llvm-6502/lib/CodeGen/VirtRegRewriter.cpp

//===-- llvm/CodeGen/Rewriter.cpp -  Rewriter -----------------------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "virtregrewriter"
#include "VirtRegRewriter.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/ADT/DepthFirstIterator.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/STLExtras.h"
#include <algorithm>
using namespace llvm;

STATISTIC(NumDSE     , "Number of dead stores elided");
STATISTIC(NumDSS     , "Number of dead spill slots removed");
STATISTIC(NumCommutes, "Number of instructions commuted");
STATISTIC(NumDRM     , "Number of re-materializable defs elided");
STATISTIC(NumStores  , "Number of stores added");
STATISTIC(NumPSpills , "Number of physical register spills");
STATISTIC(NumOmitted , "Number of reloads omited");
STATISTIC(NumAvoided , "Number of reloads deemed unnecessary");
STATISTIC(NumCopified, "Number of available reloads turned into copies");
STATISTIC(NumReMats  , "Number of re-materialization");
STATISTIC(NumLoads   , "Number of loads added");
STATISTIC(NumReused  , "Number of values reused");
STATISTIC(NumDCE     , "Number of copies elided");
STATISTIC(NumSUnfold , "Number of stores unfolded");
STATISTIC(NumModRefUnfold, "Number of modref unfolded");

namespace {
  enum RewriterName { local, trivial };
}

static cl::opt<RewriterName>
RewriterOpt("rewriter",
            cl::desc("Rewriter to use: (default: local)"),
            cl::Prefix,
            cl::values(clEnumVal(local,   "local rewriter"),
                       clEnumVal(trivial, "trivial rewriter"),
                       clEnumValEnd),
            cl::init(local));

VirtRegRewriter::~VirtRegRewriter() {}


 
/// This class is intended for use with the new spilling framework only. It
/// rewrites vreg def/uses to use the assigned preg, but does not insert any
/// spill code.
struct VISIBILITY_HIDDEN TrivialRewriter : public VirtRegRewriter {

  bool runOnMachineFunction(MachineFunction &MF, VirtRegMap &VRM,
                            LiveIntervals* LIs) {
    DOUT << "********** REWRITE MACHINE CODE **********\n";
    DOUT << "********** Function: " << MF.getFunction()->getName() << '\n';
    MachineRegisterInfo *mri = &MF.getRegInfo();

    bool changed = false;

    for (LiveIntervals::iterator liItr = LIs->begin(), liEnd = LIs->end();
         liItr != liEnd; ++liItr) {

      if (TargetRegisterInfo::isVirtualRegister(liItr->first)) {
        if (VRM.hasPhys(liItr->first)) {
          unsigned preg = VRM.getPhys(liItr->first);
          mri->replaceRegWith(liItr->first, preg);
          mri->setPhysRegUsed(preg);
          changed = true;
        }
      }
      else {
        if (!liItr->second->empty()) {
          mri->setPhysRegUsed(liItr->first);
        }
      }
    }
    
    return changed;
  }

};

// ************************************************************************ //

/// AvailableSpills - As the local rewriter is scanning and rewriting an MBB
/// from top down, keep track of which spill slots or remat are available in
/// each register.
///
/// Note that not all physregs are created equal here.  In particular, some
/// physregs are reloads that we are allowed to clobber or ignore at any time.
/// Other physregs are values that the register allocated program is using
/// that we cannot CHANGE, but we can read if we like.  We keep track of this
/// on a per-stack-slot / remat id basis as the low bit in the value of the
/// SpillSlotsAvailable entries.  The predicate 'canClobberPhysReg()' checks
/// this bit and addAvailable sets it if.
class VISIBILITY_HIDDEN AvailableSpills {
  const TargetRegisterInfo *TRI;
  const TargetInstrInfo *TII;

  // SpillSlotsOrReMatsAvailable - This map keeps track of all of the spilled
  // or remat'ed virtual register values that are still available, due to
  // being loaded or stored to, but not invalidated yet.
  std::map<int, unsigned> SpillSlotsOrReMatsAvailable;

  // PhysRegsAvailable - This is the inverse of SpillSlotsOrReMatsAvailable,
  // indicating which stack slot values are currently held by a physreg.  This
  // is used to invalidate entries in SpillSlotsOrReMatsAvailable when a
  // physreg is modified.
  std::multimap<unsigned, int> PhysRegsAvailable;

  void disallowClobberPhysRegOnly(unsigned PhysReg);

  void ClobberPhysRegOnly(unsigned PhysReg);
public:
  AvailableSpills(const TargetRegisterInfo *tri, const TargetInstrInfo *tii)
    : TRI(tri), TII(tii) {
  }

  /// clear - Reset the state.
  void clear() {
    SpillSlotsOrReMatsAvailable.clear();
    PhysRegsAvailable.clear();
  }

  const TargetRegisterInfo *getRegInfo() const { return TRI; }

  /// getSpillSlotOrReMatPhysReg - If the specified stack slot or remat is
  /// available in a physical register, return that PhysReg, otherwise
  /// return 0.
  unsigned getSpillSlotOrReMatPhysReg(int Slot) const {
    std::map<int, unsigned>::const_iterator I =
      SpillSlotsOrReMatsAvailable.find(Slot);
    if (I != SpillSlotsOrReMatsAvailable.end()) {
      return I->second >> 1;  // Remove the CanClobber bit.
    }
    return 0;
  }

  /// addAvailable - Mark that the specified stack slot / remat is available
  /// in the specified physreg.  If CanClobber is true, the physreg can be
  /// modified at any time without changing the semantics of the program.
  void addAvailable(int SlotOrReMat, unsigned Reg, bool CanClobber = true) {
    // If this stack slot is thought to be available in some other physreg, 
    // remove its record.
    ModifyStackSlotOrReMat(SlotOrReMat);

    PhysRegsAvailable.insert(std::make_pair(Reg, SlotOrReMat));
    SpillSlotsOrReMatsAvailable[SlotOrReMat]= (Reg << 1) |
                                              (unsigned)CanClobber;

    if (SlotOrReMat > VirtRegMap::MAX_STACK_SLOT)
      DOUT << "Remembering RM#" << SlotOrReMat-VirtRegMap::MAX_STACK_SLOT-1;
    else
      DOUT << "Remembering SS#" << SlotOrReMat;
    DOUT << " in physreg " << TRI->getName(Reg) << "\n";
  }

  /// canClobberPhysRegForSS - Return true if the spiller is allowed to change
  /// the value of the specified stackslot register if it desires. The
  /// specified stack slot must be available in a physreg for this query to
  /// make sense.
  bool canClobberPhysRegForSS(int SlotOrReMat) const {
    assert(SpillSlotsOrReMatsAvailable.count(SlotOrReMat) &&
           "Value not available!");
    return SpillSlotsOrReMatsAvailable.find(SlotOrReMat)->second & 1;
  }

  /// canClobberPhysReg - Return true if the spiller is allowed to clobber the
  /// physical register where values for some stack slot(s) might be
  /// available.
  bool canClobberPhysReg(unsigned PhysReg) const {
    std::multimap<unsigned, int>::const_iterator I =
      PhysRegsAvailable.lower_bound(PhysReg);
    while (I != PhysRegsAvailable.end() && I->first == PhysReg) {
      int SlotOrReMat = I->second;
      I++;
      if (!canClobberPhysRegForSS(SlotOrReMat))
        return false;
    }
    return true;
  }

  /// disallowClobberPhysReg - Unset the CanClobber bit of the specified
  /// stackslot register. The register is still available but is no longer
  /// allowed to be modifed.
  void disallowClobberPhysReg(unsigned PhysReg);

  /// ClobberPhysReg - This is called when the specified physreg changes
  /// value.  We use this to invalidate any info about stuff that lives in
  /// it and any of its aliases.
  void ClobberPhysReg(unsigned PhysReg);

  /// ModifyStackSlotOrReMat - This method is called when the value in a stack
  /// slot changes.  This removes information about which register the
  /// previous value for this slot lives in (as the previous value is dead
  /// now).
  void ModifyStackSlotOrReMat(int SlotOrReMat);

  /// AddAvailableRegsToLiveIn - Availability information is being kept coming
  /// into the specified MBB. Add available physical registers as potential
  /// live-in's. If they are reused in the MBB, they will be added to the
  /// live-in set to make register scavenger and post-allocation scheduler.
  void AddAvailableRegsToLiveIn(MachineBasicBlock &MBB, BitVector &RegKills,
                                std::vector<MachineOperand*> &KillOps);
};

// ************************************************************************ //

// ReusedOp - For each reused operand, we keep track of a bit of information,
// in case we need to rollback upon processing a new operand.  See comments
// below.
struct ReusedOp {
  // The MachineInstr operand that reused an available value.
  unsigned Operand;

  // StackSlotOrReMat - The spill slot or remat id of the value being reused.
  unsigned StackSlotOrReMat;

  // PhysRegReused - The physical register the value was available in.
  unsigned PhysRegReused;

  // AssignedPhysReg - The physreg that was assigned for use by the reload.
  unsigned AssignedPhysReg;
  
  // VirtReg - The virtual register itself.
  unsigned VirtReg;

  ReusedOp(unsigned o, unsigned ss, unsigned prr, unsigned apr,
           unsigned vreg)
    : Operand(o), StackSlotOrReMat(ss), PhysRegReused(prr),
      AssignedPhysReg(apr), VirtReg(vreg) {}
};

/// ReuseInfo - This maintains a collection of ReuseOp's for each operand that
/// is reused instead of reloaded.
class VISIBILITY_HIDDEN ReuseInfo {
  MachineInstr &MI;
  std::vector<ReusedOp> Reuses;
  BitVector PhysRegsClobbered;
public:
  ReuseInfo(MachineInstr &mi, const TargetRegisterInfo *tri) : MI(mi) {
    PhysRegsClobbered.resize(tri->getNumRegs());
  }
  
  bool hasReuses() const {
    return !Reuses.empty();
  }
  
  /// addReuse - If we choose to reuse a virtual register that is already
  /// available instead of reloading it, remember that we did so.
  void addReuse(unsigned OpNo, unsigned StackSlotOrReMat,
                unsigned PhysRegReused, unsigned AssignedPhysReg,
                unsigned VirtReg) {
    // If the reload is to the assigned register anyway, no undo will be
    // required.
    if (PhysRegReused == AssignedPhysReg) return;
    
    // Otherwise, remember this.
    Reuses.push_back(ReusedOp(OpNo, StackSlotOrReMat, PhysRegReused, 
                              AssignedPhysReg, VirtReg));
  }

  void markClobbered(unsigned PhysReg) {
    PhysRegsClobbered.set(PhysReg);
  }

  bool isClobbered(unsigned PhysReg) const {
    return PhysRegsClobbered.test(PhysReg);
  }
  
  /// GetRegForReload - We are about to emit a reload into PhysReg.  If there
  /// is some other operand that is using the specified register, either pick
  /// a new register to use, or evict the previous reload and use this reg. 
  unsigned GetRegForReload(unsigned PhysReg, MachineInstr *MI,
                           AvailableSpills &Spills,
                           std::vector<MachineInstr*> &MaybeDeadStores,
                           SmallSet<unsigned, 8> &Rejected,
                           BitVector &RegKills,
                           std::vector<MachineOperand*> &KillOps,
                           VirtRegMap &VRM);

