diff --git a/lib/CodeGen/RegAllocIterativeScan.cpp b/lib/CodeGen/RegAllocIterativeScan.cpp
index ddbb52a..0ebef5e 100644
--- a/lib/CodeGen/RegAllocIterativeScan.cpp
+++ b/lib/CodeGen/RegAllocIterativeScan.cpp
@@ -51,6 +51,7 @@
     const TargetMachine* tm_;
     const MRegisterInfo* mri_;
     LiveIntervals* li_;
+    bool *PhysRegsUsed;
     typedef std::vector<LiveInterval*> IntervalPtrs;
     IntervalPtrs unhandled_, fixed_, active_, inactive_, handled_, spilled_;
 
@@ -145,6 +146,11 @@
   tm_ = &fn.getTarget();
   mri_ = tm_->getRegisterInfo();
   li_ = &getAnalysis<LiveIntervals>();
+
+  PhysRegsUsed = new bool[mri_->getNumRegs()];
+  std::fill(PhysRegsUsed, PhysRegsUsed+mri_->getNumRegs(), false);
+  fn.setUsedPhysRegs(PhysRegsUsed);
+
   if (!prt_.get()) prt_.reset(new PhysRegTracker(*mri_));
   vrm_.reset(new VirtRegMap(*mf_));
   if (!spiller_.get()) spiller_.reset(createSpiller());
@@ -256,8 +262,10 @@
 
   for (LiveIntervals::iterator i = li_->begin(), e = li_->end(); i != e; ++i){
     unhandled_.push_back(&i->second);
-    if (MRegisterInfo::isPhysicalRegister(i->second.reg))
+    if (MRegisterInfo::isPhysicalRegister(i->second.reg)) {
+      PhysRegsUsed[i->second.reg] = true;
       fixed_.push_back(&i->second);
+    }
   }
 }
 
diff --git a/lib/CodeGen/RegAllocLinearScan.cpp b/lib/CodeGen/RegAllocLinearScan.cpp
index 0878e46..cf7a264 100644
--- a/lib/CodeGen/RegAllocLinearScan.cpp
+++ b/lib/CodeGen/RegAllocLinearScan.cpp
@@ -48,6 +48,7 @@
     const TargetMachine* tm_;
     const MRegisterInfo* mri_;
     LiveIntervals* li_;
+    bool *PhysRegsUsed;
 
     /// handled_ - Intervals are added to the handled_ set in the order of their
     /// start value.  This is uses for backtracking.
@@ -139,6 +140,10 @@
   mri_ = tm_->getRegisterInfo();
   li_ = &getAnalysis<LiveIntervals>();
 
+  PhysRegsUsed = new bool[mri_->getNumRegs()];
+  std::fill(PhysRegsUsed, PhysRegsUsed+mri_->getNumRegs(), false);
+  fn.setUsedPhysRegs(PhysRegsUsed);
+
   if (!prt_.get()) prt_.reset(new PhysRegTracker(*mri_));
   vrm_.reset(new VirtRegMap(*mf_));
   if (!spiller_.get()) spiller_.reset(createSpiller());
@@ -147,6 +152,7 @@
 
   linearScan();
 
+  // Rewrite spill code and update the PhysRegsUsed set.
   spiller_->runOnMachineFunction(*mf_, *vrm_);
 
   vrm_.reset();  // Free the VirtRegMap
@@ -170,9 +176,10 @@
          "interval sets should be empty on initialization");
 
   for (LiveIntervals::iterator i = li_->begin(), e = li_->end(); i != e; ++i) {
-    if (MRegisterInfo::isPhysicalRegister(i->second.reg))
+    if (MRegisterInfo::isPhysicalRegister(i->second.reg)) {
+      PhysRegsUsed[i->second.reg] = true;
       fixed_.push_back(std::make_pair(&i->second, i->second.begin()));
-    else
+    } else
       unhandled_.push(&i->second);
   }
 }
diff --git a/lib/CodeGen/VirtRegMap.cpp b/lib/CodeGen/VirtRegMap.cpp
index c72c18a..62f6274 100644
--- a/lib/CodeGen/VirtRegMap.cpp
+++ b/lib/CodeGen/VirtRegMap.cpp
@@ -133,13 +133,14 @@
   };
 }
 
