideredtobeadefinitionofthe declaredentity.Theentitymustnothaveweaklinkage;forexample,inC++, itcannotbeappliedtoadeclarationifadefinitionatthatlocationwouldbe consideredinline. Notalltargetssupportthisattribute.ELFtargetsupportdependsonboththe linkerandruntimelinker,andisavailableinatleastlld4.0andlater, binutils2.20.1andlater,glibcv2.11.1andlater,andFreeBSD9.1andlater. Non-ELFtargetscurrentlydonotsupportthisattribute. import_module¶ SupportedSyntaxes¶ import_module clang::import_module clang::import_module         Yes Clangsupportsthe__attribute__((import_module())) attributefortheWebAssemblytarget.Thisattributemaybeattachedtoa functiondeclaration,whereitmodifieshowthesymbolistobeimported withintheWebAssemblylinkingenvironment. WebAssemblyimportsuseatwo-levelnamespacescheme,consistingofamodule name,whichtypicallyidentifiesamodulefromwhichtoimport,andafield name,whichtypicallyidentifiesafieldfromthatmoduletoimport.By default,modulenamesforC/C++symbolsareassignedautomaticallybythe linker.Thisattributecanbeusedtooverridethedefaultbehavior,and requestaspecificmodulenamebeusedinstead. import_name¶ SupportedSyntaxes¶ import_name clang::import_name clang::import_name         Yes Clangsupportsthe__attribute__((import_name())) attributefortheWebAssemblytarget.Thisattributemaybeattachedtoa functiondeclaration,whereitmodifieshowthesymbolistobeimported withintheWebAssemblylinkingenvironment. WebAssemblyimportsuseatwo-levelnamespacescheme,consistingofamodule name,whichtypicallyidentifiesamodulefromwhichtoimport,andafield name,whichtypicallyidentifiesafieldfromthatmoduletoimport.By default,fieldnamesforC/C++symbolsarethesameastheirC/C++symbol names.Thisattributecanbeusedtooverridethedefaultbehavior,and requestaspecificfieldnamebeusedinstead. internal_linkage¶ SupportedSyntaxes¶ internal_linkage clang::internal_linkage clang::internal_linkage         Yes Theinternal_linkageattributechangesthelinkagetypeofthedeclaration tointernal.ThisissimilartoC-stylestatic,butcanbeusedonclasses andclassmethods.Whenappliedtoaclassdefinition,thisattributeaffects allmethodsandstaticdatamembersofthatclass.Thiscanbeusedtocontain theABIofaC++librarybyexcludingunwantedclassmethodsfromtheexport tables. interrupt(ARM)¶ SupportedSyntaxes¶ interrupt gnu::interrupt gnu::interrupt           ClangsupportstheGNUstyle__attribute__((interrupt("TYPE")))attributeon ARMtargets.Thisattributemaybeattachedtoafunctiondefinitionand instructsthebackendtogenerateappropriatefunctionentry/exitcodesothat itcanbeuseddirectlyasaninterruptserviceroutine. Theparameterpassedtotheinterruptattributeisoptional,butif provideditmustbeastringliteralwithoneofthefollowingvalues:“IRQ”, “FIQ”,“SWI”,“ABORT”,“UNDEF”. Thesemanticsareasfollows: IfthefunctionisAAPCS,Clanginstructsthebackendtorealignthestackto 8bytesonentry.ThisisageneralrequirementoftheAAPCSatpublic interfaces,butmaynotholdwhenanexceptionistaken.Doingthisallows otherAAPCSfunctionstobecalled. IftheCPUisM-classthisisallthatneedstobedonesincethearchitecture itselfisdesignedinsuchawaythatfunctionsobeyingthenormalAAPCSABI constraintsarevalidexceptionhandlers. IftheCPUisnotM-class,theprologueandepiloguearemodifiedtosaveall non-bankedregistersthatareused,sothatuponreturntheuser-modestate willnotbecorrupted.Notethattoavoidunnecessaryoverhead,only general-purpose(integer)registersaresavedinthisway.IfVFPoperations areneeded,thatstatemustbesavedmanually. Specifically,interruptkindsotherthan“FIQ”willsaveallcoreregisters except“lr”and“sp”.“FIQ”interruptswillsaver0-r7. IftheCPUisnotM-class,thereturninstructionischangedtooneofthe canonicalsequencespermittedbythearchitectureforexceptionreturn.Where possiblethefunctionitselfwillmakethenecessary“lr”adjustmentssothat the“preferredreturnaddress”isselected. Unfortunatelythecompilerisunabletomakethisguaranteeforan“UNDEF” handler,wheretheoffsetfrom“lr”tothepreferredreturnaddressdependson theexecutionstateofthecodewhichgeneratedtheexception.Inthiscase asequenceequivalentto“movspc,lr”willbeused. interrupt(AVR)¶ SupportedSyntaxes¶ interrupt gnu::interrupt gnu::interrupt         Yes ClangsupportstheGNUstyle__attribute__((interrupt))attributeon AVRtargets.Thisattributemaybeattachedtoafunctiondefinitionandinstructs thebackendtogenerateappropriatefunctionentry/exitcodesothatitcanbeused directlyasaninterruptserviceroutine. OntheAVR,thehardwaregloballydisablesinterruptswhenaninterruptisexecuted. ThefirstinstructionofaninterrupthandlerdeclaredwiththisattributeisaSEI instructiontore-enableinterrupts.Seealsothesignalattributethat doesnotinsertaSEIinstruction. interrupt(MIPS)¶ SupportedSyntaxes¶ interrupt gnu::interrupt gnu::interrupt         Yes ClangsupportstheGNUstyle__attribute__((interrupt("ARGUMENT")))attributeon MIPStargets.Thisattributemaybeattachedtoafunctiondefinitionandinstructs thebackendtogenerateappropriatefunctionentry/exitcodesothatitcanbeused directlyasaninterruptserviceroutine. Bydefault,thecompilerwillproduceafunctionprologueandepiloguesuitablefor aninterruptserviceroutinethathandlesanExternalInterruptController(eic) generatedinterrupt.Thisbehaviorcanbeexplicitlyrequestedwiththe“eic” argument. Otherwise,forusewithvectoredinterruptmode,theargumentpassedshouldbe oftheform“vector=LEVEL”whereLEVELisoneofthefollowingvalues: “sw0”,“sw1”,“hw0”,“hw1”,“hw2”,“hw3”,“hw4”,“hw5”.Thecompilerwill thensettheinterruptmasktothecorrespondinglevelwhichwillmaskall interruptsuptoandincludingtheargument. Thesemanticsareasfollows: TheprologueismodifiedsothattheExceptionProgramCounter(EPC)and Statuscoprocessorregistersaresavedtothestack.Theinterruptmaskis setsothatthefunctioncanonlybeinterruptedbyahigherpriority interrupt.TheepiloguewillrestorethepreviousvaluesofEPCandStatus. Theprologueandepiloguearemodifiedtosaveandrestoreallnon-kernel registersasnecessary. TheFPUisdisabledintheprologue,asthefloatingpointerregistersarenot spilledtothestack. Thefunctionreturnsequenceischangedtouseanexceptionreturninstruction. Theparametersetstheinterruptmaskforthefunctioncorrespondingtothe interruptlevelspecified.Ifnomaskisspecifiedtheinterruptmask defaultsto“eic”. interrupt(RISCV)¶ SupportedSyntaxes¶ interrupt gnu::interrupt gnu::interrupt         Yes ClangsupportstheGNUstyle__attribute__((interrupt))attributeonRISCV targets.Thisattributemaybeattachedtoafunctiondefinitionandinstructs thebackendtogenerateappropriatefunctionentry/exitcodesothatitcanbe useddirectlyasaninterruptserviceroutine. Permissiblevaluesforthisparameterareuser,supervisor, andmachine.Ifthereisnoparameter,thenitdefaultstomachine. Repeatedinterruptattributeonthesamedeclarationwillcauseawarning tobeemitted.Incaseofrepeateddeclarations,thelastoneprevails. Referto: https://gcc.gnu.org/onlinedocs/gcc/RISC-V-Function-Attributes.html https://riscv.org/specifications/privileged-isa/ TheRISC-VInstructionSetManualVolumeII:PrivilegedArchitecture Version1.10. kernel¶ SupportedSyntaxes¶ kernel             Yes __attribute__((kernel))isusedtomarkakernelfunctionin RenderScript. InRenderScript,kernelfunctionsareusedtoexpressdata-parallel computations.TheRenderScriptruntimeefficientlyparallelizeskernel functionstorunoncomputationalresourcessuchasmulti-coreCPUsandGPUs. SeetheRenderScriptdocumentationformoreinformation. lifetimebound¶ SupportedSyntaxes¶ lifetimebound clang::lifetimebound             Thelifetimeboundattributeonafunctionparameterorimplicitobject parameterindicatesthatobjectsthatarereferredtobythatparametermay alsobereferredtobythereturnvalueoftheannotatedfunction(or,fora parameterofaconstructor,bythevalueoftheconstructedobject).Itisonly supportedinC++. Bydefault,areferenceisconsideredtorefertoitsreferencedobject,a pointerisconsideredtorefertoitspointee,astd::initializer_list isconsideredtorefertoitsunderlyingarray,andaggregates(arraysand simplestructs)areconsideredtorefertoallobjectsthattheir transitivesubobjectsreferto. Clangwarnsifitisabletodetectthatanobjectorreferencerefersto anotherobjectwithashorterlifetime.Forexample,Clangwillwarnifa functionreturnsareferencetoalocalvariable,orifareferenceisboundto atemporaryobjectwhoselifetimeisnotextended.Byusingthe lifetimeboundattribute,thisdeterminationcanbeextendedtolookthrough user-declaredfunctions.Forexample: //Returnsm[key]ifkeyispresent,ordefault_valueifnot. template constU&get_or_default(conststd::map&m[[clang::lifetimebound]], constT&key,/*note,notlifetimebound*/ constU&default_value[[clang::lifetimebound]]); std::map<:string>m; //warning:temporary"bar"sthatmightbeboundtolocalreference'val' //willbedestroyedattheendofthefull-expression conststd::string&val=get_or_default(m,"foo"s,"bar"s); //Nowarninginthiscase. std::stringdef_val="bar"s; conststd::string&val=get_or_default(m,"foo"s,def_val); Theattributecanbeappliedtotheimplicitthisparameterofamember functionbywritingtheattributeafterthefunctiontype: structstring{ //Thereturnedpointershouldnotoutlive``*this``. constchar*data()const[[clang::lifetimebound]]; }; ThisattributeisinspiredbytheC++committeepaperP0936R0,butdoesnotaffectwhethertemporaryobjects havetheirlifetimesextended. long_call,far¶ SupportedSyntaxes¶ long_callfar gnu::long_callgnu::far gnu::long_callgnu::far         Yes Clangsupportsthe__attribute__((long_call)),__attribute__((far)), and__attribute__((near))attributesonMIPStargets.Theseattributesmay onlybeaddedtofunctiondeclarationsandchangethecodegenerated bythecompilerwhendirectlycallingthefunction.Thenearattribute allowscallstothefunctiontobemadeusingthejalinstruction,which requiresthefunctiontobelocatedinthesamenaturallyaligned256MB segmentasthecaller.Thelong_callandfarattributesaresynonyms andrequiretheuseofadifferentcallsequencethatworksregardless ofthedistancebetweenthefunctions. Theseattributeshavenoeffectforposition-independentcode. Theseattributestakepriorityovercommandlineswitchessuch as-mlong-callsand-mno-long-calls. malloc¶ SupportedSyntaxes¶ malloc gnu::malloc gnu::malloc restrict       Yes Themallocattributeindicatesthatthefunctionactslikeasystemmemory allocationfunction,returningapointertoallocatedstoragedisjointfromthe storageforanyotherobjectaccessibletothecaller. micromips¶ SupportedSyntaxes¶ micromips gnu::micromips gnu::micromips         Yes ClangsupportstheGNUstyle__attribute__((micromips))and __attribute__((nomicromips))attributesonMIPStargets.Theseattributes maybeattachedtoafunctiondefinitionandinstructsthebackendtogenerate ornottogeneratemicroMIPScodeforthatfunction. Theseattributesoverridethe-mmicromipsand-mno-micromipsoptions onthecommandline. mig_server_routine¶ SupportedSyntaxes¶ mig_server_routine clang::mig_server_routine clang::mig_server_routine         Yes TheMachInterfaceGeneratorrelease-on-successconventiondictates functionsthatfollowittoonlyreleaseargumentspassedtothemwhenthey return“success”(akern_return_terrorcodethatindicatesthat noerrorshaveoccurred).OtherwisethereleaseisperformedbytheMIGclient thatcalledthefunction.Theannotation__attribute__((mig_server_routine)) isappliedinordertospecifywhichfunctionsareexpectedtofollowthe convention.ThisallowstheStaticAnalyzertofindbugscausedbyviolationsof thatconvention.Theattributewouldnormallyappearontheforwarddeclaration oftheactualserverroutineintheMIGserverheader,butitmayalsobe addedtoarbitraryfunctionsthatneedtofollowthesameconvention-for example,ausercanaddthemtoauxiliaryfunctionscalledbytheserverroutine thathavetheirreturnvalueoftypekern_return_tunconditionallyreturned fromtheroutine.TheattributecanbeappliedtoC++methods,andinthiscase itwillbeautomaticallyappliedtooverridesifthemethodisvirtual.The attributecanalsobewrittenusingC++11syntax:[[mig::server_routine]]. min_vector_width¶ SupportedSyntaxes¶ min_vector_width clang::min_vector_width clang::min_vector_width         Yes Clangsupportsthe__attribute__((min_vector_width(width)))attribute.This attributemaybeattachedtoafunctionandinformsthebackendthatthis functiondesiresvectorsofatleastthiswidthtobegenerated.Target-specific maximumvectorwidthsstillapply.Thismeansevenifyouaskforsomething largerthanthetargetsupports,youwillonlygetwhatthetargetsupports. Thisattributeismeanttobeahinttocontroltargetheuristicsthatmay generatenarrowervectorsthanwhatthetargethardwaresupports. ThisiscurrentlyusedbytheX86targettoallowsomeCPUsthatsupport512-bit vectorstobelimitedtousing256-bitvectorstoavoidfrequencypenalties. Thisiscurrentlyenabledwiththe-prefer-vector-width=256commandline option.Themin_vector_widthattributecanbeusedtopreventthebackend fromtryingtosplitvectoroperationstomatchtheprefer-vector-width.All X86vectorintrinsicsfromx86intrin.halreadysetthisattribute.Additionally, useofanyoftheX86-specificvectorbuiltinswillimplicitlysetthis attributeonthecallingfunction.Theintentisthatexplicitlywritingvector codeusingtheX86intrinsicswillpreventprefer-vector-widthfrom affectingthecode. minsize¶ SupportedSyntaxes¶ minsize clang::minsize clang::minsize         Yes Thisfunctionattributeindicatesthatoptimizationpassesandcodegeneratorpasses makechoicesthatkeepthefunctioncodesizeassmallaspossible.Optimizationsmay alsosacrificeruntimeperformanceinordertominimizethesizeofthegeneratedcode. no_builtin¶ SupportedSyntaxes¶ no_builtin clang::no_builtin clang::no_builtin         Yes The__attribute__((no_builtin))issimilartothe-fno-builtinflag exceptitisspecifictothebodyofafunction.Theattributemayalsobe appliedtoavirtualfunctionbuthasnoeffectonthebehaviorofoverriding functionsinaderivedclass. Itacceptsoneormorestringscorrespondingtothespecificnamesofthe builtinstodisable(e.g.“memcpy”,“memset”). Iftheattributeisusedwithoutparametersitwilldisableallbuitinsat once. //Thecompilerisnotallowedtoaddanybuiltintofoo'sbody. voidfoo(char*data,size_tcount)__attribute__((no_builtin)){ //Thecompilerisnotallowedtoconverttheloopinto //`__builtin_memset(data,0xFE,count);`. for(size_ti=0;in){ foo(); }else{ bar(); } getspossiblymodifiedbysomeoptimizationsintocodesimilartothis: if(a>n){ nodupfunc(); foo(); }else{ nodupfunc(); bar(); } wherethecallto“nodupfunc”isduplicatedandsunkintothetwobranches ofthecondition. noinline¶ SupportedSyntaxes¶ noinline gnu::noinlineclang::noinline gnu::noinlineclang::noinline noinline __noinline__     Yes Thisfunctionattributesuppressestheinliningofafunctionatthecallsites ofthefunction. [[clang::noinline]]spellingcanbeusedasastatementattribute;other spellingsoftheattributearenotsupportedonstatements.Ifastatementis marked[[clang::noinline]]andcontainscalls,thosecallsinsidethe statementwillnotbeinlinedbythecompiler. __noinline__canbeusedasakeywordinCUDA/HIPlanguages.Thisisto avoiddiagnosticsduetousageof__attribute__((__noinline__)) with__noinline__definedasamacroas__attribute__((noinline)). intexample(void){ intr; [[clang::noinline]]foo(); [[clang::noinline]]r=bar(); returnr; } nomicromips¶ SupportedSyntaxes¶ nomicromips gnu::nomicromips gnu::nomicromips         Yes ClangsupportstheGNUstyle__attribute__((micromips))and __attribute__((nomicromips))attributesonMIPStargets.Theseattributes maybeattachedtoafunctiondefinitionandinstructsthebackendtogenerate ornottogeneratemicroMIPScodeforthatfunction. Theseattributesoverridethe-mmicromipsand-mno-micromipsoptions onthecommandline. noreturn,_Noreturn¶ SupportedSyntaxes¶   noreturn noreturn_Noreturn         Yes Afunctiondeclaredas[[noreturn]]shallnotreturntoitscaller.The compilerwillgenerateadiagnosticforafunctiondeclaredas[[noreturn]] thatappearstobecapableofreturningtoitscaller. The[[_Noreturn]]spellingisdeprecatedandonlyexiststoeasecode migrationforcodeusing[[noreturn]]afterincluding. not_tail_called¶ SupportedSyntaxes¶ not_tail_called clang::not_tail_called clang::not_tail_called         Yes Thenot_tail_calledattributepreventstail-calloptimizationonstatically boundcalls.Objective-cmethods,andfunctionsmarkedasalways_inline cannotbemarkedasnot_tail_called. Forexample,itpreventstail-calloptimizationinthefollowingcase: int__attribute__((not_tail_called))foo1(int); intfoo2(inta){ returnfoo1(a);//Notail-calloptimizationondirectcalls. } However,itdoesn’tpreventtail-calloptimizationinthiscase: int__attribute__((not_tail_called))foo1(int); intfoo2(inta){ int(*fn)(int)=&foo1; //not_tail_calledhasnoeffectonanindirectcallevenifthecallcan //beresolvedatcompiletime. return(*fn)(a); } Generally,markinganoverridingvirtualfunctionasnot_tail_calledis notuseful,becausethisattributeisapropertyofthestatictype.Calls madethroughapointerorreferencetothebaseclasstypewillrespect thenot_tail_calledattributeofthebaseclass’smemberfunction, regardlessoftheruntimedestinationofthecall: structFoo{virtualvoidf();}; structBar:Foo{ [[clang::not_tail_called]]voidf()override; }; voidcallera(Bar&bar){ Foo&foo=bar; //not_tail_calledhasnoeffectonhere,eventhoughthe //underlyingmethodisffromBar. foo.f(); bar.f();//Notail-calloptimizationonhere. } nothrow¶ SupportedSyntaxes¶ nothrow gnu::nothrow gnu::nothrow nothrow       Yes ClangsupportstheGNUstyle__attribute__((nothrow))andMicrosoftstyle __declspec(nothrow)attributeasanequivalentofnoexceptonfunction declarations.Thisattributeinformsthecompilerthattheannotatedfunction doesnotthrowanexception.Thispreventsexception-unwinding.Thisattribute isparticularlyusefulonfunctionsintheCStandardLibrarythatare guaranteedtonotthrowanexception. ns_consumed¶ SupportedSyntaxes¶ ns_consumed clang::ns_consumed clang::ns_consumed         Yes ThebehaviorofafunctionwithrespecttoreferencecountingforFoundation (Objective-C),CoreFoundation(C)andOSObject(C++)isdeterminedbyanaming convention(e.g.functionsstartingwith“get”areassumedtoreturnat +0). Itcanbeoverriddenusingafamilyofthefollowingattributes.In Objective-C,theannotation__attribute__((ns_returns_retained))appliedto afunctioncommunicatesthattheobjectisreturnedat+1,andthecaller isresponsibleforfreeingit. Similarly,theannotation__attribute__((ns_returns_not_retained)) specifiesthattheobjectisreturnedat+0andtheownershipremainswith thecallee. Theannotation__attribute__((ns_consumes_self))specifiesthat theObjective-Cmethodcallconsumesthereferencetoself,e.g.by attachingittoasuppliedparameter. Additionally,parameterscanhaveanannotation __attribute__((ns_consumed)),whichspecifiesthatpassinganownedobject asthatparametereffectivelytransferstheownership,andthecallerisno longerresponsibleforit. TheseattributesaffectcodegenerationwheninteractingwithARCcode,and theyareusedbytheClangStaticAnalyzer. InCprogramsusingCoreFoundation,asimilarsetofattributes: __attribute__((cf_returns_not_retained)), __attribute__((cf_returns_retained))and__attribute__((cf_consumed)) havethesamerespectivesemanticswhenappliedtoCoreFoundationobjects. TheseattributesaffectcodegenerationwheninteractingwithARCcode,and theyareusedbytheClangStaticAnalyzer. Finally,inC++interactingwithXNUkernel(objectsinheritingfromOSObject), thesameattributefamilyispresent: __attribute__((os_returns_not_retained)), __attribute__((os_returns_retained))and__attribute__((os_consumed)), withthesamerespectivesemantics. Similarto__attribute__((ns_consumes_self)), __attribute__((os_consumes_this))specifiesthatthemethodcallconsumes thereferenceto“this”(e.g.,whenattachingittoadifferentobjectsupplied asaparameter). Outparameters(parametersthefunctionismeanttowriteinto, eitherviapointers-to-pointersorreferences-to-pointers) maybeannotatedwith__attribute__((os_returns_retained)) or__attribute__((os_returns_not_retained))whichspecifiesthattheobject writtenintotheoutparametershould(orrespectivelyshouldnot)bereleased afteruse. Sinceoftenoutparametersmayormaynotbewrittendependingontheexit codeofthefunction, annotations__attribute__((os_returns_retained_on_zero)) and__attribute__((os_returns_retained_on_non_zero))specifythat anoutparameterat+1iswrittenifandonlyifthefunctionreturnsazero (respectivelynon-zero)errorcode. Observethatreturn-code-dependentoutparameterannotationsareonly availableforretainedoutparameters,asnon-retainedobjectdonothavetobe releasedbythecallee. TheseattributesareonlyusedbytheClangStaticAnalyzer. ThefamilyofattributesX_returns_X_retainedcanbeaddedtofunctions, C++methods,andObjective-Cmethodsandproperties. AttributesX_consumedcanbeaddedtoparametersofmethods,functions, andObjective-Cmethods. ns_consumes_self¶ SupportedSyntaxes¶ ns_consumes_self clang::ns_consumes_self clang::ns_consumes_self         Yes ThebehaviorofafunctionwithrespecttoreferencecountingforFoundation (Objective-C),CoreFoundation(C)andOSObject(C++)isdeterminedbyanaming convention(e.g.functionsstartingwith“get”areassumedtoreturnat +0). Itcanbeoverriddenusingafamilyofthefollowingattributes.In Objective-C,theannotation__attribute__((ns_returns_retained))appliedto afunctioncommunicatesthattheobjectisreturnedat+1,andthecaller isresponsibleforfreeingit. Similarly,theannotation__attribute__((ns_returns_not_retained)) specifiesthattheobjectisreturnedat+0andtheownershipremainswith thecallee. Theannotation__attribute__((ns_consumes_self))specifiesthat theObjective-Cmethodcallconsumesthereferencetoself,e.g.by attachingittoasuppliedparameter. Additionally,parameterscanhaveanannotation __attribute__((ns_consumed)),whichspecifiesthatpassinganownedobject asthatparametereffectivelytransferstheownership,andthecallerisno longerresponsibleforit. TheseattributesaffectcodegenerationwheninteractingwithARCcode,and theyareusedbytheClangStaticAnalyzer. InCprogramsusingCoreFoundation,asimilarsetofattributes: __attribute__((cf_returns_not_retained)), __attribute__((cf_returns_retained))and__attribute__((cf_consumed)) havethesamerespectivesemanticswhenappliedtoCoreFoundationobjects. TheseattributesaffectcodegenerationwheninteractingwithARCcode,and theyareusedbytheClangStaticAnalyzer. Finally,inC++interactingwithXNUkernel(objectsinheritingfromOSObject), thesameattributefamilyispresent: __attribute__((os_returns_not_retained)), __attribute__((os_returns_retained))and__attribute__((os_consumed)), withthesamerespectivesemantics. Similarto__attribute__((ns_consumes_self)), __attribute__((os_consumes_this))specifiesthatthemethodcallconsumes thereferenceto“this”(e.g.,whenattachingittoadifferentobjectsupplied asaparameter). Outparameters(parametersthefunctionismeanttowriteinto, eitherviapointers-to-pointersorreferences-to-pointers) maybeannotatedwith__attribute__((os_returns_retained)) or__attribute__((os_returns_not_retained))whichspecifiesthattheobject writtenintotheoutparametershould(orrespectivelyshouldnot)bereleased afteruse. Sinceoftenoutparametersmayormaynotbewrittendependingontheexit codeofthefunction, annotations__attribute__((os_returns_retained_on_zero)) and__attribute__((os_returns_retained_on_non_zero))specifythat anoutparameterat+1iswrittenifandonlyifthefunctionreturnsazero (respectivelynon-zero)errorcode. Observethatreturn-code-dependentoutparameterannotationsareonly availableforretainedoutparameters,asnon-retainedobjectdonothavetobe releasedbythecallee. TheseattributesareonlyusedbytheClangStaticAnalyzer. ThefamilyofattributesX_returns_X_retainedcanbeaddedtofunctions, C++methods,andObjective-Cmethodsandproperties. AttributesX_consumedcanbeaddedtoparametersofmethods,functions, andObjective-Cmethods. ns_returns_autoreleased¶ SupportedSyntaxes¶ ns_returns_autoreleased clang::ns_returns_autoreleased clang::ns_returns_autoreleased           ThebehaviorofafunctionwithrespecttoreferencecountingforFoundation (Objective-C),CoreFoundation(C)andOSObject(C++)isdeterminedbyanaming convention(e.g.functionsstartingwith“get”areassumedtoreturnat +0). Itcanbeoverriddenusingafamilyofthefollowingattributes.In Objective-C,theannotation__attribute__((ns_returns_retained))appliedto afunctioncommunicatesthattheobjectisreturnedat+1,andthecaller isresponsibleforfreeingit. Similarly,theannotation__attribute__((ns_returns_not_retained)) specifiesthattheobjectisreturnedat+0andtheownershipremainswith thecallee. Theannotation__attribute__((ns_consumes_self))specifiesthat theObjective-Cmethodcallconsumesthereferencetoself,e.g.by attachingittoasuppliedparameter. Additionally,parameterscanhaveanannotation __attribute__((ns_consumed)),whichspecifiesthatpassinganownedobject asthatparametereffectivelytransferstheownership,andthecallerisno longerresponsibleforit. TheseattributesaffectcodegenerationwheninteractingwithARCcode,and theyareusedbytheClangStaticAnalyzer. InCprogramsusingCoreFoundation,asimilarsetofattributes: __attribute__((cf_returns_not_retained)), __attribute__((cf_returns_retained))and__attribute__((cf_consumed)) havethesamerespectivesemanticswhenappliedtoCoreFoundationobjects. TheseattributesaffectcodegenerationwheninteractingwithARCcode,and theyareusedbytheClangStaticAnalyzer. Finally,inC++interactingwithXNUkernel(objectsinheritingfromOSObject), thesameattributefamilyispresent: __attribute__((os_returns_not_retained)), __attribute__((os_returns_retained))and__attribute__((os_consumed)), withthesamerespectivesemantics. Similarto__attribute__((ns_consumes_self)), __attribute__((os_consumes_this))specifiesthatthemethodcallconsumes thereferenceto“this”(e.g.,whenattachingittoadifferentobjectsupplied asaparameter). Outparameters(parametersthefunctionismeanttowriteinto, eitherviapointers-to-pointersorreferences-to-pointers) maybeannotatedwith__attribute__((os_returns_retained)) or__attribute__((os_returns_not_retained))whichspecifiesthattheobject writtenintotheoutparametershould(orrespectivelyshouldnot)bereleased afteruse. Sinceoftenoutparametersmayormaynotbewrittendependingontheexit codeofthefunction, annotations__attribute__((os_returns_retained_on_zero)) and__attribute__((os_returns_retained_on_non_zero))specifythat anoutparameterat+1iswrittenifandonlyifthefunctionreturnsazero (respectivelynon-zero)errorcode. Observethatreturn-code-dependentoutparameterannotationsareonly availableforretainedoutparameters,asnon-retainedobjectdonothavetobe releasedbythecallee. TheseattributesareonlyusedbytheClangStaticAnalyzer. ThefamilyofattributesX_returns_X_retainedcanbeaddedtofunctions, C++methods,andObjective-Cmethodsandproperties. AttributesX_consumedcanbeaddedtoparametersofmethods,functions, andObjective-Cmethods. ns_returns_not_retained¶ SupportedSyntaxes¶ ns_returns_not_retained clang::ns_returns_not_retained clang::ns_returns_not_retained           ThebehaviorofafunctionwithrespecttoreferencecountingforFoundation (Objective-C),CoreFoundation(C)andOSObject(C++)isdeterminedbyanaming convention(e.g.functionsstartingwith“get”areassumedtoreturnat +0). Itcanbeoverriddenusingafamilyofthefollowingattributes.In Objective-C,theannotation__attribute__((ns_returns_retained))appliedto afunctioncommunicatesthattheobjectisreturnedat+1,andthecaller isresponsibleforfreeingit. Similarly,theannotation__attribute__((ns_returns_not_retained)) specifiesthattheobjectisreturnedat+0andtheownershipremainswith thecallee. Theannotation__attribute__((ns_consumes_self))specifiesthat theObjective-Cmethodcallconsumesthereferencetoself,e.g.by attachingittoasuppliedparameter. Additionally,parameterscanhaveanannotation __attribute__((ns_consumed)),whichspecifiesthatpassinganownedobject asthatparametereffectivelytransferstheownership,andthecallerisno longerresponsibleforit. TheseattributesaffectcodegenerationwheninteractingwithARCcode,and theyareusedbytheClangStaticAnalyzer. InCprogramsusingCoreFoundation,asimilarsetofattributes: __attribute__((cf_returns_not_retained)), __attribute__((cf_returns_retained))and__attribute__((cf_consumed)) havethesamerespectivesemanticswhenappliedtoCoreFoundationobjects. TheseattributesaffectcodegenerationwheninteractingwithARCcode,and theyareusedbytheClangStaticAnalyzer. Finally,inC++interactingwithXNUkernel(objectsinheritingfromOSObject), thesameattributefamilyispresent: __attribute__((os_returns_not_retained)), __attribute__((os_returns_retained))and__attribute__((os_consumed)), withthesamerespectivesemantics. Similarto__attribute__((ns_consumes_self)), __attribute__((os_consumes_this))specifiesthatthemethodcallconsumes thereferenceto“this”(e.g.,whenattachingittoadifferentobjectsupplied asaparameter). Outparameters(parametersthefunctionismeanttowriteinto, eitherviapointers-to-pointersorreferences-to-pointers) maybeannotatedwith__attribute__((os_returns_retained)) or__attribute__((os_returns_not_retained))whichspecifiesthattheobject writtenintotheoutparametershould(orrespectivelyshouldnot)bereleased afteruse. Sinceoftenoutparametersmayormaynotbewrittendependingontheexit codeofthefunction, annotations__attribute__((os_returns_retained_on_zero)) and__attribute__((os_returns_retained_on_non_zero))specifythat anoutparameterat+1iswrittenifandonlyifthefunctionreturnsazero (respectivelynon-zero)errorcode. Observethatreturn-code-dependentoutparameterannotationsareonly availableforretainedoutparameters,asnon-retainedobjectdonothavetobe releasedbythecallee. TheseattributesareonlyusedbytheClangStaticAnalyzer. ThefamilyofattributesX_returns_X_retainedcanbeaddedtofunctions, C++methods,andObjective-Cmethodsandproperties. AttributesX_consumedcanbeaddedtoparametersofmethods,functions, andObjective-Cmethods. ns_returns_retained¶ SupportedSyntaxes¶ ns_returns_retained clang::ns_returns_retained clang::ns_returns_retained           ThebehaviorofafunctionwithrespecttoreferencecountingforFoundation (Objective-C),CoreFoundation(C)andOSObject(C++)isdeterminedbyanaming convention(e.g.functionsstartingwith“get”areassumedtoreturnat +0). Itcanbeoverriddenusingafamilyofthefollowingattributes.In Objective-C,theannotation__attribute__((ns_returns_retained))appliedto afunctioncommunicatesthattheobjectisreturnedat+1,andthecaller isresponsibleforfreeingit. Similarly,theannotation__attribute__((ns_returns_not_retained)) specifiesthattheobjectisreturnedat+0andtheownershipremainswith thecallee. Theannotation__attribute__((ns_consumes_self))specifiesthat theObjective-Cmethodcallconsumesthereferencetoself,e.g.by attachingittoasuppliedparameter. Additionally,parameterscanhaveanannotation __attribute__((ns_consumed)),whichspecifiesthatpassinganownedobject asthatparametereffectivelytransferstheownership,andthecallerisno longerresponsibleforit. TheseattributesaffectcodegenerationwheninteractingwithARCcode,and theyareusedbytheClangStaticAnalyzer. InCprogramsusingCoreFoundation,asimilarsetofattributes: __attribute__((cf_returns_not_retained)), __attribute__((cf_returns_retained))and__attribute__((cf_consumed)) havethesamerespectivesemanticswhenappliedtoCoreFoundationobjects. TheseattributesaffectcodegenerationwheninteractingwithARCcode,and theyareusedbytheClangStaticAnalyzer. Finally,inC++interactingwithXNUkernel(objectsinheritingfromOSObject), thesameattributefamilyispresent: __attribute__((os_returns_not_retained)), __attribute__((os_returns_retained))and__attribute__((os_consumed)), withthesamerespectivesemantics. Similarto__attribute__((ns_consumes_self)), __attribute__((os_consumes_this))specifiesthatthemethodcallconsumes thereferenceto“this”(e.g.,whenattachingittoadifferentobjectsupplied asaparameter). Outparameters(parametersthefunctionismeanttowriteinto, eitherviapointers-to-pointersorreferences-to-pointers) maybeannotatedwith__attribute__((os_returns_retained)) or__attribute__((os_returns_not_retained))whichspecifiesthattheobject writtenintotheoutparametershould(orrespectivelyshouldnot)bereleased afteruse. Sinceoftenoutparametersmayormaynotbewrittendependingontheexit codeofthefunction, annotations__attribute__((os_returns_retained_on_zero)) and__attribute__((os_returns_retained_on_non_zero))specifythat anoutparameterat+1iswrittenifandonlyifthefunctionreturnsazero (respectivelynon-zero)errorcode. Observethatreturn-code-dependentoutparameterannotationsareonly availableforretainedoutparameters,asnon-retainedobjectdonothavetobe releasedbythecallee. TheseattributesareonlyusedbytheClangStaticAnalyzer. ThefamilyofattributesX_returns_X_retainedcanbeaddedtofunctions, C++methods,andObjective-Cmethodsandproperties. AttributesX_consumedcanbeaddedtoparametersofmethods,functions, andObjective-Cmethods. numthreads¶ SupportedSyntaxes¶                 ThenumthreadsattributeappliestoHLSLshaderswhereexplcitthreadcounts arerequired.TheX,Y,andZvaluesprovidedtotheattribute dictatethethreadid.TotalnumberofthreadsexecutedisX*Y*Z. Thefulldocumentationisavailablehere:https://docs.microsoft.com/en-us/windows/win32/direct3dhlsl/sm5-attributes-numthreads objc_method_family¶ SupportedSyntaxes¶ objc_method_family clang::objc_method_family clang::objc_method_family         Yes ManymethodsinObjective-Chaveconventionalmeaningsdeterminedbytheir selectors.Itissometimesusefultobeabletomarkamethodashavinga particularconventionalmeaningdespitenothavingtherightselector,oras nothavingtheconventionalmeaningthatitsselectorwouldsuggest.Forthese usecases,weprovideanattributetospecificallydescribethe“methodfamily” thatamethodbelongsto. Usage:__attribute__((objc_method_family(X))),whereXisoneof none,alloc,copy,init,mutableCopy,ornew.This attributecanonlybeplacedattheendofamethoddeclaration: -(NSString*)initMyStringValue__attribute__((objc_method_family(none))); Userswhodonotwishtochangetheconventionalmeaningofamethod,andwho merelywanttodocumentitsnon-standardretainandreleasesemantics,should usetheretainingbehaviorattributes(ns_returns_retained, ns_returns_not_retained,etc). Queryforthisfeaturewith__has_attribute(objc_method_family). objc_requires_super¶ SupportedSyntaxes¶ objc_requires_super clang::objc_requires_super clang::objc_requires_super         Yes SomeObjective-Cclassesallowasubclasstooverrideaparticularmethodina parentclassbutexpectthattheoverridingmethodalsocallstheoverridden methodintheparentclass.Forthesecases,weprovideanattributeto designatethatamethodrequiresa“calltosuper”intheoverriding methodinthesubclass. Usage:__attribute__((objc_requires_super)).Thisattributecanonly beplacedattheendofamethoddeclaration: -(void)foo__attribute__((objc_requires_super)); Thisattributecanonlybeappliedthemethoddeclarationswithinaclass,and notaprotocol.Currentlythisattributedoesnotenforceanyplacementof wherethecalloccursintheoverridingmethod(suchasinthecaseof -deallocwherethecallmustappearattheend).Itchecksonlythatit exists. NotethatonbothOSXandiOSthattheFoundationframeworkprovidesa conveniencemacroNS_REQUIRES_SUPERthatprovidessyntacticsugarforthis attribute: -(void)fooNS_REQUIRES_SUPER; Thismacroisconditionallydefineddependingonthecompiler’ssupportfor thisattribute.Ifthecompilerdoesnotsupporttheattributethemacro expandstonothing. Operationally,whenamethodhasthisannotationthecompilerwillwarnifthe implementationofanoverrideinasubclassdoesnotcallsuper.Forexample: warning:methodpossiblymissinga[superAnnotMeth]call -(void)AnnotMeth{}; ^ optnone¶ SupportedSyntaxes¶ optnone clang::optnone clang::optnone         Yes