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

[Juneyoung Lee via llvm-dev]

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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