[PATCH] D125041: [GlobalISel] Combine G_SHL, G_ASHR, G_SHL of undef shifts to undef.

Fri May 6 12:37:12 PDT 2022

aemerson added a comment.

In D125041#3496339 <https://reviews.llvm.org/D125041#3496339>, @foad wrote:

> In D125041#3495365 <https://reviews.llvm.org/D125041#3495365>, @aemerson wrote:
>
>> Actually G_SHL with undef LHS needs to be optimized to zero, not undef.
>
> I don't understand why that would be true for SHL but not for the other shifts. What are the rules for undef in MIR? Is it like undef in IR, or like poison in IR, or neither?

I'm not exactly sure of the reasoning, it's just what the IR optimizations and SelectionDAG do. SelectionDAG optimizes the other shifts of undef to undef too.

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