[PATCH] D123453: [InstCombine] Fold mul nuw+lshr exact to a single multiplication when the latter is a factor

[chenglin.bi via Phabricator via llvm-commits]

bcl5980 added a comment.

In D123453#3441216 <https://reviews.llvm.org/D123453#3441216>, @lebedev.ri wrote:

> For example, what about this:
> ~~https://alive2.llvm.org/ce/z/aov-GB <https://alive2.llvm.org/ce/z/aov-GB>~~
> https://alive2.llvm.org/ce/z/ox4wAt (no need for `exact`, only `nuw`)



  %t0 = lshr i8 %C1, %C2
  %t1 = shl i8 %t0, %C2
  %precond = icmp eq i8 %t1, %C1
  call void @llvm.assume(i1 %precond)

This is the proof of exact. We can't remove exact check in the code. Or we need to do something like:

  if (Op0->hasOneUse()) {
    APInt NewMulC = MulC->lshr(ShAmtC);
    if (MulC->eq(NewMulC.shl(ShAmtC)))
      return BinaryOperator::CreateNUWMul(X, ConstantInt::get(Ty, NewMulC));
  }


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D123453/new/

https://reviews.llvm.org/D123453