[all-commits] [llvm/llvm-project] bb9333: [InstCombine] fold cast of right-shift if high bit...

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  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: bb9333c3504a4a02b982526ad8264d14c6ec1ad4
      https://github.com/llvm/llvm-project/commit/bb9333c3504a4a02b982526ad8264d14c6ec1ad4
  Author: Sanjay Patel <spatel at rotateright.com>
  Date:   2021-09-23 (Thu, 23 Sep 2021)

  Changed paths:
    M llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
    M llvm/test/Transforms/InstCombine/trunc-demand.ll

  Log Message:
  -----------
  [InstCombine] fold cast of right-shift if high bits are not demanded (2nd try)

The 1st try at this was reverted because it caused an infinite loop in instcombine.
That should be fixed after:
1cd6b44f267b

(masked) trunc (lshr X, C) --> (masked) lshr (trunc X), C

Narrowing the shift should be better for analysis and can lead
to follow-on transforms as shown.

Attempt at a general proof in Alive2:
https://alive2.llvm.org/ce/z/tRnnSF

Here are a couple of the specific tests:
https://alive2.llvm.org/ce/z/bCnTp-
https://alive2.llvm.org/ce/z/TfaHnb

Differential Revision: https://reviews.llvm.org/D110170