[PATCH] D104808: [clang][emscripten] Reduce alignof long double from 16 to 8 bytes

Wed Jun 23 13:38:41 PDT 2021

sbc100 added a comment.

In D104808#2836991 <https://reviews.llvm.org/D104808#2836991>, @kripken wrote:

> In D104808#2836942 <https://reviews.llvm.org/D104808#2836942>, @sunfish wrote:
>
>> Do we still intend to unify Emscripten's ABI with wasm32-unknown-unknown or wasm32-wasi eventually? This is talking a step away from that.
>
> I definitely think we should unify them as much as possible. Ideally we would all make this change.
>
> The motivation for the change is that right now we are losing either some correctness or some performance. Either is a burden, and a possible future float128 in wasm doesn't seem strong enough of a justification to me - should wasm add float128, we can consider a new ABI at that point. Does that sound reasonable?
>
>> One of the assumptions behind this is that it would be ok for malloc to be 16-byte aligned anyway, because SIMD use cases benefit from being able to call `malloc` and get a buffer aligned for SIMD. Do we have more information on how much this matters in practice?
>
> I think the discussions converged on it being ok with the spec, but perhaps a problem in practice, but portable SIMD code seems smart enough to avoid the issue. I don't think I've seen a bug report mentioning SIMD, but I may have missed one.

Indeed, we have had reports of issues with the under-alignment of values returns from malloc, but I don't think any of them have been related to SIMD,   only the expectation that malloc honors `max_align_t`.

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