llvm dev - Dec 2020 - Using poison constant as a don't-care value when emitting/optimizing vector operations

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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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
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>

-- 
Lerne, wie die Welt wirklich ist,
aber vergiss niemals, wie sie sein sollte.
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