[clang] [llvm] [LLVMABI] Handle Atomic types in the LLVM ABI library (PR #221367)

[Madhur Amilkanthwar via cfe-commits]

madhur13490 wrote:

> Uh, are we sure that what Clang is doing here is actually correct?
> 
> At least the ABI used by Clang is not the same as the one used by GCC: https://gcc.godbolt.org/z/Kec3Eqcz6 GCC seems to treat the struct of `_Atomic(float)` the same as the struct of `float`.
> 
> The x86-64 SysV ABI specification unfortunately has no hits for `_Atomic`.
> 
> cc @efriedma-quic

You're right to question it — this isn't psABI-mandated. The x86-64 psABI doesn't mention _Atomic at all; it classifies aggregates purely by size and field classes. Clang classifies struct { _Atomic(float) a, b; } as MEMORY only because its classifier doesn't look through the AtomicType wrapper (the field falls through to the default MEMORY), and the power-of-2 size inflation for over-aligned atomics is the long-standing GCC incompatibility tracked in llvm#26836. A plain reading (and GCC, which treats _Atomic as a transparent qualifier) would classify it SSE, same as struct { float a, b; }.

I think if we are mirroring current Clang, then this should be fine?

https://github.com/llvm/llvm-project/pull/221367