[HLSL][RootSignature] Add metadata generation for descriptor tables (#139633)

- prereq: Modify `RootSignatureAttr` to hold a reference to the owned
declaration
- Define and implement `MetadataBuilder` in `HLSLRootSignature`
- Integrate and invoke the builder in `CGHLSLRuntime.cpp` to generate
the Root Signature for any associated entry functions
- Add tests to demonstrate functionality in `RootSignature.hlsl`

Resolves https://github.com/llvm/llvm-project/issues/126584

Note: this is essentially just
https://github.com/llvm/llvm-project/pull/125131 rebased onto the new
approach of constructing a root signature decl, instead of holding the
elements in `AdditionalMembers`.

1 files changed, 21 insertions, 0 deletions

diff --git a/clang/lib/CodeGen/CGHLSLRuntime.cpp b/clang/lib/CodeGen/CGHLSLRuntime.cpp
index 0eb4bb0..5bc71a9 100644
--- a/clang/lib/CodeGen/CGHLSLRuntime.cpp
+++ b/clang/lib/CodeGen/CGHLSLRuntime.cpp
@@ -68,6 +68,20 @@ void addDxilValVersion(StringRef ValVersionStr, llvm::Module &M) {
   DXILValMD->addOperand(Val);
 }
 
+void addRootSignature(ArrayRef<llvm::hlsl::rootsig::RootElement> Elements,
+                      llvm::Function *Fn, llvm::Module &M) {
+  auto &Ctx = M.getContext();
+
+  llvm::hlsl::rootsig::MetadataBuilder Builder(Ctx, Elements);
+  MDNode *RootSignature = Builder.BuildRootSignature();
+  MDNode *FnPairing =
+      MDNode::get(Ctx, {ValueAsMetadata::get(Fn), RootSignature});
+
+  StringRef RootSignatureValKey = "dx.rootsignatures";
+  auto *RootSignatureValMD = M.getOrInsertNamedMetadata(RootSignatureValKey);
+  RootSignatureValMD->addOperand(FnPairing);
+}
+
 } // namespace
 
 llvm::Type *
@@ -423,6 +437,13 @@ void CGHLSLRuntime::emitEntryFunction(const FunctionDecl *FD,
   // FIXME: Handle codegen for return type semantics.
   // See: https://github.com/llvm/llvm-project/issues/57875
   B.CreateRetVoid();
+
+  // Add and identify root signature to function, if applicable
+  for (const Attr *Attr : FD->getAttrs()) {
+    if (const auto *RSAttr = dyn_cast<RootSignatureAttr>(Attr))
+      addRootSignature(RSAttr->getSignatureDecl()->getRootElements(), EntryFn,
+                       M);
+  }
 }
 
 void CGHLSLRuntime::setHLSLFunctionAttributes(const FunctionDecl *FD,