  /// GetRegForReload - Helper for the above GetRegForReload(). Add a
  /// 'Rejected' set to remember which registers have been considered and
  /// rejected for the reload. This avoids infinite looping in case like
  /// this:
  /// t1 := op t2, t3
  /// t2 <- assigned r0 for use by the reload but ended up reuse r1
  /// t3 <- assigned r1 for use by the reload but ended up reuse r0
  /// t1 <- desires r1
  ///       sees r1 is taken by t2, tries t2's reload register r0
  ///       sees r0 is taken by t3, tries t3's reload register r1
  ///       sees r1 is taken by t2, tries t2's reload register r0 ...
  unsigned GetRegForReload(unsigned PhysReg, MachineInstr *MI,
                           AvailableSpills &Spills,
                           std::vector<MachineInstr*> &MaybeDeadStores,
                           BitVector &RegKills,
                           std::vector<MachineOperand*> &KillOps,
                           VirtRegMap &VRM) {
    SmallSet<unsigned, 8> Rejected;
    return GetRegForReload(PhysReg, MI, Spills, MaybeDeadStores, Rejected,
                           RegKills, KillOps, VRM);
  }
};


// ****************** //
// Utility Functions  //
// ****************** //

/// findSinglePredSuccessor - Return via reference a vector of machine basic
/// blocks each of which is a successor of the specified BB and has no other
/// predecessor.
static void findSinglePredSuccessor(MachineBasicBlock *MBB,
                                   SmallVectorImpl<MachineBasicBlock *> &Succs) {
  for (MachineBasicBlock::succ_iterator SI = MBB->succ_begin(),
         SE = MBB->succ_end(); SI != SE; ++SI) {
    MachineBasicBlock *SuccMBB = *SI;
    if (SuccMBB->pred_size() == 1)
      Succs.push_back(SuccMBB);
  }
}

/// InvalidateKill - Invalidate register kill information for a specific
/// register. This also unsets the kills marker on the last kill operand.
static void InvalidateKill(unsigned Reg,
                           const TargetRegisterInfo* TRI,
                           BitVector &RegKills,
                           std::vector<MachineOperand*> &KillOps) {
  if (RegKills[Reg]) {
    KillOps[Reg]->setIsKill(false);
    // KillOps[Reg] might be a def of a super-register.
    unsigned KReg = KillOps[Reg]->getReg();
    KillOps[KReg] = NULL;
    RegKills.reset(KReg);
    for (const unsigned *SR = TRI->getSubRegisters(KReg); *SR; ++SR) {
      if (RegKills[*SR]) {
        KillOps[*SR]->setIsKill(false);
        KillOps[*SR] = NULL;
        RegKills.reset(*SR);
      }
    }
  }
}

/// InvalidateKills - MI is going to be deleted. If any of its operands are
/// marked kill, then invalidate the information.
static void InvalidateKills(MachineInstr &MI,
                            const TargetRegisterInfo* TRI,
                            BitVector &RegKills,
                            std::vector<MachineOperand*> &KillOps,
                            SmallVector<unsigned, 2> *KillRegs = NULL) {
  for (unsigned i = 0, e = MI.getNumOperands(); i != e; ++i) {
    MachineOperand &MO = MI.getOperand(i);
    if (!MO.isReg() || !MO.isUse() || !MO.isKill() || MO.isUndef())
      continue;
    unsigned Reg = MO.getReg();
    if (TargetRegisterInfo::isVirtualRegister(Reg))
      continue;
    if (KillRegs)
      KillRegs->push_back(Reg);
    assert(Reg < KillOps.size());
    if (KillOps[Reg] == &MO) {
      KillOps[Reg] = NULL;
      RegKills.reset(Reg);
      for (const unsigned *SR = TRI->getSubRegisters(Reg); *SR; ++SR) {
        if (RegKills[*SR]) {
          KillOps[*SR] = NULL;
          RegKills.reset(*SR);
        }
      }
    }
  }
}

/// InvalidateRegDef - If the def operand of the specified def MI is now dead
/// (since it's spill instruction is removed), mark it isDead. Also checks if
/// the def MI has other definition operands that are not dead. Returns it by
/// reference.
static bool InvalidateRegDef(MachineBasicBlock::iterator I,
                             MachineInstr &NewDef, unsigned Reg,
                             bool &HasLiveDef) {
  // Due to remat, it's possible this reg isn't being reused. That is,
  // the def of this reg (by prev MI) is now dead.
  MachineInstr *DefMI = I;
  MachineOperand *DefOp = NULL;
  for (unsigned i = 0, e = DefMI->getNumOperands(); i != e; ++i) {
    MachineOperand &MO = DefMI->getOperand(i);
    if (!MO.isReg() || !MO.isUse() || !MO.isKill() || MO.isUndef())
      continue;
    if (MO.getReg() == Reg)
      DefOp = &MO;
    else if (!MO.isDead())
      HasLiveDef = true;
  }
  if (!DefOp)
    return false;

  bool FoundUse = false, Done = false;
  MachineBasicBlock::iterator E = &NewDef;
  ++I; ++E;
  for (; !Done && I != E; ++I) {
    MachineInstr *NMI = I;
    for (unsigned j = 0, ee = NMI->getNumOperands(); j != ee; ++j) {
      MachineOperand &MO = NMI->getOperand(j);
      if (!MO.isReg() || MO.getReg() != Reg)
        continue;
      if (MO.isUse())
        FoundUse = true;
      Done = true; // Stop after scanning all the operands of this MI.
    }
  }
  if (!FoundUse) {
    // Def is dead!
    DefOp->setIsDead();
    return true;
  }
  return false;
}

/// UpdateKills - Track and update kill info. If a MI reads a register that is
/// marked kill, then it must be due to register reuse. Transfer the kill info
/// over.
static void UpdateKills(MachineInstr &MI, const TargetRegisterInfo* TRI,
                        BitVector &RegKills,
                        std::vector<MachineOperand*> &KillOps) {
  for (unsigned i = 0, e = MI.getNumOperands(); i != e; ++i) {
    MachineOperand &MO = MI.getOperand(i);
    if (!MO.isReg() || !MO.isUse() || MO.isUndef())
      continue;
    unsigned Reg = MO.getReg();
    if (Reg == 0)
      continue;
    
    if (RegKills[Reg] && KillOps[Reg]->getParent() != &MI) {
      // That can't be right. Register is killed but not re-defined and it's
      // being reused. Let's fix that.
      KillOps[Reg]->setIsKill(false);
      // KillOps[Reg] might be a def of a super-register.
      unsigned KReg = KillOps[Reg]->getReg();
      KillOps[KReg] = NULL;
      RegKills.reset(KReg);

      // Must be a def of a super-register. Its other sub-regsters are no
      // longer killed as well.
      for (const unsigned *SR = TRI->getSubRegisters(KReg); *SR; ++SR) {
        KillOps[*SR] = NULL;
        RegKills.reset(*SR);
      }

      if (!MI.isRegTiedToDefOperand(i))
        // Unless it's a two-address operand, this is the new kill.
        MO.setIsKill();
    }
    if (MO.isKill()) {
      RegKills.set(Reg);
      KillOps[Reg] = &MO;
      for (const unsigned *SR = TRI->getSubRegisters(Reg); *SR; ++SR) {
        RegKills.set(*SR);
        KillOps[*SR] = &MO;
      }
    }
  }

  for (unsigned i = 0, e = MI.getNumOperands(); i != e; ++i) {
    const MachineOperand &MO = MI.getOperand(i);
    if (!MO.isReg() || !MO.isDef())
      continue;
    unsigned Reg = MO.getReg();
    RegKills.reset(Reg);
    KillOps[Reg] = NULL;
    // It also defines (or partially define) aliases.
    for (const unsigned *SR = TRI->getSubRegisters(Reg); *SR; ++SR) {
      RegKills.reset(*SR);
      KillOps[*SR] = NULL;
    }
  }
}

/// ReMaterialize - Re-materialize definition for Reg targetting DestReg.
///
static void ReMaterialize(MachineBasicBlock &MBB,
                          MachineBasicBlock::iterator &MII,
                          unsigned DestReg, unsigned Reg,
                          const TargetInstrInfo *TII,
                          const TargetRegisterInfo *TRI,
                          VirtRegMap &VRM) {
  TII->reMaterialize(MBB, MII, DestReg, VRM.getReMaterializedMI(Reg));
  MachineInstr *NewMI = prior(MII);
  for (unsigned i = 0, e = NewMI->getNumOperands(); i != e; ++i) {
    MachineOperand &MO = NewMI->getOperand(i);
    if (!MO.isReg() || MO.getReg() == 0)
      continue;
    unsigned VirtReg = MO.getReg();
    if (TargetRegisterInfo::isPhysicalRegister(VirtReg))
      continue;
    assert(MO.isUse());
    unsigned SubIdx = MO.getSubReg();
    unsigned Phys = VRM.getPhys(VirtReg);
    assert(Phys);
    unsigned RReg = SubIdx ? TRI->getSubReg(Phys, SubIdx) : Phys;
    MO.setReg(RReg);
    MO.setSubReg(0);
  }
  ++NumReMats;
}

/// findSuperReg - Find the SubReg's super-register of given register class
/// where its SubIdx sub-register is SubReg.
static unsigned findSuperReg(const TargetRegisterClass *RC, unsigned SubReg,
                             unsigned SubIdx, const TargetRegisterInfo *TRI) {
  for (TargetRegisterClass::iterator I = RC->begin(), E = RC->end();
       I != E; ++I) {
    unsigned Reg = *I;
    if (TRI->getSubReg(Reg, SubIdx) == SubReg)
      return Reg;
  }
  return 0;
}

// ******************************** //
// Available Spills Implementation  //
// ******************************** //

/// disallowClobberPhysRegOnly - Unset the CanClobber bit of the specified
/// stackslot register. The register is still available but is no longer
/// allowed to be modifed.
void AvailableSpills::disallowClobberPhysRegOnly(unsigned PhysReg) {
  std::multimap<unsigned, int>::iterator I =
    PhysRegsAvailable.lower_bound(PhysReg);
  while (I != PhysRegsAvailable.end() && I->first == PhysReg) {
    int SlotOrReMat = I->second;
    I++;
    assert((SpillSlotsOrReMatsAvailable[SlotOrReMat] >> 1) == PhysReg &&
           "Bidirectional map mismatch!");
    SpillSlotsOrReMatsAvailable[SlotOrReMat] &= ~1;
    DOUT << "PhysReg " << TRI->getName(PhysReg)
         << " copied, it is available for use but can no longer be modified\n";
  }
}

/// disallowClobberPhysReg - Unset the CanClobber bit of the specified
/// stackslot register and its aliases. The register and its aliases may
/// still available but is no longer allowed to be modifed.
void AvailableSpills::disallowClobberPhysReg(unsigned PhysReg) {
  for (const unsigned *AS = TRI->getAliasSet(PhysReg); *AS; ++AS)
    disallowClobberPhysRegOnly(*AS);
  disallowClobberPhysRegOnly(PhysReg);
}

/// ClobberPhysRegOnly - This is called when the specified physreg changes
/// value.  We use this to invalidate any info about stuff we thing lives in it.
void AvailableSpills::ClobberPhysRegOnly(unsigned PhysReg) {
  std::multimap<unsigned, int>::iterator I =
    PhysRegsAvailable.lower_bound(PhysReg);
  while (I != PhysRegsAvailable.end() && I->first == PhysReg) {
    int SlotOrReMat = I->second;
    PhysRegsAvailable.erase(I++);
    assert((SpillSlotsOrReMatsAvailable[SlotOrReMat] >> 1) == PhysReg &&
           "Bidirectional map mismatch!");
    SpillSlotsOrReMatsAvailable.erase(SlotOrReMat);
    DOUT << "PhysReg " << TRI->getName(PhysReg)
         << " clobbered, invalidating ";
    if (SlotOrReMat > VirtRegMap::MAX_STACK_SLOT)
      DOUT << "RM#" << SlotOrReMat-VirtRegMap::MAX_STACK_SLOT-1 << "\n";
    else
      DOUT << "SS#" << SlotOrReMat << "\n";
  }
}

