[PATCH] D145008: [ControlHeightReduction] Don't combine a "poison" branch

[Kazu Hirata via Phabricator via llvm-commits]

kazu added a comment.

In D145008#4162517 <https://reviews.llvm.org/D145008#4162517>, @nikic wrote:

> In D145008#4162106 <https://reviews.llvm.org/D145008#4162106>, @davidxl wrote:
>
>> In D145008#4160793 <https://reviews.llvm.org/D145008#4160793>, @nikic wrote:
>>
>>> The value may be poison even if it's not literally poison. The problem seems to be that the condition gets swapped. CHR generates `select i1 poison, i1 %arg, i1 false` rather than the correct `select i1 %arg, i1 poison, i1 false`. Either that swapping should not occur (if it's not important to what CHR is doing) or the condition needs to be frozen.
>>
>> Correct -- kazu@ brought it up before sending the patch as well -- is there existing interface to query for propagated poison?
>
> There is `isGuaranteedNotToBeUndefOrPoison()`, but here you want to use `freeze` as a fallback.

Do you mean something like this in pseudo code?

  if (isGuaranteedNotToBeUndefOrPoison(Cond))
    Use Cond As is;
  else
    Use freeze to prevent the potential poison from propagating further;


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