diff --git a/include/llvm/CodeGen/RegisterPressure.h b/include/llvm/CodeGen/RegisterPressure.h
index bbc8ca5d28f..8a0a8f3d936 100644
--- a/include/llvm/CodeGen/RegisterPressure.h
+++ b/include/llvm/CodeGen/RegisterPressure.h
@@ -187,6 +187,9 @@ class RegPressureTracker {
   /// or RegisterPressure. If requireIntervals is false, LIS are ignored.
   bool RequireIntervals;
 
+  /// True if UntiedDefs will be populated.
+  bool TrackUntiedDefs;
+
   /// Register pressure corresponds to liveness before this instruction
   /// iterator. It may point to the end of the block or a DebugValue rather than
   /// an instruction.
@@ -198,16 +201,24 @@ class RegPressureTracker {
   /// Set of live registers.
   LiveRegSet LiveRegs;
 
+  /// Set of vreg defs that start a live range.
+  SparseSet<unsigned, VirtReg2IndexFunctor> UntiedDefs;
+  /// Live-through pressure.
+  std::vector<unsigned> LiveThruPressure;
+
 public:
   RegPressureTracker(IntervalPressure &rp) :
-    MF(0), TRI(0), RCI(0), LIS(0), MBB(0), P(rp), RequireIntervals(true) {}
+    MF(0), TRI(0), RCI(0), LIS(0), MBB(0), P(rp), RequireIntervals(true),
+    TrackUntiedDefs(false) {}
 
   RegPressureTracker(RegionPressure &rp) :
-    MF(0), TRI(0), RCI(0), LIS(0), MBB(0), P(rp), RequireIntervals(false) {}
+    MF(0), TRI(0), RCI(0), LIS(0), MBB(0), P(rp), RequireIntervals(false),
+    TrackUntiedDefs(false) {}
 
   void init(const MachineFunction *mf, const RegisterClassInfo *rci,
             const LiveIntervals *lis, const MachineBasicBlock *mbb,
-            MachineBasicBlock::const_iterator pos);
+            MachineBasicBlock::const_iterator pos,
+            bool ShouldTrackUntiedDefs = false);
 
   /// Force liveness of virtual registers or physical register
   /// units. Particularly useful to initialize the livein/out state of the
@@ -236,6 +247,17 @@ public:
   /// Finalize the region boundaries and recored live ins and live outs.
   void closeRegion();
 
+  /// Initialize the LiveThru pressure set based on the untied defs found in
+  /// RPTracker.
+  void initLiveThru(const RegPressureTracker &RPTracker);
+
+  /// Copy an existing live thru pressure result.
+  void initLiveThru(ArrayRef<unsigned> PressureSet) {
+    LiveThruPressure.assign(PressureSet.begin(), PressureSet.end());
+  }
+
+  ArrayRef<unsigned> getLiveThru() const { return LiveThruPressure; }
+
   /// Get the resulting register pressure over the traversed region.
   /// This result is complete if either advance() or recede() has returned true,
   /// or if closeRegion() was explicitly invoked.
@@ -308,6 +330,10 @@ public:
     return getDownwardPressure(MI, PressureResult, MaxPressureResult);
   }
 
+  bool hasUntiedDef(unsigned VirtReg) const {
+    return UntiedDefs.count(VirtReg);
+  }
+
   void dump() const;
 
 protected:
@@ -319,6 +345,11 @@ protected:
   void bumpUpwardPressure(const MachineInstr *MI);
   void bumpDownwardPressure(const MachineInstr *MI);
 };
+
+#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
+void dumpRegSetPressure(ArrayRef<unsigned> SetPressure,
+                        const TargetRegisterInfo *TRI);
+#endif
 } // end namespace llvm
 
 #endif
diff --git a/lib/CodeGen/MachineScheduler.cpp b/lib/CodeGen/MachineScheduler.cpp
index b088d1ad17c..badc0e56674 100644
--- a/lib/CodeGen/MachineScheduler.cpp
+++ b/lib/CodeGen/MachineScheduler.cpp
@@ -464,7 +464,7 @@ void ScheduleDAGMI::initRegPressure() {
   // Close the RPTracker to finalize live ins.
   RPTracker.closeRegion();
 
-  DEBUG(RPTracker.getPressure().dump(TRI));
+  DEBUG(RPTracker.dump());
 
   // Initialize the live ins and live outs.
   TopRPTracker.addLiveRegs(RPTracker.getPressure().LiveInRegs);
@@ -476,6 +476,13 @@ void ScheduleDAGMI::initRegPressure() {
   TopRPTracker.closeTop();
   BotRPTracker.closeBottom();
 
