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

[Amara Emerson via Phabricator via llvm-commits]

aemerson added a comment.

In D125041#3500128 <https://reviews.llvm.org/D125041#3500128>, @foad wrote:

>> Actually G_SHL with undef LHS needs to be optimized to zero, not undef.
>
> At the IR level, InstSimplify does this for all shifts (in the absence of exact/nsw/nuw flags). See SimplifyRightShift and SimplifyShlInst.

Ok I see. To be more conservative then I'll just fold to undef for the shifts only.


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