Theoptnoneattributesuppressesessentiallyalloptimizations onafunctionormethod,regardlessoftheoptimizationlevelappliedto thecompilationunitasawhole.Thisisparticularlyusefulwhenyou needtodebugaparticularfunction,butitisinfeasibletobuildthe entireapplicationwithoutoptimization.Avoidingoptimizationonthe specifiedfunctioncanimprovethequalityofthedebugginginformation forthatfunction. Thisattributeisincompatiblewiththealways_inlineandminsize attributes. os_consumed¶ SupportedSyntaxes¶ os_consumed clang::os_consumed clang::os_consumed         Yes ThebehaviorofafunctionwithrespecttoreferencecountingforFoundation (Objective-C),CoreFoundation(C)andOSObject(C++)isdeterminedbyanaming convention(e.g.functionsstartingwith“get”areassumedtoreturnat +0). Itcanbeoverriddenusingafamilyofthefollowingattributes.In Objective-C,theannotation__attribute__((ns_returns_retained))appliedto afunctioncommunicatesthattheobjectisreturnedat+1,andthecaller isresponsibleforfreeingit. Similarly,theannotation__attribute__((ns_returns_not_retained)) specifiesthattheobjectisreturnedat+0andtheownershipremainswith thecallee. Theannotation__attribute__((ns_consumes_self))specifiesthat theObjective-Cmethodcallconsumesthereferencetoself,e.g.by attachingittoasuppliedparameter. Additionally,parameterscanhaveanannotation __attribute__((ns_consumed)),whichspecifiesthatpassinganownedobject asthatparametereffectivelytransferstheownership,andthecallerisno longerresponsibleforit. TheseattributesaffectcodegenerationwheninteractingwithARCcode,and theyareusedbytheClangStaticAnalyzer. InCprogramsusingCoreFoundation,asimilarsetofattributes: __attribute__((cf_returns_not_retained)), __attribute__((cf_returns_retained))and__attribute__((cf_consumed)) havethesamerespectivesemanticswhenappliedtoCoreFoundationobjects. TheseattributesaffectcodegenerationwheninteractingwithARCcode,and theyareusedbytheClangStaticAnalyzer. Finally,inC++interactingwithXNUkernel(objectsinheritingfromOSObject), thesameattributefamilyispresent: __attribute__((os_returns_not_retained)), __attribute__((os_returns_retained))and__attribute__((os_consumed)), withthesamerespectivesemantics. Similarto__attribute__((ns_consumes_self)), __attribute__((os_consumes_this))specifiesthatthemethodcallconsumes thereferenceto“this”(e.g.,whenattachingittoadifferentobjectsupplied asaparameter). Outparameters(parametersthefunctionismeanttowriteinto, eitherviapointers-to-pointersorreferences-to-pointers) maybeannotatedwith__attribute__((os_returns_retained)) or__attribute__((os_returns_not_retained))whichspecifiesthattheobject writtenintotheoutparametershould(orrespectivelyshouldnot)bereleased afteruse. Sinceoftenoutparametersmayormaynotbewrittendependingontheexit codeofthefunction, annotations__attribute__((os_returns_retained_on_zero)) and__attribute__((os_returns_retained_on_non_zero))specifythat anoutparameterat+1iswrittenifandonlyifthefunctionreturnsazero (respectivelynon-zero)errorcode. Observethatreturn-code-dependentoutparameterannotationsareonly availableforretainedoutparameters,asnon-retainedobjectdonothavetobe releasedbythecallee. TheseattributesareonlyusedbytheClangStaticAnalyzer. ThefamilyofattributesX_returns_X_retainedcanbeaddedtofunctions, C++methods,andObjective-Cmethodsandproperties. AttributesX_consumedcanbeaddedtoparametersofmethods,functions, andObjective-Cmethods. os_consumes_this¶ SupportedSyntaxes¶ os_consumes_this clang::os_consumes_this clang::os_consumes_this           ThebehaviorofafunctionwithrespecttoreferencecountingforFoundation (Objective-C),CoreFoundation(C)andOSObject(C++)isdeterminedbyanaming convention(e.g.functionsstartingwith“get”areassumedtoreturnat +0). Itcanbeoverriddenusingafamilyofthefollowingattributes.In Objective-C,theannotation__attribute__((ns_returns_retained))appliedto afunctioncommunicatesthattheobjectisreturnedat+1,andthecaller isresponsibleforfreeingit. Similarly,theannotation__attribute__((ns_returns_not_retained)) specifiesthattheobjectisreturnedat+0andtheownershipremainswith thecallee. Theannotation__attribute__((ns_consumes_self))specifiesthat theObjective-Cmethodcallconsumesthereferencetoself,e.g.by attachingittoasuppliedparameter. Additionally,parameterscanhaveanannotation __attribute__((ns_consumed)),whichspecifiesthatpassinganownedobject asthatparametereffectivelytransferstheownership,andthecallerisno longerresponsibleforit. TheseattributesaffectcodegenerationwheninteractingwithARCcode,and theyareusedbytheClangStaticAnalyzer. InCprogramsusingCoreFoundation,asimilarsetofattributes: __attribute__((cf_returns_not_retained)), __attribute__((cf_returns_retained))and__attribute__((cf_consumed)) havethesamerespectivesemanticswhenappliedtoCoreFoundationobjects. TheseattributesaffectcodegenerationwheninteractingwithARCcode,and theyareusedbytheClangStaticAnalyzer. Finally,inC++interactingwithXNUkernel(objectsinheritingfromOSObject), thesameattributefamilyispresent: __attribute__((os_returns_not_retained)), __attribute__((os_returns_retained))and__attribute__((os_consumed)), withthesamerespectivesemantics. Similarto__attribute__((ns_consumes_self)), __attribute__((os_consumes_this))specifiesthatthemethodcallconsumes thereferenceto“this”(e.g.,whenattachingittoadifferentobjectsupplied asaparameter). Outparameters(parametersthefunctionismeanttowriteinto, eitherviapointers-to-pointersorreferences-to-pointers) maybeannotatedwith__attribute__((os_returns_retained)) or__attribute__((os_returns_not_retained))whichspecifiesthattheobject writtenintotheoutparametershould(orrespectivelyshouldnot)bereleased afteruse. Sinceoftenoutparametersmayormaynotbewrittendependingontheexit codeofthefunction, annotations__attribute__((os_returns_retained_on_zero)) and__attribute__((os_returns_retained_on_non_zero))specifythat anoutparameterat+1iswrittenifandonlyifthefunctionreturnsazero (respectivelynon-zero)errorcode. Observethatreturn-code-dependentoutparameterannotationsareonly availableforretainedoutparameters,asnon-retainedobjectdonothavetobe releasedbythecallee. TheseattributesareonlyusedbytheClangStaticAnalyzer. ThefamilyofattributesX_returns_X_retainedcanbeaddedtofunctions, C++methods,andObjective-Cmethodsandproperties. AttributesX_consumedcanbeaddedtoparametersofmethods,functions, andObjective-Cmethods. os_returns_not_retained¶ SupportedSyntaxes¶ os_returns_not_retained clang::os_returns_not_retained clang::os_returns_not_retained         Yes ThebehaviorofafunctionwithrespecttoreferencecountingforFoundation (Objective-C),CoreFoundation(C)andOSObject(C++)isdeterminedbyanaming convention(e.g.functionsstartingwith“get”areassumedtoreturnat +0). Itcanbeoverriddenusingafamilyofthefollowingattributes.In Objective-C,theannotation__attribute__((ns_returns_retained))appliedto afunctioncommunicatesthattheobjectisreturnedat+1,andthecaller isresponsibleforfreeingit. Similarly,theannotation__attribute__((ns_returns_not_retained)) specifiesthattheobjectisreturnedat+0andtheownershipremainswith thecallee. Theannotation__attribute__((ns_consumes_self))specifiesthat theObjective-Cmethodcallconsumesthereferencetoself,e.g.by attachingittoasuppliedparameter. Additionally,parameterscanhaveanannotation __attribute__((ns_consumed)),whichspecifiesthatpassinganownedobject asthatparametereffectivelytransferstheownership,andthecallerisno longerresponsibleforit. TheseattributesaffectcodegenerationwheninteractingwithARCcode,and theyareusedbytheClangStaticAnalyzer. InCprogramsusingCoreFoundation,asimilarsetofattributes: __attribute__((cf_returns_not_retained)), __attribute__((cf_returns_retained))and__attribute__((cf_consumed)) havethesamerespectivesemanticswhenappliedtoCoreFoundationobjects. TheseattributesaffectcodegenerationwheninteractingwithARCcode,and theyareusedbytheClangStaticAnalyzer. Finally,inC++interactingwithXNUkernel(objectsinheritingfromOSObject), thesameattributefamilyispresent: __attribute__((os_returns_not_retained)), __attribute__((os_returns_retained))and__attribute__((os_consumed)), withthesamerespectivesemantics. Similarto__attribute__((ns_consumes_self)), __attribute__((os_consumes_this))specifiesthatthemethodcallconsumes thereferenceto“this”(e.g.,whenattachingittoadifferentobjectsupplied asaparameter). Outparameters(parametersthefunctionismeanttowriteinto, eitherviapointers-to-pointersorreferences-to-pointers) maybeannotatedwith__attribute__((os_returns_retained)) or__attribute__((os_returns_not_retained))whichspecifiesthattheobject writtenintotheoutparametershould(orrespectivelyshouldnot)bereleased afteruse. Sinceoftenoutparametersmayormaynotbewrittendependingontheexit codeofthefunction, annotations__attribute__((os_returns_retained_on_zero)) and__attribute__((os_returns_retained_on_non_zero))specifythat anoutparameterat+1iswrittenifandonlyifthefunctionreturnsazero (respectivelynon-zero)errorcode. Observethatreturn-code-dependentoutparameterannotationsareonly availableforretainedoutparameters,asnon-retainedobjectdonothavetobe releasedbythecallee. TheseattributesareonlyusedbytheClangStaticAnalyzer. ThefamilyofattributesX_returns_X_retainedcanbeaddedtofunctions, C++methods,andObjective-Cmethodsandproperties. AttributesX_consumedcanbeaddedtoparametersofmethods,functions, andObjective-Cmethods. os_returns_retained¶ SupportedSyntaxes¶ os_returns_retained clang::os_returns_retained clang::os_returns_retained         Yes ThebehaviorofafunctionwithrespecttoreferencecountingforFoundation (Objective-C),CoreFoundation(C)andOSObject(C++)isdeterminedbyanaming convention(e.g.functionsstartingwith“get”areassumedtoreturnat +0). Itcanbeoverriddenusingafamilyofthefollowingattributes.In Objective-C,theannotation__attribute__((ns_returns_retained))appliedto afunctioncommunicatesthattheobjectisreturnedat+1,andthecaller isresponsibleforfreeingit. Similarly,theannotation__attribute__((ns_returns_not_retained)) specifiesthattheobjectisreturnedat+0andtheownershipremainswith thecallee. Theannotation__attribute__((ns_consumes_self))specifiesthat theObjective-Cmethodcallconsumesthereferencetoself,e.g.by attachingittoasuppliedparameter. Additionally,parameterscanhaveanannotation __attribute__((ns_consumed)),whichspecifiesthatpassinganownedobject asthatparametereffectivelytransferstheownership,andthecallerisno longerresponsibleforit. TheseattributesaffectcodegenerationwheninteractingwithARCcode,and theyareusedbytheClangStaticAnalyzer. InCprogramsusingCoreFoundation,asimilarsetofattributes: __attribute__((cf_returns_not_retained)), __attribute__((cf_returns_retained))and__attribute__((cf_consumed)) havethesamerespectivesemanticswhenappliedtoCoreFoundationobjects. TheseattributesaffectcodegenerationwheninteractingwithARCcode,and theyareusedbytheClangStaticAnalyzer. Finally,inC++interactingwithXNUkernel(objectsinheritingfromOSObject), thesameattributefamilyispresent: __attribute__((os_returns_not_retained)), __attribute__((os_returns_retained))and__attribute__((os_consumed)), withthesamerespectivesemantics. Similarto__attribute__((ns_consumes_self)), __attribute__((os_consumes_this))specifiesthatthemethodcallconsumes thereferenceto“this”(e.g.,whenattachingittoadifferentobjectsupplied asaparameter). Outparameters(parametersthefunctionismeanttowriteinto, eitherviapointers-to-pointersorreferences-to-pointers) maybeannotatedwith__attribute__((os_returns_retained)) or__attribute__((os_returns_not_retained))whichspecifiesthattheobject writtenintotheoutparametershould(orrespectivelyshouldnot)bereleased afteruse. Sinceoftenoutparametersmayormaynotbewrittendependingontheexit codeofthefunction, annotations__attribute__((os_returns_retained_on_zero)) and__attribute__((os_returns_retained_on_non_zero))specifythat anoutparameterat+1iswrittenifandonlyifthefunctionreturnsazero (respectivelynon-zero)errorcode. Observethatreturn-code-dependentoutparameterannotationsareonly availableforretainedoutparameters,asnon-retainedobjectdonothavetobe releasedbythecallee. TheseattributesareonlyusedbytheClangStaticAnalyzer. ThefamilyofattributesX_returns_X_retainedcanbeaddedtofunctions, C++methods,andObjective-Cmethodsandproperties. AttributesX_consumedcanbeaddedtoparametersofmethods,functions, andObjective-Cmethods. os_returns_retained_on_non_zero¶ SupportedSyntaxes¶ os_returns_retained_on_non_zero clang::os_returns_retained_on_non_zero clang::os_returns_retained_on_non_zero         Yes ThebehaviorofafunctionwithrespecttoreferencecountingforFoundation (Objective-C),CoreFoundation(C)andOSObject(C++)isdeterminedbyanaming convention(e.g.functionsstartingwith“get”areassumedtoreturnat +0). Itcanbeoverriddenusingafamilyofthefollowingattributes.In Objective-C,theannotation__attribute__((ns_returns_retained))appliedto afunctioncommunicatesthattheobjectisreturnedat+1,andthecaller isresponsibleforfreeingit. Similarly,theannotation__attribute__((ns_returns_not_retained)) specifiesthattheobjectisreturnedat+0andtheownershipremainswith thecallee. Theannotation__attribute__((ns_consumes_self))specifiesthat theObjective-Cmethodcallconsumesthereferencetoself,e.g.by attachingittoasuppliedparameter. Additionally,parameterscanhaveanannotation __attribute__((ns_consumed)),whichspecifiesthatpassinganownedobject asthatparametereffectivelytransferstheownership,andthecallerisno longerresponsibleforit. TheseattributesaffectcodegenerationwheninteractingwithARCcode,and theyareusedbytheClangStaticAnalyzer. InCprogramsusingCoreFoundation,asimilarsetofattributes: __attribute__((cf_returns_not_retained)), __attribute__((cf_returns_retained))and__attribute__((cf_consumed)) havethesamerespectivesemanticswhenappliedtoCoreFoundationobjects. TheseattributesaffectcodegenerationwheninteractingwithARCcode,and theyareusedbytheClangStaticAnalyzer. Finally,inC++interactingwithXNUkernel(objectsinheritingfromOSObject), thesameattributefamilyispresent: __attribute__((os_returns_not_retained)), __attribute__((os_returns_retained))and__attribute__((os_consumed)), withthesamerespectivesemantics. Similarto__attribute__((ns_consumes_self)), __attribute__((os_consumes_this))specifiesthatthemethodcallconsumes thereferenceto“this”(e.g.,whenattachingittoadifferentobjectsupplied asaparameter). Outparameters(parametersthefunctionismeanttowriteinto, eitherviapointers-to-pointersorreferences-to-pointers) maybeannotatedwith__attribute__((os_returns_retained)) or__attribute__((os_returns_not_retained))whichspecifiesthattheobject writtenintotheoutparametershould(orrespectivelyshouldnot)bereleased afteruse. Sinceoftenoutparametersmayormaynotbewrittendependingontheexit codeofthefunction, annotations__attribute__((os_returns_retained_on_zero)) and__attribute__((os_returns_retained_on_non_zero))specifythat anoutparameterat+1iswrittenifandonlyifthefunctionreturnsazero (respectivelynon-zero)errorcode. Observethatreturn-code-dependentoutparameterannotationsareonly availableforretainedoutparameters,asnon-retainedobjectdonothavetobe releasedbythecallee. TheseattributesareonlyusedbytheClangStaticAnalyzer. ThefamilyofattributesX_returns_X_retainedcanbeaddedtofunctions, C++methods,andObjective-Cmethodsandproperties. AttributesX_consumedcanbeaddedtoparametersofmethods,functions, andObjective-Cmethods. os_returns_retained_on_zero¶ SupportedSyntaxes¶ os_returns_retained_on_zero clang::os_returns_retained_on_zero clang::os_returns_retained_on_zero         Yes ThebehaviorofafunctionwithrespecttoreferencecountingforFoundation (Objective-C),CoreFoundation(C)andOSObject(C++)isdeterminedbyanaming convention(e.g.functionsstartingwith“get”areassumedtoreturnat +0). Itcanbeoverriddenusingafamilyofthefollowingattributes.In Objective-C,theannotation__attribute__((ns_returns_retained))appliedto afunctioncommunicatesthattheobjectisreturnedat+1,andthecaller isresponsibleforfreeingit. Similarly,theannotation__attribute__((ns_returns_not_retained)) specifiesthattheobjectisreturnedat+0andtheownershipremainswith thecallee. Theannotation__attribute__((ns_consumes_self))specifiesthat theObjective-Cmethodcallconsumesthereferencetoself,e.g.by attachingittoasuppliedparameter. Additionally,parameterscanhaveanannotation __attribute__((ns_consumed)),whichspecifiesthatpassinganownedobject asthatparametereffectivelytransferstheownership,andthecallerisno longerresponsibleforit. TheseattributesaffectcodegenerationwheninteractingwithARCcode,and theyareusedbytheClangStaticAnalyzer. InCprogramsusingCoreFoundation,asimilarsetofattributes: __attribute__((cf_returns_not_retained)), __attribute__((cf_returns_retained))and__attribute__((cf_consumed)) havethesamerespectivesemanticswhenappliedtoCoreFoundationobjects. TheseattributesaffectcodegenerationwheninteractingwithARCcode,and theyareusedbytheClangStaticAnalyzer. Finally,inC++interactingwithXNUkernel(objectsinheritingfromOSObject), thesameattributefamilyispresent: __attribute__((os_returns_not_retained)), __attribute__((os_returns_retained))and__attribute__((os_consumed)), withthesamerespectivesemantics. Similarto__attribute__((ns_consumes_self)), __attribute__((os_consumes_this))specifiesthatthemethodcallconsumes thereferenceto“this”(e.g.,whenattachingittoadifferentobjectsupplied asaparameter). Outparameters(parametersthefunctionismeanttowriteinto, eitherviapointers-to-pointersorreferences-to-pointers) maybeannotatedwith__attribute__((os_returns_retained)) or__attribute__((os_returns_not_retained))whichspecifiesthattheobject