[PATCH] D45854: [InstCombine] Support BitTests in ThreeWayComparison. Trivial case

[Max Kazantsev via Phabricator via llvm-commits]

mkazantsev created this revision.
mkazantsev added reviewers: craig.topper, lebedev.ri, spatel, reames.
mkazantsev edited the summary of this revision.

Three-way comparison matcher which is used in some of InstCombine's transforms
is intended to detect the following pattern:

  %c1 = icmp eq %a, %b
  %c2 = icmp slt %a, %b
  %select1 = select %c2, %Less, %More
  %select2 = select %c1, 0, %select2

It works fine if %a and %b are some values about which we know nothing. However
if %b is a constant zero, the situation is different. If we look closer at the example:

  %c1 = icmp eq %a, 0
  %c2 = icmp slt %a, 0
  %select1 = select %c2, %Less, %More
  %select2 = select %c1, 0, %select2

Then we can see that the pattern of `%c2` and `%select1` is actually a bit test of the
highest bit of `%a`. There are rules in InstCombine that detect this couple before we
go down to detection of the three-way comparison pattern which is only seen when we
process `select2`. These transforms may turn `%c2, %select1` into something different,
and when we process `%select2` we may not be able to recognize the three-way comparison
pattern.

This patch adds support of the following transform:

  %c2 = icmp slt %a, 0
  %select1 = select %c2, -1, %More

to

  %shift = ashr %a, 31
  %or = or ashr, %More

This is a part of fix for bug 37147.


https://reviews.llvm.org/D45854

Files:
  lib/Transforms/InstCombine/InstCombineCompares.cpp
  test/Transforms/InstCombine/three-way-comparison.ll

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