+  BotRPTracker.initLiveThru(RPTracker);
+  if (!BotRPTracker.getLiveThru().empty()) {
+    TopRPTracker.initLiveThru(BotRPTracker.getLiveThru());
+    DEBUG(dbgs() << "Live Thru: ";
+          dumpRegSetPressure(BotRPTracker.getLiveThru(), TRI));
+  };
+
   // Account for liveness generated by the region boundary.
   if (LiveRegionEnd != RegionEnd)
     BotRPTracker.recede();
@@ -579,7 +586,8 @@ void ScheduleDAGMI::schedule() {
 /// Build the DAG and setup three register pressure trackers.
 void ScheduleDAGMI::buildDAGWithRegPressure() {
   // Initialize the register pressure tracker used by buildSchedGraph.
-  RPTracker.init(&MF, RegClassInfo, LIS, BB, LiveRegionEnd);
+  RPTracker.init(&MF, RegClassInfo, LIS, BB, LiveRegionEnd,
+                 /*TrackUntiedDefs=*/true);
 
   // Account for liveness generate by the region boundary.
   if (LiveRegionEnd != RegionEnd)
diff --git a/lib/CodeGen/RegisterPressure.cpp b/lib/CodeGen/RegisterPressure.cpp
index 4175a4ff85a..b7ab138bf15 100644
--- a/lib/CodeGen/RegisterPressure.cpp
+++ b/lib/CodeGen/RegisterPressure.cpp
@@ -76,17 +76,22 @@ void RegisterPressure::decrease(unsigned Reg, const TargetRegisterInfo *TRI,
 }
 
 #if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
-static void dumpSetPressure(const std::vector<unsigned> &SetPressure,
-                            const TargetRegisterInfo *TRI) {
+void llvm::dumpRegSetPressure(ArrayRef<unsigned> SetPressure,
+                              const TargetRegisterInfo *TRI) {
+  bool Empty = true;
   for (unsigned i = 0, e = SetPressure.size(); i < e; ++i) {
-    if (SetPressure[i] != 0)
+    if (SetPressure[i] != 0) {
       dbgs() << TRI->getRegPressureSetName(i) << "=" << SetPressure[i] << '\n';
+      Empty = false;
+    }
   }
+  if (Empty)
+    dbgs() << "\n";
 }
 
 void RegisterPressure::dump(const TargetRegisterInfo *TRI) const {
   dbgs() << "Max Pressure: ";
-  dumpSetPressure(MaxSetPressure, TRI);
+  dumpRegSetPressure(MaxSetPressure, TRI);
   dbgs() << "Live In: ";
   for (unsigned i = 0, e = LiveInRegs.size(); i < e; ++i)
     dbgs() << PrintReg(LiveInRegs[i], TRI) << " ";
@@ -98,8 +103,10 @@ void RegisterPressure::dump(const TargetRegisterInfo *TRI) const {
 }
 
 void RegPressureTracker::dump() const {
-  dbgs() << "Curr Pressure: ";
-  dumpSetPressure(CurrSetPressure, TRI);
+  if (!isTopClosed() || !isBottomClosed()) {
+    dbgs() << "Curr Pressure: ";
+    dumpRegSetPressure(CurrSetPressure, TRI);
+  }
   P.dump(TRI);
 }
 #endif
@@ -200,13 +207,15 @@ void RegPressureTracker::init(const MachineFunction *mf,
                               const RegisterClassInfo *rci,
                               const LiveIntervals *lis,
                               const MachineBasicBlock *mbb,
-                              MachineBasicBlock::const_iterator pos)
+                              MachineBasicBlock::const_iterator pos,
+                              bool ShouldTrackUntiedDefs)
 {
   MF = mf;
   TRI = MF->getTarget().getRegisterInfo();
   RCI = rci;
   MRI = &MF->getRegInfo();
   MBB = mbb;
+  TrackUntiedDefs = ShouldTrackUntiedDefs;
 
   if (RequireIntervals) {
     assert(lis && "IntervalPressure requires LiveIntervals");
@@ -215,6 +224,7 @@ void RegPressureTracker::init(const MachineFunction *mf,
 
   CurrPos = pos;
   CurrSetPressure.assign(TRI->getNumRegPressureSets(), 0);
+  LiveThruPressure.clear();
 
   if (RequireIntervals)
     static_cast<IntervalPressure&>(P).reset();
@@ -226,6 +236,9 @@ void RegPressureTracker::init(const MachineFunction *mf,
   LiveRegs.PhysRegs.setUniverse(TRI->getNumRegs());
   LiveRegs.VirtRegs.clear();
   LiveRegs.VirtRegs.setUniverse(MRI->getNumVirtRegs());
+  UntiedDefs.clear();
+  if (TrackUntiedDefs)
+    UntiedDefs.setUniverse(MRI->getNumVirtRegs());
 }
 
