[PATCH] D54869: [InstCombine] Determine demanded and known bits for funnel shifts

[Nikita Popov via Phabricator via llvm-commits]

nikic created this revision.
nikic added reviewers: spatel, RKSimon.
Herald added a subscriber: llvm-commits.

Support funnel shifts in InstCombine demanded bits simplification. If the shift amount is constant, we can determine both the demanded bits of the operands, as well as the known bits of the result.

If one of the operands has no demanded bits, it will be replaced by undef and the funnel shift will be simplified into a simple shift due to the simplifications added in D54778 <https://reviews.llvm.org/D54778>.

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This change has been split off from D54666 <https://reviews.llvm.org/D54666>.

Another place where support for computing fsh demanded bits can be added is the DemandedBits analysis driving BDCE. This will allow handling funnel shifts with multiple uses. Unfortunately DemandedBits only tracks bits on a per-instruction level, so that the different demanded bits of the two operands cannot be leveraged.


Repository:
  rL LLVM

https://reviews.llvm.org/D54869

Files:
  lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
  test/Transforms/InstCombine/fsh.ll

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