[llvm-dev] Using poison constant as a don't-care value when emitting/optimizing vector operations

[Nicolai Hähnle via llvm-dev]

Thank you for doing this, it seems the right thing to me.

Cheers,
Nicolai

On Sat, Dec 19, 2020 at 10:26 PM Juneyoung Lee via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hello all,
>
> Currently, undef is used as a don't-care value for vector operations,
> which causes hard problems.
> For example, a shufflevector sometimes cannot be optimized away (
> https://bugs.llvm.org/show_bug.cgi?id=44185). The reason is that undef is
> not undefined enough.
>
> To avoid such problem, I'd like to suggest using poison constant for
> vector operations' placeholder instead.
> I made a patch that updates InstCombine's SimplifyDemandedVectorElts here,
> with a detailed description: https://reviews.llvm.org/D93586
>
> Thanks,
> Juneyoung
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>


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