[PATCH] D117365: [InstCombine] optimize icmp-ugt-ashr

[Nadav Rotem via Phabricator via llvm-commits]

nadav added a comment.

@craig.topper Thank you for the review, and for writing the proof, of course. This is my understanding of the code and of your proof.

Your proof has two checks, which I added as comments.

  ;Assume: C + 1 == ((C+1 << y) >> y)
  ;Assume ((c +1 ) << y) != 127      -- only needed for the sgt case
   

The first check is implemented with the code:

  APInt ShiftedC = (C + 1).shl(ShAmtVal) - 1;
  if ((ShiftedC + 1).ashr(ShAmtVal) == (C + 1))

And the second one with:

  !C.isMaxSignedValue() && (ShiftedC + 1).ashr(ShAmtVal) == (C + 1)

The code also has this check, which I removed, because the proof does not have this check:

  !(C + 1).shl(ShAmtVal).isMinSignedValue()

Is this also your understanding, or did I miss something.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D117365/new/

https://reviews.llvm.org/D117365