Re: [PATCH] doc: clarify semantics of vector bitwise shifts

[Alexander Monakov <amonakov@ispras.ru>]

On Fri, 2 Jun 2023, Matthias Kretz wrote:

> > Okay, I see opinions will vary here. I was thinking about our immintrin.h
> > which is partially implemented in terms of generic vectors. Imagine we
> > extend UBSan to trap on signed overflow for vector types. I expect that
> > will blow up on existing code that uses Intel intrinsics.
> 
> _mm_add_epi32 is already implemented via __v4su addition (i.e. unsigned). So 
> the intrinsic would continue to wrap on signed overflow.

Ah, if our intrinsics take care of it, that alleviates my concern.

> > I'm not sure what you consider a breaking change here. Is that the implied
> > threat to use undefinedness for range deduction and other optimizations?
> 
> Consider the stdx::simd implementation. It currently follows semantics of the 
> builtin types. So simd<char> can be shifted by 30 without UB. The 
> implementation of the shift operator depends on the current behavior, even if 
> it is target-dependent. For PPC the simd implementation adds extra code to 
> avoid the "UB". With nailing down shifts > sizeof(T) as UB this extra code now 
> needs to be added for all targets.

What does stdx::simd do on LLVM, where that has always been UB even on x86?

Alexander