[CIR] support union without field in C++ #1703
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In [libstdc++ std::variant implementation](https://github.com/gcc-mirror/gcc/blob/b0419798447ae25de2f58d1a695db6dadb5d8547/libstdc%2B%2B-v3/include/std/variant#L387-L394), union without any fields is used. According to current CodeGen logic, append 1 byte padding for this kind of union. Handle this union in `mlir::RecordType` also
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✅ With the latest revision this PR passed the undef deprecator. |
Right, we're still lacking transforming struct load/stores to their proper memcpy equivalents. |
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In [libstdc++ std::variant implementation](https://github.com/gcc-mirror/gcc/blob/b0419798447ae25de2f58d1a695db6dadb5d8547/libstdc%2B%2B-v3/include/std/variant#L387-L394), union without any fields is used. According to current CodeGen logic, append 1 byte padding for this kind of union. Handle this union in `mlir::RecordType` for getLargestMember` return nullptr also. The original LLVM IR ```llvm %union.EmptyUnion = type { i8 } @__const._Z2f0v.e = private unnamed_addr constant %union.EmptyUnion undef, align 1 define dso_local void @_Z2f0v() #0 { entry: %e = alloca %union.EmptyUnion, align 1 call void @llvm.memcpy.p0.p0.i64(ptr align 1 %e, ptr align 1 @__const._Z2f0v.e, i64 1, i1 false) ret void } ``` The CIR lowered LLVM IR ```llvm %union.EmptyUnion = type { i8 } define dso_local void @_Z2f0v() #0 { %1 = alloca %union.EmptyUnion, i64 1, align 1 store %union.EmptyUnion undef, ptr %1, align 1 ret void } ``` The major different is original use global const and memcpy, the current use store. The difference between the two is not related to this revision.
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In libstdc++ std::variant implementation, union without any fields is used.
According to current CodeGen logic, append 1 byte padding for this kind of union.
Handle this union in
mlir::RecordTypefor getLargestMember` return nullptr also.The original LLVM IR
The CIR lowered LLVM IR
The major different is original use global const and memcpy, the current use store. The difference between the two is not related to this revision.