Author: Jon Chesterfield Date: 2020-03-17T21:22:23Z New Revision: c45eaeabb77a926f4f1cf3c1e9311e9d66e0ee2a
URL: https://github.com/llvm/llvm-project/commit/c45eaeabb77a926f4f1cf3c1e9311e9d66e0ee2a DIFF: https://github.com/llvm/llvm-project/commit/c45eaeabb77a926f4f1cf3c1e9311e9d66e0ee2a.diff LOG: [Clang] Undef attribute for global variables Summary: [Clang] Attribute to allow defining undef global variables Initializing global variables is very cheap on hosted implementations. The C semantics of zero initializing globals work very well there. It is not necessarily cheap on freestanding implementations. Where there is no loader available, code must be emitted near the start point to write the appropriate values into memory. At present, external variables can be declared in C++ and definitions provided in assembly (or IR) to achive this effect. This patch provides an attribute in order to remove this reason for writing assembly for performance sensitive freestanding implementations. A close analogue in tree is LDS memory for amdgcn, where the kernel is responsible for initializing the memory after it starts executing on the gpu. Uninitalized variables in LDS are observably cheaper than zero initialized. Patch is loosely based on the cuda __shared__ and opencl __local variable implementation which also produces undef global variables. Reviewers: kcc, rjmccall, rsmith, glider, vitalybuka, pcc, eugenis, vlad.tsyrklevich, jdoerfert, gregrodgers, jfb, aaron.ballman Reviewed By: rjmccall, aaron.ballman Subscribers: Anastasia, aaron.ballman, davidb, Quuxplusone, dexonsmith, cfe-commits Tags: #clang Differential Revision: https://reviews.llvm.org/D74361 Added: clang/test/CodeGen/attr-loader-uninitialized.c clang/test/CodeGenCXX/attr-loader-uninitialized.cpp clang/test/Sema/attr-loader-uninitialized.c clang/test/Sema/attr-loader-uninitialized.cpp Modified: clang/include/clang/Basic/Attr.td clang/include/clang/Basic/AttrDocs.td clang/include/clang/Basic/DiagnosticSemaKinds.td clang/lib/AST/DeclBase.cpp clang/lib/CodeGen/CGDecl.cpp clang/lib/CodeGen/CodeGenModule.cpp clang/lib/Sema/SemaDecl.cpp clang/lib/Sema/SemaDeclAttr.cpp clang/test/Misc/pragma-attribute-supported-attributes-list.test Removed: ################################################################################ diff --git a/clang/include/clang/Basic/Attr.td b/clang/include/clang/Basic/Attr.td index 624995a2d572..a0d521d17d0f 100644 --- a/clang/include/clang/Basic/Attr.td +++ b/clang/include/clang/Basic/Attr.td @@ -3313,6 +3313,12 @@ def Uninitialized : InheritableAttr { let Documentation = [UninitializedDocs]; } +def LoaderUninitialized : Attr { + let Spellings = [Clang<"loader_uninitialized">]; + let Subjects = SubjectList<[GlobalVar]>; + let Documentation = [LoaderUninitializedDocs]; +} + def ObjCExternallyRetained : InheritableAttr { let LangOpts = [ObjCAutoRefCount]; let Spellings = [Clang<"objc_externally_retained">]; diff --git a/clang/include/clang/Basic/AttrDocs.td b/clang/include/clang/Basic/AttrDocs.td index aea574995c8e..60496694200e 100644 --- a/clang/include/clang/Basic/AttrDocs.td +++ b/clang/include/clang/Basic/AttrDocs.td @@ -4358,6 +4358,29 @@ it rather documents the programmer's intent. }]; } +def LoaderUninitializedDocs : Documentation { + let Category = DocCatVariable; + let Content = [{ +The ``loader_uninitialized`` attribute can be placed on global variables to +indicate that the variable does not need to be zero initialized by the loader. +On most targets, zero-initialization does not incur any additional cost. +For example, most general purpose operating systems deliberately ensure +that all memory is properly initialized in order to avoid leaking