[PATCH] D66965: [InstCombine] recognize bswap disguised as shufflevector

[Sanjay Patel via Phabricator via llvm-commits]

spatel created this revision.
spatel added reviewers: efriedma, lebedev.ri, craig.topper.
Herald added subscribers: hiraditya, mcrosier.
Herald added a project: LLVM.

bitcast <N x i8> (shuf X, undef, <N, N-1,...0>) to i{N*8}--> bswap (bitcast X to i{N*8})

In PR43146:
https://bugs.llvm.org/show_bug.cgi?id=43146
...we have a more complicated case where SLP is making a mess of bswap. This patch won't do anything for that currently, but we need to improve bswap recognition in instcombine, SLP, and/or a standalone pass to avoid that problem.

This is limited using the data-layout so we don't try to do this transform with actual vector types. The backend does not appear to have folds to convert in either direction, so we don't want to mess up something that is actually better lowered as a shuffle.

On x86, we're trading something like this:

  vmovd	%edi, %xmm0
  vpshufb	LCPI0_0(%rip), %xmm0, %xmm0 ## xmm0 = xmm0[3,2,1,0,u,u,u,u,u,u,u,u,u,u,u,u]
  vmovd	%xmm0, %eax

For:

  movl	%edi, %eax
  bswapl	%eax


https://reviews.llvm.org/D66965

Files:
  llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
  llvm/test/Transforms/InstCombine/bswap.ll

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