VirtRegMap.cpp

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//===-- llvm/CodeGen/VirtRegMap.cpp - Virtual Register Map ----------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements the VirtRegMap class.
//
// It also contains implementations of the the Spiller interface, which, given a
// virtual register map and a machine function, eliminates all virtual
// references by replacing them with physical register references - adding spill
// code as necessary.
//
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "spiller"
#include "VirtRegMap.h"
#include "llvm/Function.h"
#include "llvm/CodeGen/MachineFrameInfo.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/Target/TargetMachine.h"
#include "llvm/Target/TargetInstrInfo.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/Compiler.h"
#include "llvm/ADT/BitVector.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallSet.h"
#include <algorithm>
using namespace llvm;

STATISTIC(NumSpills  , "Number of register spills");
STATISTIC(NumPSpills , "Number of physical register spills");
STATISTIC(NumReMats  , "Number of re-materialization");
STATISTIC(NumDRM     , "Number of re-materializable defs elided");
STATISTIC(NumStores  , "Number of stores added");
STATISTIC(NumLoads   , "Number of loads added");
STATISTIC(NumReused  , "Number of values reused");
STATISTIC(NumDSE     , "Number of dead stores elided");
STATISTIC(NumDCE     , "Number of copies elided");
STATISTIC(NumDSS     , "Number of dead spill slots removed");
STATISTIC(NumCommutes, "Number of instructions commuted");

namespace {
  enum SpillerName { simple, local };
}

static cl::opt<SpillerName>
SpillerOpt("spiller",
           cl::desc("Spiller to use: (default: local)"),
           cl::Prefix,
           cl::values(clEnumVal(simple, "simple spiller"),
                      clEnumVal(local,  "local spiller"),
                      clEnumValEnd),
           cl::init(local));

//===----------------------------------------------------------------------===//
//  VirtRegMap implementation
//===----------------------------------------------------------------------===//

VirtRegMap::VirtRegMap(MachineFunction &mf)
  : TII(*mf.getTarget().getInstrInfo()), MF(mf), 
    Virt2PhysMap(NO_PHYS_REG), Virt2StackSlotMap(NO_STACK_SLOT),
    Virt2ReMatIdMap(NO_STACK_SLOT), Virt2SplitMap(0),
    Virt2SplitKillMap(0), ReMatMap(NULL), ReMatId(MAX_STACK_SLOT+1),
    LowSpillSlot(NO_STACK_SLOT), HighSpillSlot(NO_STACK_SLOT) {
  SpillSlotToUsesMap.resize(8);
  ImplicitDefed.resize(MF.getRegInfo().getLastVirtReg()+1-
                       TargetRegisterInfo::FirstVirtualRegister);
  grow();
}

void VirtRegMap::grow() {
  unsigned LastVirtReg = MF.getRegInfo().getLastVirtReg();
  Virt2PhysMap.grow(LastVirtReg);
  Virt2StackSlotMap.grow(LastVirtReg);
  Virt2ReMatIdMap.grow(LastVirtReg);
  Virt2SplitMap.grow(LastVirtReg);
  Virt2SplitKillMap.grow(LastVirtReg);
  ReMatMap.grow(LastVirtReg);
  ImplicitDefed.resize(LastVirtReg-TargetRegisterInfo::FirstVirtualRegister+1);
}

int VirtRegMap::assignVirt2StackSlot(unsigned virtReg) {
  assert(TargetRegisterInfo::isVirtualRegister(virtReg));
  assert(Virt2StackSlotMap[virtReg] == NO_STACK_SLOT &&
         "attempt to assign stack slot to already spilled register");
  const TargetRegisterClass* RC = MF.getRegInfo().getRegClass(virtReg);
  int SS = MF.getFrameInfo()->CreateStackObject(RC->getSize(),
                                                RC->getAlignment());
  if (LowSpillSlot == NO_STACK_SLOT)
    LowSpillSlot = SS;
  if (HighSpillSlot == NO_STACK_SLOT || SS > HighSpillSlot)
    HighSpillSlot = SS;
  unsigned Idx = SS-LowSpillSlot;
  while (Idx >= SpillSlotToUsesMap.size())
    SpillSlotToUsesMap.resize(SpillSlotToUsesMap.size()*2);
  Virt2StackSlotMap[virtReg] = SS;
  ++NumSpills;
  return SS;
}

