Don't try to analyze this "backward" case. This is overly conservative

pending a correct solution.

llvm-svn: 61589

1 files changed, 12 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index 142b82d2232..375ad5ed707 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -2905,6 +2905,11 @@ bool ScalarEvolutionsImpl::executesAtLeastOnce(const Loop *L, bool isSigned,
   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 @@ HowManyLessThans(SCEV *LHS, SCEV *RHS, const Loop *L,
     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