[PATCH] D70246: [InstCombine] remove identity shuffle simplification for mask with undefs

[Eli Friedman via Phabricator via llvm-commits]

efriedma added a comment.

My biggest concern here is the interaction with SimplifyDemandedVectorElts; previously, the way the interaction worked is that SimplifyDemandedVectorElts would change the shuffle indexes to undef, then later combines could remove those shuffles.  That doesn't work anymore because of this rule.

We can probably recover some cases by adding a little extra code to SimplifyDemandedVectorElts, to eliminate shuffles where the demanded elements correspond to an identity shuffle.


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  https://reviews.llvm.org/D70246/new/

https://reviews.llvm.org/D70246