[PATCH] D151820: [clang][WebAssembly] Fix __BIGGEST_ALIGNMENT__ under emscripten

Wed May 31 14:44:39 PDT 2023

dschuff added a comment.

> I don't think it will since `__BIGGEST_ALIGNMENT__ >= XNN_ALLOCATION_ALIGNMENT` will remain true after this change.. so this change should have no effect on that code.

I meant that when `__BIGGEST_ALIGNMENT__ >= XNN_ALLOCATION_ALIGNMENT` (which was true before and will remain true), then XNNPack uses `__builtin_alloca()` as the implementation of `XNN_SIMD_ALLOCA` (which presumably is for allocating SIMD values). This change will reduce the alignment used by `__builtin_alloca()` from 16 to 8, such that (I think) it is no longer suitable for SIMD values.

Maybe this is a bug in XNNPack (they should maybe be using XNN_ALLOCATION_ALIGNMENT with a value suitable for SIMD?) but given that BIGGEST_ALIGNMENT and alloca seem to be intended for any base type (including SIMD) it wouldn't be surprising if someone else were depending on this too.

which... maybe this is just re-litigating the previous discussion, I don't know. I wonder at what point our ABI should be treating SIMD values as "normal" rather than rare.

Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D151820/new/

https://reviews.llvm.org/D151820