wip: Index into padded arrays

This is a bit tricky - when we have a padded array we have a struct with
a slightly smaller array followed by the last element, since that last
element doesn't itself have padding. Here, we create an imaginary array
type with the padding on every element and index into that - this looks
like we index out of bounds of the smaller array for the last element
but this is okay because we only read the memory of the value that is
legitimately there.

Author: bogner

clang/lib/CodeGen/CGExpr.cpp

@@ -4681,22 +4681,27 @@ LValue CodeGenFunction::EmitArraySubscriptExpr(const ArraySubscriptExpr *E,
   } else if (E->getType().getAddressSpace() == LangAS::hlsl_constant) {
     // This is an array inside of a cbuffer.
     Addr = EmitPointerWithAlignment(E->getBase(), &EltBaseInfo, &EltTBAAInfo);
-    auto *Idx = EmitIdxAfterBase(/*Promote*/true);
 
-    // ...
-    CharUnits RowAlignedSize = getContext()
-                                   .getTypeSizeInChars(E->getType())
-                                   .alignTo(CharUnits::fromQuantity(16));
+    SmallVector<llvm::Value *, 2> Indices;
+    Indices.push_back(EmitIdxAfterBase(/*Promote*/true));
+
+    CharUnits ElementSize = getContext().getTypeSizeInChars(E->getType());
+    CharUnits RowAlignedSize = ElementSize.alignTo(CharUnits::fromQuantity(16));
 
-    llvm::Value *RowAlignedSizeVal =
-        llvm::ConstantInt::get(Idx->getType(), RowAlignedSize.getQuantity());
-    llvm::Value *ScaledIdx = Builder.CreateMul(Idx, RowAlignedSizeVal);
+    llvm::Type *EltTyToIndex = Addr.getElementType();
+    if (RowAlignedSize > ElementSize) {
+      llvm::Type *Padding = CGM.getTargetCodeGenInfo().getHLSLPadding(
+          CGM, RowAlignedSize - ElementSize);
+      EltTyToIndex = llvm::StructType::get(
+          getLLVMContext(), {EltTyToIndex, Padding}, /*isPacked=*/true);
+      Indices.push_back(llvm::ConstantInt::get(Indices[0]->getType(), 0));
+}
 
     CharUnits EltAlign =
-      getArrayElementAlign(Addr.getAlignment(), Idx, RowAlignedSize);
+      getArrayElementAlign(Addr.getAlignment(), Indices[0], RowAlignedSize);
     llvm::Value *EltPtr =
-        emitArraySubscriptGEP(*this, Int8Ty, Addr.emitRawPointer(*this),
-                              ScaledIdx, false, SignedIndices, E->getExprLoc());
+        emitArraySubscriptGEP(*this, EltTyToIndex, Addr.emitRawPointer(*this),
+                              Indices, false, SignedIndices, E->getExprLoc());
     Addr = Address(EltPtr, Addr.getElementType(), EltAlign);
   } else if (const Expr *Array = isSimpleArrayDecayOperand(E->getBase())) {
     // If this is A[i] where A is an array, the frontend will have decayed the