 /// Does this pressure result have a valid top position and live ins.
@@ -304,6 +317,25 @@ void RegPressureTracker::closeRegion() {
   // If both top and bottom are closed, do nothing.
 }
 
+/// The register tracker is unaware of global liveness so ignores normal
+/// live-thru ranges. However, two-address or coalesced chains can also lead
+/// to live ranges with no holes. Count these to inform heuristics that we
+/// can never drop below this pressure.
+void RegPressureTracker::initLiveThru(const RegPressureTracker &RPTracker) {
+  LiveThruPressure.assign(TRI->getNumRegPressureSets(), 0);
+  assert(isBottomClosed() && "need bottom-up tracking to intialize.");
+  for (unsigned i = 0, e = P.LiveOutRegs.size(); i < e; ++i) {
+    unsigned Reg = P.LiveOutRegs[i];
+    if (TargetRegisterInfo::isVirtualRegister(Reg)
+        && !RPTracker.hasUntiedDef(Reg)) {
+      const TargetRegisterClass *RC = MRI->getRegClass(Reg);
+      increaseSetPressure(LiveThruPressure, LiveThruPressure,
+                          TRI->getRegClassPressureSets(RC),
+                          TRI->getRegClassWeight(RC).RegWeight);
+    }
+  }
+}
+
 /// \brief Convenient wrapper for checking membership in RegisterOperands.
 static bool containsReg(ArrayRef<unsigned> Regs, unsigned Reg) {
   return std::find(Regs.begin(), Regs.end(), Reg) != Regs.end();
@@ -459,11 +491,20 @@ bool RegPressureTracker::recede() {
       LiveRegs.insert(Reg);
     }
   }
+  if (TrackUntiedDefs) {
+    for (unsigned i = 0, e = RegOpers.Defs.size(); i < e; ++i) {
+      unsigned Reg = RegOpers.Defs[i];
+      if (TargetRegisterInfo::isVirtualRegister(Reg) && !LiveRegs.contains(Reg))
+        UntiedDefs.insert(Reg);
+    }
+  }
   return true;
 }
 
 /// Advance across the current instruction.
 bool RegPressureTracker::advance() {
+  assert(!TrackUntiedDefs && "unsupported mode");
+
   // Check for the bottom of the analyzable region.
   if (CurrPos == MBB->end()) {
     closeRegion();
@@ -533,7 +574,8 @@ bool RegPressureTracker::advance() {
 static void computeExcessPressureDelta(ArrayRef<unsigned> OldPressureVec,
                                        ArrayRef<unsigned> NewPressureVec,
                                        RegPressureDelta &Delta,
-                                       const RegisterClassInfo *RCI) {
+                                       const RegisterClassInfo *RCI,
+                                       ArrayRef<unsigned> LiveThruPressureVec) {
   int ExcessUnits = 0;
   unsigned PSetID = ~0U;
   for (unsigned i = 0, e = OldPressureVec.size(); i < e; ++i) {
@@ -544,6 +586,9 @@ static void computeExcessPressureDelta(ArrayRef<unsigned> OldPressureVec,
       continue;
     // Only consider change beyond the limit.
     unsigned Limit = RCI->getRegPressureSetLimit(i);
+    if (!LiveThruPressureVec.empty())
+      Limit += LiveThruPressureVec[i];
+
     if (Limit > POld) {
       if (Limit > PNew)
         PDiff = 0;            // Under the limit
@@ -665,7 +710,8 @@ getMaxUpwardPressureDelta(const MachineInstr *MI, RegPressureDelta &Delta,
 
   bumpUpwardPressure(MI);
 
-  computeExcessPressureDelta(SavedPressure, CurrSetPressure, Delta, RCI);
+  computeExcessPressureDelta(SavedPressure, CurrSetPressure, Delta, RCI,
+                             LiveThruPressure);
   computeMaxPressureDelta(SavedMaxPressure, P.MaxSetPressure, CriticalPSets,
                           MaxPressureLimit, Delta);
   assert(Delta.CriticalMax.UnitIncrease >= 0 &&
@@ -755,7 +801,8 @@ getMaxDownwardPressureDelta(const MachineInstr *MI, RegPressureDelta &Delta,
 
   bumpDownwardPressure(MI);
 
-  computeExcessPressureDelta(SavedPressure, CurrSetPressure, Delta, RCI);
+  computeExcessPressureDelta(SavedPressure, CurrSetPressure, Delta, RCI,
+                             LiveThruPressure);
   computeMaxPressureDelta(SavedMaxPressure, P.MaxSetPressure, CriticalPSets,
                           MaxPressureLimit, Delta);
   assert(Delta.CriticalMax.UnitIncrease >= 0 &&