[PATCH] D66057: [InstCombine] Shift amount reassociation in bittest: trunc-of-shl (PR42399)

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri created this revision.
lebedev.ri added reviewers: spatel, nikic, xbolva00.
lebedev.ri added a project: LLVM.
Herald added a subscriber: hiraditya.

This is continuation of D63829 <https://reviews.llvm.org/D63829> / https://bugs.llvm.org/show_bug.cgi?id=42399

I thought naive pattern would solve my issue, but nope, it involved truncation,
thus more folds needed.. This isn't really the fold i'm interested in,
i need trunc-of-lshr, but i'we decided to start with `shl` because it's simpler.

In this case, no extra legality checks are needed:
https://rise4fun.com/Alive/CAb

We should be careful about not increasing instruction count,
since we need to produce `zext` because `and` is done in wider type.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D66057

Files:
  llvm/include/llvm/IR/PatternMatch.h
  llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
  llvm/test/Transforms/InstCombine/shift-amount-reassociation-in-bittest-with-truncation-shl.ll

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