Possible bug in isKnownPredicateViaConstantRanges

[tdangeti]

Summary

I have a problem with getMinusSCEV in isKnownPredicateViaConstantRanges function:

The following is the scenario:
LHS = X,
RHS = -1,

SE already has a computed SCEV (X + 1)<nuw> in the cache. This SCEV corresponds to an instruction at different scope where nuw flag is valid.

I am expecting the "Diff = getMinusSCEV(LHS, RHS)" to be simply (X + 1) instead, I get (X + 1)<nuw> and isKnownNonZero(Diff) returns true which doesn't look correct for me.

[nikic]

No wrap flags in SCEV are required to be valid independent of scope. If you see nowrap flags for which this is not the case, then the calculation of the nowrap flags is at fault.

[fhahn]

This also needs a test case. Without that it is impossible to figure out what the actual issue is.

llvm/lib/Analysis/ScalarEvolution.cpp

     if (CheckRanges(getSignedRange(LHS), getSignedRange(RHS)) ||
         CheckRanges(getUnsignedRange(LHS), getUnsignedRange(RHS)))
       return true;
-    auto *Diff = getMinusSCEV(LHS, RHS);
+    auto *Diff = getMinusSCEV(LHS, RHS, SCEV::NoWrapMask);
     return !isa<SCEVCouldNotCompute>(Diff) && isKnownNonZero(Diff);
   }