[SCEV] Compute max be count from shift operator only if all else fails

In particular, check to see if we can compute a precise trip count by
exhaustively simulating the loop first.

llvm-svn: 274199

1 files changed, 9 insertions, 6 deletions

diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index 40274f7..e28587e 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -5948,11 +5948,6 @@ ScalarEvolution::computeExitLimitFromICmp(const Loop *L,
         return ItCnt;
     }
 
-  ExitLimit ShiftEL = computeShiftCompareExitLimit(
-      ExitCond->getOperand(0), ExitCond->getOperand(1), L, Cond);
-  if (ShiftEL.hasAnyInfo())
-    return ShiftEL;
-
   const SCEV *LHS = getSCEV(ExitCond->getOperand(0));
   const SCEV *RHS = getSCEV(ExitCond->getOperand(1));
 
@@ -6018,7 +6013,15 @@ ScalarEvolution::computeExitLimitFromICmp(const Loop *L,
   default:
     break;
   }
-  return computeExitCountExhaustively(L, ExitCond, !L->contains(TBB));
+
+  auto *ExhaustiveCount =
+      computeExitCountExhaustively(L, ExitCond, !L->contains(TBB));
+
+  if (!isa<SCEVCouldNotCompute>(ExhaustiveCount))
+    return ExhaustiveCount;
+
+  return computeShiftCompareExitLimit(ExitCond->getOperand(0),
+                                      ExitCond->getOperand(1), L, Cond);
 }
 
 ScalarEvolution::ExitLimit