writtenintotheoutparametershould(orrespectivelyshouldnot)bereleased afteruse. Sinceoftenoutparametersmayormaynotbewrittendependingontheexit codeofthefunction, annotations__attribute__((os_returns_retained_on_zero)) and__attribute__((os_returns_retained_on_non_zero))specifythat anoutparameterat+1iswrittenifandonlyifthefunctionreturnsazero (respectivelynon-zero)errorcode. Observethatreturn-code-dependentoutparameterannotationsareonly availableforretainedoutparameters,asnon-retainedobjectdonothavetobe releasedbythecallee. TheseattributesareonlyusedbytheClangStaticAnalyzer. ThefamilyofattributesX_returns_X_retainedcanbeaddedtofunctions, C++methods,andObjective-Cmethodsandproperties. AttributesX_consumedcanbeaddedtoparametersofmethods,functions, andObjective-Cmethods. overloadable¶ SupportedSyntaxes¶ overloadable clang::overloadable clang::overloadable         Yes ClangprovidessupportforC++functionoverloadinginC.Functionoverloading inCisintroducedusingtheoverloadableattribute.Forexample,one mightprovideseveraloverloadedversionsofatgsinfunctionthatinvokes theappropriatestandardfunctioncomputingthesineofavaluewithfloat, double,orlongdoubleprecision: #include float__attribute__((overloadable))tgsin(floatx){returnsinf(x);} double__attribute__((overloadable))tgsin(doublex){returnsin(x);} longdouble__attribute__((overloadable))tgsin(longdoublex){returnsinl(x);} Giventhesedeclarations,onecancalltgsinwithafloatvalueto receiveafloatresult,withadoubletoreceiveadoubleresult, etc.FunctionoverloadinginCfollowstherulesofC++functionoverloading topickthebestoverloadgiventhecallarguments,withafewC-specific semantics: Conversionfromfloatordoubletolongdoubleisrankedasa floating-pointpromotion(perC99)ratherthanasafloating-pointconversion (asinC++). AconversionfromapointeroftypeT*toapointeroftypeU*is consideredapointerconversion(withconversionrank)ifTandUare compatibletypes. AconversionfromtypeTtoavalueoftypeUispermittedifT andUarecompatibletypes.Thisconversionisgiven“conversion”rank. Ifnoviablecandidatesareotherwiseavailable,weallowaconversionfroma pointeroftypeT*toapointeroftypeU*,whereTandUare incompatible.Thisconversionisrankedbelowallothertypesofconversions. Pleasenote:UlackingqualifiersthatarepresentonTissufficient forTandUtobeincompatible. Thedeclarationofoverloadablefunctionsisrestrictedtofunction declarationsanddefinitions.Ifafunctionismarkedwiththeoverloadable attribute,thenalldeclarationsanddefinitionsoffunctionswiththatname, exceptforatmostone(seethenotebelowaboutunmarkedoverloads),musthave theoverloadableattribute.Inaddition,redeclarationsofafunctionwith theoverloadableattributemusthavetheoverloadableattribute,and redeclarationsofafunctionwithouttheoverloadableattributemustnot havetheoverloadableattribute.e.g., intf(int)__attribute__((overloadable)); floatf(float);//error:declarationof"f"musthavethe"overloadable"attribute intf(int);//error:redeclarationof"f"musthavethe"overloadable"attribute intg(int)__attribute__((overloadable)); intg(int){}//error:redeclarationof"g"mustalsohavethe"overloadable"attribute inth(int); inth(int)__attribute__((overloadable));//error:declarationof"h"mustnot //havethe"overloadable"attribute Functionsmarkedoverloadablemusthaveprototypes.Therefore,the followingcodeisill-formed: inth()__attribute__((overloadable));//error:hdoesnothaveaprototype However,overloadablefunctionsareallowedtouseaellipsisevenifthere arenonamedparameters(asispermittedinC++).Thisfeatureisparticularly usefulwhencombinedwiththeunavailableattribute: voidhoneypot(...)__attribute__((overloadable,unavailable));//callingmeisanerror Functionsdeclaredwiththeoverloadableattributehavetheirnamesmangled accordingtothesamerulesasC++functionnames.Forexample,thethree tgsinfunctionsinourmotivatingexamplegetthemanglednames _Z5tgsinf,_Z5tgsind,and_Z5tgsine,respectively.Therearetwo caveatstothisuseofnamemangling: FutureversionsofClangmaychangethenamemanglingoffunctionsoverloaded inC,soyoushouldnotdependonanspecificmangling.Tobecompletely safe,westronglyurgetheuseofstaticinlinewithoverloadable functions. TheoverloadableattributehasalmostnomeaningwhenusedinC++, becausenameswillalreadybemangledandfunctionsarealreadyoverloadable. However,whenanoverloadablefunctionoccurswithinanextern"C" linkagespecification,it’snamewillbemangledinthesamewayasit wouldinC. Forthepurposeofbackwardscompatibility,atmostonefunctionwiththesame nameasotheroverloadablefunctionsmayomittheoverloadable attribute.Inthiscase,thefunctionwithouttheoverloadableattribute willnothaveitsnamemangled. Forexample: //NoteswithmanglednamesassumeItaniummangling. intf(int); intf(double)__attribute__((overloadable)); voidfoo(){ f(5);//Emitsacalltof(not_Z1fi,asitwouldwithanoverloadthat //wasmarkedwithoverloadable). f(1.0);//Emitsacallto_Z1fd. } Supportforunmarkedoverloadsisnotpresentinsomeversionsofclang.Youmay queryforitusing__has_extension(overloadable_unmarked). Queryforthisattributewith__has_attribute(overloadable). patchable_function_entry¶ SupportedSyntaxes¶ patchable_function_entry gnu::patchable_function_entry gnu::patchable_function_entry         Yes __attribute__((patchable_function_entry(N,M)))isusedtogenerateMNOPs beforethefunctionentryandN-MNOPsafterthefunctionentry.Thisattribute takesprecedenceoverthecommandlineoption-fpatchable-function-entry=N,M. Mdefaultsto0ifomitted. Thisattributeisonlysupportedon aarch64/aarch64-be/riscv32/riscv64/i386/x86-64targets. preserve_access_index¶ SupportedSyntaxes¶ preserve_access_index clang::preserve_access_index clang::preserve_access_index         Yes Clangsupportsthe__attribute__((preserve_access_index)) attributefortheBPFtarget.Thisattributemaybeattachedtoa structoruniondeclaration,whereif-gisspecified,itenables preservingstructorunionmemberaccessdebuginfoindicesofthis structorunion,similartoclang__builtin_preserve_access_index(). reinitializes¶ SupportedSyntaxes¶ reinitializes clang::reinitializes             Thereinitializesattributecanbeappliedtoanon-static,non-constC++ memberfunctiontoindicatethatthismemberfunctionreinitializestheentire objecttoaknownstate,independentofthepreviousstateoftheobject. Thisattributecanbeinterpretedbystaticanalyzersthatwarnaboutusesofan objectthathasbeenleftinanindeterminatestatebyamoveoperation.Ifa memberfunctionmarkedwiththereinitializesattributeiscalledona moved-fromobject,theanalyzercanconcludethattheobjectisnolongerinan indeterminatestate. Atypicalexamplewherethisattributewouldbeusedisonfunctionsthatclear acontainerclass: template classContainer{ public: ... [[clang::reinitializes]]voidClear(); ... }; release_capability,release_shared_capability¶ SupportedSyntaxes¶ release_capabilityrelease_shared_capabilityrelease_generic_capabilityunlock_function clang::release_capabilityclang::release_shared_capabilityclang::release_generic_capabilityclang::unlock_function             Marksafunctionasreleasingacapability. retain¶ SupportedSyntaxes¶ retain gnu::retain gnu::retain           Thisattribute,whenattachedtoafunctionorvariabledefinition,prevents sectiongarbagecollectioninthelinker.Itdoesnotpreventotherdiscard mechanisms,suchasarchivememberselection,andCOMDATgroupresolution. Ifthecompilerdoesnotemitthedefinition,e.g.becauseitwasnotusedin thetranslationunitorthecompilerwasabletoeliminatealloftheuses, thisattributehasnoeffect.Thisattributeistypicallycombinedwiththe usedattributetoforcethedefinitiontobeemittedandpreservedintothe finallinkedimage. ThisattributeisonlynecessaryonELFtargets;othertargetspreventsection garbagecollectionbythelinkerwhenusingtheusedattributealone. Usingtheattributestogethershouldresultinconsistentbehavioracross targets. ThisattributerequiresthelinkertosupporttheSHF_GNU_RETAINextension. ThissupportisavailableinGNUldandgoldasofbinutils2.36,as wellasinld.lld13. shader¶ SupportedSyntaxes¶                 TheshadertypeattributeappliestoHLSLshaderentryfunctionsto identifytheshadertypefortheentryfunction. Thesyntaxis: ``[shader(string-literal)]`` wherethestringliteralisoneof:“pixel”,“vertex”,“geometry”,“hull”, “domain”,“compute”,“raygeneration”,“intersection”,“anyhit”,“closesthit”, “miss”,“callable”,“mesh”,“amplification”.Normallytheshadertypeisset byshadertargetwiththe-Toptionlike-Tps_6_1.Whencompilingtoa librarytargetlikelib_6_3,theshadertypeattributecanhelpthe compilertoidentifytheshadertype.ItismostlyusedbyRaytracingshaders whereshadersmustbecompiledintoalibraryandlinkedatruntime. short_call,near¶ SupportedSyntaxes¶ short_callnear gnu::short_callgnu::near gnu::short_callgnu::near         Yes Clangsupportsthe__attribute__((long_call)),__attribute__((far)), __attribute__((short__call)),and__attribute__((near))attributes onMIPStargets.Theseattributesmayonlybeaddedtofunctiondeclarations andchangethecodegeneratedbythecompilerwhendirectlycalling thefunction.Theshort_callandnearattributesaresynonymsand allowcallstothefunctiontobemadeusingthejalinstruction,which requiresthefunctiontobelocatedinthesamenaturallyaligned256MBsegment asthecaller.Thelong_callandfarattributesaresynonymsand requiretheuseofadifferentcallsequencethatworksregardless ofthedistancebetweenthefunctions. Theseattributeshavenoeffectforposition-independentcode. Theseattributestakepriorityovercommandlineswitchessuch as-mlong-callsand-mno-long-calls. signal¶ SupportedSyntaxes¶ signal gnu::signal gnu::signal         Yes ClangsupportstheGNUstyle__attribute__((signal))attributeon AVRtargets.Thisattributemaybeattachedtoafunctiondefinitionandinstructs thebackendtogenerateappropriatefunctionentry/exitcodesothatitcanbeused directlyasaninterruptserviceroutine. Interrupthandlerfunctionsdefinedwiththesignalattributedonotre-enableinterrupts. speculative_load_hardening¶ SupportedSyntaxes¶ speculative_load_hardening clang::speculative_load_hardening clang::speculative_load_hardening         Yes Thisattributecanbeappliedtoafunctiondeclarationinordertoindicate thatSpeculativeLoadHardening shouldbeenabledforthefunctionbody.Thiscanalsobeappliedtoamethod inObjectiveC.Thisattributewilltakeprecedenceoverthecommandlineflagin thecasewhere-mno-speculative-load-hardeningisspecified. SpeculativeLoadHardeningisabest-effortmitigationagainst informationleakattacksthatmakeuseofcontrolflow miss-speculation-specificallymiss-speculationofwhetherabranch istakenornot.Typicallyvulnerabilitiesenablingsuchattacksare classifiedas“Spectrevariant#1”.Notably,thisdoesnotattemptto mitigateagainstmiss-speculationofbranchtarget,classifiedas “Spectrevariant#2”vulnerabilities. Wheninlining,theattributeissticky.Inliningafunctionthat carriesthisattributewillcausethecallertogainthe attribute.Thisisintendedtoprovideamaximallyconservativemodel wherethecodeinafunctionannotatedwiththisattributewillalways (evenafterinlining)enduphardened. sycl_kernel¶ SupportedSyntaxes¶ sycl_kernel clang::sycl_kernel clang::sycl_kernel           Thesycl_kernelattributespecifiesthatafunctiontemplatewillbeused tooutlinedevicecodeandtogenerateanOpenCLkernel. HereisacodeexampleoftheSYCLprogram,whichdemonstratesthecompiler’s outliningjob: intfoo(intx){return++x;} usingnamespacecl::sycl; queueQ; buffera(range<1>{1024}); Q.submit([&](handler&cgh){ autoA=a.get_access<:mode::write>(cgh); cgh.parallel_for(range<1>{1024},[=](id<1>index){ A[index]=index[0]+foo(42); }); } AC++functionobjectpassedtotheparallel_foriscalleda“SYCLkernel”. ASYCLkerneldefinestheentrypointtothe“devicepart”ofthecode.The compilerwillemitallsymbolsaccessiblefroma“kernel”.Inthiscode example,thecompilerwillemit“foo”function.Moredetailsaboutthe compilationoffunctionsforthedevicepartcanbefoundintheSYCL1.2.1 specificationSection6.4. Toshowtothecompilerentrypointtothe“devicepart”ofthecode,theSYCL runtimecanusethesycl_kernelattributeinthefollowingway: namespacecl{ namespacesycl{ classhandler{ template __attribute__((sycl_kernel))voidsycl_kernel_function(KernelTypeKernelFuncObj){ //... KernelFuncObj(); } template voidparallel_for(rangeNumWorkItems,KernelTypeKernelFunc){ #ifdef__SYCL_DEVICE_ONLY__ sycl_kernel_function(KernelFunc); #else //Hostimplementation #endif } }; }//namespacesycl }//namespacecl ThecompilerwillalsogenerateanOpenCLkernelusingthefunctionmarkedwith thesycl_kernelattribute. HereisthelistofSYCLdevicecompilerexpectationswithregardtothe functionmarkedwiththesycl_kernelattribute: Thefunctionmustbeatemplatewithatleasttwotypetemplateparameters. ThecompilergeneratesanOpenCLkernelandusesthefirsttemplateparameter asauniquenameforthegeneratedOpenCLkernel.Thehostapplicationuses thisuniquenametoinvoketheOpenCLkernelgeneratedfortheSYCLkernel specializedbythisnameandsecondtemplateparameterKernelType(which mightbeanunnamedfunctionobjecttype). Thefunctionmusthaveatleastoneparameter.Thefirstparameteris requiredtobeafunctionobjecttype(namedorunnamedi.e.lambda).The compilerusesfunctionobjecttypefieldstogenerateOpenCLkernel parameters. Thefunctionmustreturnvoid.Thecompilerreusesthebodyofmarkedfunctionsto generatetheOpenCLkernelbody,andtheOpenCLkernelmustreturnvoid. TheSYCLkernelinthepreviouscodesamplemeetstheseexpectations. target¶ SupportedSyntaxes¶ target gnu::target gnu::target         Yes ClangsupportstheGNUstyle__attribute__((target("OPTIONS")))attribute. Thisattributemaybeattachedtoafunctiondefinitionandinstructs thebackendtousedifferentcodegenerationoptionsthanwerepassedonthe commandline. Thecurrentsetofoptionscorrespondtotheexisting“subtargetfeatures”for thetargetwithorwithouta“-mno-”infrontcorrespondingtotheabsence ofthefeature,aswellasarch="CPU"whichwillchangethedefault“CPU” forthefunction. ForX86,theattributealsoallowstune="CPU"tooptimizethegenerated codeforthegivenCPUwithoutchangingtheavailableinstructions. ForAArch64,theattributealsoallowsthe“branch-protection=”option, wherethepermissibleargumentsandtheireffectoncodegenerationarethesame asforthecommand-lineoption-mbranch-protection. Example“subtargetfeatures”fromthex86backendinclude:“mmx”,“sse”,“sse4.2”, “avx”,“xop”andlargelycorrespondtothemachinespecificoptionshandledby thefrontend. Additionally,thisattributesupportsfunctionmultiversioningforELFbased x86/x86-64targets,whichcanbeusedtocreatemultipleimplementationsofthe samefunctionthatwillberesolvedatruntimebasedonthepriorityoftheir targetattributestrings.Afunctionisconsideredamultiversionedfunction ifeithertwodeclarationsofthefunctionhavedifferenttargetattribute strings,orifithasatargetattributestringofdefault.For example: __attribute__((target("arch=atom"))) voidfoo(){}//willbecalledon'atom'processors. __attribute__((target("default"))) voidfoo(){}//willbecalledonanyotherprocessors. Allmultiversionedfunctionsmustcontainadefault(fallback) implementation,otherwiseusagesofthefunctionareconsideredinvalid. Additionally,afunctionmaynotbecomemultiversionedafteritsfirstuse. target_clones¶ SupportedSyntaxes¶ target_clones gnu::target_clones gnu::target_clones         Yes Clangsupportsthetarget_clones("OPTIONS")attribute.Thisattributemaybe attachedtoafunctiondeclarationandcausesfunctionmultiversioning,where multipleversionsofthefunctionwillbeemittedwithdifferentcode generationoptions.Additionally,theseversionswillberesolvedatruntime basedonthepriorityoftheirattributeoptions.Alltarget_clonefunctions areconsideredmultiversionedfunctions. Allmultiversionedfunctionsmustcontainadefault(fallback) implementation,otherwiseusagesofthefunctionareconsideredinvalid. Additionally,afunctionmaynotbecomemultiversionedafteritsfirstuse. Theoptionstotarget_clonescaneitherbeatarget-specificarchitecture (specifiedasarch=CPU),oroneofalistofsubtargetfeatures. Example“subtargetfeatures”fromthex86backendinclude:“mmx”,“sse”,“sse4.2”, “avx”,“xop”andlargelycorrespondtothemachinespecificoptionshandledby thefrontend. Theversionscaneitherbelistedasacomma-separatedsequenceofstring literalsorasasinglestringliteralcontainingacomma-separatedlistof versions.ForcompatibilitywithGCC,thetwoformatscanbemixed.For example,thefollowingwillemit4versionsofthefunction: __attribute__((target_clones("arch=atom,avx2","arch=ivybridge","default"))) voidfoo(){} try_acquire_capability,try_acquire_shared_capability¶ SupportedSyntaxes¶ try_acquire_capabilitytry_acquire_shared_capability clang::try_acquire_capabilityclang::try_acquire_shared_capability             Marksafunctionthatattemptstoacquireacapability.Thisfunctionmayfailto actuallyacquirethecapability;theyacceptaBooleanvaluedetermining whetheracquiringthecapabilitymeanssuccess(true),orfailingtoacquire thecapabilitymeanssuccess(false). used¶ SupportedSyntaxes¶ used gnu::used gnu::used           Thisattribute,whenattachedtoafunctionorvariabledefinition,indicates thattheremaybereferencestotheentitywhicharenotapparentinthesource code.Forexample,itmaybereferencedfrominlineasm,oritmaybe foundthroughadynamicsymbolorsectionlookup. Thecompilermustemitthedefinitionevenifitappearstobeunused,andit mustnotapplyoptimizationswhichdependonfullyunderstandinghowtheentity isused. Whetherthisattributehasanyeffectonthelinkerdependsonthetargetand thelinker.Mostlinkerssupportthefeatureofsectiongarbagecollection (--gc-sections),alsoknownas“deadstripping”(ld64-dead_strip)or discardingunreferencedsections(link.exe/OPT:REF).OnCOFFandMach-O targets(WindowsandAppleplatforms),theusedattributepreventssymbols frombeingremovedbylinkersectionGC.OnELFtargets,ithasnoeffectonits