[HLSL][SPIR-V] Implement vk::push_constant

Implements initial support for vk::push_constant.
As is, this allows handling simple push constants, but has one
main issue: layout can be incorrect (See #168401). The layout
issue being not only push-constant related, it's ignored for this PR.

The frontend part of the implementation is straightforward:
 - adding a new attribute
 - when targeting vulkan/spirv, we process it
 - global variables with this attribute gets a new AS:
   hlsl_push_constant

The IR has nothing specific, only some RO globals in this new AS.

On the SPIR-V side, we not convert this AS into a PushConstant storage
class. But this creates some issues: the variables in this storage class
must have a specific set of decoration to define their layout.

Current infra to create the SPIR-V types lacks the context required to
make this decision: no indication on the AS or context around the type
being created. Refactoring this would be a heavy task as it would
require getting this information in every place using the GR for type
creation.

Instead, we do something similar to CBuffers:
 - find all globals with this address space, and change their type to
   a target-specific type.
 - insert a new intrinsic in place of every reference to this global
   variable.

This allow the backend to handle both layout variables loads and type
lowering independently.

Type lowering has nothing specific: when we encounter a target extension
type with spirv.PushConstant, we lower this to the correct SPIR-V type
with the proper offset & block decorations.

As for the intrinsic, it's mostly a no-op, but required since we have
this target-specific type.

Author: Keenuts

clang/include/clang/Basic/AddressSpaces.h

@@ -62,6 +62,7 @@ enum class LangAS : unsigned {
   hlsl_private,
   hlsl_device,
   hlsl_input,
+  hlsl_push_constant,
 
   // Wasm specific address spaces.
   wasm_funcref,

clang/include/clang/Basic/Attr.td

@@ -5164,6 +5164,14 @@ def HLSLVkExtBuiltinInput : InheritableAttr {
   let Documentation = [HLSLVkExtBuiltinInputDocs];
 }
 
+def HLSLVkPushConstant : InheritableAttr {
+  let Spellings = [CXX11<"vk", "push_constant">];
+  let Args = [];
+  let Subjects = SubjectList<[GlobalVar], ErrorDiag>;
+  let LangOpts = [HLSL];
+  let Documentation = [HLSLVkPushConstantDocs];
+}
+
 def HLSLVkConstantId : InheritableAttr {
   let Spellings = [CXX11<"vk", "constant_id">];
   let Args = [IntArgument<"Id">];

clang/include/clang/Basic/AttrDocs.td

@@ -8794,6 +8794,11 @@ https://github.com/microsoft/hlsl-specs/blob/main/proposals/0011-inline-spirv.md
   }];
 }
 
+def HLSLVkPushConstantDocs : Documentation {
+  let Category = DocCatVariable;
+  let Content = [{ FIXME }];
+}
+
 def AnnotateTypeDocs : Documentation {
   let Category = DocCatType;
   let Heading = "annotate_type";

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -13216,6 +13216,9 @@ def err_hlsl_attr_invalid_type : Error<
    "attribute %0 only applies to a field or parameter of type '%1'">;
 def err_hlsl_attr_invalid_ast_node : Error<
    "attribute %0 only applies to %1">;
+def err_hlsl_attr_incompatible
+    : Error<"%0 attribute is not compatible with %1 attribute">;
+
 def err_hlsl_entry_shader_attr_mismatch : Error<
    "%0 attribute on entry function does not match the target profile">;
 def err_hlsl_numthreads_argument_oor : Error<"argument '%select{X|Y|Z}0' to numthreads attribute cannot exceed %1">;
@@ -13333,6 +13336,9 @@ def err_hlsl_incomplete_resource_array_in_function_param: Error<
 def err_hlsl_assign_to_global_resource: Error<
   "assignment to global resource variable %0 is not allowed">;
 
+def err_hlsl_push_constant_unique
+    : Error<"cannot have more than one push constant block">;
+
 // Layout randomization diagnostics.
 def err_non_designated_init_used : Error<
   "a randomized struct can only be initialized with a designated initializer">;

clang/include/clang/Basic/HLSLRuntime.h

@@ -14,6 +14,7 @@
 #ifndef CLANG_BASIC_HLSLRUNTIME_H
 #define CLANG_BASIC_HLSLRUNTIME_H
 
+#include "clang/Basic/AddressSpaces.h"
 #include "clang/Basic/LangOptions.h"
 #include <cstdint>
 
@@ -30,6 +31,10 @@ getStageFromEnvironment(const llvm::Triple::EnvironmentType &E) {
   return static_cast<ShaderStage>(Pipeline);
 }
 
+constexpr bool isInitializedByPipeline(LangAS AS) {
+  return AS == LangAS::hlsl_input || AS == LangAS::hlsl_push_constant;
+}
+
 #define ENUM_COMPARE_ASSERT(Value)                                             \
   static_assert(                                                               \
       getStageFromEnvironment(llvm::Triple::Value) == ShaderStage::Value,      \

clang/include/clang/Sema/SemaHLSL.h

@@ -188,6 +188,7 @@ class SemaHLSL : public SemaBase {
   void handleSemanticAttr(Decl *D, const ParsedAttr &AL);
 
   void handleVkExtBuiltinInputAttr(Decl *D, const ParsedAttr &AL);
+  void handleVkPushConstantAttr(Decl *D, const ParsedAttr &AL);
 
   bool CheckBuiltinFunctionCall(unsigned BuiltinID, CallExpr *TheCall);
   QualType ProcessResourceTypeAttributes(QualType Wrapped);
@@ -237,6 +238,8 @@ class SemaHLSL : public SemaBase {
 
   IdentifierInfo *RootSigOverrideIdent = nullptr;
 
+  bool HasDeclaredAPushConstant = false;
+
   // Information about the current subtree being flattened.
   struct SemanticInfo {
     HLSLParsedSemanticAttr *Semantic;

clang/lib/AST/Type.cpp

@@ -101,6 +101,7 @@ bool Qualifiers::isTargetAddressSpaceSupersetOf(LangAS A, LangAS B,
          (A == LangAS::Default && B == LangAS::hlsl_private) ||
          (A == LangAS::Default && B == LangAS::hlsl_device) ||
          (A == LangAS::Default && B == LangAS::hlsl_input) ||
+         (A == LangAS::Default && B == LangAS::hlsl_push_constant) ||
          // Conversions from target specific address spaces may be legal
          // depending on the target information.
          Ctx.getTargetInfo().isAddressSpaceSupersetOf(A, B);

clang/lib/AST/TypePrinter.cpp

@@ -2694,6 +2694,8 @@ std::string Qualifiers::getAddrSpaceAsString(LangAS AS) {
     return "hlsl_device";
   case LangAS::hlsl_input:
     return "hlsl_input";
+  case LangAS::hlsl_push_constant:
+    return "hlsl_push_constant";
   case LangAS::wasm_funcref:
     return "__funcref";
   default:

clang/lib/Basic/TargetInfo.cpp

@@ -52,6 +52,7 @@ static const LangASMap FakeAddrSpaceMap = {
     15, // hlsl_private
     16, // hlsl_device
     17, // hlsl_input
+    18, // hlsl_push_constant
     20, // wasm_funcref
 };
 

clang/lib/Basic/Targets/AArch64.h

@@ -48,6 +48,7 @@ static const unsigned ARM64AddrSpaceMap[] = {
     0, // hlsl_private
     0, // hlsl_device
     0, // hlsl_input
+    0, // hlsl_push_constant
     // Wasm address space values for this target are dummy values,
     // as it is only enabled for Wasm targets.
     20, // wasm_funcref