Re-apply r124518 with fix. Watch out for invalidated iterator.

llvm-svn: 124526

1 files changed, 28 insertions, 3 deletions

diff --git a/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp b/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
index 5d3af3759a0..acaea195e71 100644
--- a/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
+++ b/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
@@ -509,7 +509,32 @@ void llvm::FindFunctionBackedges(const Function &F,
       // Go up one level.
       InStack.erase(VisitStack.pop_back_val().first);
     }
-  } while (!VisitStack.empty());
-  
-  
+  } while (!VisitStack.empty()); 
+}
+
+/// FoldReturnIntoUncondBranch - This method duplicates the specified return
+/// instruction into a predecessor which ends in an unconditional branch. If
+/// the return instruction returns a value defined by a PHI, propagate the
+/// right value into the return. It returns the new return instruction in the
+/// predecessor.
+ReturnInst *llvm::FoldReturnIntoUncondBranch(ReturnInst *RI, BasicBlock *BB,
+                                             BasicBlock *Pred) {
+  Instruction *UncondBranch = Pred->getTerminator();
+  // Clone the return and add it to the end of the predecessor.
+  Instruction *NewRet = RI->clone();
+  Pred->getInstList().push_back(NewRet);
+      
+  // If the return instruction returns a value, and if the value was a
+  // PHI node in "BB", propagate the right value into the return.
+  for (User::op_iterator i = NewRet->op_begin(), e = NewRet->op_end();
+       i != e; ++i)
+    if (PHINode *PN = dyn_cast<PHINode>(*i))
+      if (PN->getParent() == BB)
+        *i = PN->getIncomingValueForBlock(Pred);
+      
+  // Update any PHI nodes in the returning block to realize that we no
+  // longer branch to them.
+  BB->removePredecessor(Pred);
+  UncondBranch->eraseFromParent();
+  return cast<ReturnInst>(NewRet);
 }