[PATCH] D123337: Possible bug in isKnownPredicateViaConstantRanges

[Dangeti Tharun kumar via Phabricator via llvm-commits]

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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  = (X + 1) instead, I get (X + 1)<nuw> and isKnownNonZero(Diff) returns true.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D123337

Files:
  llvm/lib/Analysis/ScalarEvolution.cpp


Index: llvm/lib/Analysis/ScalarEvolution.cpp
===================================================================
--- llvm/lib/Analysis/ScalarEvolution.cpp
+++ llvm/lib/Analysis/ScalarEvolution.cpp
@@ -10534,7 +10534,7 @@
     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);
   }



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