[PATCH] D88287: [NARY-REASSOCIATE] Support reassociation of min/max

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri added a comment.

In D88287#2589378 <https://reviews.llvm.org/D88287#2589378>, @ebrevnov wrote:

> I think this is an issue of verification itself. In the first case max(0, undef)=>any and max(any, max_int)=>max_int. In the second case max(max_int, undef)=>x03002006. I believe the behavior of the verifier is inconsistent in these two cases and max(max_int, undef) should be evaluated to max_int as well. We can do the following trivial transformations to prove that:  max(max_int, undef) is trivially equal to max(max_int, max(undef, undef)) and max(undef, undef) should be evaluated to 'any' since max(0, undef) is evaluated to 'any' in the first case. Thus we get max(max_int, any) which is evaluated to 'max_int' in the first case. So max(max_int, undef) should be evaluated to 'max_int' but not 'x03002006'.
>
> Makes sense?

Note that given

  %a = undef
  %b = %a

, `%a` and `%b` have undefined values, and there are no guarantees that they are equal/not equal.
Since you emitted `icmp`+`select`, you 'read' from `undef`ined `%c` twice, and you are free to get different result each time.


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https://reviews.llvm.org/D88287