[HLSL][SPIRV] Add vk::constant_id attribute.

The vk::constant_id attribute is used to indicate that a global const variable
represents a specialization constant in SPIR-V. This PR adds this
attribute to clang.

The documetation for the attribute is [here](https://github.com/microsoft/DirectXShaderCompiler/blob/main/docs/SPIR-V.rst#specialization-constants).

The strategy is to to modify the initializer to get the value of a
specialize constant for a builtin defined in the SPIR-V backend.

Author: s-perron

clang/include/clang/Basic/Attr.td

@@ -4993,6 +4993,14 @@ def HLSLVkExtBuiltinInput : InheritableAttr {
   let Documentation = [HLSLVkExtBuiltinInputDocs];
 }
 
+def HLSLVkConstantId : InheritableAttr {
+  let Spellings = [CXX11<"vk", "constant_id">];
+  let Args = [IntArgument<"Id">];
+  let Subjects = SubjectList<[ExternalGlobalVar]>;
+  let LangOpts = [HLSL];
+  let Documentation = [VkConstantIdDocs];
+}
+
 def RandomizeLayout : InheritableAttr {
   let Spellings = [GCC<"randomize_layout">];
   let Subjects = SubjectList<[Record]>;

clang/include/clang/Basic/AttrDocs.td

@@ -8252,6 +8252,21 @@ and https://microsoft.github.io/hlsl-specs/proposals/0013-wave-size-range.html
   }];
 }
 
+def VkConstantIdDocs : Documentation {
+  let Category = DocCatFunction;
+  let Content = [{
+The ``vk::constant_id`` attribute specify the id for a SPIR-V specialization
+constant. The attribute applies to const global scalar variables. The variable must be initialized with a C++11 constexpr.
+In SPIR-V, the
+variable will be replaced with an `OpSpecConstant` with the given id.
+The syntax is:
+
+.. code-block:: text
+
+  ``[[vk::constant_id(<Id>)]] const T Name = <Init>``
+}];
+}
+
 def RootSignatureDocs : Documentation {
   let Category = DocCatFunction;
   let Content = [{

clang/include/clang/Basic/Builtins.td

@@ -5065,6 +5065,19 @@ def HLSLGroupMemoryBarrierWithGroupSync: LangBuiltin<"HLSL_LANG"> {
   let Prototype = "void()";
 }
 
+class HLSLScalarTemplate
+    : Template<["bool", "char", "short", "int", "long long int",
+                "unsigned short", "unsigned int", "unsigned long long int",
+                "__fp16", "float", "double"],
+               ["_bool", "_char", "_short", "_int", "_longlong", "_ushort",
+                "_uint", "_ulonglong", "_half", "_float", "_double"]>;
+
+def HLSLGetSpirvSpecConstant : LangBuiltin<"HLSL_LANG">, HLSLScalarTemplate {
+  let Spellings = ["__builtin_get_spirv_spec_constant"];
+  let Attributes = [NoThrow, Const, Pure];
+  let Prototype = "T(unsigned int, T)";
+}
+
 // Builtins for XRay.
 def XRayCustomEvent : Builtin {
   let Spellings = ["__xray_customevent"];

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -12919,6 +12919,18 @@ def err_spirv_enum_not_int : Error<
 def err_spirv_enum_not_valid : Error<
    "invalid value for %select{storage class}0 argument">;
 
+def err_specialization_const_lit_init
+    : Error<"variable with 'vk::constant_id' attribute cannot have an "
+            "initializer that is not a constexpr">;
+def err_specialization_const_missing_initializer
+    : Error<
+          "variable with 'vk::constant_id' attribute must have an initializer">;
+def err_specialization_const_missing_const
+    : Error<"variable with 'vk::constant_id' attribute must be const">;
+def err_specialization_const_is_not_int_or_float
+    : Error<"variable with 'vk::constant_id' attribute must be an enum, bool, "
+            "integer, or floating point value">;
+
 // errors of expect.with.probability
 def err_probability_not_constant_float : Error<
    "probability argument to __builtin_expect_with_probability must be constant "

clang/include/clang/Sema/SemaHLSL.h

@@ -98,6 +98,8 @@ class SemaHLSL : public SemaBase {
   HLSLWaveSizeAttr *mergeWaveSizeAttr(Decl *D, const AttributeCommonInfo &AL,
                                       int Min, int Max, int Preferred,
                                       int SpelledArgsCount);
+  HLSLVkConstantIdAttr *
+  mergeVkConstantIdAttr(Decl *D, const AttributeCommonInfo &AL, int Id);
   HLSLShaderAttr *mergeShaderAttr(Decl *D, const AttributeCommonInfo &AL,
                                   llvm::Triple::EnvironmentType ShaderType);
   HLSLParamModifierAttr *
@@ -122,6 +124,7 @@ class SemaHLSL : public SemaBase {
   void handleRootSignatureAttr(Decl *D, const ParsedAttr &AL);
   void handleNumThreadsAttr(Decl *D, const ParsedAttr &AL);
   void handleWaveSizeAttr(Decl *D, const ParsedAttr &AL);
+  void handleVkConstantIdAttr(Decl *D, const ParsedAttr &AL);
   void handleSV_DispatchThreadIDAttr(Decl *D, const ParsedAttr &AL);
   void handleSV_GroupThreadIDAttr(Decl *D, const ParsedAttr &AL);
   void handleSV_GroupIDAttr(Decl *D, const ParsedAttr &AL);
@@ -156,7 +159,7 @@ class SemaHLSL : public SemaBase {
   QualType getInoutParameterType(QualType Ty);
 
