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...r, if %x is poison and %c is 0, the original program has no UB, and the transformed program does. It would then execute the udiv unconditionally, with a divisor of poison. > > Inserting a freeze could fix this. However, where does the freeze go? %y is an input to the if-conversion pattern as SimpifyCFG knows it, however freezing %y doesn't help. Instead, the thing that needs to be frozen is %x, because that was the input to the KnownBits analysis that said it was safe. It would seem that either KnownBits would need to record the values it looked at, or there'd have to be a way to recomput...

Hi, Over the past few years we've been trying to kill poison somehow. There have been a few proposals, but they've all failed to pass the bar and/or to gather significant support (my own proposals included). We (David, Gil, John, Juneyoung, Sanjoy, Youngju, Yoonseung, and myself) have a new proposal to kill undef instead and replace it with poison + a new 'freeze' instruction. We