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Bitcast versus GEP in LLVM output #1061

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bcardosolopes opened this issue Nov 5, 2024 · 1 comment
Open

Bitcast versus GEP in LLVM output #1061

bcardosolopes opened this issue Nov 5, 2024 · 1 comment
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good first issue Good for newcomers IR difference A difference in ClangIR-generated LLVM IR that could complicate reusing original CodeGen tests

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bcardosolopes commented Nov 5, 2024 •
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From @smeenai:

gep is emitted here in the original codeden, but we don't do it so far - we have a missing feature guard

Originally posted by @gitoleg in #1041 (comment)
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cc @smeenai

@bcardosolopes bcardosolopes added good first issue Good for newcomers IR difference A difference in ClangIR-generated LLVM IR that could complicate reusing original CodeGen tests labels Nov 5, 2024
@gitoleg gitoleg mentioned this issue Nov 13, 2024
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bcardosolopes pushed a commit that referenced this issue Nov 14, 2024
@gitoleg
[CIR][ABI][Lowering] Fixes calls with union type (#1119)
d1ad076 
This PR handles calls with unions passed by value in the calling
convention pass.

#### Implementation
As one may know, data layout for unions in CIR and in LLVM differ one
from another. In CIR we track all the union members, while in LLVM IR
only the largest one.

And here we need to take this difference into account: we need to find a
type of the largest member and treat it as the first (and only) union
member in order to preserve all the logic from the original codegen.

There is a method `StructType::getLargestMember` - but looks like it
produces different results (with the one I implemented or better to say
copy-pasted). Maybe it's done intentionally, I don't know.

The LLVM IR produced has also some difference from the original one. In
the original IR `gep` is emitted - and we can not do the same. If we
create `getMemberOp` we may fail on type checking for unions - since the
first member type may differ from the largest type. This is why we
create `bitcast` instead. Relates to the issue #1061
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good first issue Good for newcomers IR difference A difference in ClangIR-generated LLVM IR that could complicate reusing original CodeGen tests
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