   bool transformInitList(const InitializedEntity &Entity, InitListExpr *Init);
-
+  bool handleInitialization(VarDecl *VDecl, Expr *&Init);
   void deduceAddressSpace(VarDecl *Decl);
 
 private:

clang/lib/Basic/Attributes.cpp

@@ -213,7 +213,8 @@ getScopeFromNormalizedScopeName(StringRef ScopeName) {
       .Case("vk", AttributeCommonInfo::Scope::VK)
       .Case("msvc", AttributeCommonInfo::Scope::MSVC)
       .Case("omp", AttributeCommonInfo::Scope::OMP)
-      .Case("riscv", AttributeCommonInfo::Scope::RISCV);
+      .Case("riscv", AttributeCommonInfo::Scope::RISCV)
+      .Case("vk", AttributeCommonInfo::Scope::HLSL);
 }
 
 unsigned AttributeCommonInfo::calculateAttributeSpellingListIndex() const {

clang/lib/CodeGen/CGHLSLBuiltins.cpp

@@ -12,6 +12,7 @@
 
 #include "CGBuiltin.h"
 #include "CGHLSLRuntime.h"
+#include "CodeGenFunction.h"
 
 using namespace clang;
 using namespace CodeGen;
@@ -774,6 +775,77 @@ Value *CodeGenFunction::EmitHLSLBuiltinExpr(unsigned BuiltinID,
     return EmitRuntimeCall(
         Intrinsic::getOrInsertDeclaration(&CGM.getModule(), ID));
   }
+  case Builtin::BI__builtin_get_spirv_spec_constant_bool:
+  case Builtin::BI__builtin_get_spirv_spec_constant_short:
+  case Builtin::BI__builtin_get_spirv_spec_constant_ushort:
+  case Builtin::BI__builtin_get_spirv_spec_constant_int:
+  case Builtin::BI__builtin_get_spirv_spec_constant_uint:
+  case Builtin::BI__builtin_get_spirv_spec_constant_longlong:
+  case Builtin::BI__builtin_get_spirv_spec_constant_ulonglong:
+  case Builtin::BI__builtin_get_spirv_spec_constant_half:
+  case Builtin::BI__builtin_get_spirv_spec_constant_float:
+  case Builtin::BI__builtin_get_spirv_spec_constant_double: {
+    llvm::Function *SpecConstantFn = getSpecConstantFunction(E->getType());
+    llvm::Value *SpecId = EmitScalarExpr(E->getArg(0));
+    llvm::Value *DefaultVal = EmitScalarExpr(E->getArg(1));
+    llvm::Value *Args[] = {SpecId, DefaultVal};
+    return Builder.CreateCall(SpecConstantFn, Args);
+  }
   }
   return nullptr;
 }
+
+llvm::Function *clang::CodeGen::CodeGenFunction::getSpecConstantFunction(
+    const clang::QualType &SpecConstantType) {
+
+  // Find or create the declaration for the function.
+  llvm::Module *M = &CGM.getModule();
+  std::string MangledName = getSpecConstantFunctionName(SpecConstantType);
+  llvm::Function *SpecConstantFn = M->getFunction(MangledName);
+
+  if (!SpecConstantFn) {
+    llvm::Type *IntType = ConvertType(getContext().IntTy);
+    llvm::Type *RetTy = ConvertType(SpecConstantType);
+    llvm::Type *ArgTypes[] = {IntType, RetTy};
+    llvm::FunctionType *FnTy = llvm::FunctionType::get(RetTy, ArgTypes, false);
+    SpecConstantFn = llvm::Function::Create(
+        FnTy, llvm::GlobalValue::ExternalLinkage, MangledName, M);
+  }
+  return SpecConstantFn;
+}
+
+std::string clang::CodeGen::CodeGenFunction::getSpecConstantFunctionName(
+    const clang::QualType &SpecConstantType) {
+  // The parameter types for our conceptual intrinsic function.
+  ASTContext &Context = getContext();
+  QualType ClangParamTypes[] = {Context.IntTy, SpecConstantType};
+
+  // Create a temporary FunctionDecl for the builtin fuction. It won't be
+  // added to the AST.
+  FunctionProtoType::ExtProtoInfo EPI;
+  QualType FnType =
+      Context.getFunctionType(SpecConstantType, ClangParamTypes, EPI);
+  DeclarationName FuncName = &Context.Idents.get("__spirv_SpecConstant");
+  FunctionDecl *FnDeclForMangling = FunctionDecl::Create(
+      Context, Context.getTranslationUnitDecl(), SourceLocation(),
+      SourceLocation(), FuncName, FnType, /*TSI=*/nullptr, SC_Extern);
+
+  // Attach the created parameter declarations to the function declaration.
+  SmallVector<ParmVarDecl *, 2> ParamDecls;
+  for (QualType ParamType : ClangParamTypes) {
+    ParmVarDecl *PD = ParmVarDecl::Create(
+        Context, FnDeclForMangling, SourceLocation(), SourceLocation(),
+        /*IdentifierInfo*/ nullptr, ParamType, /*TSI*/ nullptr, SC_None,
+        /*DefaultArg*/ nullptr);
+    ParamDecls.push_back(PD);
+  }
+  FnDeclForMangling->setParams(ParamDecls);
+
+  // Get the mangled name.
+  std::string Name;
+  llvm::raw_string_ostream MangledNameStream(Name);
+  MangleContext *Mangler = Context.createMangleContext();
+  Mangler->mangleName(FnDeclForMangling, MangledNameStream);
+  MangledNameStream.flush();
+  return Name;
+}

