defensive patch: if CGP is merging a block with the entry block, make sure

it ends up being the entry block.

llvm-svn: 60180

1 files changed, 8 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/Scalar/CodeGenPrepare.cpp b/llvm/lib/Transforms/Scalar/CodeGenPrepare.cpp
index caf60c8c7af..9bf39911b6c 100644
--- a/llvm/lib/Transforms/Scalar/CodeGenPrepare.cpp
+++ b/llvm/lib/Transforms/Scalar/CodeGenPrepare.cpp
@@ -204,8 +204,15 @@ void CodeGenPrepare::EliminateMostlyEmptyBlock(BasicBlock *BB) {
 
   // If the destination block has a single pred, then this is a trivial edge,
   // just collapse it.
-  if (DestBB->getSinglePredecessor()) {
+  if (BasicBlock *SinglePred = DestBB->getSinglePredecessor()) {
+    // Remember if SinglePred was the entry block of the function.  If so, we
+    // will need to move BB back to the entry position.
+    bool isEntry = SinglePred == &SinglePred->getParent()->getEntryBlock();
     MergeBasicBlockIntoOnlyPred(DestBB);
+
+    if (isEntry && BB != &BB->getParent()->getEntryBlock())
+      BB->moveBefore(&BB->getParent()->getEntryBlock());
+    
     DOUT << "AFTER:\n" << *DestBB << "\n\n\n";
     return;
   }