[PATCH] D88087: [SCEV] Limited support for unsigned preds in isImpliedViaOperations

[Max Kazantsev via Phabricator via llvm-commits]

mkazantsev added inline comments.


================
Comment at: llvm/lib/Analysis/ScalarEvolution.cpp:10158
+      const SCEV *MinusOne = getNegativeSCEV(getOne(LHS->getType()));
+      if (isImpliedCondOperands(ICmpInst::ICMP_SGT, LHS, MinusOne, FoundLHS,
+                                FoundRHS) &&
----------------
mkazantsev wrote:
> reames wrote:
> > I'm confused by this check, it seems overly complicated given the comment?  Why is this simply not:
> > if (isKnownNonNegative(LHS) && isKnownNonNegative(RHS) &&
> >     isKnownNonNegative(FoundLHS) && isKnownNonNegative(FoundRHS)) {
> >    Pred = ICmpInst::getSignedPredicate(Pred);
> > }
> Because non-negativity of LHS and RHS is not known. 
We need an extra fact `SignedPred FoundLHS FoundRHS` to prove it. And we only have this fact if we know `Pred FoundLHS FoundRHS && isKnownNonNegative(FoundLHS) && isKnownNonNegative(FoundRHS)`.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D88087/new/

https://reviews.llvm.org/D88087