/// ClobberPhysReg - This is called when the specified physreg changes
/// value.  We use this to invalidate any info about stuff we thing lives in
/// it and any of its aliases.
void AvailableSpills::ClobberPhysReg(unsigned PhysReg) {
  for (const unsigned *AS = TRI->getAliasSet(PhysReg); *AS; ++AS)
    ClobberPhysRegOnly(*AS);
  ClobberPhysRegOnly(PhysReg);
}

/// AddAvailableRegsToLiveIn - Availability information is being kept coming
/// into the specified MBB. Add available physical registers as potential
/// live-in's. If they are reused in the MBB, they will be added to the
/// live-in set to make register scavenger and post-allocation scheduler.
void AvailableSpills::AddAvailableRegsToLiveIn(MachineBasicBlock &MBB,
                                        BitVector &RegKills,
                                        std::vector<MachineOperand*> &KillOps) {
  std::set<unsigned> NotAvailable;
  for (std::multimap<unsigned, int>::iterator
         I = PhysRegsAvailable.begin(), E = PhysRegsAvailable.end();
       I != E; ++I) {
    unsigned Reg = I->first;
    const TargetRegisterClass* RC = TRI->getPhysicalRegisterRegClass(Reg);
    // FIXME: A temporary workaround. We can't reuse available value if it's
    // not safe to move the def of the virtual register's class. e.g.
    // X86::RFP* register classes. Do not add it as a live-in.
    if (!TII->isSafeToMoveRegClassDefs(RC))
      // This is no longer available.
      NotAvailable.insert(Reg);
    else {
      MBB.addLiveIn(Reg);
      InvalidateKill(Reg, TRI, RegKills, KillOps);
    }

    // Skip over the same register.
    std::multimap<unsigned, int>::iterator NI = next(I);
    while (NI != E && NI->first == Reg) {
      ++I;
      ++NI;
    }
  }

  for (std::set<unsigned>::iterator I = NotAvailable.begin(),
         E = NotAvailable.end(); I != E; ++I) {
    ClobberPhysReg(*I);
    for (const unsigned *SubRegs = TRI->getSubRegisters(*I);
       *SubRegs; ++SubRegs)
      ClobberPhysReg(*SubRegs);
  }
}

/// ModifyStackSlotOrReMat - This method is called when the value in a stack
/// slot changes.  This removes information about which register the previous
/// value for this slot lives in (as the previous value is dead now).
void AvailableSpills::ModifyStackSlotOrReMat(int SlotOrReMat) {
  std::map<int, unsigned>::iterator It =
    SpillSlotsOrReMatsAvailable.find(SlotOrReMat);
  if (It == SpillSlotsOrReMatsAvailable.end()) return;
  unsigned Reg = It->second >> 1;
  SpillSlotsOrReMatsAvailable.erase(It);
  
  // This register may hold the value of multiple stack slots, only remove this
  // stack slot from the set of values the register contains.
  std::multimap<unsigned, int>::iterator I = PhysRegsAvailable.lower_bound(Reg);
  for (; ; ++I) {
    assert(I != PhysRegsAvailable.end() && I->first == Reg &&
           "Map inverse broken!");
    if (I->second == SlotOrReMat) break;
  }
  PhysRegsAvailable.erase(I);
}

// ************************** //
// Reuse Info Implementation  //
// ************************** //

/// GetRegForReload - We are about to emit a reload into PhysReg.  If there
/// is some other operand that is using the specified register, either pick
/// a new register to use, or evict the previous reload and use this reg.
unsigned ReuseInfo::GetRegForReload(unsigned PhysReg, MachineInstr *MI,
                         AvailableSpills &Spills,
                         std::vector<MachineInstr*> &MaybeDeadStores,
                         SmallSet<unsigned, 8> &Rejected,
                         BitVector &RegKills,
                         std::vector<MachineOperand*> &KillOps,
                         VirtRegMap &VRM) {
  const TargetInstrInfo* TII = MI->getParent()->getParent()->getTarget()
                               .getInstrInfo();
  
  if (Reuses.empty()) return PhysReg;  // This is most often empty.

  for (unsigned ro = 0, e = Reuses.size(); ro != e; ++ro) {
    ReusedOp &Op = Reuses[ro];
    // If we find some other reuse that was supposed to use this register
    // exactly for its reload, we can change this reload to use ITS reload
    // register. That is, unless its reload register has already been
    // considered and subsequently rejected because it has also been reused
    // by another operand.
    if (Op.PhysRegReused == PhysReg &&
        Rejected.count(Op.AssignedPhysReg) == 0) {
      // Yup, use the reload register that we didn't use before.
      unsigned NewReg = Op.AssignedPhysReg;
      Rejected.insert(PhysReg);
      return GetRegForReload(NewReg, MI, Spills, MaybeDeadStores, Rejected,
                             RegKills, KillOps, VRM);
    } else {
      // Otherwise, we might also have a problem if a previously reused
      // value aliases the new register.  If so, codegen the previous reload
      // and use this one.          
      unsigned PRRU = Op.PhysRegReused;
      const TargetRegisterInfo *TRI = Spills.getRegInfo();
      if (TRI->areAliases(PRRU, PhysReg)) {
        // Okay, we found out that an alias of a reused register
        // was used.  This isn't good because it means we have
        // to undo a previous reuse.
        MachineBasicBlock *MBB = MI->getParent();
        const TargetRegisterClass *AliasRC =
          MBB->getParent()->getRegInfo().getRegClass(Op.VirtReg);

        // Copy Op out of the vector and remove it, we're going to insert an
        // explicit load for it.
        ReusedOp NewOp = Op;
        Reuses.erase(Reuses.begin()+ro);

        // Ok, we're going to try to reload the assigned physreg into the
        // slot that we were supposed to in the first place.  However, that
        // register could hold a reuse.  Check to see if it conflicts or
        // would prefer us to use a different register.
        unsigned NewPhysReg = GetRegForReload(NewOp.AssignedPhysReg,
                                              MI, Spills, MaybeDeadStores,
                                          Rejected, RegKills, KillOps, VRM);
        
        MachineBasicBlock::iterator MII = MI;
        if (NewOp.StackSlotOrReMat > VirtRegMap::MAX_STACK_SLOT) {
          ReMaterialize(*MBB, MII, NewPhysReg, NewOp.VirtReg, TII, TRI,VRM);
        } else {
          TII->loadRegFromStackSlot(*MBB, MII, NewPhysReg,
                                    NewOp.StackSlotOrReMat, AliasRC);
          MachineInstr *LoadMI = prior(MII);
          VRM.addSpillSlotUse(NewOp.StackSlotOrReMat, LoadMI);
          // Any stores to this stack slot are not dead anymore.
          MaybeDeadStores[NewOp.StackSlotOrReMat] = NULL;            
          ++NumLoads;
        }
        Spills.ClobberPhysReg(NewPhysReg);
        Spills.ClobberPhysReg(NewOp.PhysRegReused);

        unsigned SubIdx = MI->getOperand(NewOp.Operand).getSubReg();
        unsigned RReg = SubIdx ? TRI->getSubReg(NewPhysReg, SubIdx) : NewPhysReg;
        MI->getOperand(NewOp.Operand).setReg(RReg);
        MI->getOperand(NewOp.Operand).setSubReg(0);

        Spills.addAvailable(NewOp.StackSlotOrReMat, NewPhysReg);
        --MII;
        UpdateKills(*MII, TRI, RegKills, KillOps);
        DOUT << '\t' << *MII;
        
        DOUT << "Reuse undone!\n";
        --NumReused;
        
        // Finally, PhysReg is now available, go ahead and use it.
        return PhysReg;
      }
    }
  }
  return PhysReg;
}

// ************************************************************************ //

/// FoldsStackSlotModRef - Return true if the specified MI folds the specified
/// stack slot mod/ref. It also checks if it's possible to unfold the
/// instruction by having it define a specified physical register instead.
static bool FoldsStackSlotModRef(MachineInstr &MI, int SS, unsigned PhysReg,
                                 const TargetInstrInfo *TII,
                                 const TargetRegisterInfo *TRI,
                                 VirtRegMap &VRM) {
  if (VRM.hasEmergencySpills(&MI) || VRM.isSpillPt(&MI))
    return false;

  bool Found = false;
  VirtRegMap::MI2VirtMapTy::const_iterator I, End;
  for (tie(I, End) = VRM.getFoldedVirts(&MI); I != End; ++I) {
    unsigned VirtReg = I->second.first;
    VirtRegMap::ModRef MR = I->second.second;
    if (MR & VirtRegMap::isModRef)
      if (VRM.getStackSlot(VirtReg) == SS) {
        Found= TII->getOpcodeAfterMemoryUnfold(MI.getOpcode(), true, true) != 0;
        break;
      }
  }
  if (!Found)
    return false;

  // Does the instruction uses a register that overlaps the scratch register?
  for (unsigned i = 0, e = MI.getNumOperands(); i != e; ++i) {
    MachineOperand &MO = MI.getOperand(i);
    if (!MO.isReg() || MO.getReg() == 0)
      continue;
    unsigned Reg = MO.getReg();
    if (TargetRegisterInfo::isVirtualRegister(Reg)) {
      if (!VRM.hasPhys(Reg))
        continue;
      Reg = VRM.getPhys(Reg);
    }
    if (TRI->regsOverlap(PhysReg, Reg))
      return false;
  }
  return true;
}

/// FindFreeRegister - Find a free register of a given register class by looking
/// at (at most) the last two machine instructions.
static unsigned FindFreeRegister(MachineBasicBlock::iterator MII,
                                 MachineBasicBlock &MBB,
                                 const TargetRegisterClass *RC,
                                 const TargetRegisterInfo *TRI,
                                 BitVector &AllocatableRegs) {
  BitVector Defs(TRI->getNumRegs());
  BitVector Uses(TRI->getNumRegs());
  SmallVector<unsigned, 4> LocalUses;
  SmallVector<unsigned, 4> Kills;

  // Take a look at 2 instructions at most.
  for (unsigned Count = 0; Count < 2; ++Count) {
    if (MII == MBB.begin())
      break;
    MachineInstr *PrevMI = prior(MII);
    for (unsigned i = 0, e = PrevMI->getNumOperands(); i != e; ++i) {
      MachineOperand &MO = PrevMI->getOperand(i);
      if (!MO.isReg() || MO.getReg() == 0)
        continue;
      unsigned Reg = MO.getReg();
      if (MO.isDef()) {
        Defs.set(Reg);
        for (const unsigned *AS = TRI->getAliasSet(Reg); *AS; ++AS)
          Defs.set(*AS);
      } else  {
        LocalUses.push_back(Reg);
        if (MO.isKill() && AllocatableRegs[Reg])
          Kills.push_back(Reg);
      }
    }

    for (unsigned i = 0, e = Kills.size(); i != e; ++i) {
      unsigned Kill = Kills[i];
      if (!Defs[Kill] && !Uses[Kill] &&
          TRI->getPhysicalRegisterRegClass(Kill) == RC)
        return Kill;
    }
    for (unsigned i = 0, e = LocalUses.size(); i != e; ++i) {
      unsigned Reg = LocalUses[i];
      Uses.set(Reg);
      for (const unsigned *AS = TRI->getAliasSet(Reg); *AS; ++AS)
        Uses.set(*AS);
    }

    MII = PrevMI;
  }

  return 0;
}

static
void AssignPhysToVirtReg(MachineInstr *MI, unsigned VirtReg, unsigned PhysReg) {
  for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
    MachineOperand &MO = MI->getOperand(i);
    if (MO.isReg() && MO.getReg() == VirtReg)
      MO.setReg(PhysReg);
  }
}

namespace {
  struct RefSorter {
    bool operator()(const std::pair<MachineInstr*, int> &A,
                    const std::pair<MachineInstr*, int> &B) {
      return A.second < B.second;
    }
  };
}

