[clang] [HLSL] Implement HLSL Flat casting (excluding splat cases) (PR #118842)

[Ashley Coleman via cfe-commits]

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@@ -6358,3 +6359,89 @@ RValue CodeGenFunction::EmitPseudoObjectRValue(const PseudoObjectExpr *E,
 LValue CodeGenFunction::EmitPseudoObjectLValue(const PseudoObjectExpr *E) {
   return emitPseudoObjectExpr(*this, E, true, AggValueSlot::ignored()).LV;
 }
+
+void CodeGenFunction::FlattenAccessAndType(
+    Address Addr, QualType AddrType,
+    SmallVectorImpl<std::pair<Address, llvm::Value *>> &AccessList,
+    SmallVectorImpl<QualType> &FlatTypes) {
+  // WorkList is list of type we are processing + the Index List to access
+  // the field of that type in Addr for use in a GEP
+  llvm::SmallVector<std::pair<QualType, llvm::SmallVector<llvm::Value *, 4>>,
+                    16>
+      WorkList;
+  llvm::IntegerType *IdxTy = llvm::IntegerType::get(getLLVMContext(), 32);
+  WorkList.push_back(
+      {AddrType,
+       {llvm::ConstantInt::get(
+           IdxTy,
+           0)}}); // Addr should be a pointer so we need to 'dereference' it
+
+  while (!WorkList.empty()) {
+    std::pair<QualType, llvm::SmallVector<llvm::Value *, 4>> P =
+        WorkList.pop_back_val();
+    QualType T = P.first;
+    llvm::SmallVector<llvm::Value *, 4> IdxList = P.second;
+    T = T.getCanonicalType().getUnqualifiedType();
+    assert(!isa<MatrixType>(T) && "Matrix types not yet supported in HLSL");
+    if (const auto *CAT = dyn_cast<ConstantArrayType>(T)) {
+      uint64_t Size = CAT->getZExtSize();
+      for (int64_t i = Size - 1; i > -1; i--) {
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V-FEXrt wrote:

nit: `i` should be `I` https://llvm.org/docs/CodingStandards.html#name-types-functions-variables-and-enumerators-properly

aside: At least for my setup, using a lower case, even in loops, raises a warning

https://github.com/llvm/llvm-project/pull/118842