Make ScalarEvolution less aggressive with respect to no-wrap flags.
ScalarEvolution currently lowers a subtraction recurrence to an add
recurrence with the same no-wrap flags as the subtraction. This is
incorrect because `sub nsw X, Y` is not the same as `add nsw X, -Y`
and `sub nuw X, Y` is not the same as `add nuw X, -Y`. This patch
fixes the issue, and adds two test cases demonstrating the bug.
Differential Revision: http://reviews.llvm.org/D7081
llvm-svn: 226755
diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index 6a58ce1..510523e 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -3154,8 +3154,9 @@
if (LHS == RHS)
return getConstant(LHS->getType(), 0);
- // X - Y --> X + -Y
- return getAddExpr(LHS, getNegativeSCEV(RHS), Flags);
+ // X - Y --> X + -Y.
+ // X -(nsw || nuw) Y --> X + -Y.
+ return getAddExpr(LHS, getNegativeSCEV(RHS));
}
/// getTruncateOrZeroExtend - Return a SCEV corresponding to a conversion of the
@@ -3461,12 +3462,10 @@
if (isKnownPositive(getMinusSCEV(getSCEV(GEP), Ptr)))
Flags = setFlags(Flags, SCEV::FlagNUW);
}
- } else if (const SubOperator *OBO =
- dyn_cast<SubOperator>(BEValueV)) {
- if (OBO->hasNoUnsignedWrap())
- Flags = setFlags(Flags, SCEV::FlagNUW);
- if (OBO->hasNoSignedWrap())
- Flags = setFlags(Flags, SCEV::FlagNSW);
+
+ // We cannot transfer nuw and nsw flags from subtraction
+ // operations -- sub nuw X, Y is not the same as add nuw X, -Y
+ // for instance.
}
const SCEV *StartVal = getSCEV(StartValueV);