[clang] [HLSL] Add __is_scalarized_layout_compatible (PR #102227)

[Chris B via cfe-commits]

================
@@ -1142,3 +1142,85 @@ bool SemaHLSL::CheckBuiltinFunctionCall(unsigned BuiltinID, CallExpr *TheCall) {
   }
   return false;
 }
+
+static void BuildFlattenedTypeList(QualType BaseTy,
+                                   llvm::SmallVectorImpl<QualType> &List) {
+  llvm::SmallVector<QualType, 16> WorkList;
+  WorkList.push_back(BaseTy);
+  while (!WorkList.empty()) {
+    QualType T = WorkList.pop_back_val();
+    T = T.getCanonicalType().getUnqualifiedType();
+    assert(!isa<MatrixType>(T) && "Matrix types not yet supported in HLSL");
+    if (const auto *AT = dyn_cast<ConstantArrayType>(T)) {
+      llvm::SmallVector<QualType, 16> ElementFields;
+      // Generally I've avoided recursion in this algorithm, but arrays of
+      // structs could be time-consuming to flatten and churn through on the
+      // work list. Hopefully nesting arrays of structs containing arrays
+      // of structs too many levels deep is unlikely.
+      BuildFlattenedTypeList(AT->getElementType(), ElementFields);
+      // Repeat the element's field list n times.
+      for (uint64_t Ct = 0; Ct < AT->getZExtSize(); ++Ct)
+        List.insert(List.end(), ElementFields.begin(), ElementFields.end());
+      continue;
+    }
+    // Vectors can only have element types that are builtin types, so this can
+    // add directly to the list instead of to the WorkList.
+    if (const auto *VT = dyn_cast<VectorType>(T)) {
+      List.insert(List.end(), VT->getNumElements(), VT->getElementType());
+      continue;
+    }
+    if (const auto *RT = dyn_cast<RecordType>(T)) {
+      const RecordDecl *RD = RT->getDecl();
+      if (RD->isUnion()) {
----------------
llvm-beanz wrote:

Unions are tricky... we don't actually support them in HLSL officially yet, but I should probably make this work with Unions since they are likely to be a [202y feature](https://github.com/microsoft/hlsl-specs/blob/main/proposals/0004-unions.md).

https://github.com/llvm/llvm-project/pull/102227