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[CIR] Handle empty unions in record lowering #172666

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[CIR] Handle empty unions in record lowering #172666

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2 changes: 2 additions & 0 deletions clang/lib/CIR/Lowering/DirectToLLVM/LowerToLLVM.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -2969,6 +2969,8 @@ static void prepareTypeConverter(mlir::LLVMTypeConverter &converter,
break;
// Unions are lowered as only the largest member.
case cir::RecordType::Union:
if (type.getMembers().empty())
break;
if (auto largestMember = type.getLargestMember(dataLayout))
llvmMembers.push_back(
convertTypeForMemory(converter, dataLayout, largestMember));
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10 changes: 10 additions & 0 deletions clang/test/CIR/Lowering/union.cir
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Original file line number Diff line number Diff line change
@@ -0,0 +1,10 @@
// RUN: cir-opt %s --cir-to-llvm | FileCheck %s

!u_empty = !cir.record<union {}>
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@andykaylor andykaylor Dec 17, 2025

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Can you add a check for the LLVM IR type here? I think we're probably generating the wrong thing. In the incubator, we generate this:

%union.U = type {}

Whereas classic codegen generates this:

%union.U = type { [4 x i8] }

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@andykaylor andykaylor Dec 17, 2025

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Let me amend what I said above. I didn't realize the classic codegen I was referring to was using the Windows C ABI. Classic codegen does generate %union.U = type {} for C and %union.U = type { i8 } for C++ (which the incubator also does).

So, I guess my question is whether we identify these padded forms as "empty".


module {
cir.func @empty_union(%u: !u_empty) {
// CHECK-LABEL: llvm.func @empty_union
cir.return
}
}