privileged +information from the kernel or other programs. However, some targets +do not make this guarantee, and on these targets, avoiding an unnecessary +zero-initialization can have a significant impact on load times and/or code +size. + +A declaration with this attribute is a non-tentative definition just as if it +provided an initializer. Variables with this attribute are considered to be +uninitialized in the same sense as a local variable, and the programs must +write to them before reading from them. If the variable's type is a C++ class +type with a non-trivial default constructor, or an array thereof, this attribute +only suppresses the static zero-initialization of the variable, not the dynamic +initialization provided by executing the default constructor. + }]; +} + def CallbackDocs : Documentation { let Category = DocCatFunction; let Content = [{ diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td index 7cb1eae9615b..f777e0ae4c81 100644 --- a/clang/include/clang/Basic/DiagnosticSemaKinds.td +++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td @@ -5333,6 +5333,17 @@ def ext_aggregate_init_not_constant : Extension< "initializer for aggregate is not a compile-time constant">, InGroup<C99>; def err_local_cant_init : Error< "'__local' variable cannot have an initializer">; +def err_loader_uninitialized_cant_init + : Error<"variable with 'loader_uninitialized' attribute cannot have an " + "initializer">; +def err_loader_uninitialized_trivial_ctor + : Error<"variable with 'loader_uninitialized' attribute must have a " + "trivial default constructor">; +def err_loader_uninitialized_redeclaration + : Error<"redeclaration cannot add 'loader_uninitialized' attribute">; +def err_loader_uninitialized_extern_decl + : Error<"variable %0 cannot be declared both 'extern' and with the " + "'loader_uninitialized' attribute">; def err_block_extern_cant_init : Error< "'extern' variable cannot have an initializer">; def warn_extern_init : Warning<"'extern' variable has an initializer">, diff --git a/clang/lib/AST/DeclBase.cpp b/clang/lib/AST/DeclBase.cpp index 325a306b1a75..a59873cbc9fc 100644 --- a/clang/lib/AST/DeclBase.cpp +++ b/clang/lib/AST/DeclBase.cpp @@ -454,7 +454,8 @@ ExternalSourceSymbolAttr *Decl::getExternalSourceSymbolAttr() const { } bool Decl::hasDefiningAttr() const { - return hasAttr<AliasAttr>() || hasAttr<IFuncAttr>(); + return hasAttr<AliasAttr>() || hasAttr<IFuncAttr>() || + hasAttr<LoaderUninitializedAttr>(); } const Attr *Decl::getDefiningAttr() const { @@ -462,6 +463,8 @@ const Attr *Decl::getDefiningAttr() const { return AA; if (auto *IFA = getAttr<IFuncAttr>()) return IFA; + if (auto *NZA = getAttr<LoaderUninitializedAttr>()) + return NZA; return nullptr; } diff --git a/clang/lib/CodeGen/CGDecl.cpp b/clang/lib/CodeGen/CGDecl.cpp index e4f9872c7216..034e725c0eeb 100644 --- a/clang/lib/CodeGen/CGDecl.cpp +++ b/clang/lib/CodeGen/CGDecl.cpp @@ -249,7 +249,7 @@ llvm::Constant *CodeGenModule::getOrCreateStaticVarDecl( // variables cannot have an initializer. llvm::Constant *Init = nullptr; if (Ty.getAddressSpace() == LangAS::opencl_local || - D.hasAttr<CUDASharedAttr>()) + D.hasAttr<CUDASharedAttr>() || D.hasAttr<LoaderUninitializedAttr>()) Init = llvm::UndefValue::get(LTy); else Init = EmitNullConstant(Ty); diff --git a/clang/lib/CodeGen/CodeGenModule.cpp b/clang/lib/CodeGen/CodeGenModule.cpp index eca0faa56989..4ece40bd9fe0 100644 --- a/clang/lib/CodeGen/CodeGenModule.cpp +++ b/clang/lib/CodeGen/CodeGenModule.cpp @@ -3952,6 +3952,8 @@ void CodeGenModule::EmitGlobalVarDefinition(const VarDecl *D, if (getLangOpts().CUDA && (IsCUDASharedVar || IsCUDAShadowVar || IsHIPPinnedShadowVar)) Init = llvm::UndefValue::get(getTypes().ConvertType(ASTTy)); + else if (D->hasAttr<LoaderUninitializedAttr>()) + Init = llvm::UndefValue::get(getTypes().ConvertType(ASTTy)); else if (!InitExpr) { // This is a tentative definition; tentative definitions are // implicitly initialized with { 0 }. diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp index a61ddbd290f8..7b397360c52d 100644 --- a/clang/lib/Sema/SemaDecl.cpp +++ b/clang/lib/Sema/SemaDecl.cpp @@ -2712,6 +2712,18 @@ static void checkNewAttributesAfterDef(Sema &S, Decl *New, const Decl *Old) { --E; continue; } + } else if (isa<LoaderUninitializedAttr>(NewAttribute)) { + // If there is a C definition followed by a redeclaration with this + // attribute then there are two diff erent definitions. In C++, prefer the + // standard diagnostics. + if (!S.getLangOpts().CPlusPlus) { + S.Diag(NewAttribute->getLocation(), + diag::err_loader_uninitialized_redeclaration); + S.Diag(Def->getLocation(), diag::note_previous_definition); + NewAttributes.erase(NewAttributes.begin() + I); + --E; + continue; + } } else if (isa<SelectAnyAttr>(NewAttribute) && cast<VarDecl>(New)->isInline() && !cast<VarDecl>(New)->isInlineSpecified()) { @@ -11915,6 +11927,13 @@ void Sema::AddInitializerToDecl(Decl *RealDecl, Expr *Init, bool DirectInit) { return; } + // The LoaderUninitialized attribute acts as a definition (of undef). + if (VDecl->hasAttr<LoaderUninitializedAttr>()) { + Diag(VDecl->getLocation(), diag::err_loader_uninitialized_cant_init); + VDecl->setInvalidDecl(); + return; + } + // Get the decls type and save a reference for later, since // CheckInitializerTypes may change it. QualType DclT = VDecl->getType(), SavT = DclT; @@ -12328,6 +12347,22 @@ void Sema::ActOnUninitializedDecl(Decl *RealDecl) { return; } + if (!Var->isInvalidDecl() && RealDecl->hasAttr<LoaderUninitializedAttr>()) { + if (CXXRecordDecl *RD = Var->getType()->getAsCXXRecordDecl()) { + if (!RD->hasTrivialDefaultConstructor()) { + Diag(Var->getLocation(), diag::err_loader_uninitialized_trivial_ctor); + Var->setInvalidDecl(); + return; + } + } + if (Var->getStorageClass() == SC_Extern) { + Diag(Var->getLocation(), diag::err_loader_uninitialized_extern_decl) + << Var; + Var->setInvalidDecl(); + return; + } + } + VarDecl::DefinitionKind DefKind = Var->isThisDeclarationADefinition(); if (!Var->isInvalidDecl() && DefKind != VarDecl::DeclarationOnly && Var->getType().hasNonTrivialToPrimitiveDefaultInitializeCUnion()) diff --git a/clang/lib/Sema/SemaDeclAttr.cpp b/clang/lib/Sema/SemaDeclAttr.cpp index 476c9e635b45..67b7fa6cb46f 100644 --- a/clang/lib/Sema/SemaDeclAttr.cpp +++ b/clang/lib/Sema/SemaDeclAttr.cpp @@ -7467,6 +7467,10 @@ static void ProcessDeclAttribute(Sema &S, Scope *scope, Decl *D, handleUninitializedAttr(S, D, AL); break; + case ParsedAttr::AT_LoaderUninitialized: + handleSimpleAttribute<LoaderUninitializedAttr>(S, D, AL); + break; + case ParsedAttr::AT_ObjCExternallyRetained: handleObjCExternallyRetainedAttr(S, D, AL); break; diff --git a/clang/test/CodeGen/attr-loader-uninitialized.c b/clang/test/CodeGen/attr-loader-uninitialized.c new file mode 100644 index 000000000000..c653d5ba3991 --- /dev/null +++ b/clang/test/CodeGen/attr-loader-uninitialized.c @@ -0,0 +1,24 @@ +// RUN: %clang_cc1 -emit-llvm %s -o - | FileCheck %s + +// CHECK: @tentative_attr_first = global i32 undef, align 4 +int tentative_attr_first __attribute__((loader_uninitialized)); +int tentative_attr_first; + +// CHECK: @tentative_attr_second = global i32 undef, align 4 +int tentative_attr_second; +int tentative_attr_second __attribute__((loader_uninitialized)); + +// CHECK: @array = global [16 x float] undef, align 16 +float array[16] __attribute__((loader_uninitialized)); + +typedef