Fix PR21694. r219517 added a use of SCEV divide in HowFarToZero computation. This divide can produce incorrect results as we are using an unsigned divide for what should be a modular divide. This change reverts back to a more conservative computation using trailing zeros.

llvm-svn: 223974

1 files changed, 8 insertions, 10 deletions

diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index 177bd23030a..a06cfc101b3 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -6161,16 +6161,14 @@ ScalarEvolution::HowFarToZero(const SCEV *V, const Loop *L, bool ControlsExit) {
     return ExitLimit(Distance, MaxBECount);
   }
 
-  // If the step exactly divides the distance then unsigned divide computes the
-  // backedge count.
-  const SCEV *Q, *R;
-  ScalarEvolution &SE = *const_cast<ScalarEvolution *>(this);
-  SCEVUDivision::divide(SE, Distance, Step, &Q, &R);
-  if (R->isZero()) {
-    const SCEV *Exact =
-        getUDivExactExpr(Distance, CountDown ? getNegativeSCEV(Step) : Step);
-    return ExitLimit(Exact, Exact);
-  }
+  // If Step is a power of two that evenly divides Start we know that the loop
+  // will always terminate.  Start may not be a constant so we just have the
+  // number of trailing zeros available.  This is safe even in presence of
+  // overflow as the recurrence will overflow to exactly 0.
+  const APInt &StepV = StepC->getValue()->getValue();
+  if (StepV.isPowerOf2() &&
+      GetMinTrailingZeros(getNegativeSCEV(Start)) >= StepV.countTrailingZeros())
+    return getUDivExactExpr(Distance, CountDown ? getNegativeSCEV(Step) : Step);
 
   // If the condition controls loop exit (the loop exits only if the expression
   // is true) and the addition is no-wrap we can use unsigned divide to