[PATCH] D120571: [CGP, AArch64] Replace zexts with shuffle that can be lowered using tbl.

[Dave Green via Phabricator via llvm-commits]

dmgreen added a comment.

There are quite a few places where this transform is not profitable due to it blocking fold in selection-dag. They would be more obvious, but we don't have many tests with loops in the backend. I have been looking lately about whether it makes sense to replace it with something that happens either during or after ISel. I think after ISel should work as a (larger) peephole optimization, which should then work with both SDAG and GISel and prevent us needing to try and handle it in both places. We just need to recognize the patterns of USHLL's and that way we only optimize if it turns out to really be useful.


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