diff --git a/include/llvm/CodeGen/LiveIntervalAnalysis.h b/include/llvm/CodeGen/LiveIntervalAnalysis.h
index 5610ed63c52..ee407cd8d80 100644
--- a/include/llvm/CodeGen/LiveIntervalAnalysis.h
+++ b/include/llvm/CodeGen/LiveIntervalAnalysis.h
@@ -255,9 +255,10 @@ namespace llvm {
                             const SmallVectorImpl<LiveInterval*> *SpillIs,
                             bool &isLoad);
 
-    /// intervalIsInOneMBB - Returns true if the specified interval is entirely
-    /// within a single basic block.
-    bool intervalIsInOneMBB(const LiveInterval &li) const;
+    /// intervalIsInOneMBB - If LI is confined to a single basic block, return
+    /// a pointer to that block.  If LI is live in to or out of any block,
+    /// return NULL.
+    MachineBasicBlock *intervalIsInOneMBB(const LiveInterval &LI) const;
 
     /// addKillFlags - Add kill flags to any instruction that kills a virtual
     /// register.
diff --git a/lib/CodeGen/LiveIntervalAnalysis.cpp b/lib/CodeGen/LiveIntervalAnalysis.cpp
index 1d5e3b1be75..7655cf5e930 100644
--- a/lib/CodeGen/LiveIntervalAnalysis.cpp
+++ b/lib/CodeGen/LiveIntervalAnalysis.cpp
@@ -1086,23 +1086,28 @@ LiveIntervals::isReMaterializable(const LiveInterval &li,
   return true;
 }
 
-bool LiveIntervals::intervalIsInOneMBB(const LiveInterval &li) const {
-  LiveInterval::Ranges::const_iterator itr = li.ranges.begin();
+MachineBasicBlock*
+LiveIntervals::intervalIsInOneMBB(const LiveInterval &LI) const {
+  // A local live range must be fully contained inside the block, meaning it is
+  // defined and killed at instructions, not at block boundaries. It is not
+  // live in or or out of any block.
+  //
+  // It is technically possible to have a PHI-defined live range identical to a
+  // single block, but we are going to return false in that case.
 
-  MachineBasicBlock *mbb =  indexes_->getMBBCoveringRange(itr->start, itr->end);
+  SlotIndex Start = LI.beginIndex();
+  if (Start.isBlock())
+    return NULL;
 
-  if (mbb == 0)
-    return false;
+  SlotIndex Stop = LI.endIndex();
+  if (Stop.isBlock())
+    return NULL;
 
-  for (++itr; itr != li.ranges.end(); ++itr) {
-    MachineBasicBlock *mbb2 =
-      indexes_->getMBBCoveringRange(itr->start, itr->end);
-
-    if (mbb2 != mbb)
-      return false;
-  }
-
-  return true;
+  // getMBBFromIndex doesn't need to search the MBB table when both indexes
+  // belong to proper instructions.
+  MachineBasicBlock *MBB1 = indexes_->getMBBFromIndex(Start);
+  MachineBasicBlock *MBB2 = indexes_->getMBBFromIndex(Stop);
+  return MBB1 == MBB2 ? MBB1 : NULL;
 }
 
 float