Simplify blockaddress usage before giving up in MergeBlockIntoPredecessor

Summary:
Simplify blockaddress usage before giving up in MergeBlockIntoPredecessor

This is a missing small optimization in MergeBlockIntoPredecessor.

This helps with one simplifycfg test which expects this case to be handled.

Reviewers: davide, spatel, brzycki, asbirlea

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D48284

llvm-svn: 334992

1 files changed, 8 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp b/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
index 59837042550..4b89b0435e0 100644
--- a/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
+++ b/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
@@ -119,7 +119,14 @@ bool llvm::MergeBlockIntoPredecessor(BasicBlock *BB, DominatorTree *DT,
                                      LoopInfo *LI,
                                      MemoryDependenceResults *MemDep) {
   // Don't merge away blocks who have their address taken.
-  if (BB->hasAddressTaken()) return false;
+  if (BB->hasAddressTaken()) {
+    // If the block has its address taken, it may be a tree of dead constants
+    // hanging off of it.  These shouldn't keep the block alive.
+    BlockAddress *BA = BlockAddress::get(BB);
+    BA->removeDeadConstantUsers();
+    if (!BA->use_empty())
+      return false;
+  }
 
   // Can't merge if there are multiple predecessors, or no predecessors.
   BasicBlock *PredBB = BB->getUniquePredecessor();