void VirtRegMap::assignVirt2StackSlot(unsigned virtReg, int SS) {
  assert(TargetRegisterInfo::isVirtualRegister(virtReg));
  assert(Virt2StackSlotMap[virtReg] == NO_STACK_SLOT &&
         "attempt to assign stack slot to already spilled register");
  assert((SS >= 0 ||
          (SS >= MF.getFrameInfo()->getObjectIndexBegin())) &&
         "illegal fixed frame index");
  Virt2StackSlotMap[virtReg] = SS;
}

int VirtRegMap::assignVirtReMatId(unsigned virtReg) {
  assert(TargetRegisterInfo::isVirtualRegister(virtReg));
  assert(Virt2ReMatIdMap[virtReg] == NO_STACK_SLOT &&
         "attempt to assign re-mat id to already spilled register");
  Virt2ReMatIdMap[virtReg] = ReMatId;
  return ReMatId++;
}

void VirtRegMap::assignVirtReMatId(unsigned virtReg, int id) {
  assert(TargetRegisterInfo::isVirtualRegister(virtReg));
  assert(Virt2ReMatIdMap[virtReg] == NO_STACK_SLOT &&
         "attempt to assign re-mat id to already spilled register");
  Virt2ReMatIdMap[virtReg] = id;
}

int VirtRegMap::getEmergencySpillSlot(const TargetRegisterClass *RC) {
  std::map<const TargetRegisterClass*, int>::iterator I =
    EmergencySpillSlots.find(RC);
  if (I != EmergencySpillSlots.end())
    return I->second;
  int SS = MF.getFrameInfo()->CreateStackObject(RC->getSize(),
                                                RC->getAlignment());
  if (LowSpillSlot == NO_STACK_SLOT)
    LowSpillSlot = SS;
  if (HighSpillSlot == NO_STACK_SLOT || SS > HighSpillSlot)
    HighSpillSlot = SS;
  EmergencySpillSlots[RC] = SS;
  return SS;
}

void VirtRegMap::addSpillSlotUse(int FI, MachineInstr *MI) {
  if (!MF.getFrameInfo()->isFixedObjectIndex(FI)) {
    // If FI < LowSpillSlot, this stack reference was produced by
    // instruction selection and is not a spill
    if (FI >= LowSpillSlot) {
      assert(FI >= 0 && "Spill slot index should not be negative!");
      assert((unsigned)FI-LowSpillSlot < SpillSlotToUsesMap.size()
             && "Invalid spill slot");
      SpillSlotToUsesMap[FI-LowSpillSlot].insert(MI);
    }
  }
}

void VirtRegMap::virtFolded(unsigned VirtReg, MachineInstr *OldMI,
                            MachineInstr *NewMI, ModRef MRInfo) {
  // Move previous memory references folded to new instruction.
  MI2VirtMapTy::iterator IP = MI2VirtMap.lower_bound(NewMI);
  for (MI2VirtMapTy::iterator I = MI2VirtMap.lower_bound(OldMI),
         E = MI2VirtMap.end(); I != E && I->first == OldMI; ) {
    MI2VirtMap.insert(IP, std::make_pair(NewMI, I->second));
    MI2VirtMap.erase(I++);
  }

  // add new memory reference
  MI2VirtMap.insert(IP, std::make_pair(NewMI, std::make_pair(VirtReg, MRInfo)));
}

void VirtRegMap::virtFolded(unsigned VirtReg, MachineInstr *MI, ModRef MRInfo) {
  MI2VirtMapTy::iterator IP = MI2VirtMap.lower_bound(MI);
  MI2VirtMap.insert(IP, std::make_pair(MI, std::make_pair(VirtReg, MRInfo)));
}

void VirtRegMap::RemoveMachineInstrFromMaps(MachineInstr *MI) {
  for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
    MachineOperand &MO = MI->getOperand(i);
    if (!MO.isFI())
      continue;
    int FI = MO.getIndex();
    if (MF.getFrameInfo()->isFixedObjectIndex(FI))
      continue;
    // This stack reference was produced by instruction selection and
    // is not a spill
    if (FI < LowSpillSlot)
      continue;
    assert((unsigned)FI-LowSpillSlot < SpillSlotToUsesMap.size()
           && "Invalid spill slot");
    SpillSlotToUsesMap[FI-LowSpillSlot].erase(MI);
  }
  MI2VirtMap.erase(MI);
  SpillPt2VirtMap.erase(MI);
  RestorePt2VirtMap.erase(MI);
  EmergencySpillMap.erase(MI);
}

void VirtRegMap::print(std::ostream &OS) const {
  const TargetRegisterInfo* TRI = MF.getTarget().getRegisterInfo();

