[PATCH] D122299: [InstCombine] Fix missing nsw flag when fold -(x-y) to y-x

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri added a comment.

In D122299#3402088 <https://reviews.llvm.org/D122299#3402088>, @bcl5980 wrote:

> In D122299#3402030 <https://reviews.llvm.org/D122299#3402030>, @lebedev.ri wrote:
>
>> Must we do this? I was really hoping we wouldn't.
>> Does this actually unblock some measurable improvement?
>>
>> Consider: https://alive2.llvm.org/ce/z/XYfMSm
>> Both `sub`s have `NSW`, so i believe the code will now produce: https://alive2.llvm.org/ce/z/vUseE6
>
> It comes from here https://reviews.llvm.org/D122013



> Maybe we can enable it when IsTrulyNegation ?

Oh, hm, that is also a required precondition apparently: https://alive2.llvm.org/ce/z/c9i8Go
But even then, the really big problem for `NSW` preservation is that every instruction inbetween that we manage to negate through affects the `NSW`: https://alive2.llvm.org/ce/z/4A--JC

> Abandon is also OK for me.




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https://reviews.llvm.org/D122299