Don't try to analyze this "backward" case. This is overly conservative
pending a correct solution.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@61589 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Analysis/ScalarEvolution.cpp b/lib/Analysis/ScalarEvolution.cpp
index 142b82d..375ad5e 100644
--- a/lib/Analysis/ScalarEvolution.cpp
+++ b/lib/Analysis/ScalarEvolution.cpp
@@ -2905,6 +2905,11 @@
return false;
}
+static bool isNegative(SCEVHandle X) {
+ if (SCEVConstant *C = dyn_cast<SCEVConstant>(X))
+ return C->getValue()->getValue().isNegative();
+}
+
/// potentialInfiniteLoop - Test whether the loop might jump over the exit value
/// due to wrapping around 2^n.
bool ScalarEvolutionsImpl::potentialInfiniteLoop(SCEV *Stride, SCEV *RHS,
@@ -2958,6 +2963,13 @@
if (potentialInfiniteLoop(Stride, RHS, isSigned, trueWhenEqual))
return UnknownValue;
+ // We don't handle this correctly at the moment. The problem is that when
+ // the stride is negative, we're not counting how many times 'less-than' is
+ // true as we approach it, we're counting how far away we are from wrapping
+ // around the backside.
+ if (isSigned && isNegative(Stride))
+ return UnknownValue;
+
// We know the LHS is of the form {n,+,s} and the RHS is some loop-invariant
// m. So, we count the number of iterations in which {n,+,s} < m is true.
// Note that we cannot simply return max(m-n,0)/s because it's not safe to