  OS << "********** REGISTER MAP **********\n";
  for (unsigned i = TargetRegisterInfo::FirstVirtualRegister,
         e = MF.getRegInfo().getLastVirtReg(); i <= e; ++i) {
    if (Virt2PhysMap[i] != (unsigned)VirtRegMap::NO_PHYS_REG)
      OS << "[reg" << i << " -> " << TRI->getName(Virt2PhysMap[i])
         << "]\n";
  }

  for (unsigned i = TargetRegisterInfo::FirstVirtualRegister,
         e = MF.getRegInfo().getLastVirtReg(); i <= e; ++i)
    if (Virt2StackSlotMap[i] != VirtRegMap::NO_STACK_SLOT)
      OS << "[reg" << i << " -> fi#" << Virt2StackSlotMap[i] << "]\n";
  OS << '\n';
}

void VirtRegMap::dump() const {
  print(cerr);
}


//===----------------------------------------------------------------------===//
// Simple Spiller Implementation
//===----------------------------------------------------------------------===//

Spiller::~Spiller() {}

namespace {
  struct VISIBILITY_HIDDEN SimpleSpiller : public Spiller {
    bool runOnMachineFunction(MachineFunction& mf, VirtRegMap &VRM);
  };
}

bool SimpleSpiller::runOnMachineFunction(MachineFunction &MF, VirtRegMap &VRM) {
  DOUT << "********** REWRITE MACHINE CODE **********\n";
  DOUT << "********** Function: " << MF.getFunction()->getName() << '\n';
  const TargetMachine &TM = MF.getTarget();
  const TargetInstrInfo &TII = *TM.getInstrInfo();
  const TargetRegisterInfo &TRI = *TM.getRegisterInfo();
  

  // 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
  // operands).  This is always smaller than the number of operands to the
  // current machine instr, so it should be small.
  std::vector<unsigned> LoadedRegs;

  for (MachineFunction::iterator MBBI = MF.begin(), E = MF.end();
       MBBI != E; ++MBBI) {
    DOUT << MBBI->getBasicBlock()->getName() << ":\n";
    MachineBasicBlock &MBB = *MBBI;
    for (MachineBasicBlock::iterator MII = MBB.begin(),
           E = MBB.end(); MII != E; ++MII) {
      MachineInstr &MI = *MII;
      for (unsigned i = 0, e = MI.getNumOperands(); i != e; ++i) {
        MachineOperand &MO = MI.getOperand(i);
        if (MO.isReg() && MO.getReg()) {
          if (TargetRegisterInfo::isVirtualRegister(MO.getReg())) {
            unsigned VirtReg = MO.getReg();
            unsigned SubIdx = MO.getSubReg();
            unsigned PhysReg = VRM.getPhys(VirtReg);
            unsigned RReg = SubIdx ? TRI.getSubReg(PhysReg, SubIdx) : PhysReg;
            if (!VRM.isAssignedReg(VirtReg)) {
              int StackSlot = VRM.getStackSlot(VirtReg);
              const TargetRegisterClass* RC =
                MF.getRegInfo().getRegClass(VirtReg);

              if (MO.isUse() &&
                  std::find(LoadedRegs.begin(), LoadedRegs.end(), VirtReg)
                  == LoadedRegs.end()) {
                TII.loadRegFromStackSlot(MBB, &MI, PhysReg, StackSlot, RC);
                MachineInstr *LoadMI = prior(MII);
                VRM.addSpillSlotUse(StackSlot, LoadMI);
                LoadedRegs.push_back(VirtReg);
                ++NumLoads;
                DOUT << '\t' << *LoadMI;
              }

              if (MO.isDef()) {
                TII.storeRegToStackSlot(MBB, next(MII), PhysReg, true,
                                        StackSlot, RC);
                MachineInstr *StoreMI = next(MII);
                VRM.addSpillSlotUse(StackSlot, StoreMI);
                ++NumStores;
              }
            }
            MF.getRegInfo().setPhysRegUsed(RReg);
            MI.getOperand(i).setReg(RReg);
          } else {
            MF.getRegInfo().setPhysRegUsed(MO.getReg());
          }
        }
      }

      DOUT << '\t' << MI;
      LoadedRegs.clear();
    }
  }
  return true;
}

//===----------------------------------------------------------------------===//
//  Local Spiller Implementation
//===----------------------------------------------------------------------===//

namespace {
  class AvailableSpills;

