[Clang][docs] Modify generator for HLSL semantics documentation (#157841)

HLSL semantics are split between system semantics with some kind of
spelling, and user semantics with no actual spelling. Those have been
documented as normal function attributes, but they'd benefit from a
custom section with a slightly different handling.

This will allow #152537 to land.

Verified the resulting RST file, and only diff are around HLSL
semantics.

Rendered output: 
<img width="1064" height="1035" alt="Screenshot from 2025-09-10
14-05-08"
src="https://github.com/user-attachments/assets/554b70d6-2bf8-4131-b343-8f379babaca8"
/>

Author: Keenuts

clang/include/clang/Basic/AttrDocs.td

@@ -8367,6 +8367,23 @@ flag.
   }];
 }
 
+def DocHLSLSemantics : DocumentationCategory<"HLSL Semantics"> {
+  let Content = [{
+A semantic is a string attached to a shader input or output that conveys
+information about the intended use of a parameter. Semantics are required on
+all variables passed between shader stages. The syntax for adding a semantic
+to a shader variable is shown here (Variable Syntax (DirectX HLSL)).
+
+In general, data passed between pipeline stages is completely generic and is
+not uniquely interpreted by the system; arbitrary semantics are allowed which
+have no special meaning. Parameters (in Direct3D 10 and later) which contain
+these special semantics are referred to as System-Value Semantics.
+
+More information is available here:
+https://learn.microsoft.com/en-us/windows/win32/direct3dhlsl/dx-graphics-hlsl-semantics
+  }];
+}
+
 def WaveSizeDocs : Documentation {
   let Category = DocCatFunction;
   let Content = [{
@@ -8604,7 +8621,7 @@ randomized.
 }
 
 def HLSLSV_GroupThreadIDDocs : Documentation {
-  let Category = DocCatFunction;
+  let Category = DocHLSLSemantics;
   let Content = [{
 The ``SV_GroupThreadID`` semantic, when applied to an input parameter, specifies which
 individual thread within a thread group is executing in. This attribute is
@@ -8615,7 +8632,7 @@ The full documentation is available here: https://docs.microsoft.com/en-us/windo
 }
 
 def HLSLSV_GroupIDDocs : Documentation {
-  let Category = DocCatFunction;
+  let Category = DocHLSLSemantics;
   let Content = [{
 The ``SV_GroupID`` semantic, when applied to an input parameter, specifies which
 thread group a shader is executing in. This attribute is only supported in compute shaders.
@@ -8625,7 +8642,7 @@ The full documentation is available here: https://docs.microsoft.com/en-us/windo
 }
 
 def HLSLSV_GroupIndexDocs : Documentation {
-  let Category = DocCatFunction;
+  let Category = DocHLSLSemantics;
   let Content = [{
 The ``SV_GroupIndex`` semantic, when applied to an input parameter, specifies a
 data binding to map the group index to the specified parameter. This attribute
@@ -8682,7 +8699,7 @@ The full documentation is available here: https://learn.microsoft.com/en-us/wind
 }
 
 def HLSLSV_DispatchThreadIDDocs : Documentation {
-  let Category = DocCatFunction;
+  let Category = DocHLSLSemantics;
   let Content = [{
 The ``SV_DispatchThreadID`` semantic, when applied to an input parameter,
 specifies a data binding to map the global thread offset within the Dispatch
@@ -8697,7 +8714,7 @@ The full documentation is available here: https://docs.microsoft.com/en-us/windo
 }
 
 def HLSLSV_PositionDocs : Documentation {
-  let Category = DocCatFunction;
+  let Category = DocHLSLSemantics;
   let Content = [{
 The ``SV_Position`` semantic, when applied to an input parameter in a pixel
 shader, contains the location of the pixel center (x, y) in screen space.

clang/utils/TableGen/ClangAttrEmitter.cpp

@@ -5209,6 +5209,14 @@ class SpellingList {
                              Other.Spellings[Kind].end());
     }
   }
+
+  bool hasSpelling() const {
+    for (size_t Kind = 0; Kind < NumSpellingKinds; ++Kind) {
+      if (Spellings[Kind].size() > 0)
+        return true;
+    }
+    return false;
+  }
 };
 
 class DocumentationData {
@@ -5246,6 +5254,16 @@ GetAttributeHeadingAndSpellings(const Record &Documentation,
   // documentation. This may not be a limiting factor since the spellings
   // should generally be consistently applied across the category.
 