own,andthelinkermayremovethedefinitionifitisnototherwisereferenced. ThislinkerGCcanbeavoidedbyalsoaddingtheretainattribute.Note thatretainrequiresspecialsupportfromthelinker;seethatattribute’s documentationforfurtherinformation. xray_always_instrument,xray_never_instrument,xray_log_args¶ SupportedSyntaxes¶ xray_always_instrumentxray_never_instrument clang::xray_always_instrumentclang::xray_never_instrument clang::xray_always_instrumentclang::xray_never_instrument         Yes __attribute__((xray_always_instrument))or [[clang::xray_always_instrument]]isusedtomarkmemberfunctions(inC++), methods(inObjectiveC),andfreefunctions(inC,C++,andObjectiveC)tobe instrumentedwithXRay.Thiswillcausethefunctiontoalwayshavespaceat thebeginningandexitpointstoallowforruntimepatching. Conversely,__attribute__((xray_never_instrument))or [[clang::xray_never_instrument]]willinhibittheinsertionofthese instrumentationpoints. Ifafunctionhasneitheroftheseattributes,theybecomesubjecttotheXRay heuristicsusedtodeterminewhetherafunctionshouldbeinstrumentedor otherwise. __attribute__((xray_log_args(N)))or[[clang::xray_log_args(N)]]is usedtopreserveNfunctionargumentsfortheloggingfunction.Currently, onlyN==1issupported. xray_always_instrument,xray_never_instrument,xray_log_args¶ SupportedSyntaxes¶ xray_log_args clang::xray_log_args clang::xray_log_args         Yes __attribute__((xray_always_instrument))or [[clang::xray_always_instrument]]isusedtomarkmemberfunctions(inC++), methods(inObjectiveC),andfreefunctions(inC,C++,andObjectiveC)tobe instrumentedwithXRay.Thiswillcausethefunctiontoalwayshavespaceat thebeginningandexitpointstoallowforruntimepatching. Conversely,__attribute__((xray_never_instrument))or [[clang::xray_never_instrument]]willinhibittheinsertionofthese instrumentationpoints. Ifafunctionhasneitheroftheseattributes,theybecomesubjecttotheXRay heuristicsusedtodeterminewhetherafunctionshouldbeinstrumentedor otherwise. __attribute__((xray_log_args(N)))or[[clang::xray_log_args(N)]]is usedtopreserveNfunctionargumentsfortheloggingfunction.Currently, onlyN==1issupported. zero_call_used_regs¶ SupportedSyntaxes¶ zero_call_used_regs gnu::zero_call_used_regs gnu::zero_call_used_regs         Yes Thisattribute,whenattachedtoafunction,causesthecompilertozeroa subsetofallcall-usedregistersbeforethefunctionreturns.It’susedto increaseprogramsecuritybyeithermitigatingReturn-OrientedProgramming (ROP)attacksorpreventinginformationleakagethroughregisters. Theterm“call-used”meansregisterswhicharenotguaranteedtobepreserved unchangedforthecallerbythecurrentcallingconvention.Thiscouldalsobe describedas“caller-saved”or“notcallee-saved”. Thechoiceparametersgivestheprogrammerflexibilitytochoosethesubset ofthecall-usedregisterstobezeroed: skipdoesn’tzeroanycall-usedregisters.Thischoiceoverridesany command-linearguments. usedonlyzeroscall-usedregistersusedinthefunction.Byused,we meanaregisterwhosecontentshavebeensetorreferencedinthefunction. used-gpronlyzeroscall-usedGPRregistersusedinthefunciton. used-argonlyzeroscall-usedregistersusedtopassargumentstothe function. used-gpr-argonlyzeroscall-usedGPRregistersusedtopassargumentsto thefunction. allzerosallcall-usedregisters. all-gprzerosallcall-usedGPRregisters. all-argzerosallcall-usedregistersusedtopassargumentstothe function. all-gpr-argzerosallcall-usedGPRregistersusedtopassargumentsto thefunction. Thedefaultfortheattributeiscontolledbythe-fzero-call-used-regs flag. HandleAttributes¶ Handlesareawaytoidentifyresourceslikefiles,sockets,andprocesses. Theyaremoreopaquethanpointersandwidelyusedinsystemprogramming.They havesimilarriskssuchasneverreleasingaresourceassociatedwithahandle, attemptingtouseahandlethatwasalreadyreleased,ortryingtoreleasea handletwice.Usingtheannotationsbelowitispossibletomaketheownership ofthehandlesclear:whoseresponsibilityistoreleasethem.Theycanalso aidstaticanalysistoolstofindbugs. acquire_handle¶ SupportedSyntaxes¶ acquire_handle clang::acquire_handle clang::acquire_handle         Yes Ifthisannotationisonafunctionorafunctiontypeitisassumedtoreturn anewhandle.Incasethisannotationisonanoutputparameter, thefunctionisassumedtofillthecorrespondingargumentwithanew handle.Theattributerequiresastringliteralargumentwhichusedto identifythehandlewithlaterusesofuse_handleor release_handle. //OutputargumentsfromZircon. zx_status_tzx_socket_create(uint32_toptions, zx_handle_t__attribute__((acquire_handle("zircon")))*out0, zx_handle_t*out1[[clang::acquire_handle("zircon")]]); //Returnedhandle. [[clang::acquire_handle("tag")]]intopen(constchar*path,intoflag,...); intopen(constchar*path,intoflag,...)__attribute__((acquire_handle("tag"))); release_handle¶ SupportedSyntaxes¶ release_handle clang::release_handle clang::release_handle         Yes Ifafunctionparameterisannotatedwithrelease_handle(tag)itisassumedto closethehandle.Itisalsoassumedtorequireanopenhandletoworkwith.The attributerequiresastringliteralargumenttoidentifythehandlebeingreleased. zx_status_tzx_handle_close(zx_handle_thandle[[clang::release_handle("tag")]]); use_handle¶ SupportedSyntaxes¶ use_handle clang::use_handle clang::use_handle         Yes Afunctiontakingahandlebyvaluemightclosethehandle.Ifafunction parameterisannotatedwithuse_handle(tag)itisassumedtonottochange thestateofthehandle.Itisalsoassumedtorequireanopenhandletoworkwith. Theattributerequiresastringliteralargumenttoidentifythehandlebeingused. zx_status_tzx_port_wait(zx_handle_thandle[[clang::use_handle("zircon")]], zx_time_tdeadline, zx_port_packet_t*packet); NullabilityAttributes¶ Whetheraparticularpointermaybe“null”isanimportantconcernwhenworking withpointersintheCfamilyoflanguages.Thevariousnullabilityattributes indicatewhetheraparticularpointercanbenullornot,whichmakesAPIsmore expressiveandcanhelpstaticanalysistoolsidentifybugsinvolvingnull pointers.Clangsupportsseveralkindsofnullabilityattributes:the nonnullandreturns_nonnullattributesindicatewhichfunctionor methodparametersandresulttypescanneverbenull,whilenullabilitytype qualifiersindicatewhichpointertypescanbenull(_Nullable)orcannot benull(_Nonnull). Thenullability(type)qualifiersexpresswhetheravalueofagivenpointer typecanbenull(the_Nullablequalifier),doesn’thaveadefinedmeaning fornull(the_Nonnullqualifier),orforwhichthepurposeofnullis unclear(the_Null_unspecifiedqualifier).Becausenullabilityqualifiers areexpressedwithinthetypesystem,theyaremoregeneralthanthe nonnullandreturns_nonnullattributes,allowingonetoexpress(for example)anullablepointertoanarrayofnonnullpointers.Nullability qualifiersarewrittentotherightofthepointertowhichtheyapply.For example: //Nomeaningfulresultwhen'ptr'isnull(here,ithappenstobeundefinedbehavior). intfetch(int*_Nonnullptr){return*ptr;} //'ptr'maybenull. intfetch_or_zero(int*_Nullableptr){ returnptr?*ptr:0; } //Anullablepointertonon-nullpointerstoconstcharacters. constchar*join_strings(constchar*_Nonnull*_Nullablestrings,unsignedn); InObjective-C,thereisanalternatespellingforthenullabilityqualifiers thatcanbeusedinObjective-Cmethodsandpropertiesusingcontext-sensitive, non-underscoredkeywords.Forexample: @interfaceNSView:NSResponder -(nullableNSView*)ancestorSharedWithView:(nonnullNSView*)aView; @property(assign,nullable)NSView*superview; @property(readonly,nonnull)NSArray*subviews; @end _Nonnull¶ SupportedSyntaxes¶         _Nonnull       The_Nonnullnullabilityqualifierindicatesthatnullisnotameaningful valueforavalueofthe_Nonnullpointertype.Forexample,givena declarationsuchas: intfetch(int*_Nonnullptr); acalleroffetchshouldnotprovideanullvalue,andthecompilerwill produceawarningifitseesaliteralnullvaluepassedtofetch.Note that,unlikethedeclarationattributenonnull,thepresenceof _Nonnulldoesnotimplythatpassingnullisundefinedbehavior:fetch isfreetoconsidernullundefinedbehavioror(perhapsfor backward-compatibilityreasons)defensivelyhandlenull. _Null_unspecified¶ SupportedSyntaxes¶         _Null_unspecified       The_Null_unspecifiednullabilityqualifierindicatesthatneitherthe _Nonnullnor_Nullablequalifiersmakesenseforaparticularpointer type.Itisusedprimarilytoindicatethattheroleofnullwithspecific pointersinanullability-annotatedheaderisunclear,e.g.,dueto overly-compleximplementationsorhistoricalfactorswithalong-livedAPI. _Nullable¶ SupportedSyntaxes¶         _Nullable       The_Nullablenullabilityqualifierindicatesthatavalueofthe _Nullablepointertypecanbenull.Forexample,given: intfetch_or_zero(int*_Nullableptr); acalleroffetch_or_zerocanprovidenull. _Nullable_result¶ SupportedSyntaxes¶         _Nullable_result       The_Nullable_resultnullabilityqualifiermeansthatavalueofthe _Nullable_resultpointercanbenil,justlike_Nullable.Wherethis attributediffersfrom_Nullableiswhenit’susedonaparametertoa completionhandlerinaSwiftasyncmethod.Forinstance,here: -(void)fetchSomeDataWithID:(int)identifier completionHandler:(void(^)(Data*_Nullable_resultresult,NSError*error))completionHandler; ThismethodasynchronouslycallscompletionHandlerwhenthedatais available,orcallsitwithanerror._Nullable_resultindicatestothe Swiftimporterthatthisistheuncommoncasewhereresultcangetnil evenifnoerrorhasoccurred,andwillthereforeimportitasaSwiftoptional type.Otherwise,ifresultwasannotatedwith_Nullable,theSwift importerwillassumethatresultwillalwaysbenon-nilunlessanerror occurred. nonnull¶ SupportedSyntaxes¶ nonnull gnu::nonnull gnu::nonnull           Thenonnullattributeindicatesthatsomefunctionparametersmustnotbe null,andcanbeusedinseveraldifferentways.It’soriginalusage (fromGCC) isasafunction(orObjective-Cmethod)attributethatspecifieswhich parametersofthefunctionarenonnullinacomma-separatedlist.Forexample: externvoid*my_memcpy(void*dest,constvoid*src,size_tlen) __attribute__((nonnull(1,2))); Here,thenonnullattributeindicatesthatparameters1and2 cannothaveanullvalue.Omittingtheparenthesizedlistofparameterindices meansthatallparametersofpointertypecannotbenull: externvoid*my_memcpy(void*dest,constvoid*src,size_tlen) __attribute__((nonnull)); Clangalsoallowsthenonnullattributetobeplaceddirectlyonafunction (orObjective-Cmethod)parameter,eliminatingtheneedtospecifythe parameterindexaheadoftype.Forexample: externvoid*my_memcpy(void*dest__attribute__((nonnull)), constvoid*src__attribute__((nonnull)),size_tlen); Notethatthenonnullattributeindicatesthatpassingnulltoanon-null parameterisundefinedbehavior,whichtheoptimizermaytakeadvantageofto, e.g.,removenullchecks.The_Nonnulltypequalifierindicatesthata pointercannotbenullinamoregeneralmanner(becauseitispartofthetype system)anddoesnotimplyundefinedbehavior,makingitmorewidelyapplicable. returns_nonnull¶ SupportedSyntaxes¶ returns_nonnull gnu::returns_nonnull gnu::returns_nonnull         Yes Thereturns_nonnullattributeindicatesthataparticularfunction(or Objective-Cmethod)alwaysreturnsanon-nullpointer.Forexample,a particularsystemmallocmightbedefinedtoterminateaprocesswhen memoryisnotavailableratherthanreturninganullpointer: externvoid*malloc(size_tsize)__attribute__((returns_nonnull)); Thereturns_nonnullattributeimpliesthatreturninganullpointeris undefinedbehavior,whichtheoptimizermaytakeadvantageof.The_Nonnull typequalifierindicatesthatapointercannotbenullinamoregeneralmanner (becauseitispartofthetypesystem)anddoesnotimplyundefinedbehavior, makingitmorewidelyapplicable OpenCLAddressSpaces¶ Theaddressspacequalifiermaybeusedtospecifytheregionofmemorythatis usedtoallocatetheobject.OpenCLsupportsthefollowingaddressspaces: __generic(generic),__global(global),__local(local),__private(private), __constant(constant). __constantintc=...; __genericint*foo(globalint*g){ __localint*l; privateintp; ... returnl; } MoredetailscanbefoundintheOpenCLClanguageSpecv2.0,Section6.5. [[clang::opencl_global_device]],[[clang::opencl_global_host]]¶ SupportedSyntaxes¶ opencl_global_device clang::opencl_global_device clang::opencl_global_device           Theglobal_deviceandglobal_hostaddressspaceattributesspecifythat anobjectisallocatedinglobalmemoryonthedevice/host.Ithelpsto distinguishUSM(UnifiedSharedMemory)pointersthataccessglobaldevice memoryfromthosethataccessglobalhostmemory.Thesenewaddressspacesare asubsetofthe__global/opencl_globaladdressspace,thefulladdressspace setmodelforOpenCL2.0withtheextensionlooksasfollows: generic->global->host ->device ->private ->local constant Asglobal_deviceandglobal_hostareasubsetof __global/opencl_globaladdressspacesitisallowedtoconvert global_deviceandglobal_hostaddressspacesto __global/opencl_globaladdressspaces(followingISO/IECTR180375.1.3 “Addressspacenestingandrulesforpointers”). [[clang::opencl_global_device]],[[clang::opencl_global_host]]¶ SupportedSyntaxes¶ opencl_global_host clang::opencl_global_host clang::opencl_global_host           Theglobal_deviceandglobal_hostaddressspaceattributesspecifythat anobjectisallocatedinglobalmemoryonthedevice/host.Ithelpsto distinguishUSM(UnifiedSharedMemory)pointersthataccessglobaldevice memoryfromthosethataccessglobalhostmemory.Thesenewaddressspacesare asubsetofthe__global/opencl_globaladdressspace,thefulladdressspace setmodelforOpenCL2.0withtheextensionlooksasfollows: generic->global->host ->device ->private ->local constant Asglobal_deviceandglobal_hostareasubsetof __global/opencl_globaladdressspacesitisallowedtoconvert global_deviceandglobal_hostaddressspacesto __global/opencl_globaladdressspaces(followingISO/IECTR180375.1.3 “Addressspacenestingandrulesforpointers”). __constant,constant,[[clang::opencl_constant]]¶ SupportedSyntaxes¶ opencl_constant clang::opencl_constant clang::opencl_constant   __constantconstant       Theconstantaddressspaceattributesignalsthatanobjectislocatedin aconstant(non-modifiable)memoryregion.Itisavailabletoallworkitems. Anytypecanbeannotatedwiththeconstantaddressspaceattribute.Objects withtheconstantaddressspacequalifiercanbedeclaredinanyscopeandmust haveaninitializer. __generic,generic,[[clang::opencl_generic]]¶ SupportedSyntaxes¶ opencl_generic clang::opencl_generic clang::opencl_generic   __genericgeneric       ThegenericaddressspaceattributeisonlyavailablewithOpenCLv2.0andlater. Itcanbeusedwithpointertypes.Variablesinglobalandlocalscopeand functionparametersinnon-kernelfunctionscanhavethegenericaddressspace typeattribute.Itisintendedtobeaplaceholderforanyotheraddressspace exceptfor‘__constant’inOpenCLcodewhichcanbeusedwithmultipleaddress spaces. __global,global,[[clang::opencl_global]]¶ SupportedSyntaxes¶ opencl_global clang::opencl_global clang::opencl_global   __globalglobal       Theglobaladdressspaceattributespecifiesthatanobjectisallocatedin globalmemory,whichisaccessiblebyallworkitems.Thecontentstoredinthis memoryareapersistsbetweenkernelexecutions.Pointertypestotheglobal addressspaceareallowedasfunctionparametersorlocalvariables.Starting withOpenCLv2.0,theglobaladdressspacecanbeusedwithglobal(program scope)variablesandstaticlocalvariableaswell. __local,local,[[clang::opencl_local]]¶ SupportedSyntaxes¶ opencl_local clang::opencl_local clang::opencl_local   __locallocal       Thelocaladdressspacespecifiesthatanobjectisallocatedinthelocal(work group)memoryarea,whichisaccessibletoallworkitemsinthesamework group.Thecontentstoredinthismemoryregionisnotaccessibleafter thekernelexecutionends.Inakernelfunctionscope,anyvariablecanbein thelocaladdressspace.Inotherscopes,onlypointertypestothelocaladdress spaceareallowed.Localaddressspacevariablescannothaveaninitializer. __private,private,[[clang::opencl_private]]¶ SupportedSyntaxes¶ opencl_private clang::opencl_private clang::opencl_private   __privateprivate       Theprivateaddressspacespecifiesthatanobjectisallocatedintheprivate (workitem)memory.Otherworkitemscannotaccessthesamememoryareaandits contentisdestroyedafterworkitemexecutionends.Localvariablescanbe declaredintheprivateaddressspace.Functionargumentsarealwaysinthe privateaddressspace.Kernelfunctionargumentsofapointeroranarraytype cannotpointtotheprivateaddressspace. StatementAttributes¶ #pragmaclangloop¶ SupportedSyntaxes¶           clangloopunrollnounrollunroll_and_jamnounroll_and_jam     The#pragmaclangloopdirectiveallowsloopoptimizationhintstobe specifiedforthesubsequentloop.Thedirectiveallowspipeliningtobe disabled,orvectorization,vectorpredication,interleaving,andunrollingto beenabledordisabled.Vectorwidth,vectorpredication,interleavecount, unrollingcount,andtheinitiationintervalforpipeliningcanbeexplicitly specified.Seelanguageextensions fordetails. #pragmaunroll,#pragmanounroll¶ SupportedSyntaxes¶           clangloopunrollnounrollunroll_and_jamnounroll_and_jam     Loopunrollingoptimizationhintscanbespecifiedwith#pragmaunrolland #pragmanounroll.Thepragmaisplacedimmediatelybeforeafor,while, do-while,orc++11range-basedforloop.GCC’sloopunrollinghints #pragmaGCCunrolland#pragmaGCCnounrollarealsosupportedandhave identicalsemanticsto#pragmaunrolland#pragmanounroll. Specifying#pragmaunrollwithoutaparameterdirectstheloopunrollerto attempttofullyunrolltheloopifthetripcountisknownatcompiletimeand attempttopartiallyunrolltheloopifthetripcountisnotknownatcompile time: #pragmaunroll for(...){ ... } Specifyingtheoptionalparameter,#pragmaunroll_value_,directsthe unrollertounrolltheloop_value_times.Theparametermayoptionallybe enclosedinparentheses: #pragmaunroll16 for(...){ ... } #pragmaunroll(16) for(...){ ... } Specifying#pragmanounrollindicatesthattheloopshouldnotbeunrolled: #pragmanounroll for(...){ ... } #pragmaunrolland#pragmaunroll_value_haveidenticalsemanticsto #pragmaclangloopunroll(full)and #pragmaclangloopunroll_count(_value_)respectively.