// ***************************** //
// Local Spiller Implementation  //
// ***************************** //

class VISIBILITY_HIDDEN LocalRewriter : public VirtRegRewriter {
  MachineRegisterInfo *RegInfo;
  const TargetRegisterInfo *TRI;
  const TargetInstrInfo *TII;
  BitVector AllocatableRegs;
  DenseMap<MachineInstr*, unsigned> DistanceMap;
public:

  bool runOnMachineFunction(MachineFunction &MF, VirtRegMap &VRM,
                            LiveIntervals* LIs) {
    RegInfo = &MF.getRegInfo(); 
    TRI = MF.getTarget().getRegisterInfo();
    TII = MF.getTarget().getInstrInfo();
    AllocatableRegs = TRI->getAllocatableSet(MF);
    DOUT << "\n**** Local spiller rewriting function '"
         << MF.getFunction()->getName() << "':\n";
    DOUT << "**** Machine Instrs (NOTE! Does not include spills and reloads!)"
            " ****\n";
    DEBUG(MF.dump());

    // Spills - Keep track of which spilled values are available in physregs
    // so that we can choose to reuse the physregs instead of emitting
    // reloads. This is usually refreshed per basic block.
    AvailableSpills Spills(TRI, TII);

    // Keep track of kill information.
    BitVector RegKills(TRI->getNumRegs());
    std::vector<MachineOperand*> KillOps;
    KillOps.resize(TRI->getNumRegs(), NULL);

    // SingleEntrySuccs - Successor blocks which have a single predecessor.
    SmallVector<MachineBasicBlock*, 4> SinglePredSuccs;
    SmallPtrSet<MachineBasicBlock*,16> EarlyVisited;

    // Traverse the basic blocks depth first.
    MachineBasicBlock *Entry = MF.begin();
    SmallPtrSet<MachineBasicBlock*,16> Visited;
    for (df_ext_iterator<MachineBasicBlock*,
           SmallPtrSet<MachineBasicBlock*,16> >
           DFI = df_ext_begin(Entry, Visited), E = df_ext_end(Entry, Visited);
         DFI != E; ++DFI) {
      MachineBasicBlock *MBB = *DFI;
      if (!EarlyVisited.count(MBB))
        RewriteMBB(*MBB, VRM, LIs, Spills, RegKills, KillOps);

      // If this MBB is the only predecessor of a successor. Keep the
      // availability information and visit it next.
      do {
        // Keep visiting single predecessor successor as long as possible.
        SinglePredSuccs.clear();
        findSinglePredSuccessor(MBB, SinglePredSuccs);
        if (SinglePredSuccs.empty())
          MBB = 0;
        else {
          // FIXME: More than one successors, each of which has MBB has
          // the only predecessor.
          MBB = SinglePredSuccs[0];
          if (!Visited.count(MBB) && EarlyVisited.insert(MBB)) {
            Spills.AddAvailableRegsToLiveIn(*MBB, RegKills, KillOps);
            RewriteMBB(*MBB, VRM, LIs, Spills, RegKills, KillOps);
          }
        }
      } while (MBB);

      // Clear the availability info.
      Spills.clear();
    }

    DOUT << "**** Post Machine Instrs ****\n";
    DEBUG(MF.dump());

    // Mark unused spill slots.
    MachineFrameInfo *MFI = MF.getFrameInfo();
    int SS = VRM.getLowSpillSlot();
    if (SS != VirtRegMap::NO_STACK_SLOT)
      for (int e = VRM.getHighSpillSlot(); SS <= e; ++SS)
        if (!VRM.isSpillSlotUsed(SS)) {
          MFI->RemoveStackObject(SS);
          ++NumDSS;
        }

    return true;
  }

private:

  /// OptimizeByUnfold2 - Unfold a series of load / store folding instructions if
  /// a scratch register is available.
  ///     xorq  %r12<kill>, %r13
  ///     addq  %rax, -184(%rbp)
  ///     addq  %r13, -184(%rbp)
  /// ==>
  ///     xorq  %r12<kill>, %r13
  ///     movq  -184(%rbp), %r12
  ///     addq  %rax, %r12
  ///     addq  %r13, %r12
  ///     movq  %r12, -184(%rbp)
  bool OptimizeByUnfold2(unsigned VirtReg, int SS,
                         MachineBasicBlock &MBB,
                         MachineBasicBlock::iterator &MII,
                         std::vector<MachineInstr*> &MaybeDeadStores,
                         AvailableSpills &Spills,
                         BitVector &RegKills,
                         std::vector<MachineOperand*> &KillOps,
                         VirtRegMap &VRM) {

    MachineBasicBlock::iterator NextMII = next(MII);
    if (NextMII == MBB.end())
      return false;

    if (TII->getOpcodeAfterMemoryUnfold(MII->getOpcode(), true, true) == 0)
      return false;

    // Now let's see if the last couple of instructions happens to have freed up
    // a register.
    const TargetRegisterClass* RC = RegInfo->getRegClass(VirtReg);
    unsigned PhysReg = FindFreeRegister(MII, MBB, RC, TRI, AllocatableRegs);
    if (!PhysReg)
      return false;

    MachineFunction &MF = *MBB.getParent();
    TRI = MF.getTarget().getRegisterInfo();
    MachineInstr &MI = *MII;
    if (!FoldsStackSlotModRef(MI, SS, PhysReg, TII, TRI, VRM))
      return false;

    // If the next instruction also folds the same SS modref and can be unfoled,
    // then it's worthwhile to issue a load from SS into the free register and
    // then unfold these instructions.
    if (!FoldsStackSlotModRef(*NextMII, SS, PhysReg, TII, TRI, VRM))
      return false;

    // Load from SS to the spare physical register.
    TII->loadRegFromStackSlot(MBB, MII, PhysReg, SS, RC);
    // This invalidates Phys.
    Spills.ClobberPhysReg(PhysReg);
    // Remember it's available.
    Spills.addAvailable(SS, PhysReg);
    MaybeDeadStores[SS] = NULL;

    // Unfold current MI.
    SmallVector<MachineInstr*, 4> NewMIs;
    if (!TII->unfoldMemoryOperand(MF, &MI, VirtReg, false, false, NewMIs))
      LLVM_UNREACHABLE("Unable unfold the load / store folding instruction!");
    assert(NewMIs.size() == 1);
    AssignPhysToVirtReg(NewMIs[0], VirtReg, PhysReg);
    VRM.transferRestorePts(&MI, NewMIs[0]);
    MII = MBB.insert(MII, NewMIs[0]);
    InvalidateKills(MI, TRI, RegKills, KillOps);
    VRM.RemoveMachineInstrFromMaps(&MI);
    MBB.erase(&MI);
    ++NumModRefUnfold;

    // Unfold next instructions that fold the same SS.
    do {
      MachineInstr &NextMI = *NextMII;
      NextMII = next(NextMII);
      NewMIs.clear();
      if (!TII->unfoldMemoryOperand(MF, &NextMI, VirtReg, false, false, NewMIs))
        LLVM_UNREACHABLE("Unable unfold the load / store folding instruction!");
      assert(NewMIs.size() == 1);
      AssignPhysToVirtReg(NewMIs[0], VirtReg, PhysReg);
      VRM.transferRestorePts(&NextMI, NewMIs[0]);
      MBB.insert(NextMII, NewMIs[0]);
      InvalidateKills(NextMI, TRI, RegKills, KillOps);
      VRM.RemoveMachineInstrFromMaps(&NextMI);
      MBB.erase(&NextMI);
      ++NumModRefUnfold;
      if (NextMII == MBB.end())
        break;
    } while (FoldsStackSlotModRef(*NextMII, SS, PhysReg, TII, TRI, VRM));

    // Store the value back into SS.
    TII->storeRegToStackSlot(MBB, NextMII, PhysReg, true, SS, RC);
    MachineInstr *StoreMI = prior(NextMII);
    VRM.addSpillSlotUse(SS, StoreMI);
    VRM.virtFolded(VirtReg, StoreMI, VirtRegMap::isMod);

    return true;
  }

  /// OptimizeByUnfold - Turn a store folding instruction into a load folding
  /// instruction. e.g.
  ///     xorl  %edi, %eax
  ///     movl  %eax, -32(%ebp)
  ///     movl  -36(%ebp), %eax
  ///     orl   %eax, -32(%ebp)
  /// ==>
  ///     xorl  %edi, %eax
  ///     orl   -36(%ebp), %eax
  ///     mov   %eax, -32(%ebp)
  /// This enables unfolding optimization for a subsequent instruction which will
  /// also eliminate the newly introduced store instruction.
  bool OptimizeByUnfold(MachineBasicBlock &MBB,
                        MachineBasicBlock::iterator &MII,
                        std::vector<MachineInstr*> &MaybeDeadStores,
                        AvailableSpills &Spills,
                        BitVector &RegKills,
                        std::vector<MachineOperand*> &KillOps,
                        VirtRegMap &VRM) {
    MachineFunction &MF = *MBB.getParent();
    MachineInstr &MI = *MII;
    unsigned UnfoldedOpc = 0;
    unsigned UnfoldPR = 0;
    unsigned UnfoldVR = 0;
    int FoldedSS = VirtRegMap::NO_STACK_SLOT;
    VirtRegMap::MI2VirtMapTy::const_iterator I, End;
    for (tie(I, End) = VRM.getFoldedVirts(&MI); I != End; ) {
      // Only transform a MI that folds a single register.
      if (UnfoldedOpc)
        return false;
      UnfoldVR = I->second.first;
      VirtRegMap::ModRef MR = I->second.second;
      // MI2VirtMap be can updated which invalidate the iterator.
      // Increment the iterator first.
      ++I; 
      if (VRM.isAssignedReg(UnfoldVR))
        continue;
      // If this reference is not a use, any previous store is now dead.
      // Otherwise, the store to this stack slot is not dead anymore.
      FoldedSS = VRM.getStackSlot(UnfoldVR);
      MachineInstr* DeadStore = MaybeDeadStores[FoldedSS];
      if (DeadStore && (MR & VirtRegMap::isModRef)) {
        unsigned PhysReg = Spills.getSpillSlotOrReMatPhysReg(FoldedSS);
        if (!PhysReg || !DeadStore->readsRegister(PhysReg))
          continue;
        UnfoldPR = PhysReg;
        UnfoldedOpc = TII->getOpcodeAfterMemoryUnfold(MI.getOpcode(),
                                                      false, true);
      }
    }

    if (!UnfoldedOpc) {
      if (!UnfoldVR)
        return false;

      // Look for other unfolding opportunities.
      return OptimizeByUnfold2(UnfoldVR, FoldedSS, MBB, MII,
                               MaybeDeadStores, Spills, RegKills, KillOps, VRM);
    }

    for (unsigned i = 0, e = MI.getNumOperands(); i != e; ++i) {
      MachineOperand &MO = MI.getOperand(i);
      if (!MO.isReg() || MO.getReg() == 0 || !MO.isUse())
        continue;
      unsigned VirtReg = MO.getReg();
      if (TargetRegisterInfo::isPhysicalRegister(VirtReg) || MO.getSubReg())
        continue;
      if (VRM.isAssignedReg(VirtReg)) {
        unsigned PhysReg = VRM.getPhys(VirtReg);
        if (PhysReg && TRI->regsOverlap(PhysReg, UnfoldPR))
          return false;
      } else if (VRM.isReMaterialized(VirtReg))
        continue;
      int SS = VRM.getStackSlot(VirtReg);
      unsigned PhysReg = Spills.getSpillSlotOrReMatPhysReg(SS);
      if (PhysReg) {
        if (TRI->regsOverlap(PhysReg, UnfoldPR))
          return false;
        continue;
      }
      if (VRM.hasPhys(VirtReg)) {
        PhysReg = VRM.getPhys(VirtReg);
        if (!TRI->regsOverlap(PhysReg, UnfoldPR))
          continue;
      }

