Index: llvm/trunk/lib/CodeGen/MachineBlockPlacement.cpp
===================================================================
--- llvm/trunk/lib/CodeGen/MachineBlockPlacement.cpp
+++ llvm/trunk/lib/CodeGen/MachineBlockPlacement.cpp
@@ -453,6 +453,8 @@
                   BlockFilterSet *BlockFilter = nullptr);
   bool canMoveBottomBlockToTop(const MachineBasicBlock *BottomBlock,
                                const MachineBasicBlock *OldTop);
+  bool hasViableTopFallthrough(const MachineBasicBlock *Top,
+                               const BlockFilterSet &LoopBlockSet);
   MachineBasicBlock *findBestLoopTop(
       const MachineLoop &L, const BlockFilterSet &LoopBlockSet);
   MachineBasicBlock *findBestLoopExit(
@@ -1984,6 +1986,39 @@
   return ExitingBB;
 }
 
+/// Check if there is a fallthrough to loop header Top.
+///
+///   1. Look for a Pred that can be layout before Top.
+///   2. Check if Top is the most possible successor of Pred.
+bool
+MachineBlockPlacement::hasViableTopFallthrough(
+    const MachineBasicBlock *Top,
+    const BlockFilterSet &LoopBlockSet) {
+  for (MachineBasicBlock *Pred : Top->predecessors()) {
+    BlockChain *PredChain = BlockToChain[Pred];
+    if (!LoopBlockSet.count(Pred) &&
+        (!PredChain || Pred == *std::prev(PredChain->end()))) {
+      // Found a Pred block can be placed before Top.
+      // Check if Top is the best successor of Pred.
+      auto TopProb = MBPI->getEdgeProbability(Pred, Top);
+      bool TopOK = true;
+      for (MachineBasicBlock *Succ : Pred->successors()) {
+        auto SuccProb = MBPI->getEdgeProbability(Pred, Succ);
+        BlockChain *SuccChain = BlockToChain[Succ];
+        // Check if Succ can be placed after Pred.
+        // Succ should not be in any chain, or it is the head of some chain.
+        if ((!SuccChain || Succ == *SuccChain->begin()) && SuccProb > TopProb) {
+          TopOK = false;
+          break;
+        }
+      }
+      if (TopOK)
+        return true;
+    }
+  }
+  return false;
+}
+
 /// Attempt to rotate an exiting block to the bottom of the loop.
 ///
 /// Once we have built a chain, try to rotate it to line up the hot exit block
@@ -2003,15 +2038,7 @@
   if (Bottom == ExitingBB)
     return;
 
-  bool ViableTopFallthrough = false;
-  for (MachineBasicBlock *Pred : Top->predecessors()) {
-    BlockChain *PredChain = BlockToChain[Pred];
-    if (!LoopBlockSet.count(Pred) &&
-        (!PredChain || Pred == *std::prev(PredChain->end()))) {
-      ViableTopFallthrough = true;
-      break;
-    }
-  }
+  bool ViableTopFallthrough = hasViableTopFallthrough(Top, LoopBlockSet);
 
   // If the header has viable fallthrough, check whether the current loop
   // bottom is a viable exiting block. If so, bail out as rotating will
Index: llvm/trunk/test/CodeGen/X86/bb_rotate.ll
===================================================================
--- llvm/trunk/test/CodeGen/X86/bb_rotate.ll
+++ llvm/trunk/test/CodeGen/X86/bb_rotate.ll
@@ -0,0 +1,53 @@
+; RUN: llc -mtriple=i686-linux < %s | FileCheck %s
+
+define i1 @no_viable_top_fallthrough() {
+; CHECK-LABEL: no_viable_top_fallthrough
+; CHECK: %.entry
+; CHECK: %.bb1
+; CHECK: %.bb2
+; CHECK: %.middle
+; CHECK: %.backedge
+; CHECK: %.bb3
+; CHECK: %.header
+; CHECK: %.exit
+; CHECK: %.stop
+.entry:
+  %val1 = call i1 @foo()
+  br i1 %val1, label %.bb1, label %.header, !prof !10
+
+.bb1:
+  %val2 = call i1 @foo()
+  br i1 %val2, label %.stop, label %.exit, !prof !10
+
+.header:
+  %val3 = call i1 @foo()
+  br i1 %val3, label %.bb2, label %.exit
+
+.bb2:
+  %val4 = call i1 @foo()
+  br i1 %val4, label %.middle, label %.bb3, !prof !10
+
+.middle:
+  %val5 = call i1 @foo()
+  br i1 %val5, label %.header, label %.backedge
+
+.backedge:
+  %val6 = call i1 @foo()
+  br label %.header
+
+.bb3:
+  %val7 = call i1 @foo()
+  br label %.middle
+
+.exit:
+  %val8 = call i1 @foo()
+  br label %.stop
+
+.stop:
+  %result = call i1 @foo()
+  ret i1 %result
+}
+
+declare i1 @foo()
+
+!10 = !{!"branch_weights", i32 90, i32 10}