#pragmanounroll isequivalentto#pragmaclangloopunroll(disable).See languageextensions forfurtherdetailsincludinglimitationsoftheunrollhints. __read_only,__write_only,__read_write(read_only,write_only,read_write)¶ SupportedSyntaxes¶         __read_onlyread_only__write_onlywrite_only__read_writeread_write       Theaccessqualifiersmustbeusedwithimageobjectargumentsorpipearguments todeclareiftheyarebeingreadorwrittenbyakernelorfunction. Theread_only/__read_only,write_only/__write_onlyandread_write/__read_write namesarereservedforuseasaccessqualifiersandshallnotbeusedotherwise. kernelvoid foo(read_onlyimage2d_timageA, write_onlyimage2d_timageB){ ... } IntheaboveexampleimageAisaread-only2Dimageobject,andimageBisa write-only2Dimageobject. Theread_write(or__read_write)qualifiercannotbeusedwithpipe. MoredetailscanbefoundintheOpenCLClanguageSpecv2.0,Section6.6. fallthrough¶ SupportedSyntaxes¶ fallthrough fallthroughclang::fallthroughgnu::fallthrough fallthroughgnu::fallthrough           Thefallthrough(orclang::fallthrough)attributeisused toannotateintentionalfall-through betweenswitchlabels.Itcanonlybeappliedtoanullstatementplacedata pointofexecutionbetweenanystatementandthenextswitchlabel.Itis commontomarktheseplaceswithaspecificcomment,butthisattributeis meanttoreplacecommentswithamorestrictannotation,whichcanbechecked bythecompiler.Thisattributedoesn’tchangesemanticsofthecodeandcan beusedwhereveranintendedfall-throughoccurs.Itisdesignedtomimic control-flowstatementslikebreak;,soitcanbeplacedinmostplaces wherebreak;can,butonlyiftherearenostatementsontheexecutionpath betweenitandthenextswitchlabel. Bydefault,Clangdoesnotwarnonunannotatedfallthroughfromoneswitch casetoanother.Diagnosticsonfallthroughwithoutacorrespondingannotation canbeenabledwiththe-Wimplicit-fallthroughargument. Hereisanexample: //compilewith-Wimplicit-fallthrough switch(n){ case22: case33://nowarning:nostatementsbetweencaselabels f(); case44://warning:unannotatedfall-through g(); [[clang::fallthrough]]; case55://nowarning if(x){ h(); break; } else{ i(); [[clang::fallthrough]]; } case66://nowarning p(); [[clang::fallthrough]];//warning:fallthroughannotationdoesnot //directlyprecedecaselabel q(); case77://warning:unannotatedfall-through r(); } intel_reqd_sub_group_size¶ SupportedSyntaxes¶ intel_reqd_sub_group_size             Yes Theoptionalattributeintel_reqd_sub_group_sizecanbeusedtoindicatethat thekernelmustbecompiledandexecutedwiththespecifiedsubgroupsize.When thisattributeispresent,get_max_sub_group_size()isguaranteedtoreturnthe specifiedintegervalue.Thisisimportantforthecorrectnessofmanysubgroup algorithms,andinsomecasesmaybeusedbythecompilertogeneratemoreoptimal code.Seecl_intel_required_subgroup_size fordetails. likelyandunlikely¶ SupportedSyntaxes¶   likely clang::likely           Thelikelyandunlikelyattributesareusedascompilerhints. Theattributesareusedtoaidthecompilertodeterminewhichbranchis likelyorunlikelytobetaken.Thisisdonebymarkingthebranchsubstatement withoneofthetwoattributes. Itisn’tallowedtoannotateasinglestatementwithbothlikelyand unlikely.Annotatingthetrueandfalsebranchofanif statementwiththesamelikelihoodattributewillresultinadiagnosticand theattributesareignoredonbothbranches. Inaswitchstatementit’sallowedtoannotatemultiplecaselabels orthedefaultlabelwiththesamelikelihoodattribute.Thismakes *alllabelswithoutanattributehaveaneutrallikelihood, *alllabelsmarked[[likely]]haveanequallypositivelikelihood,and *alllabelsmarked[[unlikely]]haveanequallynegativelikelihood. Theneutrallikelihoodisthemorelikelyofpathexecutionthanthenegative likelihood.Thepositivelikelihoodisthemorelikelyofpathofexecution thantheneutrallikelihood. Theseattributeshavenoeffectonthegeneratedcodewhenusing PGO(Profile-GuidedOptimization)oratoptimizationlevel0. InClang,theattributeswillbeignoredifthey’renotplacedon *thecaseordefaultlabelofaswitchstatement, *oronthesubstatementofaniforelsestatement, *oronthesubstatementofanfororwhilestatement. TheC++Standardrecommendstohonorthemoneverystatementinthe pathofexecution,butthatcanbeconfusing: if(b){ [[unlikely]]--b;//Inthepathofexecution, //thisbranchisconsideredunlikely. } if(b){ --b; if(b) return; [[unlikely]]--b;//Notinthepathofexecution, }//thebranchhasnolikelihoodinformation. if(b){ --b; foo(b); //Whetherornotthenextstatementisinthepathofexecutiondepends //onthedeclarationoffoo(): //Inthepathofexecution:voidfoo(int); //Notinthepathofexecution:[[noreturn]]voidfoo(int); //Thismeansthelikelihoodofthebranchdependsonthedeclaration //offoo(). [[unlikely]]--b; } Belowaresomeexampleusagesofthelikelihoodattributesandtheireffects: if(b)[[likely]]{//Placementonthefirststatementinthebranch. //Thecompilerwilloptimizetoexecutethecodehere. }else{ } if(b) [[unlikely]]b++;//Placementonthefirststatementinthebranch. else{ //Thecompilerwilloptimizetoexecutethecodehere. } if(b){ [[unlikely]]b++;//Placementonthesecondstatementinthebranch. }//Theattributewillbeignored. if(b)[[likely]]{ [[unlikely]]b++;//Nocontradictionsincethesecondattribute }//isignored. if(b) ; else[[likely]]{ //Thecompilerwilloptimizetoexecutethecodehere. } if(b) ; else //Thecompilerwilloptimizetoexecutethenextstatement. [[likely]]b=f(); if(b)[[likely]];//Bothbranchesarelikely.Adiagnosticisissued else[[likely]];//andtheattributesareignored. if(b) [[likely]]inti=5;//Issuesadiagnosticsincetheattribute //isn'tallowedonadeclaration. switch(i){ [[likely]]case1://Thisvalueislikely ... break; [[unlikely]]case2://Thisvalueisunlikely ... [[fallthrough]]; case3://Nolikelihoodattribute ... [[likely]]break;//Noeffect case4:[[likely]]{//attributeonsubstatementhasnoeffect ... break; } [[unlikely]]default://Allothervaluesareunlikely ... break; } switch(i){ [[likely]]case0://Thisvalueandcodepathislikely ... [[fallthrough]]; case1://Nolikelihoodattribute,codepathisneutral break;//fallingthroughhasnoeffectonthelikelihood case2://Nolikelihoodattribute,codepathisneutral [[fallthrough]]; [[unlikely]]default://Thisvalueandcodepatharebothunlikely break; } for(inti=0;i!=size;++i)[[likely]]{ ...//Theloopisthelikelypathofexecution } for(constauto&E:Elements)[[likely]]{ ...//Theloopisthelikelypathofexecution } while(i!=size)[[unlikely]]{ ...//Theloopistheunlikelypathofexecution }//Thegeneratedcodewilloptimizetoskiptheloopbody while(true)[[unlikely]]{ ...//Theattributehasnoeffect }//Clangelidesthecomparisonandgeneratesaninfinite //loop likelyandunlikely¶ SupportedSyntaxes¶   unlikely clang::unlikely           Thelikelyandunlikelyattributesareusedascompilerhints. Theattributesareusedtoaidthecompilertodeterminewhichbranchis likelyorunlikelytobetaken.Thisisdonebymarkingthebranchsubstatement withoneofthetwoattributes. Itisn’tallowedtoannotateasinglestatementwithbothlikelyand unlikely.Annotatingthetrueandfalsebranchofanif statementwiththesamelikelihoodattributewillresultinadiagnosticand theattributesareignoredonbothbranches. Inaswitchstatementit’sallowedtoannotatemultiplecaselabels orthedefaultlabelwiththesamelikelihoodattribute.Thismakes *alllabelswithoutanattributehaveaneutrallikelihood, *alllabelsmarked[[likely]]haveanequallypositivelikelihood,and *alllabelsmarked[[unlikely]]haveanequallynegativelikelihood. Theneutrallikelihoodisthemorelikelyofpathexecutionthanthenegative likelihood.Thepositivelikelihoodisthemorelikelyofpathofexecution thantheneutrallikelihood. Theseattributeshavenoeffectonthegeneratedcodewhenusing PGO(Profile-GuidedOptimization)oratoptimizationlevel0. InClang,theattributeswillbeignoredifthey’renotplacedon *thecaseordefaultlabelofaswitchstatement, *oronthesubstatementofaniforelsestatement, *oronthesubstatementofanfororwhilestatement. TheC++Standardrecommendstohonorthemoneverystatementinthe pathofexecution,butthatcanbeconfusing: if(b){ [[unlikely]]--b;//Inthepathofexecution, //thisbranchisconsideredunlikely. } if(b){ --b; if(b) return; [[unlikely]]--b;//Notinthepathofexecution, }//thebranchhasnolikelihoodinformation. if(b){ --b; foo(b); //Whetherornotthenextstatementisinthepathofexecutiondepends //onthedeclarationoffoo(): //Inthepathofexecution:voidfoo(int); //Notinthepathofexecution:[[noreturn]]voidfoo(int); //Thismeansthelikelihoodofthebranchdependsonthedeclaration //offoo(). [[unlikely]]--b; } Belowaresomeexampleusagesofthelikelihoodattributesandtheireffects: if(b)[[likely]]{//Placementonthefirststatementinthebranch. //Thecompilerwilloptimizetoexecutethecodehere. }else{ } if(b) [[unlikely]]b++;//Placementonthefirststatementinthebranch. else{ //Thecompilerwilloptimizetoexecutethecodehere. } if(b){ [[unlikely]]b++;//Placementonthesecondstatementinthebranch. }//Theattributewillbeignored. if(b)[[likely]]{ [[unlikely]]b++;//Nocontradictionsincethesecondattribute }//isignored. if(b) ; else[[likely]]{ //Thecompilerwilloptimizetoexecutethecodehere. } if(b) ; else //Thecompilerwilloptimizetoexecutethenextstatement. [[likely]]b=f(); if(b)[[likely]];//Bothbranchesarelikely.Adiagnosticisissued else[[likely]];//andtheattributesareignored. if(b) [[likely]]inti=5;//Issuesadiagnosticsincetheattribute //isn'tallowedonadeclaration. switch(i){ [[likely]]case1://Thisvalueislikely ... break; [[unlikely]]case2://Thisvalueisunlikely ... [[fallthrough]]; case3://Nolikelihoodattribute ... [[likely]]break;//Noeffect case4:[[likely]]{//attributeonsubstatementhasnoeffect ... break; } [[unlikely]]default://Allothervaluesareunlikely ... break; } switch(i){ [[likely]]case0://Thisvalueandcodepathislikely ... [[fallthrough]]; case1://Nolikelihoodattribute,codepathisneutral break;//fallingthroughhasnoeffectonthelikelihood case2://Nolikelihoodattribute,codepathisneutral [[fallthrough]]; [[unlikely]]default://Thisvalueandcodepatharebothunlikely break; } for(inti=0;i!=size;++i)[[likely]]{ ...//Theloopisthelikelypathofexecution } for(constauto&E:Elements)[[likely]]{ ...//Theloopisthelikelypathofexecution } while(i!=size)[[unlikely]]{ ...//Theloopistheunlikelypathofexecution }//Thegeneratedcodewilloptimizetoskiptheloopbody while(true)[[unlikely]]{ ...//Theattributehasnoeffect }//Clangelidesthecomparisonandgeneratesaninfinite //loop musttail¶ SupportedSyntaxes¶ musttail clang::musttail clang::musttail           Ifareturnstatementismarkedmusttail,thisindicatesthatthe compilermustgenerateatailcallfortheprogramtobecorrect,evenwhen optimizationsaredisabled.Thisguaranteesthatthecallwillnotcause unboundedstackgrowthifitispartofarecursivecycleinthecallgraph. Ifthecalleeisavirtualfunctionthatisimplementedbyathunk,thereis noguaranteeingeneralthatthethunktail-callstheimplementationofthe virtualfunction,sosuchacallinarecursivecyclecanstillresultin unboundedstackgrowth. clang::musttailcanonlybeappliedtoareturnstatementwhosevalue istheresultofafunctioncall(evenfunctionsreturningvoidmustuse return,althoughnovalueisreturned).Thetargetfunctionmusthavethe samenumberofargumentsasthecaller.Thetypesofthereturnvalueandall argumentsmustbesimilaraccordingtoC++rules(differingonlyincv qualifiersorarraysize),includingtheimplicit“this”argument,ifany. Anyvariablesinscope,includingallargumentstothefunctionandthe returnvaluemustbetriviallydestructible.Thecallingconventionofthe callerandcalleemustmatch,andtheymustnotbevariadicfunctionsorhave oldstyleK&RCfunctiondeclarations. nomerge¶ SupportedSyntaxes¶ nomerge clang::nomerge clang::nomerge         Yes Ifastatementismarkednomergeandcontainscallexpressions,thosecall expressionsinsidethestatementwillnotbemergedduringoptimization.This attributecanbeusedtopreventtheoptimizerfromobscuringthesource locationofcertaincalls.Forexample,itwillpreventtailmergingotherwise identicalcodesequencesthatraiseanexceptionorterminatetheprogram.Tail mergingnormallyreducestheprecisionofsourcelocationinformation,making stacktraceslessusefulfordebugging.Thisattributegivestheusercontrol overthetradeoffbetweencodesizeanddebuginformationprecision. nomergeattributecanalsobeusedasfunctionattributetopreventall callstothespecifiedfunctionfrommerging.Ithasnoeffectonindirect calls. opencl_unroll_hint¶ SupportedSyntaxes¶ opencl_unroll_hint               Theopencl_unroll_hintattributequalifiercanbeusedtospecifythataloop (for,whileanddoloops)canbeunrolled.Thisattributequalifiercanbe usedtospecifyfullunrollingorpartialunrollingbyaspecifiedamount. Thisisacompilerhintandthecompilermayignorethisdirective.See OpenCLv2.0 s6.11.5fordetails. suppress¶ SupportedSyntaxes¶   gsl::suppress             The[[gsl::suppress]]attributesuppressesspecific clang-tidydiagnosticsforrulesoftheC++CoreGuidelinesinaportable way.Theattributecanbeattachedtodeclarations,statements,andat namespacescope. [[gsl::suppress("Rh-public")]] voidf_(){ int*p; [[gsl::suppress("type")]]{ p=reinterpret_cast(7); } } namespaceN{ [[clang::suppress("type","bounds")]]; ... } sycl_special_class¶ SupportedSyntaxes¶ sycl_special_class clang::sycl_special_class clang::sycl_special_class         Yes SYCLdefinessomespecialclasses(accessor,sampler,andstream)whichrequire specifichandlingduringthegenerationoftheSPIRentrypoint. The__attribute__((sycl_special_class))attributeisusedinSYCL headerstoindicatethataclassorastructneedsaspecifichandlingwhen itispassedfromhosttodevice. Specialclasseswillhaveamandatory__initmethodandanoptional __finalizemethod(the__finalizemethodisusedonlywiththe streamtype).Kernelparameterstypesareextractfromthe__initmethod parameters.Thekernelfunctionargumentslistisderivedfromthe argumentsofthe__initmethod.Theargumentsofthe__initmethodare copiedintothekernelfunctionargumentlistandthe__initand __finalizemethodsarecalledatthebeginningandtheendofthekernel, respectively. The__initand__finalizemethodsmustbedefinedinsidethe specialclass. Pleasenotethatthisisanattributethatisusedasaninternal implementationdetailandnotintendedtobeusedbyexternalusers. Thesyntaxoftheattributeisasfollows: class__attribute__((sycl_special_class))accessor{}; class[[clang::sycl_special_class]]accessor{}; Thisisacodeexamplethatillustratestheuseoftheattribute: class__attribute__((sycl_special_class))SpecialType{ intF1; intF2; void__init(intf1){ F1=f1; F2=f1; } void__finalize(){} public: SpecialType()=default; intgetF2()const{returnF2;} }; intmain(){ SpecialTypeT; cgh.single_task([=]{ T.getF2(); }); } ThiswouldtriggerthefollowingkernelentrypointintheAST: void__sycl_kernel(intf1){ SpecialTypeT; T.__init(f1); ... T.__finalize() } TypeAttributes¶ __ptr32¶ SupportedSyntaxes¶         __ptr32       The__ptr32qualifierrepresentsanativepointerona32-bitsystem.Ona 64-bitsystem,apointerwith__ptr32isextendedtoa64-bitpointer.The __sptrand__uptrqualifierscanbeusedtospecifywhetherthepointer issignextendedorzeroextended.Thisqualifierisenabledunder -fms-extensions. __ptr64¶ SupportedSyntaxes¶         __ptr64       The__ptr64qualifierrepresentsanativepointerona64-bitsystem.Ona 32-bitsystem,a__ptr64pointeristruncatedtoa32-bitpointer.This qualifierisenabledunder-fms-extensions. __sptr¶ SupportedSyntaxes¶         __sptr       The__sptrqualifierspecifiesthata32-bitpointershouldbesign extendedwhenconvertedtoa64-bitpointer. __uptr¶ SupportedSyntaxes¶         __uptr       The__uptrqualifierspecifiesthata32-bitpointershouldbezero extendedwhenconvertedtoa64-bitpointer. align_value¶ SupportedSyntaxes¶ align_value             Yes Thealign_valueattributecanbeaddedtothetypedefofapointertypeorthe declarationofavariableofpointerorreferencetype.Itspecifiesthatthe pointerwillpointto,orthereferencewillbindto,onlyobjectswithat leasttheprovidedalignment.Thisalignmentvaluemustbesomepositivepower of2. typedefdouble*aligned_double_ptr__attribute__((align_value(64))); voidfoo(double&x__attribute__((align_value(128)), aligned_double_ptry){...