      // Ok, we'll need to reload the value into a register which makes
      // it impossible to perform the store unfolding optimization later.
      // Let's see if it is possible to fold the load if the store is
      // unfolded. This allows us to perform the store unfolding
      // optimization.
      SmallVector<MachineInstr*, 4> NewMIs;
      if (TII->unfoldMemoryOperand(MF, &MI, UnfoldVR, false, false, NewMIs)) {
        assert(NewMIs.size() == 1);
        MachineInstr *NewMI = NewMIs.back();
        NewMIs.clear();
        int Idx = NewMI->findRegisterUseOperandIdx(VirtReg, false);
        assert(Idx != -1);
        SmallVector<unsigned, 1> Ops;
        Ops.push_back(Idx);
        MachineInstr *FoldedMI = TII->foldMemoryOperand(MF, NewMI, Ops, SS);
        if (FoldedMI) {
          VRM.addSpillSlotUse(SS, FoldedMI);
          if (!VRM.hasPhys(UnfoldVR))
            VRM.assignVirt2Phys(UnfoldVR, UnfoldPR);
          VRM.virtFolded(VirtReg, FoldedMI, VirtRegMap::isRef);
          MII = MBB.insert(MII, FoldedMI);
          InvalidateKills(MI, TRI, RegKills, KillOps);
          VRM.RemoveMachineInstrFromMaps(&MI);
          MBB.erase(&MI);
          MF.DeleteMachineInstr(NewMI);
          return true;
        }
        MF.DeleteMachineInstr(NewMI);
      }
    }

    return false;
  }

  /// CommuteChangesDestination - We are looking for r0 = op r1, r2 and
  /// where SrcReg is r1 and it is tied to r0. Return true if after
  /// commuting this instruction it will be r0 = op r2, r1.
  static bool CommuteChangesDestination(MachineInstr *DefMI,
                                        const TargetInstrDesc &TID,
                                        unsigned SrcReg,
                                        const TargetInstrInfo *TII,
                                        unsigned &DstIdx) {
    if (TID.getNumDefs() != 1 && TID.getNumOperands() != 3)
      return false;
    if (!DefMI->getOperand(1).isReg() ||
        DefMI->getOperand(1).getReg() != SrcReg)
      return false;
    unsigned DefIdx;
    if (!DefMI->isRegTiedToDefOperand(1, &DefIdx) || DefIdx != 0)
      return false;
    unsigned SrcIdx1, SrcIdx2;
    if (!TII->findCommutedOpIndices(DefMI, SrcIdx1, SrcIdx2))
      return false;
    if (SrcIdx1 == 1 && SrcIdx2 == 2) {
      DstIdx = 2;
      return true;
    }
    return false;
  }

  /// CommuteToFoldReload -
  /// Look for
  /// r1 = load fi#1
  /// r1 = op r1, r2<kill>
  /// store r1, fi#1
  ///
  /// If op is commutable and r2 is killed, then we can xform these to
  /// r2 = op r2, fi#1
  /// store r2, fi#1
  bool CommuteToFoldReload(MachineBasicBlock &MBB,
                           MachineBasicBlock::iterator &MII,
                           unsigned VirtReg, unsigned SrcReg, int SS,
                           AvailableSpills &Spills,
                           BitVector &RegKills,
                           std::vector<MachineOperand*> &KillOps,
                           const TargetRegisterInfo *TRI,
                           VirtRegMap &VRM) {
    if (MII == MBB.begin() || !MII->killsRegister(SrcReg))
      return false;

    MachineFunction &MF = *MBB.getParent();
    MachineInstr &MI = *MII;
    MachineBasicBlock::iterator DefMII = prior(MII);
    MachineInstr *DefMI = DefMII;
    const TargetInstrDesc &TID = DefMI->getDesc();
    unsigned NewDstIdx;
    if (DefMII != MBB.begin() &&
        TID.isCommutable() &&
        CommuteChangesDestination(DefMI, TID, SrcReg, TII, NewDstIdx)) {
      MachineOperand &NewDstMO = DefMI->getOperand(NewDstIdx);
      unsigned NewReg = NewDstMO.getReg();
      if (!NewDstMO.isKill() || TRI->regsOverlap(NewReg, SrcReg))
        return false;
      MachineInstr *ReloadMI = prior(DefMII);
      int FrameIdx;
      unsigned DestReg = TII->isLoadFromStackSlot(ReloadMI, FrameIdx);
      if (DestReg != SrcReg || FrameIdx != SS)
        return false;
      int UseIdx = DefMI->findRegisterUseOperandIdx(DestReg, false);
      if (UseIdx == -1)
        return false;
      unsigned DefIdx;
      if (!MI.isRegTiedToDefOperand(UseIdx, &DefIdx))
        return false;
      assert(DefMI->getOperand(DefIdx).isReg() &&
             DefMI->getOperand(DefIdx).getReg() == SrcReg);

      // Now commute def instruction.
      MachineInstr *CommutedMI = TII->commuteInstruction(DefMI, true);
      if (!CommutedMI)
        return false;
      SmallVector<unsigned, 1> Ops;
      Ops.push_back(NewDstIdx);
      MachineInstr *FoldedMI = TII->foldMemoryOperand(MF, CommutedMI, Ops, SS);
      // Not needed since foldMemoryOperand returns new MI.
      MF.DeleteMachineInstr(CommutedMI);
      if (!FoldedMI)
        return false;

      VRM.addSpillSlotUse(SS, FoldedMI);
      VRM.virtFolded(VirtReg, FoldedMI, VirtRegMap::isRef);
      // Insert new def MI and spill MI.
      const TargetRegisterClass* RC = RegInfo->getRegClass(VirtReg);
      TII->storeRegToStackSlot(MBB, &MI, NewReg, true, SS, RC);
      MII = prior(MII);
      MachineInstr *StoreMI = MII;
      VRM.addSpillSlotUse(SS, StoreMI);
      VRM.virtFolded(VirtReg, StoreMI, VirtRegMap::isMod);
      MII = MBB.insert(MII, FoldedMI);  // Update MII to backtrack.

      // Delete all 3 old instructions.
      InvalidateKills(*ReloadMI, TRI, RegKills, KillOps);
      VRM.RemoveMachineInstrFromMaps(ReloadMI);
      MBB.erase(ReloadMI);
      InvalidateKills(*DefMI, TRI, RegKills, KillOps);
      VRM.RemoveMachineInstrFromMaps(DefMI);
      MBB.erase(DefMI);
      InvalidateKills(MI, TRI, RegKills, KillOps);
      VRM.RemoveMachineInstrFromMaps(&MI);
      MBB.erase(&MI);

      // If NewReg was previously holding value of some SS, it's now clobbered.
      // This has to be done now because it's a physical register. When this
      // instruction is re-visited, it's ignored.
      Spills.ClobberPhysReg(NewReg);

      ++NumCommutes;
      return true;
    }

    return false;
  }

  /// SpillRegToStackSlot - Spill a register to a specified stack slot. Check if
  /// the last store to the same slot is now dead. If so, remove the last store.
  void SpillRegToStackSlot(MachineBasicBlock &MBB,
                           MachineBasicBlock::iterator &MII,
                           int Idx, unsigned PhysReg, int StackSlot,
                           const TargetRegisterClass *RC,
                           bool isAvailable, MachineInstr *&LastStore,
                           AvailableSpills &Spills,
                           SmallSet<MachineInstr*, 4> &ReMatDefs,
                           BitVector &RegKills,
                           std::vector<MachineOperand*> &KillOps,
                           VirtRegMap &VRM) {

    TII->storeRegToStackSlot(MBB, next(MII), PhysReg, true, StackSlot, RC);
    MachineInstr *StoreMI = next(MII);
    VRM.addSpillSlotUse(StackSlot, StoreMI);
    DOUT << "Store:\t" << *StoreMI;

    // If there is a dead store to this stack slot, nuke it now.
    if (LastStore) {
      DOUT << "Removed dead store:\t" << *LastStore;
      ++NumDSE;
      SmallVector<unsigned, 2> KillRegs;
      InvalidateKills(*LastStore, TRI, RegKills, KillOps, &KillRegs);
      MachineBasicBlock::iterator PrevMII = LastStore;
      bool CheckDef = PrevMII != MBB.begin();
      if (CheckDef)
        --PrevMII;
      VRM.RemoveMachineInstrFromMaps(LastStore);
      MBB.erase(LastStore);
      if (CheckDef) {
        // Look at defs of killed registers on the store. Mark the defs
        // as dead since the store has been deleted and they aren't
        // being reused.
        for (unsigned j = 0, ee = KillRegs.size(); j != ee; ++j) {
          bool HasOtherDef = false;
          if (InvalidateRegDef(PrevMII, *MII, KillRegs[j], HasOtherDef)) {
            MachineInstr *DeadDef = PrevMII;
            if (ReMatDefs.count(DeadDef) && !HasOtherDef) {
              // FIXME: This assumes a remat def does not have side effects.
              VRM.RemoveMachineInstrFromMaps(DeadDef);
              MBB.erase(DeadDef);
              ++NumDRM;
            }
          }
        }
      }
    }

    LastStore = next(MII);

    // If the stack slot value was previously available in some other
    // register, change it now.  Otherwise, make the register available,
    // in PhysReg.
    Spills.ModifyStackSlotOrReMat(StackSlot);
    Spills.ClobberPhysReg(PhysReg);
    Spills.addAvailable(StackSlot, PhysReg, isAvailable);
    ++NumStores;
  }

  /// TransferDeadness - A identity copy definition is dead and it's being
  /// removed. Find the last def or use and mark it as dead / kill.
  void TransferDeadness(MachineBasicBlock *MBB, unsigned CurDist,
                        unsigned Reg, BitVector &RegKills,
                        std::vector<MachineOperand*> &KillOps,
                        VirtRegMap &VRM) {
    SmallPtrSet<MachineInstr*, 4> Seens;
    SmallVector<std::pair<MachineInstr*, int>,8> Refs;
    for (MachineRegisterInfo::reg_iterator RI = RegInfo->reg_begin(Reg),
           RE = RegInfo->reg_end(); RI != RE; ++RI) {
      MachineInstr *UDMI = &*RI;
      if (UDMI->getParent() != MBB)
        continue;
      DenseMap<MachineInstr*, unsigned>::iterator DI = DistanceMap.find(UDMI);
      if (DI == DistanceMap.end() || DI->second > CurDist)
        continue;
      if (Seens.insert(UDMI))
        Refs.push_back(std::make_pair(UDMI, DI->second));
    }

    if (Refs.empty())
      return;
    std::sort(Refs.begin(), Refs.end(), RefSorter());

    while (!Refs.empty()) {
      MachineInstr *LastUDMI = Refs.back().first;
      Refs.pop_back();

      MachineOperand *LastUD = NULL;
      for (unsigned i = 0, e = LastUDMI->getNumOperands(); i != e; ++i) {
        MachineOperand &MO = LastUDMI->getOperand(i);
        if (!MO.isReg() || MO.getReg() != Reg)
          continue;
        if (!LastUD || (LastUD->isUse() && MO.isDef()))
          LastUD = &MO;
        if (LastUDMI->isRegTiedToDefOperand(i))
          break;
      }
      if (LastUD->isDef()) {
        // If the instruction has no side effect, delete it and propagate
        // backward further. Otherwise, mark is dead and we are done.
        const TargetInstrDesc &TID = LastUDMI->getDesc();
        if (TID.mayStore() || TID.isCall() || TID.isTerminator() ||
            TID.hasUnmodeledSideEffects()) {
          LastUD->setIsDead();
          break;
        }
        VRM.RemoveMachineInstrFromMaps(LastUDMI);
        MBB->erase(LastUDMI);
      } else {
        LastUD->setIsKill();
        RegKills.set(Reg);
        KillOps[Reg] = LastUD;
        break;
      }
    }
  }

