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[CIR] support union without field in C++ #1703

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Merged
merged 4 commits into from
Jun 26, 2025
Merged

[CIR] support union without field in C++ #1703

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deb5c9f
[CIR] support union without field in C++
HerrCai0907 Jun 22, 2025
3c7b7a3
add comment
HerrCai0907 Jun 24, 2025
f1c39ae
remove undef in test
HerrCai0907 Jun 24, 2025
a033566
fix comment
HerrCai0907 Jun 25, 2025
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2 changes: 1 addition & 1 deletion clang/lib/CIR/CodeGen/CIRRecordLayoutBuilder.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -364,7 +364,7 @@ void CIRRecordLowering::lowerUnion() {
}
// If we have no storage type just pad to the appropriate size and return.
if (!StorageType)
llvm_unreachable("no-storage union NYI");
return appendPaddingBytes(LayoutSize);
// If our storage size was bigger than our required size (can happen in the
// case of packed bitfields on Itanium) then just use an I8 array.
if (LayoutSize < getSize(StorageType))
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10 changes: 7 additions & 3 deletions clang/lib/CIR/Dialect/IR/CIRTypes.cpp
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Expand Up @@ -453,8 +453,9 @@ RecordType::computeUnionSize(const mlir::DataLayout &dataLayout) const {
unsigned recordSize = 0;
llvm::Align recordAlignment{1};

auto largestMember = getLargestMember(dataLayout);
recordSize = dataLayout.getTypeSize(largestMember);
Type largestMember = getLargestMember(dataLayout);
if (largestMember)
recordSize = dataLayout.getTypeSize(largestMember);

// If the union is padded, add the padding to the size.
if (getPadded()) {
Expand Down Expand Up @@ -517,7 +518,10 @@ RecordType::computeStructAlignment(const mlir::DataLayout &dataLayout) const {

uint64_t
RecordType::computeUnionAlignment(const mlir::DataLayout &dataLayout) const {
auto largestMember = getLargestMember(dataLayout);
Type largestMember = getLargestMember(dataLayout);
// use 1 byte alignment for empty union
if (!largestMember)
return 1;
return dataLayout.getTypeABIAlignment(largestMember);
}

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22 changes: 22 additions & 0 deletions clang/test/CIR/CodeGen/union-empty.cpp
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@@ -0,0 +1,22 @@
// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -fclangir -emit-cir %s -o %t.cir
// RUN: FileCheck --input-file=%t.cir %s --check-prefix=CIR
// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -fclangir -emit-llvm %s -o %t.ll
// RUN: FileCheck --input-file=%t.ll %s --check-prefix=LLVM

union EmptyUnion {
EmptyUnion() = default;
};

void f0() {
EmptyUnion e;
};

// CIR: !rec_EmptyUnion = !cir.record<union "EmptyUnion" padded {!u8i}>
// CIR: cir.func dso_local @_Z2f0v()
// CIR: %0 = cir.alloca !rec_EmptyUnion, !cir.ptr<!rec_EmptyUnion>, ["e"] {alignment = 1 : i64}
// CIR: cir.return

// LLVM: %union.EmptyUnion = type { i8 }
// LLVM: define dso_local void @_Z2f0v()
// LLVM: %1 = alloca %union.EmptyUnion, i64 1, align 1
// LLVM: ret void
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