} Ifthepointervaluedoesnothavethespecifiedalignmentatruntime,the behavioroftheprogramisundefined. annotate_type¶ SupportedSyntaxes¶   clang::annotate_type clang::annotate_type           Thisattributeisusedtoaddannotationstotypes,typicallyforusebystatic analysistoolsthatarenotintegratedintothecoreClangcompiler(e.g., Clang-Tidychecksorout-of-treeClang-basedtools).Itisacounterparttothe annotateattribute,whichservesthesamepurpose,butfordeclarations. Theattributetakesamandatorystringliteralargumentspecifyingthe annotationcategoryandanarbitrarynumberofoptionalargumentsthatprovide additionalinformationspecifictotheannotationcategory.Theoptional argumentsmustbeconstantexpressionsofarbitrarytype. Forexample: int*[[clang::annotate("category1","foo",1)]]f(int[[clang::annotate("category2")]]*); Theattributedoesnothaveanyeffectonthesemanticsofthetypesystem, neithertypecheckingrules,norruntimesemantics.Inparticular: std::is_sameistrueforalltypes T. Itisnotpermissibleforoverloadedfunctionsortemplatespecializations todiffermerelybyanannotate_typeattribute. Thepresenceofanannotate_typeattributewillnotaffectname mangling. arm_sve_vector_bits¶ SupportedSyntaxes¶ arm_sve_vector_bits               Thearm_sve_vector_bits(N)attributeisdefinedbytheArmCLanguage Extensions(ACLE)forSVE.Itisusedtodefinefixed-length(VLST)variantsof sizelesstypes(VLAT). Forexample: #include #if__ARM_FEATURE_SVE_BITS==512 typedefsvint32_tfixed_svint32_t__attribute__((arm_sve_vector_bits(512))); #endif Createsatypefixed_svint32_tthatisafixed-lengthvariantof svint32_tthatcontainsexactly512-bits.Unlikesvint32_t,thistype canbeusedinglobals,structs,unions,andarrays,allofwhichare unsupportedforsizelesstypes. TheattributecanbeattachedtoasingleSVEvector(suchassvint32_t)or totheSVEpredicatetypesvbool_t,thisexcludestupletypessuchas svint32x4_t.Thebehavioroftheattributeisundefinedunless N==__ARM_FEATURE_SVE_BITS,theimplementationdefinedfeaturemacrothatis enabledunderthe-msve-vector-bitsflag. FormoreinformationSeeArmCLanguageExtensionsforSVEformoreinformation. btf_type_tag¶ SupportedSyntaxes¶ btf_type_tag clang::btf_type_tag clang::btf_type_tag           Clangsupportsthe__attribute__((btf_type_tag("ARGUMENT")))attributefor alltargets.Itonlyhaseffectwhen-gisspecifiedonthecommandlineand iscurrentlysilentlyignoredwhennotappliedtoapointertype(note:this scenariomaybediagnosedinthefuture). TheARGUMENTstringwillbepreservedinIRandemittedtoDWARFforthe typesusedinvariabledeclarations,functiondeclarations,ortypedef declarations. ForBPFtargets,theARGUMENTstringwillalsobeemittedto.BTFELF section. clang_arm_mve_strict_polymorphism¶ SupportedSyntaxes¶ __clang_arm_mve_strict_polymorphism clang::__clang_arm_mve_strict_polymorphism clang::__clang_arm_mve_strict_polymorphism           ThisattributeisusedintheimplementationoftheACLEintrinsicsfortheArm MVEinstructionset.ItisusedtodefinethevectortypesusedbytheMVE intrinsics. Itseffectistomodifythebehaviorofavectortypewithrespecttofunction overloading.Ifacandidatefunctionforoverloadresolutionhasaparameter typewiththisattribute,thentheselectionofthatcandidatefunctionwillbe disallowediftheactualargumentcanonlybeconvertedviaalaxvector conversion.TheaimistopreventspuriousambiguityinARMMVEpolymorphic intrinsics. voidoverloaded(uint16x8_tvector,uint16_tscalar); voidoverloaded(int32x4_tvector,int32_tscalar); uint16x8_tmyVector; uint16_tmyScalar; //myScalarispromotedtoint32_tasasideeffectoftheaddition, //soiflaxvectorconversionsareconsideredformyVector,then //thetwooverloadsareequallygood(oneargumentconversion //each).Butifthevectorhasthe__clang_arm_mve_strict_polymorphism //attribute,onlytheuint16x8_t,uint16_toverloadwillmatch. overloaded(myVector,myScalar+1); However,thisattributedoesnotprohibitlaxvectorconversionsincontexts otherthanoverloading. uint16x8_tfunction(); //Thisisstillpermittedwithlaxvectorconversionenabled,even //ifthevectortypeshave__clang_arm_mve_strict_polymorphism int32x4_tresult=function(); cmse_nonsecure_call¶ SupportedSyntaxes¶ cmse_nonsecure_call               Thisattributedeclaresanon-securefunctiontype.Whencompilingforsecure state,acalltosuchafunctionwouldswitchfromsecuretonon-securestate. Allnon-securefunctioncallsmusthappenonlythroughafunctionpointer,and anon-securefunctiontypeshouldonlybeusedasabasetypeofapointer. SeeARMv8-MSecurityExtensions:RequirementsonDevelopment Tools-EngineeringSpecificationDocumentationformoreinformation. device_builtin_surface_type¶ SupportedSyntaxes¶ device_builtin_surface_type     __device_builtin_surface_type__       Yes Thedevice_builtin_surface_typeattributecanbeappliedtoaclass templatewhendeclaringthesurfacereference.Asurfacereferencevariable couldbeaccessedonthehostsideand,onthedeviceside,mightbetranslated intoaninternalsurfaceobject,whichisestablishedthroughsurfacebindand unbindruntimeAPIs. device_builtin_texture_type¶ SupportedSyntaxes¶ device_builtin_texture_type     __device_builtin_texture_type__       Yes Thedevice_builtin_texture_typeattributecanbeappliedtoaclass templatewhendeclaringthetexturereference.Atexturereferencevariable couldbeaccessedonthehostsideand,onthedeviceside,mightbetranslated intoaninternaltextureobject,whichisestablishedthroughtexturebindand unbindruntimeAPIs. noderef¶ SupportedSyntaxes¶ noderef clang::noderef clang::noderef           Thenoderefattributecausesclangtodiagnosedereferencesofannotatedpointertypes. Thisisideallyusedwithpointersthatpointtospecialmemorywhichcannotberead fromorwrittento,butallowingforthepointertobeusedinpointerarithmetic. Thefollowingareexamplesofvalidexpressionswheredereferencesarediagnosed: int__attribute__((noderef))*p; intx=*p;//warning int__attribute__((noderef))**p2; x=**p2;//warning int*__attribute__((noderef))*p3; p=*p3;//warning structS{ inta; }; structS__attribute__((noderef))*s; x=s->a;//warning x=(*s).a;//warning Notalldereferencesmaydiagnoseawarningifthevaluedirectedbythepointermaynotbe accessed.Thefollowingareexamplesofvalidexpressionswheremaynotbediagnosed: int*q; int__attribute__((noderef))*p; q=&*p; q=*&p; structS{ inta; }; structS__attribute__((noderef))*s; p=&s->a; p=&(*s).a; noderefiscurrentlyonlysupportedforpointersandarraysandnotusable forreferencesorObjective-Cobjectpointers. objc_class_stub¶ SupportedSyntaxes¶ objc_class_stub clang::objc_class_stub clang::objc_class_stub         Yes ThisattributespecifiesthattheObjective-Cclasstowhichitappliesis instantiatedatruntime. Unlike__attribute__((objc_runtime_visible)),aclasshavingthisattribute stillhasa“classstub”thatisvisibletothelinker.Thisallowscategories tobedefined.Staticmessagesendswiththeclassasareceiveruseaspecial accesspatterntoensuretheclassislazilyinstantiatedfromtheclassstub. Classesannotatedwiththisattributecannotbesubclassedandcannothave implementationsdefinedforthem.Thisattributeisintendedforusein Swift-generatedheadersforclassesdefinedinSwift. AddingorremovingthisattributetoaclassisanABI-breakingchange. TypeSafetyChecking¶ Clangsupportsadditionalattributestoenablecheckingtypesafetyproperties thatcan’tbeenforcedbytheCtypesystem.Toseewarningsproducedbythese checks,ensurethat-Wtype-safetyisenabled.Usecasesinclude: MPIlibraryimplementations,wheretheseattributesenablecheckingthat thebuffertypematchesthepassedMPI_Datatype; forHDF5librarythereisasimilarusecasetoMPI; checkingtypesofvariadicfunctions’argumentsforfunctionslike fcntl()andioctl(). Youcandetectsupportfortheseattributeswith__has_attribute().For example: #ifdefined(__has_attribute) #if__has_attribute(argument_with_type_tag)&&\ __has_attribute(pointer_with_type_tag)&&\ __has_attribute(type_tag_for_datatype) #defineATTR_MPI_PWT(buffer_idx,type_idx)__attribute__((pointer_with_type_tag(mpi,buffer_idx,type_idx))) /*...othermacros...*/ #endif #endif #if!defined(ATTR_MPI_PWT) #defineATTR_MPI_PWT(buffer_idx,type_idx) #endif intMPI_Send(void*buf,intcount,MPI_Datatypedatatype/*,otherargsomitted*/) ATTR_MPI_PWT(1,3); argument_with_type_tag¶ SupportedSyntaxes¶ argument_with_type_tagpointer_with_type_tag clang::argument_with_type_tagclang::pointer_with_type_tag clang::argument_with_type_tagclang::pointer_with_type_tag           Use__attribute__((argument_with_type_tag(arg_kind,arg_idx, type_tag_idx)))onafunctiondeclarationtospecifythatthefunction acceptsatypetagthatdeterminesthetypeofsomeotherargument. Thisattributeisprimarilyusefulforcheckingargumentsofvariadicfunctions (pointer_with_type_tagcanbeusedinmostnon-variadiccases). Intheattributeprototypeabove: arg_kindisanidentifierthatshouldbeusedwhenannotatingall applicabletypetags. arg_idxprovidesthepositionofafunctionargument.Theexpectedtypeof thisfunctionargumentwillbedeterminedbythefunctionargumentspecified bytype_tag_idx.Inthecodeexamplebelow,“3”meansthatthetypeofthe function’sthirdargumentwillbedeterminedbytype_tag_idx. type_tag_idxprovidesthepositionofafunctionargument.Thisfunction argumentwillbeatypetag.Thetypetagwilldeterminetheexpectedtypeof theargumentspecifiedbyarg_idx.Inthecodeexamplebelow,“2”means thatthetypetagassociatedwiththefunction’ssecondargumentshouldagree withthetypeoftheargumentspecifiedbyarg_idx. Forexample: intfcntl(intfd,intcmd,...) __attribute__((argument_with_type_tag(fcntl,3,2))); //Thefunction'ssecondargumentwillbeatypetag;thistypetagwill //determinetheexpectedtypeofthefunction'sthirdargument. pointer_with_type_tag¶ SupportedSyntaxes¶ argument_with_type_tagpointer_with_type_tag clang::argument_with_type_tagclang::pointer_with_type_tag clang::argument_with_type_tagclang::pointer_with_type_tag           Use__attribute__((pointer_with_type_tag(ptr_kind,ptr_idx,type_tag_idx))) onafunctiondeclarationtospecifythatthefunctionacceptsatypetagthat determinesthepointeetypeofsomeotherpointerargument. Intheattributeprototypeabove: ptr_kindisanidentifierthatshouldbeusedwhenannotatingall applicabletypetags. ptr_idxprovidesthepositionofafunctionargument;thisfunction argumentwillhaveapointertype.Theexpectedpointeetypeofthispointer typewillbedeterminedbythefunctionargumentspecifiedby type_tag_idx.Inthecodeexamplebelow,“1”meansthatthepointeetype ofthefunction’sfirstargumentwillbedeterminedbytype_tag_idx. type_tag_idxprovidesthepositionofafunctionargument;thisfunction argumentwillbeatypetag.Thetypetagwilldeterminetheexpectedpointee typeofthepointerargumentspecifiedbyptr_idx.Inthecodeexample below,“3”meansthatthetypetagassociatedwiththefunction’sthird argumentshouldagreewiththepointeetypeofthepointerargumentspecified byptr_idx. Forexample: typedefintMPI_Datatype; intMPI_Send(void*buf,intcount,MPI_Datatypedatatype/*,otherargsomitted*/) __attribute__((pointer_with_type_tag(mpi,1,3))); //Thefunction's3rdargumentwillbeatypetag;thistypetagwill //determinetheexpectedpointeetypeofthefunction's1stargument. type_tag_for_datatype¶ SupportedSyntaxes¶ type_tag_for_datatype clang::type_tag_for_datatype clang::type_tag_for_datatype           Whendeclaringavariable,use __attribute__((type_tag_for_datatype(kind,type)))tocreateatypetagthat istiedtothetypeargumentgiventotheattribute. Intheattributeprototypeabove: kindisanidentifierthatshouldbeusedwhenannotatingallapplicable typetags. typeindicatesthenameofthetype. Clangsupportsannotatingtypetagsoftwoforms. Typetagthatisareferencetoadeclaredidentifier. Use__attribute__((type_tag_for_datatype(kind,type)))whendeclaringthat identifier: typedefintMPI_Datatype; externstructmpi_datatypempi_datatype_int __attribute__((type_tag_for_datatype(mpi,int))); #defineMPI_INT((MPI_Datatype)&mpi_datatype_int) //&mpi_datatype_intisatypetag.Itistiedtotype"int". Typetagthatisanintegralliteral. Declareastaticconstvariablewithaninitializervalueandattach __attribute__((type_tag_for_datatype(kind,type)))onthatdeclaration: typedefintMPI_Datatype; staticconstMPI_Datatypempi_datatype_int __attribute__((type_tag_for_datatype(mpi,int)))=42; #defineMPI_INT((MPI_Datatype)42) //Thenumber42isatypetag.Itistiedtotype"int". Thetype_tag_for_datatypeattributealsoacceptsanoptionalthirdargument thatdetermineshowthetypeofthefunctionargumentspecifiedbyeither arg_idxorptr_idxiscomparedagainstthetypeassociatedwiththetype tag.(Recallthatfortheargument_with_type_tagattribute,thetypeofthe functionargumentspecifiedbyarg_idxiscomparedagainstthetype associatedwiththetypetag.Alsorecallthatforthepointer_with_type_tag attribute,thepointeetypeofthefunctionargumentspecifiedbyptr_idxis comparedagainstthetypeassociatedwiththetypetag.)Therearetwosupported valuesforthisoptionalthirdargument: layout_compatiblewillcausetypestobecomparedaccordingto layout-compatibilityrules(InC++11[class.mem]p17,18,seethe layout-compatibilityrulesfortwostandard-layoutstructtypesandfortwo standard-layoutuniontypes).Thisisusefulwhencreatingatypetag associatedwithastructoruniontype.Forexample: /*Inmpi.h*/ typedefintMPI_Datatype; structinternal_mpi_double_int{doubled;inti;}; externstructmpi_datatypempi_datatype_double_int __attribute__((type_tag_for_datatype(mpi, structinternal_mpi_double_int,layout_compatible))); #defineMPI_DOUBLE_INT((MPI_Datatype)&mpi_datatype_double_int) intMPI_Send(void*buf,intcount,MPI_Datatypedatatype,...) __attribute__((pointer_with_type_tag(mpi,1,3))); /*Inusercode*/ structmy_pair{doublea;intb;}; structmy_pair*buffer; MPI_Send(buffer,1,MPI_DOUBLE_INT/*,...*/);//nowarningbecausethe //layoutofmy_pairis //compatiblewiththatof //internal_mpi_double_int structmy_int_pair{inta;intb;} structmy_int_pair*buffer2; MPI_Send(buffer2,1,MPI_DOUBLE_INT/*,...*/);//warningbecausethe //layoutofmy_int_pair //doesnotmatchthatof //internal_mpi_double_int must_be_nullspecifiesthatthefunctionargumentspecifiedbyeither arg_idx(fortheargument_with_type_tagattribute)orptr_idx(for thepointer_with_type_tagattribute)shouldbeanullpointerconstant. Thesecondargumenttothetype_tag_for_datatypeattributeisignored.For example: /*Inmpi.h*/ typedefintMPI_Datatype; externstructmpi_datatypempi_datatype_null __attribute__((type_tag_for_datatype(mpi,void,must_be_null))); #defineMPI_DATATYPE_NULL((MPI_Datatype)&mpi_datatype_null) intMPI_Send(void*buf,intcount,MPI_Datatypedatatype,...) __attribute__((pointer_with_type_tag(mpi,1,3))); /*Inusercode*/ structmy_pair{doublea;intb;}; structmy_pair*buffer; MPI_Send(buffer,1,MPI_DATATYPE_NULL/*,...*/);//warning:MPI_DATATYPE_NULL //wasspecifiedbutbuffer //isnotanullpointer Undocumented¶ ThissectionlistsattributeswhicharerecognizedbyClang,butwhichare currentlymissingdocumentation. Alignas,align,alignas,aligned¶ SupportedSyntaxes¶ aligned gnu::aligned gnu::aligned align alignas_Alignas       Nodocumentation. NSObject¶ SupportedSyntaxes¶ NSObject clang::NSObject clang::NSObject           Nodocumentation. __kindof¶ SupportedSyntaxes¶         __kindof       Nodocumentation. acquired_after¶ SupportedSyntaxes¶ acquired_after               Nodocumentation. acquired_before¶ SupportedSyntaxes¶ acquired_before               Nodocumentation. address_space¶ SupportedSyntaxes¶ address_space clang::address_space clang::address_space           Nodocumentation. alias¶ SupportedSyntaxes¶ alias gnu::alias gnu::alias         Yes Nodocumentation. amdgpu_kernel¶ SupportedSyntaxes¶ amdgpu_kernel clang::amdgpu_kernel clang::amdgpu_kernel           Nodocumentation. analyzer_noreturn¶ SupportedSyntaxes¶ analyzer_noreturn               Nodocumentation. annotate¶ SupportedSyntaxes¶ annotate clang::annotate clang::annotate         Yes Nodocumentation. assert_exclusive_lock¶ SupportedSyntaxes¶ assert_exclusive_lock               Nodocumentation. assert_shared_lock¶ SupportedSyntaxes¶ assert_shared_lock               Nodocumentation. blocks¶ SupportedSyntaxes¶ blocks clang::blocks clang::blocks           Nodocumentation. capability,shared_capability¶ SupportedSyntaxes¶ capabilityshared_capability clang::capabilityclang::shared_capability           Yes Nodocumentation. cdecl¶ SupportedSyntaxes¶ cdecl gnu::cdecl gnu::cdecl   __cdecl_cdecl       Nodocumentation. cf_audited_transfer¶ SupportedSyntaxes¶ cf_audited_transfer clang::cf_audited_transfer clang::cf_audited_transfer         Yes Nodocumentation. cf_unknown_transfer¶ SupportedSyntaxes¶ cf_unknown_transfer clang::cf_unknown_transfer clang::cf_unknown_transfer         Yes Nodocumentation. cleanup¶ SupportedSyntaxes¶ cleanup gnu::cleanup gnu::cleanup         Yes Nodocumentation. cold¶ SupportedSyntaxes¶ cold gnu::cold gnu::cold         Yes Nodocumentation. common¶ SupportedSyntaxes¶ common gnu::common gnu::common         Yes Nodocumentation. const¶ SupportedSyntaxes¶ const__const gnu::constgnu::__const gnu::constgnu::__const           Nodocumentation. constant¶ SupportedSyntaxes¶ constant     __constant__       Yes Nodocumentation. constructor¶ SupportedSyntaxes¶ constructor gnu::constructor gnu::constructor         Yes Nodocumentation. consumable_auto_cast_state¶ SupportedSyntaxes¶ consumable_auto_cast_state clang::consumable_auto_cast_state           Yes Nodocumentation. consumable_set_state_on_read¶ SupportedSyntaxes¶ consumable_set_state_on_read clang::consumable_set_state_on_read           Yes Nodocumentation. destructor¶ SupportedSyntaxes¶ destructor gnu::destructor gnu::destructor         Yes Nodocumentation. device¶ SupportedSyntaxes¶ device     __device__       Yes Nodocumentation. exclusive_locks_required,requires_capability,requires_shared_capability,shared_locks_required¶ SupportedSyntaxes¶ requires_capabilityexclusive_locks_requiredrequires_shared_capabilityshared_locks_required clang::requires_capabilityclang::exclusive_locks_requiredclang::requires_shared_capabilityclang::shared_locks_required             Nodocumentation. exclusive_trylock_function¶ SupportedSyntaxes¶ exclusive_trylock_function               Nodocumentation. ext_vector_type¶ SupportedSyntaxes¶ ext_vector_type               Nodocumentation. format_arg¶ SupportedSyntaxes¶ format_arg gnu::format_arg gnu::format_arg           Nodocumentation. global¶ SupportedSyntaxes¶ global     __global__       Yes Nodocumentation. guarded_by¶ SupportedSyntaxes¶ guarded_by               Nodocumentation. guarded_var¶ SupportedSyntaxes¶ guarded_var clang::guarded_var             Nodocumentation. host¶ SupportedSyntaxes¶ host     __host__       Yes Nodocumentation. hot¶ SupportedSyntaxes¶ hot gnu::hot gnu::hot         Yes Nodocumentation. ibaction¶ SupportedSyntaxes¶ ibaction clang::ibaction clang::ibaction         Yes Nodocumentation. iboutlet¶ SupportedSyntaxes¶ iboutlet clang::iboutlet clang::iboutlet           Nodocumentation. iboutletcollection¶ SupportedSyntaxes¶ iboutletcollection clang::iboutletcollection clang::iboutletcollection           Nodocumentation. intel_ocl_bicc¶ SupportedSyntaxes¶ intel_ocl_bicc clang::intel_ocl_bicc             Nodocumentation. interrupt¶ SupportedSyntaxes¶ interrupt               Nodocumentation. interrupt¶ SupportedSyntaxes¶ interrupt gnu::interrupt gnu::interrupt           Nodocumentation. interrupt¶ SupportedSyntaxes¶ interrupt gnu::interrupt gnu::interrupt           