If a loop iterates exactly once (has backedge count = 0) then don't

mess with it.  We'd rather peel/unroll it than convert all of its 
stores into memsets.

llvm-svn: 122711

1 files changed, 6 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Scalar/LoopIdiomRecognize.cpp b/llvm/lib/Transforms/Scalar/LoopIdiomRecognize.cpp
index 8b022bab83a..57a502b1d25 100644
--- a/llvm/lib/Transforms/Scalar/LoopIdiomRecognize.cpp
+++ b/llvm/lib/Transforms/Scalar/LoopIdiomRecognize.cpp
@@ -159,6 +159,12 @@ bool LoopIdiomRecognize::runOnLoop(Loop *L, LPPassManager &LPM) {
   const SCEV *BECount = SE->getBackedgeTakenCount(L);
   if (isa<SCEVCouldNotCompute>(BECount)) return false;
   
+  // If this loop executes exactly one time, then it should be peeled, not
+  // optimized by this pass.
+  if (const SCEVConstant *BECst = dyn_cast<SCEVConstant>(BECount))
+    if (BECst->getValue()->getValue() == 0)
+      return false;
+  
   // We require target data for now.
   TD = getAnalysisIfAvailable<TargetData>();
   if (TD == 0) return false;