[PATCH] D53037: [InstCombine] combine a shuffle and an extract subvector shuffle

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri added a comment.

In D53037#1385727 <https://reviews.llvm.org/D53037#1385727>, @efriedma wrote:

> That case is specifically interesting because when it's split, each half is lowered to a different shuffle, and both shuffles are expensive.
>
> Obviously you could try to find that specific pattern in SelectionDAG on AArch64, but I'm not sure it makes sense to expect the backend to try; instcombine probably shouldn't be creating difficult-to-solve optimization puzzles.


But can you be sure there won't ever be such a `shuffle` via some other means?
I don't know that much about AArch64, but this really sounds like something to be dealt with in the backend..


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