struct +{ + int x; + float y; +} s; + +// CHECK: @i = global %struct.s undef, align 4 +s i __attribute__((loader_uninitialized)); + +// CHECK: @private_extern_ok = hidden global i32 undef, align 4 +__private_extern__ int private_extern_ok __attribute__((loader_uninitialized)); diff --git a/clang/test/CodeGenCXX/attr-loader-uninitialized.cpp b/clang/test/CodeGenCXX/attr-loader-uninitialized.cpp new file mode 100644 index 000000000000..ec9d8a54db78 --- /dev/null +++ b/clang/test/CodeGenCXX/attr-loader-uninitialized.cpp @@ -0,0 +1,29 @@ +// RUN: %clang_cc1 -emit-llvm -o - %s | FileCheck %s + +// CHECK: @defn = global i32 undef +int defn [[clang::loader_uninitialized]]; + +// CHECK: @_ZL11defn_static = internal global i32 undef +static int defn_static [[clang::loader_uninitialized]] __attribute__((used)); + +// CHECK: @_ZZ4funcvE4data = internal global i32 undef +int* func(void) +{ + static int data [[clang::loader_uninitialized]]; + return &data; +} + +class trivial +{ + float x; +}; + +// CHECK: @ut = global %class.trivial undef +trivial ut [[clang::loader_uninitialized]]; + +// CHECK: @arr = global [32 x double] undef, align 16 +double arr[32] __attribute__((loader_uninitialized)); + +// Defining as arr2[] [[clang..]] raises the error: attribute cannot be applied to types +// CHECK: @arr2 = global [4 x double] undef, align 16 +double arr2 [[clang::loader_uninitialized]] [4]; diff --git a/clang/test/Misc/pragma-attribute-supported-attributes-list.test b/clang/test/Misc/pragma-attribute-supported-attributes-list.test index 769da310a8c4..11a70119d4c1 100644 --- a/clang/test/Misc/pragma-attribute-supported-attributes-list.test +++ b/clang/test/Misc/pragma-attribute-supported-attributes-list.test @@ -65,6 +65,7 @@ // CHECK-NEXT: InitPriority (SubjectMatchRule_variable) // CHECK-NEXT: InternalLinkage (SubjectMatchRule_variable, SubjectMatchRule_function, SubjectMatchRule_record) // CHECK-NEXT: LTOVisibilityPublic (SubjectMatchRule_record) +// CHECK-NEXT: LoaderUninitialized (SubjectMatchRule_variable_is_global) // CHECK-NEXT: Lockable (SubjectMatchRule_record) // CHECK-NEXT: MIGServerRoutine (SubjectMatchRule_function, SubjectMatchRule_objc_method, SubjectMatchRule_block) // CHECK-NEXT: MSStruct (SubjectMatchRule_record) diff --git a/clang/test/Sema/attr-loader-uninitialized.c b/clang/test/Sema/attr-loader-uninitialized.c new file mode 100644 index 000000000000..f2e78d981580 --- /dev/null +++ b/clang/test/Sema/attr-loader-uninitialized.c @@ -0,0 +1,37 @@ +// RUN: %clang_cc1 %s -verify -fsyntax-only +// See also attr-loader-uninitialized.cpp + +int good __attribute__((loader_uninitialized)); +static int local_ok __attribute__((loader_uninitialized)); +int hidden_ok __attribute__((visibility("hidden"))) __attribute__((loader_uninitialized)); + +const int can_still_be_const __attribute__((loader_uninitialized)); + +extern int external_rejected __attribute__((loader_uninitialized)); +// expected-error@-1 {{variable 'external_rejected' cannot be declared both 'extern' and with the 'loader_uninitialized' attribute}} + +int noargs __attribute__((loader_uninitialized(0))); +// expected-error@-1 {{'loader_uninitialized' attribute takes no arguments}} + +int init_rejected __attribute__((loader_uninitialized)) = 42; +// expected-error@-1 {{variable with 'loader_uninitialized' attribute cannot have an initializer}} + +int declaration_then_uninit_ok; +int declaration_then_uninit_ok __attribute__((loader_uninitialized)); + +int definition_then_uninit_rejected = 0; +int definition_then_uninit_rejected __attribute__((loader_uninitialized)); +// expected-error@-1 {{redeclaration cannot add 'loader_uninitialized' attribute}} +// expected-note@-3 {{previous definition is here}} + +int tentative_repeated_ok __attribute__((loader_uninitialized)); +int tentative_repeated_ok __attribute__((loader_uninitialized)); + +__private_extern__ int private_extern_can_be_initialised = 10; +__private_extern__ int therefore_uninit_private_extern_ok __attribute__((loader_uninitialized)); + +__private_extern__ int initialized_private_extern_rejected __attribute__((loader_uninitialized)) = 5; +// expected-error@-1 {{variable with 'loader_uninitialized' attribute cannot have an initializer}} + +extern __attribute__((visibility("hidden"))) int extern_hidden __attribute__((loader_uninitialized)); +// expected-error@-1 {{variable 'extern_hidden' cannot be declared both 'extern' and with the 'loader_uninitialized' attribute}} diff --git a/clang/test/Sema/attr-loader-uninitialized.cpp b/clang/test/Sema/attr-loader-uninitialized.cpp new file mode 100644 index 000000000000..3a330b3d5965 --- /dev/null +++ b/clang/test/Sema/attr-loader-uninitialized.cpp @@ -0,0 +1,60 @@ +// RUN: %clang_cc1 %s -verify -fsyntax-only + +int good __attribute__((loader_uninitialized)); +static int local_ok __attribute__((loader_uninitialized)); +int hidden_ok __attribute__((visibility("hidden"))) __attribute__((loader_uninitialized)); + +const int still_cant_be_const __attribute__((loader_uninitialized)); +// expected-error@-1 {{default initialization of an object of const type}} +extern int external_rejected __attribute__((loader_uninitialized)); +// expected-error@-1 {{variable 'external_rejected' cannot be declared both 'extern' and with the 'loader_uninitialized' attribute}} + +int noargs __attribute__((loader_uninitialized(0))); +// expected-error@-1 {{'loader_uninitialized' attribute takes no arguments}} + +int init_rejected __attribute__((loader_uninitialized)) = 42; +// expected-error@-1 {{variable with 'loader_uninitialized' attribute cannot have an initializer}} + +void func() __attribute__((loader_uninitialized)) +// expected-warning@-1 {{'loader_uninitialized' attribute only applies to global variables}} +{ + int local __attribute__((loader_uninitialized)); + // expected-warning@-1 {{'loader_uninitialized' attribute only applies to global variables}} + + static int sl __attribute__((loader_uninitialized)); +} + +struct s { + __attribute__((loader_uninitialized)) int field; + // expected-warning@-1 {{'loader_uninitialized' attribute only applies to global variables}} + + static __attribute__((loader_uninitialized)) int sfield; + +} __attribute__((loader_uninitialized)); +// expected-warning@-1 {{'loader_uninitialized' attribute only applies to global variables}} + +int redef_attr_first __attribute__((loader_uninitialized)); +int redef_attr_first; +// expected-error@-1 {{redefinition of 'redef_attr_first'}} +// expected-note@-3 {{previous definition is here}} + +int redef_attr_second; +int redef_attr_second __attribute__((loader_uninitialized)); +// expected-warning@-1 {{attribute declaration must precede definition}} +// expected-note@-3 {{previous definition is here}} +// expected-error@-3 {{redefinition of 'redef_attr_second'}} +// expected-note@-5 {{previous definition is here}} + +struct trivial {}; + +trivial default_ok __attribute__((loader_uninitialized)); +trivial value_rejected __attribute__((loader_uninitialized)) {}; +// expected-error@-1 {{variable with 'loader_uninitialized' attribute cannot have an initializer}} + +struct nontrivial +{ + nontrivial() {} +}; + +nontrivial needs_trivial_ctor __attribute__((loader_uninitialized)); +// expected-error@-1 {{variable with 'loader_uninitialized' attribute must have a trivial default constructor}} _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
