[PATCH] D87197: [InstSimplify] Fold degenerate abs of abs form

Sat Sep 5 19:02:56 PDT 2020

efriedma accepted this revision.
efriedma added a comment.
This revision is now accepted and ready to land.

LGTM

In D87197#2257842 <https://reviews.llvm.org/D87197#2257842>, @nikic wrote:

> In D87197#2257839 <https://reviews.llvm.org/D87197#2257839>, @efriedma wrote:
>
>> It doesn't matter that the constant is 0 specifically, does it?  Should work for any constant besides INT_MIN with the correct sign.
>
> I believe this is only valid for 0 specifically, because that's where abs(X) == -abs(X). (Well, and for INT_MIN in the non-poisoning case, as both abs and neg preserve it as well.)

Oh, right, it returns a value with the opposite sign.

> Given the still open correctness issues from https://bugs.llvm.org/show_bug.cgi?id=47322, it seems dangerous to try and generalize it further.

I think it's worth doing something here eventually, but this is fine for now, sure.

Repository:
  rG LLVM Github Monorepo

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https://reviews.llvm.org/D87197