[PATCH] D79171: [InstCombine] canonicalize bitcast after insertelement into undef

Thu Apr 30 10:41:06 PDT 2020

lebedev.ri added a comment.

In D79171#2013138 <https://reviews.llvm.org/D79171#2013138>, @craig.topper wrote:

> I messed with mmx in this area once and regretted it. See rG5ebbabc1af360756f402203ba7704bb480f279a7 <https://reviews.llvm.org/rG5ebbabc1af360756f402203ba7704bb480f279a7>
>
> Is canonicalizing towards a vector type that wasn't mentioned in the IR the right way to go? Is that cast free for legal types on all targets? I think its potentially scalarized or becomes a load/store to stack temporary for illegal types in the backend.

It kinda sounds to me like such cases should use some custom intrinsics instead of generic bitcasts.

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