[HLSL][RootSignature] Add parsing for empty RootDescriptors (#140147)

- define the RootDescriptor in-memory struct containing its type
- add test harness for testing

First part of https://github.com/llvm/llvm-project/issues/126577

clang/include/clang/Parse/ParseHLSLRootSignature.h

@@ -73,6 +73,7 @@ private:
   /// Root Element parse methods:
   std::optional<llvm::hlsl::rootsig::RootFlags> parseRootFlags();
   std::optional<llvm::hlsl::rootsig::RootConstants> parseRootConstants();
+  std::optional<llvm::hlsl::rootsig::RootDescriptor> parseRootDescriptor();
   std::optional<llvm::hlsl::rootsig::DescriptorTable> parseDescriptorTable();
   std::optional<llvm::hlsl::rootsig::DescriptorTableClause>
   parseDescriptorTableClause();

clang/lib/Parse/ParseHLSLRootSignature.cpp

@@ -47,6 +47,14 @@ bool RootSignatureParser::parse() {
         return true;
       Elements.push_back(*Table);
     }
+
+    if (tryConsumeExpectedToken(
+            {TokenKind::kw_CBV, TokenKind::kw_SRV, TokenKind::kw_UAV})) {
+      auto Descriptor = parseRootDescriptor();
+      if (!Descriptor.has_value())
+        return true;
+      Elements.push_back(*Descriptor);
+    }
   } while (tryConsumeExpectedToken(TokenKind::pu_comma));
 
   return consumeExpectedToken(TokenKind::end_of_stream,
@@ -155,6 +163,41 @@ std::optional<RootConstants> RootSignatureParser::parseRootConstants() {
   return Constants;
 }
 
+std::optional<RootDescriptor> RootSignatureParser::parseRootDescriptor() {
+  assert((CurToken.TokKind == TokenKind::kw_CBV ||
+          CurToken.TokKind == TokenKind::kw_SRV ||
+          CurToken.TokKind == TokenKind::kw_UAV) &&
+         "Expects to only be invoked starting at given keyword");
+
+  TokenKind DescriptorKind = CurToken.TokKind;
+
+  if (consumeExpectedToken(TokenKind::pu_l_paren, diag::err_expected_after,
+                           CurToken.TokKind))
+    return std::nullopt;
+
+  RootDescriptor Descriptor;
+  switch (DescriptorKind) {
+  default:
+    llvm_unreachable("Switch for consumed token was not provided");
+  case TokenKind::kw_CBV:
+    Descriptor.Type = DescriptorType::CBuffer;
+    break;
+  case TokenKind::kw_SRV:
+    Descriptor.Type = DescriptorType::SRV;
+    break;
+  case TokenKind::kw_UAV:
+    Descriptor.Type = DescriptorType::UAV;
+    break;
+  }
+
+  if (consumeExpectedToken(TokenKind::pu_r_paren,
+                           diag::err_hlsl_unexpected_end_of_params,
+                           /*param of=*/TokenKind::kw_RootConstants))
+    return std::nullopt;
+
+  return Descriptor;
+}
+
 std::optional<DescriptorTable> RootSignatureParser::parseDescriptorTable() {
   assert(CurToken.TokKind == TokenKind::kw_DescriptorTable &&
          "Expects to only be invoked starting at given keyword");

clang/unittests/Parse/ParseHLSLRootSignatureTest.cpp

@@ -344,6 +344,43 @@ TEST_F(ParseHLSLRootSignatureTest, ValidParseRootFlagsTest) {
   ASSERT_TRUE(Consumer->isSatisfied());
 }
 
+TEST_F(ParseHLSLRootSignatureTest, ValidParseRootDescriptorsTest) {
+  const llvm::StringLiteral Source = R"cc(
+    CBV(),
+    SRV(),
+    UAV()
+  )cc";
+
+  TrivialModuleLoader ModLoader;
+  auto PP = createPP(Source, ModLoader);
+  auto TokLoc = SourceLocation();
+
+  hlsl::RootSignatureLexer Lexer(Source, TokLoc);
+  SmallVector<RootElement> Elements;
+  hlsl::RootSignatureParser Parser(Elements, Lexer, *PP);
+
+  // Test no diagnostics produced
+  Consumer->setNoDiag();
+
+  ASSERT_FALSE(Parser.parse());
+
+  ASSERT_EQ(Elements.size(), 3u);
+
+  RootElement Elem = Elements[0];
+  ASSERT_TRUE(std::holds_alternative<RootDescriptor>(Elem));
+  ASSERT_EQ(std::get<RootDescriptor>(Elem).Type, DescriptorType::CBuffer);
+
+  Elem = Elements[1];
+  ASSERT_TRUE(std::holds_alternative<RootDescriptor>(Elem));
+  ASSERT_EQ(std::get<RootDescriptor>(Elem).Type, DescriptorType::SRV);
+
+  Elem = Elements[2];
+  ASSERT_TRUE(std::holds_alternative<RootDescriptor>(Elem));
+  ASSERT_EQ(std::get<RootDescriptor>(Elem).Type, DescriptorType::UAV);
+
+  ASSERT_TRUE(Consumer->isSatisfied());
+}
+
 TEST_F(ParseHLSLRootSignatureTest, ValidTrailingCommaTest) {
   // This test will checks we can handling trailing commas ','
   const llvm::StringLiteral Source = R"cc(

llvm/include/llvm/Frontend/HLSL/HLSLRootSignature.h

@@ -85,6 +85,12 @@ struct RootConstants {
   ShaderVisibility Visibility = ShaderVisibility::All;
 };
 
+using DescriptorType = llvm::dxil::ResourceClass;
+// Models RootDescriptor : CBV | SRV | UAV, by collecting like parameters
+struct RootDescriptor {
+  DescriptorType Type;
+};
+
 // Models the end of a descriptor table and stores its visibility
 struct DescriptorTable {
   ShaderVisibility Visibility = ShaderVisibility::All;
@@ -125,8 +131,8 @@ struct DescriptorTableClause {
   void dump(raw_ostream &OS) const;
 };
 
-/// Models RootElement : RootFlags | RootConstants | DescriptorTable
-///  | DescriptorTableClause
+/// Models RootElement : RootFlags | RootConstants | RootDescriptor
+///  | DescriptorTable | DescriptorTableClause
 ///
 /// A Root Signature is modeled in-memory by an array of RootElements. These
 /// aim to map closely to their DSL grammar reprsentation defined in the spec.
@@ -140,8 +146,8 @@ struct DescriptorTableClause {
 /// The DescriptorTable is modelled by having its Clauses as the previous
 /// RootElements in the array, and it holds a data member for the Visibility
 /// parameter.
-using RootElement = std::variant<RootFlags, RootConstants, DescriptorTable,
-                                 DescriptorTableClause>;
+using RootElement = std::variant<RootFlags, RootConstants, RootDescriptor,
+                                 DescriptorTable, DescriptorTableClause>;
 
 void dumpRootElements(raw_ostream &OS, ArrayRef<RootElement> Elements);