[clang] c45eaea - [Clang] Undef attribute for global variables
Jon Chesterfield via cfe-commits
cfe-commits at lists.llvm.org
Tue Mar 17 14:22:38 PDT 2020
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 at -1 {{variable 'external_rejected' cannot be declared both 'extern' and with the 'loader_uninitialized' attribute}}
+
+int noargs __attribute__((loader_uninitialized(0)));
+// expected-error at -1 {{'loader_uninitialized' attribute takes no arguments}}
+
+int init_rejected __attribute__((loader_uninitialized)) = 42;
+// expected-error at -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 at -1 {{redeclaration cannot add 'loader_uninitialized' attribute}}
+// expected-note at -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 at -1 {{variable with 'loader_uninitialized' attribute cannot have an initializer}}
+
+extern __attribute__((visibility("hidden"))) int extern_hidden __attribute__((loader_uninitialized));
+// expected-error at -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 at -1 {{default initialization of an object of const type}}
+extern int external_rejected __attribute__((loader_uninitialized));
+// expected-error at -1 {{variable 'external_rejected' cannot be declared both 'extern' and with the 'loader_uninitialized' attribute}}
+
+int noargs __attribute__((loader_uninitialized(0)));
+// expected-error at -1 {{'loader_uninitialized' attribute takes no arguments}}
+
+int init_rejected __attribute__((loader_uninitialized)) = 42;
+// expected-error at -1 {{variable with 'loader_uninitialized' attribute cannot have an initializer}}
+
+void func() __attribute__((loader_uninitialized))
+// expected-warning at -1 {{'loader_uninitialized' attribute only applies to global variables}}
+{
+ int local __attribute__((loader_uninitialized));
+ // expected-warning at -1 {{'loader_uninitialized' attribute only applies to global variables}}
+
+ static int sl __attribute__((loader_uninitialized));
+}
+
+struct s {
+ __attribute__((loader_uninitialized)) int field;
+ // expected-warning at -1 {{'loader_uninitialized' attribute only applies to global variables}}
+
+ static __attribute__((loader_uninitialized)) int sfield;
+
+} __attribute__((loader_uninitialized));
+// expected-warning at -1 {{'loader_uninitialized' attribute only applies to global variables}}
+
+int redef_attr_first __attribute__((loader_uninitialized));
+int redef_attr_first;
+// expected-error at -1 {{redefinition of 'redef_attr_first'}}
+// expected-note at -3 {{previous definition is here}}
+
+int redef_attr_second;
+int redef_attr_second __attribute__((loader_uninitialized));
+// expected-warning at -1 {{attribute declaration must precede definition}}
+// expected-note at -3 {{previous definition is here}}
+// expected-error at -3 {{redefinition of 'redef_attr_second'}}
+// expected-note at -5 {{previous definition is here}}
+
+struct trivial {};
+
+trivial default_ok __attribute__((loader_uninitialized));
+trivial value_rejected __attribute__((loader_uninitialized)) {};
+// expected-error at -1 {{variable with 'loader_uninitialized' attribute cannot have an initializer}}
+
+struct nontrivial
+{
+ nontrivial() {}
+};
+
+nontrivial needs_trivial_ctor __attribute__((loader_uninitialized));
+// expected-error at -1 {{variable with 'loader_uninitialized' attribute must have a trivial default constructor}}
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