-bool SimpleSpiller::runOnMachineFunction(MachineFunction& MF,
-                                         const VirtRegMap& VRM) {
+bool SimpleSpiller::runOnMachineFunction(MachineFunction &MF,
+                                         const VirtRegMap &VRM) {
   DEBUG(std::cerr << "********** REWRITE MACHINE CODE **********\n");
   DEBUG(std::cerr << "********** Function: "
                   << MF.getFunction()->getName() << '\n');
-  const TargetMachine& TM = MF.getTarget();
-  const MRegisterInfo& MRI = *TM.getRegisterInfo();
+  const TargetMachine &TM = MF.getTarget();
+  const MRegisterInfo &MRI = *TM.getRegisterInfo();
+  bool *PhysRegsUsed = MF.getUsedPhysregs();
 
   // LoadedRegs - Keep track of which vregs are loaded, so that we only load
   // each vreg once (in the case where a spilled vreg is used by multiple
@@ -177,6 +178,7 @@
               ++NumStores;
             }
           }
+          PhysRegsUsed[PhysReg] = true;
           MI.SetMachineOperandReg(i, PhysReg);
         }
       }
@@ -296,6 +298,8 @@
   // same stack slot, the original store is deleted.
   std::map<int, MachineInstr*> MaybeDeadStores;
 
+  bool *PhysRegsUsed = MBB.getParent()->getUsedPhysregs();
+
   for (MachineBasicBlock::iterator MII = MBB.begin(), E = MBB.end();
        MII != E; ) {
     MachineInstr &MI = *MII;
@@ -313,7 +317,9 @@
 
         if (!VRM.hasStackSlot(VirtReg)) {
           // This virtual register was assigned a physreg!
-          MI.SetMachineOperandReg(i, VRM.getPhys(VirtReg));
+          unsigned Phys = VRM.getPhys(VirtReg);
+          PhysRegsUsed[Phys] = true;
+          MI.SetMachineOperandReg(i, Phys);
         } else {
           // Is this virtual register a spilled value?
           if (MO.isUse()) {
@@ -397,7 +403,7 @@
                       }
                   }
             ContinueReload:
-
+              PhysRegsUsed[PhysReg] = true;
               MRI->loadRegFromStackSlot(MBB, &MI, PhysReg, StackSlot);
               // This invalidates PhysReg.
               ClobberPhysReg(PhysReg, SpillSlotsAvailable, PhysRegsAvailable);
@@ -429,9 +435,11 @@
 
     // Loop over all of the implicit defs, clearing them from our available
     // sets.
-    const TargetInstrDescriptor &InstrDesc = TII->get(MI.getOpcode());
-    for (const unsigned* ImpDef = InstrDesc.ImplicitDefs; *ImpDef; ++ImpDef)
+    for (const unsigned *ImpDef = TII->getImplicitDefs(MI.getOpcode());
+         *ImpDef; ++ImpDef) {
+      PhysRegsUsed[*ImpDef] = true;
       ClobberPhysReg(*ImpDef, SpillSlotsAvailable, PhysRegsAvailable);
+    }
 
     DEBUG(std::cerr << '\t' << MI);
 
@@ -516,6 +524,7 @@
           else
             PhysReg = MO.getReg();
 
+          PhysRegsUsed[PhysReg] = true;
           MRI->storeRegToStackSlot(MBB, next(MII), PhysReg, StackSlot);
           DEBUG(std::cerr << "Store:\t" << *next(MII));
           MI.SetMachineOperandReg(i, PhysReg);