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

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

On Friday, 2 June 2023 11:24:23 CEST Alexander Monakov wrote:
> > > 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?

At this point Clang/LLVM support is best effort. I did not know before that 
LLVM nailed this down as UB. Also my test suite didn't show any failures on 
shifts IIRC (but that doesn't say anything about UB, I know).

FWIW, I'm okay with saying nothing in the release notes. It might just be that 
some codes have become dependent on the existing (under-specified) behavior. 🤷

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