  /// rewriteMBB - Keep track of which spills are available even after the
  /// register allocator is done with them.  If possible, avid reloading vregs.
  void RewriteMBB(MachineBasicBlock &MBB, VirtRegMap &VRM,
                  LiveIntervals *LIs,
                  AvailableSpills &Spills, BitVector &RegKills,
                  std::vector<MachineOperand*> &KillOps) {

    DOUT << "\n**** Local spiller rewriting MBB '"
         << MBB.getBasicBlock()->getName() << "':\n";

    MachineFunction &MF = *MBB.getParent();
    
    // MaybeDeadStores - When we need to write a value back into a stack slot,
    // keep track of the inserted store.  If the stack slot value is never read
    // (because the value was used from some available register, for example), and
    // subsequently stored to, the original store is dead.  This map keeps track
    // of inserted stores that are not used.  If we see a subsequent store to the
    // same stack slot, the original store is deleted.
    std::vector<MachineInstr*> MaybeDeadStores;
    MaybeDeadStores.resize(MF.getFrameInfo()->getObjectIndexEnd(), NULL);

    // ReMatDefs - These are rematerializable def MIs which are not deleted.
    SmallSet<MachineInstr*, 4> ReMatDefs;

    // Clear kill info.
    SmallSet<unsigned, 2> KilledMIRegs;
    RegKills.reset();
    KillOps.clear();
    KillOps.resize(TRI->getNumRegs(), NULL);

    unsigned Dist = 0;
    DistanceMap.clear();
    for (MachineBasicBlock::iterator MII = MBB.begin(), E = MBB.end();
         MII != E; ) {
      MachineBasicBlock::iterator NextMII = next(MII);

      VirtRegMap::MI2VirtMapTy::const_iterator I, End;
      bool Erased = false;
      bool BackTracked = false;
      if (OptimizeByUnfold(MBB, MII,
                           MaybeDeadStores, Spills, RegKills, KillOps, VRM))
        NextMII = next(MII);

      MachineInstr &MI = *MII;

      if (VRM.hasEmergencySpills(&MI)) {
        // Spill physical register(s) in the rare case the allocator has run out
        // of registers to allocate.
        SmallSet<int, 4> UsedSS;
        std::vector<unsigned> &EmSpills = VRM.getEmergencySpills(&MI);
        for (unsigned i = 0, e = EmSpills.size(); i != e; ++i) {
          unsigned PhysReg = EmSpills[i];
          const TargetRegisterClass *RC =
            TRI->getPhysicalRegisterRegClass(PhysReg);
          assert(RC && "Unable to determine register class!");
          int SS = VRM.getEmergencySpillSlot(RC);
          if (UsedSS.count(SS))
            LLVM_UNREACHABLE("Need to spill more than one physical registers!");
          UsedSS.insert(SS);
          TII->storeRegToStackSlot(MBB, MII, PhysReg, true, SS, RC);
          MachineInstr *StoreMI = prior(MII);
          VRM.addSpillSlotUse(SS, StoreMI);
          TII->loadRegFromStackSlot(MBB, next(MII), PhysReg, SS, RC);
          MachineInstr *LoadMI = next(MII);
          VRM.addSpillSlotUse(SS, LoadMI);
          ++NumPSpills;
        }
        NextMII = next(MII);
      }

      // Insert restores here if asked to.
      if (VRM.isRestorePt(&MI)) {
        std::vector<unsigned> &RestoreRegs = VRM.getRestorePtRestores(&MI);
        for (unsigned i = 0, e = RestoreRegs.size(); i != e; ++i) {
          unsigned VirtReg = RestoreRegs[e-i-1];  // Reverse order.
          if (!VRM.getPreSplitReg(VirtReg))
            continue; // Split interval spilled again.
          unsigned Phys = VRM.getPhys(VirtReg);
          RegInfo->setPhysRegUsed(Phys);

          // Check if the value being restored if available. If so, it must be
          // from a predecessor BB that fallthrough into this BB. We do not
          // expect:
          // BB1:
          // r1 = load fi#1
          // ...
          //    = r1<kill>
          // ... # r1 not clobbered
          // ...
          //    = load fi#1
          bool DoReMat = VRM.isReMaterialized(VirtReg);
          int SSorRMId = DoReMat
            ? VRM.getReMatId(VirtReg) : VRM.getStackSlot(VirtReg);
          const TargetRegisterClass* RC = RegInfo->getRegClass(VirtReg);
          unsigned InReg = Spills.getSpillSlotOrReMatPhysReg(SSorRMId);
          if (InReg == Phys) {
            // If the value is already available in the expected register, save
            // a reload / remat.
            if (SSorRMId)
              DOUT << "Reusing RM#" << SSorRMId-VirtRegMap::MAX_STACK_SLOT-1;
            else
              DOUT << "Reusing SS#" << SSorRMId;
            DOUT << " from physreg "
                 << TRI->getName(InReg) << " for vreg"
                 << VirtReg <<" instead of reloading into physreg "
                 << TRI->getName(Phys) << "\n";
            ++NumOmitted;
            continue;
          } else if (InReg && InReg != Phys) {
            if (SSorRMId)
              DOUT << "Reusing RM#" << SSorRMId-VirtRegMap::MAX_STACK_SLOT-1;
            else
              DOUT << "Reusing SS#" << SSorRMId;
            DOUT << " from physreg "
                 << TRI->getName(InReg) << " for vreg"
                 << VirtReg <<" by copying it into physreg "
                 << TRI->getName(Phys) << "\n";

            // If the reloaded / remat value is available in another register,
            // copy it to the desired register.
            TII->copyRegToReg(MBB, &MI, Phys, InReg, RC, RC);

            // This invalidates Phys.
            Spills.ClobberPhysReg(Phys);
            // Remember it's available.
            Spills.addAvailable(SSorRMId, Phys);

            // Mark is killed.
            MachineInstr *CopyMI = prior(MII);
            MachineOperand *KillOpnd = CopyMI->findRegisterUseOperand(InReg);
            KillOpnd->setIsKill();
            UpdateKills(*CopyMI, TRI, RegKills, KillOps);

            DOUT << '\t' << *CopyMI;
            ++NumCopified;
            continue;
          }

          if (VRM.isReMaterialized(VirtReg)) {
            ReMaterialize(MBB, MII, Phys, VirtReg, TII, TRI, VRM);
          } else {
            const TargetRegisterClass* RC = RegInfo->getRegClass(VirtReg);
            TII->loadRegFromStackSlot(MBB, &MI, Phys, SSorRMId, RC);
            MachineInstr *LoadMI = prior(MII);
            VRM.addSpillSlotUse(SSorRMId, LoadMI);
            ++NumLoads;
          }

          // This invalidates Phys.
          Spills.ClobberPhysReg(Phys);
          // Remember it's available.
          Spills.addAvailable(SSorRMId, Phys);

          UpdateKills(*prior(MII), TRI, RegKills, KillOps);
          DOUT << '\t' << *prior(MII);
        }
      }

      // Insert spills here if asked to.
      if (VRM.isSpillPt(&MI)) {
        std::vector<std::pair<unsigned,bool> > &SpillRegs =
          VRM.getSpillPtSpills(&MI);
        for (unsigned i = 0, e = SpillRegs.size(); i != e; ++i) {
          unsigned VirtReg = SpillRegs[i].first;
          bool isKill = SpillRegs[i].second;
          if (!VRM.getPreSplitReg(VirtReg))
            continue; // Split interval spilled again.
          const TargetRegisterClass *RC = RegInfo->getRegClass(VirtReg);
          unsigned Phys = VRM.getPhys(VirtReg);
          int StackSlot = VRM.getStackSlot(VirtReg);
          TII->storeRegToStackSlot(MBB, next(MII), Phys, isKill, StackSlot, RC);
          MachineInstr *StoreMI = next(MII);
          VRM.addSpillSlotUse(StackSlot, StoreMI);
          DOUT << "Store:\t" << *StoreMI;
          VRM.virtFolded(VirtReg, StoreMI, VirtRegMap::isMod);
        }
        NextMII = next(MII);
      }

      /// ReusedOperands - Keep track of operand reuse in case we need to undo
      /// reuse.
      ReuseInfo ReusedOperands(MI, TRI);
      SmallVector<unsigned, 4> VirtUseOps;
      for (unsigned i = 0, e = MI.getNumOperands(); i != e; ++i) {
        MachineOperand &MO = MI.getOperand(i);
        if (!MO.isReg() || MO.getReg() == 0)
          continue;   // Ignore non-register operands.
        
        unsigned VirtReg = MO.getReg();
        if (TargetRegisterInfo::isPhysicalRegister(VirtReg)) {
          // Ignore physregs for spilling, but remember that it is used by this
          // function.
          RegInfo->setPhysRegUsed(VirtReg);
          continue;
        }

        // We want to process implicit virtual register uses first.
        if (MO.isImplicit())
          // If the virtual register is implicitly defined, emit a implicit_def
          // before so scavenger knows it's "defined".
          // FIXME: This is a horrible hack done the by register allocator to
          // remat a definition with virtual register operand.
          VirtUseOps.insert(VirtUseOps.begin(), i);
        else
          VirtUseOps.push_back(i);
      }

      // Process all of the spilled uses and all non spilled reg references.
      SmallVector<int, 2> PotentialDeadStoreSlots;
      KilledMIRegs.clear();
      for (unsigned j = 0, e = VirtUseOps.size(); j != e; ++j) {
        unsigned i = VirtUseOps[j];
        MachineOperand &MO = MI.getOperand(i);
        unsigned VirtReg = MO.getReg();
        assert(TargetRegisterInfo::isVirtualRegister(VirtReg) &&
               "Not a virtual register?");

        unsigned SubIdx = MO.getSubReg();
        if (VRM.isAssignedReg(VirtReg)) {
          // This virtual register was assigned a physreg!
          unsigned Phys = VRM.getPhys(VirtReg);
          RegInfo->setPhysRegUsed(Phys);
          if (MO.isDef())
            ReusedOperands.markClobbered(Phys);
          unsigned RReg = SubIdx ? TRI->getSubReg(Phys, SubIdx) : Phys;
          MI.getOperand(i).setReg(RReg);
          MI.getOperand(i).setSubReg(0);
          if (VRM.isImplicitlyDefined(VirtReg))
            // FIXME: Is this needed?
            BuildMI(MBB, &MI, MI.getDebugLoc(),
                    TII->get(TargetInstrInfo::IMPLICIT_DEF), RReg);
          continue;
        }
        
        // This virtual register is now known to be a spilled value.
        if (!MO.isUse())
          continue;  // Handle defs in the loop below (handle use&def here though)

        bool AvoidReload = MO.isUndef();
        // Check if it is defined by an implicit def. It should not be spilled.
        // Note, this is for correctness reason. e.g.
        // 8   %reg1024<def> = IMPLICIT_DEF
        // 12  %reg1024<def> = INSERT_SUBREG %reg1024<kill>, %reg1025, 2
        // The live range [12, 14) are not part of the r1024 live interval since
        // it's defined by an implicit def. It will not conflicts with live
        // interval of r1025. Now suppose both registers are spilled, you can
        // easily see a situation where both registers are reloaded before
        // the INSERT_SUBREG and both target registers that would overlap.
        bool DoReMat = VRM.isReMaterialized(VirtReg);
        int SSorRMId = DoReMat
          ? VRM.getReMatId(VirtReg) : VRM.getStackSlot(VirtReg);
        int ReuseSlot = SSorRMId;

