change llvm::MergeBlockIntoPredecessor to not merge two blocks BB1->BB2

when BB2 has its address taken.  Since it ends up doing BB2->rauw(BB1),
this can cause the address of the entry block to be taken.  Since it is
generally undesirable to nuke blocks whose address is taken, even when
we can, just unconditionally stop this xform.

llvm-svn: 85708

1 files changed, 7 insertions, 3 deletions

diff --git a/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp b/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
index 1f9a2031f3b..c728c0bd5f1 100644
--- a/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
+++ b/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
@@ -94,10 +94,14 @@ void llvm::DeleteDeadPHIs(BasicBlock *BB) {
 
 /// MergeBlockIntoPredecessor - Attempts to merge a block into its predecessor,
 /// if possible.  The return value indicates success or failure.
-bool llvm::MergeBlockIntoPredecessor(BasicBlock* BB, Pass* P) {
+bool llvm::MergeBlockIntoPredecessor(BasicBlock *BB, Pass *P) {
   pred_iterator PI(pred_begin(BB)), PE(pred_end(BB));
-  // Can't merge the entry block.
-  if (pred_begin(BB) == pred_end(BB)) return false;
+  // Can't merge the entry block.  Don't merge away blocks who have their
+  // address taken: this is a bug if the predecessor block is the entry node
+  // (because we'd end up taking the address of the entry) and undesirable in
+  // any case.
+  if (pred_begin(BB) == pred_end(BB) ||
+      BB->hasAddressTaken()) return false;
   
   BasicBlock *PredBB = *PI++;
   for (; PI != PE; ++PI)  // Search all predecessors, see if they are all same