clang/lib/CodeGen/CodeGenFunction.h

@@ -4850,6 +4850,17 @@ class CodeGenFunction : public CodeGenTypeCache {
   llvm::Value *EmitAMDGPUBuiltinExpr(unsigned BuiltinID, const CallExpr *E);
   llvm::Value *EmitHLSLBuiltinExpr(unsigned BuiltinID, const CallExpr *E,
                                    ReturnValueSlot ReturnValue);
+
+  // Returns a builtin function that the SPIR-V backend will expand into a spec
+  // constant.
+  llvm::Function *
+  getSpecConstantFunction(const clang::QualType &SpecConstantType);
+
+  // Returns the mangled name for a builtin function that the SPIR-V backend
+  // will expand into a spec Constant.
+  std::string
+  getSpecConstantFunctionName(const clang::QualType &SpecConstantType);
+
   llvm::Value *EmitDirectXBuiltinExpr(unsigned BuiltinID, const CallExpr *E);
   llvm::Value *EmitSPIRVBuiltinExpr(unsigned BuiltinID, const CallExpr *E);
   llvm::Value *EmitScalarOrConstFoldImmArg(unsigned ICEArguments, unsigned Idx,

clang/lib/Sema/SemaDecl.cpp

@@ -2889,6 +2889,8 @@ static bool mergeDeclAttribute(Sema &S, NamedDecl *D,
     NewAttr = S.HLSL().mergeWaveSizeAttr(D, *WS, WS->getMin(), WS->getMax(),
                                          WS->getPreferred(),
                                          WS->getSpelledArgsCount());
+  else if (const auto *CI = dyn_cast<HLSLVkConstantIdAttr>(Attr))
+    NewAttr = S.HLSL().mergeVkConstantIdAttr(D, *CI, CI->getId());
   else if (const auto *SA = dyn_cast<HLSLShaderAttr>(Attr))
     NewAttr = S.HLSL().mergeShaderAttr(D, *SA, SA->getType());
   else if (isa<SuppressAttr>(Attr))
@@ -13755,6 +13757,10 @@ void Sema::AddInitializerToDecl(Decl *RealDecl, Expr *Init, bool DirectInit) {
     return;
   }
 
+  if (getLangOpts().HLSL)
+    if (!HLSL().handleInitialization(VDecl, Init))
+      return;
+
   // Get the decls type and save a reference for later, since
   // CheckInitializerTypes may change it.
   QualType DclT = VDecl->getType(), SavT = DclT;
@@ -14215,6 +14221,14 @@ void Sema::ActOnUninitializedDecl(Decl *RealDecl) {
       }
     }
 
+    // HLSL variable with the `vk::constant_id` attribute must be initialized.
+    if (!Var->isInvalidDecl() && Var->hasAttr<HLSLVkConstantIdAttr>()) {
+      Diag(Var->getLocation(),
+           diag::err_specialization_const_missing_initializer);
+      Var->setInvalidDecl();
+      return;
+    }
+
     if (!Var->isInvalidDecl() && RealDecl->hasAttr<LoaderUninitializedAttr>()) {
       if (Var->getStorageClass() == SC_Extern) {
         Diag(Var->getLocation(), diag::err_loader_uninitialized_extern_decl)

clang/lib/Sema/SemaDeclAttr.cpp

@@ -7560,6 +7560,9 @@ ProcessDeclAttribute(Sema &S, Scope *scope, Decl *D, const ParsedAttr &AL,
   case ParsedAttr::AT_HLSLVkExtBuiltinInput:
     S.HLSL().handleVkExtBuiltinInputAttr(D, AL);
     break;
+  case ParsedAttr::AT_HLSLVkConstantId:
+    S.HLSL().handleVkConstantIdAttr(D, AL);
+    break;
   case ParsedAttr::AT_HLSLSV_GroupThreadID:
     S.HLSL().handleSV_GroupThreadIDAttr(D, AL);
     break;