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

[Nikita Popov via Phabricator via llvm-commits]

nikic added a comment.

In D145008#4162691 <https://reviews.llvm.org/D145008#4162691>, @kazu wrote:

> 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;

Yes, see the existing code in addToMergedCondition() for handling branch to select conversion. It needs to be extended for the branch case if the conditions are swapped.


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