[PATCH] D122010: [InstCombine] try to canonicalize logical shift after bswap

[Sanjay Patel via Phabricator via llvm-commits]

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When shifting by a byte-multiple:
bswap (shl X, C) --> lshr (bswap X), C
bswap (lshr X, C) --> shl (bswap X), C

This is an IR implementation of a transform suggested in D120648 <https://reviews.llvm.org/D120648>. The "swaps cancel" test models the motivating optimization from that proposal.

Alive2 checks (as noted in the other review, we could use knownbits to handle shift-by-variable-amount, but that can be an enhancement patch):
https://alive2.llvm.org/ce/z/pXUaRf
https://alive2.llvm.org/ce/z/ZnaMLf


https://reviews.llvm.org/D122010

Files:
  llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
  llvm/test/Transforms/InstCombine/bswap-fold.ll

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