[PATCH] D45065: [InstCombine] Fix PR17564: don't fold [zs]ext into phi.

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri created this revision.
lebedev.ri added reviewers: spatel, craig.topper.
Herald added a subscriber: kristof.beyls.

Motivational case is highlighted in PR17564 <https://bugs.llvm.org/show_bug.cgi?id=17564>:

  int f(int x) {
      return (x & 1) || (x & 2);
  }

If the function return type is `bool`, then the IR is just `and`+`icmp`.
But if it is `int`, then it has `select`, and whatnot.
https://godbolt.org/g/uTJVVo

If one analyzes `-print-after-all`, thing appear to go bad when we fold `zext` into `phi`.
If we don't do that, then the behavior is the same as with `i1`, and PR17564 is fixed.

Also, the second part is needed, to get rid of `select` if possible,
e.g. https://godbolt.org/g/iAYRup, but that is another issue.

Now, i'm not quite sure whether *this* fix is correct?
Two other tests changed, but those changes do not look harmful?

Testing: `ninja check-llvm`

Fixes PR17564.

If this makes sense i'll commit test changes as NFC, and rebase.


Repository:
  rL LLVM

https://reviews.llvm.org/D45065

Files:
  lib/Transforms/InstCombine/InstCombineCasts.cpp
  test/Transforms/InstCombine/ext-of-select-of-bittest.ll
  test/Transforms/InstCombine/icmp-div-constant.ll
  test/Transforms/InstCombine/icmp-mul-zext.ll

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