[llvm] r223974 - 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.

[Mark Heffernan]

Author: meheff
Date: Wed Dec 10 16:53:52 2014
New Revision: 223974

URL: http://llvm.org/viewvc/llvm-project?rev=223974&view=rev
Log:
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.

Modified:
    llvm/trunk/lib/Analysis/ScalarEvolution.cpp

Modified: llvm/trunk/lib/Analysis/ScalarEvolution.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/ScalarEvolution.cpp?rev=223974&r1=223973&r2=223974&view=diff
==============================================================================
--- llvm/trunk/lib/Analysis/ScalarEvolution.cpp (original)
+++ llvm/trunk/lib/Analysis/ScalarEvolution.cpp Wed Dec 10 16:53:52 2014
@@ -6161,16 +6161,14 @@ ScalarEvolution::HowFarToZero(const SCEV
     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