[clang] [llvm] [HLSL] Add support for user semantics (PR #153424)

[Steven Perron via llvm-commits]

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@@ -562,6 +572,69 @@ static llvm::Value *createSPIRVBuiltinLoad(IRBuilder<> &B, llvm::Module &M,
   return B.CreateLoad(Ty, GV);
 }
 
+static llvm::Value *createSPIRVLocationLoad(IRBuilder<> &B, llvm::Module &M,
+                                            llvm::Type *Ty, unsigned Location,
+                                            StringRef Name = "") {
+  auto *GV = new llvm::GlobalVariable(
+      M, Ty, /* isConstant= */ true, llvm::GlobalValue::ExternalLinkage,
+      /* Initializer= */ nullptr, /* Name= */ Name, /* insertBefore= */ nullptr,
+      llvm::GlobalVariable::GeneralDynamicTLSModel,
+      /* AddressSpace */ 7, /* isExternallyInitialized= */ true);
+  GV->setVisibility(llvm::GlobalValue::HiddenVisibility);
+  addLocationDecoration(GV, Location);
+  return B.CreateLoad(Ty, GV);
+}
+
+llvm::Value *
+CGHLSLRuntime::emitSPIRVUserSemanticLoad(llvm::IRBuilder<> &B, llvm::Type *Type,
+                                         HLSLSemanticAttr *Semantic,
+                                         std::optional<unsigned> Index) {
+  Twine BaseName = Twine(Semantic->getAttrName()->getName());
+  Twine VariableName = BaseName.concat(Twine(Index.value_or(0)));
+
+  unsigned Location = SPIRVLastAssignedInputSemanticLocation;
+
+  // DXC completely ignores the semantic/index pair. Location are assigned from
+  // the first semantic to the last.
----------------
s-perron wrote:

Can we give an reason for ignoring the semantic/index pair when assigning locations? Is the only reason we are doing for compatibility with DXC, which is a good reason.

https://github.com/llvm/llvm-project/pull/153424