[PATCH] D63829: [InstCombine] Shift amount reassociation in bittest (PR42399)

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri created this revision.
lebedev.ri added reviewers: spatel, nikic, RKSimon.
lebedev.ri added a project: LLVM.

Given pattern:
`icmp eq/ne (and ((x shift Q), (y oppositeshift K))), 0`
we should move shifts to the same hand of 'and', i.e. rewrite as
`icmp eq/ne (and (x shift (Q+K)), y), 0`  iff `(Q+K) u< bitwidth(x)`

It might be tempting to not restrict this to situations where we know
we'd fold two shifts together, but i'm not sure what rules should there be
to avoid endless combine loops.

We pick the same shift that was originally used to shift the variable we picked to shift:
https://rise4fun.com/Alive/6x1v

Should fix PR42399 <https://bugs.llvm.org/show_bug.cgi?id=42399>.


Repository:
  rL LLVM

https://reviews.llvm.org/D63829

Files:
  lib/Transforms/InstCombine/InstCombineCompares.cpp
  test/Transforms/InstCombine/shift-amount-reassociation-in-bittest.ll

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