Nodocumentation. kernel¶ SupportedSyntaxes¶         __kernelkernel       Nodocumentation. launch_bounds¶ SupportedSyntaxes¶ launch_bounds     __launch_bounds__       Yes Nodocumentation. lock_returned¶ SupportedSyntaxes¶ lock_returned               Nodocumentation. lockable¶ SupportedSyntaxes¶ lockable             Yes Nodocumentation. locks_excluded¶ SupportedSyntaxes¶ locks_excluded               Nodocumentation. matrix_type¶ SupportedSyntaxes¶ matrix_type clang::matrix_type clang::matrix_type           Nodocumentation. may_alias¶ SupportedSyntaxes¶ may_alias gnu::may_alias gnu::may_alias           Nodocumentation. mips16¶ SupportedSyntaxes¶ mips16 gnu::mips16 gnu::mips16         Yes Nodocumentation. mode¶ SupportedSyntaxes¶ mode gnu::mode gnu::mode           Nodocumentation. ms_struct¶ SupportedSyntaxes¶ ms_struct gnu::ms_struct gnu::ms_struct         Yes Nodocumentation. naked¶ SupportedSyntaxes¶ naked gnu::naked gnu::naked naked       Yes Nodocumentation. neon_polyvector_type¶ SupportedSyntaxes¶ neon_polyvector_type clang::neon_polyvector_type clang::neon_polyvector_type           Nodocumentation. neon_vector_type¶ SupportedSyntaxes¶ neon_vector_type clang::neon_vector_type clang::neon_vector_type           Nodocumentation. no_instrument_function¶ SupportedSyntaxes¶ no_instrument_function gnu::no_instrument_function gnu::no_instrument_function         Yes Nodocumentation. no_thread_safety_analysis¶ SupportedSyntaxes¶ no_thread_safety_analysis clang::no_thread_safety_analysis clang::no_thread_safety_analysis         Yes Nodocumentation. nocommon¶ SupportedSyntaxes¶ nocommon gnu::nocommon gnu::nocommon         Yes Nodocumentation. nomips16¶ SupportedSyntaxes¶ nomips16 gnu::nomips16 gnu::nomips16         Yes Nodocumentation. noreturn¶ SupportedSyntaxes¶ noreturn gnu::noreturn gnu::noreturn noreturn         Nodocumentation. objc_arc_weak_reference_unavailable¶ SupportedSyntaxes¶ objc_arc_weak_reference_unavailable clang::objc_arc_weak_reference_unavailable clang::objc_arc_weak_reference_unavailable         Yes Nodocumentation. objc_bridge¶ SupportedSyntaxes¶ objc_bridge clang::objc_bridge clang::objc_bridge         Yes Nodocumentation. objc_bridge_mutable¶ SupportedSyntaxes¶ objc_bridge_mutable clang::objc_bridge_mutable clang::objc_bridge_mutable         Yes Nodocumentation. objc_bridge_related¶ SupportedSyntaxes¶ objc_bridge_related clang::objc_bridge_related clang::objc_bridge_related         Yes Nodocumentation. objc_designated_initializer¶ SupportedSyntaxes¶ objc_designated_initializer clang::objc_designated_initializer clang::objc_designated_initializer         Yes Nodocumentation. objc_exception¶ SupportedSyntaxes¶ objc_exception clang::objc_exception clang::objc_exception         Yes Nodocumentation. objc_gc¶ SupportedSyntaxes¶ objc_gc clang::objc_gc clang::objc_gc           Nodocumentation. objc_independent_class¶ SupportedSyntaxes¶ objc_independent_class clang::objc_independent_class clang::objc_independent_class           Nodocumentation. objc_ownership¶ SupportedSyntaxes¶ objc_ownership clang::objc_ownership clang::objc_ownership           Nodocumentation. objc_precise_lifetime¶ SupportedSyntaxes¶ objc_precise_lifetime clang::objc_precise_lifetime clang::objc_precise_lifetime         Yes Nodocumentation. objc_protocol_requires_explicit_implementation¶ SupportedSyntaxes¶ objc_protocol_requires_explicit_implementation clang::objc_protocol_requires_explicit_implementation clang::objc_protocol_requires_explicit_implementation         Yes Nodocumentation. objc_requires_property_definitions¶ SupportedSyntaxes¶ objc_requires_property_definitions clang::objc_requires_property_definitions clang::objc_requires_property_definitions         Yes Nodocumentation. objc_returns_inner_pointer¶ SupportedSyntaxes¶ objc_returns_inner_pointer clang::objc_returns_inner_pointer clang::objc_returns_inner_pointer         Yes Nodocumentation. objc_root_class¶ SupportedSyntaxes¶ objc_root_class clang::objc_root_class clang::objc_root_class         Yes Nodocumentation. ownership_holds,ownership_returns,ownership_takes¶ SupportedSyntaxes¶ ownership_holdsownership_returnsownership_takes clang::ownership_holdsclang::ownership_returnsclang::ownership_takes clang::ownership_holdsclang::ownership_returnsclang::ownership_takes           Nodocumentation. packed¶ SupportedSyntaxes¶ packed gnu::packed gnu::packed           Nodocumentation. pascal¶ SupportedSyntaxes¶ pascal clang::pascal clang::pascal   __pascal_pascal       Nodocumentation. pt_guarded_by¶ SupportedSyntaxes¶ pt_guarded_by               Nodocumentation. pt_guarded_var¶ SupportedSyntaxes¶ pt_guarded_var clang::pt_guarded_var             Nodocumentation. pure¶ SupportedSyntaxes¶ pure gnu::pure gnu::pure           Nodocumentation. reqd_work_group_size¶ SupportedSyntaxes¶ reqd_work_group_size             Yes Nodocumentation. returns_twice¶ SupportedSyntaxes¶ returns_twice gnu::returns_twice gnu::returns_twice         Yes Nodocumentation. scoped_lockable¶ SupportedSyntaxes¶ scoped_lockable clang::scoped_lockable           Yes Nodocumentation. sentinel¶ SupportedSyntaxes¶ sentinel gnu::sentinel gnu::sentinel           Nodocumentation. shared¶ SupportedSyntaxes¶ shared     __shared__       Yes Nodocumentation. shared_trylock_function¶ SupportedSyntaxes¶ shared_trylock_function               Nodocumentation. sysv_abi¶ SupportedSyntaxes¶ sysv_abi gnu::sysv_abi gnu::sysv_abi           Nodocumentation. type_visibility¶ SupportedSyntaxes¶ type_visibility clang::type_visibility clang::type_visibility           Nodocumentation. unavailable¶ SupportedSyntaxes¶ unavailable clang::unavailable clang::unavailable           Nodocumentation. uuid¶ SupportedSyntaxes¶       uuid         Nodocumentation. vec_type_hint¶ SupportedSyntaxes¶ vec_type_hint             Yes Nodocumentation. vecreturn¶ SupportedSyntaxes¶ vecreturn clang::vecreturn           Yes Nodocumentation. vector_size¶ SupportedSyntaxes¶ vector_size gnu::vector_size gnu::vector_size           Nodocumentation. visibility¶ SupportedSyntaxes¶ visibility gnu::visibility gnu::visibility           Nodocumentation. warn_unused¶ SupportedSyntaxes¶ warn_unused gnu::warn_unused gnu::warn_unused         Yes Nodocumentation. weak_import¶ SupportedSyntaxes¶ weak_import clang::weak_import clang::weak_import           Nodocumentation. weakref¶ SupportedSyntaxes¶ weakref gnu::weakref gnu::weakref         Yes Nodocumentation. work_group_size_hint¶ SupportedSyntaxes¶ work_group_size_hint             Yes Nodocumentation. VariableAttributes¶ always_destroy¶ SupportedSyntaxes¶ always_destroy clang::always_destroy           Yes Thealways_destroyattributespecifiesthatavariablewithstaticorthread storagedurationshouldhaveitsexit-timedestructorrun.Thisattributeisthe defaultunlessclangwasinvokedwith-fno-c++-static-destructors. called_once¶ SupportedSyntaxes¶ called_once clang::called_once clang::called_once         Yes Thecalled_onceattributespecifiesthattheannotatedfunctionormethod parameterisinvokedexactlyonceonallexecutionpaths.Itonlyapplies toparameterswithfunction-liketypes,i.e.functionpointersorblocks.This conceptisparticularlyusefulforasynchronousprograms. Clangimplementsacheckforcalled_onceparameters, -Wcalled-once-parameter.Itisonbydefaultandfindsthefollowing violations: Parameterisnotcalledatall. Parameteriscalledmorethanonce. Parameterisnotcalledononeoftheexecutionpaths. Inthelattercase,Clangpinpointsthepathwhereparameterisnotinvoked byshowingthecontrol-flowstatementwherethepathdiverges. voidfooWithCallback(void(^callback)(void)__attribute__((called_once))){ if(somePredicate()){ ... callback(); }esle{ callback();//OK:callbackiscalledoneverypath } } voidbarWithCallback(void(^callback)(void)__attribute__((called_once))){ if(somePredicate()){ ... callback();//note:previouscallishere } callback();//warning:callbackiscalledtwice } voidfoobarWithCallback(void(^callback)(void)__attribute__((called_once))){ if(somePredicate()){//warning:callbackisnotcalledwhenconditionisfalse ... callback(); } } ThisattributeisusefulforAPIdeveloperswhowanttodouble-checkifthey implementedtheirmethodcorrectly. dllexport¶ SupportedSyntaxes¶ dllexport gnu::dllexport gnu::dllexport dllexport       Yes The__declspec(dllexport)attributedeclaresavariable,function,or Objective-Cinterfacetobeexportedfromthemodule.Itisavailableunderthe -fdeclspecflagforcompatibilitywithvariouscompilers.Theprimaryuse isforCOFFobjectfileswhichexplicitlyspecifywhatinterfacesareavailable forexternaluse.SeethedllexportdocumentationonMSDNformore information. dllimport¶ SupportedSyntaxes¶ dllimport gnu::dllimport gnu::dllimport dllimport       Yes The__declspec(dllimport)attributedeclaresavariable,function,or Objective-Cinterfacetobeimportedfromanexternalmodule.Itisavailable underthe-fdeclspecflagforcompatibilitywithvariouscompilers.The primaryuseisforCOFFobjectfileswhichexplicitlyspecifywhatinterfaces areimportedfromexternalmodules.SeethedllimportdocumentationonMSDN formoreinformation. Notethatadllimportfunctionmaystillbeinlined,ifitsdefinitionis availableanditdoesn’treferenceanynon-dllimportfunctionsorglobal variables. init_priority¶ SupportedSyntaxes¶ init_priority gnu::init_priority           Yes InC++,theorderinwhichglobalvariablesareinitializedacrosstranslation unitsisunspecified,unliketheorderingwithinasingletranslationunit.The init_priorityattributeallowsyoutospecifyarelativeorderingforthe initializationofobjectsdeclaredatnamespacescopeinC++.Thepriorityis givenasanintegerconstantexpressionbetween101and65535(inclusive). Prioritiesoutsideofthatrangearereservedforusebytheimplementation.A lowervalueindicatesahigherpriorityofinitialization.Notethatonlythe relativeorderingofvaluesisimportant.Forexample: structSomeType{SomeType();}; __attribute__((init_priority(200)))SomeTypeObj1; __attribute__((init_priority(101)))SomeTypeObj2; Obj2willbeinitializedbeforeObj1despitetheusualorderof initializationbeingtheopposite. ThisattributeisonlysupportedforC++andObjective-C++andisignoredin otherlanguagemodes.Currently,thisattributeisnotimplementedonz/OS. init_seg¶ SupportedSyntaxes¶           init_seg     Theattributeappliedbypragmainit_seg()controlsthesectioninto whichglobalinitializationfunctionpointersareemitted.Itisonly availablewith-fms-extensions.Typically,thisfunctionpointeris emittedinto.CRT$XCUonWindows.Theusercanchangetheorderof initializationbyusingadifferentsectionnamewiththesame .CRT$XCprefixandasuffixthatsortslexicographicallybeforeor afterthestandard.CRT$XCUsections.Seetheinit_seg documentationonMSDNformoreinformation. leaf¶ SupportedSyntaxes¶ leaf gnu::leaf gnu::leaf         Yes Theleafattributeisusedasacompilerhinttoimprovedataflowanalysis inlibraryfunctions.Functionsmarkedwiththeleafattributearenotallowed tojumpbackintothecaller’stranslationunit,whetherthroughinvokinga callbackfunction,anexternalfunctioncall,useoflongjmp,orothermeans. Therefore,theycannotuseormodifyanydatathatdoesnotescapethecallerfunction’s compilationunit. Formoreinformationsee gccdocumentation loader_uninitialized¶ SupportedSyntaxes¶ loader_uninitialized clang::loader_uninitialized clang::loader_uninitialized         Yes Theloader_uninitializedattributecanbeplacedonglobalvariablesto indicatethatthevariabledoesnotneedtobezeroinitializedbytheloader. Onmosttargets,zero-initializationdoesnotincuranyadditionalcost. Forexample,mostgeneralpurposeoperatingsystemsdeliberatelyensure thatallmemoryisproperlyinitializedinordertoavoidleakingprivileged informationfromthekernelorotherprograms.However,sometargets donotmakethisguarantee,andonthesetargets,avoidinganunnecessary zero-initializationcanhaveasignificantimpactonloadtimesand/orcode size. Adeclarationwiththisattributeisanon-tentativedefinitionjustasifit providedaninitializer.Variableswiththisattributeareconsideredtobe uninitializedinthesamesenseasalocalvariable,andtheprogramsmust writetothembeforereadingfromthem.Ifthevariable’stypeisaC++class typewithanon-trivialdefaultconstructor,oranarraythereof,thisattribute onlysuppressesthestaticzero-initializationofthevariable,notthedynamic initializationprovidedbyexecutingthedefaultconstructor. maybe_unused,unused¶ SupportedSyntaxes¶ unused maybe_unusedgnu::unused gnu::unusedmaybe_unused           Whenpassingthe-WunusedflagtoClang,entitiesthatareunusedbythe programmaybediagnosed.The[[maybe_unused]](or __attribute__((unused)))attributecanbeusedtosilencesuchdiagnostics whentheentitycannotberemoved.Forinstance,alocalvariablemayexist solelyforuseinanassert()statement,whichmakesthelocalvariable unusedwhenNDEBUGisdefined. Theattributemaybeappliedtothedeclarationofaclass,atypedef,a variable,afunctionormethod,afunctionparameter,anenumeration,an enumerator,anon-staticdatamember,oralabel. no_destroy¶ SupportedSyntaxes¶ no_destroy clang::no_destroy           Yes Theno_destroyattributespecifiesthatavariablewithstaticorthread storagedurationshouldn’thaveitsexit-timedestructorrun.Annotatingevery staticandthreaddurationvariablewiththisattributeisequivalentto invokingclangwith-fno-c++-static-destructors. Ifavariableisdeclaredwiththisattribute,clangdoesn’taccesscheckor generatethetype’sdestructor.Ifyouhaveatypethatyouonlywanttobe annotatedwithno_destroy,youcanthereforedeclarethedestructorprivate: structonly_no_destroy{ only_no_destroy(); private: ~only_no_destroy(); }; [[clang::no_destroy]]only_no_destroyglobal;//fine! Notethatdestructorsarestillrequiredforsubobjectsofaggregatesannotated withthisattribute.Thisisbecausepreviouslyconstructedsubobjectsneedto bedestroyedifanexceptiongetsthrownbeforetheinitializationofthe completeobjectiscomplete.Forinstance: voidf(){ try{ [[clang::no_destroy]] staticonly_no_destroyarray[10];//error,only_no_destroyhasaprivatedestructor. }catch(...){ //Handletheerror } } Here,iftheconstructionofarray[9]failswithanexception,array[0..8] willbedestroyed,sotheelement’sdestructorneedstobeaccessible. nodebug¶ SupportedSyntaxes¶ nodebug gnu::nodebug gnu::nodebug         Yes Thenodebugattributeallowsyoutosuppressdebugginginformationfora functionormethod,foravariablethatisnotaparameteroranon-static datamember,orforatypedeforusingdeclaration. noescape¶ SupportedSyntaxes¶ noescape clang::noescape clang::noescape         Yes noescapeplacedonafunctionparameterofapointertypeisusedtoinform thecompilerthatthepointercannotescape:thatis,noreferencetotheobject thepointerpointstothatisderivedfromtheparametervaluewillsurvive afterthefunctionreturns.Usersareresponsibleformakingsureparameters annotatedwithnoescapedonotactuallyescape.Callingfree()onsuch aparameterdoesnotconstituteanescape. Forexample: int*gp; voidnonescapingFunc(__attribute__((noescape))int*p){ *p+=100;//OK. } voidescapingFunc(__attribute__((noescape))int*p){ gp=p;//NotOK. } Additionally,whentheparameterisablockpointer ,thesamerestriction appliestocopiesoftheblock.Forexample: typedefvoid(^BlockTy)(); BlockTyg0,g1; voidnonescapingFunc(__attribute__((noescape))BlockTyblock){ block();//OK. } voidescapingFunc(__attribute__((noescape))BlockTyblock){ g0=block;//NotOK. g1=Block_copy(block);//NotOKeither. } nosvm¶ SupportedSyntaxes¶ nosvm             Yes OpenCL2.0supportstheoptional__attribute__((nosvm))qualifierfor pointervariable.Itinformsthecompilerthatthepointerdoesnotrefer toasharedvirtualmemoryregion.SeeOpenCLv2.0s6.7.2fordetails. SinceitisnotwidelyusedandhasbeenremovedfromOpenCL2.1,itisignored byClang. objc_externally_retained¶ SupportedSyntaxes¶ objc_externally_retained clang::objc_externally_retained clang::objc_externally_retained         Yes Theobjc_externally_retainedattributecanbeappliedtostronglocal variables,functions,methods,orblockstooptinto externally-retainedsemantics. Whenappliedtothedefinitionofafunction,method,orblock,everyparameter ofthefunctionwithimplicitstrongretainableobjectpointertypeis consideredexternally-retained,andbecomesconst.Byexplicitlyannotating aparameterwith__strong,youcanoptbackintothedefault non-externally-retainedbehaviorforthatparameter.Forinstance, first_paramisexternally-retainedbelow,butnotsecond_param: __attribute__((objc_externally_retained)) voidf(NSArray*first_param,__strongNSArray*second_param){ //... } Likewise,whenappliedtoastronglocalvariable,thatvariablebecomes constandisconsideredexternally-retained. Whencompiledwithout-fobjc-arc,thisattributeisignored. pass_object_size,pass_dynamic_object_size¶ SupportedSyntaxes¶ pass_object_sizepass_dynamic_object_size clang::pass_object_sizeclang::pass_dynamic_object_size clang::pass_object_sizeclang::pass_dynamic_object_size         Yes Note Themanglingoffunctionswithparametersthatareannotatedwith pass_object_sizeissubjecttochange.Youcangetaroundthisby using__asm__("foo")toexplicitlynameyourfunctions,thuspreserving yourABI;also,non-overloadableCfunctionswithpass_object_sizeare notmangled. Thepass_object_size(Type)attributecanbeplacedonfunctionparametersto instructclangtocall__builtin_object_size(param,Type)ateachcallsite ofsaidfunction,andimplicitlypasstheresultofthiscallinasaninvisible argumentoftypesize_tdirectlyaftertheparameterannotatedwith pass_object_size.Clangwillalsoreplaceanycallsto __builtin_object_size(param,Type)inthefunctionbysaidimplicit parameter. Exampleusage: intbzero1(char*constp__attribute__((pass_object_size(0)))) __attribute__((noinline)){ inti=0; for(/**/;i