        // Check to see if this stack slot is available.
        unsigned PhysReg = Spills.getSpillSlotOrReMatPhysReg(SSorRMId);

        // If this is a sub-register use, make sure the reuse register is in the
        // right register class. For example, for x86 not all of the 32-bit
        // registers have accessible sub-registers.
        // Similarly so for EXTRACT_SUBREG. Consider this:
        // EDI = op
        // MOV32_mr fi#1, EDI
        // ...
        //       = EXTRACT_SUBREG fi#1
        // fi#1 is available in EDI, but it cannot be reused because it's not in
        // the right register file.
        if (PhysReg && !AvoidReload &&
            (SubIdx || MI.getOpcode() == TargetInstrInfo::EXTRACT_SUBREG)) {
          const TargetRegisterClass* RC = RegInfo->getRegClass(VirtReg);
          if (!RC->contains(PhysReg))
            PhysReg = 0;
        }

        if (PhysReg && !AvoidReload) {
          // This spilled operand might be part of a two-address operand.  If this
          // is the case, then changing it will necessarily require changing the 
          // def part of the instruction as well.  However, in some cases, we
          // aren't allowed to modify the reused register.  If none of these cases
          // apply, reuse it.
          bool CanReuse = true;
          bool isTied = MI.isRegTiedToDefOperand(i);
          if (isTied) {
            // Okay, we have a two address operand.  We can reuse this physreg as
            // long as we are allowed to clobber the value and there isn't an
            // earlier def that has already clobbered the physreg.
            CanReuse = !ReusedOperands.isClobbered(PhysReg) &&
              Spills.canClobberPhysReg(PhysReg);
          }
          
          if (CanReuse) {
            // If this stack slot value is already available, reuse it!
            if (ReuseSlot > VirtRegMap::MAX_STACK_SLOT)
              DOUT << "Reusing RM#" << ReuseSlot-VirtRegMap::MAX_STACK_SLOT-1;
            else
              DOUT << "Reusing SS#" << ReuseSlot;
            DOUT << " from physreg "
                 << TRI->getName(PhysReg) << " for vreg"
                 << VirtReg <<" instead of reloading into physreg "
                 << TRI->getName(VRM.getPhys(VirtReg)) << "\n";
            unsigned RReg = SubIdx ? TRI->getSubReg(PhysReg, SubIdx) : PhysReg;
            MI.getOperand(i).setReg(RReg);
            MI.getOperand(i).setSubReg(0);

            // The only technical detail we have is that we don't know that
            // PhysReg won't be clobbered by a reloaded stack slot that occurs
            // later in the instruction.  In particular, consider 'op V1, V2'.
            // If V1 is available in physreg R0, we would choose to reuse it
            // here, instead of reloading it into the register the allocator
            // indicated (say R1).  However, V2 might have to be reloaded
            // later, and it might indicate that it needs to live in R0.  When
            // this occurs, we need to have information available that
            // indicates it is safe to use R1 for the reload instead of R0.
            //
            // To further complicate matters, we might conflict with an alias,
            // or R0 and R1 might not be compatible with each other.  In this
            // case, we actually insert a reload for V1 in R1, ensuring that
            // we can get at R0 or its alias.
            ReusedOperands.addReuse(i, ReuseSlot, PhysReg,
                                    VRM.getPhys(VirtReg), VirtReg);
            if (isTied)
              // Only mark it clobbered if this is a use&def operand.
              ReusedOperands.markClobbered(PhysReg);
            ++NumReused;

            if (MI.getOperand(i).isKill() &&
                ReuseSlot <= VirtRegMap::MAX_STACK_SLOT) {

              // The store of this spilled value is potentially dead, but we
              // won't know for certain until we've confirmed that the re-use
              // above is valid, which means waiting until the other operands
              // are processed. For now we just track the spill slot, we'll
              // remove it after the other operands are processed if valid.

              PotentialDeadStoreSlots.push_back(ReuseSlot);
            }

            // Mark is isKill if it's there no other uses of the same virtual
            // register and it's not a two-address operand. IsKill will be
            // unset if reg is reused.
            if (!isTied && KilledMIRegs.count(VirtReg) == 0) {
              MI.getOperand(i).setIsKill();
              KilledMIRegs.insert(VirtReg);
            }

            continue;
          }  // CanReuse
          
          // Otherwise we have a situation where we have a two-address instruction
          // whose mod/ref operand needs to be reloaded.  This reload is already
          // available in some register "PhysReg", but if we used PhysReg as the
          // operand to our 2-addr instruction, the instruction would modify
          // PhysReg.  This isn't cool if something later uses PhysReg and expects
          // to get its initial value.
          //
          // To avoid this problem, and to avoid doing a load right after a store,
          // we emit a copy from PhysReg into the designated register for this
          // operand.
          unsigned DesignatedReg = VRM.getPhys(VirtReg);
          assert(DesignatedReg && "Must map virtreg to physreg!");

          // Note that, if we reused a register for a previous operand, the
          // register we want to reload into might not actually be
          // available.  If this occurs, use the register indicated by the
          // reuser.
          if (ReusedOperands.hasReuses())
            DesignatedReg = ReusedOperands.GetRegForReload(DesignatedReg, &MI, 
                                 Spills, MaybeDeadStores, RegKills, KillOps, VRM);
          
          // If the mapped designated register is actually the physreg we have
          // incoming, we don't need to inserted a dead copy.
          if (DesignatedReg == PhysReg) {
            // If this stack slot value is already available, reuse it!
            if (ReuseSlot > VirtRegMap::MAX_STACK_SLOT)
              DOUT << "Reusing RM#" << ReuseSlot-VirtRegMap::MAX_STACK_SLOT-1;
            else
              DOUT << "Reusing SS#" << ReuseSlot;
            DOUT << " from physreg " << TRI->getName(PhysReg)
                 << " for vreg" << VirtReg
                 << " instead of reloading into same physreg.\n";
            unsigned RReg = SubIdx ? TRI->getSubReg(PhysReg, SubIdx) : PhysReg;
            MI.getOperand(i).setReg(RReg);
            MI.getOperand(i).setSubReg(0);
            ReusedOperands.markClobbered(RReg);
            ++NumReused;
            continue;
          }
          
          const TargetRegisterClass* RC = RegInfo->getRegClass(VirtReg);
          RegInfo->setPhysRegUsed(DesignatedReg);
          ReusedOperands.markClobbered(DesignatedReg);
          TII->copyRegToReg(MBB, &MI, DesignatedReg, PhysReg, RC, RC);

          MachineInstr *CopyMI = prior(MII);
          UpdateKills(*CopyMI, TRI, RegKills, KillOps);

          // This invalidates DesignatedReg.
          Spills.ClobberPhysReg(DesignatedReg);
          
          Spills.addAvailable(ReuseSlot, DesignatedReg);
          unsigned RReg =
            SubIdx ? TRI->getSubReg(DesignatedReg, SubIdx) : DesignatedReg;
          MI.getOperand(i).setReg(RReg);
          MI.getOperand(i).setSubReg(0);
          DOUT << '\t' << *prior(MII);
          ++NumReused;
          continue;
        } // if (PhysReg)
        
        // Otherwise, reload it and remember that we have it.
        PhysReg = VRM.getPhys(VirtReg);
        assert(PhysReg && "Must map virtreg to physreg!");

        // Note that, if we reused a register for a previous operand, the
        // register we want to reload into might not actually be
        // available.  If this occurs, use the register indicated by the
        // reuser.
        if (ReusedOperands.hasReuses())
          PhysReg = ReusedOperands.GetRegForReload(PhysReg, &MI, 
                                 Spills, MaybeDeadStores, RegKills, KillOps, VRM);
        
        RegInfo->setPhysRegUsed(PhysReg);
        ReusedOperands.markClobbered(PhysReg);
        if (AvoidReload)
          ++NumAvoided;
        else {
          if (DoReMat) {
            ReMaterialize(MBB, MII, PhysReg, VirtReg, TII, TRI, VRM);
          } else {
            const TargetRegisterClass* RC = RegInfo->getRegClass(VirtReg);
            TII->loadRegFromStackSlot(MBB, &MI, PhysReg, SSorRMId, RC);
            MachineInstr *LoadMI = prior(MII);
            VRM.addSpillSlotUse(SSorRMId, LoadMI);
            ++NumLoads;
          }
          // This invalidates PhysReg.
          Spills.ClobberPhysReg(PhysReg);

          // Any stores to this stack slot are not dead anymore.
          if (!DoReMat)
            MaybeDeadStores[SSorRMId] = NULL;
          Spills.addAvailable(SSorRMId, PhysReg);
          // Assumes this is the last use. IsKill will be unset if reg is reused
          // unless it's a two-address operand.
          if (!MI.isRegTiedToDefOperand(i) &&
              KilledMIRegs.count(VirtReg) == 0) {
            MI.getOperand(i).setIsKill();
            KilledMIRegs.insert(VirtReg);
          }

          UpdateKills(*prior(MII), TRI, RegKills, KillOps);
          DOUT << '\t' << *prior(MII);
        }
        unsigned RReg = SubIdx ? TRI->getSubReg(PhysReg, SubIdx) : PhysReg;
        MI.getOperand(i).setReg(RReg);
        MI.getOperand(i).setSubReg(0);
      }

      // Ok - now we can remove stores that have been confirmed dead.
      for (unsigned j = 0, e = PotentialDeadStoreSlots.size(); j != e; ++j) {
        // This was the last use and the spilled value is still available
        // for reuse. That means the spill was unnecessary!
        int PDSSlot = PotentialDeadStoreSlots[j];
        MachineInstr* DeadStore = MaybeDeadStores[PDSSlot];
        if (DeadStore) {
          DOUT << "Removed dead store:\t" << *DeadStore;
          InvalidateKills(*DeadStore, TRI, RegKills, KillOps);
          VRM.RemoveMachineInstrFromMaps(DeadStore);
          MBB.erase(DeadStore);
          MaybeDeadStores[PDSSlot] = NULL;
          ++NumDSE;
        }
      }


      DOUT << '\t' << MI;


      // If we have folded references to memory operands, make sure we clear all
      // physical registers that may contain the value of the spilled virtual
      // register
      SmallSet<int, 2> FoldedSS;
      for (tie(I, End) = VRM.getFoldedVirts(&MI); I != End; ) {
        unsigned VirtReg = I->second.first;
        VirtRegMap::ModRef MR = I->second.second;
        DOUT << "Folded vreg: " << VirtReg << "  MR: " << MR;

        // MI2VirtMap be can updated which invalidate the iterator.
        // Increment the iterator first.
        ++I;
        int SS = VRM.getStackSlot(VirtReg);
        if (SS == VirtRegMap::NO_STACK_SLOT)
          continue;
        FoldedSS.insert(SS);
        DOUT << " - StackSlot: " << SS << "\n";
        
        // If this folded instruction is just a use, check to see if it's a
        // straight load from the virt reg slot.
        if ((MR & VirtRegMap::isRef) && !(MR & VirtRegMap::isMod)) {
          int FrameIdx;
          unsigned DestReg = TII->isLoadFromStackSlot(&MI, FrameIdx);
          if (DestReg && FrameIdx == SS) {
            // If this spill slot is available, turn it into a copy (or nothing)
            // instead of leaving it as a load!
            if (unsigned InReg = Spills.getSpillSlotOrReMatPhysReg(SS)) {
              DOUT << "Promoted Load To Copy: " << MI;
              if (DestReg != InReg) {
                const TargetRegisterClass *RC = RegInfo->getRegClass(VirtReg);
                TII->copyRegToReg(MBB, &MI, DestReg, InReg, RC, RC);
                MachineOperand *DefMO = MI.findRegisterDefOperand(DestReg);
                unsigned SubIdx = DefMO->getSubReg();
                // Revisit the copy so we make sure to notice the effects of the
                // operation on the destreg (either needing to RA it if it's 
                // virtual or needing to clobber any values if it's physical).
                NextMII = &MI;
                --NextMII;  // backtrack to the copy.
                // Propagate the sub-register index over.
                if (SubIdx) {
                  DefMO = NextMII->findRegisterDefOperand(DestReg);
                  DefMO->setSubReg(SubIdx);
                }