+  if (Cat == "HLSL Semantics") {
+    if (!Attribute.getName().starts_with("HLSL"))
+      PrintFatalError(Attribute.getLoc(),
+                      "HLSL semantic attribute name must start with HLSL");
+
+    assert(Attribute.getName().size() > 4);
+    std::string Name = Attribute.getName().substr(4).str();
+    return std::make_pair(std::move(Name), SpellingList());
+  }
+
   std::vector<FlattenedSpelling> Spellings = GetFlattenedSpellings(Attribute);
   if (Spellings.empty())
     PrintFatalError(Attribute.getLoc(),
@@ -5296,37 +5314,39 @@ static void WriteDocumentation(const RecordKeeper &Records,
     OS << ".. _" << Label << ":\n\n";
   OS << Doc.Heading << "\n" << std::string(Doc.Heading.length(), '-') << "\n";
 
-  // List what spelling syntaxes the attribute supports.
-  // Note: "#pragma clang attribute" is handled outside the spelling kinds loop
-  // so it must be last.
-  OS << ".. csv-table:: Supported Syntaxes\n";
-  OS << "   :header: \"GNU\", \"C++11\", \"C23\", \"``__declspec``\",";
-  OS << " \"Keyword\", \"``#pragma``\", \"HLSL Annotation\", \"``#pragma "
-        "clang ";
-  OS << "attribute``\"\n\n   \"";
-  for (size_t Kind = 0; Kind != NumSpellingKinds; ++Kind) {
-    SpellingKind K = (SpellingKind)Kind;
-    // TODO: List Microsoft (IDL-style attribute) spellings once we fully
-    // support them.
-    if (K == SpellingKind::Microsoft)
-      continue;
+  if (Doc.SupportedSpellings.hasSpelling()) {
+    // List what spelling syntaxes the attribute supports.
+    // Note: "#pragma clang attribute" is handled outside the spelling kinds
+    // loop so it must be last.
+    OS << ".. csv-table:: Supported Syntaxes\n";
+    OS << "   :header: \"GNU\", \"C++11\", \"C23\", \"``__declspec``\",";
+    OS << " \"Keyword\", \"``#pragma``\", \"HLSL Annotation\", \"``#pragma "
+          "clang ";
+    OS << "attribute``\"\n\n   \"";
+    for (size_t Kind = 0; Kind != NumSpellingKinds; ++Kind) {
+      SpellingKind K = (SpellingKind)Kind;
+      // TODO: List Microsoft (IDL-style attribute) spellings once we fully
+      // support them.
+      if (K == SpellingKind::Microsoft)
+        continue;
 
-    bool PrintedAny = false;
-    for (StringRef Spelling : Doc.SupportedSpellings[K]) {
-      if (PrintedAny)
-        OS << " |br| ";
-      OS << "``" << Spelling << "``";
-      PrintedAny = true;
+      bool PrintedAny = false;
+      for (StringRef Spelling : Doc.SupportedSpellings[K]) {
+        if (PrintedAny)
+          OS << " |br| ";
+        OS << "``" << Spelling << "``";
+        PrintedAny = true;
+      }
+
+      OS << "\",\"";
     }
 
-    OS << "\",\"";
+    if (getPragmaAttributeSupport(Records).isAttributedSupported(
+            *Doc.Attribute))
+      OS << "Yes";
+    OS << "\"\n\n";
   }
 
-  if (getPragmaAttributeSupport(Records).isAttributedSupported(
-          *Doc.Attribute))
-    OS << "Yes";
-  OS << "\"\n\n";
-
   // If the attribute is deprecated, print a message about it, and possibly
   // provide a replacement attribute.
   if (!Doc.Documentation->isValueUnset("Deprecated")) {