[PATCH] D148963: [X86] Support `X86ISD::PCMPEQ` and `X86ISD::PCMPGT` in ComputeKnownBits

Fri Apr 28 01:25:32 PDT 2023

nikic added a comment.

In D148963#4304568 <https://reviews.llvm.org/D148963#4304568>, @RKSimon wrote:

> In D148963#4304551 <https://reviews.llvm.org/D148963#4304551>, @nikic wrote:
>
>> Shouldn't this be a DAGCombine instead? It seems pointless to make computeKnownBits() recurse past an instruction that can only return either unknown or constant. If it returns constant, we want to just replace the instruction, and otherwise we get zero information.
>
> At least part of this is only matching cases because it has access to the DemandedElts mask

Hm, through which pathway does this actually end up folding in the non-trivial DemandedElts case? I'd have guessed SimplifyDemandedBits, but it doesn't seem to have a generic constant known bits to constant combine.

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  https://reviews.llvm.org/D148963/new/

https://reviews.llvm.org/D148963