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

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri added a comment.

In D88287#2589791 <https://reviews.llvm.org/D88287#2589791>, @nlopes wrote:

> 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?
>
> Just to add to what Roman wrote, thinking of the code as max(,) is misleading. The code is doing `icmp sgt INT_MAX, undef` which can evaluate to true or false. But we cannot assume that undef from now on is equal to INT_MAX just because the comparison evaluated to true.
> The are 2 possible fixes:
>
> 1. only do the optimization if `%c` is known non-undef (use ValueTracking's utility), e.g., https://alive2.llvm.org/ce/z/sLeOE0
> 2. freeze `%c`, e.g., https://alive2.llvm.org/ce/z/ZX557G

0. fix SCEV's https://github.com/llvm/llvm-project/blob/00fe10c6a65173c9c578babd19f8fee44d07a761/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp#L1788-L1789 to emit proper intrinsics?


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