[LoopUnrollRuntime] Use SCEV exit count for calculating trip count. NFCI

Instead of getBackEdgeTakenCount, use getExitCount on the latch exiting block
(which is proven to be the only exiting block in the loop to be unrolled).

llvm-svn: 306410

1 files changed, 5 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/Utils/LoopUnrollRuntime.cpp b/llvm/lib/Transforms/Utils/LoopUnrollRuntime.cpp
index 5f85e17..d915831 100644
--- a/llvm/lib/Transforms/Utils/LoopUnrollRuntime.cpp
+++ b/llvm/lib/Transforms/Utils/LoopUnrollRuntime.cpp
@@ -495,7 +495,11 @@ bool llvm::UnrollRuntimeLoopRemainder(Loop *L, unsigned Count,
 
   // Only unroll loops with a computable trip count, and the trip count needs
   // to be an int value (allowing a pointer type is a TODO item).
-  const SCEV *BECountSC = SE->getBackedgeTakenCount(L);
+  // We calculate the backedge count by using getExitCount on the Latch block,
+  // which is proven to be the only exiting block in this loop. This is same as
+  // calculating getBackedgeTakenCount on the loop (which computes SCEV for all
+  // exiting blocks).
+  const SCEV *BECountSC = SE->getExitCount(L, Latch);
   if (isa<SCEVCouldNotCompute>(BECountSC) ||
       !BECountSC->getType()->isIntegerTy())
     return false;