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

[Matthias Kretz <m.kretz@gsi.de>]

On Friday, 2 June 2023 09:49:26 CEST Alexander Monakov wrote:
> > simd<int> x = ...;
> > bool t = all_of(x < x + 1); // unconditionally true or not?
> > 
> > I'd expect t to be unconditionally true. Because simd<int> simply is a
> > data- parallel version of int.
> 
> 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.

> > > Revised patch below.
> > 
> > This can be considered a breaking change. Does it need a mention in the
> > release notes?
> 
> 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.

- Matthias

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 Dr. Matthias Kretz                           https://mattkretz.github.io
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