Index: lib/CodeGen/MachineBlockPlacement.cpp
===================================================================
--- lib/CodeGen/MachineBlockPlacement.cpp
+++ lib/CodeGen/MachineBlockPlacement.cpp
@@ -74,6 +74,13 @@
              "post dominator, out of line."),
     cl::init(false), cl::Hidden);
 
+static cl::opt<bool> PlaceLastSuccessor(
+    "place-last-successor",
+    cl::desc("When selecting a non-successor block, choose the last block to "
+             "have been a successor. This represents the block whose "
+             "predecessor was most recently placed."),
+    cl::init(false), cl::Hidden);
+
 namespace {
 class BlockChain;
 /// \brief Type for our function-wide basic block -> block chain mapping.
@@ -223,11 +230,11 @@
                            const BlockFilterSet *BlockFilter = nullptr);
   MachineBasicBlock *selectBestSuccessor(MachineBasicBlock *BB,
                                          BlockChain &Chain,
-                                         const BlockFilterSet *BlockFilter);
-  MachineBasicBlock *
-  selectBestCandidateBlock(BlockChain &Chain,
-                           SmallVectorImpl<MachineBasicBlock *> &WorkList,
-                           const BlockFilterSet *BlockFilter);
+                                         const BlockFilterSet *BlockFilter,
+                                         bool &HasUnmergedSuccessors);
+  MachineBasicBlock *selectBestCandidateBlock(
+      BlockChain &Chain, SmallVectorImpl<MachineBasicBlock *> &WorkList,
+      const BlockFilterSet *BlockFilter, bool HasUnmergedSuccessors);
   MachineBasicBlock *
   getFirstUnplacedBlock(MachineFunction &F, const BlockChain &PlacedChain,
                         MachineFunction::iterator &PrevUnplacedBlockIt,
@@ -343,7 +350,8 @@
 MachineBasicBlock *
 MachineBlockPlacement::selectBestSuccessor(MachineBasicBlock *BB,
                                            BlockChain &Chain,
-                                           const BlockFilterSet *BlockFilter) {
+                                           const BlockFilterSet *BlockFilter,
+                                           bool &HasUnmergedSuccessors) {
   const BranchProbability HotProb(4, 5); // 80%
 
   MachineBasicBlock *BestSucc = nullptr;
@@ -370,6 +378,8 @@
       continue;
     }
 
+    HasUnmergedSuccessors = true;
+
     uint32_t SuccWeight = MBPI->getEdgeWeight(BB, Succ);
     BranchProbability SuccProb(SuccWeight / WeightScale, SumWeight);
 
@@ -436,7 +446,25 @@
 /// \returns The best block found, or null if none are viable.
 MachineBasicBlock *MachineBlockPlacement::selectBestCandidateBlock(
     BlockChain &Chain, SmallVectorImpl<MachineBasicBlock *> &WorkList,
-    const BlockFilterSet *BlockFilter) {
+    const BlockFilterSet *BlockFilter, bool HasUnmergedSuccessors) {
+  if (PlaceLastSuccessor && HasUnmergedSuccessors) {
+    // If we're just placing the last successor as the best candidate, the
+    // logic is super simple. We skip the already placed entries on the
+    // worklist and return the most recently added entry that isn't placed.
+    while (!WorkList.empty()) {
+      MachineBasicBlock *SuccBB = WorkList.pop_back_val();
+      BlockChain &SuccChain = *BlockToChain.lookup(SuccBB);
+      if (&SuccChain == &Chain) {
+        DEBUG(dbgs() << "    " << getBlockName(SuccBB)
+                     << " -> Already merged!\n");
+        continue;
+      }
+      assert(SuccChain.LoopPredecessors == 0 && "Found CFG-violating block");
+      return SuccBB;
+    }
+
+    return nullptr;
+  }
   // Once we need to walk the worklist looking for a candidate, cleanup the
   // worklist of already placed entries.
   // FIXME: If this shows up on profiles, it could be folded (at the cost of
@@ -513,13 +541,16 @@
 
     // Look for the best viable successor if there is one to place immediately
     // after this block.
-    MachineBasicBlock *BestSucc = selectBestSuccessor(BB, Chain, BlockFilter);
+    bool HasUnmergedSuccessors = false;
+    MachineBasicBlock *BestSucc =
+        selectBestSuccessor(BB, Chain, BlockFilter, HasUnmergedSuccessors);
 
     // If an immediate successor isn't available, look for the best viable
     // block among those we've identified as not violating the loop's CFG at
     // this point. This won't be a fallthrough, but it will increase locality.
     if (!BestSucc)
-      BestSucc = selectBestCandidateBlock(Chain, BlockWorkList, BlockFilter);
+      BestSucc = selectBestCandidateBlock(Chain, BlockWorkList, BlockFilter,
+                                          HasUnmergedSuccessors);
 
     if (!BestSucc) {
       BestSucc =