[PATCH] D93882: [SCEV] recognize logical and/or pattern

[Nikita Popov via Phabricator via llvm-commits]

nikic added inline comments.


================
Comment at: llvm/lib/Analysis/ScalarEvolution.cpp:7618
+      return EL0;
+    }
   }
----------------
I think we can still do slightly better here: Even if the exact count is non-constant, we should still be able to take the min of the max (constant) counts in any case, right? In which case I would move this check into the `EitherMayExit` branch below and only adjust the BECount calculation, while keeping MaxBECount the same.

I think the condition you use here may also need more comment, as I would have expected the check to be only `!isa<SCEVConstant>(EL1.ExactNotTaken)` initially. Is the reasoning here that if EL0.ExactNotTaken is constant but EL1.ExactNotTaken is not, then either EL0.ExactNotTaken is zero, in which case umin will fold to zero and we won't use the second (potentially poison exit count), or it is non-zero, in which case we know that the other condition will be checked at least once and thus can't be poison.

So basically the general condition here would be `if (isKnownNotPoison(EL1.ExactNotTaken) || isKnownNonZero(EL0.ExactNotTaken) || isConstantZero(EL0.ExactNotTaken)`, where the last case is only safe if we do fold down to zero.


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  https://reviews.llvm.org/D93882/new/

https://reviews.llvm.org/D93882