[clang] [llvm] [HLSL][RootSignature] Implement parsing of a DescriptorTable with empty clauses (PR #133302)

[Justin Bogner via llvm-commits]

================
@@ -0,0 +1,188 @@
+#include "clang/Parse/ParseHLSLRootSignature.h"
+
+#include "llvm/Support/raw_ostream.h"
+
+using namespace llvm::hlsl::rootsig;
+
+namespace clang {
+namespace hlsl {
+
+static std::string FormatTokenKinds(ArrayRef<TokenKind> Kinds) {
+  std::string TokenString;
+  llvm::raw_string_ostream Out(TokenString);
+  bool First = true;
+  for (auto Kind : Kinds) {
+    if (!First)
+      Out << ", ";
+    switch (Kind) {
+#define TOK(X, SPELLING)                                                       \
+  case TokenKind::X:                                                           \
+    Out << SPELLING;                                                           \
+    break;
+#include "clang/Lex/HLSLRootSignatureTokenKinds.def"
+    }
+    First = false;
+  }
+
+  return TokenString;
+}
+
+// Parser Definitions
+
+RootSignatureParser::RootSignatureParser(SmallVector<RootElement> &Elements,
+                                         RootSignatureLexer &Lexer,
+                                         Preprocessor &PP)
+    : Elements(Elements), Lexer(Lexer), PP(PP), CurToken(SourceLocation()) {}
+
+bool RootSignatureParser::Parse() {
+  // Iterate as many RootElements as possible
+  while (TryConsumeExpectedToken(TokenKind::kw_DescriptorTable)) {
+    bool Error = false;
+    // Dispatch onto parser method.
+    // We guard against the unreachable here as we just ensured that CurToken
+    // will be one of the kinds in the while condition
+    switch (CurToken.Kind) {
+    case TokenKind::kw_DescriptorTable:
+      Error = ParseDescriptorTable();
+      break;
+    default:
+      llvm_unreachable("Switch for consumed token was not provided");
+    }
+
+    if (Error)
+      return true;
+
+    if (!TryConsumeExpectedToken(TokenKind::pu_comma))
+      break;
+  }
+
+  return ConsumeExpectedToken(TokenKind::end_of_stream, diag::err_expected);
+}
+
+bool RootSignatureParser::ParseDescriptorTable() {
+  assert(CurToken.Kind == TokenKind::kw_DescriptorTable &&
+         "Expects to only be invoked starting at given keyword");
+
+  DescriptorTable Table;
+
+  if (ConsumeExpectedToken(TokenKind::pu_l_paren, diag::err_expected_after,
+                           CurToken.Kind))
+    return true;
+
+  // Iterate as many Clauses as possible
+  while (TryConsumeExpectedToken({TokenKind::kw_CBV, TokenKind::kw_SRV,
+                                  TokenKind::kw_UAV, TokenKind::kw_Sampler})) {
+    if (ParseDescriptorTableClause())
+      return true;
+
+    Table.NumClauses++;
+
+    if (!TryConsumeExpectedToken(TokenKind::pu_comma))
+      break;
+  }
+
+  if (ConsumeExpectedToken(TokenKind::pu_r_paren, diag::err_expected_after,
+                           CurToken.Kind))
+    return true;
+
+  Elements.push_back(Table);
+  return false;
+}
+
+bool RootSignatureParser::ParseDescriptorTableClause() {
+  assert((CurToken.Kind == TokenKind::kw_CBV ||
+          CurToken.Kind == TokenKind::kw_SRV ||
+          CurToken.Kind == TokenKind::kw_UAV ||
+          CurToken.Kind == TokenKind::kw_Sampler) &&
+         "Expects to only be invoked starting at given keyword");
+
+  DescriptorTableClause Clause;
+  switch (CurToken.Kind) {
+  default:
+    break; // Unreachable given Try + assert pattern
----------------
bogner wrote:

Use `llvm_unreachable` when something is unreachable. Arguably you could do that instead of having the explicit assert at the top of the function, as it should have the same effect in this case.

https://github.com/llvm/llvm-project/pull/133302