  /// LocalSpiller - This spiller does a simple pass over the machine basic
  /// block to attempt to keep spills in registers as much as possible for
  /// blocks that have low register pressure (the vreg may be spilled due to
  /// register pressure in other blocks).
  class VISIBILITY_HIDDEN LocalSpiller : public Spiller {
    MachineRegisterInfo *RegInfo;
    const TargetRegisterInfo *TRI;
    const TargetInstrInfo *TII;
    DenseMap<MachineInstr*, unsigned> DistanceMap;
  public:
    bool runOnMachineFunction(MachineFunction &MF, VirtRegMap &VRM) {
      RegInfo = &MF.getRegInfo(); 
      TRI = MF.getTarget().getRegisterInfo();
      TII = MF.getTarget().getInstrInfo();
      DOUT << "\n**** Local spiller rewriting function '"
           << MF.getFunction()->getName() << "':\n";
      DOUT << "**** Machine Instrs (NOTE! Does not include spills and reloads!)"
              " ****\n";
      DEBUG(MF.dump());

      for (MachineFunction::iterator MBB = MF.begin(), E = MF.end();
           MBB != E; ++MBB)
        RewriteMBB(*MBB, VRM);

      // 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;
          }

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

      return true;
    }
  private:
    void TransferDeadness(MachineBasicBlock *MBB, unsigned CurDist,
                          unsigned Reg, BitVector &RegKills,
                          std::vector<MachineOperand*> &KillOps);
    bool PrepForUnfoldOpti(MachineBasicBlock &MBB,
                           MachineBasicBlock::iterator &MII,
                           std::vector<MachineInstr*> &MaybeDeadStores,
                           AvailableSpills &Spills, BitVector &RegKills,
                           std::vector<MachineOperand*> &KillOps,
                           VirtRegMap &VRM);
    bool CommuteToFoldReload(MachineBasicBlock &MBB,
                             MachineBasicBlock::iterator &MII,
                             unsigned VirtReg, unsigned SrcReg, int SS,
                             BitVector &RegKills,
                             std::vector<MachineOperand*> &KillOps,
                             const TargetRegisterInfo *TRI,
                             VirtRegMap &VRM);
    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);
    void RewriteMBB(MachineBasicBlock &MBB, VirtRegMap &VRM);
  };
}

/// AvailableSpills - As the local spiller is scanning and rewriting an MBB from
/// top down, keep track of which spills 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.
namespace {
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) {
  }
  
  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, MachineInstr *MI, 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";
  }

  /// canClobberPhysReg - 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 canClobberPhysReg(int SlotOrReMat) const {
    assert(SpillSlotsOrReMatsAvailable.count(SlotOrReMat) &&
           "Value not available!");
    return SpillSlotsOrReMatsAvailable.find(SlotOrReMat)->second & 1;
  }

  /// 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);
};
}

/// 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);
}

/// 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);
}



/// InvalidateKills - MI is going to be deleted. If any of its operands are
/// marked kill, then invalidate the information.
static void InvalidateKills(MachineInstr &MI, 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())
      continue;
    unsigned Reg = MO.getReg();
    if (TargetRegisterInfo::isVirtualRegister(Reg))
      continue;
    if (KillRegs)
      KillRegs->push_back(Reg);
    assert(Reg < KillOps.size());
    if (KillOps[Reg] == &MO) {
      RegKills.reset(Reg);
      KillOps[Reg] = NULL;
    }
  }
}

/// InvalidateKill - A MI that defines the specified register is being deleted,
/// invalidate the register kill information.
static void InvalidateKill(unsigned Reg, BitVector &RegKills,
                           std::vector<MachineOperand*> &KillOps) {
  if (RegKills[Reg]) {
    KillOps[Reg]->setIsKill(false);
    KillOps[Reg] = NULL;
    RegKills.reset(Reg);
  }
}

/// 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.isDef()) {
      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, BitVector &RegKills,
                        std::vector<MachineOperand*> &KillOps,
                        const TargetRegisterInfo* TRI) {
  const TargetInstrDesc &TID = MI.getDesc();
  for (unsigned i = 0, e = MI.getNumOperands(); i != e; ++i) {
    MachineOperand &MO = MI.getOperand(i);
    if (!MO.isReg() || !MO.isUse())
      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] = NULL;
      RegKills.reset(Reg);
      if (i < TID.getNumOperands() &&
          TID.getOperandConstraint(i, TOI::TIED_TO) == -1)
        // Unless it's a two-address operand, this is the new kill.
        MO.setIsKill();
    }
    if (MO.isKill()) {
      RegKills.set(Reg);
      KillOps[Reg] = &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 *AS = TRI->getAliasSet(Reg); *AS; ++AS) {
      RegKills.reset(*AS);
      KillOps[*AS] = 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);
  }
  ++NumReMats;
}


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

    /// 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);
    }
  };
}

/// PrepForUnfoldOpti - 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 LocalSpiller::PrepForUnfoldOpti(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