[clang] [llvm] Explicit types in cbuffer layouts (PR #156919)

[Justin Bogner via llvm-commits]

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@@ -4678,6 +4678,26 @@ LValue CodeGenFunction::EmitArraySubscriptExpr(const ArraySubscriptExpr *E,
         emitArraySubscriptGEP(*this, Int8Ty, Addr.emitRawPointer(*this),
                               ScaledIdx, false, SignedIndices, E->getExprLoc());
     Addr = Address(EltPtr, OrigBaseElemTy, EltAlign);
+  } 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));
+
+    llvm::Value *RowAlignedSizeVal =
+        llvm::ConstantInt::get(Idx->getType(), RowAlignedSize.getQuantity());
+    llvm::Value *ScaledIdx = Builder.CreateMul(Idx, RowAlignedSizeVal);
+
+    CharUnits EltAlign =
+      getArrayElementAlign(Addr.getAlignment(), Idx, RowAlignedSize);
+    llvm::Value *EltPtr =
+        emitArraySubscriptGEP(*this, Int8Ty, Addr.emitRawPointer(*this),
+                              ScaledIdx, false, SignedIndices, E->getExprLoc());
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bogner wrote:

I've tried this out in https://github.com/llvm/llvm-project/pull/156919/commits/92bd225443368a88319e97305b6d8a5f997ad37b (which still needs test updates). This looks mostly reasonable with the caveat that we do need a bit of a fictional type for this to work (the array with padding on all elements including the last one). Since we don't actually read the padding this is probably fine.

https://github.com/llvm/llvm-project/pull/156919