                // Mark is killed.
                MachineOperand *KillOpnd = NextMII->findRegisterUseOperand(InReg);
                KillOpnd->setIsKill();

                BackTracked = true;
              } else {
                DOUT << "Removing now-noop copy: " << MI;
                // Unset last kill since it's being reused.
                InvalidateKill(InReg, TRI, RegKills, KillOps);
                Spills.disallowClobberPhysReg(InReg);
              }

              InvalidateKills(MI, TRI, RegKills, KillOps);
              VRM.RemoveMachineInstrFromMaps(&MI);
              MBB.erase(&MI);
              Erased = true;
              goto ProcessNextInst;
            }
          } else {
            unsigned PhysReg = Spills.getSpillSlotOrReMatPhysReg(SS);
            SmallVector<MachineInstr*, 4> NewMIs;
            if (PhysReg &&
                TII->unfoldMemoryOperand(MF, &MI, PhysReg, false, false, NewMIs)) {
              MBB.insert(MII, NewMIs[0]);
              InvalidateKills(MI, TRI, RegKills, KillOps);
              VRM.RemoveMachineInstrFromMaps(&MI);
              MBB.erase(&MI);
              Erased = true;
              --NextMII;  // backtrack to the unfolded instruction.
              BackTracked = true;
              goto ProcessNextInst;
            }
          }
        }

        // If this reference is not a use, any previous store is now dead.
        // Otherwise, the store to this stack slot is not dead anymore.
        MachineInstr* DeadStore = MaybeDeadStores[SS];
        if (DeadStore) {
          bool isDead = !(MR & VirtRegMap::isRef);
          MachineInstr *NewStore = NULL;
          if (MR & VirtRegMap::isModRef) {
            unsigned PhysReg = Spills.getSpillSlotOrReMatPhysReg(SS);
            SmallVector<MachineInstr*, 4> NewMIs;
            // We can reuse this physreg as long as we are allowed to clobber
            // the value and there isn't an earlier def that has already clobbered
            // the physreg.
            if (PhysReg &&
                !ReusedOperands.isClobbered(PhysReg) &&
                Spills.canClobberPhysReg(PhysReg) &&
                !TII->isStoreToStackSlot(&MI, SS)) { // Not profitable!
              MachineOperand *KillOpnd =
                DeadStore->findRegisterUseOperand(PhysReg, true);
              // Note, if the store is storing a sub-register, it's possible the
              // super-register is needed below.
              if (KillOpnd && !KillOpnd->getSubReg() &&
                  TII->unfoldMemoryOperand(MF, &MI, PhysReg, false, true,NewMIs)){
                MBB.insert(MII, NewMIs[0]);
                NewStore = NewMIs[1];
                MBB.insert(MII, NewStore);
                VRM.addSpillSlotUse(SS, NewStore);
                InvalidateKills(MI, TRI, RegKills, KillOps);
                VRM.RemoveMachineInstrFromMaps(&MI);
                MBB.erase(&MI);
                Erased = true;
                --NextMII;
                --NextMII;  // backtrack to the unfolded instruction.
                BackTracked = true;
                isDead = true;
                ++NumSUnfold;
              }
            }
          }

          if (isDead) {  // Previous store is dead.
            // If we get here, the store is dead, nuke it now.
            DOUT << "Removed dead store:\t" << *DeadStore;
            InvalidateKills(*DeadStore, TRI, RegKills, KillOps);
            VRM.RemoveMachineInstrFromMaps(DeadStore);
            MBB.erase(DeadStore);
            if (!NewStore)
              ++NumDSE;
          }

          MaybeDeadStores[SS] = NULL;
          if (NewStore) {
            // Treat this store as a spill merged into a copy. That makes the
            // stack slot value available.
            VRM.virtFolded(VirtReg, NewStore, VirtRegMap::isMod);
            goto ProcessNextInst;
          }
        }

        // If the spill slot value is available, and this is a new definition of
        // the value, the value is not available anymore.
        if (MR & VirtRegMap::isMod) {
          // Notice that the value in this stack slot has been modified.
          Spills.ModifyStackSlotOrReMat(SS);
          
          // If this is *just* a mod of the value, check to see if this is just a
          // store to the spill slot (i.e. the spill got merged into the copy). If
          // so, realize that the vreg is available now, and add the store to the
          // MaybeDeadStore info.
          int StackSlot;
          if (!(MR & VirtRegMap::isRef)) {
            if (unsigned SrcReg = TII->isStoreToStackSlot(&MI, StackSlot)) {
              assert(TargetRegisterInfo::isPhysicalRegister(SrcReg) &&
                     "Src hasn't been allocated yet?");

              if (CommuteToFoldReload(MBB, MII, VirtReg, SrcReg, StackSlot,
                                      Spills, RegKills, KillOps, TRI, VRM)) {
                NextMII = next(MII);
                BackTracked = true;
                goto ProcessNextInst;
              }

              // Okay, this is certainly a store of SrcReg to [StackSlot].  Mark
              // this as a potentially dead store in case there is a subsequent
              // store into the stack slot without a read from it.
              MaybeDeadStores[StackSlot] = &MI;

              // If the stack slot value was previously available in some other
              // register, change it now.  Otherwise, make the register
              // available in PhysReg.
              Spills.addAvailable(StackSlot, SrcReg, MI.killsRegister(SrcReg));
            }
          }
        }
      }

      // Process all of the spilled defs.
      for (unsigned i = 0, e = MI.getNumOperands(); i != e; ++i) {
        MachineOperand &MO = MI.getOperand(i);
        if (!(MO.isReg() && MO.getReg() && MO.isDef()))
          continue;

        unsigned VirtReg = MO.getReg();
        if (!TargetRegisterInfo::isVirtualRegister(VirtReg)) {
          // Check to see if this is a noop copy.  If so, eliminate the
          // instruction before considering the dest reg to be changed.
          // Also check if it's copying from an "undef", if so, we can't
          // eliminate this or else the undef marker is lost and it will
          // confuses the scavenger. This is extremely rare.
          unsigned Src, Dst, SrcSR, DstSR;
          if (TII->isMoveInstr(MI, Src, Dst, SrcSR, DstSR) && Src == Dst &&
              !MI.findRegisterUseOperand(Src)->isUndef()) {
            ++NumDCE;
            DOUT << "Removing now-noop copy: " << MI;
            SmallVector<unsigned, 2> KillRegs;
            InvalidateKills(MI, TRI, RegKills, KillOps, &KillRegs);
            if (MO.isDead() && !KillRegs.empty()) {
              // Source register or an implicit super/sub-register use is killed.
              assert(KillRegs[0] == Dst ||
                     TRI->isSubRegister(KillRegs[0], Dst) ||
                     TRI->isSuperRegister(KillRegs[0], Dst));
              // Last def is now dead.
              TransferDeadness(&MBB, Dist, Src, RegKills, KillOps, VRM);
            }
            VRM.RemoveMachineInstrFromMaps(&MI);
            MBB.erase(&MI);
            Erased = true;
            Spills.disallowClobberPhysReg(VirtReg);
            goto ProcessNextInst;
          }

          // If it's not a no-op copy, it clobbers the value in the destreg.
          Spills.ClobberPhysReg(VirtReg);
          ReusedOperands.markClobbered(VirtReg);
   
          // Check to see if this instruction is a load from a stack slot into
          // a register.  If so, this provides the stack slot value in the reg.
          int FrameIdx;
          if (unsigned DestReg = TII->isLoadFromStackSlot(&MI, FrameIdx)) {
            assert(DestReg == VirtReg && "Unknown load situation!");

            // If it is a folded reference, then it's not safe to clobber.
            bool Folded = FoldedSS.count(FrameIdx);
            // Otherwise, if it wasn't available, remember that it is now!
            Spills.addAvailable(FrameIdx, DestReg, !Folded);
            goto ProcessNextInst;
          }
              
          continue;
        }

        unsigned SubIdx = MO.getSubReg();
        bool DoReMat = VRM.isReMaterialized(VirtReg);
        if (DoReMat)
          ReMatDefs.insert(&MI);

        // The only vregs left are stack slot definitions.
        int StackSlot = VRM.getStackSlot(VirtReg);
        const TargetRegisterClass *RC = RegInfo->getRegClass(VirtReg);

        // If this def is part of a two-address operand, make sure to execute
        // the store from the correct physical register.
        unsigned PhysReg;
        unsigned TiedOp;
        if (MI.isRegTiedToUseOperand(i, &TiedOp)) {
          PhysReg = MI.getOperand(TiedOp).getReg();
          if (SubIdx) {
            unsigned SuperReg = findSuperReg(RC, PhysReg, SubIdx, TRI);
            assert(SuperReg && TRI->getSubReg(SuperReg, SubIdx) == PhysReg &&
                   "Can't find corresponding super-register!");
            PhysReg = SuperReg;
          }
        } else {
          PhysReg = VRM.getPhys(VirtReg);
          if (ReusedOperands.isClobbered(PhysReg)) {
            // Another def has taken the assigned physreg. It must have been a
            // use&def which got it due to reuse. Undo the reuse!
            PhysReg = ReusedOperands.GetRegForReload(PhysReg, &MI, 
                                 Spills, MaybeDeadStores, RegKills, KillOps, VRM);
          }
        }

        assert(PhysReg && "VR not assigned a physical register?");
        RegInfo->setPhysRegUsed(PhysReg);
        unsigned RReg = SubIdx ? TRI->getSubReg(PhysReg, SubIdx) : PhysReg;
        ReusedOperands.markClobbered(RReg);
        MI.getOperand(i).setReg(RReg);
        MI.getOperand(i).setSubReg(0);

        if (!MO.isDead()) {
          MachineInstr *&LastStore = MaybeDeadStores[StackSlot];
          SpillRegToStackSlot(MBB, MII, -1, PhysReg, StackSlot, RC, true,
                            LastStore, Spills, ReMatDefs, RegKills, KillOps, VRM);
          NextMII = next(MII);

          // Check to see if this is a noop copy.  If so, eliminate the
          // instruction before considering the dest reg to be changed.
          {
            unsigned Src, Dst, SrcSR, DstSR;
            if (TII->isMoveInstr(MI, Src, Dst, SrcSR, DstSR) && Src == Dst) {
              ++NumDCE;
              DOUT << "Removing now-noop copy: " << MI;
              InvalidateKills(MI, TRI, RegKills, KillOps);
              VRM.RemoveMachineInstrFromMaps(&MI);
              MBB.erase(&MI);
              Erased = true;
              UpdateKills(*LastStore, TRI, RegKills, KillOps);
              goto ProcessNextInst;
            }
          }
        }    
      }
    ProcessNextInst:
      DistanceMap.insert(std::make_pair(&MI, Dist++));
      if (!Erased && !BackTracked) {
        for (MachineBasicBlock::iterator II = &MI; II != NextMII; ++II)
          UpdateKills(*II, TRI, RegKills, KillOps);
      }
      MII = NextMII;
    }

  }

};

llvm::VirtRegRewriter* llvm::createVirtRegRewriter() {
  switch (RewriterOpt) {
  default: LLVM_UNREACHABLE("Unreachable!");
  case local:
    return new LocalRewriter();
  case trivial:
    return new TrivialRewriter();
  }
}